draft-ietf-mmusic-sip-08.txt   draft-ietf-mmusic-sip-09.txt 
Internet Engineering Task Force MMUSIC WG Internet Engineering Task Force MMUSIC WG
Internet Draft Handley/Schulzrinne/Schooler/Rosenberg Internet Draft Handley/Schulzrinne/Schooler/Rosenberg
ietf-mmusic-sip-08.txt ISI/Columbia U./Caltech/Bell Labs. ietf-mmusic-sip-09.txt ISI/Columbia U./Caltech/Bell Labs.
August 7, 1998 September 18, 1998
Expires: February 1999 Expires: February 1999
SIP: Session Initiation Protocol SIP: Session Initiation Protocol
STATUS OF THIS MEMO STATUS OF THIS MEMO
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts. working documents as Internet-Drafts.
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or calls. These multimedia sessions include multimedia conferences, or calls. These multimedia sessions include multimedia conferences,
distance learning, Internet telephony and similar applications. SIP distance learning, Internet telephony and similar applications. SIP
can invite both persons and "robots", such as a media storage can invite both persons and "robots", such as a media storage
service. SIP can invite parties to both unicast and multicast service. SIP can invite parties to both unicast and multicast
sessions; the initiator does not necessarily have to be a member of sessions; the initiator does not necessarily have to be a member of
the session to which it is inviting. Media and participants can be the session to which it is inviting. Media and participants can be
added to an existing session. added to an existing session.
SIP can be used to initiate sessions as well as invite members to SIP can be used to initiate sessions as well as invite members to
sessions that have been advertised and established by other means. sessions that have been advertised and established by other means.
Sessions may be advertised using multicast protocols such as SAP, Sessions can be advertised using multicast protocols such as SAP,
electronic mail, news groups, web pages or directories (LDAP), among electronic mail, news groups, web pages or directories (LDAP), among
others. others.
SIP transparently supports name mapping and redirection services, SIP transparently supports name mapping and redirection services,
allowing the implementation of ISDN and Intelligent Network telephony allowing the implementation of ISDN and Intelligent Network telephony
subscriber services. These facilities also enable personal mobility subscriber services. These facilities also enable personal mobility
services, this is defined as: "Personal mobility is the ability of services, this is defined as: "Personal mobility is the ability of
end users to originate and receive calls and access subscribed end users to originate and receive calls and access subscribed
telecommunication services on any terminal in any location, and the telecommunication services on any terminal in any location, and the
ability of the network to identify end users as they move. Personal ability of the network to identify end users as they move. Personal
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User availability: determination of the willingness of the called User availability: determination of the willingness of the called
party to engage in communications; party to engage in communications;
Call setup: "ringing", establishment of call parameters at both Call setup: "ringing", establishment of call parameters at both
called and calling party; called and calling party;
Call handling: including transfer and termination of calls. Call handling: including transfer and termination of calls.
SIP can also initiate multi-party calls using a multipoint control SIP can also initiate multi-party calls using a multipoint control
unit (MCU) or fully-meshed interconnection instead of multicast. unit (MCU) or fully-meshed interconnection instead of multicast.
Internet telephony gateways that connect PSTN parties may also use Internet telephony gateways that connect PSTN parties can also use
SIP to set up calls between them. SIP to set up calls between them.
SIP is designed as part of the overall IETF multimedia data and SIP is designed as part of the overall IETF multimedia data and
control architecture currently incorporating protocols such as RSVP control architecture currently incorporating protocols such as RSVP
(RFC 2205 [2]) for reserving network resources, the real-time (RFC 2205 [2]) for reserving network resources, the real-time
transport protocol (RTP) (RFC 1889 [3]) for transporting real-time transport protocol (RTP) (RFC 1889 [3]) for transporting real-time
data and providing QOS feedback, the real-time streaming protocol data and providing QOS feedback, the real-time streaming protocol
(RTSP) (RFC 2326 [4]) for controlling delivery of streaming media, (RTSP) (RFC 2326 [4]) for controlling delivery of streaming media,
the session announcement protocol (SAP) for advertising multimedia the session announcement protocol (SAP) for advertising multimedia
sessions via multicast and the session description protocol (SDP) sessions via multicast and the session description protocol (SDP)
(RFC 2327 [5]) for describing multimedia sessions. However, the (RFC 2327 [5]) for describing multimedia sessions. However, the
functionality and operation of SIP does not depend on any of these functionality and operation of SIP does not depend on any of these
protocols. protocols.
SIP may also be used in conjunction with other call setup and SIP can also be used in conjunction with other call setup and
signaling protocols. In that mode, an end system uses SIP exchanges signaling protocols. In that mode, an end system uses SIP exchanges
to determine the appropriate end system address and protocol from a to determine the appropriate end system address and protocol from a
given address that is protocol-independent. For example, SIP could be given address that is protocol-independent. For example, SIP could be
used to determine that the party may be reached via H.323, obtain the used to determine that the party can be reached via H.323, obtain the
H.245 gateway and user address and then use H.225.0 to establish the H.245 gateway and user address and then use H.225.0 to establish the
call. call.
In another example, it may be used to determine that the callee is In another example, SIP might be used to determine that the callee is
reachable via the public switched telephone network (PSTN) and reachable via the public switched telephone network (PSTN) and
indicate the phone number to be called, possibly suggesting an indicate the phone number to be called, possibly suggesting an
Internet-to-PSTN gateway to be used. Internet-to-PSTN gateway to be used.
SIP does not offer conference control services such as floor control SIP does not offer conference control services such as floor control
or voting and does not prescribe how a conference is to be managed, or voting and does not prescribe how a conference is to be managed,
but SIP can be used to introduce conference control protocols. SIP but SIP can be used to introduce conference control protocols. SIP
does not allocate multicast addresses. does not allocate multicast addresses.
SIP can invite users to sessions with and without resource SIP can invite users to sessions with and without resource
reservation. SIP does not reserve resources, but may convey to the reservation. SIP does not reserve resources, but can convey to the
invited system the information necessary to do this. Quality-of- invited system the information necessary to do this.
service guarantees, if required, may depend on knowing the full
membership of the session; this information may or may not be known
to the agent performing session invitation.
1.2 Terminology 1.2 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [6] and and "OPTIONAL" are to be interpreted as described in RFC 2119 [6] and
indicate requirement levels for compliant SIP implementations. indicate requirement levels for compliant SIP implementations.
1.3 Definitions 1.3 Definitions
This specification uses a number of terms to refer to the roles This specification uses a number of terms to refer to the roles
played by participants in SIP communications. The definitions of played by participants in SIP communications. The definitions of
client, server and proxy are similar to those used by the Hypertext client, server and proxy are similar to those used by the Hypertext
Transport Protocol (HTTP) (RFC 2068 [7]). The following terms have Transport Protocol (HTTP) (RFC 2068 [7]). The terms URI and URL are
special significance for SIP. defined in [8]. The following terms have special significance for
SIP.
Call: A call consists of all participants in a conference invited by Call: A call consists of all participants in a conference invited by
a common source. A SIP call is identified by a globally unique a common source. A SIP call is identified by a globally unique
call-id (Section 6.12). Thus, if a user is, for example, invited call-id (Section 6.12). Thus, if a user is, for example, invited
to the same multicast session by several people, each of these to the same multicast session by several people, each of these
invitations will be a unique call. A point-to-point Internet invitations will be a unique call. A point-to-point Internet
telephony conversation maps into a single SIP call. In a MCU- telephony conversation maps into a single SIP call. In a MCU-
based call-in conference, each participant uses a separate call based call-in conference, each participant uses a separate call
to invite himself to the MCU. to invite himself to the MCU.
Call leg: A call leg is identified by the combination of Call-ID, To Call leg: A call leg is identified by the combination of Call-ID, To
and From. and From.
Client: An application program that establishes connections for the Client: An application program that establishes connections for the
purpose of sending requests. Clients may or may not interact purpose of sending requests. Clients may or may not interact
directly with a human user. User agents and proxies contain directly with a human user. User agents and proxies contain
clients (and servers). clients (and servers).
Conference: A multimedia session (see below), identified by a common Conference: A multimedia session (see below), identified by a common
session description. A conference may have zero or more members session description. A conference can have zero or more members
and includes the cases of a multicast conference, a full-mesh and includes the cases of a multicast conference, a full-mesh
conference and a two-party "telephone call", as well as conference and a two-party "telephone call", as well as
combinations of these. Any number of calls may be used to combinations of these. Any number of calls can be used to
create a conference. create a conference.
Downstream: Requests sent in the direction from the caller to the Downstream: Requests sent in the direction from the caller to the
callee. callee.
Final response: A response that terminates a SIP transaction, as Final response: A response that terminates a SIP transaction, as
opposed to a provisional response that does not. All 2xx, 3xx, opposed to a provisional response that does not. All 2xx, 3xx,
4xx, 5xx and 6xx responses are final. 4xx, 5xx and 6xx responses are final.
Initiator, calling party, caller: The party initiating a conference Initiator, calling party, caller: The party initiating a conference
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Isomorphic request or response: Two requests or responses are defined Isomorphic request or response: Two requests or responses are defined
to be isomorphic for the purposes of this document if they have to be isomorphic for the purposes of this document if they have
the same values for the Call-ID, To, From and CSeq header the same values for the Call-ID, To, From and CSeq header
fields. In addition, requests have to have the same Request-URI. fields. In addition, requests have to have the same Request-URI.
Location server: See location service Location server: See location service
Location service: A location service is used by a SIP redirect or Location service: A location service is used by a SIP redirect or
proxy server to obtain information about a callee's possible proxy server to obtain information about a callee's possible
location(s). Location services are offered by location servers. location(s). Location services are offered by location servers.
Location servers may be co-located with a SIP server, but the Location servers MAY be co-located with a SIP server, but the
manner in which a SIP server requests location services is manner in which a SIP server requests location services is
beyond the scope of this document. beyond the scope of this document.
Parallel search: In a parallel search, a proxy issues several Parallel search: In a parallel search, a proxy issues several
requests to possible user locations upon receiving an incoming requests to possible user locations upon receiving an incoming
request. Rather than issuing one request and then waiting for request. Rather than issuing one request and then waiting for
the final response before issuing the next request as in a the final response before issuing the next request as in a
sequential search , a parallel search issues requests without sequential search , a parallel search issues requests without
waiting for the result of previous requests. waiting for the result of previous requests.
Provisional response: A response used by the server to indicate Provisional response: A response used by the server to indicate
progress, but that does not terminate a SIP transaction. 1xx progress, but that does not terminate a SIP transaction. 1xx
responses are provisional, other responses are considered final responses are provisional, other responses are considered final
Proxy, proxy server: An intermediary program that acts as both a Proxy, proxy server: An intermediary program that acts as both a
server and a client for the purpose of making requests on behalf server and a client for the purpose of making requests on behalf
of other clients. Requests are serviced internally or by passing of other clients. Requests are serviced internally or by passing
them on, possibly after translation, to other servers. A proxy them on, possibly after translation, to other servers. A proxy
must interpret, and, if necessary, rewrite a request message interprets, and, if necessary, rewrites a request message before
before forwarding it. forwarding it.
Redirect server: A redirect server is a server that accepts a SIP Redirect server: A redirect server is a server that accepts a SIP
request, maps the address into zero or more new addresses and request, maps the address into zero or more new addresses and
returns these addresses to the client. Unlike a proxy server , returns these addresses to the client. Unlike a proxy server ,
it does not initiate its own SIP request. Unlike a user agent it does not initiate its own SIP request. Unlike a user agent
server , it does not accept calls. server , it does not accept calls.
Registrar: A registrar is server that accepts REGISTER requests. A Registrar: A registrar is server that accepts REGISTER requests. A
registrar is typically co-located with a proxy or redirect registrar is typically co-located with a proxy or redirect
server and may offer location services. server and MAY offer location services.
Ringback: Ringback is the signaling tone produced by the calling Ringback: Ringback is the signaling tone produced by the calling
client's application indicating that a called party is being client's application indicating that a called party is being
alerted (ringing). alerted (ringing).
Server: A server is an application program that accepts requests in Server: A server is an application program that accepts requests in
order to service requests and sends back responses to those order to service requests and sends back responses to those
requests. Servers are either proxy, redirect or user agent requests. Servers are either proxy, redirect or user agent
servers or registrars. servers or registrars.
Session: "A multimedia session is a set of multimedia senders and Session: "A multimedia session is a set of multimedia senders and
receivers and the data streams flowing from senders to receivers and the data streams flowing from senders to
receivers. A multimedia conference is an example of a multimedia receivers. A multimedia conference is an example of a multimedia
session." (RFC 2327 [5]) (A session as defined for SDP may session." (RFC 2327 [5]) (A session as defined for SDP can
comprise one or more RTP sessions.) As defined, a callee may be comprise one or more RTP sessions.) As defined, a callee can be
invited several times, by different calls, to the same session. invited several times, by different calls, to the same session.
If SDP is used, a session is defined by the concatenation of the If SDP is used, a session is defined by the concatenation of the
user name , session id , network type , address type and address user name , session id , network type , address type and address
elements in the origin field. elements in the origin field.
(SIP) transaction: A SIP transaction occurs between a client and a (SIP) transaction: A SIP transaction occurs between a client and a
server and comprises all messages from the first request sent server and comprises all messages from the first request sent
from the client to the server up to a final (non-1xx) response from the client to the server up to a final (non-1xx) response
sent from the server to the client. A transaction is identified sent from the server to the client. A transaction is identified
by the CSeq sequence number (Section 6.16) within a single call by the CSeq sequence number (Section 6.17) within a single call
leg The ACK request has the same CSeq number as the leg The ACK request has the same CSeq number as the
corresponding INVITE request, but comprises a transaction of its corresponding INVITE request, but comprises a transaction of its
own. own.
Upstream: Responses sent in the direction from the called client to Upstream: Responses sent in the direction from the called client to
the caller. the caller.
URL-encoded: A character string encoded according to RFC 1738, URL-encoded: A character string encoded according to RFC 1738,
Section 2.2 [8]. Section 2.2 [9].
User agent client (UAC), calling user agent: A user agent client is a User agent client (UAC), calling user agent: A user agent client is a
client application that initiates the SIP request. client application that initiates the SIP request.
User agent server (UAS), called user agent: A user agent server is a User agent server (UAS), called user agent: A user agent server is a
server application that contacts the user when a SIP request is server application that contacts the user when a SIP request is
received and that returns a response on behalf of the user. The received and that returns a response on behalf of the user. The
response may accept, reject or redirect the request. response accepts, rejects or redirects the request.
An application program may be capable of acting both as a client and An application program MAY be capable of acting both as a client and
a server. For example, a typical multimedia conference control a server. For example, a typical multimedia conference control
application would act as a user agent client to initiate calls or to application would act as a user agent client to initiate calls or to
invite others to conferences and as a user agent server to accept invite others to conferences and as a user agent server to accept
invitations. The properties of the different SIP server types are invitations. The properties of the different SIP server types are
summarized in Table 1. summarized in Table 1.
property redirect proxy user agent registrar property redirect proxy user agent registrar
server server server server server server
__________________________________________________________________________ __________________________________________________________________________
also acts as a SIP client no yes no no also acts as a SIP client no yes no no
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SIP request may be redirected or may trigger a chain of new SIP SIP request may be redirected or may trigger a chain of new SIP
requests by proxies (Section 1.4.5). Users can register their requests by proxies (Section 1.4.5). Users can register their
location(s) with SIP servers (Section 4.2.6). location(s) with SIP servers (Section 4.2.6).
1.4.1 SIP Addressing 1.4.1 SIP Addressing
The "objects" addressed by SIP are users at hosts, identified by a The "objects" addressed by SIP are users at hosts, identified by a
SIP URL. The SIP URL takes the form similar to a mailto or telnet SIP URL. The SIP URL takes the form similar to a mailto or telnet
URL, i.e., user@host user part is a user name, a civil name or a URL, i.e., user@host user part is a user name, a civil name or a
telephone number. The host part is either a domain name having a DNS telephone number. The host part is either a domain name having a DNS
SRV (RFC 2052 [9]), MX (RFC 974 [10], CNAME or A record (RFC 1035 SRV (RFC 2052 [10]), MX (RFC 974 [11], CNAME or A record (RFC 1035
[11]), or a numeric network address. [12]), or a numeric network address.
A user's SIP address can be obtained out-of-band, can be learned via A user's SIP address can be obtained out-of-band, can be learned via
existing media agents, can be included in some mailers' message existing media agents, can be included in some mailers' message
headers, or can be recorded during previous invitation interactions. headers, or can be recorded during previous invitation interactions.
In many cases, a user's SIP URL can be guessed from his email In many cases, a user's SIP URL can be guessed from his email
address. address.
Examples of SIP URLs include: Examples of SIP URLs include:
sip:mjh@metro.isi.edu sip:mjh@metro.isi.edu
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guessable from the person's name and organizational affiliation, the guessable from the person's name and organizational affiliation, the
traditional method of ensuring privacy by having an unlisted "phone" traditional method of ensuring privacy by having an unlisted "phone"
number is compromised. However, unlike traditional telephony, SIP number is compromised. However, unlike traditional telephony, SIP
offers authentication and access control mechanisms and can avail offers authentication and access control mechanisms and can avail
itself of lower-layer security mechanisms, so that client software itself of lower-layer security mechanisms, so that client software
can reject unauthorized or undesired call attempts. can reject unauthorized or undesired call attempts.
1.4.2 Locating a SIP Server 1.4.2 Locating a SIP Server
When a client wishes to send a request, the client either sends it to When a client wishes to send a request, the client either sends it to
locally configured SIP proxy server (as in HTTP), independent of the a locally configured SIP proxy server (as in HTTP), independent of
Request-URI, or sends it to the IP address and port corresponding to the Request-URI, or sends it to the IP address and port corresponding
the Request-URI. For the latter case, the client performs the to the Request-URI. For the latter case, the client performs the
following steps to obtain the server's IP address. following steps to obtain the server's IP address.
A SIP client MUST follow the following steps to resolve the host part A SIP client MUST follow the following steps to resolve the host part
of the Request-URI. If a client supports only TCP or UDP, but not of the Request-URI. If a client supports only TCP or UDP, but not
both, the client omits the respective address type. If the SIP both, the client omits the respective address type. If the SIP
address contains a port number, that number is to be used, otherwise, address contains a port number, that number is to be used, otherwise,
the default port number 5060 is to be used. The default port number the default port number 5060 is to be used. The default port number
is the same for UDP and TCP. In all cases, the client first attempts is the same for UDP and TCP. In all cases, the client first attempts
to contact the server using UDP, then TCP. to contact the server using UDP, then TCP.
A client SHOULD rely on ICMP "Port Unreachable" messages rather than A client SHOULD rely on ICMP "Port Unreachable" messages rather than
time-outs to determine that a server is not reachable at a particular time-outs to determine that a server is not reachable at a particular
address. (For socket-based programs: For TCP, connect() returns address. (For socket-based programs: For TCP, connect() returns
ECONNREFUSED if there is no server at the designated address; for ECONNREFUSED if there is no server at the designated address; for
UDP, the socket should be bound to the destination address using UDP, the socket needs to be bound to the destination address using
connect() rather than sendto() or similar so that a second write() connect() rather than sendto() or similar so that a second write()
fails with ECONNREFUSED. ) fails with ECONNREFUSED. )
If the SIP address contains a numeric IP address, the client contacts If the SIP address contains a numeric IP address, the client contacts
the SIP server at that address. Otherwise, the client follows the the SIP server at that address. Otherwise, the client follows the
steps below. steps below.
1. If there is a SRV DNS resource record (RFC 2052 [9]) of 1. If there is a SRV DNS resource record (RFC 2052 [10]) of
type sip.udp, contact the listed SIP servers in the order type sip.udp or type sip.tcp, order all such records by
of the preference values contained in those resource their priority value and attempt to contact the servers in
records, using UDP as a transport protocol at the port that order. If a port number is explicitly specified in the
number given in the URL or, if none provided, the one SIP URL, it overrides the port number in the SRV record. It
listed in the DNS resource record. is RECOMMENDED that DNS zone files give higher weight to
servers running UDP than those running TCP. If a server
2. If there is a SRV DNS resource record (RFC 2052 [9]) of responds, skip the remaining steps below.
type sip.tcp, contact the listed SIP servers in the order
of the preference value contained in those resource
records, using TCP as a transport protocol at the port
number given in the URL or, if none provided, the one
listed in the DNS resource record.
3. If there is a DNS MX record (RFC 974 [10]), contact the 2. If there is a DNS MX record (RFC 974 [11]), contact the
hosts listed in their order of preference at the port hosts listed in their order of preference at the port
number listed in the URL or the default SIP port number if number listed in the URL or the default SIP port number if
none. For each host listed, first try to contact the SIP none. For each host listed, first try to contact the SIP
server using UDP, then TCP. server using UDP, then TCP. If a server responds, skip the
remaining steps.
4. Finally, check if there is a DNS CNAME or A record for the 3. Finally, check if there is a DNS CNAME or A record for the
given host and try to contact a SIP server at the one or given host and try to contact a SIP server at the one or
more addresses listed, again trying first UDP, then TCP. more addresses listed, again trying first UDP, then TCP. If
a server responds, skip the remaining step.
If all of the above methods fail to locate a server, the caller MAY 4. If all of the above methods fail to locate a server, the
contact an SMTP server at the user's host and use the SMTP EXPN caller MAY contact an SMTP server at the user's host and
command to obtain an alternate address and repeat the steps above. As use the SMTP EXPN command to obtain an alternate address
a last resort, a client MAY choose to deliver the session description and repeat the steps above. As a last resort, a client MAY
to the callee using electronic mail. choose to deliver the session description to the callee
using electronic mail.
A client MAY cache the result of the reachability steps for a A client MAY cache the result of the reachability steps for a
particular address and retry that host address for the next request. particular address and retry that host address for the next request.
If the client does not find a SIP server at the cached address, it If the client does not find a SIP server at the cached address, it
MUST start the search at the beginning of the sequence. MUST start the search at the beginning of the sequence.
This sequence is modeled after that described for SMTP, This sequence is modeled after that described for SMTP,
where MX records are to be checked before A records (RFC where MX records are to be checked before A records (RFC
1123 [12]). 1123 [13]).
1.4.3 SIP Transaction 1.4.3 SIP Transaction
Once the host part has been resolved to a SIP server, the client Once the host part has been resolved to a SIP server, the client
sends one or more SIP requests to that server and receives one or sends one or more SIP requests to that server and receives one or
more responses from the server. A request (and its retransmissions) more responses from the server. A request (and its retransmissions)
together with the responses triggered by that request make up a SIP together with the responses triggered by that request make up a SIP
transaction. The ACK request following an INVITE is not part of the transaction. The ACK request following an INVITE is not part of the
transaction since it may traverse a different set of hosts. transaction since it may traverse a different set of hosts.
If TCP is used, request and responses within a single SIP transaction If TCP is used, request and responses within a single SIP transaction
are carried over the same TCP connection (see Section 10). Several are carried over the same TCP connection (see Section 10). Several
SIP requests from the same client to the same server may use the same SIP requests from the same client to the same server MAY use the same
TCP connection or may open a new connection for each request. TCP connection or MAY open a new connection for each request.
If the client sent the request via unicast UDP, the response is sent If the client sent the request via unicast UDP, the response is sent
to the address contained in the next Via header field (Section 6.40) to the address contained in the next Via header field (Section 6.40)
of the response. If the request is sent via multicast UDP, the of the response. If the request is sent via multicast UDP, the
response is directed to the same multicast address and destination response is directed to the same multicast address and destination
port. For UDP, reliability is achieved using retransmission (Section port. For UDP, reliability is achieved using retransmission (Section
10). 10).
The SIP message format and operation is independent of the transport The SIP message format and operation is independent of the transport
protocol. protocol.
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particular conference or establish a two-party conversation. After particular conference or establish a two-party conversation. After
the callee has agreed to participate in the call, the caller confirms the callee has agreed to participate in the call, the caller confirms
that it has received that response by sending an ACK (Section 4.2.2) that it has received that response by sending an ACK (Section 4.2.2)
request. If the caller no longer wants to participate in the call, it request. If the caller no longer wants to participate in the call, it
sends a BYE request instead of an ACK. sends a BYE request instead of an ACK.
The INVITE request typically contains a session description, for The INVITE request typically contains a session description, for
example written in SDP (RFC 2327 [5]) format, that provides the example written in SDP (RFC 2327 [5]) format, that provides the
called party with enough information to join the session. For called party with enough information to join the session. For
multicast sessions, the session description enumerates the media multicast sessions, the session description enumerates the media
types and formats that may be distributed to that session. For a types and formats that are allowed to be distributed to that session.
unicast session, the session description enumerates the media types For a unicast session, the session description enumerates the media
and formats that the caller is willing to receive and where it wishes types and formats that the caller is willing to receive and where it
the media data to be sent. In either case, if the callee wishes to wishes the media data to be sent. In either case, if the callee
accept the call, it responds to the invitation by returning a similar wishes to accept the call, it responds to the invitation by returning
description listing the media it wishes to receive. For a multicast a similar description listing the media it wishes to receive. For a
session, the callee should only return a session description if it is multicast session, the callee SHOULD only return a session
unable to receive the media indicated in the caller's description or description if it is unable to receive the media indicated in the
wants to receive data via unicast. caller's description or wants to receive data via unicast.
The protocol exchanges for the INVITE method are shown in Fig. 1 for The protocol exchanges for the INVITE method are shown in Fig. 1 for
a proxy server and in Fig. 2 for a redirect server. (Note that the a proxy server and in Fig. 2 for a redirect server. (Note that the
messages shown in the figures have been abbreviated slightly.) In messages shown in the figures have been abbreviated slightly.) In
Fig. 1, the proxy server accepts the INVITE request (step 1), Fig. 1, the proxy server accepts the INVITE request (step 1),
contacts the location service with all or parts of the address (step contacts the location service with all or parts of the address (step
2) and obtains a more precise location (step 3). The proxy server 2) and obtains a more precise location (step 3). The proxy server
then issues a SIP INVITE request to the address(es) returned by the then issues a SIP INVITE request to the address(es) returned by the
location service (step 4). The user agent server alerts the user location service (step 4). The user agent server alerts the user
(step 5) and returns a success indication to the proxy server (step (step 5) and returns a success indication to the proxy server (step
skipping to change at page 12, line 20 skipping to change at page 12, line 14
server (step 6). In the example, the call succeeds (step 7). The server (step 6). In the example, the call succeeds (step 7). The
caller and callee complete the handshake with an ACK (step 8). caller and callee complete the handshake with an ACK (step 8).
The next section discusses what happens if the location service The next section discusses what happens if the location service
returns more than one possible alternative. returns more than one possible alternative.
1.4.5 Locating a User 1.4.5 Locating a User
A callee may move between a number of different end systems over A callee may move between a number of different end systems over
time. These locations can be dynamically registered with the SIP time. These locations can be dynamically registered with the SIP
server (Sections 1.4.7, 4.2.6). A location server may also use one or server (Sections 1.4.7, 4.2.6). A location server MAY also use one or
more other protocols, such as finger (RFC 1288 [13]), rwhois (RFC more other protocols, such as finger (RFC 1288 [14]), rwhois (RFC
2167 [14]), LDAP (RFC 1777 [15]), multicast-based protocols [16] or 2167 [15]), LDAP (RFC 1777 [16]), multicast-based protocols [17] or
operating-system dependent mechanisms to actively determine the end operating-system dependent mechanisms to actively determine the end
system where a user might be reachable. A location server may return system where a user might be reachable. A location server MAY return
several locations because the user is logged in at several hosts several locations because the user is logged in at several hosts
simultaneously or because the location server has (temporarily) simultaneously or because the location server has (temporarily)
inaccurate information. The SIP server combines the results to yield inaccurate information. The SIP server combines the results to yield
a list of a zero or more locations. It is recommended that each a list of a zero or more locations. It is recommended that each
location server sorts results according to the likelihood of success. location server sorts results according to the likelihood of success.
The action taken on receiving a list of locations varies with the The action taken on receiving a list of locations varies with the
type of SIP server. A SIP redirect server returns the list to the type of SIP server. A SIP redirect server returns the list to the
client as Location headers (Section 6.22). A SIP proxy server can client as Contact headers (Section 6.13). A SIP proxy server can
sequentially or in parallel try the addresses until the call is sequentially or in parallel try the addresses until the call is
successful (2xx response) or the callee has declined the call (6xx successful (2xx response) or the callee has declined the call (6xx
response). With sequential attempts, a proxy server can implement an response). With sequential attempts, a proxy server can implement an
"anycast" service. "anycast" service.
If a proxy server forwards a SIP request, it MUST add itself to the If a proxy server forwards a SIP request, it MUST add itself to the
end of the list of forwarders noted in the Via (Section 6.40) end of the list of forwarders noted in the Via (Section 6.40)
headers. The Via trace ensures that replies can take the same path headers. The Via trace ensures that replies can take the same path
back, ensuring correct operation through compliant firewalls and back, ensuring correct operation through compliant firewalls and
avoiding request loops. On the response path, each host MUST remove avoiding request loops. On the response path, each host MUST remove
its Via, so that routing internal information is hidden from the its Via, so that routing internal information is hidden from the
callee and outside networks. A proxy server MUST check that it does callee and outside networks. A proxy server MUST check that it does
not generate a request to a host listed in the Via sent-by, via- not generate a request to a host listed in the Via sent-by, via-
received or via-maddr parameters (Section 6.40). (Note: If a host received or via-maddr parameters (Section 6.40). (Note: If a host has
has several names or network addresses, this may not always work. several names or network addresses, this does not always work. Thus,
Thus, each host also checks if it is part of the Via list.) each host also checks if it is part of the Via list.)
A SIP invitation may traverse more than one SIP proxy server. If one
of these "forks" the request, i.e., issues more than one request in
response to receiving the invitation request, it is possible that a
client is reached, independently, by more than one copy of the
invitation request. Each of these copies bears the same Call-ID. The
user agent MUST return the appropriate status response. Duplicate
+....... cs.columbia.edu .......+ +....... cs.columbia.edu .......+
: : : :
: (~~~~~~~~~~) : : (~~~~~~~~~~) :
: ( location ) : : ( location ) :
: ( service ) : : ( service ) :
: (~~~~~~~~~~) : : (~~~~~~~~~~) :
: ^ | : : ^ | :
: | hgs@play : : | hgs@play :
: 2| 3| : : 2| 3| :
: | | : : | | :
skipping to change at page 14, line 4 skipping to change at page 14, line 4
| : ( ) : | : ( ) :
| 8: ACK : ( ) : | 8: ACK : ( ) :
======================================================> (~~~~~~) : ======================================================> (~~~~~~) :
+...............................+ +...............................+
====> SIP request ====> SIP request
....> SIP response ....> SIP response
----> non-SIP protocols ----> non-SIP protocols
Figure 2: Example of SIP redirect server Figure 2: Example of SIP redirect server
A SIP invitation may traverse more than one SIP proxy server. If one
of these "forks" the request, i.e., issues more than one request in
response to receiving the invitation request, it is possible that a
client is reached, independently, by more than one copy of the
invitation request. Each of these copies bears the same Call-ID. The
user agent MUST return the appropriate status response. Duplicate
requests are not an error. requests are not an error.
1.4.6 Changing an Existing Session 1.4.6 Changing an Existing Session
In some circumstances, it may be necessary to change the parameters In some circumstances, it is desirable to change the parameters of an
of an existing session. For example, two parties may have been existing session. For example, two parties may have been conversing
conversing and then want to add a third party, switching to multicast and then want to add a third party, switching to multicast for
for efficiency. One of the participants invites the third party with efficiency. One of the participants invites the third party with the
the new multicast address and simultaneously sends an INVITE to the new multicast address and simultaneously sends an INVITE to the
second party, with the new multicast session description, but with second party, with the new multicast session description, but with
the old call identifier. the old call identifier.
1.4.7 Registration Services 1.4.7 Registration Services
The REGISTER request allows a client to let a proxy or redirect The REGISTER request allows a client to let a proxy or redirect
server know at which address(es) it may be reached. A client may also server know at which address(es) it can be reached. A client MAY also
use it to install call handling features at the server. use it to install call handling features at the server.
1.5 Protocol Properties 1.5 Protocol Properties
1.5.1 Minimal State 1.5.1 Minimal State
A single conference session or call may involve one or more SIP A single conference session or call involves one or more SIP
request-response transactions. Proxy servers do not have to keep request-response transactions. Proxy servers do not have to keep
state for a particular call, however, they MAY maintain state for a state for a particular call, however, they MAY maintain state for a
single SIP transaction, as discussed in Section 12. single SIP transaction, as discussed in Section 12. For efficiency, a
server MAY cache the results of location service requests.
For efficiency, a server may cache the results of location service
requests.
1.5.2 Lower-Layer-Protocol Neutral 1.5.2 Lower-Layer-Protocol Neutral
SIP makes minimal assumptions about the underlying transport and SIP makes minimal assumptions about the underlying transport and
network-layer protocols. The lower-layer may provide either a packet network-layer protocols. The lower-layer can provide either a packet
or a byte stream service, with reliable or unreliable service. or a byte stream service, with reliable or unreliable service.
In an Internet context, SIP is able to utilize both UDP and TCP as In an Internet context, SIP is able to utilize both UDP and TCP as
transport protocols, among others. UDP allows the application to more transport protocols, among others. UDP allows the application to more
carefully control the timing of messages and their retransmission, to carefully control the timing of messages and their retransmission, to
perform parallel searches without requiring TCP connection state for perform parallel searches without requiring TCP connection state for
each outstanding request, and to use multicast. Routers can more each outstanding request, and to use multicast. Routers can more
readily snoop SIP UDP packets. TCP allows easier passage through readily snoop SIP UDP packets. TCP allows easier passage through
existing firewalls, and given the similar protocol design, allows existing firewalls, and given the similar protocol design, allows
common servers for SIP, HTTP and the Real Time Streaming Protocol common servers for SIP, HTTP and the Real Time Streaming Protocol
(RTSP) (RFC 2326 [4]). (RTSP) (RFC 2326 [4]).
When TCP is used, SIP can use one or more connections to attempt to When TCP is used, SIP can use one or more connections to attempt to
contact a user or to modify parameters of an existing conference. contact a user or to modify parameters of an existing conference.
Different SIP requests for the same SIP call may use different TCP Different SIP requests for the same SIP call MAY use different TCP
connections or a single persistent connection, as appropriate. connections or a single persistent connection, as appropriate.
For concreteness, this document will only refer to Internet For concreteness, this document will only refer to Internet
protocols. However, SIP may also be used directly with protocols protocols. However, SIP MAY also be used directly with protocols
such as ATM AAL5, IPX, frame relay or X.25. The necessary naming such as ATM AAL5, IPX, frame relay or X.25. The necessary naming
conventions are beyond the scope of this document. User agents SHOULD conventions are beyond the scope of this document. User agents SHOULD
implement both UDP and TCP transport, proxy and redirect servers implement both UDP and TCP transport, proxy and redirect servers
MUST. MUST.
1.5.3 Text-Based 1.5.3 Text-Based
SIP is text-based, using ISO 10646 in UTF-8 encoding throughout. This SIP is text-based, using ISO 10646 in UTF-8 encoding throughout. This
allows easy implementation in languages such as Java, Tcl and Perl, allows easy implementation in languages such as Java, Tcl and Perl,
allows easy debugging, and most importantly, makes SIP flexible and allows easy debugging, and most importantly, makes SIP flexible and
extensible. As SIP is used for initiating multimedia conferences extensible. As SIP is used for initiating multimedia conferences
rather than delivering media data, it is believed that the additional rather than delivering media data, it is believed that the additional
overhead of using a text-based protocol is not significant. overhead of using a text-based protocol is not significant.
2 SIP Uniform Resource Locators 2 SIP Uniform Resource Locators
SIP URLs are used within SIP messages to indicate the originator SIP URLs are used within SIP messages to indicate the originator
(From), current destination (Request-URI) and final recipient (To) of (From), current destination (Request-URI) and final recipient (To) of
a SIP request, and to specify redirection addresses (Location). A SIP a SIP request, and to specify redirection addresses (Contact). A SIP
URL can also be embedded in web pages or other hyperlinks to indicate URL can also be embedded in web pages or other hyperlinks to indicate
that a user or service may be called. that a particular user or service can be called via SIP. When used
as a hyperlink, the SIP URL indicates the use of the INVITE method.
Because interaction with some resources may require message headers The SIP URL scheme is defined to allow setting SIP request-header
or message bodies to be specified as well as the SIP address, the SIP fields and the SIP message-body.
URL scheme is defined to allow setting SIP request-header fields and
the SIP message-body.
A SIP URL follows the guidelines of RFC 1630 [17], as revised [18], This corresponds to the use of mailto: URLs. It makes it
and has the syntax shown in Fig. 3. Note that reserved characters possible, for example, to specify the subject, urgency or
have to be escaped. media types of calls initiated through a web page or as
part of an email message.
A SIP URL follows the guidelines of RFC 2396 [18] and has the syntax
shown in Fig. 3. Note that reserved characters have to be escaped.
The URI character classes referenced above are described in Section The URI character classes referenced above are described in Section
C. The URI specification is currently being revised. It is C.
anticipated that future versions of this specification will reference
the revised edition. Note that all URL reserved characters MUST be userinfo: The SIP scheme MAY use the format "user:password" in the
userinfo field. The use of passwords in the userinfo is NOT
RECOMMENDED, because the passing of authentication information
in clear text (such as URIs) has proven to be a security risk in
almost every case where it has been used.
SIP-URL = "sip:" [ userinfo "@" ] hostport SIP-URL = "sip:" [ userinfo "@" ] hostport
url-parameters [ headers ] url-parameters [ headers ]
userinfo = user [ ":" password ] userinfo = user [ ":" password ]
user = *( unreserved | escaped user = *( unreserved | escaped
| "&" | "=" | "+" | "$" | "," ) | "&" | "=" | "+" | "$" | "," )
password = *( unreserved | escaped password = *( unreserved | escaped
| "&" | "=" | "+" | "$" | "," ) | "&" | "=" | "+" | "$" | "," )
hostport = host [ ":" port ] hostport = host [ ":" port ]
host = hostname | IPv4address host = hostname | IPv4address
hostname = *( domainlabel "." ) toplabel [ "." ] hostname = *( domainlabel "." ) toplabel [ "." ]
domainlabel = alphanum | alphanum *( alphanum | "-" ) alphanum domainlabel = alphanum | alphanum *( alphanum | "-" ) alphanum
toplabel = alpha | alpha *( alphanum | "-" ) alphanum toplabel = alpha | alpha *( alphanum | "-" ) alphanum
IPv4address = 1*digit "." 1*digit "." 1*digit "." 1*digit IPv4address = 1*digit "." 1*digit "." 1*digit "." 1*digit
port = *digit port = *digit
url-parameters = *( ";" url-parameter ) url-parameters = *( ";" url-parameter )
url-parameter = transport-param | user-param url-parameter = transport-param | user-param | method-param
| ttl-param | maddr-param | tag-param | other-param | ttl-param | maddr-param | other-param
transport-param = "transport=" ( "udp" | "tcp" ) transport-param = "transport=" ( "udp" | "tcp" )
user-param = "user=" ( "phone" | "ip" )
method-param = "method=" Method
ttl-param = "ttl=" ttl ttl-param = "ttl=" ttl
ttl = 1*3DIGIT ; 0 to 255 ttl = 1*3DIGIT ; 0 to 255
maddr-param = "maddr=" host maddr-param = "maddr=" host
user-param = "user=" ( "phone" $|$ "ip" )
tag-param = "tag=" UUID
UUID = 1*( hex | "-" )
other-param = *uric other-param = *uric
headers = "?" header *( "&" header ) headers = "?" header *( "&" header )
header = hname "=" hvalue header = hname "=" hvalue
hname = *uric hname = *uric
hvalue = *uric hvalue = *uric
uric = reserved | unreserved | escaped uric = reserved | unreserved | escaped
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" | reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
"$" | "," "$" | ","
digits = 1*DIGIT digits = 1*DIGIT
Figure 3: SIP URL syntax Figure 3: SIP URL syntax
If the host is an Internet telephony gateway, the user field MAY also
encode a telephone number using the notation of telephone-subscriber
(Fig. 4). The telephone number is a special case of a user name and
cannot be distinguished by a BNF. Thus, a URL parameter, user, is
added to distinguish telephone numbers from user names. The phone
identifier is to be used when connecting to a telephony gateway. Even
without this parameter, recipients of SIP URLs MAY interpret the
pre-@ part as a phone number if local restrictions on the name space
for user name allow it.
telephone-subscriber = global-phone-number | local-phone-number
global-phone-number = "+" 1*phonedigit [isdn-subaddress]
[post-dial]
local-phone-number = 1*(phonedigit | dtmf-digit |
pause-character) [isdn-subaddress]
[post-dial]
isdn-subaddress = ";isub=" 1*phonedigit
post-dial = ";postd=" 1*(phonedigit | dtmf-digit
| pause-character)
phonedigit = DIGIT | visual-separator
visual-separator = "-" | "."
pause-character = one-second-pause | wait-for-dial-tone
one-second-pause = "p"
wait-for-dial-tone = "w"
dtmf-digit = "*" | "#" | "A" | "B" | "C" | "D"
Figure 4: SIP URL syntax; telephone subscriber Figure 4: SIP URL syntax; telephone subscriber
encoded. If a server handles SIP addresses for another domain, it MUST URL-
encode the "@" character (%40). The ";" character MUST be URL-
encoded, as otherwise it is not possible to distinguish, in one
parsing pass, the case host;parameter and user;moreuser@host
host: The mailto: URL and RFC 822 email addresses require that host: The mailto: URL and RFC 822 email addresses require that
numeric host addresses ("host numbers") are enclosed in square numeric host addresses ("host numbers") are enclosed in square
brackets (presumably, since host names might be numeric), while brackets (presumably, since host names might be numeric), while
host numbers without brackets are used for all other URLs. The host numbers without brackets are used for all other URLs. The
SIP URL requires the latter form, without brackets. SIP URL requires the latter form, without brackets.
userinfo: The SIP scheme MAY use the format "user:password" in the port: If missing, the port number is assumed to be the SIP default
userinfo field. The use of passwords in the userinfo is NOT port, 5060.
RECOMMENDED, because the passing of authentication information
in clear text (such as URIs) has proven to be a security risk in
almost every case where it has been used.
If the host is an Internet telephony gateway, the userinfo field can
also encode a telephone number using the notation of telephone-
subscriber (Fig. 4). The telephone number is a special case of a
user name and cannot be distinguished by a BNF. Thus, a URL
parameter, user, is added to distinguish telephone numbers from user
names. The phone identifier is to be used when connecting to a
telephony gateway. Even without this parameter, recipients of SIP
URLs MAY interpret the pre-@ part as a phone number if local
restrictions on the name space for user name allow it.
If a server handles SIP addresses for another domain, it MUST URL-
encode the "@" character (%40). The ";" character MUST be URL-
encoded, as otherwise it is not possible to distinguish, in one
parsing pass, the case host;parameter and user;moreuser@host
URL parameters: SIP URLs can define specific parameters of the URL parameters: SIP URLs can define specific parameters of the
request: The transport parameter determines the the transport request. URL parameters are added after the host component and
mechanism (UDP or TCP). UDP is to be assumed when no explicit are separated by semi-colons. The transport parameter determines
transport parameter is included. The maddr parameter provides the the transport mechanism (UDP or TCP). UDP is to be assumed
the server address to be contacted for this user, overriding the when no explicit transport parameter is included. The maddr
address supplied in the host field. This address is typically a parameter provides the server address to be contacted for this
multicast address, but could also be the address of a backup user, overriding the address supplied in the host field. This
server. The ttl parameter determines the time-to-live value of address is typically a multicast address, but could also be the
the UDP multicast packet and MUST only be used if maddr is a address of a backup server. The ttl parameter determines the
multicast address and the transport protocol is UDP. The user time-to-live value of the UDP multicast packet and MUST only be
parameter was described above, the tag parameter is described in used if maddr is a multicast address and the transport protocol
its own section below. URL parameters are added after the host is UDP. The user parameter was described above. For example, to
component and are separated by semi-colons. For example, to
specify to call j.doe@big.com using multicast to 239.255.255.1 specify to call j.doe@big.com using multicast to 239.255.255.1
with a ttl of 15, the following URL would be used: with a ttl of 15, the following URL would be used:
sip:j.doe@big.com;maddr=239.255.255.1;ttl=15 sip:j.doe@big.com;maddr=239.255.255.1;ttl=15
The transport, maddr, and ttl parameters MUST NOT be used in the From The transport, maddr, and ttl parameters MUST NOT be used in the From
and To header fields and the Request-URI; they are ignored if and To header fields and the Request-URI; they are ignored if
present. present.
Headers: Headers of the SIP request can be defined with the "?" Headers: Headers of the SIP request can be defined with the "?"
mechanism within a SIP URL. The special hname "body" indicates mechanism within a SIP URL. The special hname "body" indicates
that the associated hvalue is the message-body of the SIP INVITE that the associated hvalue is the message-body of the SIP INVITE
request. Headers MUST NOT be used in the From and To header request. Headers MUST NOT be used in the From and To header
fields and the Request-URI; they are ignored if present. fields and the Request-URI; they are ignored if present.
Tag: The tag parameter serves as a general mechanism to distinguish Method: The method of the SIP request can be specified with the
multiple instances of a user identified by a single SIP URL. method parameter. This parameter MUST NOT be used in the From
Such distinction is needed in two cases. First, as proxies can and To header fields and the Request-URI; they are ignored if
fork requests, the same request can reach multiple instances of present.
a user (mobile and home phones, for example). As each can
respond, there needs to be a means to distinguish the responses
from each at the caller. The situation also arises with
multicast requests. The tag serves to distinguish responses at
the UAC. It MUST be placed in the To field of the response by
each instance when there is a possibility that the request was
forked at an intermediate proxy. This, in general, means that
the Tag MUST be inserted when the URL in the To does not refer
to a fully qualified hostname. The tag MUST be added by UAS,
registrars and redirect servers, but MUST NOT be inserted into
responses forwarded upstream by proxies. The Tag is added for
all responses for all methods. All subsequent transactions
between two entities MUST include the Tag parameter, as
described in Section 11.
Secondly, the tag MAY appear in the From field of a call invitation. Table 2 summarizes where the components of the SIP URL can be used
This is needed when it is anticipated that two instances of a user and what default values they assume if not present.
sharing a SIP address may make call invitations with the same Call-
ID. In this case, it MUST be present.
The use of version-1 (time based) or version-4 (random) UUID [19] is default Request-URI To From Contact external
OPTIONAL. The tag value is designed to be globally unique and user -- x x x x x
cryptographically random with at least 32 bits of randomness. It password -- x x x x
SHOULD NOT be included in long-lived SIP URLs, e.g., those found on host mandatory x x x x x
web pages or user databases. A single user maintains the same tag port 5060 x x x x x
throughout the call identified by the Call-ID. The tag parameter in user-param ip x x x x x
To headers is ignored when matching responses to requests that did method INVITE x x
not contain a tag in their To header. (See Section 6.37.) maddr-param -- x x
ttl-param 1 x x
transport-param -- x x
headers -- x x
Table 2 summarizes where the components of the SIP URL can be used. Table 2: Use and default values of URL components for SIP headers,
Request-URI and references
Examples of SIP URLs are: Examples of SIP URLs are:
sip:j.doe@big.com sip:j.doe@big.com
sip:j.doe:secret@big.com;transport=tcp sip:j.doe:secret@big.com;transport=tcp
sip:j.doe@big.com?subject=project sip:j.doe@big.com?subject=project
sip:+1-212-555-1212:1234@gateway.com;user=phone sip:+1-212-555-1212:1234@gateway.com;user=phone
Request-URI To From Location external
user x x x x x
password x x x
host x x x x x
user-param x x x x x
tag-param x x x x
maddr-param x x
ttl-param x x
transport-param x x
headers x x
Table 2: Use of URL components for SIP headers, Request-URI and
references
sip:1212@gateway.com sip:1212@gateway.com
sip:alice@10.1.2.3 sip:alice@10.1.2.3
sip:alice@example.com;tag=f81d4fae-7dec-11d0-a765-00a0c91e6bf6 sip:alice@example.com;tag=f81d4fae-7dec-11d0-a765-00a0c91e6bf6
sip:alice sip:alice
sip:alice@registrar.com;method=REGISTER
Within a SIP message, URLs are used to indicate the source and Within a SIP message, URLs are used to indicate the source and
intended destination of a request, redirection addresses and the intended destination of a request, redirection addresses and the
current destination of a request. Normally all these fields will current destination of a request. Normally all these fields will
contain SIP URLs. contain SIP URLs.
SIP URLs are case-insensitive, so that for example the two URLs SIP URLs are case-insensitive, so that for example the two URLs
sip:j.doe@example.com and SIP:J.Doe@Example.com are equivalent. All sip:j.doe@example.com and SIP:J.Doe@Example.com are equivalent. All
URL parameters are included when comparing SIP URLs for equality. URL parameters are included when comparing SIP URLs for equality.
In some circumstances a non-SIP URL may be used in a SIP message. An SIP header fields MAY contain non-SIP URLs. As an example, if a call
example might be making a call from a telephone which is relayed by a from a telephone is relayed to the Internet via SIP, the SIP From
gateway onto the internet as a SIP request. In such a case, the header field might contain a phone URL.
source of the call is really the telephone number of the caller, and
so a SIP URL is inappropriate and a phone URL might be used instead.
To allow for this flexibility, SIP headers that specify user
addresses allow these addresses to be SIP and non-SIP URLs.
Clearly not all URLs are appropriate to be used in a SIP message as a
user address. The correct behavior when an unknown scheme is
encountered by a SIP server is defined in the context of each of the
header fields that use a SIP URL.
3 SIP Message Overview 3 SIP Message Overview
SIP is a text-based protocol and uses the ISO 10646 character set in SIP is a text-based protocol and uses the ISO 10646 character set in
UTF-8 encoding (RFC 2279 [20]). Lines are terminated by CRLF, but UTF-8 encoding (RFC 2279 [20]). Lines are terminated by CRLF, but
receivers should be prepared to also interpret CR and LF by receivers MUST also interpret CR and LF by themselves as line
themselves as line terminators. terminators.
Except for the above difference in character sets, much of the Except for the above difference in character sets, much of the
message syntax is identical to HTTP/1.1; rather than repeating it message syntax is identical to HTTP/1.1; rather than repeating it
here we use [HX.Y] to refer to Section X.Y of the current HTTP/1.1 here we use [HX.Y] to refer to Section X.Y of the current HTTP/1.1
specification (RFC 2068 [7]). In addition, we describe SIP in both specification (RFC 2068 [7]). In addition, we describe SIP in both
prose and an augmented Backus-Naur form (BNF) [H2.1] described in prose and an augmented Backus-Naur form (BNF) [H2.1] described in
detail in RFC 2234 [21]. detail in RFC 2234 [21].
Unlike HTTP, SIP MAY use UDP. When sent over TCP or UDP, multiple SIP Unlike HTTP, SIP MAY use UDP. When sent over TCP or UDP, multiple SIP
transactions can be carried in a single TCP connection or UDP transactions can be carried in a single TCP connection or UDP
datagram. UDP datagrams, including all headers, should not normally datagram. UDP datagrams, including all headers, SHOULD NOT be larger
be larger than the path maximum transmission unit (MTU) if the MTU is than the path maximum transmission unit (MTU) if the MTU is known, or
known, or 1400 bytes if the MTU is unknown. 1400 bytes if the MTU is unknown.
The 1400 bytes accommodates lower-layer packet headers The 1400 bytes accommodates lower-layer packet headers
within the "typical" MTU of around 1500 bytes. Recent within the "typical" MTU of around 1500 bytes. Recent
studies [22] indicate that an MTU of 1500 bytes is a studies [22] indicate that an MTU of 1500 bytes is a
reasonable assumption. The next lower common MTU values are reasonable assumption. The next lower common MTU values are
1006 bytes for SLIP and 296 for low-delay PPP (RFC 1191 1006 bytes for SLIP and 296 for low-delay PPP (RFC 1191
[23]). Thus, another reasonable value would be a message [23]). Thus, another reasonable value would be a message
size of 950 bytes, to accommodate packet headers within the size of 950 bytes, to accommodate packet headers within the
SLIP MTU without fragmentation. SLIP MTU without fragmentation.
skipping to change at page 20, line 42 skipping to change at page 20, line 20
SIP-message ___ Request | Response SIP-message ___ Request | Response
Both Request (section 4) and Response (section 5) messages use the Both Request (section 4) and Response (section 5) messages use the
generic-message format of RFC 822 [24] for transferring entities (the generic-message format of RFC 822 [24] for transferring entities (the
body of the message). Both types of messages consist of a start-line, body of the message). Both types of messages consist of a start-line,
one or more header fields (also known as "headers"), an empty line one or more header fields (also known as "headers"), an empty line
(i.e., a line with nothing preceding the carriage-return line-feed (i.e., a line with nothing preceding the carriage-return line-feed
(CRLF)) indicating the end of the header fields, and an optional (CRLF)) indicating the end of the header fields, and an optional
message-body. To avoid confusion with similar-named headers in HTTP, message-body. To avoid confusion with similar-named headers in HTTP,
we refer to the header describing the message body as entity headers. we refer to the headers describing the message body as entity
These components are described in detail in the upcoming sections. headers. These components are described in detail in the upcoming
sections.
generic-message = start-line generic-message = start-line
*message-header *message-header
CRLF CRLF
[ message-body ] [ message-body ]
start-line = Request-Line | Section 4.1 start-line = Request-Line | Section 4.1
Status-Line Section 5.1 Status-Line Section 5.1
message-header = ( general-header message-header = ( general-header
| request-header | request-header
| response-header | response-header
| entity-header ) | entity-header )
In the interest of robustness, any leading empty line(s) MUST be In the interest of robustness, any leading empty line(s) MUST be In
ignored. In other words, if the Request or Response message begins other words, if the Request or Response message begins with a CRLF,
with a CRLF, CR, or LF, these characters should be ignored. CR, or LF, these characters MUST be ignored.
4 Request 4 Request
The Request message format is shown below:
Request = Request-Line ; Section 4.1
*( general-header
| request-header
| entity-header )
CRLF
[ message-body ] ; Section 8
4.1 Request-Line
The Request-Line begins with a method token, followed by the
Request-URI and the protocol version, and ending with CRLF. The
elements are separated by SP characters. No CR or LF are allowed
except in the final CRLF sequence.
Request-Line = Method SP Request-URI SP SIP-Version CRLF
4.2 Methods
general-header = Call-ID ; Section 6.12 general-header = Call-ID ; Section 6.12
| CSeq ; Section 6.16 | Contact ; Section 6.13
| Date ; Section 6.17 | CSeq ; Section 6.17
| Encryption ; Section 6.18 | Date ; Section 6.18
| Expires ; Section 6.19 | Encryption ; Section 6.19
| From ; Section 6.20 | Expires ; Section 6.20
| From ; Section 6.21
| Record-Route ; Section 6.29 | Record-Route ; Section 6.29
| Timestamp ; Section 6.36 | Timestamp ; Section 6.36
| To ; Section 6.37 | To ; Section 6.37
| Via ; Section 6.40 | Via ; Section 6.40
entity-header = Content-Encoding ; Section 6.13 entity-header = Content-Encoding ; Section 6.14
| Content-Length ; Section 6.14 | Content-Length ; Section 6.15
| Content-Type ; Section 6.15 | Content-Type ; Section 6.16
request-header = Accept ; Section 6.7 request-header = Accept ; Section 6.7
| Accept-Encoding ; Section 6.8 | Accept-Encoding ; Section 6.8
| Accept-Language ; Section 6.9 | Accept-Language ; Section 6.9
| Authorization ; Section 6.11 | Authorization ; Section 6.11
| Hide ; Section 6.21 | Contact ; Section 6.13
| Location ; Section 6.22 | Hide ; Section 6.22
| Max-Forwards ; Section 6.23 | Max-Forwards ; Section 6.23
| Organization ; Section 6.24 | Organization ; Section 6.24
| Priority ; Section 6.25 | Priority ; Section 6.25
| Proxy-Authorization ; Section 6.27 | Proxy-Authorization ; Section 6.27
| Proxy-Require ; Section 6.28 | Proxy-Require ; Section 6.28
| Route ; Section 6.33 | Route ; Section 6.33
| Require ; Section 6.30 | Require ; Section 6.30
| Response-Key ; Section 6.31 | Response-Key ; Section 6.31
| Subject ; Section 6.35 | Subject ; Section 6.35
| User-Agent ; Section 6.39 | User-Agent ; Section 6.39
response-header = Allow ; Section 6.10 response-header = Allow ; Section 6.10
| Location ; Section 6.22
| Proxy-Authenticate ; Section 6.26 | Proxy-Authenticate ; Section 6.26
| Retry-After ; Section 6.32 | Retry-After ; Section 6.32
| Server ; Section 6.34 | Server ; Section 6.34
| Unsupported ; Section 6.38 | Unsupported ; Section 6.38
| Warning ; Section 6.41 | Warning ; Section 6.41
| WWW-Authenticate ; Section 6.42 | WWW-Authenticate ; Section 6.42
Table 3: SIP headers Table 3: SIP headers
The Request message format is shown below:
Request = Request-Line ; Section 4.1
*( general-header
| request-header
| entity-header )
CRLF
[ message-body ] ; Section 8
4.1 Request-Line
The Request-Line begins with a method token, followed by the
Request-URI and the protocol version, and ending with CRLF. The
elements are separated by SP characters. No CR or LF are allowed
except in the final CRLF sequence.
Request-Line = Method SP Request-URI SP SIP-Version CRLF
4.2 Methods
The methods are defined below. Methods that are not supported by a The methods are defined below. Methods that are not supported by a
proxy or redirect server are treated by that server as if they were proxy or redirect server are treated by that server as if they were
an OPTIONS method and forwarded accordingly. Methods that are not an OPTIONS method and forwarded accordingly. Methods that are not
supported by a user agent server or registrar cause a 501 (Not supported by a user agent server or registrar cause a 501 (Not
Implemented) response to be returned (Section 7). Implemented) response to be returned (Section 7).
Method = "ACK" | "BYE" | "CANCEL" | "INVITE" Method = "ACK" | "BYE" | "CANCEL" | "INVITE"
| "OPTIONS" | "REGISTER" | "OPTIONS" | "REGISTER"
4.2.1 INVITE 4.2.1 INVITE
The INVITE method indicates that the user or service is being invited The INVITE method indicates that the user or service is being invited
to participate in a session. The message body contains a description to participate in a session. The message body contains a description
of the session to which the callee is being invited. For two-party of the session to which the callee is being invited. For two-party
calls, the caller indicates the type of media it is able to receive calls, the caller indicates the type of media it is able to receive
as well as their parameters such as network destination. If the as well as their parameters such as network destination. A success
session description format allows this, it may also indicate "send- response indicates in its message body which media the callee wishes
only" media. A success response indicates in its message body which to receive.
media the callee wishes to receive.
A server MAY automatically respond to an invitation for a conference A server MAY automatically respond to an invitation for a conference
the user is already participating in, identified either by the SIP the user is already participating in, identified either by the SIP
Call-ID or a globally unique identifier within the session Call-ID or a globally unique identifier within the session
description, with a 200 (OK) response. description, with a 200 (OK) response.
If a user agent receives an INVITE request for an existing Call-ID If a user agent receives an INVITE request for an existing Call-ID
with a higher CSeq sequence number than any previous INVITE for the with a higher CSeq sequence number than any previous INVITE for the
same Call-ID, it MUST check any version identifiers in the session same Call-ID, it MUST check any version identifiers in the session
description or, if there are no version identifiers, the content of description or, if there are no version identifiers, the content of
the session description to see if it has changed. It MUST also the session description to see if it has changed. It MUST also
inspect any other header fields for changes and act accordingly. If inspect any other header fields for changes and act accordingly. If
the session description has changed, the user agent server MUST the session description has changed, the user agent server MUST
adjust the session parameters accordingly, possibly after asking the adjust the session parameters accordingly, possibly after asking the
user for confirmation. (Versioning of the session description may be user for confirmation. (Versioning of the session description can be
used to accommodate the capabilities of new arrivals to a conference, used to accommodate the capabilities of new arrivals to a conference,
add or delete media or change from a unicast to a multicast add or delete media or change from a unicast to a multicast
conference.) conference.)
This method MUST be supported by SIP proxy, redirect and user agent This method MUST be supported by SIP proxy, redirect and user agent
servers as well as clients. servers as well as clients.
4.2.2 ACK 4.2.2 ACK
The ACK request confirms that the client has received a final The ACK request confirms that the client has received a final
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receive the response. The Via is always initialized to the host that receive the response. The Via is always initialized to the host that
originates the ACK request, i.e., the client user agent after a 2xx originates the ACK request, i.e., the client user agent after a 2xx
response or the first proxy to receive a non-2xx final response. The response or the first proxy to receive a non-2xx final response. The
ACK request is forwarded as the corresponding INVITE request, based ACK request is forwarded as the corresponding INVITE request, based
on its Request-URI. See Section 10 for details. on its Request-URI. See Section 10 for details.
The ACK request MAY contain a message body with the final session The ACK request MAY contain a message body with the final session
description to be used by the callee. If the ACK message body is description to be used by the callee. If the ACK message body is
empty, the callee uses the session description in the INVITE request. empty, the callee uses the session description in the INVITE request.
A proxy server receiving an ACK request after having sent a 3xx, 4xx,
5xx, or 6xx response must make a determination about whether the ACK
is for it, or for some user agent or proxy server further downstream.
This determination is made by examining the tag in the To field. If
the tag in the ACK To header field matches the tag in the To header
field of the response, the ACK is meant for the proxy server.
Otherwise, the ACK SHOULD be proxied downstream as any other request.
It is possible for a user agent client or proxy server to
receive multiple 3xx, 4xx, 5xx, and 6xx responses to a
request along a single branch. This can happen under
various error conditions, typically when a forking proxy
transitions from stateful to stateless before receiving all
responses. The various responses will all be identical,
except for the tag in the To field, which is different for
each one. It can therefore be used as a means to
disambiguate them.
This method MUST be supported by SIP proxy, redirect and user agent This method MUST be supported by SIP proxy, redirect and user agent
servers as well as clients. servers as well as clients.
4.2.3 OPTIONS 4.2.3 OPTIONS
The server is being queried as to its capabilities. A server that The server is being queried as to its capabilities. A server that
believes it can contact the user, such as a user agent where the user believes it can contact the user, such as a user agent where the user
is logged in and has been recently active, MAY respond to this is logged in and has been recently active, MAY respond to this
request with a capability set. A called user agent MAY return a request with a capability set. A called user agent MAY return a
status reflecting how it would have responded to an invitation, e.g., status reflecting how it would have responded to an invitation, e.g.,
600 (Busy). Such a server SHOULD return an Allow header field 600 (Busy). Such a server SHOULD return an Allow header field
indicating the methods that it supports. Proxy and redirect servers indicating the methods that it supports. Proxy and redirect servers
simply forward the request without indicating their capabilities. simply forward the request without indicating their capabilities.
This method MUST be supported by SIP proxy, redirect and user agent This method MUST be supported by SIP proxy, redirect and user agent
servers, registrars and clients. servers, registrars and clients.
4.2.4 BYE 4.2.4 BYE
The user agent client uses BYE to indicate to the server that it The user agent client uses BYE to indicate to the server that it
wishes to abort the call. A BYE request is forwarded like an INVITE wishes to release the call. A BYE request is forwarded like an INVITE
request. A caller SHOULD issue a BYE request before aborting a call request and MAY be issued by either caller or callee. A party to a
("hanging up"). Note that a BYE request may also be issued by the call SHOULD issue a BYE request before releasing a call ("hanging
callee. up"). A party receiving a BYE request MUST cease transmitting media
streams specifically directed at the party issuing the BYE request.
If the INVITE request contained a Location header, the callee MAY If the INVITE request contained a Contact header, the callee MAY send
send a BYE request to that address rather than the From address. a BYE request to that address rather than the From address.
This method MUST be supported by proxy servers and SHOULD be This method MUST be supported by proxy servers and SHOULD be
supported by redirect and user agent SIP servers. supported by redirect and user agent SIP servers.
4.2.5 CANCEL 4.2.5 CANCEL
The CANCEL request cancels a pending request with the same Call-ID, The CANCEL request cancels a pending request with the same Call-ID,
To, From and CSeq (sequence number only) header values, but does not To, From and CSeq (sequence number only) header field values, but
affect a completed request. (A request is considered completed if the does not affect a completed request. (A request is considered
server has returned a final status response.) completed if the server has returned a final status response.)
A user agent client or proxy client MAY issue a CANCEL request at any A user agent client or proxy client MAY issue a CANCEL request at any
time. A proxy, in particular, MAY choose to send a CANCEL to time. A proxy, in particular, MAY choose to send a CANCEL to
destinations that have not yet returned a final response after it has destinations that have not yet returned a final response after it has
received a 2xx or 6xx response for one or more of the parallel-search received a 2xx or 6xx response for one or more of the parallel-search
requests. A proxy that receives a CANCEL request forwards the request requests. A proxy that receives a CANCEL request forwards the request
to all destinations with pending requests. to all destinations with pending requests.
The Call-ID, To, the numeric part of CSeq and From headers in the The Call-ID, To, the numeric part of CSeq and From headers in the
CANCEL request are identical to those in the original request. This CANCEL request are identical to those in the original request. This
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However, to allow the client to distinguish responses to the CANCEL However, to allow the client to distinguish responses to the CANCEL
from those to the original request, the CSeq Method component is set from those to the original request, the CSeq Method component is set
to CANCEL. The Via header field is initialized to the proxy issuing to CANCEL. The Via header field is initialized to the proxy issuing
the CANCEL request. (Thus, responses to this CANCEL request only the CANCEL request. (Thus, responses to this CANCEL request only
reach the issuing proxy.) reach the issuing proxy.)
Once a user agent server has received a CANCEL, it MUST NOT issue a Once a user agent server has received a CANCEL, it MUST NOT issue a
2xx response for the cancelled original request. 2xx response for the cancelled original request.
A redirect or user agent server receiving a CANCEL request responds A redirect or user agent server receiving a CANCEL request responds
with a status of 200 (OK) if the Call-ID exists and a status of 481 with a status of 200 (OK) if the transaction exists and a status of
(Invalid Call-ID) if not, but takes no further action. In particular, 481 (Transaction Does Not Exist) if not, but takes no further action.
any existing call is unaffected. In particular, any existing call is unaffected.
The BYE request cannot be used to cancel branches of a The BYE request cannot be used to cancel branches of a
parallel search, since several branches may, through parallel search, since several branches may, through
intermediate proxies, find the same user agent server and intermediate proxies, find the same user agent server and
then terminate the call. To terminate a call instead of then terminate the call. To terminate a call instead of
just pending searches, the UAC must use BYE instead of or just pending searches, the UAC must use BYE instead of or
in addition to CANCEL. While CANCEL can terminate any in addition to CANCEL. While CANCEL can terminate any
pending request other than ACK or CANCEL, it is typically pending request other than ACK or CANCEL, it is typically
useful only for INVITE. 200 responses to INVITE and 200 useful only for INVITE. 200 responses to INVITE and 200
responses to CANCEL are distinguished by the method in the responses to CANCEL are distinguished by the method in the
Cseq header field, so there is no ambiguity. Cseq header field, so there is no ambiguity.
This method MUST be supported by proxy servers and SHOULD be This method MUST be supported by proxy servers and SHOULD be
supported by all other SIP server types. supported by all other SIP server types.
4.2.6 REGISTER 4.2.6 REGISTER
A client uses the REGISTER method to register the address listed in A client uses the REGISTER method to register the address listed in
the To header with a SIP server. the To header field with a SIP server.
A user agent MAY register with a local server on startup by sending a A user agent MAY register with a local server on startup by sending a
REGISTER request to the well-known "all SIP servers" multicast REGISTER request to the well-known "all SIP servers" multicast
address "sip.mcast.net" (224.0.1.75), with a time-to-live value of 1. address "sip.mcast.net" (224.0.1.75), with a time-to-live value of 1.
SIP user agents on the same subnet MAY listen to that address and use SIP user agents on the same subnet MAY listen to that address and use
it to become aware of the location of other local users [16]; it to become aware of the location of other local users [17];
however, they do not respond to the request. A user agent MAY also however, they do not respond to the request. A user agent MAY also
be configured with the address of a registrar server to which it be configured with the address of a registrar server to which it
sends a REGISTER request upon startup. sends a REGISTER request upon startup.
The meaning of the REGISTER request-header fields is defined as The meaning of the REGISTER request-header fields is defined as
follows. We define "address-of-record" as the SIP address that the follows. We define "address-of-record" as the SIP address that the
registry knows the registrand, typically of the form "user@domain" registry knows the registrand, typically of the form "user@domain"
rather than "user@host". In third-party registration, the entity rather than "user@host". In third-party registration, the entity
issuing the request is different from the entity being registered. issuing the request is different from the entity being registered.
To: The To header field contains the address-of-record whose To: The To header field contains the address-of-record whose
registration is to be created or updated. registration is to be created or updated.
From: The From header field contains the address-of-record of the From: The From header field contains the address-of-record of the
person responsible for the registration. For first-party person responsible for the registration. For first-party
registration, it is identical to the To header field value. registration, it is identical to the To header field value.
Request-URI: The Request-URI names the destination of the Request-URI: The Request-URI names the destination of the
registration request, i.e., the domain of the registrar. The registration request, i.e., the domain of the registrar. The
user name MUST be empty. Generally, the domains in the Request- user name MUST be empty. Generally, the domains in the Request-
URI and the To header have the same value; however, it is URI and the To header field have the same value; however, it is
possible to register as a "visitor", while maintaining one's possible to register as a "visitor", while maintaining one's
name. For example, a traveller sip:alice@acme.com (To) may name. For example, a traveller sip:alice@acme.com (To) might
register under the Request-URI sip:@atlanta.ayh.org , with the register under the Request-URI sip:@atlanta.ayh.org , with the
former as the To field and the latter as the Request-URI. The former as the To header field and the latter as the Request-URI.
request is no longer forwarded once it reached the server whose The request is no longer forwarded once it reached the server
authoritative domain is the one listed in the Request-URI. whose authoritative domain is the one listed in the Request-URI.
Location: The request MAY contain a Location header field; future
non-REGISTER requests for the URI given in the To field will be
directed to the address(es) given in the Location header. It is
RECOMMENDED that user agents include SIP URLs with both UDP and
TCP transport parameters in their registration. If the
registration contains a Location field whose URL includes a
transport parameter, future requests will use that protocol.
Otherwise, requests use the same transport protocol as used by
the registration. However, a multicast REGISTER request still
causes future requests to be unicast unless the maddr URL
parameter explicitly requests otherwise. If the Location header
does not contain a port number, the default SIP port number is
used for future requests.
We cannot require that registration and subsequent INVITE
requests use the same transport protocol, as multicast
registrations may be quite useful.
If the request does not contain a Location header, the registration Contact: The request MAY contain a Contact header field; future non-
remains unchanged. Registrations that differ in one or more of host, REGISTER requests for the URI given in the To header field will
port, transport or maddr from an existing registration are added to be directed to the address(es) given in the Contact header.
the list of registrations. If these parameters are the same as an
existing registration, the new registration replaces the old one if
it has a higher q value or, for the same value of q, if the ttl value
is higher. All current locations MUST share the same action value.
If the request does not contain a Contact header, the registration
remains unchanged. Registrations using SIP URIs that differ in one or
more of host, port, transport-param or maddr-param from an existing
registration are added to the list of registrations. Other URI types
are compared according to the standard URI equivalency rules for the
URI schema. If the URIs are equivalent to that of an existing
registration, the new registration replaces the old one if it has a
higher q value or, for the same value of q, if the ttl value is
higher. All current registrations MUST share the same action value.
Registrations that have a different action than current registrations Registrations that have a different action than current registrations
for the same user are rejected with status of 409 (Conflict). for the same user are rejected with status of 409 (Conflict).
A proxy server ignores the q parameter in processing non-REGISTER A proxy server ignores the q parameter when processing non-REGISTER
requests, while a redirect server simply returns the parameter in its requests, while a redirect server simply returns that parameter in
Location header. its Contact response header field.
Having the proxy server interpret the q parameter is not Having the proxy server interpret the q parameter is not
sufficient to guide proxy behavior, as it is not clear, for sufficient to guide proxy behavior, as it is not clear, for
example, how long it should wait between trying addresses. example, how long it is supposed to wait between trying
addresses.
If the registration is changed while a user agent or proxy server If the registration is changed while a user agent or proxy server
processes an invitation, the new information should be used. processes an invitation, the new information SHOULD be used.
This allows a service known as "directed pick-up". This allows a service known as "directed pick-up".
A server SHOULD silently drop the registration after one hour, unless A server SHOULD silently drop the registration after one hour, unless
refreshed by the client. A client may request a lower or higher refreshed by the client. A client MAY request a lower or higher
refresh interval through the Expires header (Section 6.19). Based on refresh interval through the Expires header (Section 6.20). Based on
this request and its configuration, the server chooses the expiration this request and its configuration, the server chooses the expiration
interval and indicates it through the Expires header in the response. interval and indicates it through the Expires header field in the
A single address (if host-independent) may be registered from several response. A single address (if host-independent) MAY be registered
different clients. from several different clients.
A client cancels an existing registration by sending a REGISTER A client cancels an existing registration by sending a REGISTER
request with an expiration time (Expires) of zero seconds for a request with an expiration time (Expires) of zero seconds for a
particular Location or the wildcard Location designated by a "*" for particular Contact or the wildcard Contact designated by a "*" for
all registrations. Registrations are matched based on the user, host, all registrations. Registrations are matched based on the user, host,
port and maddr parameters. port and maddr parameters.
The server SHOULD return the current list of registrations in the 200 The server SHOULD return the current list of registrations in the 200
response as Location header fields. response as Contact header fields.
It is particularly important that REGISTER requests are authenticated It is particularly important that REGISTER requests are authenticated
since they allow to redirect future requests. since they allow to redirect future requests.
Beyond its use as a simple location service, this method is Beyond its use as a simple location service, this method is
needed if there are several SIP servers on a single host. needed if there are several SIP servers on a single host.
In that case, only one of the servers can use the default In that case, only one of the servers can use the default
port number. Each server that cannot would register with a port number. Each server that cannot registers with a
server for the administrative domain. Since a client may server for the administrative domain. Since clients do not
not have easy access to the host address or port number, always have easy access to the host address or port number,
using the source address and port from the request itself using the source address and port from the request itself
seems simpler. seems simpler.
Support of this method is RECOMMENDED. Support of this method is RECOMMENDED.
4.3 Request-URI 4.3 Request-URI
The Request-URI field is a SIP URL as described in Section 2 or a The Request-URI is a SIP URL as described in Section 2 or a general
general URI. It indicates the user or service to which this request URI. It indicates the user or service to which this request is being
is being addressed. Unlike the To field, the Request-URI field may be addressed. Unlike the To field, the Request-URI MAY be re-written by
re-written by proxies. proxies.
When used as a Request-URI, a SIP-URL MUST NOT contain the When used as a Request-URI, a SIP-URL MUST NOT contain the
transport-param, maddr-param, ttl-param, or headers elements. A transport-param, maddr-param, ttl-param, or headers elements. A
server that receives a SIP-URL with these elements removes them server that receives a SIP-URL with these elements removes them
before further processing. before further processing.
Typically, the UAC sets the Request-URI and To to the same Typically, the UAC sets the Request-URI and To to the same
SIP URL, presumed to remain unchanged over long time SIP URL, presumed to remain unchanged over long time
periods. However, if the UAC has cached a more direct path periods. However, if the UAC has cached a more direct path
to the callee, e.g., from the Location header of a response to the callee, e.g., from the Contact header field of a
to a previous request, the To would still contain the response to a previous request, the To would still contain
long-term, "public" address, while the Request-URI would be the long-term, "public" address, while the Request-URI
set to the cached address. would be set to the cached address.
Proxy and redirect servers may use the information in the Request-URI Proxy and redirect servers MAY use the information in the Request-URI
and request header fields to handle the request and possibly rewrite and request header fields to handle the request and possibly rewrite
the Request-URI. For example, a request addressed to the generic the Request-URI. For example, a request addressed to the generic
address sip:sales@acme.com might be proxied to the particular person, address sip:sales@acme.com is proxied to the particular person, e.g.,
e.g., sip:bob@ny.acme.com , with the To remaining as sales@acme.com sip:bob@ny.acme.com , with the To remaining as sales@acme.com
ny.acme.com , Bob may have designated Alice as the temporary ny.acme.com , Bob then designates Alice as the temporary substitute.
substitute.
The host part of the Request-URI typically agrees with one of the The host part of the Request-URI typically agrees with one of the
host names of the server. If it does not, the server SHOULD proxy the host names of the server. If it does not, the server SHOULD proxy the
request to the address indicated or return a 404 (Not Found) response request to the address indicated or return a 404 (Not Found) response
if it is unwilling or unable to do so. For example, the Request-URI if it is unwilling or unable to do so. For example, the Request-URI
and server host name may disagree in the case of a firewall proxy and server host name can disagree in the case of a firewall proxy
that handles outgoing calls. This mode of operation similar to that that handles outgoing calls. This mode of operation similar to that
of HTTP proxies. of HTTP proxies.
If a SIP server receives a request with a URI indicating a scheme If a SIP server receives a request with a URI indicating a scheme
other than SIP which that server does not understand, the server MUST other than SIP which that server does not understand, the server MUST
return a 400 (Bad Request) response. It MUST do this even if the To return a 400 (Bad Request) response. It MUST do this even if the To
field contains a scheme it does understand. header field contains a scheme it does understand.
4.3.1 SIP Version 4.3.1 SIP Version
Both request and response messages include the version of SIP in use, Both request and response messages include the version of SIP in use,
and basically follow [H3.1], with HTTP replaced by SIP. To be and basically follow [H3.1], with HTTP replaced by SIP. To be
conditionally compliant with this specification, applications sending conditionally compliant with this specification, applications sending
SIP messages MUST include a SIP-Version of "SIP/2.0". SIP messages MUST include a SIP-Version of "SIP/2.0".
4.4 Option Tags 4.4 Option Tags
Option tags are unique identifiers used to designate new options in Option tags are unique identifiers used to designate new options in
SIP. These tags are used in Require (Section 6.30) and Unsupported SIP. These tags are used in Require (Section 6.30) and Unsupported
(Section 6.38) fields. (Section 6.38) fields.
Syntax: Syntax:
option-tag ___ 1*uric option-tag ___ 1*uric
The creator of a new SIP option MUST either prefix the option with a
The creator of a new SIP option should either prefix the option with reverse domain name or register the new option with the Internet
a reverse domain name or register the new option with the Internet
Assigned Numbers Authority (IANA). For example, Assigned Numbers Authority (IANA). For example,
"com.foo.mynewfeature" is an apt name for a feature whose inventor "com.foo.mynewfeature" is an apt name for a feature whose inventor
can be reached at "foo.com". Options registered with IANA have the can be reached at "foo.com". Options registered with IANA have the
prefix "org.ietf.sip.", options described in RFCs have the prefix prefix "org.ietf.sip.", options described in RFCs have the prefix
"org.ietf.rfc.N", where N is the RFC number. Option tags are case- "org.ietf.rfc.N", where N is the RFC number. Option tags are case-
insensitive. insensitive.
4.4.1 Registering New Option Tags with IANA 4.4.1 Registering New Option Tags with IANA
When registering a new SIP option, the following information should When registering a new SIP option, the following information MUST be
be provided: provided:
o Name and description of option. The name may be of any length, o Name and description of option. The name MAY be of any length,
but SHOULD be no more than twenty characters long. The name but SHOULD be no more than twenty characters long. The name
MUST NOT contain any spaces, control characters or periods. MUST NOT contain any spaces, control characters or periods.
o Indication of who has change control over the option (for o Indication of who has change control over the option (for
example, IETF, ISO, ITU-T, other international standardization example, IETF, ISO, ITU-T, other international standardization
bodies, a consortium or a particular company or group of bodies, a consortium or a particular company or group of
companies); companies);
o A reference to a further description, if available, for o A reference to a further description, if available, for
example (in order of preference) an RFC, a published paper, a example (in order of preference) an RFC, a published paper, a
skipping to change at page 31, line 21 skipping to change at page 31, line 6
definitions in Section 7. The first digit of the Status-Code defines definitions in Section 7. The first digit of the Status-Code defines
the class of response. The last two digits do not have any the class of response. The last two digits do not have any
categorization role. SIP/2.0 allows 6 values for the first digit: categorization role. SIP/2.0 allows 6 values for the first digit:
1xx: Informational -- request received, continuing to process the 1xx: Informational -- request received, continuing to process the
request; request;
2xx: Success -- the action was successfully received, understood, and 2xx: Success -- the action was successfully received, understood, and
accepted; accepted;
3xx: Redirection -- further action must be taken in order to complete 3xx: Redirection -- further action needs to be taken in order to
the request; complete the request;
4xx: Client Error -- the request contains bad syntax or cannot be 4xx: Client Error -- the request contains bad syntax or cannot be
fulfilled at this server; fulfilled at this server;
5xx: Server Error -- the server failed to fulfill an apparently valid 5xx: Server Error -- the server failed to fulfill an apparently valid
request; request;
6xx: Global Failure -- the request is invalid at any server. 6xx: Global Failure -- the request is invalid at any server.
Figures 5 through 9 present the individual values of the numeric Figures 5 through 9 present the individual values of the numeric
skipping to change at page 32, line 9 skipping to change at page 31, line 41
x00 response code of that class, with the exception that an x00 response code of that class, with the exception that an
unrecognized response MUST NOT be cached. For example, if a client unrecognized response MUST NOT be cached. For example, if a client
receives an unrecognized response code of 431, it can safely assume receives an unrecognized response code of 431, it can safely assume
that there was something wrong with its request and treat the that there was something wrong with its request and treat the
response as if it had received a 400 (Bad Request) response code. In response as if it had received a 400 (Bad Request) response code. In
such cases, user agents SHOULD present to the user the message body such cases, user agents SHOULD present to the user the message body
returned with the response, since that message body is likely to returned with the response, since that message body is likely to
include human-readable information which will explain the unusual include human-readable information which will explain the unusual
status. status.
6 Header Field Definitions
SIP header fields are similar to HTTP header fields in both syntax
and semantics [H4.2, H14]. In general, the ordering of the header
fields is not of importance (with the exception of Via fields, see
below). The only requirement is that header fields which are hop-by-
Informational = "100" ; Trying Informational = "100" ; Trying
| "180" ; Ringing | "180" ; Ringing
| "181" ; Call Is Being Forwarded | "181" ; Call Is Being Forwarded
| "182" ; Queued | "182" ; Queued
Success = "200" ; OK Success = "200" ; OK
Figure 5: Informational and success status codes Figure 5: Informational and success status codes
Redirection = "300" ; Multiple Choices Redirection = "300" ; Multiple Choices
| "301" ; Moved Permanently | "301" ; Moved Permanently
| "302" ; Moved Temporarily | "302" ; Moved Temporarily
| "303" ; See Other | "303" ; See Other
| "305" ; Use Proxy | "305" ; Use Proxy
| "380" ; Alternative Service | "380" ; Alternative Service
Figure 6: Redirection status codes Figure 6: Redirection status codes
6 Header Field Definitions
SIP header fields are similar to HTTP header fields in both syntax
and semantics [H4.2, H14]. In general, the ordering of the header
fields is not of importance (with the exception of Via fields, see
below). The only requirement is that header fields which are hop-by-
hop MUST appear before any header fields which are end-to-end. hop MUST appear before any header fields which are end-to-end.
Proxies MUST NOT reorder or otherwise modify header fields other than Proxies MUST NOT reorder or otherwise modify header fields other than
by adding a new Via or other hop-by-hop field. Proxies MUST NOT, for by adding a new Via or other hop-by-hop field. Proxies MUST NOT, for
example, change how header fields are broken across lines. This example, change how header fields are broken across lines. This
allows an authentication field to be added after the Via fields that allows an authentication field to be added after the Via header
will not be invalidated by proxies. fields that will not be invalidated by proxies.
The header fields required, optional and not applicable for each
method are listed in Table 4 and Table 5. The table uses "o" to
indicate optional, "m" mandatory and "-" for not applicable. A "*"
indicates that the header fields are needed only if message body is
not empty: The Content-Type and Content-Length header fields are
required when there is a valid message body (of non-zero length)
associated with the message (Section 8).
The "where" column describes the request and response types with
which the header field can be used. "R" refers to header fields that
can be used in requests (that is, request and general header fields).
"r" designates a response or general-header field as applicable to
all responses, while a list of numeric values indicates the status
codes with which the header field can be used. "g" and "e" designate
general (Section 6.1) and entity header (Section 6.2) fields,
respectively. If a header field is marked "c", it is copied from the
Client-Error = "400" ; Bad Request Client-Error = "400" ; Bad Request
| "401" ; Unauthorized | "401" ; Unauthorized
| "402" ; Payment Required | "402" ; Payment Required
| "403" ; Forbidden | "403" ; Forbidden
| "404" ; Not Found | "404" ; Not Found
| "405" ; Method Not Allowed | "405" ; Method Not Allowed
| "406" ; Not Acceptable | "406" ; Not Acceptable
| "407" ; Proxy Authentication Required | "407" ; Proxy Authentication Required
| "408" ; Request Timeout | "408" ; Request Timeout
| "409" ; Conflict | "409" ; Conflict
| "410" ; Gone | "410" ; Gone
| "411" ; Length Required | "411" ; Length Required
| "413" ; Request Message Body Too Large | "413" ; Request Message Body Too Large
| "414" ; Request-URI Too Large | "414" ; Request-URI Too Large
| "415" ; Unsupported Media Type | "415" ; Unsupported Media Type
| "420" ; Bad Extension | "420" ; Bad Extension
| "480" ; Temporarily not available | "480" ; Temporarily not available
| "481" ; Invalid Call-ID | "481" ; Call Leg/Transaction Does Not Exist
| "482" ; Loop Detected | "482" ; Loop Detected
| "483" ; Too Many Hops | "483" ; Too Many Hops
| "484" ; Address Incomplete | "484" ; Address Incomplete
| "485" ; Ambiguous | "485" ; Ambiguous
| "486" ; Busy Here
Figure 7: Client error status codes Figure 7: Client error status codes
Server-Error = "500" ; Internal Server Error Server-Error = "500" ; Internal Server Error
| "501" ; Not Implemented | "501" ; Not Implemented
| "502" ; Bad Gateway | "502" ; Bad Gateway
| "503" ; Service Unavailable | "503" ; Service Unavailable
| "504" ; Gateway Timeout | "504" ; Gateway Timeout
| "505" ; SIP Version not supported | "505" ; SIP Version not supported
Figure 8: Server error status codes Figure 8: Server error status codes
The header fields required, optional and not applicable for each request to the response.
method are listed in Table 4. The table uses "o" to indicate
optional, "m" mandatory and "-" for not applicable. A "*" indicates The "enc." column describes whether this message header field MAY be
that the header fields are needed only if message body is not empty: encrypted end-to-end. A "n" designates fields that MUST NOT be
The Content-Type and Content-Length headers are required when there encrypted, while "c" designates fields that SHOULD be encrypted if
is a valid message body (of non-zero length) associated with the Global-Failure | "600" ; Busy Everywhere
Global-Failure | "600" ; Busy
| "603" ; Decline | "603" ; Decline
| "604" ; Does not exist anywhere | "604" ; Does not exist anywhere
| "606" ; Not Acceptable | "606" ; Not Acceptable
Figure 9: Global failure status codes Figure 9: Global failure status codes
message (Section 8).
The "type" column describes the request and response types for which
the header field may be used. A numeric value indicates the status
code for a response, while "R" refers to any request header, "r" to
any response header. "g" and "e" designate general (Section 6.1) and
entity header (Section 6.2) fields, respectively.
The "enc." column describes whether this message header may be
encrypted end-to-end. A "n" designates fields that MUST NOT be
encrypted, while "c" designates fields that SHOULD be encrypted if
encryption is used. encryption is used.
The "e-e" column has a value of "e" for end-to-end and a value of "h" The "e-e" column has a value of "e" for end-to-end and a value of "h"
for hop-by-hop headers. for hop-by-hop header fields.
Other headers may be added as required; a server MAY ignore optional where enc. e-e ACK BYE CAN INV OPT REG
headers that it does not understand. A compact form of these header ____________________________________________________________________________
fields is also defined in Section 9 for use over UDP when the request Accept R e - - - o o o
has to fit into a single packet and size is an issue. Accept-Encoding R e - - - o o o
Accept-Language R n e - o o o o o
Allow 200 e - - - - m -
Allow 405 e o o o o o o
Authorization R e o o o o o o
Call-ID gc n e m m m m m m
Contact R e o - - o o o
Contact 1xx e - - - o o -
Contact 2xx e - - - o o o
Contact 3xx e - o - o o o
Contact 485 e - o - o o o
Content-Encoding e e * - - * * *
Content-Length e e o - - o o o
Content-Type e e * - - * * *
CSeq gc n e m m m m m m
Date g e o o o o o o
Encryption g n e o o o o o o
Expires g e - - - o - o
From gc n e m m m m m m
Hide R n h o o o o o o
Max-Forwards R n e o o o o o o
Organization g c h - - - o o o
Table 5 in Appendix A indicates which system components should be Table 4: Summary of header fields, A--O
capable of parsing which header fields. where enc. e-e ACK BYE CAN INV OPT REG
_____________________________________________________________________________________
Proxy-Authenticate 407 n h o o o o o o
Proxy-Authorization R n h o o o o o o
Proxy-Require R n h o o o o o o
Priority R c e - - - o - -
Require R e o o o o o o
Retry-After R c e - - - - - o
Retry-After 404,480,486 c e o o o o o o
503 c e o o o o o o
600,603 c e o o o o o o
Response-Key R c e - o o o o o
Record-Route R h o o o o o o
Record-Route 2xx h o o o o o o
Route R h - o o o o o
Server r c e o o o o o o
Subject R c e - - - o - -
Timestamp g e o o o o o o
To gc(1) n e m m m m m m
Unsupported 420 e o o o o o o
User-Agent g c e o o o o o o
Via gc(2) n e m m m m m m
Warning r e o o o o o o
WWW-Authenticate 401 c e o o o o o o
Table 5: Summary of header fields, P--Z; (1): copied with possible
addition of tag; (2): UAS removes first Via header field
Other header fields can be added as required; a server MAY ignore
optional header fields that it does not understand. A compact form of
these header fields is also defined in Section 9 for use over UDP
when the request has to fit into a single packet and size is an
issue.
Table 6 in Appendix A lists those header fields that different client
and server types MUST be able to parse.
6.1 General Header Fields 6.1 General Header Fields
General header fields apply to both request and response messages. General header fields apply to both request and response messages.
The general-header field names can be extended reliably only in The general-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or combination with a change in the protocol version. However, new or
experimental header fields may be given the semantics of general experimental header fields may be given the semantics of general
header fields if all parties in the communication recognize them to header fields if all parties in the communication recognize them to
be general-header fields. Unrecognized header fields are treated as be general-header fields. Unrecognized header fields are treated as
entity-header fields. entity-header fields.
skipping to change at page 35, line 4 skipping to change at page 36, line 6
General header fields apply to both request and response messages. General header fields apply to both request and response messages.
The general-header field names can be extended reliably only in The general-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or combination with a change in the protocol version. However, new or
experimental header fields may be given the semantics of general experimental header fields may be given the semantics of general
header fields if all parties in the communication recognize them to header fields if all parties in the communication recognize them to
be general-header fields. Unrecognized header fields are treated as be general-header fields. Unrecognized header fields are treated as
entity-header fields. entity-header fields.
6.2 Entity Header Fields 6.2 Entity Header Fields
The entity-header fields define meta-information about the message- The entity-header fields define meta-information about the message-
body or, if no body is present, about the resource identified by the body or, if no body is present, about the resource identified by the
request. The term "entity header" is an HTTP 1.1 term where the request. The term "entity header" is an HTTP 1.1 term where the
response body may contain a transformed version of the message body. response body can contain a transformed version of the message body.
The original message body is referred to as the "entity". We retain The original message body is referred to as the "entity". We retain
the same terminology for header fields but usually refer to the the same terminology for header fields but usually refer to the
"message body" rather then the entity as the two are the same in SIP. "message body" rather then the entity as the two are the same in SIP.
6.3 Request Header Fields 6.3 Request Header Fields
The request-header fields allow the client to pass additional The request-header fields allow the client to pass additional
information about the request, and about the client itself, to the information about the request, and about the client itself, to the
server. These fields act as request modifiers, with semantics server. These fields act as request modifiers, with semantics
equivalent to the parameters of a programming language method equivalent to the parameters of a programming language method
skipping to change at page 35, line 43 skipping to change at page 36, line 46
Response-header field names can be extended reliably only in Response-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or combination with a change in the protocol version. However, new or
experimental header fields MAY be given the semantics of response- experimental header fields MAY be given the semantics of response-
header fields if all parties in the communication recognize them to header fields if all parties in the communication recognize them to
be response-header fields. Unrecognized header fields are treated as be response-header fields. Unrecognized header fields are treated as
entity-header fields. entity-header fields.
6.5 End-to-end and Hop-by-hop Headers 6.5 End-to-end and Hop-by-hop Headers
End-to-end headers must be transmitted unmodified across all proxies, End-to-end headers MUST be transmitted unmodified across all proxies,
while hop-by-hop headers may be modified or added by proxies. while hop-by-hop headers MAY be modified or added by proxies.
6.6 Header Field Format 6.6 Header Field Format
Header fields (general-header, request-header, response-header, and Header fields (general-header, request-header, response-header, and
entity-header) follow the same generic header format as that given in entity-header) follow the same generic header format as that given in
Section 3.1 of RFC 822 [24]. Each header field consists of a name Section 3.1 of RFC 822 [24]. Each header field consists of a name
followed by a colon (":") and the field value. Field names are case- followed by a colon (":") and the field value. Field names are case-
type enc. e-e ACK BYE CAN INV OPT REG insensitive. The field value MAY be preceded by any amount of leading
_____________________________________________________________________________________________
Accept R e - - - o o o
Accept-Encoding R e - - - o o o
Accept-Language R n e - o o o o o
Allow 200 e - - - - m -
Allow 405 e o o o o o o
Authorization R e o o o o o o
Call-ID g n e m m m m m m
Content-Encoding e e * - - * * *
Content-Length e e m - - m m m
Content-Type e e * - - * * *
CSeq g n e m m m m m m
Date g e o o o o o o
Encryption g n e o o o o o o
Expires g e - - - o - o
From g n e m m m m m m
Hide R n h o o o o o o
Location R e o - - o o o
Location 1xx e - - - o o -
Location 2xx e - - - o o o
Location 3xx e - o - o o o
Location 485 e - o - o o o
Max-Forwards R n e o o o o o o
Organization g c h - - - o o o
Proxy-Authenticate 407 n h o o o o o o
Proxy-Authorization R n h o o o o o o
Proxy-Require R n h o o o o o o
Priority R c e - - - o - -
Require R e o o o o o o
Retry-After R c e - - - - - o
Retry-After 404,480,503,600,603 c e o o o o o o
Response-Key R c e - o o o o o
Record-Route R h o o o o o o
Record-Route 2xx h o o o o o o
Route R h - o o o o o
Server r c e o o o o o o
Subject R c e - - - o - -
Timestamp g e o o o o o o
To g n e m m m m m m
Unsupported 420 e o o o o o o
User-Agent g c e o o o o o o
Via g n e m m m m m m
Warning r e o o o o o o
WWW-Authenticate 401 c e o o o o o o
Table 4: Summary of header fields
insensitive. The field value may be preceded by any amount of leading
white space (LWS), though a single space (SP) is preferred. Header white space (LWS), though a single space (SP) is preferred. Header
fields can be extended over multiple lines by preceding each extra fields can be extended over multiple lines by preceding each extra
line with at least one SP or horizontal tab (HT). Applications SHOULD line with at least one SP or horizontal tab (HT). Applications SHOULD
follow HTTP "common form" when generating these constructs, since follow HTTP "common form" when generating these constructs, since
there might exist some implementations that fail to accept anything there might exist some implementations that fail to accept anything
beyond the common forms. beyond the common forms.
message-header = field-name ":" [ field-value ] CRLF message-header = field-name ":" [ field-value ] CRLF
field-name = token field-name = token
field-value = *( field-content | LWS ) field-value = *( field-content | LWS )
field-content = < the OCTETs making up the field-value field-content = < the OCTETs making up the field-value
and consisting of either *TEXT and consisting of either *TEXT
or combinations of token, or combinations of token,
tspecials, and quoted-string> tspecials, and quoted-string>
The order in which header fields are received is not significant if The relative order of header fields with different field names is not
the header fields have different field names. Multiple header fields significant. Multiple header fields with the same field-name may be
with the same field-name may be present in a message if and only if present in a message if and only if the entire field-value for that
the entire field-value for that header field is defined as a comma- header field is defined as a comma-separated list (i.e., #(values)).
separated list (i.e., #(values)). It MUST be possible to combine the It MUST be possible to combine the multiple header fields into one
multiple header fields into one "field-name: field-value" pair, "field-name: field-value" pair, without changing the semantics of the
without changing the semantics of the message, by appending each message, by appending each subsequent field-value to the first, each
subsequent field-value to the first, each separated by a comma. The separated by a comma. The order in which header fields with the same
order in which header fields with the same field-name are received is field-name are received is therefore significant to the
therefore significant to the interpretation of the combined field interpretation of the combined field value, and thus a proxy MUST NOT
value, and thus a proxy MUST NOT change the order of these field change the order of these field values when a message is forwarded.
values when a message is forwarded.
Field names are not case-sensitive, although their values may be. Field names are not case-sensitive, although their values may be.
6.7 Accept 6.7 Accept
See [H14.1] for syntax. This request-header field is used only with See [H14.1] for syntax. This request-header field is used only with
the INVITE, OPTIONS and REGISTER request methods to indicate what the INVITE, OPTIONS and REGISTER request methods to indicate what
media types are acceptable in the response. media types are acceptable in the response.
Example: Example:
skipping to change at page 38, line 4 skipping to change at page 38, line 6
See [H14.1] for syntax. This request-header field is used only with See [H14.1] for syntax. This request-header field is used only with
the INVITE, OPTIONS and REGISTER request methods to indicate what the INVITE, OPTIONS and REGISTER request methods to indicate what
media types are acceptable in the response. media types are acceptable in the response.
Example: Example:
Accept: application/sdp;level=1, application/x-private, text/html Accept: application/sdp;level=1, application/x-private, text/html
6.8 Accept-Encoding 6.8 Accept-Encoding
The Accept-Encoding request-header field is similar to Accept, but The Accept-Encoding request-header field is similar to Accept, but
restricts the content-codings [H3.4.1] that are acceptable in the restricts the content-codings [H3.4.1] that are acceptable in the
response. See [H14.3]. response. See [H14.3].
6.9 Accept-Language 6.9 Accept-Language
See [H14.4] for syntax. The Accept-Language request header can be See [H14.4] for syntax. The Accept-Language request-header field can
used to allow the client to indicate to the server in which language be used to allow the client to indicate to the server in which
it would prefer to receive reason phrases, session descriptions or language it would prefer to receive reason phrases, session
status responses carried as message bodies. A proxy may use this descriptions or status responses carried as message bodies. A proxy
field to help select the destination for the call, for example, a MAY use this field to help select the destination for the call, for
human operator conversant in a language spoken by the caller. example, a human operator conversant in a language spoken by the
caller.
Example: Example:
Accept-Language: da, en-gb;q=0.8, en;q=0.7 Accept-Language: da, en-gb;q=0.8, en;q=0.7
6.10 Allow 6.10 Allow
See [H14.7]. The Allow entity-header field lists the set of methods See [H14.7]. The Allow entity-header field lists the set of methods
supported by the resource identified by the Request-URI. The purpose supported by the resource identified by the Request-URI. The purpose
of this field is strictly to inform the recipient of valid methods of this field is strictly to inform the recipient of valid methods
skipping to change at page 38, line 43 skipping to change at page 38, line 47
A user agent that wishes to authenticate itself with a server -- A user agent that wishes to authenticate itself with a server --
usually, but not necessarily, after receiving a 401 response -- MAY usually, but not necessarily, after receiving a 401 response -- MAY
do so by including an Authorization request-header field with the do so by including an Authorization request-header field with the
request. The Authorization field value consists of credentials request. The Authorization field value consists of credentials
containing the authentication information of the user agent for the containing the authentication information of the user agent for the
realm of the resource being requested. realm of the resource being requested.
6.12 Call-ID 6.12 Call-ID
The Call-ID general header uniquely identifies a particular The Call-ID general-header field uniquely identifies a particular
invitation or all registrations of a particular client. Note that a invitation or all registrations of a particular client. Note that a
single multimedia conference may give rise to several calls with single multimedia conference can give rise to several calls with
different Call-IDs, e.g., if a user invites a single individual different Call-IDs, e.g., if a user invites a single individual
several times to the same (long-running) conference. several times to the same (long-running) conference.
For an INVITE request, a callee user agent server SHOULD NOT alert For an INVITE request, a callee user agent server SHOULD NOT alert
the user if the user has responded previously to the Call-ID in the the user if the user has responded previously to the Call-ID in the
INVITE request. If the user is already a member of the conference and INVITE request. If the user is already a member of the conference and
the conference parameters contained in the session description have the conference parameters contained in the session description have
not changed, a callee user agent server MAY silently accept the call, not changed, a callee user agent server MAY silently accept the call,
regardless of the Call-ID. An invitation for an existing Call-ID or regardless of the Call-ID. An invitation for an existing Call-ID or
session may change the parameters of the conference. A client session can change the parameters of the conference. A client
application MAY decide to simply indicate to the user that the application MAY decide to simply indicate to the user that the
conference parameters have been changed and accept the invitation conference parameters have been changed and accept the invitation
automatically or it MAY require user confirmation. automatically or it MAY require user confirmation.
A user may be invited to the same conference or call using several A user may be invited to the same conference or call using several
different Call-IDs. If desired, the client may use identifiers within different Call-IDs. If desired, the client MAY use identifiers within
the session description to detect this duplication. For example, SDP the session description to detect this duplication. For example, SDP
contains a session id and version number in the origin (o) field. contains a session id and version number in the origin (o) field.
The REGISTER and OPTIONS methods use the Call-ID value to The REGISTER and OPTIONS methods use the Call-ID value to
unambiguously match requests and responses. All REGISTER requests unambiguously match requests and responses. All REGISTER requests
issued by a single client MUST use the same Call-ID. issued by a single client MUST use the same Call-ID.
Since the Call-ID is generated by and for SIP, there is no Since the Call-ID is generated by and for SIP, there is no
reason to deal with the complexity of URL-encoding and reason to deal with the complexity of URL-encoding and
case-ignoring string comparison. case-ignoring string comparison.
Call-ID = ( "Call-ID" | "i" ) ":" local-id "@" host Call-ID = ( "Call-ID" | "i" ) ":" local-id "@" host
local-id = *uric local-id = *uric
host MUST be either a fully qualified domain name or a globally host MUST be either a fully qualified domain name or a globally
routable IP address, while the local-id is a random identifier routable IP address, while the local-id is a random identifier
consisting of URI characters that is unique within host. It MUST NOT consisting of URI characters that is unique within host. It MUST NOT
be reused for a different call. Call-IDs are case-sensitive. The be reused for a different call. Call-IDs are case-sensitive. The use
use of a UUID as local-id is OPTIONAL. The UUID is a version-4 of a UUID as local-id is OPTIONAL. The UUID is a version-4 (random)
(random) UUID [19]. UUID [19].
Using cryptographically random identifiers provides some Using cryptographically random identifiers provides some
protection against session hijacking. Call-ID, To and From protection against session hijacking. Call-ID, To and From
are needed to identify a call leg calls with third-party are needed to identify a call leg call leg matters in calls
control. with third-party control.
Example: Example:
Call-ID: f81d4fae-7dec-11d0-a765-00a0c91e6bf6@foo.bar.com Call-ID: f81d4fae-7dec-11d0-a765-00a0c91e6bf6@foo.bar.com
6.13 Content-Encoding 6.13 Contact
The Contact general-header field can appear in requests, 1xx, 2xx
responses and 3xx responses.
INVITE and ACK requests: INVITE and ACK requests MAY contain Contact
headers indicating from which location the request is
originating.
This allows the callee to send a BYE directly to the caller
instead of through a series of proxies. The Via header is
not sufficient since the desired address may be that of a
proxy.
INVITE 2xx responses: A user agent server sending a definitive,
positive response (2xx) MAY insert a Contact response header
field indicating the SIP address under which it is reachable
most directly for future SIP requests, such as ACK, within the
same Call-ID. The Contact header field contains the address of
the server itself or that of a proxy, e.g., if the host is
behind a firewall. The value of this Contact header is copied
into the Request-URI of subsequent requests for this call.
The Contact value SHOULD NOT be cached across calls, as it
may not represent the most desirable location for a
particular destination address.
INVITE 1xx responses: A UAS sending a provisional response (1xx) MAY
insert a Contact response header. It has the same semantics in a
1xx response as a 2xx INVITE response. Note that CANCEL
requests MUST NOT be sent to that address, but rather follow the
same path as the original request.
REGISTER requests: REGISTER requests MAY contain a Contact header
field indicating at which locations the user is reachable. The
REGISTER request defines a wildcard Contact field, "*", which
MUST only be used with Expires: 0 to remove all registrations
for a particular user. An optional expires parameter indicates
the desired expiration time of the registration. If a Contact
entry does not have an expires parameter, the Expires header
field is used as the default value. If neither of these
mechanisms is used, SIP URIs are assumed to expire after one
hour. Other URI schemes have no expiration times.
REGISTER 2xx responses: A REGISTER response MAY return all locations
at which the user is currently reachable. An optional expires
parameter indicates the expiration time of the registration. If
a Contact entry does not have an expires parameter, the value of
the Expires header field indicates the expiration time. If
neither mechanism is used, the expiration time specified in the
request, explicitly or by default, is used.
3xx and 485 responses: The Contact response-header field can be used
with a 3xx or 485 (Ambiguous) response codes to indicate one or
more alternate addresses to try. It can appear in responses to
BYE, INVITE and OPTIONS methods. The Contact header field
contains URIs giving the new locations or user names to try, or
may simply specify additional transport parameters. A 300
(Multiple Choices), 301 (Moved Permanently), 302 (Moved
Temporarily) or 485 (Ambiguous) response SHOULD contain a
Contact field containing URIs of new addresses to be tried. A
301 or 302 response may also give the same location and username
that was being tried but specify additional transport parameters
such as a different server or multicast address to try or a
change of SIP transport from UDP to TCP or vice versa. The
client copies the user, password, host, port and user-param
elements of the Contact URI into the Request-URI of the
redirected request and directs the request to the address
specified by the maddr and port parameters, using the transport
protocol given in the transport parameter. If maddr is a
multicast address, the value of ttl is used as the time-to-live
value.
Note that the Contact header field MAY also refer to a different
entity than the one originally called. For example, a SIP call
connected to GSTN gateway may need to deliver a special information
announcement such as "The number you have dialed has been changed."
A Contact response header field can contain any suitable URI
indicating where the called party can be reached, not limited to SIP
URLs. For example, it can contain a phone or fax,
mailto: (RFC 2368, [25]) or irc: URL.
The following parameters are defined. Additional parameters may be
defined in other specifications.
q: The qvalue indicates the relative preference among the locations
given. qvalue values are decimal numbers from 0.0 to 1.0, with
higher values indicating higher preference.
action: The action parameter is only used when registering with the
REGISTER request. It indicates whether the client wishes that
the server proxy or redirect future requests intended for the
client. If this parameter is not specified the action taken
depends on server configuration. In its response, the registrar
SHOULD indicate the mode used. This parameter is ignored for
other requests.
expires: The expires parameter indicates how long the URI is valid.
The parameter is either a number indicating seconds or a quoted
string containing an HTTP-date. If this parameter is not
provided, the value of the Expires header field determines how
long the URI is valid.
Contact = ( "Contact" | "m" ) ":" ("*" | (1# ( address-spec
[ *( ";" contact-params ) ] [ comment ] ))
contact-params = "q" "=" qvalue
| "action" "=" "proxy" | "redirect"
| "expires" "=" delta-seconds | <"> HTTP-date <">
| extension-attribute
extension-attribute = extension-name [ "=" & extension-value ]
The Contact header field fulfills functionality similar to
the Location header field in HTTP. However, the HTTP header
only allows one address, unquoted. Since URIs can contain
commas and semicolons as reserved characters, they can be
mistaken for header or parameter delimiters, respectively.
The current syntax corresponds to that for the To and From
header, which also allows the use of display names.
Example:
Contact: "Mr. Watson" <sip:watson@worcester.bell-telephone.com>
;q=0.7; expires=3600,
"Mr. Watson" <mailto:watson@bell-telephone.com> ;q=0.1
6.14 Content-Encoding
The Content-Encoding entity-header field is used as a modifier to the The Content-Encoding entity-header field is used as a modifier to the
media-type. When present, its value indicates what additional content media-type. When present, its value indicates what additional content
codings have been applied to the entity-body, and thus what decoding codings have been applied to the entity-body, and thus what decoding
mechanisms MUST be applied in order to obtain the media-type mechanisms MUST be applied in order to obtain the media-type
referenced by the Content-Type header field. Content-Encoding is referenced by the Content-Type header field. Content-Encoding is
primarily used to allow a document to be compressed without losing primarily used to allow a document to be compressed without losing
the identity of its underlying media type. See [H14.12]. the identity of its underlying media type. See [H14.12].
6.14 Content-Length 6.15 Content-Length
The Content-Length entity-header field indicates the size of the The Content-Length entity-header field indicates the size of the
message-body, in decimal number of octets, sent to the recipient. message-body, in decimal number of octets, sent to the recipient.
Content-Length = "Content-Length" ":" 1*DIGIT Content-Length = "Content-Length" ":" 1*DIGIT
An example is An example is
Content-Length: 3495 Content-Length: 3495
Applications SHOULD use this field to indicate the size of the Applications SHOULD use this field to indicate the size of the
message-body to be transferred, regardless of the media type of the message-body to be transferred, regardless of the media type of the
entity. Any Content-Length greater than or equal to zero is a valid entity. Any Content-Length greater than or equal to zero is a valid
value. If no body is present in a message, then the Content-Length value. If no body is present in a message, then the Content-Length
header MUST be set to zero. If a server receives a UDP request header field MUST be set to zero. If a server receives a UDP request
without Content-Length, it MUST assume that the request encompasses without Content-Length, it MUST assume that the request encompasses
the remainder of the packet. If a response does not contain a the remainder of the packet. If a response does not contain a
Content-Length, the client assumes that it encompasses the remainder Content-Length, the client assumes that it encompasses the remainder
of the UDP packet or the data until the TCP connection is closed, as of the UDP packet or the data until the TCP connection is closed, as
applicable. Section 8 describes how to determine the length of the applicable. Section 8 describes how to determine the length of the
message body. message body.
6.15 Content-Type 6.16 Content-Type
The Content-Type entity-header field indicates the media type of the The Content-Type entity-header field indicates the media type of the
message-body sent to the recipient. The media-type element is defined message-body sent to the recipient. The media-type element is defined
in [H3.7]. in [H3.7].
Content-Type = ( "Content-Type" ":" media-type Content-Type = ( "Content-Type" ":" media-type
Examples of this header field are Examples of this header field are
Content-Type: application/sdp Content-Type: application/sdp
Content-Type: text/html; charset=ISO-8859-4 Content-Type: text/html; charset=ISO-8859-4
6.16 CSeq 6.17 CSeq
Clients MUST add the CSeq (command sequence) general-header field to Clients MUST add the CSeq (command sequence) general-header field to
every request. A CSeq request header field contains a single decimal every request. A CSeq header field in a request contains the request
sequence number chosen by the requesting client, unique within a method and a single decimal sequence number chosen by the requesting
single value of Call-ID. The sequence number MUST be expressible as a client, unique within a single value of Call-ID. The sequence number
32-bit unsigned integer. The initial value of the sequence number is MUST be expressible as a 32-bit unsigned integer. The initial value
arbitrary, but MUST be less than 2**31. Consecutive requests that of the sequence number is arbitrary, but MUST be less than 2**31.
differ in request method, headers or body, but have the same Call-ID Consecutive requests that differ in request method, headers or body,
MUST contain strictly monotonically increasing and contiguous but have the same Call-ID MUST contain strictly monotonically
sequence numbers; sequence numbers do not wrap around. increasing and contiguous sequence numbers; sequence numbers do not
Retransmissions of the same request carry the same sequence number, wrap around. Retransmissions of the same request carry the same
but an INVITE with a different message body or different header sequence number, but an INVITE with a different message body or
fields (a "re-invitation") acquires a new, higher sequence number. A different header fields (a "re-invitation") acquires a new, higher
server MUST echo the CSeq value from the request in its response. If sequence number. A server MUST echo the CSeq value from the request
the Method value is missing, the server fills it in appropriately. in its response. If the Method value is missing, the server fills it
in appropriately.
The ACK and CANCEL requests MUST contain the same CSeq value as the The ACK and CANCEL requests MUST contain the same CSeq value as the
INVITE request that it refers to, while a BYE request cancelling an INVITE request that it refers to, while a BYE request cancelling an
invitation MUST have a higher sequence number. invitation MUST have a higher sequence number.
A user agent server MUST remember the highest sequence number for any A user agent server MUST remember the highest sequence number for any
INVITE request with the same Call-ID value. The server MUST respond INVITE request with the same Call-ID value. The server MUST respond
to, but ignore any INVITE request with a lower sequence number. to, but ignore any INVITE request with a lower sequence number.
All requests spawned in a parallel search have the same CSeq value as All requests spawned in a parallel search have the same CSeq value as
skipping to change at page 42, line 26 skipping to change at page 45, line 21
With a length of 32 bits, a server could generate, within a single With a length of 32 bits, a server could generate, within a single
call, one request a second for about 136 years before needing to wrap call, one request a second for about 136 years before needing to wrap
around. The initial value of the sequence number is chosen so that around. The initial value of the sequence number is chosen so that
subsequent requests within the same call will not wrap around. A subsequent requests within the same call will not wrap around. A
non-zero initial value allows to use a time-based initial sequence non-zero initial value allows to use a time-based initial sequence
number, which protects against ambiguities when clients are re- number, which protects against ambiguities when clients are re-
invited to the same call after rebooting. A client could, for invited to the same call after rebooting. A client could, for
example, choose the 31 most significant bits of a 32-bit second clock example, choose the 31 most significant bits of a 32-bit second clock
as an initial sequence number. as an initial sequence number.
Forked requests must have the same CSeq as there would be ambiguity Forked requests MUST have the same CSeq as there would be ambiguity
otherwise between these forked requests and later BYE issued by the otherwise between these forked requests and later BYE issued by the
client user agent. client user agent.
Example: Example:
CSeq: 4711 INVITE CSeq: 4711 INVITE
6.17 Date 6.18 Date
General header field. See [H14.19]. General-header field. See [H14.19].
The Date header field reflects the time when the request or response
is first sent. Thus, retransmissions have the same Date header field
value as the original.
The Date header field can be used by simple end systems The Date header field can be used by simple end systems
without a battery-backed clock to acquire a notion of without a battery-backed clock to acquire a notion of
current time. current time.
6.18 Encryption 6.19 Encryption
The Encryption general-header field specifies that the content has The Encryption general-header field specifies that the content has
been encrypted. Section 13 describes the overall SIP security been encrypted. Section 13 describes the overall SIP security
architecture and algorithms. This header field is intended for end- architecture and algorithms. This header field is intended for end-
to-end encryption of requests and responses. Requests are encrypted to-end encryption of requests and responses. Requests are encrypted
with a public key belonging to the entity named in the To header with a public key belonging to the entity named in the To header
field. Responses are encrypted with the public key conveyed in the field. Responses are encrypted with the public key conveyed in the
Response-Key header field. Response-Key header field.
SIP chose not to adopt HTTP's Content-Transfer-Encoding SIP chose not to adopt HTTP's Content-Transfer-Encoding
header because the encrypted body may contain additional header field because the encrypted body may contain
SIP header fields as well as the body of the message. additional SIP header fields as well as the body of the
message.
For any encrypted message, at least the message body and possibly For any encrypted message, at least the message body and possibly
other message header fields are encrypted. An application receiving a other message header fields are encrypted. An application receiving a
request or response containing an Encryption header field decrypts request or response containing an Encryption header field decrypts
the body and then concatenates the plaintext to the request line and the body and then concatenates the plaintext to the request line and
headers of the original message. Message headers in the decrypted headers of the original message. Message headers in the decrypted
part completely replace those with the same field name in the part completely replace those with the same field name in the
plaintext part. (Note: If only the body of the message is to be plaintext part. (Note: If only the body of the message is to be
encrypted, the body has to be prefixed with CRLF to allow proper encrypted, the body has to be prefixed with CRLF to allow proper
concatenation.) Note that the request method and Request-URI cannot concatenation.) Note that the request method and Request-URI cannot
skipping to change at page 44, line 19 skipping to change at page 47, line 19
X9dOVj3CMjCP66RSHa/ea0wYTRRNYA/G+kdP8DSUcqYAAAE/hZPX6nFIqk7AVnf6 X9dOVj3CMjCP66RSHa/ea0wYTRRNYA/G+kdP8DSUcqYAAAE/hZPX6nFIqk7AVnf6
IpWHUPTelNUJpzUp5Ou+q/5P7ZAsn+cSAuF2YWtVjCf+SQmBR13p2EYYWHoxlA2/ IpWHUPTelNUJpzUp5Ou+q/5P7ZAsn+cSAuF2YWtVjCf+SQmBR13p2EYYWHoxlA2/
GgKADYe4M3JSwOtqwU8zUJF3FIfk7vsxmSqtUQrRQaiIhqNyG7KxJt4YjWnEjF5E GgKADYe4M3JSwOtqwU8zUJF3FIfk7vsxmSqtUQrRQaiIhqNyG7KxJt4YjWnEjF5E
WUIPhvyGFMJaeQXIyGRYZAYvKKklyAJcm29zLACxU5alX4M25lHQd9FR9Zmq6Jed WUIPhvyGFMJaeQXIyGRYZAYvKKklyAJcm29zLACxU5alX4M25lHQd9FR9Zmq6Jed
wbWvia6cAIfsvlZ9JGocmQYF7pcuz5pnczqP+/yvRqFJtDGD/v3s++G2R+ViVYJO wbWvia6cAIfsvlZ9JGocmQYF7pcuz5pnczqP+/yvRqFJtDGD/v3s++G2R+ViVYJO
z/lxGUZaM4IWBCf+4DUjNanZM0oxAE28NjaIZ0rrldDQmO8V9FtPKdHxkqA5iJP+ z/lxGUZaM4IWBCf+4DUjNanZM0oxAE28NjaIZ0rrldDQmO8V9FtPKdHxkqA5iJP+
6vGOFti1Ak4kmEz0vM/Nsv7kkubTFhRl05OiJIGr9S1UhenlZv9l6RuXsOY/EwH2 6vGOFti1Ak4kmEz0vM/Nsv7kkubTFhRl05OiJIGr9S1UhenlZv9l6RuXsOY/EwH2
z8X9N4MhMyXEVuC9rt8/AUhmVQ== z8X9N4MhMyXEVuC9rt8/AUhmVQ==
=bOW+ =bOW+
Since proxies may base their forwarding decision on any combination Since proxies can base their forwarding decision on any combination
of SIP header fields, there is no guarantee that an encrypted request of SIP header fields, there is no guarantee that an encrypted request
"hiding" header fields will reach the same destination as an "hiding" header fields will reach the same destination as an
otherwise identical un-encrypted request. otherwise identical un-encrypted request.
6.19 Expires 6.20 Expires
The Expires entity-header field gives the date and time after which The Expires entity-header field gives the date and time after which
the message content expires. the message content expires.
This header field is currently defined only for the REGISTER and This header field is currently defined only for the REGISTER and
INVITE methods. For REGISTER, it is a request and response-header INVITE methods. For REGISTER, it is a request and response-header
field. In a REGISTER request, the client indicates how long it wishes field. In a REGISTER request, the client indicates how long it wishes
the registration to be valid. In the response, the server indicates the registration to be valid. In the response, the server indicates
the earliest expiration time of all registrations. The server MAY the earliest expiration time of all registrations. The server MAY
choose a shorter time interval than that requested by the client, but choose a shorter time interval than that requested by the client, but
SHOULD NOT choose a longer one. SHOULD NOT choose a longer one.
For INVITE, it is a request and response-header field. In a request, For INVITE requests, it is a request and response-header field. In a
the callee can limit the validity of an invitation. For example, if a request, the callee can limit the validity of an invitation, for
client wants to limit how long a search should take at most or when a example, if a client wants to limit the time duration of a search or
conference invitation is time-limited. A user interface may take this a conference invitation. A user interface MAY take this as a hint to
as a hint to leave the invitation window on the screen even if the leave the invitation window on the screen even if the user is not
user is not currently at the workstation. This also limits the currently at the workstation. This also limits the duration of a
duration of a search. If the request expires before the search search. If the request expires before the search completes, the proxy
completes, the proxy returns a 408 (Request Timeout) status. In a 302 returns a 408 (Request Timeout) status. In a 302 (Moved Temporarily)
(Moved Temporarily) response, a server can advise the client of the response, a server can advise the client of the maximal duration of
maximal duration of the redirection. the redirection.
The value of this field can be either an HTTP-date or an integer The value of this field can be either an HTTP-date or an integer
number of seconds (in decimal), measured from the receipt of the number of seconds (in decimal), measured from the receipt of the
request. The latter approach is preferable for short durations, as it request. The latter approach is preferable for short durations, as it
does not depend on clients and servers sharing a synchronized clock. does not depend on clients and servers sharing a synchronized clock.
Expires = "Expires" ":" ( HTTP-date | delta-seconds ) Expires = "Expires" ":" ( HTTP-date | delta-seconds )
Two examples of its use are Two examples of its use are
Expires: Thu, 01 Dec 1994 16:00:00 GMT Expires: Thu, 01 Dec 1994 16:00:00 GMT
Expires: 5 Expires: 5
6.20 From 6.21 From
Requests and responses MUST contain a From general-header field, Requests and responses MUST contain a From general-header field,
indicating the initiator of the request. The From field MAY contain indicating the initiator of the request. The From field MAY contain
the Tag parameter. The server copies the To and From header fields the tag parameter. The server copies the From header field from the
from the request to the response and MUST add the tag parameter to request to the response. The optional display-name is meant to be
the To field in the response if the URL in the To field is not a
fully qualified hostname . The optional display-name is meant to be
rendered by a human-user interface. rendered by a human-user interface.
The SIP-URL MUST NOT contain the transport-param, maddr-param, ttl- The SIP-URL MUST NOT contain the transport-param, maddr-param, ttl-
param, or headers elements. A server that receives a SIP-URL with param, or headers elements. A server that receives a SIP-URL with
these elements removes them before further processing. these elements removes them before further processing.
Even if the display-name is empty, the name-addr form MUST be used if
the addr-spec contains a comma or semicolon.
From = ( "From" | "f" ) ":" ( name-addr | addr-spec ) From = ( "From" | "f" ) ":" ( name-addr | addr-spec )
*( ";" addr-params )
name-addr = [ display-name ] "<" addr-spec ">" name-addr = [ display-name ] "<" addr-spec ">"
addr-spec = SIP-URL | URI addr-spec = SIP-URL | URI
display-name = *token | quoted-string display-name = *token | quoted-string
addr-params = tag-param
tag-param = "tag=" UUID
UUID = 1*( hex | "-" )
Examples: Examples:
From: A. G. Bell <sip:agb@bell-telephone.com> From: "A. G. Bell" <sip:agb@bell-telephone.com>
From: sip:+12125551212@server.phone2net.com From: sip:+12125551212@server.phone2net.com
From: Anonymous <sip:c8oqz84zk7z@privacy.org> From: Anonymous <sip:c8oqz84zk7z@privacy.org>
The tag MAY appear in the From field of a request. It MUST be present
when it is possible that two instances of a user sharing a SIP
address can make call invitations with the same Call-ID.
Call-ID, To and From are needed to identify a call leg The use of version-1 (time based) or version-4 (random) UUID [19] is
matters in calls with third-party control. The format is OPTIONAL. The tag value is designed to be globally unique and
similar to the equivalent RFC 822 [24] header, but with a cryptographically random with at least 32 bits of randomness. A
URI instead of just an email address. single user maintains the same tag throughout the call identified by
the Call-ID.
6.21 Hide Call-ID, To and From are needed to identify a call leg leg
matters in calls with multiple responses to a forked
request. The format is similar to the equivalent RFC 822
[24] header, but with a URI instead of just an email
address.
The Hide request header field indicates that the path comprised of 6.22 Hide
the Via header fields (Section 6.40) should be hidden from subsequent
proxies and user agents. It can take two forms: Hide: route and A client uses the Hide request header field to indicate that it wants
Hide: hop. Hide header fields are typically added by the client user the path comprised of the Via header fields (Section 6.40) to be
agent, but MAY be added by any proxy along the path. hidden from subsequent proxies and user agents. It can take two
forms: Hide: route and Hide: hop. Hide header fields are typically
added by the client user agent, but MAY be added by any proxy along
the path.
If a request contains the "Hide: route" header field, all following If a request contains the "Hide: route" header field, all following
proxies SHOULD hide their previous hop. If a request contains the proxies SHOULD hide their previous hop. If a request contains the
"Hide: hop" header field, only the next proxy SHOULD hide the "Hide: hop" header field, only the next proxy SHOULD hide the
previous hop and then remove the Hide option unless it also wants to previous hop and then remove the Hide option unless it also wants to
remain anonymous. remain anonymous.
A server hides the previous hop by encrypting the host and port parts A server hides the previous hop by encrypting the host and port parts
of the top-most Via header with an algorithm of its choice. Servers of the top-most Via header field with an algorithm of its choice.
SHOULD add additional "salt" to the host and port information prior Servers SHOULD add additional "salt" to the host and port information
to encryption to prevent malicious downstream proxies from guessing prior to encryption to prevent malicious downstream proxies from
earlier parts of the path based on seeing identical encrypted Via guessing earlier parts of the path based on seeing identical
headers. Hidden Via fields are marked with the hidden Via option, as encrypted Via headers. Hidden Via fields are marked with the hidden
described in Section 6.40. Via option, as described in Section 6.40.
A server that is capable of hiding Via headers MUST attempt to A server that is capable of hiding Via headers MUST attempt to
decrypt all Via headers marked as "hidden" to perform loop detection. decrypt all Via headers marked as "hidden" to perform loop detection.
Servers that are not capable of hiding can ignore hidden Via fields Servers that are not capable of hiding can ignore hidden Via fields
in their loop detection algorithm. in their loop detection algorithm.
If hidden headers were not marked, a proxy would have to If hidden headers were not marked, a proxy would have to
decrypt all headers to detect loops, just in case one was decrypt all headers to detect loops, just in case one was
encrypted, as the Hide: Hop option may have been removed encrypted, as the Hide: Hop option may have been removed
along the way. along the way.
A host MUST NOT add such a "Hide: hop" header field unless it can A host MUST NOT add such a "Hide: hop" header field unless it can
guarantee it will only send a request for this destination to the guarantee it will only send a request for this destination to the
same next hop. The reason for this is that it is possible that the same next hop. The reason for this is that it is possible that the
request will loop back through this same hop from a downstream proxy. request will loop back through this same hop from a downstream proxy.
The loop will be detected by the next hop if the choice of next hop The loop will be detected by the next hop if the choice of next hop
is fixed, but could loop an arbitrary number of times otherwise. is fixed, but could loop an arbitrary number of times otherwise.
A client requesting "Hide: route" can only rely on keeping the A client requesting "Hide: route" can only rely on keeping the
request path private if it sends the request to a trusted proxy. request path private if it sends the request to a trusted proxy.
Hiding the route of a SIP request may be of limited value if the Hiding the route of a SIP request is of limited value if the request
request results in data packets being exchanged directly between the results in data packets being exchanged directly between the calling
calling and called user agent. and called user agent.
The use of Hide header fields is discouraged unless path privacy is The use of Hide header fields is discouraged unless path privacy is
truly needed; Hide fields impose extra processing costs and truly needed; Hide fields impose extra processing costs and
restrictions for proxies and can cause requests to generate 482 (Loop restrictions for proxies and can cause requests to generate 482 (Loop
Detected) responses that could otherwise be avoided. Detected) responses that could otherwise be avoided.
The encryption of Via header fields is described in more detail in The encryption of Via header fields is described in more detail in
Section 13. Section 13.
The Hide header field has the following syntax: The Hide header field has the following syntax:
Hide = "Hide" ":" ( "route" | "hop" ) Hide = "Hide" ":" ( "route" | "hop" )
6.22 Location
The Location general-header field can appear in requests, 1xx, 2xx
responses and 3xx responses.
REGISTER requests: REGISTER requests MAY contain a Location header
field indicating at which locations the user may be reachable.
The REGISTER request defines a wildcard Location field, "*",
which MUST only be used with Expires: 0 to remove all
registrations for a particular user.
INVITE and ACK requests: INVITE and ACK requests SHOULD contain
Location headers indicating from which location the request is
originating.
This allows the callee to send a BYE directly to the caller
instead of through a series of proxies. The Via header is
not sufficient since the desired address may be that of a
proxy.
INVITE 2xx responses: A user agent server sending a definitive,
positive response (2xx) MAY insert a Location response header
indicating the SIP address under which it is reachable most
directly for future SIP requests, such as ACK, within the same
Call-ID. This may be the address of the server itself or that of
a proxy, e.g., if the host is behind a firewall. The value of
this Location header is copied into the Request-URI of
subsequent requests for this call.
The Location value MUST NOT be cached across calls, as it
may not represent the most desirable location for a
particular destination address.
INVITE 1xx responses: A UAS sending a provisional response (1xx) MAY
insert a Location response header. It has the same semantics in
a 1xx response as a 2xx INVITE response.
REGISTER 2xx responses: Similarly, a REGISTER response MAY return all
locations at which the user is currently reachable.
3xx and 485 responses: The Location response-header field can be used
with a 3xx or 485 response codes to indicate one or more
alternate addresses to try. It can appear in responses to BYE,
INVITE and OPTIONS methods. The Location header field contains
URIs giving the new locations or user names to try, or may
simply specify additional transport parameters. A 300 (Multiple
Choices), 301 (Moved Permanently), 302 (Moved Temporarily) or
485 (Ambiguous) response SHOULD contain a Location field
containing URIs of new addressed to be tried. A 301 or 302
response may also give the same location and username that was
being tried but specify additional transport parameters such as
a different server or multicast address to try or a change of
SIP transport from UDP to TCP or vice versa. The client copies
the host, user and tag elements of the Location URI into the
Request-URI of the redirected request and directs the request to
the address specified in the maddr and port parameter, using the
transport protocol given in the transport parameter. If maddr is
a multicast address, the value of ttl is used as the time-to-
live value.
Note that the Location header may also refer to a different entity
than the one originally called. For example, a SIP call connected to
GSTN gateway may need to deliver a special information announcement
such as "The number you have dialed has been changed."
A Location response header may contain any suitable URI indicating
where the called party may be reached, not limited to SIP URLs. For
example, it may contain a phone or fax
a mailto: (RFC 2368, [25]) or irc: URL.
The following parameters are defined. Additional parameters may be
defined in other specifications.
q: The qvalue indicates the relative preference among the locations
given. qvalue values are decimal numbers from 0.0 to 1.0, with
higher values indicating higher preference.
action: The action is only used when registering with the REGISTER
request. It indicates whether the client wishes that the server
proxies or redirects future requests intended for the client.
The action taken if this parameter is not specified depends on
server configuration. In its response, the registrar SHOULD
indicate the mode used. This parameter is ignored for other
requests.
Location = ( "Location" | "m" ) ":"
("*" | (1# (( SIP-URL | URI )
[ LWS *( ";" location-params ) ] [ comment ] ))
location-params = "q" "=" qvalue
| "action" "=" "proxy" | "redirect"
| extension-attribute
extension-attribute = extension-name [ "=" extension-value ]
Example:
Location: sip:watson@worcester.bell-telephone.com ;q=0.7,
mailto:watson@bell-telephone.com ;q=0.1
6.23 Max-Forwards 6.23 Max-Forwards
The Max-Forwards request-header field may be used with any SIP method The Max-Forwards request-header field may be used with any SIP method
to limit the number of proxies or gateways that can forward the to limit the number of proxies or gateways that can forward the
request to the next inbound server. This can also be useful when the request to the next downstream server. This can also be useful when
client is attempting to trace a request chain which appears to be the client is attempting to trace a request chain which appears to be
failing or looping in mid-chain. [H14.31] failing or looping in mid-chain. [H14.31]
Max-Forwards = "Max-Forwards" ":" 1*DIGIT Max-Forwards = "Max-Forwards" ":" 1*DIGIT
The Max-Forwards value is a decimal integer indicating the remaining The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message may be forwarded. number of times this request message is allowed to be forwarded.
Each proxy or gateway recipient of a request containing a Max- Each proxy or gateway recipient of a request containing a Max-
Forwards header field MUST check and update its value prior to Forwards header field MUST check and update its value prior to
forwarding the request. If the received value is zero (0), the forwarding the request. If the received value is zero (0), the
recipient MUST NOT forward the request. Instead, for the OPTIONS and recipient MUST NOT forward the request. Instead, for the OPTIONS and
REGISTER methods, it MUST respond as the final recipient. For all REGISTER methods, it MUST respond as the final recipient. For all
other methods, the server returns 483 (Too many hops). other methods, the server returns 483 (Too many hops).
If the received Max-Forwards value is greater than zero, then the If the received Max-Forwards value is greater than zero, then the
forwarded message MUST contain an updated Max-Forwards field with a forwarded message MUST contain an updated Max-Forwards field with a
value decremented by one (1). value decremented by one (1).
Example: Example:
Max-Forwards: 6 Max-Forwards: 6
6.24 Organization 6.24 Organization
The Organization general-header field conveys the name of the The Organization general-header field conveys the name of the
organization to which the entity issuing the request or response organization to which the entity issuing the request or response
belongs. It may also be inserted by proxies at the boundary of an belongs. It MAY also be inserted by proxies at the boundary of an
organization and may be used by client software to filter calls. organization.
The field MAY be used by client software to filter calls.
Organization = "Organization" ":" *text Organization = "Organization" ":" *text
6.25 Priority 6.25 Priority
The Priority request header signals the urgency of the call to the The Priority request-header field indicates the urgency of the
callee. request as perceived by the client.
Priority = "Priority" ":" priority-value Priority = "Priority" ":" priority-value
priority-value = "emergency" | "urgent" | "normal" priority-value = "emergency" | "urgent" | "normal"
| "non-urgent" | "non-urgent"
The value of "emergency" should only be used when life, limb or The value of "emergency" MUST only be used when life, limb or
property are in imminent danger. property are in imminent danger.
Examples: Examples:
Subject: A tornado is heading our way! Subject: A tornado is heading our way!
Priority: emergency Priority: emergency
Subject: Weekend plans Subject: Weekend plans
Priority: non-urgent Priority: non-urgent
These are the values of RFC 2076 [26], with the addition of These are the values of RFC 2076 [26], with the addition of
"emergency". "emergency".
6.26 Proxy-Authenticate 6.26 Proxy-Authenticate
The Proxy-Authenticate response-header field MUST be included as part The Proxy-Authenticate response-header field MUST be included as part
of a 407 (Proxy Authentication Required) response. The field value of a 407 (Proxy Authentication Required) response. The field value
skipping to change at page 51, line 44 skipping to change at page 52, line 48
The Proxy-Authorization request-header field allows the client to The Proxy-Authorization request-header field allows the client to
identify itself (or its user) to a proxy which requires identify itself (or its user) to a proxy which requires
authentication. The Proxy-Authorization field value consists of authentication. The Proxy-Authorization field value consists of
credentials containing the authentication information of the user credentials containing the authentication information of the user
agent for the proxy and/or realm of the resource being requested. See agent for the proxy and/or realm of the resource being requested. See
[H14.34] for further details. [H14.34] for further details.
6.28 Proxy-Require 6.28 Proxy-Require
The Proxy-Require header is used to indicate proxy-sensitive features The Proxy-Require header field is used to indicate proxy-sensitive
that MUST be supported by the proxy. Any Proxy-Require header features that MUST be supported by the proxy. Any Proxy-Require
features that are not supported by the proxy MUST be negatively header field features that are not supported by the proxy MUST be
acknowledged by the proxy to the client if not supported. Servers negatively acknowledged by the proxy to the client if not supported.
treat this field identically to the Require field. Servers treat this field identically to the Require field.
See Section 6.30 for more details on the mechanics of this message See Section 6.30 for more details on the mechanics of this message
and a usage example. and a usage example.
6.29 Record-Route 6.29 Record-Route
The Record-Route request and response header field is added to an The Record-Route request and response header field is added to a
INVITE request by any proxy that insists on being in the path of request by any proxy that insists on being in the path of subsequent
subsequent ACK and BYE requests for the same call. It contains a requests for the same call leg. It contains a globally reachable
globally reachable Request-URI that identifies the proxy server. Each Request-URI that identifies the proxy server. Each proxy server adds
proxy server adds its Request-URI to the beginning of the list. its Request-URI to the beginning of the list.
The server copies the Record-Route header unchanged into the The server copies the Record-Route header field unchanged into the
response. (Record-Route is only relevant for 2xx responses.) response. (Record-Route is only relevant for 2xx responses.)
The calling user agent client copies the Record-Route header into a The calling user agent client copies the Record-Route header into a
Route header of subsequent requests, reversing the order of requests, Route header field of subsequent requests within the same call leg,
so that the first entry is closest to the caller. If the response reversing the order of requests, so that the first entry is closest
contained a Location header field, the calling user agent adds its to the user agent client. If the response contained a Contact header
content as the last Route header. Unless this would cause a loop, any field, the calling user agent adds its content as the last Route
client MUST send any subsequent requests for this Call-ID to the header. Unless this would cause a loop, any client MUST send any
first Request-URI in the Route request header and remove that entry. subsequent requests for this call leg to the first Request-URI in the
Route request header field and remove that entry.
The calling user agent MUST NOT use the Record-Route header field in
requests that contain Route header fields.
Some proxies, such as those controlling firewalls or in an Some proxies, such as those controlling firewalls or in an
automatic call distribution (ACD) system, need to maintain automatic call distribution (ACD) system, need to maintain
call state and thus need to receive any BYE and ACK packets call state and thus need to receive any BYE and ACK packets
for the call. for the call.
The Record-Route header field has the following syntax: The Record-Route header field has the following syntax:
Record-Route = "Record-Route" ":" 1# SIP-URL Record-Route = "Record-Route" ":" 1# name-addr
Proxy servers SHOULD use the maddr URL parameter containing their
address to ensure that subsequent requests are guaranteed to reach
exactly the same server.
Example for a request that has traversed the hosts ieee.org and Example for a request that has traversed the hosts ieee.org and
bell-telephone.com , in that order: bell-telephone.com , in that order:
Record-Route: sip:a.g.bell@bell-telephone.com, sip:a.bell@ieee.org Record-Route: sip:a.g.bell@bell-telephone.com, sip:a.bell@ieee.org
6.30 Require 6.30 Require
The Require request header is used by clients to tell user agent The Require request-header field is used by clients to tell user
servers about options that the client expects the server to support agent servers about options that the client expects the server to
in order to properly process the request. If a server does not support in order to properly process the request. If a server does
understand the option, it MUST respond by returning status code 420 not understand the option, it MUST respond by returning status code
(Bad Extension) and list those options it does not understand in the 420 (Bad Extension) and list those options it does not understand in
Unsupported header. the Unsupported header.
Require = "Require" ":" 1#option-tag Require = "Require" ":" 1#option-tag
Example: Example:
C->S: INVITE sip:watson@bell-telephone.com SIP/2.0 C->S: INVITE sip:watson@bell-telephone.com SIP/2.0
Require: com.example.billing Require: com.example.billing
Payment: sheep_skins, conch_shells Payment: sheep_skins, conch_shells
S->C: SIP/2.0 420 Bad Extension S->C: SIP/2.0 420 Bad Extension
skipping to change at page 53, line 32 skipping to change at page 54, line 41
understood (as in the example above). For a well-matched understood (as in the example above). For a well-matched
client-server pair, the interaction proceeds quickly, client-server pair, the interaction proceeds quickly,
saving a round-trip often required by negotiation saving a round-trip often required by negotiation
mechanisms. In addition, it also removes ambiguity when the mechanisms. In addition, it also removes ambiguity when the
client requires features that the server does not client requires features that the server does not
understand. Some features, such as call handling fields, understand. Some features, such as call handling fields,
are only of interest to end systems. are only of interest to end systems.
Proxy and redirect servers MUST ignore features that are not Proxy and redirect servers MUST ignore features that are not
understood. If a particular extension requires that intermediate understood. If a particular extension requires that intermediate
devices support it, the extension should be tagged in the Proxy- devices support it, the extension MUST be tagged in the Proxy-Require
Require field instead (see Section 6.28). field instead (see Section 6.28).
6.31 Response-Key 6.31 Response-Key
The Response-Key request header field can be used by a client to The Response-Key request-header field can be used by a client to
request the key that the called user agent SHOULD use to encrypt the request the key that the called user agent SHOULD use to encrypt the
response with. The syntax is: response with. The syntax is:
Response-Key = "Response-Key" ":" key-scheme 1*SP #key-param Response-Key = "Response-Key" ":" key-scheme 1*SP #key-param
key-scheme = token key-scheme = token
key-param = token "=" ( token | quoted-string ) key-param = token "=" ( token | quoted-string )
The key-scheme gives the type of encryption to be used for the The key-scheme gives the type of encryption to be used for the
response. Section 13 describes security schemes. response. Section 13 describes security schemes.
If the client insists that the server return an encrypted response, If the client insists that the server return an encrypted response,
it includes a it includes a
Require: org.ietf.sip.encrypt-response Require: org.ietf.sip.encrypt-response
header field in its request. If the client cannot encrypt for header field in its request. If the client cannot encrypt for
whatever reason, it MUST follow normal Require header field whatever reason, it MUST follow normal Require header field
procedures and return a 420 (Bad Extension) response. If this Require procedures and return a 420 (Bad Extension) response. If this Require
header is not present, a client SHOULD still encrypt, but MAY return header field is not present, a client SHOULD still encrypt, but MAY
an unencrypted response if unable to. return an unencrypted response if unable to.
6.32 Retry-After 6.32 Retry-After
The Retry-After response header field can be used with a 503 (Service The Retry-After general-header field can be used with a 503 (Service
Unavailable) response to indicate how long the service is expected to Unavailable) response to indicate how long the service is expected to
be unavailable to the requesting client and with a 404 (Not Found), be unavailable to the requesting client and with a 404 (Not Found),
600 (Busy), or 603 (Decline) response to indicate when the called 600 (Busy), or 603 (Decline) response to indicate when the called
party may be available again. The value of this field can be either party anticipates being available again. The value of this field can
an HTTP-date or an integer number of seconds (in decimal) after the be either an HTTP-date or an integer number of seconds (in decimal)
time of the response. after the time of the response.
A REGISTER request may include this header field when deleting A REGISTER request MAY include this header field when deleting
registrations with Location: *; Expires: 0. The Retry-After value registrations with Contact: * ;expires: 0. The Retry-After value then
then indicates when the user might again be reachable. The registrar indicates when the user might again be reachable. The registrar MAY
MAY then include this information in responses to future calls. then include this information in responses to future calls.
An optional comment can be used to indicate additional information An optional comment can be used to indicate additional information
about the time of callback. An optional duration parameter indicates about the time of callback. An optional duration parameter indicates
how long the called party will be reachable starting at the initial how long the called party will be reachable starting at the initial
time of availability. If no duration parameter is given, the service time of availability. If no duration parameter is given, the service
is assumed to be available indefinitely. is assumed to be available indefinitely.
Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds ) Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds )
[ comment ] [ ";duration" "=" delta-seconds ] [ comment ] [ ";duration" "=" delta-seconds ]
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Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds ) Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds )
[ comment ] [ ";duration" "=" delta-seconds ] [ comment ] [ ";duration" "=" delta-seconds ]
Examples of its use are Examples of its use are
Retry-After: Mon, 21 Jul 1997 18:48:34 GMT (I'm in a meeting) Retry-After: Mon, 21 Jul 1997 18:48:34 GMT (I'm in a meeting)
Retry-After: Mon, 1 Jan 9999 00:00:00 GMT Retry-After: Mon, 1 Jan 9999 00:00:00 GMT
(Dear John: Don't call me back, ever) (Dear John: Don't call me back, ever)
Retry-After: Fri, 26 Sep 1997 21:00:00 GMT;duration=3600 Retry-After: Fri, 26 Sep 1997 21:00:00 GMT;duration=3600
Retry-After: 120 Retry-After: 120
In the third example, the callee is reachable for one hour starting In the third example, the callee is reachable for one hour starting
at 21:00 GMT. In the last example, the delay is 2 minutes. at 21:00 GMT. In the last example, the delay is 2 minutes.
6.33 Route 6.33 Route
The Route request header determines the route taken by a request. The Route request-header field determines the route taken by a
Each host removes the first entry and then proxies the request to the request. Each host removes the first entry and then proxies the
host listed in that entry, also using it as the Request-URI. The request to the host listed in that entry, also using it as the
operation is further described in Section 6.29. Request-URI. The operation is further described in Section 6.29.
The Route header field has the following syntax: The Route header field has the following syntax:
Route = "Route" ":" 1# request-uri Route = "Route" ":" 1# addr-spec
6.34 Server 6.34 Server
The Server response-header field contains information about the The Server response-header field contains information about the
software used by the user agent server to handle the request. See software used by the user agent server to handle the request. See
[H14.39]. [H14.39].
6.35 Subject 6.35 Subject
This is intended to provide a summary, or to indicate the nature, of This is intended to provide a summary, or to indicate the nature, of
the call, allowing call filtering without having to parse the session the call, allowing call filtering without having to parse the session
description. (Also, the session description may not necessarily use description. (Also, the session description does not have to use the
the same subject indication as the invitation.) same subject indication as the invitation.)
Subject = ( "Subject" | "s" ) ":" *text Subject = ( "Subject" | "s" ) ":" *text
Example: Example:
Subject: Tune in - they are talking about your work! Subject: Tune in - they are talking about your work!
6.36 Timestamp 6.36 Timestamp
The timestamp general header describes when the client sent the The timestamp general-header field describes when the client sent the
request to the server. The value of the timestamp is of significance request to the server. The value of the timestamp is of significance
only to the client and may use any timescale. The server MUST echo only to the client and MAY use any timescale. The server MUST echo
the exact same value and MAY, if it has accurate information about the exact same value and MAY, if it has accurate information about
this, add a floating point number indicating the number of seconds this, add a floating point number indicating the number of seconds
that have elapsed since it has received the request. The timestamp is that have elapsed since it has received the request. The timestamp is
used by the client to compute the round-trip time to the server so used by the client to compute the round-trip time to the server so
that it can adjust the timeout value for retransmissions. that it can adjust the timeout value for retransmissions.
Timestamp = "Timestamp" ":" *(DIGIT) [ "." *(DIGIT) ] [ delay ] Timestamp = "Timestamp" ":" *(DIGIT) [ "." *(DIGIT) ] [ delay ]
delay = *(DIGIT) [ "." *(DIGIT) ] delay = *(DIGIT) [ "." *(DIGIT) ]
6.37 To 6.37 To
The To general-header field specifies recipient of the request, with The To general-header field specifies recipient of the request, with
the same SIP URL syntax as the From field. the same SIP URL syntax as the From field.
To = ( "To" | "t" ) ":" ( name-addr | addr-spec ) To = ( "To" | "t" ) ":" ( name-addr | addr-spec ) addr-params
The UAS copies the To header into its response, and MUST add a tag Requests and responses MUST contain a To general-header field,
parameter if the URL is not a full qualified hostname. indicating the desired recipient of the request. The optional
display-name is meant to be rendered by a human-user interface. The
UAS or redirect server copies the To header field into its response,
and MUST add a tag parameter if the URL in the To header field is not
a full qualified hostname.
The SIP-URL MUST NOT contain the transport-param, maddr-param, ttl-
param, or headers elements. A server that receives a SIP-URL with
these elements removes them before further processing.
The tag parameter serves as a general mechanism to distinguish
multiple instances of a user identified by a single SIP URL. As
proxies can fork requests, the same request can reach multiple
instances of a user (mobile and home phones, for example). As each
can respond, there needs to be a means to distinguish the responses
from each at the caller. The situation also arises with multicast
requests. The tag in the To header field serves to distinguish
responses at the UAC. It MUST be placed in the To field of the
response by each instance when there is a possibility that the
request was forked at an intermediate proxy. This, in general, means
that the tag MUST be inserted when the URL in the To does not refer
to a fully qualified hostname. The tag MUST be added by UAS,
registrars and redirect servers, but MUST NOT be inserted into
responses forwarded upstream by proxies. The tag is added for all
definitive responses for all methods, and MAY be added for
informational responses from a UAS or redirect server. All subsequent
transactions between two entities MUST include the tag parameter, as
described in Section 11.
The use of version-1 (time based) or version-4 (random) UUID [19] is
OPTIONAL. The tag value is designed to be globally unique and
cryptographically random with at least 32 bits of randomness. A
single user maintains the same tag throughout the call identified by
the Call-ID.
The tag parameter in To headers is ignored when matching responses to
requests that did not contain a tag in their To header.
A SIP server returns a 400 (Bad Request) response if it receives a A SIP server returns a 400 (Bad Request) response if it receives a
request with a To header field containing a URI with a scheme it does request with a To header field containing a URI with a scheme it does
not recognize. not recognize.
Example: Example:
To: The Operator <sip:operator@cs.columbia.edu> To: The Operator <sip:operator@cs.columbia.edu>;tag=287447
To: sip:+12125551212@server.phone2net.com To: sip:+12125551212@server.phone2net.com
Call-ID, To and From are needed to identify a call leg Call-ID, To and From are needed to identify a call leg leg
matters in calls with third-party control. The tag is added matters in calls with multiple responses from a forked
to the To header in the response to allow forking of future request. The tag is added to the To header field in the
requests for the same call by proxies, while addressing response to allow forking of future requests for the same
only one of the possibly several responding user agent call by proxies, while addressing only one of the possibly
servers. It also allows several instances of the callee to several responding user agent servers. It also allows
send requests that can be distinguished. several instances of the callee to send requests that can
be distinguished.
6.38 Unsupported 6.38 Unsupported
The Unsupported response-header field lists the features not
The Unsupported response header lists the features not supported by supported by the server. See Section 6.30 for a usage example and
the server. See Section 6.30 for a usage example and motivation. motivation.
6.39 User-Agent 6.39 User-Agent
The User-Agent general-header field contains information about the The User-Agent general-header field contains information about the
client user agent originating the request. See [H14.42]. client user agent originating the request. See [H14.42].
6.40 Via 6.40 Via
The Via field indicates the path taken by the request so far. This The Via field indicates the path taken by the request so far. This
prevents request looping and ensures replies take the same path as prevents request looping and ensures replies take the same path as
the requests, which assists in firewall traversal and other unusual the requests, which assists in firewall traversal and other unusual
routing situations. routing situations.
6.40.1 Requests 6.40.1 Requests
The client originating the request MUST insert into the request a Via The client originating the request MUST insert into the request a Via
field containing its host name or network address and, if not the field containing its host name or network address and, if not the
default port number, the port number at which it wishes to receive default port number, the port number at which it wishes to receive
responses. (Note that this port number may differ from the UDP source responses. (Note that this port number can differ from the UDP source
port number of the request.) A fully-qualified domain name is port number of the request.) A fully-qualified domain name is
RECOMMENDED. Each subsequent proxy server that sends the request RECOMMENDED. Each subsequent proxy server that sends the request
onwards MUST add its own additional Via field before any existing Via onwards MUST add its own additional Via field before any existing Via
fields. A proxy that receives a redirection (3xx) response and then fields. A proxy that receives a redirection (3xx) response and then
searches recursively, MUST use the same Via headers as on the searches recursively, MUST use the same Via headers as on the
original request. original request.
A proxy SHOULD check the top-most Via header to ensure that it A proxy SHOULD check the top-most Via header field to ensure that it
contains the sender's correct network address, as seen from that contains the sender's correct network address, as seen from that
proxy. If the sender's address is incorrect, the proxy should add an proxy. If the sender's address is incorrect, the proxy MUST add an
additional received attribute, as described 6.40.2. additional received attribute, as described 6.40.2.
A host behind a network address translator (NAT) or A host behind a network address translator (NAT) or
firewall may not be able to insert a network address into firewall may not be able to insert a network address into
the Via header that can be reached by the next hop beyond the Via header that can be reached by the next hop beyond
the NAT. Hosts behind NATs or NAPTs should insert the local the NAT. Hosts behind NATs or NAPTs MUST insert the local
port number of the outgoing socket, rather than the port port number of the outgoing socket, rather than the port
number for incoming requests, as NAPTs assume that number for incoming requests, as NAPTs assume that
responses return with reversed source and destination responses return with reversed source and destination
ports. ports.
A proxy sending a request to a multicast address MUST add the maddr A proxy sending a request to a multicast address MUST add the maddr
parameter to its Via header field, and SHOULD add the ttl parameter. parameter to its Via header field, and SHOULD add the ttl parameter.
If a server receives a request which contained an maddr parameter in If a server receives a request which contained an maddr parameter in
the topmost Via field, it should SHOULD send the response to the the topmost Via field, it SHOULD send the response to the multicast
multicast address listed in the maddr parameter. address listed in the maddr parameter.
If a proxy server receives a request which contains its own address, If a proxy server receives a request which contains its own address,
it MUST respond with a 482 (Loop Detected) status code. it MUST respond with a 482 (Loop Detected) status code.
This prevents a malfunctioning proxy server from causing This prevents a malfunctioning proxy server from causing
loops. Also, it cannot be guaranteed that a proxy server loops. Also, it cannot be guaranteed that a proxy server
can always detect that the address returned by a location can always detect that the address returned by a location
service refers to a host listed in the Via list, as a service refers to a host listed in the Via list, as a
single host may have aliases or several network interfaces. single host may have aliases or several network interfaces.
6.40.2 Receiver-tagged Via Fields 6.40.2 Receiver-tagged Via Fields
Normally, every host that sends or forwards a SIP message adds a Via Normally, every host that sends or forwards a SIP message adds a Via
field indicating the path traversed. However, it is possible that field indicating the path traversed. However, it is possible that
Network Address Translators (NAT) may change the source address of Network Address Translators (NAT) changes the source address of the
the request (e.g., from net-10 to a globally routable address), in request (e.g., from net-10 to a globally routable address), in which
which case the Via field cannot be relied on to route replies. To case the Via field cannot be relied on to route replies. To prevent
prevent this, a proxy SHOULD check the top-most Via header to ensure this, a proxy SHOULD check the top-most Via header field to ensure
that it contains the sender's correct network address, as seen from that it contains the sender's correct network address, as seen from
that proxy. If the sender's address is incorrect, the proxy should that proxy. If the sender's address is incorrect, the proxy MUST add
add a received tag to the Via field inserted by the previous hop. a received tag to the Via field inserted by the previous hop. Such a
Such a modified Via field is known as a receiver-tagged Via field. An modified Via field is known as a receiver-tagged Via field. An
example is: example is:
Via: SIP/2.0/UDP erlang.bell-telephone.com:5060 Via: SIP/2.0/UDP erlang.bell-telephone.com:5060
Via: SIP/2.0/UDP 10.0.0.1:5060 ;received=199.172.136.3 Via: SIP/2.0/UDP 10.0.0.1:5060 ;received=199.172.136.3
In this example, the message originated from 10.0.0.1 and traversed a In this example, the message originated from 10.0.0.1 and traversed a
NAT with the external address border.ieee.org (199.172.136.3) to NAT with the external address border.ieee.org (199.172.136.3) to
reach erlang.bell-telephone.com and tagged the previous hop's Via reach erlang.bell-telephone.com and tagged the previous hop's Via
field with the address that it actually came from. field with the address that it actually came from.
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field (Section 6.40.2), send the message to the address in field (Section 6.40.2), send the message to the address in
the received tag, using the port present in sent-by, or the received tag, using the port present in sent-by, or
port 5060 if none is present. port 5060 if none is present.
4. Otherwise, send the message to the address indicated by 4. Otherwise, send the message to the address indicated by
sent-by in the second Via field. sent-by in the second Via field.
5. If there is no second Via field, this response is destined 5. If there is no second Via field, this response is destined
for this client. for this client.
A user agent server or redirect server should send a response by A user agent server or redirect server sends a response by pretending
pretending to insert the received tag into the topmost Via header in to insert the received tag into the topmost Via header field in the
the request, and treating this header as the second Via in the above request, and treating this header field as the second Via in the
procedure. above procedure.
These rules ensure that a client only has to check the first Via These rules ensure that a client only has to check the first Via
field in a response to see if it needs processing. field in a response to see if it needs processing.
6.40.4 Syntax 6.40.4 Syntax
The format for a Via header is shown in Fig. 10. The format for a Via header field is shown in Fig. 10.
The defaults for "protocol-name" and "transport" are "SIP" and "UDP",
respectively. The "maddr" parameter, designating the multicast
address, and the "ttl" parameter, designating the time-to-live (TTL)
value, are included only if the request was sent via multicast. The
"received" parameter is added only for receiver-added Via fields
(Section 6.40.2). For reasons of privacy, a client or proxy may wish
to hide its Via information by encrypting it (see Section 6.22). The
"hidden" parameter is included if this header field was hidden by the
upstream proxy (see 6.22). Note that privacy of the proxy relies on
the cooperation of the next hop, as the next-hop proxy will, by
necessity, know the IP address and port number of the source host.
The "branch" parameter is included by every forking proxy. The token
MUST be unique for each distinct request generated when a proxy
forks. When a response arrives at the proxy it can use the branch
value to figure out which branch the response corresponds to. A proxy
which generates a single request (non-forking) MAY also insert the
"branch" parameter. The identifier has to be unique only within a set
of isomorphic requests.
Via = ( "Via" $|$ "v") ":" 1#( sent-protocol sent-by Via = ( "Via" $|$ "v") ":" 1#( sent-protocol sent-by
*( ";" via-params ) [ comment ] ) *( ";" via-params ) [ comment ] )
via-params = via-hidden | via-ttl | via-maddr via-params = via-hidden | via-ttl | via-maddr
| via-received | via-branch | via-received | via-branch
via-hidden = "hidden" via-hidden = "hidden"
via-ttl = "ttl" "=" ttl via-ttl = "ttl" "=" ttl
via-maddr = "maddr" "=" maddr via-maddr = "maddr" "=" maddr
via-received = "received" "=" host via-received = "received" "=" host
via-branch = "branch" "=" token via-branch = "branch" "=" token
sent-protocol = [ protocol-name "/" ] protocol-version sent-protocol = protocol-name "/" protocol-version "/" transport
[ "/" transport ]
protocol-name = "SIP" $|$ token protocol-name = "SIP" $|$ token
protocol-version = token protocol-version = token
transport = "UDP" $|$ "TCP" $|$ token transport = "UDP" $|$ "TCP" $|$ token
sent-by = ( host [ ":" port ] ) $|$ ( concealed-host ) sent-by = ( host [ ":" port ] ) $|$ ( concealed-host )
concealed-host = token concealed-host = token
ttl = 1*3DIGIT ; 0 to 255 ttl = 1*3DIGIT ; 0 to 255
Figure 10: Syntax of Via header field Figure 10: Syntax of Via header field
The defaults for "protocol-name" and "transport" are "SIP" and "UDP",
respectively. The "maddr" parameter, designating the multicast
address, and the "ttl" parameter, designating the time-to-live (TTL)
value, are included only if the request was sent via multicast. The
"received" parameter is added only for receiver-added Via fields
(Section 6.40.2). For reasons of privacy, a client or proxy may wish
to hide its Via information by encrypting it (see Section 6.21). The
"hidden" parameter is included if this header was hidden by the
upstream proxy (see 6.21). Note that privacy of the proxy relies on
the cooperation of the next hop, as the next-hop proxy will, by
necessity, know the IP address and port number of the source host.
The "branch" parameter is included by every forking proxy. The token
MUST be unique for each distinct request generated when a proxy
forks. When a response arrives at the proxy it can use the branch
value to figure out which branch the response corresponds to. A proxy
which generates a single request (non-forking) MAY also insert the
"branch" parameter. The identifier has to be unique only within a set
of isomorphic requests.
Via: SIP/2.0/UDP first.example.com:4000;ttl=16 Via: SIP/2.0/UDP first.example.com:4000;ttl=16
;maddr=224.2.0.1 (Example) ;maddr=224.2.0.1 (Example)
Via: SIP/2.0/UDP adk8 Via: SIP/2.0/UDP adk8
6.41 Warning 6.41 Warning
The Warning response-header field is used to carry additional The Warning response-header field is used to carry additional
information about the status of a response. Warning headers are sent information about the status of a response. Warning headers are sent
with responses and have the following format: with responses and have the following format:
Warning = "Warning" ":" 1#warning-value Warning = "Warning" ":" 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text warning-value = warn-code SP warn-agent SP warn-text
warn-code = 3DIGIT warn-code = 3DIGIT
warn-agent = ( host [ ":" port ] ) | pseudonym warn-agent = ( host [ ":" port ] ) | pseudonym
; the name or pseudonym of the server adding ; the name or pseudonym of the server adding
; the Warning header, for use in debugging ; the Warning header, for use in debugging
warn-text = quoted-string warn-text = quoted-string
A response may carry more than one Warning header. A response MAY carry more than one Warning header.
The warn-text should be in a natural language that is most likely to The warn-text should be in a natural language that is most likely to
be intelligible to the human user receiving the response. This be intelligible to the human user receiving the response. This
decision may be based on any available knowledge, such as the decision can be based on any available knowledge, such as the
location of the cache or user, the Accept-Language field in a location of the cache or user, the Accept-Language field in a
request, the Content-Language field in a response, etc. The default request, or the Content-Language field in a response. The default
language is English. language is English.
Any server may add Warning headers to a response. Proxy servers MUST Any server MAY add Warning headers to a response. Proxy servers MUST
place additional Warning headers before any Authorization headers. place additional Warning headers before any Authorization headers.
Within that constraint, Warning headers MUST be added after any Within that constraint, Warning headers MUST be added after any
existing Warning headers not covered by a signature. A proxy server existing Warning headers not covered by a signature. A proxy server
MUST NOT delete any Warning header that it received with a response. MUST NOT delete any Warning header field that it received with a
response.
When multiple Warning headers are attached to a response, the user When multiple Warning headers are attached to a response, the user
agent SHOULD display as many of them as possible, in the order that agent SHOULD display as many of them as possible, in the order that
they appear in the response. If it is not possible to display all of they appear in the response. If it is not possible to display all of
the warnings, the user agent first displays warnings that appear the warnings, the user agent first displays warnings that appear
early in the response. Systems that generate multiple Warning headers early in the response.
should order them with this user agent behavior in mind.
The warn-code consists of three digits. The first digit indicates the The warn-code consists of three digits. A first digit of "3"
significance of the warning, with 3xx indicating a warning that did indicates warnings specific to SIP.
not cause the request to fail and 4xx indicating a fatal error
condition that contributed to the failure of the request.
This is a list of the currently-defined warn-codes, each with a This is a list of the currently-defined warn-codes, each with a
recommended warn-text in English, and a description of its meaning. recommended warn-text in English, and a description of its meaning.
Additional warn-codes may be defined through IANA. Note that these Note that these warnings describe failures induced by the session
warnings describe failures induced by the session description. description.
x01 Insufficient bandwidth: The bandwidth specified in the session Warnings 300 through 329 are reserved for indicating problems with
description or defined by the media exceeds that known to be keywords in the session description, 330 through 339 are warnings
available. related to basic network services requested in the session
description, 370 through 379 are warnings related to quantitative QoS
parameters requested in the session description, and 390 through 399
are miscellaneous warnings that do not fall into one of the above
categories.
x02 Incompatible transport protocol: One or more transport protocols 300 Incompatible network protocol: One or more network protocols
described in the request are not available. contained in the session description are not available.
x03 Incompatible network protocol: One or more network protocols 301 Incompatible network address formats: One or more network address
described in the request are not available. formats contained in the session description are not available.
x04 Incompatible network address formats: One or more network address 302 Incompatible transport protocol: One or more transport protocols
formats described in the request are not available. described in the session description are not available.
x05 Incompatible media format: One or more media formats described in 303 Incompatible bandwidth units: One or more bandwidth measurement
the request are not available. units contained in the session description were not understood.
x06 Incompatible bandwidth description: One or more bandwidth 304 Media type not available: One or more media types contained in
descriptions contained in the request were not understood. the session description are not available.
x07 Multicast not available: The site where the user is located does 305 Incompatible media format: One or more media formats contained in
the session description available.
306 Attribute not understood: One or more of the media attributes in
the session description are not supported.
307 Session description parameter not understood: A parameter other
than those listed above was not understood.
330 Multicast not available: The site where the user is located does
not support multicast. not support multicast.
x08 Unicast not available: The site where the user is located does 331 Unicast not available: The site where the user is located does
not support unicast communication (usually due to the presence not support unicast communication (usually due to the presence
of a firewall). of a firewall).
x09 Media type not available: One or more media types contained in 370 Insufficient bandwidth: The bandwidth specified in the session
the request are not available. description or defined by the media exceeds that known to be
available.
x10 Attribute not understood: One or more of the media attributes in
the request are not supported.
x09 Session description parameter not understood: A parameter other
than those listed above was not understood.
x99 Miscellaneous warning: The warning text may include arbitrary 399 Miscellaneous warning: The warning text can include arbitrary
information to be presented to a human user, or logged. A system information to be presented to a human user, or logged. A system
receiving this warning MUST NOT take any automated action. receiving this warning MUST NOT take any automated action.
1xx and 2xx have been taken by HTTP/1.1. 1xx and 2xx have been taken by HTTP/1.1.
Additional warn-codes, as in the example below, can be defined
through IANA.
Examples: Examples:
Warning: 309 isi.edu "Session parameter 'foo' not understood" Warning: 307 isi.edu "Session parameter 'foo' not understood"
Warning: 404 isi.edu "Incompatible network address type 'E.164'" Warning: 301 isi.edu "Incompatible network address type 'E.164'"
6.42 WWW-Authenticate 6.42 WWW-Authenticate
The WWW-Authenticate response-header field MUST be included in 401 The WWW-Authenticate response-header field MUST be included in 401
(Unauthorized) response messages. The field value consists of at (Unauthorized) response messages. The field value consists of at
least one challenge that indicates the authentication scheme(s) and least one challenge that indicates the authentication scheme(s) and
parameters applicable to the Request-URI. See [H14.46] and [27]. parameters applicable to the Request-URI. See [H14.46] and [27].
The content of the realm parameter SHOULD be displayed to the user. A The content of the realm parameter SHOULD be displayed to the user. A
user agent SHOULD cache the authorization credentials for a given user agent SHOULD cache the authorization credentials for a given
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In addition to the "basic" and "digest" authentication schemes In addition to the "basic" and "digest" authentication schemes
defined in the specifications cited above, SIP defines a new scheme, defined in the specifications cited above, SIP defines a new scheme,
PGP (RFC 2015, [28]), Section 14. Other schemes, such as S-MIME, are PGP (RFC 2015, [28]), Section 14. Other schemes, such as S-MIME, are
for further study. for further study.
7 Status Code Definitions 7 Status Code Definitions
The response codes are consistent with, and extend, HTTP/1.1 response The response codes are consistent with, and extend, HTTP/1.1 response
codes. Not all HTTP/1.1 response codes are appropriate, and only codes. Not all HTTP/1.1 response codes are appropriate, and only
those that are appropriate are given here. Other HTTP/1.1 response those that are appropriate are given here. Other HTTP/1.1 response
codes should not be used. Response codes not defined by HTTP/1.1 have codes SHOULD NOT be used. Response codes not defined by HTTP/1.1 have
codes x80 upwards to avoid clashes with future HTTP response codes. codes x80 upwards to avoid clashes with future HTTP response codes.
Also, SIP defines a new class, 6xx. The default behavior for unknown Also, SIP defines a new class, 6xx. The default behavior for unknown
response codes is given for each category of codes. response codes is given for each category of codes.
7.1 Informational 1xx 7.1 Informational 1xx
Informational responses indicate that the server or proxy contacted Informational responses indicate that the server or proxy contacted
is performing some further action and does not yet have a definitive is performing some further action and does not yet have a definitive
response. The client SHOULD wait for a further response from the response. The client SHOULD wait for a further response from the
server, and the server SHOULD send such a response without further server, and the server SHOULD send such a response without further
prompting. Typically a server should send a 1xx response if it prompting. A server SHOULD send a 1xx response if it expects to take
expects to take more than 200 ms to obtain a final response. A more than 200 ms to obtain a final response. A server MAY issue zero
server can issue zero or more 1xx responses, with no restriction on or more 1xx responses, with no restriction on their ordering or
their ordering or uniqueness. Note that 1xx responses are not uniqueness. Note that 1xx responses are not transmitted reliably,
transmitted reliably, that is, they do not cause the client to send that is, they do not cause the client to send an ACK. Servers are
an ACK. Servers are free to retransmit informational responses and free to retransmit informational responses and clients can inquire
clients can inquire about the current state of call processing by about the current state of call processing by re-sending the request.
re-sending the request.
7.1.1 100 Trying 7.1.1 100 Trying
Some unspecified action is being taken on behalf of this call (e.g., Some unspecified action is being taken on behalf of this call (e.g.,
a database is being consulted), but the user has not yet been a database is being consulted), but the user has not yet been
located. located.
7.1.2 180 Ringing 7.1.2 180 Ringing
The called user agent has located a possible location where the user The called user agent has located a possible location where the user
has registered recently and is trying to alert the user. has registered recently and is trying to alert the user.
7.1.3 181 Call Is Being Forwarded 7.1.3 181 Call Is Being Forwarded
A proxy server MAY use this status code to indicate that the call is A proxy server MAY use this status code to indicate that the call is
being forwarded to a different set of destinations. The new being forwarded to a different set of destinations. The new
destinations are listed in Location headers. Proxies SHOULD be destinations are listed in Contact headers. Proxies SHOULD be
configurable not to reveal this information. configurable not to reveal this information.
7.1.4 182 Queued 7.1.4 182 Queued
The called party is temporarily unavailable, but the callee has The called party is temporarily unavailable, but the callee has
decided to queue the call rather than reject it. When the callee decided to queue the call rather than reject it. When the callee
becomes available, it will return the appropriate final status becomes available, it will return the appropriate final status
response. The reason phrase MAY give further details about the status response. The reason phrase MAY give further details about the status
of the call, e.g., "5 calls queued; expected waiting time is 15 of the call, e.g., "5 calls queued; expected waiting time is 15
minutes". The server MAY issue several 182 responses to update the minutes". The server MAY issue several 182 responses to update the
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OPTIONS: The callee has agreed to share its capabilities, included in OPTIONS: The callee has agreed to share its capabilities, included in
the message body. the message body.
REGISTER: The registration has succeeded. The client treats the REGISTER: The registration has succeeded. The client treats the
message body according to its Content-Type. message body according to its Content-Type.
7.3 Redirection 3xx 7.3 Redirection 3xx
3xx responses give information about the user's new location, or 3xx responses give information about the user's new location, or
about alternative services that may be able to satisfy the call. They about alternative services that might be able to satisfy the call.
SHOULD terminate an existing search, and MAY cause the initiator to They SHOULD terminate an existing search, and MAY cause the initiator
begin a new search if appropriate. to begin a new search if appropriate.
Any redirection (3xx) response MUST NOT suggest any of the addresses Any redirection (3xx) response MUST NOT suggest any of the addresses
in the Via (Section 6.40) path of the request in the Location header in the Via (Section 6.40) path of the request in the Contact header
field. (Addresses match if their host and port number match.) field. (Addresses match if their host and port number match.)
To avoid forwarding loops, a user agent client or proxy MUST check To avoid forwarding loops, a user agent client or proxy MUST check
whether the address returned by a redirect server equals an address whether the address returned by a redirect server equals an address
tried earlier. tried earlier.
7.3.1 300 Multiple Choices 7.3.1 300 Multiple Choices
The address in the request resolved to several choices, each with its The address in the request resolved to several choices, each with its
own specific location, and the user (or user agent) can select a own specific location, and the user (or user agent) can select a
preferred communication end point and redirect its request to that preferred communication end point and redirect its request to that
location. location.
The response SHOULD include an entity containing a list of resource The response SHOULD include an entity containing a list of resource
characteristics and location(s) from which the user or user agent can characteristics and location(s) from which the user or user agent can
choose the one most appropriate, if allowed by the Accept request choose the one most appropriate, if allowed by the Accept request
header. The entity format is specified by the media type given in the header. The entity format is specified by the media type given in the
Content-Type header field. The choices SHOULD also be listed as Content-Type header field. The choices SHOULD also be listed as
Location fields (Section 6.22). Unlike HTTP, the SIP response may Contact fields (Section 6.13). Unlike HTTP, the SIP response MAY
contain several Location fields or a list of addresses in a Location contain several Contact fields or a list of addresses in a Contact
field. User agents MAY use the Location field value for automatic field. User agents MAY use the Contact header field value for
redirection or MAY ask the user to confirm a choice. However, this automatic redirection or MAY ask the user to confirm a choice.
specification does not define any standard for such automatic However, this specification does not define any standard for such
selection. automatic selection.
This header is appropriate if the callee can be reached at This status response is appropriate if the callee can be
several different locations and the server cannot or reached at several different locations and the server
prefers not to proxy the request. cannot or prefers not to proxy the request.
7.3.2 301 Moved Permanently 7.3.2 301 Moved Permanently
The user can no longer be found at the address in the Request-URI and The user can no longer be found at the address in the Request-URI and
the requesting client should retry at the new address given by the the requesting client SHOULD retry at the new address given by the
Location header field (Section 6.22). The caller SHOULD update any Contact header field (Section 6.13). The caller SHOULD update any
local directories, address books and user location caches with this local directories, address books and user location caches with this
new value and redirect future requests to the address(es) listed. new value and redirect future requests to the address(es) listed.
7.3.3 302 Moved Temporarily 7.3.3 302 Moved Temporarily
The requesting client should retry the request at the new address(es) The requesting client SHOULD retry the request at the new address(es)
given by the Location header field (Section 6.22). The duration of given by the Contact header field (Section 6.13). The duration of the
the redirection can be indicated through an Expires (Section 6.19) redirection can be indicated through an Expires (Section 6.20)
header. header.
7.3.4 380 Alternative Service 7.3.4 380 Alternative Service
The call was not successful, but alternative services are possible. The call was not successful, but alternative services are possible.
The alternative services are described in the message body of the The alternative services are described in the message body of the
response. response.
7.4 Request Failure 4xx 7.4 Request Failure 4xx
4xx responses are definite failure responses from a particular 4xx responses are definite failure responses from a particular
server. The client SHOULD NOT retry the same request without server. The client SHOULD NOT retry the same request without
modification (e.g., adding appropriate authorization). However, the modification (e.g., adding appropriate authorization). However, the
same request to a different server may be successful. same request to a different server might be successful.
7.4.1 400 Bad Request 7.4.1 400 Bad Request
The request could not be understood due to malformed syntax. The request could not be understood due to malformed syntax.
7.4.2 401 Unauthorized 7.4.2 401 Unauthorized
The request requires user authentication. The request requires user authentication.
7.4.3 402 Payment Required 7.4.3 402 Payment Required
Reserved for future use. Reserved for future use.
7.4.4 403 Forbidden 7.4.4 403 Forbidden
The server understood the request, but is refusing to fulfill it. The server understood the request, but is refusing to fulfill it.
Authorization will not help, and the request should not be repeated. Authorization will not help, and the request SHOULD not be repeated.
7.4.5 404 Not Found 7.4.5 404 Not Found
The server has definitive information that the user does not exist at The server has definitive information that the user does not exist at
the domain specified in the Request-URI. This status is also returned the domain specified in the Request-URI. This status is also returned
if the domain in the Request-URI does not match any of the domains if the domain in the Request-URI does not match any of the domains
handled by the recipient of the request. handled by the recipient of the request.
7.4.6 405 Method Not Allowed 7.4.6 405 Method Not Allowed
The method specified in the Request-Line is not allowed for the The method specified in the Request-Line is not allowed for the
address identified by the Request-URI. The response MUST include an address identified by the Request-URI. The response MUST include an
Allow header containing a list of valid methods for the indicated Allow header field containing a list of valid methods for the
address. indicated address.
7.4.7 406 Not Acceptable 7.4.7 406 Not Acceptable
The resource identified by the request is only capable of generating The resource identified by the request is only capable of generating
response entities which have content characteristics not acceptable response entities which have content characteristics not acceptable
according to the accept headers sent in the request. according to the accept headers sent in the request.
7.4.8 407 Proxy Authentication Required 7.4.8 407 Proxy Authentication Required
This code is similar to 401 (Unauthorized), but indicates that the This code is similar to 401 (Unauthorized), but indicates that the
client MUST first authenticate itself with the proxy. The proxy MUST client MUST first authenticate itself with the proxy. The proxy MUST
return a Proxy-Authenticate header field (section 6.26) containing a return a Proxy-Authenticate header field (section 6.26) containing a
challenge applicable to the proxy for the requested resource. The challenge applicable to the proxy for the requested resource. The
client MAY repeat the request with a suitable Proxy-Authorization client MAY repeat the request with a suitable Proxy-Authorization
header field (section 6.27). SIP access authentication is explained header field (section 6.27). SIP access authentication is explained
in section 13.2 and [H11]. in section 13.2 and [H11].
This status code should be used for applications where access to the This status code is used for applications where access to the
communication channel (e.g., a telephony gateway) rather than the communication channel (e.g., a telephony gateway) rather than the
callee herself requires authentication. callee herself requires authentication.
7.4.9 408 Request Timeout 7.4.9 408 Request Timeout
The server could not produce a response, e.g., a user location, The server could not produce a response, e.g., a user location,
within the time indicated in the Expires request-header field. The within the time indicated in the Expires request-header field. The
client MAY repeat the request without modifications at any later client MAY repeat the request without modifications at any later
time. time.
7.4.10 409 Conflict 7.4.10 409 Conflict
The request could not be completed due to a conflict with the current The request could not be completed due to a conflict with the current
state of the resource. This response is returned is the action state of the resource. This response is returned is the action
parameter in a REGISTER request conflicts with existing parameter in a REGISTER request conflicts with existing
registrations. registrations.
7.4.11 414 Request-URI Too Long 7.4.11 410 Gone
The requested resource is no longer available at the server and no
forwarding address is known. This condition is expected to be
considered permanent. If the server does not know, or has no facility
to determine, whether or not the condition is permanent, the status
code 404 (Not Found) SHOULD be used instead.
7.4.12 411 Length Required
The server refuses to accept the request without a defined Content-
Length. The client MAY repeat the request if it adds a valid
Content-Length header field containing the length of the message-body
in the request message.
7.4.13 413 Request Entity Too Large
The server is refusing to process a request because the request
entity is larger than the server is willing or able to process. The
server MAY close the connection to prevent the client from continuing
the request.
If the condition is temporary, the server SHOULD include a Retry-
After header field to indicate that it is temporary and after what
time the client MAY try again.
7.4.14 414 Request-URI Too Long
The server is refusing to service the request because the Request-URI The server is refusing to service the request because the Request-URI
is longer than the server is willing to interpret. is longer than the server is willing to interpret.
7.4.12 415 Unsupported Media Type 7.4.15 415 Unsupported Media Type
The server is refusing to service the request because the message The server is refusing to service the request because the message
body of the request is in a format not supported by the requested body of the request is in a format not supported by the requested
resource for the requested method. resource for the requested method.
7.4.13 420 Bad Extension 7.4.16 420 Bad Extension
The server did not understand the protocol extension specified in a The server did not understand the protocol extension specified in a
Require (Section 6.30) header field. Require (Section 6.30) header field.
7.4.14 480 Temporarily Unavailable 7.4.17 480 Temporarily Unavailable
The callee's end system was contacted successfully but the callee is The callee's end system was contacted successfully but the callee is
currently unavailable (e.g., not logged in or logged in in such a currently unavailable (e.g., not logged in or logged in in such a
manner as to preclude communication with the callee). The response manner as to preclude communication with the callee). The response
may indicate a better time to call in the Retry-After header. The MAY indicate a better time to call in the Retry-After header. The
user may also be available elsewhere (unbeknownst to this host), user could also be available elsewhere (unbeknownst to this host),
thus, this response does not terminate any searches. The reason thus, this response does not terminate any searches. The reason
phrase SHOULD indicate a more precise cause as to why the callee is phrase SHOULD indicate a more precise cause as to why the callee is
unavailable. This value SHOULD be setable by the user agent. unavailable. This value SHOULD be setable by the user agent. Status
486 (Busy Here) MAY be used to more precisely indicate a particular
reason for the call failure.
7.4.15 481 Invalid Call-ID 7.4.18 481 Call Leg/Transaction Does Not Exist
The server received a BYE or CANCEL request with a Call-ID (Section This status is returned under two conditions: The server received a
6.12) value it does not recognize. (A server simply discards an ACK BYE request that does not match any existing call leg or the server
with an invalid Call-ID.) received a CANCEL request that does not match any existing
transaction. (A server simply discards an ACK referring to an unknown
transaction.)
7.4.16 482 Loop Detected 7.4.19 482 Loop Detected
The server received a request with a Via (Section 6.40) path The server received a request with a Via (Section 6.40) path
containing itself. containing itself.
7.4.17 483 Too Many Hops 7.4.20 483 Too Many Hops
The server received a request that contains more Via entries (hops) The server received a request that contains more Via entries (hops)
(Section 6.40) than allowed by the Max-Forwards (Section 6.23) header (Section 6.40) than allowed by the Max-Forwards (Section 6.23) header
field. field.
7.4.18 484 Address Incomplete 7.4.21 484 Address Incomplete
The server received a request with a To (Section 6.37) address or The server received a request with a To (Section 6.37) address or
Request-URI that was incomplete. Additional information should be Request-URI that was incomplete. Additional information SHOULD be
provided. provided.
This status code allows overlapped dialing. With overlapped This status code allows overlapped dialing. With overlapped
dialing, the client does not know the length of the dialing dialing, the client does not know the length of the dialing
string. It sends strings of increasing lengths, prompting string. It sends strings of increasing lengths, prompting
the user for more input, until it no longer receives a 484 the user for more input, until it no longer receives a 484
status response. status response.
7.4.19 485 Ambiguous 7.4.22 485 Ambiguous
The callee address provided in the request was ambiguous. The The callee address provided in the request was ambiguous. The
response MAY contain a listing of possible unambiguous addresses in response MAY contain a listing of possible unambiguous addresses in
Location headers. Contact headers.
Revealing alternatives may infringe on privacy concerns of the user Revealing alternatives can infringe on privacy concerns of the user
or the organization. It MUST be possible to configure a server to or the organization. It MUST be possible to configure a server to
respond with status 404 (Not Found) or to suppress the listing of respond with status 404 (Not Found) or to suppress the listing of
possible choices if the request address was ambiguous. possible choices if the request address was ambiguous.
Example response to a request with the URL lee@example.com : Example response to a request with the URL lee@example.com :
485 Ambiguous SIP/2.0 485 Ambiguous SIP/2.0
Location: sip:carol.lee@example.com (Carol Lee) Contact: Carol Lee <sip:carol.lee@example.com>
Location: sip:p.lee@example.com (Ping Lee) Contact: Ping Lee <sip:p.lee@example.com>
Location: sip:lee.foote@example.com (Lee M. Foote) Contact: Lee M. Foote <sip:lee.foote@example.com>
Some email and voice mail systems provide this Some email and voice mail systems provide this
functionality. A status code separate from 3xx is used functionality. A status code separate from 3xx is used
since the semantics are different: for 300, it is assumed since the semantics are different: for 300, it is assumed
that the same person or service will be reached by the that the same person or service will be reached by the
choices provided. While an automated choice or sequential choices provided. While an automated choice or sequential
search makes sense for a 3xx response, user intervention is search makes sense for a 3xx response, user intervention is
required for a 485 response. required for a 485 response.
7.4.23 486 Busy Here
The callee's end system was contacted successfully but the callee is
currently not willing or able to take additional calls. The response
MAY indicate a better time to call in the Retry-After header. The
user could also be available elsewhere, such as through a voice mail
service, thus, this response does not terminate any searches. Status
600 (Busy Everywhere) SHOULD be used if the client knows that no
other end system will be able to accept this call.
7.5 Server Failure 5xx 7.5 Server Failure 5xx
5xx responses are failure responses given when a server itself has 5xx responses are failure responses given when a server itself has
erred. They are not definitive failures, and MUST NOT terminate a erred. They are not definitive failures, and MUST NOT terminate a
search if other possible locations remain untried. search if other possible locations remain untried.
7.5.1 500 Server Internal Error 7.5.1 500 Server Internal Error
The server encountered an unexpected condition that prevented it from The server encountered an unexpected condition that prevented it from
fulfilling the request. fulfilling the request.
7.5.2 501 Not Implemented 7.5.2 501 Not Implemented
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The server, while acting as a gateway or proxy, received an invalid The server, while acting as a gateway or proxy, received an invalid
response from the downstream server it accessed in attempting to response from the downstream server it accessed in attempting to
fulfill the request. fulfill the request.
7.5.4 503 Service Unavailable 7.5.4 503 Service Unavailable
The server is currently unable to handle the request due to a The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server. The implication temporary overloading or maintenance of the server. The implication
is that this is a temporary condition which will be alleviated after is that this is a temporary condition which will be alleviated after
some delay. If known, the length of the delay may be indicated in a some delay. If known, the length of the delay MAY be indicated in a
Retry-After header. If no Retry-After is given, the client MUST Retry-After header. If no Retry-After is given, the client MUST
handle the response as it would for a 500 response. handle the response as it would for a 500 response.
Note: The existence of the 503 status code does not imply that a Note: The existence of the 503 status code does not imply that a
server has to use it when becoming overloaded. Some servers may wish server has to use it when becoming overloaded. Some servers MAY wish
to simply refuse the connection. to simply refuse the connection.
7.5.5 504 Gateway Timeout 7.5.5 504 Gateway Timeout
The server, while acting as a gateway, did not receive a timely The server, while acting as a gateway, did not receive a timely
response from the server (e.g., a location server) it accessed in response from the server (e.g., a location server) it accessed in
attempting to complete the request. attempting to complete the request.
7.5.6 505 Version Not Supported 7.5.6 505 Version Not Supported
The server does not support, or refuses to support, the SIP protocol The server does not support, or refuses to support, the SIP protocol
version that was used in the request message. The server is version that was used in the request message. The server is
indicating that it is unable or unwilling to complete the request indicating that it is unable or unwilling to complete the request
using the same major version as the client, other than with this using the same major version as the client, other than with this
error message. The response SHOULD contain an entity describing why error message. The response SHOULD contain an entity describing why
that version is not supported and what other protocols are supported that version is not supported and what other protocols are supported
by that server. by that server.
7.6 Global Failures 6xx 7.6 Global Failures 6xx
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that version is not supported and what other protocols are supported that version is not supported and what other protocols are supported
by that server. by that server.
7.6 Global Failures 6xx 7.6 Global Failures 6xx
6xx responses indicate that a server has definitive information about 6xx responses indicate that a server has definitive information about
a particular user, not just the particular instance indicated in the a particular user, not just the particular instance indicated in the
Request-URI. All further searches for this user are doomed to failure Request-URI. All further searches for this user are doomed to failure
and pending searches SHOULD be terminated. and pending searches SHOULD be terminated.
7.6.1 600 Busy 7.6.1 600 Busy Everywhere
The callee's end system was contacted successfully but the callee is The callee's end system was contacted successfully but the callee is
busy and does not wish to take the call at this time. The response busy and does not wish to take the call at this time. The response
may indicate a better time to call in the Retry-After header. If the MAY indicate a better time to call in the Retry-After header. If the
callee does not wish to reveal the reason for declining the call, the callee does not wish to reveal the reason for declining the call, the
callee should use status code 603 (Decline) instead. callee uses status code 603 (Decline) instead. This status response
is returned only if the client knows that no other end point (such as
a voice mail system) will answer the request. Otherwise, 486 (Busy
Here) should be returned.
7.6.2 603 Decline 7.6.2 603 Decline
The callee's machine was successfully contacted but the user The callee's machine was successfully contacted but the user
explicitly does not wish to or cannot participate. The response may explicitly does not wish to or cannot participate. The response MAY
indicate a better time to call in the Retry-After header. indicate a better time to call in the Retry-After header.
7.6.3 604 Does Not Exist Anywhere 7.6.3 604 Does Not Exist Anywhere
The server has authoritative information that the user indicated in The server has authoritative information that the user indicated in
the To request field does not exist anywhere. Searching for the user the To request field does not exist anywhere. Searching for the user
elsewhere will not yield any results. elsewhere will not yield any results.
7.6.4 606 Not Acceptable 7.6.4 606 Not Acceptable
The user's agent was contacted successfully but some aspects of the The user's agent was contacted successfully but some aspects of the
session description such as the requested media, bandwidth, or session description such as the requested media, bandwidth, or
addressing style were not acceptable. addressing style were not acceptable.
A 606 (Not Acceptable) response means that the user wishes to A 606 (Not Acceptable) response means that the user wishes to
communicate, but cannot adequately support the session described. The communicate, but cannot adequately support the session described. The
606 (Not Acceptable) response MAY contain a list of reasons in a 606 (Not Acceptable) response MAY contain a list of reasons in a
Warning header or headers describing why the session described cannot Warning header field describing why the session described cannot be
be supported. Reasons are listed in Section 6.41. It is hoped that supported. Reasons are listed in Section 6.41. It is hoped that
negotiation will not frequently be needed, and when a new user is negotiation will not frequently be needed, and when a new user is
being invited to join an already existing conference, negotiation may being invited to join an already existing conference, negotiation may
not be possible. It is up to the invitation initiator to decide not be possible. It is up to the invitation initiator to decide
whether or not to act on a 606 (Not Acceptable) response. whether or not to act on a 606 (Not Acceptable) response.
8 SIP Message Body 8 SIP Message Body
8.1 Body Inclusion 8.1 Body Inclusion
For a request message, the presence of a body is signaled by the Requests MAY contain message bodies unless otherwise noted. Within
inclusion of a Content-Length header. Only ACK, INVITE, OPTIONS and this specification, the BYE request MUST NOT contain a message body.
REGISTER requests may contain message bodies. For ACK, INVITE and For ACK, INVITE and OPTIONS, the message body is always a session
OPTIONS, the message body is always a session description. The use of description. The use of message bodies for REGISTER requests is for
message bodies for REGISTER requests is for further study. further study.
For response messages, whether or not a body is included is dependent For response messages, the request method and the response status
on both the request method and the response message's response code. code determine the type and interpretation of any message body. All
All responses MAY include a body, although it may be of zero length. responses MAY include a body. Message bodies for 1xx responses
Message bodies for 1xx responses contain advisory information about contain advisory information about the progress of the request. 2xx
the progress of the request. 2xx responses contain session responses to INVITE requests contain session descriptions. In 3xx
descriptions. In 3xx respones, the message body MAY contain the respones, the message body MAY contain the description of alternative
description of alternative destinations or services, as described in destinations or services, as described in Section 7.3. For responses
Section 7.3. For responses with status 400 or greater, the message with status 400 or greater, the message body MAY contain additional,
body MAY contain additional, human-readable information about the human-readable information about the reasons for failure. It is
reasons for failure. It is RECOMMENDED that information in 1xx and RECOMMENDED that information in 1xx and 300 and greater responses be
300 and greater responses be of type text/plain or text/html of type text/plain or text/html
8.2 Message Body Type 8.2 Message Body Type
The Internet media type of the message body MUST be given by the The Internet media type of the message body MUST be given by the
Content-Type header field, If the body has undergone any encoding Content-Type header field, If the body has undergone any encoding
(such as compression) then this MUST be indicated by the Content- (such as compression) then this MUST be indicated by the Content-
Encoding header field, otherwise Content-Encoding MUST be omitted. If Encoding header field, otherwise Content-Encoding MUST be omitted. If
applicable, the character set of the message body is indicated as applicable, the character set of the message body is indicated as
part of the Content-Type header-field value. part of the Content-Type header-field value.
8.3 Message Body Length 8.3 Message Body Length
The body length in bytes SHOULD be given by the Content-Length header The body length in bytes SHOULD be given by the Content-Length header
field. Section 6.22 describes the behavior in detail. field. Section 6.15 describes the behavior in detail.
The "chunked" transfer encoding of HTTP/1.1 MUST NOT be used for SIP. The "chunked" transfer encoding of HTTP/1.1 MUST NOT be used for SIP.
(Note: The chunked encoding modifies the body of a message in order (Note: The chunked encoding modifies the body of a message in order
to transfer it as a series of chunks, each with its own size to transfer it as a series of chunks, each with its own size
indicator.) indicator.)
9 Compact Form 9 Compact Form
When SIP is carried over UDP with authentication and a complex When SIP is carried over UDP with authentication and a complex
session description, it may be possible that the size of a request or session description, it may be possible that the size of a request or
response is larger than the MTU. To reduce this problem, a more response is larger than the MTU. To reduce this problem, a more
compact form of SIP is also defined by using alternative names for compact form of SIP is also defined by using alternative names for
common header fields. These short forms are NOT abbreviations, they common header fields. These short forms are NOT abbreviations, they
are field names. No other header field abbreviations are allowed. are field names. No other header field abbreviations are allowed.
short field name long field name note short field name long field name note
c Content-Type c Content-Type
e Content-Encoding e Content-Encoding
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response is larger than the MTU. To reduce this problem, a more response is larger than the MTU. To reduce this problem, a more
compact form of SIP is also defined by using alternative names for compact form of SIP is also defined by using alternative names for
common header fields. These short forms are NOT abbreviations, they common header fields. These short forms are NOT abbreviations, they
are field names. No other header field abbreviations are allowed. are field names. No other header field abbreviations are allowed.
short field name long field name note short field name long field name note
c Content-Type c Content-Type
e Content-Encoding e Content-Encoding
f From f From
i Call-ID i Call-ID
m Contact from "moved"
l Content-Length l Content-Length
m Location from "moved"
s Subject s Subject
t To t To
v Via v Via
Thus, the header in section 15.2 could also be written: Thus, the header in section 15.2 could also be written:
INVITE sip:schooler@vlsi.caltech.edu SIP/2.0 INVITE sip:schooler@vlsi.caltech.edu SIP/2.0
v:SIP/2.0/UDP 131.215.131.131;maddr=239.128.16.254;ttl=16 v:SIP/2.0/UDP 131.215.131.131;maddr=239.128.16.254;ttl=16
v:SIP/2.0/UDP 128.16.64.19 v:SIP/2.0/UDP 128.16.64.19
f:sip:mjh@isi.edu f:sip:mjh@isi.edu
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s=Mbone Audio s=Mbone Audio
i=Discussion of Mbone Engineering Issues i=Discussion of Mbone Engineering Issues
e=mbone@somewhere.com e=mbone@somewhere.com
c=IN IP4 224.2.0.1/127 c=IN IP4 224.2.0.1/127
t=0 0 t=0 0
m=audio 3456 RTP/AVP 0 m=audio 3456 RTP/AVP 0
Mixing short field names and long field names is allowed, but not Mixing short field names and long field names is allowed, but not
recommended. Servers MUST accept both short and long field names for recommended. Servers MUST accept both short and long field names for
requests. Proxies MUST NOT translate a request between short and long requests. Proxies MUST NOT translate a request between short and long
forms if authentication fields are present. forms.
10 Behavior of SIP Clients and Servers 10 Behavior of SIP Clients and Servers
10.1 General Remarks 10.1 General Remarks
SIP is defined so it can use either UDP (unicast or multicast) or TCP SIP is defined so it can use either UDP (unicast or multicast) or TCP
as a transport protocol; it provides its own reliability mechanism. as a transport protocol; it provides its own reliability mechanism.
10.1.1 Requests 10.1.1 Requests
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After receiving a CANCEL request from an upstream client, a stateful After receiving a CANCEL request from an upstream client, a stateful
proxy server MAY send a CANCEL on all branches where it has not yet proxy server MAY send a CANCEL on all branches where it has not yet
received a final response. received a final response.
When a user agent receives a request, it checks the Call-ID against When a user agent receives a request, it checks the Call-ID against
those of in-progress calls. If the Call-ID was found, it compares the those of in-progress calls. If the Call-ID was found, it compares the
tag value of To with the user's tag and rejects the request if the tag value of To with the user's tag and rejects the request if the
two do not match. If the From header, including any tag value, two do not match. If the From header, including any tag value,
matches the value for an existing call leg, the server compares the matches the value for an existing call leg, the server compares the
CSeq header value. If less than or equal to the current sequence CSeq header field value. If less than or equal to the current
number, the request is a retransmission. Otherwise, it is a new sequence number, the request is a retransmission. Otherwise, it is a
request. If the From header does not match an existing call leg, a new request. If the From header does not match an existing call leg,
new call leg is created. a new call leg is created.
If the Call-ID was not found, a new call leg is created, with entries If the Call-ID was not found, a new call leg is created, with entries
for the To, From and Call-ID headers. In this case, the To header for the To, From and Call-ID headers. In this case, the To header
should not have contained a tag. The server returns a response field should not have contained a tag. The server returns a response
containing the same To value, but with a unique tag added. The tag containing the same To value, but with a unique tag added. The tag
MAY be omitted if the To refers to a fully qualified host name. MAY be omitted if the To refers to a fully qualified host name.
10.1.2 Responses 10.1.2 Responses
A server MAY issue one or more provisional responses at any time A server MAY issue one or more provisional responses at any time
before sending a final response. If a stateful proxy, user agent before sending a final response. If a stateful proxy, user agent
server, redirect server or registrar cannot respond to a request with server, redirect server or registrar cannot respond to a request with
a final response within 200 ms, it MUST issue a provisional (1xx) a final response within 200 ms, it MUST issue a provisional (1xx)
response as soon as possible. Stateless proxies MUST NOT issue response as soon as possible. Stateless proxies MUST NOT issue
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10.2 Source Addresses, Destination Addresses and Connections 10.2 Source Addresses, Destination Addresses and Connections
10.2.1 Unicast UDP 10.2.1 Unicast UDP
Responses are returned to the address listed in the Via header field Responses are returned to the address listed in the Via header field
(Section 6.40), not the source address of the request. (Section 6.40), not the source address of the request.
10.2.2 Multicast UDP 10.2.2 Multicast UDP
Requests may be multicast; multicast requests likely feature a host- Requests MAY be multicast; multicast requests likely feature a host-
independent Request-URI. Multicast requests SHOULD have a time-to- independent Request-URI. Multicast requests SHOULD have a time-to-
live value of no greater than one, i.e., be restricted to the local live value of no greater than one, i.e., be restricted to the local
network. network.
A client receiving a multicast query does not have to check whether A client receiving a multicast query does not have to check whether
the host part of the Request-URI matches its own host or domain name. the host part of the Request-URI matches its own host or domain name.
If the request was received via multicast, the response is also If the request was received via multicast, the response is also
returned via multicast. Responses to multicast requests are multicast returned via multicast. Responses to multicast requests are multicast
with the same TTL as the request, where the TTL is derived from the with the same TTL as the request, where the TTL is derived from the
ttl parameter in the Via header (Section 6.40). ttl parameter in the Via header (Section 6.40).
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CANCEL, instead, provides a simpler and more standard way CANCEL, instead, provides a simpler and more standard way
to perform response suppression. It is for this reason that to perform response suppression. It is for this reason that
the use of CANCEL here is a SHOULD the use of CANCEL here is a SHOULD
10.3 TCP 10.3 TCP
A single TCP connection can serve one or more SIP transactions. A A single TCP connection can serve one or more SIP transactions. A
transaction contains zero or more provisional responses followed by transaction contains zero or more provisional responses followed by
one or more final responses. (Typically, transactions contain exactly one or more final responses. (Typically, transactions contain exactly
one final response, but there are exceptional circumstances, where, one final response, but there are exceptional circumstances, where,
for example, multiple 200 responses may be generated.) for example, multiple 200 responses can be generated.)
The client MAY close the connection at any time, but SHOULD keep the The client MAY close the connection at any time, but SHOULD keep the
connection open at least until the first final response arrives. The connection open at least until the first final response arrives. The
server SHOULD NOT close the TCP connection until it has sent its server SHOULD NOT close the TCP connection until it has sent its
final response, at which point it MAY close the TCP connection if it final response, at which point it MAY close the TCP connection if it
wishes to. However, normally it is the client's responsibility to wishes to. However, normally it is the client's responsibility to
close the connection. close the connection.
If the server leaves the connection open, and if the client so If the server leaves the connection open, and if the client so
desires it may re-use the connection for further SIP requests or for desires it MAY re-use the connection for further SIP requests or for
requests from the same family of protocols (such as HTTP or stream requests from the same family of protocols (such as HTTP or stream
control commands). control commands).
If a client closes a connection or the connection is reset (e.g., If a client closes a connection or the connection is reset (e.g.,
because the client has crashed and rebooted), the server treats this because the client has crashed and rebooted), the server treats this
as equivalent to having received a CANCEL request. as equivalent to having received a CANCEL request for all pending
transactions.
If a server needs to return a response to a client and no longer has If a server needs to return a response to a client and no longer has
a connection open to that client, it MAY open a connection to the a connection open to that client, it MAY open a connection to the
address listed in the Via header. Thus, a proxy or user agent MUST be address listed in the Via header. Thus, a proxy or user agent MUST be
prepared to receive both requests and responses on a "passive" prepared to receive both requests and responses on a "passive"
connection. connection.
10.4 Reliability for BYE, CANCEL, OPTIONS, REGISTER Requests 10.4 Reliability for BYE, CANCEL, OPTIONS, REGISTER Requests
10.4.1 UDP 10.4.1 UDP
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100 seconds to avoid having to, for example, contact the user or 100 seconds to avoid having to, for example, contact the user or
location server again upon receiving a retransmission. location server again upon receiving a retransmission.
Each server in a proxy chain generates its own final response to a Each server in a proxy chain generates its own final response to a
CANCEL request. The server responds immediately upon receipt of the CANCEL request. The server responds immediately upon receipt of the
CANCEL request rather than not waiting until it has received final CANCEL request rather than not waiting until it has received final
responses from the CANCEL requests it generates. responses from the CANCEL requests it generates.
BYE and OPTIONS final responses are generated by redirect and user BYE and OPTIONS final responses are generated by redirect and user
agent servers; REGISTER final responses are generated by registrars. agent servers; REGISTER final responses are generated by registrars.
Note that responses to these commands are not acknowledged via ACK. Note that in contrast to the reliability mechanism described in
Section 10.5, responses to these requests are not retransmitted and
not acknowledged via ACK.
The value of the initial retransmission timer is smaller The value of the initial retransmission timer is smaller
than that that for TCP since it is expected that network than that that for TCP since it is expected that network
paths suitable for interactive communications have round- paths suitable for interactive communications have round-
trip times smaller than 1 second. To avoid flooding the trip times smaller than 1 second. To avoid flooding the
network with packets every second even if the destination network with packets every second even if the destination
network is unreachable, the retransmission count has to be network is unreachable, the retransmission count has to be
bounded. Given that most transactions should consist of one bounded. Given that most transactions are expected to
request and a few responses, round-trip time estimation is consist of one request and a few responses, round-trip time
not likely to be very useful. If RTT estimation is desired estimation is not likely to be very useful. If RTT
to more quickly discover a missing final response, each estimation is desired to more quickly discover a missing
request retransmission needs to be labeled with its own final response, each request retransmission needs to be
Timestamp (Section 6.36), returned in the response. The labeled with its own Timestamp (Section 6.36), returned in
server caches the result until it can be sure that the the response. The server caches the result until it can be
client will not retransmit the same request again. sure that the client will not retransmit the same request
again.
10.4.2 TCP 10.4.2 TCP
Clients using TCP do not need to retransmit requests. Clients using TCP do not need to retransmit requests.
10.5 Reliability for ACK Requests 10.5 Reliability for INVITE Requests
The ACK request does not generate responses. It is only retransmitted
when a response to an INVITE request arrives. This behavior is
independent of the transport protocol. Note that the ACK request MAY
take a different path than the original INVITE request, and even may
cause a new TCP connection to be opened in order to send it.
10.6 Reliability for INVITE Requests
Special considerations apply for the INVITE method. Special considerations apply for the INVITE method.
1. After receiving an invitation, considerable time may elapse 1. After receiving an invitation, considerable time can elapse
before the server can determine the outcome. For example, before the server can determine the outcome. For example,
the called party may be "rung" or extensive searches may be if the called party is "rung" or extensive searches are
performed, so delays between the request and a definitive performed, delays between the request and a definitive
response can reach several tens of seconds. If either response can reach several tens of seconds. If either
caller or callee are automated servers not directly caller or callee are automated servers not directly
controlled by a human being, a call attempt may be controlled by a human being, a call attempt could be
unbounded in time. unbounded in time.
2. If a telephony user interface is modeled or if we need to 2. If a telephony user interface is modeled or if we need to
interface to the PSTN, the caller's user interface will interface to the PSTN, the caller's user interface will
provide "ringback", a signal that the callee is being provide "ringback", a signal that the callee is being
alerted. (The status response 180 (Ringing) may be used to alerted. (The status response 180 (Ringing) MAY be used to
initiate ringback.) Once the callee picks up, the caller initiate ringback.) Once the callee picks up, the caller
needs to know so that it can enable the voice path and stop needs to know so that it can enable the voice path and stop
ringback. The callee's response to the invitation could get ringback. The callee's response to the invitation could get
lost. Unless the response is transmitted reliably, the lost. Unless the response is transmitted reliably, the
caller will continue to hear ringback while the callee caller will continue to hear ringback while the callee
assumes that the call exists. assumes that the call exists.
3. The client has to be able to terminate an on-going request, 3. The client has to be able to terminate an on-going request,
e.g., because it is no longer willing to wait for the e.g., because it is no longer willing to wait for the
connection or search to succeed. The server will have to connection or search to succeed. The server will have to
wait several round-trip times to interpret the lack of wait several round-trip times to interpret the lack of
request retransmissions as the end of a call. If the call request retransmissions as the end of a call. If the call
succeeds shortly after the caller has given up, the callee succeeds shortly after the caller has given up, the callee
will "pick up the phone" and not be "connected". will "pick up the phone" and not be "connected".
10.6.1 UDP 10.5.1 UDP
For UDP, A SIP client SHOULD retransmit a SIP INVITE request For UDP, A SIP client SHOULD retransmit a SIP INVITE request
periodically with timer T1 until it receives a response. If the periodically with timer T1 until it receives a response. If the
client receives no response, it ceases retransmission after 20 client receives no response, it ceases retransmission after 20
attempts. If the response is provisional, the client continues to attempts. If the response is provisional, the client continues to
retransmit the request, albeit less frequently, using timer T3. The retransmit the request, albeit less frequently, using timer T3. The
default values of timer T1 and T3 are 1 and 30 seconds, respectively. default values of timer T1 and T3 are 1 and 30 seconds, respectively.
The value of T3 was chosen so that for most normal phone The value of T3 was chosen so that for most normal phone
calls, only one INVITE request will be issued. Typically, a calls, only one INVITE request will be issued. Typically, a
phone switches to an answering machine or voice mail after phone switches to an answering machine or voice mail after
about 20--22 seconds. The number of retransmissions after about 20--22 seconds. The number of retransmissions after
receiving a provisional response is unlimited to allow call receiving a provisional response is unlimited to allow call
queueing. Clients may limit the number of invitations sent queueing. Clients MAY limit the number of invitations sent
for each call attempt. for each call attempt.
For 2xx final responses, only the user agent client generates an ACK. Only the user agent client generates an ACK for 2xx final responses,
If the response contained a Location header, the ACK MAY be sent to If the response contained a Contact header field, the ACK MAY be sent
the address listed in that Location header field. If the response to the address listed in that Contact header field. If the response
did not contain a Location header, the client uses the same To header did not contain a Contact header, the client uses the same To header
field and Request-URI as for the INVITE request and sends the ACK to field and Request-URI as for the INVITE request and sends the ACK to
the same destination as the original INVITE request. ACKs for final the same destination as the original INVITE request. ACKs for final
responses other than 2xx are sent to the same server that the responses other than 2xx are sent to the same server that the
original request was sent to, using the same Request-URI as the original request was sent to, using the same Request-URI as the
original request. Note, however, that the To field in the ACK is original request. Note, however, that the To header field in the ACK
copied from the response being acknowledged, not the request, and is copied from the response being acknowledged, not the request, and
thus may additionally contain the tag parameter. Also note than thus MAY additionally contain the tag parameter. Also note than
unlike 2xx final responses, a proxy generates an ACK for non-2xx unlike 2xx final responses, a proxy generates an ACK for non-2xx
final responses. final responses.
The server retransmits the final response at intervals of T4 (default The server retransmits the final response at intervals of T4 (default
value of T4 = 2 seconds) until it receives an ACK request for the value of T4 = 2 seconds) until one of the following conditions is
same Call-ID and CSeq from the same From source or until it has true:
retransmitted the final response 10 times. The ACK request MUST NOT
be acknowledged to prevent a response-ACK feedback loop.
Fig. 11 and 12 show the client and server state diagram for 1. An ACK request for the same transaction is received;
invitations.
2. a BYE request for the same call leg is received;
3. a CANCEL request for the same call leg is received and the
final response status was equal or greater to 300;
4. the response has been retransmitted 10 times.
The ACK request MUST NOT be acknowledged to prevent a response-ACK
feedback loop. Fig. 11 and 12 show the client and server state
diagram for invitations.
The mechanism in Sec. 10.4 would not work well for INVITE The mechanism in Sec. 10.4 would not work well for INVITE
because of the long delays between INVITE and a final because of the long delays between INVITE and a final
+===========+
* *
...........>* Initial *<;;;;;;;;;;
: 20*T1 * * ;
: +===========+ ;
: | ;
: | - ;
: | INVITE ;
: | ;
: v ;
: ************* ;
: T1 <--* * ;
: INVITE -->* Calling *--------+ ;
: * * | ;
: ************* | ;
: : | | ;
:.............: | 1xx xxx | ;
| - ACK | ;
| | ;
v | ;
************* | ;
T3 <--* * | ;
INVITE -->* Ringing *<->1xx | ;
* * | ;
************* | ;
| | ;
|<-------------+ ;
| ;
v ;
************* ;
xxx <--* * ;
ACK -->* Completed * ;
* * ;
************* ;
; 10*T4 ;
;;;;;;;;;;;;;;;;;;
event (xxx=status)
message
Figure 11: State transition diagram of client for INVITE method
response. If the 200 response were to get lost, the callee response. If the 200 response were to get lost, the callee
would believe the call to exist, but the voice path would would believe the call to exist, but the voice path would
be dead since the caller does not know that the callee has be dead since the caller does not know that the callee has
10*T4 +===============+
+-------------->* *
| * Initial *<...............
|;;;;;;;;;;;;;;>* *
|; +===============+ :
|; CANCEL ! :
|; 200 ! :
|; ! INVITE :
|; ! 1xx :
|; ! :
|; v :
|; ***************** BYE :
|; INVITE -->* * 200 :
|; 1xx <--* Call proceed. *..............>:
|; * * :
|;;;;;;;;;;;;;;;***************** :
|; ! ! :
|: ! ! :
|; failure ! ! picks up :
|; >= 300 ! ! 200 :
|; +-------+ +-------+ :
|; v v :
|; *********** *********** :
|;INVITE<* *< T4 ->* *>INVITE :
|;status>* failure *>status<-* success *<status :
|; * * * * :
|;;;;;;;;*********** *********** :
| ! : | | ! : :
| ! : | | ! : :
+-------------!-:-+------------+ ! : :
! :.................!..:.........>:
! ! BYE :
+---------+---------+ 200 :
! ACK :
! :
v :
***************** :
V---* * :
ACK * Confirmed * :
|-->* * :
***************** .
: :
:......................>:
event
message sent
Figure 12: State transition diagram of server for INVITE method
picked up. Thus, the INVITE retransmission interval would picked up. Thus, the INVITE retransmission interval would
have to be on the order of a second or two to limit the have to be on the order of a second or two to limit the
duration of this state confusion. Retransmitting the duration of this state confusion. Retransmitting the
response a fixed number of times increases the probability response a fixed number of times increases the probability
of success, but at the cost of significantly higher of success, but at the cost of significantly higher
processing and network load. processing and network load.
10.6.2 TCP 10.5.2 TCP
+===========+
| Initial |
+===========+
|
|
| -
| ------
| INVITE
+------v v
T1 +-----------+
------ | Calling |--------+
INVITE +-----------+ |
+------| | | |
+----------------+ | |
| | 1xx | >= 200
| | --- | ------
| | - | ACK
| | |
| +------v v v-----| |
| T3 +-----------+ 1xx |
| ------ | Ringing | --- |
| INVITE +-----------+ - |
| +------| | |-----+ |
| | |
| 2xx | |
| --- | 2xx |
| ACK | --- |
| | ACK |
+----------------+ | |
+------v | v |
xxx +-----------+ |
--- | Completed |<-------+
ACK +-----------+
+------|
event
-------
message
Figure 11: State transition diagram of client for INVITE method
A client using TCP MUST NOT retransmit requests, but uses the same A client using TCP MUST NOT retransmit requests, but uses the same
algorithm as for UDP (Section 10.6.1) to retransmit responses until algorithm as for UDP (Section 10.5.1) to retransmit responses until
it receives an ACK. (An implementation can simply set T1 and T3 to it receives an ACK. (An implementation can simply set T1 and T3 to
infinity and otherwise maintain the same state diagram.) infinity and otherwise maintain the same state diagram.)
+===========+
| Initial |<-------------+
+===========+ |
| |
| |
| INVITE |
| ------ |
| 1xx |
+------v v |
INVITE +-----------+ |
------ | Searching | |
1xx +-----------+ |
+------| | | +---------------->+
| | |
failure | | callee picks up |
------- | | --------------- |
>= 300 | | 200 |
| | | BYE
+------v v v v-----| | ---
INVITE +-----------+ T4 | 200
------ | Answered | ------ |
status +-----------+ status |
+------| | | |-----+ |
| +---------------->+
| |
| ACK |
| --- |
| - |
| |
+------v v |
ACK +-----------+ |
--- | Connected | |
- +-----------+ |
+------| | |
+-----------------+
event
-------
message
Figure 12: State transition diagram of server for INVITE method
It is necessary to retransmit 2xx responses as their It is necessary to retransmit 2xx responses as their
reliability is assured end-to-end only. If the chain of reliability is assured end-to-end only. If the chain of
proxies has a UDP link in the middle, it could lose the proxies has a UDP link in the middle, it could lose the
response, with no possibility of recovery. For simplicity, response, with no possibility of recovery. For simplicity,
we also retransmit non-2xx responses, although that is not we also retransmit non-2xx responses, although that is not
strictly necessary. strictly necessary.
10.6 Reliability for ACK Requests
The ACK request does not generate responses. It is only generated
when a response to an INVITE request arrives (see Section 10.5). This
behavior is independent of the transport protocol. Note that the ACK
request MAY take a different path than the original INVITE request,
and MAY even cause a new TCP connection to be opened in order to send
it.
11 Behavior of SIP User Agents 11 Behavior of SIP User Agents
This section describes the rules for user agent client and servers This section describes the rules for user agent client and servers
for generating and processing requests and responses. for generating and processing requests and responses.
11.1 Caller Issues Initial INVITE Request 11.1 Caller Issues Initial INVITE Request
When a user agent client desires to initiate a call, it formulates an When a user agent client desires to initiate a call, it formulates an
INVITE request. The To field in the request contains the address of INVITE request. The To field in the request contains the address of
the callee. The Request-URI contains the same address. The From field the callee. The Request-URI contains the same address. The From field
contains the address of the caller. If the From address can appear contains the address of the caller. If the From address can appear
in requests generated by other user agent clients for the same call, in requests generated by other user agent clients for the same call,
the caller should insert the tag parameter in the From field. A UAC the caller MUST insert the tag parameter in the From field. A UAC MAY
MAY optionally add a Location header containing an address where it optionally add a Contact header containing an address where it would
would like to be contacted for transactions from the callee back to like to be contacted for transactions from the callee back to the
the caller. caller.
11.2 Callee Issues Response 11.2 Callee Issues Response
When the initial INVITE request is received at the callee, the callee When the initial INVITE request is received at the callee, the callee
may accept, redirect, or reject the call. In all of these cases, it can accept, redirect, or reject the call. In all of these cases, it
formulates a response. The response MUST copy the To, From, Call-ID, formulates a response. The response MUST copy the To, From, Call-ID,
CSeq and Via fields from the request. Additionally, the responding CSeq and Via fields from the request. Additionally, the responding
UAS MUST add the tag parameter to the To field in the response if the UAS MUST add the tag parameter to the To field in the response if the
To field in the request was not the fully-qualified hostname of the To field in the request was not the fully-qualified hostname of the
UAS. Since a request from a UAC may fork and arrive at multiple UAS. Since a request from a UAC may fork and arrive at multiple
hosts, the tag parameter serves to distinguish, at the UAC, multiple hosts, the tag parameter serves to distinguish, at the UAC, multiple
responses from different UAS's. The UAS MAY add a Location header in responses from different UAS's. The UAS MAY add a Contact header
the response. It contains an address where the callee would like to field in the response. It contains an address where the callee would
be contacted for subsequent transactions, including the ACK for the like to be contacted for subsequent transactions, including the ACK
current INVITE. The UAS stores the values of the To and From field, for the current INVITE. The UAS stores the values of the To and From
including any tags. These become the local and remote addresses of field, including any tags. These become the local and remote
the call leg, respectively. addresses of the call leg, respectively.
11.3 Caller Receives Response to Initial Request 11.3 Caller Receives Response to Initial Request
Multiple responses may arrive at the UAC for a single INVITE request, Multiple responses may arrive at the UAC for a single INVITE request,
due to a forking proxy. Each response is distinguished by the "tag" due to a forking proxy. Each response is distinguished by the "tag"
parameter in the To header field, and each represents a distinct call parameter in the To header field, and each represents a distinct call
leg. The caller may choose to acknowledge or terminate the call with leg. The caller MAY choose to acknowledge or terminate the call with
each responding UAS. To acknowledge, it sends an ACK request, and to each responding UAS. To acknowledge, it sends an ACK request, and to
terminate it sends a BYE request. The To field in the ACK or BYE MUST terminate it sends a BYE request. The To header field in the ACK or
be the same as the To field in the 200 response, including any tag. BYE MUST be the same as the To field in the 200 response, including
The From header field MUST be the same as the From field in the 200 any tag. The From header field MUST be the same as the From header
response, including any tag. The Request-URI of the ACK or BYE field in the 200 (OK) response, including any tag. The Request-URI of
request MAY be set to whatever address was found in the Location the ACK or BYE request MAY be set to whatever address was found in
field in the 200 response, if present. Alternately, a UAC may copy the Contact header field in the 200 (OK) response, if present.
the address from the To field into the Request-URI. The UAC also Alternately, a UAC may copy the address from the To header field into
notes the value of the To and From field in each response. For each the Request-URI. The UAC also notes the value of the To and From
call leg, the To header field becomes the remote address, and the header fields in each response. For each call leg, the To header
From header field becomes the local address. field becomes the remote address, and the From header field becomes
the local address.
11.4 Caller or Callee Generate Subsequent Requests 11.4 Caller or Callee Generate Subsequent Requests
Once the call has been established, either the caller or callee may Once the call has been established, either the caller or callee MAY
generate either INVITE or BYE requests to change or terminate the generate INVITE or BYE requests to change or terminate the call.
call. Regardless of whether the caller or callee is generating the Regardless of whether the caller or callee is generating the new
new request, the fields in the request are set as follows. For the request, the header fields in the request are set as follows. For the
desired call leg, the To header field is set to the remote address, desired call leg, the To header field is set to the remote address,
and the From header field is set to the local address (both including and the From header field is set to the local address (both including
any tags). The Location header field MAY be different than the any tags). The Contact header field MAY be different than the Contact
Location header field sent in a previous response or request. The header field sent in a previous response or request. The Request-URI
Request-URI MAY be set to the value of the Location header field MAY be set to the value of the Contact header field received in a
received in a previous request or response from the remote party, or previous request or response from the remote party, or to the value
to the value of the remote address. of the remote address.
11.5 Receiving Subsequent Requests 11.5 Receiving Subsequent Requests
When a request is received subsequently, the following checks are When a request is received subsequently, the following checks are
made: made:
1. If the Call-ID is new, the request is for a new call, 1. If the Call-ID is new, the request is for a new call,
regardless of the values of the To and From header fields. regardless of the values of the To and From header fields.
2. If the Call-ID exists, the request is for an existing call. 2. If the Call-ID exists, the request is for an existing call.
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receiving a request, the server gathers the list of alternative receiving a request, the server gathers the list of alternative
locations and returns a final response of class 3xx or it refuses the locations and returns a final response of class 3xx or it refuses the
request. For CANCEL requests, it SHOULD also return a 2xx response. request. For CANCEL requests, it SHOULD also return a 2xx response.
This response ends the SIP transaction. The redirect server maintains This response ends the SIP transaction. The redirect server maintains
transaction state for the whole SIP transaction. It is up to the transaction state for the whole SIP transaction. It is up to the
client to detect forwarding loops between redirect servers. client to detect forwarding loops between redirect servers.
12.2 User Agent Server 12.2 User Agent Server
User agent servers behave similarly to redirect servers, except that User agent servers behave similarly to redirect servers, except that
they may also accept requests and return a response of class 2xx. they also accept requests and can return a response of class 2xx.
12.3 Proxy Server 12.3 Proxy Server
This section outlines processing rules for proxy servers. A proxy This section outlines processing rules for proxy servers. A proxy
server can either be stateful or stateless. When stateful, a proxy server can either be stateful or stateless. When stateful, a proxy
remembers the incoming request which generated outgoing requests, and remembers the incoming request which generated outgoing requests, and
the outgoing requests. A stateless proxy forgets all information once the outgoing requests. A stateless proxy forgets all information once
an outgoing request is generated. A forking proxy SHOULD be stateful. an outgoing request is generated. A forking proxy SHOULD be stateful.
A stateful proxy MAY become stateless at any time, but SHOULD remain A stateful proxy MAY become stateless at any time, but SHOULD remain
stateful until it sends a definitive response upstream. stateful until it sends a definitive response upstream.
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Furthermore, ACK's for 200 responses to INVITE's are always proxied Furthermore, ACK's for 200 responses to INVITE's are always proxied
downstream towards the UAS, as they would be for a stateless proxy. downstream towards the UAS, as they would be for a stateless proxy.
A stateless proxy does not act as a virtual UAS/UAC (as this would A stateless proxy does not act as a virtual UAS/UAC (as this would
require state). Rather, a stateless proxy forwards every request it require state). Rather, a stateless proxy forwards every request it
receives downstream, and every response it receives upstream. receives downstream, and every response it receives upstream.
12.3.1 Proxying Requests 12.3.1 Proxying Requests
To prevent loops, a server MUST check if its own address is already To prevent loops, a server MUST check if its own address is already
contained in the Via header of the incoming request. contained in the Via header field of the incoming request.
The To, From, Call-ID, and Location tags are copied exactly from the The To, From, Call-ID, and Contact tags are copied exactly from the
original request. The proxy SHOULD change the Request-URI to indicate original request. The proxy SHOULD change the Request-URI to indicate
the server where it intends to send the request. the server where it intends to send the request.
A proxy server always inserts a Via header field containing its own A proxy server always inserts a Via header field containing its own
address into those requests that are caused by an incoming request. address into those requests that are caused by an incoming request.
Each proxy MUST insert a "branch" parameter (Section 6.40). Each proxy MUST insert a "branch" parameter (Section 6.40).
12.3.2 Proxying Responses 12.3.2 Proxying Responses
A proxy only processes a response if the topmost Via field matches A proxy only processes a response if the topmost Via field matches
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A stateless proxy removes its own Via field, and checks the address A stateless proxy removes its own Via field, and checks the address
in the next Via field. If the field indicates TCP as the transport in the next Via field. If the field indicates TCP as the transport
protocol, the proxy checks to see if it has a connection currently protocol, the proxy checks to see if it has a connection currently
open to that address. If so, the response is sent on that connection. open to that address. If so, the response is sent on that connection.
Otherwise, a new TCP connection is opened to the address and port in Otherwise, a new TCP connection is opened to the address and port in
the Via field, and the response is sent there. In the case of UDP, the Via field, and the response is sent there. In the case of UDP,
the response is sent to the address listed in the "maddr" tag if the response is sent to the address listed in the "maddr" tag if
present, otherwise to the "received" tag if present, and finally to present, otherwise to the "received" tag if present, and finally to
the address in the "sent-by" field. Note that this implies that a UAC the address in the "sent-by" field. Note that this implies that a UAC
or proxy must be prepared to receive responses on the incoming side or proxy MUST be prepared to receive responses on the incoming side
of a TCP connection. of a TCP connection.
A stateless proxy MUST NOT generate its own provisional responses. A stateless proxy MUST NOT generate its own provisional responses.
12.3.4 Stateful Proxy: Receiving Requests 12.3.4 Stateful Proxy: Receiving Requests
When a stateful proxy receives a request, it checks the To, From When a stateful proxy receives a request, it checks the To, From
(including tags), Call-ID and CSeq against existing request records. (including tags), Call-ID and CSeq against existing request records.
If the tuple exists, the request is a retransmission. The provisional If the tuple exists, the request is a retransmission. The provisional
or final response sent previously is retransmitted, as per the server or final response sent previously is retransmitted, as per the server
state machine. If the tuple does not exist, the request corresponds state machine. If the tuple does not exist, the request corresponds
to a new transaction, and the request should be proxied. to a new transaction, and the request should be proxied.
A stateful proxy server MAY generate its own provisional (1xx) A stateful proxy server MAY generate its own provisional (1xx)
responses. responses.
12.3.5 Stateful Proxy: Receiving ACKs 12.3.5 Stateful Proxy: Receiving ACKs
When an ACK request is received, it must either be processed locally When an ACK request is received, it is either processed locally or
or proxied. To make this determination, the To, From, CSeq and Call- proxied. To make this determination, the To, From, CSeq and Call-ID
ID fields are compared against those in previous requests. If there fields are compared against those in previous requests. If there is
is no match, the ACK request is proxied as if it were an INVITE no match, the ACK request is proxied as if it were an INVITE request.
request. If there is a match, and if the server had ever sent a 200 If there is a match, and if the server had ever sent a 200 response
response upstream, the ACK is proxied. If the server had never sent upstream, the ACK is proxied. If the server had never sent any
any responses upstream, the ACK is also proxied. If the server had responses upstream, the ACK is also proxied. If the server had sent a
sent a 3xx, 4xx, 5xx or 6xx response, but no 2xx response, the ACK is 3xx, 4xx, 5xx or 6xx response, but no 2xx response, the ACK is
processed locally, as it is acknowledging the response generated by processed locally, as it is acknowledging the response generated by
the proxy. the proxy.
12.3.6 Stateful Proxy: Receiving Responses 12.3.6 Stateful Proxy: Receiving Responses
When a proxy server receives a response that has passed the Via When a proxy server receives a response that has passed the Via
checks, the proxy server checks the To (without the tag), From checks, the proxy server checks the To (without the tag), From
(including the tag), Call-ID and CSeq against values seen in previous (including the tag), Call-ID and CSeq against values seen in previous
requests. If there is no match, the response is forwarded upstream to requests. If there is no match, the response is forwarded upstream to
the address listed in the Via field. If there is a match, the the address listed in the Via field. If there is a match, the
"branch" tag in the Via field is examined. If it matches a known "branch" tag in the Via field is examined. If it matches a known
branch identifier, the response is for the given branch, and branch identifier, the response is for the given branch, and
processed by the virtual client for the given branch. Otherwise, the processed by the virtual client for the given branch. Otherwise, the
response is dropped. response is dropped.
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response is dropped. response is dropped.
A stateful proxy should obey the rules in Section 12.4 to determine A stateful proxy should obey the rules in Section 12.4 to determine
if the response should be proxied upstream. If it is to be proxied, if the response should be proxied upstream. If it is to be proxied,
the same rules for stateless proxies above are followed. the same rules for stateless proxies above are followed.
12.3.7 Stateless, Non-Forking Proxy 12.3.7 Stateless, Non-Forking Proxy
Proxies in this category issue at most a single unicast request for Proxies in this category issue at most a single unicast request for
each incoming SIP request, that is, they do not "fork" requests. each incoming SIP request, that is, they do not "fork" requests.
However, servers may choose to always operate in a mode that allows However, servers MAY choose to always operate in a mode that allows
issuing of several requests, as described in Section 12.4. issuing of several requests, as described in Section 12.4.
The server can forward the request and any responses. It does not The server can forward the request and any responses. It does not
have to maintain any state for the SIP transaction. Reliability is have to maintain any state for the SIP transaction. Reliability is
assured by the next redirect or stateful proxy server in the server assured by the next redirect or stateful proxy server in the server
chain. chain.
A proxy server SHOULD cache the result of any address translations A proxy server SHOULD cache the result of any address translations
and the response to speed forwarding of retransmissions. After the and the response to speed forwarding of retransmissions. After the
cache entry has been expired, the server cannot tell whether an cache entry has been expired, the server cannot tell whether an
incoming request is actually a retransmission of an older request. incoming request is actually a retransmission of an older request.
The server will treat it as a new request and commence another The server will treat it as a new request and commence another
search. search.
12.4 Forking Proxy 12.4 Forking Proxy
The server MUST respond to the request immediately with a 100 The server MUST respond to the request immediately with a 100
(Trying) response. (Trying) response.
Successful responses to an INVITE request SHOULD contain a Location Successful responses to an INVITE request SHOULD contain a Contact
header so that the following ACK or BYE bypasses the proxy search header field so that the following ACK or BYE bypasses the proxy
mechanism. If the proxy requires future requests to be routed through search mechanism. If the proxy requires future requests to be routed
it, it adds a Record-Route header to the request (Section 6.29). through it, it adds a Record-Route header to the request (Section
6.29).
The following pseudo-code describes the behavior of a proxy server The following pseudo-code describes the behavior of a proxy server
issuing several requests in response to an incoming INVITE request. issuing several requests in response to an incoming INVITE request.
The function request(r, a, b) sends a SIP request of type r to The function request(r, a, b) sends a SIP request of type r to
address a, with branch id b. await_response() waits until a response address a, with branch id b. await_response() waits until a response
is received and returns the response. close(a) closes the TCP is received and returns the response. close(a) closes the TCP
connection to client with address a. response(r) sends a response to connection to client with address a. response(r) sends a response to
the client. ismulticast() returns 1 if the location is a multicast the client. ismulticast() returns 1 if the location is a multicast
address and zero otherwise. The variable timeleft indicates the address and zero otherwise. The variable timeleft indicates the
amount of time left until the maximum response time has expired. The amount of time left until the maximum response time has expired. The
variable recurse indicates whether the server will recursively try variable recurse indicates whether the server will recursively try
addresses returned through a 3xx response. A server MAY decide to addresses returned through a 3xx response. A server MAY decide to
recursively try only certain addresses, e.g., those which are within recursively try only certain addresses, e.g., those which are within
the same domain as the proxy server. Thus, an initial multicast the same domain as the proxy server. Thus, an initial multicast
request may trigger additional unicast requests. request can trigger additional unicast requests.
/* request type */ /* request type */
typedef enum {INVITE, ACK, BYE, OPTIONS, CANCEL, REGISTER} Method; typedef enum {INVITE, ACK, BYE, OPTIONS, CANCEL, REGISTER} Method;
process_request(Method R, int N, address_t address[]) process_request(Method R, int N, address_t address[])
{ {
struct { struct {
address_t address; /* address */ address_t address; /* address */
int branch; /* branch id */ int branch; /* branch id */
int done; /* has responded */ int done; /* has responded */
skipping to change at page 87, line 22 skipping to change at page 91, line 4
} }
/* CANCEL: respond, fork and wait for responses */ /* CANCEL: respond, fork and wait for responses */
else if (class < 0) { else if (class < 0) {
best.status = 200; best.status = 200;
response(best); response(best);
for (i = 0; i < N; i++) { for (i = 0; i < N; i++) {
request(CANCEL, address[i], outgoing[i].branch); request(CANCEL, address[i], outgoing[i].branch);
} }
best.status = -1; best.status = -1;
} }
if (class == 2) { if (class == 2) {
if (r.status < best) best = r; if (r.status < best) best = r;
break; break;
} }
else if (class == 3) { else if (class == 3) {
/* A server may optionally recurse. The server MUST check /* A server MAY optionally recurse. The server MUST check
* whether it has tried this location before and whether the * whether it has tried this location before and whether the
* location is part of the Via path of the incoming request. * location is part of the Via path of the incoming request.
* This check is omitted here for brevity. Multicast locations * This check is omitted here for brevity. Multicast locations
* MUST NOT be returned to the client if the server is not * MUST NOT be returned to the client if the server is not
* recursing. * recursing.
*/ */
if (recurse) { if (recurse) {
multicast = 0; multicast = 0;
N += r.locations; N += r.locations;
for (i = 0; i < r.locations; i++) { for (i = 0; i < r.locations; i++) {
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proxy MAY send a CANCEL to all branches and return a 408 (Timeout) to proxy MAY send a CANCEL to all branches and return a 408 (Timeout) to
the client after 60 seconds or more. the client after 60 seconds or more.
1xx: The proxy MAY forward the response upstream towards the client. 1xx: The proxy MAY forward the response upstream towards the client.
2xx: The proxy MUST forward the response upstream towards the client, 2xx: The proxy MUST forward the response upstream towards the client,
without sending an ACK downstream. After receiving a 2xx, the without sending an ACK downstream. After receiving a 2xx, the
server MAY terminate all other pending requests by sending a server MAY terminate all other pending requests by sending a
CANCEL request and closing the TCP connection, if applicable. CANCEL request and closing the TCP connection, if applicable.
(Terminating pending requests is advisable as searches consume (Terminating pending requests is advisable as searches consume
resources. Also, INVITE requests may "ring" on a number of resources. Also, INVITE requests could "ring" on a number of
workstations if the callee is currently logged in more than workstations if the callee is currently logged in more than
once.) once.)
3xx: The proxy MUST send an ACK and MAY recurse on the listed 3xx: The proxy MUST send an ACK and MAY recurse on the listed Contact
Location addresses. Otherwise, the lowest-numbered response is addresses. Otherwise, the lowest-numbered response is returned
returned if there were no 2xx responses. if there were no 2xx responses.
Location lists are not merged as that would prevent Location lists are not merged as that would prevent
forwarding of authenticated responses. Also, some responses forwarding of authenticated responses. Also, responses can
may have message bodies, so that merging is not feasible. have message bodies, so that merging is not feasible.
4xx, 5xx: The proxy MUST send an ACK and remember the response if it 4xx, 5xx: The proxy MUST send an ACK and remember the response if it
has a lower status code than any previous 4xx and 5xx responses. has a lower status code than any previous 4xx and 5xx responses.
On completion, the lowest-numbered response is returned if there On completion, the lowest-numbered response is returned if there
were no 2xx or 3xx responses. were no 2xx or 3xx responses.
6xx: The proxy MUST forward the response to the client and send an 6xx: The proxy MUST forward the response to the client and send an
ACK. Other pending requests MAY be terminated with CANCEL as ACK. Other pending requests MAY be terminated with CANCEL as
described for 2xx responses. described for 2xx responses.
A proxy server forwards any response for Call-IDs for which it does A proxy server forwards any response for Call-IDs for which it does
not have a pending transaction according to the response's Via not have a pending transaction according to the response's Via
header. User agent servers respond to BYE requests with an unknown header. User agent servers respond to BYE requests for unknown call
Call-ID with status code 481 (Invalid Call-ID). ACK requests are legs with status code 481 (Transaction Does Not Exist); they drop ACK
silently dropped. requests with unknown call legs silently.
Special considerations apply for choosing forwarding destinations for Special considerations apply for choosing forwarding destinations for
ACK and BYE requests. In most cases, these requests will bypass ACK and BYE requests. In most cases, these requests will bypass
proxies and reach the desired party directly, keeping proxies from proxies and reach the desired party directly, keeping proxies from
having to make forwarding decisions. having to make forwarding decisions.
A proxy MAY maintain call state for a period of its choosing. If a A proxy MAY maintain call state for a period of its choosing. If a
proxy still has list of destinations that it forwarded the last proxy still has list of destinations that it forwarded the last
INVITE to, it SHOULD direct ACK requests only to those downstream INVITE to, it SHOULD direct ACK requests only to those downstream
servers. servers.
13 Security Considerations 13 Security Considerations
13.1 Confidentiality and Privacy: Encryption 13.1 Confidentiality and Privacy: Encryption
13.1.1 End-to-End Encryption 13.1.1 End-to-End Encryption
SIP requests and responses can contain sensitive information about SIP requests and responses can contain sensitive information about
the communication patterns and communication content of individuals the communication patterns and communication content of individuals.
and thus should be protected against eavesdropping. The SIP message The SIP message body MAY also contain encryption keys for the session
body may also contain encryption keys for the session itself. itself. SIP supports three complementary forms of encryption to
protect privacy:
SIP supports three complementary forms of encryption to protect
privacy:
o End-to-end encryption of the SIP message body and certain o End-to-end encryption of the SIP message body and certain
sensitive header fields; sensitive header fields;
o hop-by-hop encryption to prevent eavesdropping that tracks who o hop-by-hop encryption to prevent eavesdropping that tracks who
is calling whom; is calling whom;
o hop-by-hop encryption of Via fields to hide the route a o hop-by-hop encryption of Via fields to hide the route a
request has taken. request has taken.
Not all of the SIP request or response can be encrypted end-to-end Not all of the SIP request or response can be encrypted end-to-end
because header fields such as To and Via need to be visible to because header fields such as To and Via need to be visible to
proxies so that the SIP request can be routed correctly. Hop-by-hop proxies so that the SIP request can be routed correctly. Hop-by-hop
encryption encrypts the entire SIP request or response on the wire encryption encrypts the entire SIP request or response on the wire so
(the request may already have been end-to-end encrypted) so that that packet sniffers or other eavesdroppers cannot see who is calling
packet sniffers or other eavesdroppers cannot see who is calling whom. Hop-by-hop encryption can also encrypt requests and responses
whom. Note that proxies can still see who is calling whom, and this that have been end-to-end encrypted. Note that proxies can still see
information may also be deducible by performing a network traffic who is calling whom, and this information is also deducible by
analysis, so this provides a very limited but still worthwhile degree performing a network traffic analysis, so this provides a very
of protection. limited but still worthwhile degree of protection.
SIP Via fields are used to route a response back along the path taken SIP Via fields are used to route a response back along the path taken
by the request and to prevent infinite request loops. However, the by the request and to prevent infinite request loops. However, the
information given by them may also provide useful information to an information given by them can also provide useful information to an
attacker. Section 6.21 describes how a sender can request that Via attacker. Section 6.22 describes how a sender can request that Via
fields be encrypted by cooperating proxies without compromising the fields be encrypted by cooperating proxies without compromising the
purpose of the Via field. purpose of the Via field.
End-to-end encryption relies on keys shared by the two user agents End-to-end encryption relies on keys shared by the two user agents
involved in the request. Typically, the message is sent encrypted involved in the request. Typically, the message is sent encrypted
with the public key of the recipient, so that only that recipient can with the public key of the recipient, so that only that recipient can
read the message. SIP does not limit the security mechanisms that may read the message. All implementations SHOULD support PGP-based
be used, but all implementations SHOULD support PGP-based encryption. encryption and MAY implement other schemes.
A SIP request (or response) is end-to-end encrypted by splitting the A SIP request (or response) is end-to-end encrypted by splitting the
message to be sent into a part to be encrypted and a short header message to be sent into a part to be encrypted and a short header
that will remain in the clear. Some parts of the SIP message, namely that will remain in the clear. Some parts of the SIP message, namely
the request line, the response line and certain header fields marked the request line, the response line and certain header fields marked
with "n" in the "enc." column in Table 4 need to be read and returned with "n" in the "enc." column in Table 4 and 5 need to be read and
by proxies and thus MUST NOT be encrypted end-to-end. Possibly returned by proxies and thus MUST NOT be encrypted end-to-end.
sensitive information that needs to be made available as plaintext Possibly sensitive information that needs to be made available as
include destination address (To) and the forwarding path (Via) of the plaintext include destination address (To) and the forwarding path
call. The Authorization header MUST remain in the clear if it (Via) of the call. The Authorization header field MUST remain in the
contains a digital signature as the signature is generated after clear if it contains a digital signature as the signature is
encryption, but MAY be encrypted if it contains "basic" or "digest" generated after encryption, but MAY be encrypted if it contains
authentication. The From header field SHOULD normally remain in the "basic" or "digest" authentication. The From header field SHOULD
clear, but MAY be encrypted if required, in which case some proxies normally remain in the clear, but MAY be encrypted if required, in
MAY return a 401 (Unauthorized) status if they require a From field. which case some proxies MAY return a 401 (Unauthorized) status if
they require a From field.
Other header fields MAY be encrypted or MAY travel in the clear as Other header fields MAY be encrypted or MAY travel in the clear as
desired by the sender. The Subject, Allow, Call-ID, and Content-Type desired by the sender. The Subject, Allow, Call-ID, and Content-Type
header fields will typically be encrypted. The Accept, Accept- header fields will typically be encrypted. The Accept, Accept-
Language, Date, Expires, Priority, Require, Cseq, and Timestamp Language, Date, Expires, Priority, Require, Cseq, and Timestamp
header fields will remain in the clear. header fields will remain in the clear.
All fields that will remain in the clear MUST precede those that will All fields that will remain in the clear MUST precede those that will
be encrypted. The message is encrypted starting with the first be encrypted. The message is encrypted starting with the first
character of the first header field that will be encrypted and character of the first header field that will be encrypted and
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blank line as a normal SIP message body would be. blank line as a normal SIP message body would be.
Upon receipt by the called user agent possessing the correct Upon receipt by the called user agent possessing the correct
decryption key, the message body as indicated by the Content-Length decryption key, the message body as indicated by the Content-Length
field is decrypted, and the now-decrypted body is appended to the field is decrypted, and the now-decrypted body is appended to the
clear-text header fields. There is no need for an additional clear-text header fields. There is no need for an additional
Content-Length header field within the encrypted body because the Content-Length header field within the encrypted body because the
length of the actual message body is unambiguous after decryption. length of the actual message body is unambiguous after decryption.
Had no SIP header fields required encryption, the message would have Had no SIP header fields required encryption, the message would have
been as below. Note that the encrypted body must then include a blank been as below. Note that the encrypted body MUST then include a blank
line (start with CRLF) to disambiguate between any possible SIP line (start with CRLF) to disambiguate between any possible SIP
header fields that might have been present and the SIP message body. header fields that might have been present and the SIP message body.
INVITE sip:watson@boston.bell-telephone.com SIP/2.0$ INVITE sip:watson@boston.bell-telephone.com SIP/2.0$
Via: SIP/2.0/UDP 169.130.12.5$ Via: SIP/2.0/UDP 169.130.12.5$
To: T. A. Watson <sip:watson@bell-telephone.com>$ To: T. A. Watson <sip:watson@bell-telephone.com>$
From: A. Bell <a.g.bell@bell-telephone.com>$ From: A. Bell <a.g.bell@bell-telephone.com>$
Encryption: PGP version=5.0$ Encryption: PGP version=5.0$
Content-Type: application/sdp$ Content-Type: application/sdp$
Content-Length: 107$ Content-Length: 107$
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************************************************* *************************************************
* $ * * $ *
* v=0$ * * v=0$ *
* o=bell 53655765 2353687637 IN IP4 128.3.4.5$ * * o=bell 53655765 2353687637 IN IP4 128.3.4.5$ *
* c=IN IP4 135.180.144.94$ * * c=IN IP4 135.180.144.94$ *
* m=audio 3456 RTP/AVP 0 3 4 5$ * * m=audio 3456 RTP/AVP 0 3 4 5$ *
************************************************* *************************************************
13.1.2 Privacy of SIP Responses 13.1.2 Privacy of SIP Responses
SIP requests may be sent securely using end-to-end encryption and SIP requests can be sent securely using end-to-end encryption and
authentication to a called user agent that sends an insecure authentication to a called user agent that sends an insecure
response. This is allowed by the SIP security model, but is not a response. This is allowed by the SIP security model, but is not a
good idea. However, unless the correct behaviour is explicit, it good idea. However, unless the correct behaviour is explicit, it
would not always be possible for the called user agent to infer what would not always be possible for the called user agent to infer what
a reasonable behaviour was. Thus when end-to-end encryption is used a reasonable behaviour was. Thus when end-to-end encryption is used
by the request originator, the encryption key to be used for the by the request originator, the encryption key to be used for the
response SHOULD be specified in the request. If this were not done, response SHOULD be specified in the request. If this were not done,
it might be possible for the called user agent to incorrectly infer it might be possible for the called user agent to incorrectly infer
an appropriate key to use in the response. Thus, to prevent key- an appropriate key to use in the response. Thus, to prevent key-
guessing becoming an acceptable strategy, we specify that a called guessing becoming an acceptable strategy, we specify that a called
user agent receiving a request that does not specify a key to be used user agent receiving a request that does not specify a key to be used
for the response SHOULD send that response unencrypted. for the response SHOULD send that response unencrypted.
Any SIP header fields that were encrypted in a request should also be Any SIP header fields that were encrypted in a request SHOULD also be
encrypted in an encrypted response. Location response fields MAY be encrypted in an encrypted response. Contact response fields MAY be
encrypted if the information they contain is sensitive, or MAY be encrypted if the information they contain is sensitive, or MAY be
left in the clear to permit proxies more scope for localized left in the clear to permit proxies more scope for localized
searches. searches.
13.1.3 Encryption by Proxies 13.1.3 Encryption by Proxies
Normally, proxies are not allowed to alter end-to-end header fields Normally, proxies are not allowed to alter end-to-end header fields
and message bodies. Proxies MAY, however, encrypt an unsigned request and message bodies. Proxies MAY, however, encrypt an unsigned request
or response with the key of the call recipient. or response with the key of the call recipient.
Proxies may need to encrypt a SIP request if the end system Proxies need to encrypt a SIP request if the end system
cannot perform encryption or to enforce organizational cannot perform encryption or to enforce organizational
security policies. security policies.
13.1.4 Hop-by-Hop Encryption 13.1.4 Hop-by-Hop Encryption