draft-ietf-sip-symmetric-response-00.txt   draft-ietf-sip-symmetric-response-01.txt 
Internet Engineering Task Force SIP WG SIP J. Rosenberg
Internet Draft J. Rosenberg Internet-Draft dynamicsoft
dynamicsoft Expires: December 29, 2003 H. Schulzrinne
J. Weinberger Columbia University
dynamicsoft June 30, 2003
H. Schulzrinne
Columbia U.
draft-ietf-sip-symmetric-response-00.txt
September 27, 2002
Expires: March 2003
An Extension to the Session Initiation Protocol (SIP) An Extension to the Session Initiation Protocol (SIP) for Symmetric
for Symmetric Response Routing Response Routing
draft-ietf-sip-symmetric-response-01
STATUS OF THIS MEMO Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
The Session Initiation Protocol (SIP) operates over UDP and TCP. When The Session Initiation Protocol (SIP) operates over UDP and TCP. When
used with UDP, responses to requests are returned to the source used with UDP, responses to requests are returned to the source
address the request came from, and to the port written into the address the request came from, and to the port written into the
topmost Via header field value of the request. This behavior is not topmost Via header field value of the request. This behavior is not
desirable in many cases, most notably, when the client is behind a desirable in many cases, most notably, when the client is behind a
Network Address Translator (NAT). This extension defines a new Network Address Translator (NAT). This extension defines a new
parameter for the Via header field, called "rport", that allows a parameter for the Via header field, called "rport", that allows a
client to request that the server send the response back to the client to request that the server send the response back to the
source IP address and port where the request came from. source IP address and port where the request came from.
Table of Contents Table of Contents
1 Introduction ........................................ 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Terminology ......................................... 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Client Behavior ..................................... 3 3. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 5
4 Server Behavior ..................................... 4 4. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 6
5 Syntax .............................................. 5 5. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6 Example ............................................. 5 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7 Security Considerations ............................. 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8 IANA Considerations ................................. 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9 IAB Considerations .................................. 7 9. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 11
9.1 Problem Definition .................................. 8 9.1 Problem Definition . . . . . . . . . . . . . . . . . . . . . . 12
9.2 Exit Strategy ....................................... 8 9.2 Exit Strategy . . . . . . . . . . . . . . . . . . . . . . . . 12
9.3 Brittleness Introduced by this Specification ........ 8 9.3 Brittleness Introduced by this Specification . . . . . . . . . 13
9.4 Requirements for a Long Term Solution ............... 9 9.4 Requirements for a Long Term Solution . . . . . . . . . . . . 14
9.5 Issues with Existing NAPT Boxes ..................... 10 9.5 Issues with Existing NAPT Boxes . . . . . . . . . . . . . . . 14
10 Acknowledgements .................................... 10 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
11 Author's Addresses .................................. 11 Normative References . . . . . . . . . . . . . . . . . . . . . 16
12 Normative References ................................ 11 Informative References . . . . . . . . . . . . . . . . . . . . 17
13 Informative References .............................. 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . 18
1 Introduction 1. Introduction
The Session Initiation Protocol (SIP) [1] operates over UDP and TCP. The Session Initiation Protocol (SIP) [1] operates over UDP and TCP.
When used with UDP, responses to requests are returned to the source When used with UDP, responses to requests are returned to the source
address the request came from, and to the port written into the address the request came from, and to the port written into the
topmost Via header field value of the request. This results in a topmost Via header field value of the request. This results in a
"hybrid" way of computing the destination of the response. Half of "hybrid" way of computing the destination of the response. Half of
the information (specifically, the IP address) is taken from the IP the information (specifically, the IP address) is taken from the IP
packet headers, and the other half (specifically, the port) from the packet headers, and the other half (specifically, the port) from the
SIP message headers. SIP operates in this manner so that a server can SIP message headers. SIP operates in this manner so that a server can
listen for all messages, both requests and responses, on a single listen for all messages, both requests and responses, on a single IP
socket. This helps improve scalability. However, this behavior is not address and port. This helps improve scalability. However, this
desirable in many cases, most notably, when the client is behind a behavior is not desirable in many cases, most notably, when the
NAT. In that case, the response will not properly traverse the NAT, client is behind a NAT. In that case, the response will not properly
since it will not match the binding established with the request. traverse the NAT, since it will not match the binding established
with the request.
Furthermore, there is currently no way for a client to examine a Furthermore, there is currently no way for a client to examine a
response and determine the source port that the server saw in the response and determine the source port that the server saw in the
corresponding request. Currently, SIP does provide the client with corresponding request. Currently, SIP does provide the client with
the source IP address that the server saw in the request, but not the the source IP address that the server saw in the request, but not the
port. This information is conveyed in the "received" parameter in the port. This information is conveyed in the "received" parameter in the
topmost Via header field value of the response. This information has topmost Via header field value of the response. This information has
proved useful for basic NAT traversal, debugging purposes, and proved useful for basic NAT traversal, debugging purposes, and
support of multi-homed hosts. However, it is incomplete without the support of multi-homed hosts. However, it is incomplete without the
port information. port information.
This extension defines a new parameter for the Via header field, This extension defines a new parameter for the Via header field,
called "rport", that allows a client to request that the server send called "rport", that allows a client to request that the server send
the response back to the source IP address and port where the request the response back to the source IP address and port where the request
came from. The "rport" parameter is analagous to the "received" came from. The "rport" parameter is analagous to the "received"
parameter, except "rport" contains a port number, not the IP address. parameter, except "rport" contains a port number, not the IP address.
2 Terminology 2. Terminology
In this document, the key words "MUST", "MUSTNOT", "REQUIRED", In this document, the key words "MUST", "MUSTNOT", "REQUIRED",
"SHALL", "SHALLNOT", "SHOULD", "SHOULDNOT", "RECOMMENDED", "MAY", and "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
"OPTIONAL" are to be interpreted as described in RFC 2119 [2] and and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
indicate requirement levels for compliant SIP implementations. indicate requirement levels for compliant implementations.
3 Client Behavior 3. Client Behavior
The client behavior specified here affects the transport processing The client behavior specified here affects the transport processing
defined in Section 18.1 of SIP (RFC 3261) [1]. defined in Section 18.1 of SIP (RFC 3261) [1].
A client compliant to this specification (clients include UACs and A client compliant to this specification (clients include UACs and
proxies) MAY include an "rport" parameter in the top Via header field proxies) MAY include an "rport" parameter in the top Via header field
value of requests it generates. This parameter MUST have no value; it value of requests it generates. This parameter MUST have no value; it
serves as a flag to indicate to the server that this extension is serves as a flag to indicate to the server that this extension is
supported and requested for the transaction. supported and requested for the transaction.
When the client sends the request, if the request is sent using UDP, When the client sends the request, if the request is sent using UDP,
the client MUST be prepared to receive the response on the same the client MUST be prepared to receive the response on the same IP
socket the request was sent on. Specifically, it MUST be prepared to address and port it used to populate the source IP address and source
receive the response on the same IP address and port present in the port of the request. For backwards compatibility, the client MUST
source IP address and source port of the request. For backwards still be prepared to receive a response on the port indicated in the
compatibility, the client MUST still be prepared to receive a sent-by field of the topmost Via header field value, as specified in
response on the port indicated in the sent-by field of the topmost Section 18.1.1 of SIP [1].
Via header field value, as specified in Section 18.1.1 of SIP [1].
When there is a NAT between the client and server, the request will When there is a NAT between the client and server, the request will
create (or refresh) a binding in the NAT. This binding must remain in create (or refresh) a binding in the NAT. This binding must remain in
existence for the duration of the transaction in order for the client existence for the duration of the transaction in order for the client
to receive the response. Most UDP NAT bindings appear to have a to receive the response. Most UDP NAT bindings appear to have a
timeout of about one minute. This exceeds the duration of non-INVITE timeout of about one minute. This exceeds the duration of non-INVITE
transactions. Therefore, responses to a non-INVITE request will be transactions. Therefore, responses to a non-INVITE request will be
received while the binding is still in existence. INVITE transactions received while the binding is still in existence. INVITE transactions
can take an arbitrarily long amount of time to complete. As a result, can take an arbitrarily long amount of time to complete. As a result,
the binding may expire before a final response is received. To keep the binding may expire before a final response is received. To keep
the binding fresh, the client SHOULD retransmit its INVITE every 20 the binding fresh, the client SHOULD retransmit its INVITE every 20
seconds or so. These retransmissions will need to take place even seconds or so. These retransmissions will need to take place even
after receiving a provisional response. after receiving a provisional response.
4 Server Behavior A UA MAY execute the binding lifetime discovery algorithm in Section
10.2 of RFC 3489 [4] to determine the actual binding lifetime in the
NAT. If it is longer than 1 minute, the client SHOULD increase the
interval for request retransmissions up to half of the discovered
lifetime. If it is shorter than one minute, it SHOULD decrease the
interval for request retransmissions to half of the discovered
lifetime. Note that discovery of binding lifetimes can be unreliable.
See Section 14.3 of RFC 3489 [4].
4. Server Behavior
The server behavior specified here affects the transport processing The server behavior specified here affects the transport processing
defined in Section 18.2 of SIP [1]. defined in Section 18.2 of SIP [1].
When a server compliant to this specification (which can be a proxy When a server compliant to this specification (which can be a proxy
or UAS) receives a request, it examines the topmost Via header field or UAS) receives a request, it examines the topmost Via header field
value. If this Via header field value contains an "rport" parameter value. If this Via header field value contains an "rport" parameter
with no value, it MUST set the value of the parameter to the source with no value, it MUST set the value of the parameter to the source
port of the request. This is analagous to the way in which a server port of the request. This is analagous to the way in which a server
will insert the "received" parameter into the topmost Via header will insert the "received" parameter into the topmost Via header
field value. In fact, the server MUST insert a "received" parameter field value. In fact, the server MUST insert a "received" parameter
containing the source IP address that the request came from, even if containing the source IP address that the request came from, even if
it is identical to the value of the "sent-by" component. Note that it is identical to the value of the ``sent-by'' component. Note that
this processing takes place independent of the transport protocol. this processing takes place independent of the transport protocol.
When a server attempts to send a response, it examines the topmost When a server attempts to send a response, it examines the topmost
Via header field value of that response. If the "sent-protocol" Via header field value of that response. If the "sent-protocol"
component indicates an unreliable unicast transport protocol, such as component indicates an unreliable unicast transport protocol, such as
UDP, and there is no "maddr" parameter, but there is both a UDP, and there is no "maddr" parameter, but there is both a
"received" parameter and an "rport" parameter, the response MUST be "received" parameter and an "rport" parameter, the response MUST be
sent to the IP address listed in the "received" parameter, and the sent to the IP address listed in the "received" parameter, and the
port in the "rport" parameter. The response MUST be sent from the port in the "rport" parameter. The response MUST be sent from the
same address and port that the corresponding request was received on. same address and port that the corresponding request was received on.
skipping to change at page 5, line 21 skipping to change at page 7, line 5
will need to remember the one on which the request was received. For will need to remember the one on which the request was received. For
a stateful proxy, storing this information for the duration of the a stateful proxy, storing this information for the duration of the
transaction is not an issue. However, a stateless proxy does not transaction is not an issue. However, a stateless proxy does not
store state between a request and its response, and therefore cannot store state between a request and its response, and therefore cannot
remember the address and port on which a request was received. To remember the address and port on which a request was received. To
properly implement this specification, a stateless proxy can encode properly implement this specification, a stateless proxy can encode
the destination address and port of a request into the Via header the destination address and port of a request into the Via header
field value that it inserts. When the response arrives, it can field value that it inserts. When the response arrives, it can
extract this information and use it to forward the response. extract this information and use it to forward the response.
5 Syntax 5. Syntax
The syntax for the "rport" parameter is: The syntax for the "rport" parameter is:
response-port = "rport" [EQUAL 1*DIGIT] response-port = "rport" [EQUAL 1*DIGIT]
This extends the existing definition of the Via header field This extends the existing definition of the Via header field
parameters, so that its BNF now looks like: parameters, so that its BNF now looks like:
via-params = via-ttl / via-maddr via-params = via-ttl / via-maddr
/ via-received / via-branch / via-received / via-branch
/ response-port / via-extension / response-port / via-extension
6 Example 6. Example
Consider an example. A client sends an INVITE to a proxy server which Consider an example. A client sends an INVITE to a proxy server which
looks like, in part: looks like, in part:
INVITE sip:user@domain SIP/2.0 INVITE sip:user@example.com SIP/2.0
Via: SIP/2.0/UDP 10.1.1.1:4540;rport;branch=z9hG4bKkjshdyff Via: SIP/2.0/UDP 10.1.1.1:4540;rport;branch=z9hG4bKkjshdyff
This INVITE is sent with a source port of 4540 and a source IP This INVITE is sent with a source port of 4540 and a source IP
address of 10.1.1.1. The proxy is at 68.44.10.3, listening on both address of 10.1.1.1. The proxy is at 192.0.2.2, listening on both
port 5060 and 5070. The client sends the request to port 5060. The port 5060 and 5070. The client sends the request to port 5060. The
request passes through a NAT, so that to the proxy server, the source request passes through a NAT, so that to the proxy server, the source
IP address appears as 68.44.20.1 and the source port as 9988. The IP address appears as 192.0.2.1 and the source port as 9988. The
proxy forwards the request, but not before appending a value to the proxy forwards the request, but not before appending a value to the
"rport" parameter in the proxied request: "rport" parameter in the proxied request:
INVITE sip:user@domain2 SIP/2.0 INVITE sip:user@example.com SIP/2.0
Via: SIP/2.0/UDP proxy.domain.com;branch=z9hG4bKkjsh77 Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bKkjsh77
Via: SIP/2.0/UDP 10.1.1.1:4540;received=68.44.20.1;rport=9988 Via: SIP/2.0/UDP 10.1.1.1:4540;received=192.0.2.1;rport=9988
;branch=z9hG4bKkjshdyff ;branch=z9hG4bKkjshdyff
This request generates a response, which arrives at the proxy: This request generates a response, which arrives at the proxy:
SIP/2.0 200 OK SIP/2.0 200 OK
Via: SIP/2.0/UDP proxy.domain.com;branch=z9hG4bKkjsh77 Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bKkjsh77
Via: SIP/2.0/UDP 10.1.1.1:4540;received=68.44.20.1;rport=9988 Via: SIP/2.0/UDP 10.1.1.1:4540;received=192.0.2.1;rport=9988
;branch=z9hG4bKkjshdyff ;branch=z9hG4bKkjshdyff
The proxy strips its top Via header field value, and then examines The proxy strips its top Via header field value, and then examines
the next one. It contains both a "received" parameter, and an "rport" the next one. It contains both a "received" parameter, and an "rport"
parameter. The server follows the rules specified in Section 4 and parameter. The server follows the rules specified in Section 4 and
sends the response to IP address 68.44.20.1, port 9988, and sends it sends the response to IP address 192.0.2.1, port 9988, and sends it
from port 5060 on 68.44.10.3: from port 5060 on 192.0.2.2
SIP/2.0 200 OK SIP/2.0 200 OK
Via: SIP/2.0/UDP 10.1.1.1:4540;received=68.44.20.1;rport=9988 Via: SIP/2.0/UDP 10.1.1.1:4540;received=192.0.2.1;rport=9988
;branch=z9hG4bKkjshdyff ;branch=z9hG4bKkjshdyff
This packet matches the binding created by the initial request. This packet matches the binding created by the initial request.
Therefore, the NAT rewrites the destination address of this packet Therefore, the NAT rewrites the destination address of this packet
back to 10.1.1.1, and the destination port back to 4540. It forwards back to 10.1.1.1, and the destination port back to 4540. It forwards
this response to the client, which is listening for the response on this response to the client, which is listening for the response on
that address and port. The client properly receives the response. that address and port. The client properly receives the response.
7 Security Considerations 7. Security Considerations
Since this extension merely adds source port information to the When a server uses this specification, responses that it sends will
source IP address information already present in SIP, it does not now include the source port where the request came from. In some
appear to add any additional security considerations. instances, the source address and port of a request are sensitive
information. If they are sensitive, requests SHOULD be protected by
using SIP over TLS [1]. In such a case, this specification does not
provide any response routing functions (as these only work with TCP);
it merely provides the client with information about the source port
as seen by the server.
8 IANA Considerations It is possible that an attacker might try to disrupt service to a
client by acting as a man-in-the-middle, modifying the rport
parameter in a Via header in a request sent by a client. Removal of
this parameter will prevent clients from behind NATs from receiving
service. Addition of the parameter will generally have no impact. Of
course, if an attacker is capable of launching a man-in-the-middle
attack, there are many other ways of denying service, such as merely
discarding the request. Therefore, this attack does not seem
significant.
8. IANA Considerations
There are no IANA considerations associated with this specification. There are no IANA considerations associated with this specification.
9 IAB Considerations 9. IAB Considerations
The IAB has studied a class of protocols referred to as Unilateral The IAB has studied a class of protocols referred to as Unilateral
Self Address Fixing (UNSAF) protocols [3]. These protocols allow a Self Address Fixing (UNSAF) protocols [5]. These protocols allow a
client behind a NAT to learn the IP address and port that a NAT will client behind a NAT to learn the IP address and port that a NAT will
allocate for a particular request, in order to use this information allocate for a particular request, in order to use this information
in application layer protocols. An example of an UNSAF protocol is in application layer protocols. An example of an UNSAF protocol is
the Simple Traversal of UDP Through NATs (STUN) protocol [4]. the Simple Traversal of UDP Through NATs (STUN) [4].
Any protocol is an UNSAF protocol if it reveals, to a client, the Any protocol is an UNSAF protocol if it reveals, to a client, the
source IP address and port of a packet sent through that NAT. source IP address and port of a packet sent through that NAT.
Although not designed for that purpose, this specification can be Although not designed for that purpose, this specification can be
used as an UNSAF protocol. Using the "rport" parameter (defined here) used as an UNSAF protocol. Using the "rport" parameter (defined here)
and the "received" parameter (defined in RFC 3261 [1]) in the topmost and the "received" parameter (defined in RFC 3261 [1]) in the topmost
Via header field value of a response, a client sending a request can Via header field value of a response, a client sending a request can
learn its address as it was seen by the server which sent the learn its address as it was seen by the server which sent the
response. response.
There are two uses of this information. The first is for There are two uses of this information. The first is for
registrations. Consider a client behind a NAT wishing to register registrations. Consider a client behind a NAT wishing to register
with a proxy on the other side of the NAT. The client must provide, with a proxy/registrar on the other side of the NAT. The client must
in its registration, the address at which it should receive incoming provide, in its registration, the address at which it should receive
SIP requests from the proxy. However, since the client is natted, incoming SIP requests from the proxy. However, since the client is
none of the addresses on any of its interfaces will be reachable from natted, none of the addresses on any of its interfaces will be
the proxy. However, if the client can provide the proxy with an reachable from the proxy. However, if the client can provide the
address that the proxy can reach, the client can receive incoming proxy with an address that the proxy can reach, the client can
requests. Using this specification, a client behind a NAT can learn receive incoming requests. Using this specification, a client behind
its address and port as seen by the proxy which receives a REGISTER a NAT can learn its address and port as seen by the proxy which
request. The client can then perform an additional registration, receives a REGISTER request. The client can then perform an
using this address in a Contact header. This would allow a client to additional registration, using this address in a Contact header. This
receive incoming requests, such as INVITE, on the socket through would allow a client to receive incoming requests, such as INVITE, on
which the registration was sent. the IP address and port it used to populate the source IP address and
port of the registration it sent. This approach will only work when
servers send requests to a UA from the same address and port on which
the REGISTER itself was received.
In many cases, the server to whom the registration is sent won't be
the registrar itself, but rather, a proxy which then sends the
request to the registrar. In such a case, any incoming requests for
the client must traverse the proxy to whom the registration was
directly sent. The Path header extension to SIP [3] allows the proxy
to indicate that it must be on the path of such requests.
The second usage is for record routing, to address the same problem The second usage is for record routing, to address the same problem
as above, but between two proxies. A proxy behind a NAT which as above, but between two proxies. A proxy behind a NAT which
forwards a request to a server can use OPTIONS, for example, to learn forwards a request to a server can use OPTIONS, for example, to learn
its address as seen by that server. This address can be placed into its address as seen by that server. This address can be placed into
the Record-Route header field of requests sent to that server. This the Record-Route header field of requests sent to that server. This
would allow the proxy to receive requests from that server on the would allow the proxy to receive requests from that server on the
same socket used to send it requests. same IP address and port it used to populate the source IP address
and port of the OPTIONS request.
Because of this potential usage, this document must consider the Because of this potential usage, this document must consider the
issues raised in [3]. issues raised in [5].
9.1 Problem Definition 9.1 Problem Definition
From [3], any UNSAF proposal must provide: From [5], any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem Precise definition of a specific, limited-scope problem that is to
that is to be solved with the UNSAF proposal. A short term be solved with the UNSAF proposal. A short term fix should not
fix should not be generalized to solve other problems; this be generalized to solve other problems; this is why "short term
is why "short term fixes usually aren't". fixes usually aren't".
This specification is primarily aimed at allowing SIP responses to be This specification is primarily aimed at allowing SIP responses to be
received when a request is sent through a NAT. In this primary received when a request is sent through a NAT. In this primary
application, this specification is not an UNSAF proposal. However, as application, this specification is not an UNSAF proposal. However, as
a side effect of this capability, this specification can be used as a side effect of this capability, this specification can be used as
an UNSAF protocol. In that usage, it would address two issues: an UNSAF protocol. In that usage, it would address two issues:
o Provide a client with an address that it could use in the o Provide a client with an address that it could use in the Contact
Contact header of a REGISTER request when it is behind a NAT. header of a REGISTER request when it is behind a NAT.
o Provide a proxy with an address that it could use in a o Provide a proxy with an address that it could use in a
Record-Route header in a request, when it is behind a NAT. Record-Route header in a request, when it is behind a NAT.
9.2 Exit Strategy 9.2 Exit Strategy
From [3], any UNSAF proposal must provide: From [5], any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better Description of an exit strategy/transition plan. The better short
short term fixes are the ones that will naturally see less term fixes are the ones that will naturally see less and less use
and less use as the appropriate technology is deployed. as the appropriate technology is deployed.
The SIP working group has recognized that the usage of this The SIP working group has recognized that the usage of this
specification to support registrations and record-routing through specification to support registrations and record-routing through
NATs is not appropriate. It has a number of known problems which are NATs is not appropriate. It has a number of known problems which are
documented below. The way to eliminate potential usage of this documented below. The way to eliminate potential usage of this
specification for address fixing is to provide a proper solution to specification for address fixing is to provide a proper solution to
the problems that might motivate the usage of this specification for the problems that might motivate the usage of this specification for
address fixing. Specifically, appropriate solutions for registrations address fixing. Specifically, appropriate solutions for registrations
and record-routing in the presence of NATs need to be developed. and record-routing in the presence of NATs need to be developed.
These solutions would not rely on address fixing. These solutions would not rely on address fixing.
Requirements for such solutions are already under development [5]. Requirements for such solutions are already under development [6].
Implementors of this specification are encouraged to follow this work Implementors of this specification are encouraged to follow this work
for better solutions for registrations and record-routing through for better solutions for registrations and record-routing through
NAT. NAT.
9.3 Brittleness Introduced by this Specification 9.3 Brittleness Introduced by this Specification
From [3], any UNSAF proposal must provide:
From [5], any UNSAF proposal must provide:
Discussion of specific issues that may render systems more Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data "brittle". For example, approaches that involve using data at
at multiple network layers create more dependencies, multiple network layers create more dependencies, increase
increase debugging challenges, and make it harder to debugging challenges, and make it harder to transition.
transition.
This specification, if used for address fixing, introduces several This specification, if used for address fixing, introduces several
points of brittleness into a SIP system: points of brittleness into a SIP system:
o If used for registrations, a client will need to frequently o If used for registrations, a client will need to frequently
re-register in order to keep the NAT bindings fresh. In many re-register in order to keep the NAT bindings fresh. In many
cases, these registrations will need to take place nearly one cases, these registrations will need to take place nearly one
hundred times more frequently than the typical refresh hundred times more frequently than the typical refresh interval of
interval of a registration. This introduces load into the a registration. This introduces load into the system and hampers
system and hampers scalability. scalability.
o A client cannot accurately determine the binding lifetime of a o A client cannot accurately determine the binding lifetime of a NAT
NAT it is registering (or record-routing) through. Therefore, it is registering (or record-routing) through. Therefore, there
there may be periods of unreachability that occur between the may be periods of unreachability that occur between the time a
time a binding expires and the next registration or OPTIONS binding expires and the next registration or OPTIONS refresh is
refresh is sent. This may result in missed calls, messages, or sent. This may result in missed calls, messages, or other
other information. information.
o If the NAT is of the symmetric variety [4], a client will only o If the NAT is of the symmetric variety [4], a client will only be
be able to use its address to receive requests from the server able to use its address to receive requests from the server it has
it has sent the request to. If that server is one of many sent the request to. If that server is one of many servers in a
servers in a cluster, the client may not be able to receive cluster, the client may not be able to receive requests from other
requests from other servers in the cluster. This may result in servers in the cluster. This may result in missed calls, messages,
missed calls, messages, or other information. or other information.
o If the NAT is of the symmetric variety [4], a client will only be
able to use its address to receive requests if the server sends
requests to the client from the same address and port the server
received the registrations on. This server behavior is not
mandated by RFC 3261 [1], although it appears to be common in
practice.
o If the registrar and the server to whom the client sent its
REGISTER request are not the same, the approach will only work if
the server uses the Path header field [3]. There is not an easy
and reliable way for the server to determine that the Path header
should be used for a registration. Using Path when the address in
the topmost Via header field is a private address will usually
work, but may result in usage of Path when it is not actually
needed.
9.4 Requirements for a Long Term Solution 9.4 Requirements for a Long Term Solution
From [3], any UNSAF proposal must provide: From [5], any UNSAF proposal must provide:
Identify requirements for longer term, sound technical Identify requirements for longer term, sound technical solutions
solutions -- contribute to the process of finding the right -- contribute to the process of finding the right longer term
longer term solution. solution.
The brittleness described in Section 9.3 has led us to the following The brittleness described in Section 9.3 has led us to the following
requirements for a long term solution: requirements for a long term solution:
The client should not need to specify its address. The client should not need to specify its address. Registrations and
Registrations and record routing require the client to record routing require the client to specify the address at which
specify the address at which it should receive requests. A it should receive requests. A sound technical solution should
sound technical solution should allow a client to allow a client to explicitly specify that it wants to receive
explicitly specify that it wants to receive incoming incoming requests on the connection over which the outgoing
requests on the connection over which the outgoing request request was sent. In this way, the client does not need to specify
was sent. In this way, the client does not need to specify
its address. its address.
The solution must deal with clusters of servers. In many The solution must deal with clusters of servers. In many commerically
commerically deployed SIP systems, there will be multiple deployed SIP systems, there will be multiple servers, each at
servers, each at different addresses and ports, handling different addresses and ports, handling incoming requests for a
incoming requests for a client. The solution must client. The solution must explicitly consider this case.
explicitly consider this case.
The solution must not require increases in network load. There The solution must not require increases in network load. There cannot
cannot be a penalty for a sound technical solution. be a penalty for a sound technical solution.
9.5 Issues with Existing NAPT Boxes 9.5 Issues with Existing NAPT Boxes
From [3], any UNSAF proposal must provide: From [5], any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports. existing, deployed NA[P]Ts and experience reports.
To our knowledge, at the time of writing, there is only very limited To our knowledge, at the time of writing, there is only very limited
usage of this specification for address fixing. Therefore, no usage of this specification for address fixing. Therefore, no
specific practical issues have been raised. specific practical issues have been raised.
10 Acknowledgements 10. Acknowledgements
The authors would like to thank Rohan Mahy for his comments and The authors would like to thank Rohan Mahy and Allison Manking for
contributions to this work. their comments and contributions to this work.
Full Copyright Statement Normative References
Copyright (c) The Internet Society (2002). All Rights Reserved. [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
This document and translations of it may be copied and furnished to [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
others, and derivative works that comment on or otherwise explain it Levels", BCP 14, RFC 2119, March 1997.
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be [3] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
revoked by the Internet Society or its successors or assigns. Extension Header Field for Registering Non-Adjacent Contacts",
RFC 3327, December 2002.
This document and the information contained herein is provided on an [4] Rosenberg, J., Weinberger, J., Huitema, C. and R. Mahy, "STUN -
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING Simple Traversal of User Datagram Protocol (UDP) Through Network
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING Address Translators (NATs)", RFC 3489, March 2003.
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
11 Author's Addresses Informative References
[5] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[6] Mahy, R., "Requirements for Connection Reuse in the Session
Initiation Protocol (SIP)",
draft-ietf-sipping-connect-reuse-reqs-00 (work in progress),
October 2002.
Authors' Addresses
Jonathan Rosenberg Jonathan Rosenberg
dynamicsoft dynamicsoft
72 Eagle Rock Avenue 600 Lanidex Plaza
First Floor Parsippany, NJ 07054
East Hanover, NJ 07936 US
email: jdrosen@dynamicsoft.com
Joel Weinberger Phone: +1 973 952-5000
dynamicsoft EMail: jdrosen@dynamicsoft.com
72 Eagle Rock Avenue URI: http://www.jdrosen.net
First Floor
East Hanover, NJ 07936
email: jweinberger@dynamicsoft.com
Henning Schulzrinne Henning Schulzrinne
Columbia University Columbia University
M/S 0401 M/S 0401
1214 Amsterdam Ave. 1214 Amsterdam Ave.
New York, NY 10027-7003 New York, NY 10027
email: schulzrinne@cs.columbia.edu US
12 Normative References
[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
[2] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
13 Informative References EMail: schulzrinne@cs.columbia.edu
URI: http://www.cs.columbia.edu/~hgs
[3] L. Daigle, "IAB considerations for UNilateral self-address fixing Intellectual Property Statement
(UNSAF) across network address translation," Internet Draft, Internet
Engineering Task Force, July 2002. Work in progress.
[4] J. Rosenberg, J. Weinberger, C. Huitema, and R. Mahy, "STUN - The IETF takes no position regarding the validity or scope of any
simple traversal of UDP through network address translators," intellectual property or other rights that might be claimed to
Internet Draft, Internet Engineering Task Force, Aug. 2002. Work in pertain to the implementation or use of the technology described in
progress. this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
[5] R. Mahy, "Requirements for connection reuse in the session The IETF invites any interested party to bring to its attention any
initiation protocol (SIP)," Internet Draft, Internet Engineering Task copyrights, patents or patent applications, or other proprietary
Force, June 2002. Work in progress. rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement Full Copyright Statement
Copyright (c) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than followed, or as required to translate it into languages other than
English. English.
The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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