draft-ietf-sip-outbound-01.txt   draft-ietf-sip-outbound-02.txt 
SIP WG C. Jennings, Ed. Network Working Group C. Jennings, Ed.
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Expires: April 26, 2006 R. Mahy, Ed. Updates: 3261,3327 (if approved) R. Mahy, Ed.
SIP Edge LLC Expires: September 6, 2006 Plantronics
October 23, 2005 March 5, 2006
Managing Client Initiated Connections in the Session Initiation Protocol Managing Client Initiated Connections in the Session Initiation Protocol
(SIP) (SIP)
draft-ietf-sip-outbound-01 draft-ietf-sip-outbound-02
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2006).
Abstract Abstract
Session Initiation Protocol (SIP) allows proxy servers to initiate Session Initiation Protocol (SIP) allows proxy servers to initiate
TCP connections and send asynchronous UDP datagrams to User Agents in TCP connections and send asynchronous UDP datagrams to User Agents in
order to deliver requests. However, many practical considerations, order to deliver requests. However, many practical considerations,
such as the existence of firewalls and NATs, prevent servers from such as the existence of firewalls and Network Address Translators
connecting to User Agents in this way. Even when a proxy server can (NATs), prevent servers from connecting to User Agents in this way.
open a TCP connection to a User Agent, most User Agents lack a Even when a proxy server can open a TCP connection to a User Agent,
certificate suitable to act as a TLS server. This specification most User Agents lack a certificate suitable to act as a TLS
defines behaviors for User Agents, registrars and proxy servers that (Transport Layer Security) server. This specification defines
allow requests to be delivered on existing connections established by behaviors for User Agents, registrars and proxy servers that allow
the User Agent. It also defines keep alive behaviors needed to keep requests to be delivered on existing connections established by the
NAT bindings open and specifies the usage of multiple connections for User Agent. It also defines keep alive behaviors needed to keep NAT
high availability systems. bindings open and specifies the usage of multiple connections.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Summary of Mechanism . . . . . . . . . . . . . . . . . . . 4 3.1. Summary of Mechanism . . . . . . . . . . . . . . . . . . . 5
3.2. Single Registrar and UA . . . . . . . . . . . . . . . . . 5 3.2. Single Registrar and UA . . . . . . . . . . . . . . . . . 6
3.3. Multiple Connections from a User Agent . . . . . . . . . . 6 3.3. Multiple Connections from a User Agent . . . . . . . . . . 7
3.4. Edge Proxies . . . . . . . . . . . . . . . . . . . . . . . 8 3.4. Edge Proxies . . . . . . . . . . . . . . . . . . . . . . . 9
3.5. Keep Alive Technique . . . . . . . . . . . . . . . . . . . 9 3.5. Keep Alive Technique . . . . . . . . . . . . . . . . . . . 10
4. User Agent Mechanisms . . . . . . . . . . . . . . . . . . . . 10 4. User Agent Mechanisms . . . . . . . . . . . . . . . . . . . . 10
4.1. Forming Flows . . . . . . . . . . . . . . . . . . . . . . 10 4.1. Instance ID Creation . . . . . . . . . . . . . . . . . . . 10
4.1.1. Request without GRUU . . . . . . . . . . . . . . . . . 11 4.2. Initial Registrations . . . . . . . . . . . . . . . . . . 12
4.2. Detecting Flow Failure . . . . . . . . . . . . . . . . . . 11 4.2.1. Registration by Other Instances . . . . . . . . . . . 13
4.3. Flow Failure Recovery . . . . . . . . . . . . . . . . . . 12 4.3. Sending Requests . . . . . . . . . . . . . . . . . . . . . 13
4.4. Registration by Other Instances . . . . . . . . . . . . . 13 4.3.1. Selecting the First Hop . . . . . . . . . . . . . . . 13
5. Registrar Mechanisms . . . . . . . . . . . . . . . . . . . . . 13 4.3.2. Forming Flows . . . . . . . . . . . . . . . . . . . . 13
5.1. Processing Register Requests . . . . . . . . . . . . . . . 13 4.4. Detecting Flow Failure . . . . . . . . . . . . . . . . . . 14
5.2. Forwarding Requests . . . . . . . . . . . . . . . . . . . 14 4.4.1. Keep Alive with STUN . . . . . . . . . . . . . . . . . 14
6. Edge Proxy Mechanisms . . . . . . . . . . . . . . . . . . . . 15 4.4.2. Keep Alive with Double CRLF . . . . . . . . . . . . . 15
6.1. Processing Register Requests . . . . . . . . . . . . . . . 15 4.5. Flow Recovery . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Forwarding Requests . . . . . . . . . . . . . . . . . . . 16 5. Edge Proxy Mechanisms . . . . . . . . . . . . . . . . . . . . 16
7. Mechanisms for All Servers . . . . . . . . . . . . . . . . . . 17 5.1. Processing Register Requests . . . . . . . . . . . . . . . 16
7.1. STUN Processing . . . . . . . . . . . . . . . . . . . . . 17 5.2. Generating Flow Tokens . . . . . . . . . . . . . . . . . . 16
7.2. Pin-Route Processing . . . . . . . . . . . . . . . . . . . 17 5.3. Forwarding Requests . . . . . . . . . . . . . . . . . . . 17
8. Example Message Flow . . . . . . . . . . . . . . . . . . . . . 18 6. Registrar and Location Server Mechanisms . . . . . . . . . . . 17
9. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 6.1. Processing Register Requests . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 6.2. Forwarding Requests . . . . . . . . . . . . . . . . . . . 19
11. Security Considerations . . . . . . . . . . . . . . . . . . . 22 7. Mechanisms for All Servers (Proxys, Registars, UAS) . . . . . 19
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 23 7.1. STUN Processing . . . . . . . . . . . . . . . . . . . . . 19
13. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 24 7.2. Double CRLF Processing . . . . . . . . . . . . . . . . . . 20
14. Changes from 00 Version . . . . . . . . . . . . . . . . . . . 24 8. Example Message Flow . . . . . . . . . . . . . . . . . . . . . 20
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25 9. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
16.1. Normative References . . . . . . . . . . . . . . . . . . . 25 10.1. Contact Header Field . . . . . . . . . . . . . . . . . . . 24
16.2. Informative References . . . . . . . . . . . . . . . . . . 26 10.2. SIP/SIPS URI Paramters . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27 10.3. SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 28 10.4. Media Feature Tag . . . . . . . . . . . . . . . . . . . . 25
11. Security Considerations . . . . . . . . . . . . . . . . . . . 26
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 26
13. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 27
14. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
14.1. Changes from 01 Version . . . . . . . . . . . . . . . . . 27
14.2. Changes from 00 Version . . . . . . . . . . . . . . . . . 27
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
16.1. Normative References . . . . . . . . . . . . . . . . . . . 28
16.2. Informative References . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . . . . 31
1. Introduction 1. Introduction
There are many environments for SIP deployments in which the User There are many environments for SIP [5] deployments in which the User
Agent (UA) can form a connection to a Registrar or Proxy but in which Agent (UA) can form a connection to a Registrar or Proxy but in which
the connections in the reverse direction to the UA are not possible. the connections in the reverse direction to the UA are not possible.
This can happen for several reasons. Connection to the UA can be This can happen for several reasons. Connection to the UA can be
blocked by a firewall device between the UA and the proxy or blocked by a firewall device between the UA and the proxy or
registrar, which will only allow new connections in the direction of registrar, which will only allow new connections in the direction of
the UA to the Proxy. Similarly there may be a NAT, which are only the UA to the Proxy. Similarly there may be a NAT, which are only
capable of allowing new connections from the private address side to capable of allowing new connections from the private address side to
the public side. This specification allows SIP registration when the the public side. This specification allows SIP registration when the
UA is behind a firewall or NAT. UA is behind such a firewall or NAT.
Most IP phones and personal computers get their network Most IP phones and personal computers get their network
configurations dynamically via a protocol such as DHCP. These configurations dynamically via a protocol such as DHCP (Dynamic Host
systems typically do not have a useful name in DNS, and they Configuration Protocol). These systems typically do not have a
definitely do not have a long-term, stable DNS name that is useful name in the Domain Name System (DNS), and they definitely do
appropriate for binding to a certificate. It is impractical for them not have a long-term, stable DNS name that is appropriate for binding
to have a certificate that can be used as a client-side TLS to a certificate. It is impractical for them to have a certificate
certificate for SIP. However, these systems can still form TLS that can be used as a client-side TLS certificate for SIP. However,
connections to a proxy or registrar such that the UA authenticates these systems can still form TLS connections to a proxy or registrar
the server certificate, and the server authenticates the UA using a which authenticates with a server certificate. The server can
shared secret in a digest challenge over that TLS connection. authenticate the UA using a shared secret in a digest challenge over
that TLS connection.
The key idea of this specification is that when a UA sends a REGISTER The key idea of this specification is that when a UA sends a REGISTER
request, the proxy can later use this same connection, be it UDP, request, the proxy can later use this same network "flow"--whether
TCP, or another transport protocol, to forward any requests that need this is a bidirectional stream of UDP datagrams, a TCP connection, or
to go to this UA. For a UA to receive incoming requests, the UA has an analogous concept of another transport protocol--to forward any
to connect to the server. Since the server can't connect to the UA, requests that need to go to this UA. For a UA to receive incoming
the UA has to make sure that a connection is always active. This requests, the UA has to connect to a server. Since the server can't
requires the UA to detect when a connection fails. Since, such connect to the UA, the UA has to make sure that a flow is always
detection takes time and leaves a window of opportunity for missed active. This requires the UA to detect when a flow fails. Since,
incoming requests, this mechanism allows the UA to use multiple such detection takes time and leaves a window of opportunity for
connections, referred to as "flows", to the proxy or registrar and missed incoming requests, this mechanism allows the UA to use
using a keep alive mechanism on each flow so that the UA can detect multiple flows to the proxy or registrar. This mechanism also uses a
when a flow has failed. keep alive mechanism over each flow so that the UA can detect when a
flow has failed.
2. Conventions and Terminology 2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2]. document are to be interpreted as described in RFC 2119 [4].
2.1. Definitions 2.1. Definitions
Edge Proxy: An Edge Proxy is any proxy that is located topologically Edge Proxy: An Edge Proxy is any proxy that is located topologically
between the registering User Agent and the registrar. between the registering User Agent and the registrar.
flow: A Flow is a network protocol layer connection between two hosts flow: A Flow is a network protocol layer (layer 4) association
that is represented by the network address of both ends and the between two hosts that is represented by the network address and
protocol. For TCP and UDP this would include the IP addresses and port number of both ends and by the protocol. For TCP, a flow is
ports of both ends and the protocol (TCP or UDP). With TCP, a equivalent to a TCP connection. For UDP a flow is a bidirectional
flow would often have a one to one correspondence with a single stream of datagrams between a single pair of IP addresses and
file descriptor in the operating system. ports of both peers. With TCP, a flow often has a one to one
flow-id: This refers to the value of a new header field parameter correspondence with a single file descriptor in the operating
value for the contact header. When a UA register multiple times, system.
each registration gets a unique flow-id value. This does not reg-id: This refers to the value of a new header field parameter
refer to flow. value for the Contact header field. When a UA registers multiple
times, each simultaneous registration gets a unique reg-id value.
instance-id: This specification uses the word instance-id to refer to instance-id: This specification uses the word instance-id to refer to
the value of the "sip.instance" media feature tag in the Contact the value of the "sip.instance" media feature tag in the Contact
header field as defined in [1]. This is a URN that uniquely header field. This is a Uniform Resource Name (URN) that uniquely
identifies the UA. identifies this specific UA instance.
outbound-proxy-set A configured set of SIP URIs (Uniform Resource
Identifiers) that represents each of the outbound proxies (often
Edge Proxies) with which the UA will attempt to maintain a direct
flow.
3. Overview 3. Overview
Several scenarios in which this technique is useful are discussed Several scenarios in which this technique is useful are discussed
below, including the simple collocated registrar and proxy, a User below, including the simple co-located registrar and proxy, a User
Agent desiring multiple connections to a resource (for redundancy for Agent desiring multiple connections to a resource (for redundancy for
example), and a system that uses Edge Proxies. example), and a system that uses Edge Proxies.
3.1. Summary of Mechanism 3.1. Summary of Mechanism
The overall approach is fairly simple. Each UA has a unique The overall approach is fairly simple. Each UA has a unique
instance-id (found in the GRUU[1]) that stays the same for this UA instance-id that stays the same for this UA even if the UA reboots or
even if the UA reboots or is power cycled. Each UA can register is power cycled. Each UA can register multiple times over different
multiple times for the same AOR to achieve high reliability. Each connections for the same SIP Address of Record (AOR) to achieve high
registration includes the instance-id for the UA and a flow-id label reliability. Each registration includes the instance-id for the UA
that is different for each connection. and a reg-id label that is different for each flow. The registrar
can use the instance-id to recognize that two different registrations
UAs use STUN as the keep alive mechanism to keep their flow to the both reach the same UA. The registrar can use the reg-id label to
proxy or registrar alive. A UA can create more than one flow using recognize that a UA is registering after a reboot.
multiple registrations for the same AOR. The instance-id parameter
is used by the proxy to identify which UA a flow is associated with.
The flow-id is used by the proxy and registrar to tell the difference
between a UA re-registering and one that is registering over an
additional flow. The proxies keep track of the flows used for
successful registrations.
When a proxy goes to route a message to a UA for which it has a When a proxy goes to route a message to a UA for which it has a
binding, it can use any one of the flows on which a successful binding, it can use any one of the flows on which a successful
registration has been completed. A failure on a particular flow can registration has been completed. A failure on a particular flow can
be tried again on an alternate flow. Proxies can determine which be tried again on an alternate flow. Proxies can determine which
flows go to the same UA by looking at the instance-id. Proxies can flows go to the same UA by comparing the instance-id. Proxies can
tell that a flow replaces a previously abandoned flow by looking at tell that a flow replaces a previously abandoned flow by looking at
the flow-id. the reg-id.
UAs use the STUN (Simple Traversal of UDP through NATs) protocol as
the keep alive mechanism to keep their flow to the proxy or registrar
alive.
3.2. Single Registrar and UA 3.2. Single Registrar and UA
In this example there a single server is acting as both a registrar In this example, a single server is acting as both a registrar and
and proxy. proxy.
+-----------+ +-----------+
| Registrar | | Registrar |
| Proxy | | Proxy |
+-----+-----+ +-----+-----+
| |
| |
+----+--+ +----+--+
| User | | User |
| Agent | | Agent |
+-------+ +-------+
User Agents forming only a single connection continue to register User Agents which form only a single flow continue to register
normally but include the instance-id as described in the GRUU [1] normally but include the instance-id as described in Section 4.1.
specification and can also add a flow-id parameter to the Contact The UA can also include a reg-id parameter is used to allow the
header field value. The flow-id parameter is used to allow the
registrar to detect and avoid using invalid contacts when a UA registrar to detect and avoid using invalid contacts when a UA
reboots or reconnects after its old connection has failed for some reboots or reconnects after its old connection has failed for some
reason. reason.
For clarity, here is an example. Bob's UA creates a new TCP flow to For clarity, here is an example. Bob's UA creates a new TCP flow to
the registrar and sends the following REGISTER request. the registrar and sends the following REGISTER request.
REGISTER sip:example.com SIP/2.0 REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bK-bad0ce-11-1036 Via: SIP/2.0/TCP 192.0.2.1;branch=z9hG4bK-bad0ce-11-1036
Max-Forwards: 70 Max-Forwards: 70
From: Bob <sip:bob@example.com>;tag=d879h76 From: Bob <sip:bob@example.com>;tag=d879h76
To: Bob <sip:bob@example.com> To: Bob <sip:bob@example.com>
Call-ID: 8921348ju72je840.204 Call-ID: 8921348ju72je840.204
CSeq: 1 REGISTER CSeq: 1 REGISTER
Contact: <sip:line1@192.168.0.2>; flow-id=1; Supported: path
Contact: <sip:line1@192.168.0.2>; reg-id=1;
;+sip.instance="<urn:uuid:00000000-0000-0000-0000-000A95A0E128>" ;+sip.instance="<urn:uuid:00000000-0000-0000-0000-000A95A0E128>"
Content-Length: 0 Content-Length: 0
Note: Implementors often ask why the value of the sip.instance is
inside angle brackets. This is a requirement of RFC 3840 [7]
which defines media feature tags in SIP. Feature tags which are
strings are compared by case sensitive string comparison. To
differentiate these tags from tokens (which are not case
sensitive), case sensitive parameters such as the sip.instance
media feature tag are placed inside angle brackets.
The registrar challenges this registration to authenticate Bob. When The registrar challenges this registration to authenticate Bob. When
the registrar adds an entry for this contact under the AOR for Bob, the registrar adds an entry for this contact under the AOR for Bob,
the registrar also keeps track of the connection over which it the registrar also keeps track of the connection over which it
received this registration. received this registration.
The registrar saves the instance-id (as defined in [1]) and flow-id The registrar saves the instance-id and reg-id along with the rest of
(as defined in Section 9) along with the rest of the Contact header the Contact header field. If the instance-id and reg-id are the same
field. If the instance-id and flow-id are the same as a previous as a previous registration for the same AOR, the proxy uses the most
registration for the same AOR, the proxy uses the most recently recently created registration first. This allows a UA that has
created registration first. This allows a UA that has rebooted to rebooted to replace its previous registration for each flow with
replace its previous registration for each flow with minimal impact minimal impact on overall system load.
on overall system load.
Later when Alice sends a request to Bob, his proxy selects the target When Alice sends a request to Bob, his proxy selects the target set.
set. The proxy forwards the request to elements in the target set The proxy forwards the request to elements in the target set based on
based on the proxy's policy. The proxy looks at the the target set the proxy's policy. The proxy looks at the target set and uses the
and uses the instance-id to understand that two targets both end up instance-id to understand that two targets both end up routing to the
routing to the same UA. When the proxy goes to forward a request to same UA. When the proxy goes to forward a request to a given target,
a given target, it looks and finds the flows that received the it looks and finds the flows that received the registration. The
registration. The proxy then forwards the request on that flow proxy then forwards the request on that flow instead of trying to
instead of trying to form a new flow to that contact. This allows form a new flow to that contact. This allows the proxy to forward a
the proxy to forward a request to a particular contact down the same request to a particular contact over the same flow that the UA used
flow that did the registration for this AOR. If the proxy had to register this AOR. If the proxy has multiple flows that all go to
multiple flows that all went to this UA, it would choose any one of this UA, it can choose any one of registration bindings for this AOR
registration bindings that it had for this AOR and that had the same that has the same instance-id as the selected UA. In general, if two
instance-id as the selected UA. In general, if two registrations registrations have the same reg-id and instance-id, the proxy will
have the same flow-id and instance-id, the proxy would favor the most favor the most recently registered flow. This is so that if a UA
recently registered flow. This is so that if a UA reboots, the proxy reboots, the proxy will prefer to use the most recent flow that goes
will prefer to use the most recent flow that goes to this UA instead to this UA instead of trying one of the old flows which would
of trying one of the old flows which will presumably fail. presumably fail.
3.3. Multiple Connections from a User Agent 3.3. Multiple Connections from a User Agent
There are various ways to deploy SIP to build a reliable and There are various ways to deploy SIP to build a reliable and scalable
scaleable system. This section discusses one such design that is system. This section discusses one such design that is possible with
possible with the mechanisms in this draft. Other designs are also the mechanisms in this specification. Other designs are also
possible. possible.
In this example system, the logical proxy/registrar for the domain is In this example system, the logical proxy/registrar for the domain is
running on two hosts that share the appropriate state and can both running on two hosts that share the appropriate state and can both
provide registrar and proxy functionality for the domain. The UA provide registrar and proxy functionality for the domain. The UA
will form connections to two of the physical hosts that can perform will form connections to two of the physical hosts that can perform
the proxy/registrar function for the domain. Reliability is achieved the proxy/registrar function for the domain. Reliability is achieved
by having the UA form two connections to the domain. Scaleability is by having the UA form two TCP connections to the domain. Scalability
achieved by using DNS SRV to load balance the primary connection is achieved by using DNS SRV to load balance the primary connection
across a set of machines that can service the primary connection and across a set of machines that can service the primary connection and
also using DNS SRV to load balance across a separate set of machines also using DNS SRV to load balance across a separate set of machines
that can service the backup connection. The deployment here requires that can service the backup connection. The deployment here requires
that DNS be configured with an entry that resolves to all the primary that DNS is configured with one entry that resolves to all the
hosts and another that resolves to all the backup hosts. Designs primary hosts and another entry that resolves to all the backup
having only one set were also considered but in this case, there hosts. Designs having only one set were also considered, but in this
would have to be some way to ensure that the two connection did not case there would have to be some way to ensure that the two
accidentally resolve to the same host. Various approaches for this connection did not accidentally resolve to the same host. Various
are possible but all probably require extensions to the SIP protocol approaches for this are possible but all probably require extensions
so they were not included in this specification. This approach can to the SIP protocol so they were not included in this specification.
work with the disadvantage that slightly more configuration of DNS is This approach can work with the disadvantage that slightly more
required. configuration of DNS is required.
+-------------------+ +-------------------+
| Domain | | Domain |
| Logical Proxy/Reg | | Logical Proxy/Reg |
| | | |
|+-----+ +-----+| |+-----+ +-----+|
||Host1| |Host2|| ||Host1| |Host2||
|+-----+ +-----+| |+-----+ +-----+|
+---\------------/--+ +---\------------/--+
\ / \ /
\ / \ /
\ / \ /
\ / \ /
+------+ +------+
| User | | User |
| Agent| | Agent|
+------+ +------+
The UA is configured with a primary and backup registration URI. The UA is configured with a primary and backup registration URI.
These URIs are configured into the UA through whatever the normal These URIs are configured into the UA through whatever the normal
mechanism is to configure the proxy or registrar for the UA. They mechanism is to configure the proxy or registrar address in the UA.
might look something like "sip:primary.example.com;sip-stun" and If the AOR is Alice@example.com, the outbound-proxy-set might look
"sip:backup.example.com;sip-stun" if the domain was example.com. The something like "sip:primary.example.com;sip-stun" and "sip:
"sip-stun" tag indicates that they support STUN as described later in backup.example.com;sip-stun". The "sip-stun" tag indicates that a
this specification. Note that each of them could resolve to several SIP server supports STUN and SIP muxed over the same flow, as
different hosts. The administrative domain that created these URIs described later in this specification. Note that each URI in the
MUST ensure that the two URIs resolve to separate hosts. These URIs outbound-proxy-set could resolve to several different physical hosts.
have normal SIP processing so things like SRV can be used to do load The administrative domain that created these URIs should ensure that
balancing across a proxy farm. the two URIs resolve to separate hosts. These URIs are handled
according to normal SIP processing rules, so things like SRV can be
used to do load balancing across a proxy farm.
The User Agent would get a GRUU from the domain to use at its The domain also needs to ensure that a request for the UA sent to
contact. The GRUU would refer to the domain, not host1 or host2. host1 or host2 is then sent across the appropriate flow to the UA.
Regardless of which host received a request to GRUU, the domain would The domain might choose to use the Path header (as described in the
need to ensure that the request got sent to host1 or host2 and then next section) approach to store this internal routing information on
sent across the appropriate flow to the UA. The domain might choose host1 or host2.
to use the Path header (as described in the next section) approach to
form this internal routing to host1 or host2.
When a single server fails, all the UAs that have a registration with When a single server fails, all the UAs that have a flow through it
it will detect this and try to reconnect. This can cause large loads will detect a flow failure and try to reconnect. This can cause
on the server and is referred to as the avalanche restart problem large loads on the server. When large numbers of hosts reconnect
further discussed in Section 4.3. The multiple flows to many servers nearly simultaneously, this is referred to as the avalanche restart
help reduce the load caused by the avalanche restart. If a UA has problem, and is further discussed in Section 4.5. The multiple flows
multiple flows, and one of the servers fails, it can delay some to many servers help reduce the load caused by the avalanche restart.
significant time before trying to form a new connection to replace If a UA has multiple flows, and one of the servers fails, it can
the flow to the server that failed. By spreading out the time used delay some significant time before trying to form a new connection to
for all the UAs to reconnect to a server, the load on the server is replace the flow to the server that failed. By spreading out the
reduced. time used for all the UAs to reconnect to a server, the load on the
server farm is reduced.
3.4. Edge Proxies 3.4. Edge Proxies
Some SIP deployments use edge proxies such that the UA sends the Some SIP deployments use edge proxies such that the UA sends the
REGISTER to an Edge Proxy that then forwards the REGISTER to the REGISTER to an Edge Proxy that then forwards the REGISTER to the
Registrar. The Edge Proxy includes a Path header [10] so that when Registrar. The Edge Proxy includes a Path header [12] so that when
the registrar later forwards a request to this UA, the request is the registrar later forwards a request to this UA, the request is
routed through the Edge Proxy. There could be a NAT for FW between routed through the Edge Proxy. There could be a NAT or firewall
the UA and the Edge Proxy and there could also be one between the between the UA and the Edge Proxy.
Edge Proxy and the Registrar. This second case typically happens
when the Edge Proxy is in an enterprise the Registrar is located at a
service provider.
+---------+ +---------+
|Registrar| |Registrar|
|Proxy | |Proxy |
+---------+ +---------+
/ \ / \
----------------------------NAT/FW / \
/ \ / \
+-----+ +-----+ +-----+ +-----+
|Edge1| |Edge2| |Edge1| |Edge2|
+-----+ +-----+ +-----+ +-----+
\ / \ /
\ / \ /
----------------------------NAT/FW ----------------------------NAT/FW
\ / \ /
\ / \ /
+------+ +------+
|User | |User |
|Agent | |Agent |
+------+ +------+
These systems can use effectively the same mechanism as described in These systems can use effectively the same mechanism as described in
the previous sections but need to use the Path header. When the Edge the previous sections but need to use the Path header. When the Edge
Proxy receives a registration, it needs to create an identifier value Proxy receives a registration, it needs to create an identifier value
that is unique to this flow (and not a subsequent flow with the same that is unique to this flow (and not a subsequent flow with the same
addresses) and put this identifier in the path header. This is done addresses) and put this identifier in the Path header URI. This can
by putting the value in the user portion of a loose route in the path be done by putting the value in the user portion of a loose route in
header. If the registration succeeds, the Edge Proxy needs to map the path header. If the registration succeeds, the Edge Proxy needs
future requests that are routed to the identifier value that was put to map future requests that are routed to the identifier value from
in the Path header to the associated flow. the Path header, to the associated flow.
The term Edge Proxy is often used to refer to deployments where the The term Edge Proxy is often used to refer to deployments where the
the Edge Proxy is in the same administrative domain as the Registrar. Edge Proxy is in the same administrative domain as the Registrar.
However, in this specification we use the term to refer to any proxy However, in this specification we use the term to refer to any proxy
between the UA and the Registrar. For example the Edge Proxy may be between the UA and the Registrar. For example the Edge Proxy may be
inside an enterprise that requires its use and the registrar could be inside an enterprise that requires its use and the registrar could be
a service provider with no relationship to the enterprise. a service provider with no relationship to the enterprise.
Regardless if they are in the same administrative domain, this Regardless if they are in the same administrative domain, this
specification requires that Registrars and Edge proxies support the specification requires that Registrars and Edge proxies support the
Path header mechanism in RFC 3327 [10]. Path header mechanism in RFC 3327 [12].
3.5. Keep Alive Technique 3.5. Keep Alive Technique
A keep alive mechanism needs to detect both failure of a connection A keep alive mechanism needs to detect failure of a connection and
and changes to the NAT public mapping as well as keeping any NAT changes to the NAT public mapping, as well as keeping any NAT
bindings refreshed. This specification uses STUN [5] over the same bindings refreshed. This specification uses STUN [7] over the same
flow as the SIP traffic to perform the keep alive. A flow definition flow as the SIP traffic to perform the keep alive. A flow definition
could change because a NAT device in the network path reboots and the could change because a NAT device in the network path reboots and the
resulting public IP address or port mapping for the UA changes. To resulting public IP address or port mapping for the UA changes. To
detect this, requests are sent over the connection that is being used detect this, requests are sent over the same flow that is being used
for the SIP traffic. The proxy or registrar acts as a STUN server on for the SIP traffic. The proxy or registrar acts as a STUN server on
the SIP signaling port. the SIP signaling port.
Note: The STUN mechanism is very robust and allows the detection Note: The STUN mechanism is very robust and allows the detection
of a changed IP address. Many other options were considered. It of a changed IP address. Many other options were considered. It
may also be possible to do this with OPTIONS messages and rport; may also be possible to detect a changes flow with OPTIONS
although this approach has the advantage of being backwards messages and the rport parameter. Although the OPTIONS approach
compatible, it also increases the load on the proxy or registrar has the advantage of being backwards compatible, it also
server. The TCP KEEP_ALIVE mechanism is not used because most significantly increases the load on the proxy or registrar server.
operating systems do not allow the time to be set on a per The TCP KEEP_ALIVE mechanism was not used because most operating
connection basis. Linux, Solaris, OS X, and Windows all allow systems do not allow the time to be set on a per connection basis.
KEEP_ALIVEs to be turned on or off on a single socket using the Linux, Solaris, OS X, and Windows all allow KEEP_ALIVEs to be
SO_KEEPALIVE socket options but can not change the duration of the turned on or off on a single socket using the SO_KEEPALIVE socket
timer for an individual socket. The length of the timer typically options but can not change the duration of the timer for an
defaults to 7200 seconds. The length of the timer can be changed individual socket. The length of the timer typically defaults to
to a smaller value by setting a kernel parameter but that affects 7200 seconds. The length of the timer can be changed to a smaller
all TCP connections on the host and thus is not appropriate to value by setting a kernel parameter but that affects all TCP
use. connections on the host and thus is not appropriate to use.
If the UA detects that the connection has failed or that the flow When the UA detects that a flow has failed or that the flow
definition has changed, it MUST re-register and MUST use the back-off definition has changed, the UA needs to re-register and will use the
mechanism described in Section 4 in order to provide congestion back-off mechanism described in Section 4 to provide congestion
relief when a large number of agents simultaneously reboot. relief when a large number of agents simultaneously reboot.
4. User Agent Mechanisms 4. User Agent Mechanisms
The UA behavior is divided up into sections. The first describes 4.1. Instance ID Creation
what a client must do when forming a new connection, the second when
detecting failure of a connection, and the third on failure recovery.
4.1. Forming Flows Each UA MUST have an Instance Identifer URN that uniquely identifies
the device. Usage of a URN provides a persistent and unique name for
the UA instance. It also provides an easy way to guarantee
uniqueness within the AOR. This URN MUST be persitant across power
cylces of the device.
When a User Agent initiates a dialog, it MUST provide a Contact URI A UA SHOULD use a UUID URN [9]. The UUID URN allows for non-
which has GRUU properties if it is in possession of an appropriate centralized computation of a URN based on time, unique names (such as
GRUU. If it can not provide a GRUU, it needs to follow the procedure a MAC address), or a random number generator.
specified in Section 4.1.1.
A device like a soft-phone, when first installed, can generate a
UUID [9] and then save this in persistent storage for all future
use. For a device such as a hard phone, which will only ever have
a single SIP UA present, the UUID can include the MAC address and
be generated at any time because it is guaranteed that no other
UUID is being generated at the same time on that physical device.
This means the value of the time component of the UUID can be
arbitrarily selected to be any time less than the time when the
device was manufactured. A time of 0 (as shown in the example in
Section 3.2) is perfectly legal as long as the device knows no
other UUIDs were generated at this time.
If a URN scheme other than UUID is used, the URN MUST be selected
such that the instance can be certain that no other instance
registering against the same AOR would choose the same URN value. An
example of a URN that would not meet the requirements of this
specification is the national bibliographic number [15]. Since there
is no clear relationship between a SIP UA instance and a URN in this
namespace, there is no way a selection of a value can be performed
that guarantees that another UA instance doesn't choose the same
value.
The UA SHOULD include a "sip.instance" media feature tag as a UA
characteristic [10] in requests and responses. As described in [10],
this media feature tag will be encoded in the Contact header field as
the "+sip.instance" Contact header field parameter. The value of
this parameter MUST be a URN [3]. One case where a UA may not want
to include the URN in the sip.instance media feature tag is when it
is making an anoymous request or some other privacy concern requires
that the UA not reveal its identity.
RFC 3840 [10] defines equality rules for callee capabilities
parameters, and according to that specification, the
"sip.instance" media feature tag will be compared by case-
sensitive string comparison. This means that the URN will be
encapsulated by angle brackets ("<" and ">") when it is placed
within the quoted string value of the +sip.instance Contact header
field parameter. The case-sensitive matching rules apply only to
the generic usages defined in RFC 3840 [10] and in the caller
preferences specification [2]. When the instance ID is used in
this specification, it is effectively "extracted" from the value
in the "sip.instance" media feature tag. Thus, equality
comparisons are performed using the rules for URN equality that
are specific to the scheme in the URN. If the element performing
the comparisons does not understand the URN scheme, it performs
the comparisons using the lexical equality rules defined in RFC
2141 [3]. Lexical equality may result in two URNs being
considered unequal when they are actually equal. In this specific
usage of URNs, the only element which provides the URN is the SIP
UA instance identified by that URN. As a result, the UA instance
SHOULD provide lexically equivalent URNs in each registration it
generates. This is likely to be normal behavior in any case;
clients are not likely to modify the value of the instance ID so
that it remains functionally equivalent yet lexigraphically
different to previous registrations.
4.2. Initial Registrations
UAs are configured with one or more SIP URIs representing the default UAs are configured with one or more SIP URIs representing the default
outbound proxies with which to register. A UA MUST support sets with outbound-proxy-set. The specification assumes the set is determined
at least two outbound proxy URIs (primary and backup) and SHOULD via configuration but future specifications may define other
support sets with up to four URIs. For each outbound proxy URI in mechanisms such as using DNS to discover this set. How the UA is
the set, the UA MUST send a REGISTER in the normal way using this URI configured is outside the scope of this specification. However, a UA
as the default outbound proxy. Forming the route set for the request MUST support sets with at least two outbound proxy URIs (primary and
is discussed in [15] but typically results in sending the REGISTER backup) and SHOULD support sets with up to four URIs. For each
with the Route header field containing a loose route to the outbound outbound proxy URI in the set, the UA MUST send a REGISTER in the
proxy URI. The UA MUST include the instance-id as described in [1]. normal way using this URI as the default outbound proxy. Forming the
The UA MUST also add a distinct flow-id parameter to the Contact route set for the request is outside the scope of this document, but
header field. The UA SHOULD use a flow-id value of 1 for the first typically results in sending the REGISTER such that the topmost Route
URI in the set, and a flow-id value of 2 for the second, and so on. header field contains a loose route to the outbound proxy URI. Other
Each one of these registrations will form a new flow from the UA to issues related to outbound route construction are discussed in [20].
the proxy. The flow-id sequence does not have to be exactly 1,2,3
but it does have to be exactly the same flow-id sequence each time
the device power cycles or reboots so that the flow-id values will
collide with the previously used flow-id values and the proxy can
realize that the older registrations are probably not useful.
If the 200 response to a REGISTER contains a Service Route header Registration requests, other than those described in Section 4.2.1,
field value as defined in RFC 3608 [16], then whichever proxy sends MUST include the instance-id media feature tag as specified in
the 200 response last will affect where all future requests from this Section 4.1.
UA are directed.
Note that the UA needs to honor 503 responses to registrations as These ordinary registration requests MUST also add a distinct reg-id
described in RFC 3261 and RFC 3263 [4]. In particular, implementors parameter to the Contact header field. Each one of these
should note that when receiving a 503 with a Retry-After, the UA registrations will form a new flow from the UA to the proxy. The
should wait the indicated amount of time and retry the registration. reg-id sequence does not have to be sequential but MUST be exactly
A Retry-After header field value of 0 is valid and indicates the UA the same reg-id sequence each time the device power cycles or reboots
should retry the REGISTER immediately. Implementations need to so that the reg-id values will collide with the previously used
ensure that when retrying the REGISTER they redo the DNS resolution reg-id values. This is so the proxy can realize that the older
process such that if multiple hosts are reachable from the URI, there registrations are probably not useful.
is a chance that the UA will select an alternate host from the one it
chose the previous time the URI was resolved.
Note on Instance-ID Selection: The instance-id needs to be a URN but The UAC MUST indicate that it supports the Path header [12]
there are many ways one can be generated. A particularly simple way mechanism, by including the 'path' option-tag in a Supported header
for both "hard" phones and "soft" phones is to use a UUID as defined field value in its REGISTER requests. Other than optionally
in [6]. A device like a soft-phone, when first installed, should examining the Path vector in the response, this is all that is
generate a UUID [6] and then save this in persistent storage for all required of the UAC to support Path.
future use. For a device such as a hard phone, which will only ever
have a single SIP UA present, the UUID can be generated at any time
because it is guaranteed that no other UUID is being generated at the
same time on that physical device. This means the value of the time
component of the UUID can be arbitrarily selected to be any time less
than the time when the device was manufactured. A time of 0 (as
shown in the example in Section 3.2) is perfectly legal as long as
the device knows no other UUIDs were generated at this time.
4.1.1. Request without GRUU The UAC MAY examine successful registrations for the presence of an
'outbound' option-tag in a Supported header field value. Presence of
this option-tag indicates that the registrar is compliant with this
specification.
If the UA does not have a GRUU, it MUST send the request with a Note that the UA needs to honor 503 responses to registrations as
Contact header field containing a +sip.instance media feature described in RFC 3261 and RFC 3263 [6]. In particular, implementors
parameter, and it MUST include the "pin-route" option-tag in both a should note that when receiving a 503 response with a Retry-After
Proxy-Require and a Require header field value. A User Agent header field, the UA should wait the indicated amount of time and
compliant with this specification MUST NOT initiate a dialog with an retry the registration. A Retry-After header field value of 0 is
INVITE without a GRUU in the Contact header field. (At the time of valid and indicates the UA should retry the REGISTER immediately.
this writing this is allowed only for dialogs initiated with the Implementations need to ensure that when retrying the REGISTER they
SUBSCRIBE method.) revisit the DNS resolution results such that the UA can select an
alternate host from the one chosen the previous time the URI was
resolved.
This mechanism without a GRUU is not reliable if any of the proxies 4.2.1. Registration by Other Instances
on the path fail so it SHOULD not be used for long lived
subscriptions. Once a UA acquires an appropriate GRUU, it should
terminate these subscriptions and re-subscribe using the normal GRUU
based approach.
4.2. Detecting Flow Failure A User Agent MUST NOT include an instance-id or reg-id in the Contact
header field of a registration if the registering UA is not the same
instance as the UA referred to by the target Contact header field.
(This practice is occasionally used to install forwarding policy into
registrars.)
The UA needs to detect if a given flow has failed, and if it has Note that a UAC also MUST NOT include an instance-id or reg-id
failed, follow the procedures in Section 4.1 to form a new flow to parameter in a request to deregister all Contacts (a single Contact
replace the failed one. header field value with the value of "*").
User Agents that form flows MUST check if the configured URI they are 4.3. Sending Requests
connecting to has the "sip-stun" tag (defined in Section 10) and, if
the tag is present, then the UA needs to periodically perform STUN
[5] requests over the flow. The time between STUN requests when
using UDP SHOULD be a random number between 24 and 29 seconds while
for other transport protocols it SHOULD be a random number between 95
and 120 seconds. The times MAY be configurable.
Note on selection of time values: For UDP, the upper bound of 29 As described in Section 4.1, all requests need to include the
seconds was selected so that multiple STUN packets would be sent instance-id media feature tag unless privacy concerns require
before 30 seconds based on information that some NATs had UDP otherwise.
timeouts as low as 30 seconds. The 24 second lower bound was
selected so that after 10 minutes the jitter this introduce would
have unsyncronized the STUN requests from different devices to evenly
spread the load on the servers. For TCP, the 120 seconds was chosen
based on the idea that for a good user experience, failures would be
detected in this time and a new connection set up. Operators that
wish to change the relationship between load on servers and the
expected time that a user may not receive inbound communications will
probably adjust this time widely. The 95 seconds lower bound was
chosen so that the jitter introduced would result in a relatively
even load on the servers after 30 minutes.
If the mapped address in the STUN response changes, the UA must treat 4.3.1. Selecting the First Hop
this as a failure on the flow. Any time a SIP message is sent and
the proxy does not respond, this is also considered a failure, the
flow is discarded and the procedures in Section 4.3 are followed to
form a new flow.
4.3. Flow Failure Recovery When an UA is about to send a request, it first performs normal
processing to select the next hop URI. The UA can use a variety of
techniques to compute the route set and accordingly the next hop URI.
Discussion of these techniques is outside the scope of this document
but could include mechanisms specified in RFC 3608 [21] (Service
Route) and [20].
When a flow to a particular URI in the proxy set fails, the UA needs 4.3.2. Forming Flows
to form a new flow to replace it. The new flow MUST have the same The UA performs normal DNS resolution on the next hop URI (as
flow-id as the flow it is replacing. This is done in much the same described in RFC 3263 [6]) to find a protocol, IP address, and port.
way as the flows are described as being formed in Section 4.1; For non TLS protocols, if the UA has an existing flow to this IP
however, if there is a failure in forming this flow, the UA needs to address, and port with the correct protocol, then the UA MUST use the
wait a certain amount of time before retrying to form a flow to this existing connection. For TLS protocols, the existing flow is only
particular URI in the proxy set. The time to wait is computed in the used if, in addition to matching the IP address, port, and protocol,
following way. If all of the flows to every URI in the proxy set the host production in the next hop URI MUST match one of the URIs
have failed, the base time is set to 30 seconds; otherwise, in the contained in the subjectAltName in the peer certificate. If the UA
case where at least one of the flows has not failed, the base time is cannot use one of the existing flows, then it SHOULD form a new flow
set to 90 seconds. The wait time is computed by taking the base time by sending a datagram or opening a new connection to the next hop, as
multiplied by two to power of the number of consecutive registration appropriate for the transport protocol.
failures to that URI up to a maximum of 1800 seconds.
wait-time = min( 1800, (base-time * (2 ^ consecutive-failures))) 4.4. Detecting Flow Failure
These three times SHOULD be configurable in the UA. The three times The UA needs to detect when a specific flow fails. If a flow has
are the max-time with a default of 1800 seconds, the base-time-all- failed, the UA follows the procedures in Section 4.2 to form a new
fail with a default of 30 seconds, and the base-time-not-failed with flow to replace the failed one. The UA proactively tries to detect
a default of 60 seconds. For example if the base time was 30 failure by periodically sending keep alive messages using one of the
seconds, and there had been three failures, then the wait time would techniques described in this section.
be min(1800,30*(2^3)) or 240 seconds. The delay time is computed by
selecting a uniform random time between 50 and 100 percent of the the
wait time. The UA MUST wait for the value of the delay time before
trying another registration to form a new flow for that URI.
To be explicitly clear on the boundary conditions: when the UA boots The time between keep alive requests when using UDP based transports
it immediately tries to register. If this fails and no registration SHOULD be a random number between 24 and 29 seconds while for TCP
on other flows had succeeded, the first retry would happen somewhere based transports it SHOULD be a random number between 95 and 120
between 30 and 60 seconds after the failure of the first registration seconds. These times MAY be configurable.
request. If the number of consecutive-failures is large enough that
the maximum of 1800 seconds is being reached, then the UA keep trying
forever with a random time between 900 and 1800 seconds between the
attempts.
SIP dialogs can be used for one or more "usages". For example, a o Note on selection of time values: For UDP, the upper bound of 29
session created with INVITE (a session "usage") and a subscription (a seconds was selected so that multiple STUN packets could be sent
subscription "usage") can share a dialog. On failure of a flow, a before 30 seconds based on information that many NATs have UDP
User Agent might wish to resynchronizing the state of any active timeouts as low as 30 seconds. The 24 second lower bound was
usages on any dialogs using the flow. For example, the User Agent selected so that after 10 minutes the jitter introduced by
could send a new subscription for each subscription usage and an different timers will the keep alive requests unsynchronized to
INVITE with replaces for each session usage. Note that when a flow evenly spread the load on the servers. For TCP, the 120 seconds
was obtained via a REGISTER request, the flow might be used by many was chosen based on the idea that for a good user experience,
dialogs and dialog usages. A flow obtained via another request (e.g. failures should be detected in this amount of time and a new
a SUBSCRIBE request) only has usages from a single dialog. The only connection set up. Operators that wish to change the relationship
reason to do this is that a message may have been lost while the flow between load on servers and the expected time that a user may not
was being reestablished. The GRUU will ensure that any future receive inbound communications will probably adjust this time.
messages are still delivered to the UA even if it does not re- The 95 seconds lower bound was chosen so that the jitter
subscribe, re-INVITE, or otherwise refresh the usage. Deployments introduced will result in a relatively even load on the servers
need to carefully consider the implications of these sorts of after 30 minutes.
operations. This approach only helps in a very narrow corner case
and it will cause a huge load on the system if a single proxy
crashes. In some deployments, this will cause more harm than good.
4.4. Registration by Other Instances 4.4.1. Keep Alive with STUN
A User Agent MUST NOT include an instance-id or flow-id in the User Agents that form flows MUST check if the configured URI they are
Contact header field of a registration if the registering UA is not connecting to has the "sip-stun" URI parameter (defined in
the same instance as the UA referred to by the target Contact header Section 10). If the parameter is present, the UA needs to
field. (This practice is occasionally used to install forwarding periodically perform keep alive checks by sending a STUN [7] Binding
policy into registrars.) Requests over the flow.
5. Registrar Mechanisms If the XOR-MAPPED-ADDRESS in the STUN Binding Response changes, the
UA MUST treat this event as a failure on the flow.
5.1. Processing Register Requests 4.4.2. Keep Alive with Double CRLF
Registrars which implement this specification, MUST support the Path User Agents that form flows MUST check if the configured URI they are
header mechanism[10] and processes REGISTER requests as described in connecting to has the "crlf-ping" URI parameter (defined in
Section 10 of RFC 3261 with the following change. Any time the Section 10). If the parameter is present, the UA needs to send keep
registrar checks if a new contact matches an existing contact in the alive requests by sending a CRLF over the flow.
location database, it MUST also check and see if both the instance-id
and flow-id match. If they do not both match, then they are not the
same contact. Additionally, if the both the instance-id and flow-id
are present and do match, then it is considered a match regardless of
if the value of the contact header field value matches. The
registrar MUST be prepared to receive some registrations that use
instance-id and flow-id and some that do not, simultaneously for the
same AOR.
In addition to the normal information stored in the binding record, If the UA does not receive any data back over the flow within 7
some additional information MUST be stored for any registration that seconds of sending the CRLF, then it MUST consider the lack of
contains a flow-id header parameter in the Contact header field response to be a flow failure.
value. The registrar MUST store enough information to uniquely
identify the network flow over which the request arrived. For common
operating systems with TCP, this would typically just be the file
descriptor. For common operating systems with UDP this would
typically be the file descriptor for the local socket that received
the request, the local interface, and the IP address and port number
of the remote side that sent the request.
The registrar MUST also store all the Contact header field 4.5. Flow Recovery
information including the flow-id and instance-id and SHOULD also
store the time at which the binding was last updated. If a Path
header field is present RFC 3327 [10] requires this to be stored and
the registrar MUST store the Path header field value with the binding
record. Any time a messages is forwarded over the flow that created
this binding, this stored Path header field value will be used to
route the message. If the registrar receives a re-registration, it
MUST update the information that uniquely identifies the network flow
over which the request arrived and SHOULD update the time the binding
was last updated.
The REGISTRAR MAY be configured with local policy to reject any When a flow to a particular URI in the outbound-proxy-set fails, the
registrations that do not include the instance-id and flow-id to UA needs to form a new flow to replace the old flow and replace any
eliminate the amplification attack described in [14]. registrations that were previously sent over this flow. Each new
registration MUST have the same reg-id as the registration it
replaces. This is done in much the same way as forming a brand new
flow as described in Section 4.3.2; however, if there is a failure in
forming this flow, the UA needs to wait a certain amount of time
before retrying to form a flow to this particular next hop.
5.2. Forwarding Requests The time to wait is computed in the following way. If all of the
flows to every URI in the proxy set have failed, the base time is set
to 30 seconds; otherwise, in the case where at least one of the flows
has not failed, the base time is set to 90 seconds. The wait time is
computed by taking two raised to power of the number of consecutive
registration failures for that URI, and multiplying this by the base
time, up to a maximum of 1800 seconds.
wait-time = min( 1800, (base-time * (2 ^ consecutive-failures)))
When a proxy uses the location service to look up a registration These three times MAY be configurable in the UA. The three times are
binding and then proxies a request to a particular contact, it the max-time with a default of 1800 seconds, the base-time-all-fail
selects a contact to use normally, with a few additional rules: with a default of 30 seconds, and the base-time-not-failed with a
default of 60 seconds. For example if the base time was 30 seconds,
and there had been three failures, then the wait time would be
min(1800,30*(2^3)) or 240 seconds. The delay time is computed by
selecting a uniform random time between 50 and 100 percent of the
wait time. The UA MUST wait for the value of the delay time before
trying another registration to form a new flow for that URI.
o The proxy MUST NOT populate the target set with more than one To be explicitly clear on the boundary conditions: when the UA boots
contact with the same AOR and instance-id at a time. If a request it immediately tries to register. If this fails and no registration
for a particular AOR and instance-id fails with a 410 response, on other flows succeed, the first retry happens somewhere between 30
the proxy SHOULD replace the failed branch with another target and 60 seconds after the failure of the first registration request.
with the same AOR and instance-id, but a different flow-id.
o If two bindings have the same instance-id and flow-id, it SHOULD
prefer the contact that was most recently updated.
Note that if the request URI is a GRUU, the proxy will only select If the number of consecutive-failures is large enough that the
contacts with the AOR and instance-id associated with the GRUU. The maximum of 1800 seconds is reached, the UA will keep trying forever
rules above still apply to a GRUU. This allows a request routed to a with a random time between 900 and 1800 seconds between the attempts.
GRUU to first try one of the flows to a UA, then if that fails, try
another flow to the same UA instance.
The proxy uses normal forwarding rules looking at the Route of the 5. Edge Proxy Mechanisms
message and any values of the of the stored Path header field value
in the registration binding to decide how to forward the request and
populate the Route header in the request. Additionally, when the
proxy forwards a request to a binding that contains a flow-id, the
proxy MUST send the request over the same network flow that was saved
with the binding. This means that for TCP, the request MUST be sent
on the same TCP socket that received the REGISTER request. For UDP,
the request MUST be sent from the same local IP address and port over
which the registration was received to the same IP address and port
from which the REGISTER was received.
If a proxy or registrar receives an indication from the network that 5.1. Processing Register Requests
indicates that no future messages on this flow will work, then it
MUST remove all the bindings that use that flow (regardless of AOR).
Examples of this are a TCP socket closing or receiving a destination
unreachable ICMP error on a UDP flow. Similarly, if a proxy closes a
file descriptor, it MUST remove all the bindings that use that flow.
6. Edge Proxy Mechanisms When an Edge Proxy receives a registration request with a
sip.instance media feature tag in the Contact header field, it MUST
form a flow identifier token that is unique to this network flow.
The Edge Proxy MUST insert this token into a URI referring to this
proxy and place this URI into a Path header field as described in RFC
3327 [12]. The token MAY be placed in the userpart of the URI.
6.1. Processing Register Requests 5.2. Generating Flow Tokens
When an Edge Proxy receives a registration request it MUST form a A trivial but impractical way to satisfy the flow token requirement
flow identifier token that is unique to this network flow and use Section 5.1 involves storing a mapping between an incrementing
this token as the user part of the URI that this proxy inserts into counter and the connection information; however this would require
the Path header. Edge proxies MUST use a Path header. A trivial way the Edge Proxy to keep an impractical amount of state. It is unclear
to satisfy this requirement involves storing a mapping between an when this state could be removed and the approach would have problems
incrementing counter and the connection information; however this if the proxy crashed and lost the value of the counter. Two
would require the Edge Proxy to keep an impractical amount of state. stateless examples are provided below. A proxy can use any algorithm
It is unclear when this state could be removed and the approach would it wants as long as the flow token is unique to a flow, the flow can
have problems if the proxy crashed and lost the value of the counter. be recovered from the token, and the token can not be modified by
Two stateless examples are provided below. A proxy can use any attackers.
algorithm it wants as long as the flow token is unique to a flow, the
flow can be recovered from the token, and the token can not be
modified by attackers.
Algorithm 1: The proxy generates a flow token for connection-oriented Algorithm 1: The proxy generates a flow token for connection-oriented
transports by concatenating the file descriptor (or equivalent) transports by concatenating the file descriptor (or equivalent)
with the NTP time the connection was created, and base64 encoding with the NTP time the connection was created, and base64 encoding
the result. This results in an approximately 16 octet identifier. the result. This results in an approximately 16 octet identifier.
The proxy generates a flow token for UDP by concatenating the file The proxy generates a flow token for UDP by concatenating the file
descriptor and the remote IP address and port, then base64 descriptor and the remote IP address and port, then base64
encoding the result. encoding the result. This algorithm MUST NOT be used unless all
messages between the Edge proxy and Registrar use a SIPS protected
transport. If the SIPS level of integrity protection is not
available, an attacker can hijack another user's calls.
Algorithm 2: When the proxy boots it selects a 20 byte crypto random Algorithm 2: When the proxy boots it selects a 20 byte crypto random
key called K that only the Edge Proxy knows. A byte array, called key called K that only the Edge Proxy knows. A byte array, called
S, is formed that contains the following information about the S, is formed that contains the following information about the
flow the request was received on: an enumeration indicating the flow the request was received on: an enumeration indicating the
protocol, the local IP address and port, the remote IP address and protocol, the local IP address and port, the remote IP address and
port. The HMAC of S is computed using the key K and the HMAC- port. The HMAC of S is computed using the key K and the HMAC-
SHA1-80 algorithm, as defined in [8]. The concatenation of the SHA1-80 algorithm, as defined in [16]. The concatenation of the
HMAC and S are base64 encoded, as defined in [9], and used as the HMAC and S are base64 encoded, as defined in [18], and used as the
flow identifier. With IPv4 address, this will result in a 32 flow identifier. When using IPv4 addresses, this will result in a
octet identifier. 32 octet identifier.
Algorithm 1 MUST NOT be used unless the REGISTER request is over a 5.3. Forwarding Requests
SIPS protected transport. If the SIPS level of integrity protection
is not available, an attacker can hijack another user's calls.
6.2. Forwarding Requests When the Edge Proxy receives a request, it applies normal routing
procedures with the following addition. If the top-most Route header
refers to the Edge Proxy and contains a valid flow identifier token
created by this proxy, the proxy MUST forward the request over the
flow that received the REGISTER request that caused the flow
identifier token to be created. For connection-oriented transports,
if the flow no longer exists the proxy SHOULD send a 410 response to
the request.
When the Edge Proxy receives a request it applies normal routing The advantage to a stateless approach to managing the flow
procedures with the addition that it is routed to a URI with a flow information is that there is no state on the edge proxy that
identifier token that this proxy created, then the proxy MUST forward requires clean up or that has to be synchronized with the
the request over the flow that received the REGISTER request that registrar.
caused the flow identifier token to be created. For connection-
oriented transports, if the flow no longer exists the proxy SHOULD Proxies which used one of the two algorithms described in this
send a 410 response to the request. The advantage to a stateless document to form a flow token follow the procedures below to
approach to managing the flow information is that there is no state determine the correct flow.
on the edge proxy that requires clean up that has to be synchronized
with the registrar.
Algorithm 1: The proxy base64 decodes the user part of the Route Algorithm 1: The proxy base64 decodes the user part of the Route
header. For TCP, if a connection specified by the file descriptor header. For TCP, if a connection specified by the file descriptor
is present and the creation time of the file descriptor matches is present and the creation time of the file descriptor matches
the creation time encoded in the Route header, the proxy forwards the creation time encoded in the Route header, the proxy forwards
the request over that connection. For UDP, the proxy forwards the the request over that connection. For UDP, the proxy forwards the
request from the encoded file descriptor to the source IP address request from the encoded file descriptor to the source IP address
and port. and port.
Algorithm 2: To decode the flow token take the flow identifier in the Algorithm 2: To decode the flow token take the flow identifier in the
user portion of the URI, and base64 decode it, then verity the user portion of the URI, and base64 decode it, then verify the
HMAC is correct by recomputing the HMAC and checking it matches. HMAC is correct by recomputing the HMAC and checking it matches.
If the HMAC is not correct, the proxy SHOULD send a 403 response. If the HMAC is not correct, the proxy SHOULD send a 403 response.
If the HMAC was correct then the proxy should forward the request If the HMAC was correct then the proxy should forward the request
on the flow that was specified by the information in the flow on the flow that was specified by the information in the flow
identifier. If this flow no longer exists, the proxy SHOULD send identifier. If this flow no longer exists, the proxy SHOULD send
a 410 response to the request. a 410 response to the request.
Edge Proxies MUST Record-Route so that mid-dialog requests still are Note that techniques to ensure that mid-dialog requests are routed
routed over the correct flow. over an existing flow are out of scope and therefore not part of this
specification. However, an approach such as having the Edge Proxy
Record-Route with a flow token is one way to ensure that mid-dialog
requests are routed over the correct flow.
7. Mechanisms for All Servers 6. Registrar and Location Server Mechanisms
7.1. STUN Processing 6.1. Processing Register Requests
TODO: This section needs to be brought into sync with the STUN draft This specification updates the definition of a binding in RFC 3261
and check there are not issues for SIP and STUN on TCP or UDP [5] Section 10 and RFC 3327 [12] Section 5.3.
connections.
When no instance-id is present in a Contact header field value in a
REGISTER request, the corresponding binding is still between an AOR
and the URI from that Contact header field value. When an
instance-id is present in a Contact header field value in a REGISTER
request, the corresponding binding is between an AOR and the
combination of instance-id and reg-id. For a binding with an
instance-id, the registrar still stores the Contact header field
value URI with the binding, but does not consider the Contact URI for
comparison purposes (the Contact URI is not part of the "key" for the
binding). The registrar MUST be prepared to receive, simultaneously
for the same AOR, some registrations that use instance-id and reg-id
and some that do not.
Registrars which implement this specification, MUST support the Path
header mechanism [12].
In addition to the normal information stored in the binding record,
some additional information MUST be stored for any registration that
contains a reg-id header parameter in the Contact header field value.
The registrar MUST store enough information to uniquely identify the
network flow over which the request arrived. For common operating
systems with TCP, this would typically just be the file descriptor.
For common operating systems with UDP this would typically be the
file descriptor for the local socket that received the request, the
local interface, and the IP address and port number of the remote
side that sent the request.
The registrar MUST also store all the Contact header field
information including the reg-id and instance-id parameters and
SHOULD also store the time at which the binding was last updated. If
a Path header field is present, RFC 3327 [12] requires the registrar
to store this information as well. If the registrar receives a re-
registration, it MUST update the information that uniquely identifies
the network flow over which the request arrived and SHOULD update the
time the binding was last updated.
The Registrar MUST include the 'outbound' option-tag in a Supported
header field value in its responses to REGISTER requests. The
Registrar MAY be configured with local policy to reject any
registrations that do not include the instance-id and reg-id to
eliminate the amplification attack described in [19]. Note that the
requirements in this section applies to both REGISTER requests
received from an Edge Proxy as well as requests received directly
from the UAC.
6.2. Forwarding Requests
When a proxy uses the location service to look up a registration
binding and then proxies a request to a particular contact, it
selects a contact to use normally, with a few additional rules:
o The proxy MUST NOT populate the target set with more than one
contact with the same AOR and instance-id at a time. If a request
for a particular AOR and instance-id fails with a 410 response,
the proxy SHOULD replace the failed branch with another target (if
one is available) with the same AOR and instance-id, but a
different reg-id.
o If two bindings have the same instance-id and reg-id, the proxy
SHOULD prefer the contact that was most recently updated.
The proxy uses normal forwarding rules looking at the Route of the
message and the value of any stored Path header field vector in the
registration binding to decide how to forward the request and
populate the Route header in the request. Additionally, when the
proxy forwards a request to a binding that contains a reg-id, the
proxy MUST send the request over the same network flow that was saved
with the binding. This means that for TCP, the request MUST be sent
on the same TCP socket that received the REGISTER request. For UDP,
the request MUST be sent from the same local IP address and port over
which the registration was received, to the same IP address and port
from which the REGISTER was received.
If a proxy or registrar receives information from the network that
indicates that no future messages will be delivered on a specific
flow, then the proxy MUST invalidate all the bindings that use that
flow (regardless of AOR). Examples of this are a TCP socket closing
or receiving a destination unreachable ICMP error on a UDP flow.
Similarly, if a proxy closes a file descriptor, it MUST invalidate
all the bindings with flows that use that file descriptor.
7. Mechanisms for All Servers (Proxys, Registars, UAS)
A SIP device that receives SIP messages directly from a UA needs to A SIP device that receives SIP messages directly from a UA needs to
behave as specified in this section. Such devices would generally behave as specified in this section. Such devices would generally
include a Registrar and an Edge Proxy, as they both receive register include a Registrar and an Edge Proxy, as they both receive register
requests directly from a UA. requests directly from a UA.
7.1. STUN Processing
This document defines a new STUN usage for inband connectivity
checks. The only STUN messages required by this usage are Binding
Requests, Binding Responses, and Error Responses. The UAC sends
Binding Requests over the same UDP flow, TCP connection, or TLS
channel used for sending SIP messages, once a SIP registration has
been successfully processed on that flow. These Binding Requests do
not require any STUN attributes. The UAS responds to a valid Binding
Request with a Binding Response which MUST include the XOR-MAPPED-
ADDRESS attribute. After a successful STUN response is received over
TCP or TLS over TCP, the underlying TCP connection is left in the
active state.
If the server receives SIP requests on a given interface and port, it If the server receives SIP requests on a given interface and port, it
MUST also provide a limited version of a STUN server on the same MUST also provide a limited version of a STUN server on the same
interface and port. Specifically it MUST be capable of receiving and interface and port. Specifically it MUST be capable of receiving and
responding to STUN requests with the exception that it does not need responding to STUN Binding Requests.
to support STUN requests with the changed port or changed address
flag set. This allows the STUN server to run with only one port and
IP address.
It is easy to distinguish STUN and SIP packets because the first It is easy to distinguish STUN and SIP packets because the first
octet of a STUN packet has a value of 0 or 1 while the first octet of octet of a STUN packet has a value of 0 or 1 while the first octet
a SIP message is never a 0 or 1. of a SIP message is never a 0 or 1.
When a URI is created that refers to a SIP device that supports STUN When a URI is created that refers to a SIP device that supports STUN
as described in this section, the URI parameter "sip-stun", as as described in this section, the URI parameter "sip-stun", as
defined in Section 10 MUST be added to the URI. This allows a UA to defined in Section 10 MUST be added to the URI. This allows a UA to
inspect the URI to decide if it should attempt to send STUN requests inspect the URI to decide if it should attempt to send STUN requests
to this location. The sip-stun tag would typically show up in the to this location. The sip-stun tag typically would be present in the
URI in the Route header field value of a REGISTER request and would URI in the Route header field value of a REGISTER request and not be
not be in the request URI. in the Request URI.
7.2. Pin-Route Processing
A sip device receives a request with the "pin-route" options tag set 7.2. Double CRLF Processing
in the Proxy-Require header field or the Require header field needs
to follow the procedures in this section.
A UAS that receives a request with the "pin-route" option tag in the If the SIP server is acting as the TCP client and initiated the TCP
Require header MUST either reject the request if pin-route is not connection (meaning that this host did the active open), then the SIP
supported, or if pin-route is supported by this UAS, the UAS MUST server MUST NOT perform any of the processing in this section. The
ensure that any message send in the dialog formed by this request is following only applies when the SIP server is acting as the TCP
sent on the same flow as the initial request. This specification server (meaning that this host did the passive open).
does not mandate that all UAs support this option but certain UAs,
such as the NOTIFIER in the configuration framework, will want to
support this so they can form subscriptions with devices that do not
have a GRUU.
A proxy that receives a request with the "pin-route" option tag in When the server receives a CRLF before the start line of a message on
the Proxy-Require header MUST add a record-route header field value a flow, it MUST send some data back on that same flow within 3
that resolves to this proxy and it MUST ensure that any future seconds. If no message is actively being sent, it SHOULD send back a
requests or responses in this dialog are forwarded on the same flow CRLF after waiting at least 1 second. The reason for waiting at
as the original request. The suggested way to do this is to form a least 1 second is that if the other end has an incorrect
flow identifier token in the same way that an Edge Proxy would form implementation and incorrectly echoes the CRLF, this will stop the
this for the Path header and insert this flow identifier token in the flow from going into a live-lock state.
user portion of the URI used in the record route header field value.
8. Example Message Flow 8. Example Message Flow
The following call flow shows a basic registration and an incoming The following call flow shows a basic registration and an incoming
call. Part way through the call, the flow to the Primary proxy is call. Part way through the call, the flow to the Primary proxy is
lost. The BYE message for the call is rerouted to the callee via the lost. The BYE message for the call is rerouted to the callee via the
Backup proxy. When connectivity to the primary proxy is established, Backup proxy. When connectivity to the primary proxy is established,
the Callee registers again to replace the lost flow as shown in the Callee registers again to replace the lost flow as shown in
message 15. message 15.
skipping to change at page 20, line 12 skipping to change at page 22, line 12
"sip:backup.example.com;lr;sip-stun". The Callee REGISTER in message "sip:backup.example.com;lr;sip-stun". The Callee REGISTER in message
(1) looks like: (1) looks like:
REGISTER sip:example.com SIP/2.0 REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7 Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7
Max-Forwards: 70 Max-Forwards: 70
From: Callee <sip:callee@example.com>;tag=a73kszlfl From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com> To: Callee <sip:callee@example.com>
Call-ID: 1j9FpLxk3uxtm8tn@10.0.1.1 Call-ID: 1j9FpLxk3uxtm8tn@10.0.1.1
CSeq: 1 REGISTER CSeq: 1 REGISTER
Supported: path
Route: <sip:primary.example.com;lr;sip-stun> Route: <sip:primary.example.com;lr;sip-stun>
Contact: <sip:callee@10.0.1.1> Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>" ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;flow-id=1 ;reg-id=1
Content-Length: 0 Content-Length: 0
In the message, note that the Route is set and the Contact header In the message, note that the Route is set and the Contact header
field value contains the instance-id and flow-id. The response to field value contains the instance-id and reg-id. The response to the
the REGISTER in message (2) would look like: REGISTER in message (2) would look like:
SIP/2.0 200 OK SIP/2.0 200 OK
Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7 Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7
From: Callee <sip:callee@example.com>;tag=a73kszlfl From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com> ;tag=b88sn To: Callee <sip:callee@example.com> ;tag=b88sn
Call-ID: 1j9FpLxk3uxtm8tn@10.0.1.1 Call-ID: 1j9FpLxk3uxtm8tn@10.0.1.1
CSeq: 1 REGISTER CSeq: 1 REGISTER
Supported: outbound
Contact: <sip:callee@10.0.1.1> Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>" ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;flow-id=1 ;reg-id=1
;expires=3600 ;expires=3600
Content-Length: 0 Content-Length: 0
The second registration in message 3 and 4 are similar other than the The second registration in message 3 and 4 are similar other than the
Call-ID has changed, the flow-id is 2, and the route is set to the Call-ID has changed, the reg-id is 2, and the route is set to the
backup instead of the primary. They look like: backup instead of the primary. They look like:
REGISTER sip:example.com SIP/2.0 REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7 Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7
Max-Forwards: 70 Max-Forwards: 70
From: Callee <sip:callee@example.com>;tag=a73kszlfl From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com> To: Callee <sip:callee@example.com>
Call-ID: 1j9FpLxk3uxtm8tn-2@10.0.1.1 Call-ID: 1j9FpLxk3uxtm8tn-2@10.0.1.1
CSeq: 1 REGISTER CSeq: 1 REGISTER
Supported: path
Route: <sip:backup.example.com;lr;sip-stun> Route: <sip:backup.example.com;lr;sip-stun>
Contact: <sip:callee@10.0.1.1> Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>" ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;flow-id=2 ;reg-id=2
Content-Length: 0 Content-Length: 0
SIP/2.0 200 OK SIP/2.0 200 OK
Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7 Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7
From: Callee <sip:callee@example.com>;tag=a73kszlfl From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com> ;tag=b88sn To: Callee <sip:callee@example.com> ;tag=b88sn
Call-ID: 1j9FpLxk3uxtm8tn-2@10.0.1.1 Call-ID: 1j9FpLxk3uxtm8tn-2@10.0.1.1
Supported: outbound
CSeq: 1 REGISTER CSeq: 1 REGISTER
Contact: <sip:callee@10.0.1.1> Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>" ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;flow-id=1 ;reg-id=1
;expires=3600 ;expires=3600
Contact: <sip:callee@10.0.1.1> Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>" ;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;flow-id=2 ;reg-id=2
;expires=3600 ;expires=3600
Content-Length: 0 Content-Length: 0
The messages in the call flow are very normal. The only interesting The messages in the call flow are very normal. The only interesting
thing to note is that the INVITE in message 6 will have a: thing to note is that the INVITE in message 6 contains the following
Record-Route header field:
Record-Route: <sip:example.com;lr> Record-Route: <sip:example.com;lr>
Message 11 seems seams strange in that it goes to the backup instead Message 11 seems seams strange in that it goes to the backup instead
of the primary. The Caller actually sends the message to the domain of the primary. The Caller actually sends the message to the domain
of the callee based on the GRUU that the callee provided in their of the callee to a host (primary or backup) that is currently
Contact header field value when the dialog was formed and the domain available. How the domain does this is an implementation detail up
selected a host (primary or backup) that was currently available. to the domain and not part of this specification.
How the domain does this is an implementation detail up to the
domain.
The registrations in message 15 and 16 are the same as message 1 and The registrations in message 15 and 16 are the same as message 1 and
2 other than the Call-ID has changed. 2 other than the Call-ID has changed.
9. Grammar 9. Grammar
This specification defines a new Contact header field parameter, This specification defines new Contact header field parameters,
flow-id. The grammar for DIGIT and EQUAL is obtained from RFC 3261 reg-id and +sip.instance. The grammar includes the definitions from
[3]. RFC 3261 [5] and includes the definition of uric from RFC 2396 [11].
The ABNF[8] is:
contact-params = c-p-q / c-p-expires / c-p-flow / contact-extension contact-params = c-p-q / c-p-expires / c-p-flow / c-p-instance
c-p-flow = "flow-id" EQUAL 1*DIGIT / contact-extension
The value of the flow-id MUST NOT be 0 and MUST be less than 2**31.
c-p-flow = "reg-id" EQUAL 1*DIGIT ; 1 to 2**31
c-p-instance = "+sip.instance" EQUAL LDQUOT "<"
instance-val ">" RDQUOT
instance-val = *uric ; defined in RFC 2396
The value of the reg-id MUST NOT be 0 and MUST be less than 2**31.
10. IANA Considerations 10. IANA Considerations
10.1. Contact Header Field
This specification defines a new Contact header field parameter This specification defines a new Contact header field parameter
called flow-id in the "Header Field Parameters and Parameter Values" called reg-id in the "Header Field Parameters and Parameter Values"
sub-registry as per the registry created by [11] at sub-registry as per the registry created by [13] . The required
http://www.iana.org/assignments/sip-parameters. The required
information is: information is:
Header Field Parameter Name Predefined Reference Header Field Parameter Name Predefined Reference
Values Values
____________________________________________________________________ ____________________________________________________________________
Contact flow-id Yes [RFC AAAA] Contact reg-id Yes [RFC AAAA]
[NOTE TO RFC Editor: Please replace AAAA with [NOTE TO RFC Editor: Please replace AAAA with
the RFC number of this specification.] the RFC number of this specification.]
This specification defines a new value in the "SIP/SIPS URI 10.2. SIP/SIPS URI Paramters
Parameters" sub-registry as per the registry created by [12] at
http://www.iana.org/assignments/sip-parameters. The required This specification arguments the "SIP/SIPS URI Parameters" sub-
registry as per the registry created by [14] . The required
information is: information is:
Parameter Name Predefined Values Reference Parameter Name Predefined Values Reference
____________________________________________ ____________________________________________
sip-stun No [RFC AAAA] sip-stun No [RFC AAAA]
crlf-ping No [RFC AAAA]
[NOTE TO RFC Editor: Please replace AAAA with [NOTE TO RFC Editor: Please replace AAAA with
the RFC number of this specification.] the RFC number of this specification.]
TODO: Add IANA section for "pin-route" option tag. 10.3. SIP Option Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of RFC 3261.
Name: outbound
Description: This option-tag is used to identify Registrars which
support extensions for Client Initiated Connections. A Registrar
places this option-tag in a Supported header to communicate to the
registering User Agent the Registrars support for this extension.
10.4. Media Feature Tag
This section registers a new media feature tag, per the procedures
defined in RFC 2506 [1]. The tag is placed into the sip tree, which
is defined in RFC 3840 [10].
Media feature tag name: sip.instance
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature tag
contains a string containing a URN that indicates a unique identifier
associated with the UA instance registering the Contact.
Values appropriate for use with this feature tag: String.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms: This
feature tag is most useful in a communications application, for
describing the capabilities of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a specific device.
Related standards or documents: RFC XXXX
[[Note to IANA: Please replace XXXX with the RFC number of this
specification.]]
Security Considerations: This media feature tag can be used in ways
which affect application behaviors. For example, the SIP caller
preferences extension [23] allows for call routing decisions to be
based on the values of these parameters. Therefore, if an attacker
can modify the values of this tag, they may be able to affect the
behavior of applications. As a result, applications which utilize
this media feature tag SHOULD provide a means for ensuring its
integrity. Similarly, this feature tag should only be trusted as
valid when it comes from the user or user agent described by the tag.
As a result, protocols for conveying this feature tag SHOULD provide
a mechanism for guaranteeing authenticity.
11. Security Considerations 11. Security Considerations
One of the key security concerns in this work is making sure that an One of the key security concerns in this work is making sure that an
attacker cannot hijack the sessions of a valid user and cause all attacker cannot hijack the sessions of a valid user and cause all
calls destined to that user to be sent to the attacker. calls destined to that user to be sent to the attacker.
The simple case is when there are no edge proxies. In this case, the The simple case is when there are no edge proxies. In this case, the
only time an entry can be added to the routing for a given AOR is only time an entry can be added to the routing for a given AOR is
when the registration succeeds. SIP protects against attackers being when the registration succeeds. SIP already protects against
able to successfully register, and this scheme relies on that attackers being able to successfully register, and this scheme relies
security. Some implementers have considered the idea of just saving on that security. Some implementers have considered the idea of just
the instance-id without relating it to the AOR with which it saving the instance-id without relating it to the AOR with which it
registered. This idea will not work because an attacker's UA can registered. This idea will not work because an attacker's UA can
impersonate a valid user's instance-id and hijack that user's calls. impersonate a valid user's instance-id and hijack that user's calls.
The more complex case involves one or more edge proxies. When a UA The more complex case involves one or more edge proxies. When a UA
sends a REGISTER request through an Edge Proxy on to the registrar, sends a REGISTER request through an Edge Proxy on to the registrar,
the Edge Proxy inserts a Path header field value. If the the Edge Proxy inserts a Path header field value. If the
registration is successfully authenticated, the proxy stores the registration is successfully authenticated, the proxy stores the
value of the Path header field. Later when the registrar forwards a value of the Path header field. Later when the registrar forwards a
request destined for the UA, it copies the stored value of the Path request destined for the UA, it copies the stored value of the Path
header field into the route header field of the request and forwards header field into the route header field of the request and forwards
skipping to change at page 23, line 31 skipping to change at page 26, line 44
has been successful; and the registration will only be successful if has been successful; and the registration will only be successful if
the UA can correctly authenticate. Even if an attacker has spoofed the UA can correctly authenticate. Even if an attacker has spoofed
some bad information in the path header sent to the registrar, the some bad information in the path header sent to the registrar, the
attacker will not be able to get the registrar to accept this attacker will not be able to get the registrar to accept this
information for an AOR that does not belong to the attacker. The information for an AOR that does not belong to the attacker. The
registrar will not hand out this bad information to others, and registrar will not hand out this bad information to others, and
others will not be misled into contacting the attacker. others will not be misled into contacting the attacker.
12. Open Issues 12. Open Issues
Service Route: The current interaction of this draft and Do we want to include the Double CRLF keep alive option?
draft-rosenberg-sip-route-construct [15] does not work. Currently
the Service Route specification, RCFC 3608, suggests that the service
route is appended to the outbound proxy set. That will work with
this specification. However the [15] draft is suggesting to change
the behavior so that the Service Route replaces the outbound proxy.
This is basically so that SIP can be used to make configuration
changes to the UA. The problem is that this specification requires
two or more URIs for the outbound configuration (so that reliability
is possible) and the Service Route would only be able to provide a
single URI. If it is desirable to use Service Route this way, it
probably needs to be modified in many ways including allowing it to
return different Service Routes to different devices registering for
the same AOR.
Record Routing Edge Proxies: If an Edge Proxy record routes with a
name that resolves explicitly to it and then crashes, all future
requests in that dialog will fail. If an Edge Proxy record routes
with a name that resolves to many edge proxies or does not record
route at all, then requests that do not have GRUU as a contact will
not work. A suggested resolution to this is to require GRUU for long
lived dialogs and have the Edge proxies use path headers and not
record route.
SUBSCRIBEs without a GRUU. Earlier version of draft assumed that a Are thre any deployments that could use Algorithm 1 and if not can we
REGISTER was always the first message. However the configuration remove it?
framework[13] needs to perform a SUBSCRIBE to get the configuration
that will allow the UA to register. This specification needs to deal
with situations where there is a SUBSCRIBE but no REGISTER. The
current resolution is to record route for these special cases and
mitigate the reliability implications of this by not allowing these
dialogs to be long lived.
The terminology of flow, flow-id, connection is confusing. Do we We should change syntax from "sip-stun" to "keep-alive=sip-stun".
want to change it?
13. Requirements 13. Requirements
This specification was developed to meet the following requirements: This specification was developed to meet the following requirements:
1. Must be able to detect that a UA supports these mechanisms. 1. Must be able to detect that a UA supports these mechanisms.
2. Support UAs behind NATs. 2. Support UAs behind NATs.
3. Support TLS to a UA without a stable DNS name or IP. 3. Support TLS to a UA without a stable DNS name or IP address.
4. Detect failure of connection and be able to correct for this. 4. Detect failure of connection and be able to correct for this.
5. Support many UAs simultaneously rebooting. 5. Support many UAs simultaneously rebooting.
6. Support a NAT rebooting or resetting. 6. Support a NAT rebooting or resetting.
7. Support proxy farms with multiple hosts for scaling and 7. Minimize initial startup load on a proxy.
reliability purposes. 8. Support architectures with edge proxies.
8. Minimize initial startup load on a proxy.
9. Support proxies that provide geographic redundancy.
10. Support architectures with edge proxies.
11. Must be able to receive notifications over the same flow used to
send a subscription, even before any registrations have been
established. This ensures compatibility with the SIP
configuration framework [13].
14. Changes from 00 Version 14. Changes
Note to RFC Editor: Please remove this whole section.
14.1. Changes from 01 Version
Moved definition of instance-id from GRUU[17] draft to this draft.
Added tentative text about Double CRLF Keep Alive
Removed pin-route stuff
Changed the name of "flow-id" to "reg-id"
Reorganized document flow
Described the use of STUN as a proper STUN usage
Added 'outbound' option-tag to detect if registrar supports outbound
14.2. Changes from 00 Version
Moved TCP keep alive to be STUN. Moved TCP keep alive to be STUN.
Allowed SUBSCRIBE to create flow mappings. Added pin-route option Allowed SUBSCRIBE to create flow mappings. Added pin-route option
tags to support this. tags to support this.
Added text about updating dialog state on each usage after a Added text about updating dialog state on each usage after a
connection failure. connection failure.
15. Acknowledgments 15. Acknowledgments
skipping to change at page 25, line 25 skipping to change at page 28, line 21
Ganesh Jayadevan provided input and text. Nils Ohlmeier provided Ganesh Jayadevan provided input and text. Nils Ohlmeier provided
many fixes and initial implementation experience. In addition, many fixes and initial implementation experience. In addition,
thanks to the following folks for useful comments: Francois Audet, thanks to the following folks for useful comments: Francois Audet,
Flemming Andreasen, Mike Hammer, Dan Wing, Srivatsa Srinivasan, and Flemming Andreasen, Mike Hammer, Dan Wing, Srivatsa Srinivasan, and
Lyndsay Campbell. Lyndsay Campbell.
16. References 16. References
16.1. Normative References 16.1. Normative References
[1] Rosenberg, J., "Obtaining and Using Globally Routable User Agent [1] Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
(UA) URIs (GRUU) in the Session Initiation Protocol (SIP)", Registration Procedure", BCP 31, RFC 2506, March 1999.
draft-ietf-sip-gruu-04 (work in progress), July 2005.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement [2] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[3] Moats, R., "URN Syntax", RFC 2141, May 1997.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [5] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[4] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol [6] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002. (SIP): Locating SIP Servers", RFC 3263, June 2002.
[5] Rosenberg, J., "Simple Traversal of UDP Through Network Address [7] Rosenberg, J., "Simple Traversal of UDP Through Network Address
Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-02 (work Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-02
in progress), July 2005. (work in progress), July 2005.
[6] Leach, P., Mealling, M., and R. Salz, "A Universally Unique
IDentifier (UUID) URN Namespace", RFC 4122, July 2005.
[7] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User [8] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Agent Capabilities in the Session Initiation Protocol (SIP)", Specifications: ABNF", RFC 2234, November 1997.
RFC 3840, August 2004.
16.2. Informative References [9] Leach, P., Mealling, M., and R. Salz, "A Universally Unique
IDentifier (UUID) URN Namespace", RFC 4122, July 2005.
[8] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing [10] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
for Message Authentication", RFC 2104, February 1997. User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[9] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", [11] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
RFC 3548, July 2003. Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[10] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP) [12] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
Extension Header Field for Registering Non-Adjacent Contacts", Extension Header Field for Registering Non-Adjacent Contacts",
RFC 3327, December 2002. RFC 3327, December 2002.
[11] Camarillo, G., "The Internet Assigned Number Authority (IANA) [13] Camarillo, G., "The Internet Assigned Number Authority (IANA)
Header Field Parameter Registry for the Session Initiation Header Field Parameter Registry for the Session Initiation
Protocol (SIP)", BCP 98, RFC 3968, December 2004. Protocol (SIP)", BCP 98, RFC 3968, December 2004.
[12] Camarillo, G., "The Internet Assigned Number Authority (IANA) [14] Camarillo, G., "The Internet Assigned Number Authority (IANA)
Uniform Resource Identifier (URI) Parameter Registry for the Uniform Resource Identifier (URI) Parameter Registry for the
Session Initiation Protocol (SIP)", BCP 99, RFC 3969, Session Initiation Protocol (SIP)", BCP 99, RFC 3969,
December 2004. December 2004.
[13] Petrie, D., "A Framework for Session Initiation Protocol User 16.2. Informative References
Agent Profile Delivery", draft-ietf-sipping-config-framework-07
(work in progress), July 2005.
[14] Lawrence, S., Hawrylyshen, A., and R. Sparks, "Problems with [15] Hakala, J., "Using National Bibliography Numbers as Uniform
Resource Names", RFC 3188, October 2001.
[16] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[17] Rosenberg, J., "Obtaining and Using Globally Routable User
Agent (UA) URIs (GRUU) in the Session Initiation Protocol
(SIP)", draft-ietf-sip-gruu-04 (work in progress), July 2005.
[18] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
[19] Lawrence, S., Hawrylyshen, A., and R. Sparks, "Problems with
Max-Forwards Processing (and Potential Solutions)", Max-Forwards Processing (and Potential Solutions)",
October 2005. October 2005.
[15] Rosenberg, J., "Clarifying Construction of the Route Header [20] Rosenberg, J., "Clarifying Construction of the Route Header
Field in the Session Initiation Protocol (SIP)", Field in the Session Initiation Protocol (SIP)",
draft-rosenberg-sip-route-construct-00 (work in progress), draft-rosenberg-sip-route-construct-00 (work in progress),
July 2005. July 2005.
[16] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP) [21] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
Extension Header Field for Service Route Discovery During Extension Header Field for Service Route Discovery During
Registration", RFC 3608, October 2003. Registration", RFC 3608, October 2003.
Authors' Addresses Authors' Addresses
Cullen Jennings (editor) Cullen Jennings (editor)
Cisco Systems Cisco Systems
170 West Tasman Drive 170 West Tasman Drive
Mailstop SJC-21/2 Mailstop SJC-21/2
San Jose, CA 95134 San Jose, CA 95134
USA USA
Phone: +1 408 902-3341 Phone: +1 408 902-3341
Email: fluffy@cisco.com Email: fluffy@cisco.com
Rohan Mahy (editor) Rohan Mahy (editor)
SIP Edge LLC Plantronics
5617 Scotts Valley Drive, Suite 200 345 Encincal St
Scotts Valley, CA 95066 Santa Cruz, CA 95060
USA USA
Email: rohan@ekabal.com Email: rohan@ekabal.com
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
skipping to change at page 28, line 41 skipping to change at page 31, line 41
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights. except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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