SIP WG C. Jennings, Ed.
Internet-Draft Cisco Systems
Expires: April 26, 2006 R. Mahy, Ed.
SIP Edge LLC
October 23, 2005
Managing Client Initiated Connections in the Session Initiation Protocol
(SIP)
draft-ietf-sip-outbound-01
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 26, 2006.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Session Initiation Protocol (SIP) allows proxy servers to initiate
TCP connections and send asynchronous UDP datagrams to User Agents in
order to deliver requests. However, many practical considerations,
such as the existence of firewalls and NATs, prevent servers from
connecting to User Agents in this way. Even when a proxy server can
open a TCP connection to a User Agent, most User Agents lack a
Jennings & Mahy Expires April 26, 2006 [Page 1]
Internet-Draft Client Initiated Connections in SIP October 2005
certificate suitable to act as a TLS server. This specification
defines behaviors for User Agents, registrars and proxy servers that
allow requests to be delivered on existing connections established by
the User Agent. It also defines keep alive behaviors needed to keep
NAT bindings open and specifies the usage of multiple connections for
high availability systems.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Summary of Mechanism . . . . . . . . . . . . . . . . . . . 4
3.2. Single Registrar and UA . . . . . . . . . . . . . . . . . 5
3.3. Multiple Connections from a User Agent . . . . . . . . . . 6
3.4. Edge Proxies . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Keep Alive Technique . . . . . . . . . . . . . . . . . . . 9
4. User Agent Mechanisms . . . . . . . . . . . . . . . . . . . . 10
4.1. Forming Flows . . . . . . . . . . . . . . . . . . . . . . 10
4.1.1. Request without GRUU . . . . . . . . . . . . . . . . . 11
4.2. Detecting Flow Failure . . . . . . . . . . . . . . . . . . 11
4.3. Flow Failure Recovery . . . . . . . . . . . . . . . . . . 12
4.4. Registration by Other Instances . . . . . . . . . . . . . 13
5. Registrar Mechanisms . . . . . . . . . . . . . . . . . . . . . 13
5.1. Processing Register Requests . . . . . . . . . . . . . . . 13
5.2. Forwarding Requests . . . . . . . . . . . . . . . . . . . 14
6. Edge Proxy Mechanisms . . . . . . . . . . . . . . . . . . . . 15
6.1. Processing Register Requests . . . . . . . . . . . . . . . 15
6.2. Forwarding Requests . . . . . . . . . . . . . . . . . . . 16
7. Mechanisms for All Servers . . . . . . . . . . . . . . . . . . 17
7.1. STUN Processing . . . . . . . . . . . . . . . . . . . . . 17
7.2. Pin-Route Processing . . . . . . . . . . . . . . . . . . . 17
8. Example Message Flow . . . . . . . . . . . . . . . . . . . . . 18
9. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
11. Security Considerations . . . . . . . . . . . . . . . . . . . 22
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 23
13. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 24
14. Changes from 00 Version . . . . . . . . . . . . . . . . . . . 24
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
16.1. Normative References . . . . . . . . . . . . . . . . . . . 25
16.2. Informative References . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . . . 28
Jennings & Mahy Expires April 26, 2006 [Page 2]
Internet-Draft Client Initiated Connections in SIP October 2005
1. Introduction
There are many environments for SIP deployments in which the User
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.
This can happen for several reasons. Connection to the UA can be
blocked by a firewall device between the UA and the proxy or
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
capable of allowing new connections from the private address side to
the public side. This specification allows SIP registration when the
UA is behind a firewall or NAT.
Most IP phones and personal computers get their network
configurations dynamically via a protocol such as DHCP. These
systems typically do not have a useful name in DNS, and they
definitely do not have a long-term, stable DNS name that is
appropriate for binding to a certificate. It is impractical for them
to have a certificate that can be used as a client-side TLS
certificate for SIP. However, these systems can still form TLS
connections to a proxy or registrar such that the UA authenticates
the server certificate, and the server authenticates 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
request, the proxy can later use this same connection, be it UDP,
TCP, or another transport protocol, to forward any requests that need
to go to this UA. For a UA to receive incoming requests, the UA has
to connect to the server. Since the server can't connect to the UA,
the UA has to make sure that a connection is always active. This
requires the UA to detect when a connection fails. Since, such
detection takes time and leaves a window of opportunity for missed
incoming requests, this mechanism allows the UA to use multiple
connections, referred to as "flows", to the proxy or registrar and
using a keep alive mechanism on each flow so that the UA can detect
when a flow has failed.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2].
2.1. Definitions
Jennings & Mahy Expires April 26, 2006 [Page 3]
Internet-Draft Client Initiated Connections in SIP October 2005
Edge Proxy: An Edge Proxy is any proxy that is located topologically
between the registering User Agent and the registrar.
flow: A Flow is a network protocol layer connection between two hosts
that is represented by the network address of both ends and the
protocol. For TCP and UDP this would include the IP addresses and
ports of both ends and the protocol (TCP or UDP). With TCP, a
flow would often have a one to one correspondence with a single
file descriptor in the operating system.
flow-id: This refers to the value of a new header field parameter
value for the contact header. When a UA register multiple times,
each registration gets a unique flow-id value. This does not
refer to flow.
instance-id: This specification uses the word instance-id to refer to
the value of the "sip.instance" media feature tag in the Contact
header field as defined in [1]. This is a URN that uniquely
identifies the UA.
3. Overview
Several scenarios in which this technique is useful are discussed
below, including the simple collocated registrar and proxy, a User
Agent desiring multiple connections to a resource (for redundancy for
example), and a system that uses Edge Proxies.
3.1. Summary of Mechanism
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
even if the UA reboots or is power cycled. Each UA can register
multiple times for the same AOR to achieve high reliability. Each
registration includes the instance-id for the UA and a flow-id label
that is different for each connection.
UAs use STUN as the keep alive mechanism to keep their flow to the
proxy or registrar alive. A UA can create more than one flow using
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
binding, it can use any one of the flows on which a successful
registration has been completed. A failure on a particular flow can
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
Jennings & Mahy Expires April 26, 2006 [Page 4]
Internet-Draft Client Initiated Connections in SIP October 2005
tell that a flow replaces a previously abandoned flow by looking at
the flow-id.
3.2. Single Registrar and UA
In this example there a single server is acting as both a registrar
and proxy.
+-----------+
| Registrar |
| Proxy |
+-----+-----+
|
|
+----+--+
| User |
| Agent |
+-------+
User Agents forming only a single connection continue to register
normally but include the instance-id as described in the GRUU [1]
specification and can also add a flow-id parameter to the Contact
header field value. The flow-id parameter is used to allow the
registrar to detect and avoid using invalid contacts when a UA
reboots or reconnects after its old connection has failed for some
reason.
For clarity, here is an example. Bob's UA creates a new TCP flow to
the registrar and sends the following REGISTER request.
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bK-bad0ce-11-1036
Max-Forwards: 70
From: Bob <sip:bob@example.com>;tag=d879h76
To: Bob <sip:bob@example.com>
Call-ID: 8921348ju72je840.204
CSeq: 1 REGISTER
Contact: <sip:line1@192.168.0.2>; flow-id=1;
;+sip.instance="<urn:uuid:00000000-0000-0000-0000-000A95A0E128>"
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
Jennings & Mahy Expires April 26, 2006 [Page 5]
Internet-Draft Client Initiated Connections in SIP October 2005
media feature tag are placed inside angle brackets.
The registrar challenges this registration to authenticate Bob. When
the registrar adds an entry for this contact under the AOR for Bob,
the registrar also keeps track of the connection over which it
received this registration.
The registrar saves the instance-id (as defined in [1]) and flow-id
(as defined in Section 9) along with the rest of the Contact header
field. If the instance-id and flow-id are the same as a previous
registration for the same AOR, the proxy uses the most recently
created registration first. This allows a UA that has rebooted to
replace its previous registration for each flow with minimal impact
on overall system load.
Later when Alice sends a request to Bob, his proxy selects the target
set. The proxy forwards the request to elements in the target set
based on the proxy's policy. The proxy looks at the the target set
and uses the instance-id to understand that two targets both end up
routing to the same UA. When the proxy goes to forward a request to
a given target, it looks and finds the flows that received the
registration. The proxy then forwards the request on that flow
instead of trying to form a new flow to that contact. This allows
the proxy to forward a request to a particular contact down the same
flow that did the registration for this AOR. If the proxy had
multiple flows that all went to this UA, it would choose any one of
registration bindings that it had for this AOR and that had the same
instance-id as the selected UA. In general, if two registrations
have the same flow-id and instance-id, the proxy would favor the most
recently registered flow. This is so that if a UA reboots, the proxy
will prefer to use the most recent flow that goes to this UA instead
of trying one of the old flows which will presumably fail.
3.3. Multiple Connections from a User Agent
There are various ways to deploy SIP to build a reliable and
scaleable system. This section discusses one such design that is
possible with the mechanisms in this draft. Other designs are also
possible.
In this example system, the logical proxy/registrar for the domain is
running on two hosts that share the appropriate state and can both
provide registrar and proxy functionality for the domain. The UA
will form connections to two of the physical hosts that can perform
the proxy/registrar function for the domain. Reliability is achieved
by having the UA form two connections to the domain. Scaleability is
achieved by using DNS SRV to load balance the primary connection
across a set of machines that can service the primary connection and
Jennings & Mahy Expires April 26, 2006 [Page 6]
Internet-Draft Client Initiated Connections in SIP October 2005
also using DNS SRV to load balance across a separate set of machines
that can service the backup connection. The deployment here requires
that DNS be configured with an entry that resolves to all the primary
hosts and another that resolves to all the backup hosts. Designs
having only one set were also considered but in this case, there
would have to be some way to ensure that the two connection did not
accidentally resolve to the same host. Various approaches for this
are possible but all probably require extensions to the SIP protocol
so they were not included in this specification. This approach can
work with the disadvantage that slightly more configuration of DNS is
required.
+-------------------+
| Domain |
| Logical Proxy/Reg |
| |
|+-----+ +-----+|
||Host1| |Host2||
|+-----+ +-----+|
+---\------------/--+
\ /
\ /
\ /
\ /
+------+
| User |
| Agent|
+------+
The UA is configured with a primary and backup registration URI.
These URIs are configured into the UA through whatever the normal
mechanism is to configure the proxy or registrar for the UA. They
might look something like "sip:primary.example.com;sip-stun" and
"sip:backup.example.com;sip-stun" if the domain was example.com. The
"sip-stun" tag indicates that they support STUN as described later in
this specification. Note that each of them could resolve to several
different hosts. The administrative domain that created these URIs
MUST ensure that the two URIs resolve to separate hosts. These URIs
have normal SIP processing 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
contact. The GRUU would refer to the domain, not host1 or host2.
Regardless of which host received a request to GRUU, the domain would
need to ensure that the request got sent to host1 or host2 and then
sent across the appropriate flow to the UA. The domain might choose
to use the Path header (as described in the next section) approach to
form this internal routing to host1 or host2.
Jennings & Mahy Expires April 26, 2006 [Page 7]
Internet-Draft Client Initiated Connections in SIP October 2005
When a single server fails, all the UAs that have a registration with
it will detect this and try to reconnect. This can cause large loads
on the server and is referred to as the avalanche restart problem
further discussed in Section 4.3. The multiple flows to many servers
help reduce the load caused by the avalanche restart. If a UA has
multiple flows, and one of the servers fails, it can delay some
significant time before trying to form a new connection to replace
the flow to the server that failed. By spreading out the time used
for all the UAs to reconnect to a server, the load on the server is
reduced.
3.4. Edge Proxies
Some SIP deployments use edge proxies such that the UA sends 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
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
the UA and the Edge Proxy and there could also be one between the
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|
|Proxy |
+---------+
/ \
----------------------------NAT/FW
/ \
+-----+ +-----+
|Edge1| |Edge2|
+-----+ +-----+
\ /
\ /
----------------------------NAT/FW
\ /
\ /
+------+
|User |
|Agent |
+------+
These systems can use effectively the same mechanism as described in
the previous sections but need to use the Path header. When the Edge
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
addresses) and put this identifier in the path header. This is done
Jennings & Mahy Expires April 26, 2006 [Page 8]
Internet-Draft Client Initiated Connections in SIP October 2005
by putting the value in the user portion of a loose route in the path
header. If the registration succeeds, the Edge Proxy needs to map
future requests that are routed to the identifier value that was put
in the Path header to the associated flow.
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.
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
inside an enterprise that requires its use and the registrar could be
a service provider with no relationship to the enterprise.
Regardless if they are in the same administrative domain, this
specification requires that Registrars and Edge proxies support the
Path header mechanism in RFC 3327 [10].
3.5. Keep Alive Technique
A keep alive mechanism needs to detect both failure of a connection
and changes to the NAT public mapping as well as keeping any NAT
bindings refreshed. This specification uses STUN [5] over the same
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
resulting public IP address or port mapping for the UA changes. To
detect this, requests are sent over the connection that is being used
for the SIP traffic. The proxy or registrar acts as a STUN server on
the SIP signaling port.
Note: The STUN mechanism is very robust and allows the detection
of a changed IP address. Many other options were considered. It
may also be possible to do this with OPTIONS messages and rport;
although this approach has the advantage of being backwards
compatible, it also increases the load on the proxy or registrar
server. The TCP KEEP_ALIVE mechanism is not used because most
operating systems do not allow the time to be set on a per
connection basis. Linux, Solaris, OS X, and Windows all allow
KEEP_ALIVEs to be turned on or off on a single socket using the
SO_KEEPALIVE socket options but can not change the duration of the
timer for an individual socket. The length of the timer typically
defaults to 7200 seconds. The length of the timer can be changed
to a smaller value by setting a kernel parameter but that affects
all TCP connections on the host and thus is not appropriate to
use.
If the UA detects that the connection has failed or that the flow
definition has changed, it MUST re-register and MUST use the back-off
mechanism described in Section 4 in order to provide congestion
relief when a large number of agents simultaneously reboot.
Jennings & Mahy Expires April 26, 2006 [Page 9]
Internet-Draft Client Initiated Connections in SIP October 2005
4. User Agent Mechanisms
The UA behavior is divided up into sections. The first describes
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
When a User Agent initiates a dialog, it MUST provide a Contact URI
which has GRUU properties if it is in possession of an appropriate
GRUU. If it can not provide a GRUU, it needs to follow the procedure
specified in Section 4.1.1.
UAs are configured with one or more SIP URIs representing the default
outbound proxies with which to register. A UA MUST support sets with
at least two outbound proxy URIs (primary and backup) and SHOULD
support sets with up to four URIs. For each outbound proxy URI in
the set, the UA MUST send a REGISTER in the normal way using this URI
as the default outbound proxy. Forming the route set for the request
is discussed in [15] but typically results in sending the REGISTER
with the Route header field containing a loose route to the outbound
proxy URI. The UA MUST include the instance-id as described in [1].
The UA MUST also add a distinct flow-id parameter to the Contact
header field. The UA SHOULD use a flow-id value of 1 for the first
URI in the set, and a flow-id value of 2 for the second, and so on.
Each one of these registrations will form a new flow from the UA to
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
field value as defined in RFC 3608 [16], then whichever proxy sends
the 200 response last will affect where all future requests from this
UA are directed.
Note that the UA needs to honor 503 responses to registrations as
described in RFC 3261 and RFC 3263 [4]. In particular, implementors
should note that when receiving a 503 with a Retry-After, the UA
should wait the indicated amount of time and retry the registration.
A Retry-After header field value of 0 is valid and indicates the UA
should retry the REGISTER immediately. Implementations need to
ensure that when retrying the REGISTER they redo the DNS resolution
process such that if multiple hosts are reachable from the URI, there
is a chance that the UA will select an alternate host from the one it
chose the previous time the URI was resolved.
Jennings & Mahy Expires April 26, 2006 [Page 10]
Internet-Draft Client Initiated Connections in SIP October 2005
Note on Instance-ID Selection: The instance-id needs to be a URN but
there are many ways one can be generated. A particularly simple way
for both "hard" phones and "soft" phones is to use a UUID as defined
in [6]. A device like a soft-phone, when first installed, should
generate a UUID [6] 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 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
If the UA does not have a GRUU, it MUST send the request with a
Contact header field containing a +sip.instance media feature
parameter, and it MUST include the "pin-route" option-tag in both a
Proxy-Require and a Require header field value. A User Agent
compliant with this specification MUST NOT initiate a dialog with an
INVITE without a GRUU in the Contact header field. (At the time of
this writing this is allowed only for dialogs initiated with the
SUBSCRIBE method.)
This mechanism without a GRUU is not reliable if any of the proxies
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
The UA needs to detect if a given flow has failed, and if it has
failed, follow the procedures in Section 4.1 to form a new flow to
replace the failed one.
User Agents that form flows MUST check if the configured URI they are
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
seconds was selected so that multiple STUN packets would be sent
before 30 seconds based on information that some NATs had UDP
Jennings & Mahy Expires April 26, 2006 [Page 11]
Internet-Draft Client Initiated Connections in SIP October 2005
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
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 a flow to a particular URI in the proxy set fails, the UA needs
to form a new flow to replace it. The new flow MUST have the same
flow-id as the flow it is replacing. This is done in much the same
way as the flows are described as being formed in Section 4.1;
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 URI in the proxy set. 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 the base time
multiplied by two to power of the number of consecutive registration
failures to that URI up to a maximum of 1800 seconds.
wait-time = min( 1800, (base-time * (2 ^ consecutive-failures)))
These three times SHOULD be configurable in the UA. The three times
are the max-time with a default of 1800 seconds, the base-time-all-
fail 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 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
it immediately tries to register. If this fails and no registration
Jennings & Mahy Expires April 26, 2006 [Page 12]
Internet-Draft Client Initiated Connections in SIP October 2005
on other flows had succeeded, the first retry would happen somewhere
between 30 and 60 seconds after the failure of the first registration
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
session created with INVITE (a session "usage") and a subscription (a
subscription "usage") can share a dialog. On failure of a flow, a
User Agent might wish to resynchronizing the state of any active
usages on any dialogs using the flow. For example, the User Agent
could send a new subscription for each subscription usage and an
INVITE with replaces for each session usage. Note that when a flow
was obtained via a REGISTER request, the flow might be used by many
dialogs and dialog usages. A flow obtained via another request (e.g.
a SUBSCRIBE request) only has usages from a single dialog. The only
reason to do this is that a message may have been lost while the flow
was being reestablished. The GRUU will ensure that any future
messages are still delivered to the UA even if it does not re-
subscribe, re-INVITE, or otherwise refresh the usage. Deployments
need to carefully consider the implications of these sorts of
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
A User Agent MUST NOT include an instance-id or flow-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.)
5. Registrar Mechanisms
5.1. Processing Register Requests
Registrars which implement this specification, MUST support the Path
header mechanism[10] and processes REGISTER requests as described in
Section 10 of RFC 3261 with the following change. Any time the
registrar checks if a new contact matches an existing contact in the
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
Jennings & Mahy Expires April 26, 2006 [Page 13]
Internet-Draft Client Initiated Connections in SIP October 2005
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,
some additional information MUST be stored for any registration that
contains a flow-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 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
registrations that do not include the instance-id and flow-id to
eliminate the amplification attack described in [14].
5.2. 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
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
contacts with the AOR and instance-id associated with the GRUU. The
rules above still apply to a GRUU. This allows a request routed to a
Jennings & Mahy Expires April 26, 2006 [Page 14]
Internet-Draft Client Initiated Connections in SIP October 2005
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
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
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
6.1. Processing Register Requests
When an Edge Proxy receives a registration request it MUST form a
flow identifier token that is unique to this network flow and use
this token as the user part of the URI that this proxy inserts into
the Path header. Edge proxies MUST use a Path header. A trivial way
to satisfy this requirement involves storing a mapping between an
incrementing counter and the connection information; however this
would require the Edge Proxy to keep an impractical amount of state.
It is unclear when this state could be removed and the approach would
have problems if the proxy crashed and lost the value of the counter.
Two stateless examples are provided below. A proxy can use any
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
transports by concatenating the file descriptor (or equivalent)
with the NTP time the connection was created, and base64 encoding
the result. This results in an approximately 16 octet identifier.
The proxy generates a flow token for UDP by concatenating the file
descriptor and the remote IP address and port, then base64
encoding the result.
Jennings & Mahy Expires April 26, 2006 [Page 15]
Internet-Draft Client Initiated Connections in SIP October 2005
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
S, is formed that contains the following information about the
flow the request was received on: an enumeration indicating the
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-
SHA1-80 algorithm, as defined in [8]. The concatenation of the
HMAC and S are base64 encoded, as defined in [9], and used as the
flow identifier. With IPv4 address, this will result in a 32
octet identifier.
Algorithm 1 MUST NOT be used unless the REGISTER request is over a
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 addition that it is routed to a URI with a flow
identifier token that this proxy created, then 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. The advantage to a stateless
approach to managing the flow information is that there is no state
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
header. For TCP, if a connection specified by the file descriptor
is present and the creation time of the file descriptor matches
the creation time encoded in the Route header, the proxy forwards
the request over that connection. For UDP, the proxy forwards the
request from the encoded file descriptor to the source IP address
and port.
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
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 was correct then the proxy should forward the request
on the flow that was specified by the information in the flow
identifier. If this flow no longer exists, the proxy SHOULD send
a 410 response to the request.
Edge Proxies MUST Record-Route so that mid-dialog requests still are
routed over the correct flow.
Jennings & Mahy Expires April 26, 2006 [Page 16]
Internet-Draft Client Initiated Connections in SIP October 2005
7. Mechanisms for All Servers
7.1. STUN Processing
TODO: This section needs to be brought into sync with the STUN draft
and check there are not issues for SIP and STUN on TCP or UDP
connections.
A SIP device that receives SIP messages directly from a UA needs to
behave as specified in this section. Such devices would generally
include a Registrar and an Edge Proxy, as they both receive register
requests directly from a UA.
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
interface and port. Specifically it MUST be capable of receiving and
responding to STUN requests with the exception that it does not need
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
octet of a STUN packet has a value of 0 or 1 while the first octet of
a SIP message is never a 0 or 1.
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
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
to this location. The sip-stun tag would typically show up in the
URI in the Route header field value of a REGISTER request and would
not be in the request URI.
7.2. Pin-Route Processing
A sip device receives a request with the "pin-route" options tag set
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
Require header MUST either reject the request if pin-route is not
supported, or if pin-route is supported by this UAS, the UAS MUST
ensure that any message send in the dialog formed by this request is
sent on the same flow as the initial request. This specification
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.
Jennings & Mahy Expires April 26, 2006 [Page 17]
Internet-Draft Client Initiated Connections in SIP October 2005
A proxy that receives a request with the "pin-route" option tag in
the Proxy-Require header MUST add a record-route header field value
that resolves to this proxy and it MUST ensure that any future
requests or responses in this dialog are forwarded on the same flow
as the original request. The suggested way to do this is to form a
flow identifier token in the same way that an Edge Proxy would form
this for the Path header and insert this flow identifier token in the
user portion of the URI used in the record route header field value.
8. Example Message Flow
The following call flow shows a basic registration and an incoming
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
Backup proxy. When connectivity to the primary proxy is established,
the Callee registers again to replace the lost flow as shown in
message 15.
Jennings & Mahy Expires April 26, 2006 [Page 18]
Internet-Draft Client Initiated Connections in SIP October 2005
[-----example.com domain -------------------]
Caller Backup Primary Callee
| | | (1) REGISTER |
| | |<-----------------|
| | |(2) 200 OK |
| | |----------------->|
| | | (3) REGISTER |
| |<------------------------------------|
| |(4) 200 OK | |
| |------------------------------------>|
|(5) INVITE | | |
|----------------------------------->| |
| | |(6) INVITE |
| | |----------------->|
| | | (7) 200 OK |
| | |<-----------------|
| | (8) 200 OK | |
|<-----------------------------------| |
|(9) ACK | | |
|----------------------------------->| |
| | |(10) ACK |
| | |----------------->|
| | CRASH X |
|(11) BYE | |
|---------------->| |
| | (12) BYE |
| |------------------------------------>|
| | (13) 200 OK |
| |<------------------------------------|
| (14) 200 OK | |
|<----------------| REBOOT | |
| | | (15) REGISTER |
| | |<-----------------|
| | |(16) 200 OK |
| | |----------------->|
This call flow assumes that the Callee has been configured with a
proxy set that consists of "sip:primary.example.com;lr;sip-stun" and
"sip:backup.example.com;lr;sip-stun". The Callee REGISTER in message
(1) looks like:
Jennings & Mahy Expires April 26, 2006 [Page 19]
Internet-Draft Client Initiated Connections in SIP October 2005
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7
Max-Forwards: 70
From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com>
Call-ID: 1j9FpLxk3uxtm8tn@10.0.1.1
CSeq: 1 REGISTER
Route: <sip:primary.example.com;lr;sip-stun>
Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;flow-id=1
Content-Length: 0
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
the REGISTER in message (2) would look like:
SIP/2.0 200 OK
Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7
From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com> ;tag=b88sn
Call-ID: 1j9FpLxk3uxtm8tn@10.0.1.1
CSeq: 1 REGISTER
Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;flow-id=1
;expires=3600
Content-Length: 0
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
backup instead of the primary. They look like:
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7
Max-Forwards: 70
From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com>
Call-ID: 1j9FpLxk3uxtm8tn-2@10.0.1.1
CSeq: 1 REGISTER
Route: <sip:backup.example.com;lr;sip-stun>
Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;flow-id=2
Jennings & Mahy Expires April 26, 2006 [Page 20]
Internet-Draft Client Initiated Connections in SIP October 2005
Content-Length: 0
SIP/2.0 200 OK
Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7
From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com> ;tag=b88sn
Call-ID: 1j9FpLxk3uxtm8tn-2@10.0.1.1
CSeq: 1 REGISTER
Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;flow-id=1
;expires=3600
Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;flow-id=2
;expires=3600
Content-Length: 0
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:
Record-Route: <sip:example.com;lr>
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 callee based on the GRUU that the callee provided in their
Contact header field value when the dialog was formed and the domain
selected a host (primary or backup) that was currently available.
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
2 other than the Call-ID has changed.
9. Grammar
This specification defines a new Contact header field parameter,
flow-id. The grammar for DIGIT and EQUAL is obtained from RFC 3261
[3].
contact-params = c-p-q / c-p-expires / c-p-flow / contact-extension
c-p-flow = "flow-id" EQUAL 1*DIGIT
Jennings & Mahy Expires April 26, 2006 [Page 21]
Internet-Draft Client Initiated Connections in SIP October 2005
The value of the flow-id MUST NOT be 0 and MUST be less than 2**31.
10. IANA Considerations
This specification defines a new Contact header field parameter
called flow-id in the "Header Field Parameters and Parameter Values"
sub-registry as per the registry created by [11] at
http://www.iana.org/assignments/sip-parameters. The required
information is:
Header Field Parameter Name Predefined Reference
Values
____________________________________________________________________
Contact flow-id Yes [RFC AAAA]
[NOTE TO RFC Editor: Please replace AAAA with
the RFC number of this specification.]
This specification defines a new value in the "SIP/SIPS URI
Parameters" sub-registry as per the registry created by [12] at
http://www.iana.org/assignments/sip-parameters. The required
information is:
Parameter Name Predefined Values Reference
____________________________________________
sip-stun No [RFC AAAA]
[NOTE TO RFC Editor: Please replace AAAA with
the RFC number of this specification.]
TODO: Add IANA section for "pin-route" option tag.
11. Security Considerations
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
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
only time an entry can be added to the routing for a given AOR is
when the registration succeeds. SIP protects against attackers being
able to successfully register, and this scheme relies on that
security. Some implementers have considered the idea of just saving
the instance-id without relating it to the AOR with which it
Jennings & Mahy Expires April 26, 2006 [Page 22]
Internet-Draft Client Initiated Connections in SIP October 2005
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.
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,
the Edge Proxy inserts a Path header field value. If the
registration is successfully authenticated, the proxy stores the
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
header field into the route header field of the request and forwards
the request to the Edge Proxy.
The only time an Edge Proxy will route over a particular flow is when
it has received a route header that has the flow identifier
information that it has created. An incoming request would have
gotten this information from the registrar. The registrar will only
save this information for a given AOR if the registration for the AOR
has been successful; and the registration will only be successful if
the UA can correctly authenticate. Even if an attacker has spoofed
some bad information in the path header sent to the registrar, the
attacker will not be able to get the registrar to accept this
information for an AOR that does not belong to the attacker. The
registrar will not hand out this bad information to others, and
others will not be misled into contacting the attacker.
12. Open Issues
Service Route: The current interaction of this draft and
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
Jennings & Mahy Expires April 26, 2006 [Page 23]
Internet-Draft Client Initiated Connections in SIP October 2005
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
REGISTER was always the first message. However the configuration
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
want to change it?
13. Requirements
This specification was developed to meet the following requirements:
1. Must be able to detect that a UA supports these mechanisms.
2. Support UAs behind NATs.
3. Support TLS to a UA without a stable DNS name or IP.
4. Detect failure of connection and be able to correct for this.
5. Support many UAs simultaneously rebooting.
6. Support a NAT rebooting or resetting.
7. Support proxy farms with multiple hosts for scaling and
reliability purposes.
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
Moved TCP keep alive to be STUN.
Allowed SUBSCRIBE to create flow mappings. Added pin-route option
tags to support this.
Added text about updating dialog state on each usage after a
connection failure.
Jennings & Mahy Expires April 26, 2006 [Page 24]
Internet-Draft Client Initiated Connections in SIP October 2005
15. Acknowledgments
Jonathan Rosenberg provided many comments and useful text. Dave Oran
came up with the idea of using the most recent registration first in
the proxy. Alan Hawrylyshen co-authored the draft that formed the
initial text of this specification. Additionally, many of the
concepts here originated at a connection reuse meeting at IETF 60
that included the authors, Jon Peterson, Jonathan Rosenberg, Alan
Hawrylyshen, and Paul Kyzivat. The TCP design team consisting of
Chris Boulton, Scott Lawrence, Rajnish Jain, Vijay K. Gurbani, and
Ganesh Jayadevan provided input and text. Nils Ohlmeier provided
many fixes and initial implementation experience. In addition,
thanks to the following folks for useful comments: Francois Audet,
Flemming Andreasen, Mike Hammer, Dan Wing, Srivatsa Srinivasan, and
Lyndsay Campbell.
16. References
16.1. Normative References
[1] 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.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[4] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
[5] Rosenberg, J., "Simple Traversal of UDP Through Network Address
Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-02 (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
Agent Capabilities in the Session Initiation Protocol (SIP)",
RFC 3840, August 2004.
Jennings & Mahy Expires April 26, 2006 [Page 25]
Internet-Draft Client Initiated Connections in SIP October 2005
16.2. Informative References
[8] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[9] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
[10] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
Extension Header Field for Registering Non-Adjacent Contacts",
RFC 3327, December 2002.
[11] Camarillo, G., "The Internet Assigned Number Authority (IANA)
Header Field Parameter Registry for the Session Initiation
Protocol (SIP)", BCP 98, RFC 3968, December 2004.
[12] Camarillo, G., "The Internet Assigned Number Authority (IANA)
Uniform Resource Identifier (URI) Parameter Registry for the
Session Initiation Protocol (SIP)", BCP 99, RFC 3969,
December 2004.
[13] Petrie, D., "A Framework for Session Initiation Protocol User
Agent Profile Delivery", draft-ietf-sipping-config-framework-07
(work in progress), July 2005.
[14] Lawrence, S., Hawrylyshen, A., and R. Sparks, "Problems with
Max-Forwards Processing (and Potential Solutions)",
October 2005.
[15] Rosenberg, J., "Clarifying Construction of the Route Header
Field in the Session Initiation Protocol (SIP)",
draft-rosenberg-sip-route-construct-00 (work in progress),
July 2005.
[16] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
Extension Header Field for Service Route Discovery During
Registration", RFC 3608, October 2003.
Jennings & Mahy Expires April 26, 2006 [Page 26]
Internet-Draft Client Initiated Connections in SIP October 2005
Authors' Addresses
Cullen Jennings (editor)
Cisco Systems
170 West Tasman Drive
Mailstop SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 902-3341
Email: fluffy@cisco.com
Rohan Mahy (editor)
SIP Edge LLC
5617 Scotts Valley Drive, Suite 200
Scotts Valley, CA 95066
USA
Email: rohan@ekabal.com
Jennings & Mahy Expires April 26, 2006 [Page 27]
Internet-Draft Client Initiated Connections in SIP October 2005
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Jennings & Mahy Expires April 26, 2006 [Page 28]
