SIPCORE Working Group C. Holmberg
Internet-Draft Ericsson
Intended status: Standards Track August 5, 2010
Expires: February 6, 2011
Indication of support for keep-alive
draft-ietf-sipcore-keep-05.txt
Abstract
This specification defines a new Session Initiation Protocol (SIP)
Via header field parameter, "keep", which allows adjacent SIP
entities to explicitly negotiate usage of the Network Address
Translation (NAT) keep-alive mechanisms defined in SIP Outbound, in
cases where SIP Outbound is not supported, cannot be applied, or
where usage of keep-alives is not implicitly negotiated as part of
the SIP Outbound negotiation.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on February 6, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Holmberg Expires February 6, 2011 [Page 1]
Internet-Draft STUN-keep August 2010
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Use-case: Session from non-registered UAs . . . . . . . . 3
1.2. Use-case: SIP Outbound not supported . . . . . . . . . . . 3
1.3. Use-case: SIP dialog initiated Outbound flows . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. User Agent and Proxy behavior . . . . . . . . . . . . . . . . 4
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Lifetime of keep-alives . . . . . . . . . . . . . . . . . 5
4.2.1. General . . . . . . . . . . . . . . . . . . . . . . . 5
4.2.2. Keep-alives associated with registration . . . . . . . 5
4.2.3. Keep-alives associated with dialog . . . . . . . . . . 6
4.3. Behavior of a SIP entity willing to send keep-alives . . . 6
4.4. Behavior of a SIP entity willing to receive keep-alives . 7
5. Keep-alive frequency . . . . . . . . . . . . . . . . . . . . . 8
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Keep-alive negotiation associated with registration:
UA-proxy . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.3. Keep-alive negotiation associated with dialog: UA-proxy . 10
6.4. Keep-alive negotiation associated with dialog: UA-UA . . . 12
7. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8.1. keep . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9. Security Considerations . . . . . . . . . . . . . . . . . . . 14
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
11. Normative References . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
Holmberg Expires February 6, 2011 [Page 2]
Internet-Draft STUN-keep August 2010
1. Introduction
Section 3.5 of SIP Outbound [RFC5626] defines two keep-alive
mechanisms. Even though the keep-alive mechanisms are separated from
the rest of the SIP Outbound mechanism, SIP Outbound does not define
a mechanism to explicitly negotiate usage of the keep-alive
mechanisms. In some cases usage of keep-alives can be implicitly
negotiated as part of the SIP Outbound negotiation.
However, there are SIP Outbound use-cases where usage of keep-alives
is not implicitly negotiated as part of the SIP Outbound negotiation.
In addition, there are cases where SIP Outbound is not supported, or
where it cannot be applied, but where there is still a need to be
able to negotiate usage of keep-alives.
This specification defines a new Session Initiation Protocol (SIP)
[RFC3261] Via header field parameter, "keep", which allows adjacent
SIP entities to explicitly negotiate usage of the NAT keep-alive
mechanisms defined in SIP Outbound. The "keep" parameter allows SIP
entities to indicate willingness to send keep-alives, to indicate
willingness to receive keep-alives, and for SIP entities willing to
receive keep-alives to provide a recommended keep-alive frequency.
The following sections describe use-cases where a mechanism to
explicitly negotiate usage of keep-alives is needed.
1.1. Use-case: Session from non-registered UAs
In some cases a User Agent Client (UAC) does not register itself
before it establishes a session, but in order to maintain NAT
bindings open during the session it still needs to be able to
negotiate sending of keep-alives towards its adjacent upstream SIP
entity. A typical example is an emergency call, where a registration
is not always required in order to make the call.
1.2. Use-case: SIP Outbound not supported
In some cases all SIP entities that need to be able to negotiate the
usage of keep-alives might not support SIP Outbound. However, they
might still support the keep-alive mechanisms defined in SIP
Outbound, and need to be able to negotiate usage of them.
1.3. Use-case: SIP dialog initiated Outbound flows
SIP Outbound allows the establishment of flows using the initial
request for a dialog. As specified in [RFC5626], usage of keep-
alives is not implicitly negotiated for such flows.
Holmberg Expires February 6, 2011 [Page 3]
Internet-Draft STUN-keep August 2010
2. Conventions
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 BCP 14, RFC 2119
[RFC2119].
3. Definitions
Edge proxy: As defined in [RFC5626], a SIP proxy that is located
topologically between the registering User Agent (UA) and the
Authoritative Proxy.
NOTE: In some deployments the edge proxy might physically be located
in the same entity as the Authoritative Proxy.
Keep-alives: Refers to keep-alive messages as defined in SIP Outbound
[RFC5626].
"keep" parameter: A SIP Via header field parameter that a SIP entity
can insert in its Via header field of a request to explicitly
indicate willingness to send keep-alives towards its adjacent
downstream SIP entity. A SIP entity can also insert the header field
in a response to explicitly indicate willingness to receive keep-
alives from its adjacent upstream SIP entity.
SIP entity: SIP User Agent (UA), or proxy, as defined in [RFC3261].
Adjacent downstream SIP entity: The adjacent SIP entity in the
direction towards which a SIP request is sent.
Adjacent upstream SIP entity: The adjacent SIP entity in the
direction from which a SIP request is received.
4. User Agent and Proxy behavior
4.1. General
This section describes how SIP UAs and proxies negotiate usage of
keep-alives associated with a registration, or a dialog, which types
of SIP requests can be used in order to negotiate the usage, and the
lifetime of the negotiated keep-alives.
SIP entities indicate willingness to send keep-alives towards the
adjacent downstream SIP entity using SIP requests. The associated
responses are used by SIP entities to indicate willingness to receive
Holmberg Expires February 6, 2011 [Page 4]
Internet-Draft STUN-keep August 2010
keep-alives. SIP entities that indicate willingness to receive keep-
alives can provide a recommended keep-alive frequency.
The procedures to negotiate usage of keep-alives are identical for
SIP UAs and proxies.
NOTE: Usage of keep-alives is negotiated per direction. If a SIP
entity has indicated willingness to receive keep-alives from an
adjacent SIP entity, sending of keep-alives towards the same SIP
entity needs to be separately negotiated.
NOTE: Since there are SIP entities that already use a combination of
Carriage Return and Line Feed (CRLF) as keep-alive messages, and SIP
entities are expected to be able to receive those, this specification
does not forbid the sending of CRLF keep-alive messages towards an
adjacent SIP entity even if usage of keep-alives with that SIP entity
has not been negotiated. However, the "keep" parameter is still
important in order for a SIP entity to indicate that it supports
sending of CRLF keep-alive messages, so that the adjacent downstream
SIP entity does not use other mechanisms (e.g. short registration
refresh intervals) in order to keep NAT bindings open.
4.2. Lifetime of keep-alives
4.2.1. General
The lifetime of negotiated keep-alives depends on whether the keep-
alives are associated with a registration or a dialog. This section
describes the lifetime of negotiated keep-alives.
4.2.2. Keep-alives associated with registration
SIP entities use a registration request in order to negotiate usage
of keep-alives associated with a registration. Usage of keep-alives
can be negotiated when the registration is established, or later
during the lifetime of the registration. Once negotiated, keep-
alives are sent until the registration is terminated, or until a
subsequent registration refresh request is sent or forwarded. When a
subsequent registration refresh request is sent or forwarded, if a
SIP entity is willing to continue sending keep-alives associated with
the registration, usage of keep-alives MUST be re-negotiated. If
usage is not successfully re-negotiated, the SIP entity MUST cease
sending of keep-alives associated with the registration.
In case a SIP entity establishes multiple registration flows
[RFC5626], usage of keep-alives needs to be negotiated separately for
each individual registration flow. A SIP entity MUST NOT send keep-
alives associated with a registration flow for which usage of keep-
Holmberg Expires February 6, 2011 [Page 5]
Internet-Draft STUN-keep August 2010
alives has not been negotiated.
4.2.3. Keep-alives associated with dialog
SIP entities use an initial request for a dialog, or a mid-dialog
target refresh request [RFC3261], in order to negotiate sending and
receiving of keep-alives associated with a dialog. Usage of keep-
alives can be negotiated when the dialog is established, or later
during the lifetime of the dialog. Once negotiated, keep-alives MUST
be sent for the lifetime of the dialog, until the dialog is
terminated. Once usage of keep-alives associated with a dialog has
been negotiated, it is not possible to re-negotiate the usage
associated with the dialog.
4.3. Behavior of a SIP entity willing to send keep-alives
As defined in [RFC5626], a SIP entity that supports sending of keep-
alives must act as a Session Traversal Utilities for NAT (STUN)
client [RFC5389]. The SIP entity must support those aspects of STUN
that are required in order to apply the STUN keep-alive mechanism
defined in [RFC5626], and it must support the CRLF keep-alive
mechanism defined in [RFC5626].
When a SIP entity sends or forwards a request, if it wants to
negotiate the sending of keep-alives for the lifetime of a
registration, or a dialog, it MUST insert a "keep" parameter in its
Via header field of the request to indicate willingness to send keep-
alives.
When the SIP entity receives the associated response, if the "keep"
parameter in its Via header field of the response contains a "keep"
parameter value, it MUST start to send keep-alives towards the same
destination where it would send a subsequent request (e.g. REGISTER
requests and initial requests for dialog) associated with the
registration (if the keep-alive negotiation is for a registration),
or where it would send subsequent mid-dialog requests (if the keep-
alive negotiation is for a dialog). Subsequent mid-dialog requests
are addressed based on the dialog route set.
Once a SIP entity has negotiated sending of keep-alives associated
with a dialog towards an adjacent SIP entity, it MUST NOT insert a
"keep" parameter in any subsequent SIP requests, associated with the
dialog, towards that adjacent SIP entity. Such "keep" parameter MUST
be ignored, if received.
Since an ACK request does not have an associated response, it can not
be used to negotiate usage of keep-alives. Therefore, a SIP entity
MUST NOT insert a "keep" parameter in its Via header field of an ACK
Holmberg Expires February 6, 2011 [Page 6]
Internet-Draft STUN-keep August 2010
request. Such "keep" parameter MUST be ignored, if received.
A SIP entity MUST NOT indicates willingness to send keep-alives
associated with a dialog, unless it has also inserted itself in the
dialog route set [RFC3261].
NOTE: When a SIP entity sends an initial request for a dialog, if the
adjacent downstream SIP entity does not insert itself in the dialog
route set using a Record-Route header field [RFC3261], the adjacent
downstream SIP entity will change once the dialog route set has been
established. If a SIP entity inserts a "keep" parameter in its Via
header field of an initial request for a dialog, and the "keep"
parameter in the associated response does not contain a parameter
value, the SIP entity might choose to insert a "keep" parameter in
its Via header field of a subsequent SIP request associated with the
dialog, in case the new adjacent SIP downstream entity (based on the
dialog route set) is willing to receive keep-alives (in which case it
will add a parameter value to the "keep" parameter).
If an INVITE request is used to indicate willingness to send keep-
alives, as long as at least one response (provisional or final) to
the INVITE request contains a "keep" parameter with a parameter
value, it is seen as an indication that the adjacent downstream SIP
entity is willing to receive keep-alives associated with the dialog
on which the response is received.
A SIP entity that has negotiated sending of keep-alives associated
with a registration MUST be prepared to receive, from the receiver of
the keep-alives, SIP messages (requests and responses) associated
with the registration on the same IP port:address from where the
keep-alives are sent. If the SIP entity sends the keep-alives over a
connection-oriented transport protocol, it MUST be prepared to
receive SIP messages associated with the registration over the same
connection. The same rules apply for SIP messages associated with a
dialog, if the SIP entity has negotiated sending of keep-alives
associated with a dialog.
4.4. Behavior of a SIP entity willing to receive keep-alives
As defined in [RFC5626], a SIP entity that supports receiving of
keep-alives must act as a STUN server [RFC5389]. The SIP entity must
support those aspects of STUN that are required in order to apply the
STUN keep-alive mechanism defined in [RFC5626], and it must support
the CRLF keep-alive mechanism defined in [RFC5626].
When a SIP entity sends or forwards a response, and the adjacent
upstream SIP entity indicated willingness to send keep-alives, if the
SIP entity is willing to receive keep-alives associated with the
Holmberg Expires February 6, 2011 [Page 7]
Internet-Draft STUN-keep August 2010
registration, or the dialog, from the adjacent upstream SIP entity it
MUST add a parameter value to the "keep" parameter, before sending or
forwarding the response. The parameter can contain a recommended
keep-alive frequency, or a zero value.
When a SIP entity indicates willingness to receive keep-alives in a
response to an INVITE request, it MUST insert a "keep" parameter in
at least one reliable response to the request. The SIP entity MAY
insert an identical "keep" parameter value in other responses to the
same request. The SIP entity MUST NOT insert "keep" parameters with
differing values in responses to a single INVITE request. The SIP
entity SHOULD indicate the willingness to receive keep-alives as soon
as possible.
A SIP entity MUST NOT indicates willingness to receive keep-alives
associated with a dialog, unless it has also inserted itself in the
dialog route set [RFC3261].
A SIP entity that has negotiated receiving of keep-alives associated
with a registration MUST send, towards the sender of the keep-alives,
SIP messages (requests and responses) associated with the
registration using the IP address:port from where the keep-alives are
received. If the SIP entity receives the keep-alives over a
connection-oriented transport protocol, it MUST send SIP messages
associated with the registration over the same connection, rather
than creating a new connection. The same rules apply for SIP
messages associated with a dialog, if the SIP entity has negotiated
receiving of keep-alives associated with a dialog.
5. Keep-alive frequency
If a SIP entity receives a SIP response, where its Via header field
contains a "keep" parameter with a non-zero value that indicates a
recommended keep-alive frequency, it MUST use the procedures defined
for the Flow-Timer header field [RFC5626]. According to the
procedures, the SIP entity must send keep-alives at least as often as
the indicated recommended keep-alive frequency, and if the SIP entity
uses the recommended keep-alive frequency then it should send its
keep-alives so that the interval between each keep-alive is randomly
distributed between 80% and 100% of the recommended keep-alive
frequency.
If the received "keep" parameter value is zero, the SIP entity can
send keep-alives at its discretion. [RFC5626] provides additional
guidance on selecting the keep-alive frequency in case a recommended
keep-alive frequency is not provided.
Holmberg Expires February 6, 2011 [Page 8]
Internet-Draft STUN-keep August 2010
If a SIP entity that uses the "keep" parameter to indicate
willingness to receive keep-alives also inserts a Flow-Timer header
field (that can happen if the SIP entity is using both the Outbound
mechanism and the keep-alive mechanism) in the same SIP message, the
header field value and the "keep" parameter value MUST be identical.
SIP Outbound uses the Flow-Timer header field to indicate the server-
recommended keep-alive frequency. However, it will only be sent
between a UA and an edge proxy. Using the "keep" parameter, however,
the sending and receiving of keep-alives might be negotiated between
multiple entities on the signalling path. In addition, since the
server-recommended keep-alive frequency might vary between different
SIP entities, a single Flow-Timer header field can not be used to
indicate all the different frequency values, without forcing entities
to re-write the value of the Flow-Timer header field.
6. Examples
6.1. General
This section shows example flows where usage of keep-alives,
associated with a registration and a dialog, is negotiated between
different SIP entities.
6.2. Keep-alive negotiation associated with registration: UA-proxy
The figure shows an example where Alice sends an REGISTER request.
She indicates willingness of sending keep-alive by inserting a "keep"
parameter in her Via header field of the request. The edge proxy
(P1) forwards the request towards the registrar.
P1 is willing to receive keep-alives from Alice for the duration of
the registration, so When P1 receives the associated response it adds
a keep parameter value, which indicates a recommended keep-alive
frequency of 30 seconds, to Alice's Via header field, before it
forwards the response towards Alice.
When Alice receives the response, she determines from her Via header
field that P1 is willing to receive keep-alives associated with the
registration. For the lifetime of the registration, Alice then sends
periodic keep-alives (in this example using the STUN keep-alive
technique) towards P1, using the recommended keep-alive frequency
indicated by the keep parameter value.
Holmberg Expires February 6, 2011 [Page 9]
Internet-Draft STUN-keep August 2010
Alice P1 REGISTRAR
| | |
|--- REGISTER------------->| |
| Via: Alice;keep | |
| |--- REGISTER-------------->|
| | Via: P1 |
| | Via: Alice;keep |
| | |
| |<-- 200 OK ----------------|
| | Via: P1 |
| | Via: Alice;keep |
|<-- 200 OK ---------------| |
| Via: Alice;keep=30 | |
| | |
| | |
| *** Timeout *** |
| | |
|=== STUN request ========>| |
|<== STUN response ========| |
| | |
| *** Timeout *** |
| | |
|=== STUN request ========>| |
|<== STUN response ========| |
| | |
Figure 1: Example call flow
6.3. Keep-alive negotiation associated with dialog: UA-proxy
The figure shows an example where Alice sends an initial INVITE
request for a dialog. She indicates willingness to send keep-alive
by inserting a "keep" parameter in her Via header field of the
request. The edge proxy (P1) adds itself to the dialog route set by
adding itself to a Record-Route header field, before it forwards the
request towards Bob.
P1 is willing to receive keep-alives from Alice for the duration of
the dialog, so When P1 receives the associated response it adds a
keep parameter value, which indicates a recommended keep-alive
frequency of 30 seconds, to Alice's Via header field, before it
forwards the response towards Alice.
When Alice receives the response, she determines from her Via header
field that P1 is willing to receive keep-alives associated with the
dialog. For the lifetime of the dialog, Alice then sends periodic
Holmberg Expires February 6, 2011 [Page 10]
Internet-Draft STUN-keep August 2010
keep-alives (in this example using the STUN keep-alive technique)
towards P1, using the recommended keep-alive frequency indicated by
the keep parameter value.
Alice P1 Bob
| | |
|--- INVITE -------------->| |
| Via: Alice;keep | |
| |--- INVITE --------------->|
| | Via: P1 |
| | Via: Alice;keep |
| | Record-Route: P1 |
| | |
| |<-- 200 OK ----------------|
| | Via: P1 |
| | Via: Alice;keep |
| | Record-Route: P1 |
|<-- 200 OK ---------------| |
| Alice: UAC;keep=30 | |
| Record-Route: P1 | |
| | |
|--- ACK ----------------->| |
| | |
| |--- ACK ------------------>|
| | |
| *** Timeout *** |
| | |
|=== STUN request ========>| |
|<== STUN response ========| |
| | |
| *** Timeout *** |
| | |
|=== STUN request ========>| |
|<== STUN response ========| |
| | |
| | |
|--- BYE ----------------->| |
| | |
| |--- BYE ------------------>|
| | |
| |<-- 200 OK ----------------|
| | |
Figure 2: Example call flow
Holmberg Expires February 6, 2011 [Page 11]
Internet-Draft STUN-keep August 2010
6.4. Keep-alive negotiation associated with dialog: UA-UA
The figure shows an example where Alice sends an initial INVITE
request for a dialog. She indicates willingness to send keep-alive
by inserting a "keep" parameter in her Via header field of the
request. The edge proxy (P1) does not add itself to the dialog route
set, by adding itself to a Record-Route header field, before it
forwards the request towards Bob. .
When Alice receives the response, she determines from her Via header
field that P1 is not willing to receive keep-alives associated with
the dialog from her. When the dialog route set has been established,
Alice sends a mid-dialog UPDATE request towards Bob (since P1 did not
insert itself in the dialog route set), and she once again indicates
willingness to send keep-alives by inserting a "keep" parameter in
her Via header field of the request. Bob supports the keep-alive
mechanism, and is willing to receive keep-alives associated with the
dialog from Alice, so he creates a response and adds a keep parameter
value, which indicates a recommended keep-alive frequency of 30
seconds, to Alice's Via header field, before he forwards the response
towards Alice.
When Alice receives the response, she determines from her Via header
field that P1 is willing to receive keep-alives associated with the
dialog. For the lifetime of the dialog, Alice then sends periodic
keep-alives (in this example using the STUN keep-alive technique)
towards Bob, using the recommended keep-alive frequency indicated by
the keep parameter value.
Holmberg Expires February 6, 2011 [Page 12]
Internet-Draft STUN-keep August 2010
Alice P1 Bob
| | |
|--- INVITE -------------->| |
| Via: Alice;keep | |
| |--- INVITE --------------->|
| | Via: P1 |
| | Via: Alice:keep |
| | |
| |<-- 200 OK ----------------|
| | Via: P1 |
| | Via: Alice;keep |
|<-- 200 OK ---------------| |
| Via: Alice;keep | |
| | |
| |
|--- ACK --------------------------------------------->|
| |
|--- UPDATE ------------------------------------------>|
| Via: Alice;keep |
| |
|<-- 200 OK ------------------------------------------>|
| Via: UAC;keep=30 |
| |
| |
| *** Timeout *** |
| |
|=== STUN request ====================================>|
|<== STUN response ====================================|
| |
| *** Timeout *** |
| |
|=== STUN request ====================================>|
|<== STUN response ====================================|
| |
| |
|--- BYE --------------------------------------------->|
| |
|<-- 200 OK -------------------------------------------|
| |
Figure 3: Example call flow
7. Grammar
This specification defines a new Via header field parameter, "keep".
The grammar includes the definitions from [RFC5626].
Holmberg Expires February 6, 2011 [Page 13]
Internet-Draft STUN-keep August 2010
The ABNF [RFC5234] is:
via-params =/ keep
keep = "keep" [ EQUAL 1*(DIGIT) ]
8. IANA Considerations
8.1. keep
This specification defines a new Via header field parameter called
keep in the "Header Field Parameters and Parameter Values" sub-
registry as per the registry created by [RFC5626]. The syntax is
defined in Section 7. The required information is:
Predefined
Header Field Parameter Name Values Reference
---------------------- --------------------- ---------- ---------
Via keep No [RFCXXXX]
9. Security Considerations
This specification does not introduce security considerations in
addition to those specified in [RFC5626].
10. Acknowledgements
Thanks to Staffan Blau, Francois Audet, Hadriel Kaplan, Sean Schneyer
and Milo Orsic for their comments on the initial draft. Thanks to
Juha Heinaenen, Jiri Kuthan, Dean Willis, John Elwell and Paul
Kyzivat for their comments on the list. Thanks to Vijay Gurbani for
providing text (later removed) about the relationship with the
connect-reuse specification.
11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
Holmberg Expires February 6, 2011 [Page 14]
Internet-Draft STUN-keep August 2010
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5626] Jennings, C., Mahy, R., and F. Audet, "Managing Client-
Initiated Connections in the Session Initiation Protocol
(SIP)", RFC 5626, October 2009.
Author's Address
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
Holmberg Expires February 6, 2011 [Page 15]
