SIP -- Session Initiation Protocol D. Willis
Working Group B. Campbell
Internet-Draft dynamicsoft Inc.
Expires: April 12, 2004 October 13, 2003
Session Initiation Protocol Extension to Assure Congestion Safety
draft-ietf-sip-congestsafe-02
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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 12, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The Session Initiation Protocol allows the use of UDP for transport
of SIP messages. The use of UDP inherently risks network congestion
problems, as UDP itself does not define congestion prevention,
avoidance, detection, or correction mechanisms. This problem is
aggravated by large SIP messages which fragment at the UDP level.
Transport protocols in SIP are also negotiated on a per-hop basis, at
the SIP level, so SIP proxies may convert from TCP to UDP and so
forth. This document defines by which a SIP User Agent may require
that its requests are not sent over UDP or other transports having
congestion-related characteristics similar to those of UDP.
Willis & Campbell Expires April 12, 2004 [Page 1]
Internet-Draft SIP Congestion Safety October 2003
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scope of Work . . . . . . . . . . . . . . . . . . . . . . . 4
4. Assuring Transitive Congestion-Managed Transport with
Require and Proxy-Require . . . . . . . . . . . . . . . . . 5
5. New Behaviors at SIP Nodes . . . . . . . . . . . . . . . . . 5
5.1 Behavior at the UAC . . . . . . . . . . . . . . . . . . . . 5
5.1.1 Sending a Request . . . . . . . . . . . . . . . . . . . . . 5
5.1.2 Receiving a 514 Response to a Request . . . . . . . . . . . 6
5.1.3 Receiving a 515 Response to a Request . . . . . . . . . . . 6
5.1.4 Receiving a 516 Response to a Request . . . . . . . . . . . 6
5.2 Behavior at the Proxy . . . . . . . . . . . . . . . . . . . 6
5.2.1 Proxy Rejects Request Requiring Congestion Management
When Route with Congestion Management Not Available . . . . 7
5.2.2 Proxy Rejects Request Not Requiring Congestion
Management When Forwarding That Request Would Induce
Fragmentation . . . . . . . . . . . . . . . . . . . . . . . 7
5.2.3 Forwarding of Responses . . . . . . . . . . . . . . . . . . 7
5.3 Behavior at the UAS . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
Normative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . 11
Willis & Campbell Expires April 12, 2004 [Page 2]
Internet-Draft SIP Congestion Safety October 2003
1. 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 [RFC2119].
2. Background
The Session Initiation Protocol [4] provides application support over
multiple transport protocols, including UDP and TCP. Extensions to
support SCTP are under consideration, and other transport protocols
may be proposed for future use. Transport negotiation is not "end to
end" with SIP. Instead, each SIP hop individually determines which
transport to use towards the next hop. For example, a User Agent
Client (UAC) may use TCP to talk to a proxy, that proxy my use UDP to
talk to another proxy, and that second proxy may use SCTP to talk to
a destination User Agent Server (UAS).
UDP has inherent issues with congestion management or reliability.
The protocol has no explicit mechanisms for avoiding, detecting, or
adapting to network congestion. SIP attempts to deal with this in two
ways:
1. Retransmission timers with exponential back offs.
2. Attempting to limit the size of transmissions over UDP to reduce
the effects of fragmentation.
The fundamental problem with UDP is that it provides no feedback
mechanism to allow a sender to pace its transmissions against the
real performance of the network. While this tends to have no
significant effect on extremely low-volume sender-receiver pairs, the
impact of high-volume relationships on the network can be severe.
Consider the following scenario, wherein the traffic between multiple
UAs is funnelled through a single proxy-proxy relationship.
Example of large-fan out/fan-in likely to encounter congestion:
UA1----\ /----UA10
UA2-----\ /-----UA11
UA3------\ /------UA12
UA4-------\ /-------UA13
UA5--------P1------P2--------UA14
UA6-------/ \-------UA15
UA7------/ \------UA16
UA8-----/ \-----UA17
UA9----/ \----UA18
Willis & Campbell Expires April 12, 2004 [Page 3]
Internet-Draft SIP Congestion Safety October 2003
Figure 1
In this scenario, any requests from UA(1..9) to UA(10..18) traverse
the proxy-proxy link P1<-->P2. Assuming current SIP practices, if
this link is UDP and every UA emits a request simultaneously, each
proxy will insert nine (one for each UA) requests, resulting in
eighteen simultaneous requests on the P1<-->P2 link. Each request
may require retransmissions, and large requests may require
fragmentation to fit the link MTU -- at the worst case, producing
more than one hundred packets per request, or approximately 2,000
simultaneously expressed packets in this scenario. If the capacity of
link P1<-->P2 is inadequate to deliver these messages within the
SIP retransmission window, the originating UAs (or the proxies, if
acting in transaction-stateful mode) generate retransmissions,
further compounding the problem into a "retransmission storm".
Real-world scenarios may scale far more seriously. It is not
unreasonable to assume that there may be tens of thousands of UAs on
each side of the network.
It should be noted that the fundamental problem not just between UAs
and proxies, but whenever there is a high fan-out or fan-in ratio. If
in the above example, each UA were behind a "residential proxy", the
problem would occur in similar fashion.
3. Scope of Work
One solution might be to deprecate UDP entirely for SIP. However,
there is a large installed base using UDP, and there are legitimately
places where UDP appears to be quite useful such as tiny mobile
phones and in extremely high-volume proxies connecting over dedicated
networks.
As an alternative, this draft defines mechanisms whereby:
1. a UAC may require that any proxy processing its requests transmit
those requests over a transport protocol providing congestion
management.
2. a UAC may inform a UAS receiving its requests that those requests
were transmitted over a route supporting congestion management,
and require that that UAS respond in similar fashion.
3. A proxy may reject requests that require congestion-managed
transport when that proxy finds that the only route it has to the
next hop is over transport that does not support congestion
management.
4. A proxy may reject requests that would be fragmented, even for
requests that do not indicate a requirement for
congestion-managed transport.
5. A UAS may reject requests that would result in responses that
require congestion-managed transport if the originating request
Willis & Campbell Expires April 12, 2004 [Page 4]
Internet-Draft SIP Congestion Safety October 2003
did not require congestion-managed transport.
Note that SIP has no fundamental mechanism whereby a proxy may reject
a response. This precludes requiring congestion management for
responses being processed by a proxy except as provided by the
original request. If, due to an issue of network topology change or
similar event between the processing of the request and the
processing of the response by a proxy the only path available to the
proxy is not congestion managed, the proxy has no choice but to send
the response over that path. It's not perfect, but seems to be all we
can do at this time.
4. Assuring Transitive Congestion-Managed Transport with Require and
Proxy-Require
SIP provides mechanisms whereby a user agent making a request can be
assured that any proxy servicing or UAS responding to that request
support a specific extension or set of behavior.
To be assured that a proxy servicing the request meets the
requirements, the UAC includes a "Proxy-Require" header field with a
value indicating a tag for the specific extension or behavior
required. As per [4], proxies not recognizing a specific tag or
unwilling to support the associated behavior reject a request
referencing that tag with a 420 response, which has the semantic "Bad
Extension".
To be assured that a UAS responding to a request meets the
requirements, the UAC includes a "Require" header field with a value
indicating a tag for the specific extension or behavior required. As
per [4], UASs not recognizing a specific tag or unwilling to support
the associated behavior reject a request referencing that tag with a
420 response, which has the semantic "Bad Extension".
We herein define a an option-tag value of "congestion-managed".
There is an IANA registration process for these tags defined in [4],
and the "IANA Considerations" of this document fulfills the
requirements of the IANA registration process.
5. New Behaviors at SIP Nodes
5.1 Behavior at the UAC
5.1.1 Sending a Request
A UAC exercising this extension adds a Require header field and a
Proxy-Require header field value including the option tag
"congestion-managed" to each request.
Willis & Campbell Expires April 12, 2004 [Page 5]
Internet-Draft SIP Congestion Safety October 2003
For any request that exercises this extension (i.e., contains the
"congestion-managed" option tags), the UAC MUST transmit the request
using a protocol that supports congestion maangement.
Any UA supporting this extension SHOULD exercise this extension on
all initial requests.
5.1.2 Receiving a 514 Response to a Request
A 514 response (semantic "No available route with congestion
management) indicates that an intermediate proxy found that its only
vailable routes toward the required next hop did not support
congestion management. A UA receiving a 514 response has the options
of giving up, trying the request without the "Proxy-Require:
congestion-management" (which will likely return a 516) or trying a
different set of proxies, presumably through using a different
pre-loaded Route header field.
5.1.3 Receiving a 515 Response to a Request
A 515 response (semantic "Response requires congestion management")
indicates that the response generated by the UAS responding to the
request is larger than the UAS' understanding of path MTU and that
the UAS does not know that the route indicated by the VIA headers is
over congestion-managed transport. A UAC receiving a 515 to a
request may either retry the request in a congestion-managed manner
(adding the "congestion-managed" option tag to Require and
Proxy-Require)) or abandon the request.
5.1.4 Receiving a 516 Response to a Request
A 516 response (semantic "Proxying of request would induce
fragmentation") indicates that a proxy forwarding the request
detected that the request was larger than the next hop link MTU from
that proxy and that the transport protocol toward that next hop does
not support congestion management. A UAS receiving a 516 response may
retry the request with a "Proxy-Require: congestion-management" added
(which will probably return a 514), retry the request using an
alternate route, or abandon the request.
5.2 Behavior at the Proxy
A proxy forwarding a request containing a Proxy-Require with this tag
value MUST trasmit that request using a transport protocol (such as
TCP) supporting congestion-management. All proxies SHOULD attempt to
reduce fragmentation following the procedure described below.
Willis & Campbell Expires April 12, 2004 [Page 6]
Internet-Draft SIP Congestion Safety October 2003
5.2.1 Proxy Rejects Request Requiring Congestion Management When Route
with Congestion Management Not Available
When a SIP proxy processing a request marked with a Proxy-Require
header field containing the value "congestion-managed" determines
that the next hop is reachable only via a transport proocol not
supporting congestion management (such as UDP) the proxy MUST reject
that request with a 514 response.
5.2.2 Proxy Rejects Request Not Requiring Congestion Management When
Forwarding That Request Would Induce Fragmentation
When a SIP proxy supporting this extension and processing a request
not marked with a Proxy-Require header field containing the value
"congestion-managed" determines that the next hop is reachable only
via a transport protocol not supporting congestion management (such
as UDP) and the size of the request is larger than the MTU of the
interface towards that next hop, the proxy MUST reject that request
with a 516 response.
5.2.3 Forwarding of Responses
When any proxy supporting this extension forwards a request or
response and there is a choice of transport protocols toward the next
hop, the proxy SHOULD choose a transport protocol supporting
congestion management if one is available.
When a proxy supporting this extension forwards a response containing
a Proxy-Require header field with the option-tag "congestion-managed"
as a value and the relevant Via header field value allows for a
choice of transport protocols, the proxy MUST select a transport
supporting congestion management if such a transport is available.
SIP provides no mechanism whereby a proxy may reject a response.
Consequently, proxies may receive responses that require
fragmentation over a transport not supporting congestion management.
One example of a situation where this might be expected to occur is
as follows: A UAC not supporting this extension makes a request via
UDP. This request transits the proxy in question without inducing
fragmentation. The responding UAS generates a response that is larger
than the request. When the proxy prepares to send the request, it
finds that the increase in size now requires fragmentation.
Discarding the response would result in a timeout and retransmission
of the request and response, thereby doing more harm than good. There
seems to be nothing that the proxy can do to correct the situation,
so it MUST forward the response as specified in [4].
Willis & Campbell Expires April 12, 2004 [Page 7]
Internet-Draft SIP Congestion Safety October 2003
5.3 Behavior at the UAS
A user agent server server (UAS) receiving a SIP request generates a
response to that request. Delivery of this response may raise issues
of congestion management. Because SIP requires that responses
traverse exactly the reverse of the route taken by the request
(recorded in the Via: header field values), the server has no options
about routing the response. If the request was delivered in a
congestion-managed manner, it is likely that the response will also
be returned in a congestion-managed manner, as it must traverse
exactly this recorded route. However, if the request was NOT received
in a congestion-managed manner, the server cannot negotiate a
congestion-managed path for the response, as the response must follow
the path of the request.
When a UAS supporting this extension responds to a request over a
route supporting congestion management (as indicated by the presence
of the congestion-managed option tag in the request), the UAS MUST
include the congestion-managed option tag in a "Proxy-Require" header
field in the response. Furthermore, it MUST transmit that response
using a protocol supporting congestion management. If it is unable to
transmit the response using a protocol supporting congestion
management, it MUST reject the request and return an error response
using response code 515, which has the semantic of "Response requires
congestion management."
When a UAS supporting this extension generates a response to a
request that is larger than the UAS' understanding of path MTU and
that request was not received over a congestion-managed route (as
indicated by the presence of a "Require: congestion-managed"), it
cannot be assumed that the response can be safely transmitted. As the
UAS cannot respond safely, it SHOULD reject the request and return an
error response using response code 515, which has the semantic of
"Response requires congestion management". Note that this does not
absolutely preclude fragmentation of the response, as the request may
be fragmented by intervening routers. However, this sort of
fragmentation is outside of the UAS' capacity to detect or control.
6. IANA Considerations
This document defines the SIP option tag "congestion-managed" which
IANA will add to the registry of SIP option tags defined in [4].
This document defines the SIP response code 514, with the semantic
"No congestion-managed route available" which IANA will add to the
registry of SIP response codes defined in [4] in the section for 5xx
clase response codes.
Willis & Campbell Expires April 12, 2004 [Page 8]
Internet-Draft SIP Congestion Safety October 2003
This document defines the SIP response code 515, with the semantic
"Response requires congestion management" which IANA will add to the
registry of SIP response codes defined in [4] in the section for 5xx
clase response codes.
This document defines the SIP response code 516, with the semantic
"Proxying of request would induce fragmentation" which IANA will add
to the registry of SIP response codes defined in [4] in the section
for 5xx clase response codes.
The following is the registration for the congestion-managed option
tag:
RFC Number: RFCXXXX [Note to IANA: Fill in with the RFC number of
this specification.]
Option Tag: congestion-managed
The following is the registration for the SIP response code 514:
RFC Number: RFCXXXX [Note to IANA: Fill in with the RFC number of
this specification.]
Response Code: 514 No available route with congestion management
The following is the registration for the SIP response code 515:
RFC Number: RFCXXXX [Note to IANA: Fill in with the RFC number of
this specification.]
Response Code: 515 Response requires congestion management
The following is the registration for the SIP response code 516:
RFC Number: RFCXXXX [Note to IANA: Fill in with the RFC number of
this specification.]
Response Code: 516 Proxying of request would induce fragmentation
7. Acknowledgements
This document is a product of the SIP Working Group and contains
input from many contributors in that group. The named authors of this
document claim no personal contribution to the content excecpt as
provided in their capacity as participants in the working group.
Rather, they have attempted to act only in an editorial fashion,
documenting the consensus of the working group as it emerged.
Somebody had to do the typing.
Willis & Campbell Expires April 12, 2004 [Page 9]
Internet-Draft SIP Congestion Safety October 2003
Normative References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Postel, J. and J. Reynolds, "Instructions to RFC Authors", RFC
2223, October 1997.
[4] 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.
[5] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J. and B.
Rosen, "Change Process for the Session Initiation Protocol
(SIP)", BCP 67, RFC 3427, December 2002.
Authors' Addresses
Dean Willis
dynamicsoft Inc.
5100 Tennyson Parkway
Suite 1200
Plano, TX 75028
US
Phone: +1 972 473 5455
EMail: dean.willis@softarmor.com
URI: http://www.dynamicsoft.com/
Ben Campbell
dynamicsoft Inc.
5100 Tennyson Parkway
Suite 1200
Plano, TX 75028
US
Phone: +1 972 473 5452
EMail: bcampbell@dynamicsoft.com
URI: http://www.dynamicsoft.com/
Willis & Campbell Expires April 12, 2004 [Page 10]
Internet-Draft SIP Congestion Safety October 2003
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property 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; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Willis & Campbell Expires April 12, 2004 [Page 11]
Internet-Draft SIP Congestion Safety October 2003
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Willis & Campbell Expires April 12, 2004 [Page 12]
