Subject: [sip-overload] [Fwd: [dispatch] Chartering SIP Overload]

From: Volker Hilt <[email-masked]>
Date: Fri, 15 Jan 2010 08:35:08 -0500

Forwarding this email from DISPATCH (please copy the DISPATCH list on 
responses!).

Any comments?

Volker

Subject: [dispatch] Chartering SIP Overload
From: Gonzalo Camarillo <[email-masked]>
To: DISPATCH <[email-masked]>
Date: Fri, 15 Jan 2010 05:23:04 -0600
List-Id: DISPATCH Working Group Mail List <dispatch.ietf.org>

Folks,

as you know, we agreed to charter the SIP overload work a while ago. 
Below you can find the proposed charter.

We have gotten some comments back from the IESG. There are some concerns 
about the effects of the proposed solutions in real networks. It seems 
that those concerns would disappear if we chartered the WG to develop 
Experimental proposals, at least at the beginning. Those proposals would 
be able to eventually move to the standards track once we got 
operational experience with them. Comments?

Thanks,

Gonzalo

SIP Overload Control WG Charter
-------------------------------

As with any network element, a Session Initiation Protocol (SIP) server
can suffer from overload when the number of SIP messages it receives
exceeds the number of messages it can process. Overload is said to occur
when a SIP server does not have sufficient resources to process all
incoming SIP messages.  These resources can include CPU, memory, network
bandwidth, input/output, or disk resources.

Overload can pose a serious problem for a network of SIP servers. During
periods of overload, the throughput of a network of SIP servers can be
significantly degraded.  In fact, overload may lead to a situation in
which the throughput drops down to zero or a small fraction of the
original processing capacity.  This is often called congestion collapse.

The objective of a SIP overload control mechanism is to enable SIP
servers to operate close to their capacity limit during times of overload.

The SIP protocol provides a limited mechanism for overload control
through its 503 (Service Unavailable) response code and the Retry-After
header.  However, this mechanism cannot fully prevent overload of a SIP
server and it cannot prevent congestion collapse.  In fact, its on/off
behavior may cause traffic to oscillate and shift between SIP servers
and thereby worsen an overload condition.  A detailed discussion of the
SIP overload problem, the problems with the 503 (Service Unavailable)
response code and the Retry-After header and the requirements for a SIP
overload control mechanism can be found in RFC5390.

The objective of the proposed working group is to develop mechanisms for
SIP overload control. Two complementary approaches exist for handling
overload situations:
- The first mechanism aims at preventing overload in SIP servers by
adjusting the incoming load to a level that is acceptable for this
server. This mechanism uses implicit and/or explicit feedback to
determine when an overload condition occurs in a SIP server and a
reduction in load is required.
- The second mechanism aims at preventing overload in SIP servers by
distributing load control filters to SIP servers that throttle calls
based on their destination domain, telephone number prefix or for a
specific user. Such filters can be used, for example, to throttle calls
to a hotline during a ticket-giveaway event.

Specifically, the proposed working group will develop the following
deliverables:
1. A document describing key design considerations for a SIP overload
control mechanism.
2. A specification for an SIP overload control mechanism based on
implicit/explicit feedback.
3. A specification for a SIP load filtering mechanism.

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