Subject: [sip-overload] Revised Charter Proposal for SIP Overload

From: Volker Hilt <[email-masked]>
Date: Thu, 20 Aug 2009 09:52:27 -0400

All,

below is an revised SIP Overload Control charter proposal. I have 
received a few comments that are reflected in the new proposal.

All comments, thoughts, feedback is very welcome!

Thanks,

Volker

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.