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

From: Volker Hilt <[email-masked]>
Date: Tue, 21 Jul 2009 17:09:59 -0400

All,

below is a charter proposal for a WG on SIP Overload Control.

All comments, thoughts, feedback is very welcome!

Thanks,

Volker

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

As with any network element, a Session Initiation Protocol (SIP) 
[RFC3261] 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 a small fraction of the original 
processing capacity.  This is often called congestion collapse.

To objective of a SIP overload control mechanism is to enable SIP 
servers to perform 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 prevent overload of a SIP server 
and it cannot prevent congestion collapse.  In fact, it may cause 
traffic to oscillate and to 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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From: "NOEL, ERIC C, ATTLABS" <[email-masked]>
Date: Thu, 23 Jul 2009 11:19:33 -0400

Please ignore