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

From: Volker Hilt <[email-masked]>
Date: Tue, 21 Jul 2009 19:45:38 -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.

---

From: "Moran, Timothy L." <[email-masked]>
Date: Wed, 22 Jul 2009 12:36:12 -0400

Volker,

Two tweaks indicated below.

While the requirements have been defined, is it documented anywhere why
existing mechanisms (such as TCP, ECN, etc.) are insufficient?  If not,
I would propose a deliverable of such a doc - e.g. an informational RFC.

Br,
Tim Moran

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]] On Behalf Of Volker Hilt
Sent: Tuesday, July 21, 2009 7:46 PM
To: SIP-OVERLOAD
Subject: [sip-overload] Charter Proposal for SIP Overload

All,

Thanks,

Volker

To objective of a SIP overload control mechanism is to enable SIP
[tlm->] Suggest Changing "To" to "The"

servers to perform close to their capacity limit during times of
overload.

[tlm->] Suggest Changing determine to indicate

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