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

From: "JOHNSON, CAROLYN R, ATTLABS" <[email-masked]>
Date: Wed, 09 Sep 2009 01:23:58 -0000

Volker,
Sorry for the delay, this got stuck in my drafts folder while I was out
on vacation.

One comment on the last dashed item:
- 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.

I suggest replacing "telephone number" with "destination routing" so as
not to limit to applications that use telephone prefixes. I believe it
will be more general without losing the applicability to telephone IDs.

The charter is clearly laid out. 
Regards,

Carolyn

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]] On Behalf Of Volker Hilt
Sent: Thursday, August 20, 2009 9:52 AM
To: DISPATCH; SIP-OVERLOAD
Subject: [sip-overload] Revised Charter Proposal for SIP Overload

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.

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---

From: "NOEL, ERIC C, ATTLABS" <[email-masked]>
Date: Thu, 1 Oct 2009 11:17:37 -0400

Volker,

Is there a conference call this morning? I was away during last meeting
and may have missed the schedule.

Thanks,

Eric