Requirements for Management of Overload in the Session Initiation Protocol
RFC 5390
Network Working Group J. Rosenberg
Request for Comments: 5390 Cisco
Category: Informational December 2008
Requirements for Management of Overload in the
Session Initiation Protocol
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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Abstract
Overload occurs in Session Initiation Protocol (SIP) networks when
proxies and user agents have insufficient resources to complete the
processing of a request. SIP provides limited support for overload
handling through its 503 response code, which tells an upstream
element that it is overloaded. However, numerous problems have been
identified with this mechanism. This document summarizes the
problems with the existing 503 mechanism, and provides some
requirements for a solution.
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RFC 5390 Overload Requirements December 2008
Table of Contents
1. Introduction ....................................................2
2. Causes of Overload ..............................................2
3. Terminology .....................................................4
4. Current SIP Mechanisms ..........................................4
5. Problems with the Mechanism .....................................5
5.1. Load Amplification .........................................5
5.2. Underutilization ...........................................9
5.3. The Off/On Retry-After Problem .............................9
5.4. Ambiguous Usages ..........................................10
6. Solution Requirements ..........................................10
7. Security Considerations ........................................13
8. Acknowledgements ...............................................13
9. References .....................................................14
9.1. Normative Reference .......................................14
9.2. Informative References ....................................14
1. Introduction
Overload occurs in Session Initiation Protocol (SIP) [RFC3261]
networks when proxies and user agents have insufficient resources to
complete the processing of a request or a response. SIP provides
limited support for overload handling through its 503 response code.
This code allows a server to tell an upstream element that it is
overloaded. However, numerous problems have been identified with
this mechanism.
This document describes the general problem of SIP overload and
reviews the current SIP mechanisms for dealing with overload. It
then explains some of the problems with these mechanisms. Finally,
the document provides a set of requirements for fixing these
problems.
2. Causes of Overload
Overload occurs when an element, such as a SIP user agent or proxy,
has insufficient resources to successfully process all of the traffic
it is receiving. Resources include all of the capabilities of the
element used to process a request, including CPU processing, memory,
I/O, or disk resources. It can also include external resources such
as a database or DNS server, in which case the CPU, processing,
memory, I/O, and disk resources of those servers are effectively part
of the logical element processing the request. Overload can occur
for many reasons, including:
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RFC 5390 Overload Requirements December 2008
Poor Capacity Planning: SIP networks need to be designed with
sufficient numbers of servers, hardware, disks, and so on, in
order to meet the needs of the subscribers they are expected to
serve. Capacity planning is the process of determining these
needs. It is based on the number of expected subscribers and the
types of flows they are expected to use. If this work is not done
properly, the network may have insufficient capacity to handle
predictable usages, including regular usages and predictably high
ones (such as high voice calling volumes on Mother's Day).
Dependency Failures: A SIP element can become overloaded because a
resource on which it is dependent has failed or become overloaded,
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