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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.

Copyright Notice

   Copyright (c) 2008 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (http://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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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.

Rosenberg                    Informational                      [Page 1]
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:

Rosenberg                    Informational                      [Page 2]
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

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