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Versions: 00                                                            
Network Working Group                                          P. Jones
Internet Draft                                             G. Salgueiro
Intended status: Standards Track                          Cisco Systems
Expires: January 1, 2011                                     V. Pascual
                                                            Acme Packet
                                                           July 1, 2010



   Transmission of a Session Capacity Estimate (SCE) to Prevent Session
                 Initiation Protocol (SIP) Server Overload
                    draft-jones-sip-overload-sce-00.txt


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with
   the provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 1, 2011.

Copyright Notice

   Copyright (c) 2010 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
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Abstract

   This memo describes a method of preventing overload of Session
   Initiation Protocol (SIP) servers through the calculation,
   conveyance, and usage of a value referred to as the Session Capacity
   Estimate (SCE), a value that summarily represents the capacity of a
   SIP server.

Table of Contents

   1. Introduction...................................................2
   2. Conventions used in this document..............................3
   3. Session Capacity Estimate......................................3
   4. Computing the Session Capacity Estimate........................3
   5. Conveyance of the Session Capacity Estimate....................4
   6. Utilizing the Session Capacity Estimate........................4
   7. Security Considerations........................................4
   8. IANA Considerations............................................4
   9. Acknowledgments................................................5
   10. References....................................................5
      10.1. Normative References.....................................5
   Author's Addresses................................................5

1. Introduction

   Traditionally, reporting resource capacity has entailed reporting
   information related to discrete resources, such as available DS0s,
   free memory, or CPU utilization.  The problem with this approach is
   that, more often than not, the only useful information is the number
   of available DS0s, as that equates to physical components in a
   system that are clearly a limited resource.  Exhaustion of such
   tangible resources has definite meaning, unlike changes in the
   reported values of memory or CPU utilization.  While there is
   clearly a point at which memory or CPU utilization will have an
   effect on the ability of a network element to process communication
   requests, it is impossible for other entities in the network to
   effectively use that information directly, since 90% CPU
   utilization, for example, does not necessarily mean that the device
   has more or less capacity than a device reporting 50% CPU
   utilization.

   Herewith, we introduce a different method for reporting capacity.
   This method is one that, at a high level, entails reporting an
   estimation of the number of additional communication sessions a
   device can accept.  This value may be used by other SIP devices to
   facilitate the selection of a SIP server to which to direct traffic
   in an attempt to avoid overloading a server.




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2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [2].

3. Session Capacity Estimate

   Any element within a communication network has the ability to
   determine when it is nearing an overloaded state.  That
   determination might be based on, as examples, CPU utilization,
   memory utilization, bandwidth, or a combination of all of these.

   Rather than reporting raw values for discrete elements of the
   system, a better method for reporting capacity and preventing system
   overload is to report an estimated value of the number of additional
   communication sessions a network element can accept.  This
   calculation would be locally computed based on these discrete values
   and is most useful for network elements that do not have a fixed
   number of resources, such as SIP proxies or Session Border
   Controllers (SBCs).  We refer to this calculation as the Session
   Capacity Estimate (SCE).

4. Computing the Session Capacity Estimate

   This Session Capacity Estimate (SCE) value might be computed using a
   variety of input variables, usually based on measurements of
   discrete resources.  A SIP server may locally monitor memory
   utilization, CPU usage, and bandwidth, as examples, and use that
   data to arrive at an estimate for the number of additional sessions
   it can accept.

   A means of computing the Session Capacity Estimate (SCE) might be to
   observe how much of any given resource is consumed based on the
   resources already consumed for the currently active sessions.  For
   example, assume each communication session consumes 5% of available
   resources, whatever those resources might be.  In this case, the
   maximum number of sessions that could be supported would be 20.
   With each additional session, the SCE value would be reduced by one.

   Most of the time, however, a given session consumes more or less
   resources than another session and, as such, the SCE value is
   dynamic in nature.  For example, an SBC that processes media may be
   able to processes fewer video sessions than voice sessions, perhaps
   due to bandwidth restrictions or perhaps DSP resources required to
   perform transcoding of media.  As each new session is introduced in
   the system, a new SCE value can be computed that reflects the
   element's best guess as to how much capacity remains, taking into
   consideration measurable resources like memory, CPU, bandwidth, etc.


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5. Conveyance of the Session Capacity Estimate

   The SCE value is conveyed through normal SIP signaling exchanges
   between devices.

   The SCE value reported by the SIP entity sending a response upstream
   is included in the Via header as a new parameter called "sce".  For
   example, a SIP server might include a Via header like the following
   to indicate that it is capable of accepting an additional 275
   sessions:

      Via: SIP/2.0/UDP 192.168.1.10:5060;
           branch=z9hG4bK776asdhds;sce=275

   During periods when no other SIP exchanges take place between two
   servers, a SIP User Agent might send an OPTIONS "ping" message to a
   peer entity to determine the status of that peer as per draft-jones-
   sip-options-ping-02 [1].  The response to that OPTIONS message would
   include the SCE value in addition to other information that might be
   returned.

6. Utilizing the Session Capacity Estimate

   A SIP server or SIP User Agent attempting to identify a next hop to
   which it may direct a session can use the SCE information collected
   from its peers in order to select a next hop that has the highest
   likelihood of successfully accepting the sessions.  To do this, it
   merely compares the SCE values reported by its peers.

   The actual selection might be based on the highest SCE value
   reported by peer elements.  Alternatively, the server might use a
   round-robin approach to cycle through peer elements that report an
   SCE value at or above some minimum value or above the average value
   reported by peer entities.

   Whatever method is used is a matter for implementation.  In any
   case, the objective is to identify a device that is least likely to
   return a 503 "Service Unavailable" status code, indicating the peer
   device has exceeded capacity limits.

7. Security Considerations

   TBD

8. IANA Considerations

   There are no IANA considerations associated with this document.




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9. Acknowledgments

   This document was prepared using 2-Word-v2.0.template.dot.

10. References

10.1. Normative References

   [1]   Jones, et al., "Using OPTIONS to Query for Operational Status
         in the Session Initiation Protocol (SIP)", draft-jones-sip-
         options-ping-02, July 2010.

   [2]   Rosenberg, J., et al., "SIP: Session Initiation Protocol", RFC
         3261, June 2002.

   [3]   Bradner, S., "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 14, RFC 2119, March 1997.

Author's Addresses

   Paul E. Jones
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709
   USA

   Phone: +1 919 476 2048
   Email: paulej@packetizer.com
   IM: xmpp:paulej@packetizer.com


   Victor Pascual
   Acme Packet, Inc.
   Anabel Segura 10, Madrid 28108
   Spain

   Phone: +34 911 75 32 28
   Email: vpascual@acmepacket.com
   IM: xmpp:victor.pascual.avila@gmail.com












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   Gonzalo Salgueiro
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709
   USA

   Phone: +1 919 392 3266
   Email: gsalguei@cisco.com
   IM: xmpp:gsalguei@cisco.com










































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