draft-dreibholz-ipv4-flowlabel-06.txt

IPv6协议中flow_label的相关RFC

Network Working Group                                       T. Dreibholz
Internet-Draft                              University of Duisburg-Essen
Intended status: Standards Track                            June 5, 2007
Expires: December 7, 2007


                        An IPv4 Flowlabel Option
                 draft-dreibholz-ipv4-flowlabel-06.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).














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Abstract

   This draft defines an IPv4 option containing a flowlabel that is
   compatible to IPv6.  It is required for simplified usage of IntServ
   and interoperability with IPv6.














































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1.  Introduction

1.1.  Terminology

   This document uses the following terms:

   o  IntServ (Integrated Services): Reservation of network resources
      (bandwidth) on a per-flow basis.  See [2], [4], [5], [6], [7], [8]
      and [9] for details.

   o  Flow: An IntServ reservation between two endpoints.

   o  Flow Label: The Flow Label field of the IPv6 header and the IPv4
      option header defined in this draft.  It is used for marking a
      packet to use a specific IntServ reservation.  See [3] for a
      detailed description.

1.2.  Abbreviations

   o  RSVP: ReSource Reservation Protocol

   o  SCTP: Stream Control Transmission Protocol

   o  TCP: Transmission Control Protocol

   o  QoS: Quality of Service

   o  UDP: User Datagram Protocol

1.3.  Conventions

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD.
   SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
   they appear in this document, are to be interpreted as described in
   [11].
















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2.  A Flow Label Option for IPv4

2.1.  Motivation

   This section describes the motivation to add a flow label option to
   the IPv4 protocol.

2.1.1.  The Flow Label Field of IPv6

   The Flow Label field of the IPv6 header (see [10] and [3]) is a 20-
   bit pseudo-random number.  All packets from the same source address
   having the same flow label MUST contain the same destination address.
   Therefore, the flow label combined with the source address is a
   network- unique identification for a specific packet flow.  The idea
   behind the flow label is marking specific flows for IntServ.  That
   is, the routers on the path from source to destination keep e.g.
   reservation states for the flows.  The flow label provides easy
   identification and utilizes efficient lookup, e.g. using a hash
   function on the 3-tuple (source address, destination address, flow
   label).

   Using the IPv6 flow label, packets can be mapped easily to specific
   flows, with the following features:

   o  Transport Layer Protocol Independence: Since the mapping is
      directly specified in the IP header, all possible layer 4
      protocols are supported, even protocols to be specified in a far
      future.

   o  Support for Network Layer Encryption: The mapping is independent
      of payload encryption (e.g. by IPsec).

   o  Support for Fragmentation: If fragmentation of a large IP packet
      is necessary, all fragments contain the same flow label.
      Therefore, fragmentation does not cause any flow-marking problem.

   o  Flow Sharing: By marking packets with a flow label, it is possible
      to share a single flow (IntServ reservation) with several
      communication associations from host A to host B. For example, a
      video stream via UDP and a HTTP download via TCP could share a
      single reservation.  For the user, flow sharing has the advantage
      that if one of its communication associations temporarily requires
      lower bandwidth than expected, other associations sharing the same
      flow may use the remaining bandwidth.  That is, his possibly
      expensive reservation is fully utilized.  Flow sharing also helps
      keeping the total number of reservations a router has to handle
      small, reducing their CPU and memory requirements and therefore
      cost.



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   o  Multi-Flow Connections: One communication association can divide
      up its packets to several flows, simply by marking packets with
      different flow labels.  This technique can be used for layered
      transmission.  That is, a stream (e.g. a video) is divided up into
      several parts (called layers).  For example, the first layer (base
      layer) of a video contains a low-quality version, the second (1st
      enhancement layer) the data to generate a higher-quality version,
      etc..  Now, the first layer can be mapped to a high-quality
      reservation (guaranteed bandwidth, low loss rate) at higher cost,
      but the following layers can be mapped to lower-quality
      reservations (e.g. higher loss rate) or even best effort at lower
      cost.  Research shows that the total transmission cost can be
      highly reduced using layered transmission (see [12] for details).

2.1.2.  The Limitations of IntServ via IPv4

   Using IntServ with IPv4, there are several problems that can only be
   solved with high management effort:

   o  No Transport Layer Protocol Independence: It is necessary to mark
      the packets within the layer 4 protocol header.  For example, the
      TCP, UDP or SCTP port numbers can be used to mark flows (with
      limitations, see below).  But for new protocols (e.g.
      experimental, new standards, proprietary), software updates for
      *all* IntServ routers are necessary to recognize the packet flow!

   o  No Support for Network Layer Encryption: Since it is necessary to
      read fields of the layer 4 protocol header, it may not be
      encrypted.  Therefore, e.g. the usage of IPsec is impossible.

   o  Support for Fragmentation: Only the first fragment of a large
      packet contains the layer 4 header necessary to map the packet to
      a flow.  Mapping other fragments would require the hops to
      remember packet identities and try to map fragments to packet
      identities.  Due to the management effort and memory requirements,
      this is not realistic for high-bandwidth backbone routers;
      especially when packet reordering must be considered.
      Furthermore, load sharing or traffic distribution would be
      impossible.

   o  No Flow Sharing: It is usually impossible for two different
      communication associations to share the same flow, e.g. if TCP
      flows are recognized using port numbers.  This makes it necessary
      to reserve an IntServ flow for each communication association.
      This implies an increased number of flow states for routers to
      keep and maintain.  Furthermore, if one association temporarily
      uses a lower bandwidth, the free bandwidth of its flow cannot
      easily be borrowed to another association.



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