RFC 3697 (rfc3697) - Page 2 of 9

IPv6 Flow Label Specification

RFC 3697             IPv6 Flow Label Specification            March 2004


   Traditionally, flow classifiers have been based on the 5-tuple of the
   source and destination addresses, ports, and the transport protocol
   type.  However, some of these fields may be unavailable due to either
   fragmentation or encryption, or locating them past a chain of IPv6
   option headers may be inefficient.  Additionally, if classifiers
   depend only on IP layer headers, later introduction of alternative
   transport layer protocols will be easier.

   The usage of the 3-tuple of the Flow Label and the Source and
   Destination Address fields enables efficient IPv6 flow
   classification, where only IPv6 main header fields in fixed positions
   are used.

   The minimum level of IPv6 flow support consists of labeling the
   flows.  IPv6 source nodes supporting the flow labeling MUST be able
   to label known flows (e.g., TCP connections, application streams),
   even if the node itself would not require any flow-specific
   treatment.  Doing this enables load spreading and receiver oriented
   resource reservations, for example.  Node requirements for flow
   labeling are given in section 3.

   Specific flow state establishment methods and the related service
   models are out of scope for this specification, but the generic
   requirements enabling co-existence of different methods in IPv6 nodes
   are set forth in section 4.  The associated scaling characteristics
   (such as nodes involved in state establishment, amount of state
   maintained by them, and state growth function) will be specific to
   particular service models.

   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 BCP 14, RFC 2119
   [KEYWORDS].

2.  IPv6 Flow Label Specification

   The 20-bit Flow Label field in the IPv6 header [IPv6] is used by a
   source to label packets of a flow.  A Flow Label of zero is used to
   indicate packets not part of any flow.  Packet classifiers use the
   triplet of Flow Label, Source Address, and Destination Address fields
   to identify which flow a particular packet belongs to.  Packets are
   processed in a flow-specific manner by the nodes that have been set
   up with flow-specific state.  The nature of the specific treatment
   and the methods for the flow state establishment are out of scope for
   this specification.

   The Flow Label value set by the source MUST be delivered unchanged to
   the destination node(s).



Rajahalme, et al.           Standards Track