RFC 2598 (rfc2598) - Page 2 of 11

An Expedited Forwarding PHB

RFC 2598              An Expedited Forwarding PHB              June 1999


   Loss, latency and jitter are all due to the queues traffic
   experiences while transiting the network.  Therefore providing low
   loss, latency and jitter for some traffic aggregate means ensuring
   that the aggregate sees no (or very small) queues. Queues arise when
   (short-term) traffic arrival rate exceeds departure rate at some
   node.  Thus a service that ensures no queues for some aggregate is
   equivalent to bounding rates such that, at every transit node, the
   aggregate's maximum arrival rate is less than that aggregate's
   minimum departure rate.

   Creating such a service has two parts:

      1) Configuring nodes so that the aggregate has a well-defined
         minimum departure rate. ("Well-defined" means independent of
         the dynamic state of the node.  In particular, independent of
         the intensity of other traffic at the node.)

      2) Conditioning the aggregate (via policing and shaping) so that
         its arrival rate at any node is always less than that node's
         configured minimum departure rate.

   The EF PHB provides the first part of the service.  The network
   boundary traffic conditioners described in [RFC 2475] provide the
   second part.

   The EF PHB is not a mandatory part of the Differentiated Services
   architecture, i.e., a node is not required to implement the EF PHB in
   order to be considered DS-compliant.  However, when a DS-compliant
   node claims to implement the EF PHB, the implementation must conform
   to the specification given in this document.

   The next sections describe the EF PHB in detail and give examples of
   how it might be implemented.  The keywords "MUST", "MUST NOT",
   "REQUIRED", "SHOULD", "SHOULD NOT", and "MAY" that appear in this
   document are to be interpreted as described in [Bradner97].

2. Description of EF per-hop behavior

   The EF PHB is defined as a forwarding treatment for a particular
   diffserv aggregate where the departure rate of the aggregate's
   packets from any diffserv node must equal or exceed a configurable
   rate.  The EF traffic SHOULD receive this rate independent of the
   intensity of any other traffic attempting to transit the node.  It
   SHOULD average at least the configured rate when measured over any
   time interval equal to or longer than the time it takes to send an
   output link MTU sized packet at the configured rate.  (Behavior at
   time scales shorter than a packet time at the configured rate is




Jacobson, et al.            Standards Track