RFC 2598 (rfc2598) - Page 2 of 11
An Expedited Forwarding PHB
Alternative Format: Original Text Document
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