RFC 3247 (rfc3247) - Page 3 of 24

Supplemental Information for the New Definition of the EF PHB (Expedited Forwarding Per-Hop Behavior)

RFC 3247                Supplemental Information              March 2002


   and loss in sections 3 and 4.  In section 5 we discuss the impact of
   known scheduling architectures on the critical parameters of the new
   definition.  We also discuss the impact of deviation of real devices
   from the ideal output-buffered model on the magnitude of the critical
   parameters in the definition.

2. Definition of EF PHB

2.1. The formal definition

   An intuitive explanation of the new EF definition is described in
   [6].  Here we restate the formal definition from [6] verbatim.

   A node that supports EF on an interface I at some configured rate R
   MUST satisfy the following equations:

      d_j <= f_j + E_a for all j>0                                (eq_1)

   where f_j is defined iteratively by

      f_0 = 0, d_0 = 0

      f_j = max(a_j, min(d_j-1, f_j-1)) + l_j/R,  for all j > 0   (eq_2)

   In this definition:

      -  d_j is the time that the last bit of the j-th EF packet to
         depart actually leaves the node from the interface I.

      -  f_j is the target departure time for the j-th EF packet to
         depart from I, the "ideal" time at or before which the last bit
         of that packet should leave the node.

      -  a_j is the time that the last bit of the j-th EF packet
         destined to the output I actually arrives at the node.

      -  l_j is the size (bits) of the j-th EF packet to depart from I.
         l_j is measured on the IP datagram (IP header plus payload) and
         does not include any lower layer (e.g. MAC layer) overhead.

      -  R is the EF configured rate at output I (in bits/second).

      -  E_a is the error term for the treatment of the EF aggregate.
         Note that E_a represents the worst case deviation between
         actual departure time of an EF packet and ideal departure time
         of the same packet, i.e. E_a provides an upper bound on (d_j -
         f_j) for all j.




Charny, et. al.              Informational