RFC 2836 (rfc2836) - Page 3 of 7

Per Hop Behavior Identification Codes

RFC 2836         Per Hop Behavior Identification Codes          May 2000


   In another approach, the ATM Forum has a requirement to indicate
   desired IP QOS treatments in ATM signaling, so that ATM switches can
   be just as supportive of the desired service as are IP forwarders.
   To do so the Forum is defining a new VC call setup information
   element is which will carry PHB identification codes (although will
   be generalized to do more if needed).

2. Encoding

   PHBs and sets of PHBs are encoded in an unsigned 16 bit binary field.

   The 16 bit field is arranged as follows:

   Case 1: PHBs defined by standards action, as per [RFC 2474].

   The encoding for a single PHB is the recommended DSCP value for that
   PHB, left-justified in the 16 bit field, with bits 6 through 15 set
   to zero.  Note that the recommended DSCP value MUST be used, even if
   the network in question has chosen a different mapping.

   The encoding for a set of PHBs is the numerically smallest of the set
   of encodings for the various PHBs in the set, with bit 14 set to 1.
   (Thus for the AF1x PHBs, the encoding is that of the AF11 PHB, with
   bit 14 set to 1.)

       0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     |         DSCP          | 0   0   0   0   0   0   0   0   X   0 |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   Case 2: PHBs not defined by standards action, i.e. experimental or
   local use PHBs as allowed by [RFC 2474]. In this case an arbitrary 12
   bit PHB identification code, assigned by the IANA, is placed left-
   justified in the 16 bit field. Bit 15 is set to 1, and bit 14 is zero
   for a single PHB or 1 for a set of PHBs.  Bits 12 and 13 are zero.

       0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     |                      PHB id code              | 0   0   X   1 |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   Bits 12 and 13 are reserved either for expansion of the PHB
   identification code, or for other use, at some point in the future.








Brim, et al.                Standards Track