RFC 2836 (rfc2836) - Page 3 of 7
Per Hop Behavior Identification Codes
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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