RFC 2996 (rfc2996) - Page 3 of 9

Format of the RSVP DCLASS Object

RFC 2996            Format of the RSVP DCLASS Object       November 2000


   towards the receiver.  No DCLASS is required.

1.1 Additional Uses of the DCLASS Object

   The DCLASS object is intended to be a general tool for conveying DSCP
   information in RSVP messages.  This may be useful in a number of
   situations.  We give one further example here as motivation.

   In this example, we assume that the decision about the appropriate
   behavior aggregate for a RSVP-mediated traffic flow is made at the DS
   network egress router (or a related Policy Decision Point) by
   observing RSVP PATH and RESV messages and other necessary
   information.  However, the actual packet marking must be done at the
   ingress of the network. The DCLASS object can be used to carry the
   needed marking information between egress and ingress routers.

2. Format of the DCLASS Object

   The DCLASS object has the following format:

            0       |       1       |       2       |       3
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Length (>= 8)            |   C-Num (225) |      1        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Unused                               | 1st DSCP  |   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Unused                               | 2nd DSCP  |   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Unused                               | . . . .   |   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The first word contains the standard RSVP object header (the Class
   Num for the DCLASS object is 225).  The length field indicates the
   total object length in bytes.  The object header is followed by one
   or more 32-bit words, each containing a DSCP in the six high-order
   bits of the least significant byte.  The length field in the object
   header indicates the number of DSCPs included in the object.
   Specifically, the number of DCLASS objects present is equal to
   (Length - 4) / 4.

   The network may return multiple DSCPs in the DCLASS object in order
   to enable the host to discriminate sub-flows within a behavior
   aggregate. For example, in the case of the AF PHB group [AF], the
   network may return the DSCPs 001010, 001100, and 001110 corresponding
   to increasing levels of drop precedence within Class 1 of the AF PHB
   group.  Note that this document makes no statements regarding the
   significance of the order of the returned DSCPs.  Further
   interpretation of DSCP sets is dependent on the specific service



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