RFC 2996 (rfc2996) - Page 3 of 9
Format of the RSVP DCLASS Object
Alternative Format: Original Text Document
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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