Subject: "preemptable" and "expendable" packets

From: [email-masked] (John Krawczyk)
Date: Mon, 12 Dec 94 12:16:17 EST

A quick question somewhat related to the decision last week regarding
filterspecs and MMGs.

This has to do with a section of RFC 1633:

   3.3 Packet Dropping

      So far, we have implicitly assumed that all packets within a flow
      were equally important.  However, in many audio and video streams,
      some packets are more valuable than others.  We therefore propose
      augmenting the service model with a "preemptable" packet service,
      whereby some of the packets within a flow could be marked as
      preemptable.  When the network was in danger of not meeting some
      of its quantitative service commitments, it could exercise a
      certain packet's "preemptability option" and discard the packet
      (not merely delay it, since that would introduce out-of-order
      problems).  By discarding these preemptable packets, a router can
      reduce the delays of the not-preempted packets.

      Furthermore, one can define a class of packets that is not subject
      to admission control.  In the scenario described above where
      preemptable packets are dropped only when quantitative service
      commitments are in danger of being violated, the expectation is
      that preemptable packets will almost always be delivered and thus
      they must included in the traffic description used in admission
      control.  However, we can extend preemptability to the extreme
      case of "expendable" packets (the term expendable is used to
      connote an extreme degree of preemptability), where the
      expectation is that many of these expendable packets may not be
      delivered.  One can then exclude expendable packets from the
      traffic description used in admission control; i.e., the packets
      are not considered part of the flow from the perspective of
      admission control, since there is no commitment that they will be
      delivered.

I haven't seen any discussion on how this would be implemented.  Given
that we don't have the bits in the IP4/6 header to mark a packet as
such, how can we do this?  Are the preemptable / expendable packets
sent via a different group address (taking the same tack as
hierarchical encoding), asking for a similar, but preemptable level of
service (or making no reservation, in the case of expendable)?

-jj