Subject: discardable, discarded definition

From: "David Newman" <[email-masked]>
Date: Fri, 19 Nov 1999 12:53:27 -0800

At the Washington bmwg meeting, Jeff Dunn and Cynthia Martin asked if I
would supply definitions for discardable and discarded traffic for their
frame relay draft.

Here are the definitions. These are straw men -- critiques, subtractions,
and additions welcome.

dn

1.2.X  Discardable frames

Definition: Frames identified as being eligible to be dropped in the event
of congestion.

Discussion: The discard eligible field in the frame relay header is the
correct -- and by far the most common -- means of indicating which frames
may be dropped in the event of congestion. However, DE is not the only means
of identifying which frames may be dropped. There are at least two other
cases that apply.

In the first case, network devices may prioritize frame relay traffic by
non-DE means. For example, many service providers prioritize traffic on a
per-PVC basis. In this instance, any traffic from a given DLCI (data link
channel identifier) may be dropped during congestion, regardless of whether
DE is set.

In the second case, some implementations use upper-layer criteria, such as
IP addresses or TCP or UDP port numbers, to prioritize traffic within a
single PVC. In this instance, the network device may evaluate discard
eligibility based on upper-layer criteria rather than the presence or
absence of a DE bit.

Specification: None

1.2.X Discarded frames

Definition: Those frames dropped by a network device.

Discussion: Discardable and discarded frames are not synonymous. Some
implementations may ignore DE bits or other criteria, even though they
supposedly use such criteria to determine which frames to drop in the event
of congestion.

In other cases, a frame with its DE bit set may not be dropped. One example
of this is in cases where congestion clears before the frame can be
evaluated.

Specification: None