On Oct 4, 2011, at 10:38 PM, Lisi wrote:
> I understand (I hope!) that the differentiated services field tells you a
> packet's priority, but I can't work out how to read it. Does nothing but
> zeros (e.g. DSCP 0x00) mean that this particular trace has no priority set?
> Or does it mean the reverse? (Top priority.)
RFC 2474 "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers" says
The DS field structure is presented below:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| DSCP | CU |
+---+---+---+---+---+---+---+---+
DSCP: differentiated services codepoint
CU: currently unused
and
4.1 A Default PHB
A "default" PHB MUST be available in a DS-compliant node. This is
the common, best-effort forwarding behavior available in existing
routers as standardized in [RFC1812]. When no other agreements are
in place, it is assumed that packets belong to this aggregate. Such
packets MAY be sent into a network without adhering to any particular
rules and the network will deliver as many of these packets as
possible and as soon as possible, subject to other resource policy
constraints. A reasonable implementation of this PHB would be a
queueing discipline that sends packets of this aggregate whenever the
output link is not required to satisfy another PHB. A reasonable
policy for constructing services would ensure that the aggregate was
not "starved". This could be enforced by a mechanism in each node
that reserves some minimal resources (e.g, buffers, bandwidth) for
Default behavior aggregates. This permits senders that are not
differentiated services-aware to continue to use the network in the
same manner as today. The impact of the introduction of
differentiated services into a domain on the service expectations of
its customers and peers is a complex matter involving policy
decisions by the domain and is outside the scope of this document.
The RECOMMENDED codepoint for the Default PHB is the bit pattern '
000000'; the value '000000' MUST map to a PHB that meets these
specifications. The codepoint chosen for Default behavior is
compatible with existing practice [RFC791]. Where a codepoint is not
mapped to a standardized or local use PHB, it SHOULD be mapped to the
Default PHB.
A packet initially marked for the Default behavior MAY be re-marked
with another codepoint as it passes a boundary into a DS domain so
that it will be forwarded using a different PHB within that domain,
possibly subject to some negotiated agreement between the peering
domains.
so, at least by default, it means "no special priority". (In this context, PHB means "per-hop behavior":
http://en.wikipedia.org/wiki/Per-Hop_Behaviour
not "pointy-haired boss":
http://en.wikipedia.org/wiki/Pointy-Haired_Boss
.)
