Wireshark-users: Re: [Wireshark-users] Differentiated services field (kindergarten question again

From: Lisi <lisi.reisz@xxxxxxxxx>
Date: Wed, 5 Oct 2011 21:29:10 +0100
On Wednesday 05 October 2011 08:35:42 Guy Harris wrote:
> 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

Thanks, Guy.  That is very helpful of you.

Lisi