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