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Re: Differentiated services field (kindergarten question again :-( )
From: Guy Harris <guy () alum mit edu>
Date: Wed, 5 Oct 2011 00:35:42 -0700
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 .) ___________________________________________________________________________ Sent via: Wireshark-users mailing list <wireshark-users () wireshark org> Archives: http://www.wireshark.org/lists/wireshark-users Unsubscribe: https://wireshark.org/mailman/options/wireshark-users mailto:wireshark-users-request () wireshark org?subject=unsubscribe
Current thread:
- Differentiated services field (kindergarten question again :-( ) Lisi (Oct 04)
- Re: Differentiated services field (kindergarten question again :-( ) Guy Harris (Oct 05)