7.4.4. “Expert” Packet List Column (optional)

Figure 7.4. The “Expert” packet list column

An optional “Expert Info Severity” packet list column is available that displays the most significant severity of a packet or stays empty if everything seems OK. This column is not displayed by default but can be easily added using the Preferences Columns page described in 

Definition of the Differentiated Services Field

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.