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Re: Question Regarding HDLC [7:60337] posted 01/05/2003
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HDLC sequences numbers aren't in data frames. They are in separate keepalive
frames. They aren't like TCP sequence numbers, which sequence the data. They
aren't in the header of the data frame. They are in separate frames in the
control plane.

Which, to make a long and winding story short, probably supports our theory
that the Cisco HDLC sequence numbers are not compressed. But it's not
possible to tell from Cisco documentation and sniffing is difficult because
most of us can't afford a WAN sniffer. But I think that what the original
poster is seeing may prove the point also.

Priscilla

The Long and Winding Road wrote:
> 
> ""Priscilla Oppenheimer""  wrote in
> message
> news:200301052042.UAA32468@xxxxxxxxxxxxxxxxx
> > WAN compression usually compresses the data payload. The HDLC
> sequence
> > numbers are not in data packets; they are in keepalive
> packets. They are
> in
> > the control plane, not the user plane.
> >
> > I can't say for sure, but my guess is that they are not
> compressed. If
> they
> > were, the interfaces wouldn't have a way to monitor the
> status of the
> link.
> > What does Cisco documentation say? Sorry I'm in a rush and
> can't look it
> up.
> 
> 
> I checked the 12.1 docs. no help there. both the config guide
> and the
> command reference talk about "frame compression" I "believe"
> the implication
> is that the data itself is compressed, but the frame header is
> not.
> Separately, there are sections on RTP header compression, but
> that is a
> different issue.
> 
> Let me step out on a limb here and postulate that in terms of
> process, the
> router first compresses the data, then attaches the HDLC frame.
> Much the
> same way that when compression is uese on the LAN, the data is
> compressed,
> but the L2 header is not.
> 
> Anyone snifed this and have a scientific answer?
> 
> 
> 
> 
> >
> > Good question! Thanks for returning the forum to something
> meaningfull.
> >
> > Priscilla
> >
> > Simmi Singla wrote:
> > >
> > > Hi All,
> > > I have question regarding HDLC,a silly question but still a
> > > doubt.See I have HDLC connection back to back.on one
> interface
> > > I configure compression and other interface on other router
> no
> > > compression.Now when I debug the my seq and mine seen no.s
> are
> > > in sync I mean same they inncrease ,that different no layer
> 3
> > > communication I am able to do.If such case occurs how from
> > > debug commands I will come to know.
> > > I enabled debug serial interface but it only shows me that
> the
> > > no get increment that too in proper order but I have read
> that
> > > in Karl Solie book Vol1 that it will stop increment the
> > > keepalives.
> > > So can anybody guide me.
> 
> 




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