[e2e] SPAR, comment to: Re: Retransmission Timouts revisited

Detlef Bosau detlef.bosau at web.de
Thu Aug 25 15:15:51 PDT 2005

I just received a mail from Wes:

> On Thu, Aug 25, 2005 at 09:13:38PM +0200, Detlef Bosau wrote:
> > 
> > Some problems are:
> > 
> > 1. A few years ago, I read a paper that packet order distortions cannot
> > be neglected in the Internet. Thus, the Internet does no longer
> > performe "Sequencing Positive Acknowledgement Retnramission (SPAR)". I
> > did not yet udnerstand all the details, but Edge explicitely maks
> > use of the SPAR assumption several times in his paper. Thus, I´m not yet
> > convinced that his rationale will hold, if this assumption
> > is violated.
> > 
> My interpretation of SPAR is "TCP without SACK", i.e. only the last
> in-sequence segment is acknowledged, wheras PAR is "TCP with SACK" where
> segments are acked as they arrive.  So these are properties of the
> transport protocol, not of the network that delivers segments to the
> transport, so network reordering does not violate any assumption.
> Edge's paper states that his algorithm works for both SPAR and PAR
> anyways, so I'm not sure you can call this a "problem".
> -Wes

O.k. the central point is packet reordering here. I just read his
remakrs on SPAR and PAR, and in fact it seems, Wes is right here.
It´s even just on the fist page of Edge´s paper :-|

However, I did not yet completely understand the rationale here, it´s
not easy. When I stated some "possible problems", that it may be that I
just did not understand these details. Therefore, please allow me to

Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
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