From: partho@cs.umd.edu (partho pratim mishra)
Newsgroups: comp.dcom.cell-relay
Subject: Hop by hop vs End to end flow control
Date: 17 Nov 92 15:35:32 GMT
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742

craig@sics.se (Craig Partridge) writes


>>"Allen Robel" <cell@mythos.ucs.indiana.edu> writes:

> they compared
> their hop by hop (HBH) scheme of flow control to TCP as implemented in
> 4.3-Tahoe BSD (they also compare HBH with the modified window adjustment
> in the 4.3 BSD Reno version of TCP).  Their results seem to indicate
> that, in a network with a high bandwidth-delay product, TCP exhibits
> very large oscillations in end-to-end delay, and takes about 100
> round trip times (losing about 190 packets) before a link becomes
> fully utilized.  In contrast, HBH loses NO packets and  maintains
> a very even end-to-end delay variance.  Note that I'm not condoning
> their scheme.  I'm just saying that there are schemes out there that
> do better in the area of flow control than current implementations of
> TCP in high speed networks.
 
>>    I'd just like to note that the jury is still out on these results.
>>In general, the literature (both theory, simulation and practice) shows 
>>end-to-end flow control outperforming hop-by-hop.  And I've heard folks
>>question whether the representation of TCP-IP in the model was accurate.
>>Certainly, given the weight of results one way, it is not clear that one
>>dissenting paper should be sufficient to lead us.
 
and later Rob Warnock writes 

>The oscillation is real, and quite large; I have seen it during FTPs with
>"hash" turned on. But also note that Jon Crowcroft (et al?) published a paper
>some time ago which proposed a simple change to TCP slow-start that smooths
>out most of the oscillation. I wouldn't count TCP (or end-to-end congestion
>control) out yet...


There seems to be some misunderstanding about the "accuracy of the
TCP model". We used the 4.3 BSD Tahoe version of TCP as our model.
In fact the simulator we used was written by Lixia Zhang and used in several
previous studies of TCP (which appeared in CCR and Sigcomm).  What
we found was there were many reasons why 4.3 BSD TCP would perform
badly over high speed links (45 Mb/s and faster). For lack of space
we had to leave out most of the explanation in our Sigcomm paper,
although a companion Technical Memorandum examines the issues in some
more detail. 4.3 BSD Reno and selective acks togther do solve some of the 
problems though not all.  I would be glad to discuss with interested people
what it does and what it does not fix.

As far as the hop by hop vs end to end argument goes I believe we are 
entering the domain of religion so I will hold my horses. However, I
would like to disagree with the claim that "the literature (both theory, 
simulation and practice) shows end-to-end flow control outperforming 
hop-by-hop".  One has to compare specfic mechanisms before making such
a sweeping statement.

-Partho Mishra
-- 
                               |  Systems Design and Analysis Group
      Partho Pratim Mishra     |  Department of Computer Science    
         partho@cs.umd.edu     |  University of Maryland         
                               |  College Park, MD 20742      
Newsgroups: comp.dcom.cell-relay