Subject: [IMRG] Re: [e2e] TCP Traffic Measurement Studies

From: Filipe Abrantes <[email-masked]>
Date: Fri, 20 Jan 2006 14:59:04 +0000

Hello Detlef, Vijay, all:

(Inline)

Detlef Bosau wrote:
> Lloyd Wood wrote:
> 
>>On Sat, 24 Dec 2005, Detlef Bosau wrote:
>>
>>
>>>Vijay Erramilli wrote:
>>>
>>>
>>>>I am trying to get a sense of what typical ratios of forward/reverse
>>>>byte traffic is in TCP.  This will of course vary depending on the
>>>
>>>Is there a reason why this should be different from two simplex
>>>connections, at least as long as there is no asymmetric bandwidth / load
>>>somehwere in the network?
>>
>>shared media - traditional Ethernet (now rare),
> 
>                                       that´s why I forgot about it :-)
> 
> However, I think Vijay´s question was more related to the transport
> layer.
> 
> And I don´t see a "pure L4" reason for different throughput in both
> directions of a TCP connection.

Well, the loss of an ACK pkt is forgotten by receiving the subsequent 
ACK, while the loss of a data pkt cannot be forgotten - it always halves 
the cwnd. This means that if the bottleneck queue is in the downlink, 
uploads will not slow down (because only some of their ACKs are lost), 
while downloads do slowdown (because these flows lose data pkts). This 
might be OK in duplex links, but in shared-media (and non-duplex imagine 
802.11) this causes the starvation of downloads when the media reaches 
saturation (this can only happen if the "next" link has higher capacity 
than the shared media). Well, at least this is what I could experience 
through some simulations, I need to check if real-world TCP 
implementations also ignore ACK losses if they receive an higher seqno 
ACK pkt.

Does this make sense? I could share a simulation script that shows this 
phenomenon.

Filipe

> 
> (I admit: My computer is connected to my DSL Router via Ethernet ;-) And
> due to my somewhat dated hub, this is actually CSMA/CD ;-))
> 
> Detlef

-- 
Filipe Lameiro Abrantes
INESC Porto
Campus da FEUP
Rua Dr. Roberto Frias, 378
4200-465 Porto
Portugal

Phone: +351 22 209 4266
E-mail: [email-masked]

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