[e2e] TCP Performance over WWAN

[Detlef Bosau]

Hi to all,

this is not basically a new topic - however, it might be  useful to look 
at the problem from a somewhat different perspective.

Some time ago, Dominik Kaspar posted some surprisingly huge RTOs and 
RTTs which he observed in network paths which included WWAN links.

IIRC, many of us were quite surprised about these results and did not 
really get a clue where these latencies came from. Although it is not 
too difficult to name some few reasons, e.g.,
- huge recovery latencies (one poster pointed out that, e.g., 802.11 
LANs provide for up to 254 sending attempts, actually this may be even 
larger, I have to look it up, but I think one could even achieve an 
unlimited number of transmission attempts),
- excessive MAC latencies resulting from huge numbers of retransmission 
which lead to an excessive competition for shared media,
- fluctuations in the net bit rate when the line coding, channel coding 
etc. is adapted to accommodate varying channel properties,
- transient "link outages", actually, e.g., suspended flows when a 
channel becomes too bad, or consequences from opportunistic scheduling,
- mixed traffic best effort / QoS,
- roaming,
- varying cell load,
only to name a few. The list is certainly to be completed.

However, my interest is not to find out all the reasons for 
disturbances, but I'm interested in whether these things are really 
annoying or not and in whether these things cause grief or not. So we'll 
find those disturbances, which cause harm to TCP - and get them out of 
harm's way. (Or at least something similar ;-))

It would be of great help, if I could obtain some real world latency 
traces or some real world packet/block-corruption ratio traces for this 
purpose.

Unfortunately, I cannot provide appropriate traces on my own, so I ask 
whether there is existing material available.

Certainly, getting in touch with colleagues who work in this area would 
be helpful as well.

Thanks.

Detlef

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Detlef Bosau
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