[e2e] Delays / service times / delivery times in wireless networks.

David P. Reed dpreed at reed.com
Fri Feb 23 09:02:56 PST 2007

It's important to remember that TCP was never intended to be optimal in 
any scenario.   E.g. it wasn't supposed to be a protcol that met the 
"Shannon Limit" for the weird and wonderful "catenet channel" that is 
created when when one tries to unify all networks on a "best efforts" basis.

Of course, there will always be theorists who try to make silk purses 
out of decomposing sow's ears, and produce some wonderful algorithm that 
works based on some theoretical assumptions they can write down and 
convince a whole series of conferences (and DARPA) are the *one and only 
true way that networks work*.

So I wouldn't spend a lot of time trying to "control the packet loss 
rate" in the GPRS channel to optimize TCP, as you seem to be suggesting 

A simple rule of thumb is that TCP likes small bandwidth x end-to-end 
delay, and it likes low error-caused packet loss rates.   Thus, 
retransmission more than once on a link ( efforts that are >100% coding 
overhead) is probably too much effort to deal with link errors.   It's 
just as reasonable to use FEC on the link, which has an overhead far 
less than 100%.

It's sad that 802.11 frequently retries up to 256 times, but that's not 
comparable, exactly since it isn't bit errors, but arbitration errors 
that cause that.

Srinivasan Seshan wrote:
> Actually, how smart is not too hard to estimate - assuming that we are 
> really just doing this for TCP.
> As far as packet loss rates are concerned, the target probably 
> shouldn't be something fixed like 10^-3. It really depends on the link 
> speed. What you want to probably do is ensure that the packet 
> corruption rate is an order of magnitude less than the drop rate due 
> to congestion. You can get the congestion drop rate by estimating the 
> average RTT for flows and the raw speed of the link. Apply some TCP 
> modeling magic and you should be able to pull out a loss rate. Note 
> that I assumed a single flow here, which is the worst case. Multiple 
> flows will raise the congestion loss rate. So, if you can assume that 
> you have more flows, you can accommodate higher corruption loss rates.
>    Srini
> Detlef Bosau wrote:
>> David P. Reed wrote:
>>> Once the folks who ran IP networks over frame relay realized that 
>>> you should never provision reliable delivery if you were running IP, 
>>> this stopped happening.
>>> So the story is that GPRS can, if it tries to provide QoS in the 
>>> form of never dropping a frame, screw up TCP.
>>> But this has nothing to do with mobility per se.   It has to do with 
>>> GPRS, just as the old problems had to do with Frame Relay, not with 
>>> high speed data.   The architecture of the GPRS network is too smart.
>> How smart is "too smart"?
>> And how much smartness is necessary?
>> Some authors note that the IP packet delivery time in mobile networks 
>> is in fact a random variable, because the information rate in 
>> wireless networs sometimes changes several times _within_ one packet. 
>> The reasons are manifold and as a computer scientist, I have only a 
>> rough understanding of some of the relevant issues here.
>> To my understanding, the basic question is: Which packet corrution 
>> rate can be accepted by an IP network?
>> This is perhaps no fixed number but there is some tolerance in it. 
>> However, I think we can agree that a packet corruption rate less or 
>> equal to  10^-3 does not really cause grief. On the other hand, when 
>> the rate of successful transmussions is less or equal to 10^-3, the 
>> network is quite unlikely to be used.
>> So the truth is perhaps not out there but somewhere in between ;-)
>> Detlef

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