[e2e] TCP over mobile networks: Are there any problems left?

[András Veres (IJ/ETH)]

I would say yes and no. GPRS and UMTS spurious timeouts are not frequent because the estimated RTO is typically beyond the jitter induced by these technologies. And even if timeout kicks in, the bandwidth-delay product is not large, so speeding back does not take long time. Nevertheless, I would not simplify wireless "problems" solely to spurious timeouts or real losses.

There are potentially issues in future wireless that are unique to wireless and as speeds go higher may become problems. Currently I would not say that there will be problems, but I have not seen any detailed analysis on this topic. 

What issues are there? 

Mobility: handoffs may become quite frequent, e.g., once a minute. When the host goes between cells or APs, it has to reconnect, which may have some bad consequences. For example, there may be an outage time, or the available capacity in the target cell may be drastically different from the originating cell due to different technology or simply due to different traffic in the cells. Imagine a mobile with a download rate of 100Mbps entering a cell with just 10Mbps free capacity. Mobility may become also an issue in the fixed transport network, because traffic flows may be rerouted to different paths during mobility.

Capacity/coverage: In wired networks capacity changes are not typical. In wireless networks capacity is not constant, but change with time and place. Your WLAN connection may fluctuate quite heavily depending on how you place your hand or body...

Today the access is the typical bottleneck (and the server side as second.) in DSL, but with wireless becoming more common, sharing between several people (1 - 100?) will also become common. So a congestion control protocol that is good alone on a high speed link, may not be so good on an access link shared by (1 - 100?) users.

There have been some nice work on how TCP performs under dynamic conditions (at Stanford for example), and the results show quite mixed performance. I am missing very much a good, widely accepted evaluation metric of TCP goodness... Then we could have an agreed upon analysis method to use for wireless or fixed simulations and tell whether there are going to be serious problems or not.

Andras






>>

This question may sound somewhat stupid, but after having read about
quite a few claimed "problems" with TCP over mobile network, I´m finally
not sure whether there exist any or not.

As one example, the frequently discussed spurious timeouts turn out to
be a myth, when we look at 
"TCP Spurious Timeout estimation in an operational GPRS/UMTS network" by
Francesco Vacirca, Thomas Ziegler and Eduard Hasenleithner.

I´m somewhat puzzled here. I read about alleged problems here in quite a
few papers for years, but when I have a closer look
at them, most of the alleged problems are not really sound.

So, I tend to say provokingly: "TCP over mobile networks works just fine
as it is. There are no known problems left."
Is this true? Or am I wrong here?
-- 
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