Thursday, October 2, 2008

Modeling Wireless Links for Transport Protocols

Design and evaluation of transport protocols can be improved by using good models for wireless links that provide control over realism, generality and detail. Despite the vast differences in the different wireless technologies compared to the wired networks, the paper argues that it is still advantageous to use the same protocol stack as it enables interoperability. For example, delay variation models vary depending on real-world scenarios and have a strong impact on the transport protocol’s functioning. This paper considers WLAN, cellular and satellite links – wireless technologies used in practice. These have different characteristics in terms of latency and throughput, and are preferred in different settings because of their range.

The paper devotes a section to explaining the flaws in the current simulation models and parameters. It points out very important points about packet loss rate, retransmission timer, TPC flavors etc. While I am not well-versed with the references in this paper, I do sometimes feel that some “flawed” models are totally good enough for the particular problem in hand. If a certain aspect of the model is unrealistic, then probably the authors of the model have concluded with good reasoning that aspect is not expected to influence the results. On the other hand, having a model with all the knobs inevitably results in people not knowing how to set the values of the knobs accurately. For wide usage, some pre-built models have to be provided and it would obviously come along with the risk of being unrealistic in certain cases.

Error-based losses, delay variation, packet reordering, on-demand resource allocation (and associated latencies), bandwidth variation and asymmetry in bandwidth and latency are the characteristics of wireless links considered. The paper also talks about queue management and mobility modeling. The paper says that modeling handoffs and their associated delays etc. will be a more realistic model in the way wireless networks will be adopted (I agree!).

It finally gets to the chicken-and-egg equivalent problem in wireless networks: do transport protocols change or link layer technologies change? I find this to be a dangerous argument because if link layer technologies change to become more reliable (and essentially TCP-ish), then the very purpose of a transport protocol seems lost! Also, this is totally unfair to applications that bank on UDP for performance reasons. Clouding link-layer technology development with views from the TCP-world is not good.

1 comment:

Akhil Dhar said...

Link-layer technologies do not have to become more reliable; they merely have to maintain compatibility with transport protocols such as TCP. For example, suppressing corruption-related duplicate packets before congestion control reacts should not interfere with UDP operation. That said, the layering model is delicate and there is definitely a fine line here.