Saturday, September 12, 2009

10. Congestion Control for High Bandwidth-Delay Product Networks

This paper proposed a new protocol in replace of TCP. The new protocol, eXplicit Control Protocol (XCP), addressed the issues of TCP by putting a congestion level field called Explicit Congestion Notification Proposal (ECN) in the packet headers. This XCP protocol does not maintain a per-flow state in routers and thus, requires less CPU cycles in busy routers.

TCP doesn't react well with high bandwidth, high latency links such as satellite and wireless. Under such circumstances, TCP tends to be unstable and oscillate. For example, TCP sends one packet per RTT to get more bandwidth, and it can takes thousands of RTTs to make good use of the high speed network.

Robustness to congestion should be independent of number of flows, according to the authors. And packet loss shouldn't be a signal of network congestion. Thus, XCP introduces decoupling utilization control from fairness control, which adjusts its aggressiveness according to the spare bandwidth in the network and the feedback delay. It also has a per-flow control in the protocol, rather than in routers, to achieve fairness. Furthermore, XCP distinguishes error loss pack-dropping from congestion, which is quite useful on wireless connections.

Comments:
  1. I like the fact that in XCP, packet drops caused by congestion are highly uncommon because it separates drops caused by link errors from network congestion. These two factors should be viewed independently in the protocol and acted upon accordingly.
  2. Like every new stuff in our lives, backward compatibility is a big issue. XCP is just like IPv6 where everyone knows about it, but no one seems to be using it. Would it be possible to implement XCP concepts one level up in OSI model in order to be compatible with TCP?

1 comment:

  1. So the paper does suggest an adoption path by which TCP and XCP could co-exist with a probe packet going along the path to figure out if it's all XCP enabled or if there's a need to fall back to TCP. The main problem I see with this approach is that it takes only one router along the way to prevent XCP from being used, so it would be a long time before any actual XCP links were in operation.

    Still, we may yet see an explosion of IPv6, XCP links as the Internet continues to grow.

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