Transmission Control Protocol (tcp)

TCP

TCP (Transmission Control Protocol) is one of the main protocols in TCP/IP networks. Whereas the IP protocol deals only with packets, TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent.

You work for AT&T; your client hosts connect directly to your routers. You know some hosts are using experimental TCPs and suspect some may be using a "greedy" TCP with no congestion control. What measurements might you make at your router to establish that a client was not using slow start at all? Whenever a TCP segment has been sent and the sender's retransmission timer expires, the sender is forced to retransmit the segment, which the sender assumes has been lost. However, it is possible that between the time when the segment was initially sent and the time when the retransmission window expired, other segments in the window may have been sent after the lost segment. It is also possible that these later segments arrived at the receiver and is simply queued awaiting the missing segment so they can be properly re-ordered. The receiver has no way of informing the sender that it has received other segments because of the requirement to acknowledgement only the contiguous bytes it has received.

There are a number of techniques which are worth the time to examine even though they are not directly implemented within TCP software. For example, whenever a router drops a packet, it in effect is providing a signal to the TCP sender to slow down by causing a retransmission timer to expire. If routers could use some advanced pocket drop techniques, they may be able to better control network congestion through the implicit signals TCP senders detect. Also, by implementing a system where a cost is associated with network transmission, end stations may adjust their transmission rate up or down based on the value of performance they may require (Kristoff, 2000).

The goal is to provide some level playing field for all participants, and to avoid the greedy or "eager" TCP senders to make room for low bandwidth connections.

References

Kristoff,

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