5.5 Summary

This chapter has described four very different end-to-end protocols. The first protocol we considered is a simple demultiplexer, as typified by UDP. All such a protocol does is dispatch messages to the appropriate application process based on a port number. It does not enhance the best-effort service model of the underlying network in any way—it simply offers an unreliable, connectionless datagram service to application programs.

The second type is a reliable byte-stream protocol, and the specific example of this type that we looked at is TCP. The challenges faced with such a protocol are to recover from messages that may be lost by the network, to deliver messages in the same order in which they are sent, and to allow the receiver to do flow control on the sender. TCP uses the basic sliding window algorithm, enhanced with an advertised window, to implement this functionality. The other item of note for this protocol is the importance of an accurate timeout/retransmission mechanism. Interestingly, even though TCP is a single protocol, we saw that it employs at least five different algorithms—sliding window, Nagle, three-way handshake, Karn/Partridge, and Jacobson/Karels—all of which can be of value to any end-to-end protocol.

The third type of transport protocol we looked at is request/reply protocols that form the basis for RPC. Such protocols must dispatch requests to the correct remote procedures and match replies to the corresponding requests. They may additionally provide reliability, such as at-most-once semantics, or support large message sizes by message fragmentation and reassembly.

Finally, we looked at transport protocols for the class of applications that involve multimedia data (such as audio and video) and that have a requirement for real-time delivery. Such a transport protocol needs to provide help with recovering the timing information of a single media stream and with synchronizing multiple media streams. It also needs to provide information to upper layers (e.g., the application layer) about lost data (since there will normally not be enough time to retransmit lost packets) so that appropriate application-specific recovery and congestion avoidance methods can be employed. The protocol that has been developed to meet these needs is RTP, which includes a companion control protocol called RTCP.

Further Reading

There is no doubt that TCP is a complex protocol and that in fact it has subtleties not illuminated in this chapter; therefore, the recommended reading list for this chapter includes the original TCP specification. Our motivation for including this specification is not so much to fill in the missing details as to expose you to what an honest-to-goodness protocol specification looks like. The next paper, by Birrell and Nelson, is the seminal paper on RPC. Third, the paper by Clark and Tennenhouse on protocol architecture introduced the concept of Application Layer Framing which inspired the design of RTP; this paper provides considerable insight into the challenges of designing protocols as application needs change.

  • USC-ISI. Transmission Control Protocol. Request for Comments 793, September 1981.

  • Birrell, A., and B. Nelson. Implementing remote procedure calls. ACM Transactions on Computer Systems 2(1):39-59, February 1984.

  • Clark, D., and D. Tennenhouse. Architectural considerations for a new generation of protocols. Proceedings of the SIGCOMM '90 Symposium, pages 200-208, September 1990.

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