RDT Caveats

A Web-Based Introduction to Computer Networks for Non-Majors

Reliable Data Transfer Protocols

Caveats and References

This applet illustrates some of the key aspects of how reliable data transfer can be implemented at the transport layer in the protocol stack. In the Internet protocol stack, there are two transport layer protocols: the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). Of these only TCP guarantees reliable data transfer. The reliable data transfer protocol that TCP uses is a pipelined protocol that is a combination of Selective Repeat and Go-Back-N. Thus, the applet only approximates the specifies of how TCP implements reliable data transfer. The references below provide more detail about how TCP implements reliable data transfer. Here are a few of the simplifications.

The sequence number used in the acknowledgment packets is not actually the number of the next expected data packet. Instead the bytes sent in each packet is kept track of and the sequence number used is the number of the expected first byte of the next data packet.
The trigger of retransmission is not necessarily a pure timeout. For example, a certain number, say three, of duplicate acknowledgment packets received in a row could trigger a retransmission.
An error case that the applet does not consider is when a data packet or acknowledgement is neither lost nor corrupted but is delayed such that that data packet's timout timer expires. In this case, the data packet is retransmitted and the receiver resends a duplicate acknowledgement.
TCP has a number of features for flow control; that is, stopping the sender from sending data packets if the network or the receiver are too busy. The flow control features combine with error control to determine when the sender sends data packets or retransmits data packets.

References

[1] L. Cassel and R. Austing, Computer Networks and Open Systems, An Applications Perspective, Jones and Bartlett, 2000.

[2]J.F. Kurose and K.W. Ross, Computer Networking: A Top-Down Approach Featuring the Internet, Second Edition, Addison Wesley, 2003.

[3] L.L. Peterson and B.S. Davie, Computer Networks: A Systems Approach, Third Edition, Morgan Kaufmann, 2003.

Acknowledgements and Developer Information

Current Version Date: February 2004