This section focuses on the effects of mobility on the transport layer. The scope of this section includes the effects of the transport layer in a mobile environment. The transport layer provides end-to-end connectivity. Note that the UDP/IP service allows dropped or unordered packets, while the TCP/IP service guarantees packet delivery. An open issue concerns whether TCP should be terminated at the EN on the fixed network side, allowing for a modified and more robust protocol across the wireless link. Initially, TCP/IP will not be terminated at the EN, but continue end-to-end.
Because of congestion control, current TCP implementations may suffer unacceptable delays during handoffs . The handoff delay will cause the TCP window size to drop, a slow start algorithm will begin, and the retransmission timer will will be set to a backoff interval that doubles with each consecutive timeout. The result is much larger drop in throughput and delay than simply the time required to handoff. The Virtual Network Configuration algorithm, discussed in , provides a solution to this problem. Because the network configuration system runs ahead of real time, throughput should be as least as good as that shown for overlapping cells in .
A solution to help ease congestion of TCP/IP over ATM is given in . TCP/IP headers are compressed using a differential method first developed by Van Jacobsen.