MOBILE TCP

The M-TCP (mobile TCP)1 approach has the same goals as I-TCP and snooping TCP: to prevent the sender window from shrinking if bit errors or disconnection but not congestion cause current problems. M-TCP wants to improve overall throughput, to lower the delay, to maintain end-to-end semantics of TCP, and to provide a more efficient handover. Additionally, M-TCP is especially adapted to the problems arising from lengthy or frequent disconnections (Brown, 1997).

 M-TCP splits the TCP connection into two parts as I-TCP does. An unmodified TCP is used on the standard host-supervisory host (SH) connection, while an optimized TCP is used on the SH-MH connection. The supervisory host is responsible for exchanging data between both parts similar to the proxy in ITCP

The advantages of M-TCP are the following: 

● It maintains the TCP end-to-end semantics. The SH does not send any ACK itself but forwards the ACKs from the MH. 

● If the MH is disconnected, it avoids useless retransmissions, slow starts or breaking connections by simply shrinking the sender‟s window to 0. Since it does not buffer data in the SH as I-TCP does, it is not necessary to forward buffers to a new SH. Lost packets will be automatically retransmitted to the new SH. The lack of buffers and changing TCP on the wireless part also has some disadvantages: 

● As the SH does not act as proxy as in I-TCP, packet loss on the wireless link due to bit errors is propagated to the sender. M-TCP assumes low bit error rates, which is not always a valid assumption. 

● A modified TCP on the wireless link not only requires modifications to the MH protocol software but also new network elements like the bandwidth manager.