This question appears in Mumbai University > Mobile Communication and Computing subject

Marks: 10 M

Year: Dec 2015

• With the advent of WLANs, a lot of research went into increasing the performance of TCP in wireless and mobile environments, some of its outcome are I-TCP and SNOOP-TCP, Mobile-TCP etc. Let us study Mobile-TCP.

• M-TCP (mobile TCP) has the same goals as similar to its variants i.e. I-TCP and Snoop-TCP. It too wants to improve overall throughput, to lower the delay, to main end-to-end semantics of TCP.

• But, it is mainly enhanced to address problems related to lengthy or frequent disconnections.

Basic TCP methodology:

• When a node does not receive an acknowledgement back form the host, it carries out retransmission.
• A TCP sender tries to retransmit data controlled by retransmission timer which doubles up with each unsuccessful attempt. (upto a maximum of one minute)
• A sender tries to retransmit an unacknowledged packet every one minute and gives up after 12 minutes.
• If in I-TCP, the mobile host is disconnected, then in such a situation, the FA will keep of buffering more and more data packets.
• In case of a handover following this disconnection, we have more data to be transmitted to new FA.
• Snoop-TCP also suffers from similar such problems.

Mobile TCP:

• The M-TCP splits up the connection into two parts:

1. An unmodified TCP is used on the Standard host-Supervisory Host section
2. An optimised TCP is used on the Supervisory Host- Mobile Host section.

• The Supervisory Host (SH) adorns the same role as the proxy (Foreign Agent) in I-TCP.
• The SH is responsible for exchanging data to both the Standard host and the Mobile host.
• Here in this approach, we assume that the error bit rate is less as compared to other wireless links.
• So if any packet is lost, the retransmission has to occur from the original sender and not by the SH. (This also maintains the end-to-end TCP semantic)
• The SH monitors the ACKs (ACK means acknowledgement) being sent by the MH. If for a long period ACKs have not been received, then the SH assumes that the MH has been disconnected (maybe due to failure or moved out of range, etc...).
• If so the SH chokes the sender by setting its window size to 0.
• Because of this the sender goes into persistent mode i.e. the sender’s state will not change no matter how long the receiver is disconnected.
• This means that the sender will not try to retransmit the data.
• Now when the SH detects a connectivity established again with the MH (the old SH or new SH if handover), the window of the sender is restored to original value.

Advantages:

• Maintains the TCP end-to-end semantics. (No failed packet retransmission is done by the SH .All job handled by original sender)

• Does not require the change in the sender’s TCP.

• If MH disconnected, it doesn’t waste time in useless transmissions and shrinks the window size to 0.

• No need to send old buffer data to new SH in case of handover (as in I-TCP).

Disadvantages:

• M-TCP assumes low bit error which is not always true. So, any packet loss due to bit-errors occurring, then its propagated to the sender.

• Modifications are required for the MH protocol software.