Subject: Mobile Communication and Computing

Topic: Mobile Network

Difficulty: High

![Snooping TCP The main drawback of l-TCP is the segmentation of the single TCP connection into two TCP connections, which loses the original end-to-end TCP semantic. A new enhancement, which leaves the TCP connection intact and is completely transparent, is Snooping TCP. The main function is to buffer data close to the mobile host to perform fast local re transmission in case of packet loss.

Here, the foreign agent buffers all packets with destination mobile host and additionally 'snoops' the packet flow in both directions to recognize acknowledgements. The foreign agent buffers every packet until it receives an acknowledgement from the mobile host. If the FA does not receive an acknowledgement from the mobile host within a certain amount of time, either the packet or the acknowledgement has been lost. Alternatively, the foreign agent could receive a duplicate ACK which also shows the loss of a packet. Now, the FA re-transmits the packet directly from the buffer thus performing a faster re-transmission compared to the CH. For transparency, the FA does not acknowledge data to the CH, which would violate end-to-end semantic in case of a FA failure. The foreign agent can filter the duplicate acknowledgements to avoid unnecessary re-transmissions of data from the correspondent host.

If the foreign agent now crashes, the time-out of the correspondent host still works and triggers a re-transmission. The foreign agent may discard duplicates of packets already re-transmitted locally and acknowledged by the mobile host. This avoids unnecessary traffic on the wireless link.

For data transfer from the mobile host with destination correspondent host, the FA snoops into the packet stream to detect gaps in the sequence numbers of TCP. As soon as the foreign agent detects a missing packet, it returns a negative acknowledgement (NACK) to the mobile host. The mobile host can now re-transmit the missing packet immediately. Reordering of packets is done automatically at the correspondent host byTCP.

Advantage of snooping TCP

1. The end-to-end TCP semantic is preserved.
2. Most of the enhancements are done in the foreign agent itself which keeps correspondent host unchanged.
3. Handover of state is not required as soon as the mobile host moves to another foreign agent. Even though packets are present in the buffer, time out at the CH occurs and the packets are transmitted to the new COA.
4. No problem arises if the new foreign agent uses the enhancement or not. If not, the approach automatically falls back to the standard solution.

Disadvantages of snooping TCP

1. Snooping TCP does not isolate the behavior of the wireless link as well as l-TCP. Transmission errors may propagate till CH.
2. Using negative acknowledgements between the foreign agent and the mobile host assumes additional mechanisms on the mobile host. This approach is no longer transparent for arbitrary mobile hosts.
3. Snooping and buffering data may be useless if certain encryption schemes are applied end- to-end between the correspondent host and mobile host. If encryption is used above the transport layer, (eg. SSL/TLS), snooping TCP can be used. ][1]