Snooping TCP

Snooping TCP

• The access point snoops into the traffic and buffers packets for fast re-transmission.
• Transparent extension of TCP within the foreign agent
• Changes of TCP only within the foreign agent
• Buffering of packets sent to the mobile host
• Lost packets on the wireless link (both directions!) will be retransmitted immediately by the mobile host or foreign agent, respectively (so called “local” retransmission)
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  The foreign agent therefore “snoops” the packet flow and recognizes acknowledgements in both directions, it also filters ACKs
• Data transfer to the mobile host
• FA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated ACKs or time-out
• Fast retransmission possible, transparent for the fixed network
• Data transfer from the mobile host
• FA detects packet loss on the wireless link via sequence numbers, FA answers directly with a NACK to the MH
• MH can now retransmit data with only a very short delay

Advantages

• End-to-end semantics is preserved.
• Handover is easy. I-TCP requires a careful handover of the system state. Here it falls back to the standard solution if no enhancements.

Problems

• Snooping TCP does not isolate the wireless link as good as I-TCP
• Snooping might be useless depending on encryption schemes
• Data is transmitted twice in case of a paket loss. Once from the FA to the MH and the second time when the ACK finally reaches the CN.