TCP in a wireless environmentCS 215 Winter 2001

• TCP and wireless handoff• TCP Snoop for wireless loss protection• TCP and MAC layer interaction

Caceres, Mobile Handoff

• MH switches to new cell when it hears “beacon”• To switch cells, MH sends “greeting packet” to

new MSS, and changes own routing table to it• MH then requests new MSS to notify old MSS of

handoff• Only then will old MSS forwards pkts (from SH) to

new MSS• TCP packets are lost in the transition:

(a) MH sends to old (unreachable) MSS until it gets beacon from new MSS

(b) Old MSS forwards to unreachable MH

Caceres, Mobile Handoff

• Zero Rendez-vous delay (instant beacon after cell crossing): a full window of TCP packets is lost because of control packet exchange latencies

• One timeout follows• 1 sec Rendez-vous delay: two consecutive

timeouts

Performance impact:• throughput degradation• Interactive response latency (pauses can last up

to several seconds after the handoff completes• very annoying in wireless indoor LANs with

frequent handoffs ( eg, 10 m Bluetooth spacing)

Caceres, Mobile Handoff

• Possible remedies to the “hard handoff” problem:

• Smooth handoff: the old MSS buffers packets and retransmits them to new MSS upon notification (similar to TCP Snoop,discussed next)

• Fast retransmissions: • upon noticing switchover, the MH transmits 3

duplicate Acks corresponding to the last received packet ( Mobile IP notifies TCP in the MH ..)

• the SH TCP is thus “tricked” into fast retx, skipping time out; latency reduced from 1 s to 50 ms