6610-l15- - TCP over Wireless Networks Click to edit Master...

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Click to edit Master subtitle style 11 TCP over Wireless Networks
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Puzzle o o A always speaks the truth, and believes all true propositions (say 2+2=4) to be true, and all false propositions (say 2+2=3) to be false o B always lies, and believes all true propositions to be false, and all false propositions to be true o You meet one of the twins. How many questions do you need to identify which twin he is?
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33 Recap o TCP over wireless networks n Wireless losses n Other losses n Large and varying delay n Path asymmetry n Low bandwidths n Large bandwidth delay product n Slow start o Approaches to improve TCP performance …
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44 Approaches o Reliable link layers o TCP-aware link layers o Split connection protocols
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55 Reliable Link Layers o Help in recovering from losses that occur on the wireless link o Can potentially hide such losses from the TCP layer o Can be implemented without requiring any changes at all to the sender and the receiver
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66 Reliable Link Layer (contd.) o Losses can still occur o Retransmission can interfere with TCP retransmissions worsening the performance n TCP timeouts similar LL timeouts n DUPACKs o Can introduce variations in TCP’s rtt estimation increasing the RTO
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77 Snoop Module o Resides in the base-station o Caches packets sent from fixed host to mobile host o TCP-aware functionality for retransmissions, and ACK suppression o Improves on the performance of reliable link layers
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88 Snoop (Contd.) o Two modules: n Snoop_data() – for processing data packets on the forward path (from FH – MH) n Snoop_ACK() – for processing ACK packets on the reverse path (from MH – FH)
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99 Snoop_data() o 3 scenarios n Normal packet in sequence o Cache packet o Forward to MH o Timestamp if necessary (once per window) – for RTOs
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1010 Snoop_data() o Scenario 2 n Out of sequence (S) but already cached o If highestACK < S n Forward packet o Else n Generate an ACK from the snoop module for the highestACK
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1111 Snoop_data() o Scenario 3 n Out of sequence (S) and has not been cached earlier o Either out-of-order or packet that was lost earlier o Heuristically assume retransmission o Packet forwarded, and marked as having been retransmitted due to congestion loss
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1212 Snoop_ACK() o Scenario 1 n New ACK o Clean snoop cache o Round-trip estimate updated n Spurious ACK o ACK# less than highestACK# o Discard ACK
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1313 Snoop_ACK() o Duplicate ACK (DUPACK) n If sender retransmitted packet or packet not in cache, forward ACK to sender n Unexpected DUPACK (loss between BS and MH) o Retransmit packet o Keep track of maximum number of relevant DUPACKs possible n Expected DUPACKs o Suppress
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1414 Mobile host to Fixed host o Cannot use only base-station based mechanisms o Need to change the mobile host o NACKs from the base-station to the mobile- station o Mobile-station retransmits immediately and does not perform window reduction for NACKed losses
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1515 Split Connection Approaches Fixed Host Base Station Mobile Host TC P Wireless aware TP
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This note was uploaded on 12/14/2009 for the course ECE 6610 taught by Professor Staff during the Fall '08 term at Georgia Tech.

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6610-l15- - TCP over Wireless Networks Click to edit Master...

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