Wireless Transport

Puzzle Assume A & B A flips a coin. If the result is heads, A multiplies 2 90 digit prime numbers, and if the result is tails, A multiplies 3 60 digit prime numbers A gives B the result of the multiplication B calls the toss A gives B the numbers How can a toss be called over the phone (without requiring trust)?

Recap TCP/IP/MAC Overview of wireless systems Wireless MAC Wireless Scheduling Mobility Support Wireless Transport …

Outline TCP over wireless networks TCP assumptions Wireless network characteristics Impact on TCP performance Approaches to improve TCP performance Link layer approaches TCP-aware link layer approaches Split connection approaches End-to-end approaches TCP over satellite and ad-hoc networks

TCP Congestion control Window based Slow start LIMD Congestion detection Self-clocking Window limitation

TCP (Contd.) Reliability Cumulative ACKs DUPACKs and Timeouts Timeout calculation RTTavg + 4 * RTTmdev Coupling between congestion control and reliability

Wireless Networks Wide-area wireless Metropolitan-area wireless Local-area wireless Ad-hoc wireless Satellite wireless

Wireless Characteristics Low bandwidths 10-20Kbps WWANs, 1-10Mbps WLANs, 100-500Kbps satellite Random wireless losses Upto 10% packet loss rates Hand-offs Depending on cell coverage and user mobility (30m/s in an 802.11 network will result in a hand-off every 10-15 seconds)

Wireless Characteristics (Contd.) Black-outs Fading, temporary disconnections etc. Can last from a few seconds to less than a minute Large and Varying Delay WWANs have typical RTTs of 400ms and deviations of up-to a few seconds Path asymmetry Reverse path characteristics different from forward path characteristics (e.g. satellite, WWANs)

Characteristics (Contd.) Local-area wireless Frequent hand-offs when mobile Ad-hoc wireless Routers mobile! Frequent disconnections Network partitions? Satellite wireless Large bandwidth delay products (why?)

TCP on Wireless – Random Losses TCP uses losses as indication of congestion Reduces congestion window by half (LIMD) when it experiences congestion Even when no congestion, if packets are dropped due to random losses, TCP will cut down its rate (is this right?)

Other Losses TCP will interpret hand-offs related losses also as congestion based losses Hence, it will reduce the congestion window every time hand-offs related losses occur Black-outs will further result in TCP experiencing multiple timeouts of increasing granularity

Large and Varying Delay TCP uses RTTavg + 4 * RTTmdev as the retransmission timeout If there is large variance in delay, mean deviation is high resulting in inflated timeout values Hence, if there are burst losses resulting in a timeout, the sender would take longer time to detect losses and recover

Path Asymmetry TCP relies on ACK arrivals for congestion window progression If path asymmetry exists, a TCP connection’s performance will be influenced by the reverse path characteristics also Indirect effects of path asymmetry (ACK bunching)

Low Bandwidths TCP uses window based congestion control If there is free space in the congestion window, TCP will transmit Hence, TCP’s output can be bursty This coupled with the low bandwidths can result in queue build-ups in the network adversely affecting RTT calculations and causing packet drops

Large Bandwidth Delay Products TCP’s header has 16 bits allocated for receiver window advertisement Maximum of 64KB can be advertised Consider a satellite link with 1Mbps bandwidth and 1 second RTT (BDP = 1Mb) But, TCP can only achieve 500Kbps (resulting in only 50% utilization)

Slow-start TCP uses slow-start to ramp up rate to the available capacity Whenever timeouts occur, TCP uses slow-start If hand-offs, black-outs, or route re-computations are frequent, TCP will constantly be in slow-start, lowering performance

Recap TCP over wireless networks Several factors contribute to the performance degradation of TCP when used in a wireless environment Approaches to improve TCP performance …

Approaches Reliable link layers TCP-aware link layers Split connection protocols

Reliable Link Layers Help in recovering from losses that occur on the wireless link Can potentially hide such losses from the TCP layer Can be implemented without requiring any changes at all to the sender and the receiver

Reliable Link Layer (contd.) Losses can still occur Retransmission can interfere with TCP retransmissions worsening the performance TCP timeouts similar LL timeouts DUPACKs Can introduce variations in TCP’s rtt estimation increasing the RTO

Snoop Module Resides in the base-station Caches packets sent from fixed host to mobile host TCP-aware functionality for retransmissions, and ACK suppression Improves on the performance of reliable link layers

Snoop (Contd.) Two modules: Snoop_data() – for processing data packets on the forward path (from FH – MH) Snoop_ACK() – for processing ACK packets on the reverse path (from MH – FH)

Snoop_data() 3 scenarios Normal packet in sequence Cache packet Forward to MH Timestamp if necessary (once per window) – for RTOs

Snoop_data() Scenario 2 Out of sequence (S) but already cached If highestACK < S Forward packet Else Generate an ACK from the snoop module for the highestACK

Snoop_data() Scenario 3 Out of sequence (S) and has not been cached earlier Either out-of-order or packet that was lost earlier Heuristically assume retransmission Packet forwarded, and marked as having been retransmitted due to congestion loss

Snoop_ACK() Scenario 1 New ACK Spurious ACK Clean snoop cache Round-trip estimate updated Spurious ACK ACK# less than highestACK# Discard ACK

Snoop_ACK() Duplicate ACK (DUPACK) If sender retransmitted packet or packet not in cache, forward ACK to sender Unexpected DUPACK (loss between BS and MH) Retransmit packet Keep track of maximum number of relevant DUPACKs possible Expected DUPACKs Suppress

Mobile host to Fixed host Cannot use only base-station based mechanisms Need to change the mobile host NACKs from the base-station to the mobile-station Mobile-station retransmits immediately and does not perform window reduction for NACKed losses

Split Connection Approaches Fixed Host Base Station Mobile Host TCP Wireless aware TP

Advantages Wireless aware congestion control and flow control Wireless link characteristics completely decoupled from the progress of TCP on the wired leg Better service to mobile applications possible Potentially simpler protocol stack at mobile Improved performance

Cons Application layer re-linking End-to-end semantics Software overheads Hand-off latency

Recap Wireless networks and TCP Reliable link layers Snoop module Indirect TCP (split connection) WTCP …

Puzzle You have a deck of 52 cards You draw out 5 cards randomly and look at the cards You can now show 4 of the cards to a friend, and the friend should identify the 5th card How do you do this?