1. Wireless Transport

2. 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)?

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

4. 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

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

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

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

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

9. 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)

10. 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?)

11. 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?)

12. 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

13. 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

14. 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)

15. 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

16. 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)

17. 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

18. 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 …

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

20. 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

21. 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

22. 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

23. 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)

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

25. 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

26. 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

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

28. 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

29. 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

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

31. 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

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

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

34. 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?