1. Impact of transmission errors on TCP performance

2. Tutorial Outline Wireless technologies TCP basics
Impact of transmission errors on TCP performance
Approaches to improve TCP performance
Classification
Discussion of selected approaches

3. Random Errors
If number of errors is small, they may be corrected by an error correcting code
Excessive bit errors result in a packet being discarded, possibly before it reaches the transport layer

4. Random Errors May Cause Fast Retransmit40 39 37 38 36 34
Example assumes delayed ack - every other packet ack’d

5. Random Errors May Cause Fast Retransmit41 40 39 38 34 36
Example assumes delayed ack - every other packet ack’d

6. Random Errors May Cause Fast Retransmit42 41 40 39 36 36
dupack
Duplicate acks are not delayed

7. Random Errors May Cause Fast Retransmit43 42 41 40 36 36 36
Duplicate acks

8. Random Errors May Cause Fast Retransmit44 43 42 41 36 36 36
3 duplicate acks trigger
fast retransmit at sender

9. Random Errors May Cause Fast Retransmit
Fast retransmit results in
retransmission of lost packet
reduction in congestion window
Reducing congestion window in response to errors is unnecessary
Reduction in congestion window reduces the throughput

10. Sometimes Congestion Response May be Appropriate in Response to Errors
On a CDMA channel, errors occur due to interference from other user, and due to noise [Karn99pilc]
Interference due to other users is an indication of congestion. If such interference causes transmission errors, it is appropriate to reduce congestion window
If noise causes errors, it is not appropriate to reduce window
When a channel is in a bad state for a long duration, it might be better to let TCP backoff, so that it does not unnecessarily attempt retransmissions while the channel remains in the bad state [Padmanabhan99pilc]

11. Various Schemes Link level mechanisms Split connection approach
TCP-Aware link layer
TCP-Unaware approximation of TCP-aware link layer
Explicit notification
Receiver-based discrimination
Sender-based discrimination
For a brief overview, see [Dawkins99,Montenegro99]

12. Split Connection Approach

13. Split Connection Approach
End-to-end TCP connection is broken into one connection on the wired part of route and one over wireless part of the route
A single TCP connection split into two TCP connections
if wireless link is not last on route, then more than two TCP connections may be needed

14. Split Connection Approach
Connection between wireless host MH and fixed host FH goes through base station BS
FH-MH = FH-BS BS-MH FH BS MH
Fixed Host
Base Station
Mobile Host

15. Split Connection Approach
Split connection results in independent flow control for the two parts
Flow/error control protocols, packet size, time-outs, may be different for each part FH BS MH
Fixed Host
Base Station
Mobile Host

16. Split Connection Approach
Per-TCP connection state
TCP connection
TCP connection
wireless
physical
link
network
transport
application
rxmt

17. Split Connection Approach Indirect TCP [Bakre95,Bakre97]
FH - BS connection : Standard TCP
BS - MH connection : Standard TCP

18. Split Connection Approach Selective Repeat Protocol (SRP) [Yavatkar94]
FH - BS connection : standard TCP
BS - FH connection : selective repeat protocol on top of UDP
Performance better than Indirect-TCP (I-TCP), because wireless portion of the connection can be tuned to wireless behavior

19. Split Connection Approach : Other Variations
Asymmetric transport protocol (Mobile-TCP) [Haas97icc]
Low overhead protocol at wireless hosts, and higher overhead protocol at wired hosts
smaller headers used on wireless hop (header compression)
simpler flow control - on/off for MH to BS transfer
MH only does error detection, BS does error correction too
No congestion control over wireless hop

20. Split Connection Approach : Other Variations
Mobile-End Transport Protocol [Wang98infocom]
Terminate the TCP connection at BS
TCP connection runs only between BS and FH
BS pretends to be MH (MH’s IP functionality moved to BS)
BS guarantees reliable ordered delivery of packets to MH
BS-MH link can use any arbitrary protocol optimized for wireless link
Idea similar to [Yavatkar94]

21. Split Connection Approach : Classification
Hides transmission errors from sender
Primary responsibility at base station
If specialized transport protocol used on wireless, then wireless host also needs modification

22. Split Connection Approach : Advantages
BS-MH connection can be optimized independent of FH-BS connection
Different flow / error control on the two connections
Local recovery of errors
Faster recovery due to relatively shorter RTT on wireless link
Good performance achievable using appropriate BS-MH protocol
Standard TCP on BS-MH performs poorly when multiple packet losses occur per window (timeouts can occur on the BS-MH connection, stalling during the timeout interval)
Selective acks improve performance for such cases

23. Split Connection Approach : Disadvantages
End-to-end semantics violated
ack may be delivered to sender, before data delivered to the receiver
May not be a problem for applications that do not rely on TCP for the end-to-end semantics FH MH BS 40 39 37 38 36

24. Split Connection Approach : Disadvantages
BS retains hard state
BS failure can result in loss of data (unreliability)
If BS fails, packet 40 will be lost
Because it is ack’d to sender, the sender does not buffer 40 FH MH BS 40 39 37 38 36

25. Split Connection Approach : Disadvantages
BS retains hard state
Hand-off latency increases due to state transfer
Data that has been ack’d to sender, must be moved to new base station FH MH BS 40 39 37 38 36
New base station
Hand-off

26. Split Connection Approach : Disadvantages
Buffer space needed at BS for each TCP connection
BS buffers tend to get full, when wireless link slower (one window worth of data on wired connection could be stored at the base station, for each split connection)
Window on BS-MH connection reduced in response to errors
may not be an issue for wireless links with small delay-bw product

27. Split Connection Approach : Disadvantages
Extra copying of data at BS
copying from FH-BS socket buffer to BS-MH socket buffer
increases end-to-end latency
May not be useful if data and acks traverse different paths (both do not go through the base station)
Example: data on a satellite wireless hop, acks on a dial-up channel
data FH MH
ack