TCP over Wireless Link

1
TCP over Wireless Link

• Tzi-cker Chiueh

2
TCP Basics

• Abstraction Reliable in-order byte stream
• Key mechanisms
• Retransmission
• Reassembly
• Flow control
• Congestion control

3
TCP Congestion Control

• Its all about congestion window
• AIMD principle
• Slow start (multiplicative)
• Congestion avoidance (additive)
• Fast retransmit
• Fast Recovery

4
Newer Congestion Control Mechanisms (1)

• Limited transmit keep as many ACKs in the
  pipeline as possible to avoid spurious timeout
• SACK selectively ACK packets that arrive at the
  receiver out of order
• D-SACK inform the sender that duplicated packets
  are received ? symptom of spurious retransmit
  timeout or fast retransmit
• Restore congestion window and its operating mode
• Adjust duplicate ACK threshold or timeout value

5
Newer Congestion Control Mechanisms (2)

• Packet loss Congestion?
• Explicit congestion notification (ECN)
• Trigger fast retransmit before 3 consecutive
  duplicate ACKs
• Explicit loss (corruption) notification (ELN)
• Undo congestion window modification
• Active queue management
• Random early discard vs. drop-tail
• Link utilization, tolerance of packet burst
• Wasted bandwidth and high RTT deviation

6
Performance Problems of TCP on Wireless Links

• Packet loss is not necessarily an indication of
  congestion, and therefore congestion window
  should not be decreased
• Solutions
• No modification to the end points TCP stack
• Closer interaction between transport layer and
  the network/link layer

7
Wireless Link Error Characteristics

• WaveLAN (802.11)
• 1.55 Frame Error Rate (FER) for 1400-byte frames
  over a 85-ft distance bit errors tend to be
  bursty
• Half FER when the packet size is reduced to 300
  bytes
• Mobility increases FER by about 30
• TCP throughput is significantly lower (22)
  because
• ACK traffic
• Incorrect congestion window adjustment
• Hand-off delay

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Solutions (1)

• Transport layer approach
• Explicit handoff notification
• Shrink the receiver window
• Indirect TCP fooling the sender by splitting a
  TCP connection into two halves violation of
  TCPs end-to-end semantics
• Detection of spurious packet retransmission

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Solutions (2)

• Link layer approach
• Link-layer retransmission timer Snooping on
  duplicate ACKs local retransmission and
  suppression of duplicate ACKs
• Reliable link layer has to be able to finish
  link-layer transmission before fast
  retransmission is triggered

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Discussion

• What is the right thing to do when a wireless
  link is error-prone
• Transient
• Persistent is it congestion?
• What about wireless links in the middle of a path?

11
Network Asymmetry Effects

• Classification of network asymmetry
• Bandwidth asymmetry
• Media access asymmetry
• Loss rate asymmetry
• What is the problem
• TCP is ACK clocking
• No ACK, no progress Why ACK compression and
  bursty ACK loss cause problems

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Solutions (1)

• Let ACKs through on the return path
• TCP header compression
• ACK filtering
• Compression
• ACK congestion control
• Receiver-side ACK filtering
• ACK-first scheduling
• Prioritization

13
Solutions (2)

• Avoid adverse impacts of infrequent ACKs
• TCP sender adaptation
• Count the number of bytes ACKed rather than
  number of ACKs received
• ACK reconstruction
• Generate ACKs by interpolation
• Space out these reconstructed ACKs

14
TCP on 802.11-based Multi-hop Networks

• Link-layer retransmission failure ? route failure
  ? slow start even a single TCP connection can
  hurt itself (instability problem)
• Why transmission failure ?
• Inability to complete RTS/CTS transaction due to
  exposed node problem
• Unfairness in random backoffs
• Maximum window size makes a difference
• Interference range gt communication range

15
Unfairness from MAC to TCP

• Disjoint communicating node sets can still
  interfere with one another
• Inability to distinguish between repeated send
  failures and loss of connectivity
• Mostly happen to small frames control frames or
  small data frames