计算机网络（五）

Reliable Transport Protocol Design

• Goal: Deliver a reliable, in-order byte stream over an unreliable IP network
• Key mechanisms:
  • Checksums → detect bit errors
  • Sequence numbers (byte offsets)
  • Sliding window for flow control
  • Cumulative acknowledgements (like GBN)
  • Receiver buffers out-of-sequence packets (like SR)
  • Single retransmission timer
  • Extra features: fast retransmit, timeout estimation algorithms

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TCP Abstraction

• Reliable: Retransmits lost packets until acknowledged or connection shutdown
• In-order delivery: Only delivers contiguous byte sequences to applications
• Byte-stream service: Treats data as a continuous stream, not discrete messages

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TCP Header Basics

• Minimum 20 bytes, fields include:
  • Source port, Destination port
  • Sequence number (first byte offset in the segment)
  • Acknowledgement number (next expected byte)
  • Header length (in 4-byte words)
  • Flags: SYN, ACK, FIN, RST, PSH, URG
  • Checksum (over pseudo-header and data)
  • Urgent pointer (optional)
• Options: e.g., MSS, window scaling

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Segmentation

• IP Packet: Cannot exceed MTU (1500B for Ethernet, 9000B for jumbo frames)
• TCP Segment: IP packet containing TCP header + TCP data
  • MSS = MTU – IP header – TCP header
  • Typically ≈ 1460B data payload

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Sequence Numbers & Acknowledgements

• Sequence number = first byte in segment
• Acknowledgement number = next expected byte
• Cumulative ACKs:
  • If all bytes up to X are received, ACK = X+1
  • If a packet is missing, ACK “stalls” at the missing sequence

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Example of Cumulative ACKs

Sent sequence numbers: 100, 200, 300, 400, 500, 600...

• If segment with seqno 500 is lost:
  • ACKs received: 200, 300, 400, 500, 500, 500...

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Fast Retransmit

• Triggered after 3 duplicate ACKs
• Retransmits missing packet early (before timeout)
• After retransmission:
  • Option 1: Advance window by number of dup ACKs (faster, riskier)
  • Option 2: Wait for ACK of retransmitted packet (slower, correct)
• TCP chooses correctness (Option 2)

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Retransmission Timeout (RTO)

• Challenges:
  • Too long → low throughput
  • Too short → redundant retransmissions
• Solution: RTO proportional to measured RTT

RTT Estimation

• Exponentially weighted average:

  EstimatedRTT = (1 - α) * EstimatedRTT + α * SampleRTT
  

• Typical α = 0.125

Karn/Partridge Algorithm

• Ignore SampleRTT for retransmitted packets
• Use exponential backoff on timeout
  • RTO = 2 × EstimatedRTT
  • On repeated timeouts: RTO ← 2 × RTO (up to ≥ 60s)

Jacobson/Karels Algorithm

• Includes RTT deviation for robustness:

  RTO = EstimatedRTT + 4 × Deviation
  

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TCP Connection Establishment

• Initial Sequence Number (ISN): Avoids old packet confusion, helps prevent spoofing
• Three-Way Handshake:
  1. Client → Server: SYN(ISN_A)
  2. Server → Client: SYN(ISN_B), ACK(ISN_A+1)
  3. Client → Server: ACK(ISN_B+1)

SYN Loss

• If SYN lost → no SYN-ACK arrives
• Client retransmits after default timeout (3–6 seconds)
• User may perceive delay (slow web page load)

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TCP Connection Teardown

• Graceful (FIN):
  1. One side sends FIN (consumes 1 sequence number)
  2. Other side ACKs
  3. Other side sends FIN
  4. First side ACKs → enters TIME_WAIT to avoid packet reincarnation
• Simultaneous FIN: Both send FIN nearly together → both close
• Abrupt termination (RST):
  • Sent when application crashes or rejects connection
  • Not acknowledged; any further traffic elicits another RST

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Comparison: Go-Back-N (GBN) vs Selective Repeat (SR) vs TCP

Feature	GBN	SR	TCP
Sequence numbers	Per packet	Per packet	Per byte offset
Acknowledgement	Cumulative ACK only	Selective ACK	Cumulative ACK (default); optional SACK
Receiver buffering	No (discards out-of-order)	Yes (stores out-of-order)	Yes (stores out-of-order like SR)
Retransmission timer	Conceptually one per packet	One per packet	Single timer (for oldest unACKed packet)
Retransmission strategy	Lost packet → retransmit that + all subsequent	Retransmit only lost packet	Retransmit like GBN, but adds Fast Retransmit
Efficiency	Low (1 loss = many retransmits)	High	Balanced: cumulative ACK + buffering + fast retransmit
Use case	Theory model	Theory model	Practical protocol

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Why GBN Needs a Timer for Each Packet

• GBN uses cumulative ACKs and does not buffer out-of-order packets
• If a packet in the middle is lost:
  • Receiver discards later packets
  • Sender cannot rely on ACK progress to detect which one was lost
  • Therefore, conceptually, each unACKed packet requires a timer
• TCP optimization:
  • Uses cumulative ACKs + buffers out-of-order packets
  • Adds fast retransmit (dup ACKs indicate which packet is lost)
  • Thus, TCP only needs one timer (for the oldest unACKed packet)

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Summary

• TCP combines ideas from GBN and SR, plus its own enhancements:
  • Like GBN: cumulative ACKs
  • Like SR: receiver buffers out-of-order packets
  • Unique: one retransmission timer, fast retransmit, adaptive timeout algorithms
• Connection establishment: three-way handshake
• Connection termination: FIN (graceful), RST (abrupt)
• Next topics: Flow control and Congestion control