TCP Options (Cont.)

• Single-byte options

• • 0: End of options list, not necessary?
  • 1: No-op, used for padding

• Multiple-byte options

• • 2: Maximum segment size (MSS)
  • • 4 bytes, used for path MTU discovery
  • 3: Window scale
  • • 3 bytes, used to increase TCP’s sliding window beyond 16 bits
  • 4,5: SACK
  • • Length varies, used for Selective Acknowledgements
  • 8: Timestamp
  • • 10 bytes, used for round trip time (RTT) estimation and to detect wrapped sequence numbers
    • RTT is hard to measure accurately due to packet loss