传输层

• Application-level multiplexing("ports")

• Error detection, reliability,etc.

UDP

• Unreliable and unordered datagram service

• Adds multiplexing, checksum on whole packet

• No flow control,reliability, or order guarantees

• Endpoints identified by ports

• Checksum aids in error detection

Checksum algorithms

• Good checksum algorithms

  • Should detect errors that are likely to happen (E.g., should detect any single bit error)

  • Should be efficient to compute

• IP, UDP, and TCP use 1s complement sum:

  • Set checksum field to 0
  • Sum all 16-bit words in pkt
  • Add any carry bits back in(So 0x8000 + 0x8000 = 0x0001)
  • Flip bits (sum = ~sum;) to get checksum (0xf0f0 → 0x0f0f),Unless sum is 0xffff, then checksum just 0xffff
  • To check: Sum whole packet (including sum), should get 0xffff