UDP

• Source port, destination port
• Length
• Checksum
  • Add all 16 bit integers
  • Drop overflow
  • Flip bits

TCP

• Point-to-point
• Reliable, in-order
• Full duplex (bi-directional)
• Connection oriented
• Flow control

Structure

• 20 byte basic header
  • source, dest port
  • sequence number
  • acknowledgement number
  • etc.

Reliable Data Transfer (rdt)

• Single sender, receiver
• Various misbehaviors
  • Data corruption
  • Data lost
  • Out-of-order
  • Duplicate copies
  • Long delayed delivery
• Develop sender, receiver
  • Consider only unidirectional
  • States, actions
• RDT 1.0, assuming reliable channel
• RDT 2.0, channel with bit errors
  • How to detect/recover from errors?
    • Checksum
    • ACK (packet received)
    • NAK (packet not recieved)
    • Send retransmits upon NAK
  • But what if ACK or NAK are corrupted?
  • We don’t know whether to retransmit or not
  • Solution: SEQ
  • 1 bit SEQ is enough (0 again, 1 new)
  • NAK-free: send ACK for last received packet
• RDT 3.0, channel with bit errors and loss
  • Just timeout, resend
  • What if duplicate (just heavily delayed), SEQ takes care