Content

1. UDP v.s. TCP
2. User Datagram Protocol (UDP)
   1. UDP Header
   2. Internet checksum

UDP v.s. TCP

• UDP
  • No guarantee on data delivery
  • No guarantee on arrival order of segments
  • Optional error checking
• TCP
  • Reliable data transfer
    • In order & correct data transfer
    • Error checking, acknowledgement, sequencing, retransmission
  • Flow control
    • Not overflowing buffer for receiving side
  • Congestion control
    • Not sending excessive data and overflow buffer for packet switches
  • Connection management
• Services not provided
  • Delay guarantees
  • Bandwidth guarantees

User Datagram Protocol (UDP)

• Minimal transport layer services provided
• Multiplexing & demultiplexing
  • Allow process-to-process delivery (different from raw IP host-to-host)
  • (destnation IP, destination port) for demultiplexing, not caring about source
• Connectionless
  • No connection setup needed
  • Less overhead managing the logical communication channel
  • Not able to precompute optimized data routes
• Independent data segment handled
  • Need to provide sender/receiver's address in the header of each segment
• Small UDP header
• No flow/congestion control

UDP Header

• length includes header
  • 8 bytes ≤ length ≤ 216

Internet checksum

• Sending side
  1. View the message of k-bit strings as a sequence of 16-bit strings, add with one's complement addition -> X
     • i.e. carryout added to least significant bit
  2. Take one's complement -> checksum ~X
  3. Append to original message -> M + ~X = M'
• Receiving side
  1. Add the message with one's complement addition -> M' = M + ~X = X + ~X = all 1s
  2. Take one's complement -> 0