UDP and TCP

UDP

• unreliable, just add de-multiplexing and error checking on data than IP.
• Best effort datagram(数据报) service
• Multiplexing enables sharing of IP datagram service
• Simple transmitter & receiver
  • Connectionless: no handshaking & no connection state
  • Low header overhead
  • No flow control, no error control, no congestion control
  • UDP datagrams can be lost or out-of-order
• Applications
  • multimedia (e.g. RTP)
  • network services (e.g. DNS, RIP, SNMP)

UDP Datagram

• 0-255
  • Well-known ports
• 256-1023
  • Less well-known ports
• 1024-65536
  • Ephemeral(短暂的) client ports
• Source and destination port numbers
  • Client ports are ephemeral(短暂的)
  • Server ports are well-known
  • Max number is 65,535
• UDP length
  • Total number of bytes in datagram (including header)
  • 8 bytes ≤ length ≤ 65,535
• UDP Checksum
  • Optionally detects errors in UDP datagram(Because the error packet will be discard)

UDP De-Multiplexing(解复用)

• All UDP datagrams arriving to IP address B and destination port number n are delivered to the same process
• Source port number is not used in demultiplexing
  

UDP Checksum Calculation

• UDP checksum detects for end-to-end errors
• Covers pseudoheader(伪报头) followed by UDP datagram
• IP addresses included to detect against** misdelivery(错误传输)**
• The use of UDP checksums is optional
• But hosts are required to have checksums enabled

TCP

• Reliable byte-stream service
• More complex transmitter & receiver
  • Connection-oriented: full-duplex(全双工) unicast connection between client & server processes
  • Connection setup, connection state, connection release
  • Higher header overhead
  • Error control, flow control, and congestion(拥塞) control
  • Higher delay than UDP
• Most applications use TCP
  • HTTP, SMTP, FTP, TELNET, POP3, …

TCP Multiplexing

• A TCP connection is specified by a 4-tuple(数组)
  • (source IP address, source port, destination IP address, destination port)
• TCP allows multiplexing of multiple connections between end systems to support multiple applications simultaneously
  

Reliable Byte-Stream Service

• Stream Data Transfer：transfers a contiguous stream of bytes across the network, with no indication of boundaries
• groups bytes into segments(部分)
• transmits segments as convenient (Push function defined)
  Reliability: error control to deal with IP transfer impairments(损害)

TCP Segment Format

TCP Header

Window Size：TCP protocol need ACK to ensure the packet be transported, if we receive ACK after every packet arriving, it will waste a lot of time.So we need the Window Size to tell us how many packet we can send one time.

• 16 bits to advertise window size
• Used for flow control
• Sender will accept bytes with SN from ACK to ACK + window
• Maximum win size 65535 bytes
• TCP Checksum
• Internet checksum method
• TCP pseudoheader + TCP segment