Communication Networks (8)

Chapter 3: Transport Layer

Transport services and protocols

• transport protocols run in end systems
• send side: breaks app messages into segments, passes to network layer
• rcv side: reassembles segments into messages, passes to app layer
• TCP / UDP

TCP and UDP:

• TCP: reliable, in-order delivery
• UDP: unreliable, unordered delivery

Transport vs. Network Layer

• network layer: logical communication between hosts
• transport layer: logical communication between processes

Multiplexing/Demultiplexing

• multiplexing at sender
• demultiplexing at receiver

host uses IP addresses & port numbers to direct segment to appropriate socket

TCP/UDP segment format

In Connectionless Demultiplexing (UDP)

In UDP, IP datagrams with same dest. port #, but different source IP addresses and/or source port numbers will be directed to same socket at dest.

In Connection-Oriented Demux (TCP)

TCP socket identified by 4-tuple:

• source IP address
• source port number
• dest IP address
• dest port number

demux: receiver uses all four values to direct segment to appropriate socket

• server have different sockets for each connecting client
• non-persistent HTTP will have different socket for each request

UDP: User Datagram Protocol [RFC 768]

• UDP segments may be: lost or delivered out-of-order to app
• connectionless
  • no handshaking between UDP sender, receiver
  • each UDP segment handled independently of others
• used in
  • streaming multimedia apps (loss tolerant, rate sensitive)
  • DNS
  • SNMP

UDP segment format

checksum is used to detect errors.