Introduction to Transport Layer

1
Introduction to Transport Layer
2
Transport Layer Motivation
Web Browser
Web Server
FTP Client
B
FTP Server
A
R1
R2
C

• Recall that NL is responsible for forwarding a
  packet from one HOST to another HOST
• But it is the applications that communicate!
• How do you make applications on HOSTs to
  communicate?
• Need a new layer, called the Transport Layer
• Responsible for providing communication between
  applications running in different hosts
• A Web Browser talking to a Web Server

3
Transport services and protocols

• provide logical communication between app
  processes running on different hosts
• 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
• more than one transport protocol available to
  apps
• Internet UDP, TCP, SCTP

4
Transport vs. network layer

• Household analogy
• 12 kids sending letters to 12 kids
• processes kids
• app messages letters in envelopes
• hosts houses
• transport protocol Ann and Bill
• network-layer protocol postal service

• network layer logical communication between
  hosts
• transport layer logical communication between
  processes
• relies on, enhances, network layer services

5
Transport Layer Functions

• Demultiplexing to upper layer -- Obligatory
• Deliver an incoming packet to the correct
  application
• Define Delivery Sematics and Implement them
• Reliable vs. unreliable
• Unicast vs. multicast
• Ordered vs. unordered
• Flow control -- Optional
• Do not allow sender to overrun receivers buffer
  resources
• Congestion control -- Optional
• Do not allow the sender to overrun the network
  capacity

6
Internet transport-layer protocols

• User Datagram Protocol (UDP)
• unreliable (best-effort),
• unordered
• unicast or multicast delivery
• no flow, no congestion control
• Transmission Control Protocol (TCP)
• reliable
• in-order
• unicast
• flow congestion control
• Stream Control Transport Protocol (SCTP) (will
  not cover in class)
• RFC 2960
• reliable
• optional ordering
• unicast
• flow congestion control

7
Multiplexing/demultiplexing Why?

• Why need demultiplexing?
• Assume you are running 3 network applications all
  using the same transport protocol, e.g., TCP
• FTP Server, Telnet Client, Web Browser
• When a packet arrives at a host, it moves up the
  protocol stack until it reaches the transport
  layer, e.g., TCP
• Now, the transport layer needs a way to determine
  which application the packet needs to be
  delivered. This is the demultiplexing problem.
• Recall that all protocol layers perform
  multiplexing/demultiplexing
• e.g., IP needs to determine which transport
  protocol a given packet needs to be delivered,
  UDP or TCP?

transport
network
link
physical
Demultiplexing
transport
network
link
physical
Multiplexing
8
Demultiplexing How?

• host receives IP datagrams
• each datagram has source IP address, destination
  IP address
• each datagram carries 1 transport-layer segment
• each segment has source, destination port number
  (well-known port numbers for specific
  applications)
• Port s 16 bits
• 65535 ports
• 0-1023 are called well-known and are reserved
• HTTP uses port 80
• Telnet uses port 23
• RFC 1700 lists the reserved ports

32 bits
source port
dest port
other header fields
application data (message)
TCP/UDP segment format