Computer Networking Lecture Notes

25 September 2012 • The Transport Layer

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Outline

• Way finding.
• Transport services & communication.
• Transport abstraction & services.
• Transport protocols.
• Addressing

Where We Were

Where We’ll Be

What It Is

Transport Services

• A transport service: the end-to-end communication abstraction assumed by the higher layers.
  • Session and application in particular.
• The transport service's most valuable property is end-to-end communication.
  • Others include efficiency, reliability, cost-effectiveness, …
• Implemented in hardware, the OS, libraries, or user space.

Example Services

• Datagram Congestion Controlled Protocol (DCCP): message sends with congestion control.
• Multiplex light-weight streams over a connection:
  • Stream Control Transmission Protocol, SCTP (rfc 4960).
  • Structured Stream Transport (SST).
• Transactional TCP, T/TCP (rfc 1644): Reliable message streams.

Transport Communication

• End-to-end communication properties vary based on service offered:
  • Message preserving or streaming.
  • Reliable or not.
  • Unicast or multicast, and so on.
• The transport implementer may choose, or leave it to the higher levels.
  • More generality, higher complexity and cost all around.

Transport Abstraction

• The transport abstraction decouples services from underlaying networks.
  • Decoupled, not divorced.
• Transport-service properties may not be evident in lower services.
  • Reliable communication over unreliable networks.
  • At both implementation and run-time costs.
• Transport services are flexible and change tolerant.

Transport Services

• A minimal API for a transport service:

  connect()	make a connection request
  listen()	respond to a connection request
  send()	send data over a connection
  receive()	receive data over a connection
  disconnect()	end a connection

• Parts missing:

  End-point addresses.
  Connection accept-reject.
  Stream-message choice.
  Other property choices.

Example

client()
  connection c
  c.connect(server addr, message) 
  if c.connected
    c.send(msg)
    r = c.receive()
    c.disconnect()

server()
  connection c = listen(*)
  if ok(c)
    c.connect(c)
    msg = c.receive()
    c.send(f(msg))
  c.disconnect()

Transport States

Transport Protocols

• The transport protocol has the job of implementing the transport services.
  • With the help of lower layers.
• Problems to solve include error control, sequencing, and flow control.
• Although similar to other networking layers, scale complicates transport protocols.
  • Larger demands, more resources, heterogeneity.

Addressing

• End points need a name, an address by which they can be identified.
• A transport service defines the Transport Service Access Point (TSAP) for end-point addresses.
• In IP, the TSAP is an (address, port) pair.
  • Although the address can be thought of as a network address too.

Summary

• The transport layer implements the end-to-end communication abstraction.
• A transport service is a mix ‘n’ match combination of communication properties.
  • Reliability, streams, flow control, …
• The Transport Service Access Point (TSAP) identifies end-points.

References

• T/TCP: TCP for Transactions by Mark Stacey, Ivan Griffin, and John Nelson in Linux Journal, February 2000.
• SCTP: A Proposed Standard for Robust Internet Data Transport by Armando Caro, Janardhan Iyengar, Paul Amer, Sourabh Ladha, Gerard Heinz and Keyur Shah in IEEE Computer, November 2003.

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This page last modified on 2012 September 26.