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30. TCP and UDP (LAYER 4 PROTOCOLS)

BASICS OF LAYER 4

  • Provides TRANSPARENT transfer of DATA between END HOSTS (Host To Host communication)

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  • Provides (or DOESN’T provide) various SERVICES to APPLICATIONS:
    • Reliable DATA Transfer
    • Error Recovery
    • Data Sequencing
    • Flow Control
  • Provides LAYER 4 ADDRESSING (PORT numbers) - NOT the physical interfaces / ports on network devices
    • IDENTIFY the APPLICATION LAYER protocol
    • Provides SESSION multiplexing

WHAT IS A SESSION ?

  • A SESSION is an EXCHANGE of DATA between TWO or MORE communicating DEVICES

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The FOLLOWING ranges have been designated by IANA (Internet Assigned Numbers Authority)

  • Well-Known Port Numbers : 0 - 1023
  • Registered Port Numbers : 1024 - 49151
  • Ephemeral / Private / Dynamic port numbers : 49152 - 65535

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TCP (TRANSMISSION CONTROL PROTOCOL)

  • A CONNECTION-ORIENTED protocol
    • Before actually SENDING DATA to the DESTINATION HOST, the TWO HOSTS communicate to establish a CONNECTION. Once the CONNECTION is established, DATA exchange begins.

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Establishing connections

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Terminating connections

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  • TCP provides RELIABLE communication
    • The DESTINATION HOST must acknowledge that it RECEIVED each TCP SEGMENT (Layer 4 PDU)
    • If a SEGMENT isn’t ACKNOWLEDGED, it is sent again

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  • TCP provides SEQUENCING
    • SEQUENCE numbers in the TCP HEADER allow DESTINATION HOSTS to put SEGMENTS in the correct ORDER even if they arrive out of ORDER

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  • TCP provides FLOW CONTROL
    • The DESTINATION HOST can tell the SOURCE HOST to increase / decrease the RATE that DATA is sent

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UDP (USER DATAGRAM PROTOCOL)

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  • UDP is NOT a CONNECTION-ORIENTED PROTOCOL

    • The SENDING HOST does NOT establish a CONNECTION with the DESTINATION HOST before sending DATA. The DATA is simply SENT

  • UDP DOES NOT provide reliable COMMUNICATION

    • When UDP is used, ACKNOWLEDGEMENTS are NOT SENT for received SEGMENTS
    • If a SEGMENT is LOST, UDP has no mechanism to re-TRASMIT it
    • SEGMENTS are sent “best-effort”

  • UDP DOES NOT provide SEQUENCING

    • There is NO SEQUENCE NUMBER FIELD in the UDP header
    • If SEGMENTS arrive out of order, UDP has no MECHANISM to put them back in ORDER

  • UDP DOES NOT provide FLOW CONTROL

    • UDP has NO MECHANISM like TCP’s WINDOW SIZE to control the flow of DATA

  • UDP DOES provide ERROR CHECKING (via CHECKSUM)

COMPARING TCP AND UDP

Number of Fields in their Headers

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  • TCP provides MORE FEATURES than UDP but at a COST of ADDITIONAL OVERHEAD
  • For applications that require RELIABLE communications (for example, downloading a file), TCP is PREFERRED
  • For applications, like real-time voice and video, UDP is preferred
  • There are SOME applications that use UDP, but provide RELIABILITY, etc. within the APPLICATION itself.
  • Some applications use BOTH TCP and UDP, depending on the situation.

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IMPORTANT PORT NUMBERS

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