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The Transport Layer (L4). Provides an end-to-end, reliable transport service between l4 entities Reliable - error-free, in-sequence, no loss or duplication Typically connection-oriented can be connectionless depends on needs of application. ISO Transport Layer.

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the transport layer l4
The Transport Layer (L4)
  • Provides an end-to-end, reliable transport service between l4 entities
    • Reliable - error-free, in-sequence, no loss or duplication
  • Typically connection-oriented
    • can be connectionless
    • depends on needs of application
iso transport layer
ISO Transport Layer
  • ISO classify subnets as type A, B and C
    • (reliable to unreliable)
  • Define 5 different Layer 4 protocols to operate over these 3 types
  • Class 4 protocols (TP4) is defined to operate over an unreliable network
  • TP4 functionality almost identical to TCP
connection oriented service
Connection oriented service
  • typical 3 phase connection
    • establish connection
    • data transfer
    • terminate connection
  • T-Connect, T-Data, T-Disconnect services, with service primitives
    • .request
    • .indication
    • .response
    • .confirm
slide4
Parameters associated with primitives, e.g.
    • Addressing
    • Quality of Service parameters
  • Addresses are T-SAP identifiers
    • (Sockets in TCP/IP terminology)
    • Can be well known addresses
    • Can be ‘discovered’ at connection set-up time
  • T-Connect can then be directed to appropriates
    • address, T-SAP identifier (socket)
slide5
Quality of service is negotiated.
    • Source specifies requirements
    • network and destination agree or modify these.
  • Typical QoS parameters
    • throughput, end-to-end delay, error rate, etc
  • Connection requests can be denied by local or remote L4 entity
slide6
L4 has very similar functionality to L2 (link layer), but
    • L2 operates (typically) across a link
    • L4 operates end-to-end (across a network)
slide7
Main issues are:-
    • Connection establishment and termination.
    • Error and flow control
    • Buffering and multiplexing.
slide8
The problems arising from these issues are:-
    • how to guarantee ordered delivery of PDUs to the users
    • how to ensure that only legitimate connections are established
    • how to ensure that connections are broken down in a proper manner
    • how to ensure that connections are terminated correctly
    • how to separate acknowledgements from flow control, i.e.
      • Tell source everything has arrived OK
      • send / don’t send any more (yet)
slide9
Ordered delivery
    • relies on every (data) TPDU being numbered
    • SREJ typically used at L4 for error recovery, this impacts on
      • number of TPDUs which can be ‘in transit’
        • remember connection is across a network
          • connection distance is long
        • potential inefficient use of channel capacity
        • need long sequence numbers
      • buffering at destination
slide10
Long connection distance can cause other problems
    • if connectionless layer 3 in operation
      • TPDUs can take different routes
      • therefore can get delayed (lost)
    • Can’t therefore allow same sequence numbers to be reused between different connections
      • else a delayed TPDU, from a terminated call, could arrive and be accepted
    • Initial Sequence Numbers defined at connection establishment
slide11
Connection establishment phase - three-way handshake
    • CR(Connection Request) sent by source, with proposed start sequence no
      • CR(ISN = x)
    • CC (Connect Confirm) sent by destination, with ack of x, and their own start sequence no
      • CC(ISN=y, ack=x)
    • Source acks CC
      • ACK(seq = x, ack=y)
    • Only valid connections will be established
slide12
Connection Termination
    • Three way handshake also used to ensure all TPDUs have arrived before connection terminated
    • TPDUs are DR and DC, e.g.
      • DR(last sent = a, ack =b)
      • DC(last sent = b + 2, ack = a)
      • Ack(last sent = a, ack = b+2)
    • Connection will remain opern until final Ack is received
slide13
Error Control
    • Typical ARQ, but with sequence numbers constraints outlined above
      • clearly includes acks, retransmissions, etc, as appropriate
    • Can also use checksum on data
flow control
Flow Control
  • Flow Control is a more difficult problem
    • SREJ typically used for error recovery
      • essential if error control is across the network
      • essential if connectionless network (L3) under L4
    • Need to buffer out of sequence frames
    • Need large buffers at destination, to buffer out of sequence TPDUs
slide15
Hence, Ack cannot be misinterpreted as meaning
    • ‘I have acked another 2 TPDUs’
    • ‘So, you can send 2 more TPDUs’
  • Ack means TPDUs have arrived correctly
  • Permission to transmit more data must be signalled separately
    • this is a flow control issue
slide16
Mechanics of separate Acks and Flow Control
    • Acks (and certain other TPDUs) have a separate CR(credit field)
      • e.g. (AK=n, CR=m)
      • Acking upto TPDU n, permission to send m TPDUs (after n)
      • Flow control is achieved by adjusting value of m
      • m may be zero
    • Timers used (as for L2) to ensure deadlock doesn’t occur
multiplexing
Multiplexing
  • Upwards and Downwards multiplexing allowed
  • Upwards Multiplexing
    • Several transport connections using one layer 3 connection
    • Multiple calls to the same destination
  • Downwards Multiplexing
    • One L4 connection spread over multiple L3 connections
    • Necessary if L3 connect slow, and high data rate to transmit
  • One-to-one multiplexing more common
summary l4 functionality
Summary - L4 Functionality
  • To insulate application (upper layers from subnet (lower 3 layers)
  • End-to-end (connection oriented) service
    • Ordered delivery of TPDUs
      • Issues and sequence number selection and sequence number length
    • Ensuring only valid connections accepted
      • Need for 3 way hand shake
    • Ensuring connections terminated correctly