Transport Layer

Networks and Systems

Transport-layer services

Transport services and protocols

  • Provide logical communication between app processes running on different hosts, in contrast the network layer just communicates between hosts, not processes on hosts

  • Transport protocols run in end systems

    • Send side: breaks up app messages into segments, passes to network layer

    • Receive side: reassembles segments into messages, passes to app layer

  • More than one transport protocol available to apps:

    • Internet: TCP and UDP

Transport vs network layer

Network Layer

Logical communication between hosts

Transport Layer

Logical communication between processes. Relies on and enhances network layer services

Internet transport-layer protocols

TCP (Transmission Control Protocol):

  • Reliable, in-order delivery

  • Congestion control

  • Flow control, ack., timer

  • Connection setup

UDP(User Datagram Protocol):

  • Unreliable, unordered delivery

  • No-frills extension of “best-effort” IP

  • Services not available

    • Delay guarantees

    • Bandwidth guarantees

TCP and UDP extend IP delivery service between hosts to delivery service between processes $\rightarrow$ transport layer multiplexing and demultiplexing

Multiplexing and demultiplexing



In the network layer it is called a packet

In the transport layer it is called a segment

  • Host receives IP datagrams

  • Each datagram has source IP address, destination IP address

  • Each datagram carries one transport-layer segment

  • Each segment has source, destination port number

  • Host uses IP addresses and port numbers to direct segment to appropriate socket

  • Each socket has a unique identifier


Connectionless multiplexing and demultiplexing

  • All sockets have host-local port #

  • Assigned automatically, or via bind()

  • serverSocket,bind((ip, port))

  • When host receives UDP segment:

    • Checks destination port # in segment

    • Directs UDP segment to socket with that port #

If two UDP segments have different source IP addresses and/or source port numbers but same dest IP and port #, they will be directed to same process via same process via same socket as dest


Connection-oriented multiplexing and demultiplexing

TCP socket identified by 4-tuple

  • Source IP address

  • Source port number

  • Destination IP address

  • Destination port number

Demux: receiver used all four values to direct segment to appropriate socket

Server host may support many simultaneous TCP sockets:

  • Each socket identified by its own 4-tuple

  • Two arriving TCP segments with different source IP/ #Port will be directed to two different sockets


With TCP we have a different socket for each connection

Connectionless transport: UDP

  • “No frills”, “bare bones” internet transport protocol

  • “Best effort” service, UDP segments may be

    • Lost

    • Delivered out-of-order to app

  • Connectionless:

    • No handshaking between sender/receiver

    • Each UDP segment handled independently of others

  • UDP use:

    • Streaming multimedia apps (loss tolerant, rate sensitive)
  • Reliable transfer over UDP

    • Add reliability at application layer

    • Application-specific error recovery

Segment Header

UDP Segment Header

Principles of reliable data transfer

Principles reliable data transfer

Note that reliable data transfer protocol is not a standard, it is just for us to look at academically

Principles reliable data transfer

Reliable data transfer

We will:

  • Incrementally develop sender, receiver sides of reliable data transfer protocol (rdt)

  • Consider only unidirectional data transfer - but control info will flow on both directions

  • Use finite state machines (FSM) to specify sender, receiver

Reliable data transfer

rdt1.0: reliable transfer over a reliable channel

Underlying channel perfectly reliable:

  • No bit errors

  • No loss of packets

Separate FSMs for sender, receiver:

  • Sender sends data into underlying channel

  • Receiver reads data from underlying channel

RDT over reliable channel


Channel with bit errors

  • Underlying channel may flip bits in packet, checksum to detect bit errors

  • The question: how to recover from errors:

    • Acknowledgements (ACKs): receiver explicitly tells sender that packet received OK

    • Negative acknowledgements (NAKs): receiver explicitly tells sender that packet had errors

    • Sender retransmits packet on receipt of NAK

    • Using ACKs and NAKs is known as ARQ (Automatic Repeat reQuest) protocols

      • Error detection. Sender embeds extra bits in packets

      • Feedback. Receiver provide sender with feedback

      • Retransmission. Retransmit erroneous packets

  • New mechanisms in rdt2.0 (beyond rdt1.0)

    • Error detection

    • Feedback: control msgs (ACK (1), NAK(0)) from receiver to sender

FSM specification

FSM specification

Operation with no errors

Operation with no errors

Error scenario

Error scenario

Fatal flaw

What happens if ACK/NAK corrupted:

  • Sender doesn’t know what happened at receiver

  • Can’t just retransmit: possible duplicate

Handling duplicates:

  • Sender retransmits current packet if ACK/NAK corrupted

  • Sender adds sequence number to each packet

  • Receiver discards (doesn’t deliver up) duplicate packet

Stop and wait:

  • Sender sends one packet, then waits for the receiver response


Channels with errors and loss

New assumption:

  • Underlying channel can also lose packets (data, ACKs)

    • Checksum, seq. #, ACKs, retransmissions will be of help, but not enough


  • Sender waits “reasonable” amount of time for ACK

    • Retransmits if no ACK received in this time

    • If no packet (or ACK) just delayed (not lost):

      • Retransmission will be duplicate, but seq. #’s already handles this

      • Receiver must specify seq # of packet being ACKed

    • Requires countdown timer


RDT sender

In action

RDT in action

Rdt in action

Pipelined protocols

Pipelined protocols

Pipelining has the following consequences for reliable data transfer protocols:

  • Range of sequence numbers must be increased

    • Unique sequence number and there may be multiple, in-transit, unacknowledged packets
  • Multiple packet buffering at sender and/or receiver

    • Sender buffers packets that have been transmitted but not yet acknowledged

    • Buffering of correctly received packets

  • Range of sequence numbers needed and the buffering requirements will depend on the manner in which a data transfer protocol responds to lost, corrupted, and overly delayed packets

Two generic forms of pipelined protocols:

  • Sender can send multiple packets without waiting for ACK
  • Sender can have up to N unacked packets in pipeline
  • Receiver only sends cumulative ack, doesn’t ack packet if there’s a gap (so if it acks packet 4, it means it has packet 1,2,3 and 4)
  • Sender has timer for oldest unacked packet, when timer expired, retransmit all unacked packets
Selective Repeat
  • Sender can have up to N unacked packets in pipeline
  • Receiver sends individual ack for each packet
  • Sender maintains timer for each unacked packet, when timer expires, retransmit only that unacked packet

GBN(Go Back N) in action

GBN in action

Selective repeat

  • Receiver individually acknowledges all correctly receives packets. Buffers packets, as needed, for eventual in-order delivery to upper layer

  • Sender only resends packets for which ACK not received. Sender timer for each unACKed packet

  • Sender window

    • N consecutive seq #’s

    • Limits seq #s of sent, unACKed packets


  • Data from above:

    • If next available seq # in window, send packet
  • timeout(n)

    • Resend packet n, restart timer
  • Mark packet n as received

  • If n smallest unACKed packet, advance window base to next unACKed seq #


  • packet n in [rcvbase, rcvbase+N-1]

    • Send ACK(n)

    • Out-of-order: buffer

    • In-order: deliver( also deliver buffered, in-order packets), advance window to next not-yet-received packet

  • packet n in [rcvbase-N, rcvbase-1]

    • Ack(n)
  • Otherwise:

    • Ignore

Selective repeat in action

Selective repeat action

Selective repeat dilemma


  • Finite range of seq # s: 0,1,2,3

  • Window size=3

    • Receiver sees no difference in two scenarios

    • Duplicate data accepted as new in (b)

Selective repeat dilemma

Note: That curtain is there to show that there is a lack of knowledge between the sender and receiver