[Network] Transport Layer 1

Transport Layer 1

Transport services and protocols

• logical communication between application processes running on different hosts(end system)
• Sender : breaks application messages into segments (packets) and passes to network layer
• Receiver : reassembles segments into messages, passes to application layer
• Protocol : TCP, UDP, SCTP, DCCP

• Reliable, in-order delivery : TCP
  • Congestion control (control data flow even if application layer try to send lots of data)
  • Flow control (send data to receiver as they can receive)
  • Connection setup
• Unreliable, unordered delivery : UDP
  • Just send data between processes
• Services not available
  • Delay guarantees (network layer problem)
  • Bandwidth gurantees (network layer problem)

Multiplexing

• handle data from multiple sockets, add transport header

Demultiplexing

• use header info to deliver received segments to correct socket

How demultiplexing works

• Host receives IP datagrams
  • Each datagram has source IP address, destination IP address
  • Each datagram carries one transport-layer segment
  • Each segment has source(who send), destination(who receive) port numbers
• Host uses IP addresses(which computer) & port numbers(which process) to direct segment to appropriate socket

Reliable data transfer

• Sender side

  • rdt_send() : called from above(application layer), passed data to deliver to receiver upper layer
  • udt_send() : called by rdt, to transfer packet over unreliable channel to receiver

• Receiver side

  • deliver_data() : called by rdt to deliver data to upper
  • rdt_rcv(): called when packet arrives on rcv_side of channel

rdt3.0 : channels with errors and loss

• Assumptions
  • Underlying channel may flip bits in packet
  • Use checksum to detect bit errors
  • Underlying channel can also lose packets(data, ACKs)
  • checksum, sequence number, ACKs, retransmissions will be of help… but not enough
  • checksum also have error (but low probability than error in data)
• How to recover from errors
  • Retransmission
  • Feedback from receiver (ACK -> success, NAK -> corrupt)
  • Retransmisstion timer

Scenario

• no loss

• packet loss

• ACK loss

• timeout / delayed ACK

stop and wait operation