ICOM6012 Internet Infrastructure Technologies

Topic 4 Transport Layer

Actions

Sender

• Get application layer message
• Determine segment header fields values
• Create segment
• Pass segment to IP

Receiver

• Receive segment from IP
• Check header values
• Extract application layer message
• Demultiplexes message up to application via socket

Services

Provide logical communication between processes

Run in end system

• Send side
  • Break app messages into segment
  • Pass to network layer
• Receive side
  • Reassembles segments into messages
  • Pass to application layer

Protocols

TCP

• Features
  • Point-to-point
    • One sender, one receiver
  • Reliable, in-order byte system
    • No message boundaries
  • Full duplex data
    • Bi-directional data flow in same connection
    • MSS: maximum segment size (excluding header)
  • Cumulative ACKs
  • Pipelining
    • TCP congestion and flow control set window size
  • Flow Control
    • Sender will not overwhelm receiver
  • Congestion control
• Concepts
  • Segment structure
  • Sequence numbers
    • Byte stream “number” of first byte in segment’s data
  • Acknowledgements (ACKs)
    • Seq # of next byte expected from other side
    • Cumulative ACKs
• Procedure
  • Connection-oriented
    • Handshaking initializes sender, receiver state before data exchange
    • TCP socket identified by 4-tuple
      • Source IP
      • Source Port #
      • Dest IP
      • Dest Port #
    • Server host may support many simultaneous TCP sockets
    • Web servers have different sockets for each connecting client (Non-persistent HTTP will have different socket for each request)
    • Demux
      • Receiver uses all four values to direct segment to appropriate socket
  • 3-way handshake
    • Connection setup
  • Connection management
    • Handshake
      • Agree to establish connection
      • Agree on connection parameters
  • Retransmission
  • Closing a connection
• Events
  • Data received from application
    • Create segment with seq # (a byte-stream number of first data byte in segment)
    • Start timer if not already running
      • Think of timer as for oldest unACKed segment
      • Expiration interval: TimeOutInterval
  • Timeout
    • Retransmit segment that caused timeout
    • Restart timer
  • ACK received
    • Update what is known to ACKed
    • Start timer if there are still unACKed segment
• Shortcomings
  • SYN DoS Attack
    • Half-open TCP connections consume all the TCP connection resources
    • SYN packet with a spoofed source address

UDP

• Features

  • No frills
  • Bare bones
  • “Best effort”
    • Lost
    • Delivered out-of-order to app
  • No handshaking
  • Handle independently
  • Services not available
    • Delay guarantee
    • Bandwidth guarantee

• Concepts

  • Segment format
    • Source port #
    • Dest port #
    • Length
    • Checksum
      • Detect errors
        • Sender
          • Treat segement contents
          • Checksum: 1’s complement sum
          • Put checksum into UDP field value
        • Receiver
          • Compute checksum of received segment
          • Check equality
            • No: error detected
            • Yes: no error detected (not sure)
      • Checksum calculation may be disabled in order to speed up the processing

• Procedure

  • Connectionless demux
    • Create socket with unique local port #
    • Sender side: create datagram to send into UDP socket
      • Dest IP
      • Dest port #
    • Receive side: receive UDP segment (IP datagrams with same dest port #, but different source will be directed to same socket)
      • Check dest port #
      • Direct UDP segment to socket with that port #

• Utilization

  • Streaming multimedia apps
    • Loss tolerant
    • Rate sensitive
  • DNS
  • SNMP
  • HTTP/3

• Shortcomings

  • Need reliability at application layer
  • Congestion control

SCTP

DCCP

Multiplexing and Demultiplexing

Multiplexing at sender

• Handle data from multiple sockets, add transport header

Demultiplexing at receiver

• Handle data info to deliver received segments to correct socket

Host receivers IP datagram (Host uses IP & port # to redirect segment)

• Source IP
• Dest IP
• One transport layer segment
  • Source port #
  • Dest port #

Congestion Control

Cause

• Too many sources sending too much data too fast for network to handle

Manifestations

• Long delay
  • Queueing in router buffers
• Packet loss
  • Buffer overflow at network

Approaches

• End-end congestion control
  • Features
    • No explicit feedback from network
    • Congestion inferred from observed loss, delay
• Network-assisted congestion control
  • Features
    • Routers provide direct feedback to hosts with flow passing through congested router
    • May indicate congestion level or explicit set sending rate
    • TCP ECN ATM DECbit protocols

TCP Congestion Control

• AIMD
  • Sender can increase sending rate until packet loss occurs, then decrease
  • Additive increase
    • 1 MSS (maximum segment size) until loss detected
  • Multiplicative decrease
    • Cut sending rate in half at each loss event
• Detecting, reacting to loss
  • ACKs problem
    • cwnd is cut in half
    • Window grow linearly
  • Timeout event
    • cwnd is set to 1 MSS
    • Window grow exponentially to threshold, then linearly
• TCP slow start
  • Initially cwnd = 1 MSS
  • Double cwnd every RTT
  • Done by incrementing cwnd for every ACK received