ICOM6012 Topic 4 Transport Layer

Author: Arthur | 687 words, 4 minutes | 2020-10-20 | Category: Notes

hku, icom6012, internet

Translations: ZH

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

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ICOM6012 Topic 1 Overview
Arthur

Author

Arthur

Blockchain (Golang) Developer, MSc Student in Department of Computer Science @ The University of Hong Kong (HKU). Love to learn and build things. Follow me on GitHub