Manav Goyal

Transport Layer More - Computer Networks

Theory

  • The transport layer is responsible for process-to-process/end-to-end/port-to-port delivery of the entire message
  • Transport Layer
  • Header (TCP/UDP) is added to the data after it is received
  • Functionality
    • Port addressing -=> The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer
    • Segmentation and Reassembly -=> A message is divided into transmittable segments, each having a sequence number
    • Connection Control -=> The transport layer can be either connection-less or connection-oriented
    • Flow Control
      • Control the Flow at Destination
    • Error Control (Checksum)
      • The unit is divided into K sections, each of n bits
      • Section 1 and 2 are added together using one’s complement
      • Section 3 is added to the result of the previous step
      • Section 4 is added to the result of the previous step
      • The process repeats until section k is added to the result of the previous step
      • The final result is complemented to make the checksum
      • Manchester vs Differential
  • Headers

Protocols

TCP (Transmission Control Protocol)

  • Functionality
    • Byte Streaming
      • TCP creates segments (Segmentation) of bytes from the continuous stream of data (in bits) received from the application layer
      • Segment is a collection of bytes
    • Connection Oriented
      • Reliability -=> Makes sure that the number of data sent is equal to data received -=> No loss of data
      • Makes connection before sending the message
      • Uses 3-way handshaking -=> Makes request, Gets response, Sends data
      • In-order -=> Message are sent and received in order
    • Full Duplex
      • Both can transmit data at the same time
    • Piggybacking
      • Uses Go-back-N and selective repeat for sliding window
    • Error Control
      • Checksum
    • Flow Control
      • Controls the flow of data
    • Congestion Control
      • Uses AIMD algorithm to avoid congestion
    • Multiplexing/De-multiplexing
      • While sending does many-to-one, and while receiving does one-to-many of multiple data form channel to sender/receiver
  • Header
    • TCP Header
    • Source/Destination port
      • Port of the application of sender/receiver
      • Possible ports = 0 to 65535
      • Well known port numbers = 0 to 1023
        1. HTTP = 80
        2. SMTP = 25
        3. FTP = 21
    • Sequence number
      • Number given to each byte in the segment
      • Generally TCP uses random sequence number -=> Starts from a random number
    • Acknowledgement number
      • Next expected byte number
      • If the senders byte number is "x" then receiver sends the Acknowledgement "x+1"
    • Header Length (HLEN)
      • Maximum bit can be (1111) = 15 -=> 15 x 4 = 60
      • Minimum can be (0101) = 5
      • We multiply by 4 -=> Minimum should be 20
    • Flags
      • URG (Urgent) -=> If data is urgent then bit value is 1
      • ACK (Acknowledgement) -=> If data is acknowledgement then bit value is 1
      • PSH (Push) -=> Buffer (Storing data before sending) is not created
      • RST (Reset) -=> Resetting the connection
      • SYN (Synchronization) -=> First byte when making connection
      • FIN (Finish) -=> Last byte, Connection terminated
    • Window Size
      • Receiving window -=> Maximum size (Total capacity) of data that can be received
    • Checksum
    • Urgent pointer
      • Range of data (Bytes) that is urgent in the segment
    • Options & Padding
      • Extra data
      • Maximum segment size (MSS) -=> Size of 1 segment

UDP

  • Functionality
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