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Transport Layer ​

Transport Control Protocol (TCP) ​

  • Operation of TCP
    • When a sender transmits a segment, it also starts a timer.
    • When the segment arrives at the destination, the receiving TCP entity sends back a segment (with data if any) bearing an acknowledgement number equal to the next sequence number it expects to receive.
    • If the sender’s timer goes off before the acknowledgement is received, the sender transmits the segment again.
  • 3-way handshaking
    • 3-way handshaking
    • To release a connection, either party can send a TCP segment with the FIN bit set.

TCP Segments ​

TCP-Segments

Data Traffic ​

The main focus of congestion control and quality of service is data traffic.

  • In congestion control we try to avoid traffic congestion.
  • In quality of service, we try to create an appropriate environment for the traffic.
  • So, before talking about congestion control and quality of service, we discuss the data traffic itself.

Traffic Profile

Congestion Control ​

In general, we can divide congestion control mechanisms into two broad categories: open-loop congestion control (prevention) and closed-loop congestion control (removal).

  • Open-loop
    • Retransmission Policy
    • Window Policy
    • Acknowledgement Policy
    • Discard Policy
    • Admission Policy
  • Closed-loop
    • Back Pressure
    • Choke Packet
    • Implicit signaling
    • Explicit signaling

Metrics Used to Monitor a Subnet for Congestion

  • Percentage of all packets discarded for lack of buffer space;
  • Average of queue lengths;
  • Number of packets that time out and are retransmitted;
  • Average packet delay;
  • Standard deviation of packet delay, etc.