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
  • 
  • To release a connection, either party can send a TCP segment with the FIN bit set.

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.

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.