Transport Layer

TCP (Transmission Control Protocol) is a transport layer protocol

TCP

Fast Retransmit

• duplicate acknowledgements indicate something is wrong e.g. something needs to be resent
• Receiving 3 duplicate acks → resend without waiting for timeout

TCP Flow Control

Control the rate at which sender sends data, using a receive window field.

• Data in pipeline should not exceed receive buffer size → overflow
• Tell the sender the receive window (rwnd)
  • rwnd is amount of free buffer space
• Ensure $\text{LastByteSent}-\text{LastByteAcked}\le \text{rwnd}$
  • this is because in-flight data is stored in the buffer

TCP Congestion Control

Reduce rate at which sender sends data if high perceived congestion

• want to reduce congestion
• congestion occurs when $\text{input rate}>\text{output rate}$

WHY do we want to reduce congestion?

• lost packets (buffer overflow)
• long delays from queueing in router buffers?

Go Back N (GBN)

Only accept packets in order.

• Lost packets handled poorly → sending duplicate packets

Selective Repeat (SR)