TCP Flow Control

• Goal: Maximum utilization of the resources without overwhelming the receiver

Implementation

TCP uses Sliding Window algorithm:

• Allow a larger amount of data “in flight” for better channel utilization
• But should not exceed receiver’s capacity - must all fit in buffer

Receiver Buffering: Flow Control

• Receive window size (aka Advertised window)
  • Amount that can be sent without acknowledgment
• Receiver tells the sender the window
  • Tells the sender the amount of free space left
  • Use window size field in TCP header
  • This is sent during the three-way handshake and each subsequent ACK
  • If AdvertisedWindow is 0, then:
    • Sender periodically sends 1 byte packets to break the deadlock
  • If AdvertisedWindow > Maximum Segment Size (MSS), then sender sends multiple packets
  • If AdvertisedWindow < Maximum Segment Size (MSS), then sender can make packets of size up to AdvertisedWindow

For sender:

• Effective window = Advertised window - (LastByteSent - LastByteACKed)