Transport: internet transport protocols

Internet transport protocols UDP (User Datagram Protocol)

• very thin layer on top of IP

• provides: ports

• does not provide: flow/congestion control, retransmissions

• header format:

TCP (Transmission Control Protocol)

• provides reliable end-to-end byte stream (so applications can’t tell how many segments or packets were sent)

• header format:

• every data byte has its own sequence number
• window size — how much data receiver can handle, with small values leading to huge overhead
• congestion control
  • congestion window is on sender, specifying how many segments can be transmitted.
  • increase congestion window whenever ACKs arrive, until they don’t.
  • then set slow start threshold to half of congestion window and start again.
  • when threshold is crossed, switch to additive increase
• fast retransmission — three duplicate acks imply lost packet, whose identity can be infered from ack number (it’s the next packet in sequence). this packet is retransmitted right away
• in TCP Reno: