CS2005 - Lecture 6 - Transport Layer II

TCP

A connection-oriented transport protocol providing reliable, in-order and flow-controlled data delivery between processes on different hosts

TCP connection identification

By a 4-tuple: source IP, source port, destination IP, destination port

Key fields in a TCP header

Sequence/ACK numbers: For reliable data transfer

16-bit receive window: For flow control

Flags (SYN, ACK, FIN, RST): Connection management

Checksum: Error detection

MSS determination

MSS = MTU (link layer frame size) - TCP/IP header (40 bytes)

3-way handshake

Client -> Server: SYN (seq=client_isn)

Server -> Client: SYN-ACK (seq=server_isn, ack=client_isn+1)

Client -> Server: ACK (ack=server_isn+1)

TCP connection termination

One host sends FIN, other ACKs, Other host sends FIN, first host ACKs, Wait for timeout (2*MSL) to ensure all segments die

TCP reliability

Sequence numbers: Track byte stream

Cumulative ACks: Ack next expected byte

Retransmissions: On timeout or 3 duplicate ACKs (fast retransmit)

Fast retransmit trigger

Receiving 3 duplicate ACKs for the same segment

Receive window (rwnd)

Available buffer space at receiver: rwnd = rcvBuffer - (lastByteRcvd - lastByteRead)

Sender overflow avoidance

Sends only up to min(cwnd, rwnd) unacknowledged bytes

TCP congestion detection

Packet loss (timeout/duplicate ACKs) or delayed ACKs

AIMD

Additive Increase: Increase cwnd by 1 MSS/RTT

Multiplicative Decrease: Halve cwnd on loss

UDP over TCP usage

For real-time apps (low latency, tolerant to loss), no connection overhead or many clients

Key trade-offs

UDP: No reliability/flow control, low overhead

TCP: Reliable but slower (handshake, congestion control)