Connection-Oriented Transport (TCP)

COMP211 Lectures 2021-11-01

These notes are low-effort, due to catching up in this module. See the videos and slides for more detail.

TCP Features

TCP has the following features:

• Point-to-Point - one sender, one receiver
• Reliable, In-Order Byte Stream - no “message boundaries”
• Full Duplex Data:
  • Bi-directional data flow in same connection.
  • MSS - maximum segment size.
• Cumulative ACKs
• Pipelining - TCP congestion and flow control set window size.
• Connection-Oriented - Handshaking (exchange of control messages) initialises sender, receiver state before data exchange.
• Flow Controlled - Sender will not overwhelm receiver.

TCP Segment Structure

• Sequence Numbers - Count the bytes of data in the byte-stream (not the sequence of the segments).
• Acknowledgements:
  • Acknowledge the sequence number of the next byte expected from the other side.
  • Cumulative ACK.

TCP Sequence Numbers & ACKs

Here is an example of a simple telnet scenario:

sequenceDiagram
note left of A: User types 'C'
A -->> B:Seq=42, ACK=79, data='C'
note right of B: B ACKs receipt of 'C', echoes back 'C'
B -->> A:Seq=79, ACK=43, data='C'
note left of A: A ACKs receipt of echoed 'C'
A -->> B:Seq=43, ACK=80

TCP Round Trip Time & Timeout

The timeout should be longer than the round trip time (rtt) but the rtt varies:

• Too Short - Premature timeout, unnecessary re-transmission.
• Too Log - Slow reaction to segment loss.

To get a SampleRTT we can measure the time from segment transmission until ACK receipt (ignoring re-transmissions). We want to smooth this out so we sample several recent rtts.

To estimate the round trip time we use an exponential weighted moving average:

\[\text{EstimatedRTT} = (1-\alpha)\times \text{EstimatedRTT} + \alpha \times \text{SampleRTT}\]

The influence of past samples decrease exponentially.

To calculate the timeout interval we include a safety margin that takes into account the variance of the SampleRTT:

\[\text{TimeoutInterval} = \text{EstimatedRTT} + 4\times\text{DevRTT}\]

DevRRT is the safety margin. It is calculated as an exponential weighted moving average of SampleRTT deviation from EstimatedRTT:

\[\text{DevRTT} = (1-\beta)\times\text{DevRTT} + \beta\times\lvert\text{SamepleRTT}-\text{EstimatedRTT}\rvert\]

Typically $\beta=0.25$

TCP Sender (Simplified)

This is a simplified sender that ignores duplicate ACKS, flow control and congestion control.

Data received from application:

• Create segment with sequence number.
  • Sequence number is the byte-stream number of the first data byte in the segment.
• Start timer if not already running.
  • The timer is for the older unACKed segment.
  • Expiration interval is the TimeOutInterval.

Timeout:

• Re-transmit segments that caused the timeout.
• Restart the timer.

ACK Received:

• If ACK acknowledges previously unACKed segments:
  • Update what is known to be ACKed.
  • Start timer if there are still unACKed segments.

Receiver ACK Generation

This covers RFC 5681:

Event at Receiver	TCP Receiver Action
Arrival of in-order segment with expected sequence number. All data up to expected sequence number already ACKed.	Delayed ACK. Wait up to 500ms for next segment. If no segment send ACK.
Arrival of in-order segment with expected sequence number. One other segment has ACK pending.	Immediately send single cumulative ACK, ACKing both in-order segments.
Arrival of out-of-order segment higher-than-expected sequence number. Gap detected.	Immediately send duplicate ACK, indicating sequence number of next expected byte.
Arrivial of segment that partially completely fills gap.	Immediately send ACK, provided that segment start at lower end of gap.

TCP Fast Re-transmit

If a sender receives three additional ACKs fro the same data (triple duplicate ACKs), resend unACKed segment with the smallest sequence number:

• It is likely that unACKed segment was lost:
  • We can resend the data and save waiting for the timeout.