TCP Protocol: Full-Duplex, Sequence Numbers, RTT Estimation, and Loss Recovery

What does it mean when we say that TCP is a full-duplex service?

TCP allows simultaneous data transmission in both directions between two hosts.

How does TCP determine each segment's sequence number?

Each segment is assigned a unique sequence number based on the order of transmission.

What is the purpose of the acknowledgement number in TCP?

The acknowledgement number indicates the next byte that the receiver expects to receive.

How is the timeout value in TCP decided?

The timeout value must be at least as long as the expected Round Trip Time (RTT) between the two hosts.

What is SampleRTT in TCP?

SampleRTT is the time taken between when a segment is sent and when its acknowledgement (ACK) is received.

How do TCP implementations maintain an average RTT?

They use an exponential weighted moving average (EWMA) to calculate the EstimatedRTT.

What is the recommended value of alpha (α) for calculating EstimatedRTT?

The recommended value of α is 0.125.

What formula is used to estimate RTT variation in TCP?

DevRTT = (1 - β) DevRTT + β |SampleRTT - EstimatedRTT|, with β recommended as 0.25.

How is the retransmission timeout interval set in TCP?

TimeoutInterval = EstimatedRTT + 4 * DevRTT.

What happens to the timeout interval after a timeout occurs?

The timeout interval is doubled to avoid premature timeouts.

What guarantees does TCP provide for data transfer?

TCP guarantees that data is received uncorrupted, without gaps or duplication, and in sequence.

What is the role of the single timer in most TCP implementations?

The timer is used for loss recovery, tracking the oldest unacknowledged segment.

What occurs when Host A receives a duplicate ACK?

It indicates that a segment was received out of order, prompting TCP to perform a fast retransmit after three duplicate ACKs.

What is the significance of cumulative ACKs in TCP?

Cumulative ACKs acknowledge all bytes up to a certain point, allowing for efficient loss recovery.

What is the initial recommended timeout value in TCP?

The recommended initial timeout value is 1 second.

How does TCP handle a lost ACK?

If an ACK is lost, TCP will retransmit the segment after a timeout occurs.

What happens if Host A receives a cumulative ACK after a lost ACK?

Host A will not retransmit any segments if it receives a cumulative ACK for segments that were sent after the lost ACK.

What is the purpose of fast retransmit in TCP?

Fast retransmit allows for quicker recovery from packet loss by retransmitting segments upon receiving duplicate ACKs.

What are the unique qualities of TCP's loss recovery mechanism?

TCP retransmits at most one segment at a time, buffers out-of-order segments, and uses cumulative ACKs.