Question

Suppose Host A sends two TCP segments back to back to Host B over a TCP connection. The first segment has sequence number 90 ; the second has sequence number 110 . a. How much data is in the first segment? b. Suppose that the first segment is lost but the second segment arrives at B. In the acknowledgment that Host B sends to Host A, what will be the acknowledgment number?

Short answer

a. 20 bytes; b. 90.

Step-by-step solution

Calculate Data Size in First Segment

To find the amount of data in the first segment, we need to look at the sequence numbers of the two segments. The first segment has a sequence number of 90, and the second segment has a sequence number of 110. The sequence number indicates the byte number of the first byte in the segment. Therefore, the data in the first segment can be calculated by subtracting the sequence number of the first segment from the sequence number of the second segment.The data size is given by:\[ \text{Data Size in First Segment} = 110 - 90 = 20 \text{ bytes} \]

Determine Acknowledgment Number from Host B

When Host B receives the second segment but not the first one, it needs to acknowledge the data it has successfully received so far. In TCP, the acknowledgment number is the sequence number of the next byte that Host B expects to receive. Since the first segment with sequence number 90 is lost, Host B has not been able to acknowledge it. Thus, Host B's acknowledgment number will be 90, indicating that it is still waiting for the segment starting from byte 90.

Key concepts

Sequence Numbers

TCP, or Transmission Control Protocol, uses sequence numbers to ensure that data is correctly ordered and complete. These numbers are essential in keeping track of which data segments have been sent and received between hosts in a network communication.

- **Purpose**: Sequence numbers help to identify the order of bytes being sent. Each byte in a TCP connection has a sequence number, starting from an initial number that is typically derived randomly.
- **Function**: The sequence number of a segment is the sequence number of the first byte of data in that segment. For example, in the given exercise, the first segment has a sequence number of 90, signaling that it contains data starting with byte number 90.
- **Continuation**: When the second segment is sent, it continues from the last byte of the previous segment. Thus, the sequence number of the second segment would indicate the sequence number of its first byte, which is 110 here, following the end of the first segment.

Understanding sequence numbers is crucial for data reordering and loss detection. If segments arrive out of order or some segments don't arrive at all, TCP can determine how to proceed based on sequence numbers.

Acknowledgment Number

Acknowledgment numbers in TCP signify which bytes have been received and what the receiving host expects next from the sender. They play a pivotal role in ensuring reliable data transmission by confirming receipt and managing flow control.

- **Meaning**: An acknowledgment number refers to the sequence number the receiver anticipates next. Suppose Host B receives a segment. It sends back an acknowledgment number indicating the next byte expected. If the segment has a sequence number of 110, Host B would acknowledge up to that point by giving a subsequent number.
- **Example from the exercise**: When Host B misses the first segment with sequence number 90 and only receives the second segment with sequence number 110, it acknowledges its inability to receive the first. Host B sends an acknowledgment number of 90, which conveys that this segment starting from byte 90 is still expected.

This mechanism allows TCP to handle lost packets and retransmission promptly, maintaining a seamless communication stream.

Data Size Calculation

The size of the data in a TCP segment can be calculated using sequence numbers. This calculation is crucial to understand the volume of data being transmitted and managed across the network.

- **Calculating Data Size**: The difference between the sequence number of two consecutive segments signifies the amount of data in the former segment. In simpler terms, subtract the sequence number of the first segment from the second segment.
- **Application**: In our case, the sequence number for the first segment is 90, and the next segment starts at 110. Thus, the bytes transmitted in the first segment can be determined as: \[ \text{Data Size in First Segment} = 110 - 90 = 20 \, \text{bytes} \]

This calculation helps in troubleshooting network issues or optimizing data flow, providing detailed insights into the segment sizes and ensuring effective data transfer.