Problem 14 - Computer Networking: A Top-Down Approach

Question

True or false? a. Host \(A\) is sending Host \(B\) a large file over a TCP connection. Assume Host B has no data to send Host A. Host B will not send acknowledgments to Host A because Host B cannot piggyback the acknowledgments on data. True or false? a. Host \(\mathrm{A}\) is sending Host \(\mathrm{B}\) a large file over a \(\mathrm{TCP}\) connection. Assume Host B has no data to send Host A. Host B will not send acknowledgments to Host A because Host B cannot piggyback the acknowledgments on data. b. The size of the TCP rwnd never changes throughout the duration of the connection. c. Suppose Host A is sending Host B a large file over a TCP connection. The number of unacknowledged bytes that A sends cannot exceed the size of the receive buffer. d. Suppose Host \(\mathrm{A}\) is sending a large file to Host \(\mathrm{B}\) over a TCP connection. If the sequence number for a segment of this connection is \(m\), then the sequence number for the subsequent segment will necessarily be \(m+1\). e. The TCP segment has a field in its header for rwnd. f. Suppose that the last SampleRTT in a TCP connection is equal to \(1 \mathrm{sec}\). The current value of TimeoutInterval for the connection will necessarily be \(\geq 1 \mathrm{sec}\). g. Suppose Host A sends one segment with sequence number 38 and 4 bytes of data over a TCP connection to Host B. In this same segment the acknowledgment number is necessarily 42 .

Step-by-Step Solution

Answer

a. False; b. False; c. True; d. False; e. True; f. False; g. True.

Step 1: Understanding the TCP Acknowledgments

In statement a, we need to decide whether Host B will send acknowledgments even if it has no data to send back to Host A. In a TCP connection, acknowledgments are crucial for ensuring data is correctly received, and they are sent regardless of whether Host B has data to send back. Piggybacking on data is optional but not required for sending acknowledgments. Thus, Host B will send acknowledgments independently of having data to piggyback on.

Step 2: TCP Receive Window Size

For statement b, the concern is about the TCP receive window (rwnd) size changing. The receive window is dynamic and can change throughout the session to accommodate varying network conditions, like changes in available buffer space. Therefore, stating that the rwnd never changes during the connection is false.

Step 3: Maximum Unacknowledged Bytes in TCP

In statement c, the report is on how many unacknowledged bytes Host A can send. TCP ensures data transfer within limits specified by the receiver's buffer size to prevent overflow, meaning the number of unacknowledged bytes should not exceed the receive buffer size. Therefore, this statement is true.

Step 4: TCP Sequence Numbers

Statement d revolves around TCP sequence numbers. TCP sequence numbers increase by the number of bytes transmitted, not merely by one. Consequently, if a segment with sequence number \(m\) contains several bytes, the next sequence number will reflect the total number of bytes from \(m\), not \(m+1\). This statement is false.

Step 5: TCP Header Fields

In statement e, the TCP header includes several fields, including ones for flow control such as the receive window (rwnd). Therefore, it is true that TCP has a header field for rwnd.

Step 6: TCP Timeout Interval Calculation

For statement f, relating to the TimeoutInterval, it adapts based on network conditions and the computed EstimateRTT. While the last SampleRTT is equal to 1 second, the TimeoutInterval can be greater, as it typically accounts for variability and network delay factors. However, it isn't "necessarily" \(\geq 1\) sec, since parameters such as DevRTT can influence it to potentially be less.

Step 7: TCP Acknowledgment Number Calculation

Finally, statement g discusses the acknowledgment number if Host A sends 4 bytes starting from sequence number 38. The acknowledgment number should be the cumulative acknowledgment of the next expected byte, which should be 38 + 4, that is 42. Thus, the statement is true.

Key Concepts

TCP Acknowledgments

In a TCP connection, acknowledgments are vital for ensuring data integrity and reliability. Host B will send acknowledgments back to Host A even if Host B has no data to transmit. This process is independent of piggybacking, which is the practice of sending acknowledgment packets alongside data packets to optimize network usage. Piggybacking is optional; acknowledgments in a TCP connection are mandatory and operate continuously to inform the sender about successfully received packets. This mechanism ensures continuous communication and error detection, allowing retransmissions when necessary.

TCP Receive Window

The TCP receive window (rwnd) is a crucial part of flow control, dictating how much data the receiver can process at any given time. It is dynamic and adjusts in response to network conditions, such as buffer availability and network congestion. This adjustment occurs to ensure efficient data flow and prevent loss by adapting to current transmission capabilities. Because of this adaptability, the statement that the size of the TCP receive window never changes is false. Understanding rwnd helps in managing throughput and avoiding data overflow on the receiving end.

TCP Sequence Numbers

TCP sequence numbers are used to keep track of the bytes during transmission. Unlike some assumptions, TCP sequence numbers do not merely increase by one for each segment sent. Instead, the sequence number reflects the number of bytes transmitted, increasing accordingly. For example, if a segment starts with sequence number \( m \) and contains several bytes, the subsequent sequence number will reflect this total, not simply \( m + 1 \). This mechanism is part of TCP's design to ensure orderly and accurate data transmission, helping the receiver properly reconstruct the message.

TCP Header Fields

The TCP header encompasses various fields critical for connection management, including the receive window (rwnd). These fields provide essential parameters for synchronization, data flow control, and reliability. The rwnd field within the header helps manage data flow by informing the sender about the available buffer space, allowing the sender to adjust the data flow rate appropriately. This flow control is vital for maintaining efficient communication and preventing data loss due to buffer overflow. The presence of specific fields like rwnd in the TCP header illustrates the protocol's comprehensive design for managing diverse network conditions.