Question

Consider a simple protocol for transferring files over a link. After some initial negotiation, A sends data packets of size \(1 \mathrm{~KB}\) to B; B then replies with an acknowledgment. A always waits for each ACK before sending the next data packet; this is known as stop-and-wait. Packets that are overdue are presumed lost and are retransmitted. (a) In the absence of any packet losses or duplications, explain why it is not necessary to include any "sequence number" data in the packet headers. (b) Suppose that the link can lose occasional packets, but that packets that do arrive always arrive in the order sent. Is a 2-bit sequence number (that is, \(N\) mod 4) enough for \(\mathrm{A}\) and \(\mathrm{B}\) to detect and resend any lost packets? Is a 1-bit sequence number enough? (c) Now suppose that the link can deliver out of order, and that sometimes a packet can be delivered as much as 1 minute after subsequent packets. How does this change the sequence number requirements?

Short answer

No sequence numbers are needed without packet loss. A 2-bit sequence number is enough for occasional losses. For out-of-order delivery, a larger sequence number is required.

Step-by-step solution

Explain the absence of sequence numbers

Since A waits for an acknowledgment from B before sending the next packet, it is impossible to have more than one packet in transit at any time. Therefore, there is no need to distinguish between packets because only one can be sent or acknowledged at a time, eliminating the need for sequence numbers.

Analyze 2-bit sequence number in case of loss

A 2-bit sequence number allows tracking up to 4 different packet identifiers (0, 1, 2, 3). Given that packets always arrive in order and the link might occasionally lose packets, the sequence numbers help to uniquely identify each packet and acknowledge them accordingly. Therefore, a 2-bit sequence number is sufficient for resending lost packets in this scenario.

Analyze 1-bit sequence number in case of loss

A 1-bit sequence number has only 2 different packet identifiers (0 and 1). With this, it is insufficient to distinguish packets beyond the second transmission, making it inadequate for detecting and resending occasional lost packets. Therefore, a 1-bit sequence number is not enough.

Sequence number for out-of-order delivery

When out-of-order delivery is possible, the sequence number must be large enough to distinguish packets over the maximum delay period, in this case, 1 minute. To ensure unique identification within this period, the sequence number should be large enough to cover all possible in-transit packets over that time span, typically requiring more than 2 bits. The exact sequence number range would depend on the maximum possible packet rate.

Key concepts

Sequence Numbers

In the stop-and-wait protocol, sequence numbers are vital for tracking the order of packets.
Each packet gets a unique identifier, called a sequence number. These identifiers help ensure that packets are received and processed in the correct order.
Without sequence numbers, we cannot differentiate between new data and old data being retransmitted due to loss.

When there is no packet loss or duplication, sequence numbers might seem unnecessary. In this ideal scenario, since each packet waits for an acknowledgment before the next is sent, there is no risk of confusion.
However, in real-world situations, losses and duplications can occur, making sequence numbers essential.
For example:

• A 1-bit sequence number cycles between 0 and 1, which can lead to confusion after two transmissions as the same number may represent different packets.
• A 2-bit sequence number cycles between 0 and 3, providing a larger range to uniquely identify packets and minimize confusion during transmission.

Thus, sequence numbers are crucial for maintaining order and reliability in data transfer, especially when disturbances are present.

Packet Loss

Packet loss happens when data packets do not reach their destination.
This can be due to network congestion, errors, or other issues.

In the stop-and-wait protocol, detecting packet loss is crucial. When a sender, such as A, does not receive an acknowledgment from B within a certain time, it assumes the packet is lost and retransmits it.
This mechanism ensures that even when packets are lost, the data eventually reaches its destination.

Using sequence numbers helps to manage packet loss efficiently.

• With a 2-bit sequence number, up to 4 distinct sequences (0-3) can be used, making it easier to track the loss and retransmit the correct packet.
• With only a 1-bit sequence number, there are only two possible values (0 and 1), leading to ambiguity after just two packets.

Therefore, packet loss mechanisms work hand in hand with sequence numbers to ensure data integrity and reliable communication.

Out-of-Order Delivery

Out-of-order delivery occurs when packets arrive in a different order than they were sent.
This can happen due to variations in the paths packets take across a network or types of delays.

In a simple scenario with only in-order delivery, sequence numbers may not be as crucial.
However, when out-of-order delivery is possible, sequence numbers become essential.
They help to reassemble packets in the correct order at the destination.

For example:

• If a packet sent later arrives first, the sequence number helps the receiver identify and reorder the packets before processing them.
• Without sequence numbers, the receiver would be unable to detect that packets are out of order, leading to potential data corruption or errors in the transmitted file.

Furthermore, the sequence number should be large enough to cover all possible in-transit packets during the maximum delay period.
A larger sequence number range ensures each packet can be uniquely identified and correctly reordered, making the system robust against out-of-order delivery.

Acknowledgement Packets

Acknowledgment packets (ACKs) are messages sent by the receiver (B) to the sender (A) to confirm packet reception.
In the stop-and-wait protocol, the sender waits for an ACK before sending the next packet.

This process ensures each packet is received and acknowledged before proceeding.
If the sender does not receive an ACK within a certain timeframe, it assumes the packet was lost and retransmits it:

• This method reduces the likelihood of data loss as each packet must be acknowledged.
• ACKs also help in managing flow control, ensuring the sender does not overwhelm the receiver.

ACKs and sequence numbers together help maintain data integrity.
The sequence number in the ACK allows the sender to identify which packet was acknowledged and to detect any necessary retransmissions:

• For in-order delivery with occasional losses, a 2-bit sequence number allows sufficient identification for resending.
• For more complex scenarios with out-of-order delivery, larger sequence numbers and corresponding ACKs ensure proper data reconstruction.

Thus, acknowledgments play a crucial role in maintaining reliable communication under various network conditions.