Question

Suppose that between \(A\) and \(B\) there is a router \(R\). The \(A-R\) bandwidth is infinite (that is, packets are not delayed), but the R-B link introduces a bandwidth delay of 1 packet per second (that is, 2 packets take 2 seconds, etc.). Acknowledgments from \(\mathrm{B}\) to \(\mathrm{R}\), though, are sent instantaneously. \(\mathrm{A}\) sends data to \(\mathrm{B}\) over a \(\mathrm{TCP}\) connection, using slow start but with an arbitrarily large window size. R has a queue size of 1 , in addition to the packet it is sending. At each second, the sender first processes any arriving ACKs and then responds to any timeouts. (a) Assuming a fixed TimeOut period of 2 seconds, what is sent and received for \(\mathrm{T}=0,1, \ldots, 6\) seconds? Is the link ever idle due to timeouts? (b) What changes if TimeOut is 3 seconds instead?

Short answer

With a 2-second timeout, packets are resent on timeout, causing possible idle periods. With a 3-second timeout, the system can handle delays better, reducing idle periods.

Step-by-step solution

Understanding the Problem

Identify the elements in the network: Sender (A), Router (R), and Receiver (B). The R-B link introduces a delay of 1 second per packet, while the A-R link has no delay. Router R can hold a maximum of 1 packet in its queue, in addition to the packet it is sending.

Initial State and Slow Start Mechanism

At the start (T=0), initialize the TCP slow start process with an initial congestion window size of 1 packet. A sends the first packet to R.

Timing Analysis for T=0 to T=2 with 2-Second Timeout

At T=0, A sends packet 1 to R and R immediately sends it to B. At T=1, packet 1 reaches B and B sends an ACK back to A, arriving instantaneously. A responds by sending two packets (due to slow start). At T=2, R sends packet 2 (queued from T=1) to B, and R queues packet 3.

Handling TimeOut and Response for T=2 to T=4

At T=2, if no ACKs are received by T=2+2=4, A times out and resends packet 3. At T=3, packet 2 reaches B and B sends an ACK, arriving instantly at A. A processes the ACK and sends packets 4 and 5.

Continuing the Analysis for T=4 to T=6

At T=4, packet 3 successfully reaches B and ACK is sent back to A. At T=5, router R sends packet 4 to B and queues packet 5. At T=6, R sends packet 5 to B.

Analyzing Link Idle Periods

Review the timeline to find any idle periods. Due to the router's queue and the 2-second timeout, there is potential idle time if ACKs are not received on time, particularly visible around T=4 before acknowledging the successful transmission.

Changing TimeOut Period to 3 Seconds

Analyze the scenario with TimeOut set to 3 seconds instead of 2. The changes mostly affect the timeout handling. For example, A waits until T=5 before retransmitting a packet if no ACK is received, reducing the likelihood of idle periods caused by timeout triggers.

Key concepts

Network Delay

The concept of network delay is crucial when understanding TCP communication. In our given scenario, the delay is introduced by the R-B link, where each packet takes one second to travel from the router (R) to the receiver (B). This can be generalized to other network situations where data does not flow instantaneously from sender to receiver.
When data packets are sent across a network, they navigate various routes and encounter different types of delays. These delays can be:

• Transmission delay: Time taken to push all bits of the packet onto the wire
• Propagation delay: Time taken for a signal to travel through the medium
• Processing delay: Time routers take to process packet headers
• Queueing delay: Time a packet spends waiting in queue before being transmitted

In our case, the R-B link has a constant delay of 1 second per packet. Therefore, if multiple packets are queued, they take multiple seconds to reach their destination. This delay must be accounted for when calculating the timing of events like acknowledgments and timeouts. Understanding and managing these delays is crucial to ensure efficient network performance.

Packet Acknowledgment

Packet acknowledgment is an essential component of TCP's reliable data transfer mechanism. When a receiver (B) successfully receives a packet from the sender (A), it sends back an acknowledgment (ACK) to inform the sender that the packet arrived safely.
In our scenario, acknowledgments are sent instantaneously from B back to A. This means that as soon as B receives a packet, the ACK reaches A without any delay. This instant feedback is crucial for the sender to know whether it should continue sending more packets or if it needs to resend any packets due to errors or losses.
Here's how packet acknowledgment works in our given scenario:

• At T=1, the first packet reaches B, and an ACK is instantly sent back to A.
• At T=3, the second packet reaches B, and again an ACK is sent instantly to A.

By receiving ACKs, A updates its understanding of which packets have been successfully delivered and adjusts its transmission strategy accordingly. This mechanism ensures data integrity and helps manage flow control, avoiding congestion in the network.

Timeout Period

The timeout period in TCP is the duration the sender waits for an acknowledgment from the receiver before assuming the packet was lost and resending it. This period is critical in managing the network's reliability and performance.
In our exercise, we explore two different timeout periods: 2 seconds and 3 seconds. Let's examine how they impact the transmission:

• With a 2-second timeout, if A does not receive an ACK by T=2, it resends the packet. For instance, if no ACK is received by T=4 for the second packet, A resends it.
• With a 3-second timeout, A waits until T=3 before resending. This longer wait period reduces the chance of unnecessary packet resends due to momentary delays but can introduce idle periods if the ACK arrives just after the timeout.

Choosing an appropriate timeout period is a balancing act. A shorter timeout can lead to unnecessary retransmissions, increasing network load. A longer timeout can create delays, reducing overall network efficiency. Understanding how to set this parameter based on the network's characteristics and delay patterns is key to optimizing TCP performance. In practice, TCP uses an adaptive algorithm to adjust the timeout dynamically, but in academic exercises, fixed periods help illustrate the underlying principles.