Chapter 2: Problem 33
Suppose that we attempt to run the sliding window algorithm with SWS = RWS = 3 and with MaxSeqNum \(=5 .\) The Nth packet DATA[ \(N]\) thus actually contains \(N\) mod 5 in its sequence number field. Give an example in which the algorithm becomes confused; that is, a scenario in which the receiver expects DATA[5] and accepts DATA[0]-which has the same transmitted sequence number-in its stead. No packets may arrive out of order. Note this implies MaxSeqNum \(\geq 6\) is necessary as well as sufficient.
Short Answer
Expert verified
The receiver accepts DATA[0] when expecting DATA[5] because both have the same sequence number (0).
Step by step solution
01
Understand the parameters
The Sliding Window Size (SWS) and Receiver Window Size (RWS) are both set to 3. The maximum sequence number (MaxSeqNum) is given as 5.
02
Identify packet sequence numbers
Because the maximum sequence number is 5, the sequence numbers will wrap around after 4. Therefore, sequence numbers will be 0, 1, 2, 3, 4 and then wrap to 0 again.
03
Determine the packet numbers the sender wishes to send
Consider that the sender sends DATA[0], DATA[1], DATA[2] successfully, followed by DATA[3] and DATA[4]. The sequence numbers in these packets will be 0, 1, 2, 3, and 4, respectively.
04
Describe the receiver's expectation
After receiving DATA[0] to DATA[4], the receiver expects the next packet to have sequence number 0 (which corresponds to DATA[5]).
05
Describe the confusion scenario
When the sender sends DATA[0] again (since the sequence numbers wrap around), the receiver expects DATA[5] and receives DATA[0] instead because both share the same sequence number (0).
06
Conclude the example
The receiver gets confused because it cannot distinguish between DATA[5] and DATA[0] due to identical sequence numbers, which arises because the maximum sequence number is too small.
Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Sequence Numbers
In networking, a sequence number is assigned to each packet to help track and organize the transmission of data. Think of sequence numbers as a way to label packets so they can be identified uniquely and arranged correctly at the receiver's end.
For example, when the maximum sequence number (MaxSeqNum) is 5, the sequence numbers cycle through 0, 1, 2, 3, 4, and then back to 0. This cycling can create a challenging scenario.
One potential problem arises when the receiver cannot differentiate between packets sent in current and previous cycles, leading to confusion, especially when the sequence numbers repeat themselves. This process is often referred to as wrapping around because the sequence numbers wrap back to the beginning once the maximum is reached.
For example, when the maximum sequence number (MaxSeqNum) is 5, the sequence numbers cycle through 0, 1, 2, 3, 4, and then back to 0. This cycling can create a challenging scenario.
One potential problem arises when the receiver cannot differentiate between packets sent in current and previous cycles, leading to confusion, especially when the sequence numbers repeat themselves. This process is often referred to as wrapping around because the sequence numbers wrap back to the beginning once the maximum is reached.
SWS and RWS
Sliding Window Size (SWS) and Receiver Window Size (RWS) are crucial parameters that determine the number of packets the sender can transmit before requiring an acknowledgment and the number of packets the receiver can store.
In our given example, both SWS and RWS are set to 3. This means the sender can send up to 3 packets without waiting for an acknowledgment, and the receiver can accept up to 3 packets before needing to process them.
This balancing act is essential because it ensures efficient communication. However, with MaxSeqNum set to 5, there's a specific situation where the sender may resend a packet (DATA[0]) after wrapping around sequence numbers, and the receiver mistakes it for a new packet (DATA[5]).
This overlap, caused by the limited range of sequence numbers, necessitates a larger MaxSeqNum, ideally greater than or equal to 6, to avoid confusion.
In our given example, both SWS and RWS are set to 3. This means the sender can send up to 3 packets without waiting for an acknowledgment, and the receiver can accept up to 3 packets before needing to process them.
This balancing act is essential because it ensures efficient communication. However, with MaxSeqNum set to 5, there's a specific situation where the sender may resend a packet (DATA[0]) after wrapping around sequence numbers, and the receiver mistakes it for a new packet (DATA[5]).
This overlap, caused by the limited range of sequence numbers, necessitates a larger MaxSeqNum, ideally greater than or equal to 6, to avoid confusion.
Packet Transmission
Packet transmission refers to the method of sending and receiving data across a network in chunks, called packets. Each packet contains a sequence number to maintain order and ensure data integrity. In the sliding window algorithm, packets are sent sequentially, and acknowledgments (ACKs) are used to notify the sender that packets have been received successfully.
Let's consider the packets DATA[0], DATA[1], DATA[2], followed by DATA[3] and DATA[4], are transmitted using sequence numbers 0, 1, 2, 3, and 4, respectively. Once DATA[4] is sent, the next sequence number will wrap back to 0 for DATA[5].
If the receiver doesn't receive DATA[5] but instead gets DATA[0] again, it can cause confusion because both packets have the sequence number 0. To prevent this, the sliding window algorithm must be implemented with a larger MaxSeqNum to create a clearer separation of subsequent packets, thus reducing overlap and ensuring the receiver's expectation aligns correctly with what they receive.
Let's consider the packets DATA[0], DATA[1], DATA[2], followed by DATA[3] and DATA[4], are transmitted using sequence numbers 0, 1, 2, 3, and 4, respectively. Once DATA[4] is sent, the next sequence number will wrap back to 0 for DATA[5].
If the receiver doesn't receive DATA[5] but instead gets DATA[0] again, it can cause confusion because both packets have the sequence number 0. To prevent this, the sliding window algorithm must be implemented with a larger MaxSeqNum to create a clearer separation of subsequent packets, thus reducing overlap and ensuring the receiver's expectation aligns correctly with what they receive.
