Question

Two users, one using Telnet and one sending files with FTP, both send their traffic out via router \(R\). The outbound link from \(R\) is slow enough that both users keep packets in R's queue at all times. Discuss the relative performance seen by the Telnet user if \(\mathrm{R}\) 's queuing policy for these two flows is (a) round-robin service (b) fair queuing (c) modified fair queuing, where we count the cost only of data bytes, and not IP or TCP headers Consider outbound traffic only. Assume Telnet packets have 1 byte of data, FTP packets have 512 bytes of data, and all packets have 40 bytes of headers.

Short answer

Telnet: Poor with round-robin, better with fair queuing, best with modified fair queuing.

Step-by-step solution

- Understand the Packet Sizes

Identify the sizes of the packets for both Telnet and FTP. Telnet packets have 1 byte of data and 40 bytes of headers, totaling 41 bytes. FTP packets have 512 bytes of data and 40 bytes of headers, totaling 552 bytes.

- Round-Robin Service

In the round-robin queuing policy, each flow gets an equal opportunity to send packets. For every Telnet packet (41 bytes), one FTP packet (552 bytes) is also sent. Telnet user will face higher delays because FTP packets are much larger.

- Fair Queuing

In fair queuing, the system allocates equal bandwidth to each user. Since Telnet packets are smaller (41 bytes) than FTP packets (552 bytes), Telnet user will send more packets per unit time, leading to better performance for the Telnet user compared to round-robin.

- Modified Fair Queuing

If modified fair queuing is applied, and only data bytes are counted, the Telnet packets (1 byte of data) will face less competition compared to FTP packets (512 bytes of data). This gives Telnet packets even more frequent transmission opportunities, improving performance for the Telnet user significantly.

Key concepts

round-robin queuing

Round-robin queuing is a scheduling technique where each flow of data gets its turn to send a packet in a circular order. Imagine a round table with the Telnet and FTP users as participants. The router serves one packet from Telnet, then one from FTP, and so on. This method is designed to ensure a fair distribution of service time among users.
However, because Telnet packets are much smaller (41 bytes) compared to FTP packets (552 bytes), the Telnet user can experience higher delays. Every FTP packet takes longer to send, causing the Telnet user to wait more. This can be quite frustrating for the Telnet user, as their smaller packets are often stuck behind the larger FTP packets.

fair queuing

Fair queuing is a more sophisticated method compared to round-robin queuing. It aims to provide a fair share of the bandwidth to each user. Here, the system considers the size of the packets and provides service accordingly. For instance, since Telnet packets are smaller, more Telnet packets will be delivered in the same time it takes to send a few large FTP packets.
This means that the Telnet user will see better performance because their smaller packets can be transmitted more frequently. Overall, fair queuing helps to balance the service each user gets, ensuring a more equitable experience.

modified fair queuing

Modified fair queuing takes fairness a step further by focusing only on the data bytes, ignoring the headers. In TCP/IP, headers add extra bytes to each packet: 40 bytes in our example. In this method, for Telnet, only the 1 byte of data is counted and for FTP, the 512 bytes of data are considered.
This adjustment gives an edge to the Telnet user as they have very little data per packet. As a result, Telnet packets face much less competition and are transmitted more often. This greatly enhances the Telnet user's experience, providing them with quicker and more reliable service.

packet sizes

Packet size plays a crucial role in network performance. A packet contains both data and protocol headers. The sizes for our example are:

• Telnet: 1 byte data + 40 bytes headers = 41 bytes
• FTP: 512 bytes data + 40 bytes headers = 552 bytes

The larger the packet, the longer it takes for the router to send it. This difference in packet size between Telnet and FTP impacts their performance under different queuing policies. Smaller packets, such as those from Telnet, can be handled more quickly, reducing latency. This is why understanding packet sizes is so important when considering network performance and queuing policies.

network performance

Network performance is a measure of how well data is transmitted across the network. Key factors influencing it include:

• Bandwidth: The maximum rate of data transfer across a network path.
• Latency: The delay before data transfer begins following an instruction.
• Throughput: The actual amount of data successfully transferred.

Queuing policies directly impact these factors. Round-robin may increase latency for smaller packets, while fair and modified fair queuing can improve throughput and reduce latency for all users. In scenarios where packet sizes vary significantly, like between Telnet and FTP, choosing the right queuing policy can drastically improve overall network performance and user satisfaction.