Question

Consider a simple UDP-based protocol for requesting files (based somewhat loosely on the Trivial File Transport Protocol, TFTP). The client sends an initial file request, and the server answers (if the file can be sent) with the first data packet. Client and server then continue with a stop-and-wait transmission mechanism. (a) Describe a scenario by which a client might request one file but get another; you may allow the client application to exit abruptly and be restarted with the same port. (b) Propose a change in the protocol that will make this situation much less likely.

Short answer

Using a unique transaction identifier in each file request can prevent the server from confusing multiple file requests from a client.

Step-by-step solution

- Understanding Initial Conditions

The client sends a file request using a UDP-based protocol, and the server responds with a data packet transmission. This process continues with a stop-and-wait mechanism.

- Scenario Analysis for Miscommunication

Imagine the client requests a file (let's call it File A) and receives the initial data packet for File A from the server. If the client application exits abruptly and is restarted using the same port, it might send a new file request (perhaps for File B) before the server has cleared the previous state associated with the original request. The server, still thinking the ongoing transmission is for File A, might send packets of File A in response to the new request.

- Identifying the Issue

The problem arises because the server sends the file data based on the assumption that the current state is still relevant. The client, using the same port, confuses the server about which file is being requested, leading to the wrong file data being sent.

- Protocol Modification Proposal

In order to prevent this, the protocol could include a unique transaction identifier for each file request. Every request from the client would include a unique ID that the server must check before sending data packets. This way, even if the client is restarted and sends a new request with a new ID, the server will recognize it as a new transaction and not confuse it with the ongoing one.

Key concepts

Trivial File Transport Protocol (TFTP)

The Trivial File Transport Protocol (TFTP) is a simplistic protocol to transfer files using the UDP protocol. Unlike more complex protocols such as FTP, TFTP doesn't authenticate users or encrypt data.
This makes it lightweight and easy to implement, but also less secure.
Its operation is straightforward: a client sends a request for a file, and the server sends back the file in small data packets.
The primary advantage of TFTP is its simplicity, which makes it ideal for limited or embedded systems where resources are constrained.
However, it is also why TFTP is limited to simple file transfer operations and is more prone to issues compared to more robust protocols.

stop-and-wait transmission

Stop-and-wait transmission is a data communication method where the sender sends one frame at a time and waits for an acknowledgment (ACK) from the receiver before sending the next frame.
This ensures that each packet is received correctly before the sender moves on to the next.
This method is reliable because it manages errors, but it can be inefficient due to the waiting period.
In the context of file transfer, like with TFTP, the sender (server) sends a packet and waits for the client to confirm receipt.
If no ACK is received or an error is detected, the packet is retransmitted.
This process continues until all packets have been successfully transmitted and acknowledged.

unique transaction identifier

A unique transaction identifier is a distinctive ID assigned to each communication session or request.
In file transfer protocols like TFTP, it helps manage and track individual file transfer sessions.
This ensures that data packets meant for one request are not mistakenly sent in response to another request.
In the scenario where a client abruptly exits and restarts, using the same port, without a unique transaction ID, the server may get confused and continue sending data from a previous request.
Introducing unique transaction IDs in the protocol ensures that each new file request is treated as a separate transaction.
This mismatch in IDs between an old and new session prevents the server from mistaking one request for another, thereby reducing the risk of file mix-ups.