Question

Design a simple UDP-based protocol for retrieving files from a server. No authentication is to be provided. Stop-and-wait transmission of the data may be used. Your protocol should address the following issues: (a) Duplication of the first packet should not duplicate the "connection." (b) Loss of the final ACK should not necessarily leave the server in doubt as to whether the transfer succeeded. (c) A late-arriving packet from a past connection shouldn't be interpretable as part of a current connection.

Short answer

Each session has a unique ID with a nonce; each packet includes the ID and timestamp. For the final ACK, use an EOF packet with forced acknowledgment. The timestamp prevents interpreting late packets from old connections.

Step-by-step solution

Define Protocol Initiation

Start by designing a unique identifier for each file transfer session. When a client requests a file, it should send an initial request packet, including a session ID. The server should respond with the same session ID to confirm the session initiation.

Handle Duplication of the First Packet

To avoid duplication of the 'connection,' the client can include a nonce (a random number used once) in the initial request. The server should echo this nonce in its response. This way, if the first packet is duplicated, the server will recognize it from the nonce and avoid starting the connection twice.

Use Stop-and-Wait for Data Transmission

Implement stop-and-wait transmission where the client sends one packet and waits for an acknowledgment from the server before sending the next packet. Each packet should contain the session ID, packet number, and data.

Handle the Loss of the Final ACK

To ensure the server knows the transfer succeeded even if the final ACK is lost, have the client send an 'end-of-file' (EOF) packet. The server should acknowledge receipt of this EOF packet. If the client doesn't receive the acknowledgment, it resends the EOF packet. Upon receiving an acknowledgment for the EOF, the client knows the transfer is complete.

Differentiate Past and Current Connections

Include a timestamp in the session ID. When the server receives a packet with a session ID, it checks the timestamp to ensure it corresponds to the current connection. If a packet from a past connection arrives late, the server can identify and ignore it based on the outdated timestamp.

Key concepts

Protocol Initiation

The initiation of a protocol in a UDP-based file transfer set-up starts with a unique session identifier (ID) for each file transfer. The client sends an initial request packet that includes a session ID to the server. The server acknowledges this session ID, thus confirming the initiation. This process ensures that both the client and server are aware of the specific session they are engaging in. To enhance uniqueness and security, the session ID may include additional elements, such as time stamps or unique random numbers, ensuring no two sessions interfere with one another.

Stop-and-Wait Data Transmission

The stop-and-wait data transmission method is simple yet effective for reliable file transfers via UDP. In this method, the client sends one packet at a time and waits for an acknowledgment (ACK) from the server before sending the next packet. Each data packet contains critical information, including:

• Session ID
• Packet number
• Data payload

This ensures that packets are received and processed sequentially. If the client does not receive an ACK within a certain timeframe, it will resend the packet, guaranteeing that no data is missing or arrives out of order.

Session ID Management

Session ID management is crucial to maintaining the integrity of a file transfer session. By incorporating a unique session ID, possibly with a timestamp, the protocol can differentiate between ongoing and past sessions. The server uses this session ID to validate each packet it receives, ensuring it belongs to the current session.
This prevents old packets from being misinterpreted as part of a new session. Proper management of session IDs is also essential in identifying and handling duplicate packets, as well as ensuring the correct assembly of the data being transferred.

Handling Duplication

To handle the duplication of the first packet and prevent multiple connections from the same request, the client includes a nonce in the initial request packet. A nonce is a random number used only once. The server echoes this nonce back in its response.
If a duplicated packet arrives, the server will recognize it by the nonce and understand that this is not a new connection request. This approach effectively ensures that each initiated session is unique and prevents the server from starting multiple connections due to duplicated first packets.

Handling ACK Loss

Handling the loss of the final ACK is crucial for ensuring the file transfer's completeness. To address this, the client sends an 'end-of-file' (EOF) packet once the file transmission is complete. The server must acknowledge receipt of this EOF packet. If the client does not receive this acknowledgment within a specific timeframe, it will resend the EOF packet.
This process continues until the client receives the acknowledgment, ensuring that both client and server are aware that the transfer has been successfully completed. This redundancy guarantees that the transfer's completion is positively confirmed, even if the final ACK packet is lost.