Question

If host \(\mathrm{A}\) receives two SYN packets from the same port from remote host \(\mathrm{B}\), the second may be either a retransmission of the original or else, if B has crashed and rebooted, an entirely new connection request. (a) Describe the difference as seen by host A between these two cases. (b) Give an algorithmic description of what the TCP layer needs to do upon receiving a SYN packet. Consider the duplicate/new cases above, and the possibility that nothing is listening to the destination port.

Short answer

Retransmission has the same sequence number; a new connection has a different sequence number. Algorithm: check if the port is listening, compare sequence numbers, send appropriate responses based on the match.

Step-by-step solution

- Understanding the TCP Three-Way Handshake

The TCP three-way handshake is essential for establishing a connection between a client and a server. The process involves:1. SYN: Client sends a synchronization packet to initiate the connection.2. SYN-ACK: Server acknowledges the SYN packet and sends its own SYN packet.3. ACK: Client acknowledges the server’s SYN-ACK packet.Understanding this process helps in differentiating between retransmission and a new connection request.

- Identifying Retransmission vs New Connection Request

When host A receives a second SYN packet from the same port on host B, it needs to determine whether this is a retransmission or a new connection request. The key differences are:- Retransmission: Likely due to the original SYN-ACK being lost or delayed. The sequence number in the SYN packet will be identical to the original.- New Connection Request: Host B has crashed and rebooted, and the sequence number in the SYN packet will differ from the original.

- Algorithm for Handling a SYN Packet

The TCP layer on host A can follow this algorithm:1. Check if the destination port is listening: - If not, send an RST (reset) packet in response to the SYN packet.2. If the port is listening, check the sequence number in the SYN packet: - If the sequence number matches the existing connection’s initial sequence number, treat it as a retransmission. Send SYN-ACK again. - If the sequence number differs, treat it as a new connection request and initiate the TCP three-way handshake.3. Maintain a state table that logs the sequence numbers of connection attempts to help differentiate between retransmissions and new requests.

- Managing States

Host A must manage connection state information to differentiate between retransmissions and new connections:- For each connection, store the initial sequence number and other relevant information in a state table.- When a SYN packet is received, compare the sequence number with the recorded initial sequence number to determine if it is a retransmission or a new connection request.

Key concepts

SYN packet

The SYN packet is the cornerstone of the TCP three-way handshake. When initiating a connection, the client sends a SYN (synchronize) packet to the server. This packet does two essential things: it synchronizes sequence numbers, and it sets up the initial parameters for the TCP session. The SYN packet contains a sequence number, which helps in identifying and reassembling data packets in the correct order. In the context of a retransmission, the sequence number in the SYN packet will be identical to the original. However, if the remote host has crashed and rebooted, a new sequence number will be generated.
This difference in the sequence number is crucial for Host A to discern whether the second SYN packet it received is for resending due to a lost packet or for initiating a new connection.

Sequence number

Sequence numbers in TCP are vital for maintaining data integrity and ordering. Each packet sent over a TCP connection contains a sequence number, which indicates the first byte of the data contained in that packet. This allows the receiving end to reassemble the packets in the correct order.

• **Retransmission:** If Host A receives a SYN packet with the same sequence number as an already established connection, it’s likely a retransmission, possibly due to the original SYN-ACK being lost or delayed.
• **New Connection:** If Host B has crashed and rebooted, the new SYN packet will have a different sequence number.

Recognizing the difference ensures the TCP layer can effectively manage connection states, preventing confusion between old and new connection requests. A simple comparison of the incoming sequence number with existing ones helps determine the next course of action.

Connection state

Managing connection states is a critical function of the TCP layer. Upon receiving a SYN packet, Host A must correctly identify the state of the connection. Here’s an algorithm that can be followed:
1. **Check Listening Status:** If the port isn’t listening, Host A should send a Reset (RST) packet back, indicating that the connection can’t be established.
2. **Verify Sequence Number:** When the port is listening, compare the sequence number in the SYN packet:

• If it matches the existing connection’s initial sequence number, it’s a retransmission, prompting a SYN-ACK.
• If it differs, treat it as a new connection request, initiating the TCP three-way handshake.

3. **State Table Maintenance:** To manage multiple connections efficiently, Host A should maintain a state table that logs initial sequence numbers and other critical data.
This state management allows Host A to accurately differentiate between a new connection request and a retransmission, ensuring reliable and orderly communication.