Question

Consult Request for Comments 793 to find out how TCP is supposed to respond if a FIN or an RST arrives with a sequence number other than NextByteExpected. Consider both when the sequence number is within the receive window and when it is not.

Short answer

Acknowledge with NextByteExpected if within receive window; discard if outside.

Step-by-step solution

- Understand TCP Sequence Numbers

TCP (Transmission Control Protocol) uses sequence numbers to track the order of bytes. Each byte in a TCP stream has a unique sequence number, starting from an initial sequence number provided during the TCP handshake.

- Define FIN and RST Flags

The FIN flag signifies the sender's finished sending data, while the RST flag is used to reset the connection, usually due to errors or unexpected conditions.

- Review RFC 793 for Handling FIN/RST

RFC 793 specifies how TCP is to handle different segments, including those with FIN and RST flags. Find the sections in RFC 793 that describe these behaviors.

- FIN/RST with Sequence Number Other than NextByteExpected

According to RFC 793: - If a FIN or RST arrives and the sequence number is within the receive window but not exactly the NextByteExpected, an acknowledgment should be sent for the current NextByteExpected. - If the FIN or RST sequence number is outside the receive window, the segment should be discarded, and no acknowledgment should be sent other than for the current NextByteExpected.

- Handle Cases Within and Outside Receive Window

Summarize the protocols: - If within the receive window but not exactly NextByteExpected, acknowledge with NextByteExpected. - If outside the receive window, discard the segment and do not alter the current acknowledgment other than NextByteExpected.

Key concepts

TCP Protocol

TCP, or Transmission Control Protocol, is essential for ensuring reliable data transfer across networks. It breaks data into packets and ensures packets are sent, received, and reassembled correctly.
Here are some key points:

• TCP operates in a connection-oriented manner, requiring a connection to be established between the sender and receiver before data transmission.
• Each byte of data has a sequence number, allowing TCP to keep track of what has been sent and received.
• TCP uses acknowledgments (ACK) to confirm successful receipt of data.
• This protocol ensures error checking and data integrity, making it reliable for data transmission even over unstable networks.

Understanding these basics can help you grasp more advanced concepts, such as the handling of specific flags and sequence numbers in TCP connections.

FIN Flag

The FIN flag, short for 'finish,' is a control flag in the TCP protocol used to indicate that the sender has finished sending data.
Key aspects of the FIN flag:

• When a sender sets the FIN flag, it signals that it has no more data to send, concluding this side of the transmission.
• The receiver, upon encountering a FIN flag, will acknowledge it by sending an ACK and then proceed with its termination steps.
• Both sides of the connection need to exchange FIN flags to close a TCP connection completely.

If a FIN flag arrives with a sequence number different from the NextByteExpected, but within the receive window, an acknowledgment should still be sent for the current NextByteExpected.

RST Flag

The RST flag, or 'reset,' is another critical control flag in the TCP protocol.
It serves to reset a connection when an error or unexpected condition is encountered.

• Setting the RST flag immediately terminates the connection, without the need for exchanging additional flags or sequence numbers.
• This flag is typically used to recover from a situation that cannot be resolved otherwise, such as an unintentional port connection or a corrupted data stream.
• If an RST arrives and its sequence number is inside the receive window but does not match the NextByteExpected, an acknowledgment should be sent, acknowledging the current NextByteExpected.
• When the RST flag comes with a sequence number outside the receive window, the segment is discarded, and no acknowledgment is sent other than for the current NextByteExpected.

Thus, the RST flag is like an emergency stop in the TCP protocol.

RFC 793 Handling of FIN/RST Flags

RFC 793 provides the essential guidelines for managing TCP operations, including dealing with FIN and RST flags.
Here’s how RFC 793 suggests handling sequences not equivalent to the NextByteExpected:

• If a FIN or RST arrives and the sequence number is still within the receive window but not exactly matching the NextByteExpected, an acknowledgment is to be sent for the current NextByteExpected.
• If the sequence number of the FIN or RST is outside the receive window, the segment should be discarded, and no acknowledgment other than for the current NextByteExpected should be sent.
• This ensures that the integrity of the ongoing sequence is not disrupted and that erroneous or unexpected conditions are appropriately responded to.

By handling these scenarios as prescribed, TCP maintains the reliability and order of data transmission, upholding its core principle of providing a trustworthy communication channel.