Скачать или смотреть Understanding TCP Sequence Numbers: A Breakdown of Packet Ordering and Retransmission

Explore how TCP handles packet sequencing and the possibility of receiving duplicate packets with sequence numbers in a specific pattern.
---
This video is based on the question https://stackoverflow.com/q/62482492/ asked by the user 'user855' ( https://stackoverflow.com/u/211528/ ) and on the answer https://stackoverflow.com/a/62482573/ provided by the user 'David Schwartz' ( https://stackoverflow.com/u/721269/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: In TCP protocol, is it possible for receiver to get packets with sequence numbers in this pattern?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding TCP Sequence Numbers: A Breakdown of Packet Ordering and Retransmission

In the world of networking, the Transmission Control Protocol (TCP) plays a crucial role in ensuring that packets of data are transmitted correctly and in order. However, there are scenarios where a receiver may encounter packets with repeated sequence numbers. This guide delves into one such situation, examining whether it is feasible for a receiver to receive packets in the specific sequence pattern of 1, 2, 3, 1, 2, 3.

The Problem at Hand

Imagine a sender quickly dispatching packets labeled 1, 2, and 3. Due to various factors, such as network delays or issues, our receiver may not acknowledge these packets on time. Consequently, the sender may re-transmit the same packets, resulting in the receiver seeing the sequence numbers in the pattern:

[[See Video to Reveal this Text or Code Snippet]]

The fundamental question is: Is this scenario possible within TCP?

The Short Answer: Yes, It Is Possible

The answer is yes. It is indeed possible for a receiver to get packets with sequence numbers in the pattern mentioned above. To understand why let's consider how TCP operates at a deeper level.

The Role of the Internet Protocol (IP)

Before we delve into TCP specifics, it's essential to understand that TCP operates on top of the Internet Protocol (IP). IP has certain characteristics that can lead to this situation:

Out-of-Order Delivery: IP packets can arrive at their destination in any order.

Duplication: IP packets can be duplicated during transmission.

Loss: Some packets may get lost and not reach the intended recipient.

TCP's Methodology

To tackle the challenges posed by IP, TCP implements various mechanisms to ensure reliable data transmission:

Acknowledgments (ACKs): The receiver sends back acknowledgments to confirm receipt of packets.

Retransmissions: If the sender does not receive an acknowledgment for its transmitted packets within a certain timeframe, it assumes that the packets were lost and retransmits them.

Sequence Numbers: TCP assigns sequence numbers to packets to track the order of transmission and reception. This way, even if packets arrive out of order, TCP can properly reorder them.

Explanation of the Scenario

Returning to our example:

The sender transmits packets 1, 2, and 3 in quick succession.

For some reason (like network congestion or buffering issues), these packets take longer to reach the receiver.

The sender, unaware of the status of the previous packets, may decide to retransmit 1, 2, and 3.

This results in the sequence being perceived by the receiver as 1, 2, 3, 1, 2, 3.

Key Takeaways

Possible Situations: Given the unreliability of the underlying IP protocol and TCP's buffering and retransmission mechanisms, the described receipt of packets is indeed possible.

Network Dynamics: Factors like network congestion, routing issues, or even packet buffering in routers may contribute to this occurrence.

TCP’s Role: TCP provides a reliable stream by ensuring all packets are delivered and in the correct order, despite the challenges presented by the underlying IP protocol.

Conclusion

In summary, the TCP protocol is designed to handle various network challenges, including out-of-order delivery and packet loss. As demonstrated, it is entirely feasible for a receiver to encounter packets with sequence numbers in the pattern 1, 2, 3, 1, 2, 3. Understanding these nuances is essential for anyone working in networking, as it highlights the complexities and reliability mechanisms inherent in TCP/IP communications.

By keeping these concepts in mind, you can better appreciate how TCP manages to maintain coherent communicatio