Tag Archives: Network

The points of TCP retransmission you must know (1)

Published / by Chen Ye / 5 Comments on The points of TCP retransmission you must know (1)


TCP retransmission is a mechanism to ensure the data integrity and completeness especially in the bad network environment.


Binary Exponential backoff:

A simple example of TCP’s timeout and retransmission mechanism. The first retransmit occurs at time 42.954, followed by other retransmissions at times 43.374, 44.215, 45.895, and 49.255. The intervals between successive retransmissions are 206ms, 420ms, 841ms, 1.68s, and 3.36s, respectively. These times represent a doubling of the timeout between successive retransmissions of the same segment. This doubling of time between successive retransmissions is called a “binary exponential backoff”, If we measure the elapsed time between the initial request and the time at which the connection is finally aborted, the total time is about 15.5 minutes. After that, we will get the error message.

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What is duplicate packets?

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Although rare, the IP protocol may deliver a single packet more than one time. This can happen, for example, when a link-layer network protocol performs a retransmission and creates two copies of the same packet. When duplicates are created, TCP can become confused in some of the ways we have seen already. Consider the case shown in following figure in which packet number 3 has been duplicated three times.

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What is out of order packets?

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What is out of order?

IP can choose another path for traffic (e.g., that is faster) without having to worry about the consequences that doing so may cause traffic freshly injected into the network to pass ahead of older traffic, resulting in the order of packet arrivals at the receiver not matching the order of transmission at the sender.

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TCP RST flag help to trace back the protocol issues

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Performance is one of the most critical metrics for any IT environment. It directly mapped to the productivity. In order to figure out which components or entities slowing down the overall performance, we need to find out evidence to support our points. Today let’s learn one of TCP layer knowledge regarding RST to locate the performance issue.

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NTP client time out of Sync?

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We know NTP is used to synchronize time from authorized time server to the client to keep the client local time consistent with standard time.

However, what if we successfully set up the NTP servers, but the client is still out of sync with NTP server time to time which causes authentication issue? We need to know how to troubleshoot the related issue.


Knowledge Prerequisite:

Before troubleshooting the NTP related issues, you should have following knowledge:

  • What is a reference clock?
  • How will NTP use a reference clock?
  • How will NTP know about Time Sources?
  • What happens if the Reference Time changes?
  • How is Time synchronized?
  • Which Network Protocols are used by NTP?
  • When are the Servers polled?
  • How frequently will the System Clock be updated?
  • How frequently are Correction Values updated?

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What is MTU and How to use it to locate network issues

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It is normal to see the TCP retransmission most of time which will cause lots of different network issue, packet drop etc. Today let’s have a short discussion about improper size of MTU caused packet drop during the transmission.



PDU Protocol Data Unit (describes a message at some protocol layer; sometimes used interchangeably and informally with packet, frame, datagram, segment, or message)

MTU Maximum Transmission unitThere is a limit on the size of the frame available for carrying the PDUs of higher-layer protocols in many link-layer networks such as Ethernet. For the Ethernet, usually limits the number of payload bytes to about 1500.

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