Monthly Archives: February 2017

What is out of order packets?

Published / by Chen Ye / 3 Comments on What is out of order packets?

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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Starter:

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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Is Host truly representing everything within Windows SPN specs?

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Although in our mind we know if machine joined to the AD domain, it will by default register two records by default HOST/NetBIOSName and HOST/FQDN, and we also know those two SPN represent various services principal names following something called catch-all mechanism.

That means if a Windows client wants to access the SMB share, it could ask KDC to get this machine’s SPN as cifs/FQDN format, if they want to access web services(we assume the machine provide this service), it could ask KDC to get this machine’s SPN as http/FQDN format.

But do you have the inner voice echoing which passage I based upon, at least I ask myself this question and spent half day searching for the source of the catch-all mechanism, however, I could find nothing from Microsoft official website to decipher how/when/why it works 🙁

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NTP client time out of Sync?

Published / by Chen Ye / 3 Comments on NTP client time out of Sync?

Starter:

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.

 

Definition

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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