Disabling UDP Checksums
Tom Lancaster

Sometimes VoIP engineers find themselves in a situation where they need to shave every last bit of fat out of their VoIP network. Sometimes this is because they're right on the borderline between acceptable and unacceptable performance. Other times, the engineers just like tweaking their network, the way hot-rod buffs like tweaking their cars. If you find yourself in either boat, you should investigate a couple of bytes that are often overlooked: the UDP checksum.

In most networks, the UDP checksum performs an important function. It verifies the UDP payload has been transmitted without corruption. This is important because the UDP checksum is end-to-end. That is, it goes from source to destination, while the checksum in the IP header only watches the IP header itself, and the Layer 2 checksum is only relevant to the local data-link. Therefore, if a UDP payload were corrupted inside a router as it was being passed from one data-link to another, the previous data-link checksum would not catch the error and a new checksum would be calculated for the corrupted data and the packet would be sent on its way.

However, many network equipment manufacturers will allow you to disable the UDP checksum, saving two bytes of bandwidth and the time it takes for the source and destination to calculate the checksum itself. This may not sound like a lot (and it isn't) but with VoIP, every little bit helps.

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Disabling UDP checksums is a good idea if you have almost no errors on your network. VoIP traffic is particularly sensitive to this issue because many CODECs were designed to compensate for lost packets by sending duplicates or by interpolating samples. But packets with corrupted data may really affect voice quality.

One last point of interest is that some manufacturers disable UDP checksums by default. So if you're experiencing bizarre VoIP quality issues, deploy a protocol analyzer. If it shows corrupted data in the UDP payload (for example, it shows unexpected values when decoding the RTP header), then you should turn ON the UDP checksums.


Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.

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Related Book

Voice over IP Fundamentals
Author : Jonathan Davidson and James Peters
Publisher : Cisco Press
Published : Mar 2000
Summary :
The main focus of Voice over IP Fundamentals is to explain the basic concepts of VoIP technology. In order for the reader to understand each of the technologies surrounding packet voice (VoIP) it is important to begin with a base understanding of how the current telephony system works today (PSTN). The book will attempt to explain in detail how the modern telephone system works. Once the reader has a good understanding of basic telephony fundamentals, packet voice technologies will then be introduced. This book will contain technical details on each component of Voice over IP and how they all work together to create integrated voice/data networks. Comparisons between today's PSTN and tomorrows integrated network will be made. Case studies will also be used to show real world examples of the technology in use as well as next-generation applications.


This was first published in June 2001

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