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IP Checksum

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Thomas Arts

Testing IP checksum implementations

In RFC 1071 efficient algorithms are discussed for computing the internet checksum, also known as IP checksum. Whenever you implement efficient algorithms, an error may sneak through.

This article is meant to be used as test driven development specification for anyone that wants to implement one of the algorithms of RFC 1071 or even a new one to compute the IP checksum. The article can also be read as an example of specifying something without revealing its implementation details; a good example of using QuickCheck specifications.

Whether you write your code in Erlang, C or Java, we assume that you can build an interface to a module called ip_checksum.erl in which Erlang functions either are the functions under test or call the functions under test. You need access to QuickCheck, download a trial version from Quviq if you do not have a valid licence.

IP Checksum

The IP checksum is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header.

Ones complement

Ones complement is a way of representing negative numbers (see WikiPedia for more details).

The IP checksum uses base 16, that is 1 word or 2 bytes. In 16 bits you can represent the numbers 0 to 65535. The idea with ones complement is to use half the numbers in this interval for representing negative numbers. Thus, 0 up to 32767 are the positive numbers and 65535 is -0, or an alternative representation of zero. The number 65534 is -1 etc. Until 32768 which is -32767. Hence the interval -32767 up to 32767 can be represented.

In the remainder of this article we will present properties for functions that you probably would like to test. The properties are always parametrized by the base and have a special version for base 16. One could implement the functions for other bases as well.

(To be continued)