RFC 1071 (rfc1071) - Page 1 of 24

Computing the Internet checksum

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Network Working Group                                         R.  Braden
Request for Comments: 1071                                           ISI
                                                              D.  Borman
                                                           Cray Research
                                                            C. Partridge
                                                        BBN Laboratories
                                                          September 1988


                    Computing the Internet Checksum


Status of This Memo

   This memo summarizes techniques and algorithms for efficiently
   computing the Internet checksum.  It is not a standard, but a set of
   useful implementation techniques.  Distribution of this memo is
   unlimited.

1.  Introduction

   This memo discusses methods for efficiently computing the Internet
   checksum that is used by the standard Internet protocols IP, UDP, and
   TCP.

   An efficient checksum implementation is critical to good performance.
   As advances in implementation techniques streamline the rest of the
   protocol processing, the checksum computation becomes one of the
   limiting factors on TCP performance, for example.  It is usually
   appropriate to carefully hand-craft the checksum routine, exploiting
   every machine-dependent trick possible; a fraction of a microsecond
   per TCP data byte can add up to a significant CPU time savings
   overall.

   In outline, the Internet checksum algorithm is very simple:

   (1)  Adjacent octets to be checksummed are paired to form 16-bit
        integers, and the 1's complement sum of these 16-bit integers is
        formed.

   (2)  To generate a checksum, the checksum field itself is cleared,
        the 16-bit 1's complement sum is computed over the octets
        concerned, and the 1's complement of this sum is placed in the
        checksum field.

   (3)  To check a checksum, the 1's complement sum is computed over the
        same set of octets, including the checksum field.  If the result
        is all 1 bits (-0 in 1's complement arithmetic), the check
        succeeds.

        Suppose a checksum is to be computed over the sequence of octets



Braden, Borman, & Partridge
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