RFC 1141 (rfc1141) - Page 2 of 2
Incremental updating of the Internet checksum
Alternative Format: Original Text Document
RFC 1141 Incremental Updating January 1990 can bundle updating and checking the ttl. Compiler mileage may vary. Here is a more general and possibly more helpful example which updates the ttl by n seconds: UpdateTTL(iph,n) struct ip_hdr *ipptr; unsigned char n; { unsigned long sum; unsigned short old; old = ntohs(*(unsigned short *)&ipptr->ttl); ipptr->ttl -= n; sum = old + (~ntohs(*(unsigned short *)&ipptr->ttl) & 0xffff); sum += ntohs(ipptr->Checksum); sum = (sum & 0xffff) + (sum>>16); ipptr->Checksum = htons(sum + (sum>>16)); } Security Considerations Security issues are not addressed in this memo. Authors' Addresses Tracy Mallory BBN Communications Corporation 50 Moulton Street Cambridge, MA 02238 Phone: (617) 873-3193 EMail: tmallory@CCV.BBN.COM A. Kullberg BBN Communications Corporation 50 Moulton Street Cambridge, MA 02238 Phone: (617) 873-4000 EMail: akullberg@BBN.COM Mallory & Kullberg
