Comments inline ""Patricia Leeb-Hart"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > Oh, my. One's head reels! > > >>> "Phil Barker" 11/14/2001 12:55:16 PM >>> > Just been reading the above RFC. To quote > > "Checksum is the 16-bit one's complement of the one's > complement sum of a pseudo header of information from > the IP header, the UDP header, and the data, padded > with zero octets at the end (if necessary) to make a > multiple of two octets. > > The pseudo header conceptually prefixed to the UDP > header contains the source address, the destination > address, the protocol, and the UDP length. This > information gives protection against misrouted > datagrams. This checksum procedure is the same as is > used in TCP." > > > > > This business about a pseudo header is bothering me. > If the IP header has already had a checksum that runs > across the source and destination address then why > does UDP do this again ? It also creates an obvious > dependancy of UDP on IP also.
You're correct, it isn't necessary, and this has been acknowledged by the standards writers. Some TCP/IP implementations ignore the UDP checksum ( they will not reject a packet that does not have a UDP checksum, as long as the IP header checksum is OK). > > Another question about padding making a multiple of > two octets. I seem to remember Novell IPX requiring > even numbered packets (2.X) maybe. Is this something > to do with 16 bit processors working more efficiently > with an even packet sizes i.e reading two bytes at a > time was less expensive processor-wise than reading an > odd byte ? > > Phil. > > > __________________________________________________ > Do You Yahoo!? > Everything you'll ever need on one web page from News and Sport to Email and > Music Charts > http://uk.my.yahoo.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=26318&t=26283 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
