Path: utzoo!attcan!uunet!ncrlnk!ncr-sd!hp-sdd!hplabs!ucbvax!YKTVMH.BITNET!PERSHNG From: PERSHNG@YKTVMH.BITNET ("John A. Pershing Jr.") Newsgroups: comp.protocols.ibm Subject: (none) Message-ID: <8809221350.AA14186@jade.berkeley.edu> Date: 22 Sep 88 13:34:36 GMT Sender: daemon@ucbvax.BERKELEY.EDU Reply-To: "John A. Pershing Jr."Organization: The Internet Lines: 14 No, you're not missing anything. The reliability provided by the SNA DLC layer is carefully preserved by all higher layers, so that additional CRCs are probably redundant. There is probably a tacit "assumption" that the various nodes are reliable -- that is, that they won't introduce any bit errors without detecting the fault (e.g., via a machine check). As I remember (it's been a long time), TCP doesn't make many assumptions about the reliability of the lower layers; therefore, it needs some sort of checksum to provide reliable transport. If, in fact, the lower layers *are* reliable then TCP probably doesn't need the checksum; however, a proper TCP implementation cannot make such an assumption. John Pershing IBM Research, Yorktown Heights