Date: Tue, 28 Dec 1999 18:04:27 -0800 From: "Justin C. Walker" <justin@apple.com> To: Fernando Ariel Gont <fgont@softhome.net> Cc: freebsd-net@freebsd.org Subject: Re: "Identification field" at the IP header Message-ID: <199912290204.SAA01331@walker3.apple.com>
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> From: Fernando Ariel Gont <fgont@softhome.net> > Date: 1999-12-28 17:51:28 -0800 > To: freebsd-net@FreeBSD.ORG > Subject: "Identification field" at the IP header > Delivered-to: freebsd-net@freebsd.org > X-Sender: fgont@pop.softhome.net > X-Mailer: QUALCOMM Windows Eudora Pro Version 4.2.0.58 > X-Loop: FreeBSD.org > > Hi! > > I've read that in some implementations, the "Identification field" at the IP > header is incremented by the TCP layer, and NOT by the IP layer. > I've read that the "idea" of the TCP/IP protocol suit is to have several layers, > which are "independent" of each other. That means, the application (FTP, for > example) passes its data to the next layer (TCP), then TCP encapsulates it, > adding its header, then TCP passes its data to the IP layer, which encapsulates > its.... and so on... > But, keeping in mind what I mentioned above, it seems to me that that "type of > implementation" goes against the former idea of the TCP/IP protocol suite. > I mean, I'd found it more logical if the "Identification field" was incremented > by the IP layer, and NOT by the TCP layer > Am I wrong? I'm not sure where you read this, or what implementations do it. I'm not aware of any. Normally, the 'id' is assigned by the IP layer. This value is used to permit receiving IP stacks to distinguish between copies of a datagram that have been floating around in the ether and different versions of the "same datagram" (as sent), e.g., for retransmission. It's also used to keep book on fragments as they arive at the ultimate destination. The "identification" is supposed to be unique to a given datagram. Having it assigned by another agent than the IP layer makes this either difficult or an excercise in semantics (e.g., the TCP could specify it, using a value provided by the IP layer). Regards, Justin -- Justin C. Walker, Curmudgeon-At-Large * Institute for General Semantics | Manager, CoreOS Networking | Men are from Earth. Apple Computer, Inc. | Women are from Earth. 2 Infinite Loop | Deal with it. Cupertino, CA 95014 | *-------------------------------------*-------------------------------* To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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