From owner-freebsd-net Thu Dec 30 3: 2:49 1999 Delivered-To: freebsd-net@freebsd.org Received: from tango.SoftHome.net (tango.SoftHome.net [204.144.231.49]) by hub.freebsd.org (Postfix) with SMTP id 58DBD14E7C for ; Thu, 30 Dec 1999 03:02:46 -0800 (PST) (envelope-from fgont@softhome.net) Received: (qmail 24025 invoked by uid 417); 30 Dec 1999 11:02:41 -0000 Received: from unknown (HELO over) (200.51.58.193) by smtpb.softhome.net with SMTP; 30 Dec 1999 11:02:41 -0000 Message-Id: <.19991229205625.009e34d0@pop.softhome.net> X-Sender: fgont@pop.softhome.net X-Mailer: QUALCOMM Windows Eudora Pro Version 4.2.0.58 Date: Wed, 29 Dec 1999 21:11:54 -0300 To: justin@apple.com From: Fernando Ariel Gont Subject: Re: "Identification field" at the IP header Cc: freebsd-net@FreeBSD.ORG In-Reply-To: <199912290204.SAA01331@walker3.apple.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Sender: owner-freebsd-net@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.org At 18:04 28/12/1999 -0800, Justin C. Walker wrote: >I'm not sure where you read this, or what implementations do it. I read that from Richard Stevens' "TCP/IP Illustrated, Volume I" (Addison-Wesley), on page 36. It says: ".... RFC 791 says that the identification field should be chosen by the upper layer that is having IP send the datagram. This implies that two consecutive IP datagrams, one generated by TCP and one generated by UDP, can have the same identification field. While this is OK (the reassembly algorithm handles this), most Berkeley-derived implementations have the IP layer increment a kernel variable each time an IP datagram is sent, regardless of which layer passed the data to IP to send. This kernel variable is initialized to a value based on the time-of-day when the system is bootstraped" >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). What I understand from Stevens' words is that the Identification is chosen by the upper layer (say TCP or UDP)... :( I don't understand why it is possible that the Identification number is chosen by TCP or UDP, as if a packet is fragmented, neither TCP nor UDP are aware of it. Best regards, Fernando Ariel Gont E-mail: fgont@softhome.net web site: http://members.xoom.com/gont/ --- "Con las computadoras crearemos una civilizacion de estupidos tecnologicos, y una elite se ira quedando con todo. Cuando digo elite me refiero a gente como yo, que puede leer." - Ray Bradbury, escritor To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message