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Date:      Wed, 29 Dec 1999 21:11:54 -0300
From:      Fernando Ariel Gont <>
Cc:        freebsd-net@FreeBSD.ORG
Subject:   Re: "Identification field" at the IP header
Message-ID:  <>
In-Reply-To: <>

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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

web site:

--- "Con las computadoras crearemos una civilizacion de estupidos tecnologicos,
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