Subject: new issue [LL24] Weaken the PRN generated address requirement from MUST to MAY

From: Erik Guttman <[email-masked]>
Date: Tue, 11 Feb 2003 15:22:05 +0100 (CET)

The new issue follows.  It may also be found under
http://www.spybeam.org/ll24.html

I reluctantly post this issue since I do not yet understand its
justification.

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Description of Issue  Weaken the PRN generated address requirement from
MUST to MAY    
Submitter Name  Subrata Goswami    
Submitter Email Address  [email-masked]    
Date first submitted  11 Feb 03
Reference  none
Comment Type ['T'echnical | 'E'ditorial]  T
Priority ['S' Must fix | '1' Should fix | '2' May fix ]  2    
Section 2.1

Rationale/Explanation of issue: 

The algorithm that a LL node uses to determine its LL address does not
need to be always PRN based, as ARP can be used validate if it is unique
in the Link. Lengthy description of problem: Section 2.1 says,

"When a host wishes to configure a link-local address, it selects
an address using a pseudo-random number generator with a uniform
distribution in the range from 169.254.1.0 to 169.254.254.255."

".. The pseudo-random number generation algorithm MUST be chosen so that
different hosts do not generate the same sequence of numbers."

Here text should be changed to indicted that the node can use other
means to select a LL address than PRN, but has to varify uniqueness by
ARPing before it intends to use that adderess on a link the first time
(or after absence).

Requested Change: 

".. The pseudo-random number generation algorithm MAY be chosen so that
different hosts do not generate the same sequence of numbers."

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From: Keith Moore <[email-masked]>
Date: Tue, 11 Feb 2003 09:49:07 -0500

I oppose this change.  the overhead of ARP is only acceptable if collisions
are rare; using a random number generator of some sort (perhaps not a PRN) is
necessary to avoid collisions.

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From: Brad Hards <[email-masked]>
Date: Wed, 19 Feb 2003 23:30:33 +1100

> I reluctantly post this issue since I do not yet understand its
Reject.

The basis is not well supported, and the wording change is broken, since it 
(appears to) say that the implementation may elect to choose an algorithm 
that does generate the same sequence of numbers.
This can lead to a pathological case where two or more machines get into 
lock-step, each trying to claim an address, only to collide and move onto the 
next number in an identical sequence, only to collide, and so on. One of the 
I-Ds describes this.

Brad

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From: Keith Moore <[email-masked]>
Date: Wed, 19 Feb 2003 12:35:53 -0500

I disagree that MAY is sufficient.  SHOULD would be okay with me.

the consequence of more address collisions from the PRNG is more network
traffic.  the current draft tries to minimize network load, which is highly
desirable.