Subject: WG Action - 1 week last call [LL30] Use of link-local configuration on more than one link is NOT RECOMMENDED

From: Erik Guttman <[email-masked]>
Date: Thu, 3 Apr 2003 18:20:20 +0200 (CEST)

This item is entering a 1 week last call discussion period, ending Apr
10, 2003.  Please send a comment to the working group list indicating
whether you accept or reject the proposal, including more information if
appropriate.  Please use the subject line:

  [LL30] Use of link-local configuration on more than one link is NOT
RECOMMENDED

see also http://www.spybeam.org/ll30.html

Description of Issue  Use of link-local configuration on more than one
link is NOT RECOMMENDED    
Submitter Name  Erik Guttman    
Submitter Email Address  [email-masked]    
Date first submitted 03 Apr 2003
Reference       
Comment Type ['T'echnical | 'E'ditorial]  T    
Priority ['S' Must fix | '1' Should fix | '2' May fix ]  1    
Section  3 (and all subsections of 3)    
Rationale/Explanation of issue: 

Several technical issues remain unresolved regarding the use of
link-local autoconfiguration of multiple interfaces. We should recommend
that this not done, or if it is, that these are not easily solved
problems.  Lengthy description of problem: The specification currently
discusses use of the protocol on multihomed hosts. 

Section 3 offers only a few complex solutions to the problems it raises,
each requiring sophisticated use of application programming interfaces
which this specification has no control over. Further, this mechanism
(binding TCP connections to interfaces explicitely, and keeping
interface, source and destination tuples associated in applications)
will require applications to be rewritten. This is a major concern (see
issues LL4 and LL5). 

For the sake of clarity, brevity and the best chance that stable
interoperable protocol implementations will result, we should remove
this section all together and replace it with one which simply recommens
that ipv4 LL autoconfiguration be done on a single interface unless
mechanisms can be used to alleviate some known problems. 

Note that the 'big issue' of address ambiguity is not addressed by this
issue.  A host which is multihomed may encounter hosts on different
links which have the same address, whether or not the host supports LL
configuration on those distinct interfaces. Consider

H1(a)-------(b)H2(c)------(a)H3

H1 and H3 are both configured with LL address (a). They are on distinct
links. H2 has configured (non-link-local) addresses. H2 knows to send to
link-local destinations directly. (H2 will not forward packets to a
router which have link-local prefix destinations. Rather it will use ARP
and then send the packet directly to the destination.) 

H2 uses SLP to discover a service on both links it is connected to. Both
H1 and H3 support the service and respond. H2 now has locators for
address (a) which are ambiguous - unless the response to the SLP request
keeps the interface information (from which the request originated).
These issues are spelled out and addressed in RFC 3111). 

So - unfortunately, this issue does not eliminate the scoped address
problem. 

Requested Change: 

Replace section 3 and subsections with the following text:

It is RECOMMENDED that configuration of link-local addresses be done on
only one active interface at a time. This avoids a number of known
problems: 

1. Address ambiguity: A host H1 has addresses A1 and A2 configured on
its interfaces to L1 and L2 respectively. There is a host H2 with
address A2 on L1. H1 cannot easily determine whether A2 should be used
to communicate with itself or with the host H2. 

2. Complexity in address selection: A multihomed host may be attached to
the same link with more than interface. This complicates the claim and
defend algorithm somewhat since a host may defend against itself if the
algorithm is implemented in a flawed manner. 

Erik Guttman
Zeroconf WG Chair

---

From: Erik Guttman <[email-masked]>
Date: Thu, 3 Apr 2003 19:23:29 +0200 (CEST)

The title of this issue is unfortunate - it should have been "Use of
link-local configuration on more than one interface is NOT RECOMMENDED"

Sorry,

Erik

---

From: Erik Guttman <[email-masked]>
Date: Fri, 11 Apr 2003 14:55:20 +0200 (CEST)

Action: Rejected

Note:  We may revisit this changing 'NOT RECOMMENDED' to 'NOT DEFINED'
if there is support for this on the list.

Erik

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> algorithm is implemented in a flawed manner. 

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