Subject: (IPng 7448) RE: simple source address selection

From: Richard Draves <[email-masked]>
Date: Thu, 29 Apr 1999 11:02:38 -0700

> http://www.isoc.org/inet98/proceedings/6b/6b_3.htm

Kazu sent me a pointer to the KAME source address selection algorithm. (See
section 3.1, and particularly 3.1.5, in the above document.)

I would summarize the main differences between the KAME algorithm and the
algorithm I proposed as follows:

1. To handle the loopback address, it uses a node-local scope in addition to
link-local, site-local, and global scope.

2. It's much stricter - it will never choose a deprecated source address, or
a source address with scope different than the destination address.

3. There's no use of longest-matching-prefix as a tie-breaker.

4. It's sort of a mix between weak model and strong model - it first tries
to pick a suitable source address assigned to the outgoing interface, but if
there isn't one available it will consider source addresses assigned to
other interfaces. (The details imply that KAME assumes that all interfaces
on a node will belong to the same site.)

5. Not clear how v4-compatible addresses or other format prefixes are
handled.

Responding to this:

I need to add something for loopback to my draft.

I think it is a mistake to completely preclude the choice of a deprecated
source address or a source address of scope different than the destination.
For example, suppose I'm using a machine trying to diagnose why it didn't
get configured with a site-local address. The machine does have link-local
and global addresses. I try to telnet over to look at another machine in my
site, but I only remember its site-local address. If the telnet fails, I'd
be pretty peeved. Surely the implementation should just pick the global
address for the source address. And I can imagine similar scenarios where
I'm trying to figure out why a machine only has deprecated addresses.

I think longest-matching-prefix is a good tiebreaker and it should be
encouraged.

Thanks,
Rich
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