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List:       freebsd-net
Subject:    [RFC] Don't embed scope zone id into IPv6 addresses
From:       "Andrey V. Elsukov" <ae () FreeBSD ! org>
Date:       2013-09-24 18:54:01
Message-ID: 5241DFC9.6070802 () FreeBSD ! org
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Hi All,

As many of you know, FreeBSD uses IPv6 stack derived from the KAME
project. The KAME project was started so long ago, when many RFCs and
standards weren't written yet. Also the struct sockaddr_in6 did not have
the sin6_scope_id field and link-local addresses had to be disambiguated
in an implementation-dependent manner. The KAME-specific workaround to
this problem was use of an embedded form. I.e. in the kernel we keep
scope zone id in the second 16-bit field of IPv6 address structure.
Also we have a bunch of helper functions in the kernel to maintain
embedded form of addresses:
 in6_setscope(), in6_clearscope(), sa6_recoverscope(), sa6_embedscope().
Conversion to embedded form and back can be done several times per
packet [1]. This is basically useless operations. And it is worth
mentioning that some operations with embedded form use locking, and also
changing an address on packet flow has bad effect for caching.

My proposal is remove this code from our kernel. From my investigation
this needs the following changes:

1. Do not keep link-local addresses in the routing tables.
We already have list and hash table with all IPv6 addresses, so we can
use sin6_scope_id to disambiguate an address and determine outgoing
interface.

2. Add scope zone id field to the struct in_endpoints and hc_metrics.
This also leads to changes in the PCB code.

3. Rework ip6_input
 * Don't use auxiliary data
 * Use ifaddr hash to check against own addresses
 * Remove route and lle lookups from input path

4. Rework ip6_output
 * Introduce new IPV6_USEROIF flag and use ifpp argument to specify
outgoing interface
 * Rework route lookup part of ip6_output() to properly handle the
fact, that now we don't keep routes for link-local addresses.

It is the biggest changes, also there are many of small changes that
should be done. I would like to see your opinions about first point.

[1] http://tinyurl.com/figure2-12

-- 
WBR, Andrey V. Elsukov
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