Subject: [homenet] Comments on src-based routing in general

From: Alexandru Petrescu <[email-masked]>
Date: Mon, 29 Jul 2013 10:44:27 +0200

Hello,

I wanted to make a few informal comments following the J. Chroboczek
presentation of "Source-specific Routing"
draft-boutier-homenet-source-specific-routing-00
today at Berlin IETF Homenet WG mtg.

It is tempting to formalize the prefix and IPv6 addresses into sets and
subsets and relationships between them.  But that relies on an
assumption that 'longest-prefix match' algorithm is precisely known.  It
is not, there is no RFC about it AFAIK.  There are many fuzzy aspects in
the implementation of longest-prefix match (e.g. to disambiguate between
ifaces).

Given that, it is next to impossible to come up with a specification
which is src-based routing (instead of dst-based).

About the problem: I heard said the problem being to ISPs present in the
same homenet, and need to forward the packet to the right ISP based on
the src address.  If that is so, then a first step to realize it would
be to send _some_ traffic to an ISP and the rest to the other.  That
could be still achieved with dst-based routing, but with specific routes
(i.e. for services specific to that ISP use that dst-based specific route).

About the question whether this shows up somewhere else than in
homenets.  One other protocol which uses mandatory src-based routing is
Proxy Mobile IPv6 (the MAG needs it).  One would make sure that way of
doing src-based routing is compatible to homenet.

Two-cents worth,

Alex

---

From: Juliusz Chroboczek <[email-masked]>
Date: Mon, 29 Jul 2013 19:33:26 +0200

> is not, there is no RFC about it AFAIK.

It's not particularly explicit, but RFC 1519 Section 4.1 seems clear
to me.  RFC 4632 is somewhat less clear.

> There are many fuzzy aspects in the implementation of longest-prefix
> match (e.g. to disambiguate between ifaces).

Please clarify.

> which is src-based routing (instead of dst-based).

I most respectfully disagree.

> (i.e. for services specific to that ISP use that dst-based specific route).

Noted.  I'll try to make the difference clear in the next version of
the draft.

> doing src-based routing is compatible to homenet.

Thanks for the pointer.  Do you have a reference I should check?

-- Juliusz

---

From: Alexandru Petrescu <[email-masked]>
Date: Mon, 29 Jul 2013 19:46:01 +0200

Le 29/07/2013 19:33, Juliusz Chroboczek a écrit :
>> It is tempting to formalize the prefix and IPv6 addresses into
>> sets and subsets and relationships between them.  But that relies
>> on an assumption that 'longest-prefix match' algorithm is
>> precisely known.  It is not, there is no RFC about it AFAIK.
>

Well yes, but they're both IPv4, not IPv6.

We know yes IPv4 is same as IPv6 in certain respects, but in this as well?

>> There are many fuzzy aspects in the implementation of
>> longest-prefix match (e.g. to disambiguate between ifaces).
>
> Please clarify.

For example if two routes of same prefix but different next-hops will
need something to disambiguate, right?

>> Given that, it is next to impossible to come up with a
>> specification which is src-based routing (instead of dst-based).
>
>> About the problem: I heard said the problem being to ISPs present
>> in the same homenet, and need to forward the packet to the right
>> ISP based on the src address.  If that is so, then a first step to
>> realize it would be to send _some_ traffic to an ISP and the rest
>> to the other.  That could be still achieved with dst-based
>> routing, but with specific routes (i.e. for services specific to
>> that ISP use that dst-based specific route).
>
>> homenets.  One other protocol which uses mandatory src-based
>> routing is Proxy Mobile IPv6 (the MAG needs it).  One would make
>> sure that way of doing src-based routing is compatible to homenet.
>

The NETEXT WG discussion would be good at it.  The RFC is RFC5213.  I
know the details are intricate, but one of relevant text is the following:

"  The mobile access gateway acts as the default router on the point-to-
    point link shared with the mobile node.  Any packet that the mobile
    node sends to any correspondent node will be received by the mobile
    access gateway and will be sent to its local mobility anchor through
    the bi-directional tunnel."

To me, that means that when the MAG receives a packet from a certain
address (MN's), it forwards that packet to a certain tunnel interface.
That is src-based routing.

Alex