[e2e] Re: [Sigtran] Routing for multi-homed host traffic

[Brian F. G. Bidulock]

Xiaoming,

Your question should be posted on tsvwg at ietf.org rather than sigtran.

The Strong ES model has to do with routing, not source address selection
for multhomed hosts.  The Strong ES model determines the gateway by
considering the source address.  Under Linux you can turn on a number of
advanced routing options which takes the source IP address into
consideration when routing a packet.  This has nothing to do with 2)
which is selecting a source address to place in the packet.

Now, RFC3257 considers only a Weak ES model.  This is because,
regardless of RFC1122, our A-D deflected Strong ES model considerations
for SCTP stating that they were "the stuff of research".  So, in
RFC3257, when it talks about source address selection, it is indicating
that the source address in the outgoing packet should be an address of
the interface resulting from the Weak ES Model routing algorithm
(subject to binding constraints: meaning that the address must be a
valid source address for the association).

What you are looking for, I believe is really as follows:  SCTP provides
in the ULP interface a mechanism whereby the ULP can provide a
destination address associated with a given transmission.  The sockets
sendmsg(2) interface supports this.  The OpenGroup STREAM XTI/TPI also
supports this with the T_OPTDATA_REQ.  Therefore, one can associate a
destination address with your QoS message in some implementations.
Also, IP_OPTIONS can be specified with sendmsg(2)  and t_optdata() to
provide for constrained routing or at least to provide the first-hop
gateway address (not generating any options in the outgoing packet).
So, by specifying the destination address and first-hop gateway address
in the sendmsg(2) call an implementation could be made to do what you
want it to do: select the first gateway address and route a specific
packet to its destination via that address.

The short of it is, I believe, that you don't want to specify the source
address, you want to specify the destination and gateway address.  You
can still select the source address of the outbound interface selected
to the Weak ES Model, it is just that you bypass gateway selection by
specifying the first-hop gateway address.

--brian


Xiaoming Fu wrote:                                       (Mon, 06 May 2002 01:31:07)
> Hi all,
> 
> I'm wondering how a multi-homed host can deliver such "special" data
> through an alternative path while "normal" data through a default path
> toward the same destination. This has been originated by the following
> problems:
> 
> 1)	how to send a QoS signaling message (to check/re-establish QoS
>	status) through a potential path while the normal data traffic
>	of the mobile node is still through the old path (suppose the MN
>	already obtains a new Care-of-Adress from the new subnet). With
>	our trial based on MIPL Mobile IPv6 for Linux, specifying the
>	source IP address proved unable to achieve this. Our
>	(temporary?) way was to use a routing header (first destination:
>	the new Access Router - nAR) but apparently it introduces
>	certain overhead;
> 
> 2)	how to send a heartbeat message in SCTP.  It is stated in
>	RFC3257 that "When the endpoint chooses a source address, it
>	should always select the source address of the packet to
>	correspond to the IP address of the Network interface where the
>	packet will be emitted subject to the binding address
>	constraint." However the routing rule in the endpoint (which is
>	not explicitly indicated in SCTP specs), may not support this.
> 
> RFC1122 sugests implementors may use either of two different
> conceptual models for multihoming host (without preference):
>
>    Strong ES Model:
>                          route(src IP addr, dest IP addr, TOS) -> gateway
>    Weak ES Model:
>                          route(dest IP addr, TOS) -> gateway, interface
> 
> It seems to me the Strong ES model holds as a (default) assumption for
> 2).  But I don't understand why 1) doesn't work that way. Could some
> one clarify me, is the Strong ES Model ever the best practice /
> solution to routing for multi-homed host traffic?
> 
> Thanks,
> Xiaoming
> 
> 
> 
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-- 
Brian F. G. Bidulock
bidulock at openss7.org
http://www.openss7.org/