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Re: [SCTP-Discussion] Multihoming


Chronological Thread 
  • From: "Brian F. G. Bidulock" <bidulock AT openss7.org>
  • To: Tuexen Michael <Michael.Tuexen AT icn.siemens.de>
  • Cc: "'discussion AT sctp.de'" <discussion AT sctp.de>
  • Subject: Re: [SCTP-Discussion] Multihoming
  • Date: Thu Feb 21 14:07:01 2002
  • Dsn-notification-to: <bidulock AT openss7.org>
  • List-id: SCTP Discussions <discussion.sctp.de>
  • Organization: http://www.openss7.org/
Michael,

Not even TCP will accept such an arrangement. Say TCP launches
a SYN from B to A_1. If A responds with a SYN from A_2 to B the
connection will not succeed.

--brian

On Thu, 21 Feb 2002, Tuexen Michael wrote:

> Dear Roland,
>
> considering the case where the SCTP end point A only announces
> A2, but A1 is used as a source IP address is definitly a
> misconfiguration of the host!
> If a SCTP end point only announces a proper subset of its IP-addresses
> it must make sure that only addresses of this subset are used as
> source IP addresses. The most simple way to make this sure is to
> include all addresses. This is the reason why you can read the
> Implemention note in RFC 2960. The sctplib also supports using this
> subset but then your forwaring on the host must be in tune with this
> configuration as stated in the documentation.
>
> Considering the other case: B sends traffic from B1 to A2. A replies
> then from A1 to B1. This also means that your forwaring setup is
> not in a proper way configured so that you can really make use of the
> multihoming support of SCTP. The problems are described in
> http://www.ietf.org/internet-drafts/draft-coene-sctp-multihome-03.txt
> Nevertheless, when an SCTP endpoint receives an INIT-ACK it should
> check if it can find a TCB based on the source IP address and the
> IP addresses listed in the INIT-ACK. This seems not to be implemented
> in pre17 and will be added in a future release. This would result
> in being able to establish the association, but you do not have the
> redundancy you think based on the number of paths.
>
> Best regards
> Michael
>
>
> Michael Tuexen Tel.: +49 89 722 47210
> Siemens AG Fax: +49 89 722 48212
> ICN WN CS SE 5 E-mail:
> Michael.Tuexen AT icn.siemens.de
>
> > -----Original Message-----
> > From: Roland Gloeckler
> > [SMTP:gloeckle AT ind.uni-stuttgart.de]
> > Sent: Thursday, February 21, 2002 1:13 PM
> > To:
> > bidulock AT openss7.org
> > Cc:
> > discussion AT sctp.de
> > Subject: Re: [SCTP-Discussion] Multihoming
> >
> > Dear Mr. Bidulock, dear all:
> >
> > My question is not completely answered by your response.
> >
> > My problem (a little bit more detailed) is the following:
> >
> > A has two interfaces with distinct IP addresses (A_1 and A_2).
> > B sends an INIT to one of them, e.g., A_2. The other interface A_1 is not
> > known to B yet.
> >
> > In the sctp implementation A sends the INIT_ACK via the interface which
> > is
> > the default entry in the IP routing table, e.g., A_1.
> >
> > Now there are two error scenarios:
> >
> > A has initialised a multihomed instance with both IP addresses.
> > For sending the INIT ACK it uses A_1 which is not known to B before it
> > reads
> > the address list (with A_1 and A_2) in this INIT ACK chunk.
> > B responds with an ABORT.
> >
> > or
> >
> > A has initialised a singlehomed instance with the address A_2, which
> > receives
> > the INIT.
> > For sending the INIT ACK it uses A_1 which is not even initialised in the
> > SCTP instance. In the cookie it uses A_2.
> > So it is obvious that SCTP has no control over the IP source address.
> > An association cannot be initialized.
> >
> > To resume:
> >
> > If the INIT_ACK chunk of an instance A uses in its IP header a source IP
> > address that is different from the destination address of the INIT chunk
> > from
> > B, B refuses to accept it.
> >
> >
> > Section 5.1.2.c of the RFC says:
> >
> > "IMPLEMENTATION NOTE: In some cases (e.g., when the implementation
> > doesn't
> > control the source IP address that is used for transmitting), an endpoint
> > might need to include in its INIT or INIT ACK all possible IP addresses
> > from
> > which packets to the peer could be transmitted."
> >
> > B would have to read the list of IP addresses in the INIT ACK before it
> > searches in its association list to know the chunk belongs to A.
> >
> > Or should the implementation force its IP layer to use the INIT
> > destination
> > address as the source address for the INIT ACK? The RFC tries to cover
> > cases
> > where this is not the case.
> >
> > Regards,
> >
> > Roland
> >
> >
> >
> > On Friday 01 February 2002 20:38, Brian F. G. Bidulock wrote:
> > > Roland,
> > >
> > > The effect your describe was discussed on TSVWG. The
> > > defficiency of SCTP was characterized as RFC2960's inability>
> > > to provide for reliable initialization of an association.
> > >
> > > The effect you describe is required by RFC 2960 section 5.1
> > > paragraph (B): "... The destination IP address of the INIT ACK
> > > MUST be set to the source IP address of the INIT to which this
> > > INIT ACK is responding."
> > >
> > > The behavior exhibited by SCTPlib is correct. The
> > > implementation should reply with an ABORT in response to the
> > > INIT ACK sent to the wrong address.
> > >
> > > --brian
> > >
> > > On Fri, 01 Feb 2002, Roland Gloeckler wrote:
> > > > Dear all,
> > > >
> > > > when I was working with SCTPlib 1 pre14 I found an
> > > > interesting behaviour.
> > > >
> > > > I had one PC (let's call it "A") with two network cards
> > > > (let's call them A_1 and A_2) connected to another one
> > > > (let's call it "B") with only one (B_1).
> > > >
> > > > The configuration was: Interfaces A_1 and B_1 were connected
> > > > to the same LAN and A_2 was connected to the LAN via a
> > > > router.
> > > >
> > > > After some minor changes to testengine.c I was able to use
> > > > test_tool to initialise an SCTP multihoming instance on A
> > > > for the addresses A_1 and A_2.
> > > >
> > > > In the script for B I tried to start an association with A,
> > > > but only included one IP address, e.g., A_2.
> > > >
> > > > B successfully sent an INIT message to A_2, which responded
> > > > with a valid INIT_ACK.
> > > >
> > > > If A responded via A_2 all went well.
> > > >
> > > > BUT: When A used the other interface A_1 to send the
> > > > INIT_ACK (which included the list of its own addresses A_1
> > > > and A_2 belonging to that instance) to B, B was not able to
> > > > find the address A_1 in its association list, since it
> > > > didn't know it yet, and answered with an ABORT message to A.
> > > >
> > > > If I change the destination address in B's script to A_1 and
> > > > change the routing table in A to send packets to B via A_2
> > > > the result is the same.
> > > >
> > > > To summarize:
> > > >
> > > > If the INIT_ACK chunk of a multihomed instance A uses a
> > > > source IP address that is different from the one B sent the
> > > > INIT chunk to, B refuses to accept it.
> > > >
> > > > When I was looking into the RFC to search for more details,
> > > > I only found out that the behaviour is according to the
> > > > rules that no INIT_ACK is to be accepted which uses an
> > > > unknown source transport address.
> > > >
> > > > Now I wonder if this is a general problem not answered by
> > > > the RFC.
> > > >
> > > > I can think of several ways to come around the problem for
> > > > my purposes, but they are not adequate for general purposes.
> > > >
> > > > Has anyone heard of this situation before? Please tell me.
> > > >
> > > > Regards,
> > > >
> > > > Roland
> > > > _______________________________________________
> > > > discussion mailing list
> > > > discussion AT sctp.de
> > > > http://www.sctp.de/mailman/listinfo/discussion
> > _______________________________________________
> > discussion mailing list
> > discussion AT sctp.de
> > http://www.sctp.de/mailman/listinfo/discussion

--
Brian F. G. Bidulock ¦ The reasonable man adapts himself to the ¦
bidulock AT openss7.org
¦ world; the unreasonable one persists in ¦
http://www.openss7.org/ ¦ trying to adapt the world to himself. ¦
¦ Therefore all progress depends on the ¦
¦ unreasonable man. -- George Bernard Shaw ¦


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