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Network Working Group                                         M. Bagnulo
Internet-Draft                                                      UC3M
Expires: June 4, 2006                                      December 2005


               Updating RFC 3484 for multihoming support
                  draft-bagnulo-ipv6-rfc3484-update-00

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   This Internet-Draft will expire on June 4, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This note describes the limitations of RFC 3484 in multihomed
   environments and proposes possible updates to the default address
   selection mechanisms in order to cope with the identified
   limitations.








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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Limitations of RFC 3484 in multihomed environments . . . . . .  3
     2.1.  Reference topology . . . . . . . . . . . . . . . . . . . .  3
       2.1.1.  RFC 3484 and the shim6 protocol  . . . . . . . . . . .  4
     2.2.  The problem: address selection after failures  . . . . . .  5
   3.  Updates to RFC 3484  . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Providing guidance to the applications for selecting
           source addresses . . . . . . . . . . . . . . . . . . . . .  7
       3.1.1.  Considered scenario  . . . . . . . . . . . . . . . . .  7
       3.1.2.  Retrying with different source addresses . . . . . . .  7
       3.1.3.  Providing an ordered list of source address  . . . . .  8
     3.2.  Modifications to the IP layer source address selection
           mechanism  . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.1.  Considered scenario  . . . . . . . . . . . . . . . . .  9
       3.2.2.  TCP sockets  . . . . . . . . . . . . . . . . . . . . .  9
       3.2.3.  UDP sockets  . . . . . . . . . . . . . . . . . . . . .  9
   4.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12





























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1.  Introduction

   A way to solve the issue of site multihoming is to have a separate
   site prefix for each connection of the site, and to derive as many
   addresses for each hosts.  This approach to multi-homing has the
   advantage of minimal impact on the inter-domain routing fabric, since
   each site prefix can be aggregated within the larger prefix of a
   specific provider; however, it opens a number of issues, that have to
   be addressed in order to provide a multihoming solution compatible
   with such addressing scheme.

   In this memo we will present the issues that such multihoming
   configuration presents with respect to the address selection
   mechanisms.  In particular, in section 2 of this memo, we describe
   the limitations of current source and destination address selection
   mechanisms specified in RFC 3484 in the described multihoming
   configuration.  In section 3 we describe possible modifications to
   RFC 3484 to cope with the identified limitations.


2.  Limitations of RFC 3484 in multihomed environments

2.1.  Reference topology

   In the following discussion, we will use this reference topology:



                /-- ( A ) ---(      )
      X (site X)             ( IPv6 ) ---(C)---(site Y)Y
                \-- ( B ) ---(      )



   The topology features two hosts, X and Y. The site of X is multihomed
   while the site of Y is single homed.  Host X has two global IPv6
   addresses, which we will note "A:X" and "B:X", formed by combining
   the prefixes allocated by ISP A and B to "site X" with the host
   identifier of X. Y has only one address "C:Y".

   We assume that Y, when it starts engaging communication with X, has
   learned the addresses A:X and B:X, for example because they were
   published in the DNS.  We do not assume that the DNS is dynamic:
   there will be situations in which both A:X and B:X are published,
   while in fact only one is reachable.  We assume that X, when it
   receives packets from Y, has only access to information contained in
   the packet coming from Y, e.g. the source address; we do not assume
   that X can retrieve by external means the set of addresses associated



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   to Y. similar assumptions are made when X is initiating the
   communication, only that in this case, a single address i.e.  C:Y is
   published in the DNS

   In this scenario, both ISPA and ISPB are performing ingress filtering
   and have not relaxed the source address checks.  So, we assume that
   an ingress filtering compatibility mechanism [2] is available in the
   multihomed site (Site X) so that packets are forwarded through the
   ISP that corresponds to the source address prefix included in the
   packet by the host.

2.1.1.  RFC 3484 and the shim6 protocol

   The shim6 working group is developing a shim protocol to preserve
   established communications through outages.  Through the shim
   protocol a pair of shim enabled communicating peers will be able to
   survive outages affecting the path used for the communication using
   alternative addresses to exchange packets.  Communications will be
   preserved because even though a different address pair is being used
   for the communications, exchanged packets are presented to the upper
   layers as containing the addresses used initially.  In order to
   perform this function, shim protocol support from both peers involved
   in the communication is required.

   The problem addressed in this note is somehow different, since the
   goal of considered mechanisms is to enable the establishment of a new
   communication after an outage.  In this case, the communication has
   not yet been established and the address pair to be used for
   exchanging packet is being determined at this very moment.  It is
   possible then to try with different source and destination addresses
   until a working address pair is discovered.  Another difference is
   that in this case, the mechanisms are located only in the multihomed
   end of the communication and no special support other than regular
   IPv6 is required from the non-multihomed peer.  Essentially , the
   proposed mechanisms are aimed to allow a node in a multihomed site
   that implements them to be able to establish a new communication
   after an outage with an external host that does not have any
   multihoming specific support mechanism.  (In the reference topology
   depicted above, the mechanisms reside in the Host X and no
   multihoming mechanisms are located in Host Y)

   It is also possible to use the mechanisms described in this note to
   establish communications between two shim enabled peers.  However,
   whether this is the best approach to follow in this case will be
   determined by the merits of the modifications to current address
   selection mechanisms proposed to overcome the limitations that
   current mechanisms exhibits in multihomed environments.  It may well
   be that in the case of two shim enabled communicating peers, it makes



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   more sense to define special mechanisms that require cooperation from
   both nodes to establish new communications after an outage.

2.2.  The problem: address selection after failures

   In case that a failure occurs in one of the ISPs of the multihomed
   site, it may not be possible to establish a new communication towards
   a destination outside the site using the addresses derived from the
   prefix of the ISP affected by the failure.  For instance, in the case
   that the link between ISPA and the Internet fails, it will not be
   possible to establish a communication between X and Y using address
   A:X. In this case, any communication involving this address will fail
   because:
   o  If Y tries to establish a communication with X using A:X as a
      destination address, packets would be discarded because there is
      no path available from the Internet to ISPA.
   o  If X tries to communicate with Y using A:X as a source address,
      packets will be routed through ISPA in order to comply with
      ingress filters, and because ISPA has no link available with the
      rest of the Internet, the packet will be discarded (it should be
      noted that even if the packet could make it to Y, reply packets
      from Y to X would contain A:X as a destination address, which is
      unreachable from Y).

   So, in order to establish a communication between X and Y when a
   failure has occurred in ISPA, the address derived from ISPA block
   i.e.  A:X, must not be used for the communication.

   The solution for this problem has to be provided by the address
   selection mechanisms.  In particular, when the communication is
   established from the host Y to the host X, the solution has to be
   provided by the destination address selection mechanism at host Y and
   when the communication is established from the host X to the host Y,
   the solution has to be provided by the source address selection
   mechanism at host X. Default address selection for IPv6 hosts is
   specified in RFC 3484 [1]

   We will next analyze the support provided by RFC 3484 when the
   communication is established from host Y to host X. In this case,
   host Y has two possible destination addresses A:X and B:X. Without
   any additional knowledge, both addresses are equivalent to host Y, so
   the default destination address selection mechanism will return a
   list of the two addresses ordered as they were returned by the
   resolver.  It may occur that A:X is first.  In this case, host Y will
   use A:X to reach host X and it will fail.  At this point, RFC 3484
   states that if there are other destination addresses available, the
   application should retry to establish the communication, using the
   next address in the list.  If the application retries with address



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   B:X, the communication will be established successfully.

   In conclusion, the current destination address selection mechanism is
   enough to deal with this situation (as long as applications retry
   with all the addresses).

   Next, we will analyze the support provided by RFC 3484 when the
   communication is established from host X to host Y. In this case,
   destination address selection performed in host X is trivial, since
   there is only one address available for Y (C:Y).  Source address
   selection mechanism as specified in RFC 3484 will not prefer any of
   the two source addresses if no additional information is available,
   so any of the addresses can be selected as source address.  In the
   case that address A:X is selected, the communication will fail.  In
   this case there are no alternative destination address to retry with,
   so the communication will definitely fail.

   In conclusion, the source address selection mechanism defined in RFC
   3484 is not enough to support this scenario.  This memo defines
   mechanisms to provide a solution for this case.


3.  Updates to RFC 3484

   RFC 3484 essentially performs two functions:
   o  It provides an ordered list of destination addresses to the
      application that are used to initiate a communication.  In
      addition RFC 3484 states that the application should iterate
      through all the addresses contained in the list until they find a
      working address
   o  In case that the application does not select a source address, the
      source address selection mechanism describes how the IP layer
      selects the source address for a given destination address.

   However, RFC 3484 does not provides support for the following
   situations:
   o  When the source address is specified by the application, the
      source address selection mechanism does not provide any guidance
      to the application about how to select the source address for
      communicating with a destination address.  In particular, RFC 3484
      does not recommend that the application should iterate through all
      available source addresses until a working address pair is found.
   o  When the source address is unspecified by the application and it
      is selected by the IP layer, the source address selection
      mechanism does not take into account that a given destination
      address may be reachable when using a certain source address and
      unreachable when using another source address.




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   The result is that when an outage occurs, current source address
   selection mechanisms specified in RFC 3484 may not be able to find a
   working source and destination address pair, even though, one exists.
   In this section, we describe some modifications to RFC 3484 that are
   aimed to cope with these issues, and enable the source address
   selection mechanism to discover the available working pairs.

   Accordingly to the structure of RFC 3484, the proposed modifications
   are divided in two components:
   o  A set of rules that provide guidance to the application when it
      decides to select the source address itself, similar to those
      already available for the selection of the destination address.
   o  A modification to the source address selection mechanism performed
      by the IP layer, so that unreachable source and destination
      address pairs are detected and alternative address pairs are tried
      for establishing a communication.

3.1.  Providing guidance to the applications for selecting source
      addresses

3.1.1.  Considered scenario

   In this case the application selects the source address to use when
   sending packet to a given destination address (e.g. using bind()).
   The stack and the source address selection mechanisms should honour
   this choice.  The goal of the proposed mechanisms is to provide
   guidance to the application in order to perform this source address
   selection.  Current RFC 3484 specification is silent in this case.
   In order to fill this void, we propose two changes as described in
   the following sections.

3.1.2.  Retrying with different source addresses

   Current RFC 3484 states that when more than one destination address
   are available, the application should iterate through them until a
   working address is found.  However, RFC 3484 is silent with respect
   to the case where multiple source addresses are available and the
   application decides to select the source address to be used.

   So, the proposed change is to update RFC 3484 to include that:
      In the case that the application decides to select the source
      address used in the communication (e.g. using bind()) the
      application should iterate through all the source and destination
      address pairs available until a working pair is found.

   In addition an additional rule must be added to the source address
   selection algorithm:




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   Rule 0: Avoid unreachable source addresses.

   If the address pair with source address SA and destination address D
   is known to be not working, then prefer SB

3.1.3.  Providing an ordered list of source address

   In addition to recommending that when the application selects the
   source address, it should try with all available address pairs to
   establish the communication, it would also make sense to provide some
   guidance about which addresses to try first.  It should be noted that
   RFC 3484 does provides an ordered list of destination addresses so
   that the application can try with the multiple available destination
   addresses in the suggested order.  A similar approach is here
   suggested for the source addresses

   Currently, there are several ways for an application to retrieve the
   list of available source addresses i.e. the addresses available in
   the local host.  A possibility would be to let the source address
   selection mechanism order that list before it is returned to the
   application.  The problem with this approach is that available calls
   to retrieve the source address set have no destination address
   information associated, and the problem being dealt here is the
   selection of a source address to use with a given destination
   address.

   A possible approach then is to define a new function to retrieve an
   ordered list of available source addresses for a given destination
   address.  In this case, the application would have an ordered list of
   destination addresses and for each of them the application would
   retrieve an ordered list of potential source addresses.  It should be
   noted that current RFC 3484 already provides an algorithm to order
   the set of source addresses, but instead of returning the ordered
   list it just uses the "best" one.  This basically means that the
   algorithm for sorting the source addresses for a given destination
   address is already available in RFC 3484.  In this case, only the new
   function that returns the ordered list of source addresses for a
   given destination address needs to be defined (of course,
   applications need to be modified so that the new function is used)

   In the approach described in the previous paragraph an application
   would obtain an ordered list of destination addresses and for each
   destination address an ordered list of source addresses.  This option
   is attractive because it does not requires major changes in the way
   source and destination address selection mechanisms described in RFC
   3484 operate (the only change required is a new function call).
   However, such approach has the drawback that the resulting order of
   address pairs to try may not be the optimal, since for each



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   destination address, all the available source address would be tried
   before moving on to the next destination address.  A possible
   workaround for this limitation would be that an ordered list of
   address pairs is returned instead of an ordered list of destination
   addresses and for each destination address, an ordered list of source
   addresses.  The drawback of this approach is that not only a major
   change in the source and destination address selection is required to
   produce a list of ordered source and destination address pairs
   instead of a list of source addresses and a list of destination
   addresses, but also application should use a new function that
   returns the ordered list of address pairs instead of the function
   currently used to retrieve the destination address list.

3.2.  Modifications to the IP layer source address selection mechanism

3.2.1.  Considered scenario

   In this case the application that is communicating has not selected
   the source address to be used (i.e. no bind() to a specific source
   address).  In this case, it is up to the IP layer to select the
   proper source address to include in the outgoing packets.  The source
   address selection is then performed at the connect() time for
   connected sockets or when each packet is sent for non-connected
   sockets.  We will next consider two different cases: TCP sockets, and
   UDP sockets.

3.2.2.  TCP sockets

   In this case, the application has selected a destination address and
   it has open a TCP socket.  Then it performs a connect().  At this
   point in time, the 3-way handshake of TCP is executed.  Normally, a
   source address is selected before performing the handshake and the
   SYN packet is sent using this selected source address.  In order to
   deal with unreachable source addresses in this case, the proposed
   approach is that if the 3-way handshake can not be completed using
   one of the source addresses, the IP layer should iterate through the
   rest of the available source addresses until a working source address
   is found and the 3-way handshake of TCP is completed.  The list of
   source addresses to try with is ordered using the source address
   selection algorithm described in the current RFC3484.

3.2.3.  UDP sockets

   In this case, it is not possible to use a similar approach to the one
   described for TCP, because there is no way to determine if a given
   source address is working or not, because there is no connection
   establishment packet exchange as in the case of TCP.  So, in this
   case, the basic action that can be performed would be to keep trace



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   of the source address that has been used and some hints if those have
   worked or not.  In particular, the proposed approach is to keep trace
   of incoming packets with a given address pair as a possible hint of a
   working address pair.  A detailed description of how this would work
   is included in [3].  In any case, the goal here is to keep track of
   the source addresses tried for each destination address and whether
   these have worked or not (according to the previous definition of
   "working").  If they have not worked, then they should be avoided as
   long as alternative addresses are available.  If they have worked,
   they should be preferred over other potential source addresses for
   that destination address.


4.  Acknowledgments

   Thanks to Pierre Baume for reviewing this document and providing
   feedback

5.  References

   [1]  Draves, R., "Default Address Selection for Internet Protocol
        version 6 (IPv6)", RFC 3484, February 2003.

   [2]  Huitema, C. and M. Bagnulo, "Ingress Filtering compatibility for
        IPv6 multihomed sites",
        ID draft-huitema-shim6-ingress-filtering-00.txt, October 2005.

   [3]  Bagnulo, M., "Address selection in multihomed environments",
        ID draft-bagnulo-shim6-addr-selection-00.txt, October 2005.






















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Author's Address

   Marcelo Bagnulo
   Universidad Carlos III de Madrid
   Av. Universidad 30
   Leganes, Madrid  28911
   SPAIN

   Phone: 34 91 6248814
   Email: marcelo@it.uc3m.es
   URI:   http://www.it.uc3m.es








































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   Internet Society.




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