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Versions: 00 01 02 03 04                                   Informational
INTERNET-DRAFT                                                  L. Coene
Internet Engineering Task Force                                  Siemens
Issued:  June 2003
Expires: December 2003

     Multihoming issues in the Stream Control Transmission Protocol

Status of this Memo

    This document is an Internet-Draft and is in full conformance
    with all provisions of Section 10 of RFC2026. Internet-Drafts are
    working documents of the Internet Engineering Task Force (IETF),
    its areas, and its working groups.  Note that other groups may
    also distribute working documents as Internet-Drafts.

    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents at
    any time.  It is inappropriate to use Internet-Drafts as reference
    material or to cite them other than as "work in progress."

    The list of current Internet-Drafts can be accessed at
    The list of Internet-Draft Shadow Directories can be accessed at


    This document describes issues of the Stream Control Transmission
    Protocol (SCTP)[RFC2960] in regard to multihoming on the
    Internet. It explores cases where through situations in the
    internet, single points-of-failure can occur even when using
    multihoming and what the impact is of multihoming on the routing
    tables of the host.

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                           Table of Contents

   Multihoming issues in the Stream Control Transmission Protocol .   ii
   Chapter 1: Introduction ........................................    2
   Chapter 2: SCTP multihoming ....................................    3
   Chapter 2.1: Architecture of SCTP multihoming ..................    3
   Chapter 2.2: Interaction with routing ..........................    3
   Chapter 2.3: SCTP multihoming and the size of routing tables ...    7
   Chapter 2.4: SCTP multihoming and Network Adress
   Translators(NAT) ...............................................    8
   Chapter 2.5: SCTP multihoming and IPsec ........................    9
   Chapter 3: Security considerations .............................    9
   Chapter 4: References and related work .........................   10
   Chapter 5: Acknowledgments .....................................   10
   Chapter 6: Author's address ....................................   11

1 Introduction

    SCTP[RFC2960] is a transport protocol that uses multihoming. This
    draft is an attempt at identifying the issues that may arise in the
    layers below SCTP when multihoming is used by SCTP. Some issues are
    already being addresses in various other WGs and this document will
    try to highlight them. If the solutions would resolve the issues
    presented in this document then the problem presented is no longer
    an issue. This document will also try to gauge the effectiveness of
    the present multihoming architecture.

1.1 Terminology

    The following terms are commonly identified in this work:

    Association: SCTP connection between two endpoints.

    Transport address: A combination of IP address and SCTP port number.

    Multihoming: Assigning more than one IP network interface to a
    single endpoint.

    TLA: Top Level Aggregation

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1.2 Contributors

    The following people contributed to the document: L. Coene(Editor),
    M.  Tuexen, G. Verwimp, J. Loughney, R.R. Stewart, Qiaobing Xie,
    M. Holdrege, M.C. Belinchon, A. Jungmaier, and L. Ong.

2 SCTP multihoming

2.1 Architecture of SCTP multihoming

    A single message transmitted over an SCTP association from the
    originating host to the destination host will be sent using a single
    destination IP address chosen from the set of destination IP
    addresses available for that association.  The paths used by the IP
    packets across the network might be different depending on the
    destination IP address.  If a message fails to reach its
    destination, SCTP may retransmit the message using a different
    destination IP address.

    SCTP does not have any way to determine whether two paths share
    links and routers when traversing the network.

    The route of a path through a network can be static(example manual
    configuration) or dynamic(via routing protocols such as OSPF,
    BGP...). The route that a path takes through the network will change
    over time according to the routing protocols or routing decisions
    employed by the IP network layer.

    If somewhere along the path a link or/and router fails then SCTP can
    detect the failure of the path via a heartbeat message(or in the
    worst case by the halting of the regular traffic). These actions
    done by SCTP are independant of the actions performed by the lower
    layers for failure detection and restoration and might de done in
    different timescales.

2.2 Interaction with routing

    For fault resilient communication between two SCTP endpoints, the
    multihoming feature needs more than one IP network interface for
    each endpoint. The number of paths used is the minimum of network
    interfaces used by any of the endpoints. It is recommended to bind
    the association to all the IP source addresses of the endpoint.
    Eeach network interface can have more than one IP address.

    Under the assumption that every IP address will have a different,
    seperate path towards the remote endpoint, (this is the

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    responsibility of the routing protocols or of manual configuration)
    , if the transport to one of the IP addresses (= 1 particular path)
    fails then the traffic can migrate to the other remaining IP address
    (= other paths) within the SCTP association.

      +------------+           *~~~~~~~~~*            +------------+
      | Endpoint A |          *   Cloud   *           | Endpoint B |
      |      1.2   +---------+ 1.1<--->3.1 +----------+ 3.2        |
      |            |         |             |          |            |
      |      2.2   +---------+ 2.1<--->4.1 +----------+ 4.2        |
      |            |          *           *           |            |
      +------------+           *~~~~~~~~~*            +------------+

            Figure 2.1.1: Two hosts with redundant networks.

    Consider figure 2.1.1, if the host routing tables look as follows
    the endpoint will achieve maximum use of the multi-homing feature:

    Endpoint A                              Endpoint B
    Destination     Gateway                 Destination      Gateway
    ------------------------                -------------------------
    3.0             1.1                     1.0              3.1
    4.0             2.1                     2.0              4.1

    Now if you consider figure 2.1.1, if the host routing table looks as
    follows, the association is subject to a single point of failure in
    that if any interface breaks, the whole association will break(See
    figure 2.1.2).

    Host A                                  Host B
    Destination     Gateway                 Destination      Gateway
    ------------------------                -------------------------
    3.0             1.1                     1.0              4.1
    4.0             2.1                     2.0              3.1

    Example: link 4.2-4.1 fails

    Primary path: link 1.2-1.1 - link 3.1-3.2
    Second Path : Link 2.2-2.1 - link 4.1-4.2

     Endpoint A
    |S= 1.2 | D= 3.2 | DATA |  ------->----- Arrives at Endpoint B

     Endpoint B answers with SACK
    |S= 4.2 | D= 1.2 | SACK | Gets lost, because send out on the failed
    +-------+--------+------+  4.1-4.2 link

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    After X time, retransmit on the other path by endpoint A

    Endpoint A
    |S= 2.2 | D= 4.2 | DATA | Is send out on link 2.2-2.1, but gets lost,
    +-------+--------+------+ as msg has to pass via failed  4.1-4.2 link

    The same scenario will play out for failures on the other links

    Note : S = Source address
           D = Destination address

    Figure 2.1.2: Single point of failure case in redundant network
                     due to routing table in host B

    When an endpoint selects its source address, careful consideration
    must be taken. If the same source address is always used, then it is
    possible that the endpoint will be subject to the same single point
    of failure illustrated above. If possible the endpoint 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.

    +------------+           *~~~~~~~~~*            +------------+
    | Endpoint A |          *   Cloud   *           | Endpoint B |
    |      1.2   +---------+ 1.1<--+     |          |            |
    |            |         |       |->3.1|----------+ 3.2        |
    |      2.2   +---------+ 2.1<--+     |          |            |
    |            |          *           *           |            |
    +------------+           *~~~~~~~~~*            +------------+

           Figure 2.1.3: Two hosts with asymmetric networks.

    In Figure 2.1.3 consider the following host routing table:

    Endpoint A                              Endpoint B
    Destination     Gateway                 Destination      Gateway
    ------------------------                -------------------------
    3.0             1.1                     1.0              3.1
                                            2.0              3.1

    In this case the fault tolerance becomes limited by two seperate
    issues. If the path between 3.1 and 3.2 breaks in both directions
    any association will break between endpoint A and endpoint B. The
    second failure will occur for the whole the association as well due
    to a breakage between 1.2 and 1.1 in both directions, since no

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    alternative route exists to 3.2 and all traffic is being routed
    through one interface.

    Now one of these issues can be remedied by the following: In Figure
    2.1.3 consider the following host routing table:

    Endpoint A                              Endpoint B
    Destination     Gateway                 Destination      Gateway
    ------------------------                -------------------------
    3.0             1.1                     1.0              3.1
    3.0             2.1                     2.0              3.1

    The SCTP implementation on Endpoint A needs to consider both the
    source and destination address when identifying the transport.  If
    it treats 1.2/3.2 as a separate transport address from 2.2/3.2,
    normal SCTP failover mechanisms work correctly.  This mechanism is
    suggested in RFC 2960.

    If the underlying OS does not support multiple routes to the same
    destination, or if the SCTP implementation can not control which
    route gets used (as is typical for user space implementations), we
    can still solve this issue by the following modification even when
    only one interface exists on endpoint B.

    +------------+           *~~~~~~~~~~*            +------------+
    | Endpoint A |          *   Cloud    *           | Endpoint B |
    |      1.2   +---------+ 1.1<---+     |          |            |
    |            |         |        +->3.1+----------+ 3.2 & 4.2  |
    |      2.2   +---------+ 2.1<---+     |          |            |
    |            |          *            *           |            |
    +------------+           *~~~~~~~~~~*            +------------+

    Figure 2.1.4: Two hosts with asymmetric networks, but symmetric

    In Figure 2.1.4 consider the following host routing table:

    Endpoint A                              Endpoint B
    Destination     Gateway                 Destination      Gateway
    ------------------------                -------------------------
    3.0             1.1                     1.0              3.1
    4.0             2.1                     2.0              3.1

    Now with the duplicate IP addresses assigned to the same interface
    and the above routing tables, even if the interface between 1.1 and
    1.2 breaks, an association will still survive this failure.

    As a practical matter, it is recommended that IP addresses in a

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    multihomed endpoint be assigned IP endpoints from different TLA's to
    ensure against network failure.

    In IP implementations the outgoing interface of multihomed hosts is
    often determined by the destination IP address. The mapping is done
    by a lookup in a routing table maintained by the operating
    system. Therefore the outgoing interface is not determined by SCTP.
    Using such implementations, it should be noted that a multihomed
    host cannot make use of the multiple local IP addresses if the peer
    is singlehomed. The multihomed host has only one path and will
    normally use only one of its interfaces to send the SCTP datagrams
    to the peer. If this physical path fails, the IP routing table in
    the multihome host has to be changed. This problem is out of scope
    for SCTP.

    SCTP will always send its traffic to a certain transport address (=
    destination address + port number combination) for as long as the
    transmission is uninterrupted (= primary). The other transport
    addresses (secondary paths) will act as a backup in case the primary
    path goes out of service. The changeover between primary and backup
    will occur without packet loss and is completely transparent to the
    application. The secondary path can also be used for
    retransmissions(per section 6.4 of [RFC2960]).

    The port number is the same for all transport addresses of that
    specific association.

    Applications directly using SCTP may choose to control the
    multihoming service themselves. The applications have then to supply
    the specific IP address to SCTP for each outbound user message. This
    might be done for reasons of load-sharing and load-balancing across
    the different paths. This might not be advisable as the throughput
    of any of the paths is not known in advance and constantly changes
    due to the actions of other associations and transport protocols
    along that particular path, would require very tight feedback of
    each of the paths to the loadsharing functions of the user.

    By sending a heartbeat chunk/message (=SCTP internal keep alive
    message) on all the multiple paths that are not used for active
    transmission of messages across the association, it is possible for
    SCTP to detect whether one or more paths have failed. SCTP will not
    use these failed paths when a changeover is required.

    The transmission rate of sending heartbeat messages should be
    modifiable and the possible loss of the heartbeat message could be
    used for the monitoring and measurements of the concerned paths.

2.3 SCTP multihoming and the size of routing tables

    As multihoming means that more than one destination address is used
    on the host, that would mean that a routing descision must be made

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    on the host in IP. The host does not know beforehand to which other
    host it is going to send something, so that would in theory require
    that all possible paths to all possible destinations should be known
    on that host. This amounts to the host being a part of the
    distribution of the routing information in the network.

    Possible solutions would require to ask only for the paths to host
    that are actually in use(meaning a association is about to be setup
    with that particular host). This is a viable solution for hosts with
    a small number of associations to different hosts.

    If the host has many associations with a lot of different host then
    then this becomes cumbersome(getting the specific paths from the
    routers and the updates and all) and leads in practice to same
    problem of distributing prefixes from the edge router(s) to the

    It might be useful to explore ways where no distribution of routing
    information to the host for using multihoming is needed or where the
    interface/link selection is not based on the use of different
    prefixes. Not all hosts have facilities for containing possible
    large routing tables/databases.

2.4 SCTP multihoming and Network Adress Translators(NAT)

    For multihoming the NAT must have a public IP address for each
    represented internal IP address. The host can preconfigure IP
    address that the NAT can substitute. Or the NAT can have internal
    Application Layer Gateway (ALG) which will intelligently translate
    the IP addresses in the INIT and INIT ACK chunks. See Figure 1.

    If Network Address Port Translation is used with a multihomed SCTP
    endpoint, then any port translation must be applied on a
    per-association basis such that an SCTP endpoint continues to
    receive the same port number for all messages within a given

    +-------+   +----------+      *~~~~~~~~~~*           +------+
    |Host A |   |    NAT   |     *   Cloud    *          |Host B|
    | 10.2  +---+ 10.1|5.2 +-----+ 1.1<+->3.1--+---------+ 1.2  |
    | 11.2  +---+ 11.1|6.2 |     |     +->4.2--+---------+ 2.2  |
    |       |   |          |      *           *          |      |
    +-------+   +----------+       *~~~~~~~~~*           +------+

                Fig 1: SCTP through NAT with multihoming

    It should be noted that the NAT box becomes a single
    point-of-failure in this case, as ALL the paths of the SCTP

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    association have to go through that single NAT box.

2.5 SCTP multihoming and IPsec

    IPsec was not designed to support multihomed connections and that
    imposes some difficulties when using IPsec with SCTP hosts that make
    use of several addresses in a single association.

    The Security Policy Database (SPD) entries should use as selectors,
    among other fields of the IP header, the source and destination
    address. The easy and expensive way to scale the SPD to multihomed
    host, is simply creating a new entry for all the possible
    combinations of source and a destination addresses. A much better
    implementation approach is to simply use groups of addresses instead
    of single ones.

    The same problem arises when identifying a Security Association
    (SA).  An SA should be identified by the extended triplet ({set of
    destination addresses}, Security Parameter Index, Security

    Moreover, when exchanging keys using the Internet Key Exchange (IKE)
    protocol there are some extra difficulties. IKE only allows the use
    of a single source and destination address, and so the initial
    solution to the problem would be creating a number S*D of SAs, where
    S is the number of source addresses, and D the number of destination
    addresses. This solution unnecessarily consumes both time and
    resources. Other more complex and suitable approaches would need
    modifications in IKE itself.

    A specific discussion about the problems that crop up when using
    IPsec with SCTP can be seen in [IPsec-SCTP]. All SCTP+IPSEC
    implementations would have to do the above to be compatible

3 Security considerations

    SCTP only tries to increase the availability of a network. SCTP does
    not contain any protocol mechanisms which are directly related to
    user message authentication, integrity and confidentiality
    functions. For such features, it depends on the IPSEC protocols and
    architecture and/or on security features of its user protocols.

    The solutions needed for allowing multihoming may provide security

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4 References and related work

    [RFC2960] Stewart, R. R., Xie, Q., Morneault, K., Sharp, C. , ,
    Schwarzbauer, H. J., Taylor, T., Rytina, I., Kalla, M., Zhang,
    L. and Paxson, V."Stream Control Transmission Protocol", RFC2960,
    October 2000.

    [RFC2663] Srisuresh, P. and Holdrege, M., "IP Network Address
    Translator (NAT) Terminology and Considerations", RFC2663, August

    [RFC2694] Srisuresh, P., Tsirtsis, G., Akkiraju, P. and Heffernan,
    A., "DNS extensions to Network Address Translators (DNS_ALG)",
    RFC2694, September 1999

    [IPsec-SCTP] Bellovin, S. M., Ioannidis, J., Keromytis, A. D. and
    Stewart, R. R., "On the Use of SCTP with IPsec",
    draft-ietf-ipsec-sctp-06.txt, work in progress

5 Acknowledgments

    This document was initially developed by a design team consisting of
    Lode Coene, John Loughney, Michel Tuexen, Randall R. Stewart,
    Qiaobing Xie, Matt Holdrege, Maria-Carmen Belinchon, Andreas
    Jungmaier, Gery Verwimp and Lyndon Ong.

    The authors wish to thank Renee Revis, I. Rytina, H.J. Schwarzbauer,
    J.P.  Martin-Flatin, T. Taylor, G. Sidebottom, K. Morneault,
    T. George, M.  Stillman, N. Makinae, S. Bradner, A. Mankin,
    G. Camarillo, H. Schulzrinne, R. Kantola, J. Rosenberg,
    I. Arias-Rodriguez, D. Lehmann, La Monte Henry Yaroll, P. Savola,
    H. Alvestrand and many others for their invaluable comments.

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6  Author's Address

    The following authors have contributed to this document.

    Lode Coene                  Phone: +32-14-252081
    Siemens Atea                EMail: lode.coene@siemens.com
    Atealaan 34
    B-2200    Herentals

Expires: December 31, 2003

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