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SHIM6 Working Group                                              M. Komu
Internet-Draft                                                      HIIT
Intended status: Informational                                M. Bagnulo
Expires: February 20, 2011                                          UC3M
                                                               K. Slavov
                                                        S. Sugimoto, Ed.
                                                                Ericsson
                                                         August 19, 2010


    Socket Application Program Interface (API) for Multihoming Shim
                 draft-ietf-shim6-multihome-shim-api-14

Abstract

   This document specifies sockets API extensions for the multihoming
   shim layer.  The API aims to enable interactions between applications
   and the multihoming shim layer for advanced locator management, and
   access to information about failure detection and path exploration.

   This document is based on an assumption that a multihomed host is
   equipped with a conceptual sub-layer (hereafter "shim") inside the IP
   layer that maintains mappings between identifiers and locators.
   Examples of the shim are SHIM6 and HIP.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   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
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on February 20, 2011.




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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  System Overview  . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Socket Options for Multihoming Shim Sub-layer  . . . . . . . .  9
     5.1.   SHIM_ASSOCIATED . . . . . . . . . . . . . . . . . . . . . 12
     5.2.   SHIM_DONTSHIM . . . . . . . . . . . . . . . . . . . . . . 13
     5.3.   SHIM_HOT_STANDBY  . . . . . . . . . . . . . . . . . . . . 14
     5.4.   SHIM_LOC_LOCAL_PREF . . . . . . . . . . . . . . . . . . . 15
     5.5.   SHIM_LOC_PEER_PREF  . . . . . . . . . . . . . . . . . . . 16
     5.6.   SHIM_LOC_LOCAL_RECV . . . . . . . . . . . . . . . . . . . 17
     5.7.   SHIM_LOC_PEER_RECV  . . . . . . . . . . . . . . . . . . . 18
     5.8.   SHIM_LOC_LOCAL_SEND . . . . . . . . . . . . . . . . . . . 18
     5.9.   SHIM_LOC_PEER_SEND  . . . . . . . . . . . . . . . . . . . 20
     5.10.  SHIM_LOCLIST_LOCAL  . . . . . . . . . . . . . . . . . . . 20
     5.11.  SHIM_LOCLIST_PEER . . . . . . . . . . . . . . . . . . . . 23
     5.12.  SHIM_APP_TIMEOUT  . . . . . . . . . . . . . . . . . . . . 23
     5.13.  SHIM_PATHEXPLORE  . . . . . . . . . . . . . . . . . . . . 24
     5.14.  SHIM_DEFERRED_CONTEXT_SETUP . . . . . . . . . . . . . . . 25
     5.15.  Applicability . . . . . . . . . . . . . . . . . . . . . . 26
     5.16.  Error Handling  . . . . . . . . . . . . . . . . . . . . . 26
   6.  Ancillary Data for Multihoming Shim Sub-layer  . . . . . . . . 26
     6.1.   Get Locator from Incoming Packet  . . . . . . . . . . . . 27
     6.2.   Set Locator for Outgoing Packet . . . . . . . . . . . . . 28
     6.3.   Notification from Application to Multihoming Shim
            Sub-layer . . . . . . . . . . . . . . . . . . . . . . . . 28
     6.4.   Applicability . . . . . . . . . . . . . . . . . . . . . . 28
   7.  Data Structures  . . . . . . . . . . . . . . . . . . . . . . . 29
     7.1.   Placeholder for Locator Information . . . . . . . . . . . 29
       7.1.1.  Handling Locator behind NAT  . . . . . . . . . . . . . 30



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     7.2.   Path Exploration Parameter  . . . . . . . . . . . . . . . 31
     7.3.   Feedback Information  . . . . . . . . . . . . . . . . . . 32
   8.  System Requirements  . . . . . . . . . . . . . . . . . . . . . 33
   9.  Relation to Existing Sockets API Extensions  . . . . . . . . . 33
   10. Operational Considerations . . . . . . . . . . . . . . . . . . 33
     10.1.  Conflict Resolution . . . . . . . . . . . . . . . . . . . 34
     10.2.  Incompatiblility between IPv4 and IPv6  . . . . . . . . . 34
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 35
   12. Protocol Constants and Variables . . . . . . . . . . . . . . . 35
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 35
     13.1.  Treatment of Unknown Locator  . . . . . . . . . . . . . . 35
       13.1.1. Treatment of Unknown Source Locator  . . . . . . . . . 35
       13.1.2. Treatment of Unknown Destination Locator . . . . . . . 36
   14. Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     14.1.  Changes from version 00 to version 01 . . . . . . . . . . 36
     14.2.  Changes from version 01 to version 02 . . . . . . . . . . 36
     14.3.  Changes from version 02 to version 03 . . . . . . . . . . 36
     14.4.  Changes from version 03 to version 04 . . . . . . . . . . 36
     14.5.  Changes from version 04 to version 05 . . . . . . . . . . 37
     14.6.  Changes from version 05 to version 06 . . . . . . . . . . 37
     14.7.  Changes from version 06 to version 07 . . . . . . . . . . 37
     14.8.  Changes from version 07 to version 08 . . . . . . . . . . 37
     14.9.  Changes from version 08 to version 09 . . . . . . . . . . 37
     14.10. Changes from version 09 to version 10 . . . . . . . . . . 37
     14.11. Changes from version 10 to version 11 . . . . . . . . . . 37
     14.12. Changes from version 11 to version 12 . . . . . . . . . . 37
     14.13. Changes from version 12 to version 13 . . . . . . . . . . 38
     14.14. Changes from version 13 to version 14 . . . . . . . . . . 38
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 38
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     16.1.  Normative References  . . . . . . . . . . . . . . . . . . 38
     16.2.  Informative References  . . . . . . . . . . . . . . . . . 39
   Appendix A.  Context Forking . . . . . . . . . . . . . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41

















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1.  Introduction

   This document defines socket API extensions by which upper layer
   protocols may be informed about and control the way in which a
   multihoming shim sub-layer in the IP layer manages the dynamic choice
   of locators.  Initially it applies to SHIM6 and HIP, but it is
   defined generically.

   The role of the multihoming shim sub-layer (hereafter called "shim
   sub-layer" in this document) is to avoid impacts to upper layer
   protocols which may be caused when the endhost changes its attachment
   point to the Internet, for instance, in the case of rehoming event
   under the multihomed environment.  The key design of the shim sub-
   layer is to treat identifier and locator separately.  Identifiers are
   presented to upper layer protocols and used as communication
   endpoints.  Locators represent toplogical location of endhosts and
   are used to route packet from the source to the destiantion.  The
   shim sub-layer maintains mapping of identifiers and locators.

   Note that the shim sub-layer may conflict with other multihoming
   mechanisms such as SCTP and multipath
   TCP[I-D.ietf-shim6-applicability].  To avoid any conflict, only one
   of SHIM6 and HIP should be in use.

   In this document, syntax and semantics of the API are given in the
   same way as the Posix standard [POSIX].  The API specifies how to use
   ancillary data (aka cmsg) to access the locator information with
   recvmsg() and/or sendmsg() I/O calls.  The API is described in C
   language and data types are defined in the Posix format; intN_t means
   a signed integer of exactly N bits (e.g. int16_t) and uintN_t means
   an unsigned integer of exactly N bits (e.g. uint32_t).

   The distinction between "connected" sockets and "unconnected" sockets
   is important when discussing the applicability of the socket API
   defined in this document.  A connected socket is bound to a given
   peer, whereas an unconnected socket is not bound to any specific
   peers.  A TCP socket becomes a connected socket when the TCP
   connection establishment is completed.  UDP sockets are unconnected,
   unless the application uses the connect() system call.

   The target readers of this document are application programmers who
   develop application software which may benefit greatly from
   multihomed environments.  In addition, this document aims to provide
   necessary information for developers of shim protocols to implement
   API for enabling advanced locator management.






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2.  Terminology

   This section provides terminology used in this document.  Basically
   most of the terms used in this document are taken from the following
   documents:

   o  SHIM6 Protocol Specification[RFC5533]
   o  HIP Architecture[RFC4423]
   o  Reachability Protocol (REAP)[RFC5534]

   In this document, the term "IP" refers to both IPv4 and IPv6, unless
   the protocol version is specifically mentioned.  The following are
   definitions of terms frequently used in this document:

   o  Endpoint identifier (EID) - The identifier used by the application
      to specify the endpoint of a given communication.  Applications
      may handle EIDs in various ways such as long-lived connections,
      callbacks, and referrals[I-D.ietf-shim6-app-refer].
      *  In the case of SHIM6, an identifier called a ULID serves as an
         EID.  A ULID is chosen from locators available on the host.
      *  In the case of HIP, an identifier called a Host Identifier
         serves as an EID.  A Host Identifier is derived from the public
         key of a given host.  For the sake of backward compatibility
         with the sockets API, the Host Identifier is represented in a
         form of hash of public key.
      *  Note that the EID appears in the standard socket API as an
         address, and does not appear in the extensions defined in this
         document, which only concern locators.
   o  Locator - The IP address actually used to deliver IP packets.
      Locators are present in the source and destination fields of the
      IP header of a packet on the wire.
      *  List of locators - A list of locators associated with an EID.
         There are two lists of locators stored in a given context.  One
         is associated with the local EID and the other is associated
         with the remote EID.  As defined in [RFC5533], the list of
         locators associated with an EID 'A' is denoted as Ls(A).
      *  Preferred locator - The (source/destination) locator currently
         used to send packets within a given context.  As defined in
         [RFC5533], the preferred locator of a host 'A' is denoted as
         Lp(A).
      *  Unknown locator - Any locator that does not appear in the
         locator list of the shim context associated with the socket.
         When there is no shim context associated with the socket, any
         source and/or destination locator requested by the application
         is considered to be unknown locator.
   o  Shim - The conceptual sub-layer inside the IP layer which
      maintains mappings between EIDs and locators.  An EID can be
      associated with more than one locator at a time when the host is



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      multihomed.  The term 'shim' does not refer to a specific protocol
      but refers to the conceptual sub-layer inside the IP layer.
   o  Identifier/locator adaptation - The adaptation performed at the
      shim sub-layer which may end up re-writing the source and/or
      destination addresses of an IP packet.  In the outbound packet
      processing, the EID pair is converted to the associated locator
      pair.  In the inbound packet processing, the locator pair is
      converted to the EID pair.
   o  Context - The state information shared by a given pair of peers,
      which stores a binding between the EID and associated locators.
      Contexts are maintained by the shim sub-layer.
   o  Reachability detection - The procedure to check reachability
      between a given locator pair.
   o  Path - The sequence of routers that an IP packet goes through to
      reach the destination.
   o  Path exploration - The procedure to explore available paths for a
      given set of locator pairs.
   o  Outage - The incident that prevents IP packets to flow from the
      source locator to the destination locator.  When there is an
      outage, it means that there is no reachability between a given
      locator pair.  The outage may be caused by various reasons, such
      as shortage of network resources, congestion, and human error
      (faulty operation).
   o  Working address pair - The address pair is considered to be
      "working" if the packet can safely travel from the source to the
      destination where the packet contains the first address from the
      pair as the source address and the second address from the pair as
      the destination address.  If reachability is confirmed in both
      directions, the address pair is considered to be working bi-
      directionally.
   o  Reachability protocol (REAP) - The protocol for detecting failure
      and exploring reachability in a multihomed environment.  REAP is
      defined in [RFC5534].


3.  System Overview

   Figure 1 illustrates the system overview.  The shim sub-layer and
   REAP component exist inside the IP layer.  Applications use the
   sockets API defined in this document to interface with the shim sub-
   layer and the transport layer for locator management, failure
   detection, and path exploration.

   It may also be possible that the shim sub-layer interacts with the
   transport layer, however, such an interaction is outside the scope of
   this document.





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                        +------------------------+
                        |       Application      |
                        +------------------------+
                           ^                 ^
              ~~~~~~~~~~~~~|~Socket Interface|~~~~~~~~~~~~~~
                           |                 v
               +-----------|------------------------------+
               |           |  Transport Layer             |
               +-----------|------------------------------+
                     ^     |
       +-------------|-----|-------------------------------------+
       |             v     v                                     |
       |   +-----------------------------+       +----------+    |  IP
       |   |            Shim             |<----->|   REAP   |    | Layer
       |   +-----------------------------+       +----------+    |
       |                       ^                      ^          |
       +-----------------------|----------------------|----------+
                               v                      v
               +------------------------------------------+
               |                Link Layer                |
               +------------------------------------------+


                         Figure 1: System overview


4.  Requirements

   The following is a list of requirements from applications:
   o  Turn on/off shim.  An application should be able to request to
      turn on or turn off the multihoming support by the shim layer:
      *  Apply shim.  The application should be able to explicitly
         request the shim sub-layer to apply multihoming support.
      *  Don't apply shim.  The application should be able to request
         the shim sub-layer not to apply the multihoming support but to
         apply normal IP processing at the IP layer.
      *  Note that this function is also required by other types of
         multihoming mechanisms such as SCTP and multipath TCP to avoid
         potential conflict with the shim sub-layer.
   o  Locator management.
      *  It should be possible to set preferred source and/or
         destination locator within a given context: Lp(local) and/or
         Lp(remote).
      *  It should be possible to get preferred source and/or
         destination locator within a given context: Lp(local) and/or
         Lp(remote).





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      *  It should be possible to set a list of source and/or
         destination locators within a given context: Ls(local) and
         Ls(remote).
      *  It should be possible to get a list of source and/or
         destination locators within a given context: Ls(local) and
         Ls(remote).
   o  Notification from applications to the shim sub-layer about the
      status of the communication.  The notification occurs in an event-
      based manner.  Applications and/or upper layer protocols may
      provide positive feedbacks or negative feedbacks to the shim sub-
      layer.  Note that these feedbacks are mentioned in [RFC5534]:
      *  Applications and/or upper layer protocols (e.g., TCP) may
         provide positive feedbacks to the shim sub-layer informing that
         the communication is going well.
      *  Applications and/or upper layer protocols (e.g., TCP) may
         provide negative feedbacks to the shim sub-layer informing that
         the communication status is not satisfactory.  TCP may detect a
         problem when it does not receive any expected ACK message from
         the peer.  The REAP module may be triggered by these negative
         feedbacks and invoke the path exploration procedure.
   o  Feedback from applications to the shim sub-layer.  Applications
      should be able to inform the shim sub-layer of the timeout values
      for detecting failures, sending keepalives, and starting the
      exploration procedure.  In particular, applications should be able
      to suppress keepalives.
   o  Hot-standby.  Applications may request the shim sub-layer for the
      hot-standby capability.  This means that, alternative paths are
      known to be working in advance of a failure detection.  In such a
      case, it is possible for the host to immediately replace the
      current locator pair with an alternative locator pair.
   o  Eagerness for locator exploration.  An application should be able
      to inform the shim sub-layer of how aggressively it wants the REAP
      mechanism to perform a path exploration (e.g., by specifying the
      number of concurrent attempts of discovery of working locator
      pairs) when an outage occurs on the path between the locator pair
      in use.
   o  Providing locator information to applications.  An application
      should be able to obtain information about the locator pair which
      was actually used to send or receive the packet.
      *  For inbound traffic, the application may be interested in the
         locator pair which was actually used to receive the packet.
      *  For outbound traffic, the application may be interested in the
         locator pair which was actually used to transmit the packet.
      In this way, applications may have additional control on the
      locator management.  For example, an application becomes able to
      verify if its preference for locator is actually applied to the
      flow or not.




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   o  Applications should be able to know if the shim-sublayer supports
      deferred context setup or not.
   o  An application should be able to know if the communication is now
      being served by the shim sub-layer or not.
   o  An application should be able to use a common interface to access
      an IPv4 locator and an IPv6 locator.


5.  Socket Options for Multihoming Shim Sub-layer

   In this section, socket options that are specific to the shim sub-
   layer are defined.

   Table 1 shows a list of the socket options that are specific to the
   shim sub-layer.  An application may use these socket options for a
   given socket either by the getsockopt() system call or by the
   setsockopt() system call.  All of these socket options are defined at
   level SOL_SHIM.

   The first column of Table 1 gives the name of the option.  The second
   and third columns indicate whether the option can be handled by the
   getsockopt() system call and/or by the setsockopt() system call.  The
   fourth column provides a brief description of the socket option.  The
   fifth column shows the type of data structure specified along with
   the socket option.  By default, the data structure type is an
   integer.

   +-----------------------------+-----+-----+-----------------+-------+
   | optname                     | get | set | description     | dtype |
   +-----------------------------+-----+-----+-----------------+-------+
   | SHIM_ASSOCIATED             | o   |     | Get the         | int   |
   |                             |     |     | parameter which |       |
   |                             |     |     | indicates       |       |
   |                             |     |     | whether the     |       |
   |                             |     |     | socket is       |       |
   |                             |     |     | associated with |       |
   |                             |     |     | any shim        |       |
   |                             |     |     | context or not. |       |
   | SHIM_DONTSHIM               | o   | o   | Get or set the  | int   |
   |                             |     |     | parameter which |       |
   |                             |     |     | indicates       |       |
   |                             |     |     | whether to      |       |
   |                             |     |     | employ the      |       |
   |                             |     |     | multihoming     |       |
   |                             |     |     | support by the  |       |
   |                             |     |     | shim sub-layer  |       |
   |                             |     |     | or not.         |       |




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   | SHIM_HOT_STANDBY            | o   | o   | Get or set the  | int   |
   |                             |     |     | parameter to    |       |
   |                             |     |     | request the     |       |
   |                             |     |     | shim sub-layer  |       |
   |                             |     |     | to prepare a    |       |
   |                             |     |     | hot-standby     |       |
   |                             |     |     | connection.     |       |
   | SHIM_LOC_LOCAL_PREF         | o   | o   | Get or set the  | *1    |
   |                             |     |     | preferred       |       |
   |                             |     |     | locator on the  |       |
   |                             |     |     | local side for  |       |
   |                             |     |     | the context     |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the socket.     |       |
   | SHIM_LOC_PEER_PREF          | o   | o   | Get or set the  | *1    |
   |                             |     |     | preferred       |       |
   |                             |     |     | locator on the  |       |
   |                             |     |     | remote side for |       |
   |                             |     |     | the context     |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the socket.     |       |
   | SHIM_LOC_LOCAL_RECV         | o   | o   | Get or set the  | int   |
   |                             |     |     | parameter which |       |
   |                             |     |     | is used to      |       |
   |                             |     |     | request the     |       |
   |                             |     |     | shim sub-layer  |       |
   |                             |     |     | to store the    |       |
   |                             |     |     | destination     |       |
   |                             |     |     | locator of the  |       |
   |                             |     |     | received IP     |       |
   |                             |     |     | packet.         |       |
   | SHIM_LOC_PEER_RECV          | o   | o   | Get or set the  | int   |
   |                             |     |     | parameter which |       |
   |                             |     |     | is used to      |       |
   |                             |     |     | request the     |       |
   |                             |     |     | shim sub-layer  |       |
   |                             |     |     | to store the    |       |
   |                             |     |     | source locator  |       |
   |                             |     |     | of the received |       |
   |                             |     |     | IP packet.      |       |
   | SHIM_LOC_LOCAL_SEND         | o   | o   | Get or set the  | *1    |
   |                             |     |     | source locator  |       |
   |                             |     |     | of outgoing IP  |       |
   |                             |     |     | packets.        |       |







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   | SHIM_LOC_PEER_SEND          | o   | o   | Get or set the  | *1    |
   |                             |     |     | destination     |       |
   |                             |     |     | locator of      |       |
   |                             |     |     | outgoing IP     |       |
   |                             |     |     | packets.        |       |
   | SHIM_LOCLIST_LOCAL          | o   | o   | Get or set the  | *2    |
   |                             |     |     | list of         |       |
   |                             |     |     | locators        |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the local EID.  |       |
   | SHIM_LOCLIST_PEER           | o   | o   | Get or set the  | *2    |
   |                             |     |     | list of         |       |
   |                             |     |     | locators        |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the peer's EID. |       |
   | SHIM_APP_TIMEOUT            | o   | o   | Get or set the  | int   |
   |                             |     |     | timeout value   |       |
   |                             |     |     | for detecting   |       |
   |                             |     |     | failure.        |       |
   | SHIM_PATHEXPLORE            | o   | o   | Get or set      | *3    |
   |                             |     |     | parameters for  |       |
   |                             |     |     | path            |       |
   |                             |     |     | exploration and |       |
   |                             |     |     | failure         |       |
   |                             |     |     | detection.      |       |
   | SHIM_CONTEXT_DEFERRED_SETUP | o   |     | Get the         | int   |
   |                             |     |     | parameter which |       |
   |                             |     |     | indicates       |       |
   |                             |     |     | whether         |       |
   |                             |     |     | deferred        |       |
   |                             |     |     | context setup   |       |
   |                             |     |     | is supported or |       |
   |                             |     |     | not.            |       |
   +-----------------------------+-----+-----+-----------------+-------+

          Table 1: Socket options for multihoming shim sub-layer

   *1: Pointer to a shim_locator which is defined in Section 7.

   *2: Pointer to an array of shim_locator.

   *3: Pointer to a shim_pathexplore which is defined in Section 7.

   Figure 2 illustrates how the shim specific socket options fit into
   the system model of socket API.  The figure shows that the shim sub-
   layer and the additional protocol components (IPv4 and IPv6) below
   the shim sub-layer are new to the system model.  As previously
   mentioned, all the shim specific socket options are defined at the



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   SOL_SHIM level.  This design choice brings the following advantages:

   1.  The existing sockets API continue to work at the layer above the
       shim sub-layer.  That is, those legacy API handle IP addresses as
       identifiers.
   2.  With newly defined socket options for the shim sub-layer, the
       application obtains additional control of locator management.
   3.  The shim specific socket options can be kept independent from
       address family (IPPROTO_IP or IPPROTO_IPV6) and transport
       protocol (IPPROTO_TCP or IPPROTO_UDP).


                            s1 s2      s3 s4
                             |  |       |  |
            +----------------|--|-------|--|----------------+
            |             +-------+   +-------+             |
            | IPPROTO_TCP |  TCP  |   |  UDP  |             |
            |             +-------+   +-------+             |
            |                |   \     /   |                |
            |                |    -----    |                |
            |                |   /     \   |                |
            |              +------+   +------+              |
            |   IPPROTO_IP | IPv4 |   | IPv6 | IPPROTO_IPV6 |
            |              +------+   +------+              |
            |                  \         /             SOL_SOCKET
            |          +--------\-------/--------+          |
            | SOL_SHIM |          shim           |          |
            |          +--------/-------\--------+          |
            |                  /         \                  |
            |              +------+   +------+              |
            |              | IPv4 |   | IPv6 |              |
            |              +------+   +------+              |
            |                  |          |                 |
            +------------------|----------|-----------------+
                               |          |
                             IPv4       IPv6
                           Datagram   Datagram


         Figure 2: System model of sockets API with shim sub-layer

5.1.  SHIM_ASSOCIATED

   The SHIM_ASSOCIATED option is used to check whether the socket is
   associated with any shim context or not.

   This option is meaningful when the locator information of the
   received IP packet does not tell whether the identifier/locator



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   adaptation is performed or not.  Note that the EID pair and the
   locator pair may be identical in some cases.

   This option can be specified by getsockopt().  Thus, the option is
   read-only and the result (0/1/2) is set in the option value (the
   fourth argument of getsockopt()).

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   The data type of the option value is an integer.  The option value
   indicates the presence of shim context.  A return value 1 means that
   the socket is associated with a shim context at the shim sub-layer.
   A return value 0 indicates that there is no shim context associated
   with the socket.  A return value 2 means that it is not known whether
   the socket is associated with a shim context or not, and this must be
   returned only when the socket is unconnected.  In other words, the
   returned value must be 0 or 1 when the socket is connected.

   For example, the option can be used by the application as follows:

       int optval;
       int optlen = sizeof(optval);

       getsockopt(fd, SOL_SHIM, SHIM_ASSOCIATED, &optval, &optlen);

5.2.  SHIM_DONTSHIM

   The SHIM_DONTSHIM option is used to request the shim layer not to
   provide the multihoming support for the communication established
   over the socket.

   The data type of the option value is an integer, and it takes 0 or 1.
   An option value 0 means that the shim sub-layer is employed if
   available.  An option value 1 means that the application does not
   want the shim sub-layer to provide the multihoming support for the
   communication established over the socket.

   Default value is set as 0, which means that the shim sub-layer
   performs identifier/locator adaptation if available.

   Any attempt to disable the multihoming shim support MUST be made by
   the application before the socket is connected.  If an application
   makes such an attempt for a connected-socket, an error code
   EOPNOTSUPP MUST be returned.

   For example, an application can request the system not to apply the
   multihoming support as follows:



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       int optval;

       optval = 1;

       setsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, sizeof(optval));

   For example, the application can check the option value as follows:

       int optval;
       int len;

       len = sizeof(optval);

       getsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, &len);

5.3.  SHIM_HOT_STANDBY

   The SHIM_HOT_STANDBY option is used to control the shim sub-layer
   whether to employ a hot-standby connection for the socket or not.  A
   hot-standby connection is an alternative working locator pair to the
   current locator pair.  This option is effective only when there is a
   shim context associated with the socket.

   The data type of the option value is an integer.

   The option value can be set by setsockopt().

   The option value can be read by getsockopt().

   By default, the value is set to 0, meaning that hot-standby
   connection is disabled.

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   For example, an application can request establishment of a hot-
   standby connection by using the socket option as follows:

       int optval;

       optval = 1;

       setsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval,
                  sizeof(optval));




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   For example, an application can get the option value by using the
   socket option as follows:

       int optval;
       int len;

       len = sizeof(optval);

       getsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval, &len);

5.4.  SHIM_LOC_LOCAL_PREF

   The SHIM_LOC_LOCAL_PREF option is used to get or set preference for a
   source locator for outbound traffic within a given context.  This
   option is effective only when there is a shim context associated with
   the socket.

   The preference of a locator is defined by a combination of priority
   and weight as per DNS SRV[RFC2782].  Note that the SHIM6 base
   protocol defines preference of locator in the same way.

   The data type of the option value is a pointer to a locator
   information data structure which is defined in Section 7.

   By default, the option value is set to NULL, meaning that the option
   is disabled.

   The preferred locator can be set by setsockopt().  The shim sub-layer
   shall verify requested locator before it updates the preferred
   locator.

   An application can get the preferred locator by getsockopt().

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   An error EINVALIDLOCATOR is returned when the validation of the
   specified locator fails.

   For example, an application can set the preferred locator by using
   the socket option as follows.  Note that some members of the
   shim_locator (lc_ifidx and lc_flags) are ignored in the set
   operation.





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       struct shim_locator lc;
       struct in6_addr ip6;

       /* ...set the locator (ip6)... */

       memset(&lc, 0, sizeof(shim_locator));
       lc.lc_family = AF_INET6;  /* IPv6 */
       lc.lc_ifidx = 0;
       lc.lc_flags = 0;
       lc.lc_prio = 1;
       lc.lc_weight = 10;
       memcpy(&lc.lc_addr, &ip6, sizeof(in6_addr));

       setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc,
                  sizeof(optval));

   For example, an application can get the preferred locator by using
   the socket option as follows.

       struct shim_locator lc;
       int len;

       len = sizeof(lc);

       getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc, &len);

5.5.  SHIM_LOC_PEER_PREF

   The SHIM_LOC_PEER_PREF option is used to get or set preference of a
   destination locator for outbound traffic within a given context.
   This option is effective only when there is a shim context associated
   with the socket.

   As defined earlier, the preference of a locator is defined by a
   combination of priority and weight as per DNS SRV[RFC2782].  When
   there are more than one candidate destination locators, the shim sub-
   layer makes selection based on the priority and weight specified for
   each locator.

   The data type of the option value is a pointer to the locator
   information data structure which is defined in Section 7.

   By default, the option value is set to NULL, meaning that the option
   is disabled.

   The preferred locator can be set by setsockopt().  The shim sub-layer
   shall verify requested locator before it updating the preferred
   locator.



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   An application can get the preferred locator by getsockopt().

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   An error EINVALIDLOCATOR is returned when the validation of the
   requested locator fails.

   The usage of the option is same as that of SHIM_LOC_LOCAL_PREF.  Note
   that some members of the shim_locator (lc_ifidx and lc_flags) are
   ignored in the set operation.

5.6.  SHIM_LOC_LOCAL_RECV

   The SHIM_LOC_LOCAL_RECV option can be used to request the shim sub-
   layer to store the destination locator of the received IP packet in
   an ancillary data object which can be accessed by recvmsg().  This
   option is effective only when there is a shim context associated with
   the socket.

   The data type of the option value is integer.  The option value
   should be binary (0 or 1).  By default, the option value is set to 0,
   meaning that the option is disabled.

   An application can set the option value by setsockopt().

   An application can get the option value by getsockopt().

   See Section 6 for the procedure to access locator information stored
   in the ancillary data objects.

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   For example, an application can request the shim sub-layer to store
   destination locator by using the socket option as follows.

       int optval;

       optval = 1;

       setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval,



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                  sizeof(optval));

   For example, an application can get the option value as follows.

       int optval;
       int len;

       len = sizeof(optval);

       getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval, &len);

5.7.  SHIM_LOC_PEER_RECV

   The SHIM_LOC_PEER_RECV option is used to request the shim sub-layer
   to store the source locator of the received IP packet in an ancillary
   data object which can be accessed by recvmsg().  This option is
   effective only when there is a shim context associated with the
   socket.

   The data type of the option value is integer.  The option value
   should be binary (0 or 1).  By default, the option value is set to 0,
   meaning that the option is disabled.

   The option value can be set by setsockopt().

   The option value can be read by getsockopt().

   See Section 6 for the procedure to access locator information stored
   in the ancillary data objects.

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   The usage of the option is same as that of SHIM_LOC_LOCAL_RECV
   option.

5.8.  SHIM_LOC_LOCAL_SEND

   The SHIM_LOC_LOCAL_SEND option is used to request the shim sub-layer
   to use a specific locator as the source locator for the IP packets to
   be sent from the socket.  This option is effective only when there is
   a shim context associated with the socket.

   The data type of option value is pointer to shim_locator data
   structure.



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   An application can set the local locator by setsockopt() providing a
   valid locator which is stored in a shim_locator data structure.  When
   a zero-filled locator is specified, pre-existing setting of local
   locator is inactivated.

   An application can get the local locator by getsockopt().

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   An error EINVALIDLOCATOR is returned when an invalid locator is
   specified.

   For example, an application can request the shim sub-layer to use a
   specific local locator by using the socket option as follows.

       struct shim_locator locator;
       struct in6_addr ia6;

       /* an IPv6 address preferred for the source locator is copied
          to the parameter ia6 */

       memset(&locator, 0, sizeof(locator));

       /* fill shim_locator data structure */
       locator.lc_family = AF_INET6;
       locator.lc_ifidx = 1;
       locator.lc_flags = 0;
       locator.lc_prio = 0;
       locator.lc_weight = 0;
       memcpy(&locator.lc_addr, &ia6, sizeof(ia6));

       setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
                  sizeof(locator));

   For example, an application can get the preferred local locator by
   using the socket option as follows.

       struct shim_locator locator;

       memset(&locator, 0, sizeof(locator));

       getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
                  sizeof(locator));




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       /* check locator */

5.9.  SHIM_LOC_PEER_SEND

   The SHIM_LOC_PEER_SEND option is used to request the shim sub-layer
   to use a specific locator for the destination locator of IP packets
   to be sent from the socket.  This option is effective only when there
   is a shim context associated with the socket.

   The data type of the option value is a pointer to shim_locator data
   structure.

   An application can set the remote locator by setsockopt() providing a
   valid locator which is stored in a shim_locator data structure.  When
   a zero-filled locator is specified, pre-existing setting of remote
   locator is inactivated.

   An application can get the specified remote locator by getsockopt().

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   An error EINVALIDLOCATOR when invalid an locator is specified.

   The usage of the option is the same as that of SHIM_LOC_LOCAL_SEND
   option.

5.10.  SHIM_LOCLIST_LOCAL

   The SHIM_LOCLIST_LOCAL option is used to get or set the locator list
   associated with the local EID of the shim context associated with the
   socket.  This option is effective only when there is a shim context
   associated with the socket.

   The data type of the option value is a pointer to the buffer in which
   a locator list is stored.  See Section 7 for the data structure for
   storing the locator information.  By default, the option value is set
   to NULL, meaning that the option is disabled.

   An application can get the locator list by getsockopt().  Note that
   the size of the buffer pointed to by the optval argument should be
   large enough to store an array of locator information.  The number of
   the locator information is not known beforehand.

   The local locator list can be set by setsockopt().  The buffer



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   pointed to by the optval argument should contain an array of locator
   structures.

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   An error EINVALIDLOCATOR is returned when the validation of any of
   the specified locators failed.

   An error ETOOMANYLOCATORS is returned when the number of locators
   specified exceeds the limit (SHIM_MAX_LOCATORS), or when the size of
   the buffer provided by the application is not large enough to store
   the locator list provided by the shim sub-layer.

   For example, an application can set a list of locators to be
   associated with the local EID by using the socket option as follows:
































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       struct shim_locator locators[SHIM_MAX_LOCATORS];
       struct sockaddr_in *sin;
       struct sockaddr_in6 *sin6;

       memset(locators, 0, sizeof(locators));

       ...

       /* obtain local IP addresses from local interfaces */

       ...

       /* first locator (an IPv6 address) */
       locators[0].lc_family = AF_INET6;
       locators[0].lc_ifidx = 0;
       locators[0].lc_flags = 0;
       locators[0].lc_prio = 1;
       locators[0].lc_weight = 0;
       memcpy(&locators[0].lc_addr, &sa6->sin6_addr,
              sizeof(sa6->sin6_addr));

       ...

       /* second locator (an IPv4 address) */
       locators[1].lc_family = AF_INET;
       locators[1].lc_ifidx = 0;
       locators[1].lc_flags = 0;
       locators[1].lc_prio = 0;
       locators[1].lc_weight = 0;
       memcpy(&locators[1].lc_addr, &sa->sin_addr,
              sizeof(sa->sin_addr));

       setsockopt(fd, SOL_SHIM, SHIM_LOCLIST_LOCAL, locators,
                  sizeof(locators));

   For example, an application can get a list of locators that are
   associated with the local EID by using the socket option as follows.

       struct shim_locator locators[SHIM_MAX_LOCATORS];

       memset(locators, 0, sizeof(locators));

       getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, locators,
                  sizeof(locators));

       /* parse locators */
       ...




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5.11.  SHIM_LOCLIST_PEER

   The SHIM_LOCLIST_PEER option is used to get or set the locator list
   associated with the peer EID of the shim context associated with the
   socket.  This option is effective only when there is a shim context
   associated with the socket.

   The data type of the option value is a pointer to the buffer where a
   locator list is stored.  See Section 7 for the data structure for
   storing the locator information.  By default, the option value is set
   to NULL, meaning that the option is disabled.

   An application can get the locator list by getsockopt().  Note that
   the size of the buffer pointed to by the optval argument should be
   large enough to store an array of locator information.  The number of
   the locator information is not known beforehand.

   An application can set the locator list by setsockopt().  The buffer
   pointed to by the optval argument should contain an array of locator
   list.

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   An error EINVALIDLOCATOR is returned when the validation of any of
   the specified locators failed.

   An error ETOOMANYLOCATORS is returned when the number of locators
   specified exceeds the limit (SHIM_MAX_LOCATORS), or when the size of
   the buffer provided by the application is not large enough to store
   the locator list provided by the shim sub-layer.

   The usage of the option is same as that of SHIM_LOCLIST_LOCAL.

5.12.  SHIM_APP_TIMEOUT

   The SHIM_APP_TIMEOUT option is used to get or set the Send Timeout
   value of the REAP protocol.  This option is effective only when there
   is a shim context associated with the socket.

   The data type of the option value is an integer.  The value indicates
   the period of timeout in seconds to send a REAP Keepalive message
   since the last outbound traffic.  By default, the option value is set
   to 0, meaning that the option is disabled.  When the option is
   disabled, the REAP mechanism follows its default value of Send



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   Timeout value as specified in [RFC5534]

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   For example, an application can set the timeout value by using the
   socket option as follows.

       int optval;

       optval = 15; /* 15 seconds */

       setsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval,
                  sizeof(optval));

   For example, an application can get the timeout value by using the
   socket option as follows.

       int optval;
       int len;

       len = sizeof(optval);

       getsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval, &len);

5.13.  SHIM_PATHEXPLORE

   The application may use this socket option to get or set parameters
   concerning path exploration.  Path exploration is a procedure to find
   an alternative locator pair to the current locator pair.  As the REAP
   specification defines, a peer may send Probe messages to find an
   alternative locator pair.

   This option is effective only when there is a shim context associated
   with the socket.

   The data type of the option value is a pointer to the buffer where a
   set of information for path exploration is stored.  The data
   structure is defined in Section 7.

   By default, the option value is set to NULL, meaning that the option
   is disabled.

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.



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   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   For example, an application can set parameters for path exploration
   by using the socket option as follows.

       struct shim6_pathexplore pe;

       pe.pe_probenum = 4;        /* times */
       pe.pe_keepaliveto = 10;    /* seconds */
       pe.pe_initprobeto = 500;   /* milliseconds */
       pe.pe_reserved = 0;

       setsockopt(fd, SOL_SHIM, SHIM_PATHEXPLORE, &pe, sizeof(pe));

   For example, an application can get parameters for path exploration
   by using the socket option as follows.

       struct shim6_pathexplore pe;
       int len;

       len = sizeof(pe);

       getsockopt(fd, SOL_SHIM, SHIM_PATHEXPLORE, &pe, &len);

5.14.  SHIM_DEFERRED_CONTEXT_SETUP

   The SHIM_DEFERRED_CONTEXT_SETUP option is used to check whether
   deferred context setup is possible or not.  Deferred context setup
   means that the context is established in parallel with the data
   communication.  Note that SHIM6 supports deferred context setup and
   HIP does not because EIDs in HIP (i.e., Host Identifiers) are non-
   routable.

   The data type for the option value is an integer.  The option value
   should be binary (0 or 1).  The option value 1 means that the shim
   sub-layer supports deferred context setup.

   When the application specifies the socket option to an unconnected
   socket, an error code EOPNOTSUPP is returned to the application.

   For example, an application can check whether deferred context setup
   is possible or not as follows:








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       int optval;
       int len;

       len = sizeof(optval);

       getsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP,
                  &optval, &len);

5.15.  Applicability

   All the socket options defined in this section except for the
   SHIM_DONTSHIM option are applicable to applications that use
   connected sockets.

   All the socket options defined in this section except for the
   SHIM_ASSOCIATED, SHIM_DONTSHIM and SHIM_CONTEXT_DEFERRED_SETUP
   options are effective only when there is a shim context associated
   with the socket.

5.16.  Error Handling

   If successful, getsockopt() and setsockopt() return 0; otherwise, the
   functions return -1 and set errno to indicate an error.

   The following are new error values defined for some shim specific
   socket options indicating that the getsockopt() or setsockopt()
   finished incompletely:

   EINVALIDLOCATOR
      This indicates that at least one of the necessary validations
      inside the shim sub-layer for the specified locator has failed.
      In case of SHIM6, there are two kinds of verifications required
      for security reasons prior to sending an IP packet to the peer's
      new locator; one is the return routability (check if the peer is
      actually willing to receive data with the specified locator) and
      the other one is the verification based on crypto identifier
      mechanisms [RFC3972], [RFC5535].


6.  Ancillary Data for Multihoming Shim Sub-layer

   This section provides definitions of ancillary data to be used for
   locator management and notification from/to the shim sub-layer to/
   from application.

   When the application performs locator management by sendmsg() or
   recvmsg(), a member of the msghdr structure (given in Figure 3)
   called msg_control holds a pointer to the buffer in which one ore



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   more shim specific ancillary data objects may be stored.  An
   ancillary data object can store a single locator.  It should be
   possible to process the shim specific ancillary data object by the
   existing macros defined in the Posix standard and [RFC3542].

        struct msghdr {
                caddr_t msg_name;       /* optional address */
                u_int   msg_namelen;    /* size of address */
                struct  iovec *msg_iov; /* scatter/gather array */
                u_int   msg_iovlen;     /* # elements in msg_iov */
                caddr_t msg_control;    /* ancillary data, see below */
                u_int   msg_controllen; /* ancillary data buffer len */
                int     msg_flags;      /* flags on received message */
        };

                        Figure 3: msghdr structure

   In the case of unconnected socket, msg_name stores the socket address
   of the peer which should be considered to be an identifier rather
   than a locator.  SHIM_LOC_PEER_RECV should be used to get the locator
   of the peer node.

   Table 2 is a list of the shim specific ancillary data which can be
   used for locator management by recvmsg() or sendmsg().  In any case,
   the value of cmsg_level must be set as SOL_SHIM.

     +---------------------+-----------+-----------+-----------------+
     | cmsg_type           | sendmsg() | recvmsg() |   cmsg_data[]   |
     +---------------------+-----------+-----------+-----------------+
     | SHIM_LOC_LOCAL_RECV |           |     o     |        *1       |
     | SHIM_LOC_PEER_RECV  |           |     o     |        *1       |
     | SHIM_LOC_LOCAL_SEND |     o     |           |        *1       |
     | SHIM_LOC_PEER_SEND  |     o     |           |        *1       |
     | SHIM_FEEDBACK       |     o     |           | shim_feedback{} |
     +---------------------+-----------+-----------+-----------------+

                   Table 2: Shim specific ancillary data

   *1: cmsg_data[] includes a single sockaddr_in{} or sockaddr_in6{} and
   padding if necessary

6.1.  Get Locator from Incoming Packet

   An application can get locator information from the received IP
   packet by specifying the shim specific socket options for the socket.
   When SHIM_LOC_LOCAL_RECV and/or SHIM_LOC_PEER_RECV socket options are
   set, the application can retrieve local and/or remote locator from
   the ancillary data.



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   When there is no shim context associated with the socket, the shim
   sub-layer MUST return zero-filled locator information to the
   application.

6.2.  Set Locator for Outgoing Packet

   An application can specify the locators to be used for transmitting
   an IP packet by sendmsg().  When the ancillary data of cmsg_type
   SHIM_LOC_LOCAL_SEND and/or SHIM_LOC_PEER_SEND are specified, the
   application can explicitly specify the source and/or the destination
   locators to be used for the communication over the socket.  If the
   specified locator pair is verified, the shim sub-layer overrides the
   locator(s) of the outgoing IP packet.  Note that the effect is
   limited to the datagram transmitted by the sendmsg().

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   An error code EINVALIDLOCATOR is returned when validation of the
   specified locator fails.

6.3.  Notification from Application to Multihoming Shim Sub-layer

   An application may provide feedbacks to the shim sub-layer about the
   communication status.  Such feedbacks are particularly useful for the
   shim sub-layer in the absence of REAP mechanism to monitor the
   reachability status of the currently used locator pair in a given
   shim context.

   The notification can be made by sendmsg() specifying a new ancillary
   data called SHIM_FEEDBACK.  The ancillary data can be handled by
   specifying SHIM_FEEDBACK option in cmsg_type.

   When there is no shim context associated with the socket, an error
   code ENOENT is returned to the application.

   See Section 7.3 for details of the data structure to be used.

   It is outside the scope of this document how the shim sub-layer would
   react when a feedback is provided by an application.

6.4.  Applicability

   All the ancillary data for the shim sub-layer is applicable to
   connected sockets.

   Care is needed when the SHIM_LOC_*_RECV socket option is used for
   stream-oriented sockets (e.g., TCP sockets) because there is no one-



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   to-one mapping between a single send or receive operation and the
   data (e.g., a TCP segment) being received.  In other words, there is
   no gurantee that the locator(s) set in the SHIM_LOC_*_RECV ancillary
   data is identical to the locator(s) that appear in the IP packets
   received.  The shim sub-layer SHOULD provide the latest locator
   information to the application in response to the SHIM_LOC_*_RECV
   socket option.


7.  Data Structures

   This section data structures for the shim sub-layer.  These data
   structures are either used as a parameter for setsockopt() or
   getsockopt() (as mentioned in Section 5) or as a parameter for
   ancillary data to be processed by sendmsg() or recvmsg() (as
   mentioned in Section 6).

7.1.  Placeholder for Locator Information

   As defined in Section 5, the SHIM_LOC_*_PREF, SHIM_LOC_*_SEND, and
   SHIM_LOCLIST_* socket options need to handle one or more locator
   information.  Locator information includes not only the locator
   itself but also additional information about the locator which is
   useful for locator management.  A new data structure is defined to
   serve as a placeholder for the locator information.

   Figure 4 illustrates the data structure called shim_locator which
   stores a locator information.

        struct shim_locator {
                uint8_t    lc_family;       /* address family */
                uint8_t    lc_proto;        /* protocol */
                uint16_t   lc_port;         /* port number */
                uint16_t   lc_prio;         /* preference value */
                uint16_t   lc_weight;       /* weight */
                uint32_t   lc_ifidx;        /* interface index */
                struct in6_addr lc_addr;    /* address */
                uint16_t   lc_flags;        /* flags */
        };

                     Figure 4: shim locator structure

   lc_family
      Address family of the locator (e.g.  AF_INET, AF_INET6).  It is
      required that the parameter contains non-zero value indicating the
      exact address family of the locator.





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   lc_proto
      Internet Protocol number for the protocol which is used to handle
      locator behind NAT.  Typically, this value is set as UDP (17) when
      the locator is a UDP encapsulation interface.
   lc_port
      Port number which is used for handling locator behind NAT.
   lc_prio
      The priority of the locator.  The range is 0-65535.  The lowest
      priority value means the highest priority.
   lc_weight
      The weight value indicates a relative weight for locators with the
      same priority value.  The range is 0-65535.
   lc_ifidx
      Interface index of the network interface to which the locator is
      assigned.  This field should be valid only in a read
      (getsockopt()) operation.
   lc_addr
      Contains the locator.  In the case where a locator whose size is
      smaller than 16 bytes, an encoding rule should be provided for
      each locator of a given address family.  For instance, in case of
      AF_INET (IPv4), the locator should be in the format of an IPv4-
      mapped IPv6 address as defined in [RFC4291].
   lc_flags
      Each bit of the flags represents a specific characteristics of the
      locator.  Hash Based Address (HBA) is defined as 0x01.
      Cryptographically Generated Address (CGA) is defined as 0x02.

7.1.1.  Handling Locator behind NAT

   Note that the locator information MAY contain a locator behind a
   Network Address Translator (NAT).  Such a situation may arise when
   the host is behind the NAT and uses a local address as a source
   locator to communicate with the peer.  Note that a NAT traversal
   mechanism for HIP is defined, which allows HIP host to tunnel control
   and data traffic over UDP[I-D.ietf-hip-nat-traversal].  Note also
   that the locator behind NAT is not necessarily an IPv4 address but it
   can be an IPv6 address.  Below is an example where the application
   sets a UDP encapsulation interface as a source locator when sending
   IP packets.












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          struct shim_locator locator;
          struct in6_addr ia6;

          /* copy the private IPv4 address to the ia6 as an IPv4-mapped
             IPv6 address */

          memset(&locator, 0, sizeof(locator));

          /* fill shim_locator data structure */
          locator.lc_family = AF_INET;
          locator.lc_proto = IPPROTO_UDP;
          locator.lc_port = 50500;
          locator.lc_flags = 0;
          locator.lc_prio = 0;
          locator.lc_weight = 0;
          locator.lc_ifidx = 3;

          memcpy(&locator.lc_addr, &ia6, sizeof(ia6));

          setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
                     sizeof(locator));

                   Figure 5: Handling locator behind NAT

7.2.  Path Exploration Parameter

   As defined in Section 5, SHIM_PATHEXPLORE allows application to set
   or read the parameters for path exploration and failure detection.  A
   new data structure called shim_pathexplore is defined to store the
   necessary parameters.  Figure 6 illustrates the data structure.  The
   data structure can be passed to getsockopt() or setsockopt() as an
   argument.

        struct shim_pathexplore {
                uint8_t   pe_probenum;      /* # of initial probe */
                uint8_t   pe_keepaliveto;   /* Keepalive Timeout */
                uint16_t  pe_initprobeto;   /* Initial Probe Timeout */
                uint32_t  pe_reserved;      /* reserved */
        };

                     Figure 6: path explore structure

   pe_probenum
      Indicates the number of initial probe messages to be sent.
      Default value of this parameter should follow what is specified in
      [RFC5534].





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   pe_keepaliveto
      Indicates timeout value for detecting a failure when the host does
      not receive any packets for a certain period of time while there
      is outbound traffic.  When the timer expires, path exploration
      procedure will be carried out by sending a REAP Probe message.
      Default value of this parameter should follow what is specified in
      [RFC5534].
   pe_initprobeto
      Indicates retransmission timer of REAP Probe message in
      milliseconds.  Note that this timer is applied before exponential
      back-off is started.  A REAP Probe message for the same locator
      pair may be retransmitted.  Default value of this parameter should
      follow what is specified in [RFC5534].
   pe_reserved
      A reserved field for future extension.  By default, the field
      should be initialized to zero.

7.3.  Feedback Information

   As mentioned in Section 6.3, applications can inform the shim sub-
   layer about the status of unicast reachability of the locator pair
   currently in use.  The feedback information can be handled by using
   ancillary data called SHIM_FEEDBACK.  A new data structure named
   shim_feedback is illustrated in Figure 7.

        struct shim_feedback {
                uint8_t   fb_direction;    /* direction of traffic */
                uint8_t   fb_indicator;    /* indicator (1-3) */
                uint16_t  fb_reserved;     /* reserved */
        };

                 Figure 7: feedback information structure

   direction
      Indicates direction of reachability between a locator pair in
      question.  A value 0 indicates outbound and a value 1 indicates
      inbound direction.
   indicator
      A value indicating the degree of satisfaction of a unidirectional
      reachability for a given locator pair.
      *  0: Default value.  Whenever this value is specified the
         feedback information must not be processed by the shim sub-
         layer.
      *  1: Unable to connect.  There is no unidirectional reachability
         between the locator pair in question.
      *  2: Unsatisfactory.  The application is not satisfied with the
         unidirectional reachability between the locator pair in
         question.



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      *  3: Satisfactory.  There is satisfactory unidirectional
         reachability between the locator pair in question.
   reserved
      Reserved field.  Must be ignored by the receiver.


8.  System Requirements

   As addressed in Section 5, most of the socket options and ancillary
   data defined in this document are applicable to connected sockets.
   It is assumed that the kernel is capable of maintaining the
   association between a connected socket and a shim context.  This
   requirement is considered to be reasonable because a pair of source
   and destination IP addresses is bound to a connected socket.


9.  Relation to Existing Sockets API Extensions

   This section explains relation between the sockets API defined in
   this document and the existing sockets API extensions.

   As mentioned in Section 5, the basic assumption is that the existing
   sockets API continues to work above the shim sub-layer.  This means
   that, the existing sockets API deals with identifiers, and the
   sockets API defined in this document deals with locators.

   SHIM_LOC_LOCAL_SEND and SHIM_LOC_PEER_SEND socket options are
   semantically similar to the IPV6_PKTINFO socket API in the sense that
   both provide a means for application to set the source IP address of
   outbound IP packets.

   SHIM_LOC_LOCAL_RECV and SHIM_LOC_PEER_RECV socket options are
   semantically similar to the IP_RECVDSTADDR and IPV6_PKTINFO socket
   APIs in the sense that both provides a means for application to get
   the source and/or destination IP address of inbound IP packets.

   getsockname() and getpeername() enable application to get 'name' of
   the communication endpoints which is represented by a pair of IP
   address and port number assigned to the socket. getsockname() gives
   IP address and port number assigned to the socket on the local side,
   and getpeername() gives IP address and port number of the peer side.


10.  Operational Considerations

   This section gives operational considerations of the sockets API
   defined in this document.




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10.1.  Conflict Resolution

   There may be a conflicting situation when different applications
   specify difference preference for the same shim context.  For
   instance, application A and B may establish communication with the
   same EID pair while both applications have different preference in
   their choice of local locator.  The notion of context forking in
   SHIM6 can resolve the conflicting situation.

   Socket options defined in Section 5 may cause conflicting situation
   when the target context is shared by multiple applications.  In such
   a case, the socket handler should inform the shim sub-layer that
   context forking is required.  In SHIM6, when a context is forked, an
   unique identifier called Forked Instance Identifier (FII) is assigned
   to the newly forked context.  The forked context is then exclusively
   associated with the socket through which non-default preference value
   was specified.  The forked context is maintained by the shim sub-
   layer during the lifetime of associated socket instance.  When the
   socket is closed, the shim sub-layer SHOULD delete associated
   context.

   When the application specifies SHIM_LOC_*_SEND specifying a different
   source or destination locator which does not have the highest
   priority and weight specified by the SHIM_LOC_*_PREF, the shim sub-
   layer SHOULD supersede the request made by SHIM_LOC_*_SEND over the
   preference specified by SHIM_LOC_*_PREF.

   When the peer provides preferences of the locators (e.g., a SHIM6
   peer may send a locator with a Locator Preferences Option) which
   conflict with preference specified by the applications either by
   SHIM_LOC_PEER_SEND or SHIM_LOC_PEER_PREF, the shim sub-layer SHOULD
   supersede the preference made by the application over the preference
   specified by the peer.

10.2.  Incompatiblility between IPv4 and IPv6

   The shim sub-layer performs identifier/locator adaptation.
   Therefore, in some cases, the whole IP header can be replaced with
   new IP header of a different address family (e.g. conversion from
   IPv4 to IPv6 or vice versa).  Hence, there is an issue how to make
   the conversion with minimum impact.  Note that this issue is common
   in other protocol conversion techniques
   [RFC2765][I-D.ietf-behave-v6v4-xlate].

   As studied in the previous works on protocol
   conversion[RFC2765][I-D.ietf-behave-v6v4-xlate], some of the features
   (IPv6 routing headers, hop-by-hop extension headers, and destination
   headers) from IPv6 are not convertible to IPv4.  In addition, notion



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   of source routing is not exactly the same in IPv4 and IPv6.  This
   means that an error may occur during the conversion of identifier and
   locator.  It is ffs exactly how the shim sub-layer should behave in
   such erroneous cases.


11.  IANA Considerations

   This document contains no IANA consideration.


12.  Protocol Constants and Variables

   This section defines protocol constants and variables.
   SHIM_MAX_LOCATORS  The maximum number of the locators to be included
      in a locator list. 32.


13.  Security Considerations

   This section gives security considerations of the API defined in this
   document.

13.1.  Treatment of Unknown Locator

   When sending IP packets, application may request use of unknown
   locator for the source and/or destination locators.  Note that
   treatment of unknown locator can be a subject of security
   considerations because use of invalid source and/or destination
   locator may cause redirection attack.

13.1.1.  Treatment of Unknown Source Locator

   The shim sub-layer checks if the requested locator is available on
   any of the local interface.  If not, the shim sub-layer MUST reject
   the request and return an error message with the EINVALIDLOCATOR code
   to the application.  If the locator is confirmed to be available, the
   shim sub-layer SHOULD initiate the procedure to update the locator
   list.

   Use of the following socket options and ancillary data may require
   treatment of unknown source locator:
   o  SHIM_LOC_LOCAL_SEND
   o  SHIM_LOC_LOCAL_PREF
   o  SHIM_LOCLIST_LOCAL






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13.1.2.  Treatment of Unknown Destination Locator

   If the shim sub-layer turns out to be SHIM6, the SHIM6 implementation
   MUST reject the request for using unknown destination locator.

   If the shim sub-layer turns out to be HIP, the HIP implementation MAY
   accept the request for using unknown destination locator.

   Use of the following socket options and ancillary data may require
   treatment of unknown destination locator:
   o  SHIM_LOC_PEER_SEND
   o  SHIM_LOC_PEER_PREF
   o  SHIM_LOCLIST_PEER


14.  Changes

14.1.  Changes from version 00 to version 01

   o  Define shim_locator{} data type which is a placeholder for
      locator.
   o  Define shim_pathexplore{} data type in which a set of REAP
      parameters are stored.
   o  Remove descriptions about "stickiness" of socket options.
   o  Deprecate SHIM_IF_RECV and SHIM_IF_SEND socket options.
   o  Give default value and how to disable given socket option.

14.2.  Changes from version 01 to version 02

   o  Add section describing context forking.
   o  Rephrase conclusion section.
   o  Separate normative references from informative references.
   o  Remove texts from discussion section that are not relevant to the
      contents of the document.
   o  Add section describing change history (this section).

14.3.  Changes from version 02 to version 03

   o  Add an Appendix section describing the issue of context forking.

14.4.  Changes from version 03 to version 04

   o  Updated reference.
   o  Correct typo and grammatical errors.







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14.5.  Changes from version 04 to version 05

   o  Added definition of SHIM_FEEDBACK ancillary data.
   o  Added an example of code using the SHIM_LOCLIST_LOCAL
   o  Added SHIM_LOC_LOCAL_SEND and SHIM_LOC_PEER_SEND socket options.

14.6.  Changes from version 05 to version 06

   o  Updated references.

14.7.  Changes from version 06 to version 07

   o  Resolved editorial issues.

14.8.  Changes from version 07 to version 08

   No changes are made except for updates of the references.

14.9.  Changes from version 08 to version 09

   o  Updated texts for Section 1 and Section 5 according to the
      comments provided by Samu Varjonen.
   o  Made it clear that downgrading the multihoming shim support (i.e.,
      specifying value 1 with the SHIM_DONTSHIM socket option) is only
      allowed before the socket is connected.
   o  Updated locator information (shim_locator{}) so that it can
      contain a locator behind NAT.

14.10.  Changes from version 09 to version 10

   o  Addressed applicability of socket options and ancillary data for
      the shim sub-layer.
   o  Addressed system requirements.
   o  Removed unnecessary description about deprecated socket option
      (SHIM_IF_RECV).

14.11.  Changes from version 10 to version 11

   o  Added short descriptions about connected sockets and unconnected
      sockets.
   o  Relaxed applicability of the socket options.
   o  Relaxed applicability of the ancillary data.
   o  Added notification about locator change.

14.12.  Changes from version 11 to version 12






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   o  Reflected comments from Brian Karpenter.
   o  Reflected comments from Michael Scharf.

14.13.  Changes from version 12 to version 13

   o  Reflected comments from Sebastien Barre.
   o  Removed the description about the notification from the shim sub-
      layer to applications.
   o  Narrowed down the scope of the applicability of the socket options
      and the ancillary data.

14.14.  Changes from version 13 to version 14

   o  No change was made.  The draft was re-submitted to avoid
      expiration.


15.  Acknowledgments

   Authors would like to thank Jari Arkko who participated in the
   discussion that lead to the first version of this document, and
   Tatuya Jinmei who thoroughly reviewed the early version of this draft
   and provided detailed comments on sockets API related issues.  Thomas
   Henderson provided valuable comments especially from HIP
   perspectives.

   Authors sincerely thank to the following people for their helpful
   comments to the document: Samu Varjonen, Dmitriy Kuptsov, Brian
   Carpenter, Michael Scharf, and Sebastien Barre


16.  References

16.1.  Normative References

   [POSIX]    "IEEE Std. 1003.1-2001 Standard for Information Technology
              -- Portable Operating System Interface (POSIX). Open group
              Technical Standard: Base Specifications, Issue 6,
              http://www.opengroup.org/austin", December 2001.

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, May 2003.

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, May 2006.

   [RFC5533]  Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim



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              protocol", RFC 5533, June 2009.

   [RFC5534]  Arkko, J. and I. Beijnum, "Failure Detection and Locator
              Pair Exploration Protocol for IPv6 Multihoming", RFC 5534,
              June 2009.

16.2.  Informative References

   [I-D.ietf-behave-v6v4-xlate]
              Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", draft-ietf-behave-v6v4-xlate-21 (work in
              progress), August 2010.

   [I-D.ietf-hip-nat-traversal]
              Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A.
              Keranen, "Basic HIP Extensions for Traversal of Network
              Address Translators", Internet
              Draft draft-ietf-hip-nat-traversal-09, October 2009.

   [I-D.ietf-shim6-app-refer]
              Nordmark, E., "Shim6 Application Referral Issues",
              draft-ietf-shim6-app-refer-00 (work in progress),
              July 2005.

   [I-D.ietf-shim6-applicability]
              Abley, J., Bagnulo, M., and A. Garcia-Martinez,
              "Applicability Statement for the Level 3 Multihoming Shim
              Protocol (Shim6)", draft-ietf-shim6-applicability-05 (work
              in progress), February 2010.

   [RFC2765]  Nordmark, E., "Stateless IP/ICMP Translation Algorithm
              (SIIT)", RFC 2765, February 2000.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC5535]  Bagnulo, M., "Hash Based Addresses (HBA)", RFC 5535,
              June 2009.






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Appendix A.  Context Forking

   In this section, an issue concerning context forking and its relation
   to the multihoming shim API are discussed.

   SHIM6 supports a notion of context forking.  A peer may decide to
   fork a context for certain reason (e.g. upper layer protocol prefers
   to use different locator pair than the one defined in available
   context).  The procedure of forking context is done similar to the
   normal context establishment, performing the 4-way message exchange.
   A peer who has decided to fork a context initiates the context
   establishment.  Hereafter, we call this peer the "initiator".  The
   peer of the initiator is called the "responder".

   Once the forked context is established between the peers, on the
   initiator side, it is possible to apply forked context to the packet
   flow since the system maintains an association between the forked
   context and the socket owned by the application that has requested
   the context forking.  How this association is maintained is an
   implementation specific issue.  However, on the responder side, there
   is a question how the outbound packet can be multiplexed by the shim
   sub-layer because there are more than one SHIM6 contexts that match
   with the ULID pair of the packet flow.  There is a need to
   differentiate packet flows not only by the ULID pairs but by some
   other information and associate a given packet flow with a specific
   context.

   Figure 8 gives an example of a scenario where two communicating peers
   fork a context.  Initially, there has been a single transaction
   between the peers, by the application 1 (App1).  Accordingly, another
   transaction is started, by application 2 (App2).  Both of the
   transactions are made based on the same ULID pair.  The first context
   pair (Ctx1) is established for the transaction of App1.  Given the
   requests from App2, the shim sub-layer on Peer 1 decides to fork a
   context.  Accordingly, a forked context (Ctx2) is established between
   the peers, which should be exclusively applied to the transaction of
   App2.  Ideally, multiplexing and demultiplexing of packet flows that
   relate to App1 and App2 should be done as illustrated in Figure 8.
   However, as mentioned earlier, the responder needs to multiplex
   outbound flows of App1 and App2 somehow.  Note that if a context
   forking occurs on the initiator side, a context forking needs to
   occur also on the responder side.









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              Peer 1                                 Peer 2
            (initiator)                            (responder)

       +----+         +----+                  +----+         +----+
       |App1|         |App2|                  |App1|         |App2|
       +----+         +----+                  +----+         +----+
         |^             |^                      ^|             ^|
         v|             v|                      |v             |v
    -----S1-------------S2-----            -----S1-------------S2-----
         ||             ||                      ||             ||
         ||             ||                      ||             ||

        Ctx1           Ctx2                    Ctx1           Ctx2
    ULID:<A1,B1>   ULID:<A1,B1>            ULID:<B1,A1>    ULID:<B1,A1>
    Loc: <A1,B2>   Loc: <A1,B3>            Loc: <B2,A1>    Loc: <B3,A1>
    FII: 0         FII: 100                FII: 0          FII: 100

         |^             |^                      ^|             ^|
         ||             ||                      ||             ||
         ||             ||                      ||             ||
         \..............||....................../|             ||
          \.............||......................./             ||
                        ||                                     ||
                        \|...................................../|
                         \....................................../

                         Figure 8: context forking

   Any solution is needed to overcome the problem mentioned above.


Authors' Addresses

   Miika Komu
   Helsinki Institute for Information Technology
   Tammasaarenkatu 3
   Helsinki
   Finland

   Phone: +358503841531
   Fax:   +35896949768
   Email: miika@iki.fi
   URI:   http://www.hiit.fi/








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

   Phone: +34 91 6248837
   Email: marcelo@it.uc3m.es
   URI:   http://it.uc3m.es/marcelo


   Kristian Slavov
   Ericsson Research Nomadiclab
   Hirsalantie 11
   Jorvas  FI-02420
   Finland

   Phone: +358 9 299 3286
   Email: kristian.slavov@ericsson.com


   Shinta Sugimoto (editor)
   Nippon Ericsson K.K.
   Koraku Mori Building
   1-4-14, Koraku, Bunkyo-ku
   Tokyo  112-0004
   Japan

   Phone: +81 3 3830 2241
   Email: shinta@sfc.wide.ad.jp





















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