SHIM6 Working Group                                              M. Komu
Internet-Draft                                                      HIIT
Intended status: Informational                                M. Bagnulo
Expires: August 26, 2008 February 28, 2009                                          UC3M
                                                               K. Slavov
                                                        S. Sugimoto, Ed.
                                                                Ericsson
                                                       February 23,
                                                         August 27, 2008

    Socket Application Program Interface (API) for Multihoming Shim
                 draft-ietf-shim6-multihome-shim-api-05
                 draft-ietf-shim6-multihome-shim-api-06

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This document specifies a socket API for the multihoming shim layer.
   The API aims to enable interactions between the 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 sublayer (here after "shim") inside the IP
   layer that maintains mappings between identifiers and locators.
   Examples of the shim are SHIM6 and HIP.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  System Overview  . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Socket Options for Multihoming Shim Layer  . . . . . . . . . .  9
     5.1.  SHIM_ASSOCIATED  . . . . . . . . . . . . . . . . . . . . . 12
     5.2.  SHIM_DONTSHIM  . . . . . . . . . . . . . . . . . . . . . . 13
     5.3.  SHIM_HOT_STANDBY . . . . . . . . . . . . . . . . . . . . . 13
     5.4.  SHIM_PATHEXPLORE . . . . . . . . . . . . . . . . . . . . . 14
     5.5.  SHIM_LOC_LOCAL_PREF  . . . . . . . . . . . . . . . . . . . 15
     5.6.  SHIM_LOC_PEER_PREF . . . . . . . . . . . . . . . . . . . . 16
     5.7.  SHIM_LOC_LOCAL_RECV  . . . . . . . . . . . . . . . . . . . 17
     5.8.  SHIM_LOC_PEER_RECV . . . . . . . . . . . . . . . . . . . . 18
     5.9.  SHIM_LOC_LOCAL_SEND  . . . . . . . . . . . . . . . . . . . 18
     5.10. SHIM_LOC_PEER_SEND . . . . . . . . . . . . . . . . . . . . 19
     5.11. SHIM_LOCLIST_LOCAL . . . . . . . . . . . . . . . . . . . . 20
     5.12. SHIM_LOCLIST_PEER  . . . . . . . . . . . . . . . . . . . . 22
     5.13. SHIM_APP_TIMEOUT . . . . . . . . . . . . . . . . . . . . . 22
     5.14. SHIM_DEFERRED_CONTEXT_SETUP  . . . . . . . . . . . . . . . 23
     5.15. Error Handling . . . . . . . . . . . . . . . . . . . . . . 24
   6.  Ancillary Data for Multihoming Shim  . . . . . . . . . . . . . 24
     6.1.  Get Locator Information from Incoming Packet . . . . . . . 26
     6.2.  Specify Locator Information for Outgoing Packet  . . . . . 26
     6.3.  Notification from Application to Multihoming Shim  . . . . 26
   7.  Data Structures  . . . . . . . . . . . . . . . . . . . . . . . 27
     7.1.  Placeholder for Locator Information  . . . . . . . . . . . 27
     7.2.  Path Exploration Parameter . . . . . . . . . . . . . . . . 28
     7.3.  Feedback Information . . . . . . . . . . . . . . . . . . . 29
   8.  Implications for Existing Socket API Extensions  . . . . . . . 29
   9.  Resolving Conflicts with Preference Values . . . . . . . . . . 30
     9.1.  Implicit Forking . . . . . . . . . . . . . . . . . . . . . 30
   10. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     10.1. Naming at Socket Layer . . . . . . . . . . . . . . . . . . 31
     10.2. Additional Requirements from Application . . . . . . . . . 31
     10.3. Issues of Header Conversion among Different Address
           Family . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     10.4. Handling of Unknown Locator Provided by Application  . . . 32
   11. Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     11.1. Changes from version 00 to version 01  . . . . . . . . . . 32
     11.2. Changes from version 01 to version 02  . . . . . . . . . . 33
     11.3. Changes from version 02 to version 03  . . . . . . . . . . 33
     11.4. Changes from version 03 to version 04  . . . . . . . . . . 33
     11.5. Changes from version 04 to version 05  . . . . . . . . . . 33
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 33
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 33
   14. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 33
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 34
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 34
     16.2. Informative References . . . . . . . . . . . . . . . . . . 35
   Appendix A.  Context Forking . . . . . . . . . . . . . . . . . . . 35
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
   Intellectual Property and Copyright Statements . . . . . . . . . . 39

1.  Introduction

   HIP and SHIM6 have a commonality in their protocol design separation
   of identifier and locator (hereafter identifier/locator separation).
   Both protocols aim to solve problems that are specific to multihoming
   environment in a host centric approach.  In these protocols, a sub-
   layer within the IP layer maintains mappings of identifiers and
   locators.

   The shim layer is useful in a sense that the IP layer can maintain
   the mapping of an identifier to corresponding locators.  Under a
   multihomed environment, typically, a host has more than one IP
   address at a time.  During a given transaction, a host may be
   required to switch the IP address used for the communication to
   another IP address to preserve the communication.  The protocol stack
   should take care of isolating the upper layer from disruption by the
   address update.  The shim layer can make this locator update
   transparent to the upper layer protocols.

   In a system which is based on identifier/locator separation, upper
   layer protocols are expected to deal with identifiers for
   establishing and handling the communications.  If an application
   wants to have a multihoming support by the shim layer, the IP
   addresses specified as source and destination addresses must be
   identifiers.  However, this does not necessarily mean that
   applications are prohibited to choose specific locators in its
   communication.  It may be useful for applications, in some situation,
   to specify a preferred locator for the flow.

   This document recommends that the identifier/locator adaptation is
   done only once inside the network stack of a host.  That is, if
   multiple shim sublayers exist at the IP layer, any one of them should
   be applied exclusively for a given flow.

   As this document specifies socket API, it is written so that the
   contents are in line with Posix standard [POSIX] as much as possible.
   The API specified in this document defines how to use ancillary data
   (aka cmsg) to access locator information with recvmsg() and/or
   sendmsg() I/O calls.  Definition of API is presented in C language
   and data types follow Posix format; intN_t means a singed 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 target readers of this document are application programmers who
   develop application software which may benefit greatly from
   multihomed environment.  In addition, this document should be of
   interest for the developers of a given shim protocol, as the shim
   layer should provide the interface to the application.

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[I-D.ietf-shim6-proto]
   o  HIP Architecture[RFC4423]
   o  Reachability Protocol (REAP)[I-D.ietf-shim6-failure-detection]

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

   o  Endpoint Identifier (EID) - An identifier used by the application
      to specify the endpoint of a given communication.  Applications
      may handle EID 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 an ULID serves as an
         EID.  An ULID is chosen from locators available on the host.
      *  In the case of HIP, an identifier which specifies communication
         endpoints is derived from the public key of the host, which is
         called a Host Identifier.  For the sake of backward
         compatibility of the socket API, the Host Identifier is
         represented in a form of hash of public key.
   o  Locator - An IP address actually used to deliver IP packets.
      Locators should be 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 [I-D.ietf-shim6-proto], the
         list of locators associated with an EID 'A' can be denoted as
         Ls(A).
      *  Preferred Locator - The (source/destination) locator currently
         used to send packets within a given context.  As defined in
         [I-D.ietf-shim6-proto], the preferred locator of a host 'A' is
         denoted as Lp(A).
   o  Shim - A conceptual (sub-)layer inside the IP Layer which
      maintains mappings of EIDs and locators.  An EID can be associated
      with more than one locators at a time when the host is multihomed.
      The term 'shim' does not refer to a specific protocol but refers
      to the conceptual sublayer inside the IP layer.
   o  identifier/locator adaptation - An adaptation performed at the
      shim layer between EIDs and locators within a given context.  The
      adaptation may end up re-writing the source and destination
      addresses of the IP packet.  In the outbound packet processing,
      the EID pair is converted to the associated locator pair, while
      the locator pair is converted to the EID pair in the inbound
      packet processing.
   o  Context - State information shared by a given pair of peers, which
      stores a binding between the EIDs and associated locators.  The
      context is maintained at the shim layer.
   o  Reachability Detection - A procedure to check reachability between
      a given locator pair.
   o  Path - A sequence of routers that an IP packet goes through to
      reach the destination.
   o  Path Exploration - A procedure to explore available paths for a
      given set of locator pairs.
   o  Outage - An 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 can be caused by various reasons, such
      as shortage of network resources, congestion, and human error
      (faulty operation).
   o  Working Address Pair - An address pair is said to be working if
      the packet containing the first address from the pair as source
      address and the second address from the pair as destination
      address can safely travel from the source to the destination.  If
      the reachability is confirmed in both directions, the address
      pairs is said to be bi-directional.  Otherwise, it's
      unidirectional.
   o  Reachability Protocol (REAP) - A protocol for detecting failure
      and exploring reachability in a multihomed environment.  REAP is
      defined in [I-D.ietf-shim6-failure-detection].

3.  System Overview

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

   It is also possible that the shim layer interacts with transport
   layers, but the interactions are outside the scope of this document.

                        +------------------------+
                        |       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 the list of requirements from the application
   perspective:
   o  Locator management.  The shim layer selects a pair of locators for
      sending IP packets within a given context.  The selection is made
      by taking miscellaneous conditions into account such as
      reachability of the path, application's preference, and
      characteristics of path.  From the application's perspective:
      *  It should be possible to obtain the lists of locators of a
         given context: Ls(local) and Ls(remote).
      *  It should be possible to obtain the preferred locators of a
         given context: Lp(local) and Lp(remote).
   o  Notification from the application to the shim layer about the
      status of the communication.  Note that the notification is made
      in an event based manner.  There are mainly two aspects of the
      feedback that application or upper layer protocol may provide for
      the shim layer, positive and negative feedbacks [NOTE: These
      feedbacks are mentioned in [I-D.ietf-shim6-failure-detection]]:
      *  Positive feedback could be given by the application or upper
         layer protocol (e.g.  TCP) to the shim layer informing that the
         communication is going well.

      *  Negative feedback could be given by the application or upper
         layer protocol (e.g.  TCP) to the shim layer informing that the
         communication status is not satisfactory.  TCP could detect a
         problem when it does not receive expected ACK from the peer.
         ICMP error messages delivered to the upper layer protocol could
         be a clue for application to detect potential problems.  REAP
         module may be triggered by these negative feedbacks and invoke
         procedure of path exploration.
   o  Feedback from application to shim layer.  The application should
      be able to inform the shim layer of the timeout values for
      detecting failures, for sending keepalives, for starting the
      exploration procedure.  In particular, the application should be
      able to suppress the keepalives.
   o  Hot-standby.  The application may request the shim layer for hot-
      standby capabilities.  In this case, alternative paths are known
      to be working before a failure is detected.  Hence it is possible
      for the host to immediately replace the current locator pair with
      an alternative locator pair.  Hot-standby may allow applications
      to achieve better failover.
   o  Eagerness of locator exploration.  The application should be able
      to inform the shim layer how aggressive it wants REAP mechanism to
      perform path exploration (e.g. specifying the number of concurrent
      attempts of discovering working locator pair) when an outage
      occurs on the path between the currently selected locator pair.
   o  Providing locator information to application.  The 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, the application may have additional control on the
      locator management.  For example, the application can verify if
      its preference of locator is actually applied to the flow or not.
   o  The application should be able to specify if it wants to defer the
      context setup or if it wants context establishment to be started
      immediately in case there is no available context.  With deferred
      context setup, there should be no additional delay imposed by
      context establishment in initiation of communication.
   o  Turn on/off shim.  The application should be able to request to
      turn on/off the multihoming support by the shim layer:
      *  Apply shim.  The application should be able to explicitly
         request the shim layer to apply multihoming support.
      *  Don't apply shim.  The application should be able to request
         the shim layer not to apply the multihoming support but to
         apply normal IP processing at the IP layer.

   o  The application should be able to know if the communication is now
      served by the shim layer or not.
   o  The application should be able to access locator information
      regardless of its address family.  In other words, no matter
      whether the target locator is IPv4 or IPv6, the application should
      be able to use common interface to access the locator information.

5.  Socket Options for Multihoming Shim Layer

   In this section, socket options that are specific to multihomed shim
   are defined.

   Table 1 provides a list of the socket options that are specific to
   multihoming shim layer.  These socket options can be used by either
   getsockopt() or setsockopt() system call for a given socket.  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
   getsockopt() and/or setsockopt(), respectively.  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   |     | Check if the    | int   |
   |                             |     |     | socket is       |       |
   |                             |     |     | associated with |       |
   |                             |     |     | any shim        |       |
   |                             |     |     | context or not. |       |
   | SHIM_DONTSHIM               | o   | o   | Request the     | int   |
   |                             |     |     | shim layer not  |       |
   |                             |     |     | to apply any    |       |
   |                             |     |     | multihoming     |       |
   |                             |     |     | support for the |       |
   |                             |     |     | communication.  |       |
   | SHIM_HOT_STANDBY            | o   | o   | Request the     | int   |
   |                             |     |     | shim layer to   |       |
   |                             |     |     | prepare a       |       |
   |                             |     |     | hot-standby     |       |
   |                             |     |     | connection (in  |       |
   |                             |     |     | addition to the |       |
   |                             |     |     | current path).  |       |
   | 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   | Request for the | int   |
   |                             |     |     | destination     |       |
   |                             |     |     | locator of the  |       |
   |                             |     |     | received IP     |       |
   |                             |     |     | packet.         |       |
   | SHIM_LOC_PEER_RECV          | o   | o   | Request for the | int   |
   |                             |     |     | source locator  |       |
   |                             |     |     | of the received |       |
   |                             |     |     | IP packet.      |       |
   | SHIM_LOC_LOCAL_SEND         | o   | o   | Request use of  | *2    |
   |                             |     |     | specific        |       |
   |                             |     |     | locator as      |       |
   |                             |     |     | source locator  |       |
   |                             |     |     | of outgoing IP  |       |
   |                             |     |     | packets.        |       |
   | SHIM_LOC_PEER_SEND          | o   | o   | Request use of  | *2    |
   |                             |     |     | specific        |       |
   |                             |     |     | locator as      |       |
   |                             |     |     | destination     |       |
   |                             |     |     | locator of      |       |
   |                             |     |     | outgoing IP     |       |
   |                             |     |     | packets.        |       |
   | SHIM_LOCLIST_LOCAL          | o   | o   | Get or set a    | *3    |
   |                             |     |     | list of         |       |
   |                             |     |     | locators        |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the local EID.  |       |
   | SHIM_LOCLIST_PEER           | o   | o   | Get or set a    | *3    |
   |                             |     |     | list of         |       |
   |                             |     |     | locators        |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the peer's EID. |       |
   | SHIM_APP_TIMEOUT            | o   | o   | Inform the shim | int   |
   |                             |     |     | layer of a      |       |
   |                             |     |     | timeout value   |       |
   |                             |     |     | for detecting   |       |
   |                             |     |     | failure.        |       |
   | SHIM_PATHEXPLORE            | o   | o   | Specify         | *4    |
   |                             |     |     | behavior of     |       |
   |                             |     |     | path            |       |
   |                             |     |     | exploration and |       |
   |                             |     |     | failure         |       |
   |                             |     |     | detection.      |       |
   | SHIM_CONTEXT_DEFERRED_SETUP | o   | o   | Specify if the  | int   |
   |                             |     |     | context setup   |       |
   |                             |     |     | can be deferred |       |
   |                             |     |     | or not.         |       |
   +-----------------------------+-----+-----+-----------------+-------+

               Table 1: Socket options for multihoming shim

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

   *2: Pointer to shim_locator data structure.

   *3: Pointer to an array of shim_locator.

   *4: 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.  In the figure, it can be seen that
   the shim layer and the additional protocol components (IPv4 and IPv6)
   below the shim layer are new to the system model.  As previously
   mentioned, all the shim specific socket options are defined at
   SOL_SHIM level.  This design choice brings the following advantages:

   1.  It is assured that the existing socket API continue to work at
       the layer above the shim layer.  That is, those legacy API deal
       with 'identifier' aspect of the IP addresses.
   2.  With newly defined socket options for the shim layer, the
       application obtains additional control on locator management.
   3.  The shim specific socket options are not specific to any address
       family (IPPROTO_IP or IPPROTO_IPV6) or any 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 socket API with shim layer

5.1.  SHIM_ASSOCIATED

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

   This option is particularly meaningful in a case where the locator
   information of the received IP packet does not tell whether the
   identifier/locator adaptation is performed or not.  Note that the EID
   pair and locator pair may be identical in some case.

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

   Data type of the option value is integer.  The option value indicates
   presence of shim context.  A returned value 1 means that the socket
   is associated with a certain shim context at the shim layer, while a
   return value 0 indicates that there is no context associated with the
   socket.

   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 can be used to request the shim layer to not
   apply the multihoming support for the communication established over
   the socket.

   Data type of the option value is integer.  The option value indicates
   whether the multihoming shim support is deprecated or not.  The
   option value is binary (0 or 1).  By default, the value is set to 0,
   meaning that the shim layer applies identifier/locator adaptation for
   the communication.  In order to disable the socket option, the
   application should call setsockopt() with optval set as 0.

   For example, the option can be disabled by the application as
   follows.

       int optval;

       optval = 0;

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

   For example, the option value can be checked by the application 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 can be used to check if the shim layer
   uses hot-standby connection or not for the communication established
   over the socket.  Hot-standby connection is another working locator
   pair than the current locator pair.  Hence this option is effective
   only when there is a shim context associated with the socket.

   Data type of the option value is 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.

   For example, the option can be activated by the application as
   follows.

       int optval;

       optval = 1;

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

   For example, the option value can be checked by the application as
   follows.

       int optval;
       int len;

       len = sizeof(optval);

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

5.4.  SHIM_PATHEXPLORE

   This option can be used to specify behavior of path exploration to be
   carried out.  Path exploration is a procedure to find an alternative
   locator pair when the host finds any problem with current locator
   pair.  A message used for finding an alternative locator pair is
   called a Probe message and it is sent per locator pair.  Default
   value is defined for Initial Probe Timeout (0.5 seconds) and Initial
   Probe (4 times) in the REAP specification.

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

   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 as NULL, meaning that the option
   is disabled.

   An error ENOENT will be returned when there is no context associated
   with the socket.

   For example, the parameters for the path exploration can be set 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, the parameters for the path exploration can be read as
   follows.

       struct shim6_pathexplore pe;
       int len;

       len = sizeof(pe);

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

5.5.  SHIM_LOC_LOCAL_PREF

   The SHIM_LOC_LOCAL_PREF option can be used to read or set preferred
   locator on local side within a given context.  Hence this option is
   effective only when there is a shim context associated with the
   socket.

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

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

   The preferred locator can be set by setsockopt().  Verification of
   the locator shall be done by the shim layer before updating the
   preferred locator.

   The preferred locator can be read by getsockopt().

   An error ENOENT will be returned when there is no context associated
   with the socket.

   An error EINVALIDLOCATOR will be returned when the validation of the
   specified locator failed.

   For example, a preferred locator can be set as follows.  It should be
   noted that some members of the shim_locator (lc_ifidx and lc_flags)
   are ignored in the write operation.

       struct shim_locator lc;
       struct in6_addr ip6;

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

       bzero(&lc, sizeof(shim_locator));
       lc.lc_family = AF_INET6;  /* IPv6 */
       lc.lc_ifidx = 0;
       lc.lc_flags = 0;
       lc.lc_preference = 255;
       memcpy(lc.lc_addr, &ip6, sizeof(in6_addr));

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

   For example, the preferred locator of the context can be read by
   application as follows.

       struct shim_locator lc;
       int len;

       len = sizeof(lc);

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

5.6.  SHIM_LOC_PEER_PREF

   The SHIM_LOC_PEER_PREF option can be used to read or set preferred
   locator on peer side within a given context.  Hence this option is
   effective only when there is a shim context associated with the
   socket.

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

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

   The preferred locator can be set by setsockopt().  Necessary
   verification of the locator shall be done by the shim layer before
   updating the preferred locator.

   The preferred locator can be read by getsockopt().

   An error ENOENT will be returned when there is no context associated
   with the socket.

   An error EINVALIDLOCATOR will be returned when the validation of the
   specified locator failed.

   For example, a preferred locator can be set as follows.  It should be
   noted that some members of the shim_locator (lc_ifidx and lc_flags)
   are ignored in the write operation.

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

5.7.  SHIM_LOC_LOCAL_RECV

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

   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.

   An error ENOENT will be returned when there is no context associated
   with the socket.

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

       int optval;

       optval = 1;

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

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

       int optval;
       int len;

       len = sizeof(optval);

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

5.8.  SHIM_LOC_PEER_RECV

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

   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.

   An error ENOENT will be returned when there is no context associated
   with the socket.

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

5.9.  SHIM_LOC_LOCAL_SEND

   The SHIM_LOC_LOCAL_SEND option can be used to request the shim layer
   to use specific locator for the source locator of IP packets to be
   sent from the socket.  Hence this option is effective only when there
   is a shim context associated with the socket.

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

   The local locator can be specified 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 deactivated.

   The local locator specified can be obtained by getsockopt().  The
   locator can be obtained from the option value.

   An error ENOENT will be returned when there is no context associated
   with the socket.

   An error EINVALIDLOCATOR when invalid locator is specified.

   For example, a preferred local locator can be specified 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_preference = 0;
       memcpy(&locator.lc_addr, &ia6, sizeof(ia6));

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

   For example, a preferred local locator can be read as follows.

       struct shim_locator locator;

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

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

       /* check locator */

5.10.  SHIM_LOC_PEER_SEND

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

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

   The remote locator can be specified 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 deactivated.

   The remote locator specified can be obtained by getsockopt().  The
   locator can be obtained from the option value.

   An error ENOENT will be returned when there is no context associated
   with the socket.

   An error EINVALIDLOCATOR when invalid locator is specified.

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

5.11.  SHIM_LOCLIST_LOCAL

   The SHIM_LOCLIST_LOCAL option can be used to read or set the locator
   list associated with the local EID of the shim context associated
   with the socket.  Hence this option is effective only when there is a
   shim context associated with the socket.

   Data type of option value is 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 as NULL,
   meaning that the option is disabled.

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

   The locator list can be set by setsockopt().  The buffer pointed by
   optval argument should contain an array of locator list.

   An error ENOENT will be returned when there is no context associated
   with the socket.

   An error EINVALIDLOCATOR will be returned when the validation of the
   specified locator failed.

   For example, a list of locators to be associated with the local EID
   can be specified as follows:

      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_preference = 1;
      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_preference = 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, a list of locators that are associated with the local
   EID can be obtained 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 */
       ...

5.12.  SHIM_LOCLIST_PEER

   The SHIM_LOCLIST_PEER option can be used to read or set the locator
   list associated with the peer EID of the shim context associated with
   the socket.  Hence this option is effective only when there is a shim
   context associated with the socket.

   Data type of option value is 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 as NULL,
   meaning that the option is disabled.

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

   The locator list can be set by setsockopt().  The buffer pointed by
   optval argument should contain an array of locator list.

   An error ENOENT will be returned when there is no context associated
   with the socket.

   An error EINVALIDLOCATOR will be returned when the validation of the
   specified locator failed.

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

5.13.  SHIM_APP_TIMEOUT

   The SHIM_APP_TIMEOUT option indicates timeout value for application
   to detect failure.  Hence this option is effective only when there is
   a shim context associated with the socket.

   Data type of the option value is 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 as 0,
   meaning that the option is disabled.  When the option is disabled,
   the REAP mechanism follows its default value of Send Timeout value as
   specified in [I-D.ietf-shim6-failure-detection]

   If the timeout value specified is longer than the Send Timeout
   configured in the REAP component, the REAP Keepalive message should
   be suppressed.

   An error ENOENT will be returned when there is no context associated
   with the socket.

   For example, a specific timeout value can be configured by the
   application as follows:

       int optval;

       optval = 15; /* 15 seconds */

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

   For example, the option value namely the period of timeout can be
   checked by the application as follows:

       int optval;
       int len;

       len = sizeof(optval);

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

5.14.  SHIM_DEFERRED_CONTEXT_SETUP

   The SHIM_DEFERRED_CONTEXT_SETUP option indicates how initiation of
   context setup is made in terms of timing (before or after) the
   initial communication flow.  Deferred context means that the
   establishment of context does not put additional delay for an initial
   transaction.

   Data type for the option value is integer.  The option value should
   binary (0 or 1).  By default, the value is set as 1, meaning that the
   context setup is deferred.  In order to disable the option, the
   application should call setsockopt() with option value set as 0.

   However, it should be noted that in some case, deferred context setup
   is not possible; given EID is non-routable address and there is no
   way to transmit any IP packet unless there is a context providing the
   locators.  In such case, context should be established prior to the
   communication.

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

       int optval;

       optval = 0;

       setsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP,
                  &optval, sizeof(optval));
   For example, the option value can be checked by the application as
   follows:

       int optval;
       int len;

       len = sizeof(optval);

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

5.15.  Error Handling

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

   The followings are errno codes newly 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 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 return routability (check if the peer is actually
      willing to receive data with the specified locator) and the other
      is verifications based on given crypto identifier mechanisms
      [RFC3972], [I-D.ietf-shim6-hba].

6.  Ancillary Data for Multihoming Shim

   In this section, definition and usage of the ancillary data which is
   specific to multihoming shim are provided.

   As defined in Posix standard, sendmsg() and recvmsg() take msghdr
   structure as its argument and they can additionally handle control
   information along with data.  Figure 22 shows the msghdr structure
   which is defined in <sys/socket.h>. msg_control member holds a
   pointer to the buffer where the shim specific ancillary data objects
   can be stored in addition to other ancillary data objects.

        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 22: msghdr structure

   The buffer pointed from the msg_control member of the msghdr
   structure may contain a locator information which is a single locator
   and it should be possible to process them with the existing macros
   defined in Posix and [RFC3542].  Each cmsghdr{} should be followed by
   data which stores a single locator.

   In case of non-connected socket, msg_name member stores the socket
   address of the peer which should be considered as an identifier
   rather than a locator.  The locator of the peer node should be
   retrieved by SHIM_LOC_PEER_RECV as specified below.

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

     +---------------------+-----------+-----------+-----------------+
     | 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[] should include padding (if necessary) and a single
   sockaddr_in{}/sockaddr_in6{}.

   It should be noted that the above ancillary data can only be handled
   in UDP and raw sockets, not in TCP sockets because there is no one-
   to-one mapping of send/receive operations and the TCP segments being
   transmitted/received.

6.1.  Get Locator Information from Incoming Packet

   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.

6.2.  Specify Locator Information for Outgoing Packet

   Application can specify the locators to be used for transmitting an
   IP packet by sendmsg().  When ancillary data of cmsg_type
   SHIM_LOC_LOCAL_SEND and/or SHIM_LOC_PEER_SEND are specified, the
   application can explicitly specify source and/or destination locators
   to be used for the communication over the socket.

   In addition, the application can specify the outgoing interface by
   SHIM_IF_SEND ancillary data.  The ancillary data should contain the
   interface identifier of the physical interface over which the
   application expects the packet to be transmitted.

   Note that the effect is limited to the datagram transmitted by the
   sendmsg().

   If the specified locator pair seems to be valid, the shim layer
   overrides the locator of the IP packet as requested.

   An error EINVALIDLOCATOR will be returned when validation of the
   specified locator failed.

6.3.  Notification from Application to Multihoming Shim

   Application may provide feedback to the shim layer about the
   communication status.  Such feedback is particularly useful for the
   shim layer in the absence of REAP mechanism to monitor the
   reachability status of 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.

   An error ENOENT will be returned when there is no context associated
   with the socket.

   See Section 7.3 for details of the data structure to be used.  Note
   that this specification does not specify the exact behavior of the
   shim layer when a feedback information is given by an application.

7.  Data Structures

   In this section, data structures specifically defined for the
   multihoming shim layer are introduced.  Those data structure are
   either used as a parameter for setsockopt()/getsockopt() (as
   mentioned in Section 5) or a parameter for ancillary data to be
   processed by sendmsg()/recvmsg() (as mentioned in Section 6).

7.1.  Placeholder for Locator Information

   As defined in Section 5, the SHIM_LOC_LOCAL_PREF, SHIM_LOC_PEER_PREF,
   SHIM_LOCLIST_LOCAL, and SHIM_LOCLIST_PEER 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 23 illustrates the data structure called shim_locator which
   stores a locator information.

        struct shim_locator {
                uint8_t   lc_family;     /* address family */
                uint8_t   lc_ifidx;      /* interface index */
                uint8_t   lc_flags;      /* flags */
                uint8_t   lc_preference; /* preference value */
                uint8_t   lc_addr[16];   /* address data */
        };

                     Figure 23: 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.
   lc_ifidx
      Interface index of the network interface to which the locator is
      assigned.  This field should be valid only in read (getsockopt())
      operation.
   lc_flags
      Each bit of the flags represents a specific characteristics of the
      locator.  HBA is defined as 0x01.  CGA is defined as 0x02.  The
      other bits are TBD.
   lc_preference
      Indicates preference of the locator.  The preference is
      represented by integer.

   lc_addr
      Contains the locator.  For the cases where a locator whose size is
      smaller than 16 bytes, encoding rule should be provided for each
      locator of a given address family.  For instance, in case of
      AF_INET (IPv4), the first 4 bytes of lc_addr should contain the
      IPv4 address.

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 24 illustrates the data structure.  The
   data structure can be used by 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 24: 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
      [I-D.ietf-shim6-failure-detection].
   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
      [I-D.ietf-shim6-failure-detection].
   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 [I-D.ietf-shim6-failure-detection].
   pe_reserved
      A reserved field for future extension.  By default, the field
      should be initialized with zero.

7.3.  Feedback Information

   As mentioned in Section 6.3, applications can inform the shim 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 25).

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

                 Figure 25: 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.
   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 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.
      *  3: Satisfactory.  There is satisfactory unidirectional
         reachability between the locator pair in question.
   reserved
      Reserved field.  Must be ignored by the receiver.

8.  Implications for Existing Socket API Extensions

   Some of the socket options defined in this document have some
   overlapping with existing socket API and care should be made for the
   usage not to confuse the features.

   The socket options for requesting specific locators to be used for a
   given transaction (SHIM_LOC_LOCAL_PREF and SHIM_LOC_PEER_PREF) are
   semantically similar to the existing socket API (IPV6_PKTINFO).  The
   socket options for obtaining the locator information from the
   received IP packet (SHIM_LOC_LOCAL_RECV and SHIM_LOC_PEER_RECV) are
   semantically similar to the existing socket API (IP_RECVDSTADDR and
   IPV6_PKTINFO).

   In IPv4, application can obtain the destination IP address of the
   received IP packet (IP_RECVDSTADDR).  If the shim layer performs
   identifier/locator adaptation for the received packet, the
   destination EID should be stored in the ancillary data
   (IP_RECVDSTADDR).

   In IPv6, [RFC3542] defines that IPV6_PKTINFO can be used to specify
   source IPv6 address and the outgoing interface for outgoing packets,
   and retrieve destination IPv6 address and receiving interface for
   incoming packets.  This information is stored in ancillary data being
   IPV6_PKTINFO specified as cmsg_type.  Existing socket API should
   continue to work above the shim layer, that is, the IP addresses
   handled in IPV6_PKTINFO should be EIDs, not the locators.

   Baseline is that the above existing socket API (IP_RECVDSTADDR and
   IPV6_PKTINFO) is assumed to work above the multihoming shim layer.
   In other words, the IP addresses those socket options deal with are
   EIDs rather than locators.

9.  Resolving Conflicts with Preference Values

   Since the multihoming shim API allows application to specify
   preference value for the context which is associated with the socket
   instance, there may be a conflict with preference values specified by
   different applications.  For instance, application A and B may
   establish communication over the same EID pair while each application
   have different preference in their choice of local locator.

   SHIM6 supports a notion of forking context in which a context is
   split when there is a conflict with preference values specified by
   multiple applications.  Thus, context forking can simply resolve the
   conflicting situation which may be caused by the use of socket
   options for multihoming shim layer.

9.1.  Implicit Forking

   Socket options defined in Section 5 may cause conflicting situation
   when the target context is shared by multiple applications.  In such
   case, socket handler and the multihoming shim layer should react as
   follows; socket handler should inform the shim 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 multihoming
   shim layer during the lifetime of associated socket instance.  When
   the socket is closed, the multihoming shim layer SHOULD delete
   associated context.  In this way, garbage collection can be carried
   out to cleanup unused forked contexts.  Upon garbage collection,
   every forked context SHOULD be checked if there is no socket
   (process) associated with the context.  If there is none, the forked
   context should be deleted.  When a forked context is torn down, SHIM6
   should notify the peer about the deletion of forked context.

   As opposed to socket options, context forking MUST NOT be triggered
   by any use of ancillary data that are specific to multihoming shim
   defined in Section 6.

10.  Discussion

   In this section, open issues are introduced.

10.1.  Naming at Socket Layer

   getsockname() and getpeername() system calls are used to obtain the
   'name' of endpoint which is actually a pair of IP address and port
   number assigned to a given socket. getsockname() is used when an
   application wants to obtain the local IP address and port number
   assigned for a given socket instance. getpeername() is used when an
   application wants to obtain the remote IP address and port number.

   The above is based on a traditional system model of the socket API
   where an IP address is expected to play both the role of identifier
   and the role of locator.

   In a system model where a shim layer exists inside the IP layer, both
   getsockname() and getpeername() deal with identifiers, namely EIDs.
   In this sense, the shim layer serves to (1) hide locators and (2)
   provide access to the identifier for the application over the legacy
   socket APIs.

10.2.  Additional Requirements from Application

   At the moment, it is not certain if following requirements are common
   in all the multihomed environments (SHIM6 and HIP).  These are mainly
   identified during discussions made on SHIM6 WG mailing list.
   o  The application should be able to set preferences for the
      locators, local and remote one and also to the preferences of the
      local locators that will be passed to the peer.

10.3.  Issues of Header Conversion among Different Address Family

   The shim layer performs identifier/locator adaptation.  Therefore, in
   some case, 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 such as SIIT[RFC2765].

   As addressed in SIIT specification, some of the features (IPv6
   routing headers, hop-by-hop extension headers, or destination
   headers) from IPv6 are not convertible to IPv4.  In addition, notion
   of source routing is not exactly the same in IPv4 and IPv6.  Hence,
   there is certain limitation in protocol conversion between IPv4 and
   IPv6.

   The question is how should the shim layer behave when it is face with
   limitation problem of protocol conversion.  Should we introduce new
   error something like ENOSUITABLELOCATOR ?

10.4.  Handling of Unknown Locator Provided by Application

   There might be a case where application provides the shim layer new
   locator with the SHIM_LOC_*_PREF socket options or SHIM_LOC_*_SEND
   ancillary data.  Then there is a question how should the shim layer
   treat the new locator informed by the application.

   In principle, locator information are exchanged by the shim protocol.
   However, there might be a case where application acquires information
   about the locator and prefers to use it for its communication.

11.  Changes

11.1.  Changes from version 00 to version 01

   The followings are 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.

11.2.  Changes from version 01 to version 02

   The followings are 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).

11.3.  Changes from version 02 to version 03

   The followings are changes from version 02 to version 03:
   o  Add an Appendix section describing the issue of context forking.

11.4.  Changes from version 03 to version 04

   The followings are changes from version 03 to version 04:
   o  Updated reference.
   o  Correct typo and grammatical errors.

11.5.  Changes from version 04 to version 05

   The followings are 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.

12.  IANA Considerations

   This document contains no IANA consideration.

13.  Security Considerations

   This document does not specify any security mechanism for the shim
   layer.  Fundamentally, the shim layer has a potential to impose
   security threats, as it changes the source and/or destination IP
   addresses of the IP packet being sent or received.  Therefore, the
   basic assumption is that the security mechanism defined in each
   protocol of the shim layer is strictly applied.

14.  Conclusion

   In this document, the Application Program Interface (API) for
   multihoming shim layer is specified.  The socket API allows
   applications to have additional control of the locator management and
   interface to the REAP mechanism inside the multihoming shim layer.

   Socket options for multihoming shim layer can be used by getsockopt()
   and/or setsockopt() system calls.  Besides, applications can use some
   ancillary data that are specific to multihoming shim layer to get
   locator from received packet or to set locator for outgoing packet.

   From an architectural point of view, the socket API provides extends
   the existing socket API framework in the face of ID/Locator
   separation.  With regard to API that relate to IP address management,
   it is assured that existing socket API continue to work above the
   shim layer dealing with identifiers, while multihoming shim API deals
   with locators.

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 socket API related issues.  Thomas
   Henderson provided valuable comments especially from HIP
   perspectives.

16.  References

16.1.  Normative References

   [I-D.ietf-shim6-failure-detection]
              Arkko, J. and I. Beijnum, "Failure Detection and Locator
              Pair Exploration Protocol for IPv6  Multihoming",
              draft-ietf-shim6-failure-detection-10
              draft-ietf-shim6-failure-detection-13 (work in progress),
              January
              June 2008.

   [I-D.ietf-shim6-proto]
              Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
              protocol", draft-ietf-shim6-proto-09 draft-ietf-shim6-proto-10 (work in progress),
              October
              February 2007.

   [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.

16.2.  Informative References

   [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-hba]
              Bagnulo, M., "Hash Based Addresses (HBA)",
              draft-ietf-shim6-hba-05 (work in progress), December 2007.

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

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

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 initiator.

   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
   implementation specific issue.  However, on the responder side, there
   is a question on how the outbound packet can be multiplexed by the
   shim layer.  Since 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 some other
   information and associate a given packet flow with specific context.

   Figure 26 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 the same ULID pair.  The first context
   pair (Ctx1) is established for the transaction of App1.  Given the
   requests from App2, the shim 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 26.
   However, as mentioned earlier, on the responder side, there is a
   problem with multiplexing the outbound packet flows of App1 and App2.

           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 26: context forking

   To overcome the problem mentioned above, there are some solutions.

   One viable approach is to let the system implicitly maintain an
   association between the socket and the associated context by keeping
   the record of inbound packet processing.  That is, the system stores
   the information about the context on which the inbound packet flow
   was demultiplexed.  The information comprises the ULID pair and FII
   of the context and is stored in the socket instance.  Later, the
   system can use the information to identify the associated context in
   outbound packet processing.  This approach should be feasible as far
   as there is bi-directional user traffic.

   Another viable approach is to extend SHIM6 protocol by adding
   capability of exchanging additional information to identify the
   packet flow from others which needs to be handled by a newly forked
   context.  The information exchange can be done during the context
   establishment.  The initiator appends 5 tuple of the packet flow to
   be handled by the newly forked context.  Note that the additional
   information provided by the 5 tuple are source and destination port
   numbers and upper layer protocol.  The information is later used by
   the shim layer to multiplex the outbound packet flow on the responder
   side.

   The socket options for multihoming shim can be used by the
   application to trigger the context forking in implicit manner.  The
   peer becomes an initiator in the establishment of the forked context.
   Once the forked context is established between the peers, application
   on each end can influence the preference on context by utilizing the
   multihoming shim API.

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/
   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.sugimoto@ericsson.com

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