SHIM6 Working Group                                              M. Komu
Internet-Draft                                                      HIIT
Expires: August 5, 2006                                       M. Bagnulo
                                                                    UC3M
                                                               K. Slavov
                                                        S. Sugimoto, Ed.
                                                                Ericsson
                                                           February 2006

    Socket Application Program Interface (API) for Multihoming Shim
                 draft-ietf-shim6-multihome-shim-api-00
                 draft-ietf-shim6-multihome-shim-api-01

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of 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 August 5, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

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 multhomed 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  . . . . . . . . . . . . . . . . . . . 14 15
     5.6.  SHIM_LOC_PEER_PREF . . . . . . . . . . . . . . . . . . . . 15 16
     5.7.  SHIM_LOC_LOCAL_RECV  . . . . . . . . . . . . . . . . . . . 15 17
     5.8.  SHIM_LOC_PEER_RECV . . . . . . . . . . . . . . . . . . . . 16 18
     5.9.  SHIM_LOCLIST_LOCAL . . . . . . . . . . . . . . . . . . . . 16 18
     5.10. SHIM_LOCLIST_PEER  . . . . . . . . . . . . . . . . . . . . 17 19
     5.11. SHIM_APP_TIMEOUT . . . . . . . . . . . . . . . . . . . . . 17 19
     5.12. SHIM_DEFERRED_CONTEXT_SETUP  . . . . . . . . . . . . . . . 18 20
     5.13. Error Handling . . . . . . . . . . . . . . . . . . . . . . 19 21
   6.  Ancillary Data for Multihoming Shim  . . . . . . . . . . . . . 19 21
     6.1.  Get Locator Information from Incoming Packet . . . . . . . 21 23
     6.2.  Specify Locator Information for Outgoing Packet  . . . . . 21 23
     6.3.  Notification from Application to Multihomign Multihoming Shim  . . . . 21 23
       6.3.1.  SHIM_FEEDBACK_POSITIVE . . . . . . . . . . . . . . . . 21 24
       6.3.2.  SHIM_FEEDBACK_NEGATIVE . . . . . . . . . . . . . . . . 22 24
   7.  Data Structures  . . . . . . . . . . . . . . . . . . . . . . . 22 24
     7.1.  Placeholder for Locator Information  . . . . . . . . . . . 22
       7.1.1.  Locator Information Stored in Control Message  . . . . 22
       7.1.2.  Locator Information Handled by getsockopt() and
               setsockopt() . . . . . . . . . . . . . . . . . . . . . 22 24
     7.2.  Parameters of  Path Exploration Parameter . . . . . . . . . . . . . . 23 . . 25
   8.  Implications for Existing Socket API Extensions  . . . . . . . 23 26
   9.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 23 27
     9.1.  Issues with a Context Shared by Applications . . . . . . . 23 27
     9.2.  Issues with Shim Unaware Application . . . . . . . . . . . 24 27
       9.2.1.  Initial Contact with Multiple Locator Pairs  . . . . . 24 27
       9.2.2.  Naming at Socket Layer . . . . . . . . . . . . . . . . 25 29
     9.3.  Additional Requirements from Application . . . . . . . . . 26 29
     9.4.  Issues of Header Conversion among Different Address
           Family . . . . . . . . . . . . . . . . . . . . . . . . . . 26 29
     9.5.  Handling of Unknown Locator Provided by Application  . . . 26 30
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27 30
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 27 30
   12. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 27 30
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 27 31
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 31
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 28 31
     14.2. Informative References . . . . . . . . . . . . . . . . . . 28 32
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30 33
   Intellectual Property and Copyright Statements . . . . . . . . . . 31 34

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.  A care is needed
   to not disrupt the upper layer protocols 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[I-D.ietf-hip-arch]
   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, congestions, 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 addressed mentioned in section 4.3 and section 5.2 of REAP
      specification]: [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, the socket options for the interface between the
   application and the multihomed shim layer are defined.  These options
   can be used either by getsockopt() or setsockopt() system call for an
   open socket.  Table 1 provides a list of the socket options.  Note
   that all socket options are defined at level SOL_SHIM.

   The first column of the table gives the name of the option.  The
   second and third columns indicate whether the option is for
   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 with the socket option,
   which can store an argument for setsockopt() and result for
   getsockopt().  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_LOCLIST_LOCAL          | o   | o   | Get or set a    | *2    |
   |                             |     |     | list of         |       |
   |                             |     |     | locators        |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the local EID.  |       |
   | SHIM_LOCLIST_PEER           | o   | o   | Get or set a    | *2    |
   |                             |     |     | 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 how         | *3    |
   |                             |     |     | path            |       |
   |                             |     |     | exploration behavior of     |       |
   |                             |     |     | should be path            |       |
   |                             |     |     | performed in exploration and |       |
   |                             |     |     | case of failure         |       |
   |                             |     |     | failure. detection.      |       |
   | SHIM_CONTEXT_DEFERRED_SETUP | o   | o   | Specify if the  | int   |
   |                             |     |     | context setup   |       |
   |                             |     |     | can be deferred |       |
   |                             |     |     | or not.         |       |
   +-----------------------------+-----+-----+-----------------+-------+

        Table 1: Shim specific socket options for getsockopt() and
                               setsockopt()

   *1: Pointer to the buffer (TBD) in which a single locator information shim_locator which is stored. defined in Section 7.

   *2: Pointer to the buffer (TBD) in which a list an array of locator
   information is stored. shim_locator.

   *3: Pointer to the buffer (TBD) in which a set of parameters of path
   exploration shim_pathexplore which is stored. 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

   This

   The SHIM_ASSOCIATED option can be specified by getsockopt() used to check if whether the socket is
   associated with a 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
   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.

   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.

   For example,

   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

   This

   The SHIM_DONTSHIM option indicates whether can be used to request the shim layer applies to not
   apply the multihoming support for the communication established over
   the socket 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 can be overwritten by setsockopt() and can be
   checked by getsockopt().  The optval should be 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:
   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:
   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 indicates whether 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. another working locator pair than

   Data type of the
   current locator pair. option value is integer.

   The option value can be overwritten set by
   setsockopt() and setsockopt().

   The option value can be checked 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:
   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:
   follows.

       int optval;
       int len;

       len = sizeof(optval);

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

5.4.  SHIM_PATHEXPLORE

   The

   This option indicates how aggressive the application wants can be used to specify behavior of path exploration to be performed
   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 case of failure.  Hence this the REAP specification.

   The option is effective only when there is a shim context associated
   with the socket.  The option value can be overwritten by setsockopt() and can
   be checked by getsockopt().  The

   Data type of the option value contains is a pointer to the buffer where information of path exploration (the number a set
   of
   attempts information for path exploration, frequency of the path exploration, and
   so on) 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.

   Example is TBD.

5.5.  SHIM_LOC_LOCAL_PREF

   The option value contains

   For example, the preferred 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 context associated with the socket. given context.  Hence this option is
   effective only when there is a shim context associated with the
   socket.  The
   option value holds a single locator information.  The option value
   can be overwritten by setsockopt() and can be checked by
   getsockopt().  When

   Data type of the option value is changed by the application by
   setsockopt(), a pointer to the shim layer shall accordingly update a specific data
   structure which stores the preferred locator within the context associated with the socket. 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.

   Example is TBD.

   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 value contains the can be used to read or set preferred
   locator on remote peer side within a context associated with the socket. given context.  Hence this option is
   effective only when there is a shim context associated with the
   socket.  The
   option value holds a single locator information.  The option value
   can be overwritten by setsockopt() and can be checked by
   getsockopt().  When

   Data type of the option value is changed by the application by
   setsockopt(), a pointer to the shim layer shall accordingly update a specific data
   structure which stores the preferred locator within the context associated with the socket. 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.

   Example

   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 TBD. same as that of SHIM_LOC_LOCAL_PREF.

5.7.  SHIM_LOC_LOCAL_RECV

   With this option, the application

   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 overwritten set by setsockopt() and setsockopt().

   The option value can be checked read by getsockopt().

   See section Section 7 for the data structure 6 for
   storing the procedure to access locator information. 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

   With this option, the application

   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 overwritten set by setsockopt() and setsockopt().

   The option value can be checked read by getsockopt().

   See section Section 7 for the data structure 6 for
   storing the procedure to access locator information. 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 almost identical to same as that of SHIM_LOC_LOCAL_RECV
   option.

5.9.  SHIM_LOCLIST_LOCAL

   With this option, the application

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

   Data type of option value
   contains a is pointer to the buffer where the a locator
   list is stored.  See section 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.  By getsockopt(), the application can get the

   The locator
   list.  By setsockopt(), the application list can request be read by getsockopt().  Note that the shim layer size of
   the buffer pointed by optval argument should be large enough to
   update its store
   an array of locator list that is associated with a local EID.  See
   section Section 7 for the data structure for storing information.  The number of the locator
   information.
   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.

   Example is TBD.

5.10.  SHIM_LOCLIST_PEER

   With this option, the application

   The SHIM_LOCLIST_LOCAL option can request be used to read or set the shim layer for a locator
   list of locators which is currently associated with the remote peer EID
   within a of the shim context. context associated with
   the socket.  Hence this option is effective only when there is a shim
   context associated with the socket.  The

   Data type of option value
   contains a is pointer to the buffer where the a locator
   list is stored.  See section 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.  By getsockopt(), the application can get the locator
   list.  By setsockopt(), the application can request the shim layer to
   update its

   The locator list can be read by getsockopt().  Note that is associated with a remote EID.  See
   section Section 7 for the data structure for storing 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.

   Example

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

5.11.  SHIM_APP_TIMEOUT

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

   Data type of timeout in seconds.  Accordingly, the shim layer shall
   update the strategy for reachability test.  In particular, this is
   efficient in a case where the informed timeout option value is shorter than integer.  The value indicates the
   period of the keepalive timer.  In such case, keepalives timeout in seconds to be
   performed by send a REAP may be suppressed. Keepalive message since
   the last outbound traffic.  By default, the option value is set to 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 = 4; 15; /* 4 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);

5.12.  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
   bi
   binary (0 or 1).  By default, the value is set to as 1, meaning that the
   context setup is deferred.  In order to disable the option, the
   application should call setsockopt() with option value set to 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 establishment should be made 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);

5.13.  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 multihiming 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 14 18 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 14: 18: 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_POSITIVE | o         |           | TBD         |
     | SHIM_FEEDBACK_NEGATICE | o         |           | TBD         |
     +------------------------+-----------+-----------+-------------+

                   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.  As explained in
   [RFC3542], there is no one-to-one mapping of send/receive operations
   and the TCP segments being transmitted/received.  In case of TCP,
   application may use setsockopt() or getsockopt() to access or specify
   some of locator information provided by the shim layer.

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 Multihomign Multihoming Shim

   Application may provide feedback to the shim layer in accordance with
   its communication status.  The notification can be made by specifying
   shim specific ancillary data in sendmsg() call.  Note that this
   notification is dynamic rather than static.

6.3.1.  SHIM_FEEDBACK_POSITIVE

   The application can simply inform the shim layer that its
   communication is going well.

   Ancillary data object

   Data type is TBD.

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

6.3.2.  SHIM_FEEDBACK_NEGATIVE

   The application can inform the shim layer that its communication is
   not going well.

   Ancillary data object

   Data type is TBD.

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

7.  Data Structures

   In this section, data structures newly specifically defined for socket options for the
   multihoming shim layer are introduced.  Those data structures are

7.1.  Placeholder for Locator Information

   Some of

   As defined in Section 5, the SHIM_LOC_LOCAL_PREF, SHIM_LOC_PEER_PREF,
   SHIM_LOCLIST_LOCAL, and SHIM_LOCLIST_PEER socket options defined in this document need to
   handle one or more locator information.  Locator information could be a single locator or an
   array of locators.  An important requirement is that includes
   not only the locator itself but also additional information should be handled in a protocol independent manner.  In
   other words, an interface to about the
   locator information should not be
   dependent on any address family.

7.1.1.  Locator Information Stored in Control Message

   When either SHIM_LOC_LOCAL_* or SHIM_LOC_PEER_* socket option which is
   specified, sendmsg() or recvmsg() should handle useful for locator information management.  A new data structure
   is defined to serve as a control message.  The placeholder for the locator information is stored in an
   ancillary information.

   Figure 19 illustrates the data object structure called shim_locator which consists of
   stores a common header (cmsghdr{}), 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];   /* locator */
        };

   Figure 19: shim locator structure
   lc_family
      Address family of the data, and locator (e.g.  AF_INET, AF_INET6).  It is
      required that the padding if necessary.  In parameter contains non-zero value indicating the case when
      exact address family of the locator.
   lc_ifidx
      Interface index of the network interface to which the locator is IPv4, the cmsg_data[]
      assigned.  This field should contain sockaddr_in{}.  In be valid only in read (getsockopt())
      operation.
   lc_flags
      Each bit of the case
   when 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 IPv6,
      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 cmsg_data[] last 4 bytes of lc_addr should contain
   sockaddr_in6{}.

7.1.2.  Locator Information Handled by getsockopt() and setsockopt()

   SHIM_LOC_LOCAL_PREF and SHIM_LOC_PEER_PREF socket options the
      IPv4 address.

7.2.  Path Exploration Parameter

   As defined in Section Section 5 () require getsockopt() or setsockopt() 5, SHIM_PATHEXPLORE allows application to handle a
   single locator information.  The set
   or read the parameters for path exploration and failure detection.  A
   new data structure for storing the
   locator information called shim_pathexplore is TBD.

   SHIM_LOCLIST_LOCAL and SHIM_LOCLIST_PEER defined in Section Section 5
   () require getsockopt() or setsockopt() to handle a set of locator
   information (aka locator list). store the
   necessary parameters.  Figure 20 illustrates the data structure.  The
   data structure for storing the
   locator information is TBD.

7.2.  Parameters of Path Exploration

   SHIM_PATHEXPLORE requires 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 20: path explore structure

   pe_probenum
      Indicates the number of initial probe messages to handle a
   set be sent.
      Default value of parameters 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.  The data structure exploration
      procedure will be carried out by sending a REAP Probe message.
      Default value of this parameter should follow what is
   TBD. 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.

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.  Discussion

   In this section, open discussion issues are noted.

9.1.  Issues with a Context Shared by Applications

   A context is by definition, system-wide.  This means that a context
   could be shared by applications whose communications are using the
   same EID pair.

   When a context is shared by applications, there may be some problems
   when the shim layer needs to handle feedbacks from the multiple
   applications.  As mentioned in Section Section 6, an application may
   provide the shim layer feedback about timeout values from its own
   settings.  This implies that there is potentially a race condition at
   the shim layer.

   First of all, the socket options must be used with a proper
   privilege.  Feedback from the application which is run under a root
   privilege must always override the feedback provided by application
   which is run under normal user privilege.

   For other cases, one could rely on a kind of heuristics of the
   configuration.  For instance, prioritizing feedback with higher
   demand (e.g. timeout value 300 seconds are more demanding then
   timeout value 600 seconds) may make sense in some cases.  However, it
   is still an open issue what kind of timer value could be handled in
   this way.

   Further discussions are needed how the shim layer can accommodate
   feedbacks from multiple applications within a same context.

9.2.  Issues with Shim Unaware Application

   In multihomed environment where either of the peers or both of the
   peers have multiple locators, there are some issues with shim unaware
   application which uses legacy socket API.

9.2.1.  Initial Contact with Multiple Locator Pairs

   In a connection oriented communication, the connect() system call is
   used to make the initial contact to the peer, which typically
   requires IP address and port number to specify the endpoint.  Hence,
   name-to-address resolution should be performed prior to connect().

   The application needs to resolve the FQDN of the peer to an IP
   address by any available name-to-address conversion method.

   In typical case, the application receives information from the
   resolver.  If the application ends up with receiving multiple IP
   addresses to reach the peer, it should iterate through each
   destination address one-by-one.  It should be noted that the host may
   also have multiple source addresses.

   The different resulting address pairs may have different reachability
   status so, in order to find a working address pair, it may be
   required to explore all the available address pairs (as opposed to
   explore all available destination addresses).

   In normal case, the application issues a connect() by specifying the
   resolved IP address of the peer.  If the connect() fails, it iterates
   through the available IP addresses one by one sequentially until
   working pair is found.  Another approach is to initiate concurrent
   connect() with every locator of the peer. connect() can also be
   called in a sequence which would probably require more time to find
   the working pair.

   There is a case where involvement of the shim layer is expected for
   handling initial contact.  In such case, behavior of the shim layer
   will depend on presence of the required context.  This case occurs
   when there exists a context for the EID specified in connect(), the
   initial contact can be made in accordance with the context
   information.  Otherwise, the shim layer should invoke context
   establishment with the peer EID specified in the argument for
   connect().

   Additional efforts would be required in a case where the peer cannot
   be reachable through the EID (for example, EID is non-routable or
   non-IP reachable) but it can be reached through alternative locator.
   In particular, the shim layer should somehow discover the alternate
   locator for the EID to establish context.  [I-D.nordmark-shim6-esd]
   addresses the possible approach to perform reverse DNS lookup from
   EID to FQDN, then perform forward lookup again to find the full-set
   of locators and EID.

   In HIP, resolving HITs to IP addresses using DNS is not feasible
   because HITs do not contain any hierarchical information.  To
   mitigate this problem, there are a few alternatives.  Firstly,
   resolver library on end-host can be modified to provide HIT-to-IP
   mappings for HIP software module.  Secondly, a distributed hash table
   (DHT) service can be used for storing and looking up the mappings
   because it supports non-hierarchical identifiers, such as HITs
   [I-D.ietf-hip-arch].  Thirdly, it is possible to use IP addresses in
   legacy applications as described in [I-D.henderson-hip-applications].

9.2.2.  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.

9.3.  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.

9.4.  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 ?

9.5.  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.

10.  IANA Considerations

   This document contains no IANA consideration.

11.  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.

12.  Conclusion

   In this document, the Application Program Interface (API) for
   multihomed shim layer is specified.  The socket API allows
   applications to have additional control on the locator management and
   interface to the REAP mechanism inside the shim layer.  The socket
   API is expected to be useful for applications that may greatly
   benefit from multihomed environment.  From the architectural
   perspective, the socket API enhances software development environment
   in a sense that it allows separate treatment of identifier and
   locator at the IP layer.  The API is designed with a care not to
   break the semantics of existing socket API and minimize the impact to
   the legacy applications.

   Multihoming shim socket options defined in this document can be used
   by getsockopt() and/or setcokopt() system calls, which allow
   applications to have control of locator management.  Additionally,
   applications can specify locator information for outgoing packet and
   get locator information from incoming packet by using ancillary data
   objects that are specific to the multihoming shim layer.

13.  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.

14.  References

14.1.  Normative References

   [I-D.henderson-hip-applications]
              Henderson, T. and P. Nikander, "Using HIP with Legacy
              Applications", draft-henderson-hip-applications-03 (work
              in progress), May 2006.

   [I-D.ietf-hip-arch]
              Moskowitz, R. and P. Nikander, "Host Identity Protocol
              Architecture", draft-ietf-hip-arch-03 (work in progress),
              August 2005.

   [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-03
              draft-ietf-shim6-failure-detection-06 (work in progress),
              December 2005.
              September 2006.

   [I-D.ietf-shim6-proto]
              Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
              protocol", draft-ietf-shim6-proto-03 draft-ietf-shim6-proto-05 (work in progress),
              December 2005.
              May 2006.

   [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.

14.2.  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-01
              draft-ietf-shim6-hba-02 (work in progress), October 2005. 2006.

   [I-D.nordmark-shim6-esd]
              Nordmark, E., "Extended Shim6 Design for ID/loc split and
              Traffic Engineering", draft-nordmark-shim6-esd-00 (work in
              progress), February 2006.

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

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

Authors' Addresses

   Miika Komu
   Helsinki Institue 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

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.