SHIM6 Working Group                                              M. Komu
Internet-Draft                                                      HIIT
Intended status: Informational                                M. Bagnulo
Expires: November 8, 2009 January 14, 2010                                           UC3M
                                                               K. Slavov
                                                        S. Sugimoto, Ed.
                                                                Ericsson
                                                             May 7,
                                                           July 13, 2009

    Socket Application Program Interface (API) for Multihoming Shim
                 draft-ietf-shim6-multihome-shim-api-08
                 draft-ietf-shim6-multihome-shim-api-09

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Abstract

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

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  System Overview  . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Socket Options for Multihoming Shim Layer  . . Sub-Layer  . . . . . . . .  9
     5.1.  SHIM_ASSOCIATED  . . . . . . . . . . . . . . . . . . . . . 12 13
     5.2.  SHIM_DONTSHIM  . . . . . . . . . . . . . . . . . . . . . . 13 14
     5.3.  SHIM_HOT_STANDBY . . . . . . . . . . . . . . . . . . . . . 13 15
     5.4.  SHIM_PATHEXPLORE . . . . . . . . . . . . . . . . . . . . . 14 15
     5.5.  SHIM_LOC_LOCAL_PREF  . . . . . . . . . . . . . . . . . . . 15 16
     5.6.  SHIM_LOC_PEER_PREF . . . . . . . . . . . . . . . . . . . . 16 17
     5.7.  SHIM_LOC_LOCAL_RECV  . . . . . . . . . . . . . . . . . . . 17 18
     5.8.  SHIM_LOC_PEER_RECV . . . . . . . . . . . . . . . . . . . . 18 19
     5.9.  SHIM_LOC_LOCAL_SEND  . . . . . . . . . . . . . . . . . . . 18 19
     5.10. SHIM_LOC_PEER_SEND . . . . . . . . . . . . . . . . . . . . 19 21
     5.11. SHIM_LOCLIST_LOCAL . . . . . . . . . . . . . . . . . . . . 20 21
     5.12. SHIM_LOCLIST_PEER  . . . . . . . . . . . . . . . . . . . . 22 23
     5.13. SHIM_APP_TIMEOUT . . . . . . . . . . . . . . . . . . . . . 22 23
     5.14. SHIM_DEFERRED_CONTEXT_SETUP  . . . . . . . . . . . . . . . 23 24
     5.15. Error Handling . . . . . . . . . . . . . . . . . . . . . . 24 25
   6.  Ancillary Data for Multihoming Shim  . . . . . . . . . . . . . 24 25
     6.1.  Get Locator Information from Incoming Packet . . . . . . . 26 27
     6.2.  Specify Locator Information for Outgoing Packet  . . . . . 26 27
     6.3.  Notification from Application to Multihoming Shim  . . . . 26 27
   7.  Data Structures  . . . . . . . . . . . . . . . . . . . . . . . 27 28
     7.1.  Placeholder for Locator Information  . . . . . . . . . . . 27 28
       7.1.1.  Handling Locator behind NAT  . . . . . . . . . . . . . 29
     7.2.  Path Exploration Parameter . . . . . . . . . . . . . . . . 28 30
     7.3.  Feedback Information . . . . . . . . . . . . . . . . . . . 29 30
   8.  Implications for Existing Socket API Extensions  . . . . . . . 29 31
   9.  Resolving Conflicts with Preference Values . . . . . . . . . . 30 32
     9.1.  Implicit Forking . . . . . . . . . . . . . . . . . . . . . 30 32
   10. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 31 33
     10.1. Naming at Socket Layer . . . . . . . . . . . . . . . . . . 31 33
     10.2. Additional Requirements from Applications  . . . . . . . . 31 33
     10.3. Issues of Header Conversion among Different Address
           Family . . . . . . . . . . . . . . . . . . . . . . . . . . 32 33
     10.4. Handling of Unknown Locator Provided by Application  . . . 32 34
   11. Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 34
     11.1. Changes from version 00 to version 01  . . . . . . . . . . 32 34
     11.2. Changes from version 01 to version 02  . . . . . . . . . . 33 34
     11.3. Changes from version 02 to version 03  . . . . . . . . . . 33 35
     11.4. Changes from version 03 to version 04  . . . . . . . . . . 33 35
     11.5. Changes from version 04 to version 05  . . . . . . . . . . 33 35
     11.6. Changes from version 05 to version 06  . . . . . . . . . . 33 35
     11.7. Changes from version 06 to version 07  . . . . . . . . . . 33 35
     11.8. Changes from version 07 to version 08  . . . . . . . . . . 33 35
     11.9. Changes from version 08 to version 09  . . . . . . . . . . 35
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 33 35
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 34 36
   14. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 34 36
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 34 36
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34 37
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 34 37
     16.2. Informative References . . . . . . . . . . . . . . . . . . 35 37
   Appendix A.  Context Forking . . . . . . . . . . . . . . . . . . . 35 38
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38 40

1.  Introduction

   HIP and SHIM6 have a commonality in their protocol design in the
   sense that the semantic roles of an IP address as address, i.e., an identifier
   and a locator locator, are clearly distinguished.  Hereafter this design principle  Separation of identifier and
   locator is called
   "identifier/locator separation" in this document.  Both protocols aim
   to solve problems that are specific to multihoming environment in an
   endhost centric approach.  In these protocols, done by introducing a sub-layer within "shim" inside the IP layer which
   maintains mappings mapping of identifiers the identifier and associated locators.

   The  This
   design principle is called "identifier/locator separation" and the
   shim layer is useful referred to as a "shim sub-layer" in this document.

   The shim sub-layer provides a sense that nice property to present a stable
   communication endpoints (i.e., identifiers) to the IP upper layer
   protocols.  An on-going session can maintain be maintained even when the mapping of an identifier to
   locator associated with the corresponding locators.  Under identifier is changed, for instance, upon
   a
   multihomed environment, typically, re-homing event under a host has multihomed environment.  Therefore, upper
   layer protocols, especially connection-oriented applications are no
   more than one IP
   address at a time.  During the transaction, annoyed by the host may be required locator change thanks to switch the IP address in use to another IP address to preserve identifier/locator
   separation mechanism.

   While the
   communication.  Such an address update should be kept hidden from the
   upper layer protocols to avoid communication disruption.  The shim
   layer aims to make the address 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 multihoming support from the shim layer, the IP
   addresses specified as source and destination addresses must be
   identifiers.  However, this separation removes negative impact of
   locator change, it does not necessarily mean that applications are prohibited
   always ignorant about locators.  We rather think that applications
   may want to choose specific have a control of locators for its
   communication.  It may be useful for in some applications cases.  For instance,
   an application may want to specify use a
   preferred specific locator for to send IP packets.
   Such a given flow. control of locators is referred to as "locator management" in
   this document.  Besides, applications may want to turn on or off the
   identifier/locator separation mechanism.  This document defines API
   that provides locator management and additional control of shim sub-
   layer for applications.

   This document recommends that the switching of identifier and locator
   is done only once inside the TCP/IP stack of an endhost.  That is, if
   multiple shim sub-layers exist at the IP layer, any one of them
   should be applied exclusively for a given flow.

   As this document specifies sockets API extensions, it is written so
   that the syntax and semantics are in line with the Posix standard
   [POSIX] as much as possible.  The API specified in this document
   defines how to use ancillary data (aka cmsg) to access the locator
   information with recvmsg() and/or sendmsg() I/O calls.  The
   definition of API is presented in C language and data types follow
   the 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 environments.  In addition, this document aims to provide
   necessary information for developers of multihoming shim protocols to
   implement API for enabling advanced locator management.

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 following are
   definitions of terms frequently used in this document:

   o  Endpoint identifier (EID) - The identifier used by the application
      to specify the endpoint of a given communication.  Applications
      may handle EIDs in various ways such as long-lived connections,
      callbacks, and referrals[I-D.ietf-shim6-app-refer].
      *  In the case of SHIM6, an identifier called a ULID serves as an
         EID.  A ULID is chosen from locators available on the host.
      *  In the case of HIP, an identifier called a Host Identifier
         serves as an EID.  A Host Identifier is derived from the public
         key of a given host.  For the sake of backward compatibility
         with the sockets API, the Host Identifier is represented in a
         form of hash of public key.
   o  Locator - The IP address actually used to deliver IP packets.
      Locators are present in the source and destination fields of the
      IP header of a packet on the wire.
      *  List of locators - A list of locators associated with an EID.
         There are two lists of locators stored in a given context.  One
         is associated with the local EID and the other is associated
         with the remote EID.  As defined in [I-D.ietf-shim6-proto], the
         list of locators associated with an EID 'A' is denoted as
         Ls(A).
      *  Preferred locator - The (source/destination) locator currently
         used to send packets within a given context.  As defined in
         [I-D.ietf-shim6-proto], the preferred locator of a host 'A' is
         denoted as Lp(A).
   o  Shim - The conceptual sub-layer inside the IP layer which
      maintains mappings between EIDs and locators.  An EID can be
      associated with more than one locator at a time when the host is
      multihomed.  The term 'shim' does not refer to a specific protocol
      but refers to the conceptual sub-layer inside the IP layer.

   o  Identifier/locator adaptation - The adaptation performed at the
      shim layer sub-layer which may end up re-writing the source and/or
      destination addresses of an IP packet.  In the outbound packet
      processing, the EID pair is converted to the associated locator
      pair.  In the inbound packet processing, the locator pair is
      converted to the EID pair.
   o  Context - The state information shared by a given pair of peers,
      which stores a binding between the EID and associated locators.
      Contexts are maintained by the shim layer. sub-layer.
   o  Reachability detection - The procedure to check reachability
      between a given locator pair.
   o  Path - The sequence of routers that an IP packet goes through to
      reach the destination.
   o  Path exploration - The procedure to explore available paths for a
      given set of locator pairs.
   o  Outage - The incident that prevents IP packets to flow from the
      source locator to the destination locator.  When there is an
      outage, it means that there is no reachability between a given
      locator pair.  The outage may be caused by various reasons, such
      as shortage of network resources, congestion, and human error
      (faulty operation).
   o  Working address pair - The address pair is considered to be
      "working" if the packet can safely travel from the source to the
      destination where the packet contains the first address from the
      pair as the source address and the second address from the pair as
      the destination address.  If reachability is confirmed in both
      directions, the address pair is considered to be working bi-
      directionally.
   o  Reachability protocol (REAP) - The protocol for detecting failure
      and exploring reachability in a multihomed environment.  REAP is
      defined in [I-D.ietf-shim6-failure-detection].

3.  System Overview

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

   It may also be possible that the shim layer sub-layer interacts with the
   transport layer, however, such an interaction is 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 a list of requirements from applications:
   o  Locator management.  The shim layer selects
      *  It should be possible to set preferred source and/or
         destination locator within a pair of locators for
      sending IP packets given context: Lp(local) and/or
         Lp(remote).
      *  It should be possible to get preferred source and/or
         destination locator 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 applications' perspective: context: Lp(local) and/or
         Lp(remote).
      *  It should be possible to obtain the lists set a list of source and/or
         destination locators of within a given context: Ls(local) and
         Ls(remote).
      *  It should be possible to obtain the preferred locators get a list of source and/or
         destination locators within a given context: Lp(local) Ls(local) and Lp(remote).
         Ls(remote).
   o  Notification from applications to the shim layer sub-layer about the
      status of the communication.  The notification occurs in an event-based event-
      based manner.  Applications and/or upper layer protocols may
      provide positive feedbacks or negative feedbacks to the shim sub-
      layer.
      [NOTE: These  Note that these feedbacks are mentioned in
      [I-D.ietf-shim6-failure-detection]]:

      *  Applications and/or upper layer protocols (e.g., TCP) may
         provide positive feedbacks to the shim layer sub-layer informing that
         the communication is going well.
      *  Applications and/or upper layer protocols (e.g., TCP) may
         provide negative feedbacks to the shim layer sub-layer informing that
         the communication status is not satisfactory.  TCP may detect a
         problem when it does not receive any expected ACK message from
         the peer.  Besides, a receipt of an ICMP error message could be
         a clue for the application to detect problems.  The REAP module
         may be triggered by these negative feedbacks and invoke the
         path exploration procedure.
   o  Feedback from applications to the shim layer. sub-layer.  Applications
      should be able to inform the shim layer sub-layer of the timeout values
      for detecting failures, sending keepalives, and starting the
      exploration procedure.  In particular, applications should be able
      to suppress keepalives.
   o  Hot-standby.  Applications may request the shim layer sub-layer for the hot-
      standby
      hot-standby capability.  This means that, alternative paths are
      known to be working in advance of a failure detection.  In such a
      case, it is possible for the host to immediately replace the
      current locator pair with an alternative locator pair.
   o  Eagerness for locator exploration.  An application should be able
      to inform the shim layer sub-layer of how aggressively it wants the REAP
      mechanism to perform a path exploration (e.g., by specifying the
      number of concurrent attempts of discovery of working locator
      pairs) when an outage occurs on the path between the locator pair
      in use.
   o  Providing locator information to applications.  An application
      should be able to obtain information about the locator pair which
      was actually used to send or receive the packet.
      *  For inbound traffic, the application may be interested in the
         locator pair which was actually used to receive the packet.
      *  For outbound traffic, the application may be interested in the
         locator pair which was actually used to transmit the packet.
      In this way, applications may have additional control on the
      locator management.  For example, an application becomes able to
      verify if its preference for locator is actually applied to the
      flow or not.
   o  Applications should be able to specify if they want to defer the
      context setup, or if they want context establishment to be started
      immediately in the case where there is no available context.  A
      deferred context setup means that the initiation of communication
      should not be blocked to wait for completion of the context
      establishment.
   o  Turn on/off shim.  An application should be able to request to
      turn on or turn off the multihoming support by the shim layer:

      *  Apply shim.  The application should be able to explicitly
         request the shim layer sub-layer to apply multihoming support.
      *  Don't apply shim.  The application should be able to request
         the shim layer sub-layer not to apply the multihoming support but to
         apply normal IP processing at the IP layer.
   o  An application should be able to know if the communication is now
      being served by the shim layer sub-layer or not.
   o  An application should be able to use a common interface to access
      an IPv4 locator and an IPv6 locator.

5.  Socket Options for Multihoming Shim Layer Sub-Layer

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

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

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

   +-----------------------------+-----+-----+-----------------+-------+
   | optname                     | get | set | description     | dtype |
   +-----------------------------+-----+-----+-----------------+-------+
   | SHIM_ASSOCIATED             | o   |     | Check if Get the         | int   |
   |                             |     |     | parameter which |       |
   |                             |     |     | indicates       |       |
   |                             |     |     | whether if the  |       |
   |                             |     |     | socket is       |       |
   |                             |     |     | associated with |       |
   |                             |     |     | any shim        |       |
   |                             |     |     | context or not. |       |
   | SHIM_DONTSHIM               | o   | o   | Request Get or set the  | int   |
   |                             |     |     | shim layer not parameter which |       |
   |                             |     |     | indicates       |       |
   |                             |     |     | whether to apply any      |       |
   |                             |     |     | employ the      |       |
   |                             |     |     | multihoming     |       |
   |                             |     |     | support for by the  |       |
   |                             |     |     | communication. shim sub-layer  |       |
   |                             |     |     | or not.         |       |
   | SHIM_HOT_STANDBY            | o   | o   | Request Get or set the  | int   |
   |                             |     |     | shim layer parameter to    |       |
   |                             |     |     | prepare a request the     |       |
   |                             |     |     | hot-standby shim sub-layer  |       |
   |                             |     |     | connection (in to prepare a    |       |
   |                             |     |     | addition to the hot-standby     |       |
   |                             |     |     | current path). connection.     |       |
   | SHIM_LOC_LOCAL_PREF         | o   | o   | Get or set the  | *1    |
   |                             |     |     | preferred       |       |
   |                             |     |     | locator on the  |       |
   |                             |     |     | local side for  |       |
   |                             |     |     | the context     |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the socket.     |       |
   | SHIM_LOC_PEER_PREF          | o   | o   | Get or set the  | *1    |
   |                             |     |     | preferred       |       |
   |                             |     |     | locator on the  |       |
   |                             |     |     | remote side for |       |
   |                             |     |     | the context     |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the socket.     |       |
   | SHIM_LOC_LOCAL_RECV         | o   | o   | Request for Get or set the  | int   |
   |                             |     |     | destination parameter which |       |
   |                             |     |     | locator of the is used to      |       |
   |                             |     |     | received IP request the     |       |
   |                             |     |     | packet. shim sub-layer  |       |
   | SHIM_LOC_PEER_RECV                             | o     | o     | Request for to store the    | int       |
   |                             |     |     | source locator destination     |       |
   |                             |     |     | locator of the received  |       |
   |                             |     |     | received IP     |       |
   |                             |     |     | packet.         |       |
   | SHIM_LOC_LOCAL_SEND SHIM_LOC_PEER_RECV          | o   | o   | Request Get or set the use  | *2 int   |
   |                             |     |     | of specific parameter which |       |
   |                             |     |     | locator as is used to      |       |
   |                             |     |     | request the     |       |
   |                             |     |     | shim sub-layer  |       |
   |                             |     |     | to store the    |       |
   |                             |     |     | source locator  |       |
   |                             |     |     | of outgoing IP the received |       |
   |                             |     |     | packets. IP packet.      |       |
   | SHIM_LOC_PEER_SEND SHIM_LOC_LOCAL_SEND         | o   | o   | Request Get or set the use  | *2    |
   |                             |     |     | source locator  |       |
   |                             |     |     | of specific outgoing IP  |       |
   |                             |     |     | locator as packets.        |       |
   | SHIM_LOC_PEER_SEND          | o   | o   | Get or set the  | *2    |
   |                             |     |     | destination     |       |
   |                             |     |     | locator of      |       |
   |                             |     |     | outgoing IP     |       |
   |                             |     |     | packets.        |       |
   | SHIM_LOCLIST_LOCAL          | o   | o   | Get or set the  | *3    |
   |                             |     |     | list of         |       |
   |                             |     |     | locators        |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the local EID.  |       |
   | SHIM_LOCLIST_PEER           | o   | o   | Get or set the  | *3    |
   |                             |     |     | list of         |       |
   |                             |     |     | locators        |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the peer's EID. |       |
   | SHIM_APP_TIMEOUT            | o   | o   | Inform Get or set the shim  | int   |
   |                             |     |     | layer of the    |       |
   |                             |     |     | timeout value   |       |
   |                             |     |     | for detecting   |       |
   |                             |     |     | failure.        |       |
   | SHIM_PATHEXPLORE            | o   | o   | Specify Get or set      | *4    |
   |                             |     |     | behavior of parameters for  |       |
   |                             |     |     | path            |       |
   |                             |     |     | exploration and |       |
   |                             |     |     | failure         |       |
   |                             |     |     | detection.      |       |
   | SHIM_CONTEXT_DEFERRED_SETUP | o   | o   | Specify if Get or set the  | int   |
   |                             |     |     | context setup parameter which |       |
   |                             |     |     | indicates       |       |
   |                             |     |     | whether         |       |
   |                             |     |     | can be deferred        |       |
   |                             |     |     | context setup   |       |
   |                             |     |     | is supported 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.  The figure shows that the shim sub-
   layer and the additional protocol components (IPv4 and IPv6) below
   the shim
   layer sub-layer are new to the system model.  As previously
   mentioned, all the shim specific socket options are defined at the
   SOL_SHIM level.  This design choice brings the following advantages:

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

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

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

5.1.  SHIM_ASSOCIATED

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

   This option is particularly meaningful in the case where when 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 the
   locator pair may be identical in some case. cases.

   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()).

   The data type of the option value is an integer.  The option value
   indicates the presence of shim context.  A returned value 1 means
   that the socket is associated with a shim context at the shim layer,
   while a sub-
   layer.  A return value 0 indicates that there is no shim 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 is used to request the shim layer to not
   apply to
   provide the multihoming support for the communication established
   over the socket.

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

   Default value is set to as 0, which means that the multihoming shim sub-
   layer applies performs identifier/locator adaptation for the flow.  In order if available.

   Any attempt to disable the socket option, multihoming shim support MUST be made by
   the application should call setsockopt() with optval set to 0. before the socket is connected.  If an application
   makes such an attempt for a connected-socket, an error code
   EOPNOTSUPP MUST be returned.

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

       int optval;

       optval = 0; 1;

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

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

       int optval;
       int len;

       len = sizeof(optval);

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

5.3.  SHIM_HOT_STANDBY

   The SHIM_HOT_STANDBY option can be is used to check if control the shim layer
   uses sub-layer
   whether to employ a hot-standby connection or not for the communication established
   over the socket. socket or not.  A
   hot-standby connection is based on an alternative working locator pair to the
   current locator pair.  This option is effective only when there is a
   shim context associated with the socket.

   The data type of the option value is an integer.

   The option value can be set by setsockopt().

   The option value can be read by getsockopt().

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

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

       int optval;

       optval = 1;

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

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

       int optval;
       int len;

       len = sizeof(optval);

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

5.4.  SHIM_PATHEXPLORE

   The application may specify use this socket option to specify specify
   behavior of parameters
   concerning path exploration.  Path exploration is a procedure to find
   an alternative locator pair when the host finds any problem with to the current locator pair.  The message used for finding an
   alternative locator pair is called  As the Probe message and it is sent
   per locator pair.  The REAP
   specification defines the default values
   for Initial defines, a peer may send Probe Timeout and Initial Probe. messages to find an
   alternative locator pair.

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

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

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

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

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

       struct shim6_pathexplore pe;

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

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

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

       struct shim6_pathexplore pe;
       int len;

       len = sizeof(pe);

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

5.5.  SHIM_LOC_LOCAL_PREF

   The SHIM_LOC_LOCAL_PREF option can be is used to read get 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.

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

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

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

   The

   An application can get 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 an application can be set the preferred locator by using
   the socket option as follows.  It should be
   noted  Note that some members of the
   shim_locator (lc_ifidx and lc_flags) are ignored in the write set
   operation.

       struct shim_locator lc;
       struct in6_addr ip6;

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

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

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

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

       struct shim_locator lc;
       int len;

       len = sizeof(lc);

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

5.6.  SHIM_LOC_PEER_PREF

   The SHIM_LOC_PEER_PREF option can be is used to read get 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.

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

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

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

   The

   An application can get 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
   requested locator can be set fails.

   The usage of the option is same as follows.  It should be
   noted that of SHIM_LOC_LOCAL_PREF.  Note
   that some members of the shim_locator (lc_ifidx and lc_flags) are
   ignored in the write set 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 sub-
   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.

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

   The option value

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

   The

   An application can get 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 an application can be activated request the shim sub-layer to store
   destination locator by using the application socket option as
   follows: follows.

       int optval;

       optval = 1;

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

   For example, an application can get the option value can be checked by the application as
   follows: 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 is used to request the shim layer sub-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.

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

   The option value can be set by setsockopt().

   The option value can be read by getsockopt().

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

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

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

   The

   An application can set 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 inactivated.

   The

   An application can get 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 will be returned when invalid locator is
   specified.

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

       struct shim_locator locator;
       struct in6_addr ia6;

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

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

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

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

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

       struct shim_locator locator;

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

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

       /* check locator */

5.10.  SHIM_LOC_PEER_SEND

   The SHIM_LOC_PEER_SEND option can be is used to request the shim layer sub-layer
   to use a 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.

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

   The

   An application can set 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 inactivated.

   The

   An application can get the specified 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 is used to read get 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.

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

   The

   An application can get 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 local 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, an application can set a list of locators to be
   associated with the local EID
   can be specified by using the socket option 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, an application can get a list of locators that are
   associated with the local EID can be obtained by using the socket option as follows: 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 is used to read get 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.

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

   The

   An application can get 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

   An application can be set the locator list 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 is used to get or set the timeout value
   for application to detect failure.  Hence this option is effective
   only when there is a shim context associated with the socket.

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

       int optval;

       optval = 15; /* 15 seconds */

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

   For example, an application can get the option value namely the period of timeout can be
   checked value by using the application
   socket option as follows: 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 is used to specify whether to
   enable deferred context setup is made in terms of timing (before or after) the
   initial communication flow. not.  Deferred context setup means
   that the
   establishment of context is established in parallel with the data
   communication.  Note that SHIM6 supports deferred context setup and
   HIP does not put additional delay for an initial
   transaction. because EIDs in HIP (i.e., Host Identifiers) are non-
   routable.

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

   However, it should be noted that deferred context setup may not be
   possible in some cases.  For instance, an EID may be non-routable
   address (e.g., Host Identifier in HIP) and there is no way to
   transmit any IP packet unless there is a context providing the
   locators.  In such a case, a context application should be established prior call setsockopt() with option value
   set to
   the communication. 0.

   For example, the option an application can be disabled disable the deferred context setup by
   using the application socket option as follows:

       int optval;

       optval = 0;

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

   For example, an application can get the option value can be checked by the application as
   follows: 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 an error.

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

   EINVALIDLOCATOR
      This indicates that at least one of the necessary validations
      inside the shim layer sub-layer for the specified locator has failed.
      In case of SHIM6, there are two kinds of verifications required
      for security reasons prior to sending an IP packet to the peer's
      new locator; one is the return routability (check if the peer is
      actually willing to receive data with the specified locator) and
      the other one is the verification based on crypto identifier
      mechanisms [RFC3972], [I-D.ietf-shim6-hba].

6.  Ancillary Data for Multihoming Shim

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

   As defined in the Posix standard, sendmsg() and recvmsg() input a
   msghdr structure as their arguments.  These system calls can handle
   control information along with data.  Figure 3 shows the msghdr
   structure which is defined in <sys/socket.h>.  The member msg_control
   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 3: msghdr structure

   The buffer pointed by the member msg_control of the msghdr structure
   may contain 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
   by a UDP or a raw socket and not by a TCP socket.  This is because
   there is no one-to-one mapping between a single send/receive
   operation and a TCP segment being transmitted/received.

6.1.  Get Locator Information from Incoming Packet

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

6.2.  Specify Locator Information for Outgoing Packet

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

   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 is verified, the shim layer sub-layer
   overrides the locators of the IP packet.

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

6.3.  Notification from Application to Multihoming Shim

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

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

   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 sub-layer when a feedback is given by an application.

7.  Data Structures

   In this section, data structures specifically defined for the
   multihoming shim layer sub-layer are introduced.  These data structures are
   either used as a parameter for setsockopt()/getsockopt() (as
   mentioned in Section 5) or as 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 4 illustrates the data structure called shim_locator which
   stores a locator information.

        struct shim_locator {
                uint8_t    lc_family;       /* address family */
                uint8_t   lc_ifidx;    lc_proto;        /* interface index protocol */
                uint8_t
                uint16_t   lc_port;         /* port number */
                uint16_t   lc_flags;        /* flags */
                uint8_t   lc_preference;
                uint16_t   lc_pref;         /* preference value */
                uint8_t   lc_addr[16];
                uint32_t   lc_ifidx;        /* interface index */
                struct in6_addr lc_addr;    /* address data */
        };

                     Figure 4: shim locator structure

   lc_family
      Address family of the locator (e.g.  AF_INET, AF_INET6).  It is
      required that the parameter contains non-zero value indicating the
      exact address family of the locator.
   lc_ifidx
      Interface index of
   lc_proto
      Internet Protocol number for the network interface to protocol which is used to handle
      locator behind NAT.  Typically, this value is set as UDP (17) when
      the locator is
      assigned.  This field should be valid only in a read
      (getsockopt()) operation. UDP encapsulation interface.
   lc_port
      Port number which is used for handling locator behind NAT.
   lc_flags
      Each bit of the flags represents a specific characteristics of the
      locator.  Hash Based Address (HBA) is defined as 0x01.
      Cryptographically Generated Address (CGA) is defined as 0x02.
   lc_preference
      Indicates a preference

   lc_pref
      Preference of the locator.  The preference is represented by an
      integer.
   lc_ifidx
      Interface index of the network interface to which the locator is
      assigned.  This field should be valid only in a read
      (getsockopt()) operation.
   lc_addr
      Contains the locator.  In the case where a case where a locator whose size is
      smaller than 16 bytes, an encoding rule should be provided for
      each locator of a given address family.  For instance, in case of
      AF_INET (IPv4), the locator should be in the format of an IPv4-
      mapped IPv6 address as defined in [RFC4291].

7.1.1.  Handling Locator behind NAT

   Note that the locator information MAY contain a locator behind a
   Network Address Translator (NAT).  Such a situation may arise when
   the host is behind the NAT and uses a local address as a source
   locator to communicate with the peer.  Note that a NAT traversal
   mechanism for HIP is defined, which allows HIP host to tunnel control
   and data traffic over UDP[I-D.ietf-hip-nat-traversal].  Note also
   that the locator whose size behind NAT is
      smaller than 16 bytes, not necessarily an encoding rule should be provided for
      each locator of a given IPv4 address family.  For instance, in case of
      AF_INET (IPv4), but it
   can be an IPv6 address.  Below is an example where the application
   sets a UDP encapsulation interface as a source locator should be in when sending
   IP packets.

          struct shim_locator locator;
          struct in6_addr ia6;

          /* copy the format of private IPv4 address to the ia6 as an IPv4-
      mapped IPv4-mapped
             IPv6 address as defined in RFC 4291[RFC4291]. */

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

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

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

          setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
                     sizeof(locator));
                   Figure 5: Handling locator behind NAT

7.2.  Path Exploration Parameter

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

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

                     Figure 5: 6: path explore structure

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

7.3.  Feedback Information

   As mentioned in Section 6.3, applications can inform the shim sub-
   layer about the status of unicast reachability of the locator pair
   currently in use.  The feedback information can be handled by using
   ancillary data called SHIM_FEEDBACK.  A new data structure named
   shim_feedback is illustrated in Figure 6. 7.

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

                 Figure 6: 7: feedback information structure

   direction
      Indicates direction of reachability between a locator pair in
      question.  A value 0 indicates outbound and a value 1 indicates
      inbound direction.
   indicator
      A value indicating the degree of satisfaction of a unidirectional
      reachability for a given locator pair.
      *  0: Default value.  Whenever this value is specified the
         feedback information must not be processed by the shim sub-
         layer.
      *  1: Unable to connect.  There is no unidirectional reachability
         between the locator pair in question.
      *  2: Unsatisfactory.  The application is not satisfied with the
         unidirectional reachability between the locator pair in
         question.
      *  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 are overlapping
   with existing sockets API and care should be taken for the usage not
   to confuse with the overlapping 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 sockets 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 sockets 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 sub-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 sockets API should
   continue to work above the shim layer, sub-layer, that is, the IP addresses
   handled in IPV6_PKTINFO should be EIDs, not the locators.

   Baseline is that the above existing sockets API (IP_RECVDSTADDR and
   IPV6_PKTINFO) is assumed to work above the multihoming shim sub-
   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 with the same EID pair while both
   applications have different preference in their choice of local
   locator.

   SHIM6 supports a notion of context forking 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. sub-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 sub-layer that context
   forking is required.  In SHIM6, when a context is forked, an unique
   identifier called Forked Instance Identifier (FII) is assigned to the
   newly forked context.  The forked context is then exclusively
   associated with the socket through which non-default preference value
   was specified.  The forked context is maintained by the multihoming
   shim layer sub-layer during the lifetime of associated socket instance.
   When the socket is closed, the multihoming shim layer sub-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 is specific to multihoming shim as
   defined in Section 6.

10.  Discussion

   In this section, open issues are introduced.

10.1.  Naming at Socket Layer

   The getsockname() and getpeername() system calls are used to obtain
   the 'name' of an 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 obtains the remote IP address and port number.

   The above is based on a traditional system model of the sockets 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 sub-layer exists inside the IP layer,
   both getsockname() and getpeername() deal with identifiers, namely
   EIDs.  In this sense, the shim layer sub-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 Applications

   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 ones, 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 sub-layer performs identifier/locator adaptation.
   Therefore, in some case, cases, the whole IP header can be replaced with
   new IP header of a different address family (e.g. conversion from
   IPv4 to IPv6 or vice versa).  Hence, there is an issue how to make
   the conversion with minimum impact.  Note that this issue is common
   in other protocol conversion 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 a certain limitation in protocol conversion between IPv4 and
   IPv6.

   The question is how should the shim layer sub-layer behave when it is face faces
   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 sub-
   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.

11.6.  Changes from version 05 to version 06

   The followings are changes from version 04 to version 05:
   o  Updated references.

11.7.  Changes from version 06 to version 07

   The followings are changes from version 06 to version 07:
   o  Resolved editorial issues.

11.8.  Changes from version 07 to version 08

   No changes are made except for updates of the references.

11.9.  Changes from version 08 to version 09

   The followings are changes from version 08 to version 09:
   o  Updated texts for Section 1 and Section 5 according to the
      comments provided by Samu Varjonen.
   o  Made it clear that downgrading the multihome shim support (i.e.,
      specifying value 1 with the SHIM_DONTSHIM socket option) is only
      allowed before the socket is connected.
   o  Updated locator information (shim_locator{}) so that it can
      contain a locator behind NAT.

12.  IANA Considerations

   This document contains no IANA consideration.

13.  Security Considerations

   This document does not specify any security mechanism for the shim
   layer.
   sub-layer.  Fundamentally, the shim layer sub-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 sub-layer is strictly applied.

14.  Conclusion

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

   Socket options for multihoming shim layer sub-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
   sub-layer to get locator from received packet or to set locator for
   outgoing packet.

   From an architectural point of view, the sockets API provides extends
   the existing sockets API framework in the face of ID/Locator
   separation.  With regard to API that relate to IP address management,
   it is assured that existing sockets API continue to work above the
   shim layer sub-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 sockets API related issues.  Thomas
   Henderson provided valuable comments especially from HIP
   perspectives.

   Authors sincerely thank to the following people for their help to
   improve this document: Samu Varjonen and Dmitriy Kuptsov.

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-13 (work in progress),
              June 2008.

   [I-D.ietf-shim6-proto]
              Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
              protocol", draft-ietf-shim6-proto-12 (work in progress),
              February 2009.

   [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-hip-nat-traversal]
              Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A.
              Keranen, "Basic HIP Extensions for Traversal of Network
              Address Translators", Internet
              Draft draft-ietf-hip-nat-traversal-08, June 2009.

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

   [I-D.ietf-shim6-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.

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

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. sub-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 7 8 gives an example of a scenario where two communicating peers
   fork a context.  Initially, there has been a single transaction
   between the peers, by the application 1 (App1).  Accordingly, another
   transaction is started, by application 2 (App2).  Both of the
   transactions are made based the same ULID pair.  The first context
   pair (Ctx1) is established for the transaction of App1.  Given the
   requests from App2, the shim layer sub-layer on Peer 1 decides to fork a
   context.  Accordingly, a forked context (Ctx2) is established between
   the peers, which should be exclusively applied to the transaction of
   App2.  Ideally, multiplexing and demultiplexing of packet flows that
   relate to App1 and App2 should be done as illustrated in Figure 7. 8.
   However, as mentioned earlier, the responder needs to multiplex
   outbound flows of App1 and App2 somehow.  Note that if a context
   forking occurs on the initiator side, a context forking needs to
   occur also on the responder side.

              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 7: 8: 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 sub-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