SHIM6 Working Group M. Komu Internet-Draft HIIT Intended status: Informational M. Bagnulo Expires:June 10,July 13, 2010 UC3M K. Slavov S. Sugimoto, Ed. EricssonDecember 7, 2009January 9, 2010 Socket Application Program Interface (API) for Multihoming Shimdraft-ietf-shim6-multihome-shim-api-11draft-ietf-shim6-multihome-shim-api-12 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire onJune 10,July 13, 2010. Copyright Notice Copyright (c)20092010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. 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 Sub-layer . . . . . . . . 9 5.1. SHIM_ASSOCIATED . . . . . . . . . . . . . . . . . . . . . 13 5.2. SHIM_DONTSHIM . . . . . . . . . . . . . . . . . . . . . . 14 5.3. SHIM_HOT_STANDBY . . . . . . . . . . . . . . . . . . . . 15 5.4. SHIM_PATHEXPLORE . . . . . . . . . . . . . . . . . . . . 15 5.5. SHIM_LOC_LOCAL_PREF . . . . . . . . . . . . . . . . . . . 16 5.6. SHIM_LOC_PEER_PREF . . . . . . . . . . . . . . . . . . . 17 5.7. SHIM_LOC_LOCAL_RECV . . . . . . . . . . . . . . . . . . . 18 5.8. SHIM_LOC_PEER_RECV . . . . . . . . . . . . . . . . . . . 19 5.9. SHIM_LOC_LOCAL_SEND . . . . . . . . . . . . . . . . . . . 19 5.10. SHIM_LOC_PEER_SEND . . . . . . . . . . . . . . . . . . . 21 5.11. SHIM_LOCLIST_LOCAL . . . . . . . . . . . . . . . . . . . 21 5.12. SHIM_LOCLIST_PEER . . . . . . . . . . . . . . . . . . . . 23 5.13. SHIM_APP_TIMEOUT . . . . . . . . . . . . . . . . . . . . 23 5.14. SHIM_DEFERRED_CONTEXT_SETUP . . . . . . . . . . . . . . . 24 5.15. Applicability . . . . . . . . . . . . . . . . . . . . . . 25 5.16. Error Handling . . . . . . . . . . . . . . . . . . . . . 25 6. Ancillary Data for Multihoming Shim Sub-layer . . . . . . . . 26 6.1. Get Locator from Incoming Packet . . . . . . . . . . . . 27 6.2. Set Locator for Outgoing Packet . . . . . . . . . . . . . 27 6.3. Notification from Application to Multihoming Shim Sub-layer . . . . . . . . . . . . . . . . . . . . . . . . 27 6.4. Notification from Multihoming Shim Sub-layer to Application . . . . . . . . . . . . . . . . . . . . . . . 28 6.5. Applicability . . . . . . . . . . . . . . . . . . . . . .2928 7. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 29 7.1. Placeholder for Locator Information . . . . . . . . . . . 29 7.1.1. Handling Locator behind NAT . . . . . . . . . . . . . 30 7.2. Path Exploration Parameter . . . . . . . . . . . . . . . 31 7.3. Feedback Information . . . . . . . . . . . . . . . . . . 32 8. System Requirements . . . . . . . . . . . . . . . . . . . . . 33 9.Implications forRelation to ExistingSocketSockets API Extensions . . . . . . . . . 33 10.Resolving Conflicts with Preference ValuesOperational Considerations . . . . . . . . . . . . . . . . . . 34 10.1.Implicit Forking .Conflict Resolution . . . . . . . . . . . . . . . . . . . 3411. Discussion10.2. Incompatiblility between IPv4 and IPv6 . . . . . . . . . 35 11. IANA Considerations . . . . . . . . . . . . . . . . . .35 11.1. Naming at Socket Layer. . . 35 12. Protocol Constants and Variables . . . . . . . . . . . . . . . 3511.2. Additional Requirements from Applications13. Security Considerations . . . . . . . .35 11.3. Issues of Header Conversion among Different Address Family. . . . . . . . . . . 35 13.1. Treatment of Unknown Locator . . . . . . . . . . . . . .36 11.4. Handling35 13.1.1. Treatment of Unknown Source LocatorProvided by Application. . . . . . . . . 35 13.1.2. Treatment of Unknown Destination Locator . . . . . . . 3612.14. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 3612.1.14.1. Changes from version 00 to version 01 . . . . . . . . . . 3612.2.14.2. Changes from version 01 to version 02 . . . . . . . . . .37 12.3.36 14.3. Changes from version 02 to version 03 . . . . . . . . . . 3712.4.14.4. Changes from version 03 to version 04 . . . . . . . . . . 3712.5.14.5. Changes from version 04 to version 05 . . . . . . . . . . 3712.6.14.6. Changes from version 05 to version 06 . . . . . . . . . . 3712.7.14.7. Changes from version 06 to version 07 . . . . . . . . . . 3712.8.14.8. Changes from version 07 to version 08 . . . . . . . . . . 3712.9.14.9. Changes from version 08 to version 09 . . . . . . . . . .38 12.10.37 14.10. Changes from version 09 to version 10 . . . . . . . . . .38 12.11.37 14.11. Changes from version 10 to version 11 . . . . . . . . . . 3813. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38 14. Security Considerations . . . . . . . . .14.12. Changes from version 11 to version 12 . . . . . . . . . . 38 15.Conclusion . . .Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 38 16.Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39 17.References . . . . . . . . . . . . . . . . . . . . . . . . . .39 17.1.38 16.1. Normative References . . . . . . . . . . . . . . . . . .39 17.2.38 16.2. Informative References . . . . . . . . . . . . . . . . .4039 Appendix A. Context Forking . . . . . . . . . . . . . . . . . . .4039 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .4341 1. IntroductionHIPThis document defines socket API extensions by which upper layer protocols may be informed about andSHIM6 have a commonality in their protocol design in the sense thatcontrol thesemantic roles of an IP address, i.e., an identifier and a locator, are distinguished. Separation of identifier and locator is done by introducingway in which a"shim" insidemultihoming shim sub-layer in the IP layerwhich maintains mapping ofmanages theidentifier and associateddynamic choice of locators.This design principle is called "identifier/locator separation"Initially it applies to SHIM6 and HIP, but it is defined generically. The role of the multihoming shimis referred to as asub-layer (hereafter called "shim sub-layer" in thisdocument. The shim sub-layer provides a nice propertydocument) is topresent a stable communication endpoints (i.e., identifiers)avoid impacts totheupper layerprotocols. An on-going session canprotocols which may bemaintained evencaused when thelocator associated withendhost changes its attachment point to theidentifier is changed,Internet, for instance,upon a re-homingin the case of rehoming event underathe multihomed environment.Therefore, upper layer protocols, especially connection-oriented applications are no more annoyed byThe key design of thelocator change thanksshim sub- layer is tothe identifier/locator separation mechanism. While the identifier/locator separation removes negative impact oftreat identifier and locatorchange, it does not necessarily mean that applicationsseparately. Identifiers arealways ignorant about locators. We rather think that applications may want to have a control of locators in some cases. For instance, an application may want to use a specific locatorpresented tosend IP packets. Such a controlupper layer protocols and used as communication endpoints. Locators represent toplogical location oflocators is referredendhosts and are used toas "locator management" in this document. Besides, applications may wantroute packet from the source toturn on or offtheidentifier/locator separation mechanism. This document defines API that provides locator management and additional control ofdestiantion. The shimsub- layer for applications. This document recommends that the switchingsub-layer maintains mapping ofidentifieridentifiers andlocator is done only once insidelocators. Note that theTCP/IP stack of an endhost. That is, if multipleshimsub-layers exist at the IP layer,sub-layer may conflict with other multihoming mechanisms such as SCTP and multipath TCP[I-D.ietf-shim6-applicability]. To avoid any conflict, only one ofthemSHIM6 and HIP should beapplied exclusively for a given flow. Asin use. In thisdocument specifies sockets API extensions, it is written so that thedocument, syntax and semantics of the API are given inline withthe same way as the Posix standard[POSIX] as much as possible.[POSIX]. The APIspecified in this document definesspecifies how to use ancillary data (aka cmsg) to access the locator information with recvmsg() and/or sendmsg() I/O calls. Thedefinition ofAPI ispresenteddescribed in C language and data typesfolloware defined in 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 distinction between "connected" sockets and "unconnected" sockets is important when discussing the applicability of the socket API defined in this document. A connected socket is bound to a given peer, whereas an unconnected socket is not bound to any specific peers.That is, the destination ofA TCP socket becomes a connected socket when theuser dataTCP connection establishment isnot known until the application writes data to an unconnected socket. TCP sockets are connected, by definition.completed. UDP sockets are unconnected, unless the application uses the connect() system call. The target readers of this document are application programmers who develop application software which may benefit greatly from multihomed environments. In addition, this document aims to provide necessary information for developers ofmultihomingshim 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[RFC5533] o HIP Architecture[RFC4423] o Reachability Protocol (REAP)[RFC5534] In this document, the term "IP" refers to both IPv4 and IPv6, unless the protocol version is specifically mentioned. The following are definitions of terms frequently used in this document: o Endpoint identifier (EID) - The identifier used by the application to specify the endpoint of a given communication. Applications may handle EIDs in various ways such as long-lived connections, callbacks, and referrals[I-D.ietf-shim6-app-refer]. * In the case of SHIM6, an identifier called a ULID serves as an EID. A ULID is chosen from locators available on the host. * In the case of HIP, an identifier called a Host Identifier serves as an EID. A Host Identifier is derived from the public key of a given host. For the sake of backward compatibility with the sockets API, the Host Identifier is represented in a form of hash of public key. * Note that the EID appears in the standard socket API as an address, and does not appear in the extensions defined in this document, which only concern locators. o Locator - The IP address actually used to deliver IP packets. Locators are present in the source and destination fields of the IP header of a packet on the wire. * List of locators - A list of locators associated with an EID. There are two lists of locators stored in a given context. One is associated with the local EID and the other is associated with the remote EID. As defined in [RFC5533], the list of locators associated with an EID 'A' is denoted as Ls(A). * Preferred locator - The (source/destination) locator currently used to send packets within a given context. As defined in [RFC5533], the preferred locator of a host 'A' is denoted as Lp(A). * Unknown locator - Any locator that does not appear in the locator list of the shim context associated with the socket. When there is no shim context associated with the socket, any source and/or destination locator requested by the application is considered to be unknown locator. o Shim - The conceptual sub-layer inside the IP layer which maintains mappings between EIDs and locators. An EID can be associated with more than one locator at a time when the host is multihomed. The term 'shim' does not refer to a specific protocol but refers to the conceptual sub-layer inside the IP layer. o Identifier/locator adaptation - The adaptation performed at the shim sub-layer which may end up re-writing the source and/or destination addresses of an IP packet. In the outbound packet processing, the EID pair is converted to the associated locator pair. In the inbound packet processing, the locator pair is converted to the EID pair. o Context - The state information shared by a given pair of peers, which stores a binding between the EID and associated locators. Contexts are maintained by the shim sub-layer. o Reachability detection - The procedure to check reachability between a given locator pair. o Path - The sequence of routers that an IP packet goes through to reach the destination. o Path exploration - The procedure to explore available paths for a given set of locator pairs. o Outage - The incident that prevents IP packets to flow from the source locator to the destination locator. When there is an outage, it means that there is no reachability between a given locator pair. The outage may be caused by various reasons, such as shortage of network resources, congestion, and human error (faulty operation). o Working address pair - The address pair is considered to be "working" if the packet can safely travel from the source to the destination where the packet contains the first address from the pair as the source address and the second address from the pair as the destination address. If reachability is confirmed in both directions, the address pair is considered to be working bi- directionally. o Reachability protocol (REAP) - The protocol for detecting failure and exploring reachability in a multihomed environment. REAP is defined in [RFC5534]. 3. System Overview Figure 1 illustrates the system overview. The shim sub-layer and REAP component exist inside the IP layer. Applications use the sockets API defined in this document to interface with the shim sub- layer and the transport layer for locator management, failure detection, and path exploration. It may also be possible that the shim sub-layer interacts with the transport layer, however, such an interaction is outside the scope of this document. +------------------------+ | Application | +------------------------+ ^ ^ ~~~~~~~~~~~~~|~Socket Interface|~~~~~~~~~~~~~~ | v +-----------|------------------------------+ | | Transport Layer | +-----------|------------------------------+ ^ | +-------------|-----|-------------------------------------+ | v v | | +-----------------------------+ +----------+ | IP | | Shim |<----->| REAP | | Layer | +-----------------------------+ +----------+ | | ^ ^ | +-----------------------|----------------------|----------+ v v +------------------------------------------+ | Link Layer | +------------------------------------------+ Figure 1: System overview 4. Requirements The following is a list of requirements from applications: o Turn on/off shim. An application should be able to request to turn on or turn off the multihoming support by the shim layer: * Apply shim. The application should be able to explicitly request the shim sub-layer to apply multihoming support. * Don't apply shim. The application should be able to request the shim sub-layer not to apply the multihoming support but to apply normal IP processing at the IP layer. * Note that this function is also required by other types of multihoming mechanisms such as SCTP and multipath TCP to avoid potential conflict with the shim sub-layer. o Locator management. * It should be possible to set preferred source and/or destination locator within a given context: Lp(local) and/or Lp(remote). * It should be possible to get preferred source and/or destination locator within a given context: Lp(local) and/or Lp(remote). * It should be possible to set a list of source and/or destination locators within a given context: Ls(local) and Ls(remote). * It should be possible to get a list of source and/or destination locators within a given context: Ls(local) and Ls(remote). o Notification from applications to the shim sub-layer about the status of the communication. The notification occurs in an event- based manner. Applications and/or upper layer protocols may provide positive feedbacks or negative feedbacks to the shim sub- layer. Note that these feedbacks are mentioned in[RFC5534]]:[RFC5534]: * Applications and/or upper layer protocols (e.g., TCP) may provide positive feedbacks to the shim sub-layer informing that the communication is going well. * Applications and/or upper layer protocols (e.g., TCP) may provide negative feedbacks to the shim sub-layer informing that the communication status is not satisfactory. TCP may detect a problem when it does not receive any expected ACK message from the peer. 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 sub-layer. Applications should be able to inform the shim sub-layer of the timeout values for detecting failures, sending keepalives, and starting the exploration procedure. In particular, applications should be able to suppress keepalives. o Hot-standby. Applications may request the shim sub-layer for the hot-standby capability. This means that, alternative paths are known to be working in advance of a failure detection. In such a case, it is possible for the host to immediately replace the current locator pair with an alternative locator pair. o Eagerness for locator exploration. An application should be able to inform the shim sub-layer of how aggressively it wants the REAP mechanism to perform a path exploration (e.g., by specifying the number of concurrent attempts of discovery of working locator pairs) when an outage occurs on the path between the locator pair in use. o Providing locator information to applications. An application should be able to obtain information about the locator pair which was actually used to send or receive the packet. * For inbound traffic, the application may be interested in the locator pair which was actually used to receive the packet. * For outbound traffic, the application may be interested in the locator pair which was actually used to transmit the packet. In this way, applications may have additional control on the locator management. For example, an application becomes able to verify if its preference for locator is actually applied to the flow or not. 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. oTurn on/off shim.An application should be able torequest to turn on or turn offknow if themultihoming supportcommunication is now being served by the shimlayer: * Apply shim. The application should be able to explicitly request the shim sub-layer to apply multihoming support. * Don't apply shim. The application should be able to request the shimsub-layernot 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 sub-layer or not.or not. o An application should be able to use a common interface to access an IPv4 locator and an IPv6 locator. 5. Socket Options for Multihoming Shim Sub-layer In this section, socket options that are specific to the shim sub- layer are defined. Table 1 shows a list of the socket options that are specific to themultihomingshim sub-layer. An application may use these socket options for a given socket either by the getsockopt() system call or by the setsockopt() system call. All of these socket options are defined at level SOL_SHIM. The first column of Table 1 gives the name of the option. The second and third columns indicate whether the option can be handled by the getsockopt() system call and/or by the setsockopt() system call. The fourth column provides a brief description of the socket option. The fifth column shows the type of data structure specified along with the socket option. By default, the data structure type is an integer. +-----------------------------+-----+-----+-----------------+-------+ | optname | get | set | description | dtype | +-----------------------------+-----+-----+-----------------+-------+ | SHIM_ASSOCIATED | o | | Get the | int | | | | | parameter which | | | | | | indicates | | | | | | whether if the | | | | | | socket is | | | | | | associated with | | | | | | any shim | | | | | | context or not. | | | SHIM_DONTSHIM | o | o | Get or set the | int | | | | | parameter which | | | | | | indicates | | | | | | whether to | | | | | | employ the | | | | | | multihoming | | | | | | support by the | | | | | | shim sub-layer | | | | | | or not. | | | SHIM_HOT_STANDBY | o | o | Get or set the | int | | | | | parameter to | | | | | | request the | | | | | | shim sub-layer | | | | | | to prepare a | | | | | | hot-standby | | | | | | connection. | | | SHIM_LOC_LOCAL_PREF | o | o | Get or set the | *1 | | | | | preferred | | | | | | locator on the | | | | | | local side for | | | | | | the context | | | | | | associated with | | | | | | the socket. | | | SHIM_LOC_PEER_PREF | o | o | Get or set the | *1 | | | | | preferred | | | | | | locator on the | | | | | | remote side for | | | | | | the context | | | | | | associated with | | | | | | the socket. | | | SHIM_LOC_LOCAL_RECV | o | o | Get or set the | int | | | | | parameter which | | | | | | is used to | | | | | | request the | | | | | | shim sub-layer | | | | | | to store the | | | | | | destination | | | | | | locator of the | | | | | | received IP | | | | | | packet. | | | SHIM_LOC_PEER_RECV | o | o | Get or set the | int | | | | | parameter which | | | | | | is used to | | | | | | request the | | | | | | shim sub-layer | | | | | | to store the | | | | | | source locator | | | | | | of the received | | | | | | IP packet. | | | SHIM_LOC_LOCAL_SEND | o | o | Get or set the | *2 | | | | | source locator | | | | | | of outgoing IP | | | | | | 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 | Get or set the | int | | | | | timeout value | | | | | | for detecting | | | | | | failure. | | | SHIM_PATHEXPLORE | o | o | Get or set | *4 | | | | | parameters for | | | | | | path | | | | | | exploration and | | | | | | failure | | | | | | detection. | | | SHIM_CONTEXT_DEFERRED_SETUP | o | o | Get or set the | int | | | | | parameter which | | | | | | indicates | | | | | | whether | | | | | | deferred | | | | | | context setup | | | | | | is supported or | | | | | | not. | | +-----------------------------+-----+-----+-----------------+-------+ Table 1: Socket options for multihoming shim sub-layer *1: Pointer to a shim_locator which is defined in Section 7. *2: Pointer to 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 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 sub-layer. That is, those legacy API handle IP addresses as identifiers. 2. With newly defined socket options for the shim sub-layer, the application obtains additional control of locator management. 3. The shim specific socket options can be kept independent from address family (IPPROTO_IP or IPPROTO_IPV6) and transport protocol (IPPROTO_TCP or IPPROTO_UDP). s1 s2 s3 s4 | | | | +----------------|--|-------|--|----------------+ | +-------+ +-------+ | | IPPROTO_TCP | TCP | | UDP | | | +-------+ +-------+ | | | \ / | | | | ----- | | | | / \ | | | +------+ +------+ | | IPPROTO_IP | IPv4 | | IPv6 | IPPROTO_IPV6 | | +------+ +------+ | | \ / SOL_SOCKET | +--------\-------/--------+ | | SOL_SHIM | shim | | | +--------/-------\--------+ | | / \ | | +------+ +------+ | | | IPv4 | | IPv6 | | | +------+ +------+ | | | | | +------------------|----------|-----------------+ | | IPv4 IPv6 Datagram Datagram Figure 2: System model of sockets API with shim sub-layer 5.1. SHIM_ASSOCIATED The SHIM_ASSOCIATED option is used to check whether the socket is associated with any shim context or not. This option is meaningful when the locator information of the received IP packet does not tell whether the identifier/locator adaptation is performed or not. Note that the EID pair and the locator pair may be identical in some cases. This option can be specified by getsockopt(). Thus, the option is read-only and the result (0/1/2) is set in the option value (the fourth argument of getsockopt()). The data type of the option value is an integer. The option value indicates the presence of shim context. A return value 1 means that the socket is associated with a shim context at the shim sub-layer. A return value 0 indicates that there is no shim context associated with the socket. A return value 2 means that it is not known whether the socket is associated with a shim context or not, and this must be returned only when the socket is unconnected. In other words, the returned value must be 0 or 1 when the socket is connected. For example, the option can be used by the application as follows: int optval; int optlen = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_ASSOCIATED, &optval, &optlen); 5.2. SHIM_DONTSHIM The SHIM_DONTSHIM option is used to request the shim layer not to provide the multihoming support for the communication established over the socket. The data type of the option value is an integer, and it takes 0 or 1. An option value 0 means that themultihomingshim sub-layer is employed if available. An option value 1 means that the application does not want themultihomingshim sub-layer to provide the multihoming support for the communication established over the socket. Default value is set as 0, which means that themultihomingshimsub- layersub-layer performs identifier/locator adaptation if available. Any attempt to disable the multihoming shim support MUST be made by the application before the socket is connected. If an application makes such an attempt for a connected-socket, an error code EOPNOTSUPP MUST be returned. For example, an application can request the system not to apply the multihoming support as follows: int optval; optval = 1; setsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, sizeof(optval)); For example, the application can check the option value as follows: int optval; int len; len = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, &len); 5.3. SHIM_HOT_STANDBY The SHIM_HOT_STANDBY option is used to control the shim sub-layer whether to employ a hot-standby connection for the socket or not. A hot-standby connection is an alternative working locator pair to the current locator pair. This option is effective only when there is a shim context associated with the socket. The data type of the option value is an integer. The option value can be set by setsockopt(). The option value can be read by getsockopt(). By default, the value is set to 0, meaning that hot-standby connection is disabled. For example, an application can request establishment of a hot- standby connection by using the socket option as follows: int optval; optval = 1; setsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval, sizeof(optval)); For example, an application can get the option value by using the socket option as follows: int optval; int len; len = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval, &len); 5.4. SHIM_PATHEXPLORE The application may use this socket option to specify parameters concerning path exploration. Path exploration is a procedure to find an alternative locator pair to the current locator pair. As the REAP specification defines, a peer may send Probe messages to find an alternative locator pair. 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 ENOENTwill beis returned when there is no context associated with the socket. For example, an application can set parameters for path exploration by using the socket option as follows. struct shim6_pathexplore pe; pe.pe_probenum = 4; /* times */ pe.pe_keepaliveto = 10; /* seconds */ pe.pe_initprobeto = 500; /* milliseconds */ pe.pe_reserved = 0; setsockopt(fd, SOL_SHIM, SHIM_PATHEXPLORE, &pe, sizeof(pe)); For example, an application can get parameters for path exploration by using the socket option as follows. struct shim6_pathexplore pe; int len; len = sizeof(pe); getsockopt(fd, SOL_SHIM, SHIM_PATHEXPLORE, &pe, &len); 5.5. SHIM_LOC_LOCAL_PREF The SHIM_LOC_LOCAL_PREF option is used to 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 a locator information data structure which is defined in Section 7. By default, the option value is set to NULL, meaning that the option is disabled. The preferred locator can be set by setsockopt(). The shim sub-layer shall verify requested locator before it updating the preferred locator. An application can get the preferred locator by getsockopt(). An error ENOENTwill beis returned when there is no context associated with the socket. An error EINVALIDLOCATORwill beis returned when the validation of the specified locator failed. For example, an application can set the preferred locator by using the socket option as follows. Note that some members of the shim_locator (lc_ifidx and lc_flags) are ignored in the set operation. 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 by using the socket option as follows. struct shim_locator lc; int len; len = sizeof(lc); getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc, &len); 5.6. SHIM_LOC_PEER_PREF The SHIM_LOC_PEER_PREF option is used to 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 locator information data structure which is defined in Section 7. By default, the option value is set to NULL, meaning that the option is disabled. The preferred locator can be set by setsockopt(). The shim sub-layer shall verify requested locator before it updating the preferred locator. An application can get the preferred locator by getsockopt(). An error ENOENTwill beis returned when there is no context associated with the socket. An error EINVALIDLOCATORwill beis returned when the validation of the requested locator fails. The usage of the option is same as that of SHIM_LOC_LOCAL_PREF. Note that some members of the shim_locator (lc_ifidx and lc_flags) are ignored in the set operation. 5.7. 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. An application can set the option value by setsockopt(). An application can get the option value by getsockopt(). See Section 6 for the procedure to access locator information stored in the ancillary data objects. An error ENOENTwill beis returned when there is no context associated with the socket. For example, an application can request the shim sub-layer to store destination locator by using the socket option as follows. int optval; optval = 1; setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval, sizeof(optval)); For example, an application can get the option value as follows. int optval; int len; len = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval, &len); 5.8. SHIM_LOC_PEER_RECV The SHIM_LOC_PEER_RECV option is used to request the shim sub-layer to store the source locator of the received IP packet in an ancillary data object which can be accessed by recvmsg(). 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 ENOENTwill beis 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 is used to request the shim sub-layer to use a specific locator as the source locator for the IP packets to be sent from the socket. 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. An application can set the local locator by setsockopt() providing a valid locator which is stored in a shim_locator data structure. When a zero-filled locator is specified, pre-existing setting of local locator is inactivated. An application can get the local locator by getsockopt(). An error ENOENTwill beis returned when there is no context associated with the socket. An error EINVALIDLOCATORwill beis returned when invalid locator is specified. For example, an application can request the shim sub-layer to use a specific local locator by using the socket option as follows. struct shim_locator locator; struct in6_addr ia6; /* an IPv6 address preferred for the source locator is copied to the parameter ia6 */ memset(&locator, 0, sizeof(locator)); /* fill shim_locator data structure */ locator.lc_family = AF_INET6; locator.lc_ifidx = 1; locator.lc_flags = 0; locator.lc_preference = 0; memcpy(&locator.lc_addr, &ia6, sizeof(ia6)); setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator, sizeof(locator)); For example, an application can get the preferred local locator by using the socket option as follows. struct shim_locator locator; memset(&locator, 0, sizeof(locator)); getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator, sizeof(locator)); /* check locator */ 5.10. SHIM_LOC_PEER_SEND The SHIM_LOC_PEER_SEND option is used to request the shim sub-layer to use a specific locator for the destination locator of IP packets to be sent from the socket. 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. An application can set the remote locator by setsockopt() providing a valid locator which is stored in a shim_locator data structure. When a zero-filled locator is specified, pre-existing setting of remote locator is inactivated. An application can get the specified remote locator by getsockopt(). An error ENOENTwill beis 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 is used to 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 in which a locator list is stored. See Section 7 for the data structure for storing the locator information. By default, the option value is set to NULL, meaning that the option is disabled. An application can get the locator list by getsockopt(). Note that the size of the buffer pointed 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 ENOENTwill beis returned when there is no context associated with the socket. An error EINVALIDLOCATORwill beis returned when the validation of the specified locator failed. An error ETOOMANYLOCATORS is returned when the number of locators specified exceeds the limit (SHIM_MAX_LOCATORS). For example, an application can set a list of locators to be associated with the local EID 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 by using the socket option as follows. struct shim_locator locators[SHIM_MAX_LOCATORS]; memset(locators, 0, sizeof(locators)); getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, locators, sizeof(locators)); /* parse locators */ ... 5.12. SHIM_LOCLIST_PEER The SHIM_LOCLIST_PEER option is used to get or set the locator list associated with the peer EID of the shim context associated with the socket. 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. An application can get the locator list 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. An application can set the locator list by setsockopt(). The buffer pointed by optval argument should contain an array of locator list. An error ENOENTwill beis returned when there is no context associated with the socket. An error EINVALIDLOCATORwill beis returned when the validation of the specified locator failed. An error ETOOMANYLOCATORS is returned when the number of locators specified exceeds the limit (SHIM_MAX_LOCATORS). The usage of the option is same as that of SHIM_LOCLIST_LOCAL. 5.13. SHIM_APP_TIMEOUT The SHIM_APP_TIMEOUT option 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 [RFC5534] 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 ENOENTwill beis returned when there is no context associated with the socket. For example, an application can set the timeout value by using the socket option as follows. int optval; optval = 15; /* 15 seconds */ setsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval, sizeof(optval)); For example, an application can get the timeout value by using the socket option as follows. int optval; int len; len = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval, &len); 5.14. SHIM_DEFERRED_CONTEXT_SETUP The SHIM_DEFERRED_CONTEXT_SETUP option is used to specify whether to enable deferred context setup or not. Deferred context setup means that the context is established in parallel with the data communication. Note that SHIM6 supports deferred context setup and HIP does not because EIDs in HIP (i.e., Host Identifiers) are non- routable. The data type for the option value is an integer. The option value should be binary (0 or 1). 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. Note that HIP does not support deferred context setup, by default. When the application requests to enable deferred context setup in case of HIP, it may mean that the application allows the system to start TCP handshake even when there is no IPsec security association with the peer. Such a usage of the SHIM_DEFERRED_CONTEXT_SETUP option should be considered as experimental and left for further study. For example, an application can disable the deferred context setup by using the 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 as follows. int optval; int len; len = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP, &optval, &len); 5.15. Applicability All the socket options for themultihomingshim sub-layer except for SHIM_HOT_STANDBY are applicableforto both connected and unconnected sockets.The reasonSHIM_HOT_STANDBY socket optioncannot be used for an unconnected socketisthatapplicable only to connected sockets. This is because themultihomingshim sub-layer cannot initiate context establishment to create a hot standby connectionbecausewhen the peer's IP address is not known until the application writes data to the unconnected socket. 5.16. Error Handling If successful, getsockopt() and setsockopt() return 0; otherwise, the functions return -1 and set errno to indicate an error. The following are new error values defined for some shim specific socket options indicating that the getsockopt() or setsockopt() finished incompletely: EINVALIDLOCATOR This indicates that at least one of the necessary validations inside the shim sub-layer for the specified locator has failed. In case of SHIM6, there are two kinds of verifications required for security reasons prior to sending an IP packet to the peer's new locator; one is the return routability (check if the peer is actually willing to receive data with the specified locator) and the other one is the verification based on crypto identifier mechanisms [RFC3972], [RFC5535]. 6. Ancillary Data for Multihoming Shim Sub-layerIn this section, the definition and the usageThis section provides definitions oftheancillary dataspecifictomultihomingbe used for locator management and notification from/to the shim sub-layerare provided. As defined into/ from application. When thePosix standard,application performs locator management by sendmsg()and recvmsg() inputor recvmsg(), amsghdr structure as their arguments. These system calls can handle control information along with data. Figure 3 showsmember of the msghdr structurewhich is defined(given in<sys/socket.h>. The memberFigure 3) called msg_control holds a pointer to the bufferwhere thein which one ore more shim specific ancillary data objects may be stored. An ancillary data object can store a single locator. It should bestored in additionpossible tootherprocess the shim specific ancillary dataobjects.object by the existing macros defined in the Posix standard and [RFC3542]. struct msghdr { caddr_t msg_name; /* optional address */ u_int msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ u_int msg_iovlen; /* # elements in msg_iov */ caddr_t msg_control; /* ancillary data, see below */ u_int msg_controllen; /* ancillary data buffer len */ int msg_flags; /* flags on received message */ }; Figure 3: msghdr structureThe buffer pointed byIn themember msg_controlcase of unconnected socket, msg_name stores themsghdr structure may contain locator informationsocket address of the peer whichis a single locator and itshould bepossibleconsidered toprocess 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 shouldbeconsidered asan identifier rather than a locator.TheSHIM_LOC_PEER_RECV should be used to get the locator of the peernode should be retrieved by SHIM_LOC_PEER_RECV as specified below.node. Table 2 is a list of the shim specific ancillary data which can be used for locator management by recvmsg() or sendmsg(). In any case,SOL_SHIMthe value of cmsg_level must be set ascmsg_level.SOL_SHIM. +---------------------+-----------+-----------+-----------------+ | cmsg_type | sendmsg() | recvmsg() | cmsg_data[] | +---------------------+-----------+-----------+-----------------+ | SHIM_LOC_LOCAL_RECV | | o | *1 | | SHIM_LOC_PEER_RECV | | o | *1 | | SHIM_LOC_LOCAL_SEND | o | | *1 | | SHIM_LOC_PEER_SEND | o | | *1 | | SHIM_FEEDBACK | o | | shim_feedback{} | +---------------------+-----------+-----------+-----------------+ Table 2: Shim specific ancillary data *1: cmsg_data[]should include padding (if necessary) andincludes a singlesockaddr_in{}/sockaddr_in6{}.sockaddr_in{} or sockaddr_in6{} and padding if necessary 6.1. Get Locator from Incoming Packet An application can get locator information from the received IP packet by specifying the shim specific socket options for the socket. When SHIM_LOC_LOCAL_RECV and/or SHIM_LOC_PEER_RECV socket options are set, the application can retrieve local and/or remote locator from the ancillary data. 6.2. Set Locator for Outgoing Packet An application can specify the locators to be used for transmitting an IP packet by sendmsg(). When the ancillary data of cmsg_type SHIM_LOC_LOCAL_SEND and/or SHIM_LOC_PEER_SEND are specified, the application can explicitly specify the source and/or the destination locators to be used for the communication over the socket. Note that the effect is limited to the datagram transmitted by the sendmsg(). If the specified locator pair is verified, the shim sub-layer overrides the locators of the IP packet. An error EINVALIDLOCATORwill beis returned when validation of the specified locator failed. 6.3. Notification from Application to Multihoming Shim Sub-layer An application may provide feedbacks to the shim sub-layer about the communication status. Such feedbacks are particularly useful for the shim sub-layer in the absence of REAP mechanism to monitor the reachability status of the currently used locator pair in a given shim context. The notification can be made by sendmsg() specifying a new ancillary data called SHIM_FEEDBACK. The ancillary data can be handled by specifying SHIM_FEEDBACK option in cmsg_type. An error ENOENTwill beis returned when there is no context associated with the socket. See Section 7.3 for details of the data structure to be used. It is outside the scope of this document how the shim sub-layer would react when a feedback is provided by an application. 6.4. Notification from Multihoming Shim Sub-layer to Application Themultihomingshim sub-layer MAY provide notification about a locator change within a multihome shim context to applications that have concern with the context. Such a notificationismay be useful, for example, for an application which is sensitive topath characteristics.the characteristics of the current path. A locator change is caused when either of local or peer's locator (or both) is changed. Note that locators discussed here are the ones that appear in the IP packet header, and not the ones that are included in the locator list. A locator change may take place asynchronously. The notification is handled as an out-of-band data by the application. 1. Application calls the select() system call by setting non-NULL value for the fourth argument. 2. When there is a notification, the application reads out-of-band data from the socket by callingthe recvmsg() system call.recvmsg(). 3. The application checks the flag in the msghdr (msg_flags) to see if there is any notification about locator change delivered. If the MSG_SHIM_LOCATOR_CHANGE flag is set, applicationparseparses the chain of control message to read a pair of ancillary data objects which contains the source locator and the destination locator. Note that the direction of locator change is distinguished by the value of cmsg_type; SHIM_LOC_*_RECV is used for inbound locator change, and SHIM_LOC_*_SEND is used for outbound locator change. There is no restriction in terms of applicability of the notification about locator change. The notification can be delivered tosockets regardlessany type ofif it is connectedsocket (connected or unconnected, stream-oriented ordatagram-oriented.datagram- oriented). 6.5. Applicability All the ancillary data for the shim sub-layer is applicable to both connected and unconnected sockets. A care is needed when the SHIM_LOC_*_RECV socket option is used for stream-oriented sockets (e.g., TCP sockets) because there is no one- to-one mapping between a single send or receive operation and the data (e.g., a TCP segment) being received. In other words, there is no gurantee that the locator(s) set in the SHIM_LOC_*_RECV ancillary data is identical to the locator(s) that appear in the IP packets received. Themultihomingshim sub-layer SHOULD provide the latest locator information to the application in response to the SHIM_LOC_*_RECV socket option. 7. Data StructuresIn this section,This section data structuresspecifically definedfor themultihomingshimsub-layer are introduced.sub-layer. These data structures are either used as a parameter forsetsockopt()/getsockopt()setsockopt() or getsockopt() (as mentioned in Section 5) or as a parameter for ancillary data to be processed bysendmsg()/recvmsg()sendmsg() or 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_proto; /* protocol */ uint16_t lc_port; /* port number */ uint16_t lc_flags; /* flags */ uint16_t lc_pref; /* preference value */ uint32_t lc_ifidx; /* interface index */ struct in6_addr lc_addr; /* address */ }; 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_proto Internet Protocol number for the protocol which is used to handle locator behind NAT. Typically, this value is set as UDP (17) when the locator is a UDP encapsulation interface. lc_port Port number which is used for handling locator behind NAT. lc_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_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 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 behind NAT is not necessarily an IPv4 address but it can be an IPv6 address. Below is an example where the application sets a UDP encapsulation interface as a source locator when sending IP packets. struct shim_locator locator; struct in6_addr ia6; /* copy the private IPv4 address to the ia6 as an IPv4-mapped IPv6 address */ memset(&locator, 0, sizeof(locator)); /* fill shim_locator data structure */ locator.lc_family = AF_INET; locator.lc_proto = IPPROTO_UDP; locator.lc_port = 50500; locator.lc_flags = 0; locator.lc_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 6 illustrates the data structure. The data structure can be passed to getsockopt() or setsockopt() as an argument. struct shim_pathexplore { uint8_t pe_probenum; /* # of initial probe */ uint8_t pe_keepaliveto; /* Keepalive Timeout */ uint16_t pe_initprobeto; /* Initial Probe Timeout */ uint32_t pe_reserved; /* reserved */ }; Figure 6: path explore structure pe_probenum Indicates the number of initial probe messages to be sent. Default value of this parameter should follow what is specified in [RFC5534]. pe_keepaliveto Indicates timeout value for detecting a failure when the host does not receive any packets for a certain period of time while there is outbound traffic. When the timer expires, path exploration procedure will be carried out by sending a REAP Probe message. Default value of this parameter should follow what is specified in [RFC5534]. pe_initprobeto Indicates retransmission timer of REAP Probe message in milliseconds. Note that this timer is applied before exponential back-off is started. A REAP Probe message for the same locator pair may be retransmitted. Default value of this parameter should follow what is specified in [RFC5534]. pe_reserved A reserved field for future extension. By default, the field should be initialized to zero. 7.3. Feedback Information As mentioned in Section 6.3, applications can inform the shim sub- layer about the status of unicast reachability of the locator pair currently in use. The feedback information can be handled by using ancillary data called SHIM_FEEDBACK. A new data structure named shim_feedback is illustrated in Figure 7. struct shim_feedback { uint8_t fb_direction; /* direction of traffic */ uint8_t fb_indicator; /* indicator (1-3) */ uint16_t fb_reserved; /* reserved */ }; Figure 7: feedback information structure direction Indicates direction of reachability between a locator pair in question. A value 0 indicates outbound and a value 1 indicates inbound direction. indicator A value indicating the degree of satisfaction of a unidirectional reachability for a given locator pair. * 0: Default value. Whenever this value is specified the feedback information must not be processed by the shim sub- layer. * 1: Unable to connect. There is no unidirectional reachability between the locator pair in question. * 2: Unsatisfactory. The application is not satisfied with the unidirectional reachability between the locator pair in question. * 3: Satisfactory. There is satisfactory unidirectional reachability between the locator pair in question. reserved Reserved field. Must be ignored by the receiver. 8. System Requirements This section gives system requirements posed by the sockets API defined in this document. There exist requirements for the system (kernel) to maintain the association between sockets and shim contexts. Asdiscussedaddressed in Section 5, all the socket optionsfor multihoming shim sub-layerand ancillary data defined in this document except for the SHIM_HOT_STANDBY socket option are applicable to both connected and unconnected sockets.The implications of this design choice on system requirements should be noted.There are less system requirements to enable support for applications that use connected sockets. This is because the kernel caneasilymaintain the association between a connected socket and amultihomingshimcontext. Note that the multihomingcontext in a static manner because a connected socket is bound to a source and destination IP addresses (identifiers). The kernel should be able to identify shimcontexts are identifiedcontext associated with a connected socket byan EID pair. In contrast, there are more system requirements to enable supportsearching shim context keyed by the pair of source and destination identifiers. However, this is not the case forapplications that use unconnectedunnconnected sockets. The kernel needs to dynamically resolve association between an unconnected socket and amultihomingshim context, if any, upon packet processing. As to outbound packet processing, the kernel needs to check if there is anymultihomingshim context whose EID pair matches with the source and destination IP addresses of the user data originating from an unconnected socket. If a matching context is found, themultihomingshim sub-layer performs packet processing taking the application's preference into account. Note that themultihomingshim sub-layer should be able to backtrack the socket from which the user data was originated. As to inbound packet processing, themultihomingshim sub-layer needs to check not only the IP header but also the transport layer protocol header to resolve the destination socket. If the destination socket is resolved and it contains any values concerning themultihomingshim sub-layer socket options, themultihomingshim sub-layer processes the IP packet as requested (e.g., set locator information of received packet in the ancillary data). 9.Implications forRelation to ExistingSocketSockets API ExtensionsSome ofThis section explains relation between thesocket optionssockets API defined in this documentare overlapping withand the existing sockets APIand care should be taken forextensions. As mentioned in Section 5, theusage notbasic assumption is that the existing sockets API continues toconfusework above the shim sub-layer. This means that, the existing sockets API deals with identifiers, and theoverlapping features. Thesockets API defined in this document deals with locators. SHIM_LOC_LOCAL_PREF and SHIM_LOC_PEER_PREF socket optionsfor requesting specific locators to be used for a given transaction (SHIM_LOC_LOCAL_PREF and SHIM_LOC_PEER_PREF)are semantically similar to theexisting sockets API (IPV6_PKTINFO). TheIPV6_PKTINFO socketoptions for obtainingAPI in thelocator information fromsense that both provide a means for application to set thereceivedsource IPpacket (SHIM_LOC_LOCAL_RECVaddress of outbound IP packets. SHIM_LOC_LOCAL_RECV andSHIM_LOC_PEER_RECV)SHIM_LOC_PEER_RECV socket options are semantically similar to theexisting sockets API (IP_RECVDSTADDRIP_RECVDSTADDR andIPV6_PKTINFO). In IPv4,IPV6_PKTINFO socket APIs in the sense that both provides a means for applicationcan obtainto get the source and/or destination IP address ofthe receivedinbound IPpacket (IP_RECVDSTADDR). If the shim sub-layer performs identifier/locator adaptation for the received packet, the destination EID should be stored inpackets. getsockname() and getpeername() enable application to get 'name' of theancillary data (IP_RECVDSTADDR). In IPv6, [RFC3542] defines that IPV6_PKTINFO can be usedcommunication endpoints which is represented by a pair of IP address and port number assigned tospecify source IPv6the socket. getsockname() gives IP address and port number assigned to theoutgoing interface for outgoing packets,socket on the local side, andretrieve destination IPv6getpeername() gives IP address andreceiving interface for incoming packets.port number of the peer side. 10. Operational Considerations Thisinformation is stored in ancillary data being IPV6_PKTINFO specified as cmsg_type. Existingsection gives operational considerations of the sockets APIshould continue to work above the shim sub-layer, that is, the IP addresses handleddefined inIPV6_PKTINFO shouldthis document. 10.1. Conflict Resolution There may beEIDs, 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. 10. Resolving Conflicts with Preference Values Since the multihoming shim API allows application toa conflicting situation when different applications specify difference preferencevaluefor thecontext which is associated with the socket instance, there may be a conflict with preference values specified by different applications.same shim context. For instance, application A and B may establish communication with the same EID pair while both applications have different preference in their choice of local locator.SHIM6 supports aThe notion of context forking inwhich a context is split when there is a conflict with preference values specified by multiple applications. Thus, context forkingSHIM6 cansimplyresolve the conflictingsituation which may be caused by the use of socket options for multihoming shim sub-layer. 10.1. Implicit Forkingsituation. Socket options defined in Section 5 may cause conflicting situation when the target context is shared by multiple applications. In such a case, the socket handler should inform the shim sub-layer that context forking is required. In SHIM6, when a context is forked, an unique identifier called Forked Instance Identifier (FII) is assigned to the newly forked context. The forked context is then exclusively associated with the socket through which non-default preference value was specified. The forked context is maintained by themultihomingshimsub-layersub- layer during the lifetime of associated socket instance. When the socket is closed, themultihomingshim 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, the SHIM6 implementation 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 sub-layer as defined in Section 6. 11. Discussion In this section, open issues are introduced. 11.1. 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 sub-layer exists inside the IP layer, both getsockname() and getpeername() deal with identifiers, namely EIDs. In this sense, the shim sub-layer serves to (1) hide locators and (2) provide access to the identifier for the application over the legacy socket APIs. 11.2. 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,10.2. Incompatiblility between IPv4 andalso to the preferences of the local locators that will be passed to the peer. 11.3. Issues of Header Conversion among Different Address FamilyIPv6 The shim sub-layer performs identifier/locator adaptation. Therefore, in some cases, the whole IP header can be replaced with new IP header of a different address family (e.g. conversion from IPv4 to IPv6 or vice versa). Hence, there is an issue how to make the conversion with minimum impact. Note that this issue is common in other protocol conversion such as SIIT[RFC2765]. As addressed in the 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 ofsource routing is not exactlysource routing is not exactly the same in IPv4 and IPv6. This means that an error may occur during the conversion of identifier and locator. It is ffs exactly how the shim sub-layer should behave in such erroneous cases. 11. IANA Considerations This document contains no IANA consideration. 12. Protocol Constants and Variables This section defines protocol constants and variables. SHIM_MAX_LOCATORS The maximum number of the locators to be included in a locator list. 32. 13. Security Considerations This section gives security considerations of the API defined in this document. 13.1. Treatment of Unknown Locator When sending IP packets, application may request use of unknown locator for thesame in IPv4 and IPv6. Hence, there issource and/or destination locators. Note that treatment of unknown locator can be acertain limitation in protocol conversion between IPv4 and IPv6.subject of security considerations because use of invalid source and/or destination locator may cause redirection attack. 13.1.1. Treatment of Unknown Source Locator Thequestionshim sub-layer checks if the requested locator ishow shouldavailable on any of the local interface. If not, the shim sub-layerbehave when it faces with limitation problem of protocol conversion. Should we introduce newMUST reject the request and return an errorsomething like ENOSUITABLELOCATOR ? 11.4. Handling of Unknown Locator Provided by Application There mightmessage with the EINVALIDLOCATOR code to the application. If the locator is confirmed to bea case where application providesavailable, the shimlayer newsub-layer SHOULD initiate the procedure to update the locatorwithlist. Use of theSHIM_LOC_*_PREFfollowing socket optionsor SHIM_LOC_*_SENDand ancillarydata. Then there is a question how shoulddata may require treatment of unknown source locator: o SHIM_LOC_LOCAL_SEND o SHIM_LOC_LOCAL_PREF o SHIM_LOC_LIST_LOCAL 13.1.2. Treatment of Unknown Destination Locator If the shimsub- layer treatsub-layer turns out to be SHIM6, thenew locator informed bySHIM6 implementation MUST reject theapplication. In principle, locator information are exchanged byrequest for using unknown destination locator. If the shimprotocol. However, there mightsub-layer turns out to bea case where application acquires information aboutHIP, thelocator and prefers to use itHIP implementation MAY accept the request forits communication. 12.using unknown destination locator. Use of the following socket options and ancillary data may require treatment of unknown destination locator: o SHIM_LOC_PEER_SEND o SHIM_LOC_PEER_PREF o SHIM_LOC_LIST_PEER 14. Changes12.1.14.1. Changes from version 00 to version 01The 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.12.2.14.2. Changes from version 01 to version 02The 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).12.3.14.3. Changes from version 02 to version 03The followings are changes from version 02 to version 03:o Add an Appendix section describing the issue ofcontext forking. 12.4. Changes from version 03 to version 04 The followings are changescontext forking. 14.4. Changes from version 03 to version04:04 o Updated reference. o Correct typo and grammatical errors.12.5.14.5. Changes from version 04 to version 05The followings are changes from version 04 to version 05:o Added definition of SHIM_FEEDBACK ancillary data. o Added an example of code using the SHIM_LOCLIST_LOCAL o Added SHIM_LOC_LOCAL_SEND and SHIM_LOC_PEER_SEND socket options.12.6.14.6. Changes from version 05 to version 06The followings are changes from version 04 to version 05:o Updated references.12.7.14.7. Changes from version 06 to version 07The followings are changes from version 06 to version 07:o Resolved editorial issues.12.8.14.8. Changes from version 07 to version 08 No changes are made except for updates of the references.12.9.14.9. Changes from version 08 to version 09The 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 multihoming shim support (i.e., specifying value 1 with the SHIM_DONTSHIM socket option) is only allowed before the socket is connected. o Updated locator information (shim_locator{}) so that it can contain a locator behind NAT.12.10.14.10. Changes from version 09 to version 10The followings are changes from version 09 to version 10:o Addressed applicability of socket options and ancillary data for themultihomingshim sub-layer. o Addressed system requirements. o Removed unnecessary description about deprecated socket option (SHIM_IF_RECV).12.11.14.11. Changes from version 10 to version 11The followings are changes from version 10 to version 11:o Added short descriptions about connected sockets and unconnected sockets. oRelaxRelaxed applicability of the socket options. oRelaxRelaxed applicability of the ancillary data. o Added notification about locator change.13. IANA Considerations This document contains no IANA consideration. 14. Security Considerations This document does not specify any security mechanism for the shim sub-layer. Fundamentally, the shim 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 sub-layer is strictly applied. 15. Conclusion In this document, the Application Program Interface (API) for multihoming shim 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 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 sub-layer to get locator14.12. Changes fromreceived 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 continueversion 11 towork above the shim sub-layer dealing with identifiers, while multihoming shim API deals with locators. 16.version 12 o Reflected comments from Brian Karpenter o Reflected comments from Michael Scharf 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.17.16. References17.1.16.1. Normative References [POSIX] "IEEE Std. 1003.1-2001 Standard for Information Technology -- Portable Operating System Interface (POSIX). Open group Technical Standard: Base Specifications, Issue 6, http://www.opengroup.org/austin", December 2001. [RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, "Advanced Sockets Application Program Interface (API) for IPv6", RFC 3542, May 2003. [RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol (HIP) Architecture", RFC 4423, May 2006. [RFC5533] Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim protocol", RFC 5533, June 2009. [RFC5534] Arkko, J. and I. Beijnum, "Failure Detection and Locator Pair Exploration Protocol for IPv6 Multihoming", RFC 5534, June 2009.17.2.16.2. 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-09, October 2009. [I-D.ietf-shim6-app-refer] Nordmark, E., "Shim6 Application Referral Issues", draft-ietf-shim6-app-refer-00 (work in progress), July 2005. [I-D.ietf-shim6-applicability] Abley, J., Bagnulo, M., and A. Garcia-Martinez, "Applicability Statement for the Level 3 Multihoming Shim Protocol (Shim6)", draft-ietf-shim6-applicability-04 (work in progress), November 2009. [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. [RFC5535] Bagnulo, M., "Hash Based Addresses (HBA)", RFC 5535, June 2009. 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 peerinitiator.the "initiator". The peer of the initiator is called the "responder". Once the forked context is established between the peers, on the initiator side, it is possible to apply forked context to the packet flow since the system maintains an association between the forked context and the socket owned by the application that has requested the context forking. How this association is maintained is an implementation specific issue. However, on the responder side, there is a questiononhow the outbound packet can be multiplexed by the shimsub-layer. Sincesub-layer because there are more than one SHIM6 contexts that match with the ULID pair of the packet flow. There is a need to differentiate packet flows not only by the ULID pairs but by some other information and associate a given packet flow with a specific context. Figure 8 gives an example of a scenario where two communicating peers fork a context. Initially, there has been a single transaction between the peers, by the application 1 (App1). Accordingly, another transaction is started, by application 2 (App2). Both of the transactions are made based on the same ULID pair. The first context pair (Ctx1) is established for the transaction of App1. Given the requests from App2, the shim sub-layer on Peer 1 decides to fork a context. Accordingly, a forked context (Ctx2) is established between the peers, which should be exclusively applied to the transaction of App2. Ideally, multiplexing and demultiplexing of packet flows that relate to App1 and App2 should be done as illustrated in Figure 8. However, as mentioned earlier, the responder needs to multiplex outbound flows of App1 and App2 somehow. Note that if a context forking occurs on the initiator side, a context forking needs to occur also on the responder side. Peer 1 Peer 2 (initiator) (responder) +----+ +----+ +----+ +----+ |App1| |App2| |App1| |App2| +----+ +----+ +----+ +----+ |^ |^ ^| ^| v| v| |v |v -----S1-------------S2----- -----S1-------------S2----- || || || || || || || || Ctx1 Ctx2 Ctx1 Ctx2 ULID:<A1,B1> ULID:<A1,B1> ULID:<B1,A1> ULID:<B1,A1> Loc: <A1,B2> Loc: <A1,B3> Loc: <B2,A1> Loc: <B3,A1> FII: 0 FII: 100 FII: 0 FII: 100 |^ |^ ^| ^| || || || || || || || || \..............||....................../| || \.............||......................./ || || || \|...................................../| \....................................../ Figure 8: context forkingToAny solution is needed to overcome the problem mentionedabove, 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 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.above. Authors' Addresses Miika Komu Helsinki Institute for Information Technology Tammasaarenkatu 3 Helsinki Finland Phone: +358503841531 Fax: +35896949768 Email: miika@iki.fi URI: http://www.hiit.fi/ 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.comshinta@sfc.wide.ad.jp