SHIM6 Working Group M. Komu Internet-Draft HIIT Intended status: Informational M. Bagnulo Expires: August 26, 2008 UC3M K. Slavov S. Sugimoto, Ed. Ericsson February 23, 2008 Socket Application Program Interface (API) for Multihoming Shim draft-ietf-shim6-multihome-shim-api-05 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 26, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract This document specifies a socket API for the multihoming shim layer. The API aims to enable interactions between the applications and the multihoming shim layer for advanced locator management and access to information about failure detection and path exploration. Komu, et al. Expires August 26, 2008 [Page 1] Internet-Draft Multihoming Shim API February 2008 This document is based on an assumption that a multihomed host is equipped with a conceptual sublayer (here after "shim") inside the IP layer that maintains mappings between identifiers and locators. Examples of the shim are SHIM6 and HIP. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. System Overview . . . . . . . . . . . . . . . . . . . . . . . 6 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7 5. Socket Options for Multihoming Shim Layer . . . . . . . . . . 9 5.1. SHIM_ASSOCIATED . . . . . . . . . . . . . . . . . . . . . 12 5.2. SHIM_DONTSHIM . . . . . . . . . . . . . . . . . . . . . . 13 5.3. SHIM_HOT_STANDBY . . . . . . . . . . . . . . . . . . . . . 13 5.4. SHIM_PATHEXPLORE . . . . . . . . . . . . . . . . . . . . . 14 5.5. SHIM_LOC_LOCAL_PREF . . . . . . . . . . . . . . . . . . . 15 5.6. SHIM_LOC_PEER_PREF . . . . . . . . . . . . . . . . . . . . 16 5.7. SHIM_LOC_LOCAL_RECV . . . . . . . . . . . . . . . . . . . 17 5.8. SHIM_LOC_PEER_RECV . . . . . . . . . . . . . . . . . . . . 18 5.9. SHIM_LOC_LOCAL_SEND . . . . . . . . . . . . . . . . . . . 18 5.10. SHIM_LOC_PEER_SEND . . . . . . . . . . . . . . . . . . . . 19 5.11. SHIM_LOCLIST_LOCAL . . . . . . . . . . . . . . . . . . . . 20 5.12. SHIM_LOCLIST_PEER . . . . . . . . . . . . . . . . . . . . 22 5.13. SHIM_APP_TIMEOUT . . . . . . . . . . . . . . . . . . . . . 22 5.14. SHIM_DEFERRED_CONTEXT_SETUP . . . . . . . . . . . . . . . 23 5.15. Error Handling . . . . . . . . . . . . . . . . . . . . . . 24 6. Ancillary Data for Multihoming Shim . . . . . . . . . . . . . 24 6.1. Get Locator Information from Incoming Packet . . . . . . . 26 6.2. Specify Locator Information for Outgoing Packet . . . . . 26 6.3. Notification from Application to Multihoming Shim . . . . 26 7. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 27 7.1. Placeholder for Locator Information . . . . . . . . . . . 27 7.2. Path Exploration Parameter . . . . . . . . . . . . . . . . 28 7.3. Feedback Information . . . . . . . . . . . . . . . . . . . 29 8. Implications for Existing Socket API Extensions . . . . . . . 29 9. Resolving Conflicts with Preference Values . . . . . . . . . . 30 9.1. Implicit Forking . . . . . . . . . . . . . . . . . . . . . 30 10. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 31 10.1. Naming at Socket Layer . . . . . . . . . . . . . . . . . . 31 10.2. Additional Requirements from Application . . . . . . . . . 31 10.3. Issues of Header Conversion among Different Address Family . . . . . . . . . . . . . . . . . . . . . . . . . . 32 10.4. Handling of Unknown Locator Provided by Application . . . 32 11. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 11.1. Changes from version 00 to version 01 . . . . . . . . . . 32 11.2. Changes from version 01 to version 02 . . . . . . . . . . 33 Komu, et al. Expires August 26, 2008 [Page 2] Internet-Draft Multihoming Shim API February 2008 11.3. Changes from version 02 to version 03 . . . . . . . . . . 33 11.4. Changes from version 03 to version 04 . . . . . . . . . . 33 11.5. Changes from version 04 to version 05 . . . . . . . . . . 33 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 13. Security Considerations . . . . . . . . . . . . . . . . . . . 33 14. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 33 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34 16.1. Normative References . . . . . . . . . . . . . . . . . . . 34 16.2. Informative References . . . . . . . . . . . . . . . . . . 35 Appendix A. Context Forking . . . . . . . . . . . . . . . . . . . 35 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37 Intellectual Property and Copyright Statements . . . . . . . . . . 39 Komu, et al. Expires August 26, 2008 [Page 3] Internet-Draft Multihoming Shim API February 2008 1. Introduction HIP and SHIM6 have a commonality in their protocol design separation of identifier and locator (hereafter identifier/locator separation). Both protocols aim to solve problems that are specific to multihoming environment in a host centric approach. In these protocols, a sub- layer within the IP layer maintains mappings of identifiers and locators. The shim layer is useful in a sense that the IP layer can maintain the mapping of an identifier to corresponding locators. Under a multihomed environment, typically, a host has more than one IP address at a time. During a given transaction, a host may be required to switch the IP address used for the communication to another IP address to preserve the communication. The protocol stack should take care of isolating the upper layer from disruption by the address update. The shim layer can make this locator update transparent to the upper layer protocols. In a system which is based on identifier/locator separation, upper layer protocols are expected to deal with identifiers for establishing and handling the communications. If an application wants to have a multihoming support by the shim layer, the IP addresses specified as source and destination addresses must be identifiers. However, this does not necessarily mean that applications are prohibited to choose specific locators in its communication. It may be useful for applications, in some situation, to specify a preferred locator for the flow. This document recommends that the identifier/locator adaptation is done only once inside the network stack of a host. That is, if multiple shim sublayers exist at the IP layer, any one of them should be applied exclusively for a given flow. As this document specifies socket API, it is written so that the contents are in line with Posix standard [POSIX] as much as possible. The API specified in this document defines how to use ancillary data (aka cmsg) to access locator information with recvmsg() and/or sendmsg() I/O calls. Definition of API is presented in C language and data types follow Posix format; intN_t means a singed integer of exactly N bits (e.g. int16_t) and uintN_t means an unsigned integer of exactly N bits (e.g. uint32_t). The target readers of this document are application programmers who develop application software which may benefit greatly from multihomed environment. In addition, this document should be of interest for the developers of a given shim protocol, as the shim layer should provide the interface to the application. Komu, et al. Expires August 26, 2008 [Page 4] Internet-Draft Multihoming Shim API February 2008 2. Terminology This section provides terminology used in this document. Basically most of the terms used in this document are taken from the following documents: o SHIM6 Protocol Specification[I-D.ietf-shim6-proto] o HIP Architecture[RFC4423] o Reachability Protocol (REAP)[I-D.ietf-shim6-failure-detection] In this document, the term "IP" refers to both IPv4 and IPv6, unless the protocol version is specifically mentioned. The followings are definitions of the terms that are frequently used in this document: o Endpoint Identifier (EID) - An identifier used by the application to specify the endpoint of a given communication. Applications may handle EID in various ways such as long-lived connections, callbacks, and referrals[I-D.ietf-shim6-app-refer]. * In the case of SHIM6, an identifier called an ULID serves as an EID. An ULID is chosen from locators available on the host. * In the case of HIP, an identifier which specifies communication endpoints is derived from the public key of the host, which is called a Host Identifier. For the sake of backward compatibility of the socket API, the Host Identifier is represented in a form of hash of public key. o Locator - An IP address actually used to deliver IP packets. Locators should be present in the source and destination fields of the IP header of a packet on the wire. * List of Locators - A list of locators associated with an EID. There are two lists of locators stored in a given context, one is associated with the local EID and the other is associated with the remote EID. As defined in [I-D.ietf-shim6-proto], the list of locators associated with an EID 'A' can be denoted as Ls(A). * Preferred Locator - The (source/destination) locator currently used to send packets within a given context. As defined in [I-D.ietf-shim6-proto], the preferred locator of a host 'A' is denoted as Lp(A). o Shim - A conceptual (sub-)layer inside the IP Layer which maintains mappings of EIDs and locators. An EID can be associated with more than one locators at a time when the host is multihomed. The term 'shim' does not refer to a specific protocol but refers to the conceptual sublayer inside the IP layer. o identifier/locator adaptation - An adaptation performed at the shim layer between EIDs and locators within a given context. The adaptation may end up re-writing the source and destination addresses of the IP packet. In the outbound packet processing, the EID pair is converted to the associated locator pair, while Komu, et al. Expires August 26, 2008 [Page 5] Internet-Draft Multihoming Shim API February 2008 the locator pair is converted to the EID pair in the inbound packet processing. o Context - State information shared by a given pair of peers, which stores a binding between the EIDs and associated locators. The context is maintained at the shim layer. o Reachability Detection - A procedure to check reachability between a given locator pair. o Path - A sequence of routers that an IP packet goes through to reach the destination. o Path Exploration - A procedure to explore available paths for a given set of locator pairs. o Outage - An incident that prevents IP packets to flow from the source locator to the destination locator. When there is an outage, it means that there is no reachability between a given locator pair. The outage can be caused by various reasons, such as shortage of network resources, congestion, and human error (faulty operation). o Working Address Pair - An address pair is said to be working if the packet containing the first address from the pair as source address and the second address from the pair as destination address can safely travel from the source to the destination. If the reachability is confirmed in both directions, the address pairs is said to be bi-directional. Otherwise, it's unidirectional. o Reachability Protocol (REAP) - A protocol for detecting failure and exploring reachability in a multihomed environment. REAP is defined in [I-D.ietf-shim6-failure-detection]. 3. System Overview Figure 1 illustrates the system overview. The shim layer and REAP component exist inside the IP layer. Applications can use the socket API defined in this document to interface the shim layer and transport layer for locator management and failure detection and path exploration. It is also possible that the shim layer interacts with transport layers, but the interactions are outside the scope of this document. Komu, et al. Expires August 26, 2008 [Page 6] Internet-Draft Multihoming Shim API February 2008 +------------------------+ | Application | +------------------------+ ^ ^ ~~~~~~~~~~~~~|~Socket Interface|~~~~~~~~~~~~~~ | v +-----------|------------------------------+ | | Transport Layer | +-----------|------------------------------+ ^ | +-------------|-----|-------------------------------------+ | v v | | +-----------------------------+ +----------+ | IP | | Shim |<----->| REAP | | Layer | +-----------------------------+ +----------+ | | ^ ^ | +-----------------------|----------------------|----------+ v v +------------------------------------------+ | Link Layer | +------------------------------------------+ Figure 1: System overview 4. Requirements The following is the list of requirements from the application perspective: o Locator management. The shim layer selects a pair of locators for sending IP packets within a given context. The selection is made by taking miscellaneous conditions into account such as reachability of the path, application's preference, and characteristics of path. From the application's perspective: * It should be possible to obtain the lists of locators of a given context: Ls(local) and Ls(remote). * It should be possible to obtain the preferred locators of a given context: Lp(local) and Lp(remote). o Notification from the application to the shim layer about the status of the communication. Note that the notification is made in an event based manner. There are mainly two aspects of the feedback that application or upper layer protocol may provide for the shim layer, positive and negative feedbacks [NOTE: These feedbacks are mentioned in [I-D.ietf-shim6-failure-detection]]: * Positive feedback could be given by the application or upper layer protocol (e.g. TCP) to the shim layer informing that the communication is going well. Komu, et al. Expires August 26, 2008 [Page 7] Internet-Draft Multihoming Shim API February 2008 * Negative feedback could be given by the application or upper layer protocol (e.g. TCP) to the shim layer informing that the communication status is not satisfactory. TCP could detect a problem when it does not receive expected ACK from the peer. ICMP error messages delivered to the upper layer protocol could be a clue for application to detect potential problems. REAP module may be triggered by these negative feedbacks and invoke procedure of path exploration. o Feedback from application to shim layer. The application should be able to inform the shim layer of the timeout values for detecting failures, for sending keepalives, for starting the exploration procedure. In particular, the application should be able to suppress the keepalives. o Hot-standby. The application may request the shim layer for hot- standby capabilities. In this case, alternative paths are known to be working before a failure is detected. Hence it is possible for the host to immediately replace the current locator pair with an alternative locator pair. Hot-standby may allow applications to achieve better failover. o Eagerness of locator exploration. The application should be able to inform the shim layer how aggressive it wants REAP mechanism to perform path exploration (e.g. specifying the number of concurrent attempts of discovering working locator pair) when an outage occurs on the path between the currently selected locator pair. o Providing locator information to application. The application should be able to obtain information about the locator pair which was actually used to send or receive the packet. * For inbound traffic, the application may be interested in the locator pair which was actually used to receive the packet. * For outbound traffic, the application may be interested in the locator pair which was actually used to transmit the packet. In this way, the application may have additional control on the locator management. For example, the application can verify if its preference of locator is actually applied to the flow or not. o The application should be able to specify if it wants to defer the context setup or if it wants context establishment to be started immediately in case there is no available context. With deferred context setup, there should be no additional delay imposed by context establishment in initiation of communication. o Turn on/off shim. The application should be able to request to turn on/off the multihoming support by the shim layer: * Apply shim. The application should be able to explicitly request the shim layer to apply multihoming support. * Don't apply shim. The application should be able to request the shim layer not to apply the multihoming support but to apply normal IP processing at the IP layer. Komu, et al. Expires August 26, 2008 [Page 8] Internet-Draft Multihoming Shim API February 2008 o The application should be able to know if the communication is now served by the shim layer or not. o The application should be able to access locator information regardless of its address family. In other words, no matter whether the target locator is IPv4 or IPv6, the application should be able to use common interface to access the locator information. 5. Socket Options for Multihoming Shim Layer In this section, socket options that are specific to multihomed shim are defined. Table 1 provides a list of the socket options that are specific to multihoming shim layer. These socket options can be used by either getsockopt() or setsockopt() system call for a given socket. All of these socket options are defined at level SOL_SHIM. The first column of Table 1 gives the name of the option. The second and third columns indicate whether the option can be handled by getsockopt() and/or setsockopt(), respectively. The fourth column provides a brief description of the socket option. The fifth column shows the type of data structure specified along with the socket option. By default, the data structure type is an integer. +-----------------------------+-----+-----+-----------------+-------+ | optname | get | set | description | dtype | +-----------------------------+-----+-----+-----------------+-------+ | SHIM_ASSOCIATED | o | | Check if the | int | | | | | socket is | | | | | | associated with | | | | | | any shim | | | | | | context or not. | | | SHIM_DONTSHIM | o | o | Request the | int | | | | | shim layer not | | | | | | to apply any | | | | | | multihoming | | | | | | support for the | | | | | | communication. | | | SHIM_HOT_STANDBY | o | o | Request the | int | | | | | shim layer to | | | | | | prepare a | | | | | | hot-standby | | | | | | connection (in | | | | | | addition to the | | | | | | current path). | | Komu, et al. Expires August 26, 2008 [Page 9] Internet-Draft Multihoming Shim API February 2008 | SHIM_LOC_LOCAL_PREF | o | o | Get or set the | *1 | | | | | preferred | | | | | | locator on the | | | | | | local side for | | | | | | the context | | | | | | associated with | | | | | | the socket. | | | SHIM_LOC_PEER_PREF | o | o | Get or set the | *1 | | | | | preferred | | | | | | locator on the | | | | | | remote side for | | | | | | the context | | | | | | associated with | | | | | | the socket. | | | SHIM_LOC_LOCAL_RECV | o | o | Request for the | int | | | | | destination | | | | | | locator of the | | | | | | received IP | | | | | | packet. | | | SHIM_LOC_PEER_RECV | o | o | Request for the | int | | | | | source locator | | | | | | of the received | | | | | | IP packet. | | | SHIM_LOC_LOCAL_SEND | o | o | Request use of | *2 | | | | | specific | | | | | | locator as | | | | | | source locator | | | | | | of outgoing IP | | | | | | packets. | | | SHIM_LOC_PEER_SEND | o | o | Request use of | *2 | | | | | specific | | | | | | locator as | | | | | | destination | | | | | | locator of | | | | | | outgoing IP | | | | | | packets. | | | SHIM_LOCLIST_LOCAL | o | o | Get or set a | *3 | | | | | list of | | | | | | locators | | | | | | associated with | | | | | | the local EID. | | | SHIM_LOCLIST_PEER | o | o | Get or set a | *3 | | | | | list of | | | | | | locators | | | | | | associated with | | | | | | the peer's EID. | | Komu, et al. Expires August 26, 2008 [Page 10] Internet-Draft Multihoming Shim API February 2008 | SHIM_APP_TIMEOUT | o | o | Inform the shim | int | | | | | layer of a | | | | | | timeout value | | | | | | for detecting | | | | | | failure. | | | SHIM_PATHEXPLORE | o | o | Specify | *4 | | | | | behavior of | | | | | | path | | | | | | exploration and | | | | | | failure | | | | | | detection. | | | SHIM_CONTEXT_DEFERRED_SETUP | o | o | Specify if the | int | | | | | context setup | | | | | | can be deferred | | | | | | or not. | | +-----------------------------+-----+-----+-----------------+-------+ Table 1: Socket options for multihoming shim *1: Pointer to a shim_locator which is defined in Section 7. *2: Pointer to shim_locator data structure. *3: Pointer to an array of shim_locator. *4: Pointer to a shim_pathexplore which is defined in Section 7. Figure 2 illustrates how the shim specific socket options fit into the system model of socket API. In the figure, it can be seen that the shim layer and the additional protocol components (IPv4 and IPv6) below the shim layer are new to the system model. As previously mentioned, all the shim specific socket options are defined at SOL_SHIM level. This design choice brings the following advantages: 1. It is assured that the existing socket API continue to work at the layer above the shim layer. That is, those legacy API deal with 'identifier' aspect of the IP addresses. 2. With newly defined socket options for the shim layer, the application obtains additional control on locator management. 3. The shim specific socket options are not specific to any address family (IPPROTO_IP or IPPROTO_IPV6) or any transport protocol (IPPROTO_TCP or IPPROTO_UDP). Komu, et al. Expires August 26, 2008 [Page 11] Internet-Draft Multihoming Shim API February 2008 s1 s2 s3 s4 | | | | +----------------|--|-------|--|----------------+ | +-------+ +-------+ | | IPPROTO_TCP | TCP | | UDP | | | +-------+ +-------+ | | | \ / | | | | ----- | | | | / \ | | | +------+ +------+ | | IPPROTO_IP | IPv4 | | IPv6 | IPPROTO_IPV6 | | +------+ +------+ | | \ / SOL_SOCKET | +--------\-------/--------+ | | SOL_SHIM | shim | | | +--------/-------\--------+ | | / \ | | +------+ +------+ | | | IPv4 | | IPv6 | | | +------+ +------+ | | | | | +------------------|----------|-----------------+ | | IPv4 IPv6 Datagram Datagram Figure 2: System model of socket API with shim layer 5.1. SHIM_ASSOCIATED The SHIM_ASSOCIATED option can be used to check whether the socket is associated with any shim context or not. This option is particularly meaningful in a case where the locator information of the received IP packet does not tell whether the identifier/locator adaptation is performed or not. Note that the EID pair and locator pair may be identical in some case. This option can be specified by getsockopt(). Thus, the option is read-only and the result (0 or 1) is set in the option value (the fourth argument of getsockopt()). Data type of the option value is integer. The option value indicates presence of shim context. A returned value 1 means that the socket is associated with a certain shim context at the shim layer, while a return value 0 indicates that there is no context associated with the socket. Komu, et al. Expires August 26, 2008 [Page 12] Internet-Draft Multihoming Shim API February 2008 For example, the option can be used by the application as follows: int optval; int optlen = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_ASSOCIATED, &optval, &optlen); 5.2. SHIM_DONTSHIM The SHIM_DONTSHIM option can be used to request the shim layer to not apply the multihoming support for the communication established over the socket. Data type of the option value is integer. The option value indicates whether the multihoming shim support is deprecated or not. The option value is binary (0 or 1). By default, the value is set to 0, meaning that the shim layer applies identifier/locator adaptation for the communication. In order to disable the socket option, the application should call setsockopt() with optval set as 0. For example, the option can be disabled by the application as follows. int optval; optval = 0; setsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, sizeof(optval)); For example, the option value can be checked by the application as follows. int optval; int len; len = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, &len); 5.3. SHIM_HOT_STANDBY The SHIM_HOT_STANDBY option can be used to check if the shim layer uses hot-standby connection or not for the communication established over the socket. Hot-standby connection is another working locator pair than the current locator pair. Hence this option is effective only when there is a shim context associated with the socket. Data type of the option value is integer. Komu, et al. Expires August 26, 2008 [Page 13] Internet-Draft Multihoming Shim API February 2008 The option value can be set by setsockopt(). The option value can be read by getsockopt(). By default, the value is set to 0, meaning that hot-standby connection is disabled. For example, the option can be activated by the application as follows. int optval; optval = 1; setsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval, sizeof(optval)); For example, the option value can be checked by the application as follows. int optval; int len; len = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval, &len); 5.4. SHIM_PATHEXPLORE This option can be used to specify behavior of path exploration to be carried out. Path exploration is a procedure to find an alternative locator pair when the host finds any problem with current locator pair. A message used for finding an alternative locator pair is called a Probe message and it is sent per locator pair. Default value is defined for Initial Probe Timeout (0.5 seconds) and Initial Probe (4 times) in the REAP specification. The option is effective only when there is a shim context associated with the socket. Data type of the option value is a pointer to the buffer where a set of information for path exploration is stored. The data structure is defined in Section 7. By default, the option value is set as NULL, meaning that the option is disabled. An error ENOENT will be returned when there is no context associated Komu, et al. Expires August 26, 2008 [Page 14] Internet-Draft Multihoming Shim API February 2008 with the socket. For example, the parameters for the path exploration can be set as follows. struct shim6_pathexplore pe; pe.pe_probenum = 4; /* times */ pe.pe_keepaliveto = 10; /* seconds */ pe.pe_initprobeto = 500; /* milliseconds */ pe.pe_reserved = 0; setsockopt(fd, SOL_SHIM, SHIM_PATHEXPLORE, &pe, sizeof(pe)); For example, the parameters for the path exploration can be read as follows. struct shim6_pathexplore pe; int len; len = sizeof(pe); getsockopt(fd, SOL_SHIM, SHIM_PATHEXPLORE, &pe, &len); 5.5. SHIM_LOC_LOCAL_PREF The SHIM_LOC_LOCAL_PREF option can be used to read or set preferred locator on local side within a given context. Hence this option is effective only when there is a shim context associated with the socket. Data type of the option value is a pointer to the specific data structure which stores the locator information. The data structure is defined in Section 7. By default, the option value is set as NULL, meaning that the option is disabled. The preferred locator can be set by setsockopt(). Verification of the locator shall be done by the shim layer before updating the preferred locator. The preferred locator can be read by getsockopt(). An error ENOENT will be returned when there is no context associated with the socket. An error EINVALIDLOCATOR will be returned when the validation of the Komu, et al. Expires August 26, 2008 [Page 15] Internet-Draft Multihoming Shim API February 2008 specified locator failed. For example, a preferred locator can be set as follows. It should be noted that some members of the shim_locator (lc_ifidx and lc_flags) are ignored in the write operation. struct shim_locator lc; struct in6_addr ip6; /* ...set the locator (ip6)... */ bzero(&lc, sizeof(shim_locator)); lc.lc_family = AF_INET6; /* IPv6 */ lc.lc_ifidx = 0; lc.lc_flags = 0; lc.lc_preference = 255; memcpy(lc.lc_addr, &ip6, sizeof(in6_addr)); setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc, sizeof(optval)); For example, the preferred locator of the context can be read by application as follows. struct shim_locator lc; int len; len = sizeof(lc); getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc, &len); 5.6. SHIM_LOC_PEER_PREF The SHIM_LOC_PEER_PREF option can be used to read or set preferred locator on peer side within a given context. Hence this option is effective only when there is a shim context associated with the socket. Data type of the option value is a pointer to the specific data structure which stores the locator information. The data structure is defined in Section 7. By default, the option value is set as NULL, meaning that the option is disabled. The preferred locator can be set by setsockopt(). Necessary verification of the locator shall be done by the shim layer before updating the preferred locator. Komu, et al. Expires August 26, 2008 [Page 16] Internet-Draft Multihoming Shim API February 2008 The preferred locator can be read by getsockopt(). An error ENOENT will be returned when there is no context associated with the socket. An error EINVALIDLOCATOR will be returned when the validation of the specified locator failed. For example, a preferred locator can be set as follows. It should be noted that some members of the shim_locator (lc_ifidx and lc_flags) are ignored in the write operation. The usage of the option is same as that of SHIM_LOC_LOCAL_PREF. 5.7. SHIM_LOC_LOCAL_RECV The SHIM_LOC_LOCAL_RECV option can be used to request the shim layer to store the destination locator of the received IP packet in an ancillary data object which can be accessed by recvmsg(). Hence this option is effective only when there is a shim context associated with the socket. Data type of the option value is integer. The option value should be binary (0 or 1). By default, the option value is set to 0, meaning that the option is disabled. The option value can be set by setsockopt(). The option value can be read by getsockopt(). See Section 6 for the procedure to access locator information stored in the ancillary data objects. An error ENOENT will be returned when there is no context associated with the socket. For example, the option can be activated by the application as follows: int optval; optval = 1; setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval, sizeof(optval)); For example, the option value can be checked by the application as follows: Komu, et al. Expires August 26, 2008 [Page 17] Internet-Draft Multihoming Shim API February 2008 int optval; int len; len = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval, &len); 5.8. SHIM_LOC_PEER_RECV The SHIM_LOC_PEER_RECV option can be used to request the shim layer to store the source locator of the received IP packet in an ancillary data object which can be accessed by recvmsg(). Hence this option is effective only when there is a shim context associated with the socket. Data type of the option value is integer. The option value should be binary (0 or 1). By default, the option value is set to 0, meaning that the option is disabled. The option value can be set by setsockopt(). The option value can be read by getsockopt(). See Section 6 for the procedure to access locator information stored in the ancillary data objects. An error ENOENT will be returned when there is no context associated with the socket. The usage of the option is same as that of SHIM_LOC_LOCAL_RECV option. 5.9. SHIM_LOC_LOCAL_SEND The SHIM_LOC_LOCAL_SEND option can be used to request the shim layer to use specific locator for the source locator of IP packets to be sent from the socket. Hence this option is effective only when there is a shim context associated with the socket. Data type of option value is pointer to shim_locator data structure. The local locator can be specified by setsockopt() providing a valid locator which is stored in a shim_locator data structure. When a zero-filled locator is specified, pre-existing setting of local locator is deactivated. The local locator specified can be obtained by getsockopt(). The locator can be obtained from the option value. Komu, et al. Expires August 26, 2008 [Page 18] Internet-Draft Multihoming Shim API February 2008 An error ENOENT will be returned when there is no context associated with the socket. An error EINVALIDLOCATOR when invalid locator is specified. For example, a preferred local locator can be specified as follows. struct shim_locator locator; struct in6_addr ia6; /* an IPv6 address preferred for the source locator is copied to the parameter ia6 */ memset(&locator, 0, sizeof(locator)); /* fill shim_locator data structure */ locator.lc_family = AF_INET6; locator.lc_ifidx = 1; locator.lc_flags = 0; locator.lc_preference = 0; memcpy(&locator.lc_addr, &ia6, sizeof(ia6)); setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator, sizeof(locator)); For example, a preferred local locator can be read as follows. struct shim_locator locator; memset(&locator, 0, sizeof(locator)); getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator, sizeof(locator)); /* check locator */ 5.10. SHIM_LOC_PEER_SEND The SHIM_LOC_PEER_SEND option can be used to request the shim layer to use specific locator for the destination locator of IP packets to be sent from the socket. Hence this option is effective only when there is a shim context associated with the socket. Data type of option value is pointer to shim_locator data structure. The remote locator can be specified by setsockopt() providing a valid locator which is stored in a shim_locator data structure. When a zero-filled locator is specified, pre-existing setting of remote Komu, et al. Expires August 26, 2008 [Page 19] Internet-Draft Multihoming Shim API February 2008 locator is deactivated. The remote locator specified can be obtained by getsockopt(). The locator can be obtained from the option value. An error ENOENT will be returned when there is no context associated with the socket. An error EINVALIDLOCATOR when invalid locator is specified. The usage of the option is as the same as that of SHIM_LOC_LOCAL_SEND option. 5.11. SHIM_LOCLIST_LOCAL The SHIM_LOCLIST_LOCAL option can be used to read or set the locator list associated with the local EID of the shim context associated with the socket. Hence this option is effective only when there is a shim context associated with the socket. Data type of option value is pointer to the buffer where a locator list is stored. See Section 7 for the data structure for storing the locator information. By default, the option value is set as NULL, meaning that the option is disabled. The locator list can be read by getsockopt(). Note that the size of the buffer pointed by optval argument should be large enough to store an array of locator information. The number of the locator information is not known beforehand. The locator list can be set by setsockopt(). The buffer pointed by optval argument should contain an array of locator list. An error ENOENT will be returned when there is no context associated with the socket. An error EINVALIDLOCATOR will be returned when the validation of the specified locator failed. For example, a list of locators to be associated with the local EID can be specified as follows: Komu, et al. Expires August 26, 2008 [Page 20] Internet-Draft Multihoming Shim API February 2008 struct shim_locator locators[SHIM_MAX_LOCATORS]; struct sockaddr_in *sin; struct sockaddr_in6 *sin6; memset(locators, 0, sizeof(locators)); ... /* obtain local IP addresses from local interfaces */ ... /* first locator (an IPv6 address) */ locators[0].lc_family = AF_INET6; locators[0].lc_ifidx = 0; locators[0].lc_flags = 0; locators[0].lc_preference = 1; memcpy(&locators[0].lc_addr, &sa6->sin6_addr, sizeof(sa6->sin6_addr)); ... /* second locator (an IPv4 address) */ locators[1].lc_family = AF_INET; locators[1].lc_ifidx = 0; locators[1].lc_flags = 0; locators[1].lc_preference = 0; memcpy(&locators[1].lc_addr, &sa->sin_addr, sizeof(sa->sin_addr)); setsockopt(fd, SOL_SHIM, SHIM_LOCLIST_LOCAL, locators, sizeof(locators)); For example, a list of locators that are associated with the local EID can be obtained as follows: struct shim_locator locators[SHIM_MAX_LOCATORS]; memset(locators, 0, sizeof(locators)); getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, locators, sizeof(locators)); /* parse locators */ ... Komu, et al. Expires August 26, 2008 [Page 21] Internet-Draft Multihoming Shim API February 2008 5.12. SHIM_LOCLIST_PEER The SHIM_LOCLIST_PEER option can be used to read or set the locator list associated with the peer EID of the shim context associated with the socket. Hence this option is effective only when there is a shim context associated with the socket. Data type of option value is pointer to the buffer where a locator list is stored. See Section 7 for the data structure for storing the locator information. By default, the option value is set as NULL, meaning that the option is disabled. The locator list can be read by getsockopt(). Note that the size of the buffer pointed by optval argument should be large enough to store an array of locator information. The number of the locator information is not known beforehand. The locator list can be set by setsockopt(). The buffer pointed by optval argument should contain an array of locator list. An error ENOENT will be returned when there is no context associated with the socket. An error EINVALIDLOCATOR will be returned when the validation of the specified locator failed. The usage of the option is same as that of SHIM_LOCLIST_LOCAL. 5.13. SHIM_APP_TIMEOUT The SHIM_APP_TIMEOUT option indicates timeout value for application to detect failure. Hence this option is effective only when there is a shim context associated with the socket. Data type of the option value is integer. The value indicates the period of timeout in seconds to send a REAP Keepalive message since the last outbound traffic. By default, the option value is set as 0, meaning that the option is disabled. When the option is disabled, the REAP mechanism follows its default value of Send Timeout value as specified in [I-D.ietf-shim6-failure-detection] If the timeout value specified is longer than the Send Timeout configured in the REAP component, the REAP Keepalive message should be suppressed. An error ENOENT will be returned when there is no context associated with the socket. Komu, et al. Expires August 26, 2008 [Page 22] Internet-Draft Multihoming Shim API February 2008 For example, a specific timeout value can be configured by the application as follows: int optval; optval = 15; /* 15 seconds */ setsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval, sizeof(optval)); For example, the option value namely the period of timeout can be checked by the application as follows: int optval; int len; len = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval, &len); 5.14. SHIM_DEFERRED_CONTEXT_SETUP The SHIM_DEFERRED_CONTEXT_SETUP option indicates how initiation of context setup is made in terms of timing (before or after) the initial communication flow. Deferred context means that the establishment of context does not put additional delay for an initial transaction. Data type for the option value is integer. The option value should binary (0 or 1). By default, the value is set as 1, meaning that the context setup is deferred. In order to disable the option, the application should call setsockopt() with option value set as 0. However, it should be noted that in some case, deferred context setup is not possible; given EID is non-routable address and there is no way to transmit any IP packet unless there is a context providing the locators. In such case, context should be established prior to the communication. For example, the option can be disabled by the application as follows: int optval; optval = 0; setsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP, &optval, sizeof(optval)); Komu, et al. Expires August 26, 2008 [Page 23] Internet-Draft Multihoming Shim API February 2008 For example, the option value can be checked by the application as follows: int optval; int len; len = sizeof(optval); getsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP, &optval, &len); 5.15. Error Handling If successful, getsockopt() and setsockopt() return 0; otherwise, the functions return -1 and set errno to indicate error. The followings are errno codes newly defined for some shim specific socket options indicating that the getsockopt() or setsockopt() finished incompletely: EINVALIDLOCATOR This indicates that at least one of the necessary validations inside the shim layer for the specified locator has failed. In case of SHIM6, there are two kinds of verifications required for security reasons prior to sending an IP packet to the peer's new locator; one is return routability (check if the peer is actually willing to receive data with the specified locator) and the other is verifications based on given crypto identifier mechanisms [RFC3972], [I-D.ietf-shim6-hba]. 6. Ancillary Data for Multihoming Shim In this section, definition and usage of the ancillary data which is specific to multihoming shim are provided. As defined in Posix standard, sendmsg() and recvmsg() take msghdr structure as its argument and they can additionally handle control information along with data. Figure 22 shows the msghdr structure which is defined in . msg_control member holds a pointer to the buffer where the shim specific ancillary data objects can be stored in addition to other ancillary data objects. Komu, et al. Expires August 26, 2008 [Page 24] Internet-Draft Multihoming Shim API February 2008 struct msghdr { caddr_t msg_name; /* optional address */ u_int msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ u_int msg_iovlen; /* # elements in msg_iov */ caddr_t msg_control; /* ancillary data, see below */ u_int msg_controllen; /* ancillary data buffer len */ int msg_flags; /* flags on received message */ }; Figure 22: msghdr structure The buffer pointed from the msg_control member of the msghdr structure may contain a locator information which is a single locator and it should be possible to process them with the existing macros defined in Posix and [RFC3542]. Each cmsghdr{} should be followed by data which stores a single locator. In case of non-connected socket, msg_name member stores the socket address of the peer which should be considered as an identifier rather than a locator. The locator of the peer node should be retrieved by SHIM_LOC_PEER_RECV as specified below. Table 2 is a list of the shim specific ancillary data which can be used for recvmsg() or sendmsg(). In any case, SOL_SHIM must be set as cmsg_level. +---------------------+-----------+-----------+-----------------+ | cmsg_type | sendmsg() | recvmsg() | cmsg_data[] | +---------------------+-----------+-----------+-----------------+ | SHIM_LOC_LOCAL_RECV | | o | *1 | | SHIM_LOC_PEER_RECV | | o | *1 | | SHIM_LOC_LOCAL_SEND | o | | *1 | | SHIM_LOC_PEER_SEND | o | | *1 | | SHIM_FEEDBACK | o | | shim_feedback{} | +---------------------+-----------+-----------+-----------------+ Table 2: Shim specific ancillary data *1: cmsg_data[] should include padding (if necessary) and a single sockaddr_in{}/sockaddr_in6{}. It should be noted that the above ancillary data can only be handled in UDP and raw sockets, not in TCP sockets because there is no one- to-one mapping of send/receive operations and the TCP segments being transmitted/received. Komu, et al. Expires August 26, 2008 [Page 25] Internet-Draft Multihoming Shim API February 2008 6.1. Get Locator Information from Incoming Packet Application can get locator information from the received IP packet by specifying the shim specific socket options for the socket. When SHIM_LOC_LOCAL_RECV and/or SHIM_LOC_PEER_RECV socket options are set, the application can retrieve local and/or remote locator from the ancillary data. 6.2. Specify Locator Information for Outgoing Packet Application can specify the locators to be used for transmitting an IP packet by sendmsg(). When ancillary data of cmsg_type SHIM_LOC_LOCAL_SEND and/or SHIM_LOC_PEER_SEND are specified, the application can explicitly specify source and/or destination locators to be used for the communication over the socket. In addition, the application can specify the outgoing interface by SHIM_IF_SEND ancillary data. The ancillary data should contain the interface identifier of the physical interface over which the application expects the packet to be transmitted. Note that the effect is limited to the datagram transmitted by the sendmsg(). If the specified locator pair seems to be valid, the shim layer overrides the locator of the IP packet as requested. An error EINVALIDLOCATOR will be returned when validation of the specified locator failed. 6.3. Notification from Application to Multihoming Shim Application may provide feedback to the shim layer about the communication status. Such feedback is particularly useful for the shim layer in the absence of REAP mechanism to monitor the reachability status of currently used locator pair in a given shim context. The notification can be made by sendmsg() specifying a new ancillary data called SHIM_FEEDBACK. The ancillary data can be handled by specifying SHIM_FEEDBACK option in cmsg_type. An error ENOENT will be returned when there is no context associated with the socket. See Section 7.3 for details of the data structure to be used. Note that this specification does not specify the exact behavior of the shim layer when a feedback information is given by an application. Komu, et al. Expires August 26, 2008 [Page 26] Internet-Draft Multihoming Shim API February 2008 7. Data Structures In this section, data structures specifically defined for the multihoming shim layer are introduced. Those data structure are either used as a parameter for setsockopt()/getsockopt() (as mentioned in Section 5) or a parameter for ancillary data to be processed by sendmsg()/recvmsg() (as mentioned in Section 6). 7.1. Placeholder for Locator Information As defined in Section 5, the SHIM_LOC_LOCAL_PREF, SHIM_LOC_PEER_PREF, SHIM_LOCLIST_LOCAL, and SHIM_LOCLIST_PEER socket options need to handle one or more locator information. Locator information includes not only the locator itself but also additional information about the locator which is useful for locator management. A new data structure is defined to serve as a placeholder for the locator information. Figure 23 illustrates the data structure called shim_locator which stores a locator information. struct shim_locator { uint8_t lc_family; /* address family */ uint8_t lc_ifidx; /* interface index */ uint8_t lc_flags; /* flags */ uint8_t lc_preference; /* preference value */ uint8_t lc_addr[16]; /* address data */ }; Figure 23: shim locator structure lc_family Address family of the locator (e.g. AF_INET, AF_INET6). It is required that the parameter contains non-zero value indicating the exact address family of the locator. lc_ifidx Interface index of the network interface to which the locator is assigned. This field should be valid only in read (getsockopt()) operation. lc_flags Each bit of the flags represents a specific characteristics of the locator. HBA is defined as 0x01. CGA is defined as 0x02. The other bits are TBD. lc_preference Indicates preference of the locator. The preference is represented by integer. Komu, et al. Expires August 26, 2008 [Page 27] Internet-Draft Multihoming Shim API February 2008 lc_addr Contains the locator. For the cases where a locator whose size is smaller than 16 bytes, encoding rule should be provided for each locator of a given address family. For instance, in case of AF_INET (IPv4), the first 4 bytes of lc_addr should contain the IPv4 address. 7.2. Path Exploration Parameter As defined in Section 5, SHIM_PATHEXPLORE allows application to set or read the parameters for path exploration and failure detection. A new data structure called shim_pathexplore is defined to store the necessary parameters. Figure 24 illustrates the data structure. The data structure can be used by getsockopt() or setsockopt() as an argument. struct shim_pathexplore { uint8_t pe_probenum; /* # of initial probe */ uint8_t pe_keepaliveto; /* Keepalive Timeout */ uint16_t pe_initprobeto; /* Initial Probe Timeout */ uint32_t pe_reserved; /* reserved */ }; Figure 24: path explore structure pe_probenum Indicates the number of initial probe messages to be sent. Default value of this parameter should follow what is specified in [I-D.ietf-shim6-failure-detection]. pe_keepaliveto Indicates timeout value for detecting a failure when the host does not receive any packets for a certain period of time while there is outbound traffic. When the timer expires, path exploration procedure will be carried out by sending a REAP Probe message. Default value of this parameter should follow what is specified in [I-D.ietf-shim6-failure-detection]. pe_initprobeto Indicates retransmission timer of REAP Probe message in milliseconds. Note that this timer is applied before exponential back-off is started. A REAP Probe message for the same locator pair may be retransmitted. Default value of this parameter should follow what is specified in [I-D.ietf-shim6-failure-detection]. pe_reserved A reserved field for future extension. By default, the field should be initialized with zero. Komu, et al. Expires August 26, 2008 [Page 28] Internet-Draft Multihoming Shim API February 2008 7.3. Feedback Information As mentioned in Section 6.3, applications can inform the shim layer about the status of unicast reachability of the locator pair currently in use. The feedback information can be handled by using ancillary data called SHIM_FEEDBACK. A new data structure named shim_feedback is illustrated in Figure 25). struct shim_feedback { uint8_t fb_direction; /* direction of traffic */ uint8_t fb_indicator; /* indicator (1-3) */ uint16_t fb_reserved; /* reserved */ }; Figure 25: feedback information structure direction Indicates direction of reachability between a locator pair in question. A value 0 indicates outbound and a value 1 indicates inbound. indicator A value indicating the degree of satisfaction of a unidirectional reachability for a given locator pair. * 0: Default value. Whenever this value is specified the feedback information must not be processed by the shim layer. * 1: Unable to connect. There is no unidirectional reachability between the locator pair in question. * 2: Unsatisfactory. The application is not satisfied with the unidirectional reachability between the locator pair in question. * 3: Satisfactory. There is satisfactory unidirectional reachability between the locator pair in question. reserved Reserved field. Must be ignored by the receiver. 8. Implications for Existing Socket API Extensions Some of the socket options defined in this document have some overlapping with existing socket API and care should be made for the usage not to confuse the features. The socket options for requesting specific locators to be used for a given transaction (SHIM_LOC_LOCAL_PREF and SHIM_LOC_PEER_PREF) are semantically similar to the existing socket API (IPV6_PKTINFO). The socket options for obtaining the locator information from the received IP packet (SHIM_LOC_LOCAL_RECV and SHIM_LOC_PEER_RECV) are semantically similar to the existing socket API (IP_RECVDSTADDR and Komu, et al. Expires August 26, 2008 [Page 29] Internet-Draft Multihoming Shim API February 2008 IPV6_PKTINFO). In IPv4, application can obtain the destination IP address of the received IP packet (IP_RECVDSTADDR). If the shim layer performs identifier/locator adaptation for the received packet, the destination EID should be stored in the ancillary data (IP_RECVDSTADDR). In IPv6, [RFC3542] defines that IPV6_PKTINFO can be used to specify source IPv6 address and the outgoing interface for outgoing packets, and retrieve destination IPv6 address and receiving interface for incoming packets. This information is stored in ancillary data being IPV6_PKTINFO specified as cmsg_type. Existing socket API should continue to work above the shim layer, that is, the IP addresses handled in IPV6_PKTINFO should be EIDs, not the locators. Baseline is that the above existing socket API (IP_RECVDSTADDR and IPV6_PKTINFO) is assumed to work above the multihoming shim layer. In other words, the IP addresses those socket options deal with are EIDs rather than locators. 9. Resolving Conflicts with Preference Values Since the multihoming shim API allows application to specify preference value for the context which is associated with the socket instance, there may be a conflict with preference values specified by different applications. For instance, application A and B may establish communication over the same EID pair while each application have different preference in their choice of local locator. SHIM6 supports a notion of forking context in which a context is split when there is a conflict with preference values specified by multiple applications. Thus, context forking can simply resolve the conflicting situation which may be caused by the use of socket options for multihoming shim layer. 9.1. Implicit Forking Socket options defined in Section 5 may cause conflicting situation when the target context is shared by multiple applications. In such case, socket handler and the multihoming shim layer should react as follows; socket handler should inform the shim layer that context forking is required. In SHIM6, when a context is forked, an unique identifier called Forked Instance Identifier (FII) is assigned to the newly forked context. The forked context is then exclusively associated with the socket through which non-default preference value was specified. The forked context is maintained by the multihoming Komu, et al. Expires August 26, 2008 [Page 30] Internet-Draft Multihoming Shim API February 2008 shim layer during the lifetime of associated socket instance. When the socket is closed, the multihoming shim layer SHOULD delete associated context. In this way, garbage collection can be carried out to cleanup unused forked contexts. Upon garbage collection, every forked context SHOULD be checked if there is no socket (process) associated with the context. If there is none, the forked context should be deleted. When a forked context is torn down, SHIM6 should notify the peer about the deletion of forked context. As opposed to socket options, context forking MUST NOT be triggered by any use of ancillary data that are specific to multihoming shim defined in Section 6. 10. Discussion In this section, open issues are introduced. 10.1. Naming at Socket Layer getsockname() and getpeername() system calls are used to obtain the 'name' of endpoint which is actually a pair of IP address and port number assigned to a given socket. getsockname() is used when an application wants to obtain the local IP address and port number assigned for a given socket instance. getpeername() is used when an application wants to obtain the remote IP address and port number. The above is based on a traditional system model of the socket API where an IP address is expected to play both the role of identifier and the role of locator. In a system model where a shim layer exists inside the IP layer, both getsockname() and getpeername() deal with identifiers, namely EIDs. In this sense, the shim layer serves to (1) hide locators and (2) provide access to the identifier for the application over the legacy socket APIs. 10.2. Additional Requirements from Application At the moment, it is not certain if following requirements are common in all the multihomed environments (SHIM6 and HIP). These are mainly identified during discussions made on SHIM6 WG mailing list. o The application should be able to set preferences for the locators, local and remote one and also to the preferences of the local locators that will be passed to the peer. Komu, et al. Expires August 26, 2008 [Page 31] Internet-Draft Multihoming Shim API February 2008 10.3. Issues of Header Conversion among Different Address Family The shim layer performs identifier/locator adaptation. Therefore, in some case, the whole IP header can be replaced with new IP header of a different address family (e.g. conversion from IPv4 to IPv6 or vice versa). Hence, there is an issue how to make the conversion with minimum impact. Note that this issue is common in other protocol conversion such as SIIT[RFC2765]. As addressed in SIIT specification, some of the features (IPv6 routing headers, hop-by-hop extension headers, or destination headers) from IPv6 are not convertible to IPv4. In addition, notion of source routing is not exactly the same in IPv4 and IPv6. Hence, there is certain limitation in protocol conversion between IPv4 and IPv6. The question is how should the shim layer behave when it is face with limitation problem of protocol conversion. Should we introduce new error something like ENOSUITABLELOCATOR ? 10.4. Handling of Unknown Locator Provided by Application There might be a case where application provides the shim layer new locator with the SHIM_LOC_*_PREF socket options or SHIM_LOC_*_SEND ancillary data. Then there is a question how should the shim layer treat the new locator informed by the application. In principle, locator information are exchanged by the shim protocol. However, there might be a case where application acquires information about the locator and prefers to use it for its communication. 11. Changes 11.1. Changes from version 00 to version 01 The followings are changes from version 00 to version 01: o Define shim_locator{} data type which is a placeholder for locator. o Define shim_pathexplore{} data type in which a set of REAP parameters are stored. o Remove descriptions about "stickiness" of socket options. o Deprecate SHIM_IF_RECV and SHIM_IF_SEND socket options. o Give default value and how to disable given socket option. Komu, et al. Expires August 26, 2008 [Page 32] Internet-Draft Multihoming Shim API February 2008 11.2. Changes from version 01 to version 02 The followings are changes from version 01 to version 02: o Add section describing context forking. o Rephrase conclusion section. o Separate normative references from informative references. o Remove texts from discussion section that are not relevant to the contents of the document. o Add section describing change history (this section). 11.3. Changes from version 02 to version 03 The followings are changes from version 02 to version 03: o Add an Appendix section describing the issue of context forking. 11.4. Changes from version 03 to version 04 The followings are changes from version 03 to version 04: o Updated reference. o Correct typo and grammatical errors. 11.5. Changes from version 04 to version 05 The followings are changes from version 04 to version 05: o Added definition of SHIM_FEEDBACK ancillary data. o Added an example of code using the SHIM_LOCLIST_LOCAL o Added SHIM_LOC_LOCAL_SEND and SHIM_LOC_PEER_SEND socket options. 12. IANA Considerations This document contains no IANA consideration. 13. Security Considerations This document does not specify any security mechanism for the shim layer. Fundamentally, the shim layer has a potential to impose security threats, as it changes the source and/or destination IP addresses of the IP packet being sent or received. Therefore, the basic assumption is that the security mechanism defined in each protocol of the shim layer is strictly applied. 14. Conclusion In this document, the Application Program Interface (API) for multihoming shim layer is specified. The socket API allows Komu, et al. Expires August 26, 2008 [Page 33] Internet-Draft Multihoming Shim API February 2008 applications to have additional control of the locator management and interface to the REAP mechanism inside the multihoming shim layer. Socket options for multihoming shim layer can be used by getsockopt() and/or setsockopt() system calls. Besides, applications can use some ancillary data that are specific to multihoming shim layer to get locator from received packet or to set locator for outgoing packet. From an architectural point of view, the socket API provides extends the existing socket API framework in the face of ID/Locator separation. With regard to API that relate to IP address management, it is assured that existing socket API continue to work above the shim layer dealing with identifiers, while multihoming shim API deals with locators. 15. Acknowledgments Authors would like to thank Jari Arkko who participated in the discussion that lead to the first version of this document, and Tatuya Jinmei who thoroughly reviewed the early version of this draft and provided detailed comments on socket API related issues. Thomas Henderson provided valuable comments especially from HIP perspectives. 16. References 16.1. Normative References [I-D.ietf-shim6-failure-detection] Arkko, J. and I. Beijnum, "Failure Detection and Locator Pair Exploration Protocol for IPv6 Multihoming", draft-ietf-shim6-failure-detection-10 (work in progress), January 2008. [I-D.ietf-shim6-proto] Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim protocol", draft-ietf-shim6-proto-09 (work in progress), October 2007. [POSIX] "IEEE Std. 1003.1-2001 Standard for Information Technology -- Portable Operating System Interface (POSIX). Open group Technical Standard: Base Specifications, Issue 6, http://www.opengroup.org/austin", December 2001. [RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei, "Advanced Sockets Application Program Interface (API) for Komu, et al. Expires August 26, 2008 [Page 34] Internet-Draft Multihoming Shim API February 2008 IPv6", RFC 3542, May 2003. [RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol (HIP) Architecture", RFC 4423, May 2006. 16.2. Informative References [I-D.ietf-shim6-app-refer] Nordmark, E., "Shim6 Application Referral Issues", draft-ietf-shim6-app-refer-00 (work in progress), July 2005. [I-D.ietf-shim6-hba] Bagnulo, M., "Hash Based Addresses (HBA)", draft-ietf-shim6-hba-05 (work in progress), December 2007. [RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm (SIIT)", RFC 2765, February 2000. [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", RFC 3972, March 2005. Appendix A. Context Forking In this section, an issue concerning context forking and its relation to the multihoming shim API are discussed. SHIM6 supports a notion of context forking. A peer may decide to fork a context for certain reason (e.g. upper layer protocol prefers to use different locator pair than the one defined in available context). The procedure of forking context is done similar to the normal context establishment, performing the 4-way message exchange. A peer who has decided to fork a context initiates the context establishment. Hereafter, we call this peer initiator. Once the forked context is established between the peers, on the initiator side, it is possible to apply forked context to the packet flow since the system maintains an association between the forked context and the socket owned by the application that has requested the context forking. How this association is maintained is implementation specific issue. However, on the responder side, there is a question on how the outbound packet can be multiplexed by the shim layer. Since there are more than one SHIM6 contexts that match with the ULID pair of the packet flow. There is a need to differentiate packet flows not only by the ULID pairs but some other information and associate a given packet flow with specific context. Komu, et al. Expires August 26, 2008 [Page 35] Internet-Draft Multihoming Shim API February 2008 Figure 26 gives an example of a scenario where two communicating peers fork a context. Initially, there has been a single transaction between the peers, by the application 1 (App1). Accordingly, another transaction is started, by application 2 (App2). Both of the transactions are made based the same ULID pair. The first context pair (Ctx1) is established for the transaction of App1. Given the requests from App2, the shim layer on Peer 1 decides to fork a context. Accordingly, a forked context (Ctx2) is established between the peers, which should be exclusively applied to the transaction of App2. Ideally, multiplexing and demultiplexing of packet flows that relate to App1 and App2 should be done as illustrated in Figure 26. However, as mentioned earlier, on the responder side, there is a problem with multiplexing the outbound packet flows of App1 and App2. Peer 1 Peer 2 (initiator) (responder) +----+ +----+ +----+ +----+ |App1| |App2| |App1| |App2| +----+ +----+ +----+ +----+ |^ |^ ^| ^| v| v| |v |v -----S1-------------S2----- -----S1-------------S2----- || || || || || || || || Ctx1 Ctx2 Ctx1 Ctx2 ULID: ULID: ULID: ULID: Loc: Loc: Loc: Loc: FII: 0 FII: 100 FII: 0 FII: 100 |^ |^ ^| ^| || || || || || || || || \..............||........................../| || \.............||.........................../ || || || \|........................................./| \........................................../ Figure 26: context forking To overcome the problem mentioned above, there are some solutions. One viable approach is to let the system implicitly maintain an association between the socket and the associated context by keeping the record of inbound packet processing. That is, the system stores the information about the context on which the inbound packet flow Komu, et al. Expires August 26, 2008 [Page 36] Internet-Draft Multihoming Shim API February 2008 was demultiplexed. The information comprises the ULID pair and FII of the context and is stored in the socket instance. Later, the system can use the information to identify the associated context in outbound packet processing. This approach should be feasible as far as there is bi-directional user traffic. Another viable approach is to extend SHIM6 protocol by adding capability of exchanging additional information to identify the packet flow from others which needs to be handled by a newly forked context. The information exchange can be done during the context establishment. The initiator appends 5 tuple of the packet flow to be handled by the newly forked context. Note that the additional information provided by the 5 tuple are source and destination port numbers and upper layer protocol. The information is later used by the shim layer to multiplex the outbound packet flow on the responder side. The socket options for multihoming shim can be used by the application to trigger the context forking in implicit manner. The peer becomes an initiator in the establishment of the forked context. Once the forked context is established between the peers, application on each end can influence the preference on context by utilizing the multihoming shim API. Authors' Addresses Miika Komu Helsinki Institute for Information Technology Tammasaarenkatu 3 Helsinki Finland Phone: +358503841531 Fax: +35896949768 Email: miika@iki.fi URI: http://www.hiit.fi/ Komu, et al. Expires August 26, 2008 [Page 37] Internet-Draft Multihoming Shim API February 2008 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 Komu, et al. Expires August 26, 2008 [Page 38] Internet-Draft Multihoming Shim API February 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Komu, et al. Expires August 26, 2008 [Page 39]