Internet Draft Network Working Group T. Bradley INTERNET-DRAFT Avici Systems, Inc. Obsoletes: 1293 C. Brown <draft-ietf-ion-inarp-update-01.txt> Fore Systems, Inc. A. Malis Cascade Communications Corp. May 7, 1997 Expires November 7, 1997 Inverse Address Resolution Protocol 1. Status of this Memo This document is an Internet-Draft. 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.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). This draft specifies an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited. 2. Abstract This memo describes additions to ARP that will allow a station to request a protocol address corresponding to a given hardware address. Specifically, this applies to Frame Relay stations that may have a Data Link Connection Identifier (DLCI), the Frame Relay equivalent of a hardware address, associated with an established Permanent Virtual Circuit (PVC), but do not know the protocol address of the station on the other side of this connection. It will also apply to other networks with similar circumstances. 3. Conventions Bradley, Brown, Malis Expires November 7, 1997 [Page 1] INTERNET-DRAFT Inverse ARP May 7, 1997 The following language conventions are used in the items of specification in this document: o Must, Will, Shall or Mandatory -- the item is an absolute requirement of the specification. o Should or Recommended -- the item should generally be followed for all but exceptional circumstances. o May or Optional -- the item is truly optional and may be followed or ignored according to the needs of the implementor. 4. Introduction This document will rely heavily on Frame Relay as an example of how the Inverse Address Resolution Protocol (InARP) can be useful. It is not, however, intended that InARP be used exclusively with Frame Relay. InARP may be used in any network that provides destination hardware addresses without indicating corresponding protocol addresses. 5. Motivation The motivation for the development of Inverse ARP is a result of the desire to make dynamic address resolution within Frame Relay both possible and efficient. Permanent virtual circuits (PVCs) and eventually switched virtual circuits (SVCs) are identified by a Data Link Connection Identifier (DLCI). These DLCIs define a single virtual connection through the wide area network (WAN) and may be thought of as the Frame Relay equivalent to a hardware address. Periodically, through the exchange of signaling messages, a network may announce a new virtual circuit with its corresponding DLCI. Unfortunately, protocol addressing is not included in the announcement. The station receiving such an indication will learn of the new connection, but will not be able to address the other side. Without a new configuration or a mechanism for discovering the protocol address of the other side, this new virtual circuit is unusable. Other resolution methods were considered to solve the problems, but were rejected. Reverse ARP [4], for example, seemed like a good candidate, but the response to a request is the protocol address of the requesting station, not the station receiving the request. IP specific mechanisms were limiting since they would not allow resolution of other protocols other than IP. For this reason, the ARP protocol was expanded. Bradley, Brown, Malis Expires November 7, 1997 [Page 2] INTERNET-DRAFT Inverse ARP May 7, 1997 Inverse Address Resolution Protocol (InARP) will allow a Frame Relay station to discover the protocol address of a station associated with the virtual circuit. It is more efficient than sending ARP messages on every VC for every address the system wants to resolve and it is more flexible than relying on static configuration. 6. Packet Format Inverse ARP is an extension of the existing ARP. Therefore, it has the same format as standard ARP. ar$hrd 16 bits Hardware type ar$pro 16 bits Protocol type ar$hln 8 bits Byte length of each hardware address (n) ar$pln 8 bits Byte length of each protocol address (m) ar$op 16 bits Operation code ar$sha nbytes source hardware address ar$spa mbytes source protocol address ar$tha nbytes target hardware address ar$tpa mbytes target protocol address Possible values for hardware and protocol types are the same as those for ARP and may be found in the current Assigned Numbers RFC [2]. Length of the hardware and protocol address are dependent on the environment in which InARP is running. For example, if IP is running over Frame Relay, the hardware address length is either 2, 3, or 4, and the protocol address length is 4. The operation code indicates the type of message, request or reply. InARP request = 8 InARP reply = 9 These values were chosen so as not to conflict with other ARP extensions. 7. Protocol Operation Basic InARP operates essentially the same as ARP with the exception that InARP does not broadcast requests. This is because the hardware address of the destination station is already known. When an interface supporting InARP becomes active, it should initiate the InARP protocol and format InARP requests for each active PVC for which InARP is active. To do this, a requesting station simply formats a request by inserting its source hardware, source protocol addresses and the known target hardware address. It then zero fills Bradley, Brown, Malis Expires November 7, 1997 [Page 3] INTERNET-DRAFT Inverse ARP May 7, 1997 the target protocol address field. Finally, it will encapsulate the packet for the specific network and send it directly to the target station. Upon receiving an InARP request, a station may put the requester's protocol address/hardware address mapping into its ARP cache as it would any ARP request. Unlike other ARP requests, however, the receiving station may assume that any InARP request it receives is destined for it. For every InARP request, the receiving station should format a proper reply using the source addresses from the request as the target addresses of the reply. If the station is unable or unwilling to reply, it ignores the request. When the requesting station receives the InARP reply, it may complete the ARP table entry and use the provided address information. Note: as with ARP, information learned via InARP may be aged or invalidated under certain circumstances. 7.1. Operation with Multi-Addressed Hosts In the context of this discussion, a multi-addressed host will refer to a host that has multiple protocol addresses assigned to a single interface. If such a station receives an InARP request, it must choose one address with which to respond. To make such a selection, the receiving station must first look at the protocol address of the requesting station, and then respond with the protocol address corresponding to the network of the requester. For example, if the requesting station is probing for an IP address, the responding multi-addressed station should respond with an IP address which corresponds to the same subnet as the requesting station. If the station does not have an address that is appropriate for the request it should not respond. In the IP example, if the receiving station does not have an IP address assigned to the interface that is a part of the requested subnet, the receiving station would not respond. A multi-addressed host should send an InARP request for each of the addresses defined for the given interface. It should be noted, however, that the receiving side may answer some or none of the requests depending on its configuration. 7.2. Protocol Operation Within Frame Relay One case where Inverse ARP can be used is on a frame relay interface which supports signaling of DLCIs via a data link management interface. An InARP equipped station connected to such an interface will format an InARP request and address it to the new virtual circuit. If the other side supports InARP, it may return a reply indicating the protocol address requested. Bradley, Brown, Malis Expires November 7, 1997 [Page 4] INTERNET-DRAFT Inverse ARP May 7, 1997 In a frame relay environment, InARP packets are encapsulated using the NLPID/SNAP format defined in [3] which indicates the ARP protocol. Specifically, the packet encapsulation will be as follows: +----------+----------+ | Q.922 address | +----------+----------+ |ctrl 0x03 | pad 00 | +----------+----------+ |nlpid 0x80| oui 0x00 | +----------+ + | oui (cont) 0x00 00 | +----------+----------+ | pid 0x08 06 | +----------+----------+ | . | | . | The format for an InARP request itself is defined by the following: ar$hrd - 0x000F the value assigned to Frame Relay ar$pro - protocol type for which you are searching (i.e. IP = 0x0800) ar$hln - 2,3, or 4 byte addressing length ar$pln - byte length of protocol address for which you are searching (for IP = 4) ar$op - 8; InARP request ar$sha - Q.922 address of requesting station ar$spa - protocol address of requesting station ar$tha - Q.922 addressed of newly announced virtual circuit ar$tpa - 0; This is what is being requested The InARP response will be completed similarly. ar$hrd - 0x000F the value assigned to Frame Relay ar$pro - protocol type for which you are searching (i.e. IP = 0x0800) ar$hln - 2,3, or 4 byte addressing length ar$pln - byte length of protocol address for which you are searching (for IP = 4) ar$op - 9; InARP response ar$sha - Q.922 address of responding station ar$spa - protocol address requested ar$tha - Q.922 address of requesting station ar$tpa - protocol address of requesting station Note that the Q.922 addresses specified have the C/R, FECN, BECN, and Bradley, Brown, Malis Expires November 7, 1997 [Page 5] INTERNET-DRAFT Inverse ARP May 7, 1997 DE bits set to zero. Procedures for using InARP over a Frame Relay network are identical to those for using ARP and RARP discussed in [3]. 8. References [1] Plummer, D., "An Ethernet Address Resolution Protocol - or - Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware", STD 37, RFC 826, MIT, November 1982. [2] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, USC/Information Sciences Institute, October 1994 [3] Bradley, T., Brown, C., Malis, A., "Multiprotocol Interconnect over Frame Relay", RFC 1490, July 1993. [4] Finlayson, R., Mann, R., Mogul, J., and M. Theimer, "A Reverse Address Resolution Protocol", STD 38, RFC 903, Stanford University, June 1984. 9. Security Considerations Security issues are not addressed in this memo. 10. Authors' Addresses Terry Bradley Avici Systems, Inc. 12 Elizabeth Drive Chelmsford, MA 01824 Phone: (508) 250-3344 Email: tbradley@avici.com Caralyn Brown FORE Systems, Inc. 1 Corporate Drive Andover, MA 01810 Phone: (508) 689-2400 x133 Email: cbrown@fore.com Andrew Malis Cascade Communications Corp. 5 Carlisle Road Westford, MA 01886 Phone: (508) 952-7414 Email: malis@casc.com Bradley, Brown, Malis Expires November 7, 1997 [Page 6] INTERNET-DRAFT Inverse ARP May 7, 1997 Bradley, Brown, Malis Expires November 7, 1997 [Page 7]