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Network Working Group Mark Laubach INTERNET DRAFT Hewlett-Packard Laboratories Expires April 1, 1994 October 1, 1993 <draft-ietf-atm-classic-ip-04.txt> Classical IP and ARP over ATM Status of this Memo This memo 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress". Please check the lid-abstracts.txt listing contained in the internet-drafts shadow directories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net, ftp.nisc.src.com, or munnari.oz.au to learn the current status of any Internet Draft. 1. Abstract This memo defines an initial application of classical IP, ARP, and Inverse ARP in an Asynchronous Transfer Mode (ATM) network environment configured as a Logical IP Subnetwork (LIS) as described below and in [1]. This memo does not preclude the subsequent development of ATM technology into areas other than a LIS; specifically, as single ATM networks grow to replace many ethernet local LAN segments and as these networks become globally connected, the application of IP and ARP will be treated differently. This memo considers only the application of ATM as a direct replacement for the "wires" and local LAN segments connecting IP end-stations ("members") and routers. Issues raised by MAC level bridging and LAN emulation are beyond the scope of this paper. This memo introduces general ATM technology and nomenclature. Readers are encouraged to review the ATM Forum and ITU-TS (formerly CCITT) references for more detailed information about ATM implementation agreements and standards. Laubach [Page 1] DRAFT Classical IP and ARP over ATM October 1993 3. Acknowledgment This memo could not have come into being without the critical review from Jim Forster of Cisco Systems, Drew Perkins of FORE Systems, and Bryan Lyles, Steve Deering, and Berry Kercheval of XEROX PARC. The concepts and models presented in [1], written by Dave Piscitello and Joseph Lawrence, laid the structural groundwork for this work. This document could have not been completed without the expertise of the IP over ATM Working Group of the IETF and the ad hoc PVC committee at the Amsterdam IETF meeting. 4. Conventions The following language conventions are used in the items of specification in this document: o MUST, SHALL, or MANDATORY -- the item is an absolute requirement of the specification. o SHOULD or RECOMMEND -- this 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. 5. Introduction The goal of this specification is to allow compatible and interoperable implementations for transmitting IP datagrams, ARP and InARP requests and replies over ATM Adaptation Layer 5 (AAL5)[6]. Note: this memo defines only the operation of IP and ARP over ATM, and is not meant to describe the operation of ATM networks. Any reference to virtual connections, permanent virtual connections, or switched virtual connections applies only to virtual channel connections used to support IP and ARP over ATM, and thus are assumed to be using AAL5. This memo places no restrictions or requirements on virtual connections used for other purposes. Initial deployment of ATM provides a LAN segment replacement for local area networks (e.g., Ethernets, Token Rings and FDDI), as a local-area backbone between existing (non-ATM) LANs, and as replacement for dedicated circuits or frame relay PVCs between IP routers. In the former case, local IP routers with one or more ATM interfaces will connect islands of ATM networks. In the latter case, public or private ATM Wide Area networks will be used to connect IP routers, which in turn may or may not connect to local ATM networks. ATM WANs and LANs may be interconnected. Laubach [Page 2] DRAFT Classical IP and ARP over ATM October 1993 Private ATM networks (local or wide area) will use the private ATM address structure specified in the ATM Forum UNI specification [9]. This structure is modeled after the format of an OSI Network Service Access Point Address. A private ATM address uniquely identifies an ATM endpoint. Public networks will use either the address structure specified in ITU-TS recommendation E.164 or the private network ATM address structure. An E.164 address uniquely identifies an interface to a public network. The characteristics and features of ATM networks are different than those found in LANs: o ATM provides a Virtual Connection (VC) switched environment. VC setup may be done on either a Permanent Virtual Connection (PVC) or dynamic Switched Virtual Connection (SVC) basis. SVC call management signalling is performed via implementations of the Q.93B protocol [7,9]. o Data to be passed by a VC is segmented into 53 octet quantities called cells (5 octets of ATM header and 48 octets of data). o The function of mapping user PDUs (Protocol Data Unit) into the information field of the ATM cell and vice versa is performed in the ATM Adaptation Layer (AAL). When a VC is created a specific AAL type is associated with the VC. There are four different AAL types, which are referred to individually as "AAL1", "AAL2", "AAL3/4", and "AAL5". (Note: this memo concerns itself with the mapping of IP and ARP over AAL5 only. The other AAL types are mentioned for introductory purposes only.) The AAL type is known by the VC end points via the call setup mechanism and is not carried in the ATM cell header. For PVCs the AAL type is administratively configured at the end points when the Connection (circuit) is set up. For SVCs, the AAL type is communicated along the VC path via Q.93B as part of call setup establishment and the end points use the signaled information for configuration. ATM switches generally do not care about the AAL type of VCs. The AAL5 format specifies a packet format with a maximum size of (64K - 1) octets of user data. Cells for an AAL5 PDU are transmitted first to last, the last cell indicating the end of the PDU. ATM standards guarantee that on a given VC, cell ordering is preserved end-to-end. NOTE: AAL5 provides a non- assured data transfer service - it is up to higher-level protocols to provide retransmission. o ATM Forum signalling defines point-to-point and point-to- multipoint Connection setup [9]. Multipoint-to-multipoint VCs are not yet specified by ITU-TS or ATM Forum. Laubach [Page 3] DRAFT Classical IP and ARP over ATM October 1993 o An ATM Forum ATM endpoint address is either encoded as an NSAP, or is an E.164 Public-UNI address [9]. In some cases, both an ATM endpoint address and an E.164 Public UNI address are needed by an ARP client to reach another host or router. Since the use of ATM endpoint addresses and E.164 public UNI addresses by ARP are analogous to the use of Ethernet addresses, the notion of "hardware address" is extended to encompass ATM addresses in the context of ARP, even though ATM addresses need not have hardware significance. ATM Forum NSAPs use the same basic format as U.S. GOSIP NSAPs [11]. Note: ATM Forum addresses should not be construed as being U.S. GOSIP NSAPs. They are not, the administration is different, which fields get filled out are different, etc. This memo describes the initial deployment of ATM within "classical" IP networks as a direct replacement for local area networks (ethernets) and for IP links which interconnect routers, either within or between administrative domains. The "classical" model here refers to the treatment of the ATM host adapter as a networking interface to the IP protocol stack. Characteristics of the classical model are: o The same maximum transmission unit (MTU) size is used for all VCs in a LIS. o Default LLC/SNAP encapsulation of IP packets. o End-to-end IP routing architecture stays the same. o IP addresses are resolved to ATM addresses by use of an ARP service within the LIS - ARPs stay within the LIS. From a client's perspective, the ARP architecture stays essentially the same, consistent with current model. o One IP subnet is used for many hosts and routers. Each VC directly connects two IP members within the same LIS. Future memos will describe the deployment of ATM on a global scale. The "global" ATM model will be an evolution of the classical model where the ATM network becomes more fully deployed and globally available. In this model, the traditional protocol stack architecture also evolves allowing applications to map directly onto VCs (e.g., TCP and UDP ports map directly onto VCs). ARPs are no longer bound to LIS boundaries; queries and replies may traverse the globe. IP will evolve to take advantage of the network services provided by the global ATM network. Laubach [Page 4] DRAFT Classical IP and ARP over ATM October 1993 Characteristics of the global model are: o MTU size is negotiated per VC via ATM signalling. o IP encapsulation is negotiated per VC via ATM signalling; this requires common signalling to be implemented. o Applications may map directly to VCs, requiring changes to TCP/UDP/IP implementations to allow ports to map directly on to VCs o ARPs may be global, ARP architecture needs to change to support a robust global client/server model. o Differing quality of service (QOS) guarantees can be established on a per application and per VC basis. The deployment of ATM into the Internet community is just beginning and will take many years to complete. During the early part of this period, we expect deployment to follow traditional IP subnet boundaries for the following reasons: o Administrators and managers of IP subnetworks will tend to initially follow the same models as they currently have deployed. The mindset of the community will change slowly over time as ATM increases its coverage and builds its credibility. o Policy administration practices rely on the security, access, routing, and filtering capability of IP Internet gateways: i.e. firewalls. ATM will not be allowed to "back-door" around these mechanisms until ATM provides better management capability than the existing services and practices. o Standards for global IP over ATM will take some time to complete and deploy. This memo details the treatment of the classical model of IP and ARP over ATM. There are those who would like to have IP-over-ATM begin by breaking the classical model - this memo represents the view that we MUST walk before we can run. This memo does not preclude the subsequent evolution of ATM networks as they become more globally deployed and interconnected and the evolution of TCP and IP to take advantage of these changes - these will be the subject of future documents. This memo does not address issues related to transparent data link layer interoperability. Laubach [Page 5] DRAFT Classical IP and ARP over ATM October 1993 6. IP Subnetwork Configuration In the LIS scenario, each separate administrative entity configures its hosts and routers within a closed logical IP subnetwork. Each LIS operates and communicates independently of other LISs on the same ATM network. Hosts connected to ATM communicate directly to other hosts within the same LIS. Communication to hosts outside of the local LIS is provided via an IP router. This router is an ATM Endpoint attached to the ATM network that is configured as a member of one or more LISs. This configuration may result in a number of disjoint LISs operating over the same ATM network. Hosts of differing IP subnets MUST communicate via an intermediate IP router even though it may be possible to open a direct VC between the two IP members over the ATM network. The requirements for IP members (hosts, routers) operating in an ATM LIS configuration are: o All members have the same IP network/subnet number and address mask [8]. o All members within a LIS are directly connected to the ATM network. o All members outside of the LIS are accessed via a router. o All members of a LIS MUST have a mechanism for resolving IP addresses to ATM addresses via ARP [3] and vice versa via InARP[12] when using SVCs. o All members of a LIS MUST have a mechanism for resolving VCs to IP addresses via InARP [12] when using PVCs. o All members within a LIS MUST be able to communicate via ATM with all other members in the same LIS; i.e., the virtual Connection topology underlying the intercommunication among the members is fully meshed. The following list identifies a set of ATM specific parameters that MUST be implemented in each IP station connected to the ATM network: o ATM Hardware Address (atm$ha). The ATM address of the individual IP station. o ATM ARP Request Address (atm$arp-req). atm$arp-req is the ATM address of an individual ARP server located within the LIS. In an SVC environment, ARP requests are sent to this address for the resolution of target protocol addresses to target ATM addresses. Laubach [Page 6] DRAFT Classical IP and ARP over ATM October 1993 That server MUST have authoritative responsibility for resolving ARP requests of all IP members within the LIS. Note: if the LIS is operating with PVCs only, then this parameter may be set to null and the IP station is not required to send ARP requests to the ARP server. It is RECOMMENDED that routers providing LIS functionality over the ATM network also support the ability to interconnect multiple LISs. Routers that wish to provide interconnection of differing LISs MUST be able to support multiple sets of these parameters (one set for each connected LIS) and be able to associate each set of parameters with a specific IP network/ subnet number. In addition, it is RECOMMENDED that a router be able to provide this multiple LIS support with a single physical ATM interface that may have one or more individual ATM endpoint addresses. Note: this does not necessarily mean different ESIs (IEEE MAC addresses) when NSAPS are used. The last octet of the NSAP is the "Selector" field which can be used to differentiate up to 256 different LISs. 7. Packet Format Implementations MUST support IEEE 802.2 LLC/SNAP encapsulation as described in [2]. LLC/SNAP encapsulation is the default packet format for IP datagrams. This memo recognizes that other encapsulation methods may be used however, in the absence of other knowledge or agreement, LLC/SNAP encapsulation is the default. This memo recognizes the future deployment of end-to-end signalling within ATM that will allow negotiation of encapsulation method on a per-VC basis. Signalling negotiations are beyond the scope of this memo. 8. MTU Size The default MTU size for IP members operating over the ATM network SHALL be 9180 octets. The LLC/SNAP header is 8 octets, therefore the default ATM AAL5 protocol data unit size is 9188 octets. In classical IP subnets, values other than the default can only be used if and only if all members in the LIS have been configured to use the non-default value. This memo recognizes the future deployment of end-to-end signalling within ATM that will allow negotiation of MTU size on a per-VC basis. Signalling negotiations are beyond the scope of this document. Laubach [Page 7] DRAFT Classical IP and ARP over ATM October 1993 9. ADDRESS RESOLUTION Address resolution within an ATM logical IP subnet SHALL make use of the Address Resolution Protocol (ARP) [3] and the Inverse Address Resolution Protocol (InARP) [12]. All IP stations are required to support these protocols as updated and extended in this memo. Use of these protocols differ depending on whether PVCs or SVCs are used. Permanent Virtual Connections An IP station MUST have a mechanism (eg. manual configuration) for determining what PVCs it has, and in particular which PVCs are being used with LLC/SNAP encapsulation. The details of the mechanism are beyond the scope of this memo. All IP members supporting PVCs are required to use the Inverse Address Resolution Protocol (InARP) as defined in [12] on those VCs using LLC/SNAP encapsulation. In a strict PVC environment, the receiver SHALL infer the relevant VC from the VC on which the InARP request (InARP_REQUEST) or response (InARP_REPLY) was received. When the ATM source and/or target address is unknown, the corresponding ATM address length in the InARP packet MUST be set to zero (0) indicating a null length, otherwise the appropriate address field should be filled in and the corresponding length set appropriately. InARP packet format details are presented later in this memo. Directly from [12]: "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." It is the responsibility of each IP station supporting PVCs to re-validate ARP table entries as part of the aging process. See the section below on "ARP Table Aging". Switched Virtual Connections SVCs require support for ARP in the non-broadcast, non-multicast environment that ATM networks currently provide. To meet this need a single ARP Server MUST be located within the LIS. This server MUST have authoritative responsibility for resolving the ARP requests of all IP members within the LIS. The server itself does not actively establish connections. It depends on the clients in the LIS to initiate the ARP registration procedure. An individual client connects to the ARP server using a point-to-point VC. The server, upon the completion of an ATM call/connection of a new VC specifying LLC/SNAP encapsulation, will transmit an InARP request to determine the IP address of the client. Laubach [Page 8] DRAFT Classical IP and ARP over ATM October 1993 The InARP reply from the client contains the information necessary for the ARP Server to build its ARP table cache. This information is used to generate replies to the ARP requests it receives. The ARP Server mechanism requires that each client be administratively configured with the ATM address of the ARP Server atm$arp-req as defined earlier in this memo. There is to be one and only one ARP Server operational per logical IP subnet. It is RECOMMENDED that the ARP Server also be an IP station. This station MUST be administratively configured to operate and recognize itself as the ARP Server for a LIS. The ARP Server MUST be configured with an IP address for each logical IP subnet it is serving to support InARP requests. This memo recognizes that a single ARP Server is not as robust as multiple servers which synchronize their databases correctly. This document is defining the client-server interaction by using a simple, single server approach as a reference model, and does not prohibit more robust approaches which use the same client-server interface. ARP Server Operational Requirements The ARP server accepts ATM calls/connections from other ATM end points. At call setup and if the VC supports LLC/SNAP encapsulation, the ARP server will transmit to the originating ATM station an InARP request (InARP_REQUEST) for each logical IP subnet the server is configured to serve. After receiving an InARP reply (InARP_REPLY), the server will examine the IP address and the ATM address. The server will add (or update) the <ATM address, IP address> map entry and timestamp into its ARP table. If the InARP IP address duplicates a table entry IP address and the InARP ATM address does not match the table entry ATM address and there is an open VC associated with that table entry, the InARP information is discarded and no modifications to the table are made. ARP table entries persist until aged or invalidated. VC call tear down does not remove ARP table entries. The ARP server, upon receiving an ARP request (ARP_REQUEST), will generate the corresponding ARP reply (ARP_REPLY) if it has an entry in its ARP table. Otherwise it will generate a negative ARP reply (ARP_NAK). The ARP_NAK response is an extension to the ARP protocol and is used to improve the robustness of the ARP server mechanism. With ARP_NAK, a client can determine the difference between a catastrophic server failure and an ARP table lookup failure. The ARP_NAK packet format is the same as the received ARP_REQUEST packet format with the operation code set to ARP_NAK, i.e., the ARP_REQUEST packet data is merely copied for transmission with the ARP_REQUEST operation code reset to ARP_NAK. Laubach [Page 9] DRAFT Classical IP and ARP over ATM October 1993 Updating the ARP table information timeout, the short form: when the server receives an ARP request over a VC, where the source IP and ATM address match the association already in the ARP table and the ATM address matches that associated with the VC, the server may update the timeout on the source ARP table entry: i.e., if the client is sending ARP requests to the server over the same VC that it used to register its ARP entry, the server should examine the ARP requests and note that the client is still "alive" by updating the timeout on the client's ARP table entry. Adding robustness to the address resolution mechanism using ARP: when the server receives an ARP_REQUEST over a VC, it examines the source information. If there is no IP address associated with the VC over which the ARP request was received and if the source IP address is not associated with any other connection, then the server will add the <ATM address, IP address> entry and timestamp into its ARP table and associate the entry with this VC. ARP Client Operational Requirements The ARP client is responsible for contacting the ARP server to register its own ARP information and to gain and refresh its own ARP entry/information about other IP members. This means, as noted above, that ARP clients MUST be configured with the ATM address of the ARP server. ARP clients MUST: 1. Initiate the VC connection to the ARP server for transmitting and receiving ARP and InARP packets. 2. Respond to ARP_REQUEST and InARP_REQUEST packets received on any VC appropriately. (Refer to Section 7, "Protocol Operation" in [12].) 3. Generate and transmit ARP_REQUEST packets to the ARP server and to process ARP_REPLY and ARP_NAK packets from the server appropriately. ARP_REPLY packets should be used to build/refresh its own client ARP table entries. 4. Generate and transmit InARP_REQUEST packets as needed and to process InARP_REPLY packets appropriately. InARP_REPLY packets should be used to build/refresh its own client ARP table entries. (Refer to Section 7, "Protocol Operation" in [12].) 5. Provide an ARP table aging function to remove its own old client ARP tables entries after a convenient period of time. Note: if the client does not maintain an open VC to the server, the client MUST refresh its ARP information with the server at least once Laubach [Page 10] DRAFT Classical IP and ARP over ATM October 1993 every 20 minutes. This is done by opening a VC to the server and exchanging the initial InARP packets. ARP Table Aging An ARP client or server MUST have knowledge of any open VCs it has (permanent or switched), their association with an ARP table entry, and in particular, which VCs support LLC/SNAP encapsulation. Client ARP table entries are valid for a maximum time of 15 minutes. Server ARP table entries are valid for a minimum time of 20 minutes. Prior to aging (removing) an ARP table entry, all members MUST generate an InARP_REQUEST on any open VC associated with that entry. If an InARP_REPLY is received, that table entry is updated and not deleted. If there is no open VC associated with the table entry, the entry is deleted. ARP and InARP Packet Format Internet addresses are assigned independently of ATM addresses. Each host implementation MUST know its own IP and ATM address(es) and MUST respond to address resolution requests appropriately. IP members MUST also use ARP and InARP to resolve IP addresses to ATM addresses when needed. The ARP and InARP protocol has several fields that have the following format and values: Data: ar$hrd 16 bits Hardware type ar$pro 16 bits Protocol type ar$shtl 8 bits Type & length of source ATM number (q) ar$sstl 8 bits Type & length of source ATM subaddress (r) ar$op 16 bits Operation code (request or reply) ar$spln 8 bits Length of source protocol address (s) ar$thtl 8 bits Type & length of target ATM number (x) ar$tstl 8 bits Type & length of target ATM subaddress (y) ar$tpln 8 bits Length of target protocol address (z) ar$sha qoctets source ATM number ar$ssa roctets source ATM subaddress ar$spa soctets source protocol address ar$tha xoctets target ATM number ar$tsa yoctets target ATM subaddress ar$tpa zoctets target protocol address Laubach [Page 11] DRAFT Classical IP and ARP over ATM October 1993 Where: ar$hrd - assigned to ATM Forum address family and is dd decimal (0x00nn) [4]. ar$pro - see Assigned Numbers for protocol type number for the protocol using ARP. (IP is 0x0800). ar$op - The operation type value (decimal): ARP_REQUEST = 1 ARP_REPLY = 2 InARP_REQUEST = 8 InARP_REPLY = 9 ARP_NAK = ?? ar$spln - length in octets of the source protocol address. For IP ar$spln is 4. ar$tpln - length in octets of the target protocol address. For IP ar$tpln is 4. ar$sha - source ATM number (E.164 or ATM Forum NSAP) ar$ssa - source ATM subaddress (ATM Forum NSAP) ar$spa - source protocol address ar$tha - target ATM number (E.164 or ATM Forum NSAP) ar$tha - target ATM subaddress (ATM Forum NSAP) ar$tpa - target protocol address The encoding of the 8-bit type and length value for ar$shtl, ar$sstl, ar$thtl, and ar$tstl is as follows: MSB 8 7 6 5 4 3 2 1 LSB +-----+-----+-----+-----+-----+-----+-----+-----+ | 0 | 1/0 | Octet length of address | +-----+-----+-----+-----+-----+-----+-----+-----+ Where: bit.8 (reserved) = 0 (for future use) bit.7 (type) = 0 ATM Forum NSAP format = 1 E.164 format bit.6-1 (length) = 6 bit unsigned octet length of address (MSB = bit.6, LSB = bit.1) Laubach [Page 12] DRAFT Classical IP and ARP over ATM October 1993 ATM addresses in Q.93B (as defined by the ATM Forum UNI 3.0 signalling specification [9]) include a "Calling Party Number Information Element" and a "Calling Party Subaddress Information Element". These Information Elements (IEs) SHOULD map to ARP/InARP source ATM number and source ATM subaddress respectively. Furthermore, ATM Forum defines a "Called Party Number Information Element" and a "Called Party Subaddress Information Element". These IEs map to ARP/InARP target ATM number and target ATM subaddress respectively. The ATM Forum defines three structures for the combined use of number and subaddress [9]: ATM Number ATM Subaddress -------------- -------------- Structure 1 ATM Forum NSAP null Structure 2 E.164 null Structure 3 E.164 ATM Forum NSAP IP members MUST register with their ARP server their ATM endpoint address using the ATM address structure appropriate for their ATM network connection: i.e., LISs implemented over ATM LANs following ATM Forum UNI 3.0 should register using Structure 1; LISs implemented over an E.164 "public" ATM network should register using Structure 2. A LIS implemented over a combination of ATM LANs and public ATM networks may need to register using Structure 3. Implementations based on this memo MUST support all three ATM address structures. ARP and InARP requests and replies for ATM address structures 1 and 2 MUST indicate a null ATM subaddress; i.e. ar$sstl.type = 1 and ar$sstl.length = 0 and ar$tstl.type = 1 and ar$tstl.length = 0. Background information: the ARP packet format presented in this memo is general in nature in that the ATM number and ATM subaddress fields SHOULD map directly to the corresponding Q.93B fields used for ATM call/connection setup signalling messages. The IP over ATM Working Group expects ATM Forum NSAPs numbers (Structure 1) to predominate over E.164 numbers (Structure 2) as ATM endpoint identifiers within ATM LANs. The ATM Forum's VC Routing specification is not complete at this time and therefore its impact on the operational use of ATM Address Structure 3 is undefined. The ATM Forum will be defining this relationship in the future. It is for this reason that IP members need to support all three ATM address structures. Laubach [Page 13] DRAFT Classical IP and ARP over ATM October 1993 ARP/InARP Packet Encapsulation ARP and InARP packets are to be encoded in AAL5 PDUs using LLC/SNAP encapsulation. The format of the AAL5 CPCS-SDU payload field for ARP/InARP PDUs is: Payload Format for ARP/InARP PDUs: +------------------------------+ | LLC 0xAA-AA-03 | +------------------------------+ | OUI 0x00-00-00 | +------------------------------+ | Ethertype 0x08-06 | +------------------------------+ | | | ARP/InARP Packet | | | +------------------------------+ The LLC value of 0xAA-AA-03 (3 octets) indicates the presence of a SNAP header. The OUI value of 0x00-00-00 (3 octets) indicates that the following two-bytes is an ethertype. The Ethertype value of 0x08-06 (2 octets) indicates ARP [4]. The total size of the LLC/SNAP header is fixed at 8-octets. This aligns the start of the ARP packet on a 64-bit boundary relative to the start of the AAL5 CPCS-SDU. The LLC/SNAP encapsulation for ARP/InARP presented here is consistent with the treatment of multiprotocol encapsulation of IP over ATM AAL5 as specified in [2] and in the format of ARP over IEEE 802 networks as specified in [5]. Traditionally, ARP requests are broadcast to all directly connected IP members within a LIS. It is conceivable in the future that larger scaled ATM networks may handle ARP requests to destinations outside the originating LIS, perhaps even globally; issues raised by ARP'ing outside the LIS or by a global ARP mechanism are beyond the scope of this memo. 10. IP Broadcast Address ATM does not support broadcast addressing, therefore there are no mappings available from IP broadcast addresses to ATM broadcast services. Note: this lack of mapping does not restrict members from Laubach [Page 14] DRAFT Classical IP and ARP over ATM October 1993 transmitting or receiving IP datagrams specifying any of the four standard IP broadcast address forms as described in [8]. Members, upon receiving an IP broadcast or IP subnet broadcast for their LIS, MUST process the packet as if addressed to that station. 11. IP Multicast Address ATM does not support multicast address services, therefore there are no mappings available from IP multicast addresses to ATM multicast services. Current IP multicast implementations (i.e., MBONE and IP tunneling, see [10]) will continue to operate over ATM based logical IP subnets if operated in the WAN configuration. This memo recognizes the future development of ATM multicast service addressing by the ATM Forum. When available and widely implemented, the roll-over from the current IP multicast architecture to this new ATM architecture will be straightforward. 12. Security Not all of the security issues relating to IP over ATM are clearly understood at this time, due to the fluid state of ATM specifications, newness of the technology, and other factors. It is believed that ATM and IP facilities for authenticated call management, authenticated end-to-end communications, and data encryption will be needed in globally connected ATM networks. Such future security facilities and their use by IP networks are beyond the scope of this memo. There are known security issues relating to host impersonation via the address resolution protocols used in the Internet [13]. No special security mechanisms have been added to the address resolution mechanism defined here for use with networks using IP over ATM. 13. Open Issues o The ARP hardware address type value for ATM Forum and the ARP_NAK operation type value need yet to be assigned by the Internet Assigned Numbers Authority (IANA) o Well known ATM address(es) for ARP servers? It would be very handy if we came up with a set of well known ATM addresses for ARP services. We should probably have well-known PVC port numbers for a non-SVC environment also. o Interim Local Management Interface (ILMI) services will not be generally implemented by some providers and vendors and will not Laubach [Page 15] DRAFT Classical IP and ARP over ATM October 1993 be used to obtain the ATM address network prefix from the network [9]. Meta-signalling does provide some of this functionality and in the future we need to document the options. o There are many VC management issues which have not yet been addressed by this specification and which await the unwary implementor. For example, one problem that has not yet been resolved is how two IP members decide which of duplicate VCs can be released without causing VC thrashing. If two IP stations simultaneously established VCs to each other, it is tempting to allow only one of these VCs to be established, or to release one of these VCs immediately after it is established. If both IP stations simultaneously decide to release opposite VCs, a thrashing effect can be created where VCs are repeatedly established and immediately released. For the time being, the safest strategy is to allow duplicate VCs to be established and simply age them like any other VCs. REFERENCES [1] Piscitello, D., and Lawrence, J., "IP and ARP over the SMDS Service", RFC1209, USC/Information Sciences Institute, March 1991. [2] Heinanen, Juha, "Multiprotocol Encapsulation over ATM Adaptation Layer 5", RFC1483, USC/Information Sciences Institute, July 1993. [3] Plummer, D., "An Ethernet Address Resolution Protocol - or - Converting Network Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware", RFC 826, MIT, November 1982. [4] Reynolds, J., and Postel, J., "Assigned Numbers", RFC1340, USC/ Information Sciences Institute, July 1992. [5] Postel, J., and Reynolds, J., "A Standard for the Transmission of IP Datagrams over IEEE 802 Networks", RFC1042, USC/Information Sciences Institute, February 1988. [6] CCITT, "Draft Recommendation I.363", CCITT Study Group XVIII, Geneva, 19-29 January 1993. [7] CCITT, "Draft text for Q.93B", CCITT Study Group XI, 23 September - 2 October 1992. [8] Braden, R., "Requirements for Internet Hosts -- Communication Layers", RFC1122, USC/Information Sciences Institute, October 1989. Laubach [Page 16] DRAFT Classical IP and ARP over ATM October 1993 [9] ATM Forum, "ATM User-Network Interface Specification Version 3.0.", ATM Forum, 480 San Antonio Road, Suite 100, Mountain View, CA 94040, June 1993. [10] Deering, S, "Host Extensions for IP Multicasting", RFC1112, USC/Information Sciences Institute, August 1989. [11] Colella, Richard, and Gardner, Ella, and Callon, Ross, "Guidelines for OSI NSAP Allocation in the Internet", RFC1237, USC/Information Sciences Institute, July 1991. [12] Bradely, T., and Brown, C., "Inverse Address Resolution Protocol", RFC1293, USC/Information Sciences Institute, January 1992. [13] Bellovin, Steven M., "Security Problems in the TCP/IP Protocol Suite", ACM Computer Communications Review, Vol. 19, Issue 2, pp. 32-48, 1989. Author's Address Mark Laubach Hewlett-Packard Laboratories 1501 Page Mill Road Palo Alto, CA 94304 Phone: 415.857.3513 FAX: 415.857.8526 EMail: laubach@hpl.hp.com Laubach [Page 17]