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Network Working Group                                      D. PiscitelloRequest for Comments: 1209                                   J. Lawrence                                            Bell Communications Research                                                              March 1991The Transmission of IP Datagrams over the SMDS ServiceStatus of this Memo   This memo defines a protocol for the transmission of IP and ARP   packets over a Switched Multi-megabit Data Service Network configured   as a logical IP subnetwork.  This RFC 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.Abstract   This memo describes an initial use of IP and ARP in an SMDS service   environment configured as a logical IP subnetwork, LIS (described   below).  The encapsulation method used is described, as well as   various service-specific issues.  This memo does not preclude   subsequent treatment of the SMDS Service in configurations other than   LIS; specifically, public or inter-company, inter-enterprise   configurations may be treated differently and will be described in   future documents.  This document considers only directly connected IP   end-stations or routers; issues raised by MAC level bridging are   beyond the scope of this paper.Acknowledgment   This memo draws heavily in both concept and text from [4], written by   Jon Postel and Joyce K. Reynolds of ISI and [5], written by David   Katz of Merit, Inc.  The authors would also like to acknowledge the   contributions of the IP Over SMDS Service working group of the   Internet Engineering Task Force.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.IP over SMDS Working Group                                      [Page 1]

RFC 1209            IP and ARP over the SMDS Service          March 1991      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.Introduction   The goal of this specification is to allow compatible and   interoperable implementations for transmitting IP datagrams and ARP   requests and replies.   The characteristics of the SMDS Service and the SMDS Interface   Protocol (SIP) are presented in [3], [6], and in [7].  Briefly, the   SMDS Service is a connectionless, public, packet-switched data   service.  The operation and features of the SMDS Service are similar   to those found in high-speed data networks such as LANs:      o The SMDS Service provides a datagram packet transfer, where each        data unit is handled and switched separately without the prior        establishment of a network connection.      o The SMDS Service exhibits high throughput and low delay, and        provides the transparent transport and delivery of up to 9188        octets of user information in a single transmission.      o No explicit flow control mechanisms are provided; instead, the        rate of information transfer on the access paths is controlled        both in the subscriber-to-network direction and in the network-        to-subscriber direction through the use of an access class        enforcement mechanism.      o Both individually and group-addressed (multicast) packets can        be transferred.      o In addition to these LAN-like features, a set of addressing-        related service features (source address validation, source and        destination address screening) are provided to enable a        subscriber or set of subscribers to create a logical private        network, or closed user group, over the SMDS Service.  The        access control provided by the closed user group mechanism is        supplied by the SMDS provider according to the specifications        stated in [3].      o SMDS addresses are 60 bits plus a 4 bit Address Type.  The        Address Type subfield occupies the 4 most significant bits of        the destination and source address fields of the SIP Level 3        Protocol Data Unit (PDU).  It contains the value 1100 toIP over SMDS Working Group                                      [Page 2]

RFC 1209            IP and ARP over the SMDS Service          March 1991        indicate an individual address and the value 1110 for a 60-bit        group address.   The SMDS Interface Protocol is based on the IEEE Standard 802.6,   Distributed Queue Dual Bus (DQDB) Connectionless MAC protocol [8].   The SMDS service layer corresponds to the IEEE 802 MAC sublayer.  The   remainder of the Data Link Service is provided by the IEEE 802.2   Logical Link Control (LLC) service [9].  The resulting stack of   services is illustrated in Figure 1:                           +--------------------+                           |      IP/ARP        |                           +--------------------+                           |IEEE 802.2 LLC/SNAP |                           +--------------------+                           | SIP LEVEL 3 (MAC)  |                           +--------------------+                           | SIP LEVELS 1 & 2   |                           +--------------------+            Figure 1.  Protocol stack for IP over SMDS Service   This memo describes an initial use of IP and ARP in an SMDS Service   environment configured as a logical IP subnetwork (described below).   It does not preclude subsequent treatment of SMDS Service in   configurations other than logical IP subnetworks; specifically,   public or inter-company, inter-enterprise configurations may be   treated differently and will be described in future documents.  This   document does not address issues related to transparent data link   layer interoperability.Logical IP Subnetwork Configuration   This section describes the scenario for an SMDS Service that is   configured with multiple logical IP subnetworks, LIS (described   below).  The scenario considers only directly connected IP end-   stations or routers; issues raised by MAC level bridging are beyond   the scope of this paper.   In the LIS scenario, each separate administrative entity configures   its hosts within a closed logical IP subnetwork.  Each LIS operates   and communicates independently of other LISs over the same network   providing SMDS.  Hosts connected to SMDS communicate directly to   other hosts within the same LIS.  Communication to hosts outside of   an individual LIS is provided via an IP router.  This router would   simply be a station attached to the SMDS Service that has been   configured to be a member of both logical IP subnetworks.  This   configuration results in a number of disjoint LISs operating over theIP over SMDS Working Group                                      [Page 3]

RFC 1209            IP and ARP over the SMDS Service          March 1991   same network supporting the SMDS Service.  It is recognized that with   this configuration, hosts of differing IP networks would communicate   via an intermediate router even though a direct path over the SMDS   Service may be possible.   It is envisioned that the service will evolve to provide a more   public interconnection, allowing machines directly connected to the   SMDS Service to communicate without an intermediate router.  However,   the issues raised by such a large public interconnection, such as   scalability of address resolution or propagation of routing updates,   are beyond the scope of this paper.  We anticipate that future RFCs    will address these issues.   The following is a list of the requirements for a LIS configuration:      o All members have the same IP network/subnet number.      o All stations within a LIS are accessed directly over SMDS.      o All stations outside of the LIS are accessed via a router.      o For each LIS a single SMDS group address has been configured        that identifies all members of the LIS.  Any packet transmitted        with this address is delivered by SMDS Service to all members        of the LIS.   The following list identifies a set of SMDS Service specific   parameters that MUST be implemented in each IP station which would   connect to the SMDS Service.  The parameter values will be determined   at SMDS subscription time and will be different for each LIS.  Thus   these parameters MUST be user configurable.      o SMDS Hardware Address (smds$ha).  The SMDS Individual address        of the IP station as determined at subscription time.  Each        host MUST be configured to accept datagrams destined for this        address.      o SMDS LIS Group Address(smds$lis-ga).  The SMDS Group address        that has been configured at subscription time to identify the        SMDS Subscriber Network Interfaces (SNI) of all members of the        LIS connected to the SMDS Service.  All members of the LIS MUST        be prepared to accept datagrams addressed to smds$lis-ga.      o SMDS Arp Request Address (smds$arp-req).  The SMDS address        (individual or group) to which arp requests are to be sent.  In        the initial LIS configuration this value is set to smds$lis-ga.        It is conceivable that in other configurations this value would        be set to some address other than that of smds$lis-ga (seeIP over SMDS Working Group                                      [Page 4]

RFC 1209            IP and ARP over the SMDS Service          March 1991        section on Address Resolution).   It is RECOMMENDED that routers providing LIS functionality over the   SMDS service also support the ability to interconnect differing 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 SMDS interface that may have one or   more individual SMDS addresses.   The following list identifies LIS specific parameters that MUST be   configured in the network supporting the SMDS Service.  For each LIS,   the IP network administrator MUST request the configuration of these   parameters at subscription time.  The administrator of each LIS MUST   update these parameters as each new station is added to the LIS.      o SMDS LIS Group Address(smds$lis-ga).  An SMDS Group address MUST        be configured at subscription time to identify the SMDS        Subscriber Network Interfaces (SNI) of all members of the LIS        connected to the SMDS Service.      o SMDS Address Screening Tables (Source and Destination).  The use        of SMDS screening tables is not necessary for the operation of        IP over SMDS Service.  If the SMDS screening tables are to be        used, both source and destination tables for each SNI MUST be        configured to allow, at minimum, both the direct communication        between all hosts in the same LIS and the use of the SMDS LIS        Group Address.Packet Format      Service SHALL be encapsulated within the IEEE 802.2 LLC and IEEE      802.1A Sub-Network Access Protocol (SNAP) [10] Data Link layers      and the 3-level SIP.  The SNAP MUST be used with an      Organizationally Unique Identifier Code indicating that the SNAP      header contains the EtherType code as listed in Assigned Numbers      [11] (see Figure 2).  Note that values specified in this document      follow Internet conventions: multi-byte fields are described in      big-endian order and bits within bytes are described as most      significant first [11].IP over SMDS Working Group                                      [Page 5]

RFC 1209            IP and ARP over the SMDS Service          March 1991                                                       +-------+                                                       |IP/ARP | IP/ARP                              +----+----+----+----+----+-------+                              |   Org Code   |Ethertype|       | SNAP               +----+----+----+----+----+----+----+----+-------+               |DSAP|SSAP|Ctrl|                                | LLC+-----+----+-+-+----+----+----+----+----+----+----+----+-------+|SIP..|HLPI|...|                                               | SIP L3+-----+----+-+-+----+----+----+----+----+----+----+----+-------+                    Figure 2.  Data Link Encapsulation      o The value of HLPI in the SIP L3 Header is 1.      o The total length of the LLC Header and the SNAP header is 8        octets.      o The value of DSAP and SSAP in the LLC header is 170 (decimal),        AA (Internet hexadecimal).      o The Ctrl (Control) value in the LLC header is 3 (Indicates Type        One Unnumbered Information).      o The Org Code in the SNAP header is zero (000000 Internet        hexadecimal).      o The EtherType for IP is 2048 (decimal), 0800 (Internet        hexadecimal).  The EtherType for ARP is 2054 (decimal), 0806        (Internet hexadecimal).   IEEE 802.2 LLC Type One Unnumbered Information (UI) communication   (which must be implemented by all conforming IEEE 802.2 stations) is   used exclusively.  The Higher Layer Protocol Id (HLPI) field in the   SIP L3_PDU header MUST be set to the IEEE 802.6 assigned Protocol Id   value for LLC (decimal 1) [8].  All frames MUST be transmitted in   standard IEEE 802.2 LLC Type 1 Unnumbered Information format, with   the DSAP and the SSAP fields of the IEEE 802.2 header set to the   assigned global SAP value for SNAP (decimal 170) [10].  The 24-bit   Org Code (Organizationally Unique Identifier Code) in the SNAP MUST   be set to a value of zero, and the remaining 16 bits are set to the   EtherType value from Assigned Numbers [11] (2048 for IP, 2054 for   ARP).   The data link encapsulation for IP packets is shown in Figure 3 and   for ARP in Figure 4.  All values shown are in Internet hexadecimal   format.IP over SMDS Working Group                                      [Page 6]

RFC 1209            IP and ARP over the SMDS Service          March 1991     +--------------+---------------------------------------+-------+     |      SIP     |             LLC / SNAP                |  IP   |     |              |                                       |       |     |SIP..|HLPI|...|DSAP|SSAP|Ctrl|   Org Code   |Ethertype|       |     +-----+----+-+-+----+----+----+----+----+----+----+----+-------+     |SIP..| 01 |...| AA | AA | 03 |    000000    |  0800   | IP... |     +-----+----+-+-+----+----+----+----+----+----+----+----+-------+             Figure 3.  IP Data Link Encapsulation and Values     +--------------+---------------------------------------+-------+     |      SIP     |             LLC / SNAP                |  ARP  |     |              |                                       |       |     |SIP..|HLPI|...|DSAP|SSAP|Ctrl|   Org Code   |Ethertype|       |     +-----+----+-+-+----+----+----+----+----+----+----+----+-------+     |SIP..| 01 |...| AA | AA | 03 |    000000    |  0806   | ARP...|     +-----+----+-+-+----+----+----+----+----+----+----+----+-------+             Figure 4.  ARP Data Link Encapsulation and ValuesAddress Resolution   The dynamic mapping of 32-bit Internet addresses to SMDS addresses   SHALL be done via the dynamic discovery procedure of the Address   Resolution Protocol (ARP) [2].   Internet addresses are assigned independent of SMDS addresses.  Each   host implementation MUST know its own Internet address and SMDS   address and respond to Address Resolution requests appropriately.   Hosts MUST also use ARP to map Internet addresses to SMDS addresses   when needed.   The ARP protocol has several fields that parameterize its use in any   specific context [2].  These fields are:           ar$hrd   16 - bits     The Hardware Type Code           ar$pro   16 - bits     The Protocol Type Code           ar$hln    8 - bits     Octets in each hardware address           ar$pln    8 - bits     Octets in each protocol address           ar$op    16 - bits     Operation Code      o The hardware type code assigned to SMDS addresses is 14        (decimal), 0E (Internet hexadecimal) [11].      o The protocol type code for IP is 2048 (decimal), 0800        (Internet hexadecimal) [11].IP over SMDS Working Group                                      [Page 7]

RFC 1209            IP and ARP over the SMDS Service          March 1991      o The hardware address length for SMDS is 8.      o The protocol address length for IP is 4.      o The operation code is 1 for request and 2 for reply.   The SMDS hardware addresses in ARP packets (ar$sha, ar$tha) MUST be   carried in SMDS native address format, with the most significant bit   of the Address Type sub-field as the high order bit of the first   octet.  Although outside the scope of this document, it is   RECOMMENDED that SMDS addresses be represented in this format in all   higher layer Internet protocols (e.g., SNMP).   Traditionally, ARP requests are broadcast to all directly connected   stations.  For the SMDS Service, the ARP request packet is   transmitted to the smds$arp-req hardware address.  In the LIS   configuration, the smds$arp-req address is set to smds$lis-ga, (the   SMDS group address that identifies all members of the LIS).  It is   conceivable that in a larger scale, public configuration, the   smds$arp-req address would be configured to the address of some ARP-   server(s) instead of the group address that identifies the entire   LIS.IP Broadcast Address   There is no facility for complete hardware broadcast addressing over   the SMDS Service.  As discussed in the "LIS Configuration" section,   an SMDS group address (smds$lis-ga) SHALL be configured to include   all stations in the same LIS.  The broadcast Internet address (the   address on that network with a host part of all binary ones) MUST be   mapped to smds$lis-ga (see also [12]).IP Multicast Support   A method of supporting IP multicasting is specified in [13].  It   would be desirable to fully utilize the SMDS group address   capabilities to support IP multicasting.  However, the method in [13]   requires a Network Service Interface which provides multicast-like   ability to provide dynamic access to the local network service   interface operations:      o JoinLocalGroup (group-address)      o LeaveLocalGroup (group-address)   The SMDS group address ability does not currently support dynamic   subscription and removal from group address lists.  Therefore, it is   RECOMMENDED that in the LIS configuration, if IP multicasting is toIP over SMDS Working Group                                      [Page 8]

RFC 1209            IP and ARP over the SMDS Service          March 1991   be supported, the method of IP multicasting described for pure   broadcast media, such as the Experimental Ethernet, be used.  For   this method, all Multicast IP addresses are mapped to the same SMDS   address which the broadcast Internet address is mapped for a given   LIS.  Thus all Multicast IP addresses are mapped to smds$lis-ga.   Filtering of multicast packets MUST be performed in the destination   host.Trailer Formats   Some versions of Unix 4.x BSD use a different encapsulation method in   order to get better network performance with the VAX virtual memory   architecture.  Trailers SHALL not be used over the SMDS Service.Byte Order   As described inAppendix B of the Internet Protocol specification   [1], the IP datagram is transmitted over the SMDS Service as a series   of 8-bit bytes.  The byte order of the IP datagram shall be mapped   directly onto the native SMDS byte order.MAC Sublayer DetailsPacket Size   The SMDS Service defines a maximum service data unit size of 9188   information octets.  This leaves 9180 octets for user data after the   LLC/SNAP header is taken into account.  Therefore, the MTU for IP   stations operating over the network supporting the SMDS Service SHALL   be 9180 octets.   There is no minimum packet size restriction defined for the SMDS   Service.Other MAC Sublayer Issues   The SMDS Service requires that the publicly administered 60-bit   address plus 4-bit type field format SHALL be used in both source and   destination address fields of the SIP L3_PDU [3].IEEE 802.2 Details   While not necessary for supporting IP and ARP, all implementations   MUST support IEEE 802.2 standard Class I service in order to be   compliant with IEEE 802.2.  Some of the functions are not related   directly to the support of the SNAP SAP (e.g., responding to XID and   TEST commands directed to the null or global SAP addresses), but are   part of a general LLC implementation.  Both [4] and [5] describe theIP over SMDS Working Group                                      [Page 9]

RFC 1209            IP and ARP over the SMDS Service          March 1991   minimum functionality necessary for a conformant station.   Implementors should also consult IEEE Std. 802.2 [14] for details.REFERENCES    1. Postel, J., "Internet Protocol",RFC 791, USC/Information       Sciences Institute, September 1981.    2. Plummer, D., "An Ethernet Address Resolution Protocol - or -       Converting Network Protocol Addresses to 48.bit Ethernet Address       for Transmission on Ethernet Hardware",RFC 826, MIT, November       1982.    3. "Generic Systems Requirements in support of Switched Multi-       megabit Data Service", Technical Advisory TA-TSY-000772, Bellcore       Technical Advisory, Issue 3, October 1989.    4. Postel, J., and J. Reynolds, "A Standard for the Transmission of       IP Datagrams over IEEE 802 Networks",RFC 1042, USC/Information       Sciences Institute, February 1988.    5. Katz, D., "A Proposed Standard for the Transmission of IP       Datagrams over FDDI Networks",RFC 1188, Merit/NSFNET, October       1990.    6. Dix, F., Kelly, M., and R. Klessig, "Access to a Public Switched       Multi-Megabit Data Service Offering", ACM SIGCOMM CCR, July 1990.    7. Hemrick, C. and L. Lang, "Introduction to Switched Multi-megabit       Data Service (SMDS), an Early Broadband Service", publication       pending in the Proceedings of the XIII International Switching       Symposium (ISS 90), May 27, 1990 - June 1, 1990.    8. Institute of Electrical & Electronic Engineers, Inc. IEEE       Standard 802.6, "Distributed Queue Dual Bus (DQDB) Subnetwork of       a Metropolitan Area Network (MAN) Standard", December 1990.    9. IEEE, "IEEE Standards for Local Area Networks: Logical Link       Control", IEEE, New York, New York, 1985.   10. IEEE, "Draft Standard P802.1A--Overview and Architecture", 1989.   11. Reynolds, J., and J. Postel, "Assigned Numbers",RFC 1060,       USC/Information Sciences Institute, March 1990.   12. Braden, R., and J. Postel, "Requirements for Internet Gateways",RFC 1009, USC/Information Sciences Institute, June 1987.IP over SMDS Working Group                                     [Page 10]

RFC 1209            IP and ARP over the SMDS Service          March 1991   13. Deering, S., "Host Extensions for IP Multicasting",RFC 1112,       Stanford University, August 1989.   14. IEEE,"ANSI/IEEE Std 802.2-1985, ISO Draft International Standard       8802/2", IEEE, New York, New York, 1985.Security Considerations   Security issues are not discussed in this memo.Authors' Addresses   Dave Piscitello   Bell Communications Research   331 Newman Springs Road   Red Bank, NJ  07701   Phone: (908) 758-2286   EMail: dave@sabre.bellcore.com   Joseph Lawrence   Bell Communications Research   331 Newman Springs Road   Red Bank, NJ  07701   Phone: (908) 758-4146   EMail: jcl@sabre.bellcore.comIP over SMDS Working Group                                     [Page 11]