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Network Working Group                                           A. MalisRequest for Comments: 1356                            BBN CommunicationsObsoletes: RFC877                                           D. Robinson                                      Computervision Systems Integration                                                              R. Ullmann                                            Process Software Corporation                                                             August 1992Multiprotocol Interconnecton X.25 and ISDN in the Packet ModeStatus of this Memo   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 document specifies the encapsulation of IP and other network   layer protocols over X.25 networks, in accordance and alignment with   ISO/IEC and CCITT standards.  It is a replacement forRFC 877, "A   Standard for the Transmission of IP Datagrams Over Public Data   Networks" [1].   It was written to correct several ambiguities in the Internet   Standard for IP/X.25 (RFC 877), to align it with ISO/IEC standards   that have been written followingRFC 877, to allow interoperable   multiprotocol operation between routers and bridges over X.25, and to   add some additional remarks based upon practical experience with the   specification over the 8 years since that RFC.   The substantive change to the IP encapsulation is an increase in the   allowed IP datagram Maximum Transmission Unit from 576 to 1600, to   reflect existing practice.   This document also specifies the Internet encapsulation for   protocols, including IP, on the packet mode of the ISDN.  It applies   to the use of Internet protocols on the ISDN in the circuit mode only   when the circuit is established as an end-to-end X.25 connection.Malis, Robinson, & Ullmann                                      [Page 1]

RFC 1356           Multiprotocol Interconnect on X.25        August 1992AcknowledgementsRFC 877 was written by J. T. Korb of Purdue University, and this   document follows that RFC's format and builds upon its text as   appropriate.  This document was produced under the auspices of the IP   over Large Public Data Networks Working Group of the IETF.1. Conventions   The following language conventions are used in the items of   specification in this document:   o MUST -- the item is an absolute requirement of the specification.     MUST is only used where it is actually required for interoperation,     not to try to impose a particular method on implementors where not     required for interoperability.   o SHOULD -- the item should 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.   The words "should" and "may" are also used, in lower case, in their   more ordinary senses.2. IntroductionRFC 877 was written to document the method CSNET and the VAN Gateway   had adopted to transmit IP datagrams over X.25 networks.  Its success   is evident in its current wide use and the inclusion of its IP   protocol identifier in ISO/IEC TR 9577, "Protocol Identification in   the Network Layer" [2], which is administered by ISO/IEC and CCITT.   However, due to changes in the scope of X.25 and the protocols that   it can carry, several inadequacies have become evident in the RFC,   especially in the areas of IP datagram Maximum Transmission Unit   (MTU) size, X.25 maximum data packet size, virtual circuit   management, and the interoperable encapsulation, over X.25, of   protocols other than IP between multiprotocol routers and bridges.   As withRFC 877, one or more X.25 virtual circuits are opened on   demand when datagrams arrive at the network interface for   transmission.  A virtual circuit is closed after some period of   inactivity (the length of the period depends on the cost associated   with an open virtual circuit).  A virtual circuit may also be closed   if the interface runs out of virtual circuits.Malis, Robinson, & Ullmann                                      [Page 2]

RFC 1356           Multiprotocol Interconnect on X.25        August 19923. Standards3.1 Protocol Data Units (PDUs) are sent as X.25 "complete packet    sequences".  That is, PDUs begin on X.25 data packet boundaries and    the M bit ("more data") is used to fragment PDUs that are larger    than one X.25 data packet in length.    In the IP encapsulation the PDU is the IP datagram.3.2 The first octet in the Call User Data (CUD) Field (the first data    octet in the Call Request packet) is used for protocol    demultiplexing, in accordance with the Subsequent Protocol    Identifier (SPI) in ISO/IEC TR 9577.  This field contains a one-    octet Network Layer Protocol Identifier (NLPID), which identifies    the network layer protocol encapsulated over the X.25 virtual    circuit.  The CUD field MAY contain more than one octet of    information, and receivers MUST ignore all extraneous octets in the    field.    In the following discussion, the most significant digit of the    binary numbers is left-most.    For the Internet community, the NLPID has four relevant values:    The value hex CC (binary 11001100, decimal 204) is IP [6].    Conformance with this specification requires that IP be supported.    Seesection 5.1 for a diagram of the packet formats.    The value hex 81 (binary 10000001, decimal 129) identifies ISO/IEC    8473 (CLNP) [4].  ISO/IEC TR 9577 specifically allows other ISO/IEC    connectionless-protocol packets, such as ES-IS and IS-IS, to also be    carried on the same virtual circuit as CLNP.  Conformance with this    specification does not require that CLNP be supported.  Seesection5.2 for a diagram of the packet formats.    The value hex 82 (binary 10000010, decimal 130) is used specifically    for ISO/IEC 9542 (ES-IS) [5].  If there is already a circuit open to    carry CLNP, then it is not necessary to open a second circuit to    carry ES-IS.  Conformance with this specification does not require    that ES-IS be supported.    The value hex 80 (binary 10000000, decimal 128) identifies the use    of IEEE Subnetwork Access Protocol (SNAP) [3] to further encapsulate    and identify a single network-layer protocol.  The SNAP-encapsulated    protocol is identified by including a five-octet SNAP header in the    Call Request CUD field immediately following the hex 80 octet.  SNAP    headers are not included in the subsequent X.25 data packets.  Only    one SNAP-encapsulated protocol may be carried over a virtual circuitMalis, Robinson, & Ullmann                                      [Page 3]

RFC 1356           Multiprotocol Interconnect on X.25        August 1992    opened using this encoding.  The receiver SHOULD accept the incoming    call only if it can support the particular SNAP-identified protocol.    Conformance with this specification does not require that this SNAP    encoding be supported.  Seesection 5.3 for a diagram of the packet    formats.    The value hex 00 (binary 00000000, decimal 0) identifies the Null    encapsulation, used to multiplex multiple network layer protocols    over the same circuit.  This encoding is further discussed insection 3.3 below.    The "Assigned Numbers" RFC [7] contains one other non-CCITT and    non-ISO/IEC value that has been in active use for Internet X.25    encapsulation identification, namely hex C5 (binary 11000101,    decimal 197) for Blacker X.25.  This value MAY continue to be used,    but only by prior preconfiguration of the sending and receiving X.25    interfaces to support this value.  The value hex CD (binary    11001101, decimal 205), listed in "Assigned Numbers" for "ISO-IP",    is also used by Blacker and also can only be used by prior    preconfiguration of the sending and receiving X.25 interfaces.    Each system MUST only accept calls for protocols it can process;    every Internet system MUST be able to accept the CC encapsulation    for IP datagrams.  A system MUST NOT accept calls, and then    immediately clear them.  Accepting the call indicates to the calling    system that the protocol encapsulation is supported; on some    networks, a call accepted and cleared is charged, while a call    cleared in the request state is not charged.    Systems that support NLPIDs other than hex CC (for IP) SHOULD allow    their use to be configured on a per-peer address basis.  The use of    hex CC (for IP) MUST always be allowed between peers and cannot be    configured.3.3 The NLPID encodings discussed insection 3.2 only allow a single    network layer protocol to be sent over a circuit.  The Null    encapsulation, identified by a NLPID encoding of hex 00, is used in    order to multiplex multiple network layer protocols over one    circuit.    When the Null encapsulation is used, each X.25 complete packet    sequence sent on the circuit begins with a one-octet NLPID, which    identifies the network layer protocol data unit contained only in    that particular complete packet sequence.  Further, if the SNAP    NLPID (hex 80) is used, then the NLPID octet is immediately followed    by the five-octet SNAP header, which is then immediately followed by    the encapsulated PDU.  The encapsulated network layer protocol MAY    differ from one complete packet sequence to the next over the sameMalis, Robinson, & Ullmann                                      [Page 4]

RFC 1356           Multiprotocol Interconnect on X.25        August 1992    circuit.    When a receiver is presented with an Incoming Call identifying the    Null encapsulation, the receiver MUST accept the call if it supports    the Null encapsulation for any network layer protocol.  The receiver    MAY then silently discard a multiplexed PDU if it cannot support    that particular encapsulated protocol.  Seesection 5.4 for a    diagram of the packet formats.    Use of the single network layer protocol circuits described insection 3.2 is more efficient in terms of bandwidth if only a    limited number of protocols are supported by a system.  It also    allows each system to determine exactly which protocols are    supported by its communicating partner.  Other advantages include    being able to use X.25 accounting to detail each protocol and    different quality of service or flow control windows for different    protocols.    The Null encapsulation, for multiplexing, is useful when a system,    for any reason (such as implementation restrictions or network cost    considerations), may only open a limited number of virtual circuits    simultaneously.  This is the method most likely to be used by a    multiprotocol router, to avoid using an unreasonable number of    virtual circuits.    If performing IEEE 802.1d bridging across X.25 is desired, then the    Null encapsulation MUST be used.  Seesection 4.2 for a further    discussion.    Conformance with this specification does not require that the Null    encapsulation be supported.    Systems that support the Null encapsulation SHOULD allow its use to    be configured on a per-peer address basis.3.4 For compatibility with existing practice, andRFC 877 systems, IP    datagrams MUST, by default, be encapsulated on a virtual circuit    opened with the CC CUD.    Implementations MAY also support up to three other possible    encapsulations of IP:   o IP may be contained in multiplexed data packets on a circuit using     the Null (multiplexed) encapsulation.  Such data packets are     identified by a NLPID of hex CC.   o IP may be encapsulated within the SNAP encapsulation on a circuit.     This encapsulation is identified by containing, in the 5-octet SNAPMalis, Robinson, & Ullmann                                      [Page 5]

RFC 1356           Multiprotocol Interconnect on X.25        August 1992     header, an Organizationally Unique Identifier (OUI) of hex 00-00-00     and Protocol Identifier (PID) of hex 08-00.   o On a circuit using the Null encapsulation, IP may be contained     within the SNAP encapsulation of IP in multiplexed data packets.    If an implementation supports the SNAP, multiplexed, and/or    multiplexed SNAP encapsulations, then it MUST accept the encoding of    IP within the supported encapsulation(s), MAY send IP using those    encapsulation(s), and MUST allow the IP encapsulation to send to be    configured on a per-peer address basis.3.5 The negotiable facilities of X.25 MAY be used (e.g., packet and    window size negotiation).  Since PDUs are sent as complete packet    sequences, any maximum X.25 data packet size MAY be configured or    negotiated between systems and their network service providers.  Seesection 4.5 for a discussion of maximum X.25 data packet size and    network performance.    There is no implied relationship between PDU size and X.25 packet    size (i.e., the method of setting IP MTU based on X.25 packet size    inRFC 877 is not used).3.6 Every system MUST be able to receive and transmit PDUs up to at    least 1600 octets in length.    For compatibility with existing practice, as well as    interoperability withRFC 877 systems, the default transmit MTU for    IP datagrams SHOULD default to 1500, and MUST be configurable in at    least the range 576 to 1600.    This is done with a view toward a standard default IP MTU of 1500,    used on both local and wide area networks with no fragmentation at    routers. Actually redefining the IP default MTU is, of course,    outside the scope of this specification.    The PDU size (e.g., IP MTU) MUST be configurable, on at least a    per-interface basis.  The maximum transmitted PDU length SHOULD be    configurable on a per-peer basis, and MAY be configurable on a per-    encapsulation basis as well.  Note that the ability to configure to    send IP datagrams with an MTU of 576 octets and to receive IP    datagrams of 1600 octets is essential to interoperate with existing    implementations ofRFC 877 and implementations of this    specification.    Note that on circuits using the Null (multiplexed) encapsulation,    when IP packets are encapsulated using the NLPID of hex CC, then the    default IP MTU of 1500 implies a PDU size of 1501; a PDU size ofMalis, Robinson, & Ullmann                                      [Page 6]

RFC 1356           Multiprotocol Interconnect on X.25        August 1992    1600 implies an IP MTU of 1599.  When IP packets are encapsulated    using the NLPID of hex 80 followed by the SNAP header for IP, then    the default IP MTU of 1500 implies a PDU size of 1506; a PDU size of    1600 implies an IP MTU of 1594.    Of course, an implementation MAY support a maximum PDU size larger    than 1600 octets.  In particular, there is no limit to the size that    may be used when explicitly configured by communicating peers.3.7 Each ISO/IEC TR 9577 encapsulation (e.g., IP, CLNP, and SNAP)    requires a separate virtual circuit between systems.  In addition,    multiple virtual circuits for a single encapsulation MAY be used    between systems, to, for example, increase throughput (see notes insection 4.5).    Receivers SHOULD accept multiple incoming calls with the same    encapsulation from a single system.  Having done so, receivers MUST    then accept incoming PDUs on the additional circuit(s), and SHOULD    transmit on the additional circuits.    Shedding load by refusing additional calls for the same    encapsulation with a X.25 diagnostic of 0 (DTE clearing) is correct    practice, as is shortening inactivity timers to try to clear    circuits.    Receivers MUST NOT accept the incoming call, only to close the    circuit or ignore PDUs from the circuit.    Because opening multiple virtual circuits specifying the same    encapsulation is specifically allowed, algorithms to prevent virtual    circuit call collision, such as the one found insection 8.4.3.5 of    ISO/IEC 8473 [4], MUST NOT be implemented.    While allowing multiple virtual circuits for a single protocol is    specifically desired and allowed, implementations MAY choose (by    configuration) to permit only a single circuit for some protocols to    some destinations.  Only in such a case, if a colliding incoming    call is received while a call request is pending, the incoming call    shall be rejected.  Note that this may result in a failure to    establish a connection.  In such a case, each system shall wait at    least a configurable collision retry time before retrying.  Adding a    random increment, with exponential backoff if necessary, is    recommended.3.8 Either system MAY close a virtual circuit.If the virtual circuit    is closed or reset while a datagram is being transmitted, the    datagram is lost.  Systems SHOULD be able to configure a minimum    holding time for circuits to remain open as long as the endpointsMalis, Robinson, & Ullmann                                      [Page 7]

RFC 1356           Multiprotocol Interconnect on X.25        August 1992    are up. (Note that holding time, the time the circuit has been open,    differs from idle time.)3.9 Each system MUST use an inactivity timer to clear virtual circuits    that are idle for some period of time.  Some X.25 networks,    including the ISDN under present tariffs in most areas, charge for    virtual circuit holding time.  Even where they do not, the resource    SHOULD be released when idle.  The timer SHOULD be configurable; a    timer value of "infinite" is acceptable when explicitly configured.    The default SHOULD be a small number of minutes.  For IP, a    reasonable default is 90 seconds.3.10 Systems SHOULD allow calls from unconfigured calling addresses     (presumably not collect calls, however); this SHOULD be a     configuration option.  A system accepting such a call will, of     course, not transmit on that virtual circuit if it cannot determine     the protocol (e.g., IP) address of the caller.  As an example, on     the DDN this is not a restriction because IP addresses can be     determined algorithmically based upon the caller's X.121 address     [7,9].     Allowing such calls helps work around various "helpful" address     translations done by the network(s), as well as allowing     experimentation with various address resolution protocols.3.11 Systems SHOULD use a configurable hold-down timer to prevent calls     to failed destinations from being immediately retried.3.12 X.25 implementations MUST minimally support the following features     in order to conform with this specification: call setup and     clearing and complete packet sequences.  For better performance     and/or interoperability, X.25 implementations SHOULD also support:     extended frame and/or packet sequence numbering, flow control     parameter negotiation, and reverse charging.3.13 The following X.25 features MUST NOT be used: interrupt packets and     the Q bit (indicating qualified data).  Other X.25 features not     explicitly discussed in this document, such as fast select and the     D bit (indicating end-to-end significance) SHOULD NOT be used.     Use of the D bit will interfere with use of the M bit (more data     sequences) required for identification of PDUs.  In particular, as     subscription to the D bit modification facility (X.25-1988,section3.3) will prevent proper operation, this user facility MUST NOT be     subscribed.3.14 ISO/IEC 8208 [11] defines the clearing diagnostic code 249 to     signify that a requested protocol is not supported.  Systems MAYMalis, Robinson, & Ullmann                                      [Page 8]

RFC 1356           Multiprotocol Interconnect on X.25        August 1992     use this diagnostic code when clearing an incoming call because the     identified protocol is not supported.  Non-8208 systems more     typically use a diagnostic code of 0 for this function.  Supplying     a diagnostic code is not mandatory, but when it is supplied for     this reason, it MUST be either of these two values.4. General Remarks   The following remarks are not specifications or requirements for   implementations, but provide developers and users with guidelines and   the results of operational experience withRFC 877.4.1 Protocols above the network layer, such as TCP or TP4, do not    affect this standard.  In particular, no attempt is made to open    X.25 virtual circuits corresponding to TCP or TP4 connections.4.2 Both the circuit and multiplexed encapsulations of SNAP may be used    to contain any SNAP encapsulated protocol.  In particular, this    includes using an OUI of 00-00-00 and the two octets of PID    containing an Ethertype [7], or using IEEE 802.1's OUI of hex 00-    80-C2 with the bridging PIDs listed inRFC 1294, "Multiprotocol    Interconnect over Frame Relay" [8].  Note that IEEE 802.1d bridging    can only be performed over a circuit using the Null (multiplexed)    encapsulation of SNAP, because of the necessity of preserving the    order of PDUs (including 802.1d Bridged PDUs) using different SNAP    headers.4.3 Experience has shown that there are X.25 implementations that will    assign calls with CC CUD to the X.29 PAD (remote login) facility    when the IP layer is not installed, not configured properly, or not    operating (indeed, they assume that ALL calls for unconfigured or    unbound X.25 protocol IDs are for X.29 PAD sessions).  Call    originators can detect that this has occurred at the receiver if the    originator receives any X.25 data packets with the Q bit set,    especially if the first octet of these packets is hex 02, 04, or 06    (X.29 PAD parameter commands).  In this case, the originator should    clear the call, and log the occurrence so that the misconfigured    X.25 address can be corrected.  It may be useful to also use the    hold-down timer (seesection 3.11) to prevent further attempts for    some period of time.4.4 It is often assumed that a larger X.25 data packet size will result    in increased performance.  This is not necessarily true: in typical    X.25 networks it will actually decrease performance.    Many, if not most, X.25 networks completely store X.25 data packets    in each switch before forwarding them.  If the X.25 network requires    a path through a number of switches, and low-speed trunks are used,Malis, Robinson, & Ullmann                                      [Page 9]

RFC 1356           Multiprotocol Interconnect on X.25        August 1992    then negotiating and using large X.25 data packets could result in    large transit delays through the X.25 network as a result of the    time required to clock the data packets over each low-speed trunk.    If a small end-to-end window size is also used, this may also    adversely affect the end-to-end throughput of the X.25 circuit.  For    this reason, segmenting large IP datagrams in the X.25 layer into    complete packet sequences of smaller X.25 data packets allows a    greater amount of pipelining through the X.25 switches, with    subsequent improvements in end-to-end throughput.    Large X.25 data packet size combined with slow (e.g., 9.6Kbps)    physical circuits will also increase individual packet latency for    other virtual circuits on the same path; this may cause unacceptable    effects on, for example, X.29 connections.    This discussion is further complicated by the fact that X.25    networks are free to internally combine or split X.25 data packets    as long as the complete packet sequence is preserved.    The optimum X.25 data packet size is, therefore, dependent on the    network, and is not necessarily the largest size offered by that    network.4.5 Another method of increasing performance is to open multiple virtual    circuits to the same destination, specifying the same CUD.  Like    packet size, this is not always the best method.    When the throughput limitation is due to X.25 window size, opening    multiple circuits effectively multiplies the window, and may    increase performance.    However, opening multiple circuits also competes more effectively    for the physical path, by taking more shares of the available    bandwidth.  While this may be desirable to the user of the    encapsulation, it may be somewhat less desirable to the other users    of the path.    Opening multiple circuits may also cause datagram sequencing and    reordering problems in end systems with limited buffering (e.g., at    the TCP level, receiving segments out of order, when a single    circuit would have delivered them in order). This will only affect    performance, not correctness of operation.    Opening multiple circuits may also increase the cost of delivering    datagrams across a public data network.Malis, Robinson, & Ullmann                                     [Page 10]

RFC 1356           Multiprotocol Interconnect on X.25        August 19924.6 This document does not specify any method of dynamic IP to X.25 (or    X.121) address resolution.  The problem is left for further study.    Typical present-day implementations use static tables of varying    kinds, or an algorithmic transformation between IP and X.121    addresses [7,9].  There are proposals for other methods.  In    particular,RFC 1183 [10] describes Domain Name System (DNS)    resource records that may be useful either for automatic resolution    or for maintenance of static tables.  Use of these method(s) is    entirely experimental at this time.5. Packet Formats   For each protocol encoding, the diagrams outline the call request and   the data packet format. The data packet shown is the first of a   complete packet (M bit) sequence.5.1 IP Encapsulation    Call Request:    +----------------+-----------+------------+----+    | GFI, LCN, type | addresses | facilities | CC |    +----------------+-----------+------------+----+    X.25 data packets:    +----------------+------------------------+    | GFI, LCN, I    | IP datagram            |    +----------------+------------------------+5.2 CLNP, ES-IS, IS-IS Encapsulation    Call Request:    +----------------+-----------+------------+----+    | GFI, LCN, type | addresses | facilities | 81 |    +----------------+-----------+------------+----+    X.25 data packets:    +----------------+--------------------------------+    | GFI, LCN, I    | CLNP, ES-IS, or IS-IS datagram |    +----------------+--------------------------------+    (Note that these datagrams are self-identifying in their    first octet).Malis, Robinson, & Ullmann                                     [Page 11]

RFC 1356           Multiprotocol Interconnect on X.25        August 19925.3 SNAP Encapsulation    Call Request:    +----------------+-----------+------------+----+-----------------+    | GFI, LCN, type | addresses | facilities | 80 | SNAP (5 octets) |    +----------------+-----------+------------+----+-----------------+    X.25 data packets:    +----------------+-------------------------------------+    | GFI, LCN, I    | Protocol Data Unit (no SNAP header) |    +----------------+-------------------------------------+5.4 Null (Multiplexed) Encapsulation    Call Request:    +----------------+-----------+------------+----+    | GFI, LCN, type | addresses | facilities | 00 |    +----------------+-----------+------------+----+    X.25 data packets:    +----------------+-----------------+---------------------+    | GFI, LCN, I    | NLPID (1 octet) | Protocol Data Unit  |    +----------------+-----------------+---------------------+    Examples of data packets:    Multiplexed IP datagram:    +----------------+----+-----------------------+    | GFI, LCN, I    | CC | IP datagram           |    +----------------+----+-----------------------+    Multiplexed CLNP datagram:    +----------------+----+-------------------------+    | GFI, LCN, I    | 81 | CLNP datagram           |    +----------------+----+-------------------------+    Multiplexed SNAP PDU:    +----------------+----+-----------------+--------------------+    | GFI, LCN, I    | 80 | SNAP (5 octets) | Protocol Data Unit |    +----------------+----+-----------------+--------------------+Malis, Robinson, & Ullmann                                     [Page 12]

RFC 1356           Multiprotocol Interconnect on X.25        August 19926. Security Considerations   Security issues are not discussed in this memo.7. References   [1]  Korb, J., "A Standard for the Transmission of IP Datagrams Over        Public Data Networks",RFC 877, Purdue University, September        1983.   [2]  ISO/IEC TR 9577, Information technology - Telecommunications and        Information exchange between systems - Protocol Identification        in the network layer, 1990 (E) 1990-10-15.   [3]  IEEE, "IEEE Standard for Local and Metropolitan Area Networks:        Overview and Architecture", IEEE Standards 802-1990.   [4]  ISO/IEC 8473, Information processing systems - Data        communications - Protocol for providing the connectionless- mode        network service, 1988.   [5]  ISO/IEC 9542, Information processing systems -        Telecommunications and information exchange between systems -        End system to intermediate system routeing protocol for use in        conjunction with the protocol for providing the connectionless-        mode network service (ISO/IEC 8473), 1988.   [6]  Postel, J., Editor., "Internet Protocol - DARPA Internet Program        Protocol Specification",RFC 791, USC/Information Sciences        Institute, September 1981.   [7]  Reynolds, J. and J. Postel, "Assigned Numbers",RFC 1340,        USC/Information Sciences Institute, July 1992.   [8]  Bradley, T., Brown, C., and A. Malis, "Multiprotocol        Interconnect over Frame Relay",RFC 1294, Wellfleet        Communications and BBN Communications, January 1992.   [9]  "Defense Data Network X.25 Host Interface Specification",        contained in "DDN Protocol Handbook", Volume 1, DDN Network        Information Center 50004, December 1985.  [10]  Everhart, C., Mamakos, L., Ullmann, R, and P. Mockapetris,        Editors, "New DNS RR Definitions",RFC 1183, Transarc,        University of Maryland, Prime Computer, USC/Information Sciences        Institute, October 1990.  [11]  ISO/IEC 8208, Information processing systems - DataMalis, Robinson, & Ullmann                                     [Page 13]

RFC 1356           Multiprotocol Interconnect on X.25        August 1992        communications - X.25 Packet Level Protocol for Data Terminal        Equipment, 1987.8. Authors' Addresses   Andrew G. Malis   BBN Communications   150 CambridgePark Drive   Cambridge, MA 02140   USA   Phone: +1 617 873 3419   Email: malis@bbn.com   David Robinson   Computervision Systems Integration   201 Burlington Road   Bedford, MA 01730   USA   Phone: +1 617 275 1800 x2774   Email: drb@relay.prime.com   Robert L. Ullmann   Process Software Corporation   959 Concord Street   Framingham, MA 01701   USA   Phone: +1 508 879 6994   Email: ariel@process.comMalis, Robinson, & Ullmann                                     [Page 14]