Network Working Group:                                       M. McGovern
Request for Comments: 1707                              Sunspot Graphics
Category: Informational                                       R. Ullmann
                                           Lotus Development Corporation
                                                            October 1994

CATNIP: Common Architecture for the Internet

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document was submitted to the IETF IPng area in response to RFC
   1550  Publication of this document does not imply acceptance by the
   IPng area of any ideas expressed within.  Comments should be
   submitted to the big-internet@munnari.oz.au mailing list.

Executive Summary

   This paper describes a common architecture for the network layer
   protocol. The Common Architecture for Next Generation Internet
   Protocol (CATNIP) provides a compressed form of the existing network
   layer protocols. Each compression is defined so that the resulting
   network protocol data units are identical in format. The fixed part
   of the compressed format is 16 bytes in length, and may often be the
   only part transmitted on the subnetwork.

   With some attention paid to details, it is possible for a transport
   layer protocol (such as TCP) to operate properly with one end system
   using one network layer (e.g. IP version 4) and the other using some
   other network protocol, such as CLNP. Using the CATNIP definitions,
   all the existing transport layer protocols used on connectionless
   network services will operate over any existing network layer
   protocol.

   The CATNIP uses cache handles to provide both rapid identification of
   the next hop in high performance routing as well as abbreviation of
   the network header by permitting the addresses to be omitted when a
   valid cache handle is available. The fixed part of the network layer
   header carries the cache handles.

   The cache handles are either provided by feedback from the downstream
   router in response to offered traffic, or explicitly provided as part
   of the establishment of a circuit or flow through the network. When
   used for flows, the handle is the locally significant flow
   identifier.

   When used for circuits, the handle is the layer 3 peer-to-peer
   logical channel identifier, and permits a full implementation of
   network-layer connection-oriented service if the routers along the
   path provide sufficient features. At the same time, the packet format
   of the connectionless service is retained, and hop by hop fully
   addressed datagrams can be used at the same time. Any intermediate
   model between the connection oriented and the connectionless service
   can thus be provided over cooperating routers.

CATNIP Objectives

   The first objective of the CATNIP is a practical recognition of the
   existing state of internetworking, and an understanding that any
   approach must encompass the entire problem. While it is common in the
   IP Internet to dismiss the ISO with various amusing phrases, it is
   hardly realistic. As the Internet moves into the realm of providing
   real commercial infrastructure, for telephone, cable television, and
   the myriad other mundane uses, compliance with international
   standards is an imperative.

   The argument that the IETF need not (or should not) follow existing
   ISO standards will not hold. The ISO is the legal standards
   organization for the planet. Every other industry develops and
   follows ISO standards. There is (no longer) anything special about
   computer software or data networking.

   ISO convergence is both necessary and sufficient to gain
   international acceptance and deployment of IPng. Non-convergence will
   effectively preclude deployment.

   The CATNIP integrates CLNP, IP, and IPX. The CATNIP design provides
   for any of the transport layer protocols in use, for example TP4,
   CLTP, TCP, UDP, IPX and SPX to run over any of the network layer
   protocol formats: CLNP, IP (version 4), IPX, and the CATNIP.

Incremental Infrastructure Deployment

   The best use of the CATNIP is to begin to build a common Internet
   infrastructure. The routers and other components of the common system
   are able to use a single consistent addressing method, and common
   terms of reference for other aspects of the system.

   The CATNIP is designed to be incrementally deployable in the strong
   sense: you can plop a CATNIP system down in place of any existing
   network component and continue to operate normally with no
   reconfiguration.  (Note: not "just a little". None at all. The number
   of "little changes" suggested by some proposals, and the utterly
   enormous amount of documentation, training, and administrative effort
   then required, astounds the present authors.) The vendors do all of
   the work.

   There are also no external requirements; no "border routers", no
   requirement that administrators apply specific restrictions to their
   network designs, define special tables, or add things to the DNS.
   When the end users and administrators fully understand the combined
   system, they will want to operate differently, but in no case will
   they be forced. Not even in small ways. Networks and end user
   organizations operate under sufficient constraints on deployment of
   systems anyway; they do not need a new network architecture adding to
   the difficulty.

   Typically deployment will occur as part of normal upgrade revisions
   of software, and due to the "swamping" of the existing base as the
   network grows. (When the Internet grows by a factor of 5, at least
   80% will then be "new" systems.) The users of the network may then
   take advantage of the new capabilities. Some of the performance
   improvements will be automatic, others may require some
   administrative understanding to get to the best performance level.

   The CATNIP definitions provide stateless translation of network
   datagrams to and from CATNIP and, by implication, directly between
   the other network layer protocols. A CATNIP-capable system
   implementing the full set of definitions can interoperate with any
   existing protocol. Various subsets of the full capability may be
   provided by some vendors.

No Address Translation

   Note that there is no "address translation" in the CATNIP
   specification.  (While it may seem odd to state a negative objective,
   this is worth saying as people seem to assume the opposite.) There
   are no "mapping tables", no magic ways of digging translations out of
   the DNS or X.500, no routers looking up translations or asking other
   systems for them.

   Addresses are modified with a simple algorithmic mapping, a mapping
   that is no more than using specific prefixes for IP and IPX
   addresses. Not a large set of prefixes; one prefix. The entire
   existing IP version 4 network is mapped with one prefix and the IPX
   global network with one other prefix. (The IP mapping does provide

   for future assignment of other IANA/IPv4 domains that are disjoint
   from the existing one.)

   This means that there is no immediate effect on addresses embedded in
   higher level protocols.

   Higher level protocols not using the full form (those native to IP
   and IPX) will eventually be extended to use the full addressing to
   extend their usability over all of the network layers.

No Legacy Systems

   The CATNIP leaves no systems behind: with no reconfiguration, any
   system presently capable of IP, CLNP, or IPX retains at least the
   connectivity it has now.  With some administrative changes (such as
   assigning IPX domain addresses to some CLNP hosts for example) on
   other systems, unmodified systems may gain significant connectivity.
   IPX systems with registered network numbers may gain the most.

Limited Scope

   The CATNIP defines a common network layer packet format and basic
   architecture. It intentionally does not specify ES-IS methods,
   routing, naming systems, autoconfiguration and other subjects not
   part of the core Internet wide architecture. The related problems and
   their (many) solutions are not within the scope of the specification
   of the basic common network layer.

Existing Addresses and Network Numbers

   The Internet's version 4 numbering system has proven to be very
   flexible, (mostly) expandable, and simple.  In short: it works.
   However, there are two problems. Neither was considered serious when
   the CATNIP was first developed in 1988 and 1989, but both are now of
   major concern:

   o The division into network, and then subnet, is insufficient.
     Almost all sites need a network assignment large enough to
     subnet. At the top of the hierarchy, there is a need to assign
     administrative domains.

   o As bit-packing is done to accomplish the desired network
     structure, the 32-bit limit causes more and more aggravation.

   Another major addressing system used in open internetworking is the
   OSI method of specifying Network Service Access Points (NSAPs). The
   NSAP consists of an authority and format identifier, a number

   assigned to that authority, an address assigned by that authority,
   and a selector identifying the next layer (transport layer) protocol.
   This is actually a general multi-level hierarchy, often obscured by
   the details of specific profiles. (For example, CLNP doesn't specify
   20 octet NSAPs, it allows any length. But various GOSIPs profile the
   NSAP as 20 octets, and IS-IS makes specific assumptions about the
   last 1-8 octets. And so on.)

   The NSAP does not directly correspond to an IP address, as the
   selector in IP is separate from the address. The concept that does
   correspond is the NSAP less the selector, called the Network Entity
   Title or NET. (An unfortunate acronym, but one we will use to avoid
   repeating the full term.) The usual definition of NET is an NSAP with
   the selector set to 0; the NET used here omits the 0 selector.

   There is also a network numbering system used by IPX, a product of
   Novell, Inc. (referred to from here on as Novell) and other vendors
   making compatible software. While IPX is not yet well connected into
   a global network, it has a larger installed base than either of the
   other network layers.

Network Layer Address

   The network layer address looks like:

      +----------+----------+---------------+---------------+
      |  length  |   AFI    |  IDI ...      | DSP ...       |
      +----------+----------+---------------+---------------+

   The fields are named in the usual OSI terminology although that leads
   to an oversupply of acronyms. Here are more detailed descriptions of
   each field:

   length: the number of bytes (octets) in the remainder of the
           address.

   AFI: the Authority and Format Identifier. A single byte
        value, from a set of well-known values registered by
        ISO, that determines the semantics of the IDI field

   IDI: the Initial Domain Identifier, a number assigned by the
        authority named by the AFI, formatted according to the
        semantics implied by the AFI, that determines the
        authority for the remainder of the address.

   DSP: Domain Specific Part, an address assigned by the
        authority identified by the value of the IDI.

   Note that there are several levels of authority. ISO, for example,
   identifies (with the AFI) a set of numbering authorities (like X.121,
   the numbering plan for the PSPDN, or E.164, the numbering plan for
   the telephone system). Each authority numbers a set of organizations
   or individuals or other entities. (For example, E.164 assigns
   16172477959 to me as a telephone subscriber.)

   The entity then is the authority for the remainder of the address. I
   can do what I please with the addresses starting with (AFI=E.164)
   (IDI=16172477959). Note that this is a delegation of authority, and
   not an embedding of a data-link address (the telephone number) in a
   network layer address. The actual routing of the network layer
   address has nothing to do with the authority numbering.

   The domain-specific part is variable length, and can be allocated in
   whatever way the authority identified by the AFI+IDI desires.

Network Layer Datagram

   The common architecture format for network layer datagrams is
   described below. The design is a balance between use on high
   performance networks and routers, and a desire to minimize the number
   of bits in the fixed header. Using the current state of processor
   technology as a reference, the fixed header is all loaded into CPU
   registers on the first memory cycle, and it all fits within the
   operation bandwidth. The header leaves the remaining data aligned on
   the header size (128 bits); with 64 bit addresses present and no
   options it leaves the transport header 256 bit aligned.

   On very slow and low performance networks, the fixed header is still
   fairly small, and could be further compressed by methods similar to
   those used with IP version 4 on links that consider every bit
   precious. In between, it fits nicely into ATM cells and radio
   packets, leaving sufficient space for the transport header and
   application data.

      +---------------+---------------+-+-+-+-+-+-+-+-+---------------+
      |   NLPID (70)  |  Header Size  |D|S|R|M|E| MBZ | Time to Live  |
      +---------------+---------------+-+-+-+-+-+-+-+-+---------------+
      |                 Forward Cache Identifier                      |
      +---------------------------------------------------------------+
      |                      Datagram Length                          |
      +---------------------------------------------------------------+
      |     Transport Protocol        |          Checksum             |
      +---------------------------------------------------------------+
      |               Destination Address ...                         |
      +---------------------------------------------------------------+
      |               Source Address ...                              |
      +---------------------------------------------------------------+
      |               Options ...                                     |
      +---------------------------------------------------------------+

  NLPID: The first byte (the network layer protocol identifier in OSI)
         is an 8 bit constant 70 (hex). This corresponds to Internet
         Version 7.

  Header Length: The header length is a 8-bit count of the number of
         32-bit words in the header. This allows the header to
         be up to 1020 bytes in length.

  Flags: This byte is a small set of flags determining the datagram
         header format and the processing semantics. The last three bits
         are reserved, and must be set to zero. (Note that the
         corresponding bits in CLNP version 1 are 001, since this byte
         is the version field. This may be useful.)

  Destination Address Omitted: When the destination address omitted
         (DAO) flag is zero, the destination address is present as shown
         in the datagram format diagram. When a datagram is sent with
         an FCI that identifies the destination and the DAO flag is
         set, the address does not appear in the datagram.

  Source Address Omitted: The source address omitted (SAO) flag is zero
         when the source address is present in the datagram. When
         datagram is sent with an FCI that identifies the source and the
         SAO flag is set, the source address is omitted from the
         datagram.

  Report Fragmentation Done: When this bit (RFD) is set, an intermediate
         router that fragments the datagram (because it is larger than
         the next subnetwork MTU) should report the event with an ICMP
         Datagram Too Big message. (Unlike IP version 4, which uses DF
         for MTU discovery, the RFD flag allows the fragmented datagram

         to be delivered.)

  Mandatory Router Option: The mandatory router option (MRO) flag
         indicates that routers forwarding the datagram must look at the
         network header options.  If not set, an intermediate router
         should not look at the header options.  (But it may anyway;
         this is a necessary consequence of transparent network layer
         translation, which may occur anywhere.)

         The destination host, or an intermediate router doing
         translation, must look at the header options regardless of
         the setting of the MRO flag.

         A router doing fragmentation will normally only use the F
         flag in options to determine whether options should be copied
         within the fragmentation code path. (It might also recognize
         and elide null options.) If the MRO flag is not set, the router
         may not act on an option even though it copies it properly
         during fragmentation.

         If there are no options present, MRO should always be zero, so
         that routers can follow the no-option profile path in their
         implementation.  (Remember that the presence of options cannot
         be divided from the header length, since the addresses are
         variable length.)

  Error Report Suppression: The ERS flag is set to suppress the sending
         of error reports by any system (whether host or router)
         receiving or forwarding the datagram. The system may log the
         error, increment network management counters, and take any
         similar action, but ICMP error messages or CNLP error reports
         must not be sent.

         The ERS flag is normally set on ICMP messages and other network
         layer error reports. It does not suppress the normal response
         to ICMP queries or similar network layer queries (CNLP echo
         request).

         If both the RFD and ERS flags are set, the fragmentation report
         is sent.  (This definition allows a larger range of
         possibilities than simply over-riding the RFD flag would; a
         sender not desiring this behavior can see to it that RFD is
         clear.)

  Time To Live: The time to live is a 8-bit count, nominally in seconds.
         Each hop is required to decrement TTL by at least one. A hop
         that holds a datagram for an unusual amount of time (more than
         2 seconds, a typical example being a wait for a subnetwork

         connection establishment) should subtract the entire waiting
         time in seconds (rounded upward) from the TTL.

  Forward Cache Identifier: Each datagram carries a 32 bit field, called
         "forward cache identifier", that is updated (if the information
         is available) at each hop. This field's value is derived from
         ICMP messages sent back by the next hop router, a routing
         protocol (e.g., RAP), or some other method. The FCI is used to
         expedite routing decisions by preserving knowledge where
         possible between consecutive routers. It can also be used to
         make datagrams stay within reserved flows, circuits, and mobile
         host tunnels. If an FCI is not available, this field must be
         zero, the SAO and DAO flags must be clear, and both destination
         and source addresses must appear in the datagram.

  Datagram Length: The 32-bit length of the entire datagram in octets.
         A datagram can therefore be up to 4294967295 bytes in overall
         length.  Particular networks normally impose lower limits.

  Transport Protocol: The transport layer protocol. For example, TCP is
         6.

  Checksum: The checksum is a 16-bit checksum of the entire header,
         using the familiar algorithm used in IP version 4.

  Destination: The destination address, a count byte followed by the
         destination NSAP with the zero selector omitted. This field is
         present only if the DAO flag is zero. If the count field is not
         3 modulo 4 (the destination is not an integral multiple of
         32-bit words) zero bytes are added to pad to the next multiple
         of 32 bits. These pad bytes are not required to be ignored:
         routers may rely on them being zero.

  Source: The source address, in the same format as the destination.
         Present only if the SAO flag is zero. The source is padded in
         the same way as destination to arrive at a 32-bit boundary.

  Options: Options may follow. They are variable length, and always
         32-bit aligned. If the MRO flag in the header is not set,
         routers will usually not look at or take action on any option,
         regardless of the setting of the class field.

Multicasting

   The multicast-enable option permits multicast forwarding of the
   CATNIP datagram on subnetworks that directly support media layer
   multicasting. This is a vanishing species, even in 10 Mbps Ethernet,
   given the increasing prevalence of switching hubs. It also (perhaps

   more usefully) permits a router to forward the datagram on multiple
   paths when a multicast routing algorithm has established such paths.
   There is no option data.

   Note that there is no special address space for multicasting in the
   CATNIP. Multicast destination addresses can be allocated anywhere by
   any administration or authority. This supports a number of differing
   models of addressing. It does require that the transport layer
   protocol know that the destination is multicast; this is desirable in
   any case. (For example, the transport will probably want to set the
   ERS flag.)

   On an IEEE 802.x (ISO 8802.x) type media, the last 23 bits of the
   address (not including the 0 selector) are used in combination with
   the multicast group address assigned to the Internet to form the
   media address when forwarding a datagram with the multicast enable
   option from a router to an attached network provided that the
   datagram was not received on that network with either multicast or
   broadcast media addressing. A host may send a multicast datagram
   either to the media multicast address (the IP catenet model,) or
   media unicast to a router which is expected to repeat it to the
   multicast address within the entire level I area or to repeat copies
   to the appropriate end systems within the area on non-broadcast media
   (the more general CLNP model.)

Network Layer Translation

   The objective of translation is to be able to upgrade systems, both
   hosts and routers, in whatever order desired by their owners.
   Organizations must be able to upgrade any given system without
   reconfiguration or modification of any other, and existing hosts must
   be able to interoperate essentially forever. (Non-CATNIP routers will
   probably be effectively eliminated at some point, except where they
   exist in their own remote or isolated corners.)

   Each CATNIP system, whether host or router, must be able to recognize
   adjacent systems in the topology that are (only) IP version 4, CLNP,
   or IPX and call the appropriate translation routine just before
   sending the datagram.

OSI CNLP

   The translation between CLNP and the CATNIP compressed form of the
   datagrams is the simplest case for CATNIP, since the addresses are
   the same and need not be extended. The resulting CATNIP datagrams may
   omit the source and destination addresses as explained previously,
   and may be mixed with uncompressed datagrams on the same subnetwork
   link. Alternatively, a subnetwork may operate entirely in the CATNIP,

   converting all transit traffic to CATNIP datagrams, even if FCIs that
   would make the compression effective are not available.

   Similarly, all network datagram formats with CATNIP mappings may be
   compressed into the common form, providing a uniform transit network
   service, with common routing protocols (such as IS-IS).

Internet Protocol

   All existing version 4 numbers are defined as belonging to the
   Internet by using a new AFI, to be assigned to IANA by the ISO. This
   document uses 192 at present for clarity in examples; it is to be
   replaced with the assigned AFI. The AFI specifies that the IDI is two
   bytes long, containing an administrative domain number.

   The AD (Administrative Domain), identifies an administration which
   may be an international authority (such as the existing InterNIC), a
   national administration, or a large multi-organization (e.g., a
   government). The idea is that there should not be more than a few
   hundred of these at first, and eventually thousands or tens of
   thousands at most.

   AD numbers are assigned by IANA. Initially, the only assignment is
   the number 0.0, assigned to the InterNIC, encompassing the entire
   existing version 4 Internet.

   The mapping from/to version 4 IP addresses:

      +----------+----------+---------------+---------------------+
      |  length  |   AFI    |  IDI ...      | DSP ...             |
      +----------+----------+---------------+---------------------+
      |     7    |   192    |  AD number    | version 4 address   |
      +----------+----------+---------------+---------------------+

   While the address (DSP) is initially always the 4 byte, version 4
   address, it can be extended to arbitrary levels of subnetting within
   the existing Internet numbering plan. Hosts with DSPs longer than 4
   bytes will not be able to interoperate with version 4 hosts.

Novell IPX

   The Internetwork Packet Exchange protocol, developed by Novell based
   on the XNS protocol (Xerox Network System) has many of the same
   capabilities as the Internet and OSI protocols. At first look, it
   appears to confuse the network and transport layers, as IPX includes
   both the network layer service and the user datagram service of the
   transport layer, while SPX (sequenced packet exchange) includes the
   IPX network layer and provides service similar to TCP or TP4. This

   turns out to be mostly a matter of the naming and ordering of fields
   in the packets, rather than any architectural difference.

   IPX uses a 32-bit LAN network number, implicitly concatenated with
   the 48-bit MAC layer address to form an Internet address. Initially,
   the network numbers were not assigned by any central authority, and
   thus were not useful for inter-organizational traffic without
   substantial prior arrangement. There is now an authority established
   by Novell to assign unique 32-bit numbers and blocks of numbers to
   organizations that desire inter-organization networking with the IPX
   protocol.

   The Novell/IPX numbering plan uses an ICD, to be assigned, to
   designate an address as an IPX address. This means Novell uses the
   authority (AFI=47)(ICD=Novell) and delegates assignments of the
   following 32 bits.

   An IPX address in the common form looks like:

      +----------+----------+---------------+---------------------+
      |  length  |   AFI    |  IDI ...      | DSP ...             |
      +----------+----------+---------------+---------------------+
      |    13    | 47 (hex) |  Novell ICD   | network+MAC address |
      +----------+----------+---------------+---------------------+

   This will always be followed by two bytes of zero padding when it
   appears in a common network layer datagram. Note that the socket
   numbers included in the native form IPX address are part of the
   transport layer.

SIPP

   It may seem a little odd to describe the interaction with SIPP-16
   (version 6 of IP) which is another proposed candidate for the next
   generation of network layer protocols. However, if SIPP-16 is
   deployed, whether or not as the protocol of choice for replacement of
   IP version 4, there will then be four network protocols to
   accommodate.  It is prudent to investigate how SIPP-16 could then be
   integrated into the common addressing plan and datagram format.

   SIPP-16 defines 128 bit addresses, which are included in the NSAP
   addressing plan under the Internet AFI as AD number 0.1. It is not
   clear at this time what administration will hold the authority for
   the SIPP-16 numbering plan. This produces a 20 byte NSAPA, with the
   system ID field positioned exactly as expected by (e.g.) IS-IS.

      +----------+----------+---------------+---------------------+
      |  length  |   AFI    |  IDI ...      | DSP ...             |
      +----------+----------+---------------+---------------------+
      |    19    |   192    |  AD (0.1)     |   SIPP-16 address   |
      +----------+----------+---------------+---------------------+

   The SIPP-16 addressing method (the definition of the 128 bits) will
   not be described here.

   The SIPP proposal also includes an encapsulated-tunnel proposal
   called IPAE, to address some of the issues that are designed into
   CATNIP.  The CATNIP direct translation does not use the SIPP-IPAE
   packet formats. IPAE also specifies a "mapping table" for prefixes.
   This table is kept up-to-date by periodic FTP transfers from a
   "central site." The CATNIP definitions leave the problem of prefix
   selection when converting into SIPP firmly within the scope of the
   SIPP-IPAE proposal, and possible methods are not described here.

   In translating from SIPP (IPv6) to CATNIP (IPv7), the only unusual
   aspect is that SIPP defines some things that are normally considered
   options to be "payloads" overloaded onto the transport protocol
   numbering space.  Fortunately, the only one that need be considered
   is fragmentation; a fragmented SIPP datagram may need to be
   reassembled prior to conversion.  Other "payloads" such as routing
   are ignored (translated verbatim) and will normally simply fail to
   achieve the desired effect.

   Translation to SIPP is simple, except for the difficult problem of
   inventing the "prefix" if an implementation wants to support
   translating Internet AD 0.0 numbers into the SIPP addressing domain.

Internet DNS

   CATNIP addresses are represented in the DNS with the NSAP RR. The
   data in the resource record is the NSAP, including the zero selector
   at the end. The zone file syntax for the data is a string of
   hexadecimal digits, with a period "." inserted between any two octets
   where desired for readability. For example:

   The inverse (PTR) zone is .NSAP.INT, with the CATNIP address
   (reversed).  That is, like .IN-ADDR.ARPA, but with .NSAP.INT instead.
   The nibbles are represented as hexadecimal digits.

   This respects the difference in actual authority: the IANA is the
   authority for the entire space rooted in .IN-ADDR.ARPA. in the
   version 4 Internet, while in the new Internet it holds the authority
   only for 0.C.NSAP.INT. (Following the example of 192 as the AFI
   value.) The domain 0.0.0.0.0.C.NSAP.INT is to be delegated by IANA to

   the InterNIC. (Understanding that in present practice the InterNIC is
   the operator of the authoritative root.)

Security Considerations

   The CATNIP design permits the direct use of the present proposals for
   network layer security being developed in the IPSEC WG of the IETF.
   There are a number of detailed requirements; the most relevant being
   that network layer datagram translation must not affect (cannot
   affect) the transport layers, since the TPDU is mostly inaccessible
   to the router. For example, the translation into IPX will only work
   if the port numbers are shadowed into the plaintext security header.

References

   [Chapin93]      Chapin, L., and D. Piscitello, "Open Systems
                   Networking", Addison-Wesley, Reading, Massachusetts,
                   1993.

   [Perlman92]     Perlman, R., "Interconnections: Bridges and Routers"
                   Addison-Wesley, Reading, Massachusetts, 1992.

   [RFC791]        Postel, J., Editor, "Internet Protocol - DARPA
                   Internet Program Protocol Specification", STD 5, RFC
                   791 USC/Information Sciences Institute,  September
                   1981.

   [RFC792]        Postel, J., Editor, "Internet Control Message
                   Protocol - DARPA Internet Program Protocol
                   Specification", STD 5, RFC 792, USC/Information
                   Sciences Institute, September 1981.

   [RFC793]        Postel, J., Editor, "Transmission Control Protocol -
                   DARPA Internet Program Protocol Specification,
                   STD 7, RFC 793, USC/Information Sciences Institute,
                   September, 1981.

   [RFC801]        Postel, J., "NCP/TCP Transition Plan", RFC 801,
                   USC/Information Sciences Institute, November, 1981.

   [RFC1191]       Mogul, J., and S. Deering, "Path MTU Discovery",
                   RFC 1191, DECWRL, Stanford University, November,
                   1990.

   [RFC1234]       Provan, D., "Tunneling IPX Traffic Through IP
                   Networks", RFC 1234, Novell, Inc., June 1991.

   [RFC1247]       Moy, J., "OSPF Version 2", RFC 1247, Proteon, Inc.,
                   July 1991.

   [RFC1287]       Clark, D., Chapin, L., Cerf, V., Braden, R., and
                   R. Hobby, "Towards the Future Internet Architecture",
                   RFC 1287, MIT, BBN, CNRI, ISI, UCDavis, December,
                   1991.

   [RFC1335]       Wang, Z., and J. Crowcroft, "A Two-Tier Address
                   Structure for the Internet: A Solution to the
                   Problem of Address Space Exhaustion", RFC 1335,
                   University College London, May 1992.

   [RFC1338]       Fuller, V., Li, T., Yu, J., and K. Varadhan,
                   "Supernetting: an Address Assignment and Aggregation
                   Strategy", RFC 1338, BAARNet, cicso, Merit, OARnet,
                   June 1992.

   [RFC1347]       Callon, R., "TCP and UDP with Bigger Addresses
                   (TUBA), A Simple Proposal for Internet Addressing
                   and Routing", RFC 1347, DEC, June 1992.

   [RFC1466]       Gerich, E., "Guidelines for Management of IP Address
                   Space", RFC 1466, Merit, May 1993.

   [RFC1475]       Ullmann, R., "TP/IX: The Next Internet", RFC 1475,
                   Process Software Corporation, June 1993.

   [RFC1476]       Ullmann, R., "RAP: Internet Route Access Protocol",
                   RFC 1476, Process Software Corporation, June 1993.

   [RFC1561]       Piscitello, D., "Use of ISO CLNP in TUBA
                   Environments", RFC 1561, Core Competence, December
                   1993.

Authors' Addresses

   Michael McGovern
   Sunspot Graphics

   EMail: scrivner@world.std.com

   Robert Ullmann
   Lotus Development Corporation
   1 Rogers Street
   Cambridge, MA 02142

   Phone: +1 617 693 1315
   EMail: rullmann@crd.lotus.com