    Network Working Group                                         P. Francis
    Request for Comments: 1621                                           NTT
    Category: Informational                                         May 1994

                           Pip Near-term Architecture

    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.

    Preamble

       During 1992 and 1993, the Pip internet protocol, developed at
       Belclore, was one of the candidate replacments for IP.  In mid 1993,
       Pip was merged with another candidate, the Simple Internet Protocol
       (SIP), creating SIPP (SIP Plus).  While the major aspects of Pip--
       particularly its distinction of identifier from address, and its use
       of the source route mechanism to achieve rich routing capabilities--
       were preserved, many of the ideas in Pip were not.  The purpose of
       this RFC and the companion RFC "Pip Header Processing" are to record
       the ideas (good and bad) of Pip.

       This document references a number of Pip draft memos that were in
       various stages of completion.  The basic ideas of those memos are
       presented in this document, though many details are lost.  The very
       interested reader can obtain those internet drafts by requesting them
       directly from me at <francis@cactus.ntt.jp>.

       The remainder of this document is taken verbatim from the Pip draft
       memo of the same title that existed when the Pip project ended.  As
       such, any text that indicates that Pip is an intended replacement for
       IP should be ignored.

    Abstract

       Pip is an internet protocol intended as the replacement for IP
       version 4.  Pip is a general purpose internet protocol, designed to
       evolve to all forseeable internet protocol requirements.  This
       specification describes the routing and addressing architecture for
       near-term Pip deployment.  We say near-term only because Pip is
       designed with evolution in mind, so other architectures are expected
       in the future.  This document, however, makes no reference to such
       future architectures.

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    RFC 1621               Pip Near-term Architecture               May 1994

    Table of Contents

       1. Pip Architecture Overview ...................................    4
       1.1 Pip Architecture Characteristics ...........................    4
       1.2 Components of the Pip Architecture .........................    5

       2. A Simple Example ............................................    6

       3. Pip Overview ................................................    7

       4. Pip Addressing ..............................................    9
       4.1 Hierarchical Pip Addressing ................................    9
       4.1.1 Assignment of (Hierarchical) Pip Addresses ...............   12
       4.1.2 Host Addressing ..........................................   14
       4.2 CBT Style Multicast Addresses ..............................   15
       4.3 Class D Style Multicast Addresses ..........................   16
       4.4 Anycast Addressing .........................................   16

       5. Pip IDs .....................................................   17

       6. Use of DNS ..................................................   18
       6.1 Information Held by DNS ....................................   19
       6.2 Authoritative Queries in DNS ...............................   20

       7. Type-of-Service (TOS) (or lack thereof) .....................   21

       8. Routing on (Hierarchical) Pip Addresses .....................   22
       8.1 Exiting a Private Domain ...................................   23
       8.2 Intra-domain Networking ....................................   24

       9. Pip Header Server ...........................................   25
       9.1 Forming Pip Headers ........................................   25
       9.2 Pip Header Protocol (PHP) ..................................   27
       9.3 Application Interface ......................................   27

       10. Routing Algorithms in Pip ..................................   28
       10.1 Routing Information Filtering .............................   29

       11. Transition .................................................   30
       11.1 Justification for Pip Transition Scheme ...................   31
       11.2 Architecture for Pip Transition Scheme ....................   31
       11.3 Translation between Pip and IP packets ....................   33
       11.4 Translating between PCMP and ICMP .........................   34
       11.5 Translating between IP and Pip Routing Information ........   34
       11.6 Old TCP and Application Binaries in Pip Hosts .............   34
       11.7 Translating between Pip Capable and non-Pip Capable DNS
            Servers ...................................................   35

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       12. Pip Address and ID Auto-configuration ......................   37
       12.1 Pip Address Prefix Administration .........................   37
       12.2 Host Autoconfiguration ....................................   38
       12.2.1 Host Initial Pip ID Creation ............................   38
       12.2.2 Host Pip Address Assignment .............................   39
       12.2.3 Pip ID and Domain Name Assignment .......................   39

       13. Pip Control Message Protocol (PCMP) ........................   40

       14. Host Mobility ..............................................   42
       14.1 PCMP Mobile Host message ..................................   43
       14.2 Spoofing Pip IDs ..........................................   44

       15. Public Data Network (PDN) Address Discovery ................   44
       15.1 Notes on Carrying PDN Addresses in NSAPs ..................   46

       16. Evolution with Pip .........................................   46
       16.1 Handling Directive (HD) and Routing Context (RC) Evolution.   49
       16.1.1 Options Evolution .......................................   50
       References .....................................................   51
       Security Considerations ........................................   51
       Author's Address ...............................................   51

    Francis                                                         [Page 3]

    RFC 1621               Pip Near-term Architecture               May 1994

    Introduction

       Pip is an internet protocol intended as the replacement for IP
       version 4.  Pip is a general purpose internet protocol, designed to
       handle all forseeable internet protocol requirements.  This
       specification describes the routing and addressing architecture for
       near-term Pip deployment.  We say near-term only because Pip is
       designed with evolution in mind, so other architectures are expected
       in the future.  This document, however, makes no reference to such
       future architectures (except in that it discusses Pip evolution in
       general).

       This document gives an overall picture of how Pip operates.  It is
       provided primarily as a framework within which to understand the
       total set of documents that comprise Pip.

    1.  Pip Architecture Overview

       The Pip near-term architecture is an incremental step from IP.  Like
       IP, near-term Pip is datagram.  Pip runs under TCP and UDP.  DNS is
       used in the same fashion it is now used to distribute name to Pip
       Address (and ID) mappings.  Routing in the near-term Pip architecture
       is hop-by-hop, though it is possible for a host to create a domain-
       level source route (for policy reasons).

       Pip Addresses have more hierarchy than IP, thus improving scaling on
       one hand, but introducing additional addressing complexities, such as
       multiple addresses, on the other.  Pip, however, uses hierarchical
       addresses to advantage by making them provider-based, and using them
       to make policy routing (in this case, provider selection) choices.
       Pip also provides mechanisms for automatically assigning provider
       prefixes to hosts and routers in domains.  This is the main
       difference between the Pip near-term architecture and the IP
       architecture.  (Note that in the remainder of this paper, unless
       otherwise stated, the phrase "Pip architecture" refers to the near-
       term Pip architecture described herein.)

    2.  Pip Architecture Characteristics

       The proposed architecture for near-term Pip has the following
       characteristics:

       1.  Provider-rooted hierarchical addresses.

       2.  Automatic domain-wide address prefix assignment.

       3.  Automatic host address and ID assignment.

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       4.  Exit provider selection.

       5.  Multiple defaults routing (default routing, but to multiple exit
           points).

       6.  Equivalent of IP Class D style addressing for multicast.

       7.  CBT style multicast.

       8.  "Anycast" addressing (route to one of a group, usually the
           nearest).

       9.  Providers support forwarding on policy routes (but initially will
           not provide the support for sources to calculate policy routes).

       10.  Mobile hosts.

       11.  Support for routing across large Public Data Networks (PDN).

       12.  Inter-operation with IP hosts (but, only within an IP-address
            domain where IP addresses are unique).  In particular, an IP
            address can be explicitly carried in a Pip header.

       13.  Operation with existing transport and application binaries
            (though if the application contains IP context, like FTP, it may
            only work within a domain where IP addresses are unique).

       14.  Mechanisms for evolving Pip beyond the near-term architecture.

    1.2 Components of the Pip Architecture

       The Pip Architecture consists of the following five systems:

       1.  Host (source and sink of Pip packets)

       2.  Router (forwards Pip packets)

       3.  DNS

       4.  Pip/IP Translator

       5.  Pip Header Server (formats Pip headers)

       The first three systems exist in the IP architecture, and require no
       explanation here.  The fourth system, the Pip/IP Translator, is
       required solely for the purpose of inter-operating with current IP
       systems.  All Pip routers are also Pip/IP translators.

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    RFC 1621               Pip Near-term Architecture               May 1994

       The fifth system, the Pip Header Server, is new.  Its function is to
       format Pip headers on behalf of the source host (though initially
       hosts will be able to do this themselves).  This use of the Pip
       Header Server will increase as policy routing becomes more
       sophisticated (moves beyond near-term Pip Architecture capabilities).

       To handle future evolution, a Pip Header Server can be used to
       "spoon-feed" Pip headers to old hosts that have not been updated to
       understand new uses of Pip.  This way, the probability that the
       internet can evolve without changing all hosts is increased.

    2.  A Simple Example

       A typical Pip "exchange" is as follows: An application initiates an
       exchange with another host as identified by a domain name.  A request
       for one or more Pip Headers, containing the domain name of the
       destination host, goes to the Pip Header Server.  The Pip Header
       Server generates a DNS request, and receive back a Pip ID, multiple
       Pip Addresses, and possibly other information such as a mobile host
       server or a PDN address.  Given this information, plus information
       about the source host (its Pip Addresses, for instance), plus
       optionally policy information, plus optionally topology information,
       the Pip Header Server formats an ordered list of valid Pip headers
       and give these to the host.  (Note that if the Pip Header Server is
       co-resident with the host, as will be common initially, the host
       behavior is similar to that of an IP host in that a DNS request comes
       from the host, and the host forms a Pip header based on the answer
       from DNS.)

       The source host then begins to transmit Pip packets to the
       destination host.  If the destination host is an IP host, then the
       Pip packet is translated into an IP packet along the way.  Assuming
       that the destination host is a Pip host, however, the destination
       host uses the destination Pip ID alone to determine if the packet is
       destined for it.  The destination host generates a return Pip header
       based either on information in the received Pip header, or the
       destination host uses the Pip ID of the source host to query the Pip
       Header Server/DNS itself.  The latter case involves more overhead,
       but allows a more informed decision about how to return packets to
       the originating host.

       If either host is mobile, and moves to a new location, thus getting a
       new Pip Address, it informs the other host of its new address
       directly.  Since host identification is based on the Pip ID and not
       the Pip Address, this doesn't cause transport level to fail.  If both
       hosts are mobile and receive new Pip Addresses at the same time (and
       thus cannot exchange packets at all), then they can query each
       other's respective mobile host servers (learned from DNS).  Note that

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       keeping track of host mobility is completely confined to hosts.
       Routers never get involved in tracking mobile hosts (though naturally
       they are involved in host discovery and automatic host address
       assignment).

    3.  Pip Overview

       Here, a brief overview of the Pip protocol is given.  The reader is
       encouraged to read [2] for a complete description.

       The Pip header is divided into three parts:

          Initial Part
          Transit Part
          Options Part

       The Initial Part contains the following fields:

          Version Number
          Options Offset, OP Contents, Options Present (OP)
          Packet SubID
          Protocol
          Dest ID
          Source ID
          Payload Length
          Host Version
          Payload Offset
          Hop Count

       All of the fields in the Initial Part are of fixed length.  The
       Initial Part is 8 32-bit words in length.

       The Version Number places Pip as a subsequent version of IP.  The
       Options Offset, OP Contents, and Options Present (OP) fields tell how
       to process the options.  The Options Offset tells where the options
       are The OP tells which of up to 8 options are in the options part, so
       that the Pip system can efficiently ignore options that don't pertain
       to it.  The OP Contents is like a version number for the OP field.
       It allows for different sets of the (up to 8) options.

       The Packet SubID is used to relate a received PCMP message to a
       previously sent Pip packet.  This is necessary because, since routers
       in Pip can tag packets, the packet returned to a host in a PCMP
       message may not be the same as the packet sent.  The Payload Length
       and Protocol take the place of IP's Total Length and Protocol fields
       respectively.  The Dest ID identifies the destination host, and is
       not used for routing, except for where the final router on a LAN uses
       ARP to find the physical address of the host identified by the dest

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       ID.  The Source ID identifies the source of the packet.  The Host
       Version tells what control algorithms the host has implemented, so
       that routers can respond to hosts appropriately.  This is an
       evolution mechanism.  The Hop Count is similar to IP's Time-to-Live.

       The Transit Part contains the following fields:

          Transit Part Offset
          HD Contents
          Handling Directive (HD)
          Active FTIF
          RC Contents
          Routing Context (RC)
          FTIF Chain (FTIF = Forwarding Table Index Field)

       Except for the FTIF Chain, which can have a variable number of 16-bit
       FTIF fields, the fields in the Transit Part are of fixed length, and
       are three 32-bit words in length.

       The Transit Part Offset gives the length of the Transit Part.  This
       is used to determine the location of the subsequent Transit Part (in
       the case of Transit Part encapsulation).

       The Handling Directive (HD) is a set of subfields, each of which
       indicates a specific handling action that must be executed on the
       packet.  Handling directives have no influence on routing.  The HD
       Contents field indicates what subfields are in the Handling
       Directive.  This allows the definition of the set of handling
       directives to evolve over time.  Example handling directives are
       queueing priority, congestion experienced bit, drop priority, and so
       on.

       The remaining fields comprise the Routing Directive.  This is where
       the routing decision gets made.  The basic algorithm is that the
       router uses the Routing Context to choose one of multiple forwarding
       tables.  The Active FTIF indicates which of the FTIFs to retrieve,
       which is then used as an index into the forwarding table, which
       either instructs the router to look at the next FTIF, or returns the
       forwarding information.

       Examples of Routing Context uses are; to distinguish address families
       (multicast vs. unicast), to indicate which level of the hierarchy a
       packet is being routed at, and to indicate a Type of Service.  In the
       near-term architecture, the FTIF Chain is used to carry source and
       destination hierarchical unicast addresses, policy route fragments,
       multicast addresses (all-of-group), and anycast (one-of-group)
       addresses.  Like the OP Contents and HD Contents fields, the RC
       Contents field indicates what subfields are in the Routing Context.

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       This allows the definition of the Routing Context to evolve over
       time.

       The Options Part contains the options.  The options are preceded by
       an array of 8 fields that gives the offset of each of up to 8
       options.  Thus, a particular option can be found without a serial
       search of the list of options.

    4.  Pip Addressing

       Addressing is the core of any internet architecture.  Pip Addresses
       are carried in the Routing Directive (RD) of the Pip header (except
       for the Pip ID, which in certain circumstances functions as part of
       the Pip Address).  Pip Addresses are used only for routing packets.
       They do not identify the source and destination of a Pip packet.  The
       Pip ID does this.  Here we describe and justify the Pip Addressing
       types.

       There are four Pip Address types [11].  The hierarchical Pip Address
       (referred to simply as the Pip Address) is used for scalable unicast
       and for the unicast part of a CBT-style multicast and anycast.  The
       multicast part of a CBT-style multicast is the second Pip address
       type.  The third Pip address type is class-D style multicast.  The
       fourth type of Pip address is the so-called "anycast" address.  This
       address causes the packet to be forwarded to one of a class of
       destinations (such as, to the nearest DNS server).

       Bits 0 and 1 of the RC defined by RC Contents value of 1 (that is,
       for the near-term Pip architecture) indicate which of four address
       families the FTIFs and Dest ID apply to.  The values are:

          Value      Address Family
          -----      --------------
           00        Hierarchical Unicast Pip Address
           01        Class D Style Multicast Address
           10        CBT Style Multicast Address
           11        Anycast Pip Address

       The remaining bits are defined differently for different address
       families, and are defined in the following sections.

    4.1  Hierarchical Pip Addressing

       The primary purpose of a hierarchical address is to allow better
       scaling of routing information, though Pip also uses the "path"
       information latent in hierarchical addresses for making provider
       selection (policy routing) decisions.

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       The Pip Header encodes addresses as a series of separate numbers, one
       number for each level of hierarchy.  This can be contrasted to
       traditional packet encodings of addresses, which places the entire
       address into one field.  Because of Pip's encoding, it is not
       necessary to specify a format for a Pip Address as it is with
       traditional addresses (for instance, the SIP address is formatted
       such that the first so-many bits are the country/metro code, the next
       so-many bits are the site/subscriber, and so on).  Pip's encoding
       also eliminates the "cornering in" effect of running out of space in
       one part of the hierarchy even though there is plenty of room in
       another.  No "field sizing" decisions need be made at all with Pip
       Addresses.  This makes address assignment easier and more flexible
       than with traditional addresses.

       Pip Addresses are carried in DNS as a series of numbers, usually with
       each number representing a layer of the hierarchy [1], but optionally
       with the initial number(s) representing a "route fragment" (the tail
       end of a policy route--a source route whose elements are providers).
       The route fragments would be used, for instance, when the destination
       network's directly attached (local access) provider is only giving
       access to other (long distance) providers, but the important
       provider-selection policy decision has to do the long distance
       providers.

       The RC for (hierarchical) Pip Addresses is defined as:

          bits       meaning
          ----       -------
          0,1        Pip Address (= 00)
          2,3        level
          4,5        metalevel
          6          exit routing type

       The level and metalevel subfields are used to indicate what level of
       the hierarchy the packet is currently at (see section 8).  The exit
       routing type subfield is used to indicate whether host-driven (hosts
       decide exit provider) or router-driven (routers decide exit provider)
       exit routing is in effect (see section 8.1).

       Each FTIF in the FTIF Chain is 16 bits in length.  The low-order part
       of each FTIF in a (hierarchical unicast) Pip Address indicates the
       relationship of the FTIF with the next FTIF.  The three relators are
       Vertical, Horizontal, and Extension.  The Vertical and Horizontal
       relators indicate if the subsequent FTIF is hierarchically above or
       below (Vertical) or hierarchically unrelated (Horizontal).  The
       Extension relator is used to encode FTIF values longer than 16 bits.

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       FTIF values 0 - 31 are reserved for special purposes.  That is, they
       cannot be assigned to normal hierarchical elements.  FTIF value 1 is
       defined as a flag to indicate a switch from the unicast phase of
       packet forwarding to the anycast phase of packet forwarding.

       Note that Pip Addresses do not need to be seen by protocol layers
       above Pip (though layers above Pip can provide a Pip Address if
       desired).  Transport and above use the Pip ID to identify the source
       and destination of a Pip packet.  The Pip layer is able to map the
       Pip IDs (and other information received from the layer above, such as
       QOS) into Pip Addresses.

       The Pip ID can serve as the lowest level of a Pip Address.  While
       this "bends the principal" of separating Pip Addressing from Pip
       Identification, it greatly simplifies dynamic host address
       assignment.  The Pip ID also serves as a multicast ID.  Unless
       otherwise stated, the term "Pip Address" refers to just the part in
       the Routing Directive (that is, excludes the Pip ID).

       Pip Addresses are provider-rooted (as opposed to geographical).  That
       is, the top-level of a Pip Address indicates a network service
       provider (even when the service provided is not Pip).  (A
       justification of using provider-rooted rather than geographical
       addresses is given in [12].)

       Thus, the basic form of a Pip address is:

             providerPart,subscriberPart

       where both the providerPart and subscriberPart can have multiple
       layers of hierarchy internally.

       A subscriber may be attached to multiple providers.  In this case, a
       host can end up with multiple Pip Addresses by virtue of having
       multiple providerParts:

             providerPart1,subscriberPart
             providerPart2,subscriberPart
             providerPart3,subscriberPart

       This applies to the case where the subscriber network spans many
       different provider areas, for instance, a global corporate network.
       In this case, some hosts in the global corporate network will have
       certain providerParts, and other hosts will have others.  The
       subscriberPart should be assigned such that routing can successfully
       take place without a providerPart in the destination Pip Address of
       the Pip Routing Directive (see section 8.2).

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    RFC 1621               Pip Near-term Architecture               May 1994

       Note that, while there are three providerParts shown, there is only
       one subscriberPart.  Internal subscriber numbering should be
       independent of the providerPart.  Indeed, with the Pip architecture,
       it is possible to address internal packets without including any of
       the providerPart of the address.

       Top-level Pip numbers can be assigned to subscriber networks as well
       as to providers.

             privatePart,subscriberPart

       In this case, however, the top-level number (privatePart) would not
       be advertised globally.  The purpose of such an assignment is to give
       a private network "ownership" of a globally unique Pip Address space.
       Note that the privatePart is assigned as an extended FTIF (that is,
       from numbers greater than 2^15).  Because the privatePart is not
       advertised globally, and because internal packets do not need the
       prefix (above the subscriberPart), the privatePart actually never
       appears in a Pip packet header.

       Pip Addresses can be prepended with a route fragment.  That is, one
       or more Pip numbers that are all at the top of the hierarchy.

             longDistanceProvider.localAccessProvider.subscriber
                 (top-level)          (top-level)     (next level)

       This is useful, for instance, when the subscriber's directly attached
       provider is a "local access" provider, and is not advertised
       globally.  In this case, the "long distance" provider is prepended to
       the address even though the local access provider number is enough to
       provide global uniqueness.

       Note that no coordination is required between the long distance and
       local access providers to form this address.  The subscriber with a
       prefix assigned to it by the local access provider can autonomously
       form and use this address.  It is only necessary that the long
       distance provider know how to route to the local access provider.

    4.1.1  Assignment of (Hierarchical) Pip Addresses

       Administratively, Pip Addresses are assigned as follows [3].  There
       is a root Pip Address assignment authority.  Likely choices for this
       are IANA or ISOC.  The root authority assigns top-level Pip Address
       numbers.  (A "Pip Address number" is the number at a single level of
       the Pip Address hierarchy.  A Pip Address prefix is a series of
       contiguous Pip Address numbers, starting at the top level but not
       including the entire Pip Address.  Thus, the top-level prefix is the
       same thing as the top-level number.)

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       Though by-and-large, and most importantly, top-level assignments are
       made to providers, each country is given an assignment, each existing
       address space (such as E.164, X.121, IP, etc.) is given an
       assignment, and private networks can be given assignments.  Thus,
       existing addresses can be grandfathered in.  Even if the top-level
       Pip address number is an administrative rather than topological
       assignment, the routing algorithm still advertises providers at the
       top (provider) level of routing.  That is, routing will advertise
       enough levels of hierarchy that providers know how to route to each
       other.

       There must be some means of validating top-level number requests from
       providers (basically, those numbers less than 2^15).  That is, top-
       level assignments must be made only to true providers.  While
       designing the best way to do this is outside the scope of this
       document, it seems off hand that a reasonable approach is to charge
       for the top-level prefixes.  The charge should be enough to
       discourage non-serious requests for prefixes, but not so much that it
       becomes an inhibitor to entry in the market.  The charge might
       include a yearly "rent", and top-level prefixes could be reclaimed
       when they are no longer used by the provider.  Any profit made from
       this activity could be used to support the overall role of number
       assignment.  Since roughly 32,000 top-level assignments can be made
       before having to increase the FTIF size in the Pip header from 16
       bits to 32 bits, it is envisioned that top-level prefixes will not be
       viewed as a scarce resource.

       After a provider obtains a top-level prefix, it becomes an assignment
       authority with respect to that particular prefix.  The provider has
       complete control over assignments at the next level down (the level
       below the top-level).  The provider may either assign top-level minus
       one prefixes to subscribers, or preferably use that level to provide
       hierarchy within the provider's network (for instance, in the case
       where the provider has so many subscribers that keeping routing
       information on all of them creates a scaling problem).  It is
       envisioned that the subscriber will have complete control over number
       assignments made at levels below that of the prefix assigned it by
       the provider.

       Assigning top level prefixes directly to providers leaves the number
       of top-level assignments open-ended, resulting in the possibility of
       scaling problems at the top level.  While it is expected that the
       number of providers will remain relatively small (say less than 10000
       globally), this can't be guaranteed.  If there are more providers
       than top-level routing can handle, it is likely that many of these
       providers will be "local access" providers--providers whose role is
       to give a subscriber access to multiple "long-distance" providers.
       In this case, the local access providers need not appear at the top

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    RFC 1621               Pip Near-term Architecture               May 1994

       level of routing, thus mitigating the scaling problem at that level.

       In the worst case, if there are too many top-level "long-distance"
       providers for top-level routing to handle, a layer of hierarchy above
       the top-level can be created.  This layer should probably conform to
       some policy criteria (as opposed to a geographical criteria).  For
       instance, backbones with similar access restrictions or type-of-
       service can be hierarchically clustered.  Clustering according to
       policy criteria rather than geographical allows the choice of address
       to remain an effective policy routing mechanism.  Of course, adding a
       layer of hierarchy to the top requires that all systems, over time,
       obtain a new providerPart prefix.  Since Pip has automatic prefix
       assignment, and since DNS hides addresses from users, this is not a
       debilitating problem.

    4.1.2  Host Addressing

       Hosts can have multiple Pip Addresses.  Since Pip Addresses are
       topologically significant, a host has multiple Pip Addresses because
       it exists in multiple places topologically.  For instance, a host can
       have multiple Pip addresses because it can be reached via multiple
       providers, or because it has multiple physical interfaces.  The
       address used to reach the host influences the path to the host.

       Locally, Pip Addressing is similar to IP Addressing.  That is, Pip
       prefixes are assigned to subnetworks (where the term subnetwork here
       is meant in the OSI sense.  That is, it denotes a network operating
       at a lower layer than the Pip layer, for instance, a LAN).  Thus, it
       is not necessary to advertise individual hosts in routing updates--
       routers only need to advertise and store routes to subnetworks.

       Unlike IP, however, a single subnetwork can have multiple prefixes.
       (Strictly speaking, in IP a single subnetwork can have multiple
       prefixes, but a host may not be able to recognize that it can reach
       another host on the same subnetwork but with a different prefix
       without going through a router.)

       There are two styles of local Pip Addressing--one where the Pip
       Address denotes the host, and another where the Pip Address denotes
       only the destination subnetwork.  The latter style is called ID-
       tailed Pip Addressing.  With ID-tailed Pip Addresses, the Pip ID is
       used by the last router to forward the packet to the host.  It is
       expected that ID-tailed Pip Addressing is the most common, because it
       greatly eases address administration.

       (Note that the Pip Routing Directive can be used to route a Pip
       packet internal to a host.  For instance, the RD can be used to
       direct a packet to a device in a host, or even a certain memory

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    RFC 1621               Pip Near-term Architecture               May 1994

       location.  The use of the RD for this purpose is not part of this
       near-term Pip architecture.  We note, however, that this use of the
       RD could be locally done without effecting any other Pip systems.)

       When a router receives a Pip packet and determines that the packet is
       destined for a host on one of its' attached subnetworks (by examining
       the appropriate FTIF), it then examines the destination Pip ID (which
       is in a fixed position) and forwards based on that.  If it does not
       know the subnetwork address of the host, then it ARPs, using the Pip
       ID as the "address" in the ARP query.

    4.2  CBT Style Multicast Addresses

       When bits 1 and 0 of the RC defined by RC Contents = 1 are set to 10,
       the FTIF and Dest ID indicate CBT (Core Based Tree) style multicast.
       The remainder of the bits are defined as follows:

          bits       meaning
          ----       -------
          0,1        CBT Multicast (= 10)
          2,3        level
          4,5        metalevel
          6          exit routing type
          7          on-tree bit
          8,9        scoping

       With CBT (Core-based Tree) multicast, there is a single multicast
       tree connecting the members (recipients) of the multicast group (as
       opposed to Class-D style multicast, where there is a tree per
       source).  The tree emanates from a single "core" router.  To transmit
       to the group, a packet is routed to the core using unicast routing.
       Once the packet reaches a router on the tree, it is multicast using a
       group ID.

       Thus, the FTIF Chain for CBT multicast contains the (Unicast)
       Hierarchical Pip Address of the core router. The Dest ID field
       contains the group ID.

       A Pip CBT packet, then, has two phases of forwarding, a unicast phase
       and a multicast phase.  The "on-tree" bit of the RC indicates which
       phase the packet is in.  While in the unicast phase, the on-tree bit
       is set to 0, and the packet is forwarded similarly to Pip Addresses.
       During this phase, the scoping bits are ignored.

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       Once the packet reaches the multicast tree, it switches to multicast
       routing by changing the on-tree bit to 1 and using the Dest ID group
       address for forwarding.  During this phase, bits 2-6 are ignored.

    4.3  Class D Style Multicast Addresses

       When bits 1 and 0 of the RC defined by RC Contents = 1 are set to 01,
       the FTIF and Dest ID indicate Class D style multicast.  The remainder
       of the RC is defined as:

          bits       meaning
          ----       -------
          0,1        Class D Style Multicast (= 01)
          2-5        Scoping

       By "class D" style multicast, we mean multicast using the algorithms
       developed for use with Class D addresses in IP (class D addresses are
       not used per se).  This style of routing uses both source and
       destination information to route the packet (source host address and
       destination multicast group).

       For Pip, the FTIF Chain holds the source Pip Address, in order of
       most significant hierarchy level first.  The reason for putting the
       source Pip Address rather than the Source ID in the FTIF Chain is
       that use of the source Pip Address allows the multicast routing to
       take advantage of the hierarchical source address, as is being done
       with IP.  The Dest ID field holds the multicast group.  The Routing
       Context indicates Class-D style multicast.  All routers must first
       look at the FTIF Chain and Dest ID field to route the packet on the
       tree.

       Bits 2 through 5 of the RC are the scoping bits.

    4.4  Anycast Addressing

       When bits 1 and 0 of the RC defined by RC Contents = 1 are set to 11,
       the FTIF and Dest ID indicate Anycast addressing.  The remainder of
       the RC is defined as:

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          bits       meaning
          ----       -------
          0,1        Anycast Address (= 11)
          2,3        level
          4,5        metalevel
          6          exit routing type
          7          anycast active
          8,9        scoping

       With anycast routing, the packet is unicast, but to the nearest of a
       group of destinations.  This type of routing is used by Pip for
       autoconfiguration.  Other applications, such as discovery protocols,
       may also use anycast routing.

       Like CBT, Pip anycast has two phases of operation, in this case the
       unicast phase and the anycast phase.  The unicast phase is for the
       purpose of getting the packet into a certain vicinity.  The anycast
       phase is to forward the packet to the nearest of a group of
       destinations in that vicinity.

       Thus, the RC has both unicast and anycast information in it.  During
       the unicast phase, the anycast active bit is set to 0, and the packet
       is forwarded according to the rules of Pip Addressing.  The scoping
       bits are ignored.

       The switch from the unicast phase to the anycast phase is triggered
       by the presence of an FTIF of value 1 in the FTIF Chain.  When this
       FTIF is reached, the anycast active bit is set to 1, the scoping bits
       take effect, and bits 2 through 6 are ignored.  When in the anycast
       phase, forwarding is based on the Dest ID field.

    5.  Pip IDs

       The Pip ID is 64-bits in length [4].

       The basic role of the Pip ID is to identify the source and
       destination host of a Pip Packet.  (The other role of the Pip ID is
       for allowing a router to find the destination host on the destination
       subnetwork.)

       This having been said, it is possible for the Pip ID to ultimately
       identify something in addition to the host.  For instance, the Pip ID
       could identify a user or a process.  For this to work, however, the
       Pip ID has to be bound to the host, so that as far as the Pip layer
       is concerned, the ID is that of the host.  Any additional use of the
       Pip ID is outside the scope of this Pip architecture.

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       The Pip ID is treated as flat.  When a host receives a Pip packet, it
       compares the destination Pip ID in the Pip header with its' own.  If
       there is a complete match, then the packet has reached the correct
       destination, and is sent to the higher layer protocol.  If there is
       not a complete match, then the packet is discarded, and a PCMP
       Invalid Address packet is returned to the originator of the packet
       [7].

       It is something of an open issue as to whether or not Pip IDs should
       contain significant organizational hierarchy information.  Such
       information could be used for inverse DNS lookups and allowing a Pip
       packet to be associated with an organization.  (Note that the use of
       the Pip ID alone for this purpose can be easily spoofed.  By cross
       checking the Pip ID with the Pip Address prefix, spoofing is harder-
       -as hard as it is with IP--but still easy.  Section 14.2 discusses
       methods for making spoofing harder still, without requiring
       encryption.)

       However, relying on organizational information in the Pip header
       generally complicates ID assignment.  This complication has several
       ramifications.  It makes host autoconfiguration of hosts harder,
       because hosts then have to obtain an assignment from some database
       somewhere (versus creating one locally from an IEEE 802 address, for
       instance).  It means that a host has to get a new assignment if it
       changes organizations.  It is not clear what the ramifications of
       this might be in the case of a mobile host moving through different
       organizations.

       Because of these difficulties, the use of flat Pip IDs is currently
       favored.

       Blocks of Pip ID numbers have been reserved for existing numbering
       spaces, such as IP, IEEE 802, and E.164.  Pip ID numbers have been
       assigned for such special purposes such as "any host", "any router",
       "all hosts on a subnetwork", "all routers on a subnetwork", and so
       on.  Finally, 32-bit blocks of Pip ID numbers have been reserved for
       each country, according to ISO 3166 country code assignments.

    6.  Use of DNS

       The Pip near-term architecture uses DNS in roughly the same style
       that it is currently used.  In particular, the Pip architecture
       maintains the two fundamental DNS characteristics of 1) information
       stored in DNS does not change often, and 2) the information returned
       by DNS is independent of who requested it.

       While the fundamental use of DNS remains roughly the same, Pip's use
       of DNS differs from IP's use by degrees.  First, Pip relies on DNS to

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       hold more types of information than IP [1].  Second, Pip Addresses in
       DNS are expected to change more often than IP addresses, due to
       reassignment of Pip Address prefixes (the providerPart).  To still
       allow aggressive caching of DNS records in the face of more quickly
       changing addressing, Pip has a mechanism of indicating to hosts when
       an address is no longer assigned.  This triggers an authoritative
       query, which overrides DNS caches.  The mechanism consists of PCMP
       Packet Not Delivered messages that indicate explicitly that the Pip
       Address is invalid.

       In what follows, we first discuss the information contained in DNS,
       and then discuss authoritative queries.

    6.1  Information Held by DNS

       The information contained in DNS for the Pip architecture is:

       1.  The Pip ID.

       2.  Multiple Pip Addresses

       3.  The destination's mobile host address servers.

       4.  The Public Data Network (PDN) addresses through which the
           destination can be reached.

       5.  The Pip/IP Translators through which the destination (if the
           destination is IP-only) can be reached.

       6.  Information about the providers represented by the destination's
           Pip addresses.  This information includes provider name, the type
           of provider network (such as SMDS, ATM, or SIP), and access
           restrictions on the provider's network.

       The Pip ID and Addresses are the basic units of information required
       for carriage of a Pip packet.

       The mobile host address server tells where to send queries for the
       current address of a mobile Pip host. Note that usually the current
       address of the mobile host is conveyed by the mobile host itself,
       thus a mobile host server query is not usually required.

       The PDN address is used by the entry router of a PDN to learn the PDN
       address of the next hop router.  The entry router obtains the PDN
       address via an option in the Pip packet.  If there are multiple PDNs
       associated with a given Pip Address, then there can be multiple PDN
       addresses carried in the option.  Note that the option is not sent on
       every packet, and that only the PDN entry router need examine the

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       option.

       The Pip/IP translator information is used to know how to translate an
       IP address into a Pip Address so that the packet can be carried
       across the Pip infrastructure.  If the originating host is IP, then
       the first IP/Pip translator reached by the IP packet must query DNS
       for this information.

       The information about the destination's providers is used to help the
       "source" (either the source host or a Pip Header Server near the
       source host) format an appropriate Pip header with regards to
       choosing a Pip Address [14].  The choice of one of multiple Pip
       Addresses is essentially a policy routing choice.

       More detailed descriptions of the use of the information carried in
       DNS is contained in the relevant sections.

    6.2 Authoritative Queries in DNS

       In general, Pip treats addresses as more dynamic entities than does
       IP.  One example of this is how Pip Address prefixes change when a
       subscriber network attaches to a new provider.  Pip also carries more
       information in DNS, any of which can change for various reasons.
       Thus, the information in DNS is more dynamic with Pip than with IP.

       Because of the increased reliance on DNS, there is a danger of
       increasing the load on DNS.  This would be particularly true if the
       means of increasing DNS' dynamicity is by shortening the cache
       holding time by decreasing the DNS Time-to-Live (TTL).  To counteract
       this trend, Pip provides explicit network layer (Pip layer) feedback
       on the correctness of address information.  This allows Pip hosts to
       selectively over-ride cached DNS information by making an
       authoritative query.  Through this mechanism, we actually hope to
       increase the cache holding time of DNS, thus improving DNS' scaling
       characteristics overall.

       The network layer feedback is in the form of a type of PCMP Packet
       Not Delivered (PDN) message that indicates that the address used is
       known to be out-of-date.  Routers can be configured with this
       information, or it can be provided through the routing algorithm
       (when an address is decommissioned, the routing algorithm can
       indicate that this is the reason that it has become unreachable, as
       opposed to becoming "temporarily" unreachable through equipment
       failure).

       Pip hosts consider destination addresses to be in one of four states:

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       1.  Unknown, but assumed to be valid.

       2.  Reachable (and therefore valid).

       3.  Unreachable and known to be invalid.

       4.  Unreachable, but weakly assumed to be valid.

       The first state exists before a host has attempted communication with
       another host.  In this state, the host queries DNS as normal (that
       is, does not make an authoritative query).

       The second state is reached when a host has successfully communicated
       with another host.  Once a host has reached this state, it can stay
       in it for an arbitrarily long time, including after the DNS TTL has
       expired.  When in this state, there is no need to query DNS.

       A host enters the third state after a failed attempt at communicating
       with another host where the PCMP PND message indicates explicitly
       that the address is known to be invalid.  In this case, the host
       makes an authoritative query to DNS whether or not the TTL has
       expired.  It is this case that allows lengthy caching of DNS
       information while still allowing addresses to be reassigned
       frequently.

       A host enters the fourth state after a failed attempt at
       communicating with another host, but where the address is not
       explicitly known to be invalid.  In this state, the host weakly
       assumes that the address of the destination is still valid, and so
       can requery DNS with a normal (non-authoritative) query.

    7.  Type-of-Service (TOS) (or lack thereof)

       One year ago it probably would have been adequate to define a handful
       (4 or 5) of priority levels to drive a simple priority FIFO queue.
       With the advent of real-time services over the Internet, however,
       this is no longer sufficient.  Real-time traffic cannot be handled on
       the same footing as non-real-time.  In particular, real-time traffic
       must be subject to access control so that excess real-time traffic
       does not swamp a link (to the detriment of other real-time and non-
       real-time traffic alike).

       Given that a consensus solution to real- and non-real-time traffic
       management in the internet does not exist, this version of the Pip
       near-term architecture does not specify any classes of service (and
       related queueing mechanisms).  It is expected that Pip will define
       classes of service (primarily for use in the Handling Directive) as
       solutions become available.

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    8.  Routing on (Hierarchical) Pip Addresses

       Pip forwarding in a single router is done based on one or a small
       number of FTIFs.  What this means with respect to hierarchical Pip
       Addresses is that a Pip router is able to forward a packet based on
       examining only part of the Pip Address--often a single level.

       One advantage to encoding each level of the Pip Address separately is
       that it makes handling of addresses, for instance address assignment
       or managing multiple addresses, easier.  Another advantage is address
       lookup speed--the entire address does not have to be examined to
       forward a packet (as is necessary, for instance, with traditional
       hierarchical address encoding).  The cost of this, however, is
       additional complexity in keeping track of the active hierarchical
       level in the Pip header.

       Since Pip Addresses allow reuse of numbers at each level of the
       hierarchy, it is necessary for a Pip router to know which level of
       the hierarchy it is acting at when it retrieves an FTIF.  This is
       done in part with a hierarchy level indicator in the Routing Context
       (RC) field.  RC level is numbered from the top of the hierarchy down.
       Therefore, the top of the hierarchy is RC level = 0, the next level
       down is RC level = 1, and so on.

       The RC level alone, however, is not adequate to keep track of the
       appropriate level in all cases.  This is because different parts of
       the hierarchy may have different numbers of levels, and elements of
       the hierarchy (such as a domain or an area) may exist in multiple
       parts of the hierarchy.  Thus, a hierarchy element can be, say, level
       3 under one of its parents and level 2 under another.

       To resolve this ambiguity, the topological hierarchy is superimposed
       with another set of levels--metalevels [11].  A metalevel boundary
       exists wherever a hierarchy element has multiple parents with
       different numbers of levels, or may with reasonable probability have
       multiple parents with different numbers of levels in the future.

       Thus, a metalevel boundary exists between a subscriber network and
       its provider.  (Note that in general the metalevel represents a
       significant administrative boundary between two levels of the
       topological hierarchy.  It is because of this administrative boundary
       that the child is likely to have multiple parents.) Lower metalevels
       may exist, but usually will not.

       The RC, then, contains a level and a metalevel indicator.  The level
       indicates the number of levels from the top of the next higher
       metalevel.  The top of the global hierarchy is metalevel 0, level 0.
       The next level down (for instance, the level that provides a level of

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       hierarchy within a provider) is metalevel 0, level 1.  The first
       level of hierarchy under a provider is metalevel 1, level 0, and so
       on.

       To determine the RC level and RC metalevel in a transmitted Pip
       packet, a host (or Pip Header Server) must know where the metalevels
       are in its own Pip Addresses.

       The host compares its source Pip Address with the destination Pip
       Address.  The highest Pip Address component that is different between
       the two addresses determines the level and metalevel.  (No levels
       higher than this level need be encoded in the Routing Directive.)

       Neighbor routers are configured to know if there is a level or
       metalevel boundary between them, so that they can modify the RC level
       and RC metalevel in a transmitted packet appropriately.

    8.1  Exiting a Private Domain

       The near-term Pip Architecture provides two methods of exit routing,
       that is, routing inter-domain Pip packets from a source host to a
       network service provider of a private domain [12,15].  In the first
       method, called transit-driven exit routing, the source host leaves
       the choice of provider to the routers.  In the second method, called
       host-driven exit routing, the source host explicitly chooses the
       provider.  In either method, it is possible to prevent internal
       routers from having to carry external routing information.  The exit
       routing bit of the RC indicates which type of exit routing is in
       effect.

       With host-driven exit routing, it is possible for the host to choose
       a provider through which the destination cannot be reached.  In this
       case, the host receives the appropriate PCMP Packet Not Delivered
       message, and may either fallback on transit-driven exit routing or
       choose a different provider.

       When using transit-driven exit routing, there are two modes of
       operation.  The first, called destination-oriented, is used when the
       routers internal to a domain have external routing information, and
       the host has only one provider prefix.  The second, called provider-
       oriented, is used when the routers internal to a domain do not have
       any external routing information or when the host has multiple
       provider prefixes.  (With IP, this case is called default routing.
       In the case of IP, however, default routing does not allow an
       intelligent choice of multiple exit points.)

       With provider-oriented exit routing, the host arbitrarily chooses a
       source Pip Address (and therefore, a provider).  (Note that if the

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       Pip Header Server is tracking inter-domain routing, then it chooses
       the appropriate provider.) If the host chooses the wrong provider,
       then the border router will redirect the host to the correct provider
       with a PCMP Provider Redirect message.

    8.2  Intra-domain Networking

       With intra-domain networking (where both source and destination are
       in the private network), there are two scenarios of concern.  In the
       first, the destination address shares a providerPart with the source
       address, and so the destination is known to be intra-domain even
       before a packet is sent.  In the second, the destination address does
       not share a providerPart with the source address, and so the source
       host doesn't know that the destination is reachable intra-domain.
       Note that the first case is the most common, because the private
       top-level number assignment acts as the common prefix even though it
       isn't advertised globally (see section 4.1).

       In the first case, the Pip Addresses in the Routing Directive need
       not contain the providerPart.  Rather, it contains only the address
       part below the metalevel boundary.  (A Pip Address in an FTIF Chain
       always starts at a metalevel boundary).

       For instance, if the source Pip Address is 1.2.3,4.5.6 and the
       destination Pip Address is 1.2.3,4.7.8, then only 4.7.8 need be
       included for the destination address in the Routing Directive.  (The
       comma "," in the address indicates the metalevel boundary between
       providerPart and subscriberPart.) The metalevel and level are set
       accordingly.

       In the second case, it is desirable to use the Pip Header Server to
       determine if the destination is intra-domain or inter-domain.  The
       Pip Header Server can do this by monitoring intra-domain routing.
       (This is done by having the Pip Header Server run the intra-domain
       routing algorithm, but not advertise any destinations.) Thus, the Pip
       Header Server can determine if the providerPart can be eliminated
       from the address, as described in the last paragraph, or cannot and
       must conform to the rules of exit routing as described in the
       previous section.

       If the Pip Header Server does not monitor intra-domain routing,
       however, then the following actions occur.  In the case of host-
       driven exit routing, the packet will be routed to the stated
       provider, and an external path will be used to reach an internal
       destination.  (The moral here is to not use host-driven exit routing
       unless the Pip Header Server is privy to routing information, both
       internal and external.)

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    RFC 1621               Pip Near-term Architecture               May 1994

       In the case of transit-driven exit routing, the packet sent by the
       host will eventually reach a router that knows that the destination
       is intra-domain.  This router will forward the packet towards the
       destination, and at the same time send a PCMP Reformat Transit Part
       message to the host.  This message tells the host how much of the Pip
       Address is needed to route the packet.

    9.  Pip Header Server

       Two new components of the Pip Architecture are the Pip/IP Translator
       and the Pip Header Server.  The Pip/IP Translator is only used for
       transition from IP to Pip, and otherwise would not be necessary.  The
       Pip Header Server, however, is a new architectural component.

       The purpose of the Pip Header Server is to form a Pip Header.  It is
       useful to form the Pip header in a separate box because 1) in the
       future (as policy routing matures, for instance), significant amounts
       of information may be needed to form the Pip header--too much
       information to distribute to all hosts, and 2) it won't be possible
       to evolve all hosts at the same time, so the existence of a separate
       box that can spoon-feed Pip headers to old hosts is necessary.  (It
       is impossible to guarantee that no modification of Pip hosts is
       necessary for any potential evolution, but being able to form the
       header in a server, and hand it to an outdated host, is a large step
       in the right direction.)

       (Note that policy routing architectures commonly if not universally
       require the use of some kind of "route server" for calculating policy
       routes.  The Pip Header Server is, among other things, just this
       server.  Thus, the Pip Header Server does not so much result from the
       fact that Pip itself is more complex than IP or other "IPv7"
       proposals.  Rather, the Pip Header Server reflects the fact that the
       Pip Architecture has more functionality than ROAD architectures
       supported by the simpler proposals.)

       We note that for the near-term architecture hosts themselves will
       by-and-large have the capability of forming Pip headers.  The
       exception to this will be the case where the Pip Header Server wishes
       to monitor inter-domain routing to enhance provider selection.  Thus,
       the Pip Header Server role will be largely limited to evolution (see
       section 16).

    9.1  Forming Pip Headers

       Forming a Pip header is more complex than forming an IP header
       because there are many more choices to make.  At a minimum, one of
       multiple Pip Addresses (both source and destination) must be chosen
       [14].  In the near future, it will also be necessary to choose a TOS.

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    RFC 1621               Pip Near-term Architecture               May 1994

       After DNS information about the destination has been received, the
       the following information is available to the Pip header formation
       function.

       1.  From DNS: The destination's providers (either directly connected
           or nearby enough to justify making a policy decision about), and
           the names, types, and access restrictions of those providers.

       2.  From the source host: The application type (and thus, the desired
           service), and the user access restriction classes.

       3.  From local configuration: The source's providers, and the names,
           types, and access restrictions of those providers.

       4.  Optionally from inter-domain routing: The routes chosen by
           inter-domain to all top level providers.  (Note that inter-domain
           routing in the Pip near-term architecture is path-vector.
           Because of this, the Pip Header Server does not obtain enough
           information from inter-domain routing to form a policy route.
           When the technology to do this matures, it can be installed into
           Pip Header Servers.)

           The inter-domain routing information is optional.  If it is used,
           then probably a Pip Header Server is necessary, to limit this
           information to a small number of systems.

       There may also be arbitrary policy information available to the Pip
       header formation function.  This architecture does not specify any
       such information.

       The Pip header formation function then goes through a process of
       forming an ordered list of source/destination Pip Addresses to use.
       The ordering is based on knowledge of the application service
       requirements, the service provided by the source providers, guesses
       or learned information about the service provided by the destination
       providers or by source/destination provider pairs, and the cost of
       using source providers to reach destination providers.  It is assumed
       that the sophistication of forming the ordered list will grow as
       experienced is gained with internet commercialization and real-time
       services.

       The Pip Header formation function then returns the ordered pairs of
       source and destination addresses to the source host in the PHP
       response message, along with an indication of what kind of exit
       routing to use with each pair.  Any additional information, such as
       PDN Address, is also returned.  With this information, the source
       host can now establish communications and properly respond to PCMP
       messages.  Based on information received from PCMP messages,

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    RFC 1621               Pip Near-term Architecture               May 1994

       particularly PCMP Packet Not Delivered messages but also Mobile Host
       messages, the host is able to choose appropriately from the ordered
       list.

       Note that if Pip evolves to the point where the Transit Part of the
       Pip header is no longer compatible with the current Transit Part, and
       the querying host has not been updated to understand the new Transit
       Part, then the PHP response message contains a bit map of the Transit
       Part.  The host puts this bit map into the Transit Part of the
       transmitted Pip header even though it does not understand the
       semantics of the Transit Part.  The Host Version field indicates to
       the Pip Header Server what kinds of transit parts the host can
       understand.

    9.2  Pip Header Protocol (PHP)

       The Pip Header Protocol (PHP) is a simple query/response protocol
       used to exchange information between the Pip host and the Pip Header
       Server [6].  In the query, the Pip host includes (among other things)
       the domain name of the destination it wishes to send Pip packets to.
       (Thus, the PHP query serves as a substitute for the DNS query.)

       The PHP query also contains 1) User Access Restriction Classes, 2)
       Application Types, and 3) host version.  The host version tells the
       Pip Header Server what features are installed in the host.  Thus, the
       Pip Header Server is able to determine if the host can format its own
       Pip header based on DNS information, or whether the Pip Header Server
       needs to do it on behalf of the host.  In the future, the PHP query
       will also contain desired TOS (possibly in lieu of Application Type).
       (Note that this information could come from the application.  Thus,
       the application interface to PHP (the equivalent of gethostbyname())
       must pass this information.)

    9.3  Application Interface

       In order for a Pip host to generate the information required in the
       PHP query, there must be a way for the application to convey the
       information to the PHP software.  The host architecture for doing
       this is as follows.

       A local "Pip Header Client" (the source host analog to the Pip Header
       Server) is called by the application (instead of the current
       gethostbyname()).  The application provides the Pip Header Client
       with either the destination host domain name or the destination host
       Pip ID, and other pertinent information such as user access
       restriction class and TOS.  The Pip Header Client, if it doesn't have
       the information cached locally, queries the Pip Header Server and
       receives an answer.  (Remember that the Pip Header Server can be co-

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       resident with the host.)

       Once the Pip Header Client has determined what the Pip header(s) are,
       it assigns a local handle to the headers, returns the handle to the
       application, and configures the Pip packet processing engine with the
       handle and related Pip Headers.  The application then issues packets
       to the Pip layer (via intervening layers such as transport) using the
       local handle.

    10.  Routing Algorithms in Pip

       This section discusses the routing algorithm for use with
       (hierarchical) Pip Addresses.

       The architecture for operating routing algorithms in Pip reflects the
       clean partitioning of routing contexts in the Pip header.  Thus,
       routing in the Pip architecture is nicely modularized.

       Within the Hierarchical Pip Address, there are multiple hierarchical
       levels.  Wherever two routers connect, or two levels interface
       (either in a single router or between routers), two decisions must be
       made:  1) what information should be exchanged (that is, what of one
       side's routing table should be propagated to the other side), and 2)
       what routing algorithm should be used to exchange the information?
       The first decision is discussed in section 10.1 below (Routing
       Information Filtering).  The latter decision is discussed here.

       Conceptually, and to a large extent in practice, the routing
       algorithms at each level are cleanly partitioned.  This partition is
       much like the partition between "egp" and "igp" level routing in IP,
       but with Pip it exists at each level of the hierarchy.

       At the top-level of the Pip Address hierarchy, a path-vector routing
       algorithm is used.  Path-vector is more appropriate at the top level
       than link-state because path-vector does not require agreement
       between top-level entities (providers) on metrics in order to be
       loop-free.  (Agreement between the providers is likely to result in
       better paths, but the Pip Architecture does not assume such
       agreement.)

       The top-level path-vector routing algorithm is based on IDRP, but
       enhanced to handle Pip addresses and Pip idiosyncrasies such as the
       Routing Context.  At any level below the top level, it is a local
       decision as to what routing algorithm technology to run.  However,
       the path-vector routing algorithm is generalized so that it can run
       at multiple levels of the Pip Address hierarchy.  Thus, a lower level
       domain can choose to take advantage of the path-vector algorithm, or
       run another, such as a link-state algorithm.  The modified version of

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       IDRP is called MLPV [10], for Multi-Level Path-Vector (pronounced
       "milpiv").

       Normally, information is exchanged between two separate routing
       algorithms by virtue of the two algorithms co-existing in the same
       router.  For instance, a border router is likely to participate in an
       exchange of routing information with provider routers, and still run
       the routing algorithm of the internal routers.  If the two algorithms
       are different routing technologies (for instance, link-state versus
       distance-vector) then internal conversion translates information from
       one routing algorithm to the form of the other.

       In some cases, however, two routing algorithms that exchange
       information will exist in different routers, and will have to
       exchange information over a link.  If these two algorithms are
       different technologies, then they need a common means of exchanging
       routing information.  While strictly speaking this is a local matter,
       MLPV can also serve as the interface between two disparate routing
       algorithms.  Thus, all routers should be able to run MLPV, if for no
       other reason than to exchange information with other, perhaps
       proprietary, routing protocols.

       MLPV is designed to be extendible with regards to the type of routes
       that it calculates.  It uses the Pip Object parameter identification
       number space to identify what type of route is being advertised and
       calculated [9].  Thus, to add new types of routes (for instance, new
       types of service), it is only necessary to configure the routers to
       accept the new route type, define metrics for that type, and criteria
       for preferring one route of that type over another.

    10.1  Routing Information Filtering

       Of course, the main point behind having hierarchical routing is so
       that information from one part of the hierarchy can be reduced when
       passed to another.  In general, reduction (in the form of
       aggregation) takes place when passing information from the bottom of
       the hierarchy up.  However, Pip uses tunneling and exit routing to,
       if desired, allow information from the top to be reduced when it goes
       down.

       When two routers become neighbors, they can determine what
       hierarchical levels they have in common by comparing Pip Addresses.
       For instance, if two neighbor routers have Pip Addresses 1.2.3,4 and
       1.2.8,9.14 respectively, then they share levels 0 and 1, and are
       different at levels below that.  (0 is the highest level, 1 is the
       next highest, and so on.) As a general rule, these two routers
       exchange level 0, level 1, and level 2 routing information, but not
       level 3 or lower routing information.  In other words, both routers

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       know how to route to all things at the top level (level 0), how to
       route to all level 1 things with "1" as the level 0 prefix, and how
       to route to all level 2 things with "1.2" as the level 1 prefix.

       In the absence of other instructions, two routers will by default
       exchange information about all levels from the top down to the first
       level at which they have differing Pip Addresses.  In practice,
       however, this default exchange is as likely to be followed as not.
       For instance, assume that 1.2.3,4 is a provider router, and
       1.2.8,9.14 is a subscriber router.  (Note that 1.2.8 is the prefix
       given the subscriber by the provider, thus the metalevel boundary
       indicated by the comma.) Assume also that the subscriber network is
       using destination-oriented transit-driven exit routing (see section
       8.1).  Finally, assume that router 1.2.8,9.14 is the subscriber's
       only entry point into provider 1 (other routers provide entry points
       to other providers).

       In this case, 1.2.8,9.14 does not need to know about level 2 or level
       1 areas in the provider (that is, it does not need to know about
       1.2.4..., 1.2.5..., or 1.3..., 1.4..., and so on).  Thus, 1.2.8,9.14
       should be configured to inform 1.2.3,4 that it does not need level 1
       or 2 information.

       As another example, assume still that 1.2.3,4 is a provider and
       1.2.8,9.14 is a subscriber.  However, assume now that the subscriber
       network is using host-driven exit routing.  In this case, the
       subscriber does not even need to know about level 0 information,
       because all exit routing is directed to the provider of choice, and
       having level 0 information therefore does not influence that choice.

    11.  Transition

       The transition scheme for Pip has two major components, 1)
       translation, and 2) encapsulation.  Translation is required to map
       the Pip Address into the IP address and vice versa.  Encapsulation is
       used for one Pip router (or host) to exchange packets with another
       Pip router (or host) by tunneling through intermediate IP routers.

       The Pip transition scheme is basically a set of techniques that
       allows existing IP "stuff" and Pip to coexist, but within the
       limitations of IP address depletion (though not within the
       limitations of IP scaling problems).  By this I mean that an IP-only
       host can only exchange packets with other hosts in a domain where IP
       numbers are unique.  Initially this domain includes all IP hosts, but
       eventually will include only hosts within a private domain.  The IP
       "stuff" that can exist includes 1) whole IP domains, 2) individual IP
       hosts, 3) IP-oriented TCPs, and 4) IP-oriented applications.

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    11.1  Justification for Pip Transition Scheme

       Note that all transition to a bigger address require translation.  It
       cannot be avoided.  The major choices one must make when deciding on
       a translation scheme are:

       1.  Will we require a contiguous infrastructure consisting of the new
           protocol, or will we allow tunneling through whatever remains of
           the existing IP infrastructure at any point in time?

       2.  Will we allow global connectivity between IP machines after IP
           addresses are no longer globally unique, or not?  (In other words,
           will we use a NAT scheme or not? [15])

       Concerning question number 1; while it is desirable to move as
       quickly as possible to a contiguous Pip (or SIP or whatever)
       infrastructure, especially for purposes of improved scaling, it is
       fantasy to think that the whole infrastructure will cut over to Pip
       quickly.  Furthermore, during the testing stages of Pip, it is highly
       desirable to be able to install Pip in any box anywhere, and by
       tunneling through IP, create a virtual Pip internet.  Thus, it seems
       that the only reasonable answer to question number 1 is to allow
       tunneling.

       Concerning question number 2; it is highly desirable to avoid using a
       NAT scheme.  A NAT (Network Address Translation) scheme is one
       whereby any two IP hosts can communicate, even though IP addresses
       are not globally unique.  This is done by dynamically mapping non-
       unique IP addresses into unique ones in order to cross the
       infrastructure.  NAT schemes have the problems of increased
       complexity to maintain the mappings, and of translating IP addresses
       that reside within application data structures (such as the PORT
       command in FTP).

       This having been said, it is conceivable that the new protocol will
       not be far enough along when IP addresses are no longer unique, and
       therefore a NAT scheme becomes necessary.  It is possible to employ a
       NAT scheme at any time in the future without making it part of the
       intended transition scheme now.  Thus, we can plan for a NATless
       transition now without preventing the potential use of NAT if it
       becomes necessary.

    11.2  Architecture for Pip Transition Scheme

       The architecture for Pip Transition is that of a Pip infrastructure
       surrounded by IP-only "systems".  The IP-only "systems" surrounding
       Pip can be whole IP domains, individual IP hosts, an old TCP in an
       otherwise Pip host, or an old application running on top of a Pip-

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       smart TCP.

       The Pip infrastructure will initially get its internal connectivity
       by tunneling through IP.  Thus, any Pip box can be installed
       anywhere, and become part of the Pip infrastructure by configuration
       over a "virtual" IP.  Of course, it is desirable that Pip boxes be
       directly connected to other Pip boxes, but very early on this is the
       exception rather than the rule.

       Two neighbor Pip systems tunneling through IP simply view IP as a
       "link layer" protocol, and encapsulate Pip over IP just as they would
       encapsulate Pip over any other link layer protocol.  In particular,
       the hop-count field of Pip is not copied into the Time-to-Live field
       of IP.  There is no automatic configuration of neighbor Pip systems
       over IP.  Manual configuration (and careful "virtual topology"
       engineering) is required.  Note that ICMP messages from a IP router
       in a tunnel is not translated into a PCMP message and sent on.  ICMP
       messages are sinked at the translating router at the head of the
       tunnel.  The information learned from such ICMP messages, however,
       may be used to determine unreachability of the other end of the
       tunnel, and may there result in PCMP message on later packets.

       In the remainder of this section, we do not distinguish between a
       virtual Pip infrastructure on IP, and a pure Pip infrastructure.

       Given the model of a Pip infrastructure surrounded by IP, there are 5
       possible packet paths:

       1.  IP - IP

       2.  IP - Pip - IP

       3.  IP - Pip

       4.  Pip - IP

       5.  Pip - Pip

       The first three paths involve packets that originate at IP-only
       hosts.  In order for an IP host to talk to any other host (IP or
       not), the other host must be addressable within the context of the IP
       host's 32-bit IP address.  Initially, this "IP-unique" domain will
       include all IP hosts.  When IP addresses become no longer unique, the
       IP-unique domain will include a subset of all hosts.  At a minimum,
       this subset will include those hosts in the IP-host's private domain.
       However, it makes sense also to arrange for the set of all "public"
       hosts, basically anonymous ftp servers and mail gateways, to be in
       this subset.  In other words, a portion of IP address space should be

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       set aside to remain globally unique, even though other parts of the
       address space are being reused.

    11.3  Translation between Pip and IP packets

       Paths 2 and 4 involve translation from Pip to IP.  This translation
       is straightforward, as all the information needed to create the IP
       addresses is in the Pip header.  In particular, Pip IDs have an
       encoding that allows them to contain an IP address (again, one that
       is unique within an IP-unique domain).  Whenever a packet path
       involves an IP host on either end, both hosts must have IP addresses.
       Thus, translating from Pip to IP is just a matter of extracting the
       IP addresses from the Pip IDs and forming an IP header.

       Translating from an IP header to a Pip header is more difficult,
       because the 32-bit IP address must be "translated" into a 64-bit Pip
       ID and a Pip Address.  There is no algorithm for making this
       translation.  A table mapping IP addresses (or, rather, network
       numbers) to Pip IDs and Pip Addresses is required.  Since such a
       table must potentially map every IP address, we choose to use dynamic
       discovery and caching to maintain the table.  We choose also to use
       DNS as the means of discovering the mappings.

       Thus, DNS contains records that map IP address to Pip ID and Pip
       Address.  In the case where the host represented by the DNS record is
       a Pip host (packet path 3), the Pip ID and Pip Address are those of
       the host.  In the case where the host represented by the DNS record
       is an IP-only host (packet paths 2 and 4), the Pip Address is that of
       the Pip/IP translating gateway that is used to reach the IP host.
       Thus, an IP-only domain must at least be able to return Pip
       information in its DNS records (or, the parent DNS domain must be
       able to do it on behalf of the child).

       With paths 2 and 3 (IP-Pip-IP and IP-Pip), the initial translating
       gateway (IP to Pip) makes the DNS query.  It stores the IP packet
       while waiting for the answer.  The query is an inverse address (in-
       addr) using the destination IP address.  The translating gateway can
       cache the record for an arbitrarily long period, because if the
       mapping ever becomes invalid, a PCMP Invalid Address message flushes
       the cache entry.

       In the case of path 4 (Pip-IP), however, the Pip Address of the
       translating gateway is returned directly to the source host--
       piggybacked on the DNS record that is normally returned.  Thus this
       scheme incurs only a small incremental cost over the normal DNS
       query.

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    11.4  Translating between PCMP and ICMP

       The only ICMP/PCMP messages that are translated are the Destination
       Unreachable, Echo, and PTMU Exceeded messages.  The portion of the
       offending IP/Pip header that is attached to the ICMP/PCMP message is
       not translated.

    11.5  Translating between IP and Pip Routing Information

       It is not necessary to pass IP routing information into the Pip
       infrastructure.  The mapping of IP address to Pip Address in DNS
       allows Pip to find the appropriate gateway to IP in the context of
       Pip addresses only.

       It is impossible to pass Pip routing information into IP routers,
       since IP routers cannot understand Pip addresses.  IP domains must
       therefore use default routing to reach IP/Pip translators.

    11.6  Old TCP and Application Binaries in Pip Hosts

       A Pip host can be expected to have an old TCP above it for a long
       time to come, and a new (Pip-smart) TCP can be expected to have old
       application binaries running over it for a long time to come.  Thus,
       we must have some way of insuring that the TCP checksum is correctly
       calculated in the event that one or both ends is running Pip, and one
       or both ends has an old TCP binary.  In addition, we must arrange to
       allow applications to interface with TCP using a 32-bit "address"
       only, even though those 32 bits get locally translated into Pip
       Addresses and IDs.

       As stated above, in all cases where a Pip host is talking to an IP-
       only host, the Pip host has a 32-bit IP address. This address is
       embedded in the Pip ID such that it can be identified as an IP
       address from inspection of the Pip ID alone.

       The TCP pseudo-header is calculated using the Payload Length and
       Protocol fields, and some or all of the Source and Dest Pip IDs.  In
       the case where both Source and Dest Pip IDs are IP-based, only the
       32-bit IP address is included in the pseudo-header checksum
       calculation.  Otherwise, the full 64 bits are used.  (Note that using
       the full Payload Length and Protocol fields does not fail when old
       TCP binaries are being used, because the values for those fields must
       be within the 16-bit and 8-bit limits for TCP to correctly operate.)

       The reason for only using 32 bits of the Pip ID in the case of both
       ends using an IP address is that an old TCP will use only 32 bits of
       some number to form the pseudo-header.  If the entire 64 bits of the
       Pip ID were used, then there would be cases where no 32-bit number

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       could be used to insure that the correct checksum is calculated in
       all cases.

       Note that in the case of an old TCP on top of Pip, "Pip" (actually, a
       Pip daemon) must create a 32-bit number that can both be used to 1)
       allow the Pip layer to correctly associate a packet from the TCP
       layer with the right Pip header, and 2) cause the TCP layer to
       calculate the right checksum.  (This number is created when the
       application initiates a DNS query.  A Pip daemon intervenes in this
       request, calculates a 32 bit number that the application/TCP can use,
       and informs the Pip layer of the mapping between this 32 bit number
       and the full Pip header.)

       When the destination host is an IP only host, then this 32-bit number
       is nothing more than the IP address.  When the destination host is a
       Pip host, then this 32-bit number is some number generated by Pip to
       "fool" the old TCP into generating the right checksum.  This 32-bit
       number can normally be the same as the lower 32 bits of the Pip ID.
       However, it is possible that two or more active TCP connections is
       established to different hosts whose Pip IDs have the same lower 32
       bits.  In this case, the Pip layer must generate a different 32-bit
       number for each connection, but in such a way that the sum of the two
       16-bit components of the 32-bit numbers are the same as the sum of
       the two 16-bit components of the lower 32 bits of the Pip IDs.

       In the case where an old Application wants to open a socket using an
       IP address handed to it (by the user or hard-coded), and not using a
       domain name, then it must be assumed that this IP address is valid
       within the IP-unique domain.  To form a Pip ID out of this 32-bit
       number, the host appends the high-order 24 bits of its own Pip ID,
       plus the IP-address-identifier-byte value, to the 32-bit IP address.

    11.7  Translating between Pip Capable and non-Pip Capable DNS Servers

       In addition to transitioning "Pip-layer" packets, it is necessary to
       transition DNS from non-Pip capable to Pip capable.  During
       transition there will be name servers in DNS that only understand IP
       queries and those that understand both Pip and IP queries.  This
       means there must be a mechanism for Pip resolvers to detect whether a
       name server is Pip capable, and vice versa.  Also, a name server, if
       it provides recursive service, must be able to translate Pip requests
       to IP requests.  (Pip-capable means a name server understands Pip and
       existing IP queries.  It does not necessarily mean the name server
       uses the Pip protocol to communicate.)

       New resource records have been defined to hold Pip identifiers and
       addresses, and other information [1].  These resource records must be
       queried using a new opcode in the DNS query packet header.  Existing

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       resource records can be queried using both the old and new header
       formats.  Name servers that are not Pip-capable will respond with a
       format error to queries with the new opcode.  Thus, a resolver can
       determine dynamically whether a name server is Pip-capable, by
       sending it a Pip query and noting the response.  This only need be
       done once, when querying a server for the "first" time, and the
       outcome can be cached along with the name server's address.

       Using a new opcode for making Pip queries also helps name servers
       determine whether a resolver is Pip-capable (it is not always not
       obvious from the type of query made since many queries are common to
       to IP and Pip).  Determining whether a resolver is Pip-capable is
       necessary when responding with address information that is not
       explicitly requested by the query.  An important example of this is
       when a name server makes a referral to another name server in a
       response: if the request comes from a Pip resolver, name server
       addresses will be returned as Pip identifier/address resource
       records, otherwise the addresses will be returned as IP A resource
       records.

       Those servers that are Pip-capable and provide recursive service must
       translate Pip requests to IP requests when querying an IP name
       server.  For most queries, this will just mean modifying the opcode
       value in the query header to reflect an IP query, rather than a Pip
       query.  (Most queries are identical in IP and Pip.) Other queries,
       notably the query for Pip identifier/address information, must be
       translated into its IP counterpart, namely, an IP A query.  On
       receipt of an answer from an IP name server, a Pip name server must
       translate the query header and question section back to its original,
       and format the answer appropriately.  Again, for most queries, this
       will be a trivial operation, but responses containing IP addresses,
       either as a result of an explicit query or as additional information,
       must be formatted to appear as a valid Pip response.

       Pip-capable name servers that provide recursive name service should
       also translate IP address requests into Pip identifier/address
       requests when querying a Pip-capable name server.  (A host's IP
       address can be deduced from the host's Pip identifier.) This enables
       a Pip-capable name server to cache all relevant addressing
       information about a Pip host in the first address query concerning
       the host.  Caching partial information is undesirable since the name
       server, using the current DNS caching strategy, would return only the
       cached information on a future Pip request, and IP, rather than Pip,
       would be used to communicate with the destination host.

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    12.  Pip Address and ID Auto-configuration

       One goal of Pip is to make networks as easy to administer as
       possible, especially with regards to hosts.  Certain aspects of the
       Pip architecture make administration easier.  For instance, the ID
       field provides a network layer "anchor" around which address changes
       can be administered.

       This section discusses three aspects of autoconfiguration; 1)
       domain-wide Pip Address prefix assignment, 2) host Pip Address
       assignment, and 3) host Pip ID assignment.

    12.1  Pip Address Prefix Administration

       A central premise behind the use of provider-rooted hierarchical
       addresses is that domain-wide address prefix assignment and re-
       assignment is straight-forward.  This section describes that process.

       Pip Address prefix administration limits required manual prefix
       configuration to DNS and border routers.  This is the minimum
       required manual configuration possible, because both border routers
       and DNS must be configured with prefix information for other reasons.
       DNS must be configured with prefix information so that it can reply
       to address queries.  DNS files are structured so that the prefix is
       administered in only one place (that is, every host record does not
       have to be changed to create a new prefix).  Border routers must be
       configured with prefix information in order to advertise exit routes
       internally.

       Note in particular that no internal (non-border) routers or hosts
       need ever be manually configured with any externally derived
       addressing information.  All internal routers that are expected to
       fall under a common provider-prefix must, however, be configured with
       a "group ID" taken from the Pip ID space.  (This group ID is not a
       multicast ID per se.  Rather, it is an identifier that allows prefix
       updates to be targetted to a specific set of routers.)

       Each border router is configured with the following information.

       1.  The type of exit routing for the domain.  This tells the border
           router whether or not it needs to advertise external routes
           internally.

       2.  The address prefix of the providers that the border is directly
           connected to.  This prefix information includes any metalevel
           boundaries above the subscriber/provider metalevel boundary
           (called simply the subscriber metalevel).

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    RFC 1621               Pip Near-term Architecture               May 1994

       3.  Other information about the provider (provider name, type, user
           access restriction classes).

       4.  A list of common-provider-prefix group IDs that should receive the
           auto-configuration information. (The default is that only systems
           that share a group ID with the border router will receive the
           information.)

       This information is injected into the intra-domain routing algorithm.
       It is automatically spread to all routers indicated by the group ID
       list.  This way, the default behavior is for the information to be
       automatically constrained to the border router's "area".

       When a non-border router receives this information, it 1) records the
       route to the providers in its forwarding table, and 2) advertises the
       information to hosts in the router discovery protocol [8].  Thus
       hosts learn not only their complete address, but also information on
       how to do exit routing and on how to choose source addresses.

    12.2  Host Autoconfiguration

       There are three phases of host autoconfiguration:

       1.  The host locally creates a flat unique Pip ID (probably globally
           unique but at least unique on the attached subnet).

       2.  The host learns its Pip Addresses.

       3.  The host optionally obtains a hierarchical, organizationally
           meaningful Pip ID and a domain name from a Pip ID/domain name
           assignment service.  This service updates DNS.

       Item three is optional.  If Pip ID and domain name assignment
       services are not installed, then the host must obtain its domain name
       and, if necessary, Pip ID, from static configuration.  Each of the
       three phases are described below.

    12.2.1  Host Initial Pip ID Creation

       When a host boots, it can form an ID based only on local information.
       If the host has an IEEE 802 number, either from an IEEE 802 interface
       or from an internal identifier, then it can create a globally unique
       Pip ID from the IEEE 802 Pip ID type [4].  Otherwise, the host can
       create an ID from the IEEE 802 space using its subnet (link layer)
       address.  This latter ID is only guaranteed to be locally unique.

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    12.2.2  Host Pip Address Assignment

       Unless a host does not wish to use ID-tailed Pip Addresses (see
       section 4.1.2), host Pip Address assignment is trivial.  (The near-
       term Pip Architecture doesn't specify a means for a host to obtain a
       non-ID-tailed Pip Address.) When a host attaches to a subnet, it
       learns the Pip Address of the attached routers through router
       discovery.

       The host simply adopts these Pip Addresses as its own.  The Pip
       Address gets a packet to the host's subnet, and the host's Pip ID is
       used to route across the subnet.  When the routers advertise new
       addresses (for instance, because of a new provider), the host adopts
       the new addresses.

    12.2.3  Pip ID and Domain Name Assignment

       Once the host has obtained its Pip Addresses and an at-least-
       locally-unique Pip ID, it can exchange packets with an ID/Domain Name
       (ID/DN) assignment service.  If the host locally created a globally
       unique Pip ID (using an IEEE 802 number), and the organization it
       belongs to does not use organizationally structured Pip IDs (which
       should normally be the case) then it only needs to obtain a domain
       name.  The ID/DN assignment service is reachable at a well-known
       anycast address [4].  Thus, the host is able to start exchanging
       packets with the ID/DN assignment service without any additional
       configuration.

       If there is no ID/DN assignment service available, then the host must
       obtain it's organizational ID or DNS name in a non-automatic way.  If
       the ID/DN assignment service is down, the host must temporarily
       suffice with just a Pip ID and Address.  The host can periodically
       try to reach the ID/DN assignment service.

       The ID/DN assignment service must coordinate with DNS.  When the
       ID/DN assignment service creates a new ID or domain name to assign to
       a new host, it must know which IDs and domain names are available for
       assignment.  It must also update DNS with the new information.

       The design of this service is left for further study.

    Francis                                                        [Page 39]

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    13.  Pip Control Message Protocol (PCMP)

       The Pip analog to ICMP is PCMP [7].  The near-term Pip architecture
       defines the following PCMP messages:

       1.  Local Redirect

       2.  Packet Not Delivered

       3.  Echo

       4.  Parameter Problem

       5.  Router Discovery

       6.  PMTU Exceeded

       7.  Provider Redirect

       8.  Reformat Transit Part

       9.  Unknown Parameter

       10. Host Mobility

       11. Exit PDN Address

       The Local Redirect, Echo, and Parameter Problem PCMP messages operate
       almost identically to their ICMP counterparts.

       The Packet Not Delivered PCMP message serves the role of ICMP's
       Destination Unreachable.  The Packet Not Delivered, has two major
       differences.  First, it is more general in that it indicates the
       hierarchy level of unreachability (rather than explicit host, subnet,
       network unreachability as with IP).  Second, it indicates when an
       address is known to be invalid, thus allowing for more intelligent
       use of DNS (see section 6.2).

       The Router Discovery PCMP message operates as ICMP's, with the
       exception that a host derives its Pip Address from it.

       The PMTU Exceeded message operates as ICMP's, with the exception that
       the Pip header size of the offending Packet is also given.  This
       allows the source host transport to determine how much smaller the
       packet PMTU should be from the advertised subnet PMTU.  Note that if
       an occasional option, such as the PDN Address option, needs to be
       attached to one of many packets, and that this option makes the
       packet larger than the PMTU, then it is not necessary to modify the

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       MTU coming from transport.  Instead, that packet can be fragmented by
       the host's Pip forwarding engine.  (Pip specifies
       fragmentation/reassembly for hosts but not for routers.  The
       fragmentation information is in a Pip Option.)

       The Provider Redirect, Invalid Address, Reformat Transit Part,
       Unknown Parameter, Host Mobility, and Exit PDN Address PCMP messages
       are new.

       The Provider Redirect PCMP message is used to inform the source host
       of a preferable exit provider to use when provider-rooted, transit-
       driven exit routing is used (see section 8.1).

       The Invalid Address PCMP message is used to inform the source host
       that none of the IDs of the destination host match that of the Pip
       packet.  The purpose of this message is to allow for authoritative
       DNS requests (see section 6.2).

       The Reformat Transit Part PCMP message has both near-term Pip
       architecture functions and evolution functions.  Near-term, the
       Reformat Transit Part PCMP message is used to indicate to the source
       whether it has too few or too many layers of address in the Routing
       Directive (see section 8.2).  Long-term, the Reformat Transit Part
       PCMP message is able to arbitrarily modify the transit part
       transmitted by the host, as encoded by a bit string.

       The Unknown Parameter PCMP message is used to inform the source host
       that the router does not understand a parameter in either the
       Handling Directive, the Routing Context, or the Transit Options.  The
       purpose of this message is to assist evolution (see section 16.1).

       The Host Mobility PCMP message is sent by a host to inform another
       host (for instance, the host's Mobile Address Server) that it has a
       new address (see section 14).  The main use of this packet is for
       host mobility, though it can be used to manage any address changes,
       such as because of a new prefix assignment.

       The Exit PDN Address PCMP message is used to manage the function
       whereby the source host informs the PDN entry router of the PDN
       Address of the exit PDN system (see section 15).

       When a router needs to send a PCMP message, it sends it to the source
       Pip Address.  If the Pip header is in a tunnel, then the PCMP message
       is sent to the router that is the source of the tunnel.  Depending on
       the situation, this may result in another PCMP message from the
       source of the tunnel to the true source (for instance, if the source
       of the tunnel finds that the dest of the tunnel can't be reached, it
       may send a Packet Not Delivered to the source host).

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    14.  Host Mobility

       Depending on how security conscience a host is, and what security
       mechanisms a host has available, mobility can come from Pip "for
       free".  If a host is willing to accept a packet by just looking at
       source and destination Pip ID, and if the host simply records the
       source Pip Address on any packet it receives as the appropriate
       return address to the source Pip ID, then mobility comes
       automatically.

       That is, when a mobile host gets a new Pip Address, it simply puts
       that address into the next packet it sends.  When the other host
       receives it, it records the new Pip Address, and starts sending
       return packets to that address.  The security aspect of this is that
       this type of operation leads to an easy way to spoof the (internet
       level) identity of a host.  That is, absent any other security
       mechanisms, any host can write any Pip ID into a packet.  (Cross-
       checking a source Pip ID against the source Pip Address at least
       makes spoofing of this sort as hard as with IP. This is discussed
       below.)

       The above simple host mobility mechanism does not work in the case
       where source and destination hosts obtain new Pip Addresses at the
       same time and the old Pip addresses no longer work, because neither
       is able to send its new address information directly to the other.
       Furthermore, if a host wishes to be more secure about authenticating
       the source Pip ID of a packet, then the above mechanism also is not
       satisfactory.  In what follows, the complete host mobility mechanism
       is described.

       Pip uses the Mobile Host Server and the PCMP Host Mobility message to
       manage host mobility;

       The Mobile Host Server is a non-mobile host (or router acting as a
       host) that keeps track of the active address of a mobile host.  The
       Pip ID and Address of the Mobile Host Server is configured into the
       mobile host, and in DNS.  When a host X obtains information from DNS
       about a host Y, the Pip ID and Address of host Y's Mobile Host Server
       is among the information.  (Also among the information is host Y's
       "permanent" address, if host Y has one.  If host Y is so mobile that
       it doesn't have a permanent address, then no permanent address is
       returned by DNS.  In particular, note that DNS is not intended to
       keep track of a mobile host's active address.)

       Given the destination host's (Y) permanent ID and Address, and the
       Mobile Host Server's permanent IDs and Addresses, the source host (X)
       proceedes as follows.  X tries to establish communications with Y
       using one of the permanent addresses.  If this fails (or if at any

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       time X cannot contact Y), X sends a PCMP Mobile Host message to the
       Mobile Host Server requesting the active address for Y.  (Note that X
       can determine that it cannot contact Y from receipt of a PCMP
       Destination Unreachable or a PCMP Invalid Address message.)

       The Mobile Host Server responds to X with the active Pip Addresses of
       Y.  (Of course, Y must inform its Mobile Host Server(s) of its active
       Pip Addresses when it knows them.  This also is done using the PCMP
       Mobile Host message.  Y also informs any hosts that it is actively
       communicating with, using either a regular Pip packet or with a PCMP
       Mobile Host message.  Thus, usually X does not need to contact the
       Mobile Host Server to track Y's active address.)

       If the address that X already tried is among those returned by Y,
       then the source host has the option of either 1) continuing to try
       the same Pip Address, 2) trying another of Y's Pip Addresses, 3)
       waiting and querying the Mobile Host Server again, or 4) giving up.

       If the Mobile Host Server indicates that Y has new active Pip
       Addresses, then X chooses among these in the same manner that it
       chooses among multiple permanent Pip Addresses, and tries to contact
       Y.

    14.1  PCMP Mobile Host message

       There are two types of PCMP Mobile Host messages, the query and the
       response.  The query consists of the Pip ID of the host for which
       active Pip Address information is being requested.

       The response consists of a Pip ID, a sequence number, a set of Pip
       Addresses, and a signature field.  The set of Pip Addresses includes
       all currently usable addresses of the host indicated by the Pip ID.
       Thus, the PCMP Mobile Host message can be used both to indicate a
       newly obtained address, and to indicate that a previous address is no
       longer active (by that addresses' absence in the set).

       The sequence number indicates which is the most recent information.
       It is needed to deal with the case where an older PCMP Mobile Host
       response is received after a newer one.

       The signature field is a value that derives from encrypting the
       sequence number and the set of Pip Addresses.  For now, the
       encryption algorithms used, how to obtain keys, and so on are for
       further study.

    Francis                                                        [Page 43]

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    14.2  Spoofing Pip IDs

       This section discusses host mechanisms for decreasing the probability
       of Pip ID spoofing.  The mechanisms provided in this version of the
       near-term Pip architecture are no more secure than DNS itself.  It is
       hoped that mechanisms and the corresponding infrastructure needed for
       better internetwork layer security can be installed with whatever new
       IP protocol is chosen.

       After a host makes a DNS query, it knows:

       1.  The destination host's Pip ID,

       2.  The destination host's permanent Pip Addresses, and

       3.  The destination host's Mobile Host Server's Pip ID and Addresses.

       Note that the DNS query can be a normal one (based on domain name) or
       an inverse query (based on Pip ID or Pip Address, though the latter
       is more likely to succeed, since the Pip ID may be flat and therefore
       not suitable for an inverse lookup).  The inverse query is done when
       the host did not initiate the packet exchange, and therefore doesn't
       know the domain name of the remote (initiating) host.

       If the destination host is not mobile, then the source host can check
       the source Pip Address, compare it with those received from DNS, and
       reject the packet if it does not match.  This gives spoof protection
       equal to that of IP.

       If the destination host is mobile and obtains new Pip Addresses, then
       the source host can check the validity of the new Pip Address by
       sending a PCMP Mobile Host query to the Mobile Host Server learned
       from DNS.  The set of Pip Addresses learned from the Mobile Address
       Server is then used for subsequent validation.

    15.  Public Data Network (PDN) Address Discovery

       One of the problems with running Pip (or any internet protocol) over
       a PDN is that of the PDN entry Pip System discovering the PDN Address
       of the appropriate PDN exit Pip System.  This problem is solved using
       ARP in small, broadcast LANs because the broadcast mechanism is
       relatively cheap.  This solution is not available in the PDN case,
       where the number of attached systems is very large, and where
       broadcast is not available (or is not cheap if it is).

       For the case where the domain of the destination host is attached to
       a PDN, the problem is nicely solved by distributing the domain's exit
       PDN Address information in DNS, and then having the source host

    Francis                                                        [Page 44]

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       convey the exit PDN Address to the PDN entry router in a Pip option.

       The DNS of the destination host's domain contains the PDN Addresses
       for the domain.  When DNS returns a record for the destination host,
       the record associates zero or more PDN Addresses with each Pip
       Address.  There can be more than one PDN address associated with a
       given PDN, and there can be more than on PDN associated with a given
       Pip Address.  This latter case occurs when more than one hierarchical
       component of the Pip Address each represents a separate PDN.  It is
       expected that in almost all cases, there will be only one (or none)
       PDN associated with any Pip address.

       (Note that, while the returned DNS record associates the PDN
       Addresses with a single Pip Address, in general the PDN Address will
       apply to a set of Pip Addresses--those for all hosts in the domain.
       The DNS files are structured to reflect this grouping in the same way
       that a single Pip Address prefix in DNS applies to many hosts.
       Therefore, every individual host entry in the DNS files does not need
       to have separate PDN Addresses typed in with it.  This simplifies
       configuration of DNS.)

       When the source host sends the first packet to a given destination
       host, it attaches the PDN Addresses, one per PDN, to the packet in an
       option.  (Note that, because of the way that options are processed in
       Pip packets, no router other than the entry PDN router need look at
       the option.) When the entry router receives this packet, it
       determines that it is the entry router based on the result of the
       FTIF Chain lookup.

       It retrieves the PDN Address from the option, and caches it locally.
       The cache entry can later by retrieved using either the destination
       Pip ID or the destination Pip Address as the cache index.

       The entry router sends the source host a PCMP Exit PDN Address
       message indicating that it has cached the information.  If there are
       multiple exit PDN Addresses, then the source host can at this time
       inform the entry PDN router of all the PDN addresses.  The entry PDN
       router can either choose from these to setup a connection, or cache
       them to recover from the case where the existing connection breaks.

       Finally, the entry PDN router delivers the Pip packet (perhaps by
       setting up a connection) to the PDN Address indicated.

       When a PDN entry router receives a Pip packet for which it doesn't
       know the exit PDN address (and has no other means of determining it,
       such as shortcut routing), it sends a PCMP Exit PDN Address query
       message to the originating host.  This can happen if, for instance,
       routing changes and directs the packets to a new PDN entry router.

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       When the source host receives the PCMP Exit PDN Address query
       message, it transmits the PDN Addresses to the entry PDN router.

    15.1  Notes on Carrying PDN Addresses in NSAPs

       The Pip use of PDN Address carriage in the option or PCP Exit PDN
       Address message solves two significant problems associated with the
       analogous use of PDN Address-based NSAPs.

       First, there is no existing agreement (standards or otherwise) that
       the existence of of a PDN Address in an NSAP address implies that the
       identified host is reachable behind the PDN Address.  Thus, upon
       receiving such an NSAP, the entry PDN router does not know for sure,
       without explicit configuration information, whether or not the PDN
       Address can be used at the lower layer.  Solution of this problem
       requires standards body agreement, perhaps be setting aside
       additional AFIs to mean "PDN Address with topological significance".

       The second, and more serious, problem is that a PDN Address in an
       NSAP does not necessarily scale well.  This is best illustrated with
       the E.164 address.  E.164 addresses can be used in many different
       network technologies--telephone network, BISDN, SMDS, Frame Relay,
       and other ATM.  When a router receives a packet with an E.164-based
       NSAP, the E.164 address is in the most significant part of the NSAP
       address (that is, contains the highest level routing information).
       Thus, without a potentially significant amount of routing table
       information, the router does not know which network to send the
       packet to.  Thus, unless E.164 addresses are assigned out in blocks
       according to provider network, it won't scale well.

       A related problem is that of how an entry PDN router knows that the
       PDN address is meant for the PDN it is attached to or some other PDN.
       With Pip, there is a one-to-one relationship between Pip Address
       prefix and PDN, so it is always known.  With NSAPs, it is not clear
       without the potentially large routing tables discussed in the
       previous paragraph.

    16.  Evolution with Pip

       The fact that we call this architecture "near-term" implies that we
       expect it to evolve to other architectures.  Thus it is important
       that we have a plan to evolve to these architectures.  The Pip near-
       term architecture includes explicit mechanisms to support evolution.

       The key to evolution is being able to evolve any system at any time
       without destroying old functionality.  Depending on what the new
       functionality is, it may be immediately useful to any system that
       installs it, or it may not become useful until a significant number

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       or even a majority of systems install it.  None-the-less, it is
       necessary to be able to install it piece-wise.

       The Pip protocol itself supports evolution through the following
       mechanisms [2]:

       1.  Tunneling.  This allows more up-to-date routers to tunnel less
           up-to-date routers, thus allowing for incremental router
           evolution.  (Of course, by virtue of encapsulation, tunneling is
           always an evolution option, and indeed tunneling through IP is
           used in the Pip transition.  However, Pip's tunneling encoding is
           efficient because it doesn't duplicate header information.)

           The only use for Pip tunneling in the Pip near-term architecture
           is to route packets through the internal routers of a transit
           domain when the internal routers have no external routing
           information.  It is assumed that enhancements to the Pip
           Architecture that require tunneling will have their own means of
           indicating when forming a tunnel is necessary.

       2.  Host independence from routing information.  Since a host can
           receive packets without understanding the routing content of the
           packet, routers can evolve without necessarily requiring hosts to
           evolve at the same pace.

           In order to allow hosts to send Pip packets without understanding
           the contents of the routing information (in the Transit Part), the
           Pip Header Server is able to "spoon-feed" the host the Pip header.

           If the Pip Header Server determines that the host is able to form
           its own Pip header (as will usually be the case with the near-term
           Pip architecture), the Pip Header is essentially a null function.
           It accepts a query from the host, passes it on to DNS, and returns
           the DNS information to the host.

           If the Pip Header Server determines that the host is not able to
           form its own Pip header, then the Pip Header Server forms one on
           behalf of the host.  In one mode of operation, the Pip Header
           Server gives the host the values of some or all Transit Part
           fields, and the host constructs the Transit Part.  This allows for
           evolution within the framework of the current Transit Part.  In
           another mode, the Pip Header Server gives the host the Transit
           Part as a simple bit field.  This allows for evolution outside the
           framework of the current Transit Part.

           In addition to the Pip Header Server being able to spoon-feed the
           host a Transit Part, routers are also able to spoon-feed hosts a
           Transit Part, in case the original Transit Part needs to be

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           modified, using the PCMP Reformat Transit Part message.

       3.  Separation of handling from routing.  This allows one aspect to
           evolve independently of the other.

       4.  Flexible Handling Directive, Routing Context, and Options
           definition.  This allows new handling, routing, and option types
           to be added and defunct ones to be removed over time (see section
           16.1 below).

       5.  Fast and general options processing.  Options processing in Pip is
           fast, both because not every router need look at every option, and
           because once a router decides it needs to look at an option, it
           can find it quickly (does not require a serial search).  Thus the
           oft-heard argument that a new option can't be used because it will
           slow down processing in all routers goes away.

        Pip Options can be thought of as an extension of the Handling
        Directive (HD).  The HD is used when the handling type is common,
        and can be encoded in a small space.  The option is used otherwise.
        It is possible that a future option will influence routing, and thus
        the Option will be an extension of the RD as well.  The RD, however,
        is rich enough that this is unlikely.

       6.  Generalized Routing Directive.  Because the Routing Directive is
           so general, it is more likely that we can evolve routing and
           addressing semantics without having to redefine the Pip header or
           the forwarding machinery.

       7.  Host version number.  This number tells what Pip functions a host
           has, such as which PCMP messages it can handle, so that routers
           can respond appropriately to a Pip packet received from a remote
           host.  This supports the capability for routers to evolve ahead of
           hosts.  (All Pip hosts will at least be able to handle all Pip
           near-term architecture functions.)

        The Host version number is also used by the Pip Header Server to
        determine the extent to which the Pip Header Server needs to format
        a header on behalf of the host.

       8.  Generalized Route Types.  The IDRP/MLPV routing algorithm is
           generic with regards to the types of routes it can calculate.
           Thus, adding new route types is a matter of configuring routers to
           accept the new route type, defining metrics for the new route
           type, and defining criteria for selecting one route of the new
           type over another.

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       Note that none of these evolution features of Pip significantly slow
       down Pip header processing (as compared to other internet protocols).

    16.1 Handling Directive (HD) and Routing Context (RC) Evolution

       Because the HD and RC are central to handling and routing of a Pip
       packet, the evolution of these aspects deserves more discussion.

       Both the HD and the RC fields contain multiple parameters.  (In the
       case of the RC, the router treats the RC field as a single number,
       that is, ignores the fact that the RC is composed of multiple
       parameters.  This allows for fast forwarding of Pip packets.) These
       HD and RC multiple parameters may be arranged in any fashion (can be
       any length, subject to the length of the HD and RC fields themselves,
       and can fall on arbitrary bit boundaries).

       Associated with the HD and RC are "Contents" fields that indicate
       what parameters are in the HD and RC fields, and where they are.
       (The Contents fields are basically version numbers, except that a
       higher "version" number is not considered to supersede a lower one.
       Typical types of parameters are address family, TOS value, queueing
       priority, and so on.)

       The Contents field is a single number, the value of which indicates
       the parameter set.  The mapping of Contents field value to parameter
       set is configured manually.

       The procedure for establishing new HD or RC parameter sets (or,
       erasing old ones) is as follows.  Some organization defines the new
       parameter set.  This may involve defining a new parameter.  If it
       does, then the new parameter is described as a Pip Object.  A Pip
       Object is nothing more than a number space used to unambiguously
       identify a new parameter type, and a character string that describes
       it [9].

       Thus, the new parameter set is described as a list of Pip Objects,
       and the bit locations in the HD/RC that each Pip Object occupies.
       The organization that defines the parameter set submits it for an
       official Contents field value.  (It would be submitted to the
       standards body that has authority over Pip, currently the IAB.) If
       the new parameter set is approved, it is given a Contents value, and
       that value is published in a well known place (an RFC).

       Of course, network administrators are free to install or not install
       the new parameter set in their hosts and routers.  In the case of a
       new RC parameter set, installation of the new parameter set does not
       necessarily require any new software, because any Pip routing
       protocol, such as IDRP/MLPV, is able to find routes according to the

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       new parameter set by appropriate configuration of routers.

       In the case of a new HD parameter set, however, new software is
       necessary--to execute the new handling.

       For new HD and RC parameters sets, systems that do not understand the
       new parameter set can still be configured to execute one of several
       default actions on the new parameter.  These default action allow for
       some control over how new functions are introduced into Pip systems.
       The default actions are:

       1.  Ignore the unknown parameter,

       2.  Set unknown parameter to all 0's,

       3.  Set unknown parameter to all 1's,

       4.  Silently discard packet,

       5.  Discard packet with PCMP Parameter Unknown.

       Action 1 is used when it doesn't much matter if previous systems on a
       path have acted on the parameter or not.  Actions 2 and 3 are used
       when systems should know whether a previous system has not understood
       the parameter.  Actions 4 and 5 are used when something bad happens
       if not all systems understand the new parameter.

    16.1.1  Options Evolution

       The evolution of Options is very similar to that of the HD and RC.
       Associated with the Options is an Options Present field that
       indicates in a single word which of up to 8 options are present in
       the Options Part.  There is a Contents field associated with the
       Options Present field that indicates which subset of all possible
       options the Options Present field refers to.  Contents field values
       are assigned in the same way as for the HD and RC Contents fields.

       The same 5 default actions used for the HD and RC also apply to the
       Options.

    Francis                                                        [Page 50]

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    References

       [1]  Thomson, F., "Use of DNS with Pip", Work in Progress.
       [2]  Francis, P., "Pip Header Processing", Work in Progress.
       [3]  Pip Address Assignment Specification,  Work in Progress.
       [4]  Francis, P., "Pip Identifiers", Work in Progress.
       [5]  Pip Assigned Numbers, Work in Progress.
       [6]  Pip Header Protocol,  Work in Progress.
       [7]  Francis, G., "PCMP: Pip Control Message Protocol",
            Work in Progress.
       [8]  Pip Router Discovery Protocol, Work in Progress.
       [9]  Pip Objects Specification, Work in Progress.
       [10] Rajagopolan, and P. Francis, "The Multi-Level Path Vector
            Routing Scheme", Work in Progress.
       [11] Francis, P., "Pip Address Conventions", Work in Progress.
       [12] Francis, P., "On the Assignment of Provider Rooted Addresses",
            Work in Progress.
       [13] Ballardie, Francis, P., and J. Crowcroft, "Core Based Trees
            (CBT), An Architecture for Scalable Inter-Domain Multicast
            Routing", Work in Progress.
       [14] Franics, P., "Pip Host Operation", Work in Progress.
       [15] Egevang, K., and P. Francis, "The IP Network Address
            Translator (NAT)", RFC 1631, Cray Communications, NTT,
            May 1994.

    Notes on the References:

       As of the publication of this RFC, a version of [12], titled
       "Comparison of Geographic and Provider-rooted Internet Addressing,"
       was submitted to ISOC INET 94 in Prague.  Reference [13] was
       published at ACM SIGCOMM 93 in San Francisco under the title "An
       Architecture for Scalable Inter-Domain Multicast Routing".

    Security Considerations

       Security issues are not discussed in this memo.

    Author's Address:

       Paul Francis
       NTT Software Lab
       3-9-11 Midori-cho Musashino-shi
       Tokyo 180 Japan

       Phone: +81-422-59-3843
       Fax +81-422-59-3765
       EMail: francis@cactus.ntt.jp

    Francis                                                        [Page 51]