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RFC 1621

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Pip Near-term Architecture

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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.


   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 <>.

   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.


   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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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

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   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

   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.)

1.1  Pip Architecture Characteristics

   The proposed architecture for near-term Pip has the following

   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

   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

   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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   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

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
      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

   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

   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

   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

   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:


   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:


   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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   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.


   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.

             (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

   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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   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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   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

   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

   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

   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

   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

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

   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

Top      ToC       Page 20 

   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

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

Top      ToC       Page 21 
   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

   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

   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

   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

   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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   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.

(page 25 continued on part 2)

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