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 RFCand 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.
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
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
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.
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.
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
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
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.
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.
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.
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).
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.)
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
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
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.
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:
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.
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
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
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:
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.
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
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
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.)
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.
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,
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-
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 Pi