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TRANSPORT, ADDRESSING, AND ROUTING IN THE WIDEBAND NET
Wideband Net Working Note #16
Internet Experiment Note #162
John A. Pershing Jr.
Bolt Beranek and Newman Inc.
50 Moulton Street
Cambridge, Mass. 02238
(617) 491-1850
October 1980
The online version of this note does not contain 2 figures. Hardcopy
versions including the figures may be obtained from the author at the
above address, or by a request via computer mail to JPershing@BBNA.
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TRANSPORT, ADDRESSING, AND ROUTING IN THE WIDEBAND NET
This note proposes a model for addressing and routing in the
DARPA Wideband Satellite Experiment. The purpose of the
organization described herein is twofold: (1) to hide the
physical structure of the Wideband Net from The Internet and its
gateways; and (2) to unify the transport and routing functions
performed within the Wideband Net.
Certain terms, defined in a glossary at the end of this memo,
are used with specific, somewhat non-standard meanings in order
to avoid ambiguities.
1. Current Organization
The Wideband Satellite Experiment involves the development of
the PSAT satellite network, several local networks, and
connections to several existing networks such as the ARPANET.
Currently, no plan exists for the organization of these parts
into a unified communication medium. So far, the Wideband Net is
a collection of independent, sovereign networks, each with its
own transport protocol, addressing, and routing schemes. This
sovereignty of the constituent networks is a feature which should
be preserved as much as possible, so that the development of
local and satellite network technologies can proceed without
artificial constraints.
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Figure 1 illustrates a hypothetical organization sometime in
the future, after the network has expanded somewhat. Each site
has one PSAT, and perhaps a Voice Funnel and/or
mini-concentrator; additionally, sites have a number of local
networks with various interconnections. Hosts may be connected
directly to a PSAT, a Voice Funnel, or to a local network; some
hosts may have connections to two or more nets, etc.
A uniform plan for communication in the Wideband Net will avoid
ad hoc schemes involving specialized interface machines which
transform one net's local protocol and addressing into that of
another net. Using specialized gateways in combination with
source routing will not take full advantage of the topology and
dynamics of the situation. (This is particularly evident if the
link indicated by the dotted line exists, since only the Voice
Funnel at the right side of the figure knows the status of both
the dotted link and the satellite link, and only that Voice
Funnel is able to choose the appropriate link.) Such ad hoc
schemes are inflexible, inefficient in terms of manpower (since
large amounts of special-purpose code must be implemented), and
do not allow the Wideband Net to be readily integrated into the
Internet.
A "simple" approach is to consider every component in Figure 1
to be a member of The Internet, assigning an Internet Network
Number to each of the constituent nets, and relying on internet
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Figure 1: Current Organization of the Wideband Net
gateways for routing. However, even this simple diagram has
twelve nets, not counting the ARPANET (the Voice Funnels must
also behave as networks so that the directly attached hosts have
well-defined internet addresses), and there are 16 cross-net
connections requiring Internet gateways. It is probably
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unreasonable for us to consume so many network numbers from the
address space of 255; furthermore, a proliferation of Internet
Networks places an unreasonable burden on the gateways and
networks of the Internet, both in terms of table space and
routing update traffic (since all gateways must track all
networks).
2. Proposed Structure
This memo proposes that the networks within the Wideband
Satellite Experiment collectively behave as one network from the
point of view of the rest of The Internet, as illustrated in
Figure 2. All components of the Wideband Net share one Internet
network number. Internally, the Wideband Net looks like a
catenet; this structure was suggested by Vint Cerf in IEN-48 [1].
The various subnets are interconnected by gateways; each subnet
maintains its own autonomy, and hosts that are only involved in
local communication can ignore the catenet (and Internet) aspects
of the arrangement.
We will adopt an internal addressing and routing scheme which
is transparent to the Internet Protocol (IP) [2], so that all
hosts on the Wideband Net will have well-defined Internet
addresses; we adopt IP as the Wideband Net's "catenet transport
protocol", and superimpose a fine-structure on the 24-bit local
address part of the (32-bit) Internet address (see Figure 3).
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Figure 2: Proposed Organization of the Wideband Net
The high-order 8 bits specify a subnet of the Wideband Net (e.g.,
a specific LexNet), and the remaining 16 bits specify the local
address on that net.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 28. | Subnet Number | Reserved for Subnet Use |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<-------------> <--------------------------------------------->
Internet Internet Local Address
Network Number
Figure 3: Internet Address, as used in the Wideband Net
Since the model we are adopting is one that has been thoroughly
explored by the Internet community, many of the problems have
been solved and many issues have already been resolved. For
example, the gateways between these subnets are much like the
Internet Gateways. These gateways will pass IP packets among
subnets of the Wideband Net, stripping the local transport
protocol layer from incoming packets, making routing decisions,
and wrapping outgoing packets in the protocol of the next net
which the packet will have to traverse.
This scheme does not depend on the participation of all
attached hosts -- only the gateways are critical, and, as the
network becomes richly interconnected, individual gateways cease
to be critical to the proper operation of the net.
Other catenet transport protocols could be chosen, or one could
be devised for use within the Wideband Net. However, hosts would
still need to implement IP in order to communicate over The
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Internet, so such a "local" catenet protocol would be essentially
excess baggage. By adopting IP as the Wideband Net's protocol,
there is also a good chance of being able to use existing code
with only minor modifications.
The issue of what is an Internet network (with its own assigned
Internet network number) and what is a subnet is more of a
managerial problem than a technical one. The relevant issue is
the partitioning of The Internet, in terms of name space and the
burden of the routing algorithm on the gateways, as well as in
terms of managerial responsibility. The proposed scheme for the
Wideband Net is quite flexible in this regard. If one of the
subnets becomes an Internet network, then the gateways will
perform somewhat different routing with respect to that network,
and the other hosts of the Wideband Net will be mostly
indifferent to the change (except that the addresses of the hosts
on the "promoted" subnet will change). Presumably, any Wideband
Net gateways which were connected to the net will be "promoted"
to Internet gateways.
While we believe that this proposed organization should be
adopted, it has several problems. However, these problems also
exist in The Internet and are treated more fully in various
Internet Experiment Notes; they are only briefly mentioned here.
The name space is becoming crowded; using 8 bits for the subnet
number and 16 bits for the local host address is perhaps the
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wrong partition. However, this choice seems to be a good
tradeoff between the potential size of the Wideband Net (measured
in subnets) and the addressing requirements of any individual
subnet. Eventually, the issue of name-space size will have to be
addressed by the Internet community as a whole.
Since the fine structure of the Wideband Net is not known to
The Internet, it is possible that Internet Gateways will make
non-optimal routing decisions with respect to the Wideband Net.
This is the penalty which must be paid for trying to minimize the
number of Internet Networks. If the Internet Gateways which are
connected to the Wideband Net are also Wideband Net gateways
(that is, they participate in Wideband Net routing as well as
Internet routing), then they may be able to fine-tune the routing
of Internet packets through the use of advisory messages
exchanged with the other Internet Gateways.
3. Transport Protocol Layers
For purposes of discussion, we define the notion of catenet
adjacency. Two hosts on some catenet are adjacent if they are
connected to the same constituent network. Conversely, two hosts
are non-adjacent if they are connected to different constituent
networks of some catenet. Note that hosts with interfaces on
more than one network (such as a gateway) may be both adjacent
and non-adjacent to a given host; in fact, such a host is
non-adjacent to itself by this definition.
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The transport, addressing, and routing schemes suggested in
this memo are intended to be used by non-adjacent hosts on the
Wideband Net. Of course this does not prohibit adjacent hosts
from communicating with these protocols; however, such hosts have
the option of communicating using protocols which are local to
the network to which they are attached (and, for efficiency
reasons, such hosts will probably exercise this option).
Datagrams which are to be transmitted through the Wideband Net
are wrapped in a layer of catenet protocol, which is common to
the entire Wideband Net, followed by a (possibly null) layer of
protocol which is dictated by the particular network that the
datagram is traversing. The catenet protocol header is
considered to be part of the datagram, and is preserved (with
only minor changes) as the datagram traverses the various
constituent networks. The layer of local protocol is volatile,
and will be discarded as soon as the datagram exits the network
defining that particular local protocol.
For compatibility with The Internet, the catenet protocol used
is the DoD Standard Internet Protocol (IP) [2]. This permits
hosts of the Wideband Net to communicate with hosts on other
Internet Networks without resorting to yet another layer of
protocol. Initially, no internet gateways will be provided on
the Wideband Net, so that a restricted subset of IP may be
implemented by the various hosts. The restrictions of the
initial implementation follow:
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o Fragmentation and Reassembly: Not implemented. Total
length may not exceed 576 octets; we believe that all
participating hosts, gateways, and networks will support
Internet datagrams of this length without fragmentation.
The "Flags" field will always be set to '010' (binary)
to inhibit fragmentation; the "Fragment Offset" field
will always be zero.
o Source/Destination Addresses: The Internet Network
Number (first 8 bits) will always be '28' (decimal).
The Source/Destination Local Address (remaining 24 bits)
is further interpreted as being 8 bits of network number
and 16 bits of local address on that network.
o Options: Strictly optional. Any options which are
present must be accounted for by the Internet Header
Length (IHL) and Header Checksum fields; however, no
host is required to interpret any options.
The local protocol is dependent on the particular network(s) to
which a host is attached -- there is potentially a different
local protocol for every constituent network. In addition, a
single network may have more than one link-level protocols,
depending on the particular type of port to which the host is
attached. Issues of local protocols are of no concern to the
purposes of this memo; the implementor is referred to the
(possibly nonexistent) documentation on the local protocol of the
network(s) to which a particular host is to be interfaced.
4. Addressing
All hosts on the Wideband Net have at least one unique Internet
address. Since the 8-bit Internet Network Number is already
specified by the Internet protocols (the Wideband Net is number
28 decimal), this leaves a 24-bit address space for use within
the Wideband Net.
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Since the Wideband Net is organized as a catenet, the "network
number / local host number" strategy employed by The Internet is
also employed within the Wideband Net. Each distinct,
constituent network is assigned a unique, 8-bit network number;
and each host is assigned a unique, 16-bit local address on each
network to which it is attached. These two numbers are
concatenated to produce the 24-bit Wideband Net Address.
5. Routing and Gateways
Routing in the Wideband Net is implemented in the same manner
as routing in The Internet [4]. The constituent networks are
connected by gateways; these gateways implement the routing
function. One of the primary functions of the Voice Funnels is
to implement this gateway function for each network to which it
is attached. If a network is to be attached to the Wideband Net
at some place besides a Voice Funnel, then it is the
responsibility of that network to provide a gateway.
The IP server in each host needs to know very little about
routing in order to function properly. It is assumed that each
host knows its own address (that is, its network number and its
address on that network) and the address of at least one adjacent
gateway. Also, the IP server must be able to produce a local
network protocol header from a 24-bit Wideband Net address which
specifies this local network.
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When a host's IP server is called on to deliver a datagram to
an adjacent host it wraps the datagram first in an IP header,
then in a local header which is addressed to that host, and sends
the result directly to its destination. When an IP server is
called on to deliver to a non-adjacent host, it wraps the
datagram first in an IP header, then in a local header which is
addressed to an adjacent gateway and delivers it. That gateway,
in cooperation with the other gateways of the Wideband Net, will
deliver the datagram to its destination.
In addition to transmitting outgoing datagrams, IP servers will
receive datagrams from their network(s). These should be routed
internally as appropriate; this is probably dependent on the
"Protocol" field of the IP header (but note that a protocol of
'3' indicates an advisory message from a gateway directed at the
IP server itself).
If an IP server is to function optimally, it must keep track of
ALL adjacent gateways which are up. Additionally, it must
maintain a cache of those non-adjacent hosts to which it has
recently sent datagrams, along with the gateway through which
these datagrams were forwarded. When it is about to send a
datagram, the IP server first checks the cache. If the addressee
is found in the cache and if the gateway named in the cache is
still up, then the datagram is forwarded to that same gateway.
Otherwise, a gateway is chosen arbitrarily through which to
forward the datagram.
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Occasionally, a routing advisory message will be sent to the IP
server by a gateway. Currently, one of two advisories may be
received: "destination unreachable", meaning that it will be
futile to send more datagrams to some host for a while (e.g. a
couple of minutes); and "redirect", meaning that a non-optimal
gateway was used, and that further datagrams should be forwarded
through the gateway specified in the redirect message. When an
IP server receives a routing advisory, it should update its
cache, and perhaps notify one or more processes as appropriate.
6. Unresolved Issues
Since the currently planned constituents of the Wideband Net
(the PSAT Net, LexNets, and Voice Funnels) are all capable of
supporting broadcasting, it might be worthwhile to consider some
form of broadcasting as a basic Wideband Net service. This would
probably take two forms: group addressing, essentially extending
the group concept of the PSAT Net to allow broadcasting to a
designated group of hosts of the Wideband Net; and general
broadcasting, directed at all hosts on a particular subnet or at
all hosts of the Wideband Net.
The ST protocol should also be supported directly by the
Wideband Net's gateways. ST will allow the capacity of the
various subnets to be more fully utilized. It should also be
made to take advantage of the broadcast nature of the various
subnets.
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7. Glossary
In order to avert the ambiguities inherent in this two-level
structure, this memo uses a two-level nomenclature, defined as
follows:
network: A physical communication service in which all
attached hosts communicate with all other hosts
using a uniform, local set of link, addressing,
and transport protocols; i.e. the "usual" meaning
of the word. To quote Cerf [1], "the term
'local' is used in a loose sense here, since it
means 'peculiar to the particular network' rather
than 'a network of limited geographical extent'.
A satellite-based network, such as the ARPA
packet satellite network, therefore has 'local'
characteristics (e.g. broadcast operation) even
though it spans many thousands of square miles
geographically speaking." A network must appear
to be homogeneous from the "outside looking in";
however, this does not necessarily preclude an
internal structure.
Internet Network:
A communicating system of hosts and/or networks
belonging to The Internet, and which can be
uniquely identified by an 8-bit "network number"
assigned by the number czar [3]. This may
correspond to part of a network, one network, or
a concatenation of many networks. Note that the
Wideband Net is an Internet Network.
catenet: A collection of two or more networks, arbitrarily
interconnected by gateways, in which the
communicating hosts have agreed, a-priori, on
some canonical "catenet protocol" which is used
for datagram transport.
The Internet: The collection of Internet Networks, along with
their Internet Gateways. Hosts on The Internet
communicate using version 4 of the DoD Standard
Internet Protocol [2].
gateway: A logical host which is connected to two or more
networks, and which can forward "catenet
protocol" datagrams arriving from any of these
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networks to the appropriate outgoing network.
Many adjectives may be applied to "gateway" as
needed. A hidden gateway is one whose presence
is generally unknown by the hosts attached to the
networks adjacent to the gateway. A non-routing
(or static-routing) gateway makes its routing
decisions based on a-priori information; a
routing gateway exchanges information with other
routing gateways in order to be able to make
dynamic adjustments to its routing information as
the conditions of the catenet change.
Internet Gateway:
A gateway connected to two or more Internet
Networks, which can forward internet datagrams.
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REFERENCES
[1] Cerf.
The Catenet Model for Internetworking.
Internet Experiment Note 48, July, 1978.
[2] Postel, ed.
The DoD Standard Internet Protocol.
Internet Experiment Note 128, January, 1980.
[3] Postel, ed.
Assigned Numbers.
Request for Comments 770, September, 1980.
[4] Strasisar.
How to Build a Gateway.
Internet Experiment Note 109, August, 1979.
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