Network Working Group A. Oppenheimer
Request for Comments: 1504 Apple Computer
August 1993
Appletalk Update-Based Routing Protocol:
Enhanced Appletalk Routing
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Introduction
This memo is being distributed to members of the Internet community
to fully document an Apple protocol that may be running over the
Internet. While the issues discussed may not be directly relevant to
the research problems of the Internet, they may be interesting to a
number of researchers and implementers.
About This Document
This document provides detailed information about the AppleTalk
Update-based Routing Protocol (AURP) and wide area routing. AURP
provides wide area routing enhancements to the AppleTalk routing
protocols and is fully compatible with AppleTalk Phase 2. The
organization of this document has as its basis the three major
components of AURP:
AppleTalk tunneling, which allows AppleTalk data to pass through
foreign networks and over point-to-point links
the propagation of AppleTalk routing information between internet
routers connected through foreign networks or over point-to-point
links
the presentation of AppleTalk network information by an internet
router to nodes and other Phase 2-compatible routers on its local
internet
What This Document Contains
The chapters of this document contain the following information:
Chapter 1, "Introduction to the AppleTalk Update-Based Routing
Protocol," introduces the three major components of AURP and the
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key wide area routing enhancements that AURP provides to the
AppleTalk routing protocols.
Chapter 2, "Wide Area AppleTalk Connectivity," provides
information about AppleTalk tunneling through IP internets and over
point-to-point links.
Chapter 3, "Propagating Routing Information With the AppleTalk
Update-Based Routing Protocol," describes the essential elements of
AURP, including the architectural model for update-based routing.
This chapter provides detailed information about the methods that
AURP uses to propagate routing information between internet routers
connected through tunnels.
Chapter 4, "Representing Wide Area Network Information," describes
optional features of AURP-some of which can also be implemented on
routers that use RTMP rather than AURP for routing-information
propagation. It gives detailed information about how an exterior
router represents imported network information to its local
internet and to other exterior routers. It describes network
hiding, device hiding, network-number remapping, clustering, loop
detection, hop-count reduction, hop-count weighting, and backup
paths.
The Appendix, "Implementation Details," provides information about
implementing AURP.
What You Need to Know
This document is intended for developers of AppleTalk wide area
routing products. It assumes familiarity with the AppleTalk network
system, internet routing, and wide area networking terms and
concepts.
Format of This RFC Document
The text of this document has been quickly prepared for RFC format.
However, the art is more complex and is not yet ready in this format.
We plan to incorporate the art in the future. Consult the official
APDA document, as indicated below, for the actual art.
For More Information
The following manuals and books from Apple Computer provide
additional information about AppleTalk networks. You can obtain books
published by Addison-Wesley at your local bookstore. Contact APDA,
Apple's source for developer tools, to obtain technical reference
materials for developers:
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RFC 1504 Appletalk Update-Based Routing Protocol August 1993
APDA
Apple Computer, Inc.
20525 Mariani Avenue, M/S 33-G
Cupertino, CA 95014-6299
These manuals provide information about some AppleTalk network
products:
The Apple Ethernet NB User's Guide explains how to install and use
an Apple Ethernet NB Card and EtherTalk software on an AppleTalk
network.
The Apple InteroPoll Network Administrator's Guide describes how
to perform maintenance and troubleshooting on an AppleTalk network
using InteroPoll, a network administrator's utility program.
The Apple Internet Router Administrator's Guide explains how to
install the Apple Internet Router Basic Connectivity Package and
how to use the Router Manager application program. It provides
information about setting up the router, configuring ports to
create local area and wide area internets, monitoring and
troubleshooting router operation, and planning your internet.
Using the AppleTalk/IP Wide Area Extension explains how to install
and use the AppleTalk/IP Wide Area Extension for the Apple Internet
Router. It provides information about tunneling through TCP/IP
networks, configuring an IP Tunnel access method for an Ethernet or
Token Ring port on the Apple Internet Router, troubleshooting IP
tunneling problems, and configuring MacTCP.
The AppleTalk Remote Access User's Guide explains how to use a
Macintosh computer to communicate with another Macintosh computer
over standard telephone lines to access information and resources
at a remote location.
The Apple Token Ring 4/16 NB Card User's Guide explains how to
install and operate the card and TokenTalk software on a Token Ring
network.
The MacTCP Administrator's Guide, version 1.1, explains how to
install and configure the MacTCP driver, which implements TCP/IP
(Transmission Control Protocol/Internet Protocol) on a Macintosh
computer.
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The following books provide reference information about AppleTalk
networks:
The Advantages of AppleTalk Phase 2 provides a detailed
description of the enhanced internetworking capabilities of
AppleTalk Phase 2, and a brief guide to upgrading an AppleTalk
internet to AppleTalk Phase 2. Available from Apple Computer.
The AppleTalk Network System Overview provides a technical
introduction to the AppleTalk network system and its protocol
architecture. Published by Addison-Wesley Publishing Company.
The AppleTalk Phase 2 Introduction and Upgrade Guide is a detailed
guide to upgrading AppleTalk network hardware, drivers, and
application programs to AppleTalk Phase 2, and briefly describes
extensions to the AppleTalk network system that enhance its
support for large networks. Available from Apple Computer.
The AppleTalk Phase 2 Protocol Specification is an addendum to the
first edition of Inside AppleTalk that defines AppleTalk Phase 2
extensions to AppleTalk protocols that provide enhanced AppleTalk
addressing, routing, and naming services. Available from APDA.
Inside AppleTalk, second edition, is a technical reference that
describes the AppleTalk protocols in detail and includes
information about AppleTalk Phase 2. Published by Addison-Wesley
Publishing Company.
The Local Area Network Cabling Guide provides information about
network media, topologies, and network types. Available from Apple
Computer.
Planning and Managing AppleTalk Networks provides in-depth
information for network administrators about planning and managing
AppleTalk networks-including AppleTalk terms and concepts, and
information about network services, media, topologies, security,
monitoring and optimizing network performance, and
troubleshooting. Published by Addison-Wesley Publishing Company.
Understanding Computer Networks provides an overview of
networking-including basic information about protocol
architectures, network media, and topologies. Published by
Addison-Wesley Publishing Company.
The AppleTalk Update-Based Routing Protocol Specification is the
official Apple specification of AURP. It includes the artwork
currently missing from this document. Available from APDA.
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Table of Contents
1. Introduction to the AppleTalk Update-Based Routing Protocol 6
Wide area routing enhancements provided by AURP 6
2. Wide Area AppleTalk Connectivity 7
AppleTalk tunneling 7
IP tunneling 14
Point-to-point tunneling 17
3. Propagating Routing Information With the AppleTalk Update-Based
Routing Protocol 18
AURP architectural model 18
Maintaining current routing information with AURP 20
AURP-Tr 21
One-way connections 22
Initial information exchange 22
Reobtaining routing information 28
Updating routing information 28
Processing update events 33
Router-down notification 38
Obtaining zone information 40
Hiding local networks from remote networks 44
AURP packet format 45
Error codes 55
4. Representing Wide Area Network Information 56
Network hiding 56
Device hiding 57
Resolving network-numbering conflicts 59
Zone-name management 65
Hop-count reduction 66
Routing loops 67
Using alternative paths 71
Network management 73
Appendix. Implementation Details 75
State diagrams 75
AURP table overflow 75
A scheme for updates following initial information exchange 75
Implementation effort for different components of AURP 76
Creating free-trade zones 77
Implementation details for clustering 78
Modified RTMP algorithms for a backup path 79
Security Considerations 82
Author's Address 82
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RFC 1504 Appletalk Update-Based Routing Protocol August 1993
1. INTRODUCTION TO THE APPLETALK UPDATE-BASED ROUTING PROTOCOL
The AppleTalk Update-based Routing Protocol (AURP) provides wide area
routing enhancements to the AppleTalk routing protocols and is fully
compatible with AppleTalk Phase 2. AURP consists of three major
components:
AppleTalk tunneling through foreign network systems-for example,
TCP/IP (Transmission Control Protocol/Internet Protocol) and over
point-to-point links
the propagation of routing information between internet routers
connected through foreign network systems or over point-to-point
links
the presentation of AppleTalk network information by an internet
router to nodes or to other Phase 2-compatible routers on its local
internet-in other words, on the AppleTalk internet connected
directly to the router
Chapter 3, "Propagating Routing Information With the AppleTalk
Update-Based Routing Protocol," describes the elements of AURP that
are essential for a minimal implementation of AURP. AURP includes
many optional features for the presentation of network information.
You can implement many of these optional features on routers that use
either AURP or RTMP (Routing Table Maintenance Protocol) for
routing-information propagation.
Figure 1-1 shows how the three major components of AURP interact.
<<Figure 1-1 Major components of AURP>>
Wide Area Routing Enhancements Provided by AURP
AURP provides AppleTalk Phase 2-compatible routing for large wide
area networks (WANs). Key wide area routing enhancements provided by
AURP include:
tunneling through TCP/IP internets and other foreign network
systems
point-to-point tunneling
basic security-including device hiding and network hiding
remapping of remote network numbers to resolve numbering conflicts
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internet clustering to minimize routing traffic and routing-
information storage requirements
hop-count reduction to allow the creation of larger internets
improved use of alternate paths through hop-count weighting and
the designation of backup paths
2. WIDE AREA APPLETALK CONNECTIVITY
This chapter describes the wide area connectivity capabilities
provided by the AppleTalk Update-based Routing Protocol (AURP),
including:
AppleTalk tunneling
tunneling through TCP/IP internets
tunneling over point-to-point links
AppleTalk Tunneling
Tunneling allows a network administrator to connect two or more
native internets through a foreign network system to form a large
wide area network (WAN). For example, an AppleTalk WAN might consist
of two or more native AppleTalk internets connected through a tunnel
built on a TCP/IP internet. In such an AppleTalk WAN, native
internets use AppleTalk protocols, while the foreign network system
uses a different protocol family.
A tunnel connecting AppleTalk internets functions as a single,
virtual data link between the internets. A tunnel can be either a
foreign network system or a point-to-point link. Figure 2-1 shows an
AppleTalk tunnel.
<<Figure 2-1 AppleTalk tunnel>>
There are two types of tunnels:
dual-endpoint tunnels, which have only two routers on a tunnel-for
example, point-to-point tunnels
multiple-endpoint tunnels-herein referred to as multipoint tunnels-
which have two or more routers on a tunnel
AURP implements multipoint tunneling by providing mechanisms for data
encapsulation and the propagation of routing information to specific
routers.
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Exterior Routers
An AppleTalk router with a port that connects an AppleTalk internet
to a tunnel is an exterior router. An exterior router always sends
split-horizoned routing information to the other exterior routers on
a multipoint tunnel. That is, an exterior router on a multipoint
tunnel sends routing information for only its local internet to other
exterior routers on that tunnel. An exterior router never exports
routing information obtained from other exterior routers on the
tunnel, because the exterior routers communicate their own routing
information to one another.
As shown in Figure 2-2, the absence or presence of redundant paths,
or loops, across a tunnel changes the way an exterior router defines
its local internet. For more information about redundant paths, see
the section "Redundant Paths" in Chapter 4. If no loops exist across
a tunnel, an exterior router's local internet comprises all networks
connected directly or indirectly to other ports on the exterior
router. When loops exist across a tunnel, an exterior router's local
internet comprises only those networks for which the next internet
router is not across a tunnel. Using this definition of a local
internet, two exterior routers' local internets might overlap if
loops existed across a tunnel. For more information about routing
loops, see the section "Routing Loops" in Chapter 4.
<<Figure 2-2 An exterior router's local internet>>
An exterior router functions as an AppleTalk router within its local
internet and as an end node in the foreign network system connecting
AppleTalk internets. An exterior router uses RTMP to communicate
routing information to its local internet, and uses AURP and the
network-layer protocol of the tunnel's underlying foreign network
system to communicate with other exterior routers connected to the
tunnel. An exterior router encapsulates AppleTalk data packets using
the headers required by the foreign network system, then forwards the
packets to another exterior router connected to the tunnel.
FORWARDING DATA: When forwarding AppleTalk data packets across a
multipoint tunnel, an exterior router
encapsulates the AppleTalk data packets in the packets of the
tunnel's underlying foreign network system by adding the headers
required by that network system
adds an AURP-specific header-called a domain header-immediately
preceding each AppleTalk data packet
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A domain header contains additional addressing information-including
a source domain identifier and destination domain identifier. For
more information about domain headers, see the sections "AppleTalk
Data-Packet Format" and "AppleTalk Data-Packet Format for IP
Tunneling" later in this chapter. For detailed information about
domain identifiers, see the section "Domain Identifiers" later in
this chapter.
Before forwarding a data packet to a network in another exterior
router's local internet, an exterior router must obtain the foreign-
protocol address of the exterior router that is the next internet
router in the path to the packet's destination network. The exterior
router then sends the packet to that exterior router's foreign-
protocol address using the network-layer protocol of the foreign
network system. The exterior router need not know anything further
about how the packet traverses this virtual data link.
Once the destination exterior router receives the packet, it removes
the headers required by the foreign network system and the domain
header, then forwards the packet to its destination in the local
AppleTalk internet.
If the length of an AppleTalk data packet in bytes is greater than
that of the data field of a foreign-protocol packet, a forwarding
exterior router must fragment the AppleTalk data packet into multiple
foreign-protocol packets, then forward these packets to their
destination. Once the destination exterior router receives all of the
fragments that make up the AppleTalk data packet, it reassembles the
packet.
CONNECTING MULTIPLE TUNNELS TO AN EXTERIOR ROUTER: An exterior router
can also connect two or more multipoint tunnels. As shown in Figure
2-3, when an exterior router connects more than one multipoint
tunnel, the tunnels can be built on any of the following:
the same foreign network system
different foreign network systems
similar, but distinct foreign network systems
<<Figure 2-3 Connecting multiple tunnels to an exterior router>>
Whether the tunnels connected to an exterior router are built on
similar or different foreign network systems, each tunnel acts as an
independent, virtual data link. As shown in Figure 2-4, an exterior
router connected to multiple tunnels functions logically as though it
were two or more exterior routers connected to the same AppleTalk
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RFC 1504 Appletalk Update-Based Routing Protocol August 1993
network, with each exterior router connected to a different tunnel.
<<Figure 2-4 An exterior router connected to multiple tunnels>>
Fully Connected and Partially Connected Tunnels
An AppleTalk multipoint tunnel functions as a virtual data link. AURP
assumes full connectivity across a multipoint tunnel-that is, all
exterior routers on such a tunnel can communicate with one another.
An exterior router always sends split-horizoned routing information
to other exterior routers on a multipoint tunnel. That is, an
exterior router on a multipoint tunnel sends routing information for
only its local internet to other exterior routers on that tunnel. An
exterior router never exports routing information obtained from other
exterior routers on the tunnel, because exterior routers communicate
their routing information to one another.
If all exterior routers connected to a multipoint tunnel are aware of
and can send packets to one another, that tunnel is fully connected.
If some of the exterior routers on a multipoint tunnel are not aware
of one another, the tunnel is only partially connected. Figure 2-5
shows examples of a fully connected tunnel, a partially connected
tunnel, and two fully connected tunnels.
<<Figure 2-5 Fully connected and partially connected tunnels>>
In the second example shown in Figure 2-5, the network administrator
may have connected the tunnel partially for one of these reasons:
to prevent the local internets connected to exterior routers A and
C from communicating with one another, while providing full
connectivity between the local internets connected to exterior
router
B and the local internets connected to both exterior routers A and
C
because local internets connected to exterior routers A and C need
access only to local internets connected to exterior router B-not
to each other's local internets
because exterior routers A and C-which should be aware of one
another-were misconfigured
Generally, an exterior router cannot determine whether a multipoint
tunnel is fully connected or partially connected. In the second
example in Figure 2-5, exterior router B does not know whether
exterior routers A and C are aware of one another. However, exterior
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RFC 1504 Appletalk Update-Based Routing Protocol August 1993
router B must assume that the tunnel is fully connected, and that
exterior routers A and C can exchange routing information. An
exterior router should never forward routing information received
from other exterior routers back across the tunnel. It should always
send split-horizoned routing information to other exterior routers.
If connecting exterior routers A and C directly would be either
expensive or slow, a network administrator could instead establish
two independent multipoint tunnels-one connecting exterior routers A
and B, another connecting exterior routers B and C-as shown in the
third example in Figure 2-5. Exterior routers A and C could then
establish connectivity by routing all data packets forwarded by one
to the other through exterior router B.
Hiding Local Networks From Tunnels
When configuring a tunneling port on an exterior router, a network
administrator can provide network-level security to a network in the
exterior router's local internet by hiding that network. Hiding a
specific network in the exterior router's local internet prevents
internets across a multipoint tunnel from becoming aware of the
presence of that network. When the exterior router exchanges routing
information with other exterior routers connected to the tunnel, it
exports no information about any hidden networks to the exterior
routers from which the networks are hidden.
An administrator can specify that certain networks in the exterior
router's local internet be hidden from a specific exterior router
connected to the tunnel or from all exterior routers on the tunnel.
Nodes on the local internet of an exterior router from which a
network is hidden cannot access that network. Neither the zones on a
hidden network nor the names of devices in those zones appear in the
Chooser on computers connected to such an internet. When a network is
hidden, its nodes are also unable to access internets from which the
network is hidden. If a node on a hidden network sends a packet
across a tunnel to a node on an internet from which it is hidden,
even if the packet arrives at its destination, the receiving node
cannot respond. The exterior router connected to the receiving node's
internet does not know the return path to the node on the hidden
network. Thus, it appears to the node on the hidden network that the
node to which it sent the packet is inaccessible.
ADVANTAGES AND DISADVANTAGES OF NETWORK HIDING: Network hiding
provides the following advantages:
On large, global WANs, a network administrator can configure
network-level security for an organization's internets.
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It reduces the amount of network traffic across both a tunnel and
the internets connected to that tunnel.
Network hiding has the following disadvantages:
Nodes on hidden networks have limited access to internets across a
tunnel.
AppleTalk networking software running on a node on a hidden network
lists all of the AppleTalk zone names exported by exterior routers
connected to a tunnel, but may list the names of only some or none
of the devices in those zones. It cannot list the names of devices
that are unable to respond to Name Binding Protocol (NBP) lookups
originating from a node on a hidden network.
Domain Identifiers
Exterior routers assign a unique domain identifier to each AppleTalk
internet, or domain. Domain identifiers enable exterior routers on a
multipoint tunnel to distinguish individual AppleTalk internets in a
wide area internet from one another.
The definition of an AppleTalk domain identifier is extensible to
allow for future use when many additional types of AppleTalk tunnels
and tunneling topologies may exist:
Under the current version of AURP, each exterior router connected
to a multipoint tunnel assigns a domain identifier to its local
AppleTalk internet that uniquely identifies that internet on the
tunnel. If redundant paths connect an AppleTalk internet through
more than one exterior router on a tunnel, each exterior router can
assign a different domain identifier to that internet, or AppleTalk
domain, as shown in Figure 2-6.
Under future routing protocols, a domain identifier will define the
boundaries of an AppleTalk domain globally-for all exterior
routers. Thus, a domain identifier will be unique among all
domains in a wide area internet. All exterior routers within a wide
area internet will use the same domain identifier for a given
AppleTalk internet, as shown in Figure 2-6.
<<Figure 2-6 Domain identifiers>>
To simplify an exterior router's port configuration, a parameter that
is already administrated-such as a node address-can serve as the
basis for an exterior router's domain identifier.
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RFC 1504 Appletalk Update-Based Routing Protocol August 1993
GENERAL DOMAIN-IDENTIFIER FORMAT: Figure 2-7 shows the general form
of a domain identifier.
<<Figure 2-7 General domain-identifier format>>
The general domain identifier (DI) consists of the following fields:
Length: Byte 1 represents the length of the DI in bytes, not
including the length byte. A DI must consist of an even number of
bytes. Thus, the length byte is always an odd-numbered byte. The
length field permits tunneling through foreign network systems that
have addresses of any length-including the long addresses
characteristic of X.25 and OSI. The value of the length byte varies,
depending on the format of the DI.
Authority: Byte 2 indicates the authority that administrates the
identifier bytes of the DI. At present, Apple has defined only two
authority-byte values:
$01-indicates that the subsequent bytes correspond to a unique,
centrally administrated IP address
$00-the null DI-indicates that no additional bytes follow
All other authority-byte values are reserved and should not be used.
Identifier: The identifier field starts at byte 3 and consists of a
variable number of bytes of the type indicated by the authority byte.
NULL DOMAIN-IDENTIFIER FORMAT: The use of a null domain identifier is
appropriate only when there is no need to distinguish the domains
connected to a tunnel-for example, where a tunnel exists within a
single internet-or for a point-to-point link. Figure 2-8 shows the
null form of a domain identifier.
<<Figure 2-8 Null domain-identifier format>>
A null domain identifier consists of the following bytes:
Length: Byte 1 contains the value $01, defining the length of the
null DI as one byte.
Authority: Byte 2 contains the value $00, indicating a null DI.
AppleTalk Data-Packet Format
Part of the format of an AppleTalk data packet sent across a
multipoint tunnel or a point-to-point link depends on the underlying
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foreign network system. The headers required by a foreign-network
protocol always precede an AppleTalk data packet sent across a
multipoint tunnel. A domain header generally immediately precedes
the AppleTalk data packet. Figure 2-9 shows the format of an
AppleTalk data packet preceded by a domain header.
<<Figure 2-9 AppleTalk data-packet format with a domain header>>
A domain header consists of the following fields:
Destination DI: The length of the destination DI field in bytes
depends on the type of DI.
Source DI: The length of the source DI field in bytes depends on the
type of DI.
Version number: The version number field is two bytes in length and
currently contains the value 0001.
Reserved: The two-byte field that follows the version number field
is reserved for future use and is set to 0000.
Packet type: The two-byte packet type field contains the value 0002
to identify the data that follows as AppleTalk data-distinguishing it
from other data, such as routing data. In the future, Apple may
define other values for this field.
An AppleTalk data packet does not require a domain header if
it is sent across a multipoint tunnel or point-to-point link that
provides separate channels for data and routing packets
the domain header's destination DI and source DI fields would both
contain null DIs
Omitting a domain header reduces overhead associated with the
exchange of routing information, without any loss of routing
information. Figure 2-10 shows the format of an AppleTalk data packet
without a domain header.
<<Figure 2-10 AppleTalk data-packet format without a domain header>>
IP Tunneling
The Transmission Control Protocol/Internet Protocol (TCP/IP) protocol
suite is a widely used communications standard that provides
interoperability among computers from various vendors, including
Apple, IBM, Digital Equipment Corporation, Sun, and Hewlett-Packard.
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Descriptions of three of the most important TCP/IP protocols follow:
The Transmission Control Protocol (TCP) is a transport-layer
protocol that provides reliable data transmission between
processes-that is, between programs that communicate with one
another. This connection-oriented, byte-stream protocol ensures
error-free, sequential data delivery, without loss or duplication.
The User Datagram Protocol (UDP) is a transport-layer protocol
that provides best-effort, low-overhead interprocess data
transmission. This datagram-oriented protocol allows higher-layer
protocols that do not require reliability to transmit data without
incurring the overhead associated with TCP. UDP does no error
checking, does not acknowledge its successful receipt of data,
and does not sequence incoming messages. UDP messages may be lost,
duplicated, or improperly sequenced.
The Internet Protocol (IP) is a network-layer protocol that
provides connectionless, best-effort datagram delivery across
multiple networks. Each host on a TCP/IP network has a unique,
centrally administrated internet address, called an IP address,
that identifies the node. The header of an IP datagram contains its
source and destination IP addresses, allowing any host to route a
datagram to its destination. TCP/IP provides connectivity between
many different network types that use data frames of various sizes.
Therefore, IP can fragment a datagram before sending it across an
internet. Datagram fragments can fit into data frames of any size.
Once all of a datagram's fragments reach their destination, IP
reassembles the datagram.
Protocols in higher layers pass data to TCP or UDP for delivery to
peer processes. TCP and UDP encapsulate the data in segments, using
the appropriate headers, then pass the segments to IP. IP further
encapsulates the data in IP datagrams, determines each datagram's
path to its destination, and sends the datagrams across the internet.
Figure 2-11 shows how the TCP/IP family of protocols conforms to the
Open Systems Interconnection (OSI) model.
<<Figure 2-11 TCP/IP protocol stack and the OSI model>>
Exterior routers that connect AppleTalk internets through a TCP/IP
tunnel are configured as nodes on both an AppleTalk internet and on
the TCP/IP internet. Thus, an exterior router on a TCP/IP tunnel is
also an IP end node in the TCP/IP network system. Exterior routers
use the TCP/IP internet only to exchange AppleTalk routing
information and AppleTalk data packets with one another. An exterior
router encapsulates AppleTalk data packets in IP datagrams before
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RFC 1504 Appletalk Update-Based Routing Protocol August 1993
sending them across the TCP/IP internet to a forwarding exterior
router, which decapsulates the packets, then forwards them to their
destination AppleTalk networks.
IP Domain-Identifier Format
Under the current version of AURP, exterior routers on IP tunnels
must use domain identifiers that are based on IP addresses. An
exterior router on an IP tunnel derives its domain identifier from
its IP address. Thus, a network administrator does not need to
configure an exterior router's domain identifier. Figure 2-12 shows
the IP form of a domain identifier.
<<Figure 2-12 IP domain-identifier format>>
An IP domain identifier consists of the following fields:
Length: Byte 1 contains the value $07, defining the length of the IP
DI as seven bytes.
Authority: Byte 2 contains the value $01, indicating that the
remainder of the DI is based on an IP address.
Distinguisher: Bytes 3 and 4 are reserved for future use and are set
to 0 ($00).
IP address: Bytes 5 through 8 contain the four-byte IP address of
either the sending or the receiving exterior router.
NOTE: Future versions of AURP will allow exterior routers to
usealternative formats for domain identifiers, even on IP tunnels.
AppleTalk Data-Packet Format for IP Tunneling
The following protocol headers precede an AppleTalk data packet that
is forwarded across an IP tunnel by an exterior router:
a data-link header
an IP header
a User Datagram Protocol (UDP) header
a domain header
An exterior router encapsulates AppleTalk data packets in UDP packets
when forwarding them through its UDP port 387, across an IP tunnel,
to UDP port 387 on another exterior router. When encapsulating data
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RFC 1504 Appletalk Update-Based Routing Protocol August 1993
packets, an exterior router should always use UDP checksums. When a
destination exterior router receives the UDP packets at UDP port 387,
it decapsulates the packets.
A domain header consists of the following fields:
Destination DI: This field contains the DI of the exterior router to
which a packet is being forwarded.
Source DI: This field contains the DI of the exterior router that is
forwarding a packet.
Version number: The version number field is two bytes in length and
currently contains the value 0001.
Reserved: The two-byte field that follows the version number field
is reserved for future use and is set to 0000.
Packet type: The two-byte packet type field contains the value 0002
to identify the data that follows as AppleTalk data-distinguishing it
from other data, such as routing data.
An AppleTalk data packet consists of a domain header and AppleTalk
data. Figure 2-13 shows the format of an AppleTalk data packet
forwarded across an IP tunnel.
<<Figure 2-13 AppleTalk data packet forwarded across an IP tunnel>>
Point-to-Point Tunneling
In point-to-point tunneling, two remote AppleTalk local area networks
(LANs) connected to half-routers communicate with one another over a
point-to-point link. A point-to-point link may consist of modems
communicating over a standard telephone line or a leased line, such
as a T1 line. Figure 2-14 shows an example of point-to-point
tunneling.
<<Figure 2-14 Point-to-point tunneling>>
Generally, exterior routers use null domain identifiers on point-to-
point links, because there is no IP address to be administrated and
the opposite end of the tunnel is already uniquely identified.
However, an exterior router may use other domain-identifier formats.
Point-to-Point Protocol
The Point-to-Point Protocol (PPP) is a data-link-layer protocol that
provides a standard method of encapsulating and decapsulating
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network-layer protocol information, and transmitting that information
over point-to-point links. PPP includes an extensible Link Control
Protocol (LCP) and a suite of Network Control Protocols (NCPs) that
configure, enable, and disable various network-layer protocols.
The AppleTalk Control Protocol (ATCP) is a PPP NCP for AppleTalk
protocols. ATCP configures, enables, and disables the AppleTalk
network-layer protocol DDP on the half-router at each end of a
point-to-point link. ATCP also specifies the protocol that a half-
router uses to propagate routing information-for example, AURP. When
using AURP for routing-information propagation, a half-router uses a
specific PPP protocol type to identify AURP routing-information
packets-that is, packets preceded by a domain header. PPP provides
separate channels for AppleTalk data packets and AppleTalk routing-
information packets. Thus, a half-router can use DDP encapsulation to
send AppleTalk data packets without including their domain headers.
When using AURP, a half-router should accept both AppleTalk data
packets that are preceded by domain headers and DDP-encapsulated
packets.
NOTE: The Request for Comments (RFC) 1378, "The PPP AppleTalk
Control Protocol (ATCP)," provides a detailed specification of ATCP,
as well as information about using PPP to send AppleTalk data.
3. PROPAGATING ROUTING INFORMATION WITH THE APPLETALK UPDATE-BASED
ROUTING PROTOCOL
This chapter describes the required elements of AURP. It provides
detailed information about using the AppleTalk Update-based Routing
Protocol (AURP) to propagate routing information between AppleTalk
exterior routers connected through a foreign network or over a
point-to-point link, and includes information about
the AURP architectural model
one-way connections
exchanging routing information
updating routing information
notifying other exterior routers that an exterior router is going
down
obtaining zone information
packet formats
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error codes
AURP Architectural Model
AURP provides the functionality of the Routing Table Maintenance
Protocol (RTMP) and the Zone Information Protocol (ZIP) while
eliminating most of the routing traffic generated by these protocols.
Figure 3-1 shows the architectural model for AURP.
<<Figure 3-1 AURP architectural model>>
Generally, an AppleTalk router uses RTMP and ZIP to maintain routing
information, and sends RTMP data packets, ZIP Queries, and ZIP
Replies out its ports. However, if one of the router's ports is
connected to an AppleTalk tunnel, the architectural model for the
router's central routing module becomes more complex. Logically, the
central routing module in an exterior router communicates RTMP and
ZIP information to an RTMP/ZIP-to-AURP conversion module, which sends
AURP data packets out the tunneling port.
RTMP/ZIP-to-AURP Conversion Module
The RTMP/ZIP-to-AURP conversion module maintains split-horizoned
routing-table information and network number-to-zone name mappings
for each exterior router on the tunnel-that is, a copy of the routing
information for each exterior router's local internet. Figure 3-2
shows the architectural components of the RTMP/ZIP-to-AURP conversion
module.
<<Figure 3-2 RTMP/ZIP-to-AURP conversion module architecture>>
The AURP module of the conversion module obtains routing information
from the other exterior routers on the tunnel, then periodically
updates the routing-table information and the mappings in the
conversion module. The RTMP module passes this routing-table
information to the exterior router's central routing module.
Logically, the RTMP module generates an RTMP data packet for each
exterior router on the tunnel every ten seconds-the RTMP
retransmission time-then passes the packet to the central routing
module.
The RTMP/ZIP-to-AURP conversion module also maintains a split-
horizoned copy of the routing information maintained by the exterior
router in which it resides. Logically, the conversion module obtains
the routing information from RTMP data packets and ZIP Replies sent
by the exterior router's central routing module, then updates the
routing information in the conversion module.
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The AURP module exports routing information about its local AppleTalk
internet to other exterior routers on the tunnel.
AURP Transport Layering
AURP can propagate routing information between exterior routers using
a simple, reliable transport based on an underlying datagram
service-such as the default transport-layer service for AURP,
AURP-Tr. See the section "AURP-Tr," later in this chapter,
for more information.
a more complex transport-layer service-such as TCP
Figure 3-3 shows the AURP transport-layering model.
<<Figure 3-3 AURP transport-layering model>>
Maintaining Current Routing Information With AURP
AURP allows exterior routers to maintain current routing information
for other exterior routers on a tunnel by supporting
the reliable, initial exchange of split-horizoned routing
information - that is, the routing information for an exterior
router's local internet
reliable updates to that information whenever it changes
If an internet topology does not change, AURP generates significantly
less routing traffic than RTMP and ZIP. Thus, an administrator can
connect very large AppleTalk internets through a tunnel, and the
resulting internet generates little or no routing traffic on the
tunnel.
When an exterior router discovers another exterior router on the
tunnel-that is, a peer exterior router-it can request that exterior
router to send its routing information. In a reliable, initial
exchange of split-horizoned routing information, the peer exterior
router returns its network-number list. The peer exterior router also
returns each connected network's zone information in an unsequenced
series of zone-information packets. If the exterior router requesting
the routing information does not receive complete zone information
for a network, it must retransmit requests for zone information until
it receives the information.
Once an exterior router requesting routing information from a peer
exterior router has received that exterior router's network-number
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list and complete zone information, it typically requests the peer
exterior router to notify it of any changes to that routing
information. The peer exterior router then provides the requesting
exterior router with reliable updates to its routing information-
however, it sends no other routing information.
Notifying Other Exterior Routers of Events
If an exterior router has requested notification of changes in
another exterior router's split-horizoned routing information, that
exterior router must notify the requesting exterior router of any
event that changes its routing information. Thus, an exterior router
must send updated routing information to the requesting exterior
router whenever any of the following events occur:
the addition of a new, exported network-that is, a network that is
not hidden-to the exterior router's local internet and,
consequently, to its routing table
a change in the path to an exported network that causes the
exterior router to access that network through its local internet
rather than through a tunneling port
the removal of an exported network from the exterior router's
routing table because a network in the exterior router's local
internet has gone down
a change in the path to an exported network that causes the
exterior router to access that network through a tunneling port
rather than through its local internet
a change in the distance to an exported network
a change to a zone name in the zone list of an exported network-
an event not currently supported by ZIP or the current version of
AURP
the exterior router goes down or is shut down
Routing-information updates allow an exterior router to maintain
accurate, split-horizoned routing information for a peer exterior
router on a tunnel.
AURP-Tr
AURP-Tr, the default transport-layer service for AURP, provides a
simple, reliable transport that is based on an underlying datagram
service. When using AURP-Tr, only one sequenced transaction can be
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outstanding, or unacknowledged, at a time-greatly simplifying the
implementation of AURP, without limiting its functionality.
One-Way Connections
A one-way connection is an asymmetrical link between a data sender
and a data receiver that are using AURP-Tr, in which an exterior
router functioning as a data sender sends a sequenced, reliable,
unidirectional data stream to an exterior router functioning as a
data receiver. An exterior router can send routing information over
a one-way connection as
sequenced data
transaction data
Sequenced data is data sent in sequence by the data sender and
delivered reliably to the data receiver. Typically, the sending of
sequenced data is unprovoked-that is, it is not requested by a data
receiver. However, a data receiver can request sequenced data. Figure
3-4 shows sequenced data being sent across a one-way connection.
<<Figure 3-4 Sequenced data on a one-way connection>>
Transaction data-also referred to as out-of-band data-is data sent
unsequenced by the data sender through a linked request/response
transaction that is initiated by the data receiver.
The data receiver can use a one-way connection to request transaction
data from the data sender. If the data receiver does not receive a
response, it must retransmit its request. Figure 3-5 shows a one-way
connection on which the data receiver requests transaction data from
the data sender.
<<Figure 3-5 Request for transaction data on a one-way connection>>
Generally, communication between two exterior routers is
bidirectional-that is, two one-way connections exist between the
exterior routers, with each exterior router acting as the data sender
on one connection and the data receiver on the other. Thus, each
exterior router can send its routing information to the other.
Initial Information Exchange
When an AppleTalk exterior router discovers another exterior router
on the tunnel, it uses the underlying transport-layer service to open
a connection with that exterior router. When using AURP-Tr, an
exterior router opens this connection as a one-way connection.
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Open Request Packet
Once the data receiver opens a connection using the underlying
transport, the data receiver sends an Open Request packet, or Open-
Req, to the data sender. An Open-Req packet includes the following
information:
Send update information flags: The states of the four send update
information (SUI) flags indicate whether the data sender should send
various types of update information over the connection. Typically,
the four SUI flags are set to 1.
Version number: The version number field indicates the version of
AURP used by the data receiver. The current version number of AURP is
1.
Data field: The optional data field allows exterior routers with
capabilities beyond those described in this document to notify other
exterior routers about such options, by initiating option
negotiation. An exterior router that has similar capabilities
indicates that it accepts the options, completing option negotiation.
An exterior router that lacks such options ignores the information in
the data field.
Open Response Packet
When an exterior router receives an Open-Req, it becomes the data
sender and responds with an Open Response packet, or Open-Rsp, as
follows:
If the exterior router accepts the connection, it returns
information about its setup in the Open-Rsp. An Open-Rsp also
contains an optional data field. This data field indicates whether
the exterior router accepts the options in the data field of the
Open-Req to which it is responding.
If the exterior router cannot accept the connection-for example,
because the Open-Req does not contain the correct version number-it
returns an error in the Open-Rsp and closes the transport-layer
connection.
Figure 3-6 shows a connection-opening dialog between a data sender
and a data receiver.
<<Figure 3-6 Connection-opening dialog>>
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Routing Information Request Packet
Under AURP, once two exterior routers establish a connection, the
data receiver can request the data sender to send its routing
information by sending it a Routing Information Request packet, or
RI-Req.
Routing Information Response Packets
When the data sender receives an RI-Req, it reliably sends a sequence
of Routing Information Response packets, or RI-Rsp, to the exterior
router requesting the information.
The RI-Rsp packets provide a list of exported networks on the data
sender's local internet and the distance of each network from the
data sender. The data sender must finish sending RI-Rsp packets to
the exterior router requesting routing information before it can send
any other sequenced data over the connection. Figure 3-7 shows a
routing-information request/response dialog between a data sender and
a data receiver.
<<Figure 3-7 Routing-information request/response dialog>>
Zone Information Request Packet
The data receiver can obtain zone information for known networks on
the data sender's local internet at any time, by sending it a Zone
Information Request packet, or ZI-Req. A ZI-Req lists the numbers of
networks for which the data receiver is requesting zone information.
IMPORTANT: To prevent other exterior routers on a tunnel from sending
endless streams of ZI-Req packets across the tunnel-causing what is
referred to as a ZIP storm-an exterior router must not export
information about a network until it has a complete zone list for
that network.
Zone Information Response Packets
When the data sender receives a ZI-Req, it responds by sending
unsequenced Zone Information Response packets, or ZI-Rsp, to the data
receiver. Zone information is transaction data-thus, its reliable
delivery is not guaranteed. Figure 3-8 shows a zone-information
request/response dialog between a data sender and a data receiver.
<<Figure 3-8 Zone-information request/response dialog>>
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Recovering Lost Zone Information
A data receiver enters a network-to-zone list association in its
routing table for each network for which it receives a ZI-Rsp packet.
If a data receiver that requested zone information for a network does
not receive a complete zone list for that network, it must retransmit
ZI-Req packets, requesting zone information for that network, until
it receives that network's complete zone information.
To determine if any ZI-Rsp packets were lost, the data receiver
periodically scans its routing table for networks for which the
associated zone lists are incomplete-that is, for zone lists that do
not include all zones associated with the networks. The data receiver
sends a ZI-Req to each data sender from which it received incomplete
zone information, listing the numbers of networks for which it has
incomplete zone lists. The data sender responds to zone information
requests by sending ZI-Rsp packets containing the requested
information to the data receiver.
Using AURP-Tr for Initial Information Exchange
The following sections describe the use of AURP-Tr-the default
transport-layer service for AURP-for initial information exchange.
OPEN REQUEST PACKET: An exterior router sends an Open-Req packet to
request that an AURP-Tr one-way connection with another exterior
router be established
specify the connection ID for that connection
pass the AURP version number, SUI flags, and optional data to the
other exterior router
If the exterior router does not receive an Open-Rsp from the exterior
router to which it sent an Open-Req, it must retransmit the Open-Req.
OPEN RESPONSE PACKET: When using AURP-Tr, an exterior router sends an
Open-Rsp to
acknowledge that a one-way connection has been established
reject a connection
return information about its environment, as well as any optional
data, to the exterior router from which it received an Open-Req
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If an exterior router receives an Open-Req on a one-way connection
that is already open-that is, if it receives an Open-Req with the
same connection ID as an open one-way connection-an Open-Rsp sent
previously may have been lost. The exterior router receiving the
duplicate Open-Req should send a duplicate Open-Rsp to the sending
exterior router, unless it has already received some other packet on
the connection-such as an RI-Req-indicating the existence of a fully
established connection.
ROUTING INFORMATION RESPONSE PACKETS: When responding to a request
for routing information using AURP-Tr, an exterior router sends a
sequence of RI-Rsp packets to the exterior router requesting the
information. However, an exterior router's complete list of network
numbers often fits in a single RI-Rsp packet. Each RI-Rsp packet
contains the following information:
Connection ID: The connection ID identifies the specific one-way
connection to which a packet belongs.
Sequence number: The sequence number identifies an individual packet
on a connection. Packets on a connection are numbered starting with
the number 1.
The data sender sending routing information must wait for the data
receiver to acknowledge that it has received each RI-Rsp packet in
the sequence-by sending an RI-Ack packet-before sending the next RI-
Rsp packet. Each RI-Rsp contains a flag that indicates whether it is
the last packet in the sequence. In the last RI-Rsp in the sequence,
this flag is set to 1. If the data sender receives no acknowledgment
of an RI-Rsp from the data receiver within a specified period of
time, it must retransmit the RI-Rsp.
ROUTING INFORMATION RESPONSE PACKETS: When an exterior router
receives an RI-Rsp, it verifies the packet's connection ID and
sequence number. The connection ID must be the same as that in the
Open-Req. The sequence number must be either
the last sequence number received, indicating that the previous
acknowledgment was lost or delayed, and that this is a duplicate
RI-Rsp the next number in the sequence, indicating that this
RI-Rsp contains new routing information
If the connection ID or sequence number is invalid, the data receiver
discards the packet. Figure 3-9 shows a dialog between a data sender
and a data receiver in which the data receiver requests routing
information, the data sender responds by sending its routing
information, and the data receiver acknowledges the data sender's
response. If the data sender receives no acknowledgment, it sends
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duplicate RI-Rsp packets until the data receiver responds with an
acknowledgment.
<<Figure 3-9 Routing-information request/response/acknowledgment
dialog>>
Once the data receiver has verified the information in the RI-Rsp, it
responds with a Routing Information Acknowledgment packet, or RI-Ack,
which contains the following information:
Connection ID: The connection ID is the same as that in the RI-Rsp
packet.
Sequence number: The sequence number is the same as that in the RI-
Rsp packet.
Send zone information flag: The state of the send zone information
(SZI) flag in an RI-Ack packet indicates whether the RI-Ack packet
doubles as a ZI-Req packet. If the SZI flag is set to 1, the data
receiver sends the zone information associated with the networks
about which it sent routing information in the previous RI-Rsp.
Figure 3-10 shows a data receiver sending zone information to a data
sender in response to a ZI-Req and in response to an RI-Ack, which
optimizes the data flow.
When the data sender receives an RI-Ack, it verifies that the RI-Ack
corresponds to the outstanding RI-Rsp-that is, both packets have the
same connection ID and sequence number. Once the data sender has
verified the information in the RI-Ack, it responds by sending the
next RI-Rsp in the sequence, if any.
<<Figure 3-10 Nonoptimized and optimized flows of zone information>>
ZONE INFORMATION RESPONSE PACKETS: If the data sender receives an
RI-Ack with its SZI flag set to 1, it responds by sending ZI-Rsp
packets that contain the zone information associated with the
networks about which it sent routing information in the RI-Rsp being
acknowledged-just as it would if it received a ZI-Req for those
networks.
The data sender sends RI-Rsp and ZI-Rsp packets as independent data
streams. It sends RI-Rsp packets as sequenced data and ZI-Rsp packets
as transaction data. If the data sender receives an RI-Ack with its
SZI flag set to 1, it sends an unsequenced series of ZI-Rsp packets
that contain the following information:
Connection ID: The connection ID is the same as that in the
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associated RI-Req.
Network number and zone list tuples: The exterior router sends the
zone information associated with each network number in the
corresponding RI-Rsp.
Reobtaining Routing Information
An exterior router can reobtain another exterior router's complete
routing information at any time, by sending an RI-Req packet. An
exterior router might need to reobtain complete routing information
for a one-way connection on which it is the data receiver under the
following circumstances:
During the initial routing-information exchange, the exterior
router set the SUI flags in the Open-Req to disable updates. The
exterior router can subsequently poll the other exterior router on
the connection by sending an RI-Req to that exterior router to
determine whether any of its routing information has changed.
The exterior router set the SUI flags to request updates, but
suspects that the routing information for the other exterior router
on the connection is incorrect or obsolete. The exterior router
should send an RI-Req to the other exterior router to obtain its
complete, updated routing information.
Whenever an exterior router receives an RI-Req from an exterior
router requesting updated routing information, it responds by sending
RI-Rsp packets, just as it does when it first receives an RI-Req. The
data sender also r