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INTERNET-DRAFT       A Framework for IP over RPR            June 2001

Network Working Group                                  Albert Herrera
Internet Draft                                 Lantern Communications
Expiration Date: December 2001                             Russ White
                                                        Cisco Systems

                                                        Pankaj K. Jha
                                                Cypress Semiconductor

                                                           Raj Sharma
                                                       Sanjay Agrawal
                                                     Prasad Jogalekar
                                                          Arun Sastry
                                                    Luminous Networks

                                                          Khaled Amer

                                                            June 2001

            A Framework for IP over Resilient Packet Rings

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at

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This document discusses technical issues and requirements of running
IP over Resilient Packet Rings. It is the intent of this document to
produce a coherent description of all significant approaches, which
were and are being considered by the IPORPR working group. Selections
of specific approaches, making choices regarding engineering
tradeoffs, and detailed protocol specification, are outside of the
scope of this framework document.

Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in RFC 2119.

Table of Contents

1       Introduction  ...........................................   3
1.1     Terminology   ...........................................   3
1.2     Overview of IEEE 802.17 RPR  ............................   3

2       IPORPR Objectives........................................   5

3       Motivation for IPORPR  ..................................   5
3.1     Benefits Related to Traditional Broadcast Media Approach.   6
3.2     Benefits Related to a Client/Service Interface to RPR....   6
3.3     Benefits Related to Enhanced L3 Awareness  ..............   7

4       IPORPR Input and Proposals to 802.17 RPRWG ..............   9
4.1     No Changes to IP in a L2-Broadcast-Media Mode ...........   9
4.2     Support for a Service Interface Capability  .............   9
4.2.1   Proposed Operations    ..................................  10
4.2.2   Granularity of Provisioned BW/Path/Tunnel/Circuit    ....  11
4.3     Support for Enhanced Layer3 Operation    ................  11
4.3.1   Discovering Logical Adjacencies on the Ring    ..........  11
4.3.2   Ring Direction Indication    ............................  12
4.3.3   Fault Interaction    ....................................  12
4.3.4   Traffic Engineering    ..................................  12
4.4     Network Management Support  .............................  13

5       Acknowledgements  .......................................  13
6       Security  ...............................................  13
7       Full Copyright Statement  ...............................  14
8       References  .............................................  14
9       Author's Addresses  .....................................  15

Appendix A: Logical Adjacency TLV Format and Processes   ......... 16

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1.      Introduction

This document takes into consideration the parallel efforts that are
progressing within the IEEE and IETF. IEEE's 802.17 RPRWG is expected
to develop an RPR (Resilient Packet Rings) MAC standard that
optimizes packet transport over rings on LAN, MAN and WAN topologies.
The primary goal of IETF's IPORPR WG is to articulate a set of
features that integrates the RPR MAC functions as defined by the
802.17 RPRWG with network layer routing.

This document is based on current 802.17 RPRWG objectives and
assumptions, which are by no means final.

1.1     Terminology

- Link Layer or Layer 2 or L2: Refer to the data-link layer such as
  IEEE 802.3 / Ethernet and, in the case of this document, IEEE

- Layer 2 or L2 devices: Refer to devices that only implement Layer 2

- Layer 3 or L3: Layer 3 of the ISO 7 layer model and the use of the
  Internet Protocol (IP) at this layer.

- Layer 3 or L3 devices: Refer to devices that implement Layer 3

- The reader is assumed to be familiar with the terminology as
  defined in [1] [2] [3] [4] [5].

1.2     Overview of IEEE 802.17/RPR

Traditional metro and core networks were designed and optimized for
voice and video using the SONET/SDH circuit switch layer. Today's
dominant traffic sources are data applications that drive bandwidth
demands in both the MAN and WAN. Carrying data traffic on traditional
circuit switched networks has proven to be inefficient, complex and

The IEEE 802.17 Working Group was established to formulate a new
Resilient Packet Rings (RPR) MAC layer standard. RPR is designed to
be a packet switched technology optimized for data traffic while
maintaining carrier class operational attributes previously available
only through circuit switch services.

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Some objectives adopted by the 802.17 RPRWG include:

- Support for a dual counter-rotating ring topology
- Support for destination stripping of unicast traffic
- Support for protection switching in less than 50ms
- Support for data rates of (at least) 1 Gb/s to 10Gb/s
- Support for a MAC that is both PHY and payload agnostic
- Support for a fully distributed access method (no master node)
- Support for multi-vendor interoperability at the ring level
- Support for multicast traffic
- Support for managed objects or management "hooks" required for
  efficient operational control of the network.

These objectives define a network topology made up of closed-loop,
point to point, spans with the MAC layer creating the resulting
logical ring topology. At the physical layer, RPR looks like a set of
point-to-point spans while at the datalink layer it appears to be a
broadcast media LAN (equivalent to Ethernet, for instance). The dual
logical rings created are counter-rotating, carrying both control and
data packets. Packets are destination stripped for efficient
bandwidth utilization and prioritized for appropriate treatment
during protection situations. Protection mechanisms will meet or
better the SONET's 50ms service guarantees without the need for
dedicated or reserved bandwidth for redundancy. Data rates are
defined at 1 Gb/s to 10Gb/s but do not preclude support for lower or
higher rates in the future.

These are initial objectives passed by the 802.17 WG with more to
follow. IPORPR, further described in section 2.0, will build upon
this set of features.

This initial set of objectives provides RPR with the following key

Bandwidth Efficiency:

Unlike traditional SONET networks that require as much as 50% of ring
bandwidth for redundancy, RPR utilizes both dual counter rotating
rings for control and data traffic while still maintaining SONET APS-
like protection mechanisms. Spatial reuse is achieved by allowing a
bi-directional flow of traffic on ring spans between source and
destination nodes. The destination node strips unicast packets from
the ring. When a packet is "stripped" from the ring, it no longer
consumes bandwidth on the ring, but frees up downstream spans to be
utilized by other packets.

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RPR's objective is to provide 50ms service protection similar to
SONET APS. Approaches under consideration are to wrap and/or steer
traffic away from the fault. When 'wrapping', nodes adjacent to the
fault will 'wrap' traffic around onto the other ring (i.e. inner ring
traffic to outer) allowing flows to maintain connectivity to the
destination node by way of a long path. 'Steering' will actually
divert flows to the destination via the long path. A combination of
both is also an option where nodes beside the fault will first 'wrap'
then steer at their convenience.

This protection feature will be available while fully utilizing both
counter-rotating physical fibers. Traffic priorities ensure
appropriate handling of high priority vs. best effort traffic during
fault situations.

Simplified Service Provisioning:

One RPR objectives is for a distributed access method. This feature
together with the fast protection and automatic re-establishment of
services, provide for plug-and-play mechanisms for quick insertion
and removal of nodes.

RPR is also designed as a packet switched technology utilizing shared
bandwidth within the rings, with nodes aware of available ring
capacities. The simplicity becomes evident on full-mesh connectivity
requirements where traditional circuit-based models require O(n2)
point-to-point connections versus RPR's single service connection to
the ring.

2.      IPORPR Objectives

The primary goal of the IPORPR Working Group is to articulate a set
of features and functionality that will allow IP to integrate
smoothly with the RPR MAC functions as described in Section 1.2. This
will be presented as input to IEEE's 802.17 RPRWG for consideration.
The intent is to align the needs of the Internet and IP with the
emerging 802.17 standard with the hope that the 802.17 RPRWG will
make design choices that take these requirements into consideration
and in some instances, avoid duplication of efforts.

3.      Motivation for IPORPR

Network layer routing can treat these RPR rings as a broadcast
capable LAN media similar to Ethernet without requiring any change to
current L3 protocols. With minimal change, it could also leverage
RPR's unique features, such as dual-counter-rotating rings,
destination stripping (which effectively provide two possible paths
from a source node to a destination node) or 50ms protection
switching (capable of masking any single physical fault).

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There are three basic approaches to L2/L3 interaction presented in
this document (further described in the following sections):

-  L3 view of an RPR ring as a traditional broadcast media
-  A L3 service interface to an RPR ring for specific
   resource/services requests
-  Enhance L3 awareness and interaction with an RPR ring.

Introducing additional L2/L3 interaction will provide increased
ability to better utilize ring resources and increase efficiency and
scalability at Layer 3.

The following are projected benefits related to IPORPR.

3.1     Benefits Related to Traditional Broadcast Media Approach

In an approach where L3 treats an RPR Ring as a traditional data
link, broadcast media (e.g. Ethernet), L3 will rely on L2 to provide
the benefits of the underlying features such as protection
mechanisms, congestion control and bandwidth management within each
RPR ring domain.

This is the simplest approach to L2/L3 interaction and requires no
change whatsoever to existing L3 protocols.

3.2     Benefits Related to a Client/Service Interface to RPR

RPR rings include a distributed access management capability that
allows rapid establishment and reconfiguration of flows and/or
connections, reducing provisioning times and effectively lowering
costs while providing the means to set and guarantee SLAs and QoS
configured on either a per-flow and/or connection basis. The
projected new services for RPR rings are bandwidth on demand, point
and click establishments of circuits/paths and VLANs and/or VPNs.

A standardized interface between RPR and the service layers such as
IP will allow end-to-end internetworking of paths and flows through
these rings and will provide the benefits of RPR end-to-end even if
other networks are involved (i.e. meshes).

This service interface will allow clients to the RPR ring to signal
parameters (i.e. SLA constraints, bandwidth requirements, etc.) to L2
for specific path selection or other L2 behavioral characteristics.
The signaling and routing interface between the client (IP and
others) and the RPR ring is similar to traditional User-Network
Interface (UNI).

This allows either a dynamic and/or static set of capabilities for
service provisioning without closely coupling the underlying RPR ring
operations to the upper layers.

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This interface can support a range of clients from individual
customers to lower tier ISPs connected to a carrier's carrier for
dynamic provisioning of services or the service provider's management
console for static establishment of services.

3.3     Benefits Related to Enhanced L3 Awareness

Enhanced L3 awareness of the underlying physical topology will allow
greater L3 controls of certain ring operations/behaviour such as
choice of ring direction, alternate fault response, traffic
engineering and related QoS metrics.

Note that this document does not address issues related to enhanced
Layer 3 awareness across bridged, multi-ring domains. In topologies
such as these, it might be best for Layer 3 to simply treat these as
simple broadcast domains.

The following are related L3 components that can be enhanced when
running IP over RPR rings.

Cost Metrics Awareness:

At the MAC layer, RPR may implement its own topology discovery
mechanism within a single set of dual-ring implementation. This may
involve tracking source and destination MAC addresses and utilizing
simple metrics, such as number of hops, to determine the shortest
path and most efficient ring direction for delivery of unicast

For instance, in the network depicted below, the cost from node A to
node C would be 1 hop across the directly connected side of the ring,
while it would be two hops through node B.

                         /               |
                        C               (5)
                         \               |

However, using L3 associated costs for links between the nodes (based
on physical distance or some other link trait, for instance), the
path through node B is shorter.

Communicating the preferred ring direction to L2, based on layer 3
metrics can optimize packet delivery - especially in topologies with
dramatic differences in node distances and costs.

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Adjacency and Fault Awareness:

Given RPR's destination stripping ability, and once fault
notification is enabled between Layer 2 and 3, it would be beneficial
for Layer 3 to be aware of directly adjacent neighbors for route re-
calculation during fault situations.

Within a single ring domain, the benefits of adjacency awareness
might not be obvious since the underlying RPR mechanisms will self-
heal and send flows efficiently around or away from faults to
maintain connectivity.  However, L3 route recalculation might be
necessary to fully reflect the changed topology during these events
especially when the user deploys a mix of ring and mesh topologies.
This will allow a recalculation of paths based on established cost

For instance, in the network depicted below, a ring made up of nodes
A, B, C and D is bisected by a P2P link between D and B with a cost
of (5). Under normal ring operations, C will reach B in a counter-
clockwise direction via a single span with a cost of (5). Should a
fault occur on this span between C and B, C will reach B via D
skipping A altogether due to an expensive span cost when traversing A
on a clockwise ring direction.

                          |                |
                          |                |

Notifying L3 of fault situations from the lower layers will allow
swifter response from L3 should this be required by the user. Fault
situations that L3 should be aware of are those that would cause ring
traffic to be wrapped or redirected away from the fault. Other minor
fault situations, where ring operations continue to function, might
not have to be communicated to L3.

TE/Explicit Routing/QoS Routing Support:

Although RPR might incorporate its own topology discovery, bandwidth
management and congestion/flow control mechanisms at L2, there might
be instances wherein L3 would prefer to override these.

MPLS, used for traffic engineering, offers several benefits within an
RPR-ring topology. One benefit is the capability to establish
explicit switched paths that are not constrained by any of RPR's
forwarding paradigms. The MPLS preferred path along the ring could be
the long-path towards a destination node. Another benefit is the
capability to associate attributes to these traffic flows and to
administer these flows across diverse topologies (combination of
meshes and rings). This same mechanisms will enable L3 per flow QoS
treatment in which the path chosen for a particular stream is chosen
in response to the QoS constraints.

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4.      IPORPR Input and Proposals to 802.17 RPRWG

The IPORPR Working Group was chartered by the IESG to produce a
requirements and framework document, which can be used as input to
the IEEE 802.17 RPRWG. The intent is to help the RPRWG formulate its
requirements. Areas of particular interest to the IPORPR WG are:
Layer 2 to Layer 3 interaction in: alarm notification; fast
restoration; fast convergence; traffic engineering and quality of
service issues.

The primary requirement set forward by IPORPR is that the IEEE 802.17
WG makes design decisions that will have the least impact and require
the least amount of change to IP.

The following are the proposed requirements for L2/L3 interaction.

4.1     No Changes to IP in a L2-Broadcast-Media Mode

The first and foremost requirement for 802.17 is to allow IP to
operate without any changes, whatsoever, when treating RPR rings as a
traditional L2 broadcast media. This approach should not be any
different from how IP operates today over other data link layers such
as Ethernet. This will allow existing protocols and design constructs
to apply unchanged.

4.2     Support for a Service Interface Capability

A 'service interface' can be thought of as a User-Network Interface
(UNI). A connection, in a UNI sense, is defined by its demarcation
from the ingress node to egress node. These can be within the same
ring or might span other rings or meshes. The behaviors of these
pairings are defined by their connection attributes. A standardized
set of parameters is required for the edge-facing interfaces. This
will include service requirements and attributes during normal ring
operations and during fault situations. Specifics for these
parameters are not discussed in this current framework but should be
developed as the 802.17 standards evolve and as more detailed
features and behaviors are firmed up.

There are two approaches in terms of request handoffs from a service

One approach is to handoff requests to L2 and use L2 capabilities,
such as topology discovery, path selection and bandwidth accounting,
to service requests from the client interface. RPR's ring operations
should be capable of satisfying these requests in terms of contracts
associated with SLAs.

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The other approach is to allow L3 intelligence to meet these
requests. In cases where enhanced L3 awareness is enabled for traffic
engineering, path establishment, bandwidth accounting, QoS and
associated signaling, service parameters can be handed over to L3.
This approach will accommodate requirements for extending any
existing L3 provisioning and signaling models (i.e. on mesh networks)
to and through RPR ring topologies.

Both approaches should be supported.

4.2.1   Proposed Operations

A client upon ingress to the RPR ring may request:

- a new connection subject to policy
- a tear-down of an existing connection
- attribute modifications to an existing connection based on policy
- attribute queries on an existing connection
- status queries of issued requests

Admission control in terms of accepting or rejecting requests depends
on the underlying provisioning and signaling capabilities. Local node
policy will dictate when connections may be established and whether
these are dynamically established or administratively controlled.
Parameters and associated mechanisms are required to validate
interoperable capabilities at the ingress and egress ports of a
requested connection. Neither the connection setup mechanism nor the
method for identifying flows is addressed in this document.

Bounded delay guarantees require that every RPR node in the
connection path supports a guaranteed service feature whether within
the L2 operations itself and/or via L3 enhancements, resource
accounting and policy enforcements. Each RPR node accepting a request
for service must ensure that adequate bandwidth and packet processing
resources are available to handle the request as specified in the
client's request parameters / Traffic Specifications. This may be
implemented through active admission control based on resource
availability such as link bandwidth, port buffer space, forwarding
engine capacity or transit traffic treatments. The assumption is that
802.17 will provide the appropriate architecture to meet stringent
delay and jitter guarantees whether through appropriate queuing
mechanisms, media access management or transit traffic mechanisms.

In cases where the RPR node detects a failure on the ingress physical
port, or if the ingress port is administratively disabled, the
corresponding service connection, related mappings and reservations
may be withdrawn. Payload synchronization and maintenance alarm
requirements are outside the scope of this document.

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4.2.2   Granularity of Provisioned Bandwidth/Path/Tunnel/Circuit

As stated on the objectives, RPR rings will support at a minimum,
data rates, on the ring, of 1Gb/s to 10Gb/s with even higher rates
for consideration within 802.17 in the future. At the ingress service
interface, an upper limit equivalent to the fastest link on the ring
should be allowed, as well as lower granularities such as 1Gb/s,
10/100mb/s, 128kb/s, 56kb/s, etc.

4.3     Support for Enhanced Layer3 Operation

Upper layer protocols will require various interaction points with
RPR rings to take effective advantage of the available bandwidth (in
both directions) through the ring. Generally, these interaction
points will fall into two categories: Informing the data link layer
of ring direction choices made by Layer 3 protocols for each next hop
along the ring, and informing the Layer 3 protocols of the ring

The following are technical approaches for consideration:

4.3.1   Discovering Logical Adjacencies on the Ring

IS-IS and OSPF require knowledge of all neighbors on a ring with the
additional information of which neighbor is directly connected
upstream and directly connected downstream. Since all devices
attached to a ring may not be running layer three protocols, simply
using the physical topology discovered by the data link layer isn't
an effective way to discover all layer three topology information.
Instead, it will be more effective for RPR to provide an opaque
transport through which routing protocols can determine which
neighbors are logically attached upstream and downstream.

To this end, the RPR data link layer should be able to carry 'opaque
data' TLV's (Type/Length/Value) between nodes, accepting data from
upper layer protocols, and passing this information back up the stack
on adjacent nodes only (Refer to Appendix A: Logical Adjacency TLV
Format and Processes).

Through this process, devices running layer 3 protocols will discover
its directly adjacent downstream and directly adjacent upstream

NOTE: This is one implementation possibility used to explain the
behavior; this document is not intended to restrict or dictate the
correct implementation to achieve this behavior. This could, for
instance, be implemented using multicast between the nodes, which
layer two could then compare to the physical topology discovered
through other means, passing only the information about 'logical
neighbors' up to layer 3, or some other mechanism.

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4.3.2   Ring Direction Indication

A mechanism MUST exist for Layer 3 to override any indication by
Layer 2 as to the direction to be followed on the ring to reach any
of its nodes.  The two possible directions are "upstream" and
"downstream". Administrative policies and/or traffic engineering
parameters will influence layer 3 decisions.

If Layer 3 does not indicate one of these directions, then Layer 2
SHOULD use its own methods to determine it.

Should Layer 2 determine the ring direction, there might be instances
where the direction taken by RPR be explicitly indicated to Layer 3.
These are instances where strict latency and jitter requirements are
to be met and ring direction decisions will require indications from
Layer 2.

4.3.3   Fault Interaction

If a device on the ring or a link between two nodes on the ring
fails, the ring will wrap or steer traffic away from the fault.
During these fault situations, nodes adjacent to the fault might want
to perform route re-computation and advertise a single ring adjacency
in their LSP. A signal MUST be provided to the layer 3 protocols on
the attached nodes indicating the loss of a logical adjacency. Layer
2 MUST NOT provide information about the newly reachable neighbor
through the wrapped or re-directed ring, as this could cause layer 3
to be unable to properly converge on the new topology.

In cases where a node failure occurs, where a particular device
leaves the ring but maintains transit paths and ring connectivity in
place, normal RPR ring operations are expected to proceed as normal.
Any fault interaction is expected to be resolved at a higher layer
where procedures are in place to react a lost destination nodes or
adjacencies (e.g. hellos, keepalives, topology refresh, etc.).

4.3.4   Traffic Engineering

To take full advantage of RPR, TE capabilities should be made
available to manage bandwidth on both inner and outer rings.

Sometimes it is preferred to force a packet to follow an explicitly
chosen route different from the path chosen by a dynamic routing
protocol or different from the shortest path chosen by RPR. This
implies the capability to determine the ring direction a particular
flow or tunnel will take (long path or short path) and the capability
to establish an explicitly routed path from ingress to egress.

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RPR should be able to meet the SLA constraints of established flows
that originate from outside the ring whether from another ring or
from a mesh topology. This implies the capability to apply resource
reservations along a chosen path and the capability to recognize
precedence and class of service parameters for applicable discard or
scheduling mechanisms.

In terms of MPLS support, different approaches for label stack
encoding, to enable forwarding of labeled packets encapsulated within
RPR headers, can be referenced in [5].

This document does not suggest use of a single label distribution
protocol. Please refer to [4] for options and implications.

This document does not cover interaction between RPR protection
switching and MPLS based fault recovery mechanisms.

4.4     Network Management Support

RPR MUST support operations, administration, and maintenance
facilities that are at least as extensive as those supported in
current IP networks. Current network management and diagnostic tools
SHOULD continue to work in order to provide some backward

5.      Acknowledgments

The ideas and text in this document have been collected from a number
of sources and comments received. We would like to thank John
Coulter, Kshitij Kumar, Alvaro Retana, Don Slice, Liem Nguyen,
Thierry Paiement, Jason Fan, Vinay Bannai for their inputs and ideas.

6.      Security

Security in a network using IPORPR should be relatively similar to
security in a normal IP network. The route filtering and packet
filtering features are unchanged.

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7.      Full Copyright Statement

"Copyright (C) The Internet Society. All Rights Reserved. This
document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into."

8. References

   [1] ISO 10589, "Intermediate System to Intermediate System Intra-
       Domain Routing Exchange Protocol for use in Conjunction with
       the Protocol for Providing the Connectionless-mode Network
       Service (ISO 8473)" [Also republished as RFC 1142]

   [2] RFC 1195, "Use of OSI IS-IS for routing in TCP/IP and dual
       environments", R. W. Callon, Dec. 1990

   [3] RFC 2178, "OSPF Version 2", J. Moy. July 1997

   [4] RFC 3031, "Multiprotocol Label Switching Architecture", E.
       Rosen, A. Viswanathan, R. Callon, January 2001.

   [5] RFC 3032, "MPLS Label Stack Encoding", Rosen, E., Rekhter, Y.,
       Tappan, D., Fedorkow, G., Farinacci, D. and A. Conta,
       January 2001.

   [6] IEEE 802.17 RPR PAR and 5 Criteria, http://www.ieee802.org/17,
       November 2000.

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9.      Author's Addresses

        Albert Herrera
        Lantern Communications
        2000-1642 Merivale Road,
        Ottawa, ON K2G 4A1
        Phone: 613-727-7112
        Email: aherrera@lanterncom.com

        Russ White
        Cisco Systems, Inc.
        7025 Kit Creek Road
        Research Triangle Park, NC
        Phone: 919 392-3139
        Email: ruwhite@cisco.com

        Pankaj K Jha
        Cypress Semiconductor
        3901 N First Street
        San Jose, CA 95134
        Phone: 408 432 7091

        Raj Sharma (raj)
        Sanjay Agrawal (sanjay)
        Prasad Jogalekar (prasad)
        Arun Sastry
        Luminous Networks
        10460 Bubb Road
        Cupertino, CA 95014
        Tel: 408-342-6400

        Khaled Amer
        Email: khaledamer@usa.net

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Appendix A: Logical Adjacency TLV Format and Processes

IS-IS and OSPF require knowledge of all neighbors on a ring with the
additional information of which neighbor is directly connected
upstream and directly connected downstream. To this end, the RPR data
link layer should be able to carry 'opaque data' TLV's
(Type/Length/Value) between nodes, accepting data from upper layer
protocols, and passing this information back up the stack on adjacent
nodes only.

So, for instance, in this small sample ring, one possible
implementation of this functionality would be:

 / \
A   C
 \ /

Assume that A, B, and C are running some layer three protocol, while
D is not. A needs to know which devices on the ring are possible next
hops upstream and downstream. To facilitate this:

-  A, B, and C will inject information into a Logical Adjacency TLV
(LAT) which will be carried by the RPR data link layer both upstream
and downstream.

- When A receives B's LAT, the RPR data link processes will determine
which protocol stack to pass the opaque information within the TLV
to, and pass it up the stack with an indication of which direction is
used to reach this adjacent device.

- When D receives C's LAT, it will discover that it has no layer
three protocol running, so it will simply forward the LAT on to A.

- When A receives C's LAT, the RPR data link processes will again
determine which protocol to forward the opaque portion of the data
to, and pass it up the stack with an indication of which ring
direction is used to reach this adjacent device.

Through this process, A will discover that it's directly adjacent
downstream (clockwise) neighbor is B, while it's directly adjacent
upstream (counter clockwise) neighbor is C. A's routing processes can
use this information, in conjunction with information learned at
layer three, to make optimal routing decisions through the ring.

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INTERNET-DRAFT       A Framework for IP over RPR            June 2001

Logical Adjacency TLV Format

 | src mac| id_number | id_type | id_len | id | ...

o src mac: The MAC address of the source ring node

o id_number (1 byte): The number of this identifier within the packet

o id_type (1 byte): described below

o id_len (2 bytes): length of the opaque information contained in
                      the id section

o id: The actual opaque information provided by the layer 3
      protocol, and passed to the receiving device's layer 3

ID Type Field

While the information contained within the logical adjacency TLV's is
opaque to the RPR data link layer, the TLV's themselves need to be
identifiable in some way so the layer 3 protocol on one device
receives the correct TLV from the layer 3 protocols on connected
devices. To this end, the TLV should be encoded with a protocol
identifier of some type.

Three protocol identifiers are defined here, and others can be
identified as needed in the future.

  o OSPF:  1
  o IS-IS: 2
  o BGP:   3
  o EIGRP: 4

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