rfc2340.txt

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Network Working Group                                        B. Jamoussi
Request for Comments: 2340                                   D. Jamieson
Category: Informational                                     D. Williston
                                                                 S. Gabe
                                          Nortel (Northern Telecom) Ltd.
                                                                May 1998


           Nortel's Virtual Network Switching (VNS) Overview

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

   This document provides an overview of Virtual Network Switching
   (VNS).

   VNS is a multi-protocol switching architecture that provides COS-
   sensitive packet switching, reduces the complexity of operating
   protocols like PPP and frame relay, provides logical networks and
   traffic segregation for Virtual Private Networks (VPNs), security and
   traffic engineering, enables efficient WAN broadcasting and
   multicasting, and reduces address space requirements. VNS reduces the
   number of routing hops over the WAN by switching packets based on
   labels.

   VNS has been proven in production networks for several years.

Table of Contents

   1       Introduction ............................................   2
   2       What is VNS? ............................................   3
   3       VNS Header  .............................................   5
   4       VNS Label Distribution ..................................   7
   5     Logical Networks (LNs) ....................................   7
   6       VNS Routing .............................................   8
   7       VNS Forwarding ..........................................   9
      7.1   Unicast ................................................   9
      7.2   Multicast ..............................................   9
   8       Traffic Engineering .....................................  10



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RFC 2340        Nortel's Virtual Network Switching (VNS)        May 1998


      8.1   Equal Cost Multipaths ..................................  10
      8.2   Trunk Load Spreading ...................................  10
   9       Class of Service ........................................  11
   10      VNS Migration Strategies ................................  11
   11      Summary .................................................  11
   12      Security Considerations .................................  12
   13      Acknowledgments .........................................  12
   14      Authors' Addresses ......................................  13
   15      Full Copyright Statement ................................  14

1. Introduction

   There are several key problem areas with today's wide area backbone
   networks that carry LAN traffic: scalability, service
   differentiation, redundancy, administration, and traffic containment.

   First, scalability is becoming a major concern because of the rapid
   growth in bandwidth demand and geographical reach. As the size of the
   WAN network grows traditional point-to-point and NBMA topologies or
   network models lose their performance.

   Second, the need to provide several Classes of Service (CoS) has
   never been greater. The days of a single "best effort" service are
   over and service providers demand ways to differentiate the quality
   of the service offered to their clients based on several policies.

   Third, the WAN is often carrying mission-critical traffic and loss of
   service is not acceptable. So far, path redundancy has been addressed
   inefficiently by requiring additional links or VCs.

   Fourth, network operators demand easy and simplified network
   administration. Large NBMA topologies require extensive PVC
   provisioning until SVC  deployment becomes more ubiquitous. For
   Point-to-point models, IP address space may be used inefficiently and
   non-trivial network schemas are required to contain reserved address
   space.

   Finally, proper segregation of traffic is becoming a must. This
   requirement is being addressed today by adding leased lines or VCs
   used to separate traffic flows based on regions or interest or
   protocol.

   Nortel's Virtual Network Switching (VNS) is a technology that
   provides efficient solutions to these challenges.







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RFC 2340        Nortel's Virtual Network Switching (VNS)        May 1998


   Section 2 provides an overview of VNS. The VNS header is specified in
   Section 3. Section 4 describes the VNS label distribution mechanism.
   Section 5 defines how a VNS network can be partitioned into Logical
   Networks (LN). Section 6 outlines VNS routing. Section 7 defines both
   unicast and multicast forwarding. Section 8 describes the mechanisms
   used to engineer the traffic. Section 9 defines the COS based
   switching of VNS. Section 10 provides network migration scenarios
   using VNS. A summary of VNS is provided in Section 11.

2. What is VNS?

   Virtual Network Switching (VNS) is a CoS-sensitive multi-protocol
   label switching architecture that reduces or eliminates the number of
   layer 3 hops over the WAN by switching traffic based on labels.

   VNS makes a network of point to point links  appear to be a single
   LAN (broadcast, multiple access) media.  The network used by a
   particular instance of VNS is called a Logical Network (LN) which is
   described in more detail in Section 5.

   In reference to the ISO Network Layering Model, the Data Link Layer
   is expanded to include VNS network layer. To the ISO Network Layer,
   (e.g., IP), VNS is treated as a Data Link Layer.

           ------------------------
           | Application          |
           ------------------------
           | Presentation         |
           ------------------------
           | Session              |
           ------------------------
           | Transport            |
           ------------------------      -------------------------
           | Network (e.g., IP)   |     / Network VNS            |
           -----------------------------                         |
           | Data Link                 |--------------------------
           -----------------------------                         |
           | Physical             |     \ data link (e.g., ATM)  |
           ------------------------      -------------------------

               Figure 1. ISO Network Layering Model for VNS

   In a VNS Network, three separate nodal functions are defined.  An
   ingress node, an egress node, and a tandem node. The ingress and
   egress nodes define the boundary between an IP network and the VNS
   network. Therefore, these nodes run both IP routing and VNS routing.
   However, tandem nodes need only run VNS routing.




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RFC 2340        Nortel's Virtual Network Switching (VNS)        May 1998


   A LAN packet is encapsulated in a VNS header as it enters the LN. The
   label in the header is used to switch the packet across the LN. The
   encapsulation header contains the identifier of the last node (or
   egress node) that processes the packet as it traverses the LN. It is
   the first  node (or ingress node) that decides to which egress node
   the packet is sent. All nodes between the ingress and egress nodes
   (known as tandem nodes) decide independently the best packet
   forwarding route to the egress node identified in the packet.

   The network layer protocols view VNS as a shared broadcast media,
   where the speed to reach any node on the media is the same for all
   nodes. VNS ensures that traffic destined to other nodes is forwarded
   optimally. This transparent view of the VNS means that all the
   details of the network (for example, topology and link states) can be
   hidden from the Upper Layer Protocols (e.g. Layer 3 routing
   protocols) and their applications. VNS also ensures that changes to
   topology and link state are hidden.

   The network layer protocol on the ingress node views the network
   layer protocol on the egress node as its logical and directly
   connected neighbor. This is significant because the network layer
   protocols always decide which directly connected neighbor should
   receive a forwarded packet. The details of the actual topology
   supporting the connectionless network are managed entirely by the
   Virtual Network Switching and are hidden from the network layer
   protocols. To the network layer, VNS simply appears to be another
   Data Link Layer (or media), even though VNS is a network layer itself
   running on top of the actual Data Link Layer (for example, ATM
   trunks).

   For the ingress node to choose the egress node that provides the best
   path to the packet's final destination, it must have knowledge of the
   following:

      - the nodes that can be reached in the  network
      - the topology of the network that is using the VNS services for
        transport across the network (but not necessarily the topology
        of the full network)

   This knowledge is obtained through the network layer routing
   mechanisms such as, IP's Open Shortest Path First (OSPF) and Address
   Resolution Protocol (ARP).

   Once the network layer protocol on the ingress node has decided which
   neighbor to transmit the packet to, it is the responsibility of VNS
   forwarding, a part of VNS, to deliver the packet to that node. Once
   the packet arrives at the egress node, the packet is delivered to the
   network layer protocol, which then forwards it to its ultimate



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RFC 2340        Nortel's Virtual Network Switching (VNS)        May 1998


   destination.

   Tandem nodes have no interaction with the network layer protocols.
   They only require knowledge of the  VNS network topology. They make
   their packet forwarding decision on the egress node  identifier and
   LN identifier carried in the VNS header of the packet.

3. VNS Header

   VNS defines a unicast header shown in Figure 2 and a multicast header
   shown in Figure 3.

       3                   2                   1                   0
     1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      TTL      |      LNN            |x|LS-Key |x|DP | CmnHdr  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Protocol Type |         Destination Node Identifier           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  COS  |x x x x|         Source Node Identifier                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Network Layer Header (e.g. IP)                |
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Data                                 |
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2. Unicast VNS Header

   The unicast header includes the following fields:

   - Common Header (CmnHdr): The common header identifies the packet to