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Network Working Group                                        B. JamoussiRequest for Comments: 2340                                   D. JamiesonCategory: Informational                                     D. Williston                                                                 S. Gabe                                          Nortel (Northern Telecom) Ltd.                                                                May 1998Nortel's Virtual Network Switching (VNS) OverviewStatus 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 Contents1       Introduction ............................................22       What is VNS? ............................................33       VNS Header  .............................................54       VNS Label Distribution ..................................75     Logical Networks (LNs) ....................................76       VNS Routing .............................................87       VNS Forwarding ..........................................97.1   Unicast ................................................97.2   Multicast ..............................................98       Traffic Engineering .....................................10Jamoussi, et. al.            Informational                      [Page 1]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 19988.1   Equal Cost Multipaths ..................................108.2   Trunk Load Spreading ...................................109       Class of Service ........................................1110      VNS Migration Strategies ................................1111      Summary .................................................1112      Security Considerations .................................1213      Acknowledgments .........................................1214      Authors' Addresses ......................................1315      Full Copyright Statement ................................141. 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.Jamoussi, et. al.            Informational                      [Page 2]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 1998Section 2 provides an overview of VNS. The VNS header is specified inSection 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 inSection 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 inSection 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.Jamoussi, et. al.            Informational                      [Page 3]

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 ultimateJamoussi, et. al.            Informational                      [Page 4]

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   be a VNS encapsulated packet.   - Discard Priority: Indicates the level of congestion at which the   packet should be discarded. The value of this field is assigned on   the originating node based on policy information (seeSection 9).   - Load Spreading Key: indicates the stream to which the packet   belongs for the purposes of equal cost multipath and trunk load   spreading (seeSection 8).   - LNN: The Logical Network Number defines the logical network the   packet belongs to. This field in is used in conjunction with the   destination node identifier as the VNS switching label (seeSection5).Jamoussi, et. al.            Informational                      [Page 5]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 1998   - TTL: The Time To Live field is used to detect and discard packets   caught in temporary routing loops.   - Destination Node Identifier: This field contains an ID which   uniquely identifies the destination node.  This ID is unique to  the   physical network not just the LN. In conjunction with the LNN, this   forms a global VNS switching label.   - Protocol Type: indicates the type of Network layer protocol being   carried in the packet. Examples include IP, IPX, and Bridging. If the   packet is a multicast packet then this is indicated in this field.   - Source Node Identifier: This field contains an ID which uniquely   identifies the source node (ingress node).   - CoS: The Class of Service field is used to provide routing class of   service. The COS field also affects the Emission Priority of the   packet in the scheduler (seeSection 9).   - Reserved Fields: All the fields marked with "x" are Reserved.       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  |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    | PT = Multicast|         Destination Node Identifier           |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |  COS  |x x x x|         Source Node Identifier                |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    | Protocol Type |x x x x x x x x|    Multicast Group            |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                 Network Layer Header (e.g. IP)                |    /                                                               /    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    /                          Data                                 /    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                      Figure 3. Multicast VNS Header   The multicast header shown in Figure 3, includes all the fields of   the unicast header. In addition, the multicast header includes the   following fields:   - Multicast Group: this field is used to identify a sub-group within   the logical network that receives the multicast packets.Jamoussi, et. al.            Informational                      [Page 6]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 1998   - Protocol Type: indicates the type of Network layer protocol being   carried in the packet. Examples include IP, IPX, and Bridging.4. VNS Label Distribution   Label distribution in VNS is based on a distributed serverless   topology driven approach. Standard ARP or address gleaning is used to   distribute and map network layer addresses to VNS addresses.   A VNS Label is an 6 byte encoding of the LNN and the node ID.  VNS   Labels are treated as MAC addresses by the network layer.  This means   that labels are distributed by the same means network layers use to   distribute MAC addresses.  Thus, VNS leverages existing L2/L3 mapping   techniques and doesn't require a separate Label Distribution   Protocol.5. Logical Networks (LNs)   A logical network consists of a subset of the nodes in a network   together with a subset of the trunking facilities that link those   nodes. Logical networks partition the network into subnetworks that   serve a subset of the overall topology.   Each of the logical networks supported on any given node has a   separate routing and forwarding table (built by VNS). Therefore,   routing decisions are based on the resources available to the logical   network, not the entire network.   Each instance of VNS will discover all the trunks which are connected   to neighbors which support a matching LNN.  This provides a huge   administrative saving, since VNS provisioning is on a per-node basis,   not on a per-link basis.  VNS provisioning requires only a unique   node ID and an LNN.  Discovery of which trunks support which LNNs is   done at run time, relieving administrative effort, and allowing the   LN to dynamically adapt to topology changes.   Multiple Logical Networks provide the following benefits to the   network system:      - Logical networks allow service providers to service multiple      private networks or (Virtual Private Internets) easily over one      network.      - Logical networks can be used to limit the impact of one network      layer protocol on the others. This is particularly true for      protocols that broadcast or multicast a large percentage of either      their control or data packets.  This increases the effective      bandwidth of the trunks and allows the overall network to scaleJamoussi, et. al.            Informational                      [Page 7]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 1998      better.      - Logical networks allow for the configuration of the network to      meet individual community of interest and geographical      subnetworking needs.      - Routing control traffic has significance only in the local      subnetwork that is isolated to that subnetwork.      - Logical networks allow different instances of the same protocol      to share trunk facilities.6. VNS Routing   VNS routing is a link state routing system which uses many concepts   similar to OSPF and PNNI. One of the most significant departures from   the others is its ability to calculate shortest path trees for   routing unicast traffic and spanning trees for routing multicast   traffic within a Logical Network.   There is only one type of interface that VNS routing supports and   this is known as a VNS link. A link is a set of trunks that join two   VNS neighbor nodes. Each node in a VNS network maintains information   about the state of locally attached links. This information is   flooded throughout the network whenever there is a significant change   to the link's state or attributes (i.e. up/down, speed change,   available bandwidth change).   Each node stores and forwards the link state information received   from all other nodes. This allows each node to have the same view of   all of the nodes in the network together with all of their link state   information. This data is used to compute both the shortest path to   reach each node in the Logical Network and a spanning tree for the   Logical Network.   Logical networks are not bound to a particular trunk or link. They   are configured on a node. By default, a link will support a specific   logical network if the two nodes which it connects both are   configured to support the logical network number. This provides a   significant savings in operations over having to configure logical   networks on links or trunks.   When a link first comes into service, a protocol is run which allows   the two neighboring nodes to exchange information about the logical   networks they support. This allows the two nodes to determine if the   links are to be considered as a locally attached link for a logical   network.Jamoussi, et. al.            Informational                      [Page 8]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 19987. VNS Forwarding   VNS supports two types of forwarding: unicasting and multicasting. In   the first type, the data packet arrives on the ingress node and   unicasting forwards the data packet to a single destination (egress   node). In the second type, the data packet arrives on the ingress   node and multicasting forwards the data packet to all other nodes in   the logical network.7.1 Unicast   When a packet first enters the  LAN internetwork, the network layer   routing protocol determines the next hop of the best route for the   packet to reach its final destination. If the best route is through a   VNS Logical Network, the network layer routing protocol relies on VNS   forwarding to get the packet to the egress  node. A VNS packet header   containing the node ID (the unique ID assigned  to each  node) of the   egress node is added to the front of the packet and VNS forwarding is   invoked to deliver the packet. The network layer routing protocol   learns the egress node ID through an Address Resolution Protocol   (ARP) for IP and Source Address learning for bridging.   As the packet traverses the LN, routing decisions are made to   determine the next hop in the route to reach the destination node ID   specified in the VNS header. A forwarding table is built on each node   that assists in making the routing decision.   Each VNS instance on each  node builds and maintains a forwarding   table for its LN. Each forwarding table has an entry for every  node   that is a member of the logical network.7.2 Multicast   In addition to the unicast forwarding function, VNS also supports a   multicast forwarding service for traffic within an LN at the VNS   layer. Multicast packets are delivered to all nodes supporting the   logical network to which the multicast packet belongs. The packets   are sent along the branches of a spanning tree that is built by each   node supporting the logical network and is based on a common root   node (so that each node's view of the tree is the same as other   nodes). In other words, multicast packets are sent intelligently,   consuming a minimum of network bandwidth. If the network topology is   stable, each node receives each multicast packet only once.   Multicast packets received at any node are not acknowledged. They are   simply forwarded to the specified network layer interface and sent to   any other neighbor nodes on the spanning tree.Jamoussi, et. al.            Informational                      [Page 9]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 19988. Traffic Engineering   VNS forwarding supports two types of traffic engineering mechanisms:   equal cost multipaths and trunk load spreading.   Equal cost multipaths allows different streams (unique network layer   source and destination address pairings) to be load spread between   multiple relatively equal cost paths, through the Logical Network to   the egress node.   Trunk load spreading between two neighbors can take place when   multiple VNS  trunks are defined between neighbors. Again, the load   spreading is based on network layer streams.8.1 Equal Cost Multipaths   From any point in a logical network, there may be multiple paths to   reach a specific egress node. If VNS routing determines that more   than one of these paths are of equal cost, VNS packets will be load   spread between two of them.   Equal cost multipath forwarding is supported not only on ingress   nodes but on tandem nodes as well. Each packet on an ingress node is   tagged with an equal cost multipath key. This key is acted upon at   the ingress node and stored in the VNS header to be used on tandem   nodes.   The equal cost multipath key is calculated by running an algorithm   over the source and destination network layer addresses. This means   that, in a stable network, any given stream will always take the same   path through a Logical Network avoiding the problems that misordering   would otherwise cause.8.2 Trunk Load Spreading Between Neighbors   VNS allows multiple trunks to be configured between neighboring VNS   nodes. VNS routing considers the aggregate bandwidth of those trunks   to determine the metric between the nodes. Also, VNS load spreads its   traffic amongst those trunks.   As is the case with equal cost multipaths, the trunk load spreading   key is calculated on the ingress node from an algorithm run over the   source and destination network layer addresses. The key is then   stored in the VNS header to be used on all tandem nodes through the   Logical Network.Jamoussi, et. al.            Informational                     [Page 10]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 19989. Class of Service   At the ingress to a VNS Network, packets are classified according to   the Class of Service (Cos) policy settings. The CoS differentiation   is achieved through different  Emission and Discard priorities. The   semantics of the classification is carried in the VNS label (DP and   COS Fields described inSection 3) to be used at the ingress node as   well as all tandem points in the VNS network to affect queuing and   scheduling decisions.10. VNS Migration Strategies   VNS supports several upper layer protocols such as IP, IPX, and   Bridging. Therefore, it is a multiprotocol label switching   architecture. In addition, VNS  is not tied to a particular L2   technology. It runs on cell (e.g., ATM) trunks, frame trunks, or a   mixture of both.   VNS can be gradually introduced in a network. It can be implemented   between switching elements interconnected by point to point links.   Each of the switching nodes can run layer 3 routing simultaneously   with packet switching. VNS also allows for the interconnection of VNS   clouds through an ATM VC.   Since VNS can run on a mixture of Frame and Cell trunks, it allows   for the graceful migration of the frame links to ATM without   requiring a complete immediate overhaul.11. Summary   VNS addresses scalability problems in several ways:      1. By a generally distributed design which doesn't         require a Label Distribution Protocol, or servers of any kind.      2. By providing an efficient, distributed multicast mechanism.      3. By allowing administrators to control the size of a         Logical Network, limiting traffic to a subset of the physical         topology.      4. By reducing layer 3 address space/subnet requirements in the         WAN which reduces the routing table size.   VNS provides redundancy transparent to the network layer protocol by   managing the network of trunks independently of the network layer.   VNS will automatically discover any topology changes and re-route   traffic accordingly.Jamoussi, et. al.            Informational                     [Page 11]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 1998   VNS eases network administration by dynamically keeping track of   which trunks are available for each LNN.  Network administrators   don't have to configure VNS or network layer addresses on a per link   basis.  Network layer addresses only have to be assigned on a per   Logical Network basis.  For nodes which will only be tandem VNS   nodes, network layer addresses aren't required at all.   Since VNS traffic is constrained within an LNN, administrators have   control of where VNS traffic is allowed to flow.   Finally, VNS supports switching of several Upper Layer Protocols and   supports  several media (cell and Frame) or a mixture thereof.   Switching in the core of the WAN removes the need for routers and   improves the performance due to a reduction in the  number of fields   that need to processed.12. Security Considerations   Logical networks provide a means of restricting traffic flow for   security purposes. VNS also relies on the inherent security of the L2   media such as an ATM Virtual Circuit.13. Acknowledgments   The authors would like to acknowledge the valuable comments of Terry   Boland, Pierre Cousineau, Robert Eros, Robert Tomkins, and John   Whatman.Jamoussi, et. al.            Informational                     [Page 12]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 199814. Authors' Addresses   Bilel Jamoussi   Nortel (Northern Telecom), Ltd.   PO Box 3511 Station C   Ottawa ON K1Y 4H7   Canada   EMail: jamoussi@Nortel.ca   Dwight Jamieson   Nortel (Northern Telecom), Ltd.   PO Box 3511 Station C   Ottawa ON K1Y 4H7   Canada   EMail: djamies@Nortel.ca   Dan Williston   Nortel (Northern Telecom), Ltd.   PO Box 3511 Station C   Ottawa ON K1Y 4H7   Canada   EMail: danwil@Nortel.ca   Stephen Gabe   Nortel (Northern Telecom), Ltd.   PO Box 3511 Station C   Ottawa ON K1Y 4H7   Canada   EMail: spgabe@Nortel.caJamoussi, et. al.            Informational                     [Page 13]

RFC 2340        Nortel's Virtual Network Switching (VNS)        May 199815.  Full Copyright Statement   Copyright (C) The Internet Society (1998).  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 languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Jamoussi, et. al.            Informational                     [Page 14]