rfc2340.txt

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   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 (see Section 9).

   - Load Spreading Key: indicates the stream to which the packet
   belongs for the purposes of equal cost multipath and trunk load
   spreading (see Section 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 (see Section
   5).




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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 (see Section 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.





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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 scale



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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.




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


7. 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.




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


8. 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