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RFC2340 - Nortels Virtual Network Switching (VNS) Overview

王朝other·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

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

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.

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.

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

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 xLS-Key xDP 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 (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).

- 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 xLS-Key xDP 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.

- 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

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.

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.

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

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.

9. 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 in Section 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.

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.

14. 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.ca

15. 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.

 
 
 
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