PPVPN Terminology: Provider Provisioned Virtual Private Network (VPN) Terminology

Network Working Group                                       L. Andersson
Request for Comments: 4026                                     T. Madsen
Category: Informational                                         Acreo AB
                                                              March 2005


     Provider Provisioned Virtual Private Network (VPN) Terminology

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 (2005).

Abstract

   The widespread interest in provider-provisioned Virtual Private
   Network (VPN) solutions lead to memos proposing different and
   overlapping solutions.  The IETF working groups (first Provider
   Provisioned VPNs and later Layer 2 VPNs and Layer 3 VPNs) have
   discussed these proposals and documented specifications.  This has
   lead to the development of a partially new set of concepts used to
   describe the set of VPN services.

   To a certain extent, more than one term covers the same concept, and
   sometimes the same term covers more than one concept.  This document
   seeks to make the terminology in the area clearer and more intuitive.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  PPVPN Terminology  . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Provider Provisioned Virtual Private Network Services  . . . .  4
       3.1.  Layer 3 VPN (L3VPN)  . . . . . . . . . . . . . . . . . .  4
       3.2.  Layer 2 VPN (L2VPN)  . . . . . . . . . . . . . . . . . .  4
       3.3.  Virtual Private LAN Service (VPLS) . . . . . . . . . . .  4
       3.4.  Virtual Private Wire Service (VPWS)  . . . . . . . . . .  4
       3.5.  IP-Only LAN-Like Service (IPLS)  . . . . . . . . . . . .  5
       3.6.  Pseudo Wire (PW) . . . . . . . . . . . . . . . . . . . .  5
       3.7.  Transparent LAN Service (TLS)  . . . . . . . . . . . . .  5
       3.8.  Virtual LAN (VLAN) . . . . . . . . . . . . . . . . . . .  6
       3.9.  Virtual Leased Line Service (VLLS) . . . . . . . . . . .  6
       3.10. Virtual Private Network (VPN)  . . . . . . . . . . . . .  6
       3.11. Virtual Private Switched Network (VPSN)  . . . . . . . .  6

   4.  Classification of VPNs . . . . . . . . . . . . . . . . . . . .  7
   5.  Building Blocks  . . . . . . . . . . . . . . . . . . . . . . .  8
       5.1.  Customer Edge Device (CE)  . . . . . . . . . . . . . . .  8
             5.1.1.  Device Based CE Naming . . . . . . . . . . . . .  9
             5.1.2.  Service Based CE Naming  . . . . . . . . . . . .  9
       5.2.  Provider Edge (PE) . . . . . . . . . . . . . . . . . . . 10
             5.2.1.  Device Based PE Naming . . . . . . . . . . . . . 10
             5.2.2.  Service Based PE Naming  . . . . . . . . . . . . 10
             5.2.3.  Distribution Based PE Naming . . . . . . . . . . 11
       5.3.  Core . . . . . . . . . . . . . . . . . . . . . . . . . . 11
             5.3.1   Provider Router (P)  . . . . . . . . . . . . . . 11
       5.4.  Naming in Specific Internet Drafts . . . . . . . . . . . 11
             5.4.1.  Layer 2 PE (L2PE)  . . . . . . . . . . . . . . . 11
             5.4.2.  Logical PE (LPE) . . . . . . . . . . . . . . . . 12
             5.4.3.  PE-CLE . . . . . . . . . . . . . . . . . . . . . 12
             5.4.4.  PE-Core  . . . . . . . . . . . . . . . . . . . . 12
             5.4.5.  PE-Edge  . . . . . . . . . . . . . . . . . . . . 12
             5.4.6.  PE-POP . . . . . . . . . . . . . . . . . . . . . 12
             5.4.7.  VPLS Edge (VE) . . . . . . . . . . . . . . . . . 12
   6.  Functions  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
       6.1.  Attachment Circuit (AC)  . . . . . . . . . . . . . . . . 12
       6.2.  Backdoor Links . . . . . . . . . . . . . . . . . . . . . 13
       6.3.  Endpoint Discovery . . . . . . . . . . . . . . . . . . . 13
       6.4.  Flooding . . . . . . . . . . . . . . . . . . . . . . . . 13
       6.5.  MAC Address Learning . . . . . . . . . . . . . . . . . . 13
             6.5.1.  Qualified Learning . . . . . . . . . . . . . . . 13
             6.5.2.  Unqualified Learning . . . . . . . . . . . . . . 13
       6.6.  Signalling . . . . . . . . . . . . . . . . . . . . . . . 13
   7.  'Boxes'  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
       7.1.  Aggregation Box  . . . . . . . . . . . . . . . . . . . . 14
       7.2.  Customer Premises Equipment (CPE)  . . . . . . . . . . . 14
       7.3.  Multi-Tenant Unit (MTU)  . . . . . . . . . . . . . . . . 14
   8.  Packet Switched Network (PSN)  . . . . . . . . . . . . . . . . 14
       8.1.  Route Distinguisher (RD) . . . . . . . . . . . . . . . . 15
       8.2.  Route Reflector  . . . . . . . . . . . . . . . . . . . . 15
       8.3.  Route Target (RT)  . . . . . . . . . . . . . . . . . . . 15
       8.4.  Tunnel . . . . . . . . . . . . . . . . . . . . . . . . . 15
       8.5.  Tunnel Multiplexor . . . . . . . . . . . . . . . . . . . 16
       8.6.  Virtual Channel (VC) . . . . . . . . . . . . . . . . . . 16
       8.7.  VC Label . . . . . . . . . . . . . . . . . . . . . . . . 16
       8.8.  Inner Label  . . . . . . . . . . . . . . . . . . . . . . 16
       8.9.  VPN Routing and Forwarding (VRF) . . . . . . . . . . . . 16
       8.10. VPN Forwarding Instance (VFI)  . . . . . . . . . . . . . 16
       8.11. Virtual Switch Instance (VSI)  . . . . . . . . . . . . . 17
       8.12. Virtual Router (VR)  . . . . . . . . . . . . . . . . . . 17
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   11. Informative References . . . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 20

1.  Introduction

   A comparatively large number of memos have been submitted to the
   former PPVPN working group, and to the L2VPN, L3VPN, and PWE3 working
   groups, which all address the same problem space; provider
   provisioned virtual private networking for end customers.  The memos
   address a wide range of services, but there is also a great deal of
   commonality among the proposed solutions.

   This has led to the development of a partial set of new concepts used
   to describe this set of VPN services.  To a certain extent, more than
   one term covers the same concept, and sometimes the same term covers
   more than one concept.

   This document proposes a foundation for a unified terminology for the
   L2VPN and L3VPN working groups.  In some cases, the parallel concepts
   within the PWE3 working group are used as references.

2.  PPVPN Terminology

   The concepts and terms in this list are gathered from Internet Drafts
   sent to the L2VPN and L3VPN mailing lists (earlier the PPVPN mailing
   list) and RFCs relevant to the L2VPN and L3VPN working groups.  The
   focus is on terminology and concepts that are specific to the PPVPN
   area, but this is not strictly enforced;  e.g., some concepts and
   terms within the PWE3 and (Generalized) MPLS areas are closely
   related.  We've tried to find the earliest uses of terms and
   concepts.

   This document is intended to fully cover the concepts within the core
   documents from the L2VPN and L3VPN working groups; i.e., [L3VPN-REQ],
   [L2VPN-REQ], [L3VPN-FRAME], [L2VPN], and [RFC3809].  The intention is
   to create a comprehensive and unified set of concepts for these
   documents and, by extension, for the entire PPVPN area.  To do so, it
   is also necessary to give some of the development the concepts of the
   area have been through.

   The document is structured in four major sections.  Section 4 lists
   the different services that have been or will be specified Section 5
   lists the building blocks that are used to specify those services
   Section 6 lists the functions needed in those services.  Section 7
   lists some typical devices used in customer and provider networks.

3.  Provider Provisioned Virtual Private Network Services

   In this section, we define the terminology that relates the set of
   services to solutions specified by the L2VPN and L3VPN working
   groups.  The "pseudo wire" concept, which belongs to the PWE3 working
   group, is included for reference purposes.  For requirements in
   provider provisioned VPNs, see [L3VPN-REQ].

   All terms and abbreviations are listed together with a brief
   description of the service.  The list is structured to give the more
   general information first and the more specific later.  The names of
   services for which the IETF is working on solutions have been moved
   to the top of the list.  Older and more dated terminology has been
   pushed toward the end of the list.

3.1.  Layer 3 VPN (L3VPN)

   An L3VPN interconnects sets of hosts and routers based on Layer 3
   addresses; see [L3VPN-FRAME].

3.2.  Layer 2 VPN (L2VPN)

   Three types of L2VPNs are described in this document: Virtual Private
   Wire Service (VPWS) (Section 3.4); Virtual Private LAN Service
   (VPLS)(Section 3.3); and IP-only LAN-like Service
   (IPLS)(Section 3.5).

3.3.  Virtual Private LAN Service (VPLS)

   A VPLS is a provider service that emulates the full functionality of
   a traditional Local Area Network (LAN).  A VPLS makes it possible to
   interconnect several LAN segments over a packet switched network
   (PSN) and makes the remote LAN segments behave as one single LAN.
   For an early work on defining a solution and protocol for a VPLS, see
   [L2VPN-REQ], [VPLS-LDP], and [VPLS].

   In a VPLS, the provider network emulates a learning bridge, and
   forwarding decisions are taken based on MAC addresses or MAC
   addresses and VLAN tag.

3.4.  Virtual Private Wire Service (VPWS)

   A Virtual Private Wire Service (VPWS) is a point-to-point circuit
   (link) connecting two Customer Edge devices.  The link is established
   as a logical through a packet switched network.  The CE in the
   customer network is connected to a PE in the provider network via an
   Attachment Circuit (see Section 6.1); the Attachment Circuit is
   either a physical or a logical circuit.

   The PEs in the core network are connected via a PW.

   The CE devices can be routers, bridges, switches, or hosts.  In some
   implementations, a set of VPWSs is used to create a multi-site L2VPN
   network.  An example of a VPWS solution is described in
   [PPVPN-L2VPN].

   A VPWS differs from a VPLS (Section 3.3) in that the VPLS is point to
   multipoint, while the VPWS is point to point.  See [L2VPN].

3.5.  IP-Only LAN-Like Service (IPLS)

   An IPLS is very like a VPLS (see Section 3.3), except that

   o  it is assumed that the CE devices (see Section 5.1) are hosts or
      routers, not switches,
   o  it is assumed that the service will only have to carry IP packets,
      and supporting packets such as ICMP and ARP (otherwise layer 2
      packets that do not contain IP are not supported); and
   o  the assumption that only IP packets are carried by the service
      applies equally to IPv4 and IPv6 packets.

   While this service is a functional subset of the VPLS service, it is
   considered separately because it may be possible to provide it by
   using different mechanisms, which may allow it to run on certain
   hardware platforms that cannot support the full VPLS functionality
   [L2VPN].

3.6.  Pseudo Wire (PW)

   The PWE3 working group within the IETF specifies the pseudo wire
   technology.  A pseudo wire is an emulated point-to-point connection
   over a packet switched network that allows the interconnection of two
   nodes with any L2 technology.  The PW shares some of the building
   blocks and architecture constructs with the point-to-multipoint
   solutions; e.g., PE (see Section 5.2) and CE (see Section 5.1).  An
   early solution for PWs is described in [TRANS-MPLS].  Encapsulation
   formats readily used in VPWS, VPLS, and PWs are described in
   [ENCAP-MPLS].  Requirements for PWs are found in [RFC3916], and
   [PWE3-ARCH] presents an architectural framework for PWs.

3.7.  Transparent LAN Service (TLS)

   TLS was an early name used to describe the VPLS service.  TLS has
   been replaced by VPLS, which is the current term.

3.8.  Virtual LAN (VLAN)

   The term VLAN was specified by IEEE 802.1Q; it defines a method of
   differentiating traffic on a LAN by tagging the Ethernet frames.  By
   extension, VLAN is used to mean the traffic separated by Ethernet
   frame tagging or similar mechanisms.

3.9.  Virtual Leased Line Service (VLLS)

   The term VLLS has been replaced by term VPWS.  VLLS was used in a now
   dated document intended to create metrics by which it should have
   been possible to compare different L2VPN solutions.  This document
   has now expired, and the work has been terminated.

3.10.  Virtual Private Network (VPN)

   VPN is a generic term that covers the use of public or private
   networks to create groups of users that are separated from other
   network users and that may communicate among them as if they were on
   a private network.  It is possible to enhance the level of separation
   (e.g., by end-to-end encryption), but this is outside the scope of
   IETF VPN working group charters.  This VPN definition is from
   [RFC2764].

   In the [L3VPN-FRAME], the term VPN is used to refer to a specific set
   of sites as either an intranet or an extranet that have been
   configured to allow communication.  Note that a site is a member of
   at least one VPN and may be a member of many.

   In this document, "VPN" is also used as a generic name for all
   services listed in Section 3.

3.11.  Virtual Private Switched Network (VPSN)

   The term VPSN has been replaced by the term VPLS.  The requirements
   have been merged into the L3VPN [L3VPN-REQ] and L2VPN [L2VPN-REQ]
   requirements.

4.  Classification of VPNs

   The terminology used in [RFC3809] is defined based on the figure
   below.


                             PPVPN
               ________________|__________________
              |                                   |
            Layer 2                             Layer 3
        ______|_____                        ______|______
       |            |                      |             |
      P2P          P2M                  PE-based      CE-based
    (VPWS)     _____|____            ______|____         |
              |          |          |           |        |
             VPLS      IPLS     BGP/MPLS     Virtual    IPsec
                                 IP VPNs      Router

                           Figure 1

   The figure above presents a taxonomy of PPVPN technologies.  Some of
   the definitions are given below:


   CE-based VPN: A VPN approach in which the shared service provider
   network does not have any knowledge of the customer VPN.  This
   information is limited to CE equipment.  All the VPN-specific
   procedures are performed in the CE devices, and the PE devices are
   not aware in any way that some of the traffic they are processing is
   VPN traffic (see also [L3VPN-FRAME]).

   PE-Based VPNs: A Layer 3 VPN approach in which a service provider
   network is used to interconnect customer sites using shared
   resources.  Specifically, the PE device maintains VPN state,
   isolating users of one VPN from users of another.  Because the PE
   device maintains all required VPN states, the CE device may behave as
   if it were connected to a private network.  Specifically, the CE in a
   PE-based VPN must not require any changes or additional functionality
   to be connected to a PPVPN instead of a private network.

   The PE devices know that certain traffic is VPN traffic.  They
   forward the traffic (through tunnels) based on the destination IP
   address of the packet, and optionally based on other information in
   the IP header of the packet.  The PE devices are themselves the
   tunnel endpoints.  The tunnels may make use of various encapsulations
   to send traffic over the SP network (such as, but not restricted to,
   GRE, IP-in-IP, IPsec, or MPLS tunnels) [L3VPN-FRAME].

   Virtual Router (VR) style: A PE-based VPN approach in which the PE
   router maintains a complete logical router for each VPN that it
   supports.  Each logical router maintains a unique forwarding table
   and executes a unique instance of the routing protocols.  These VPNs
   are described in [L3VPN-VR].

   BGP/MPLS IP VPNs: A PE-based VPN approach in which the PE router
   maintains a separate forwarding environment and a separate forwarding
   table for each VPN.  In order to maintain multiple forwarding table
   instances while running only a single BGP instance, BGP/MPLS IP VPNs
   mark route advertisements with attributes that identify their VPN
   context.  These VPNs are based on the approach described in
   [RFC2547bis].

   RFC 2547 Style: The term has been used by the L3VPN to describe the
   extensions of the VPNs defined in the informational RFC 2547
   [RFC2547].  This term has now been replaced by the term BGP/MPLS IP
   VPNs.

5.  Building Blocks

   Starting with specifications of L3VPNs (e.g., the 2547 specification
   [RFC2547] and [RFC2547bis] and Virtual Routers [L3VPN-VR]), a way of
   describing the building blocks and allocation of functions in VPN
   solutions was developed.  The building blocks are often used in
   day-to-day talk as if they were physical boxes, common for all
   services.

   However, for different reasons, this is an oversimplification.  Any
   of the building blocks could be implemented across more than one
   physical box.  How common the use of such implementations will be is
   beyond the scope of this document.

5.1.  Customer Edge Device (CE)

   A CE is the name of the device with the functionality needed on the
   customer premises to access the services specified by the former
   PPVPN working group in relation to the work done on L3VPNs
   [L3VPN-FRAME].  The concept has been modified; e.g., when L2VPNs and
   CE-based VPNs were defined.  This is addressed further in the
   sub-sections of this section.

   There are two different aspects that have to be considered in naming
   CE devices.  One could start with the type of device that is used to
   implement the CE (see Section 5.1.1).  It is also possible to use the
   service the CE provides whereby the result will be a set of "prefixed
   CEs", (see Section 5.1.2).

   It is common practice to use "CE" to indicate any of these boxes, as
   it is very often unambiguous in the specific context.

5.1.1.  Device Based CE Naming

5.1.1.1.  Customer Edge Router (CE-R)

   A CE-R is a router in the customer network interfacing the provider
   network.  There are many reasons to use a router in the customer
   network; e.g., in an L3VPN using private IP addressing, this is the
   router that is able to do forwarding based on the private addresses.
   Another reason to require the use of a CE-R on the customer side is
   that one wants to limit the number of MAC-addresses that need to be
   learned in the provider network.

   A CE-R could be used to access both L2 and L3 services.

5.1.1.2.  Customer Edge Switch (CE-S)

   A CE-S is a service aware L2 switch in the customer network
   interfacing the provider network.  In a VPWS or a VPLS, it is not
   strictly necessary to use a router in the customer network; a layer 2
   switch might very well do the job.

5.1.2.  Service Based CE Naming

   The list below contains examples of how different functionality has
   been used to name CEs.  There are many examples of this type of
   naming, and we only cover the most frequently used functional names.
   As these are functional names, it is quite possible that on a single
   piece of equipment there are platforms for more than one type of
   function.  For example, a router might at the same time be both a
   L2VPN-CE and a L3VPN-CE.  It might also be that the functions needed
   for a L2VPN-CE or L3VPN-CE are distributed over more than one
   platform.

5.1.2.1.  L3VPN-CE

   An L3VPN-CE is the device or set of devices on the customer premises
   that attaches to a provider provisioned L3VPN; e.g., a 2547bis
   implementation.

5.1.2.2.  VPLS-CE

   A VPLS-CE is the device or set of devices on the customer premises
   that attaches to a provider provisioned VPLS.

5.1.2.3.  VPWS-CE

   A VPWS-CE is the device or set of devices on the customer premises
   that attaches to a provider provisioned VPWS.

5.2.  Provider Edge (PE)

   A PE is the name of the device or set of devices at the edge of the
   provider network with the functionality that is needed to interface
   with the customer.  Without further qualifications, PE is very often
   used for naming the devices since it is made unambiguous by the
   context.

   In naming PEs there are three aspects that we need to consider, the
   service they support, whether the functionality needed for service is
   distributed across more than one device and the type of device they
   are build on.

5.2.1.  Device Based PE Naming

   Both routers and switches may be used to implement PEs; however, the
   scaling properties will be radically different depending on which
   type of equipment is chosen.

5.2.1.1.  Provider Edge Router (PE-R)

   A PE-R is a L3 device that participates in the PSN (see Section 8)
   routing and forwards packets based on the routing information.

5.2.1.2.  Provider Edge Switch (PE-S)

   A PE-S is a L2 device that participates in for example a switched
   Ethernet taking forwarding decision packets based on L2 address
   information.

5.2.2.  Service Based PE Naming

5.2.2.1.  L3VPN-PE

   An L3VPN-PE is a device or set of devices at the edge of the provider
   network interfacing the customer network, with the functionality
   needed for an L3VPN.

5.2.2.2.  VPWS-PE

   A VPWS-PE is a device or set of devices at the edge of the provider
   network interfacing the customer network, with the functionality
   needed for a VPWS.

5.2.2.3.  VPLS-PE

   A VPLS-PE is a device or set of devices at the edge of the provider
   network interfacing the customer network, with the functionality
   needed for a VPLS.

5.2.3.  Distribution Based PE Naming

   For scaling reasons, in the VPLS/VPWS cases sometimes it is desired
   to distribute the functions in the VPLS/VPWS-PE across more than one
   device.  For example, is it feasible to allocate MAC address learning
   on a comparatively small and inexpensive device close to the customer
   site, while participation in the PSN signalling and setup of PE to PE
   tunnels are done by routers closer to the network core.

   When distributing functionality across devices, a protocol is needed
   to exchange information between the Network facing PE (N-PE) (see
   Section 5.2.3.1) and the User facing PE (U-PE) (see Section 5.2.3.2).

5.2.3.1.  Network Facing PE (N-PE)

   The N-PE is the device to which the signalling and control functions
   are allocated when a VPLS-PE is distributed across more than one box.

5.2.3.2.  User Facing PE (U-PE)

   The U-PE is the device to which the functions needed to take
   forwarding or switching decisions at the ingress of the provider
   network.

5.3.  Core

5.3.1.  Provider Router (P)

   The P is defined as a router in the core network that does not have
   interfaces directly toward a customer.  Therefore, a P router does
   not need to keep VPN state and is VPN unaware.

5.4.  Naming in Specific Internet Drafts

5.4.1.  Layer 2 PE (L2PE)

   L2PE is the joint name of the devices in the provider network that
   implement L2 functions needed for a VPLS or a VPWS.

5.4.2.  Logical PE (LPE)

   The term Logical PE (LPE) originates from a dated Internet Draft,
   "VPLS/LPE L2VPNs: Virtual Private LAN Services using Logical PE
   Architecture", and was used to describe a set of devices used in a
   provider network to implement a VPLS.  In a LPE, VPLS functions are
   distributed across small devices (PE-Edges/U-PE) and devices attached
   to a network core (PE-Core/N-PE).  In an LPE solution, the PE-edge
   and PE-Core can be interconnected by a switched Ethernet transport
   network or uplinks.  The LPE will appear to the core network as a
   single PE.  In this document, the devices that constitutes, the LPE
   are called N-PE and U-PE.

5.4.3.  PE-CLE

   An alternative name for the U-PE suggested in the expired Internet
   Draft, "VPLS architectures".

5.4.4.  PE-Core

   See the origins and use of this concept in Section 5.4.2.

5.4.5.  PE-Edge

   See the origins and use of this concept in Section 5.4.2.

5.4.6.  PE-POP

   An alternative name for the U-PE suggested in the expired Internet
   Draft, "VPLS architectures".

5.4.7.  VPLS Edge (VE)

   The term VE originates from a dated Internet Draft on a distributed
   transparent LAN service and was used to describe the device used by a
   provider network to hand off a VPLS to a customer.  In this document,
   the VE is called a VPLS-PE.  This name is dated.

6.  Functions

   In this section, we have grouped a number of concepts and terms that
   have to be performed to make the VPN services work.

6.1.  Attachment Circuit (AC)

   In a Layer 2 VPN the CE is attached to PE via an Attachment Circuit
   (AC).  The AC may be a physical or logical link.

6.2.  Backdoor Links

   Backdoor Links are links between CE devices that are provided by the
   end customer rather than by the SP; they may be used to interconnect
   CE devices in multiple-homing arrangements [L3VPN-FRAME].

6.3.  Endpoint Discovery

   Endpoint discovery is the process by which the devices that are aware
   of a specific VPN service will find all customer facing ports that
   belong to the same service.

   The requirements on endpoint discovery and signalling are discussed
   in [L3VPN-REQ].  It was also the topic in a now dated Internet Draft
   reporting from a design team activity on VPN discovery.

6.4.  Flooding

   Flooding is a function related to L2 services; when a PE receives a
   frame with an unknown destination MAC address, that frame is send out
   over (flooded) every other interface.

6.5.  MAC Address Learning

   MAC address learning is a function related to L2 services; when PE
   receives a frame with an unknown source MAC address, the relationship
   between that MAC-address and interface is learned for future
   forwarding purposes.  In a layer 2 VPN solution from the L2VPN WG,
   this function is allocated to the VPLS-PE.

6.5.1.  Qualified Learning

   In qualified learning, the learning decisions at the U-PE are based
   on the customer Ethernet frame's MAC address and VLAN tag, if a VLAN
   tag exists.  If no VLAN tag exists, the default VLAN is assumed.

6.5.2.  Unqualified Learning

   In unqualified learning, learning is based on a customer Ethernet
   frame's MAC address only.

6.6.  Signalling

   Signalling is the process by which the PEs that have VPNs behind them
   exchange information to set up PWs, PSN tunnels, and tunnel
   multiplexers.  This process might be automated through a protocol or
   done by manual configuration.  Different protocols may be used to
   establish the PSN tunnels and exchange the tunnel multiplexers.

7.  'Boxes'

   We list a set of boxes that will typically be used in an environment
   that supports different kinds of VPN services.  We have chosen to
   include some names of boxes that originate outside the protocol
   specifying organisations.

7.1.  Aggregation Box

   The aggregation box is typically an L2 switch that is service unaware
   and is used only to aggregate traffic to more function rich points in
   the network.

7.2.  Customer Premises Equipment (CPE)

   The CPE equipment is the box that a provider places with the
   customer.  It serves two purposes: giving the customer ports to plug
   in to and making it possible for a provider to monitor the
   connectivity to the customer site.  The CPE is typically a low cost
   box with limited functionality and, in most cases, is not aware of
   the VPN services offered by the provider network.  The CPE equipment
   is not necessarily the equipment to which the CE functions are
   allocated, but it is part of the provider network and is used for
   monitoring purposes.

   The CPE name is used primarily in network operation and deployment
   contexts and should not be used in protocol specifications.

7.3.  Multi-Tenant Unit (MTU)

   An MTU is typically an L2 switch placed by a service provider in a
   building where several customers of that service provider are
   located.  The term was introduced in an Internet Draft specifying a
   VPLS solution with function distributed between the MTU and the PE in
   the context of a [VPLS].

   The MTU device name is used primarily in network operation and
   deployment contexts and should not be used in protocol
   specifications, as it is also an abbreviation used for Maximum
   Transmit Units.

8.  Packet Switched Network (PSN)

   A PSN is the network through which the tunnels supporting the VPN
   services are set up.

8.1.  Route Distinguisher (RD)

   A Route Distinguisher [RFC2547bis] is an 8-byte value that, together
   with a 4 byte IPv4 address, identifies a VPN-IPv4 address family.  If
   two VPNs use the same IPv4 address prefix, the PEs translate these
   into unique VPN-IPv4 address prefixes.  This ensures that if the same
   address is used in two different VPNs, it is possible to install two
   completely different routes to that address, one for each VPN.

8.2.  Route Reflector

   A route reflector is a network element owned by a Service Provider
   (SP) that is used to distribute BGP routes to the SP's BGP-enabled
   routers [L3VPN-FRAME].

8.3.  Route Target (RT)

   A Route Target attribute [RFC2547bis] can be thought of as
   identifying a set of sites or, more precisely, a set of VRFs (see
   Section 8.9).

   Associating a particular Route Target with a route allows that route
   to be placed in all VRFs used for routing traffic received from the
   corresponding sites.

   A Route Target attribute is also a BGP extended community used in
   [RFC2547] and [BGP-VPN].  A Route Target community is used to
   constrain VPN information distribution to the set of VRFs.  A route
   target can be perceived as identifying a set of sites or, more
   precisely, a set of VRFs.

8.4.  Tunnel

   A tunnel is connectivity through a PSN that is used to send traffic
   across the network from one PE to another.  The tunnel provides a
   means to transport packets from one PE to another.  Separation of one
   customer's traffic from another customer's traffic is done based on
   tunnel multiplexers (see Section 8.5).  How the tunnel is established
   depends on the tunnelling mechanisms provided by the PSN; e.g., the
   tunnel could be based on the IP-header, an MPLS label, the L2TP
   Session ID, or the GRE Key field.

8.5.  Tunnel Multiplexor

   A tunnel multiplexor is an entity that is sent with the packets
   traversing the tunnel to make it possible to decide which instance of
   a service a packet belongs to and from which sender it was received.
   In [PPVPN-L2VPN], the tunnel multiplexor is formatted as an MPLS
   label.

8.6.  Virtual Channel (VC)

   A VC is transported within a tunnel and identified by its tunnel
   multiplexer.  A virtual channel is identified by a VCI (Virtual
   Channel Identifier).  In the PPVPN context, a VCI is a VC label or
   tunnel multiplexer, and in the Martini case, it is equal to the VCID.

8.7.  VC Label

   In an MPLS-enabled IP network, a VC label is an MPLS label used to
   identify traffic within a tunnel that belongs to a particular VPN;
   i.e., the VC label is the tunnel multiplexer in networks that use
   MPLS labels.

8.8.  Inner Label

   "Inner label" is another name for VC label (see Section 8.6).

8.9.  VPN Routing and Forwarding (VRF)

   In networks running 2547 VPN's [RFC2547], PE routers maintain VRFs.
   A VRF is a per-site forwarding table.  Every site to which the PE
   router is attached is associated with one of these tables.  A
   particular packet's IP destination address is looked up in a
   particular VRF only if that packet has arrived directly from a site
   that is associated with that table.

8.10.  VPN Forwarding Instance (VFI)

   VPN Forwarding Instance (VFI) is a logical entity that resides in a
   PE that includes the router information base and forwarding
   information base for a VPN instance [L3VPN-FRAME].

8.11.  Virtual Switch Instance (VSI)

   In a layer 2 context, a VSI is a virtual switching instance that
   serves one single VPLS [L2VPN].  A VSI performs standard LAN (i.e.,
   Ethernet) bridging functions.  Forwarding done by a VSI is based on
   MAC addresses and VLAN tags, and possibly on other relevant
   information on a per VPLS basis.  The VSI is allocated to VPLS-PE or,
   in the distributed case, to the U-PE.

8.12.  Virtual Router (VR)

   A Virtual Router (VR) is software and hardware based emulation of a
   physical router.  Virtual routers have independent IP routing and
   forwarding tables, and they are isolated from each other; see
   [L3VPN-VR].

9.  Security Considerations

   This is a terminology document and as such doesn't have direct
   security implications.  Security considerations will be specific to
   solutions, frameworks, and specification documents whose terminology
   is collected and discussed in this document.

10.  Acknowledgements

   Much of the content in this document is based on discussion in the
   PPVPN design teams for "auto discovery" and "l2vpn".

   Dave McDysan, Adrian Farrel, and Thomas Narten have carefully
   reviewed the document and given many useful suggestions.

   Thomas Narten converted an almost final version of this document into
   XML, after extracting an acceptable version from Word became too
   painful.  Avri Doria has been very helpful in guiding us in the use
   of XML.

11.  Informative References

   [L2VPN]       Andersson, L. and E. Rosen, "Framework for Layer 2
                 Virtual Private Networks (L2VPNs)", Work in Progress,
                 June 2004.

   [L2VPN-REQ]   Augustyn, W. and Y. Serbest, "Service Requirements for
                 Layer 2 Provider Provisioned Virtual Private
                 Networks", Work in Progress, October 2004.

   [VPLS]        Kompella, K., "Virtual Private LAN Service", Work in
                 Progress, January 2005.

   [VPLS-LDP]    Lasserre, M. and V. Kompella, "Virtual Private LAN
                 Services over MPLS", Work in Progress, September 2004.

   [BGP-VPN]     Ould-Brahim, H., Rosen, E., and Y. Rekhter, "Using BGP
                 as an Auto-Discovery Mechanism for Layer-3 and Layer-2
                 VPNs", Work in Progress, May 2004.

   [L3VPN-FRAME] Callon, R. and M. Suzuki, "A Framework for Layer 3
                 Provider Provisioned Virtual Private Networks", Work in
                 Progress, July 2003.

   [RFC3809]     Nagarajan, A., "Generic Requirements for Provider
                 Provisioned Virtual Private Networks (PPVPN)", RFC
                 3809, June 2004.

   [L3VPN-REQ]   Carugi, M. and D. McDysan, "Service requirements for
                 Layer 3 Virtual Private Networks", Work in Progress,
                 July 2004.

   [RFC2547bis]  Rosen, E., "BGP/MPLS IP VPNs", Work in Progress,
                 October 2004.

   [L3VPN-VR]    Knight, P., Ould-Brahim, H. and B. Gleeson, "Network
                 based IP VPN Architecture using Virtual Routers", Work
                 in Progress, April 2004.

   [PWE3-ARCH]   Bryant, S. and P. Pate, "PWE3 Architecture", Work in
                 Progress, March 2004.

   [RFC3916]     Xiao, X., McPherson, D., and P. Pate, "Requirements for
                 Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
                 September 2004.

   [PPVPN-L2VPN] Kompella, K., "Layer 2 VPNs Over Tunnels", Work in
                 Progress, June 2002.

   [ENCAP-MPLS]  Martini, L., "Encapsulation Methods for Transport of
                 Layer 2 Frames Over IP and MPLS  Networks", Work in
                 Progress, September 2004.

   [TRANS-MPLS]  Martini, L. and N. El-Aawar, "Transport of Layer 2
                 Frames Over MPLS", Work in Progress, June 2004.

   [RFC2547]     Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547,
                 March 1999.

   [RFC2764]     Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and
                 A. Malis, "A Framework for IP Based Virtual Private
                 Networks", RFC 2764, February 2000.

Authors' Addresses

   Loa Anderson
   Acreo AB

   EMail: loa@pi.se


   Tove Madsen
   Acreo AB

   EMail: tove.madsen@acreo.se

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