Internet Draft Document Marc Lasserre
L2VPN Working Group Xipeng Xiao
Riverstone Networks
Yetik Serbest Marc Rapoport
SBC Completel
Cesar Garrido
Telefonica
draft-ietf-l2vpn-vpls-ldp-applic-00.txt
Expires: October 2004 March 2005
VPLS Applicability
Status of this Memo
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Abstract
Virtual Private LAN Service (VPLS) is a layer 2 VPN service that
provides multipoint connectivity in the form of an Ethernet emulated
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LAN, while usual L2 VPN services are typically point-to-point. Such
emulated LANs can span across metropolitan area networks as well as
wide area networks.
[VPLS-LDP] defines a method for signaling MPLS connections between
member PEs of a VPN and a method for forwarding Ethernet frames over
such connections. This document describes the applicability of such
procedures to provide VPLS services.
This document also compares the characteristics of this solution
against the requirements specified in [L2VPN-REQ]. In summary, there
are no architectural limitations to prevent the requirements from
being met. But meeting certain requirements (e.g. QoS) is beyond
the specification of [VPLS-LDP], and requires careful planning and
precise implementation of the Service Provider (SP) networks. This
document attempts to capture such issues, presents the potential
solutions to these issues, and discusses the pros and cons of each
alternative.
This document does not cover the applicability of [VPLS-BGP].
Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119
Placement of this Memo in Sub-IP Area
RELATED DOCUMENTS
www.ietf.org/internet-drafts/draft-ietf-l2vpn-vpls-ldp-04.txt
www.ietf.org/internet-drafts/draft-ietf-l3vpn-applicability-
guidelines-00.txt
Table of Contents
1. Operation of Signaling and Data Planes..........................4
1.1. Signaling Plane...............................................4
1.2. Data Plane....................................................4
1.2.1. Ingress Processing..........................................4
1.2.2. Egress Processing...........................................4
1.2.3. Intermediate Node Processing................................5
2. VPLS vs. Alternative Approaches.................................5
2.1. Ethernet Switching............................................5
2.2. BGP/MPLS IP VPN...............................................5
3. Provisioning....................................................6
3.1. PE Auto-Discovery.............................................6
3.2. Operations and Maintenance....................................7
4. Migration Impacts...............................................7
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4.1. Interconnecting L2 Ethernet Islands with a VPLS Core..........7
4.2. Migrating an Existing L2 Ethernet Core to a VPLS Core.........8
4.3. Interconnecting VPLS networks with ATM/FR networks............9
4.4. Adding VPLS Support to an IP Routed Network...................9
5. Multi-homing....................................................9
6. Loop Prevention................................................11
7. Packet Ordering................................................12
8. Multi-Domain VPLS Service......................................12
9. Maximum Transmission Unit (MTU) Issues.........................12
10. Interoperability and Interworking.............................13
10.1. Interworking with BGP/MPLS IP VPN...........................13
10.2. Interworking With Frame Relay/ATM Attachment Circuits.......13
11. Quality of Service............................................13
12. Security......................................................14
12.1. Customer Access Control and Authentication..................14
12.2. Traffic Separation between VPLS Instances...................14
12.3. Protection of SP Networks...................................14
12.4. Protection of User Data.....................................15
13. Scalability...................................................15
13.1. Mesh topology...............................................16
13.2. Signaling...................................................16
13.3. MAC addresses and MAC learning..............................16
13.4. Packet replication..........................................16
13.5. Broadcast limiting..........................................16
13.6. Multicast...................................................17
14. Management....................................................17
VPLS Overview
The primary motivation behind Virtual Private LAN Services (VPLS) is
to provide connectivity between geographically dispersed customer
sites across MAN/WAN network(s), as if they were connected using a
LAN. The intended applications for the end-user can be divided into
the following two categories:
- Connectivity between customer routers
- Connectivity between customer Ethernet switches
In addition, VPLS can also be used by the service providers to
deliver voice, video , and data services (i.e., triple play,
aggregation of IP services over Ethernet access) to connected end-
users.
Unlike L3 VPNs such as BGP/MPLS IP VPNs [2547bis] where traffic
exchanged between customers and service providers must be IP, VPLS
only requires traffic to be Ethernet over which any protocol can be
transported, e.g. Netbios or IPX.
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The Service Provider Network is a packet switched network (PSN).
The PEs are assumed to be fully meshed (note that the mesh can be
broken with HVPLS) with transport tunnels over which customer frames
that belong to a specific VPLS instance are encapsulated and
forwarded. IP-in-IP, L2TPv3, GRE, and MPLS are examples of transport
tunnels.
Specific labels used to identify end-to-end paths over such
transport tunnels, and these end-to-end paths, which are known as
pseudo-wires (PW), are established via targeted LDP [VPLS-LDP].
VPLS defines the bridging rules required for PEs to provide an
emulated Ethernet LAN service. In particular, it defines how a loop-
free topology must be built, the forwarding rules between PEs, and
the signaling method to set up PWs between PEs. The resulting
service provides a unique broadcast domain per VPN, with the ability
to send unicast, multicast and broadcast traffic (as well as
flooding of unknown unicast traffic).
1.
Operation of Signaling and Data Planes
1.1.
Signaling Plane
As with [PWE3-ETHERNET], [VPLS-LDP] specifies the use of targeted
LDP for the signaling of PWs. PWs are established between PEs that
are part of the same VPLS instance.
1.2.
Data Plane
1.2.1.
Ingress Processing
VPLS provides an Ethernet emulated LAN service and hence customer
frames are capsulated as Ethernet frames (Ethernet DIX or 802.1).
Note that such Ethernet frames can be carried over various access
transport technologies (Frame Relay, ATM, etc). Ingress PEs will
determine which Forwarding Information Base (FIB) to look up based
on the port, VLAN or port/VLAN combination where frames come from.
This port to FIB mapping is performed at provisioning time. The
destination MAC address is then looked up to determine on which PW
this address has been learned from. If the lookup fails, i.e. if
this MAC address has not been learned yet, the frame needs to be
sent on all the PWs that are part of the corresponding VPLS
instance. If the address is known, the frame is sent only over the
associated PW. Before actually transmitting the customer frame, it
needs to be encapsulated as defined in [PWE3-ETHERNET], and is
further encapsulated with the appropriate transport header (e.g.
MPLS or GRE).
1.2.2.
Egress Processing
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Once the tunnel header has been removed, the egress PE determines
from the PW label which FIB to look up to determine the egress
interface, i.e., VLAN or port/VLAN combination. The original
Ethernet frame is then encapsulated with the proper transmission
header if necessary (e.g. Frame Relay header) and sent over the
corresponding port.
MAC addresses are learned dynamically as traffic is exchanged. New
source MAC addresses are learned on a per PW label per VPLS instance
basis. An aging timer is used to remove such bindings after a period
of time. When user topology changes occur, MAC withdrawal messages
in the signaling plane may be used to unlearn MAC addresses to
improve convergence time.
Egress PEs might also be configured to perform specific egress
encapsulation functions (e.g. VLAN translation).
1.2.3.
Intermediate Node Processing
Intermediate nodes (P routers) only act as pure forwarders based on
the outer tunnel header. Hence, they do not participate in any VPLS
related processing. Only PE routers maintain VPN specific
information. This improves the scalability of VPLS service.
2.
VPLS vs. Alternative Approaches
2.1.
Ethernet Switching
Ethernet can be used to provide multipoint connectivity within small
geographical areas such as small metropolitan networks. Pure
Ethernet based solutions have scalability issues (e.g. STP
limitations, 4095 VLAN limitations). Some enhancements such as QinQ,
STP extensions (RSTP, MSTP) provide additional scalability.
VPLS overcomes several limitations of Ethernet based solutions by
supporting large numbers of VPNs, better traffic engineering,
transport link layer independence and better quality of service.
It is not uncommon for VPLS networks to be complemented with
Ethernet switched networks as an aggregation layer.
2.2.
BGP/MPLS IP VPN
In metropolitan area networks (MANs), BGP is usually not enabled.
MANs provide a transport service to end-users. When multiple sites
need to be connected within a metro, VPLS offers the appropriate
multipoint transport solution. It is expected that a VPLS instance
supports up to O(10^2) site interfaces. When multipoint connectivity
is required for a higher number of interfaces sites, with a various
range of interface types (e.g. dial-up access, IPSec Tunnels),
BGP/MPLS IP VPNs can be more appropriate.
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Section 10.1. describes how VPLS and BGP/MPLS IP VPNs can be
complementary.
The following sections compare the characteristics of LDP-based VPLS
solution against the requirements specified in [L2VPN-REQ]. Key
deployment issues that require careful planning and precise
implementation of SP networks are highlighted.
3.
Provisioning
To provision a VPLS service for a customer, the first step is to
create a Virtual Switching Instance (VSI), and assign the customer
attachment circuit (AC) (e.g. port, port/VLAN, ATM VC with 1483b
encapsulation, etc.) and PWs (including H-VPLS spokes) to it. The
PWs interconnect VSIs at different PEs and MTUs together to form an
emulated LAN for the customer.
One challenge in doing this is, when a VPLS site needs to be added
or removed at a PE, in addition to configuring that particular PE,
the network operator needs to find out which other PEs participate
in that VPLS instance, and re-configure those PEs. PE auto-
discovery can automate this process. The pros and cons of several
auto-discovery approaches are discussed in 3.1. .
3.1.
PE Auto-Discovery
Currently there are several proposals for PE auto-discovery: the
BGP-based approach [VPLS-BGP], the RADIUS-based approach [RADIUS-
DIS], and the Provisioning System-based approach.
The BGP and RADIUS-based approaches mandate the use of BGP or
RADIUS in every PE, and rely on it to propagate the information of
which PEs participate in a VPLS instance (Signaling can
automatically happen after the other PEs belonging to the same VPLS
instance are discovered). The pros of both approaches are reduced
provisioning work and no need for a provisioning system. The con is
BGP/RADIUS has to be in every PE, which may not be the case in
reality.
With the Provisioning System-based approach, network operators do
not configure the PEs. Instead, they specify which PEs participate
in which VPLS instances at the Provisioning System. The
Provisioning System then translates such service information into PE
configuration commands and telnet/ssh to the PEs to execute such
commands. Because all information related to every VPLS instance is
centralized at the Provisioning System, PE auto-discovery is
automatically achieved. To add or remove a PE for a VPLS instance, a
network operator simply specifies it at the Provisioning System
which will then configure the PEs accordingly.
For VPLS deployments that span across multiple domains, because the
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ASBRs (autonomous system border routers) of other domains can be
treated as CEs of the current domain, these auto-discovery
approaches can all work in the multi-domain case. However, the
built-in scalability mechanism in BGP makes the BGP-based auto-
discovery more scalable in this scenario [VPLS-BGP].
3.2.
Operations and Maintenance
To meet the service level agreement (SLA) with their customers, SPs
also need to provision the following:
- Traffic management throughout the network and on customer
facing ports in particular
- Traffic Engineering
- Traffic protection (e.g. Fast reroute)
- Service management (e.g. SLA measurement, OAM, accounting,
billing, etc)
Manual provisioning for these tasks can be tedious. A provisioning
system is highly desirable. If a provisioning system is used, PE
auto-discovery may be integrated into it.
4.
Migration Impacts
Migration in this document means replacing, or more often,
supplementing, an existing metro Ethernet or ATM/Frame Relay network
with a VPLS network. There are four likely scenarios:
Interconnecting existing L2 Ethernet islands with a VPLS core
Migrating an existing L2 Ethernet core to a VPLS core;
Interconnecting a new VPLS network with existing ATM/FR networks
Adding VPLS support to an IP routed network
Migration impacts may be mitigated through the use of careful
planning when building the network. Also, consideration must be
taken when integrating with protocols such as STP/MSTP and how
control packets (BPDUs) are handled. In addition, one must also
consider ongoing standards efforts within various standards bodies
such as the IEEE [802.1ad] and the Metro Ethernet Forum to assess
future impact of any changes within the provider network.
4.1.
Interconnecting L2 Ethernet Islands with a VPLS Core
Today, many existing metro Ethernet networks are relatively small
and cover only specific districts in a metro area. Such networks may
simply backhaul traffic to a routing backbone and not interconnected
at L2. When metro Ethernet service grows and these networks need to
be interconnected at L2, one approach that may be used for a
migration strategy is to effectively utilize existing L2 (possibly
802.1Q based or QinQ) networks as "islands" attached to an MPLS
based VPLS core network. In this particular case, the L2 network
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uses predetermined Provider 802.1Q tags (P-tags) to transport a
given customers traffic. This P-tag is then utilized as a service
delimiter that is then stripped prior to being transported across
the MPLS cloud. The service delimiting P-tag is used to identify
the VPLS instance to which the traffic should be mapped.
----CE1
------- ------- / --------
CE2- / \ / PE1 / \
\ / \ / \ / \
---| QinQ \ / MPLS/ \ / QinQ |
| Domain PE VPLS PE Domain |
\ / \ Domain / \ /\
\ / \ / \ / \
------- ---------- -------- --CE3
In this scenario, when different sites of a customer have a mismatch
of 802.1Q tags, VLAN translation as defined in [802.1ad] should be
applied.
-----
/ A1 \
---- ----CE1 |
/ \ ------- ------- / | |
| A2 CE2- / \ / PE1 \ /
\ / \ / \ / \ -----
---- ---| QinQ \ / MPLS/ |
| Domain PE2 VPLS |
\ / \ Domain /
----- \ / \ /
|QinQ|_/ ------- -------
-| |
---- / ------ ----
/ \/ \ / \ CE = Customer Edge Router
| A3 CE3 --C4 A4 | PE = Provider Edge Router
\ / \ /
---- ----
4.2.
Migrating an Existing L2 Ethernet Core to a VPLS Core
CE1
------------------- ------ /
/ \ -|VPLS| /
/ \ / | PE |-
/ \ ------
/ \
| 802.1Q/QinQ |
\ /
----- \ /\ ------
|VPLS|_/ \ / \ |VPLS|
-| PE | \ / -| PE |-
/ ------ ------------------- ------ \
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/ \ \
CE3 --CE4 CE2
Providers that have already deployed VLAN based core may choose to
build a parallel VPLS core and connect it to the existing 802.1Q/Q-
in-Q core. The 802.1Q/Q-in-Q core is effectively treated as a
super-island. Then one by one, each individual Ethernet access
island is disconnected from the existing core (i.e. super-island)
and connected to the VPLS core. The migration issues then become
similar to those described in 4.1. and interoperability aspects
between .1ad network and MPLS/VPLS network need to be worked out
before such migration
A second approach consists in configuring a second VPLS control
plane in the existing QinQ PE, hence implementing two virtual
networks over a single physical infrastructure. Once the PE/MPLS
control plane is running, each customer can be separately migrated
through a reconfiguration of its corresponding access ports. For
increased stability, the dual control plane approach might require
dedicating some links or PEs to the MPLS/VPLS network.
4.3.
Interconnecting VPLS networks with ATM/FR networks
If interworking at L2 is needed, the existing ATM/FR networks would
need to carry bridge-encapsulated traffic. VPLS can support ATM and
Frame Relay (FR) attachment circuits with Ethernet bridge
encapsulation. Once the FR/ATM encapsulation has been stripped off,
the resulting Ethernet frames can be processed as if they came from
an Ethernet link. Therefore, interworking can be naturally achieved.
If the existing ATM/FR networks do not carry bridge-encapsulated
traffic, then interworking can only happen at L3. For example, if
both VPLS and ATM/FR carry IP traffic, then an IP router can be used
to interconnect the two networks.
4.4.
Adding VPLS Support to an IP Routed Network
In such a scenario, if existing PEs can support VPLS, then they can
continue to serve as PEs. Otherwise, new VPLS PEs need to be added
and existing IP routers will serve as Ps or as Layer3-only PEs.
Depending on whether the existing IP routers support MPLS or not,
MPLS or some other tunneling mechanism such as GRE can be used.
5.
Multi-homing
Multi-homing is necessary in order to remove a VPLS PE as a single
point of failure for all devices attached to it. There are two
instances of multi-homing that apply to VPLS:
- When a CE device is connected to more than one PE,
- In the case of hierarchical VPLS - when an MTU-s device is
connected to more than one PE-rs.
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In both of these cases, the concern is that a particular MAC address
will appear as a source on more than one PE device, causing other PE
devices to continuously change their FIBs with regard to the true
location of the MAC. This will cause constant table thrashing on
the remote PEs, a behavior akin to a Layer 2 switch which
participates in a loop.
It is therefore required that any Layer 2 loops, created by multi-
homing of a CE or an MTU-s, be resolved within the group of devices
participating in that loop. This group includes the multi-homed CE
or MTU-s, and all PEs to which it is attached. The PEs involved in
such a loop are connected with a full mesh of PWs per VPLS instance.
There are two approaches to resolving the loops created by the
multi-homed devices:
- Running an MSTP instance between all devices in the group. In
this case, the PEs within the group will need to utilize a P-
VLAN for the purposes of running MSTP in the group. This P-
VLAN can be re-used on non-overlapping groups of multi-homed CE
(or MTU-s) and its PEs. It must be clear that the MSTP
process discussed here is a completely different and
independent instance of STP than any STP the customer may be
running. Such customer STP is always tunneled through the VPLS
network, and is never acted upon by the PE or MTU-s devices.
- The MTU-s or the CE can designate its link to one of the PEs it
connects to as primary and only send packets for this
particular VPLS instance over that link. In this case the MTU-
s (CE) is responsible for monitoring the state of that link and
for switching to an alternate link if the primary fails. No
action is required from the PEs participating in the group,
though there should be an indication given from the MTU-s to
its connected PEs as to whether the PE is connected to the
primary or backup link. This is a very lightweight approach,
which is quite useful given the simple and known topology
between the CE (MTU-s) and its PEs. With this approach the
operator must ensure that PWs in the core remain up, as long as
the ingress PE they start from is up. This can typically be
ensured with MPLS TE tools, such as fast re-route or back-up
LSPs. If there is no available path between the ingress PE and
the Egress PEs, a mechanism that monitors the status of the PWs
to force the access connection to go down when the PWs are down
might be useful. If PWs in the core go down while their ingress
PE is up and accepting customer traffic, black-holes can occur.
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In each case, the PE nodes are most likely in two different physical
locations in the provider network providing network element
protection, last mile protection, fiber diversity and provider
facility backup. Customer STP traffic is always tunneled through the
provider network, and is never acted upon by the PE or MTU-s
devices.
Lastly, it should be observed that, since VPLS services provide
Ethernet switch-like transport level services, the customer is free
to connect any device they desire as a CE. This could be anything
from a simple host, hub, L2 switch, or a router. The operator has
to be cognizant of the different capabilities of each of those
devices to ensure loop-free environment when multi-homed.
6.
Loop Prevention
Loops in the core VPLS network are prevented by creating a full mesh
of transport circuits between PEs and by applying a split-horizon
rule. The split-horizon approach prevents a frame received from the
backbone network from being sent out anything other than the
customer facing ports belonging to that VPLS instance on the
receiving PE. The frame MUST not be forwarded out other PW
connecting the receiving PE to other PEs participating in the VPLS
instance. This provides the necessary protection, network bandwidth
optimization and scalability in the carriers' network as it does not
rely on link blocking technologies, like spanning tree type
protocols. This forwarding mechanism allows PEs to effectively
protect the core network from data loops.
Customer networks need to be able to transparently transport the
protocol information that allows their network to properly converge.
However, the provider should consider loop protection schemes
between the CE and PE that do not affect the customer functions.
This would be in addition to spanning tree when the PE connects to a
VLAN based L2 metro or when the customer is directly connected to
multiple PE nodes.
The provider should look at deploying a loop protection scheme that
would intervene automatically when it detects a loop condition on
customer access ports. This loop protection scheme serves as an
additional line of defense against protocol failures or
misconfigurations, which can result in data loops. The concern is
that a particular MAC address will appear as a source on more than
one PE device, causing other PE devices to continuously update their
tables. An external loop protection scheme adds a level of insurance
above the customer link protection schemes. Its function is to
reduce unnecessary core bandwidth usage when a loop condition occurs
in an adjacent network and provide an extra level of protection to
multi-homed networks. It is a complement but not a replacement for
traditional loop protection mechanisms, like spanning tree. Such a
loop protection scheme could be based on the monitoring of the
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number of Mac addresses moving from one attachment circuit/PW to
another circuit/PW.
With directly connected customers, careful consideration needs to be
given to backdoor connections. Backdoor connections provide an
alternate path around a single provider. If a loop detection scheme
is invoked here the customer may be forced to traverse a link that
is not desired.
7.
Packet Ordering
Normally there is only one transmission path towards a destination
with VPLS. So there is no packet re-ordering issue. But if some
load balancing mechanism is enabled or if LSPs carrying VPLS traffic
are rerouted, packets may be re-ordered inside the PSN. Note that
reordering can be avoided when load balancing flows across PWs.
Flows can be identified through a number of identifiers in the
packet, including MPLS labels, MAC addresses, IP addresses, and
UDP/TCP ports.
VPLS data packets use the encapsulation mechanism defined in [PWE3-
ETHERNET]. An optional control word which contains a sequence number
field can be used to assist in-order delivery. If the user's
applications are sensitive to packet re-ordering, this option may be
used. However, enabling sequencing usually causes forwarding
performance degradation. Another alternative is to avoid load
sharing for traffic inside a LSP and pin down LSPs to avoid
rerouting.
8.
Multi-Domain VPLS Service
As the use of VPLS grows, it is expected that customers will require
a single VPLS service delivered by different providers (e.g. either
for redundancy or because none of the SPs has the presence to
support all the sites of a customer). Different providers would then
need to interconnect their VPLS domains for these customers. [VPLS-
LDP] has provision for such a requirement, utilizing a full mesh of
LSPs among the VPLS gateways of these domains. However, experience
of such interconnection is not yet available.
9.
Maximum Transmission Unit (MTU) Issues
Because of the encapsulation and transport headers, the MTU for user
applications will be smaller than the smallest MTU of all the
physical links. In responding to path MTU discovery message, each
network device must deduct the total header size from a physical
link's MTU. Since path MTU discovery is not always used, SPs must
clearly communicate the potential MTU issue to their customers and
ask for their cooperation. In reality, most applications will work
fine but a small number of them may be affected. This is by no
means specific to VPLS. Any networks that put additional header(s)
on customer's packets will have the same issue.
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10.
Interoperability and Interworking
Interoperability should be ensured by proper implementation of the
published standards.
10.1.
Interworking with BGP/MPLS IP VPN
When interworking VPLS with BGP/MPLS IP VPN, a BGP/MPLS IP VPN (in
the backbone) is typically used to interconnect VPLS domains in
multiple metros, with such VPLS domains acting as Ethernet
aggregation networks for the IP service. In this type of scenario,
the BGP/MPLS IP VPN will carry inter-metro traffic whereas VPLS will
handle intra-metro traffic.
A useful method for interconnecting a VPLS with a BGP/MPLS IP VPN is
to use a "link" to interconnect the VSI and the VRF. Such a "link"
can be a physical port, a VLAN spanning across one or multiple
physical hops, or 2 LSPs with one in each direction, etc.
Analogously, this is like interconnecting a L2 switch with a router,
with the VSI as the switch and the VRF as the router.
Access/transport networks such as VPLS can also be interconnected
with BGP/MPLS IP VPNs using various mechanisms such as Carrier's
Carrier as defined in [RFC-2547].
10.2.
Interworking With Frame Relay/ATM Attachment Circuits
Frame Relay (FR) and ATM attachment circuits with Ethernet bridged
encapsulation can be terminated within VPLS PEs. The resulting
Ethernet frames (i.e. once the FR/ATM encapsulation has been
stripped off) are processed as standard Ethernet frames.
In order to support a complete interworking model between FR and
Ethernet or between ATM and Ethernet, mapping service profiles and
OAM traffic from one to the other are necessary. Additionally,
circuit management (e.g. LMI to PW state mapping) between the
various technologies are required. Such standards are being defined
by other standard organizations such as the MPLS-FR-ATM Alliance.
11.
Quality of Service
The provision of appropriate QoS capabilities may require any
combination of the following:
- QoS in the access network.
- Admission control by the PE router on the ingress access links.
- Classification by the PE, for traffic arriving from the CE.
Once the PE classifies a user packet, this classification needs
to be preserved in the encapsulation (MPLS EXP or IP DSCP) used
to send the packet across the backbone.
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- Traffic conditioning (policing or shaping) by the PE router on
the ingress access links.
- DSCP/EXP-based queuing and WRED in the VPLS network
- Traffic engineering in the VPLS network.
- Fast reroute in the VPLS network
None of these features are VPLS specific. The ability to support
them depends on whether the features are available on the edge and
core devices. It is up to the SPs to decide how to use such
mechanisms to provide QoS. Such mechanisms can be used to support
either the "hose model" or the "pipe model", although the hose model
is a more natural fit and is usually the support model by default.
12.
Security
12.1.
Customer Access Control and Authentication
Control of the customer access can be achieved by controlling
physical access to the CEs, the PEs and the links between them. If
multiple customers use 802.1Q service delimiting tags in the same
trunk link to access VPLS service, and the tags are put on by the
customers themselves, ACLs should be used to ensure that each
customer only puts on the tag that it is supposed to put on. Packets
with other tag(s) must be dropped.
802.1x may be used for CE device authentication.
12.2.
Traffic Separation between VPLS Instances
VPLS instances maintain separation of broadcast domains between
themselves. Traffic entering a given VPLS instance at a given PE
device does not, under any circumstances, cross the boundaries of
the VPLS into another instance. VPLS devices (PEs and MTU-s) ensure
that by maintaining a FIB table and a full mesh of PWs on a per-VPLS
instance basis.
The above statement is correct regardless of the learning mode
employed by a particular VPLS instance (qualified or unqualified),
or whether or not VLANs are treated as broadcast domain identifiers,
or simply as circuit IDs which have no significance in determining
the broadcast domain. In either of these cases, the VPLS instance
is the outer-most "envelope" which ensures that traffic within it
does not "leak" into another VPLS instance.
12.3.
Protection of SP Networks
Two types of DoS attacks are of concern with VPLS:
- Attacks against VPLS devices
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- Attacks against other devices, for which the VPLS network is a
transport.
Attacks of the first type are naturally of greater concern for a
VPLS operator, because they can destabilize the VPLS network as a
whole, and affect multiple customers. The tunneling nature of VPLS
by itself limits the possibilities for attacks via the data plane,
simply because such attacks will be tunneled through the VPLS
network, and will create the same load on the VPLS equipment as
legitimate traffic will.
Operators must watch for exception packet handling in VPLS
equipment. In many cases, exception packets are sent to the control
plane for handling. If that is the case, the operator must ensure
that such exception packets can be rate-limited in a fashion that
guarantees that the control plane will not be significantly burdened
by them. A SP should limit the amount of traffic that a customer can
flood.
The second type of DoS attacks, which use the VPLS network as a
transport, are not really a threat to the VPLS devices themselves
but are to devices behind them. VPLS PEs may be configured with
rate-limiting and rate-shaping capabilities which permit them to
limit the amount of traffic allowed into a particular VPLS instance.
This prevents a VPLS customer from consuming excessive amount of
network resources and from starving other customers. For example, it
might be useful to limit the multicast/broadcast/unknown traffic of
the customer, considering that the replication of this traffic will
create a load in the core proportional to the number of PEs
participating to the VPLS instance. Optionally, they can also be
tasked with advanced processing of the traffic they tunnel. For
example, they may impose access lists which deny traffic from
particular sources or protocols.
Such approaches however are highly vendor-specific and outside the
scope of [VPLS-LDP]. In addition, they may have significant design
and operational repercussions. Alternative approaches which hand-
off DoS protection activities to non-VPLS devices (such as customer
equipment) are a possibility.
12.4.
Protection of User Data
VPLS does not have special provisioning for ensuring user data
security. If a customer's traffic is IP traffic, that customer may
provide its own user data security by using IPsec. In fact, VPLS is
compatible with any use of security by the customer, as long as a
clear text Ethernet header is passed from CE to PE.
13.
Scalability
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As per [L2VPN-REQ], a large SP may eventually require support of up
to O(10^4) VPLS instances. In addition, some of these VPLS instances
may need to support O(10^2) sites and O(10^3) users/MACs. This
section describes the key scalability challenges and how VPLS-LDP
addresses them.
13.1.
Mesh topology
A full mesh of tunnel LSPs, over which a full mesh of PWs is
established, is created between participating PEs. When using
hierarchical VPLS constructs, the size of this full mesh can be
reduced to hub PEs aggregating point-to-point spokes as described in
section 10 of [VPLS-LDP].
This reduces the number of tunnels and PWs from O(N*N) to O(N).
13.2.
Signaling
Using HVPLS constructs also allows the total number of targeted LDP
sessions to be reduced from O(N*N) to O(N).
13.3.
MAC addresses and MAC learning
Depending on the type of CE devices used, i.e. switches or routers,
the total number of MAC addresses to be learned by VPLS PEs can vary
from one address per site to a large number of MAC addresses.
When Ethernet networks exceed a large number of MAC addresses (e.g.
hundreds), routers are introduced to limit the size of such
broadcast domains. This reduces the total number of MAC addresses to
learn to such routers only.
In the case of large flat Ethernet networks, ingress PEs must be
able to limit the number of MAC addresses that can be learned on a
per VPLS basis.
13.4.
Packet replication
With VPLS, broadcast, multicast and unknown destination frames get
replicated by the ingress PEs, i.e. close to the source of the
frame. Ideally such frames should be replicated as close to the
destination as possible to minimize bandwidth consumption. With
hierarchical VPLS, the replication process is distributed between
several ingress and egress MTUs and PEs. This helps not only
minimizing bandwidth resources but also improving multicast
performance and reducing latency.
13.5.
Broadcast limiting
Ingress MTUs or PEs may be able to rate limit the amount of
broadcast/multicast/unknown traffic generated by end users in order
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to protect core resources and to prevent a few users from using all
the bandwidth available.
13.6.
Multicast
In order to optimize the replication of multicast traffic, it is
highly desirable for PEs to support multicast snooping techniques in
order to only forward traffic where needed. In the case where the CE
device is an L2 switch, IGMP snooping would be required, however, if
the CE device is a router PIM snooping would be more applicable.
14.
Management
The following five major areas in management are discussed bellow:
- Fault
- Configuration
- Accounting
- Provisioning
- Security
VPLS introduces new configurations related to creation and removal
of VSIs, etc. VPLS also introduces new provisioning challenges
because the service needs to be delivered end-to-end and therefore
many things such as access control, QoS, etc need to be provisioned
accordingly. Achieving these via manual CLI configuration can be
error prone. Therefore, it is advisable to use a provisioning system
for configuration and provisioning.
Although VPLS-specific MIBs are still under development, accounting
information can usually be achieved via [IF-MIB] and [LSR-MIB]. The
important point is that accounting information should be available
per service basis. Such information can then be processed by an
accounting application to produce the accounting records. Security
can be achieved by the measures described in Section 0.
Managing fault with VPLS involves multi-point connectivity
verification and locating the fault if there is one. Such mechanism
is sometimes referred to as "VPLS OAM" and is discussed below.
Although VPLS OAM is still being defined, one of the approaches has
gained momentum. This approach proposes applying Ethernet OAM
mechanism that is being standardized by ITU, IEEE and the Metro
Ethernet Forum (MEF) to an VPLS environment for L2 connectivity
verification and fault locating, and applying MPLS OAM mechanism
such as [LSP-PING] or [BFD] or [VCCV] to MPLS connectivity
verification and fault locating. Of course, if IP tunnels (e.g. GRE)
are used, IP ping and traceroute can be used in the place of MPLS
OAM. VPLS OAM is therefore achieved by integrating OAM mechanisms at
different layers together.
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In summary of this section: management of VPLS services involves
many things and can be tedious. A complete suite of management
software including EMS, NMS and a provisioning system can therefore
be highly desirable.
Acknowledgments
The authors wish to thank the following people for their
constructive contributions to the text in this document:
Javier Antich
Ian Cowburn
Richard Foote
Rob Nath
Ali Sajassi
Nick Slabakov
Some text was adapted from the Applicability Statement for BGP/MPLS
IP VPNs [AS2547] document.
Copyright Notice
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to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
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specification can be obtained from the IETF on-line IPR repository
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The IETF invites any interested party to bring to its attention any
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Release Statement
By submitting this Internet-Draft, the authors accept the provisions
of Section 4 of RFC 3667.
Normative References
[VPLS-LDP] "Virtual Private LAN Services over MPLS", Marc Lasserre,
Vach Kompella, et al., draft-ietf-l2vpn-vpls-ldp-05.txt, Work in
progress, September 2004
[PWE3-ETHERNET] "Encapsulation Methods for Transport of Ethernet
Frames Over IP/MPLS Networks", draft-ietf-pwe3-ethernet-encap-
02.txt, Work in progress, February 2003.
[PWE3-CTRL] "Transport of Layer 2 Frames Over MPLS", draft-ietf-
pwe3-control-protocol-02.txt, Work in progress, February 2003.
[RFC3036] "LDP Specification", L. Andersson, et al. RFC 3036.
January 2001.
[RFC3036] "LDP Specification", L. Andersson, et al. RFC 3036.
January 2001.
[802.1D-ORIG] Original 802.1D - ISO/IEC 10038, ANSI/IEEE Std 802.1D-
1993 "MAC Bridges".
[802.1D-REV] 802.1D - "Information technology - Telecommunications
and information exchange between systems - Local and metropolitan
area networks - Common specifications - Part 3: Media Access Control
(MAC) Bridges: Revision. This is a revision of ISO/IEC 10038: 1993,
802.1j-1992 and 802.6k-1992. It incorporates P802.11c, P802.1p and
P802.12e." ISO/IEC 15802-3: 1998.
[802.1Q] 802.1Q - ANSI/IEEE Draft Standard P802.1Q/D11, "IEEE
Standards for Local and Metropolitan Area Networks: Virtual Bridged
Local Area Networks", July 1998.
Informative References
[BGP-VPN] Rosen and Rekhter, "BGP/MPLS VPNs". draft-ietf-ppvpn-
rfc2547bis-04.txt, Work in Progress, May 2003.
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[RADIUS-DISC] " Using Radius for PE-Based VPN Discovery", Juha
Heinanen, draft-heinanen-radius-pe-discovery-04.txt, Work in
Progress, June 2003.
[BGP-DISC] "Using BGP as an Auto-Discovery Mechanism for Network-
based VPNs", Ould-Brahim, et. al., draft-ietf-ppvpn-bgpvpn-auto-
05.txt, Work in Progress, May 2003.
[VPLS-BRIDGING] "Bridging and VPLS", draft-finn-ppvpn-bridging-vpls-
00.txt, Work in Progress, June 2002.
[L2VPN-SIG] "LDP-based Signaling for L2VPNs", draft-rosen-ppvpn-l2-
signaling-03.txt, Work in Progress, May 2003.
[L2FRAME] "L2VPN Framework", draft-ietf-ppvpn-l2-framework-03, Work
in Progress, February 2003.
[L2VPN-REQ] "Service Requirements for Layer 2 Provider Provisioned
Virtual Private Networks", draft-ietf-ppvpn-l2vpn-requirements-
00.txt, Work in Progress, May 2003.
[802.1ad] "IEEE standard for Provider Bridges", Work in Progress,
December 2002.
Authors' Addresses
Marc Lasserre
Riverstone Networks
Email: marc@riverstonenet.com
Xipeng Xiao
Riverstone Networks
Email: xxiao@riverstonenet.com
Yetik Serbest
SBC Communications
Yetik_serbest@labs.sbc.com
Cesar Garrido,
Telefonica
cesar.garridosanahuja@telefonica.es
Marc Rapoport
Completel
m.rapoport@completel.fr
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