Network Working Group Y. Kamite, Ed.
Internet-Draft Y. Wada
Expires: December 28, 2006 NTT Communications
Y. Serbest
AT&T
T. Morin
France Telecom
L. Fang
AT&T
Jun 26, 2006
Requirements for Multicast Support in Virtual Private LAN Services
draft-ietf-l2vpn-vpls-mcast-reqts-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document provides functional requirements for network solutions
that support multicast over Virtual Private LAN Service (VPLS). It
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specifies requirements both from the end user and service provider
standpoints. It is intended that potential solutions will use these
requirements as guidelines.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Scope of this document . . . . . . . . . . . . . . . . . . 5
2. Conventions used in this document . . . . . . . . . . . . . . 5
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 7
3. Problem Statements . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Multicast Scalability . . . . . . . . . . . . . . . . . . 7
3.3. Application Considerations . . . . . . . . . . . . . . . . 8
3.3.1. Two Perspectives of the Service . . . . . . . . . . . 8
4. General Requirements . . . . . . . . . . . . . . . . . . . . . 9
4.1. Scope of Transport . . . . . . . . . . . . . . . . . . . . 9
4.1.1. Traffic Types . . . . . . . . . . . . . . . . . . . . 9
4.1.2. Multicast Packet Types . . . . . . . . . . . . . . . . 10
4.2. Static Solutions . . . . . . . . . . . . . . . . . . . . . 11
4.3. Backward Compatibility . . . . . . . . . . . . . . . . . . 12
5. Customer Requirements . . . . . . . . . . . . . . . . . . . . 12
5.1. CE-PE protocol . . . . . . . . . . . . . . . . . . . . . . 12
5.1.1. Layer-2 Aspect . . . . . . . . . . . . . . . . . . . . 12
5.1.2. Layer-3 Aspect . . . . . . . . . . . . . . . . . . . . 12
5.2. Multicast Domain . . . . . . . . . . . . . . . . . . . . . 13
5.3. Quality of Service (QoS) . . . . . . . . . . . . . . . . . 14
5.4. SLA Parameters Measurement . . . . . . . . . . . . . . . . 14
5.5. Security . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.5.1. Isolation from Unicast . . . . . . . . . . . . . . . . 15
5.5.2. Access Control . . . . . . . . . . . . . . . . . . . . 15
5.5.3. Policing and Shaping on Multicast . . . . . . . . . . 15
5.6. Access Connectivity . . . . . . . . . . . . . . . . . . . 15
5.7. Protection and Restoration . . . . . . . . . . . . . . . . 15
5.8. Minimum MTU . . . . . . . . . . . . . . . . . . . . . . . 16
5.9. Frame Reordering Prevention . . . . . . . . . . . . . . . 16
5.10. Fate-Sharing between Unicast and Multicast . . . . . . . . 16
6. Service Provider Network Requirements . . . . . . . . . . . . 17
6.1. Scalability . . . . . . . . . . . . . . . . . . . . . . . 17
6.1.1. Trade-off of Optimality and State Resource . . . . . . 17
6.1.2. Key Metrics for Scalability . . . . . . . . . . . . . 18
6.2. Tunneling Requirements . . . . . . . . . . . . . . . . . . 19
6.2.1. Tunneling Technologies . . . . . . . . . . . . . . . . 19
6.2.2. MTU of MDTunnel . . . . . . . . . . . . . . . . . . . 19
6.3. Robustness . . . . . . . . . . . . . . . . . . . . . . . . 19
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6.4. Discovering Related Information . . . . . . . . . . . . . 19
6.5. Operation, Administration and Maintenance . . . . . . . . 20
6.5.1. Activation . . . . . . . . . . . . . . . . . . . . . . 20
6.5.2. Testing . . . . . . . . . . . . . . . . . . . . . . . 20
6.5.3. Performance Management . . . . . . . . . . . . . . . . 21
6.5.4. Fault Management . . . . . . . . . . . . . . . . . . . 21
6.6. Security . . . . . . . . . . . . . . . . . . . . . . . . . 22
6.7. Hierarchical VPLS support . . . . . . . . . . . . . . . . 23
6.8. L2VPN Wholesale . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . . . 28
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1. Introduction
1.1. Background
VPLS (Virtual Private LAN Service) is a provider service that
emulates the full functionality of a traditional Local Area Network
(LAN). VPLS interconnects several customer LAN segments over a
packet switched network (PSN) backbone, creating a multipoint-to-
multipoint Ethernet VPN. For customers, their remote LAN segments
behave as one single LAN.
In a VPLS, the provider network emulates a learning bridge, and
forwarding takes place based on Ethernet MAC learning. Hence, a VPLS
requires MAC address learning/aging on a per PW (Pseudo Wire) basis,
where forwarding decision treats the PW as a "bridge port".
VPLS is a Layer-2 service. However, it provides two applications
from the customer's point of view:
- LAN Routing application: providing connectivity between customer
routers
- LAN Switching application: providing connectivity between
customer Ethernet switches
Thus, in some cases, customers across MAN/WAN have transparent
Layer-2 connectivity while their main goal is to run Layer-3
applications within their routing domain. As a result, different
requirements arise from their variety of applications.
Originally VPLS functionality natively transports broadcast/multicast
Ethernet frames. In the current solution, a PE simply replicates all
multicast/broadcast frames over all corresponding PWs. Such a
technique has the advantage of keeping the P and PE devices
completely unaware of IP multicast-specific issues. Obviously,
however, it has quite a few scalability drawbacks in terms of
bandwidth waste, which will lead to increased cost in large-scale
deployment.
Meanwhile, there is a growing need for support of multicast-based
services such as IP TV. This commercial trend makes it necessary for
most VPLS deployment to support multicast more efficiently than
before. It is even more true, since customer routers are now likely
running IP multicast protocols and those routers and connected
switches will be handling huge amount of multicast traffic.
Therefore, it is desirable to have more efficient techniques to
support IP multicast over VPLS.
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1.2. Scope of this document
This document provides functional requirements for network solutions
that support IP multicast in VPLS [VPLS-LDP][VPLS-BGP]. It
identifies requirements that MAY apply to the existing base VPLS
architecture in order to treat IP multicast. It also complements the
generic L2 VPN requirements document [L2VPN-REQ], by specifying
additional requirements specific to the deployment of IP multicast in
VPLS.
The technical specifications are outside the scope of this document.
There is no intent to either specify solution-specific details in
this document or application-specific requirements. Also, this
document does NOT aim at expressing multicast-inferred requirements
that are not specific to VPLS. It does NOT aim at expressing any
requirements for native Ethernet specifications, either.
This document is proposed as a solution guideline and a checklist of
requirements for solutions, by which we will evaluate how each
solution satisfies the requirements.
This document clarifies the needs from both VPN client and provider
standpoints and formulates the problems that should be addressed by
technical solutions with as a key objective to stay solution
agnostic.
A technical solution and corresponding service which supports this
document's requirements are hereinafter called a "multicast VPLS".
2. Conventions used in this document
2.1. Terminology
The reader is assumed to be familiar with the terminology, reference
models and taxonomy defined in [L2VPN-FR] and [L2VPN-REQ]. For
readability purposes, we repeat some of the terms here.
Moreover, we also propose some other terms needed when IP multicast
support in VPLS is discussed.
- ASM: Any Source Multicast. One of the two multicast service models
where each corresponding service can have arbitrarily many
senders.
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- G: denotes a multicast group.
- MDTunnel: Multicast Distribution Tunnel, the means by which the
customer's multicast traffic will be conveyed across the SP
network. This is meant in a generic way: such tunnels can be
point-to-point, point-to-multipoint or multipoint-to-multipoint.
Although this definition may seem to assume that distribution
tunnels are unidirectional, but the wording encompasses bi-
directional tunnels as well.
- Multicast Channel: (S,G) in the SSM model.
- Multicast domain: an area where transmitted multicast data are
reachable. In this document, this term has a generic meaning
which can refer to Layer-2 and Layer-3. Generally, the Layer-3
multicast domain is determined by Layer-3 multicast protocol for
reaching all potential receivers in the corresponding subnet. The
Layer-2 multicast domain can be the same as the Layer-2 broadcast
domain (i.e., VLAN), but it can be smaller than that with
additional control.
- PE/CE: Provider/Customer edge Equipment.
- S: denotes a multicast source.
- SP: Service Provider.
- SSM: Source Specific Multicast. One of the two multicast service
models where each corresponding service relies upon the use of a
single source.
- U-PE/N-PE: The device closer to the customer/user is called User
facing PE (U-PE) and the device closer to the core network is
called Network facing PE (N-PE).
- VPLS instance: A service entity manageable in VPLS architecture.
All CE devices participating in a single VPLS instance appear to
be on the same LAN, composing a VPN across SP network. A VPLS
instance corresponds to a group of VSIs that are interconnected
using PWs (Pseudo Wires).
- VSI: Virtual Switching Instance. VSI is a logical entity in PE
that maps multiple ACs (Attachment Circuits) to multiple PWs
(Pseudo Wires). The VSI is populated in much the same way as a
standard bridge populates its forwarding table. Each PE device
may have a multiple VSIs, where each VSI belongs to a different
VPLS instance.
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2.2. 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 [RFC2119] .
3. Problem Statements
3.1. Motivation
Today, many kinds of IP multicast services are becoming available.
Over their Layer-2 VPN service, particularly over VPLS, customers
would often like to operate their multicast applications across
remote sites. Also, multicast service providers using IP-based
network are expecting that such Layer-2 network infrastructure will
efficiently support them.
However, VPLS has a shortcoming in multicast scalability as mentioned
below because of its replication mechanisms intrinsic to the original
architecture. Accordingly, the primary goal for technical solutions
is to solve this issue partially or completely, and provide efficient
ways to support IP multicast services over VPLS.
3.2. Multicast Scalability
In VPLS, replication occurs at ingress PE when a CE sends (1)
Broadcast, (2) Multicast or (3) Unknown destination unicast. There
are two well known issues about this:
Issue A: Replication to non-member site
In case (1) and (3), the upstream PE has to transmit packets to
all of the downstream PEs which belong to the common VPLS
instance. You cannot decrease the number of members, so this is
basically an inevitable situation for most VPLS deployment.
In case (2), however, there is an issue that multicast traffic is
sent to sites with no members. Usually this is caused when the
upstream PE does not maintain downstream membership information.
The upstream PE simply floods frames to all downstream PEs, and
the downstream PEs forward them to directly connected CEs;
however, those CEs might not be the members of any multicast
group. From the perspective of customers, they might suffer from
pressure on their own resources due to unnecessary traffic. From
the perspective of SPs, they would not like wasteful over-
provisioning to cover such traffic.
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Issue B: Replication of PWs on shared physical path
In VPLS, a VSI associated with each VPLS instance behaves as a
logical emulated bridge which can transport Ethernet across the
PSN backbone using PWs. In principle, PWs are designed for
unicast traffic.
In all cases (1), (2) and (3), Ethernet frames are replicated on
one or more PWs that belong to that VSI. This replication is
often inefficient in terms of bandwidth usage if those PWs are
traversing shared physical links in the backbone.
For instance, suppose there are 20 remote PEs belonging to a
particular VPLS instance, and all PWs happen to be traversing over
the same link from one local PE to its next-hop P. In this case,
even if a CE sends 50Mbps to the local PE, the total bandwidth of
that link will be wasted up to 1000Mbps.
Note that while traditional 802.1D Ethernet switches replicate
broadcast/multicast flows once at most per output interface, VPLS
often needs to transmit one or more flows duplicated over the same
output interface.
From the perspective of customers, there is no serious issue
because they do not know what happens in the core. However, from
the perspective of SPs, unnecessary replication brings the risk of
resource exhaustion when the number of PWs increases.
In both issue A and B, these undesirable situations will become
obvious when the wide-spread use of IP multicast applications by
customers results in frequent occurrences of case (2). Naturally the
problem will become more serious as the number of sites grows. In
other words, we have multicast scalability concerns in VPLS today.
3.3. Application Considerations
3.3.1. Two Perspectives of the Service
When it comes to IP multicast over VPLS, there are two different
aspects in terms of service provisioning. They are closely related
to the functional requirements from two technical standpoints:
Layer-2 and Layer-3.
- Native Ethernet service aspect
This is an aspect mainly from Ethernet network service operators.
Their main interest is how to deal with the issue that current
existing VPLS cannot always handle multicast/broadcast frames
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efficiently.
Today, wide-area Ethernet services are becoming popular, and VPLS
can be utilized to provide wide-area LAN services. As customers
come to use various kinds of content distributing applications
which use IP multicast (or other protocols which lead to
multicast/broadcast in Ethernet layer), the total amount of
Ethernet frames will also grow. In addition, considerations of
OAM, security and other related points in multicast in view of
Layer-2 are important as well.
In such circumstances, the native VPLS specipication would not
always be satisfactory if multicast traffic is more dominant in
total resource utilization than before. The scalability issues
mentioned in the previous section are expected to be solved.
- IP multicast service aspect
This is an aspect mainly from both IP service providers and end
users. Their main interest is how to provide IP multicast
services transparently but effectively by means of VPLS as a
network infrastructure.
There are some hopeful applications such as Triple-play (Video,
Voice, Data) and Multicast IP-VPN. SPs might expect VPLS as an
access/metro network to deliver multicast traffic in an efficient
way.
Furthermore, in some cases, IP service operators might expect
operational simplicity of VPLS. That is, they avoid direct and
detailed operation of IP routing. In this case, the multicast
delivery mechanism is expected to have not only efficiency but
also simplicity. Generally speaking, efficiency and simplicity
have trade-off relationship in terms of bandwidth usage and state
maintenance, so the best trade-off comes to be highly expected.
4. General Requirements
We assume the basic requirements for VPLS written in [L2VPN-REQ] are
fulfilled if there is no special reference in this document.
4.1. Scope of Transport
4.1.1. Traffic Types
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4.1.1.1. Multicast and Broadcast
As described before, any solution is expected to have mechanisms for
efficient transport of IP multicast. Multicast is related to both
issues A and B; however, broadcast is related to issue B only because
it does not need membership control.
- A multicast VPLS solution SHOULD attempt to solve both issues, if
possible. However, since some applications prioritize solving one
issue over the other, the solution MUST identify which issue (A or
B) it is attempting to solve. The solution SHOULD provide a basis
for evaluating how well it solves the issue(s) it is targeting, if
it is providing an approximate solution.
4.1.1.2. Unknown Destination Unicast
Unknown destination MAC unicast needs flooding, but its
characteristic in terms of service aspect is quite different from
multicast/broadcast. When the unicast MAC address is learned, the PE
changes its forwarding behavior from flooding over all PWs into
sending over one PW. Thereby it will require different technical
studies from multicast/broadcast, which is out of scope in this
document.
4.1.2. Multicast Packet Types
Ethernet multicast is used for conveying Layer-3 multicast data.
When IP multicast is encapsulated by an Ethernet frame, the IP
multicast group address is mapped to the Ethernet destination MAC
address (beginning with 01-00-5E in hex). Since the mapping between
IPv4 multicast addresses and Ethernet-layer multicast addresses is
ambiguous (i.e., multiplicity of 1 Ethernet address to 32 IP
addresses), MAC-based multicast forwarding is not totally ideal for
IP multicast.
Ethernet multicast is also used for a Layer-2 control frames. For
example, BPDU (Bridge Protocol Data Unit) for IEEE 802.1D Spanning
Tree uses multicast destination MAC address (01-80-C2-00-00-00).
Also some of IEEE 802.1ag [802.1ag] Connectivity Fault Management
(CFM) messages use a multicast destination MAC address dependent on
their message type and application. From the perspective of IP
multicast, however, it is necessary in VPLS to flood such control
frames to all participating CEs, without requiring any membership
controls.
As for a multicast VPLS solution, it can only use Ethernet-related
information, if you only stand by the strict application of the basic
requirement: "a L2VPN service SHOULD be agnostic to customer's Layer
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3 traffic [L2VPN-REQ]." This means no Layer-3 information should be
checked for transport. However, it is obvious this is an impediment
to solve Issue A.
Consequently, a multicast VPLS can be allowed to make use of some
Layer-3-related supplementary information in order to improve
transport efficiency. In fact, today's LAN switch implementations
often support such approaches to snoop upper layer protocols and
examine IP multicast memberships (e.g., PIM snooping and IGMP/MLD
snooping [RFC4541]). This will implicitly suggest that VPLS may
adopt similar techniques although this document does NOT state
Layer-3 snooping is mandatory. If such an approach is taken, careful
considerations about Layer-3 state maintenance performance are much
needed. In addition, note that snooping approaches sometimes have
disadvantages in the system's transparency; that is, one particular
protocol's snooping solution might hinder other (especially future)
protocol's working (e.g., an IGMPv2-snooping switch vs. a new IGMPv3-
snooping one). Also, note that you can take into account other
potential alternatives to snooping:
- static configuration of multicast Ethernet addresses and ports/
interfaces
- multicast control protocol based on Layer-2 technology which
signals mappings of multicast addresses to ports/interfaces, such
as GARP/GMRP[802.1D], CGMP[CGMP] and RGMP[RFC3488].
On the basis described above, general requirements about packet types
are given as follows:
- A solution SHOULD support the IP multicast of the customers with
the care of their Layer-3 multicast routing state. It MAY consult
Layer-3 information to the degree necessary, but any information
irrelevant to multicast transport SHOULD NOT be consulted.
- In a solution, Layer-2 control frames (e.g., BPDU, 802.1ag CFM)
SHOULD be flooded to all PE/CEs in a common VPLS instance. A
solution SHOULD NOT change or limit the flooding scope to remote
PE/CEs in terms of end-point reachability.
- In a solution, Layer-2 frames that encapsulate Layer-3 multicast
control packets (e.g., PIM, IGMP(for IPv4), MLD(for IPv6)) MAY be
flooded only to relevant members, with control of limiting
flooding scope. However, those which encapsulate Layer-3 other
control packets (e.g., OSPF, ISIS) SHOULD be flooded to all PE/CEs
in a VPLS instance.
4.2. Static Solutions
A solution SHOULD allow static configuration by operator's policies,
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where logical multicast topology does not change dynamically in
conjunction with customer's multicast routing.
4.3. Backward Compatibility
A solution SHOULD be backward compatible with the existing VPLS
solution. It SHOULD allow a case where a common VPLS instance is
composed of both PEs supporting the solution and PEs not supporting,
and the multicast forwarding enhancement is partially achieved by the
compliant PEs.
Note again that the existing VPLS solutions already have a simple
flooding capability. Thus this backward compatibility will give
customers and SPs the improved efficiency of multicast forwarding
incrementally as the solution is deployed.
5. Customer Requirements
5.1. CE-PE protocol
5.1.1. Layer-2 Aspect
A solution SHOULD allow transparent operation of Ethernet control
protocols employed by customers (e.g. Spanning Tree Protocol
[802.1D]) and their seamless operation with multicast data transport.
Solutions MAY examine Ethernet multicast control frames for the
purpose of efficient dynamic transport (e.g. GARP/GMRP [802.1D]).
However, solutions MUST NOT assume all CEs are always running such
protocols (typically in the case where a CE is a router not aware of
Layer-2 details).
A whole Layer-2 multicast frame (whether for data or control) SHOULD
NOT be altered from a CE to CE(s) EXCEPT for the VLAN Id field, for
its transparency. If VLAN Ids are assigned by the SP, they can be
altered. Note, however, when VLAN Ids are changed, Layer-2 protocol
may be broken in some cases, such as Multiple Spanning Tree [802.1s].
5.1.2. Layer-3 Aspect
Again, a solution MAY examine customer's Layer-3 multicast protocol
packets for the purpose of efficient and dynamic transport. If it
does, supported protocols SHOULD include:
o PIM-SM [RFC2362], PIM-SSM [PIM-SSM], bidirectional PIM [BIDIR-PIM]
and PIM-DM [RFC3973]
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o IGMP (v1[RFC1112], v2[RFC2236] and v3[RFC3376]) (for IPv4
solutions)
o Multicast Listener Discovery Protocol (MLD) (v1[RFC2710] and
v2[RFC3810]) (for IPv6 solutions).
A solution MUST NOT require any special packet processing about
Layer-3 multicast protocol by the end users. It MAY require some
configuration change as necessary though (e.g., turning explicit
tracking on/off in PIM).
A whole Layer-3 multicast packet (whether for data or control) which
is encapsulated inside Layer-2 frame SHOULD NOT be altered from a CE
to CE(s), for its transparency. However, as for Layer-3 multicast
control (like PIM Join/Prune/Hello packet), it MAY be altered to the
minimum necessary if such partial non-transparency is acceptable from
multicast service point of view.
5.2. Multicast Domain
As noted in Section 2.1., a term "multicast domain" is used in a
generic context for Layer-2 and Layer-3.
A solution SHOULD honor customer's multicast domains. It MUST ensure
that provided Ethernet multicast domain always encompass customer's
corresponding Layer-3 multicast domain.
A solution SHOULD optimize those domains' coverage sizes, i.e.,
ensure that unnecessary traffic is not sent to CEs with no members.
Ideally, provided domain size will be close to that of customer's
Layer-3 multicast membership distribution; however, it is OPTIONAL to
achieve such absolute optimality from the perspective of Layer-3.
If a customer uses VLAN and a VLAN Id as a service delimiter (i.e.,
each VPLS instance is represented by a unique customer VLAN tag
carried by a frame through the UNI port), a solution MUST support
separate multicast domains per VLAN Id. Note that if VLAN Id
translation is provided (i.e., if a customer VLAN at one site is
mapped into a different customer VLAN at a different site), multicast
domains will be created per set of VLAN Ids which are associated with
translation.
If a customer uses VLAN but a VLAN Id is not service delimiter (i.e.,
a VPN is composed in disregard of customer's VLAN Ids), a solution
MAY provide separate multicast domains per VLAN Id. A SP does not
always have to provide separate domains per VLAN IDs, but it will
definitely benefit customer's usage.
A solution MAY build multicast domains with the care of Ethernet MAC
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addresses. It MAY also build with the care of IP addresses inside
Ethernet frames. That is, PEs in each VPLS instance might control
forwarding behavior and provide different multicast frame
reachability depending on each MAC/IP destination address separately.
If IP multicast channels are fully considered in a solution, the
provided domain size will be closer to actual channel reachability.
5.3. Quality of Service (QoS)
Customers require that multicast quality of service MUST be at least
on par with what exists for unicast traffic. Moreover, as multicast
is often used to deliver high quality services such as TV broadcast,
delay/jitter/loss sensitive traffic MUST be supported over multicast
VPLS.
To accomplish this, the solution MAY have additional features to
support high QoS such as bandwidth reservation and flow admission
control. Also multicast VPLS deployment SHALL benefit from IEEE
802.1p CoS techniques [802.1D] and DiffServ [RFC2475] mechanisms.
Moreover, multicast traffic SHOULD NOT affect the QoS that unicast
traffic receives and vice versa. That is, separation of multicast
and unicast traffic in terms of QoS is necessary.
5.4. SLA Parameters Measurement
Since SLA parameters are part of the service sold to customers, they
simply want to verify their application performance by measuring
parameters SP(s) provide.
Multicast specific characteristics that may be monitored are, for
instance, multicast statistics per stream, delay and latency time
(time to start receiving a multicast group traffic across the VPN).
You can also see about variation in delivery time of a multicast
packet to different destination.
A solution SHOULD allow providing these parameters with Ethernet
multicast level granularity. (For example, multicast MAC address
will be one of those entries for classifying flows with statistics,
delay and so on.) However, if a solution aims at IP multicast
transport efficiency more, it MAY support IP multicast level
granularity. (For example, multicast IP address/channel will be
entries for latency time.)
In order to monitor them, standard interfaces SHOULD also be provided
(e.g., standard SNMP MIB Modules).
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5.5. Security
A solution MUST provide customers with architectures that give the
same level of security both for unicast and multicast.
5.5.1. Isolation from Unicast
Solutions SHOULD NOT affect any forwarding information base,
throughput, resiliency of unicast frames; that is, they SHOULD
provide isolation from unicast.
5.5.2. Access Control
A solution MAY have the mechanisms of multicast filtering
capabilities inside the activated service upon the request of each
customer (for example, MAC/VLAN filtering, IP multicast channels
filtering, and so on)
5.5.3. Policing and Shaping on Multicast
A solution SHOULD have the mechanisms of multicast policing and
shaping capabilities for a common customer. This is intended to
prevent multicast traffic from exhausting resources for unicast
inside a common VPN. This might also be beneficial for QoS
separation (see section 5.3).
5.6. Access Connectivity
First and foremost various physical connectivity types described in
[L2VPN-REQ] MUST be supported.
For particular reference here, a multicast VPLS MUST allow a
situation on which a CE is dual-homed to two different SPs via
diverse access networks -- one is supporting multicast VPLS but the
other is not supporting (existing VPLS or 802.1Q/QinQ network).
5.7. Protection and Restoration
A multicast VPLS infrastructure SHOULD allow redundant paths to
assure high availability.
Multicast forwarding restoration time MUST NOT be greater than the
time of customer's Layer-3 multicast protocols. For example, if a
customer uses PIM with default configuration, hello hold timer is 105
seconds, and solutions are required to detect a failure no later than
this period.
Moreover, if multicast forwarding was not successfully restored
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(e.g., in case of no redundant paths), a solution MAY have a raising
alarm mechanism to notify outage to customers before such hold timer
expires.
5.8. Minimum MTU
Multicast applications are often sensitive to packet fragmentation
and reassembling, so the requirement to avoid fragmentation might be
stronger than the existing VPLS solution.
A solution SHOULD provide customers with enough committed minimum MTU
(i.e., service MTU) for multicast Ethernet frames to ensure that IP
fragmentation between customer sites never occurs. It MAY give
different MTU sizes to multicast and unicast.
5.9. Frame Reordering Prevention
A solution SHOULD prevent frame reordering delivery of customers'
multicast traffic. Likewise, for unicast and unknown unicast
traffic, it SHOULD NOT increase reordering occurrence in comparison
with the existing VPLS.
5.10. Fate-Sharing between Unicast and Multicast
In native Ethernet, multicast and unicast connectivity are often
managed all together. For instance, 802.1ag CFM Continuity Check
message is forwarded by multicast technique as a periodical
heartbeat, but it is supposed to check the "whole" traffic continuity
regardless of unicast or multicast, at the same time. Hence, the
aliveness of unicast and multicast is naturally coupled (i.e., fate-
shared) in this customer's environment.
However, a multicast VPLS, by nature, is the one which allows to pick
up individually customer's multicast and unicast logically in order
to solve resource waste issue. Thus this point will bring some
customers more or less a new concern that they might have complicated
situations in case of failure in either unicast or multicast only.
Therefore, there will be an additional requirement for making both
connectivity coupled. This means that if either one of them have a
failure, the other is also made stopped. If it becomes alive again,
the other is also made activated.
- It SHOULD be identified if the solution can provide customers with
fate-sharing between unicast and multicast connectivity for their
LAN switching application. It MAY have a configurable mechanism
for SPs to provide that on behalf of customers, e.g., aliveness
synchronization, but its use is OPTIONAL.
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Note that how serious this issue is depends on each customer's stance
in Ethernet operation. For example, if all CEs are IP routers i.e.,
if VPLS is provided for LAN Routing application, such the customer
might not need to care about it because both connectivity is assured
in IP layer.
6. Service Provider Network Requirements
6.1. Scalability
The existing VPLS architecture has major advantages in scalability.
For example, P-routers are free from maintaining customers'
information owing to PSN tunnel encapsulations. Also a PW's split-
horizon technique can prevent loops, making PE routers free from
maintaining complicated spanning trees.
However, a multicast VPLS needs additional scalability considerations
related to its expected enhanced mechanisms. [RFC3809] lists common
L2VPN sizing and scalability requirements and metrics, which are
applicable in multicast VPLS too. Accordingly, this section deals
with specific requirements on the premise of it.
6.1.1. Trade-off of Optimality and State Resource
A solution needs to improve the scalability of multicast as is shown
in section 3:
Issue A: Replication to non-member site
Issue B: Replication of PWs on shared physical path
For both issues, the optimization of physical resources (i.e. link
bandwidth usage and router's duplication performance) will become a
major goal. However, there is a trade-off between optimality and
usage of state resources.
In order to solve Issue A, at least a PE might have to maintain
multicast group information of CEs which was not kept in the existing
VPLS. This will present us scalability concerns about state
resources (memory, CPU, etc.) and their maintenance complexity.
In order to solve Issue B, PE and P might have to know some kinds of
additional membership information of remote PEs, and possibly
additional tree topology information as well, when they are using
point-to-multipoint techniques (PIM tree, P2MP-LSP, etc.).
Consequently, the scalability evaluation of multicast VPLS solutions
needs careful trade-off analysis between bandwidth optimality and
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state resources.
6.1.2. Key Metrics for Scalability
(Note: This part has a number of similar characteristics to
requirements for Layer 3 Multicast VPN [MVPN-REQ].)
A multicast VPLS solution MUST be designed to scale well with an
increase in the number of any of the following metrics:
- the number of PEs
- the number of VPLS instances (total and per PE)
- the number of PEs and sites in any VPLS instance
- the number of client VLAN ids
- the number of client Layer-2 MAC multicast groups
- the number of client Layer-3 multicast channels (groups or source-
groups)
- the number of PWs and PSN Tunnels (MDTunnels) (total and per PE)
Each multicast VPLS solution SHALL document its scalability
characteristics in quantitative terms. A solution SHOULD quantify
the amount of state that a PE and P device has to support.
The characteristics considerations SHOULD include:
- the processing resources required by the control plane processing
PWs (neighborhood or session maintenance messages, keep-alives,
timers, etc.)
- the processing resources required by the control plane processing
PSN tunnels
- the memory resources needed for the control plane
- the amount of protocol information transmitted to manage a
multicast VPLS (e.g. signaling throughput)
- the amount Layer-2/Layer-3 multicast information a P/PE router
treats (e.g. traffic rate of join/leave, keep-alives etc.)
- the number of multicast IP addresses used (if IP multicast in ASM
mode is proposed as a multicast distribution tunnel)
- other particular elements inherent to each solution that impacts
scalability
Another metric for scalability is operational complexity. Operations
will naturally become more complicated if the number of managed
object (e.g., multicast groups) grows up, or topology changes more
frequently. A solution SHOULD note such the factors which lead to
operational complexity.
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6.2. Tunneling Requirements
6.2.1. Tunneling Technologies
A MDTunnel denotes a multicast distribution tunnel. This is a
generic term of tunneling that carries customer's multicast traffic
over the provider's network. In L2VPN service context, it will
correspond to a PSN tunnel.
A solution SHOULD be able to use a range of tunneling technologies,
including point-to-point (unicast oriented) and point-to-multipoint/
multipoint-to-multipoint (multicast oriented). For example, today
there are many kinds of protocols for tunneling such as L2TP, IP,
(including multicast IP trees), MPLS (including P2MP-LSP [RSVP-P2MP]
and P2MP/MP2MP-LSP [LDP-P2MP-MP2MP] ), etc.
Note that which variant, point-to-point, point-to-multipoint or
multipoint-to-multipoint, is used depends largely on the
consideration about the trade-off mentioned above and the targeted
network and applications. Therefore, this document does not mandate
any specific protocols. A solution, however, SHOULD state reasonable
criteria if it adopts a specific kind of tunneling protocol.
6.2.2. MTU of MDTunnel
From the view of SP, it is not acceptable to have fragmentation/
assembling so often while packets are traversing MDTunnel.
Therefore, a solution SHOULD support a method that provides minimum
path MTU of the MDTunnel to accommodate the service MTU.
6.3. Robustness
Multicast VPLS solutions SHOULD avoid whatever single points of
failures or propose some technical solutions making possible to
implement a failover mechanism.
6.4. Discovering Related Information
The operation of a multicast VPLS solution SHALL be as light as
possible and providing automatic configuration and discovery SHOULD
be prioritized.
Therefore, in addition to L2VPN discovery requirements shown in
[L2VPN-REQ], a multicast VPLS solution SHOULD provide a method that
dynamically allows multicast membership information to be discovered
by PEs. Such membership information is, for example, a set of
multicast addresses. Which kind of information is provided
dynamically depends on solutions.
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6.5. Operation, Administration and Maintenance
6.5.1. Activation
The activation of multicast enhancement in a solution MUST be
possible:
o with a VPLS instance granularity
o with an Attachment Circuit granularity (i.e., with a PE-CE
Ethernet port granularity, or with a VLAN Id granularity when it
is a service delimiter)
Also it SHOULD be possible:
o with a CE granularity (when multiple CEs of a same VPN are
associated with a common VPLS instance)
o with a distinction between multicast reception and emission
o with a multicast MAC address granularity
Also it MAY be possible:
o with an IP multicast group and/or channel granularity
o with a VLAN Id granularity when it is not a service delimiter
6.5.2. Testing
A solution MUST provide a mechanism for testing multicast data
connectivity and verify the associated information. Examples that
SHOULD be supported which are specific to multicast are:
- Testing connectivity per multicast MAC address
- Testing connectivity per multicast Layer-3 group/channels
- Verifying data plane and control plane integrity (e.g. PW,
MDTunnel)
- Verifying multicast membership-relevant information (e.g.
multicast MAC-addresses/PW-ports associations, Layer-3 group
associations)
Operators usually want to test if an end-to-end multicast user
connectivity is OK before and after activation. Such end-to-end
multicast connectivity checking SHOULD enable the end-to-end testing
of the data path used by that of customer's data multicast packets.
For details, end-to-end checking will have CE-to-CE path test and PE-
to-PE path test. A solution MUST support PE-to-PE path test and MAY
support CE-to-CE path test.
Also operators will want to make use of a testing mechanism for
diagnosis and troubleshooting. In particular, a solution SHOULD be
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enabled to monitor information describing how client multicast
traffic is carried over the SP network. Note that if a solution
supports frequent dynamic membership changes with optimized
transport, the SP's network will tend to incur difficulty in
troubleshooting.
6.5.3. Performance Management
Monitoring multicast specific parameters and statistics MUST be
offered to the SP.
(Note: This part has a number of similar characteristics to
requirements for Layer 3 Multicast VPN [MVPN-REQ].)
A solution MUST provide SPs with access to:
- Multicast traffic statistics (total traffic conveyed, incoming,
outgoing, dropped, etc., by period of time)
A solution SHOULD provide access to:
- Information about customer's multicast resource usage (the number
of multicast state and throughput)
- Performance information relevant to the multicast traffic usage
(one-way delay, jitter, loss, delay variations between different
destinations etc.)
- Alarms when limits are reached on such resources
- Statistics on decisions related to how client traffic is carried
on MDTunnels (e.g. "traffic switched onto a multicast tree
dedicated to such groups or channels")
- Statistics on parameters that could help the provider to evaluate
its optimality/state trade-off
All or part of this information SHOULD be made available through
standardized SNMP MIB Modules (Management Information Base).
6.5.4. Fault Management
A multicast VPLS solution needs to consider those management steps
taken by SPs below:
o Fault detection
A solution MUST provide tools that detect group membership/
reachability failure and traffic looping for multicast
transport. It is naturally anticipated that such tools are
well coordinated with testing mechanisms mentioned in 6.5.2.
In particular, such mechanisms SHOULD be able to detect
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multicast failure quickly on par with unicast cases. It needs
to obviate the cases where multicast has been in failure for
long time while unicast remains alive; such a situation, in
many cases, might end up in complicated troubles in customer
applications which use a combination of them.
However, in multicast, as there are many receivers pertaining
to a particular unidirectional traffic, possibly the number of
potential detecting points also grows, which will raise
scalability concern. Consequently, a fault detection solution
SHOULD scale well with consideration of key metrics described
in section 6.1.2.
o Fault notification
A solution MUST also provide fault notification and trouble
tracking mechanisms. (e.g. SNMP-trap and syslog.)
In case of multicast, one point of failure often affects a
number of involved downstream routers/receivers that might be
able to raise notification. Hence notification messages MAY be
summarized or compressed for operators' easy management.
o Fault isolation
A solution MUST provide diagnostic/troubleshooting tools for
multicast as well. Also it is anticipated that such tools are
well coordinated with testing mechanisms mentioned in 6.5.2.
In particular, a solution needs to identify correctly the
impacted area inside a multicast group by the failure. Then it
SHOULD be able to diagnose if an entire multicast group is
faulty or some specific destinations are still alive.
6.6. Security
A SP network MUST be invulnerable to malformed or maliciously
constructed customer traffic. This applies to data packets and
control packets both.
Moreover, because multicast, broadcast, and unknown-unicast need more
resources than unicast, a SP network MUST have high safeguards
against unwanted or malicious traffic of them. This applies to data
packets.
Specifically, a multicast VPLS solution SHOULD have measures against:
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- invalid multicast MAC addresses (always)
- invalid multicast IP addresses (if they are used for forwarding)
- malformed Ethernet multicast control protocol (if they are
examined)
- malformed IP multicast control protocol (if they are examined)
- high volume traffic of
* valid/invalid customer's control packets
* valid/invalid customer's data packets (broadcast/multicast/
unknown-unicast)
We show a few additional guidelines below.
A solution MAY allow imposing some bounds on the quantity of state
used by a VPN. It is intended to prevent out-of-state-resources
(i.e., lack of memory, CPU etc.) situations.
Also a solutions MAY allow a policing mechanism to limit the
unwanted data traffic shown above. In this case, while policing
MAY be configurable to the sum of unicast, multicast, broadcast
and unknown unicast traffic, it also MAY be configurable to each
such type of traffic individually, or to their combination. It is
intended to prevent out-of-physical-resources (i.e., lack of
bandwidth and forwarding performance) situations.
Moreover, mechanisms against customer's frequent changes of group
membership MAY be supported. For example, if the core's
distribution tunnel is tightly coupled to dynamic changes of
customer multicast domain, a kind of dampening function would be
possible.
6.7. Hierarchical VPLS support
A VPLS multicast solution SHOULD allow a service model by
hierarchical VPLS (H-VPLS) [VPLS-LDP]. In other words, a solution is
expected to be operable seamlessly with existing hub and spoke PW
connectivity.
Note that it is also important to take into account the case of
redundant spoke connections between U-PEs and N-PEs.
6.8. L2VPN Wholesale
A solution MUST allow a situation where one SP is offering L2VPN
services to another SP. One example here is a wholesale model that
one VPLS interconnects other SPs' VPLS or 802.1D network islands.
For customer SP, their multicast transport can obtain enhancement by
virtue of multicast VPLS in the wholesaler SP.
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7. Security Considerations
Security concerns and requirements for a base VPLS solution are
described in [L2VPN-REQ].
On top of that, we need additional considerations specific to
multicast VPLS. Thus a set of security issues have been identified
that MUST be addressed when considering the design and deployment of
the multicast VPLS. Such issues have been described in Section 5.5
and 6.6.
8. Acknowledgments
The authors thank the contributors of [MVPN-REQ] since the structure
and content of this document were, for some section, largely inspired
from [MVPN-REQ].
The authors also thank Yuichi Ikejiri, Jerry Ash, Bill Fenner, Vach
Kompella and Shane Amante for their valuable reviews and feedbacks.
9. References
9.1. Normative References
[L2VPN-REQ]
Augustyn, W. and Y. Serbest, "Service Requirements for
Layer-2 Provider Provisioned Virtual Private Networks,
draft-ietf-l2vpn-requirements-06.txt", Jan 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[802.1D] ISO/IEC 15802-3: 1998 ANSI/IEEE Std 802.1D, 1998 Edition
(Revision and redesignation of ISO/IEC 10038:98), "Part
3: Media Access Control (MAC) Bridges", ISO/IEC 15802-3:,
1998.
[802.1ag] IEEE, "Virtual Bridge Local Area Networks: Connectivity
Fault Management (Work in Progress)", 2006.
[802.1s] IEEE Std 802.1s-2002, "Virtual Bridged Local Area
Networks- Amendment 3: Multiple Spanning Trees", 2002.
[BIDIR-PIM]
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Handley, M., Kouvelas, I., Speakman, T., and L. Vicisanos,
"Bi-directional Protocol Independent Multicast (BIDIR-
PIM), draft-ietf-pim-bidir-08.txt", Oct 2005.
[CGMP] Farinacci, D., Tweedly, A., and T. Speakman, "Cisco Group
Management Protocol (CGMP)",
ftp://ftpeng.cisco.com/ipmulticast/specs/cgmp.txt , 1996/
1997.
[L2VPN-FR]
Andersson, L. and E. Rosen, "Framework for Layer 2 Virtual
Private Networks, draft-ietf-l2vpn-l2-framework-05.txt",
June 2004.
[LDP-P2MP-MP2MP]
Minei, I. and I. Wijnands, "Label Distribution Protocol
Extensions for Point-to-Multipoint and Multipoint-to-
Multipoint Label Switched Paths,
draft-ietf-mpls-ldp-p2mp-00.txt", Feb 2006.
[MVPN-REQ]
Morin, T., "Requirements for Multicast in L3 Provider-
Provisioned VPNs,
draft-ietf-l3vpn-ppvpn-mcast-reqts-08.txt", May 2006.
[PIM-SSM] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP, draft-ietf-ssm-arch-07.txt", Oct 2005.
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[RFC2236] Fenner, W., "Internet Group Management Protocol, Version
2", RFC 2236, November 1997.
[RFC2362] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
S., Handley, M., and V. Jacobson, "Protocol Independent
Multicast-Sparse Mode (PIM-SM): Protocol Specification",
RFC 2362, June 1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
October 1999.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
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Internet-Draft Multicast VPLS Requirements Jun 2006
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[RFC3488] Wu, I. and T. Eckert, "Cisco Systems Router-port Group
Management Protocol (RGMP)", RFC 3488, February 2003.
[RFC3809] Nagarajan, A., "Generic Requirements for Provider
Provisioned Virtual Private Networks (PPVPN)", RFC 3809,
June 2004.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3973] Adams, A., Nicholas, J., and W. Siadak, "Protocol
Independent Multicast - Dense Mode (PIM-DM): Protocol
Specification (Revised)", RFC 3973, January 2005.
[RFC4541] Christensen, M., Kimball, K., and F. Solensky,
"Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping
Switches", RFC 4541, May 2006.
[RSVP-P2MP]
Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
Yasukawa, Ed., "Extensions to RSVP-TE for Point to
Multipoint TE LSPs, draft-ietf-mpls-rsvp-te-p2mp-05.txt",
May 2006.
[VPLS-BGP]
Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-discovery and Signaling,
draft-ietf-l2vpn-vpls-bgp-08.txt", Jun 2006.
[VPLS-LDP]
Lasserre, M. and V. Kompella, "Virtual Private LAN
Services Using LDP, draft-ietf-l2vpn-vpls-ldp-09.txt",
June 2006.
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Authors' Addresses
Yuji Kamite (editor)
NTT Communications Corporation
Tokyo Opera City Tower
3-20-2 Nishi Shinjuku, Shinjuku-ku
Tokyo 163-1421
Japan
Email: y.kamite@ntt.com
Yuichiro Wada
NTT Communications Corporation
1-1-6, Uchisaiwai-cho, Chiyoda-ku
Tokyo 100-8019
Japan
Email: yuichiro.wada@ntt.com
Yetik Serbest
AT&T Labs
9505 Arboretum Blvd.
Austin, TX 78759
USA
Email: Yetik_serbest@labs.sbc.com
Thomas Morin
France Telecom R&D
2, avenue Pierre-Marzin
22307 Lannion Cedex
France
Email: thomas.morin@francetelecom.com
Luyuan Fang
AT&T Labs
200 Laurel Avenue
Middletown, NJ 07748
USA
Email: luyuanfang@att.com
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