Network Working Group Y. Kamite, Ed.
Internet-Draft Y. Wada
Intended status: Informational NTT Communications
Expires: September 6, 2007 Y. Serbest
AT&T
T. Morin
France Telecom
L. Fang
Cisco Systems, Inc.
Mar 5, 2007
Requirements for Multicast Support in Virtual Private LAN Services
draft-ietf-l2vpn-vpls-mcast-reqts-04.txt
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Copyright (C) The IETF Trust (2007).
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 . . . . . . . . . . . . . . . . . . . . 13
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 . . . . . . . . . . . . . . . . 16
5.8. Minimum MTU . . . . . . . . . . . . . . . . . . . . . . . 16
5.9. Frame Reordering Prevention . . . . . . . . . . . . . . . 16
5.10. Fate-Sharing between Unicast and Multicast . . . . . . . . 17
6. Service Provider Network Requirements . . . . . . . . . . . . 17
6.1. Scalability . . . . . . . . . . . . . . . . . . . . . . . 17
6.1.1. Trade-off of Optimality and State Resource . . . . . . 18
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 . . . . . . . . . . . . . . . . . . . 20
6.3. Robustness . . . . . . . . . . . . . . . . . . . . . . . . 20
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6.4. Discovering Related Information . . . . . . . . . . . . . 20
6.5. Operation, Administration and Maintenance . . . . . . . . 20
6.5.1. Activation . . . . . . . . . . . . . . . . . . . . . . 20
6.5.2. Testing . . . . . . . . . . . . . . . . . . . . . . . 21
6.5.3. Performance Management . . . . . . . . . . . . . . . . 22
6.5.4. Fault Management . . . . . . . . . . . . . . . . . . . 22
6.6. Security . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.7. Hierarchical VPLS support . . . . . . . . . . . . . . . . 24
6.8. L2VPN Wholesale . . . . . . . . . . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1. Normative References . . . . . . . . . . . . . . . . . . . 25
10.2. Informative References . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . . . 29
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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 decisions treat 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 PEs transport broadcast/multicast Ethernet frames by
replicating all multicast/broadcast frames received from an AC to all
PW's corresponding to a particular VSI. 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 consumption, 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 deployments to support multicast more efficiently than
before. It is also necessary as customer routers are now likely to
be running IP multicast protocols and those routers and connected to
switches that will be handling large amounts 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 [RFC4761] [RFC4762]. It identifies
requirements that MAY apply to the existing base VPLS architecture in
order to optimize IP multicast. It also complements the generic L2
VPN requirements document [RFC4665], 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 to express multicast-inferred requirements that
are not specific to VPLS. It does NOT aim to express 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 VPLS customer as well as
provider standpoints and formulates the problems that should be
addressed by technical solutions while staying 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 [RFC4664] and [RFC4665]. 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 an arbitrary
number of senders.
- G: denotes a multicast group.
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- 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, the wording encompasses bi-directional
tunnels as well.
- Multicast Channel: (S,G) in the SSM model.
- Multicast domain: An area in which multicast data is transmitted.
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 the Layer-3 multicast protocol used to establish
reachability between all potential receivers in the corresponding
domain. The Layer-2 multicast domain can be the same as the
Layer-2 broadcast domain (i.e., VLAN), but it may be restricted to
being smaller than the Layer-2 broadcast domain if an additional
control protocol is used.
- 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 closest to the customer/user is called User
facing PE (U-PE) and the device closest 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 the SP's 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 a 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 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 to remote
sites. Also, VPN service providers using an IP-based networks expect
that such Layer-2 network infrastructure will efficiently support
multicast data traffic.
However, VPLS has a shortcoming as it relates to multicast
scalability as mentioned below because of the 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 an ingress PE when a CE sends (1)
Broadcast, (2) Multicast or (3) Unknown destination unicast. There
are two well known issues with this approach:
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 deployments.
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 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 issues A and B, these undesirable situations will become
obvious with the wide-spread use of IP multicast applications by
customers. Naturally the problem will become more serious as the
number of sites grows. In other words, there are concerns over the
scalability of multicast 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 aspect mainly affects Ethernet network service operators.
Their main interest is how to deal with the issue that existing
VPLS deployments 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 distribution applications
which use IP multicast (or other protocols which lead to
multicast/broadcast in the Ethernet layer), the total amount of
traffic 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 specification 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 aspect mainly affects 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.
SPs might expect VPLS as an access/metro network to deliver
multicast traffic (such as Triple-play (Video, Voice, Data) and
Multicast IP VPNs) in an efficient way.
Furthermore, in some cases, IP service providers might expect
operational simplicity of VPLS. That is, they avoid direct and
detailed knowledge of IP routing. In this case, the multicast
delivery mechanism is expected to have not only efficiency but
also simplicity. Generally speaking, there is a trade-off between
efficiency and simplicity in terms of bandwidth usage and state
maintenance, so the optimum trade-off will vary depending on the
requirements of each IP service provider.
4. General Requirements
We assume the basic requirements for VPLS written in [RFC4665] 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 (A)
and (B), 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 requires flooding, but its
characteristics are 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 of 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 ideal for IP
multicast.
Ethernet multicast is also used for Layer-2 control frames. For
example, BPDU (Bridge Protocol Data Unit) for IEEE 802.1D Spanning
Tree uses a 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 stand by the strict application of the basic
requirement: "a L2VPN service SHOULD be agnostic to customer's Layer
3 traffic [RFC4665]." This means no Layer-3 information should be
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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 and 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
consideration of Layer-3 state maintenance is necessary. 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 there are 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 a way to facilitate IP multicast
forwarding of the customers. It MAY observe Layer-3 information
(i.e., multicast routing protocols and state) 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 the goal of limiting
flooding scope. However, Layer-2 frames that encapsulate other
Layer-3 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 to account for various
operator policies, where the logical multicast topology does not
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change dynamically in conjunction with a 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
it, 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 and is 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,
ensuring that it is transparently transported. If VLAN Ids are
assigned by the SP, they can be altered. Note, however, when VLAN
Ids are changed, Layer-2 protocols may be broken in some cases, such
as Multiple Spanning Tree [802.1s]. Also if the Layer-2 frame is
encapsulating Layer-3 multicast control packet (e.g., PIM/IGMP) and
customers allow it to be regenerated at PE (aka proxy: see section
5.1.2.), then the MAC address for that frame MAY be altered to the
minimum necessary (e.g., use PE's own MAC address as a source).
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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 [RFC4601], PIM-SSM [RFC4607], bidirectional PIM
[I-D.ietf-pim-bidir] and PIM-DM [RFC3973]
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 Layer-3 multicast protocol
packet processing by the end users. However, it MAY require some
configuration changes (e.g., turning explicit tracking on/off in
PIM).
A whole Layer-3 multicast packet (whether for data or control), which
is encapsulated inside a Layer-2 frame, SHOULD NOT be altered from a
CE to CE(s), ensuring that it is transparently transported. However,
as for Layer-3 multicast control (like PIM Join/Prune/Hello and IGMP
Query/Report packet), it MAY be altered to the minimum necessary if
such partial non-transparency is acceptable from point of view of the
multicast service. Similarly, a PE MAY consume such Layer-3
multicast control packets and regenerate an entirely new packet if
partial non-transparency is acceptable with legitimate reason for
customers (aka proxy).
5.2. Multicast Domain
As noted in Section 2.1., the term "multicast domain" is used in a
generic context for Layer-2 and Layer-3.
A solution SHOULD honor customer multicast domains. It MUST ensure
that the provided Ethernet multicast domain always encompasses the
corresponding customer Layer-3 multicast domain.
A solution SHOULD optimize those domains' coverage sizes, i.e., a
solution SHOULD ensure that unnecessary traffic is not sent to CEs
with no members. Ideally, the provided domain size will be close to
that of the 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 VLANs 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
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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 VLANs but a VLAN Id is not a service delimiter
(i.e., the VPN disregards customer VLAN Ids), a solution MAY provide
separate multicast domains per VLAN Id. A SP is not required to
provide separate multicast domains per VLAN IDs, but it may be
considered beneficial to do so.
A solution MAY build multicast domains based on Ethernet MAC
addresses. It MAY also build multicast domains based on the 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 the
parameters SP(s) provide.
Multicast specific characteristics that may be monitored are, for
instance, multicast statistics per stream (e.g. total/incoming/
outgoing/dropped traffic by period of time), one-way delay, jitter
and group join/leave delay (time to start receiving traffic from a
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multicast group across the VPN since join/leave was issued). An
operator may also wish to compare the difference in one-way delay for
a solitary multicast group/stream from a single, source PE to
multiple receiver PEs.
A solution SHOULD provide these parameters with Ethernet multicast
group 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 is aimed at IP multicast transport
efficiency, 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 for statistics
gathering SHOULD also be provided (e.g., standard SNMP MIB Modules).
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 or resiliency etc. of unicast frames; that is, they SHOULD
provide isolation from unicast.
5.5.2. Access Control
A solution MAY filter multicast traffic inside a VPLS, upon the
request of an individual customer, (for example, MAC/VLAN filtering,
IP multicast channel filtering, etc.).
5.5.3. Policing and Shaping on Multicast
A solution SHOULD support policing and shaping multicast traffic on a
per customer basis and on a per AC (Attachment Circuit) basis. This
is intended to prevent multicast traffic from exhausting resources
for unicast inside a common customer's 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
[RFC4665] MUST be supported.
For particular reference here, a multicast VPLS MUST allow a
situation in which a CE is dual-homed to two different SPs via
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diverse access networks -- one is supporting multicast VPLS but the
other is not supporting it, (because it is an 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
restoration time of a 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
(e.g., in case of no redundant paths), a solution MAY raise alarms to
provide outage notification to customers before such a hold timer
expires.
5.8. Minimum MTU
Multicast applications are often sensitive to packet fragmentation
and reassembly, 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 attempt to prevent frame reordering when delivering
customer multicast traffic. Likewise, for unicast and unknown
unicast traffic, it SHOULD attempt not to increase the likelihood of
reordering compared with existing VPLS solutions.
It is to be noted that delivery of out-of-order frames is not
avoidable in certain cases. Specifically if a solution adopts some
MDTunnels (see section 6.2.1) and dynamically selects them for
optimized delivery (e.g., switching from one aggregate tree to
another), end-to-end data delivery is prone to be out-of-order. This
fact can be considered a trade-off between bandwidth optimization and
network stability. Therefore, such a solution is expected to promote
awareness about this kind of drawback.
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5.10. Fate-Sharing between Unicast and Multicast
In native Ethernet, multicast and unicast connectivity are often
managed together. For instance, 802.1ag CFM Continuity Check message
is forwarded by multicast as a periodic 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.
A multicast VPLS solution may decouple the path that a customer's
unicast and multicast traffic follow through a SP's backbone, in
order to provide the most optimal path for multicast data traffic.
This may cause concern among some multicast VPLS customers who desire
that, during a failure in the SP's network, both unicast and
multicast traffic fail concurrently.
Therefore, there will be an additional requirement that makes both
unicast and multicast connectivity coupled. This means that if
either one of them have a failure, the other is also disabled. If
one of the services (either unicast or multicast) becomes
operational, the other is also activated simultaneously.
- 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.
Note that how serious this issue is depends on each customer's stance
in Ethernet operation. If all CEs are IP routers i.e., if VPLS is
provided for LAN routing application, the customer might not care
about it because both unicast and multicast connectivity is assured
in IP layer. If the CE routers are running an IGP (e.g., OSPF/IS-IS)
and a multicast routing protocol (e.g., PIM), then aliveness of both
the unicast and multicast paths will be detected by the CEs. This
does not guarantee that unicast and multicast traffic are to follow
the same path in the SP's backbone network, but does mitigate this
issue to some degree.
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 because customer traffic is encapsulated in PSN tunnels.
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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 related to scalability.
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 duplication performance) will become a
major goal. However, there is a trade-off between optimality and
state resource consumption.
In order to solve Issue A, a PE might have to maintain multicast
group information for CEs which was not kept in the existing VPLS
solutions. This will present scalability concerns about state
resources (memory, CPU, etc.) and their maintenance complexity.
In order to solve Issue B, PE and P routers might have to have
knowledge of additional membership information for remote PEs, and
possibly additional tree topology information, when they are using
point-to-multipoint techniques (PIM tree, P2MP-LSP, etc.).
Consequently, the scalability evaluation of multicast VPLS solutions
needs a careful trade-off analysis between bandwidth optimality and
state resource consumption.
6.1.2. Key Metrics for Scalability
(Note: This part has a number of similar characteristics to
requirements for Layer 3 Multicast VPN
[I-D.ietf-l3vpn-ppvpn-mcast-reqts].)
A multicast VPLS solution MUST be designed to scale well with an
increase in the number of any of the following metrics:
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- 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 a P device has to support.
The scalability characteristics SHOULD include:
- the processing resources required by the control plane in managing
PWs (neighborhood or session maintenance messages, keepalives,
timers, etc.)
- the processing resources required by the control plane in managing
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 of Layer-2/Layer-3 multicast information a P/PE router
consumes (e.g. traffic rate of join/leave, keepalives 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 impact
scalability
Another metric for scalability is operational complexity. Operations
will naturally become more complicated if the number of managed
objects (e.g., multicast groups) increases, or the topology changes
occur more frequently. A solution SHOULD note the factors which lead
to additional operational complexity.
6.2. Tunneling Requirements
6.2.1. Tunneling Technologies
A MDTunnel denotes a multicast distribution tunnel. This is a
generic term for tunneling where customer multicast traffic is
carried over a provider's network. In the 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
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there are many kinds of protocols for tunneling such as L2TP, IP,
(including multicast IP trees), MPLS (including P2MP-LSP
[I-D.ietf-mpls-rsvp-te-p2mp] and P2MP/MP2MP-LSP
[I-D.ietf-mpls-ldp-p2mp] ), etc.
Note that which variant, point-to-point, point-to-multipoint or
multipoint-to-multipoint, is used depends largely on the trade-offs
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 a SP, it is not acceptable to have fragmentation/
reassembly so often while packets are traversing a MDTunnel.
Therefore, a solution SHOULD support a method that provides the
minimum path MTU of the MDTunnel in order to accommodate the service
MTU.
6.3. Robustness
Multicast VPLS solutions SHOULD avoid single points of failures or
propose technical solutions that make it 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 considered a high priority.
Therefore, in addition to the L2VPN discovery requirements in
[RFC4665], 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. What information is provided dynamically is
solution specific.
6.5. Operation, Administration and Maintenance
6.5.1. Activation
The activation of multicast enhancement in a solution MUST be
possible:
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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
o with a customer IP multicast group and/or channel granularity
(when Layer-3 information is consulted)
Also it MAY be possible:
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 verifying 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/channel
- 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's
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 customer's multicast data packets.
Specifically, 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
able 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,
troubleshooting within the SP's network will tend to be difficult.
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6.5.3. Performance Management
Mechanisms to monitor 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
[I-D.ietf-l3vpn-ppvpn-mcast-reqts].)
A solution MUST provide SPs with access to:
- Multicast traffic statistics (total traffic forwarded, incoming,
outgoing, dropped, etc., by period of time)
A solution SHOULD provide access to:
- Information about a customer's multicast resource usage (the
amount of multicast state and throughput)
- Performance information related to multicast traffic usage, e.g.,
one-way delay, jitter, loss, delay variations (the difference in
one-way delay for a solitary multicast group/stream from a single,
source PE to multiple receiver PEs) etc.
- Alarms when limits are reached on such resources
- Statistics on decisions related to how client traffic is carried
on MDTunnels (e.g. "How much traffic was 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 anticipated that such tools are coordinated
with the testing mechanisms mentioned in 6.5.2.
In particular, such mechanisms SHOULD be able to detect a
multicast failure quickly, (on par with unicast cases). It
SHOULD also avoid situations where multicast traffic has been
in a failure state for a relatively long time while unicast
traffic remains operational. If such a situation were to
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occur, it would end up causing problems with customer
applications that depend on a combination of unicast and
multicast forwarding.
With multicast, there may be many receivers associated with a
particular mulitcast stream/group. As the number of receivers
increases, the number of places (typically nearest the
receivers) required to detect a fault will increase
proportionately. This raises concerns over the scalability of
fault detection in large multicast deployments. Consequently,
a fault detection solution SHOULD scale well; in particular, a
solution should consider key metrics for scalability as
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 downstream routers/receivers that might be able to
raise a notification. Hence notification messages MAY be
summarized or compressed for operators' ease of management.
o Fault isolation
A solution MUST provide diagnostic/troubleshooting tools for
multicast as well. Also it is anticipated that such tools are
coordinated with the testing mechanisms mentioned in 6.5.2.
In particular, a solution needs to correctly identify the area
inside a multicast group impacted by the failure. A solution
SHOULD be able to diagnose if an entire multicast group is
faulty or if 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 both multicast data
packets and multicast control packets.
Moreover, because multicast, broadcast, and unknown-unicast need more
resources than unicast, a SP network MUST have safeguards against
unwanted or malicious multicast traffic. This applies to both
multicast data packets and multicast control packets.
Specifically, a multicast VPLS solution SHOULD have mechanisms to
protect a SP network from:
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- invalid multicast MAC addresses (always)
- invalid multicast IP addresses (if they are used for forwarding)
- malformed Ethernet multicast control protocol frames (if they are
examined)
- malformed IP multicast control protocol packets (if they are
examined)
- high volumes of
* valid/invalid customer control packets
* valid/invalid customer data packets (broadcast/multicast/
unknown-unicast)
The following are a few additional guidelines.
A solution MAY allow some bounds on the quantity of state used by
a VPN to be imposed in order to prevent state resource exhaustion
(i.e., lack of memory, CPU etc.).
Also a solution 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 MAY also be configurable to each
such type of traffic individually, or to their combination in
order to prevent physical resource exhaustion (i.e., lack of
bandwidth and degradation of throughput).
Moreover, mechanisms to limit frequent changes of group membership
by customers MAY be supported. For example, if the core
distribution tunnel is tightly coupled to dynamic changes of
customer multicast domain, a kind of dampening function should be
possible.
6.7. Hierarchical VPLS support
A VPLS multicast solution SHOULD allow a hierarchical VPLS (H-VPLS)
[RFC4762] service model. In other words, a solution is expected to
operate 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 where
one VPLS interconnects other SPs' VPLS or 802.1D network islands.
For customer SP, their multicast forwarding can be optimized by
making use 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 [RFC4665].
In additions, there are security 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 multicast
VPLS. Such issues have been described in Section 5.5 and 6.6.
8. IANA Considerations
This document has no actions for IANA.
9. Acknowledgments
The authors thank the contributors of
[I-D.ietf-l3vpn-ppvpn-mcast-reqts] since the structure and content of
this document were, for some sections, largely inspired by
[I-D.ietf-l3vpn-ppvpn-mcast-reqts].
The authors also thank Yuichi Ikejiri, Jerry Ash, Bill Fenner, Vach
Kompella, Shane Amante, Ben Niven-Jenkins and Venu Hemige for their
valuable reviews and feedbacks.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4665] Augustyn, W. and Y. Serbest, "Service Requirements for
Layer 2 Provider-Provisioned Virtual Private Networks",
RFC 4665, September 2006.
10.2. 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.
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[802.1s] IEEE Std 802.1s-2002, "Virtual Bridged Local Area
Networks- Amendment 3: Multiple Spanning Trees", 2002.
[CGMP] Farinacci, D., Tweedly, A., and T. Speakman, "Cisco Group
Management Protocol (CGMP)",
ftp://ftpeng.cisco.com/ipmulticast/specs/cgmp.txt , 1996/
1997.
[I-D.ietf-l3vpn-ppvpn-mcast-reqts]
Morin, T., "Requirements for Multicast in L3 Provider-
Provisioned VPNs", draft-ietf-l3vpn-ppvpn-mcast-reqts-10
(work in progress), November 2006.
[I-D.ietf-mpls-ldp-p2mp]
Minei, I., "Label Distribution Protocol Extensions for
Point-to-Multipoint and Multipoint-to-Multipoint Label
Switched Paths", draft-ietf-mpls-ldp-p2mp-02 (work in
progress), October 2006.
[I-D.ietf-mpls-rsvp-te-p2mp]
Aggarwal, R., "Extensions to RSVP-TE for Point-to-
Multipoint TE LSPs", draft-ietf-mpls-rsvp-te-p2mp-07 (work
in progress), January 2007.
[I-D.ietf-pim-bidir]
Handley, M., "Bi-directional Protocol Independent
Multicast (BIDIR-PIM)", draft-ietf-pim-bidir-09 (work in
progress), February 2007.
[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.
[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.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[RFC3488] Wu, I. and T. Eckert, "Cisco Systems Router-port Group
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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.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, August 2006.
[RFC4664] Andersson, L. and E. Rosen, "Framework for Layer 2 Virtual
Private Networks (L2VPNs)", RFC 4664, September 2006.
[RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling",
RFC 4761, January 2007.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
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
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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.att.com
Thomas Morin
France Telecom R&D
2, avenue Pierre-Marzin
22307 Lannion Cedex
France
Email: thomas.morin@francetelecom.com
Luyuan Fang
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough, MA 01719
USA
Email: lufang@cisco.com
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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Kamite, et al. Expires September 6, 2007 [Page 29]
