BIER G. Shepherd
Internet-Draft Cisco
Intended status: Informational A. Dolganow
Expires: October 10, 2016 Alcatel-Lucent
A. Gulko
Thomson Reuters
April 4, 2016
Bit Indexed Explicit Replication (BIER) Problem Statement
draft-ietf-bier-problem-statement-00
Abstract
There is a need to simplify network operations for multicast
services. Current solutions require a tree-building control plane to
build and maintain end-to-end tree state per flow, impacting router
state capacity and network convergence times. Multi-point tree
building protocols are often considered complex to deploy and debug
and may include mechanics from legacy use-cases and/or assumptions
which no longer apply to the current use-cases. When multicast
services are transiting a provider network through an overlay, the
core network has a choice to either aggregate customer state into a
minimum set of core states resulting in flooding traffic to unwanted
network end-points, or to map per-customer, per-flow tree state
directly into the provider core state amplifying the network-wide
state problem.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 10, 2016.
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Deering's Multicast Model . . . . . . . . . . . . . . . . . . 5
4. Network Based Source Discovery . . . . . . . . . . . . . . . 7
5. Receiver Driven State . . . . . . . . . . . . . . . . . . . . 8
6. Multicast Virtual Private Networks . . . . . . . . . . . . . 9
7. Overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Security Considerations . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
There is a need to simplify network operations for multicast
services. Current solutions require a tree-building control plane,
to build and maintain end-to-end tree state per flow, impacting
router state capacity and network convergence times. Multi-point
tree building protocols are often considered complex to deploy and
debug and include mechanics from legacy use-cases and/or assumptions
which may no longer apply to the current use-case. When multicast
services are transiting a provider network through an overlay, the
core network has a choice to either aggregate customer state into a
minimum set of core states resulting in flooding traffic to unwanted
network end-points, or to map per-customer, per-flow tree state
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directly into the provider core state amplifying the network-wide
state problem.
This document attempts to discuss the uses, benefits and challenges
of the current multicast solutions and to put them in an historical
context to better understand why we are where we are today, and to
provide a framework for discussion around new solutions that may
address our current requirements and challenges.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Objectives
IP Multicast services have been widely adopted in networks where the
benefits of efficient, concurrent delivery of content to a
sufficiently large set of receivers outweighs the complexity and
challenges of deploying and managing the current set of multicast
protocols. These deployments are primarily dedicated multicast
islands with very little cross-domain inter-networking, and fall
short of the early dreams of a multicast enabled Internet.
Multicast began with a large set of requirements shoehorned into a
single, complex protocol. Over time, multicast protocols essentially
devolved into a set of more simple components to overcome the
original complexity, and to address a growing set of use cases. Many
of the early complexity can be avoided today by correctly selecting
your service model and protocols. But the standard set of protocols
available can still be considered overloaded for various reasons.
The current problems associated with the today's multicast solutions
can be stated as follows:
- Current multicast methods all require explicit tree building
protocols, thereby incurring a lot of state in the transit nodes.
- Receiver driven tree state uses Reverse Path Forwarding (RPF) to
build the trees toward the root which often results in multicast
forwarding following different paths than unicast forwarding
between the same two endpoints.
- Multicast convergence times are negatively impacted by tree
state. Any network transition requires unicast to first converge.
Once unicast has converged multicast must then recalculate RPF for
every tree and rebuild the trees by sending join messages toward
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the new RPF neighbor per tree. Joins toward a common RPF neighbor
can be aggregated but only up to the link MTU. In large multicast
deployments this can result in multicast convergence times of up
to a minute or more. In extreme cases the active state may time
out before all the new joins are sent and received resulting in
multicast to permanently fail after a network failure event even
though there is a restored path. This has put an upward bound on
the amount of state a multicast network can support.
- Current multicast methods, if they are to provide optimal
delivery of multicast packets, require one explicitly built tree
per multicast flow; there is no way to aggregate flows (having one
state for multiple flows) without sacrificing optimal delivery.
In the case of Multicast Virtual Private Network (MVPN)
deployments, the operator is forced to choose between unwanted
flooded traffic across an aggregate state entry and exposing
customer state in the core.
- Some multicast solutions include data-driven events. This has
required specialized capabilities to be integrated into routing
equipment to protect the control plane from the multicast data
plane increasing the cost of multicast support in routing
equipment.
- Maintaining and troubleshooting multicast networks can be very
difficult. The available solutions are so different than unicast,
often revealing unique corner cases that specialized training and
skills, and frequently dedicated staff are required just to
operate multicast services on a network.
- Current Multicast Virtual Private Networks [RFC6513](MVPN)
introduced Border Gateway Protocol[RFC4271](BGP) routes for
neighbour discovery and Protocol Independent Multicast[RFC4601]
(PIM) Join/Prune propagation. In some deployments when many
Multicast MVPNs with many Provider Edge (PE) routers exist in a
network and at least some of those MVPNs have a large number of
customer-multicast flows, the resulting tax on BGP may be deemed
undesired as millions of BGP routes can easily result from
multicast deployments. Therefore a solution that allows large
MVPN scale with large number of edge PEs and c-multicast flows per
MVPN is desired.
- With the introductions of Segment Routing, some networks may
elect to remove the Multiprotocol Lable Switching[RFC3031](MPLS)
control plane and rely on Interior Gateway Protocol-only or
Software Defined Networking-based Segment Routing. In such
networks the alternative to existing mechanisms is needed for
multicast. Removing the MPLS control plane for unicast makes
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little sense unless the multicast control plane also gets
simplified.
- The benefits of multi-point services are well understood, but
the challenges with the current solutions often result in a failed
cost/benefit analysis. Today only those networks with an
overwhelming business need have successful multicast deployments,
and the rest of the community have come to think of multicast as a
failed technology.
How did we get here? What follows is a semi-chronological tour
through the devolution of multicast protocols, solutions, and use-
cases, describing why earlier complexities and challenges existed,
and how they were overcome. This may help frame future work to
overcome our current challenges.
3. Deering's Multicast Model
The original Multicast Extensions to the Internet Protocol [RFC0966]
and Host Extensions for IP Multicasting [RFC1112]were envisioned by
Stephen Deering as part of his graduate work at Stanford University.
The need for a multi-point service model was motivated by the advent
and deployment of layer3 network topologies breaking existing layer2
applications. The need arose to create an underlay service with the
characteristics of a broadcast domain to allow these layer2
applications to continue to function without modification across a
layer3 infrastructure.
Though the community quickly saw the value and envisioned many other
uses for a multi-point service model, a broadcast domain remained the
target model for the solution and the list of requirements focused
around those of a broadcast domain. For simplicity the rules of this
underlay broadcast domain can be summed up as follows: anyone can
send packets into the domain; all members will receive all packets
sent into the domain. In order for these layer2 applications to
function across this broadcast domain overlay, all of the functions
to provide this service were loaded onto network layer.
This new multi-point model was called Multicast. The first multicast
solution adopted by the IETF was Distance Vector Multicast Routing
Protocol [RFC1075](DVMRP). As the name implies, DVMRP uses a
distance-vector routing algorithm derived from Routing Information
Protocol [RFC1058](RIP) in combination with the Truncated Reverse
Path Broadcasting (TRPB) algorithm to build and maintain tree state
and forward multicast packets along these distribution trees. The
Internet Assigned Numbers Authority (IANA) was asked to reserve a
portion of the global IPv4 address space for multicast destination
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addresses required by this model, and in response 224/4 was allocated
as the Class D address space for IP Multicast group addresses.
DVMRP has no concept of a "join" message. All new source packets for
any given group were simply flooded downstream--essentially
broadcasted--following the DVMRP topology. Each leaf of the tree was
responsible for sending Non Membership Reports (NMR--prunes) toward
the source if there were no downstream receivers for the group. This
mechanism came to be known as flood-and-prune, and is a very
primitive form of network-based source discovery that all the
contemporary applications came to depend on. These contemporary
applications were inherently many-to-many either by the nature of the
data distribution model, or at the least depended on the many-to-many
nature of the network-based-source discovery mechanism.
DVMRP also incorporated the IETF's first specification of an
encapsulated overlay. It was clear that this new model would not be
supported by every node in the path, and an encapsulation allowed
early adopters to build a global multi-point, or multicast capable
topology as an overlay.
For clarity of discussion, the functions of the Deering model can be
described as:
- Tree building and maintenance
- Network-based source discovery
- Source route information
- Overlay mechanism - tunneling
DVMRP was considered over-loaded in that it carries network source
routing information within the protocol in parallel to any existing
Interior Gateway Protocol (IGP) generated local routing table. The
next generation goal was to focus on the multi-point services needed
for the model but to use the local, native routing table as needed
for Reverse Path Check (RPF). From this came the advent of Protocol
Independent Multicast Sparse Mode [RFC4601](PIM-SM) and Protocol
Independent Multicast Dense Mode [RFC3973](PIM-DM). PIM removed any
embedded source routing function from the protocol, and instead
relied on the exiting routing table as generated from the deployed
IGP. PIM also removed any overlay functionality, but retained
network-based source discovery as a fundamental part of the protocol.
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4. Network Based Source Discovery
The Deering model introduced the concept of a Group address (G)
representing a single broadcast domain. Any source is allowed to
send to the group address and the multicast routing infrastructure
will build tree state from every source to all interested receivers.
All group members only need to signal their G membership to the
network and the network will ensure that all source traffic sending
to that same group address will arrive at all group members. The
network-based source discovery operation providing these functions
was intended to provide operational constancy with a layer2 broadcast
domain, but comes at significant cost.
Allowing any source to send to a group is an obvious security
vulnerability. Many implementations today provide various layers of
access control both at the edges and core of the network just to
overcome the security concerns for the basic operation of the
multicast network.
Network-based source discovery methods can be grouped into two types;
flood and prune (DVMRP, PIM-DM), or explicit join (PIM-SM). Both
methods depend on the arrival of data to trigger complex network
functions to build and maintain the per-source distribution of data
for every group. Multicast is often considered complex, fragile, and
difficult to troubleshoot, but it is most often the network-based
source discovery functions that are the cause of this reputation.
The majority of the use-cases for multicast today are for content
with well-know sources. The development of Internet Group Membership
Protocol [RFC3376] (IGMPv3) provided a mechanism for group members to
signal interest in a source and a group, eliminating the need for
network-based source discovery, and facilitating the advent of Source
Specific Multicast [RFC4607] (SSM). Many operators still ask how
potential SSM group members learn about the sources. The answer is
simply to use the same mechanism in which they learned about the
group - out-of-band. Source (and group) discovery mechanisms are
better served at the application layer for most use-cases. With SSM
multicast content can be forwarded and constrained to a single
source-rooted tree, or (S,G) channel which has several key benefits:
- Simplified configuration and operation
- Elimination of rouge sources 'stealing' receivers
- Elimination of rouge sources consuming network resources
- Elimination of group address resource restrictions
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5. Receiver Driven State
Today's multicast solutions are primarily receiver driven. This is a
logical approach in that it is the receiver that decides if and when
to join or leave a group or channel. Receiver driven distribution
trees built hop-by-hop are an efficient way to dynamically build and
scale very large membership fanout. It can be argued that a receiver
driven tree's radius can scale infinitely without impact to any
upstream segment or node for that tree. But it does then require
forwarding state for each tree, or pre-flow state.
The joins propagate upstream from the receiver toward the source or
root of the tree, following the unicast routing table. But this
reverse path may differ from the optimal unicast forwarding path from
the source to the receiver. The result is multicast traffic
potentially taking a different forwarding path than unicast traffic
between the same to network endpoints. This can often complicate
network and traffic engineering.
Each of the existing multicast solutions today, native or overlay,
builds and maintains forwarding state per flow, or aggregates some
flows into a subset of flow-states. On the surface this may look
like an unbounded problem, but in actuality the flow state is only
present along the branches of the tree, and no one router needs to
maintain global tree state. Router state capacity is not infinite,
and this coupling of receiver actions to network state is a potential
Denial of Service (DoS) vector. Most implementations today have
provided filtering and state-limiting capabilities to secure the
multicast infrastructure from this vulnerability.
Increasing multicast forwarding state can also negatively impact
network convergence performance. Unicast is only concerned with
topology, and any topology changes can converge in a relatively
bounded amount of time. The same topology change requires the
multicast protocol to rebuild the forwarding state for every active
flow. The resulting multicast convergence times are directly
dependent on the amount of flow state affected by the convergence
event. In extreme cases, the sending, receiving, and processing of
the join state for all active flows can exceed the flow state timers
resulting in a race condition in which convergence never occurs.
Today's implementations have had to incorporate various proprietary
solutions to improve network convergence times in large flow-state
multicast deployments.
The pros and cons of receiver driven state are as follows:
Pros:
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Infinitely scales distribution radius
Aligns with receiver driven join model
Cons:
Potential state DoS vector
Host driven network events
Unbounded per-flow state
Unicast/Multicast traffic divergence
Non-deterministic join latency
Convergence times increasing with flow-state
6. Multicast Virtual Private Networks
Multicast Virtual Private Networks [RFC6513](MVPN) are solutions
which allow a core network to transit edge network multicast flows
over a core transit network to and from only those MVPN member nodes,
without exposing the edge network addressing into the core network
forwarding state. The solutions attempt to minimize core state by
aggregating trees per-VRF/PE. But this aggregation has the side
affect of sending all multicast traffic from that VRF/PE to all other
VRF/PE members, whether or not they have down stream flow state.
Various optimizations are available to selectively de-aggregate flow
state to better constrain the traffic distribution to only those VRF/
PEs with active state. This becomes a trade off between unwanted
traffic and an increase in core flow state. These solutions are
often data driven resulting in core router state being triggered by
date and receiver events.
In addition to a potential BGP route explosion due to an MVPN
deployment scale as discussed in section 2, another issue with MVPN
relates to architectures used when MVPN deployments require both
video-distribution-like model, well served by point-to-multipoint
(P2MP) connectivity, and many-to-many model requiring Multipoint-to-
multipoint (MP2MP) connectivity. Today, if both models are deployed
in a single network, either MP2MP or a mesh of P2MP trees needs to be
established, or dual P2MP/MP2MP mLDP architecture may be used, or
MP2MP mLDP can be used for both P2MP and MP2MP connectivity. None of
those models is optimal as each requires a trade-off between
supported protocols, optimal delivery, and operational complexity.
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7. Overlay
Deering had the correct insight to assume not every node in a network
would be capable of natively transiting multicast flows. The
migration to PIM was an attempt to move to a completely native model,
which was the right direction. But in this move it also abandoned
any other solution for incorporating an underlay into the topology
for those portions of the network which for whatever reason do not
support native multicast. Early deployments of PIM often
incorporated static Generic Routing Encapsulation [RFC2784](GRE)
tunnels between PIM domains in an attempt to create an inter domain
multicast deployment.
Static tunneling has it's use cases and benefits, but it is not the
ideal tool to dynamically stitch together a large and topologically
diverse receiver population. A receiver driven distribution model
would be better served with a receiver driving overlay mechanism.
This would indicate that when overlay was removed from the tree
building protocol it should have migrated to IGMPv3 and Multicast
Listener Discovery [RFC3810](MLDv2), the membership protocol, but it
was seen as a necessary requirement at that time. To fill this
requirement today Automatic Multicast Tunnels (AMT) is being
progressed as the overlay standard for bridging multicast interested
receivers over unicast only intermediate networks.
8. Summary
Multicast began with a heavily overloaded protocol DVMRP, and has
evolved over time by removing functionality from this all-in-one
solution, and off-loading certain function to either more specialized
protocols or existing protocols and functions. Multicast has what
may be the unique distinction of starting very complex, but evolving
through more simple stages along the way. It may be time to consider
the next step in the evolution toward simplicity.
Today we depend on receiver driven joins propagating end-to-end from
receivers toward sources, and maintaining per-flow state in every
node along the path. This state crosses administrative domains.
Unicast has a simple model where local specificity stays local and
does not directly impact the global table. Multicast state has no
administrative boundaries today. It may be beneficial to consider
the autonomy of networks in the path, and their specific topology and
requirements. PIM successfully utilizes the available routing table
for RPF checks and joins. This route table may also be considered as
a source of topology information for a set of receiver nodes within a
given network.
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9. IANA Considerations
This memo includes no request to IANA.
All drafts are required to have an IANA considerations section (see
Guidelines for Writing an IANA Considerations Section in RFCs
[RFC5226] for a guide). If the draft does not require IANA to do
anything, the section contains an explicit statement that this is the
case (as above). If there are no requirements for IANA, the section
will be removed during conversion into an RFC by the RFC Editor.
10. Security Considerations
All drafts are required to have a security considerations section.
See RFC 3552 [RFC3552] for a guide.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[RFC0966] Deering, S. and D. Cheriton, "Host groups: A multicast
extension to the Internet Protocol", RFC 966, December
1985.
[RFC1058] Hedrick, C., "Routing Information Protocol", RFC 1058,
June 1988.
[RFC1075] Waitzman, D., Partridge, C., and S. Deering, "Distance
Vector Multicast Routing Protocol", RFC 1075, November
1988.
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
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[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, July
2003.
[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.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6513] Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP
VPNs", RFC 6513, February 2012.
Appendix A. Additional Stuff
This becomes an Appendix.
Authors' Addresses
Greg Shepherd (editor)
Cisco
170 W. Tasman Dr.
San Jose
US
Email: gjshep@gmail.com
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Andrew Dolganow (editor)
Alcatel-Lucent
600 March Rd.
Ottawa, Ontario K2K 2E6
Canada
Email: andrew.dolganow@alcatel-lucent.com
Arkadiy Gulko (editor)
Thomson Reuters
Email: arkadiy.gulko@thomsonreuters.com
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