Internet DRAFT - draft-zzhang-pim-sr-multicast
draft-zzhang-pim-sr-multicast
PIM Z. Zhang
Internet-Draft Juniper Networks
Intended status: Informational IJ. Wijnands
Expires: February 16, 2019 Cisco Systems
A. Dolganow
Nokia
L. Geng
China Mobile
August 15, 2018
Multicast in SR Networks
draft-zzhang-pim-sr-multicast-00
Abstract
With Segment Routing (SR) architecture [rfc8402], a unicast flow can
be routed through an SR network following an explicit path specified
in the packet, w/o the need for per-flow state in the network. As a
result, the otherwise needed protocols to signal the per-flow unicast
state can also be removed from the network. This document discusses
options for multicast in an SR network.
Status of This Memo
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This Internet-Draft will expire on February 16, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Traditional Multicast Technologies . . . . . . . . . . . . . 2
3. Bit Index Explicit Replication . . . . . . . . . . . . . . . 3
4. Multicast Trees Set up by Other Means . . . . . . . . . . . . 4
5. SR Specific Solutions . . . . . . . . . . . . . . . . . . . . 4
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
9. Informative References . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
With Segment Routing (SR) architecture [RFC8402], a unicast flow can
be routed through an SR network following an explicit path specified
in the packet, w/o the need for per-flow state in the network. As a
result, the otherwise needed protocols to signal the per-flow unicast
state can also be removed from the network.
This document summarizes options for multicast in an SR network,
including traditional multicast technologies, BIER, and various SR
specific proposals. The pros and cons of each solution are listed
for considerations by operators and vendors.
2. Traditional Multicast Technologies
Traditional multicast technologies include PIM [RFC7761], RSVP-TE
P2MP [RFC4875], and mLDP [RFC6388]. They all require per-tree state
on nodes on the tree, and the corresponding protocols to signal and
maintain the state. An incoming packet's IP header or MPLS label is
looked up, and the packet is forwarded according to the matched
state.
While SR allows simplification of state, protocols and centralized
SDN-control, the traditional methods of delivering multicast traffic
run contrary to those SR goals.
An alternative is Ingress Replication (IR) - an upstream node of a
multicast packet tunnels individual copies to some downstream nodes
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across some intermediate nodes. In an SR network, the unicast
tunnels used for IR could be traditional IP/MPLS tunnels or could be
SR tunnels.
While with IR there is no per-tree state on those intermediate nodes,
multiple copies of the same packet may be sent over the same link.
If efficient replication is required, PIM/mLDP/RSVP-TE P2MP can still
be used for multicast even in an SR network. Deploying SR in the
network does not mandate changing the solution that is in place for
Multicast.
While mLDP uses the same LDP Session (and packet encoding) as is used
by unicast and there is code sharing between LDP and mLDP with
regards to neighbor discovery, session setup and Label allocation
management, the protocol logic for setting up mLDP tunnel is
completely different. It is understood that there is a desire to
remove LDP from the network when SR is deployed, but there is really
no problem to continue to run mLDP for Multicast. RFC7473 documents
a solution where LDP can be run in mLDP-Only mode by simply not
exchanging any Unicast bindings.
Similarly, RSVP-TE P2MP tunnel setup is orthogonal from P2P tunnel
setup as well. It can be used for multicast even in an SR network if
bandwidth reservation and/or per-tunnel explicit path are required.
3. Bit Index Explicit Replication
Bit Index Explicit Replication (BIER) [RFC8279] is a new multicast
technology that achieves efficient replication as with PIM tree or
mLDP/RSVP-TE P2MP tunnels but without requiring per-tree/tunnel state
in the transit nodes as with IR.
BIER is a perfect fit for an SR network. A BIER domain can be used
as provider/underlay tunnels for MVPN/EVPN-BUM (just as traditional
PIM tree or mLDP/RSVP P2MP tunnels), or can be part of end-to-end PIM
trees [I-D.ietf-bier-pim-signaling] (similiar to mLDP inband
signaling [RFC6826]).
However, BIER uses a new forwarding plane that cannot be implemented
on many deployed routers. On the other hand, the entry point barrier
is decreasing and BIER is becoming more realistic with the following
developments:
o Several major router vendors have wide deployment of platforms
capable of BIER (with a software upgrade)
o Several ASIC vendors have BIER support on their latest/upcoming
ASIC offering
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o There have been very good discussions in BIER WG for methods of
brown field deployments ( [I-D.przygienda-bier-migration-options]
[I-D.zzhang-bier-php])
o More and more customers now have concrete plans and requirements
for BIER deployment in their networks
4. Multicast Trees Set up by Other Means
Some operators may accept that they need to maintain per-tree state
in their SR network for efficient multicast, and their applications
do not require too much per-tree multicast state. However, since
unicast no longer needs LDP or RSVP-TE protocol in an SR network,
they really want to remove PIM/LDP/RSVP-TE protocol from their
networks, and make use of controllers. In that case, the multicast
trees could be set up statically from management plane (e.g.,
netconf), or dynamically via BGP [I-D.zzhang-bess-bgp-multicast].
[I-D.zzhang-bess-bgp-multicast-controller].
5. SR Specific Solutions
There are two multicast solutions that are specifically tied to SR
technologies - "TreeSID" and "Spray"
[I-D.voyer-spring-sr-p2mp-policy]. There is also a proposal
[I-D.allan-pim-sr-mpls-multicast-framework] that uses calculated
multicast trees based on flooded membership information.
1. TreeSID essentially refers to P2MP tunnels setup not by mLDP or
RSVP-TE. It could be statically configured, or instantiated from
a controller.
2. Spray is Ingress Replication. In particular, in case of SRv6,
instead of regular IP/MPLS tunneling, an SRv6 header encodes the
"tunnel end" and the original destination multicast address.
When the packet arrives at the tunnel end, the original multicast
address is restored from the SRv6 header.
3. In [I-D.allan-pim-sr-mpls-multicast-framework], multicast
membership information is flooded, and each node will calculate
if it plays a role (as a root, leaf, or a branching point) for
each multicast tree. If yes, corresponding forwarding state is
installed to forward incoming multicast traffic accordingly.
The above three methods are really no different from existing
technologies. 1 and 3 do not use existing tree signaling protocol but
still require per-tree forwarding state on roots, leaves and
branching points (for comparison, existing technologies does have
per-tree state on all nodes on a tree - including those non-branching
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points). 3 is similar to MOSPF [RFC 1584] and requires flooded
receiver information throughout the domain. 2 is just IR with a
different encapsulation (e.g. SRv6).
6. Summary
There is no one-for-all option when it comes to multicast in an SR
network. Depending on specific deployment scenarios and
requirements, the following could be considered in order:
o If most of nodes can support it, BIER is the best choice, because
it removes state from the network and simplifies the control-plane
(like SR).
o If efficient multicast replication is required, then run mLDP/
RSVP-TE/PIM for multicast purpose if that is acceptable.
o If there is a strong desire to create multicast forwarding state
without relying on mLDP and/or RSVP-TE, one can use static
configuration or controller signaling (e.g. SR specific TreeSID,
generic BGP based signaling or netconf based provisioning).
o Use Ingress Replication with SR unicast tunnels. This includes
Spray Multicast if SRv6 is desired.
In case of MPLS based P2MP, a Binding SID could be optionally used to
direct traffic to the root of the tunnel.
Notice that there are already many protocols defined (PIM, mLDP,
RSVP-TE, BGP, MOSPF) that can be used to create multicast forwarding
state. When designing a new protocol to do the same, we need to be
sure the benefits are significant enough and out-weight the cost of
developing it.
7. Security Considerations
This document does not seem to introduce new security risks.
8. Acknowledgements
The authors thank Eric Rosen, Tony Przygienda, and John Drake for
their reviews, comments and suggestions.
9. Informative References
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[I-D.allan-pim-sr-mpls-multicast-framework]
Allan, D., Tantsura, J., and I. Duncan, "A Framework for
Computed Multicast Applied to SR-MPLS", draft-allan-pim-
sr-mpls-multicast-framework-00 (work in progress), June
2018.
[I-D.ietf-bier-pim-signaling]
Bidgoli, H., Dolganow, A., Kotalwar, J., Xu, F., mishra,
m., and Z. Zhang, "PIM Signaling Through BIER Core",
draft-ietf-bier-pim-signaling-03 (work in progress), June
2018.
[I-D.przygienda-bier-migration-options]
Przygienda, T. and Z. Zhang, "BIER Migration Frameworks",
draft-przygienda-bier-migration-options-00 (work in
progress), June 2018.
[I-D.voyer-spring-sr-p2mp-policy]
daniel.voyer@bell.ca, d., Hassen, C., Gillis, K.,
Filsfils, C., and A. Venkateswaran, "SR Replication Policy
for P2MP Service Delivery", draft-voyer-spring-sr-p2mp-
policy-00 (work in progress), June 2018.
[I-D.zzhang-bess-bgp-multicast]
Zhang, Z., Patel, K., Wijnands, I., and a.
arkadiy.gulko@thomsonreuters.com, "BGP Based Multicast",
draft-zzhang-bess-bgp-multicast-01 (work in progress),
March 2017.
[I-D.zzhang-bess-bgp-multicast-controller]
Zhang, Z., Raszuk, R., Pacella, D., and A. Gulko,
"Controller Based BGP Multicast Signaling", draft-zzhang-
bess-bgp-multicast-controller-00 (work in progress),
September 2017.
[I-D.zzhang-bier-php]
Zhang, Z., "BIER Penultimate Hop Popping", draft-zzhang-
bier-php-01 (work in progress), August 2018.
[RFC4875] Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
Yasukawa, Ed., "Extensions to Resource Reservation
Protocol - Traffic Engineering (RSVP-TE) for Point-to-
Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
DOI 10.17487/RFC4875, May 2007,
<https://www.rfc-editor.org/info/rfc4875>.
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[RFC6388] Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
Thomas, "Label Distribution Protocol Extensions for Point-
to-Multipoint and Multipoint-to-Multipoint Label Switched
Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011,
<https://www.rfc-editor.org/info/rfc6388>.
[RFC6826] Wijnands, IJ., Ed., Eckert, T., Leymann, N., and M.
Napierala, "Multipoint LDP In-Band Signaling for Point-to-
Multipoint and Multipoint-to-Multipoint Label Switched
Paths", RFC 6826, DOI 10.17487/RFC6826, January 2013,
<https://www.rfc-editor.org/info/rfc6826>.
[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <https://www.rfc-editor.org/info/rfc7761>.
[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
Authors' Addresses
Zhaohui Zhang
Juniper Networks
EMail: zzhang@juniper.net
IJsbrand Wijnands
Cisco Systems
EMail: ice@cisco.com
Andrew Dolganow
Nokia
EMail: andrew.dolganow@nokia.com
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Liang Geng
China Mobile
EMail: gengliang@chinamobile.com
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