Internet DRAFT - draft-zzhang-pim-non-source-routed-sr-mcast
draft-zzhang-pim-non-source-routed-sr-mcast
PIM Z. Zhang
Internet-Draft Juniper Networks
Intended status: Informational I. Wijnands
Expires: 5 September 2024 Arrcus
H. Bidgoli
Nokia
Y. Liu
China Mobile
4 March 2024
Non-source-routed Multicast in SR Networks
draft-zzhang-pim-non-source-routed-sr-mcast-00
Abstract
With Segment Routing (SR) architecture, a unicast flow can be source-
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. In the case of
multicast, traffic can be either source-routed or non-source-routed,
and this document discusses non-sourced-routed options for multicast
in an SR network with either MPLS or IPv6/SRv6 data plane.
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
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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 5 September 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Traditional Multicast Technologies . . . . . . . . . . . . . 3
3. Bit Index Explicit Replication . . . . . . . . . . . . . . . 3
4. Multicast Trees Set up by Other Means . . . . . . . . . . . . 4
5. SR Specific Solutions . . . . . . . . . . . . . . . . . . . . 5
6. IPv6 Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
10. Informative References . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
With Segment Routing (SR) architecture [RFC8402], a unicast flow can
be source-routed through an SR network following an explicit path
specified in the packet, without 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 non-source-routed multicast in
an SR network, including traditional multicast technologies, BIER,
and various SR specific solutions. The pros and cons of each
solution are listed for considerations by operators and vendors.
Source-routed multicast in an SRv6 network has also been discussed in
IETF [https://datatracker.ietf.org/doc/bofreq-liu-multicast-source-
routing-over-ipv6msr6/] and some work is ongoing. However, that is
outside the scope of this document.
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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
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 (referred to as SR policies), which could be MPLS/
SRv6-based.
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 protocol packet encoding)
as 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.
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Even though BIER is designed and developed independent of SR, it is a
perfect fit for an SR network because it shares with SR the number
one principle of "no per-flow state". A BIER domain can be used as
provider/underlay tunnels for MVPN/EVPN-BUM (just like 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:
* Several major ASIC makers have BIER support on their ASIC
offering.
* Several Major router vendors have BIER hardware capability across
their access/aggregation/edge/core platforms.
* Some major router vendors have had or will soon have interoperable
production support of BIER on certain platforms. Roadmaps for
BIER on more platforms also exist.
* There have been very good discussions and progress in BIER WG for
methods of brown field deployments (
[I-D.przygienda-bier-migration-options] [I-D.ietf-bier-tether]
[I-D.ietf-bier-multicast-as-a-service] [I-D.ietf-bier-php])
* 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/PCEP [I-D.ietf-bess-bgp-multicast]
[I-D.ietf-bess-bgp-multicast-controller]
[I-D.ietf-idr-sr-p2mp-policy] [I-D.ietf-pce-sr-p2mp-policy], as
described in [I-D.ietf-pim-sr-p2mp-policy].
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5. SR Specific Solutions
There are two multicast solutions that are specifically tied to SR
technologies - "SR-P2MP" [I-D.ietf-pim-sr-p2mp-policy] and "Spray".
1. SR-P2MP refers to tunnels setup not by mLDP or RSVP-TE. It could
be statically configured, or instantiated from a controller.
2. Spray is Ingress Replication with 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.
The above methods are really no different from existing technologies.
Spray is just IR with a different encapsulation (e.g. SRv6). While
SR-P2MP does not use existing tree signaling protocol, it still
requires per-tree forwarding state on the tree nodes (roots, leaves
and branching points), and it is identical to mLDP/RSVP P2MP tunnels
in the MPLS data plane on those tree nodes.
mLDP/PIM trees are typically hop-by-hop - the tree nodes are directly
connected, though tunnels can be used between mLDP, PIM and SR-P2MP
tree nodes to avoid tree states on the non-branching nodes between
tree nodes. This is important in some deployment scenarios (e.g.,
mobile user plane transport) where the replication points are
scattered with many non-replication points among them.
SR-P2MP can use both MPLS and SRv6 data plane. The next section
discusses some SRv6 aspects of SR-P2MP.
6. IPv6 Considerations
Some operators run IPv6/SRv6 networks without using MPLS at all. As
a result, Ingress Replication or SR-P2MP with MPLS data plane is not
an option and IPv6 based forwarding must be used. There are two
options in this case - traditional IPv6 multicast (signaled by either
PIM or BGP [I-D.ietf-bess-bgp-multicast]
[I-D.ietf-bess-bgp-multicast-controller]) and SRv6-P2MP (SR-P2MP with
SRv6 data plane).
SR-P2MP with MPLS data plane (SRmpls-P2MP) is identical to mLDP/RSVP-
P2MP in the data plane of the tree nodes, in that incoming multicast
traffic carries a tree-identifing label and is replicated with
outgoing label stacks. The label stack includes a label that
identifies the tree to the downstream node, and optional labels that
leads the traffic to the downstream node.
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SRv6-P2MP is very similar to SRmpls-P2MP in the data plane.
Multicast traffic has an IPv6 header and the destination address has
the LOC:FUNCT:ARGS encoding format for SRv6 [RFC8986]. The FUNCT
bits correspond to the tree-identifying label in SRmpls-P2MP, and the
LOC bits get the traffic from an upstream node to a downstream node
(which could be directly or indirectly connected).
SRv6-P2MP allows native tunneling of multicast traffic between
indirectly connected upstream and downstream nodes, but at the cost
of changing destination address at each tree node, which requires
either programmable ASIC or new generation of non-programmable ASIC.
It also requires a new control plane that involves controller-based
calculation and signaling of the tree state.
On the other hand, traditional IPv6 multicast is mature in both
control plane and data plane. PIM has been widely deployed for many
years without platform restrictions, and the PIM-SSM protocol is very
straightforward without the complexity of PIM-ASM.
Even if an operator wants to use controller based calculation and
signaling, or does not want to run PIM protocol, traditional
multicast can still be used with BGP/controller-based signaling as
specified in [I-D.ietf-bess-bgp-multicast] and
[I-D.ietf-bess-bgp-multicast-controller].
7. 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 when source-
routing is not used:
* 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 does).
* If efficient multicast replication is required, mLDP/RSVP-TE/PIM
can still be used for multicast purpose if that is acceptable.
* If the replication points are scattered with many non-replication
points between them, or if there is a strong desire to create
multicast forwarding state without relying on PIM/mLDP or RSVP-TE,
one can use static configuration (e.g., netconf based
provisioning) or controller-based tree calculation/signaling (SR-
P2MP, BGP-signaled IP multicast or mLDP).
* Use Ingress Replication with SR unicast tunnels.
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In case of MPLS based P2MP, a Binding SID could be optionally used to
direct traffic to the root of the tunnel.
8. Security Considerations
This informational document does not introduce new security risks.
9. Acknowledgements
The authors thank Eric Rosen, Tony Przygienda, and John Drake for
their reviews, comments and suggestions.
10. Informative References
[I-D.ietf-bess-bgp-multicast]
Zhang, Z. J., Giuliano, L., Patel, K., Wijnands, I.,
Mishra, M. P., and A. Gulko, "BGP Based Multicast", Work
in Progress, Internet-Draft, draft-ietf-bess-bgp-
multicast-07, 2 December 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-bess-
bgp-multicast-07>.
[I-D.ietf-bess-bgp-multicast-controller]
Zhang, Z. J., Raszuk, R., Pacella, D., and A. Gulko,
"Controller Based BGP Multicast Signaling", Work in
Progress, Internet-Draft, draft-ietf-bess-bgp-multicast-
controller-12, 30 December 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-bess-
bgp-multicast-controller-12>.
[I-D.ietf-bier-multicast-as-a-service]
Zhang, Z. J., Rosen, E. C., Awduche, D. O., Shepherd, G.,
Zhang, Z., and G. S. Mishra, "Multicast/BIER As A
Service", Work in Progress, Internet-Draft, draft-ietf-
bier-multicast-as-a-service-03, 6 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-bier-
multicast-as-a-service-03>.
[I-D.ietf-bier-php]
Zhang, Z. J., "BIER Penultimate Hop Popping", Work in
Progress, Internet-Draft, draft-ietf-bier-php-11, 6
February 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-bier-php-11>.
[I-D.ietf-bier-pim-signaling]
Bidgoli, H., Xu, F., Kotalwar, J., Wijnands, I., Mishra,
M. P., and Z. J. Zhang, "PIM Signaling Through BIER Core",
Work in Progress, Internet-Draft, draft-ietf-bier-pim-
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signaling-12, 25 July 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-bier-
pim-signaling-12>.
[I-D.ietf-bier-tether]
Zhang, Z. J., Warnke, N., Wijnands, I., and D. O. Awduche,
"Tethering A BIER Router To A BIER incapable Router", Work
in Progress, Internet-Draft, draft-ietf-bier-tether-05, 16
February 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-bier-tether-05>.
[I-D.ietf-idr-sr-p2mp-policy]
Bidgoli, H., Voyer, D., Stone, A., Parekh, R., Krier, S.,
and S. Agrawal, "Advertising p2mp policies in BGP", Work
in Progress, Internet-Draft, draft-ietf-idr-sr-p2mp-
policy-00, 27 May 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
p2mp-policy-00>.
[I-D.ietf-pce-sr-p2mp-policy]
Bidgoli, H., Voyer, D., Rajarathinam, S., Budhiraja, A.,
Parekh, R., and S. Sivabalan, "PCEP extensions for p2mp sr
policy", Work in Progress, Internet-Draft, draft-ietf-pce-
sr-p2mp-policy-05, 3 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-sr-
p2mp-policy-05>.
[I-D.ietf-pim-sr-p2mp-policy]
Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
J. Zhang, "Segment Routing Point-to-Multipoint Policy",
Work in Progress, Internet-Draft, draft-ietf-pim-sr-p2mp-
policy-07, 11 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-pim-sr-
p2mp-policy-07>.
[I-D.przygienda-bier-migration-options]
Przygienda, T. and Z. J. Zhang, "BIER Migration
Frameworks", Work in Progress, Internet-Draft, draft-
przygienda-bier-migration-options-00, 22 June 2018,
<https://datatracker.ietf.org/doc/html/draft-przygienda-
bier-migration-options-00>.
[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>.
[RFC7473] Raza, K. and S. Boutros, "Controlling State Advertisements
of Non-negotiated LDP Applications", RFC 7473,
DOI 10.17487/RFC7473, March 2015,
<https://www.rfc-editor.org/info/rfc7473>.
[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>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
Authors' Addresses
Zhaohui Zhang
Juniper Networks
Email: zzhang@juniper.net
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IJsbrand Wijnands
Arrcus
Email: ice@braindump.be
Hooman Bidgoli
Nokia
Email: hooman.bidgoli@nokia.com
Yisong Liu
China Mobile
Email: liuyisong@chinamobile.com
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