PIM Working Group Yisong Liu
Internet Draft China Mobile
Intended status: Standards Track M. McBride
Expires: August 21, 2021 Futurewei
Z. Zhang
ZTE
J. Xie
Huawei
Feb 21, 2021
Multicast-only Fast Reroute Based on Topology Independent Loop-free
Alternate Fast Reroute
draft-liu-pim-mofrr-tilfa-03
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Abstract
Multicast-only Fast Reroute (MoFRR) has been defined in [RFC7431],
but the selection of the secondary multicast next hop only according
to the loop-free alternate fast reroute, which has restrictions in
multicast deployments. This document describes a mechanism for
Multicast-only Fast Reroute by using Topology Independent Loop-free
Alternate fast reroute, which is independent of network topology and
can achieve covering more network environments.
Table of Contents
1. Introduction ................................................ 2
1.1. Requirements Language .................................. 3
1.2. Terminology ............................................ 3
2. Problem Statement ........................................... 3
3. Solution .................................................... 4
3.1. Secondary UMH Selection ................................ 5
3.2. Extension Protocol Fields Conflict ..................... 5
4. Packet Format ............................................... 6
5. IANA Considerations ......................................... 7
6. Security Consideration ..................................... 7
7. References .................................................. 7
7.1. Normative References ................................... 7
7.2. Informative References ................................. 8
8. Acknowledgments ............................................. 8
Authors' Addresses ............................................. 9
1. Introduction
As the deployment of video services, operators are paying more and
more attention to solutions that minimize the service disruption due
to faults in the IP network carrying the packets for these services.
Multicast-only Fast Reroute (MoFRR) has been defined in [RFC7431],
which can minimize multicast packet loss in a network when node or
link failures occur by making simple enhancements to multicast
routing protocols such as Protocol Independent Multicast (PIM). But
the selection of the secondary multicast next hop only according to
the loop-free alternate fast reroute in [RFC7431], and there are
limitations in multicast deployments for this mechanism. This
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document describes a new mechanism for Multicast-only Fast Reroute
using Topology Independent Loop-free Alternate (TILFA) fast reroute,
which is independent of network topology and can achieve covering
more network environments.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
This document use the terms defined in [RFC7431], and also use the
concepts defined in [RFC7490]. The specific content of each term is
not described in this document.
2. Problem Statement
In [RFC7431] section 3, the secondary Upstream Multicast Hop (UMH)
of PIM for MoFRR is a loop-free alternate (LFA). However, the
traditional LFA mechanism needs to satisfy at least one neighbor
whose next hop to the destination node is an acyclic next hop,
existing limitations in network deployments, and can only cover part
of the network topology environments. In some network topology, the
corresponding secondary UMH cannot be calculated, so PIM cannot
establish a standby multicast tree and cannot implement MoFRR
protection. Therefore, the current MoFRR of PIM is only available in
the network topology applicable to LFA.
The remote loop-free alternate (RLFA) defined in [RFC7490] is
extended from the LFA and can cover more network deployment
scenarios through the tunnel as an alternate path. The RLFA
mechanism needs to satisfy at least one node assumed to be N in the
network that the fault node is neither on the path from the source
node to the N node, nor on the path from the N node to the
destination node. RLFA only has enhancement compared to LFA but
still has limitations in network deployments.
[I-D.ietf-rtgwg-segment-routing-ti-lfa] defined a unicast FRR
solution based on the TILFA mechanism. The TILFA mechanism can
express the backup path with an explicit path, and has no constraint
on the topology, providing a more reliable FRR mechanism. The
unicast traffic can be forwarded according to the explicit path list
as an alternate path to implement unicast traffic protection, and
can achieve full coverage of various networking environments.
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The alternate path provided by the TILFA mechanism is actually a
Segment List, including one or more Adjacency SIDs of one or more
links between the P space and the Q space, and the NodeSID of P
space node. PIM can look up the corresponding node IP address in
the unicast route according to the NodeSID, and the IP addresses of
the two endpoints of the corresponding link in the unicast route
according to the Adjacency SIDs, but the multicast protocol
packets cannot be directly sent along the path of the Segment
List.
PIM join message need to be sent hop-by-hop to establish a standby
multicast tree. However, not all of the nodes and links on the
unicast alternate path are included in the Segment List. If the PIM
protocol packets are transmitted only in unicast mode, then
equivalently the PIM packets are transmitted through the unicast
tunnel like unicast traffic, and cannot pass through the
intermediate nodes of the tunnel. The intermediate nodes of the
alternate path cannot forward multicast traffic because there are no
PIM state entries on the nodes. PIM needs to create entries on the
device hop-by-hop and generate an incoming interface and an outgoing
interface list. So it can form an end-to-end complete multicast tree
for forwarding multicast traffic. Therefore, it is not possible to
send PIM packets like unicast traffic according to the Segment List
path and can only establish a standby multicast tree.
It is available in principle that the path information of the
Segment List is added to the PIM packets to guide the hop-by-hop RPF
selection. The IP address of the node corresponding to the NodeSID
can be used as the segmented root node, and the IP addresses of the
interfaces at both endpoints of the link corresponding to the
Adjacency SID can be used directly as the local upstream interface
and upstream neighbor, but there is currently no field in protocol
packet to carry the explicit path specified by the Segment List. For
the PIM protocol, the PIM RPF Vector attribute was defined in
[RFC5496], which can carry the node IP address corresponding to the
NodeSID. The Explicit RPF Vector was defined in [RFC7891], which can
carry the peer IP address corresponding to the Adjacency SID, but if
there are multiple same peer IP addresses corresponding to the
Adjacency SID (i.e. anycast IP address), the upstream neighbor of
RPF selection may be different from the actual upstream link
corresponding to the Adjacency SID, which can make the PIM join path
and the TILFA calculation path inconsistent.
3. Solution
An Upstream Multicast Hop (UMH) is a candidate next-hop that can be
used to reach the root of the tree. In This document the secondary
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UMH is based on unicast routing to find Segment List calculated by
TILFA.
This document extends the PIM protocol, to establish the standby
multicast tree according to the Segment List calculated by TILFA,
and can achieve full coverage of various networking environments for
MoFRR protection of multicast services.
Assume that the Segment List calculated by TILFA is (NodeSID(A),
AdjSID(A-B)). Node A belongs to the P Space, and node B belongs to
the Q space. The IP address corresponding to NodeSID(A) can be
looked up in the local link state database of the IGP protocol, and
can be assumed to be IP-a. The IP addresses of the two endpoints of
the link corresponding to AdjSID(A-B) can also be looked up in the
local link state database of the IGP protocol, and can be assumed to
be IP-La and IP-Lb.
3.1. Secondary UMH Selection
In the procedure of PIM, IP-a can be looked as the normal RPF vector
attribute and added to the PIM join packet. IP-La and IP-Lb can be
looked as the RPF Vector attribute of the adjacency relationship,
called Adjacency RPF Vector, which is a new type of PIM join
attributes, and added to the PIM join packet too.
The PIM protocol firstly can select the RPF incoming interface and
upstream towards IP-a, and can join hop-by-hop to establish the PIM
standby multicast tree until the node A. On the node A, IP-Lb can be
looked as one PIM neighbor. If there are multiple PIM neighbors with
the same address IP-Lb, all of the corresponding local interfaces on
the node A need to be checked. The interface that is the only one
with the IP address IP-La can be looked as the RPF incoming
interface. The node A can send the PIM join packet to the node B on
the interface of IP-La, and IP-Lb is used as the RPF upstream
address of the PIM join.
After the PIM join packet is received on the node B, the PIM
protocol can find no more join attributes and select the RPF
incoming interface and upstream towards the multicast source
directly, and then can continue to join hop-by-hop to establish the
PIM standby multicast tree until the router directly connected the
source.
3.2. Extension Protocol Fields Conflict
PIM Adjacency RPF Vector attribute is newly defined in join
attributes. If there are conflicts from multiple downstream PIM
neighbors, the mechanism in [RFC5384] Section 3.3.3 can be used to
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select a PIM downstream neighbor with a numerically smallest IP
address. If at least two neighbors have the same IP address, the
interface index MUST be used as a tie breaker.
4. Packet Format
This section describes the format of PIM join message extension
introduced by this document.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type | Rsrvd |S|W|R| Mask Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....
|F|E| Attr_Type | Length | Value
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....
|F|E| Attr_Type | Length | Value
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.....
. . .
. . .
The original PIM join attribute already has been defined in
[RFC5384]
Attr_Type:
0- Vector ;
4- Explicit RPF Vector ;
Other existing definitions are not related to RPF Vector Attribute.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|E| Type | Length | Value
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-.......
The definition of Adjacency RPF Vector attribute
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F bit: 0, indicating that the unrecognized device does not forward
the attribute
E bit: indicates the last join attribute
Type: TBD
Length: depends on the address family of Encoded-Unicast address,
including the length of 2 addresses.
Value: Encoded-Unicast Address format defined in [RFC7761] Section
4.9.1, including 2 addresses. The first one indicates the address of
the local interface, and the second one indicates the address of the
peer interface. Only the case of the same address family is
supported.
5. IANA Considerations
This document requests IANA to assign a registry for Adjacency RPF
Vector in PIM Join Attribute and the Explicit Path TLV Node Address
Sub TLV. The assignment is requested permanent for IANA when this
document is published as an RFC. The string TBD should be replaced
by the assigned values accordingly.
6. Security Considerations
For general PIM-SM protocol Security Considerations, see [RFC7761].
TBD
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5384] Boers, A., Wijnands, I., and E. Rosen, "The Protocol
Independent Multicast (PIM) Join Attribute Format",
RFC 5384, November 2008
[RFC5496] Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse Path
Forwarding (RPF) Vector TLV", RFC 5496, March 2009
[RFC7431] Karan, A., Filsfils, C., Wijnands, IJ., Ed., and B.
Decraene, "Multicast-Only Fast Reroute", RFC 7431, August
2015
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[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, April 2015
[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas,
I.,Parekh, R., Zhang, Z., and L. Zheng, "Protocol
IndependentMulticast - Sparse Mode (PIM-SM): Protocol
Specification(Revised)", RFC 7761, March 2016
[RFC7891] Asghar, J., Wijnands, IJ., Ed., Krishnaswamy, S., Karan,
A., and V. Arya, "Explicit Reverse Path Forwarding (RPF)
Vector", RFC 7891, June 2016
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017
[I-D.ietf-rtgwg-segment-routing-ti-lfa] Litkowski, S., Bashandy, A.,
Filsfils, C., Francois, P., Decraene, B., and D. Voyer,
"Topology Independent Fast Reroute using Segment Routing",
draft-ietf-rtgwg-segment-routing-ti-lfa-06, work-in-
progress, February 2021
7.2. Informative References
TBD
8. Acknowledgments
The authors would like to thank the following for their valuable
contributions of this document:
TBD
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Authors' Addresses
Yisong Liu
China Mobile
China
Email: liuyisong@chinamobile.com
Mike McBride
Futurewei Inc.
USA
Email: michael.mcbride@futurewei.com
Zheng(Sandy) Zhang
ZTE Corporation
China
Email: zhang.zheng@zte.com.cn
Jingrong Xie
Huawei Technologies
China
Email: xiejingrong@huawei.com
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