SPRING Working Group Dave Allan
Internet Draft Ericsson
Intended status: Standards Track Jeff Tantsura
Expires: October 2017
April 2017
A Framework for Computed Multicast applied to MPLS based Segment
Routing
draft-allan-spring-mpls-multicast-framework-03
Abstract
This document describes a multicast solution for Segment Routing with
MPLS data plane. It is consistent with the Segment Routing
architecture in that an IGP is augmented to distribute information in
addition to the link state. In this solution it is multicast group
membership information sufficient to synchronize state in a given
network domain. Computation is employed to determine the topology of
any loosely specified multicast distribution tree.
Status of this Memo
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with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire in October 2017.
Copyright and License Notice
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Table of Contents
1. Introduction...................................................3
1.1. Authors......................................................3
1.2. Requirements Language........................................3
2. Conventions used in this document..............................3
2.1. Terminology..................................................3
3. Solution Overview..............................................4
3.1. Mapping source specific trees onto the segment routing
architecture......................................................5
3.2. Role of the Routing System...................................6
3.3. MDT Construction Requirements................................6
3.4. Pruning - theory of operation................................6
4. Elements of Procedure..........................................7
4.1. Triggers for Computation.....................................7
4.2. FIB Determination............................................7
4.2.1. Information in the IGP.....................................7
4.2.2. Computation of individual segments.........................8
4.3. FIB Generation..............................................11
4.4. FIB installation............................................11
5. Related work..................................................12
5.1. IGP Extensions..............................................12
5.2. BGP Extensions..............................................12
6. Observations..................................................12
7. Acknowledgements..............................................13
8. Security Considerations.......................................13
9. IANA Considerations...........................................13
10. References...................................................13
10.1. Normative References.......................................13
10.2. Informative References.....................................13
11. Authors' Addresses...........................................14
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1. Introduction
This memo describes a solution for multicast for Segment Routing with
MPLS data plane in which source specific multicast distribution trees
(MDTs) are computed from information distributed via an IGP.
Computation can use information in the IGP to determine if a given
node in the network has a role as a root, leaf or replication point
in a given MDT. Unicast tunnels are employed to interconnect the
nodes determined to have a role. Therefore state only need be
installed in nodes that have one of these three roles to fully
instantiate an MDT.
Although this approach is computationally intensive, a significant
amount of computation can be avoided when the computing agent
determines that the node it is computing for has no role in a given
MDT. This permits a computed approach to multicast convergence to be
computationally tractable.
1.1. Authors
David Allan, Jeff Tantsura
1.2. 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 RFC2119 [RFC2119].
2. Changes from the last version
Clarifications in the pruning and simplification algorithm
3. Conventions used in this document
3.1. Terminology
Candidate replication point (CRP) - is a node that potentially needs
to install state to replicate multicast traffic as determined at an
intermediate step in multicast segment computation. It will either
resolve to having no role or a role as a replication point once
multicast has converged.
Candidate role - refers to any potential combination of roles on a
given multicast segment as determined at some intermediate step in
MDT computation. For example, a node with a candidate role may be a
leaf and may be a candidate replication point.
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Downstream - refers to the direction along the shortest path to one
or more leaves for a given multicast distribution tree
Multicast convergence - is when all computation and state
installation to ensure the FIB reflects the multicast information in
the IGP is complete.
MDT - multicast distribution tree. Is a tree composed of one or more
multicast segments.
Multicast segment - is a portion of the multicast tree where only the
root and the leaves have been specified, and computation based upon
the current state of the IGP database is employed to determine and
install the required state to implement the segment. For MPLS a
multicast segment is implemented as a p2mp LSP. A multicast segment
is identified by a multicast SID.
Multicast SID - Is the data plane identifier that is used to
implement a multicast segment. As per a unicast MPLS segment, the
rightmost 20 bits of a multicast SID is encoded as a label. It is
drawn from an SRGB that is global to the SR domain.
Pinned path - Is a unique shortest path extending from a leaf
upstream towards the root for a given multicast segment. Therefore is
a component of the multicast segment that it has been determined must
be there. It will not necessarily extend from the leaf all the way to
the root during intermediate computation steps. A pinned path can
result from pruning operations.
Role - refers specifically to a node that is either a root, a leaf, a
replication node, or a pinned waypoint for a given MDT.
Unicast convergence - is when all computation and state installation
to ensure the FIB reflects the unicast information in the IGP is
complete.
Upstream - refers to the direction along the shortest path to the
root of a given MDT.
4. Solution Overview
This memo describes a multicast architecture in which multicast state
is only installed in those nodes that have roles as a root, leaves,
and replication points for a given multicast segment. The a-priori
established segment routing unicast tunnels are used as interconnect
between the nodes that have a role in a given multicast SID.
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A loosely specified MDT is composed of a single multicast segment and
the routing of the MDT is delegated entirely to computation driven by
information in the IGP database.
Explicitly routed MDTs are expressed as a tree of concatenated
multicast segments where both the leaves of each segment and the
waypoints coupling a given segment to the upstream and/or downstream
segment(s) is specified in information flooded in the IGP by the
overall root of the MDT. The segments themselves will be computed as
per a loosely specified MDT.
A PE acting as an overall root for a given tree is expected to be
configured by the operator as to where to source multicast traffic
from, be it an attachment circuit, interworking function for client
technology or other. Similarly a leaf for a given tree is expected to
be configured by the operator as to the disposition of received
multicast traffic.
A computed segment is guaranteed to be loop free in a stable system.
A concatenation of segments to construct an MDT will similarly be
loop free as any collision of segments can be disambiguated in the
data plane via the SIDs.
This architecture significantly reduces the amount of state that
needs to be installed in the data plane to support multicast. This
also means that the impact of many failures in the network on
multicast traffic distribution will be recovered by unicast local
repair or unicast convergence with subsequent multicast convergence
acting in the role of network re-optimization (as opposed to
restoration).
4.1. Mapping source specific trees onto the segment routing architecture
A computed source specific tree for a given multicast group
corresponds to one or more multicast segments in the SR architecture.
Each multicast segment is assigned a SID, typically by management
configuration of the node that will be the overall root for the
source specific tree. The root node then uses the IGP to advertise
this information to all nodes in the IGP area/domain.
A multicast group is implemented as the set of source specific trees
from all nodes that have registered transmit interest to all nodes
that have registered receive interest in a multicast group.
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4.2. Role of the Routing System
The role of the IGP is to communicate topology information, multicast
capability and associated algorithm, multicast registrations, unicast
to SID bindings, multicast to SID bindings and waypoints in multi-
segment MDTs. No changes to topology or unicast to SID binding
advertisements are proposed by this memo.
The multicast registrations/bindings will be in the form of source,
group, transmit/receive interest and the SID to use for the source
specific multicast tree. Registrations are originated by any node
that has send or receive interest in a given multicast group. Nodes
will use the combination of topology and multicast registrations to
determine the nodes that have a role in each source specific tree and
the SID information to then derive the required FIB state.
4.3. MDT Construction Requirements
A multicast segment in an MDT is constructed such that between any
pair of nodes that have a role in the segment and are connected by a
unicast tunnel, there is not another node on the shortest path
between the two with a role in that segment. This ensures that copies
of a packet forwarded by an multicast segment will traverse a link
only once in a stable system.
Note that this can be satisfied by a minimum cost shortest path tree,
but is not an absolute requirement. The pruning rules specified in
this memo will meet this requirement without necessarily producing
absolutely minimum cost multicast segment (or incurring the
associated computational cost).
4.4. Pruning - theory of operation
The role of nodes in a given multicast segment is determined by first
producing an inclusive shortest path tree with all possible paths
between the root and leaves, and then applying a set of pruning rules
repeatedly until an acyclic tree is produced or no further prunes are
possible.
For the majority of multicast segments these rules will
authoritatively produce a minimum cost tree. For those segments that
have not yet been authoritatively resolved, there is a set of pruning
operations applied that are not guaranteed to produce a tree that
meets the requirements of 3.3, therefore these trees require auditing
and potential correction according to a further set of agreed rules.
This avoids the necessity of an exhaustive search of the solution
space.
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A node during computation of a segment may conclude that it will
absolutely not have a role at any of numerous points in the
computation process and abandon computation of that segment.
5. Elements of Procedure
5.1. Triggers for Computation
MDT computation is triggered by changes to the IGP database. These
are in the form of either changes in registered multicast group
interest, addition or removal of a multi-segment MDT descriptor, or
topology changes.
A change in registered interest for a group will require re-
computation of all MDTs that implement the multicast group.
A topology change will require the computation of some number of
multicast segments, the actual number will depend on the
implementation of tree computation but at a minimum will be all trees
for which there is not an optimal shortest path solution as a result
of the topology change.
5.2. FIB Determination
5.2.1. Information in the IGP
Group membership information for a multicast segment is obtained from
the IGP. This is true for single segment MDTs as well as multi-
segment MDTs. Included in the multi-segment MDT specification is the
waypoint nodes in MDT and the upstream and downstream SIDs. The
specified node is expected to cross connect the SIDs to join the
segments together acting in the role of leaf for the upstream segment
and root for the downstream segment.
When a waypoint in an MDT descriptor does not exist in the IGP, the
assumption is that the node identified by the waypoint SID has
failed. The response of the other nodes in the system in FIB
determination is to add the leaves of the downstream segment to the
upstream segment.
An example of this would be consider a node "x", and another node
"y". At some point in time, "x" advertises a tree that identifies "y"
as a waypoint that cross connects upstream SID "a" to downstream SID
"b". At some later point node "y" fails. The other nodes in the
network will compute segment "a" as if it included all leaves and
waypoints in segment "b". All apriori state installed for segment "b"
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would be removed as the failure of "y" has required "b" to be
subsumed by "a".
5.2.2. Computation of individual segments
FIB generation for a multicast segment is the result of computation,
ultimately as applied to all source specific trees in the network.
All computing nodes implement a common algorithm for tree generation,
as all MUST agree on the solution.
One algorithm is as follows:
All possible shortest paths to the set of leaves for the MDT is
determined. Then pruning rules are repeatedly applied until no
further prunes are possible.
The philosophy of the application of these rules could be expressed
as "simplify as much as possible, and prune that which cannot be".
The rules are:
1) Eliminate any links and nodes not on a potential shortest path
from the root to the leaves for the MDT under consideration.
2) Simplify via the replacement of any nodes that do not have a
potential role in the MDT with links.
This will be nodes that are not a leaf, a root or a candidate
replication point. For example:
Root---------A----------B
B is a leaf. A is not but is in a potential shortest path from root
to B. However A will have no role in the MDT that serves B as it
provides simple transit therefore is replaced with a direct
connection between the root and B.
Root--------------------B
Note that such pruning also needs to avoid the creation of
duplicate parallel links. For example:
/----------A----------\
Root B
\----------C----------/
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Where A and C have no role and the cost root-A-B = cost root-C-B,
they can be replaced with a single link from Root to B.
3) Simplify via the elimination of fewer hop paths
When for a given set of leaves, a node has multiple downstream
links that converge on a common downstream point, and that set of
leaves is only a subset of the leaves reachable on one or more of
the links, any link that only serves that subset of leaves can be
pruned.
For example:
--A---------------------------B
\ /
-----------C-----------
\
----D
Link AB is cost 2, link AC and CB are cost 1 (cost of link CD does
not affect the example).
B and D are leaves of a root upstream of A. From A, link AB can
reach leaf B. Path AC can reach leaf B and D. In this case path A-B
can be pruned from consideration. The set of leaves reachable via
link A-B is a subset of that reachable by A-C, and the paths from A
that serves that subset converges at B.
4) Prune upstream links.
The normal procedure is to determine the closest upstream leaf or
pinned path and then compare all upstream adjacencies with that
metric
a. If the upstream adjacency extends closer to the root than
the closest leaf or pinned path, then that adjacency can be
pruned.
b. If the upstream adjacency extends the same distance towards
the root then
i. If it is to a non-leaf or pinned path candidate
replication point, it can be pruned
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ii. If it is to a pinned path, where there are equal upstream
adjacencies that terminate on leaves, it can be pruned
(considered inferior).
iii. If there is more than one "equal" upstream adjacency,
that is all terminate on nodes that are on pinned paths,
or all terminate on nodes that are leaves, than one is
selected. This is via the lowest node ID.
c. If the upstream adjacency is a candidate replication point
closer than the closest leaf, and upstream from it is a node
that is a leaf or pinned path equidistant with the closest
leaf, then all adjacencies that extend to leaves ranked lower
than the leaf or pinned path behind the CRP may be pruned.
Note that an upstream adjacency that has a CRP closer than
the closest leaf or pinned path cannot be pruned.
d. When for a given node all possible upstream adjacencies that
can be pruned have been identified, each is removed, and any
simplifications that can be performed as a result of the
prune are performed. This is the equivalent of a localized
check for 2 and 3 above and is then performed iteratively in
response to changes to the graph as a result of pruning.
The procedure is to implement 1, 2 and 3 above, then loop on 4 until
such time as the MDT is fully resolved, or no further prunes are
possible. Step 4 is performed in a specific order. The nodes are
processed according to a ranking from closest to the root to the
farthest, and from lowest node ID to the highest within a given
distance from the root.
If, at the end of pruning and simplification, all leaves in a
multicast segment have a unique shortest path to the root, the tree
is considered resolved, and the computation can progress directly to
the FIB generation step.
If not all leaves have a unique shortest path, additional pruning
steps are applied. These steps are NOT guaranteed to produce a lowest
cost tree, and therefore require an additional audit and possible
modification to ensure when forwarding a maximum of one copy of a
packet will traverse an interface.
For segments not authoritatively resolved by the above rules, a prune
that will not authoritatively result in a minimum cost tree is
applied. For the purpose of interoperability, the following rule is
proposed: A computing node will select the closest node to the root
with a candidate role that does not have a unique shortest path to
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the root. Where more than one such node exists, the one with the
lowest unicast SID is selected. For that node, the best upstream link
is selected and all other upstream links pruned. The best upstream
link is defined as the link with the closest node with a candidate
role that potentially serves the highest number of leaves. Where
there is a tie, once again the node with the lowest SID is selected.
Once the links have been pruned, rules 2 through 4 are repeatedly
applied until either the tree is fully resolved, or again no further
prunes are possible, in which case the next closest remaining
unresolved node has the same prune applied.
For all segments not resolved by the initial prune rules, they are
audited to ensure all nodes that have a role in the tree do not have
a node with a role between them and their upstream node on the tree.
If they do, the old upstream adjacency is removed, and the superior
one added.
5.3. FIB Generation
The topology components that remain at the end of the pruning
operation will reflect all nodes that have a role in a given
multicast segment plus the necessary tunnels (as all intervening
multi-path scenarios will have been simplified away). From this the
FIB can be generated:
All nodes that have a role in a given multicast segment and have
nodes upstream in the segment will need to accept the SID for the MDT
from at minimum, all upstream interfaces.
All nodes that have a role in a given segment and have nodes
immediately downstream in the segment will need to replicate packets
simply labelled with the multicast SID onto those interfaces.
All nodes that have a role in a given segment and have nodes
reachable via a tunnel downstream set the FIB to push the tunnel
unicast SID for the downstream node onto any replicated copies of a
received packet, and identify the set of interfaces on the shortest
path for the tunnel SID.
5.4. FIB installation
FIB installation needs to acknowledge two aspects of the hybrid
tunnel and role model of multicast tree construction. The first is
that because of the sparse state model simple tree adds, moves, and
changes may require the installation of state where it did not
previously exist, and such changes may impact existing services. The
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second is that it is possible to retain the knowledge to prioritize
computation of those trees impacted the failure of a node with a
role.
To address this, there are three stages of state installation for
multicast convergence:
1) Immediate:
a. Installation of state for multicast segments impacted by the
failure of a node in the network, and installation of state
for segments in nodes that have not previously had a role in
the given segment.
b. Installation of state for waypoints in multi-segment MDTs.
2) After T1: Update state for nodes that both had and have a role in
a given multicast segment.
3) After T2: Removal of state for nodes that transition from having a
role to not having a role for a given multicast segment.
T1 and T2 are network wide configurable values.
6. Related work
6.1. IGP Extensions
The required IGP changes are documented in [MCAST-ISIS] and [MCAST-
OPSF].
6.2. BGP Extensions
This memo will require the specification of a new PMSI Tunnel
Attribute (SPRING P2MP tunnel, tentatively 0x09) to order to
integrate into the multicast framework documented in RFC 6514
7. Observations
This technique is not confined to segment routing, and with the
provision of a global label space (to be employed as per a multicast
SID), an MPLS-LDP network would also provide the requisite mesh of
unicast tunnels and be capable of implementing this approach to
multicast.
This memo focuses on an implementation based upon nodes that are IGP
speakers and converge independently so is written in a form that
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assumes a node, computing node and IGP speaker are one in the same.
It should be observed that the relative frugality of data plane state
would suggest that separation of computation from nodes in the data
plane combined with management or "software defined networking" based
population of the multicast FIB entries may also be useful modes of
network operation.
8. Acknowledgements
Thanks to Uma Chunduri for his detailed review and suggestions.
9. Security Considerations
For a future version of this document.
10. IANA Considerations
This document requires the allocation of a PMSI tunnel type to
identify a SPRING P2MP tunnel type from the P-Multicast Service
Interface Tunnel (PMSI Tunnel) Tunnel Types registry.
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
[MCAST-ISIS] Allan et.al., "IS-IS extensions for Computed Multicast
applied to MPLS based Segment Routing", IETF work in progress,
draft-allan-isis-spring-multicast-00, July 2016
[MCAST-OSPF] Allan et.al., "OSPF extensions for Computed Multicast
applied to MPLS based Segment Routing", IETF work in progress,
draft-allan-ospf-spring-multicast-00, July 2016
[RFC6514] Aggarwal et.al., "BGP Encodings and Procedures for Multicast
in MPLS/BGP IP VPNs", IETF RFC 6514, February 2012
[RFC7385] Andersson & Swallow "IANA Registry for P-Multicast Service
Interface (PMSI) Tunnel Type Code Points", IETF RFC 7385,
October 2014
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12. Authors' Addresses
Dave Allan (editor)
Ericsson
300 Holger Way
San Jose, CA 95134
USA
Email: david.i.allan@ericsson.com
Jeff Tantsura
Email: jefftant.ietf@gmail.com
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