Dynamic Flooding on Dense Graphs
draft-ietf-lsr-dynamic-flooding-11
| Document | Type | Active Internet-Draft (lsr WG) | |
|---|---|---|---|
| Authors | Tony Li , Tony Przygienda , Peter Psenak , Les Ginsberg , Huaimo Chen , David Cooper , Luay Jalil , Srinath Dontula , Gyan Mishra | ||
| Last updated | 2022-07-07 (Latest revision 2022-06-07) | ||
| Replaces | draft-li-lsr-dynamic-flooding | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Experimental | ||
| Formats | |||
| Stream | WG state | WG Document | |
| Document shepherd | Acee Lindem | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | Acee Lindem <acee@cisco.com> |
draft-ietf-lsr-dynamic-flooding-11
Internet Engineering Task Force T. Li, Ed.
Internet-Draft T. Przygienda
Intended status: Standards Track Juniper Networks
Expires: 9 December 2022 P. Psenak, Ed.
L. Ginsberg
Cisco Systems, Inc.
H. Chen
Futurewei
D. Cooper
CenturyLink
L. Jalil
Verizon
S. Dontula
ATT
G. Mishra
Verizon Inc.
7 June 2022
Dynamic Flooding on Dense Graphs
draft-ietf-lsr-dynamic-flooding-11
Abstract
Routing with link state protocols in dense network topologies can
result in sub-optimal convergence times due to the overhead
associated with flooding. This can be addressed by decreasing the
flooding topology so that it is less dense.
This document discusses the problem in some depth and an
architectural solution. Specific protocol changes for IS-IS, OSPFv2,
and OSPFv3 are described in this document.
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 https://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."
Li, et al. Expires 9 December 2022 [Page 1]
Internet-Draft Dynamic Flooding June 2022
This Internet-Draft will expire on 9 December 2022.
Copyright Notice
Copyright (c) 2022 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
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
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
3. Solution Requirements . . . . . . . . . . . . . . . . . . . . 5
4. Dynamic Flooding . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Leader election . . . . . . . . . . . . . . . . . . . . . 8
4.3. Computing the Flooding Topology . . . . . . . . . . . . . 9
4.4. Topologies on Complete Bipartite Graphs . . . . . . . . . 10
4.4.1. A Minimal Flooding Topology . . . . . . . . . . . . . 10
4.4.2. Xia Topologies . . . . . . . . . . . . . . . . . . . 10
4.4.3. Optimization . . . . . . . . . . . . . . . . . . . . 11
4.5. Encoding the Flooding Topology . . . . . . . . . . . . . 11
4.6. Advertising the Local Edges Enabled for Flooding . . . . 12
5. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 13
5.1. IS-IS TLVs . . . . . . . . . . . . . . . . . . . . . . . 13
5.1.1. IS-IS Area Leader Sub-TLV . . . . . . . . . . . . . . 13
5.1.2. IS-IS Dynamic Flooding Sub-TLV . . . . . . . . . . . 14
5.1.3. IS-IS Area Node IDs TLV . . . . . . . . . . . . . . . 15
5.1.4. IS-IS Flooding Path TLV . . . . . . . . . . . . . . . 16
5.1.5. IS-IS Flooding Request TLV . . . . . . . . . . . . . 17
5.1.6. IS-IS LEEF Advertisement . . . . . . . . . . . . . . 18
5.2. OSPF LSAs and TLVs . . . . . . . . . . . . . . . . . . . 18
5.2.1. OSPF Area Leader Sub-TLV . . . . . . . . . . . . . . 19
5.2.2. OSPF Dynamic Flooding Sub-TLV . . . . . . . . . . . . 20
5.2.3. OSPFv2 Dynamic Flooding Opaque LSA . . . . . . . . . 20
5.2.4. OSPFv3 Dynamic Flooding LSA . . . . . . . . . . . . . 22
5.2.5. OSPF Area Router ID TLVs . . . . . . . . . . . . . . 22
5.2.5.1. OSPFv2 Area Router ID TLV . . . . . . . . . . . . 23
5.2.5.2. OSPFv3 Area Router ID TLV . . . . . . . . . . . . 24
Li, et al. Expires 9 December 2022 [Page 2]
Internet-Draft Dynamic Flooding June 2022
5.2.6. OSPF Flooding Path TLV . . . . . . . . . . . . . . . 26
5.2.7. OSPF Flooding Request Bit . . . . . . . . . . . . . . 27
5.2.8. OSPF LEEF Advertisement . . . . . . . . . . . . . . . 28
6. Behavioral Specification . . . . . . . . . . . . . . . . . . 29
6.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 29
6.2. Flooding Topology . . . . . . . . . . . . . . . . . . . . 29
6.3. Leader Election . . . . . . . . . . . . . . . . . . . . . 30
6.4. Area Leader Responsibilities . . . . . . . . . . . . . . 30
6.5. Distributed Flooding Topology Calculation . . . . . . . . 30
6.6. Use of LANs in the Flooding Topology . . . . . . . . . . 31
6.6.1. Use of LANs in Centralized mode . . . . . . . . . . . 31
6.6.2. Use of LANs in Distributed Mode . . . . . . . . . . . 31
6.6.2.1. Partial flooding on a LAN in IS-IS . . . . . . . 31
6.6.2.2. Partial Flooding on a LAN in OSPF . . . . . . . . 32
6.7. Flooding Behavior . . . . . . . . . . . . . . . . . . . . 32
6.8. Treatment of Topology Events . . . . . . . . . . . . . . 33
6.8.1. Temporary Addition of Link to Flooding Topology . . . 33
6.8.2. Local Link Addition . . . . . . . . . . . . . . . . . 34
6.8.3. Node Addition . . . . . . . . . . . . . . . . . . . . 35
6.8.4. Failures of Link Not on Flooding Topology . . . . . . 35
6.8.5. Failures of Link On the Flooding Topology . . . . . . 36
6.8.6. Node Deletion . . . . . . . . . . . . . . . . . . . . 36
6.8.7. Local Link Addition to the Flooding Topology . . . . 36
6.8.8. Local Link Deletion from the Flooding Topology . . . 37
6.8.9. Treatment of Disconnected Adjacent Nodes . . . . . . 37
6.8.10. Failure of the Area Leader . . . . . . . . . . . . . 37
6.8.11. Recovery from Multiple Failures . . . . . . . . . . . 38
6.8.12. Rate Limiting Temporary Flooding . . . . . . . . . . 38
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
7.1. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . 39
7.2. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . 40
7.2.1. OSPF Dynamic Flooding LSA TLVs Registry . . . . . . . 41
7.2.2. OSPF Link Attributes Sub-TLV Bit Values Registry . . 42
7.3. IGP . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8. Security Considerations . . . . . . . . . . . . . . . . . . . 43
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 43
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 43
10.1. Normative References . . . . . . . . . . . . . . . . . . 43
10.2. Informative References . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46
Li, et al. Expires 9 December 2022 [Page 3]
Internet-Draft Dynamic Flooding June 2022
1. Introduction
In recent years, there has been increased focus on how to address the
dynamic routing of networks that have a bipartite (a.k.a. spine-leaf
or leaf-spine), Clos [Clos], or Fat Tree [Leiserson] topology.
Conventional Interior Gateway Protocols (IGPs, i.e., IS-IS
[ISO10589], OSPFv2 [RFC2328], and OSPFv3 [RFC5340]) under-perform,
redundantly flooding information throughout the dense topology,
leading to overloaded control plane inputs and thereby creating
operational issues. For practical considerations, network architects
have resorted to applying unconventional techniques to address the
problem, e.g., applying BGP in the data center [RFC7938]. However it
is very clear that using an Exterior Gateway Protocol as an IGP is
sub-optimal, if only due to the configuration overhead.
The primary issue that is demonstrated when conventional mechanisms
are applied is the poor reaction of the network to topology changes.
Normal link state routing protocols rely on a flooding algorithm for
state distribution within an area. In a dense topology, this
flooding algorithm is highly redundant, resulting in unnecessary
overhead. Each node in the topology receives each link state update
multiple times. Ultimately, all of the redundant copies will be
discarded, but only after they have reached the control plane and
been processed. This creates issues because significant link state
database updates can become queued behind many redundant copies of
another update. This delays convergence as the link state database
does not stabilize promptly.
In a real world implementation, the packet queues leading to the
control plane are necessarily of finite size, so if the flooding rate
exceeds the update processing rate for long enough, the control plane
will be obligated to drop incoming updates. If these lost updates
are of significance, this will further delay stabilization of the
link state database and the convergence of the network.
This is not a new problem. Historically, when routing protocols have
been deployed in networks where the underlying topology is a complete
graph, there have been similar issues. This was more common when the
underlying link layer fabric presented the network layer with a full
mesh of virtual connections. This was addressed by reducing the
flooding topology through IS-IS Mesh Groups [RFC2973], but this
approach requires careful configuration of the flooding topology.
Thus, the root problem is not limited to massively scalable data
centers. It exists with any dense topology at scale.
Li, et al. Expires 9 December 2022 [Page 4]
Internet-Draft Dynamic Flooding June 2022
This problem is not entirely surprising. Link state routing
protocols were conceived when links were very expensive and
topologies were sparse. The fact that those same designs are sub-
optimal in a dense topology should not come as a huge surprise. The
fundamental premise that was addressed by the original designs was an
environment of extreme cost and scarcity. Technology has progressed
to the point where links are cheap and common. This represents a
complete reversal in the economic fundamentals of network
engineering. The original designs are to be commended for continuing
to provide correct operation to this point, and optimizations for
operation in today's environment are to be expected.
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. Problem Statement
In a dense topology, the flooding algorithm that is the heart of
conventional link state routing protocols causes a great deal of
redundant messaging. This is exacerbated by scale. While the
protocol can survive this combination, the redundant messaging is
unnecessary overhead and delays convergence. Thus, the problem is to
provide routing in dense, scalable topologies with rapid convergence.
3. Solution Requirements
A solution to this problem must then meet the following requirements:
Requirement 1 Provide a dynamic routing solution. Reachability must
be restored after any topology change.
Requirement 2 Provide a significant improvement in convergence.
Requirement 3 The solution should address a variety of dense
topologies. Just addressing a complete bipartite
topology such as K5,8 is insufficient. Multi-stage
Clos topologies must also be addressed, as well as
topologies that are slight variants. Addressing
complete graphs is a good demonstration of generality.
Requirement 4 There must be no single point of failure. The loss of
any link or node should not unduly hinder convergence.
Li, et al. Expires 9 December 2022 [Page 5]
Internet-Draft Dynamic Flooding June 2022
Requirement 5 Dense topologies are subgraphs of much larger
topologies. Operational efficiency requires that the
dense subgraph not operate in a radically different
manner than the remainder of the topology. While some
operational differences are permissible, they should
be minimized. Changes to nodes outside of the dense
subgraph are not acceptable. These situations occur
when massively scaled data centers are part of an
overall larger wide-area network. Having a second
protocol operating just on this subgraph would add
much more complexity at the edge of the subgraph where
the two protocols would have to inter-operate.
4. Dynamic Flooding
We have observed that the combination of the dense topology and
flooding on the physical topology in a scalable network is sub-
optimal. However, if we decouple the flooding topology from the
physical topology and only flood on a greatly reduced portion of that
topology, we can have efficient flooding and retain all of the
resilience of existing protocols. A node that supports flooding on
the decoupled flooding topology is said to support dynamic flooding.
In this idea, the flooding topology is computed within an IGP area
with the dense topology either centrally on an elected node, termed
the Area Leader, or in a distributed manner on all nodes that are
supporting Dynamic Flooding. If the flooding topology is computed
centrally, it is encoded into and distributed as part of the normal
link state database. We call this the centralized mode of operation.
If the flooding topology is computed in a distributed fashion, we
call this the distributed mode of operation. Nodes within such an
IGP area would only flood on the flooding topology. On links outside
of the normal flooding topology, normal database synchronization
mechanisms (i.e., OSPF database exchange, IS-IS CSNPs) would apply,
but flooding may not. Details are described in Section 6. New link
state information that arrives from outside of the flooding topology
suggests that the sender has a different or no flooding topology
information and that the link state update should be flooded on the
flooding topology as well.
The flooding topology covers the full set of nodes within the area,
but excludes some of the links that standard flooding would employ.
Since the flooding topology is computed prior to topology changes, it
does not factor into the convergence time and can be done when the
topology is stable. The speed of the computation and its
distribution, in the case of a centralized mode, is not a significant
issue.
Li, et al. Expires 9 December 2022 [Page 6]
Internet-Draft Dynamic Flooding June 2022
If a node does not have any flooding topology information when it
receives new link state information, it should flood according to
standard flooding rules. This situation will occur when the dense
topology is first established, but is unlikely to recur.
When centralized mode is used and if, during a transient, there are
multiple flooding topologies being advertised, then nodes should
flood link state updates on all of the flooding topologies. Each
node should locally evaluate the election of the Area Leader for the
IGP area and first flood on its flooding topology. The rationale
behind this is straightforward: if there is a transient and there has
been a recent change in Area Leader, then propagating topology
information promptly along the most likely flooding topology should
be the priority.
During transients, it is possible that loops will form in the
flooding topology. This is not problematic, as the legacy flooding
rules would cause duplicate updates to be ignored. Similarly, during
transients, it is possible that the flooding topology may become
disconnected. Section 6.8.11 discusses how such conditions are
handled.
4.1. Applicability
In a complete graph, this approach is appealing because it
drastically decreases the flooding topology without the manual
configuration of mesh groups. By controlling the diameter of the
flooding topology, as well as the maximum degree node in the flooding
topology, convergence time goals can be met and the stability of the
control plane can be assured.
Similarly, in a massively scaled data center, where there are many
opportunities for redundant flooding, this mechanism ensures that
flooding is redundant, with each leaf and spine well connected, while
ensuring that no update need make too many hops and that no node
shares an undue portion of the flooding effort.
In a network where only a portion of the nodes support Dynamic
Flooding, the remaining nodes will continue to perform standard
flooding. This is not an issue for correctness, as no node can
become isolated.
Li, et al. Expires 9 December 2022 [Page 7]
Internet-Draft Dynamic Flooding June 2022
Flooding that is initiated by nodes that support Dynamic Flooding
will remain within the flooding topology until it reaches a legacy
node, which will resume legacy flooding. Standard flooding will be
bounded by nodes supporting Dynamic Flooding, which can help limit
the propagation of unnecessary flooding. Whether or not the network
can remain stable in this condition is unknown and may be very
dependent on the number and location of the nodes that support
Dynamic Flooding.
During incremental deployment of dynamic flooding an area will
consist of one or more sets of connected nodes that support dynamic
flooding and one or more sets of connected nodes that do not, i.e.,
nodes that support standard flooding. The flooding topology is the
union of these sets of nodes. Each set of nodes that does not
support dynamic flooding needs to be part of the flooding topology
and such a set of nodes may provide connectivity between two or more
sets of nodes that support dynamic flooding.
4.2. Leader election
A single node within the dense topology is elected as an Area Leader.
A generalization of the mechanisms used in existing Designated Router
(OSPF) or Designated Intermediate-System (IS-IS) elections suffices.
The elected node is known as the Area Leader.
In the case of centralized mode, the Area Leader is responsible for
computing and distributing the flooding topology. When a new Area
Leader is elected and has distributed new flooding topology
information, then any prior Area Leaders should withdraw any of their
flooding topology information from their link state database entries.
In the case of distributed mode, the distributed algorithm advertised
by the Area Leader MUST be used by all nodes that participate in
Dynamic Flooding.
Not every node needs to be a candidate to be Area Leader within an
area, as a single candidate is sufficient for correct operation. For
redundancy, however, it is strongly RECOMMENDED that there be
multiple candidates.
Li, et al. Expires 9 December 2022 [Page 8]
Internet-Draft Dynamic Flooding June 2022
4.3. Computing the Flooding Topology
There is a great deal of flexibility in how the flooding topology may
be computed. For resilience, it needs to at least contain a cycle of
all nodes in the dense subgraph. However, additional links could be
added to decrease the convergence time. The trade-off between the
density of the flooding topology and the convergence time is a matter
for further study. The exact algorithm for computing the flooding
topology in the case of the centralized computation need not be
standardized, as it is not an interoperability issue. Only the
encoding of the result needs to be documented. In the case of
distributed mode, all nodes in the IGP area need to use the same
algorithm to compute the flooding topology. It is possible to use
private algorithms to compute flooding topology, so long as all nodes
in the IGP area use the same algorithm.
While the flooding topology should be a covering cycle, it need not
be a Hamiltonian cycle where each node appears only once. In fact,
in many relevant topologies this will not be possible e.g., K5,8.
This is fortunate, as computing a Hamiltonian cycle is known to be
NP-complete.
A simple algorithm to compute the topology for a complete bipartite
graph is to simply select unvisited nodes on each side of the graph
until both sides are completely visited. If the number of nodes on
each side of the graph are unequal, then revisiting nodes on the less
populated side of the graph will be inevitable. This algorithm can
run in O(N) time, so is quite efficient.
While a simple cycle is adequate for correctness and resiliency, it
may not be optimal for convergence. At scale, a cycle may have a
diameter that is half the number of nodes in the graph. This could
cause an undue delay in link state update propagation. Therefore it
may be useful to have a bound on the diameter of the flooding
topology. Introducing more links into the flooding topology would
reduce the diameter, but at the trade-off of possibly adding
redundant messaging. The optimal trade-off between convergence time
and graph diameter is for further study.
Similarly, if additional redundancy is added to the flooding
topology, specific nodes in that topology may end up with a very high
degree. This could result in overloading the control plane of those
nodes, resulting in poor convergence. Thus, it may be optimal to
have an upper bound on the degree of nodes in the flooding topology.
Again, the optimal trade-off between graph diameter, node degree, and
convergence time, and topology computation time is for further study.
Li, et al. Expires 9 December 2022 [Page 9]
Internet-Draft Dynamic Flooding June 2022
If the leader chooses to include a multi-node broadcast LAN segment
as part of the flooding topology, all of the connectivity to that LAN
segment should be included as well. Once updates are flooded onto
the LAN, they will be received by every attached node.
4.4. Topologies on Complete Bipartite Graphs
Complete bipartite graph topologies have become popular for data
center applications and are commonly called leaf-spine or spine-leaf
topologies. In this section, we discuss some flooding topologies
that are of particular interest in these networks.
4.4.1. A Minimal Flooding Topology
We define a Minimal Flooding Topology on a complete bipartite graph
as one in which the topology is connected and each node has at least
degree two. This is of interest because it guarantees that the
flooding topology has no single points of failure.
In practice, this implies that every leaf node in the flooding
topology will have a degree of two. As there are usually more leaves
than spines, the degree of the spines will be higher, but the load on
the individual spines can be evenly distributed.
This type of flooding topology is also of interest because it scales
well. As the number of leaves increases, we can construct flooding
topologies that perform well. Specifically, for n spines and m
leaves, if m >= n(n/2-1), then there is a flooding topology that has
a diameter of four.
4.4.2. Xia Topologies
We define a Xia Topology on a complete bipartite graph as one in
which all spine nodes are bi-connected through leaves with degree
two, but the remaining leaves all have degree one and are evenly
distributed across the spines.
Constructively, we can create a Xia topology by iterating through the
spines. Each spine can be connected to the next spine by selecting
any unused leaf. Since leaves are connected to all spines, all
leaves will have a connection to both the first and second spine and
we can therefore choose any leaf without loss of generality.
Continuing this iteration across all of the spines, selecting a new
leaf at each iteration, will result in a path that connects all
spines. Adding one more leaf between the last and first spine will
produce a cycle of n spines and n leaves.
Li, et al. Expires 9 December 2022 [Page 10]
Internet-Draft Dynamic Flooding June 2022
At this point, m-n leaves remain unconnected. These can be
distributed evenly across the remaining spines, connected by a single
link.
Xia topologies represent a compromise that trades off increased risk
and decreased performance for lower flooding amplification. Xia
topologies will have a larger diameter. For m spines, the diameter
will be m + 2.
In a Xia topology, some leaves are singly connected. This represents
a risk in that in some failures, convergence may be delayed.
However, there may be some alternate behaviors that can be employed
to mitigate these risks. If a leaf node sees that its single link on
the flooding topology has failed, it can compensate by performing a
database synchronization check with a different spine. Similarly, if
a leaf determines that its connected spine on the flooding topology
has failed, it can compensate by performing a database
synchronization check with a different spine. In both of these
cases, the synchronization check is intended to ameliorate any delays
in link state propagation due to the fragmentation of the flooding
topology.
The benefit of this topology is that flooding load is easily
understood. Each node in the spine cycle will never receive an
update more than twice. For m leaves and n spines, a spine never
transmits more than (m/n +1) updates.
4.4.3. Optimization
If two nodes are adjacent on the flooding topology and there are a
set of parallel links between them, then any given update MUST be
flooded over a single one of those links. Selection of the specific
link is implementation specific.
4.5. Encoding the Flooding Topology
There are a variety of ways that the flooding topology could be
encoded efficiently. If the topology was only a cycle, a simple list
of the nodes in the topology would suffice. However, this is
insufficiently flexible as it would require a slightly different
encoding scheme as soon as a single additional link is added.
Instead, we choose to encode the flooding topology as a set of
intersecting paths, where each path is a set of connected edges.
Advertisement of the flooding topology includes support for multi-
access LANs. When a LAN is included in the flooding topology, all
edges between the LAN and nodes connected to the LAN are assumed to
be part of the flooding topology. In order to reduce the size of the
Li, et al. Expires 9 December 2022 [Page 11]
Internet-Draft Dynamic Flooding June 2022
flooding topology advertisement, explicit advertisement of these
edges is optional. Note that this may result in the possibility of
"hidden nodes" existing which are actually part of the flooding
topology but which are not explicitly mentioned in the flooding
topology advertisements. These hidden nodes can be found by
examination of the Link State database where connectivity between a
LAN and nodes connected to the LAN is fully specified.
Note that while all nodes MUST be part of the advertised flooding
topology not all multi-access LANs need to be included. Only those
LANs which are part of the flooding topology need to be included in
the advertised flooding topology.
Other encodings are certainly possible. We have attempted to make a
useful trade off between simplicity, generality, and space.
4.6. Advertising the Local Edges Enabled for Flooding
Correct operation of the flooding topology requires that all nodes
which participate in the flooding topology choose local links for
flooding which are consistent with the calculated flooding topology.
Failure to do so could result in unexpected partition of the flooding
topology and/or sub-optimal flooding reduction. As an aid to
diagnosing problems when dynamic flooding is in use, this document
defines a means of advertising what local edges are enabled for
flooding (LEEF). The protocol specific encodings are defined in
Sections 5.1.6 and 5.2.8.
The following guidelines apply:
Advertisement of LEEFs is optional.
As the flooding topology is defined by edges (not by links), in
cases where parallel adjacencies to the same neighbor exist, the
advertisement SHOULD indicate that all such links have been
enabled.
LEEF advertisements MUST NOT include edges enabled for temporary
flooding (Section 6.7).
LEEF advertisements MUST NOT be used either when calculating a
flooding topology or when determining what links to add
temporarily to the flooding topology when the flooding topology is
temporarily partitioned.
Li, et al. Expires 9 December 2022 [Page 12]
Internet-Draft Dynamic Flooding June 2022
5. Protocol Elements
5.1. IS-IS TLVs
The following TLVs/sub-TLVs are added to IS-IS:
1. A sub-TLV that an IS may inject into its LSP to indicate its
preference for becoming Area Leader.
2. A sub-TLV that an IS may inject into its LSP to indicate that it
supports Dynamic Flooding and the algorithms that it supports for
distributed mode, if any.
3. A TLV to carry the list of system IDs that compromise the
flooding topology for the area.
4. A TLV to carry a path which is part of the flooding topology
5. A TLV that requests flooding from the adjacent node
5.1.1. IS-IS Area Leader Sub-TLV
The Area Leader Sub-TLV allows a system to:
1. Indicate its eligibility and priority for becoming Area Leader.
2. Indicate whether centralized or distributed mode is to be used to
compute the flooding topology in the area.
3. Indicate the algorithm identifier for the algorithm that is used
to compute the flooding topology in distributed mode.
Intermediate Systems (nodes) that are not advertising this Sub-TLV
are not eligible to become Area Leader.
The Area Leader is the node with the numerically highest Area Leader
priority in the area. In the event of ties, the node with the
numerically highest system ID is the Area Leader. Due to transients
during database flooding, different nodes may not agree on the Area
Leader.
The Area Leader Sub-TLV is advertised as a Sub-TLV of the IS-IS
Router Capability TLV-242 that is defined in [RFC7981] and has the
following format:
Li, et al. Expires 9 December 2022 [Page 13]
Internet-Draft Dynamic Flooding June 2022
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Priority | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD1
Length: 2
Priority: 0-255, unsigned integer
Algorithm: a numeric identifier in the range 0-255 that identifies
the algorithm used to calculate the flooding topology. The
following values are defined:
- 0: Centralized computation by the Area Leader.
- 1-127: Standardized distributed algorithms. Individual values
are are to be assigned according to the "Specification
Required" policy defined in [RFC8126] (see Section 7.3).
- 128-254: Private distributed algorithms. Individual values are
are to be assigned according to the "Private Use" policy
defined in [RFC8126] (see Section 7.3).
- 255: Reserved
5.1.2. IS-IS Dynamic Flooding Sub-TLV
The Dynamic Flooding Sub-TLV allows a system to:
1. Indicate that it supports Dynamic Flooding. This is indicated by
the advertisement of this Sub-TLV.
2. Indicate the set of algorithms that it supports for distributed
mode, if any.
In incremental deployments, understanding which nodes support Dynamic
Flooding can be used to optimize the flooding topology. In
distributed mode, knowing the capabilities of the nodes can allow the
Area Leader to select the optimal algorithm.
The Dynamic Flooding Sub-TLV is advertised as a Sub-TLV of the IS-IS
Router Capability TLV (242) [RFC7981] and has the following format:
Li, et al. Expires 9 December 2022 [Page 14]
Internet-Draft Dynamic Flooding June 2022
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Algorithm... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD7
Length: 0-255; number of Algorithms
Algorithm: zero or more numeric identifiers in the range 0-255
that identifies the algorithm used to calculate the flooding
topology, as described in Section 5.1.1.
5.1.3. IS-IS Area Node IDs TLV
The IS-IS Area Node IDs TLV is only used in centralized mode.
The Area Node IDs TLV is used by the Area Leader to enumerate the
Node IDs (System ID + pseudo-node ID) that it has used in computing
the area flooding topology. Conceptually, the Area Leader creates a
list of node IDs for all nodes in the area (including pseudo-nodes
for all LANs in the topology), assigning indices to each node,
starting with index 0.
Because the space in a single TLV is limited, more than one TLV may
be required to encode all of the node IDs in the area. This TLV may
be present in multiple LSPs.
The format of the Area Node IDs TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Starting Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Reserved | Node IDs ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Node IDs continued ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD2
Length: 3 + ((System ID Length + 1) * (number of node IDs))
Starting index: The index of the first node ID that appears in
this TLV.
Li, et al. Expires 9 December 2022 [Page 15]
Internet-Draft Dynamic Flooding June 2022
L (Last): This bit is set if the index of the last node ID that
appears in this TLV is equal to the last index in the full list of
node IDs for the area.
Node IDs: A concatenated list of node IDs for the area
If there are multiple IS-IS Area Node IDs TLVs with the L bit set
advertised by the same node, the TLV which specifies the smaller
maximum index is used and the other TLV(s) with L bit set are
ignored. TLVs which specify node IDs with indices greater than that
specified by the TLV with the L bit set are also ignored.
5.1.4. IS-IS Flooding Path TLV
IS-IS Flooding Path TLV is only used in centralized mode.
The Flooding Path TLV is used to denote a path in the flooding
topology. The goal is an efficient encoding of the links of the
topology. A single link is a simple case of a path that only covers
two nodes. A connected path may be described as a sequence of
indices: (I1, I2, I3, ...), denoting a link from the system with
index 1 to the system with index 2, a link from the system with index
2 to the system with index 3, and so on.
If a path exceeds the size that can be stored in a single TLV, then
the path may be distributed across multiple TLVs by the replication
of a single system index.
Complex topologies that are not a single path can be described using
multiple TLVs.
The Flooding Path TLV contains a list of system indices relative to
the systems advertised through the Area Node IDs TLV. At least 2
indices must be included in the TLV. Due to the length restriction
of TLVs, this TLV can contain at most 126 system indices.
The Flooding Path TLV has the format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Starting Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Index 2 | Additional indices ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD3
Li, et al. Expires 9 December 2022 [Page 16]
Internet-Draft Dynamic Flooding June 2022
Length: 2 * (number of indices in the path)
Starting index: The index of the first system in the path.
Index 2: The index of the next system in the path.
Additional indices (optional): A sequence of additional indices to
systems along the path.
5.1.5. IS-IS Flooding Request TLV
The Flooding Request TLV allows a system to request an adjacent node
to enable flooding towards it on a specific link in the case where
the connection to adjacent node is not part of the existing flooding
topology.
Nodes that support Dynamic Flooding MAY include the Flooding Request
TLV in its IIH PDUs.
The Flooding Request TLV has the format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Levels |R| Scope |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| ... |
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD9
Length: 1 + number of advertised Flooding Scopes
Levels - the level(s) for which flooding is requested. Levels are
encoded as the circuit type specified in IS-IS [ISO10589]
R bit: MUST be 0 and is ignored on receipt.
Scope: Flooding Scope for which the flooding is requested as
defined by LSP Flooding Scope Identifier Registry defined by
[RFC7356]. Inclusion of flooding scopes is optional and is only
necessary if [RFC7356] is supported. Multiple flooding scopes MAY
be included.
Circuit Flooding Scope MUST NOT be sent in the Flooding Request TLV
and MUST be ignored if received.
Li, et al. Expires 9 December 2022 [Page 17]
Internet-Draft Dynamic Flooding June 2022
When the TLV is received in a level specific LAN-Hello PDU (L1-LAN-
IIH or L2-LAN-IIH) only levels which match the PDU type are valid.
Levels which do not match the PDU type MUST be ignored on receipt.
When the TLV is received in a Point-to-Point Hello (P2P-IIH) only
levels which are supported by the established adjacency are valid.
Levels which are not supported by the adjacency MUST be ignored on
receipt.
If flooding was disabled on the received link due to Dynamic
Flooding, then flooding MUST be temporarily enabled over the link for
the specified Circuit Type(s) and Flooding Scope(s) received in the
in the Flooding Request TLV. Flooding MUST be enabled until the
Circuit Type or Flooding Scope is no longer advertised in the
Flooding Request TLV or the TLV no longer appears in IIH PDUs
received on the link.
When the flooding is temporarily enabled on the link for any Circuit
Type or Flooding Scope due to received Flooding Request TLV, the
receiver MUST perform standard database synchronization for the
corresponding Circuit Type(s) and Flooding Scope(s) on the link. In
the case of IS-IS, this results in setting SRM bit for all related
LSPs on the link and sending CSNPs.
So long as the Flooding Request TLV is being received flooding MUST
NOT be disabled for any of the Circuit Types or Flooding Scopes
present in the Flooding Request TLV even if the connection between
the neighbors is removed from the flooding topology. Flooding for
such Circuit Types or Flooding Scopes MUST continue on the link and
be considered as temporarily enabled.
5.1.6. IS-IS LEEF Advertisement
In support of advertising which edges are currently enabled in the
flooding topology, an implementation MAY indicate that a link is part
of the flooding topology by advertising a bit value in the Link
Attributes sub-TLV defined by [RFC5029].
The following bit value is defined by this document:
Local Edge Enabled for Flooding (LEEF) - suggested value 4 (to be
assigned by IANA)
5.2. OSPF LSAs and TLVs
This section defines new LSAs and TLVs for both OSPFv2 and OSPFv3.
Following objects are added:
Li, et al. Expires 9 December 2022 [Page 18]
Internet-Draft Dynamic Flooding June 2022
1. A TLV that is used to advertise the preference for becoming Area
Leader.
2. A TLV that is used to indicate the support for Dynamic Flooding
and the algorithms that the advertising node supports for
distributed mode, if any.
3. OSPFv2 Opaque LSA and OSPFv3 LSA to advertise the flooding
topology for centralized mode.
4. A TLV to carry the list of Router IDs that comprise the flooding
topology for the area.
5. A TLV to carry a path which is part of the flooding topology.
6. The bit in the LLS Type 1 Extended Options and Flags requests
flooding from the adjacent node.
5.2.1. OSPF Area Leader Sub-TLV
The usage of the OSPF Area Leader Sub-TLV is identical to IS-IS and
is described in Section 5.1.1.
The OSPF Area Leader Sub-TLV is used by both OSPFv2 and OSPFv3.
The OSPF Area Leader Sub-TLV is advertised as a top-level TLV of the
RI LSA that is defined in [RFC7770] and has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Priority | Algorithm | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD4
Length: 4 octets
Priority: 0-255, unsigned integer
Algorithm: as defined in Section 5.1.1.
Li, et al. Expires 9 December 2022 [Page 19]
Internet-Draft Dynamic Flooding June 2022
5.2.2. OSPF Dynamic Flooding Sub-TLV
The usage of the OSPF Dynamic Flooding Sub-TLV is identical to IS-IS
and is described in Section 5.1.2.
The OSPF Dynamic Flooding Sub-TLV is used by both OSPFv2 and OSPFv3.
The OSPF Dynamic Flooding Sub-TLV is advertised as a top-level TLV of
the RI LSA that is defined in [RFC7770] and has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm ... | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD8
Length: number of Algorithms
Algorithm: as defined in Section 5.1.1.
5.2.3. OSPFv2 Dynamic Flooding Opaque LSA
The OSPFv2 Dynamic Flooding Opaque LSA is only used in centralized
mode.
The OSPFv2 Dynamic Flooding Opaque LSA is used to advertise
additional data related to the dynamic flooding in OSPFv2. OSPFv2
Opaque LSAs are described in [RFC5250].
Multiple OSPFv2 Dynamic Flooding Opaque LSAs can be advertised by an
OSPFv2 router. The flooding scope of the OSPFv2 Dynamic Flooding
Opaque LSA is area-local.
The format of the OSPFv2 Dynamic Flooding Opaque LSA is as follows:
Li, et al. Expires 9 December 2022 [Page 20]
Internet-Draft Dynamic Flooding June 2022
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | LS Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD5 | Opaque ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- TLVs -+
| ... |
Figure 1: OSPFv2 Dynamic Flooding Opaque LSA
The opaque type used by OSPFv2 Dynamic Flooding Opaque LSA is TBD.
The opaque type is used to differentiate the various type of OSPFv2
Opaque LSAs and is described in section 3 of [RFC5250]. The LS Type
is 10. The LSA Length field [RFC2328] represents the total length
(in octets) of the Opaque LSA including the LSA header and all TLVs
(including padding).
The Opaque ID field is an arbitrary value used to maintain multiple
Dynamic Flooding Opaque LSAs. For OSPFv2 Dynamic Flooding Opaque
LSAs, the Opaque ID has no semantic significance other than to
differentiate Dynamic Flooding Opaque LSAs originated by the same
OSPFv2 router.
The format of the TLVs within the body of the OSPFv2 Dynamic Flooding
Opaque LSA is the same as the format used by the Traffic Engineering
Extensions to OSPF [RFC3630].
The Length field defines the length of the value portion in octets
(thus a TLV with no value portion would have a length of 0). The TLV
is padded to 4-octet alignment; padding is not included in the length
field (so a 3-octet value would have a length of 3, but the total
size of the TLV would be 8 octets). Nested TLVs are also 32-bit
aligned. For example, a 1-octet value would have the length field
set to 1, and 3 octets of padding would be added to the end of the
value portion of the TLV. The padding is composed of zeros.
Li, et al. Expires 9 December 2022 [Page 21]
Internet-Draft Dynamic Flooding June 2022
5.2.4. OSPFv3 Dynamic Flooding LSA
The OSPFv3 Dynamic Flooding Opaque LSA is only used in centralized
mode.
The OSPFv3 Dynamic Flooding LSA is used to advertise additional data
related to the dynamic flooding in OSPFv3.
The OSPFv3 Dynamic Flooding LSA has a function code of TBD. The
flooding scope of the OSPFv3 Dynamic Flooding LSA is area-local. The
U bit will be set indicating that the OSPFv3 Dynamic Flooding LSA
should be flooded even if it is not understood. The Link State ID
(LSID) value for this LSA is the Instance ID. OSPFv3 routers MAY
advertise multiple Dynamic Flooding Opaque LSAs in each area.
The format of the OSPFv3 Dynamic Flooding LSA is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age |1|0|1| TBD6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- TLVs -+
| ... |
Figure 2: OSPFv3 Dynamic Flooding LSA
5.2.5. OSPF Area Router ID TLVs
In OSPF new TLVs are introduced to advertise indeces associated with
nodes and Broadcast/NBMA networks. Due to identifier differences
between OSPFv2 and OSPFv3 two different TLVs are defined as decribed
in the following sub-sections.
Li, et al. Expires 9 December 2022 [Page 22]
Internet-Draft Dynamic Flooding June 2022
The OSPF Area Router ID TLVs are used by the Area Leader to enumerate
the Router IDs that it has used in computing the flooding topology.
This includes the identifiers associated with Broadcast/NBMA networks
as defined for Network LSAs. Conceptually, the Area Leader creates a
list of Router IDs for all routers in the area, assigning indices to
each router, starting with index 0.
5.2.5.1. OSPFv2 Area Router ID TLV
This TLV is a top level TLV of the OSPFv2 Dynamic Flooding Opaque
LSA.
Because the space in a single OSPFv2 Area Router IDs TLV is limited,
more than one TLV may be required to encode all of the Router IDs in
the area. This TLV may also occur in multiple OSPFv2 Dynamic
Flooding Opaque LSAs so that all Router IDs can be advertised.
Each entry in the OSPFv2 Area Router IDs TLV represents either a node
or a Broadcast/NBMA network identifier. An entry has the following
format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Conn Type | Number of IDs | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Originating Router ID/DR Address -+
| ... |
Figure 3: OSPFv2 Router IDs TLV Entry
Conn Type: 1 byte
- The following values are defined:
- 1 - Router
- 2 - Designated Router
Number of IDs: 2 bytes
Reserved: 1 byte, MUST be transmitted as 0 and MUST be ignored on
receipt
Originating Router ID/DR Address:(4 * Number of IDs) bytes as
indicated by the ID Type
Li, et al. Expires 9 December 2022 [Page 23]
Internet-Draft Dynamic Flooding June 2022
The format of the Area Router IDs TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Starting Index |L| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- OSPFv2 Router ID TLV Entry -+
| ... |
Figure 4: OSPFv2 Area Router IDs TLV
TLV Type: 1
TLV Length: 4 + (8 * the number TLV entries)
Starting index: The index of the first Router/Designated Router ID
that appears in this TLV.
L (Last): This bit is set if the index of the last Router/
Designated ID that appears in this TLV is equal to the last index
in the full list of Rourer IDs for the area.
OSPFv2 Router ID TLV Entries: A concatenated list of Router ID TLV
Entries for the area.
If there are multiple OSPFv2 Area Router ID TLVs with the L bit set
advertised by the same router, the TLV which specifies the smaller
maximum index is used and the other TLV(s) with L bit set are
ignored. TLVs which specify Router IDs with indices greater than
that specified by the TLV with the L bit set are also ignored.
5.2.5.2. OSPFv3 Area Router ID TLV
This TLV is a top level TLV of the OSPFv3 Dynamic Flooding LSA.
Because the space in a single OSPFv3 Area Router ID TLV is limited,
more than one TLV may be required to encode all of the Router IDs in
the area. This TLV may also occur in multiple OSPFv3 Dynamic
Flooding Opaque LSAs so that all Router IDs can be advertised.
Each entry in the OSPFv3 Area Router IDs TLV represents either a
router or a Broadcast/NBMA network identifier. An entry has the
following format:
Li, et al. Expires 9 December 2022 [Page 24]
Internet-Draft Dynamic Flooding June 2022
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Conn Type | Number of IDs | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Originating ID Entry -+
| ... |
where
Conn Type - 1 byte
The following values are defined:
1 - Router
2 - Designated Router
Number of IDs - 2 bytes
Reserved - 1 byte
MUST be transmitted as 0 and MUST be ignored on receipt
Originating ID Entry takes one of the following forms:
Router:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originating Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length of Originating ID Entry is 4 * Number of IDs) bytes
Designated Router:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originating Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length of Originating ID Entry is (8 * Number of IDs) bytes
Figure 5: OSPFv3 Router ID TLV Entry
The format of the OSPFv3Area Router IDs TLV is:
Li, et al. Expires 9 December 2022 [Page 25]
Internet-Draft Dynamic Flooding June 2022
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Starting Index |L| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- OSPFv3 Router ID TLV Entry -+
| ... |
Figure 6: OSPFv3 Area Router IDs TLV
TLV Type: 1
TLV Length: 4 + sum of the lengths of all TLV entries
Starting index: The index of the first Router/Designated Router ID
that appears in this TLV.
L (Last): This bit is set if the index of the last Router/
Designated Router ID that appears in this TLV is equal to the last
index in the full list of Router IDs for the area.
OSPFv3 Router ID TLV Entries: A concatenated list of Router ID TLV
Entries for the area.
If there are multiple OSPFv3 Area Router ID TLVs with the L bit set
advertised by the same router, the TLV which specifies the smaller
maximum index is used and the other TLV(s) with L bit set are
ignored. TLVs which specify Router IDs with indices greater than
that specified by the TLV with the L bit set are also ignored.
5.2.6. OSPF Flooding Path TLV
The OSPF Flooding Path TLV is a top level TLV of the OSPFv2 Dynamic
Flooding Opaque LSAs and OSPFv3 Dynamic Flooding LSA.
The usage of the OSPF Flooding Path TLV is identical to IS-IS and is
described in Section 5.1.4.
The OSPF Flooding Path TLV contains a list of Router ID indices
relative to the Router IDs advertised through the OSPF Area Router
IDs TLV. At least 2 indices must be included in the TLV.
Li, et al. Expires 9 December 2022 [Page 26]
Internet-Draft Dynamic Flooding June 2022
Multiple OSPF Flooding Path TLVs can be advertised in a single OSPFv2
Dynamic Flooding Opaque LSA or OSPFv3 Dynamic Flooding LSA. OSPF
Flooding Path TLVs can also be advertised in multiple OSPFv2 Dynamic
Flooding Opaque LSAs or OSPFv3 Dynamic Flooding LSA, if they all can
not fit in a single LSA.
The Flooding Path TLV has the format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Starting Index | Index 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Additional Indices -+
| ... |
Figure 7: OSPF Flooding Path TLV
TLV Type: 2
TLV Length: 2 * (number of indices in the path)
Starting index: The index of the first Router ID in the path.
Index 2: The index of the next Router ID in the path.
Additional indices (optional): A sequence of additional indices to
Router IDs along the path.
5.2.7. OSPF Flooding Request Bit
A single new option bit, the Flooding-Request (FR-bit), is defined in
the LLS Type 1 Extended Options and Flags field [RFC2328]. The FR-
bit allows a router to request an adjacent node to enable flooding
towards it on a specific link in the case where the connection to
adjacent node is not part of the current flooding topology.
Nodes that support Dynamic Flooding MAY include FR-bit in its OSPF
LLS Extended Options and Flags TLV.
Li, et al. Expires 9 December 2022 [Page 27]
Internet-Draft Dynamic Flooding June 2022
If FR-bit is signalled for an area for which the flooding on the link
was disabled due to Dynamic Flooding, the flooding MUST be
temporarily enabled over such link and area. Flooding MUST be
enabled until FR-bit is no longer advertised in the OSPF LLS Extended
Options and Flags TLV or the OSPF LLS Extended Options and Flags TLV
no longer appears in the OSPF Hellos.
When the flooding is temporarily enabled on the link for any area due
to received FR-bit in OSPF LLS Extended Options and Flags TLV, the
receiver MUST perform standard database synchronization for the
corresponding area(s) on the link. If the adjacency is already in
the FULL state, mechanism specified in [RFC4811] MUST be used for
database resynchronization.
So long as the FR-bit is being received in the OSPF LLS Extended
Options and Flags TLV for an area, flooding MUST NOT be disabled in
such area even if the connection between the neighbors is removed
from the flooding topology. Flooding for such area MUST continue on
the link and be considered as temporarily enabled.
5.2.8. OSPF LEEF Advertisement
In support of advertising which edges are currently enabled in the
flooding topology, an implementation MAY indicate that a link is part
of the flooding topology. The OSPF Link Attributes Bits TLV is
defined to support this advertisement.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Attribute Bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Additional Link Attribute Bits -+
| ... |
Figure 8: OSPF Link Attributes Bits TLV
Type: TBD and specific to OSPFv2 and OSPFv3
Length: size of the Link Attribute Bits in bytes. It MUST be a
multiple of 4 bytes.
The following bits are defined:
Bit #0: - Local Edge Enabled for Flooding (LEEF)
Li, et al. Expires 9 December 2022 [Page 28]
Internet-Draft Dynamic Flooding June 2022
OSPF Link-attribute Bits TLV appears as:
1. a sub-TLV of the OSPFv2 Extended Link TLV [RFC7684]
2. a sub-TLV of the OSPFv3 Router-Link TLV [RFC8362]
6. Behavioral Specification
In this section, we specify the detailed behaviors of the nodes
participating in the IGP.
6.1. Terminology
We define some terminology here that is used in the following
sections:
A node is considered reachable if it is part of the connected
network graph. Note that this is independent of any constraints
which may be considered when performing IGP SPT calculation (e.g.,
link metrics, OL bit state, etc.). Two-way-connectivity check
MUST be performed before including an edge in the connected
network graph.
Node is connected to the flooding topology, if it has at least one
local link, which is part of the flooding topology.
Node is disconnected from the flooding topology when it is not
connected to the flooding topology.
Current flooding topology - latest version of the flooding
topology received (in case of the centralized mode) or calculated
locally (in case of the distributed mode).
6.2. Flooding Topology
The flooding topology MUST include all reachable nodes in the area.
If a node's reachability changes, the flooding topology MUST be
recalculated. In centralized mode, the Area Leader MUST advertise a
new flooding topology.
If a node becomes disconnected from the current flooding topology but
is still reachable then a new flooding topology MUST be calculated.
In centralized mode the Area Leader MUST advertise the new flooding
topology.
The flooding topology SHOULD be bi-connected.
Li, et al. Expires 9 December 2022 [Page 29]
Internet-Draft Dynamic Flooding June 2022
6.3. Leader Election
Any node that is capable MAY advertise its eligibility to become Area
Leader.
Nodes that are not reachable are not eligible as Area Leader. Nodes
that do not advertise their eligibility to become Area Leader are not
eligible. Amongst the eligible nodes, the node with the numerically
highest priority is the Area Leader. If multiple nodes all have the
highest priority, then the node with the numerically highest system
identifier in the case of IS-IS, or Router-ID in the case of OSPFv2
and OSPFv3 is the Area Leader.
6.4. Area Leader Responsibilities
If the Area Leader operates in centralized mode, it MUST advertise
algorithm 0 in its Area Leader Sub-TLV. In order for Dynamic
Flooding to be enabled it also MUST compute and advertise a flooding
topology for the area. The Area Leader may update the flooding
topology at any time, however, it should not destabilize the network
with undue or overly frequent topology changes. If the Area Leader
operates in centralized mode and needs to advertise a new flooding
topology, it floods the new flooding topology on both the new and old
flooding topologies.
If the Area Leader operates in distributed mode, it MUST advertise a
non-zero algorithm in its Area Leader Sub-TLV.
When the Area Leader advertises algorithm 0 in its Area Leader Sub-
TLV and does not advertise a flooding topology, Dynamic Flooding is
disabled for the area. Note this applies whether the Area Leader
intends to operate in centralized mode or in distributed mode.
Note that once Dynamic Flooding is enabled, disabling it risks
destabilizing the network.
6.5. Distributed Flooding Topology Calculation
If the Area Leader advertises a non-zero algorithm in its Area Leader
Sub-TLV, all nodes in the area that support Dynamic Flooding and the
value of algorithm advertised by the Area Leader MUST compute the
flooding topology based on the Area Leader's advertised algorithm.
Nodes that do not support the value of algorithm advertised by the
Area Leader MUST continue to use standard flooding mechanism as
defined by the protocol.
Li, et al. Expires 9 December 2022 [Page 30]
Internet-Draft Dynamic Flooding June 2022
Nodes that do not support the value of algorithm advertised by the
Area Leader MUST be considered as Dynamic Flooding incapable nodes by
the Area Leader.
If the value of the algorithm advertised by the Area Leader is from
the range 128-254 (private distributed algorithms), it is the
responsibility of the network operator to guarantee that all nodes in
the area have a common understanding of what the given algorithm
value represents.
6.6. Use of LANs in the Flooding Topology
Use of LANs in the flooding topology differs depending on whether the
area is operating in Centralized or Distributed mode.
6.6.1. Use of LANs in Centralized mode
As specified in Section 4.5, when a LAN is advertised as part of the
flooding topology, all nodes connected to the LAN are assumed to be
using the LAN as part of the flooding topology. This assumption is
made to reduce the size of the Flooding Topology advertisement.
6.6.2. Use of LANs in Distributed Mode
In distributed mode, the flooding topology is NOT advertised,
therefore the space consumed to advertise it is not a concern. It is
therefore possible to assign only a subset of the nodes connected to
the LAN to use the LAN as part of the flooding topology. Doing so
may further optimize flooding by reducing the amount of redundant
flooding on a LAN. However, support of flooding only by a subset of
the nodes connected to a LAN requires some modest - but backwards
compatible - changes in the way flooding is performed on a LAN.
6.6.2.1. Partial flooding on a LAN in IS-IS
Designated Intermediate System (DIS) for a LAN MUST use standard
flooding behavior.
Non-DIS nodes whose connection to the LAN is included in the flooding
topology MUST use standard flooding behavior.
Non-DIS nodes whose connection to the LAN is NOT included in the
flooding topology behave as follows:
* Received CSNPs from the DIS are ignored
* PSNPs are NOT originated on the LAN
Li, et al. Expires 9 December 2022 [Page 31]
Internet-Draft Dynamic Flooding June 2022
* LSPs received on the LAN which are newer than the corresponding
LSP present in the LSPDB are retained and flooded on all local
circuits which are part of the flooding topology (i.e., do not
discard newer LSPs simply because they were received on a LAN
which the receiving node is not using for flooding)
* LSPs received on the LAN which are older or same as the
corresponding LSP present in the LSPDB are silently discarded
* LSPs received on links other than the LAN are NOT flooded on the
LAN
NOTE: If any node connected to the LAN requests the enablement of
temporary flooding all nodes revert to standard flooding behavior.
6.6.2.2. Partial Flooding on a LAN in OSPF
Designated Router (DR) and Backup Designated Router (BDR) for LANs
MUST use standard flooding behavior.
Non-DR/BDR nodes whose connection to the LAN is included in the
flooding topology use standard flooding behavior.
Non-DR/BDR nodes whose connection to the LAN is NOT included in the
flooding topology behave as follows:
* LSAs received on the LAN are acknowledged to DR/BDR
* LSAs received on interfaces other than the LAN are NOT flooded on
the LAN
NOTE: If any node connected to the LAN requests the enablement of
temporary flooding all nodes revert to standard flooding behavior.
NOTE: The sending of LSA acks by nodes NOT using the LAN as part of
the flooding topology eliminates the need for changes on the part of
the DR/BDR, which might include nodes which do not support the
flooding optimizations.
6.7. Flooding Behavior
Nodes that support Dynamic Flooding MUST use the flooding topology
for flooding when possible, and MUST NOT revert to standard flooding
when a valid flooding topology is available.
Li, et al. Expires 9 December 2022 [Page 32]
Internet-Draft Dynamic Flooding June 2022
In some cases a node that supports Dynamic Flooding may need to add a
local link(s) to the flooding topology temporarily, even though the
link(s) is not part of the calculated flooding topology. This is
termed "temporary flooding" and is discussed in Section 6.8.1.
The flooding topology is calculated locally in the case of
distributed mode. In centralized mode the flooding topology is
advertised in the area link state database. Received link state
updates, whether received on a link that is in the flooding topology
or on a link that is not in the flooding topology, MUST be flooded on
all links that are in the flooding topology, except for the link on
which the update was received.
In centralized mode, if multiple flooding topologies are present in
the area link state database, the node SHOULD flood on each of these
topologies.
When the flooding topology changes on a node, either as a result of
the local computation in distributed mode or as a result of the
advertisement from the Area Leader in centralized mode, the node MUST
continue to flood on both the old and new flooding topology for a
limited amount of time. This is required to provide all nodes
sufficient time to migrate to the new flooding topology.
6.8. Treatment of Topology Events
In this section, we explicitly consider a variety of different
topological events in the network and how Dynamic Flooding should
address them.
6.8.1. Temporary Addition of Link to Flooding Topology
In some cases a node that supports Dynamic Flooding may need to add a
local link(s) to the flooding topology temporarily, even though the
link(s) is not part of the calculated flooding topology. We refer to
this as "temporary flooding" on the link.
When temporary flooding is enabled on the link, the flooding needs to
be enabled from both directions on the link. To achieve that, the
following steps MUST be performed:
Link State Database needs to be re-synchronised on the link. This
is done using the standard protocol mechanisms. In the case of
IS-IS, this results in setting SRM bit for all LSPs on the circuit
and sending compete set of CSNPs on it. In OSPF, the mechanism
specified in [RFC4811] is used.
Flooding is enabled locally on the link.
Li, et al. Expires 9 December 2022 [Page 33]
Internet-Draft Dynamic Flooding June 2022
Flooding is requested from the neighbor using the mechanism
specified in section Section 5.1.5 or Section 5.2.7.
The request for temporary flooding is withdrawn on the link when all
of the following conditions are met:
Node itself is connected to the current flooding topology.
Adjacent node is connected to the current flooding topology.
Any change in the flooding topology MUST result in evaluation of the
above conditions for any link on which the temporary flooding was
enabled.
Temporary flooding is stopped on the link when both adjacent nodes
stop requesting temporary flooding on the link.
6.8.2. Local Link Addition
If a local link is added to the topology, the protocol will form a
normal adjacency on the link and update the appropriate link state
advertisements for the nodes on either end of the link. These link
state updates will be flooded on the flooding topology.
In centralized mode, the Area Leader, upon receiving these updates,
may choose to retain the existing flooding topology or may choose to
modify the flooding topology. If it elects to change the flooding
topology, it will update the flooding topology in the link state
database and flood it using the new flooding topology.
In distributed mode, any change in the topology, including the link
addition, MUST trigger the flooding topology recalculation. This is
done to ensure that all nodes converge to the same flooding topology,
regardless of the time of the calculation.
Temporary flooding MUST be enabled on the newly added local link, if
at least one of the following conditions are met:
The node on which the local link was added is not connected to the
current flooding topology.
The new adjacent node is not connected to the current flooding
topology.
Note that in this case there is no need to perform a database
synchronization as part of the enablement of the temporary flooding,
because it has been part of the adjacency bring-up itself.
Li, et al. Expires 9 December 2022 [Page 34]
Internet-Draft Dynamic Flooding June 2022
If multiple local links are added to the topology before the flooding
topology is updated, temporary flooding MUST be enabled on a subset
of these links.
6.8.3. Node Addition
If a node is added to the topology, then at least one link is also
added to the topology. Section 6.8.2 applies.
A node which has a large number of neighbors is at risk for
introducing a local flooding storm if all neighbors are brought up at
once and temporary flooding is enabled on all links simultaneously.
The most robust way to address this is to limit the rate of initial
adjacency formation following bootup. This both reduces unnecessary
redundant flooding as part of initial database synchronization and
minimizes the need for temporary flooding as it allows time for the
new node to be added to the flooding topology after only a small
number of adjacencies have been formed.
In the event a node elects to bring up a large number of adjacencies
simultaneously, a significant amount of redundant flooding may be
introduced as multiple neighbors of the new node enable temporary
flooding to the new node which initially is not part of the flooding
topology.
6.8.4. Failures of Link Not on Flooding Topology
If a link that is not part of the flooding topology fails, then the
adjacent nodes will update their link state advertisements and flood
them on the flooding topology.
In centralized mode, the Area Leader, upon receiving these updates,
may choose to retain the existing flooding topology or may choose to
modify the flooding topology. If it elects to change the flooding
topology, it will update the flooding topology in the link state
database and flood it using the new flooding topology.
In distributed mode, any change in the topology, including the
failure of the link that is not part of the flooding topology MUST
trigger the flooding topology recalculation. This is done to ensure
that all nodes converge to the same flooding topology, regardless of
the time of the calculation.
Li, et al. Expires 9 December 2022 [Page 35]
Internet-Draft Dynamic Flooding June 2022
6.8.5. Failures of Link On the Flooding Topology
If there is a failure on the flooding topology, the adjacent nodes
will update their link state advertisements and flood them. If the
original flooding topology is bi-connected, the flooding topology
should still be connected despite a single failure.
If the failed local link represented the only connection to the
flooding topology on the node where the link failed, the node MUST
enable temporary flooding on a subset of its local links. This
allows the node to send its updated link state advertisement(s) and
also keep receiving link state updates from other nodes in the
network before the new flooding topology is calculated and
distributed (in the case of centralized mode).
In centralized mode, the Area Leader will notice the change in the
flooding topology, recompute the flooding topology, and flood it
using the new flooding topology.
In distributed mode, all nodes supporting dynamic flooding will
notice the change in the topology and recompute the new flooding
topology.
6.8.6. Node Deletion
If a node is deleted from the topology, then at least one link is
also removed from the topology. Section 6.8.4 and Section 6.8.5
apply.
6.8.7. Local Link Addition to the Flooding Topology
If the new flooding topology is received in the case of centralized
mode, or calculated locally in the case of distributed mode and the
local link on the node that was not part of the flooding topology has
been added to the flooding topology, the node MUST:
Re-synchronize the Link State Database over the link. This is
done using the standard protocol mechanisms. In the case of IS-
IS, this results in setting SRM bit for all LSPs on the circuit
and sending a complete set of CSNPs. In OSPF, the mechanism
specified in [RFC4811] is used.
Make the link part of the flooding topology and start flooding
over it
Li, et al. Expires 9 December 2022 [Page 36]
Internet-Draft Dynamic Flooding June 2022
6.8.8. Local Link Deletion from the Flooding Topology
If the new flooding topology is received in the case of centralized
mode, or calculated locally in the case of distributed mode and the
local link on the node that was part of the flooding topology has
been removed from the flooding topology, the node MUST remove the
link from the flooding topology.
The node MUST keep flooding on such link for a limited amount of time
to allow other nodes to migrate to the new flooding topology.
If the removed local link represented the only connection to the
flooding topology on the node, the node MUST enable temporary
flooding on a subset of its local links. This allows the node to
send its updated link state advertisement(s) and also keep receiving
link state updates from other nodes in the network before the new
flooding topology is calculated and distributed (in the case of
centralized mode).
6.8.9. Treatment of Disconnected Adjacent Nodes
Every time there is a change in the flooding topology a node MUST
check if there are any adjacent nodes that are disconnected from the
current flooding topology. Temporary flooding MUST be enabled
towards a subset of the disconnected nodes.
6.8.10. Failure of the Area Leader
The failure of the Area Leader can be detected by observing that it
is no longer reachable. In this case, the Area Leader election
process is repeated and a new Area Leader is elected.
In order to minimize disruption to Dynamic Flooding if the Area
Leader becomes unreachable, the node which has the second highest
priority for becoming Area Leader (including the system identifier/
Router-ID tie breaker if necessary) SHOULD advertise the same
algorithm in its Area Leader Sub-TLV as the Area Leader and (in
centralized mode) SHOULD advertise a flooding topology. This SHOULD
be done even when the Area Leader is reachable.
In centralized mode, the new Area Leader will compute a new flooding
topology and flood it using the new flooding topology. To minimze
disruption, the new flooding topology SHOULD have as much in common
as possible with the old flooding topology. This will minimize the
risk of over-flooding.
Li, et al. Expires 9 December 2022 [Page 37]
Internet-Draft Dynamic Flooding June 2022
In the distributed mode, the new flooding topology will be calculated
on all nodes that support the algorithm that is advertised by the new
Area Leader. Nodes that do not support the algorithm advertised by
the new Area Leader will no longer participate in Dynamic Flooding
and will revert to standard flooding.
6.8.11. Recovery from Multiple Failures
In the unlikely event of multiple failures on the flooding topology,
it may become partitioned. The nodes that remain active on the edges
of the flooding topology partitions will recognize this and will try
to repair the flooding topology locally by enabling temporary
flooding towards the nodes that they consider disconnected from the
flooding topology until a new flooding topology becomes connected
again.
Nodes where local failure was detected update their own link state
advertisements and flood them on the remainder of the flooding
topology.
In centralized mode, the Area Leader will notice the change in the
flooding topology, recompute the flooding topology, and flood it
using the new flooding topology.
In distributed mode, all nodes that actively participate in Dynamic
Flooding will compute the new flooding topology.
Note that this is very different from the area partition because
there is still a connected network graph between the nodes in the
area. The area may remain connected and forwarding may still be
effective.
6.8.12. Rate Limiting Temporary Flooding
As discussed in the previous sections, there are events which require
the introduction of temporary flooding on edges which are not part of
the current flooding topology. This can occur regardless of whether
the area is operating in centralized mode or distributed mode.
Nodes which decide to enable temporary flooding also have to decide
whether to do so on a subset of the edges which are currently not
part of the flooding topology or on all the edges which are currently
not part of the flooding topology. Doing the former risks a longer
convergence time as it is possible that the initial set of edges
enabled does not fully repair the flooding topology. Doing the
latter risks introducing a flooding storm which destablizes the
network.
Li, et al. Expires 9 December 2022 [Page 38]
Internet-Draft Dynamic Flooding June 2022
It is recommended that a node implement rate limiting on the number
of edges on which it chooses to enable temporary flooding. Initial
values for the number of edges to enable and the rate at which
additional edges may subsequently be enabled is left as an
implementation decision.
7. IANA Considerations
7.1. IS-IS
This document requests the following code points from the "sub-TLVs
for TLV 242" registry (IS-IS Router CAPABILITY TLV).
Type: TBD1
Description: IS-IS Area Leader Sub-TLV
Reference: This document (Section 5.1.1)
Type: TBD7
Description: IS-IS Dynamic Flooding Sub-TLV
Reference: This document (Section 5.1.2)
This document requests that IANA allocate and assign code points from
the "IS-IS TLV Codepoints" registry. One for each of the following
TLVs:
Type: TBD2
Description: IS-IS Area System IDs TLV
Reference: This document (Section 5.1.3)
Type: TBD3
Description: IS-IS Flooding Path TLV
Reference: This document (Section 5.1.4)
Type: TBD9
Description: IS-IS Flooding Request TLV
Reference: This document (Section 5.1.5)
Li, et al. Expires 9 December 2022 [Page 39]
Internet-Draft Dynamic Flooding June 2022
This document requests that IANA allocate a new bit value from the
"link-attribute bit values for sub-TLV 19 of TLV 22" registry.
Local Edge Enabled for Flooding (LEEF) - suggested value 4 (to be
assigned by IANA)
7.2. OSPF
This document requests the following code points from the "OSPF
Router Information (RI) TLVs" registry:
Type: TBD4
Description: OSPF Area Leader Sub-TLV
Reference: This document (Section 5.2.1)
Type: TBD8
Description: OSPF Dynamic Flooding Sub-TLV
Reference: This document (Section 5.2.2)
This document requests the following code point from the "Opaque
Link-State Advertisements (LSA) Option Types" registry:
Type: TBD5
Description: OSPFv2 Dynamic Flooding Opaque LSA
Reference: This document (Section 5.2.3)
This document requests the following code point from the "OSPFv3 LSA
Function Codes" registry:
Type: TBD6
Description: OSPFv3 Dynamic Flooding LSA
Reference: This document (Section 5.2.4)
This document requests a new bit in LLS Type 1 Extended Options and
Flags registry:
Bit Position: TBD10
Description: Flooding Request bit
Li, et al. Expires 9 December 2022 [Page 40]
Internet-Draft Dynamic Flooding June 2022
Reference: This document (Section 5.2.7)
This document requests the following code point from the "OSPFv2
Extended Link TLV Sub-TLVs" registry:
Type: TBD11
Description: OSPFv2 Link Attributes Bits Sub-TLV
Reference: This document (Section 5.2.8)
This document requests the following code point from the "OSPFv3
Extended LSA Sub-TLVs" registry:
Type: TBD12
Description: OSPFv3 Link Attributes Bits Sub-TLV
Reference: This document (Section 5.2.8)
7.2.1. OSPF Dynamic Flooding LSA TLVs Registry
This specification also requests a new registry - "OSPF Dynamic
Flooding LSA TLVs". New values can be allocated via IETF Review or
IESG Approval
The "OSPF Dynamic Flooding LSA TLVs" registry will define top-level
TLVs for the OSPFv2 Dynamic Flooding Opaque LSA and OSPFv3 Dynamic
Flooding LSAs. It should be added to the "Open Shortest Path First
(OSPF) Parameters" registries group.
The following initial values are allocated:
Type: 0
Description: Reserved
Reference: This document
Type: 1
Description: OSPF Area Router IDs TLV
Reference: This document (Section 5.2.5)
Type: 2
Description: OSPF Flooding Path TLV
Li, et al. Expires 9 December 2022 [Page 41]
Internet-Draft Dynamic Flooding June 2022
Reference: This document (Section 5.2.6)
Types in the range 32768-33023 are for experimental use; these will
not be registered with IANA, and MUST NOT be mentioned by RFCs.
Types in the range 33024-65535 are not to be assigned at this time.
Before any assignments can be made in the 33024-65535 range, there
MUST be an IETF specification that specifies IANA Considerations that
covers the range being assigned.
7.2.2. OSPF Link Attributes Sub-TLV Bit Values Registry
This specification also requests a new registry - "OSPF Link
Attributes Sub-TLV Bit Values". New values can be allocated via IETF
Review or IESG Approval
The "OSPF Link Attributes Sub-TLV Bit Values" registry defines Link
Attribute bit values for the OSPFv2 Link Attributes Sub-TLV and
OSPFv3 Link Attributes Sub-TLV. It should be added to the "Open
Shortest Path First (OSPF) Parameters" registries group.
The following initial value is allocated:
Bit Number: 0
Description: Local Edge Enabled for Flooding(LEEF)
Reference: This document (Section 5.2.8)
7.3. IGP
IANA is requested to set up a registry called "IGP Algorithm Type For
Computing Flooding Topology" under an existing "Interior Gateway
Protocol (IGP) Parameters" IANA registries.
Values in this registry come from the range 0-255.
The initial values in the IGP Algorithm Type For Computing Flooding
Topology registry are:
0: Reserved for centralized mode.
1-127: Available for standards action. Individual values are to
be assigned according to the "Specification Required" policy
defined in [RFC8126].
128-254: Reserved for private use.
Li, et al. Expires 9 December 2022 [Page 42]
Internet-Draft Dynamic Flooding June 2022
255: Reserved.
8. Security Considerations
This document introduces no new security issues. Security of routing
within a domain is already addressed as part of the routing protocols
themselves. This document proposes no changes to those security
architectures.
It is possible that an attacker could become Area Leader and
introduce a flawed flooding algorithm into the network thus
compromising the operation of the protocol. Authentication methods
as describe in [RFC5304] and [RFC5310] for IS-IS, [RFC2328] and
[RFC7474] for OSPFv2 and [RFC5340] and [RFC4552] for OSPFv3 SHOULD be
used to prevent such attack.
9. Acknowledgements
The authors would like to thank Sarah Chen for her contribution to
this work.
The authors would like to thank Zeqing (Fred) Xia, Naiming Shen, Adam
Sweeney and Olufemi Komolafe for their helpful comments.
The authors would like to thank Tom Edsall for initially introducing
them to the problem.
Advertising Local Edges Enabled for Flooding (LEEF) is based on an
idea proposed in [I-D.cc-lsr-flooding-reduction]. We wish to thank
the authors of that draft.
10. References
10.1. Normative References
[ISO10589] ISO, "Intermediate System to Intermediate System Intra-
Domain Routing Exchange Protocol for use in Conjunction
with the Protocol for Providing the Connectionless-mode
Network Service (ISO 8473)", ISO/IEC 10589:2002, October
2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
Li, et al. Expires 9 December 2022 [Page 43]
Internet-Draft Dynamic Flooding June 2022
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
<https://www.rfc-editor.org/info/rfc4552>.
[RFC5029] Vasseur, JP. and S. Previdi, "Definition of an IS-IS Link
Attribute Sub-TLV", RFC 5029, DOI 10.17487/RFC5029,
September 2007, <https://www.rfc-editor.org/info/rfc5029>.
[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
July 2008, <https://www.rfc-editor.org/info/rfc5250>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <https://www.rfc-editor.org/info/rfc5304>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <https://www.rfc-editor.org/info/rfc5310>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC7356] Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
Scope Link State PDUs (LSPs)", RFC 7356,
DOI 10.17487/RFC7356, September 2014,
<https://www.rfc-editor.org/info/rfc7356>.
[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.
[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.
Li, et al. Expires 9 December 2022 [Page 44]
Internet-Draft Dynamic Flooding June 2022
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.
10.2. Informative References
[Clos] Clos, C., "A Study of Non-Blocking Switching Networks",
The Bell System Technical Journal Vol. 32(2), DOI
10.1002/j.1538-7305.1953.tb01433.x, March 1953,
<http://dx.doi.org/10.1002/j.1538-7305.1953.tb01433.x>.
[I-D.cc-lsr-flooding-reduction]
Chen, H., Toy, M., Yang, Y., Wang, A., Liu, X., Fan, Y.,
and L. Liu, "Flooding Topology Computation Algorithm",
Work in Progress, Internet-Draft, draft-cc-lsr-flooding-
reduction-09, 5 June 2020,
<https://www.ietf.org/archive/id/draft-cc-lsr-flooding-
reduction-09.txt>.
[Leiserson]
Leiserson, C. E., "Fat-Trees: Universal Networks for
Hardware-Efficient Supercomputing", IEEE Transactions on
Computers 34(10):892-901, 1985.
[RFC2973] Balay, R., Katz, D., and J. Parker, "IS-IS Mesh Groups",
RFC 2973, DOI 10.17487/RFC2973, October 2000,
<https://www.rfc-editor.org/info/rfc2973>.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[RFC4811] Nguyen, L., Roy, A., and A. Zinin, "OSPF Out-of-Band Link
State Database (LSDB) Resynchronization", RFC 4811,
DOI 10.17487/RFC4811, March 2007,
<https://www.rfc-editor.org/info/rfc4811>.
Li, et al. Expires 9 December 2022 [Page 45]
Internet-Draft Dynamic Flooding June 2022
[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.
Authors' Addresses
Tony Li (editor)
Juniper Networks
1133 Innovation Way
Sunnyvale, California 94089
United States of America
Email: tony.li@tony.li
Tony Przygienda
Juniper Networks
1133 Innovation Way
Sunnyvale, California 94089
United States of America
Email: prz@juniper.net
Peter Psenak (editor)
Cisco Systems, Inc.
Eurovea Centre, Central 3
Pribinova Street 10
81109 Bratislava
Slovakia
Email: ppsenak@cisco.com
Les Ginsberg
Cisco Systems, Inc.
510 McCarthy Blvd.
Milpitas, California 95035
United States of America
Email: ginsberg@cisco.com
Huaimo Chen
Futurewei
Boston, Ma,
United States of America
Email: hchen@futurewei.com
Li, et al. Expires 9 December 2022 [Page 46]
Internet-Draft Dynamic Flooding June 2022
Dave Cooper
CenturyLink
1025 Eldorado Blvd
Broomfield, Colorado 80021
United States of America
Email: Dave.Cooper@centurylink.com
Luay Jalil
Verizon
Richardson, Texas 75081
United States of America
Email: luay.jalil@verizon.com
Srinath Dontula
ATT
200 S Laurel Ave
Middletown, New Jersey 07748
United States of America
Email: sd947e@att.com
Gyan S. Mishra
Verizon Inc.
13101 Columbia Pike
Silver Spring, Maryland 20904
United States of America
Phone: 301 502-1347
Email: gyan.s.mishra@verizon.com
Li, et al. Expires 9 December 2022 [Page 47]