Internet Engineering Task Force T. Li
Internet-Draft Arista Networks
Intended status: Informational P. Psenak
Expires: December 30, 2018 Cisco Systems, Inc.
June 28, 2018
Dynamic Flooding on Dense Graphs
draft-li-dynamic-flooding-05
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Solution Requirements . . . . . . . . . . . . . . . . . . . . 4
4. Dynamic Flooding . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Leader election . . . . . . . . . . . . . . . . . . . . . 7
4.3. Computing the Flooding Topology . . . . . . . . . . . . . 7
4.4. Topologies on Complete Bipartite Graphs . . . . . . . . . 8
4.4.1. A Minimal Flooding Topology . . . . . . . . . . . . . 8
4.4.2. Xia Topologies . . . . . . . . . . . . . . . . . . . 9
4.4.3. Optimization . . . . . . . . . . . . . . . . . . . . 10
4.5. Encoding the Flooding Topology . . . . . . . . . . . . . 10
4.6. Analysis of Topology Changes . . . . . . . . . . . . . . 10
4.6.1. Link Addition . . . . . . . . . . . . . . . . . . . . 10
4.6.2. Node Addition . . . . . . . . . . . . . . . . . . . . 11
4.6.3. Link Failures Off the Flooding Topology . . . . . . . 11
4.6.4. Failure of the Area Leader . . . . . . . . . . . . . 11
4.6.5. Failures on the Flooding Topology . . . . . . . . . . 11
4.6.6. Recovery from Multiple Failures . . . . . . . . . . . 12
5. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 12
5.1. IS-IS TLVs . . . . . . . . . . . . . . . . . . . . . . . 12
5.1.1. IS-IS Area Leader Sub-TLV . . . . . . . . . . . . . . 13
5.1.2. IS-IS Area System IDs TLV . . . . . . . . . . . . . . 14
5.1.3. IS-IS Flooding Path TLV . . . . . . . . . . . . . . . 15
5.2. OSPF LSAs and TLVs . . . . . . . . . . . . . . . . . . . 16
5.2.1. OSPF Area Leader Sub-TLV . . . . . . . . . . . . . . 16
5.2.2. OSPFv2 Dynamic Flooding Opaque LSA . . . . . . . . . 16
5.2.3. OSPFv3 Dynamic Flooding LSA . . . . . . . . . . . . . 18
5.2.4. OSPF Area Router IDs TLV . . . . . . . . . . . . . . 18
5.2.5. OSPF Flooding Path TLV . . . . . . . . . . . . . . . 19
6. Behavioral Specification . . . . . . . . . . . . . . . . . . 20
6.1. Leader Election . . . . . . . . . . . . . . . . . . . . . 21
6.2. Area Leader Responsibilities . . . . . . . . . . . . . . 21
6.3. Distributed Flooding Topology Calculation . . . . . . . . 21
6.4. Flooding Behavior . . . . . . . . . . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
7.1. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.2. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.2.1. OSPF Dynamic Flooding LSA TLVs Registry . . . . . . . 24
7.3. IGP . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1. Normative References . . . . . . . . . . . . . . . . . . 25
10.2. Informative References . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
In recent years, there has been increased focused 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, 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. 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
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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.
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.
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Requirement 4 There must be no single point of failure. The loss
of any link or node should not unduly hinder convergence.
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.
In this idea, the flooding topology is computed either centrally on
an elected node 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 the dense
topology 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 would not. 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.
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.
If a node does not have any flooding topology information when it
receives new link state information, it should flood according to
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legacy 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 lead node for the
dense subgraph and first flood on the topology of the lead node. The
rationale behind this is straightforward: if there is a transient and
there has been a recent change in the elected node, 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 forwarding topology may become
disconnected. To address this, nodes can perform a database
synchronization check anytime a link is added to or removed from the
flooding topology.
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 universal
flooding. This is not an issue for correctness, as no node can
become isolated.
Flooding that is initiated within the flooding topology will remain
within that flooding topology until it reaches a legacy node, which
will resume legacy flooding. Legacy flooding will be bounded by the
flooding topology, 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.
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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 node is
elected and has distributed new flooding topology information, then
the old node should withdraw its flooding topology information from
the link state database. If the old node does not return to the
topology in a timely manner, the new node may remove the old node's
information from the link state database.
In the case of distributed mode, the distributed algorithm advertised
by the Area Leader MUST be used by all routers that participate in
Dynamic Flooding.
Not every router 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.
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.
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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.
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.
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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.
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
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update more than twice. For n leaves and m spines, a spine never
transmits more than m/n updates.
4.4.3. Optimization
If two systems have multiple links between them, only one of the
links should be part of the flooding topology. Moreover, symmetric
selection of the link to use for flooding is not required.
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.
Other encodings are certainly possible. We have attempted to make a
useful trade off between simplicity, generality, and space.
4.6. Analysis of Topology Changes
In this section, we explicitly consider a variety of different
topological failures in the network and how dynamic flooding should
address them.
4.6.1. Link Addition
If a 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 routers 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, should trigger the flooding topology recalculation. This
is done to ensure that all nodes converge on the same flooding
topology, regardless of the time of the calculation.
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4.6.2. Node Addition
In centralized mode, if a node is added to the topology, then at
least one link is also added to the topology. The paragraph above
applies and the Area Leader will necessarily need to add the new node
to the flooding topology.
In distributed mode, the addition of a node should trigger flooding
topology recalculation.
Until the new node is incorporated into the flooding topology at
least a single link towards the new node MUST be added to the
flooding topology locally on all of its neighbors.
4.6.3. Link Failures Off the Flooding Topology
If a link that is not part of the flooding topology fails, then the
adjoining routers will update their link state advertisements and
flood them on the flooding topology. There is no need for changes to
the flooding topology.
4.6.4. Failure of the Area Leader
The failure of the Area Leader can be detected by observing that it
is disconnected from the area topology. In this case, the Area
Leader election process is repeated and a new Area Leader is elected.
In the centralized mode, the new Area Leader will compute a new
flooding topology and flood it using the new flooding topology.
As an optimization, applicable to centralized mode, the new Area
Leader MAY compute a new flooding topology that has as much in common
as possible with the old flooding topology. This will minimize the
risk of over-flooding.
4.6.5. Failures on the Flooding Topology
If there is a failure on the flooding topology, the adjoining routers
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.
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.
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In distributed mode, all routers supporting dynamic flooding will
notice the change in the flooding topology and recompute the new
flooding topology.
4.6.6. Recovery from Multiple Failures
In the unlikely event of multiple failures on the flooding topology,
it may become disconnected. The nodes that remain active on the
edges of the flooding topology will recognize this, update their own
link state advertisements and flood them on the remainder of the
flooding topology. At this point, nodes will be able to compute that
the flooding topology is partitioned.
Note that this is very different from partitioning the area itself.
The area may remain connected and forwarding may still be effective.
When this condition is detected, the flooding topology can no longer
be expected to deliver link state updates in a prompt manner. Nodes
on the edges of the flooding topology should perform database
synchronization on all links not on the flooding topology. Updates
received from off of the flooding topology should be flooded on the
remaining flooding topology. Any links that provide updates or
require updates that are not part of the flooding topology should
temporarily be added to the flooding topology. This should repair
the current flooding topology, albeit in a sub-optimal manner.
In centrailzed mode, the Area Leader will also detect this condition,
compute a new flooding topology, and flood it using the new flooding
topology.
In distributed mode, all routers that actively participate in Dynamic
Flooding will compute the new flooding topology.
5. Protocol Elements
5.1. IS-IS TLVs
The following TLVs are added to IS-IS:
1. A TLV that an IS may inject into its LSP to indicate its
preference for becoming Area Leader.
2. A TLV to carry the list of system IDs that compromise the
flooding topology for the area.
3. A TLV to carry the adjacency matrix for the flooding topology for
the area.
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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 (routers) that are not advertising this Sub-TLV
are not eligible to become Area Leader.
The Area Leader is the router with the numerically highest Area
Leader priority in the area. In the event of ties, the router with
the numerically highest system ID is the Area Leader. Due to
transients during database flooding, different routers 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:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type | TLV Length | Priority | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TLV Type: TBD1
TLV 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
area assigned and managed by IANA. Before any assignments can
be made, there MUST be an IETF specification that specifies
IANA allocation for any value from this range (see
Section 7.3).
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128-254: Private distributed algorithms. Values from this
range will not be registered with IANA and MUST NOT be
mentioned by RFCs.
255: Reserved
5.1.2. IS-IS Area System IDs TLV
IS-IS Area System IDs TLV is only used in centralized mode.
The Area System IDs TLV is used by the Area Leader to enumerate the
system IDs that it has used in computing the flooding topology.
Conceptually, the Area Leader creates a list of system IDs for all
routers in the area, assigning indices to each system, starting with
index 0.
Because the space in a single TLV is small, more than one TLV may be
required to encode all of the system IDs in the area. This TLV may
be present in multiple LSPs.
The format of the Area System 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type | TLV Length | Starting Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L| Reserved | System IDs ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
System IDs continued ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TLV Type: TBD2
TLV Length: 3 + (System ID length * (number of System IDs))
Starting index: The index of the first system ID that appears in
this TLV.
L (Last): This bit is set if the index of the last system ID that
appears in this TLV is equal to the last index in the full list of
system IDs for the area.
System IDs: A concatenated list of system IDs for the area.
If there are multiple IS-IS Area System IDs 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
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ignored. TLVs which specify system IDs with indices greater than
that specified by the TLV with the L bit set are also ignored.
5.1.3. 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 System 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type | TLV Length | Starting Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Index 2 | Additional indices ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TLV Type: TBD3
TLV 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.
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5.2. OSPF LSAs and TLVs
This section defines new LSAs and TLVs for both OSPFv2 and OSPFv3.
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.
5.2.2. 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:
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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 -+
| ... |
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.
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5.2.3. 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 -+
| ... |
OSPFv3 Dynamic Flooding LSA
5.2.4. OSPF Area Router IDs TLV
The OSPF Area Router IDs TLV is a top level TLV of the OSPFv2 Dynamic
Flooding Opaque LSA and OSPFv3 Dynamic Flooding LSA.
The OSPF Area Router IDs TLV is used by the Area Leader to enumerate
the Router IDs that it has used in computing the flooding topology.
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.
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Because the space in a single OSPF Area Router IDs TLV is limited,
more than one TLV may ve required to encode all of the Router IDs in
the area. This TLV may also recur in multiple OSPFv2 Dynamic
Flooding Opaque LSAs or OSPFv3 Dynamic Flooding LSA, so that all
Router IDs can be advertised.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Router IDs -+
| ... |
OSPF Area Router IDs TLV
TLV Type: 1
TLV Length: 4 + (Router ID length * (number of Router IDs))
Starting index: The index of the first Router ID that appears in
this TLV.
L (Last): This bit is set if the index of the last system ID that
appears in this TLV is equal to the last index in the full list of
Rourer IDs for the area.
Router IDs: A concatenated list of Router IDs for the area.
If there are multiple OSPF Area Router IDs 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. 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 5.1.3.
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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.
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 -+
| ... |
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.
6. Behavioral Specification
In this section, we specify the detailed behaviors of the nodes
participating in the IGP.
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6.1. 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.
6.2. Area Leader Responsibilities
If the Area Leader operates in centralized mode, it MUST advertise
algorithm 0 in its Area Leader Sub-TLV. 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.
The flooding topology MUST include all reachable nodes in the area.
If nodes become unreachable on the flooding topology, the flooding
topology MUST be recalculated. In centralized mode, the Area Leader
MUST advertise a new flooding topology.
The flooding topology MAY be bi-connected. This is strongly
RECOMMENDED but not required.
6.3. Distributed Flooding Topology Calculation
If the Area Leader advertises a non-zero algorithm in its Area Leader
Sub-TLV, all routers 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.
Routers that do not support the value of algorithm advertised by the
Area Leader MUST continue to use legacy flooding mechanism as defined
by the protocol.
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.
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6.4. Flooding Behavior
Nodes that support Dynamic Flooding MUST use the flooding topology
for flooding. 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. Link state updates
received on one link in the flooding topology MUST be flooded on all
other links in the flooding topology other than the link on which the
update has been received. Link state updates received on a link not
in the flooding topology MUST be flooded on all links in the flooding
topology.
In centralized mode, if multiple flooding topologies are present in
the area link state database, the node SHOULD flood on the union of
the 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 the union of 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.
When failures occur, nodes will learn about them from link state
updates and can compare those to the existing flooding topology. If
the flooding topology becomes disconnected, then the nodes at the
edges of the flooding topology should perform a database
synchronization on all links. While the flooding topology is
disconnected, if a new link state update is received on a link not in
the flooding topology, then the node SHOULD temporarily consider the
link as part of the flooding topology. When a new flooding topology
is received or locally calculated, this MUST be discontinued.
7. IANA Considerations
7.1. IS-IS
This document requests the following code point 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)
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This document requests that IANA allocate and assign two 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 5.1.2)
Type: TBD3
Description: IS-IS Flooding Path TLV
Reference: This document (Section 5.1.3)
7.2. OSPF
This document requests the following code point from the "OSPF Router
Information (RI) TLVs" registry:
Type: TBD4
Description: OSPF Area Leader Sub-TLV
Reference: This document (Section 5.2.1)
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 5.2.2)
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 5.2.3)
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7.2.1. OSPF Dynamic Flooding LSA TLVs Registry
This specification also requests one 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 5.2.4)
Type: 2
Description: OSPF Flooding Path TLV
Reference: This document (Section 5.2.5)
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.
7.3. 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. The registration policy
for this registry is "Standards Action" ([RFC8126] and [RFC7120]).
Values in this registry come from the range 0-255.
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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.
128-254: Reserved for private use.
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 Les Ginsberg, 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.
10. References
10.1. Normative References
[ISO10589]
International Organization for Standardization,
"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, Nov. 2002.
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[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>.
[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>.
[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>.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.
[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>.
[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>.
[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>.
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[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>.
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>.
[Leiserson]
Leiserson, C., "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>.
[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
Arista Networks
5453 Great America Parkway
Santa Clara, California 95054
USA
Email: tony.li@tony.li
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Peter Psenak
Cisco Systems, Inc.
Eurovea Centre, Central 3
Pribinova Street 10
Bratislava 81109
Slovakia
Email: ppsenak@cisco.com
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