RTGWG P. Thubert, Ed.
Internet-Draft Cisco
Intended status: Standards Track P. Bellagamba
Expires: July 26, 2015 Cisco Systems
January 22, 2015
Available Routing Constructs
draft-thubert-rtgwg-arc-03
Abstract
This draft introduces the concept of ARC, a two-edged routing
construct that forms its own fault isolation and recovery domain.
The new paradigm can be leveraged to improve the network utilization
and resiliency for unicast and multicast traffic in multiple
environments, and is optimized to compute short reroute paths in case
of breakages.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on July 26, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. ARC Set representations . . . . . . . . . . . . . . . . . . . 7
4. Lowest ARC First . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Init . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Growing Trees . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Being Safe . . . . . . . . . . . . . . . . . . . . . . . 11
4.4. Bending An ARC . . . . . . . . . . . . . . . . . . . . . 11
4.5. Orienting Links . . . . . . . . . . . . . . . . . . . . . 12
4.6. Looping or recursing . . . . . . . . . . . . . . . . . . 13
5. Forwarding Along An ARC Set . . . . . . . . . . . . . . . . . 13
5.1. Control Plane Recovery . . . . . . . . . . . . . . . . . 14
5.2. Data Plane Recovery . . . . . . . . . . . . . . . . . . . 14
5.2.1. Label Switched ARCs . . . . . . . . . . . . . . . . . 15
5.2.2. Segment Routed ARCs . . . . . . . . . . . . . . . . . 15
5.3. Flooding . . . . . . . . . . . . . . . . . . . . . . . . 16
6. ARC Signaling . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Serial ARC Representation . . . . . . . . . . . . . . . . 16
6.2. Centralized vs. Distributed computation . . . . . . . . . 16
6.3. ARC Topology Injection . . . . . . . . . . . . . . . . . 17
6.4. ARC Operations, Administration, and Maintenance . . . . . 17
7. Other ARC Operations . . . . . . . . . . . . . . . . . . . . 17
7.1. Node-Local vs. ARC-Wide reaction . . . . . . . . . . . . 17
7.2. Load Balancing . . . . . . . . . . . . . . . . . . . . . 17
7.3. Shared Risk Link Group . . . . . . . . . . . . . . . . . 18
7.4. Olympic Rings . . . . . . . . . . . . . . . . . . . . . . 19
7.5. Routing Hierarchies . . . . . . . . . . . . . . . . . . . 19
8. Manageability . . . . . . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. Normative References . . . . . . . . . . . . . . . . . . 20
12.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
Traditional routing and forwarding uses the concept of path as the
basic routing paradigm to get a packet from a source to a destination
by following an ordered sequence of arrows between intermediate
nodes. In this serial design, a path is broken as soon as a single
arrow is, and getting around a breakage can require path re-
computation, network re-convergence, and incur delays to till service
is restored.
Multiple paths can be bound together for instance to form a Directed
Acyclic Graph (DAG) to a destination, but that technique can be
difficult to balance and cannot provide a full path redundancy even
in the case of a biconnected graph. For instance, if the node that
is closest to the DAG destination has only one link to that
destination, then it does not have a alternate path to get to that
destination.
It is also possible to compute an alternate routing topology for fast
rerouting to a given destination, in which case some signaling,
tagging or labeling can be put in place to indicate whether a packet
follows the normal path or was rerouted over an alternate topology.
Once a packet is rerouted, it is bound to the alternate topology so
only one breakage can be handled with loop-free guarantees in most
practical situations.
This draft introduces the concept of an Available Routing Construct
(ARC) as a routing construct made of a bidirectional sequence of
nodes and links with 2 outgoing edges, so that, upon a single
breakage, each lively node in along ARC can still reach one of the
outgoing edges.
The routing graph to reach a certain destination is expressed as a
cascade of ARCs, each ARC providing its own independent domain of
fault isolation and recovery. Unicast traffic may enter an ARC via
any node but it may only leave the ARC through one of its two edges.
One node along the ARC is designated as the Cursor. In normal
unicast operations, the traffic inside an ARC flows away from the
Cursor towards an edge. Upon a failure, packets may bounce on the
breakage point and flow the other way along the ARC to take the other
exit.
Aa a result an ARC is resilient to any single failure, and the
recovery can be driven either from the data plane or the control
plane. A second failure occurring within a same ARC will isolate an
ARC segment. This can be further corrected from the control plane by
reversing all the incoming Edges in a process that might recurse till
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an exit is found. When ARC reversal is applied, an ARC topology is
resilient to some cases of Shared Risk Link Group (SRLG) failures.
Properties of the Maximally Redundant Tree (MRT) and ARC are compared
in [I-D.thubert-rtgwg-arc-vs-mrt]. The study shows that the reroute
path that ARC derives is generally shorter than the alternate path
that MRT computes. This property is largely due to the concept of
cursor that delineates the shortest path on both sides of an ARC.
Once a rerouted packet passes the cursor of the ARC in which it is
rerouted, it should not cross a cursor again unless there is a second
breakage later. It results that the packet follows the shortest path
for the rest of the way, staying on the right side of each downstream
ARC, when MRT would be following all subsequent eyes in the same
direction.
This draft presents the concept and provides an intuition of how ARCs
can simplify the operation and improve the network utilization and
resiliency for all sorts of traffic in multiple environments, but
defers to further documents to elaborate on the algorithms and
optimizations in the different application domains. For instance,
ARCs can also be used in datacenters for the purpose of fast-reroute,
or within a service provider network to simplify load balancing
operations or leverage optimally the ring topologies [RFC5921].
2. Terminology
The definition of the constituent parts of the "OAM" term is found in
[RFC6291].
The draft uses the following terminology:
ARC: Available Routing Construct. An ARC is a loopless ordered set
of nodes and links whereby traffic may enter via any node in the
ARC but may only leave the ARC through either one of the ARC
edges.
Comb: An ARC generalization: a Comb is a n-edged loopless set of
nodes and links with n >= 2; traffic may enter via any node in the
Comb but may only exit the Comb through one of its n edges. A
Comb comes with a walk operation that enables to attempt to exit
via every edge and to discover when all have been tried.
Cursor: A virtual point along an ARC that can be located on a node
or on a link between 2 nodes. In normal operations, the traffic
along the ARC flows away from its Cursor. If the Cursor is a
node, then traffic can be distributed on both sides. The Cursor
may be moved to change the way traffic is load balanced along an
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ARC. It may also be placed at the location of a failure to direct
traffic away from that point.
ARC Node: A Node that belongs to an ARC.
Edge ARC Node: An ARC Node at an edge of its ARC. An Edge ARC Node
is a node via wich traffic can exit the ARC.
Edge Link: A directed link outgoing from an Edge ARC Node. Traffic
can only exit from an ARC via an Edge Link. An Edge Link does not
accept traffic into an ARC.
Intermediate ARC Node: A node that is not at an edge of an ARC. A
Intermediate ARC Node node that can receive traffic and forward
traffic between its adjacent nodes.
Intermediate Link: A link between two Intermediate ARC Nodes. An
Intermediate Link is reversible, meaning that traffic is allowed
in both directions though an individual packet is constrained in
the way its direction is reversed. For stable links such as wired
links, the typical constraint is that the direction of a packet
may be reversed at most once along a given ARC.
Collapsed ARC: An ARC that is formed of a single node. This node is
altogether the Cursor and both Edge Nodes. This implies that the
node has at least 2 outgoing links to 2 different Safe Nodes.
|
|
V
C+EAN
/|\
/ | \
| V |
V V
E: Edge ARC Node -| collapsed in a single node
C: Cursor -|
Figure 1: Collapsed ARC
Infrastructure ARC: An ARC that is formed of more than one node,
which also means that the Edge Nodes are different nodes.
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| \ | |
| \ | | |
V V V |
_->IAN<---->IAN<---->IAN<---->IAN<-_ |
/ + C \ |
/ \|
V V
EAN EAN
| /|\
| / | \
| | V |
V V V
IAN: Intermediate ARC Node -|
EAN: Edge ARC Node |- All are Safe Nodes
C: Cursor -|
Figure 2: Infrastructure ARC
DAG: Directed Acyclic Graph.
ARC Set (or Cascade): A DAG with ARCs as vertices. In the DAG, an
edge between ARC A and ARC B corresponds to a link from an Edge
ARC Node in ARC A and an arbitrary ARC Node in ARC B. Note that
by definition, an ARC has at least 2 outgoing Edge Links, one per
Edge Node, and maybe more if an Edge Node has multiple outgoing
Edge Links. All vertices in the DAG have 2 forwarding solutions,
even the ARC closest to the destination.
Omega: the abstract destination (== root) of an ARC Set. Omega is
also referred as a complex destination in that it typically
comprises more than one node and/or more than one link on a node.
if Omega has a single node, then the plural interfaces on that
node are considered as as many virtual node for the sake of the
ARC computation algorithm.
ARC Height: An arbitrary distance from Omega that is associated to
an ARC. The Height of an ARC must be more than the Height of any
of the ARCs it terminates into. The order of ARC formation by a
given algorithm can be used as a Height whereby an ARC is always
strictly higher or lower than another.
Buttressing ARC: A split ARC that is merged into another ARC at one
edge. An ARC and one or more Buttressing ARCs form a Comb
construct that is resilient to additional breakages. A
Buttressing ARC may be applied to an ARC or a Comb iff traffic
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outgoing the Buttressing ARC Edge always reaches in an ARC that is
lower than this ARC, or Omega.
| \ | |
| \ | | |
V V V |
_->IAN<---->IAN<---->IAN<---->IAN<-_----->IAN<-_
/ + C \ | \
/ \| \
V V V
EAN EAN EAN
| /|\ |
| / | \ |
| | V | |
V V V V
Figure 3: Comb with Buttressing ARC
Safe Node: A node is Safe if there is no single point of failure -
apart from the node itself - on its way to Omega. From this
definition, a node is Safe if it has at least two non-congruent
paths to two different other Safe Nodes. It results that a Safe
node that is not Omega has at least two completely disjunct paths
to Omega. When an ARC has been successfully constructed, all its
nodes become safe with respect to the Omega for which the ARC was
constructed. By extension for a collapsed path Omega is deemed to
be Safe, that is any node that pertains in Omega is a Safe Node.
?-S: A node N is deemed dependent on a node S or S-dependent
(denoted as ?-S) if S is the last single point of failure along
N's shortest path to Omega.
3. ARC Set representations
An ARC Set can be represented in a number of fashions:
Graph View:
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H2<==>H<==>H1<---I--->J1<==>J--->K1<===>K
| | | | |
| | | | |
V V V V V
D1<==>D<==>D3 E1<==>E F1<==>F<==>F2 G
| | | | | | / \
| | | | | | / \
V V V V V V V V
B1<==>B2<==>B3<==>B--->A<==>A1<------C2<==>C<==>C4
| | | |
| | | |
| V V |
+--------------------> Omega <-------------------+
Figure 4: Routing Graph View
This representation is similar to a classical routing graph with
the peculiarity that some Links are marked reversible. An ARC is
represented as a sequence of reversible links. The node that
holds the Cursor is also indicated somehow.
ARC View:
+========I========+
| |
| +====J====+
| | |
+====H====+ | +=====K=====+
| | | | |
+====D====+ +====E====+ +====F====+ +====G====+
| | | | | | | |
+=========B=========+ | | +=========C=========+
| | | | | |
| +======A=======+ |
| | | |
------------------------------------------------------------------Omega
Figure 5: ARC Representation
This ARC representation abstracts a whole ARC as a single vertex.
An ARC ends in one or more other ARCs, but it has to be noted that
even if both edges of an ARC end in a same other ARC, it ends in
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fact in 2 different nodes, or Omega. This is turn can be
represented as a DAG as described in Paragraph 3.
Collapsed DAG view:
+====+ +====+ +====+ +====+
| H | <--- | I | ---> | J | ---> | K |
| \__ | ___/ |
| \ | / |
V _| V |_ V
+====+ +====+ +====+ +====+
| D | | E | | F | <--- | G |
\ \ __/ \__ __/ \__ / /
\ \ / \ / \ / /
_| _| |_ _| |_ _| |_ |_
+====+ +====+ +====+
| B | ---> | A | <--- | C |
| | | |
V V V V
------------------------------------------------------------------Omega
Figure 6: ARC DAG
In the DAG representation, an ARC is abstracted as a vertex and
links between ARCs are shown as directed edges. This way, the
reversible links are omitted and the graph is simplified. It can
be noted that even the vertex closest to Omega has 2 non-congruent
forwarding solutions, that is Heir Links to Omega.
4. Lowest ARC First
The open Lowest ARC First(oLAF) algorithm is presented below in such
a way as to help the reader figure how an ARC Set can be obtained but
not in a computer-optimized fashion that is left to be determined.
oLAF is based on Dijkstra's algorithm for Shortest Path First (SPF)
computation, and is designed in such a fashion that the reverse SPF
tree towards a destination is conserved and preferred for forwarding
along the resulting ARC Set.
We make the computation on behalf of Omega, that is an abstraction,
but could represent the node or the set of nodes that we want to
reach with an ARC Set. If Omega is instantiated as an actual
destination node, then that node may be a fine location for an ARC
Computing Engine.
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4.1. Init
So we start with an proverbial Initial Set of Nodes that are
interconnected by Links, and Omega that is the destination that we
want to reach with an ARC Set.
If there is no Heir, we're done. If there is a single Heir then the
graph is mono-connected, so we restart the computation taking that
Heir off the Set of Nodes and making it Omega.
Else, if Omega is a single Node, or if Omega is composed of multiple
nodes but we are willing to accept that both ends of an ARC terminate
in a same node in Omega, then we create virtual Omega nodes, a
minimum of two and at most one per Heir, and we make them the new
Omega. Note: we need at least two destinations because both ends of
an ARC cannot terminate in a same node.
Now we can start building an ARC Set towards the resulting Omega.
In this process, we create so-called Dependent Sets of nodes, each
owned by a Safe Node S, DSet(S). DSet(S) contains nodes that are not
determined to be Safe at the current stage of the computation and for
which S, the owner Safe Node, is the last single point of failure on
the shortest path tree to Omega. It results that a given node can be
at most in one DSet, and that a Safe Node belongs to its own DSet.
For each node S in Omega we create a DSet(S) in which we place S.
4.2. Growing Trees
And then the process goes like this:
We select the node in the Set of Nodes that is closest to Omega using
the cost towards Omega as if we were building a traditional reverse
SPF tree and we place the selected node in the same Dependent Set as
its parent in the reverse SPF tree. Note that for a Heir, the parent
might be a real node in Omega, or a virtual Omega node.
If we kept it at that, we would be building subtrees that are hanging
off a Safe Node and together would represent the reverse shortest
path tree towards Omega, each subtree being grown separately inside
DSet(S) where S is the (virtual) Safe node that is the root of the
subtree.
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4.3. Being Safe
But once we have placed the selected node in a DSet, we consider its
neighbors one by one. If at least one of the neighbors is already in
a different DSet than this node, we select the neighbor that provides
the shortest alternate path to Omega for the selected node.
Doing so, we have isolated two paths:
o one along its own shortest path that is contained within its own
Dependent Set and that leads to the owner Safe Node of this set.
o and one via the selected neighbor, along its own shortest path
within the selected neighbor's Dependent Set and that leads to the
owner Safe Node of that other set.
Because the two sets are different and have no intersection, these
paths are non-congruent. And because the two non-congruent paths
lead to two different Safe Nodes, this node is Safe.
It might happen that:
o the selected node's parent is already a Safe Node, in which case
the selected node is the Edge AN on its shortest path side.
o It might also happen that the selected neighbor is already a Safe
Node, in which case selected node is the Edge AN on its alternate
side.
If both conditions are met for a same AN, then that AN forms a
collapsed ARC by itself.
4.4. Bending An ARC
Now we form an ARC as follows:
o A height is attributed to this ARC that must be strictly more than
that of the ARCs it terminates into, if any. The order in which
the ARCs are built may be used in some cases.
o The ARC terminates in the two Safe Nodes that are the owners of
the two DSets. The normal behaviour is to make a Edge Link the
link to the Safe Node.
o If the Safe Node at one end forms a collapsed ARC by itself, it
may be absorbed in the ARC in order to build a multi-edged ARC.
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o If one of the two Safe Nodes pertains in a ARC or a Comb construct
that is higher than the other end, then this ARC may be merged at
the Safe Node with its original ARC, in order to form a Comb
construct whereby this ARC is a Buttressing ARC of the Comb. The
resulting Comb conserves the height on the original ARC or Comb
that it extends.
o The ARC is built by adjoining the two non-congruent paths that we
isolated for the selected node.
o The selected node is the node farthest from Omega in the resulting
ARC, so we make it the Cursor.
o The link between the selected node and the selected neighbor would
not have been used in a classical reverse SPF tree. Here, we have
determined that this link is in fact critical to connect 2 zones
of the network (the DSets) that can act as a back up for one
another in case of the failure of their respective single points
of failure (the Safe Nodes).
o Because the ARC can be used in both directions, each AN along the
ARC has two non-congruent paths to the Safe Nodes that the ARC
terminates into. So it is a Safe Node. We create individual
DSets for each AN and we move the AN to its own DSet.
4.5. Orienting Links
For each ARC Node along the ARC:
o any link (there can be zero for a collapsed ARC, one for an Edge
AN or two of them for a Intermediate AN) between this AN and a
next AN along this ARC is made an Intermediate Link, that is,
reversible. The normal direction, away from the Cursor, preserves
the shortest path.
o If this AN is an Edge AN for this ARC, than all links off this
node that terminate in a Safe Node are made Edge Links, that is,
outgoing but not reversible.
o All the other links left undetermined.
The nodes left in the Dependent Sets but the owner Safe Node are
still not Safe. They are moved back to the original Set of Nodes to
enable forming additional ARCs which might depend on this ARC in the
ARC Set.
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4.6. Looping or recursing
We are done processing the particular node we had picked in the
original Set of Nodes. If the Set of Nodes as it stands now is not
empty, we continue from Section 4.2.
If the Set of Nodes went empty, we are done with this pass and we
consider the Dependent Sets that we have put together. In a
biconnected graph, there should be one set per node and one node per
set, denoting that every node is a Safe Node.
If some portion of the graph is mono-connected, then each mono-
connected portion forms the Dependent Set of the Safe Node that is
its single point of failure. In order to be maximally redundant, we
need to form the ARCs again, within the Dependent Set.
To do so, we remove the Safe Node from the Dependent set and make it
Omega. We make the resulting DSet our Set of Nodes and run the
algorithm again.
This may recurse a number of times if the graph has mono-connected
zones within others.
5. Forwarding Along An ARC Set
Under normal conditions, the traffic flows away from the Cursor of
the current ARC and cascades into the next ARC on that side of the
Cursor, with the Height of the current ARC decreasing monotonically
from ARC to ARC till Omega is reached.
The same goes for a generic Comb construct. When Buttressing ARCs
are applied on a main ARC or other Buttressing ARCs, the final
construct assumes the shape of a tree. The tree may be walked in
different manners but the shortest path requires to start going down
the current ARC or Buttressing ARC to its Edge.
In case of Label forwarding, the same recursive technique is applied
and a multiple ARC label path is constructed. Each ARC has is own
set of label path per Omega, each ARC Set label path being merged
into the lower ARC label set, thus at the interconnection point. At
minimum, ARC label path should be built from the Cursor toward each
edge, but this would require label path recompilation upon Cursor
move, the proposed approach is then to build for the normal flow to
an Omega one pair of label path from edge to edge.
As this label construct maps the ARC topology with local significant
label, the Label Distribution Protocol (LDP) could be reused to
announce label association to neighbors on the ARC.
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Upon a breakage inside an ARC, until a corrective action takes place,
some traffic will be lost. The corrective action might be either
operated at the control plane or the data plane, if immediate action
and near-zero packet loss is required.
5.1. Control Plane Recovery
Upon a first breakage in an ARC, the Cursor is moved to the breakage
point, either a node or a link, so that traffic flows away from the
Cursor again.
Upon a second breakage within a same ARC, a segment of the ARC is now
isolated. Both breakage points become sinks till an additional
corrective action, such as modifying the ARC Set, takes place. All
incoming links in the isolated segment are blocked , causing the
traffic to exit at the other end of the incoming ARCs.
Blocking an Edge Link in the incoming ARC may create an isolated
segment in the incoming ARC as well if it is a second breakage there
too, or if both edges of the incoming ARC terminate in the broken
segment. In that case the process recurses and the broken zone can
be determined as the collection of the isolated segments.
If a segment of an ARC is getting isolated by a dual failure but that
ARC segment has incoming Edges then the ARC can be reversed. This
reversal is done by reversing of all the incoming Edges, which become
outgoing. The segment that was isolated now benefits from multiple
exits in a loop free fashion. This process might in turn isolate a
segment of an ARC that was incoming and the process recurses and some
links flap. If a real exit exits the process will stabilize, but a
count to infinity must be put in place to avoid a permanent flapping
when a whole ARC Subset is physically isolated. One may consider
that this process is in fact the classical link reversal technique,
as applied to the DAG of ARCs.
5.2. Data Plane Recovery
Upon a breakage inside an ARC, it is possible in the data plane to
reverse the direction of -to turn- a given packet once along the ARC
so the packets exits over the other Edge Link. But in order to avoid
loops, it is undesirable to reverse the direction of a given packet a
second time.
Note that once a given packet leaves an ARC to enter the next, it is
free to bounce again in the next ARC. In other Words, the domain
that is impacted by a turn is limited to the current ARC itself; the
ARC forms the event horizon wherein the notion that a turn happened
may cause a loop.
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So a local strategy must be put in place inside an ARC to allow a
given packet to bounce once upon a breakage, and get dropped upon a
second breakage.
In the case of IP packet forwarding, a packet can be tagged when it
bounces inside an ARC, or when it passes the Cursor, for instance by
reserving a TOS bit for that purpose. When the packet bounces, the
bit is set and when the packet leaves the ARC, the bit is reset and
may be used again in the next ARC. In the generic case of a Comb, a
strategy must be put in place to walk the structure and drop a packet
that tries all the Edges. it attempts to pass the Cursor twice in a
same direction, meaning that more than a full walk was already
accomplished.
5.2.1. Label Switched ARCs
In the case of MultiProtocol Label Switching (MPLS) forwarding, the
same result can be achieved with Label Switched ARCs (LSARCs), that
are composed of either 3 or 4 Labels Switched Paths (LSPs) along the
ARC.
3-Labels method: In this case we lay a primary LSP from the cursor
to the Edge in each direction, and a backup LSP Edge to Edge in
each direction. So a node along the way has three labels, one
primary and two backup, one in each direction. Should the primary
path fail, the packet can be placed along the backup LSP in the
other direction. We'll note that this method constraints the
location of the Cursor. Should the Cursor move, The primary LSPs
have to be recomputed, at a minimum between the old and the new
location of the Cursor where the direction is reversed.
4-Labels method: In this case we have a primary and a backup LSPs in
each direction all of them Edge to Edge, 4 labels total. The
labels are independent of the location of the Cursor, so the
Cursor can be moved from a node to the next in control plane with
no impact on labels. This method consumes an additional label but
is more amenable to load balancing techniques and allows each node
that inject a packet inside an ARC to make its own decision of the
exit edge for a given packet or flow.
5.2.2. Segment Routed ARCs
In the case of an infrastructure that is capable of Segment Routing
(SR) [I-D.ietf-spring-segment-routing], the tag in the packet is in
essence a Routing Header (RH) via the cursor. The RH forces routing
to the destination all the way back up the broken ARC and then down
on the other end. via the cursor of a broken ARC.
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Upon a breakage, the node detecting the failure reroutes the packet
towards the other edge, which means going backwards up to the cursor,
and following normal routing from there.
The Routing Header may indicate, as consumed, an entry that points on
the broken edge, if that is necessary for the cursor to figure out
which is the broken edge so as to route towards the other edge.
5.3. Flooding
ARCs probably apply to both unicast and multicast traffic, as
illustrated by [I-D.thubert-rtgwg-arc-bicast]. In particular, ARCs
enable a redundant flooding of a packet. The flooded packet is
injected at all edges ending in Omega, and from there swims upriver
along the reverse ARC direction. The packet is then forwarded from
the incoming edge of the ARC to the other edge where it is absorbed.
On the way along the ARC, the packet is copied into all the ARCs that
terminate in this ARC where the process recommences.
Since a packet is finally injected from both edges of any ARC, it
should get to all nodes in an ARC even if there is one breakage in
that ARC. in normal conditions, at least two copies of the packet
circulate in an ARC, one in each direction, and a mechanism should be
put in place to make sure that only one copy is injected in an
incoming edge.
6. ARC Signaling
6.1. Serial ARC Representation
A single ARC can be serialized as the sets of endpoints at both edges
and the ordered list of nodes in the ARC between the edges. Since
the endpoints are effectively nodes in downstream ARCs, the set of
all serialized ARCs provides a full description of the topology.
6.2. Centralized vs. Distributed computation
An ARC set can be computed with a slightly altered Shortest Path
First algorithm, as further explained in Section 4. It results that
any node, or all nodes participating to a Link State protocol, may
learn the topology and compute an ARC Set. If all nodes compute the
topology on their own and asynchronously, micro-loops will follow
till the network converges.
It makes more sense to limit the computation of an ARC Set to
specific nodes, typically a Path Computation Element (PCE), a Network
Management Entity (NME), or nodes in Omega. This is typically what
happens in a Software Define Networking (SDN) environment.
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6.3. ARC Topology Injection
Regardless of the central entity that computes the ARC set, the new
or updated ARC Set is serialized in a control message and flooded
over itself from Omega as described in Section 5.3.
The new ARC Set can be used as soon as it is received, in the
direction from which it is received, since a path along nodes in that
direction exists already, through the nodes that forwarded the
control messages. The full ARC redundancy is only available when a
control message has been received along an ARC in both directions.
In that model, there is no micro-loop.
6.4. ARC Operations, Administration, and Maintenance
Operations, Administration, and Maintenance (OAM) frames are used
within an ARC and flow periodically or asynchronously from an edge to
the other. Such frame may carry indications such as a breakage or a
congestion, and may be used to control the load balancing, or link
reversal operations.
7. Other ARC Operations
7.1. Node-Local vs. ARC-Wide reaction
ARCs enable forwarding plane reactions to breakages. In the simple
case of a single breakage in an ARC, the reaction can be immediate to
the discovery of the breakage and consists in rerouting the packet
towards the other edge across the cursor, as explained in
Section 5.2.
More complex situations require the coordination of all the nodes
along an ARC. For instance, load balancing requires the knowledge of
the congestion level at multiple points along the ARC, whereas the
solution to the Shared Risk Link Group (SRLG) problem discussed in
Section 7.3 requires all incoming edges in an isolated ARC segment to
be blocked before they can be returned. For such ARC-Wide
coordinated reactions, OAM frames are necessary to enable forwarding
plane rapid reactions.
7.2. Load Balancing
In normal conditions, only the Cursor may distribute its traffic
between the two Edge Nodes. If an Edge Node is still congested after
the Cursor forwards all its traffic towards the other Edge Node, then
the Cursor can be moved towards the congested Edge in order to derive
even more traffic towards the other Edge. If both Edges are
congested, then a back-pressure can be applied on the incoming ARCs
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so that they move their own traffic towards their own alternate Edge.
The process may recurse.
It is expected that control frames similar to those defined for MPLS
Fault Management Operations, Administration, and Maintenance (OAM)
[RFC6291] will echo from Edge Node to Edge Node provide information
such as liveliness and load. In order to establish a control loop
between the Edge Nodes and the Cursor, the OAM frame would carry at
least a logical information whether:
The Edge Node is capable of forwarding data down to the next ARC
the load may be increased (e.g. rate below threshold including
hysteresis)
the load should be decreased (e.g. congestion observed as
increased latency or buffer bloat)
If the load should be decreased towards of congested Edge Node and
the load may be increased towards the other then the Cursor may
adjust its balancing of the load, or move Cursor ownership towards
the congested Edge if it is already redirecting all the traffic
towards the non-congested Edge.
If the Cursor is balancing traffic away from the default position due
to a past congestion notification and the Edge that was congested now
reports that the load may be increased, then the reverse operation
can happen and the Cursor may balance the load back to the original
position taking the reverse steps as above.
If the OAM can not be forwarded due to a link or a node failure, then
the last node towards the broken Edge becomes Cursor and echoes the
OAM frames advertising that it is an Edge node that is blocked, not
capable of forwarding data down to the next ARC.
If both Edges are experiencing a congestion then the condition should
be reported to the Edge Nodes of all incoming ARCs. Same goes when
both Edges are blocked.
7.3. Shared Risk Link Group
Essentially, the Shared Risk Link Group (SRLG) problem is that a
physical breakage may end up breaking more than one apparently
unrelated IP links. such a breakage may end up breaking an ARC in
more than one place, effectively creating isolated segments.
The basic approach to solve that problem is the classical link
reversal technique. Since ARCs form a DAG as illustrated in
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Figure 6, it is possible to return all the incoming edges in an
isolated ARC segment so that traffic that circulates inside the
segment is actually fed back in incoming ARCs. The incoming ARCs are
considered broken on that edge so all the traffic is fed into the
other edge. If this causes the incoming ARC to be doubly broken, the
process recurses. in that incoming ARC. Over a number of iterations,
if there is an exit, it will be found and the traffic will be
funneled that way. If there is none, after a certain number of
iterations, the process counts to infinity and stops.
If the iterations are performed too quickly, the process may cause
micro-loops. OAM frames circulating within the broken segment can
solve that issue. On the way in, the OAM frame should block all
incoming ARCs, which effectively causes the edges to appear broken in
incoming ARCs. On the way back, the OAM frame returns the incoming
edges to be used the other way.
7.4. Olympic Rings
By Olympic Ring problem we mean how to optimally reuse the multiple
path opportunities that interconnecting 2 rings enable. ARCs can
simply be deployed inside a ring A to reach a connected ring B by
installing an ARC between adjacent interconnections on the rings. If
the rings only connect at one point, there is a single ARC going all
the way around the ring, with the cursor at the far side. If there
are more than one interconnection, then you always end up with as
many ARCs as there are interconnections.
A packet being forwarded inside a ring picks the side of the ring
that is away from the cursor, taking effectively the shortest path to
the next ring. If a hop is broken, then the packet is returned to
the other edge of the ARC, which is the adjacent interconnection
between the ARCs.
Note: There is no need in that model to artificially disable one hop
in the ring and re-enable it in case of breakage.
7.5. Routing Hierarchies
The ARC methods may be used to build and/or leverage routing
hierarchies, allowing high availability at multiple hierarchical
levels. In one hand, the view of an ARC Set can be simplified by
abstracting an ARC as a node in a DAG. The view of the routing
topology is thus simplified, as illustrated in Figure 6.
In the case of connected rings,abstracting a full ring as a node,
ARCs can be applied to a graph of rings, providing another level of
redundancy and an abstract end-to-end path computation, ring to ring
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to ring. ARCs may be used to make that computation resilient as
well.
8. Manageability
This specification describes a generic model. Protocols and
management will come later
9. IANA Considerations
This specification does not require IANA action.
10. Security Considerations
This specification is not found to introduce new security threat.
11. Acknowledgments
The authors wishes to thank Dirk Anteunis, Stewart Bryant, IJsbrand
Wijnands, George Swallow, Eric Osborne, Clarence Filsfils and Eric
Levy-Abegnoli for their participation and continuous support to the
work presented here.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J.,
and E. Crabbe, "Segment Routing Architecture", draft-ietf-
spring-segment-routing-00 (work in progress), December
2014.
[I-D.thubert-rtgwg-arc-bicast]
Thubert, P. and I. Wijnands, "Applying Available Routing
Constructs to bicasting", draft-thubert-rtgwg-arc-
bicast-01 (work in progress), October 2013.
[I-D.thubert-rtgwg-arc-vs-mrt]
Thubert, P., Enyedi, G., and S. Ramasubramanian,
"Available Routing Constructs", draft-thubert-rtgwg-arc-
vs-mrt-01 (work in progress), January 2014.
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[RFC5921] Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
Berger, "A Framework for MPLS in Transport Networks", RFC
5921, July 2010.
[RFC6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
D., and S. Mansfield, "Guidelines for the Use of the "OAM"
Acronym in the IETF", BCP 161, RFC 6291, June 2011.
Authors' Addresses
Pascal Thubert (editor)
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis 06254
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
Patrice Bellagamba
Cisco Systems
214 Avenue des fleurs
Saint-Raphael 83700
FRANCE
Phone: +33.6.1998.4346
Email: pbellaga@cisco.com
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