Internet Engineering Task Force (IETF) Y. Wei, Ed.
Request for Comments: 9696 Z. Zhang
Category: Informational ZTE Corporation
ISSN: 2070-1721 D. Afanasiev
Yandex
P. Thubert
Cisco Systems
T. Przygienda
Juniper Networks
December 2024
Routing in Fat Trees (RIFT) Applicability and Operational Considerations
Abstract
This document discusses the properties, applicability, and
operational considerations of Routing in Fat Trees (RIFT) in
different network scenarios with the intention of providing a rough
guide on how RIFT can be deployed to simplify routing operations in
Clos topologies and their variations.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9696.
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Table of Contents
1. Introduction
2. Terminology
3. Problem Statement of Routing in Modern IP Fabric Fat Tree
Networks
4. Applicability of RIFT to Clos IP Fabrics
4.1. Overview of RIFT
4.2. Applicable Topologies
4.2.1. Horizontal Links
4.2.2. Vertical Shortcuts
4.2.3. Generalizing to Any Directed Acyclic Graph
4.2.4. Reachability of Internal Nodes in the Fabric
4.3. Use Cases
4.3.1. Data Center Topologies
4.3.2. Metro Networks
4.3.3. Building Cabling
4.3.4. Internal Router Switching Fabrics
4.3.5. CloudCO
5. Operational Considerations
5.1. South Reflection
5.2. Suboptimal Routing on Link Failures
5.3. Black-Holing on Link Failures
5.4. Zero Touch Provisioning (ZTP)
5.5. Miscabling
5.5.1. Miscabling Examples
5.5.2. Miscabling Considerations
5.6. Multicast and Broadcast Implementations
5.7. Positive vs. Negative Disaggregation
5.8. Mobile Edge and Anycast
5.9. IPv4 over IPv6
5.10. In-Band Reachability of Nodes
5.11. Dual-Homing Servers
5.12. Fabric with a Controller
5.12.1. Controller Attached to ToFs
5.12.2. Controller Attached to Leaf
5.13. Internet Connectivity Within Underlay
5.13.1. Internet Default on the Leaf
5.13.2. Internet Default on the ToFs
5.14. Subnet Mismatch and Address Families
5.15. Anycast Considerations
5.16. IoT Applicability
5.17. Key Management
5.18. TTL/Hop Limit of 1 vs. 255 on LIEs/TIEs
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgments
Contributors
Authors' Addresses
1. Introduction
This document discusses the properties and applicability of "RIFT:
Routing in Fat Trees" [RFC9692] in different deployment scenarios and
highlights the operational simplicity of the technology compared to
traditional
classical routing solutions. It also documents special
considerations when RIFT is used with or without overlays and/or
controllers and how RIFT identifies miscablings and reroutes around
node and link failures.
2. Terminology
This document uses the terminology defined in [RFC9692]. The most
frequently used terms and their definitions from that document are
listed here.
Clos / Fat Tree:
This document uses the terms "Clos" and "Fat Tree" interchangeably
where it always refers to a folded spine-and-leaf topology with
possibly multiple Points of Delivery (PoDs) and one or multiple
Top of Fabric (ToF) planes. Several modifications such as leaf-
2-leaf shortcuts and multiple level shortcuts are possible and
described further in the document.
Crossbar:
Physical arrangement of ports in a switching matrix without
implying any further scheduling or buffering disciplines.
Directed Acyclic Graph (DAG):
A finite directed graph with no directed cycles (loops). If links
in a Clos are considered as either being all directed towards the
top or vice versa, bottom, each of two such two graphs is a DAG.
Disaggregation:
The process in which a node decides to advertise more specific
prefixes southwards, either positively to attract the
corresponding traffic or negatively to repel it. Disaggregation
is performed to prevent traffic loss and suboptimal routing to the
more specific prefixes.
Leaf:
A node without southbound adjacencies. Level 0 implies a leaf in
RIFT, but a leaf does not have to be level 0.
LIE:
This is an acronym for "Link Information Element" exchanged on all
the system's links running RIFT to form _ThreeWay_ adjacencies and
carry information used to perform RIFT Zero Touch Provisioning
(ZTP) of levels.
South Reflection:
Often abbreviated just as "reflection", South Reflection defines a
mechanism where South Node TIEs are "reflected" from the level
south back up north to allow nodes in the same level without East-
West links to be aware of each other's node Topology Information
Elements (TIEs).
Spine:
Any nodes north of leaves and south of ToF nodes. Multiple layers
of spines in a PoD are possible.
TIE:
This is an acronym for "Topology Information Element". TIEs are
exchanged between RIFT nodes to describe parts of a network such
as links and address prefixes. A TIE always has a direction and a
type. North TIEs (sometimes abbreviated as N-TIEs) are used when
dealing with TIEs in the northbound representation, and South-TIEs
(sometimes abbreviated as S-TIEs) are used for the southbound
equivalent. TIEs have different types, such as node and prefix
TIEs.
3. Problem Statement of Routing in Modern IP Fabric Fat Tree Networks
Clos [CLOS] topologies (commonly called a Fat Tree/network in modern
IP fabric considerations as a homonym to similar term for the original
definition of the term Fat Tree [FATTREE]) have gained prominence in
today's networking, primarily as a result of the paradigm shift
towards a centralized data-center-based architecture that delivers a
majority of computation and storage services.
Current routing protocols were geared towards a network with an
irregular topology with isotropic properties and a low degree of
connectivity. When applied to Fat Tree topologies:
* They tend to need extensive configuration or provisioning during
initialization and adding or removing nodes from the fabric.
* For link-state routing protocols, all nodes including spine-and-
leaf nodes learn the entire network topology and routing
information, which is actually not needed on the leaf nodes during
normal operation. They flood significant amounts of duplicate
link-state information between spine-and-leaf nodes during
topology updates and convergence events, requiring that additional
CPU and link bandwidth be consumed. This may impact the stability
and scalability of the fabric, make the fabric less reactive to
failures, and prevent the use of cheaper hardware at the lower
levels (i.e., spine-and-leaf nodes).
4. Applicability of RIFT to Clos IP Fabrics
Further content of this document assumes that the reader is familiar
with the terms and concepts used in the Open Shortest Path First
(OSPF) [RFC2328], OSPF for IPv6 [RFC5340], and Intermediate System to
Intermediate System (IS-IS) [ISO10589-Second-Edition] link-state
protocols. [RFC9692] outlines the requirements of routing in IP
fabrics and RIFT protocol concepts.
4.1. Overview of RIFT
RIFT is a dynamic routing protocol that is tailored for use in Clos,
Fat Tree, and other anisotropic topologies. Therefore, a core
property of RIFT is that its operation is sensitive to the structure
of the fabric -- it is anisotropic. RIFT acts as a link-state
protocol when "pointing north", advertising southward routes to
northward peers (parents) through flooding and database
synchronization. When "pointing south", RIFT operates hop-by-hop
like a distance-vector protocol, typically advertising a fabric
default route towards the ToF, aka superspine, to southward peers
(children).
The fabric default is typically the default route as described in
Section 6.3.8 ("Southbound Default Route Origination") of [RFC9692].
The ToF nodes may alternatively originate more specific prefixes (P')
southbound instead of the default route. In such a scenario, all
addresses carried within the RIFT domain must be contained within P',
and it is possible for a leaf that acts as gateway to the Internet to
advertise the default route instead.
RIFT floods flat link-state information northbound only so that each
level obtains the full topology of the levels that are south of it.
That information is never flooded East-West or back south again, so a
top tier node has a full set of prefixes from the Shortest Path First
(SPF) calculation.
In the southbound direction, the protocol operates like a "fully
summarizing, unidirectional" path-vector protocol or, rather, a
distance-vector with implicit split horizon. Routing information,
normally just the default route, propagates one hop south and is "re-
advertised" by nodes at next lower level.
+---------------+ +----------------+
| ToF | | ToF | LEVEL 2
+ ++------+--+--+-+ ++-+--+----+-----+
| | | | | | | | | ^
+ | | | +-------------------------+ |
Distance- | +-------------------+ | | | | |
Vector | | | | | | | | +
South | | | | +--------+ | | | Link-State
+ | | | | | | | | Flooding
| | | +----------------+ | | | North
v | | | | | | | | +
++---+-+ +------+ +-+----+ ++----++ |
|SPINE | |SPINE | | SPINE| | SPINE| | LEVEL 1
+ ++----++ ++---+-+ +-+--+-+ ++----++ |
+ | | | | | | | | | ^ N
Distance- | +-------+ | | +--------+ | | | E
Vector | | | | | | | | | +------>
South | +-------+ | | | +------+ | | | |
+ | | | | | | | | | +
v ++--++ +-+-++ ++--++ ++--++ +
|LEAF| |LEAF| |LEAF| |LEAF| LEVEL 0
+----+ +----+ +----+ +----+
Figure 1: RIFT Overview
A spine node only has information necessary for its level, which is
all destinations south of the node based on SPF calculation, the
default route, and potentially disaggregated routes.
RIFT combines the advantages of both link-state and distance-vector: distance-vector
protocols:
* Fastest possible convergence
* Automatic detection of topology
* Minimal routes/information on Top-of-Rack (ToR) switches, aka leaf
nodes
* High degree of ECMP
* Fast decommissioning of nodes
* Maximum propagation speed with flexible prefixes in an update
There are two types of link-state databases that are "north
representation" North Topology Information Elements (N-TIEs) and
"south representation" South Topology Information Elements (S-TIEs).
The N-TIEs contain a link-state topology description of lower levels,
and the S-TIEs simply carry default and disaggregated routes for the
lower levels.
RIFT also eliminates major disadvantages of link-state and distance-
vector protocols with the following:
* Reduced and balanced flooding
* Level-constrained automatic neighbor discovery
To achieve this, RIFT builds on the art of IGPs, such as OSPF, IS-IS,
Mobile Ad Hoc Network (MANET), and Internet of Things (IoT) to
provide unique features:
* Automatic (positive or negative) route disaggregation of northward
routes upon fallen leaves
* Recursive operation in the case of negative route disaggregation
* Anisotropic routing that extends a principle seen in the Routing
Protocol for Low-Power and Lossy Networks (RPL) [RFC6550] to wide
superspines
* Optimal flooding reduction that derives from the concept of a
"multipoint relay" (MPR) found in Optimized Link State Routing
(OLSR) [RFC3626] and balances the flooding load over northbound
links and nodes
Additional advantages that are unique to RIFT are listed below. The
details of these advantages can be found in RIFT [RFC9692].
* True ZTP
* Minimal blast radius on failures
* Can utilize all paths through fabric without looping
* Simple leaf implementation that can scale down to servers
* Key-value store
* Horizontal links used for protection only
4.2. Applicable Topologies
Albeit RIFT is specified primarily for "proper" Clos or Fat Tree
topologies, the protocol natively supports Points of Delivery (PoD)
concepts, which, strictly speaking, are not found in the original
Clos concept.
Further, the specification explains and supports operations of multi-
plane Clos variants where the protocol recommends the use of inter-
plane rings at the ToF level to allow the reconciliation of topology
view of different planes to make the Negative Disaggregation viable
in case of failures within a plane. These observations hold not only
in case of RIFT but also in the generic case of dynamic routing on
Clos variants with multiple planes and failures in bisectional
bandwidth, especially on the leaves.
4.2.1. Horizontal Links
RIFT is not limited to pure Clos divided into PoD and multi-planes
but supports horizontal (East-West) links below the ToF level. Those
links are used only for last resort northbound forwarding when a
spine loses all its northbound links or cannot compute a default
route through them.
A full-mesh connectivity between nodes on the same level can be
employed and that
deployed, which allows North SPF (N-SPF) to provide for any node
losing all its northbound adjacencies (as long as any of the other
nodes in the level are northbound connected) to and still participate in
northbound forwarding.
Note that a "ring" of horizontal links at any level below ToF does
not provide a "ring-based protection" scheme since the SPF
computation would have to deal with breaking of "loops", an
application for which RIFT is not intended.
4.2.2. Vertical Shortcuts
Through relaxations of the specified adjacency forming rules, RIFT
implementations can be extended to support vertical "shortcuts". The
RIFT specification itself does not provide the exact details since
the resulting solution suffers from either a much larger blast radius
with increased flooding volumes or bow tie problems in the case of
maximum aggregation routing.
4.2.3. Generalizing to Any Directed Acyclic Graph
RIFT is an anisotropic routing protocol, meaning that it has a sense
of direction (northbound, southbound, and East-West) and operates
differently depending on the direction.
Since a DAG provides a sense of north (the direction of the DAG) and
south (the reverse), it can be used to apply RIFT -- an edge in the
DAG that has only incoming vertices is a ToF node.
There are a number of caveats though:
* The DAG structure must exist before RIFT starts, so there is a
need for a companion protocol to establish the logical DAG
structure.
* A generic DAG does not have a sense of East and West. The
operation specified for East-West links and the southbound
reflection between nodes are not applicable. Also, ZTP will
derive a sense of depth that will eliminate some links.
Variations of ZTP could be derived to meet specific objectives,
e.g., make it so that most routers have at least two parents to
reach the ToF.
* RIFT applies to any Destination-Oriented DAG (DODAG) where there's
only one ToF node and the problem of disaggregation does not
exist. In that case, RIFT operates very much like RPL [RFC6550],
but uses Link State link-state information for southbound routes (downwards
in RPL's terms). For an arbitrary DAG with multiple destinations
(ToFs), the way disaggregation happens has to be considered.
* Positive Disaggregation expects that most of the ToF nodes reach
most of the leaves, so disaggregation is the exception as opposed
to the rule. When this is no longer true, it makes sense to turn
off disaggregation and route between the ToF nodes over a ring, a
full mesh, a transit network, or a form of area zero. Then again,
this operation is similar to RPL operating as a single DODAG with
a virtual root.
* In order to aggregate and disaggregate routes, RIFT requires that
all the ToF nodes share the full knowledge of the prefixes in the
fabric. This can be achieved with a ring as suggested by RIFT
[RFC9692], by some preconfiguration, or by using a synchronization
with a common repository where all the active prefixes are
registered.
4.2.4. Reachability of Internal Nodes in the Fabric
RIFT does not require that nodes have reachable addresses in the
fabric, though it is clearly desirable for operational purposes.
Under normal operating conditions, this can be easily achieved by
injecting the node's loopback address into Prefix North TIEs and South
Prefix South TIEs or other implementation-specific mechanisms.
Special considerations arise when a node loses all northbound
adjacencies but is not at the top of the fabric. If a spine node
loses all northbound links, the spine node doesn't advertise a
default route. But if the level of the spine node is auto-determined
by ZTP, it will "fall down" as depicted in Figure 8.
4.3. Use Cases
4.3.1. Data Center Topologies
4.3.1.1. Data Center Fabrics
RIFT is suited for applying underlay routing in data center (DC) IP fabrics underlay
routing,
fabrics, with the vast majority of which seem to be currently these IP fabrics being Clos
architectures (and will be for the foreseeable future) Clos architectures. future). It
significantly simplifies operation and deployment of such fabrics as
described in Section 5 for environments compared to extensive
proprietary provisioning and operational solutions.
4.3.1.2. Adaptations to Other Proposed Data Center Topologies
. +-----+ +-----+
. | | | |
.+-+ S0 | | S1 |
.| ++---++ ++---++
.| | | | |
.| | +------------+ |
.| | | +------------+ |
.| | | | |
.| ++-+--+ +--+-++
.| | | | |
.| | A0 | | A1 |
.| +-+--++ ++---++
.| | | | |
.| | +------------+ |
.| | +-----------+ | |
.| | | | |
.| +-+-+-+ +--+-++
.+-+ | | |
. | L0 | | L1 |
. +-----+ +-----+
Figure 2: Level Shortcut
RIFT is not strictly limited to Clos topologies. The protocol only
requires a sense of "compass rose directionality" either achieved
through configuration or derivation of levels. So conceptually,
shortcuts between levels could be included. Figure 2 depicts an
example of a shortcut between levels. In this example, suboptimal
routing will occur when traffic is sent from L0 to L1 via S0's
default route and back down through A0 or A1. In order to avoid
that, only default routes from A0 or A1 are used. All leaves would
be required to install each other's routes.
While various technical and operational challenges may require the
use of such modifications, discussion of those topics is outside the
scope of this document.
4.3.2. Metro Networks
The demand for bandwidth is increasing steadily, driven primarily by
environments close to content producers (server farms connection via
DC fabrics) but in proximity to content consumers as well. Consumers
are often clustered in metro areas with their own network
architectures that can benefit from simplified, regular Clos
structures. Thus, they can also benefit from RIFT.
4.3.3. Building Cabling
Commercial edifices are often cabled in topologies that are either
Clos or its isomorphic equivalents. The Clos can grow rather high
with many levels. That presents a challenge for traditional classical routing
protocols (except BGP [RFC4271] and Private Network-Network Interface
(PNNI) [PNNI], which is largely phased-out by now) that do not
support an arbitrary number of levels, which RIFT does naturally.
Moreover, due to the limited sizes of forwarding tables in network
elements of building cabling, the minimum FIB size RIFT maintains
under normal conditions is cost-effective in terms of hardware and
operational costs.
4.3.4. Internal Router Switching Fabrics
It is common in high-speed communications switching and routing
devices to use switch fabrics that are interconnection networks
inside the devices connecting the input ports to their output ports.
For example, a crossbar is one of the switch fabric techniques, even
though it is not feasible due to cost, head-of-line blocking, or size
trade-offs. Normally, such fabrics are not self-healing or rely on
1:1 or 1+1 protection schemes, but it is conceivable to use RIFT to
operate Clos fabrics that can deal effectively with interconnections
or subsystem failures in such a module. RIFT is not IP specific and
hence any link addressing connecting internal device subnets is
conceivable.
4.3.5. CloudCO
The Cloud Central Office (CloudCO) is a new stage of the telecom
Central Office. It takes the advantage of Software-Defined
Networking (SDN) and Network Function Virtualization (NFV) in
conjunction with general purpose hardware to optimize current
networks. The following figure illustrates this architecture at a
high level. It describes a single instance or macro-node of CloudCO
that provides a number of value-added services (VASes), a Broadband
Access Abstraction (BAA), and virtualized network services. An
Access I/O module faces a CloudCO access node and the Customer
Premises Equipment (CPE) behind it. A Network I/O module is facing
the core network. The two I/O modules are interconnected by a leaf
and spine spine-
and-leaf fabric [TR-384].
+---------------------+ +----------------------+
| Spine | | Spine |
| Switch | | Switch |
+------+---+------+-+-+ +--+-+-+-+-----+-------+
| | | | | | | | | | | |
| | | | | +-------------------------------+ |
| | | | | | | | | | | |
| | | | +-------------------------+ | | |
| | | | | | | | | | | |
| | +----------------------+ | | | | | | | |
| | | | | | | | | | | |
| +---------------------------------+ | | | | | | |
| | | | | | | | | | | |
| | | +-----------------------------+ | | | | |
| | | | | | | | | | | |
| | | | | +--------------------+ | | | |
| | | | | | | | | | | |
+--+ +-+---+--+ +-+---+--+ +--+----+--+ +-+--+--+ +--+
|L | | Leaf | | Leaf | | Leaf | | Leaf | |L |
|S | | Switch | | Switch | | Switch | | Switch| |S |
++-+ +-+-+-+--+ +-+-+-+--+ +--+-+--+--+ ++-+--+-+ +-++
| | | | | | | | | | | | | |
| +-+-+-+--+ +-+-+-+--+ +--+-+--+--+ ++-+--+-+ |
| |Compute | |Compute | | Compute | |Compute| |
| |Node | |Node | | Node | |Node | |
| +--------+ +--------+ +----------+ +-------+ |
| || VAS5 || || vDHCP|| || vRouter|| ||VAS1 || |
| |--------| |--------| |----------| |-------| |
| |--------| |--------| |----------| |-------| |
| || VAS6 || || VAS3 || || v802.1x|| ||VAS2 || |
| |--------| |--------| |----------| |-------| |
| |--------| |--------| |----------| |-------| |
| || VAS7 || || VAS4 || || vIGMP || ||BAA || |
| |--------| |--------| |----------| |-------| |
| +--------+ +--------+ +----------+ +-------+ |
| |
++-----------+ +---------++
|Network I/O | |Access I/O|
+------------+ +----------+
Figure 3: CloudCO Architecture Example
The Spine-Leaf architecture deployed inside CloudCO meets the network
requirements of being adaptable, agile, scalable, and dynamic.
5. Operational Considerations
RIFT presents the features for organizations building and operating
IP fabrics to simplify the operation and deployments while achieving
many desirable properties of a dynamic routing protocol on such a
substrate:
* RIFT only floods routing information to the devices that need it.
* RIFT allows for ZTP within the protocol. In its most extreme
version, RIFT does not rely on any specific addressing and can
operate using IPv6 Neighbor Discovery (ND) [RFC4861] only for IP
fabric.
* RIFT has provisions to detect common IP fabric miscabling
scenarios.
* RIFT automatically negotiates Bidirectional Forwarding Detection
(BFD) per link. This allows for IP and micro-BFD [RFC7130] to
replace Link Aggregation Groups (LAGs) that hide bandwidth
imbalances in case of constituent failures. Further automatic
link validation techniques similar to those in [RFC5357] could be
supported as well.
* RIFT inherently solves many problems associated with the use of
traditional
classical routing topologies with dense meshes and high degrees of
ECMP by including automatic bandwidth balancing, flood reduction,
and automatic disaggregation on failures while providing maximum
aggregation of prefixes in default scenarios. ECMP in RIFT
eliminates the need for more Loop-Free Alternate (LFA) procedures.
* RIFT reduces FIB size towards the bottom of the IP fabric where
most nodes reside and reside. This allows with that for cheaper hardware on the edges
and introduction of modern IP fabric architectures that
encompass, e.g., encompass
server multihoming. multihoming and other mechanisms.
* RIFT provides valley-free routing that is loop free. A valley-
free path allows for reversal of direction at most once from a
packet heading northbound to southbound while permitting traversal
of horizontal links in the northbound phase. This allows for the
use of any such valley-free path in bisectional fabric bandwidth
between two destinations irrespective of their metrics that can be
used to balance load on the fabric in different ways. Valley-free
routing eliminates the need for any specific micro-loop avoidance
procedures for RIFT.
* RIFT includes a key-value distribution mechanism that allows for
future applications such as automatic provisioning of basic
overlay services or automatic key rollovers over whole fabrics.
* RIFT is designed for minimum delay in case of prefix mobility on
the fabric. In conjunction with [RFC8505], RIFT can differentiate
anycast advertisements from mobility events and retain only the
most recent advertisement in the latter case.
* Many further operational and design points collected over many
years of routing protocol deployments have been incorporated in
RIFT such as fast flooding rates, protection of information
lifetimes, and operationally recognizable remote ends of links and
node names.
5.1. South Reflection
South reflection is a mechanism where South Node TIEs are "reflected"
back up north to allow nodes in the same level without East-West
links to "see" each other.
For example, in Figure 4, Spine111\Spine112\Spine121\Spine122
reflects Node S-TIEs from ToF21 to ToF22 separately. Respectively,
Spine111\Spine112\Spine121\Spine122 reflects Node S-TIEs from ToF22
to ToF21 separately, so ToF22 and ToF21 see each other's node
information as level 2 nodes.
In an equivalent fashion, as the result of the south reflection
between Spine121-Leaf121-Spine122 and Spine121-Leaf122-Spine122,
Spine121 and Spine 122 know each other at level 1.
5.2. Suboptimal Routing on Link Failures
+--------+ +--------+
| ToF21 | | ToF22 | LEVEL 2
++--+-+-++ ++-+--+-++
| | | | | | | +
| | | | | | | linkTS8
+------------+ | +-+linkTS3+-+ | | | +-------------+
| | | | | | + |
| +---------------------------+ | linkTS7 |
| | | | + + + |
| | | +-------+linkTS4+------------+ |
| | | + + | | |
| | | +-------------+--+ | |
| | | | | linkTS6 | |
+-+----+-+ +-+----+-+ ++--------+ +-+----+-+
|Spine111| |Spine112| |Spine121 | |Spine122| LEVEL 1
+-+---+--+ +-+----+-+ +-+---+---+ +-+----+-+
| | | | | | | |
| +-------------+ | + ++XX+linkSL6+---+ +
| | | | linkSL5 | | linkSL8
| +-----------+ | | + +---+linkSL7+-+ | +
| | | | | | | |
+-+---+-+ +--+--+-+ +-+---+-+ +--+--+-+
|Leaf111| |Leaf112| |Leaf121| |Leaf122| LEVEL 0
+-+-----+ +-+-----+ +-----+-+ +-+-----+
+ + + +
Prefix111 Prefix112 Prefix121 Prefix122
Figure 4: Suboptimal Routing Upon Link Failure Use Case
As shown in Figure 4, as the result of the south reflection between
Spine121-Leaf121-Spine122 and Spine121-Leaf122-Spine122, reflection, Spine121
and Spine 122 know each other at level 1.
Without disaggregation mechanisms, the packet from leaf121 to
prefix122 will probably go up through linkSL5 to linkTS3 when linkSL6
fails. Then, the packet will go down through linkTS4 to linkSL8 to
Leaf122 or go up through linkSL5 to linkTS6, then go down through
linkTS8 and linkSL8 to Leaf122 based on the pure default route. This
is the case of suboptimal routing or bow tying.
With disaggregation mechanisms, Spine122 will detect the failure
according to the reflected node S-TIE from Spine121 when linkSL6
fails. Based on the disaggregation algorithm provided by RIFT,
Spine122 will explicitly advertise prefix122 in Disaggregated Prefix
S-TIE PrefixTIEElement(prefix122, cost 1). The packet from leaf121
to prefix122 will only be sent to linkSL7 following a longest-prefix
match to prefix 122 directly, then it will go down through linkSL8 to
Leaf122.
5.3. Black-Holing on Link Failures
+--------+ +--------+
| ToF 21 | | ToF 22 | LEVEL 2
++-+--+-++ ++-+--+-++
| | | | | | | +
| | | | | | | linkTS8
+--------------+ | +-+linkTS3+X+ | | | +--------------+
linkTS1 | | | | | + |
+ +-----------------------------+ | linkTS7 |
| | + | + + + |
| | linkTS2 +-------+linkTS4+X+----------+ |
| + + + + | | |
| linkTS5 +-+ +------------+--+ | |
| + | | | linkTS6 | |
+-+----+-+ +-+----+-+ ++-------+ +-+-----++
|Spine111| |Spine112| |Spine121| |Spine122| LEVEL 1
+-+---+--+ ++----+--+ +-+---+--+ +-+----+-+
| | | | | | | |
+ +---------------+ | + +---+linkSL6+---+ +
linkSL1 | | | linkSL5 | | linkSL8
+ +--+linkSL3+--+ | | + +---+linkSL7+-+ | +
| | | | | | | |
+-+---+-+ +--+--+-+ +-+---+-+ +--+--+-+
|Leaf111| |Leaf112| |Leaf121| |Leaf122| LEVEL 0
+-+-----+ +-+-----+ +-----+-+ +-----+-+
+ + + +
Prefix111 Prefix112 Prefix121 Prefix122
Figure 5: Black-Holing Upon Link Failure Use Case
This scenario illustrates a case where double link failure occurs and
black-holing can happen.
Without disaggregation mechanisms, the packet from leaf111 to
prefix122 would suffer 50% black-holing based on pure default route
when linkTS3 and linkTS4 both fail. The packet is supposed to go up
through linkSL1 to linkTS1 and then go down through linkTS3 or
linkTS4 will be dropped. The packet is supposed to go up through
linkSL3 to linkTS2, then go down through linkTS3 or linkTS4 will be
dropped as well. This is the case of black-holing.
With disaggregation mechanisms, ToF22 will detect the failure
according to the reflected node S-TIE of ToF21 from Spine111\Spine112
when linkTS3 and linkTS4 both fail. Based on the disaggregation
algorithm provided by RIFT, ToF22 will explicitly originate an S-TIE
with prefix 121 and prefix 122 that is flooded to spines 111, 112,
121, and 122.
The packet from leaf111 to prefix122 will not be routed to linkTS1 or
linkTS2. The packet from leaf111 to prefix122 will only be routed to
linkTS5 or linkTS7 following a longest-prefix match to prefix122.
5.4. Zero Touch Provisioning (ZTP)
RIFT is designed to require a very minimal configuration to simplify
its operation and avoid human errors; based on that minimal
information, ZTP auto configures the key operational parameters of
all the RIFT nodes, including the System ID of the node that must be
unique in the RIFT network and the level of the node in the Fat Tree,
which determines which peers are northward "parents" and which are
southward "children".
ZTP is always on, but its decisions can be overridden when a network
administrator prefers to impose its own configuration. In that case,
it is the responsibility of the administrator to ensure that the
configured parameters are correct, i.e., ensure that the System ID of
each node is unique and that the administratively set levels truly
reflect the relative position of the nodes in the fabric. It is
recommended to let ZTP configure the network, and when not, ZTP does not
configure the network, it is recommended to configure the level of
all the nodes to avoid an undesirable interaction between ZTP and the
manual configuration.
ZTP requires that the administrator points out the ToF nodes to set
the baseline from which the fabric topology is derived. The ToF
nodes are configured with the TOP_OF_FABRIC flag, which are initial
'seeds' needed for other ZTP nodes to derive their level in the
topology. ZTP computes the level of each node based on the Highest
Available Level (HAL) of the potential parent closest to that
baseline, which represents the superspine. In a fashion, RIFT can be
seen as a distance-vector protocol that computes a set of feasible
successors towards the superspine and autoconfigures the rest of the
topology.
The autoconfiguration mechanism computes a global maximum of levels
by diffusion. The derivation of the level of each node happens then
based on LIEs received from its neighbors, whereas each node (with
possible exceptions of configured leaves) tries to attach at the
highest possible point in the fabric. This guarantees that even if
the diffusion front reaches a node from "below" faster than from
"above", it will greedily abandon already negotiated levels derived
from nodes topologically below it and properly peer with nodes above.
The achieved equilibrium can be disturbed massively by all nodes with
the highest level either leaving or entering the domain (with some
finer distinctions not explained further). It is therefore
recommended that each node is multihomed towards nodes with
respective HAL offerings. Fortunately, this is the natural state of
things for the topology variants considered in RIFT.
A RIFT node may also be configured to confine it to the leaf role
with the LEAF_ONLY flag. A leaf node can also be configured to
support leaf-2-leaf procedures with the LEAF_2_LEAF flag. In both
cases, the node cannot be TOP_OF_FABRIC and its level cannot be
configured. RIFT will fully determine the node's level after it is
attached to the topology and ensure that the node is at the "bottom
of the hierarchy" (southernmost).
5.5. Miscabling
5.5.1. Miscabling Examples
+----------------+ +-----------------+
| ToF21 | +------+ ToF22 | LEVEL 2
+-------+----+---+ | +----+---+--------+
| | | | | | | | |
| | | +----------------------------+ |
| +---------------------------+ | | | |
| | | | | | | | |
| | | | +-----------------------+ | |
| | +------------------------+ | | |
| | | | | | | | |
+-+---+--+ +-+---+--+ | +--+---+-+ +--+---+-+
|Spine111| |Spine112| | |Spine121| |Spine122| LEVEL 1
+-+---+--+ ++----+--+ | +--+---+-+ +-+----+-+
| | | | | | | | |
| +---------+ | link-M | +---------+ |
| | | | | | | | |
| +-------+ | | | | +-------+ | |
| | | | | | | | |
+-+---+-+ +--+--+-+ | +-+---+-+ +--+--+-+
|Leaf111| |Leaf112+-----+ |Leaf121| |Leaf122| LEVEL 0
+-------+ +-------+ +-------+ +-------+
Figure 6: A Single-Plane Miscabling Example
Figure 6 shows a single-plane miscabling example. It's a perfect Fat
Tree fabric except for link-M connecting Leaf112 to ToF22.
The RIFT control protocol can discover the physical links
automatically and is able to detect cabling that violates Fat Tree
topology constraints. It reacts accordingly to such miscabling
attempts, preventing adjacencies between nodes from being formed and
traffic from being forwarded on those miscabled links at a minimum.
In such scenario, Leaf112 will use link-M to derive its level (unless
it is leaf) and can report links to Spine111 and Spine112 as
miscabled unless the implementations allow horizontal links.
Figure 7 shows a multi-plane miscabling example. Since Leaf112 and
Spine121 belong to two different PoDs, the adjacency between Leaf112
and Spine121 cannot be formed. Link-W would be detected and
prevented.
+-------+ +-------+ +-------+ +-------+
|ToF A1| |ToF A2| |ToF B1| |ToF B2| LEVEL 2
+-------+ +-------+ +-------+ +-------+
| | | | | | | |
| | | +-----------------+ | | |
| +--------------------------+ | | | |
| +------+ | | | +------+ |
| | +-----------------+ | | | | |
| | | +--------------------------+ | |
| A | | B | | A | | B |
+-----+--+ +-+---+--+ +--+---+-+ +--+-----+
|Spine111| |Spine112| +---+Spine121| |Spine122| LEVEL 1
+-+---+--+ ++----+--+ | +--+---+-+ +-+----+-+
| | | | | | | | |
| +---------+ | | | +---------+ |
| | | | link-W | | | |
| +-------+ | | | | +-------+ | |
| | | | | | | | |
+-+---+-+ +--+--+-+ | +-+---+-+ +--+--+-+
|Leaf111| |Leaf112+------+ |Leaf121| |Leaf122| LEVEL 0
+-------+ +-------+ +-------+ +-------+
+--------PoD#1----------+ +---------PoD#2---------+
Figure 7: A Multiple Plane Miscabling Example
RIFT provides an optional level determination procedure in its ZTP
mode. Nodes in the fabric without their level configured determine
it automatically. However, this can have possible counter-intuitive
consequences. One extreme failure scenario is depicted in Figure 8,
and it shows that if all northbound links of Spine11 fail at the same
time, Spine11 negotiates a lower level than Leaf11 and Leaf12.
To prevent such scenario where leaves are expected to act as
switches, the LEAF_ONLY flag can be set for Leaf111 and Leaf112.
Since level -1 is invalid, Spine11 would not derive a valid level
from the topology in Figure 8. It will be isolated from the whole
fabric, and it would be up to the leaves to declare the links towards
such spine as miscabled.
+-------+ +-------+ +-------+ +-------+
|ToF A1| |ToF A2| |ToF A1| |ToF A2|
+-------+ +-------+ +-------+ +-------+
| | | | | |
| +-------+ | | |
+ + | | ====> | |
X X +------+ | +------+ |
+ + | | | |
+----+--+ +-+-----+ +-+-----+
|Spine11| |Spine12| |Spine12|
+-+---+-+ ++----+-+ ++----+-+
| | | | | |
| +---------+ | | |
| +-------+ | | +-------+ |
| | | | | |
+-+---+-+ +--+--+-+ +-----+-+ +-----+-+
|Leaf111| |Leaf112| |Leaf111| |Leaf112|
+-------+ +-------+ +-+-----+ +-+-----+
| |
| +--------+
| |
+-+---+-+
|Spine11|
+-------+
Figure 8: Fallen Spine
5.5.2. Miscabling Considerations
There are scenarios where operators may want to leverage ZTP and
implement additional cabling constraints that go beyond the
previously described topology violations. Enforcing cabling down to
specific level, node, and port combinations might make it simpler for
onsite staff to perform troubleshooting activities or replace optical
transceivers and/or cabling as the physical layout will be consistent
across the fabric. This is especially true for densely connected
fabrics where it is difficult to physically manipulate those
components. It is also easy to imagine other models, such as one
where the strict port requirement is relaxed.
Figure 9 illustrates an example where the first port on Leaf1 must
connect to the first port on Spine1, the second port on Leaf1 must
connect to the first port on Spine2, and so on. Consider a case
where (Leaf1, Port1) and (Leaf1, Port2) were reversed. RIFT would
not consider this to be miscabled by default; however, an operator
might want to.
+--------+ +--------+ +--------+ +--------+
| Spine1 | | Spine2 | | Spine3 | | Spine4 |
+-1------+ +-1------+ +-1------+ +-1------+
+ + + +
| +----------+ | |
| | | |
| | +---------------------+ |
| | | |
| | | +--------------------------------+
| | | |
| | | |
| | | |
| | | |
+ + + +
+-1--2--3--4--+
| Leaf1 | ......
+-------------+
Figure 9: Fallen Spine Additional Cabling Constraint Example
RIFT allows implementations to provide programmable plug-ins that can
adjust ZTP operation or capture information during computation.
While defining this is outside the scope of this document, such a
mechanism could be used to extend the miscabling functionality.
For other protocols to achieve this, it would require additional
operational overhead. Consider a fabric that is using unnumbered
OSPF links; it is still very likely that a miscabled link will form
an adjacency. Each attempt to move cables to the correct port may
result in the need for additional troubleshooting as other links will
become miscabled in the process. Without automation to explicitly
tell the operator which ports need to be moved where, the process
becomes manually intensive and error-prone very quickly. If the
problem goes unnoticed, it will result in suboptimal performance in
the fabric.
5.6. Multicast and Broadcast Implementations
RIFT supports both multicast and broadcast implementations. While a
multicast implementation is preferred, there might cases where a
broadcast implementation is optimal or even required. For example,
operating systems on IoT devices and embedded devices may not have
the required multicast support. Another example is containers, which
do support multicast in some cases but tend to be very CPU-
inefficient and difficult to tune.
5.7. Positive vs. Negative Disaggregation
Disaggregation is the procedure whereby RIFT [RFC9692] advertises a
more specific route southwards as an exception to the aggregated
fabric-default north. Disaggregation is useful when a prefix within
the aggregation is reachable via some of the parents but not the
others at the same level of the fabric. It is mandatory when the
level is the ToF since a ToF node that cannot reach a prefix becomes
a black hole for that prefix. The hard problem is to know which
prefixes are reachable by whom.
In the general case, RIFT [RFC9692] solves that problem by
interconnecting the ToF nodes so that the ToF nodes can exchange the
full list of prefixes that exist in the fabric and figure out when a
ToF node lacks reachability to some prefixes. This requires
additional ports at the ToF, typically two ports per ToF node to form
a ToF-spanning ring. RIFT [RFC9692] also defines the southbound
reflection procedure that enables a parent to explore the direct
connectivity of its peers, meaning their own parents and children;
based on the advertisements received from the shared parents and
children, it may enable the parent to infer the prefixes its peers
can reach.
When a parent lacks reachability to a prefix, it may disaggregate the
prefix negatively, i.e., advertise that this parent can be used to
reach any prefix in the aggregation except that one. The Negative
Disaggregation signaling is simple and functions transitively from
ToF to Top-of-Pod (ToP) and then from ToP to Leaf. However, it is
hard for a parent to figure out which prefix it needs to disaggregate
because it does not know what it does not know; it results that the
use of a spanning ring at the ToF is required to operate the Negative
Disaggregation. Also, though it is only an implementation problem,
the programming of the FIB is complex compared to normal routes and
may incur recursions.
The more classical alternative is, for the parents that can reach a
prefix that peers at the same level cannot, to advertise a more
specific route to that prefix. This leverages the normal longest
prefix match in the FIB and does not require a special
implementation. As opposed to the Negative Disaggregation, the
Positive Disaggregation is difficult and inefficient to operate
transitively.
Transitivity is not needed by a grandchild if all its parents
received the Positive Disaggregation, meaning that they shall all
avoid the black hole; when that is the case, they collectively build
a ceiling that protects the grandchild. Until then, a parent that
received the Positive Disaggregation may believe that some peers are
lacking the reachability and re-advertise too early or defer and
maintain a black hole situation longer than necessary.
In a non-partitioned fabric, all the ToF nodes see one another
through the reflection and can figure out if one is missing a child.
In that case, it is possible to compute the prefixes that the peer
cannot reach and disaggregate positively without a ToF-spanning ring.
The ToF nodes can also ascertain that the ToP nodes are each
connected to at least a ToF node that can still reach the prefix,
meaning that the transitive operation is not required.
The bottom line is that in a fabric that is partitioned (e.g., using
multiple planes) and/or where the ToP nodes are not guaranteed to
always form a ceiling for their children, it is mandatory to use
Negative Disaggregation. On the other hand, in a highly symmetrical
and fully connected fabric (e.g., a canonical Clos Network), the
Positive Disaggregation methods save the complexity and cost
associated to the ToF-spanning ring.
Note that in the case of Positive Disaggregation, the first ToF nodes
that announce a more-specific route attract all the traffic for that
route and may suffer from a transient incast. A ToP node that defers
injecting the longer prefix in the FIB, in order to receive more
advertisements and spread the packets better, also keeps on sending a
portion of the traffic to the black hole in the meantime. In the
case of Negative Disaggregation, the last ToF nodes that inject the
route may also incur an incast issue; this problem would occur if a
prefix that becomes totally unreachable is disaggregated.
5.8. Mobile Edge and Anycast
When a physical or a virtual node changes its point of attachment in
the fabric from a previous-leaf to a next-leaf, new routes must be
installed that supersede the old ones. Since the flooding flows
northwards, the nodes (if any) between the previous-leaf and the
common parent are not immediately aware that the path via the
previous-leaf is obsolete and a stale route may exist for a while.
The common parent needs to select the freshest route advertisement in
order to install the correct route via the next-leaf. This requires
that the fabric determines the sequence of the movements of the
mobile node.
On the one hand, a classical sequence counter provides a total order
for a while, but it will eventually wrap. On the other hand, a
timestamp provides a permanent order, but it may miss a movement that
happens too quickly vs. the granularity of the timing information.
It is not envisioned that an average fabric supports the Precision
Time Protocol [IEEEstd1588] in the short term nor that the precision
available with the Network Time Protocol [RFC5905] (in the order of
100 to 200 ms) may not be necessarily enough to cover, e.g., the fast
mobility of a Virtual Machine (VM).
Section 6.8.4 ("Mobility") of [RFC9692] specifies a hybrid method
that combines a sequence counter from the mobile node and a timestamp
from the network taken at the leaf when the route is injected. If
the timestamps of the concurrent advertisements are comparable (i.e.,
more distant than the precision of the timing protocol), then the
timestamp alone is used to determine the relative freshness of the
routes. Otherwise, the sequence counter from the mobile node is used
if it is available. One caveat is that the sequence counter must not
wrap within the precision of the timing protocol. Another is that
the mobile node may not even provide a sequence counter; in which
case, the mobility itself must be slower than the precision of the
timing.
Mobility must not be confused with anycast. In both cases, the same
address is injected in RIFT at different leaves. In the case of
mobility, only the freshest route must be conserved since the mobile
node changes its point of attachment for a leaf to the next. In the
case of anycast, the node may either be multihomed (attached to
multiple leaves in parallel) or reachable beyond the fabric via
multiple routes that are redistributed to different leaves. Either
way, the multiple routes are equally valid and should be conserved in
the case of anycast. Without further information from the
redistributed routing protocol, it is impossible to sort out a
movement from a redistribution that happens asynchronously on
different leaves. RIFT [RFC9692] expects that anycast addresses are
advertised within the timing precision, which is typically the case
with a low-precision timing and a multihomed node. Beyond that time
interval, RIFT interprets the lag as a mobility and only the freshest
route is retained.
When using IPv6 [RFC8200], RIFT suggests to leverage leveraging 6LoWPAN ND
[RFC8505] as the IPv6 ND interaction between the mobile node and the
leaf. This not only provides a sequence counter but also a lifetime
and a security token that may be used to protect the ownership of an
address [RFC8928]. When using 6LoWPAN ND [RFC8505], the parallel
registration of an anycast address to multiple leaves is done with
the same sequence counter, whereas the sequence counter is
incremented when the point of attachment changes. This way, it is
possible to differentiate a mobile node from a multihomed node, even
when the mobility happens within the timing precision. It is also
possible for a mobile node to be multihomed as well, e.g., to change
only one of its points of attachment.
5.9. IPv4 over IPv6
RIFT allows advertising IPv4 prefixes over an IPv6 RIFT network. An
IPv6 Address Family (AF) configures via the usual ND mechanisms and
then V4 can use V6 next-hops analogous to [RFC8950]. It is expected
that the whole fabric supports the same type of forwarding of AFs on
all the links. RIFT provides an indication whether a node is capable
of V4-forwarding and implementations are possible where different
routing tables are computed per AF as long as the computation remains
loop-free.
+-----+ +-----+
+---+---+ | ToF | | ToF |
^ +--+--+ +-----+
| | | | |
| | +-------------+ |
| | +--------+ | |
+ | | | |
V6 +-----+ +-+---+
Forwarding |Spine| |Spine|
+ +--+--+ +-----+
| | | | |
| | +-------------+ |
| | +--------+ | |
| | | | |
v +-----+ +-+---+
+---+---+ |Leaf | | Leaf|
+--+--+ +--+--+
| |
IPv4 prefixes| |IPv4 prefixes
| |
+---+----+ +---+----+
| V4 | | V4 |
| subnet | | subnet |
+--------+ +--------+
Figure 10: IPv4 over IPv6
5.10. In-Band Reachability of Nodes
RIFT doesn't precondition that nodes of the fabric have reachable
addresses, but the operational reasons to reach the internal nodes
may exist. Figure 11 shows an example that the network management
station (NMS) attaches to Leaf1.
+-------+ +-------+
| ToF1 | | ToF2 |
++---- ++ ++-----++
| | | |
| +----------+ |
| +--------+ | |
| | | |
++-----++ +--+---++
|Spine1 | |Spine2 |
++-----++ ++-----++
| | | |
| +----------+ |
| +--------+ | |
| | | |
++-----++ +--+---++
| Leaf1 | | Leaf2 |
+---+---+ +-------+
|
|NMS
Figure 11: In-Band Reachability of Nodes
If the NMS wants to access Leaf2, it simply works because the
loopback address of Leaf2 is flooded in its Prefix North TIE.
If the NMS wants to access Spine2, it also works because a spine node
always advertises its loopback address in the Prefix North TIE. The
NMS may reach Spine2 from Leaf1-Spine2 or Leaf1-Spine1-ToF1/
ToF2-Spine2.
If the NMS wants to access ToF2, ToF2's loopback address needs to be
injected into its Prefix South TIE. This TIE must be seen by all
nodes at the level below -- the spine nodes in Figure 9 11 -- that must
form a ceiling for all the traffic coming from below (south).
Otherwise, the traffic from the NMS may follow the default route to
the wrong ToF Node, e.g., ToF1.
In the case of failure between ToF2 and spine nodes, ToF2's loopback
address must be disaggregated recursively all the way to the leaves.
In a partitioned ToF, even with recursive disaggregation, a ToF node
is only reachable within its plane.
A possible alternative to recursive disaggregation is to use a ring
that interconnects the ToF nodes to transmit packets between them for
their loopback addresses only. The idea is that this is mostly
control traffic and should not alter the load-balancing properties of
the fabric.
5.11. Dual-Homing Servers
Each RIFT node may operate in ZTP mode. It has no configuration
(unless it is a ToF node at the top of the topology or the if it must
operate in the topology as a leaf and/or support leaf-2-leaf
procedures), and it will fully configure itself after being attached
to the topology.
+---+ +---+ +---+
|ToF| |ToF| |ToF| ToF
+---+ +---+ +---+
| | | | | |
| +----------------+ | |
| +----------------+ |
| | | | | |
+----------+--+ +--+----------+
| ToR1 | | ToR2 | Spine
+--+------+---+ +--+-------+--+
+---+ | | | | | | +---+
| +-----------------+ | | |
| | | +-------------+ | |
| | | | | +-----------------+ |
| | | | +--------------+ | | |
| | | | | | | |
+---+ +---+ +---+ +---+
| | | | | | | |
+---+ +---+ ............. +---+ +---+
SV(1) SV(2) SV(n-1) SV(n) Leaf
Figure 12: Dual-Homing Servers
Sometimes people may prefer to disaggregate from ToR nodes to servers
from
start on, i.e. startup, i.e., the servers have couple tens of multiple routes in the FIB from
start on beside
startup other than default routes to avoid breakages at the rack
level. Full disaggregation of the fabric could be achieved by
configuration supported by RIFT.
5.12. Fabric with a Controller
There are many different ways to deploy the controller. One
possibility is attaching a controller to the RIFT domain from ToF and
another possibility is attaching a controller from the leaf.
+------------+
| Controller |
++----------++
| |
| |
+----++ ++----+
------- | ToF | | ToF |
| +--+--+ +-----+
| | | | |
| | +-------------+ |
| | +--------+ | |
| | | | |
+-----+ +-+---+
RIFT domain |Spine| |Spine|
+--+--+ +-----+
| | | | |
| | +-------------+ |
| | +--------+ | |
| | | | |
| +-----+ +-+---+
------- |Leaf | | Leaf|
+-----+ +-----+
Figure 13: Fabric with a Controller
5.12.1. Controller Attached to ToFs
If a controller is attaching to the RIFT domain from ToF, it usually
uses dual-homing connections. The loopback prefix of the controller
should be advertised down by the ToF and spine to the leaves. If the
controller loses the link to ToF, make sure the ToF withdraws the
prefix of the controller.
5.12.2. Controller Attached to Leaf
If the controller is attaching from a leaf to the fabric, no special
provisions are needed.
5.13. Internet Connectivity Within Underlay
If global addressing is running without overlay, an external default
route needs to be advertised through the RIFT fabric to achieve
internet connectivity. For the purpose of forwarding of the entire
RIFT fabric, an internal fabric prefix needs to be advertised in the
South
Prefix South TIE by ToF and spine nodes.
5.13.1. Internet Default on the Leaf
In the case that the internet gateway is a leaf, the leaf node as the
internet gateway needs to advertise a default route in its Prefix
North TIE.
5.13.2. Internet Default on the ToFs
In the case that the internet gateway is a ToF, the ToF and spine
nodes need to advertise a default route in the Prefix South TIE.
5.14. Subnet Mismatch and Address Families
+--------+ +--------+
| | LIE LIE | |
| A | +----> <----+ | B |
| +---------------------+ |
+--------+ +--------+
X/24 Y/24
Figure 14: Subnet Mismatch
LIEs are exchanged over all links running RIFT to perform Link
(Neighbor) Discovery. A node must NOT originate LIEs on an AF if it
does not process received LIEs on that family. LIEs on the same link
are considered part of the same negotiation independent from the AF
they arrive on. An implementation must be ready to accept TIEs on
all addresses it used as the source of LIE frames.
As shown in Figure 14, an adjacency of nodes A and B may form without
further checks, but the forwarding between nodes A and B may fail
because subnet X mismatches with subnet Y.
To prevent this, a RIFT implementation should check for subnet
mismatch in a way that is similar to how IS-IS does. This can lead
to scenarios where an adjacency, despite the exchange of LIEs in both
AFs, may end up having an adjacency in a single AF only. This is
especially a consideration in scenarios relating to Section 5.9.
5.15. Anycast Considerations
+ traffic
|
v
+------+------+
| ToF |
+---+-----+---+
| | | |
+------------+ | | +------------+
| | | |
+---+---+ +-------+ +-------+ +---+---+
| | | | | | | |
|Spine11| |Spine12| |Spine21| |Spine22| LEVEL 1
+-+---+-+ ++----+-+ +-+---+-+ ++----+-+
| | | | | | | |
| +---------+ | | +---------+ |
| +-------+ | | | +-------+ | |
| | | | | | | |
+-+---+-+ +--+--+-+ +-+---+-+ +--+--+-+
| | | | | | | |
|Leaf111| |Leaf112| |Leaf121| |Leaf122| LEVEL 0
+-+-----+ ++------+ +-----+-+ +-----+-+
+ + + ^ +
PrefixA PrefixB PrefixA | PrefixC
|
+ traffic
Figure 15: Anycast
If the traffic comes from ToF to Leaf111 or Leaf121, which has
anycast prefix PrefixA, RIFT can deal with this case well. However,
if the traffic comes from Leaf122, it arrives to Spine21 or Spine22
at LEVEL 1. Additionally, Spine21 or Spine22 doesn't know another
PrefixA attaching Leaf111, so it will always get to Leaf121 and never
Leaf111. If the intention is that the traffic should be offloaded to
Leaf111, then use the policy-guided prefixes defined in RIFT
[RFC9692].
5.16. IoT Applicability
The design of RIFT inherits the anisotropic design of a default route
upwards (northwards) from RPL [RFC6550]. It also inherits the
capability to inject external host routes at the Leaf level using
Wireless ND (WiND) [RFC8505] [RFC8928] between a RIFT-agnostic host
and a RIFT router. Both the RPL and the RIFT protocols are meant for
a large scale, and WiND enables device mobility at the edge the same
way in both cases.
The main difference between RIFT and RPL is that there's a single
root with RPL, whereas RIFT has many ToF nodes. This adds huge
capabilities for leaf-2-leaf ECMP paths but additional complexity
with the need to disaggregate. Also, RIFT uses link-state flooding
northwards and is not designed for low-power operation.
Still, nothing prevents that the IP devices connected at the Leaf are
IoT devices, which typically expose their address using WiND -- this
is an upgrade from 6LoWPAN ND [RFC6775].
A network that serves high speed / high power IoT devices should
typically provide deterministic capabilities for applications such as
high speed control loops or movement detection. The Fat Tree is
highly reliable and, in normal conditions, provides an equivalent
multipath operation; however, the ECMP doesn't provide hard
guarantees for either delivery or latency. As long as the fabric is
non-blocking, the result is the same, but there can be load
unbalances resulting in incast and possibly congestion loss that will
prevent the delivery within bounded latency.
This could be alleviated with Packet Replication, Elimination, and
Ordering Functions (PREOF) [RFC8655] leaf-2-leaf, but PREOF is hard
to provide at the scale of all flows and the replication may increase
the probability of the overload that it attempts to solve.
Note that the load balancing is not RIFT's problem, but it is key to
serve IoT adequately.
5.17. Key Management
As outlined in Section 9 ("Security Considerations") of [RFC9692],
either a private shared key or a public/private key pair is used to
authenticate the adjacency. Both the key distribution and key
synchronization methods are out of scope for this document. Both
nodes in the adjacency must share the same keys, key type, and
algorithm for a given key ID. Mismatched keys will not interoperate
as their security envelopes will be unverifiable.
Key rollover while the adjacency is active may be supported. The
specific mechanism is well documented in [RFC6518]. As outlined in
9.9 ("Host Implementations") of [RFC9692], hosts as well as VMs
acting as RIFT devices are possible. Key Management Protocols
(KMPs), such as Key Value (KV) for key rollover in the fabric, use a
symmetric key that can be changed easily when compromised; in which
case, the symmetric key of a host is more likely to be compromised
than an in-fabric networking node.
5.18. TTL/Hop Limit of 1 vs. 255 on LIEs/TIEs
The use of a packet's Time to Live (TTL) (IPv4) or Hop Limit (IPv6)
to verify whether the packet was originated by an adjacent node on a
connected link has been used in RIFT. RIFT explicitly requires the
use of a TTL/HL value of 1 or 255 when sending/receiving LIEs and
TIEs so that implementers have a choice between the two.
TTL=1 or HL=1 protects against the information disseminating more
than 1 hop in the fabric and should be the default unless configured
otherwise. TTL=255 or HL=255 can lead RIFT TIE packet propagation to
more than one hop (the multicast address is already in local
subnetwork range) in case of implementation problems but does protect
against a remote attack as well, and the receiving remote router will
ignore such TIE packet unless the remote router is exactly 254 hops
away and accepts only TTL=1 or HL=1. [RFC5082] defines a Generalized
TTL Security Mechanism (GTSM). The GTSM is applicable to LIE/TIE
implementations that use a TTL or HL of 255. It provides a defense
from infrastructure attacks based on forged protocol packets from
outside the fabric.
6. Security Considerations
This document presents applicability of RIFT. As such, it does not
introduce any security considerations. However, there are a number
of security concerns in [RFC9692].
7. IANA Considerations
This document has no IANA actions.
8. References
8.1. Normative References
[ISO10589-Second-Edition]
ISO/IEC, "Information technology - Telecommunications and
information exchange between systems - Intermediate System
to Intermediate System intra-domain routeing information
exchange protocol for use in conjunction with the protocol
for providing the connectionless-mode network service (ISO
8473)", ISO/IEC 10589:2002, November 2002,
<https://www.iso.org/standard/30932.html>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
<https://www.rfc-editor.org/info/rfc5082>.
[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>.
[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, DOI 10.17487/RFC5357, October 2008,
<https://www.rfc-editor.org/info/rfc5357>.
[RFC6518] Lebovitz, G. and M. Bhatia, "Keying and Authentication for
Routing Protocols (KARP) Design Guidelines", RFC 6518,
DOI 10.17487/RFC6518, February 2012,
<https://www.rfc-editor.org/info/rfc6518>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, DOI 10.17487/RFC6775, November 2012,
<https://www.rfc-editor.org/info/rfc6775>.
[RFC7130] Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed.,
Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional
Forwarding Detection (BFD) on Link Aggregation Group (LAG)
Interfaces", RFC 7130, DOI 10.17487/RFC7130, February
2014, <https://www.rfc-editor.org/info/rfc7130>.
[RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.
[RFC8950] Litkowski, S., Agrawal, S., Ananthamurthy, K., and K.
Patel, "Advertising IPv4 Network Layer Reachability
Information (NLRI) with an IPv6 Next Hop", RFC 8950,
DOI 10.17487/RFC8950, November 2020,
<https://www.rfc-editor.org/info/rfc8950>.
[RFC9692] Przygienda, T., Ed., Head, J., Ed., Sharma, A., Thubert,
P., Rijsman, B., and D. Afanasiev, "RIFT: Routing in Fat
Trees", RFC 9692, DOI 10.17487/RFC9692, December 2024,
<https://www.rfc-editor.org/info/rfc9692>.
[TR-384] Broadband Forum Technical Report, "TR-384: Cloud Central
Office Reference Architectural Framework", TR-384, Issue
1, January 2018,
<https://www.broadband-forum.org/pdfs/tr-384-1-0-0.pdf>.
8.2. Informative References
[CLOS] Yuan, X., "On Nonblocking Folded-Clos Networks in Computer
Communication Environments", 2011 IEEE International
Parallel & Distributed Processing Symposium,
DOI 10.1109/IPDPS.2011.27, May 2011,
<https://ieeexplore.ieee.org/document/6012836>.
[FATTREE] Leiserson, C. E., "Fat-Trees: Universal Networks for
Hardware-Efficient Supercomputing", IEEE Transactions on
Computers, vol. C-34, no. 10, pp. 892-901,
DOI 10.1109/TC.1985.6312192, October 1985,
<https://ieeexplore.ieee.org/document/6312192>.
[IEEEstd1588]
IEEE, "IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems",
IEEE Std 1588-2019, DOI 10.1109/IEEESTD.2020.9120376, June
2020, <https://ieeexplore.ieee.org/document/9120376>.
[PNNI] The ATM Forum Technical Committee, "Private Network-
Network Interface - Specification Version 1.1 - (PNNI
1.1)", af-pnni-0055.001, April 2002,
<https://www.broadband-forum.org/download/af-pnni-
0055.001.pdf>.
[RFC3626] Clausen, T., Ed. and P. Jacquet, Ed., "Optimized Link
State Routing Protocol (OLSR)", RFC 3626,
DOI 10.17487/RFC3626, October 2003,
<https://www.rfc-editor.org/info/rfc3626>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8505] Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
Perkins, "Registration Extensions for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Neighbor
Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
<https://www.rfc-editor.org/info/rfc8505>.
[RFC8928] Thubert, P., Ed., Sarikaya, B., Sethi, M., and R. Struik,
"Address-Protected Neighbor Discovery for Low-Power and
Lossy Networks", RFC 8928, DOI 10.17487/RFC8928, November
2020, <https://www.rfc-editor.org/info/rfc8928>.
Acknowledgments
The authors would like to thank Jaroslaw Kowalczyk, Alvaro Retana,
Jim Guichard, and Jeffrey Zhang for providing invaluable concepts and
content for this document.
Contributors
The following people contributed substantially to the content of this
document and should be considered coauthors:
Jordan Head
Juniper Networks
Email: jhead@juniper.net
Tom Verhaeg
Juniper Networks
Email: tverhaeg@juniper.net
Authors' Addresses
Yuehua Wei (editor)
ZTE Corporation
No.50, Software Avenue
Nanjing
210012
China
Email: wei.yuehua@zte.com.cn
Zheng (Sandy) Zhang
ZTE Corporation
No.50, Software Avenue
Nanjing
210012
China
Email: zhang.zheng@zte.com.cn
Dmitry Afanasiev
Yandex
Email: fl0w@yandex-team.ru
Pascal Thubert
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
06254 Mougins - Sophia Antipolis
France
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
Tony Przygienda
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
United States of America
Email: prz@juniper.net