Networking Working Group T. Przygienda
Internet-Draft J. Drake
Intended status: Standards Track A. Atlas
Expires: July 15, 2017 Juniper Networks
January 11, 2017
RIFT: Routing in Fat Trees
draft-przygienda-rift-00
Abstract
This document outlines a specialized, dynamic routing protocol for
Clos and fat-tree network topologies. The protocol (1) deals with
automatic construction of fat-tree topologies based on detection of
links, (2) minimizes the amount of routing state held at each level,
(3) automatically prunes the topology distribution exchanges to a
sufficient subset of links, (4) supports automatic disaggregation of
prefixes on link and node failures to prevent blackholing and
suboptimal routing, (5) allows traffic steering and re-routing
policies and ultimately (6) provides mechanisms to synchronize a
limited key-value data-store that can be used after protocol
convergence to e.g. bootstrap higher levels of functionality on
nodes.
Status of This Memo
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Reference Frame . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Topology . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Requirement Considerations . . . . . . . . . . . . . . . . . 8
4. RIFT: Routing in Fat Trees . . . . . . . . . . . . . . . . . 9
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Specification . . . . . . . . . . . . . . . . . . . . . . 10
4.2.1. Transport . . . . . . . . . . . . . . . . . . . . . . 10
4.2.2. Link (Neighbor) Discovery (LIE Exchange) . . . . . . 10
4.2.3. Topology Exchange (TIE Exchange) . . . . . . . . . . 11
4.2.3.1. Topology Information Elements . . . . . . . . . . 11
4.2.3.2. South- and Northbound Representation . . . . . . 11
4.2.3.3. Flooding . . . . . . . . . . . . . . . . . . . . 13
4.2.3.4. TIE Flooding Scopes . . . . . . . . . . . . . . . 13
4.2.3.5. Initial and Periodic Database Synchronization . . 14
4.2.3.6. Purging . . . . . . . . . . . . . . . . . . . . . 14
4.2.3.7. Optional Automatic Flooding Reduction and
Partitioning . . . . . . . . . . . . . . . . . . 15
4.2.4. Automatic Disaggregation on Link & Node Failures . . 16
4.2.5. Policy-Guided Prefixes . . . . . . . . . . . . . . . 19
4.2.5.1. Ingress Filtering . . . . . . . . . . . . . . . . 20
4.2.5.2. Applying Policy . . . . . . . . . . . . . . . . . 21
4.2.5.3. Store Policy-Guided Prefix for Route Computation
and Regeneration . . . . . . . . . . . . . . . . 21
4.2.5.4. Regeneration . . . . . . . . . . . . . . . . . . 22
4.2.5.5. Overlap with Disaggregated Prefixes . . . . . . . 22
4.2.6. Reachability Computation . . . . . . . . . . . . . . 22
4.2.6.1. Specification . . . . . . . . . . . . . . . . . . 23
4.2.6.2. Further Mechanisms . . . . . . . . . . . . . . . 25
4.2.7. Key/Value Store . . . . . . . . . . . . . . . . . . . 26
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1. Normal Operation . . . . . . . . . . . . . . . . . . . . 26
5.2. Leaf Link Failure . . . . . . . . . . . . . . . . . . . . 27
5.3. Partitioned Fabric . . . . . . . . . . . . . . . . . . . 28
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6. Implementation and Operation: Further Details . . . . . . . . 30
6.1. Leaf to Leaf connection . . . . . . . . . . . . . . . . . 30
6.2. Other End-to-End Services . . . . . . . . . . . . . . . . 30
6.3. Address Family and Topology . . . . . . . . . . . . . . . 31
7. Information Elements Schema . . . . . . . . . . . . . . . . . 31
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
9. Security Considerations . . . . . . . . . . . . . . . . . . . 36
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 36
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
11.1. Normative References . . . . . . . . . . . . . . . . . . 36
11.2. Informative References . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction
Clos [CLOS] and Fat-Tree [FATTREE] have gained prominence in today's
networking, primarily as a result of a the paradigm shift towards a
centralized data-center based architecture that is poised to deliver
a majority of computation and storage services in the future. The
existing set of dynamic routing protocols was geared originally
towards a network with an irregular topology and low degree of
connectivity and consequently several attempts to adapt those have
been made. Most succesfully BGP [RFC4271] [RFC7938] has been
extended to this purpose, not as much due to its inherent suitability
to solve the problem but rather because the perceived capability to
modify it "quicker" and the immanent difficulties with link-state
[DIJKSTRA] based protocols to fulfill certain of the resulting
requirements.
In looking at the problem through the very lens of its requirements
an optimal approach does not seem to be a simple modification of
either a link-state (distributed computation) or distance-vector
(diffused computation) approach but rather a mixture of both,
colloquially best described as 'link-state towards the spine' and
'distance vector towards the leafs'. The balance of this document
details the resulting protocol.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Reference Frame
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2.1. Terminology
This section presents the terminology used in this document. It is
assumed that the reader is thoroughly familiar with the terms and
concepts used in OSPF [RFC2328] and IS-IS [RFC1142] as well as the
according graph theoretical concepts of shortest path first (SPF)
[DIJKSTRA] computation and directed acyclic graphs (DAG).
Level: Clos and Fat Tree networks are trees and 'level' denotes the
set of nodes at the same height in such a network, where the
bottom level is level 0. A node has links to nodes one level down
and/or one level up. Under some circumstances, a node may have
links to nodes at the same level. As footnote: Clos terminology
uses often the concept of "stage" but due to the folded nature of
the Fat Tree we do not use it to prevent misunderstandings.
Spine/Aggregation/Edge Levels: Traditional names for Level 2, 1 and
0 respectively. Level 0 is often called leaf as well.
Point of Delivery (PoD): A self-contained vertical slice of a Clos
or Fat Tree network containing normally only level 0 and level 1
nodes. It communicates with nodes in other PoDs via the spine.
Spine: The set of nodes that provide inter-PoD communication. These
nodes are also organized into levels (typically one, three, or
five levels). Spine nodes do not belong to any PoD and are
assigned the PoD value 0 to indicate this.
Leaf: A node at level 0.
Connected Spine: In case a spine level represents a connected graph
(discounting links terminating at different levels), we call it a
"connected spine", in case a spine level consists of multiple
partitions, we call it a "disconnected" or "partitioned spine".
In other terms, a spine without east-west links is disconnected
and is the typical configuration for Clos and Fat Tree networks.
South/Southbound and North/Northbound (Direction): When describing
protocol elements and procedures, we will be using in different
situations the directionality of the compass. I.e., 'south' or
'southbound' mean moving towards the bottom of the Clos or Fat
Tree network and 'north' and 'northbound' mean moving towards the
top of the Clos or Fat Tree network.
Northbound Link: A link to a node one level up or in other words,
one level further north.
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Southbound Link: A link to a node one level down or in other words,
one level further south.
East-West Link: A link between two nodes at the same level. East-
west links are not common in "fat-trees".
Leaf shortcuts (L2L): East-west links at leaf level will need to be
differentiated from East-west links at other levels.
Southbound representation: Information sent towards a lower level
representing only limited amount of information.
TIE: This is an acronym for a "Topology Information Element". TIEs
are exchanged between RIFT nodes to describe parts of a network
such as links and address prefixes. It can be thought of as
largely equivalent to ISIS LSPs or OSPF LSA. We will talk about
N-TIEs when talking about TIEs in the northbound representation
and S-TIEs for the southbound equivalent.
Node TIE: This is an acronym for a "Node Topology Information
Element", largely equivalent to OSPF Node LSA, i.e. it contains
all neighbors the node discovered and information about node
itself.
Prefix TIE: This is an acronym for a "Prefix Topology Information
Element" and it contains all prefixes directly attached to this
node in case of a N-TIE and in case of S-TIE the necesssary
default and de-aggregated prefixes the node passes southbound.
Policy-Guided Information: Information that is passed in either
southbound direction or north-bound direction by the means of
diffusion and can be filtered via policies. Policy-Guided
Prefixes and KV Ties are examples of Policy-Guided Information.
Key Value TIE: A S-TIE that is carrying a set of key value pairs
[DYNAMO]. It can be used to distribute information in the
southbound direction within the protocol.
TIDE: Topology Information Description Element, equivalent to CSNP
in ISIS.
TIRE: Topology Information Request Element, equivalent to PSNP in
ISIS. It can both confirm received and request missing TIEs.
PGP: Policy-Guided Prefixes allow to support traffic engineering
that cannot be achieved by the means of SPF computation or normal
node and prefix S-TIE origination. S-PGPs are propagated in south
direction only and N-PGPs follow northern direction strictly.
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Deaggregation/Disaggregation Process in which a node decides to
advertise certain prefixes it received in N-TIEs to prevent
blackholing and suboptimal routing upon link failures.
LIE: This is an acronym for a "Link Information Element", largely
equivalent to HELLOs in IGPs.
FL: Flooding Leader for a specific system has a dedicated role to
flood TIEs of that system.
2.2. Topology
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. +--------+ +--------+
. | | | | ^ N
. |Spine 21| |Spine 22| |
.Level 2 ++-+--+-++ ++-+--+-++ <-*-> E/W
. | | | | | | | | |
. P111/2| |P121 | | | | S v
. ^ ^ ^ ^ | | | |
. | | | | | | | |
. +--------------+ | +-----------+ | | | +---------------+
. | | | | | | | |
. South +-----------------------------+ | | ^
. | | | | | | | All TIEs
. 0/0 0/0 0/0 +-----------------------------+ |
. v v v | | | | |
. | | +-+ +<-0/0----------+ | |
. | | | | | | | |
.+-+----++ optional +-+----++ ++----+-+ ++-----++
.| | E/W link | | | | | |
.|Node111+----------+Node112| |Node121| |Node122|
.+-+---+-+ ++----+-+ +-+---+-+ ++---+--+
. | | | South | | | |
. | +---0/0--->-----+ 0/0 | +----------------+ |
. 0/0 | | | | | | |
. | +---<-0/0-----+ | v | +--------------+ | |
. v | | | | | | |
.+-+---+-+ +--+--+-+ +-+---+-+ +---+-+-+
.| | (L2L) | | | | Level 0 | |
.|Leaf111~~~~~~~~~~~~Leaf112| |Leaf121| |Leaf122|
.+-+-----+ +-+---+-+ +--+--+-+ +-+-----+
. + + \ / + +
. Prefix111 Prefix112 \ / Prefix121 Prefix122
. multihomed
. Prefix
.+---------- Pod 1 ---------+ +---------- Pod 2 ---------+
Figure 1: A two level spine-and-leaf topology
We will use this topology (called commonly a fat tree/network in
modern DC considerations [VAHDAT08] as homonym to the original
definition of the term [FATTREE]) in all further considerations. It
depicts a generic "fat-tree" and the concepts explained in three
levels here carry by induction for further levels and higher degrees
of connectivity.
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3. Requirement Considerations
[RFC7938] gives the original set of requirements augmented here based
upon recent experience in the operation of fat-tree networks.
REQ1: The solution should allow for minimum size routing
information base and forwarding tables at leaf level for
speed, cost and simplicity reasons. Holding excessive
amount of information away from leaf nodes simplifies
operation of the underlay when addresses are moving in the
topology.
REQ2: High degree of ECMP (and ideally non equal cost) must be
supported.
REQ3: Traffic engineering should be allowed by modification of
prefixes and/or their next-hops.
REQ4: The control protocol must discover the physical links
automatically and be able to detect cabling that violates
fat-tree topology constraints. It must react accordingly to
such miscabling attempts, at a minimum preventing
adjacencies between nodes from being formed and traffic from
being forwarded on those miscabled links. E.g. connecting
a leaf to a spine at level 2 should be detected and ideally
prevented.
REQ5: The solution should allow for access to link states of the
whole topology to allow efficient support for modern control
architectures like SPRING [RFC7855] or PCE [RFC4655].
REQ6: The solution should easily accomodate opaque data to be
carried throughout the topology to subsets of nodes. This
can be used for many purposes, one of them being a key-value
store that allows bootstrapping of nodes based right at the
time of topology discovery.
REQ7: Nodes should be taken out and introduced into production
with minimum wait-times and minimum of "shaking" of the
network, i.e. radius of propagation of necessary
information should be as small as viable.
REQ8: The protocol should allow for maximum aggregation of carried
routing information while at the same time automatically
deaggregating the prefixes to prevent blackholing in case of
failures. The deaggregation should support maximum possible
ECMP/N-ECMP remaining after failure.
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REQ9: A node without any configuration beside default values
should come up as leaf in any PoD it is introduced into.
Optionally, it must be possible to configure nodes to
restrict their participation to the PoD(s) targeted at any
level.
REQ10: Reducing the scope of communication needed throughout the
network on link and state failure, as well as reducing
advertisements of repeating, idiomatic or policy-guided
information in stable state is highly desirable since it
leads to better stability and faster convergence behavior.
REQ11: Once a packet traverses a link in a "southbound" direction,
it must not take any further "northbound" steps along its
path to delivery to its destination. Taking a path through
the spine in cases where a shorter path is available is
highly undesirable.
Following list represents possible requirements and requirements
under discussion:
PEND1: Supporting anything but point-to-point links is a non-
requirement. Questions remain: for connecting to the
leaves, is there a case where multipoint is desirable? One
could still model it as point-to-point links; it seems there
is no need for anything more than a NBMA-type construct.
PEND2: We carrry parallel links with unique identifer carried in
node TIEs. Link bundles (i.e. parallel links between same
set of nodes) must be distinguishable for SPF and traffic
engineering purposes. But further, do we rely on coalesced
links from lower layers and BFD/m-BFD detection or hello all
links ?
PEND3: BFD will obviously play a big role in fast detection of
failures and the interactions will need to be worked out.
PEND4: What is the maximum scale of number leaf prefixes we need to
carry. Is 0.5E6 enough ?
4. RIFT: Routing in Fat Trees
Derived from the above requirements we present a detailed outline of
a protocol optimized for Routing in Fat Trees (RIFT) that in most
abstract terms has many properties of a modified link-state protocol
[RFC2328][RFC1142] when "pointing north" and path-vector [RFC4271]
protocol when "pointing south". Albeit an unusual combination, it
does quite naturally exhibit the desirable properties we seek.
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4.1. Overview
The novel property of RIFT is that it floods northbound "flat" link-
state information so that each level understands the full topology of
levels south of it. In contrast, in the southbound direction the
protocol operates like a path vector protocol or rather a distance
vector with implicit split horizon since the topology constraints
make a diffused computation front propagating in all directions
unnecessary.
4.2. Specification
4.2.1. Transport
All protocol elements are carried over UDP. LIE exchange happens
over well-known multicast address with a TTL of 1. TIE exchange
mechanism uses address and port indicated by each node in the LIE
exchange with TTL of 1 as well.
All packet formats are defined in Thrift or protobuf models.
4.2.2. Link (Neighbor) Discovery (LIE Exchange)
Each node is provisioned with the level at which it is operating and
its PoD. A default level and PoD of zero are assumed, meaning that
leafs do not need to be configured with a level (or even PoD). Nodes
in the spine are configured with a PoD of zero. This information is
propagated in the LIEs exchanged. Adjacencies are formed if and only
if
a. the node is in the same PoD or either the node or the neighbor
advertises any PoD membership (PoD# = 0) AND
b. the neighboring node is at most one level away AND
c. the neighboring node is running the same MAJOR schema version AND
d. the neighbor is not member of some PoD while the node has a
northbound adjacency already joining another PoD.
A node configure with any PoD membership MUST, after building first
northbound adjacency making it participant in a PoD, advertise that
PoD as part of its LIEs.
LIEs arriving with a TTL larger than 1 MUST be ignored.
LIE exchange uses three-way handshake mechanism [RFC5303]. LIE
packets contain nonces and may contain an SHA-1 [RFC6234] over nonces
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and some of the LIE data which prevents corruption and replay
attacks. TIE flooding reuses those nonces to prevent mismatches and
can use those for security purposes in case it is using QUIC [QUIC].
4.2.3. Topology Exchange (TIE Exchange)
4.2.3.1. Topology Information Elements
Topology and reachability information in RIFT is conveyed by the
means of TIEs which have good amount of commonalities with LSAs in
OSPF. They contain sequence numbers, lifetimes and a type. Each
type has a large identifying number space and information is spread
across possibly many TIEs of a certain type by the means of a hash
function that a node or deployment can individually determine. One
extreme side of the scale is a prefix per TIE which leads to BGP-like
behavior vs. dense packing into few TIEs leading to more traditional
IGP trade-off with fewer TIEs. An implementation may even rehash at
the cost of significant amount of readvertisements of TIEs.
More information about the TIE structure can be found in the schema
in Section 7.
4.2.3.2. South- and Northbound Representation
As a central concept to RIFT, each node represents itself differently
depending on the direction in which it is advertising information.
More precisely, a spine node represents two different databases to
its neighbors depending whether it advertises TIEs to the south or to
the north/sideways. We call those differing TIE databases either
south- or northbound (S-TIEs and N-TIEs) depending on the direction
of distribution.
The N-TIEs hold all of the node's adjacencies, local prefixes and
northbound policy-guided prefixes while the S-TIEs hold only all of
the node's neighbors and the default prefix with necessary
disaggregated prefixes and southbound policy-guided prefixes. We
will explain this in detail further in Section 4.2.4 and
Section 4.2.5.
As an example to illustrate databases holding both representations,
consider the topology in Figure 1 with the optional link between
Node111 and Node 112 (so that the flooding on an east-west link can
be shown). This example assumes unnumbered interfaces. First, here
are the TIEs generated by some nodes. For simplicity, the
KeyValueElements and the PolicyGuidedPrefixesElements which may be
included in an S-TIE or an N-TIE are not shown.
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Spine21 S-TIE:
NodeElement(layer=2, neighbors((Node111, layer 1, cost 1),
(Node112, layer 1, cost 1), (Node121, layer 1, cost 1),
(Node122, layer 1, cost 1)))
SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))
Node111 S-TIE:
NodeElement(layer=1, neighbors((Spine21,layer 2,cost 1),
(Spine22, layer 2, cost 1), (Node112, layer 1, cost 1),
(Leaf111, layer 0, cost 1), (Leaf112, layer 0, cost 1)))
SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))
Node111 N-TIE:
NodeLinkElement(layer=1,
neighbors((Spine21, layer 2, cost 1, links(...)),
(Spine22, layer 2, cost 1, links(...)),
(Node112, layer 1, cost 1, links(...)),
(Leaf111, layer 0, cost 1, links(...)),
(Leaf112, layer 0, cost 1, links(...))))
NorthPrefixesElement(prefixes(Node111.loopback)
Node121 S-TIE:
NodeElement(layer=1, neighbors((Spine21,layer 2,cost 1),
(Spine22, layer 2, cost 1), (Leaf121, layer 0, cost 1),
(Leaf122, layer 0, cost 1)))
SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))
Node121 N-TIE: NodeLinkElement(layer=1,
neighbors((Spine21, layer 2, cost 1, links(...)),
(Spine22, layer 2, cost 1, links(...)),
(Leaf121, layer 0, cost 1, links(...)),
(Leaf122, layer 0, cost 1, links(...))))
NorthPrefixesElement(prefixes(Node121.loopback)
Leaf112 N-TIE:
NodeLinkElement(layer=0,
neighbors((Node111, layer 1, cost 1, links(...)),
(Node112, layer 1, cost 1, links(...))))
NorthPrefixesElement(prefixes(Leaf112.loopback, Prefix112,
Prefix_MH))
Figure 2: example TIES generated in a 2 level spine-and-leaf topology
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4.2.3.3. Flooding
The mechanism used to distribute TIEs is the well-known (albeit
modified in several respects to address fat tree requirements)
flooding mechanism used by today's link-state protocols. Albeit
initially more demanding to implement it avoids many problems with
diffused computation update style used by path vector. TIEs
themselves are transported over UDP with the ports indicates in the
LIE exchanges.
Once QUIC [QUIC] achieves the desired stability in deployments it may
prove a valuable candidate for TIE transport.
4.2.3.4. TIE Flooding Scopes
Every N-TIE is flooded northbound, providing a node at a given level
with the complete topology of the Clos or Fat Tree network underneath
it, including all specific prefixes. This means that a packet
received from a node at the same or lower level whose destination is
covered by one of those specific prefixes may be routed directly
towards the node advertising that prefix rather than sending the
packet to a node at a higher level.
It should be noted that east-west links are included in N-TIE
flooding; they need to be flooded in case the level above the current
level is disconnected from one or more nodes in the current level and
southbound SPF desires to use those links as backup in case of some
switches in the spine being partitioned in respect to some PoDs.
A node's S-TIEs, consisting of a node's adjacencies and a default IP
prefix, are flooded southbound in order to allow the nodes one level
down to see connectivity of the higher level as well as reachability
to the rest of the fabric. In order to allow a disconnected node in
a given level to receive the S-TIEs of other nodes at its level,
every *Node* S-TIE is "reflected" northbound to level from which it
was received. A node does not send an S-TIE northbound if it is from
the same or lower level. No S-TIEs are propagated southbound.
Node S-TIE "reflection" allows to support disaggregation on failures
describes in Section 4.2.4 and flooding reduction in Section 4.2.3.7.
Observe that a node does not reflood S-TIE received from the lower
level towards other southbound nodes which has implications on the
way TIDEs are generated in the southbound direction.
As an example to illustrate these rules, consider using the topology
in Figure 1, with the optional link between Node111 and Node 112, and
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the associated TIEs given in Figure 2. The flooding from particular
nodes of the TIEs is given in Table 1.
Router Neighbor TIEs
floods to
--------- -------- --------------------------------------------------
Leaf111 Node112 Leaf111 N-TIE, Node111 S-TIE
Leaf111 Node111 Leaf111 N-TIE, Node112 S-TIE
Node111 Leaf111 Node111 S-TIE
Node111 Leaf112 Node111 S-TIE
Node111 Node112 Node111 S-TIE, Node111 N-TIE, Leaf111 N-TIE,
Leaf112 N-TIE, Spine21 S-TIE, Spine22 S-TIE
Node111 Spine21 Node111 N-TIE, Node112 N-TIE, Leaf111 N-TIE,
Leaf112 N-TIE, Spine22 S-TIE
Node111 Spine22 Node111 N-TIE, Node112 N-TIE, Leaf111 N-TIE,
Leaf112 N-TIE, Spine21 S-TIE
Node121 Leaf121 Node121 S-TIE
Node121 Leaf122 Node121 S-TIE
Node121 Spine21 Node121 N-TIE, Leaf121 N-TIE, Leaf122 N-TIE,
Spine22 S-TIE
Node121 Spine22 Node121 N-TIE, Leaf121 N-TIE, Leaf122 N-TIE,
Spine22 S-TIE
Spine21 Node111 Spine21 S-TIE
Spine21 Node112 Spine21 S-TIE
Spine21 Node121 Spine21 S-TIE
Spine21 Node122 Spine21 S-TIE
Spine22 Node111 Spine22 S-TIE
Spine22 Node112 Spine22 S-TIE
Spine22 Node121 Spine22 S-TIE
Spine22 Node122 Spine22 S-TIE
Table 1: Flooding some TIEs from example topology
4.2.3.5. Initial and Periodic Database Synchronization
The initial exchange of RIFT is modelled after ISIS with TIDE being
equivalent to CSNP and TIRE playing the role of PSNP. The content of
TIDEs in north and south direction will contain obviously just the
according database variant and reflect the flooding scopes defined.
4.2.3.6. Purging
RIFT does not purge information that has been distributed by the
protocol. Purging mechanisms in other routing protocols have proven
through many years of experience to be complex and fragile. Abundant
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amounts of memory are available today even on low-end platforms. The
information will age out and all computations will deliver correct
results if a node leaves the network due to the new information
distributed by its adjacent nodes.
Once a RIFT node issues a TIE with an ID, it MUST preserve the ID in
its database until it restarts, even if the TIE looses all content.
The re-advertisement of empty TIE fullfills the purpose of purging
any information advertised in previous versions. The originator is
free to not re-originate the according empty TIE again or originate
an empty TIE with relatively short lifetime to prevent large number
of long-lived empty stubs polluting the network. Each node will
timeout and clean up the according empty TIEs independently.
4.2.3.7. Optional Automatic Flooding Reduction and Partitioning
Two nodes can, but strictly only under conditions defined below, run
a hashing function based on TIE originator value and partition
flooding between them.
Steps for flooding reduction and partitioning:
1. select all nodes in the same level for which node S-TIEs have
been received and which have precisely the same set of north and
south neighbor adjacencies and support flooding reduction and
then
2. run on the chosen set a hash algorithm using nodes flood
priorities and IDs to select flooding leader and backup per TIE
originator ID, i.e. each node floods immediately through to all
its necessary neighbors TIEs that it received with an originator
ID that makes it the flooding leader or backup for this
originator. The preference (higher is better) is computed as
XOR(TIE-ORIGINATOR-ID<<1,~OWN-SYSTEM-ID)).
Additional rules for flooding reduction and partitioning:
a. A node always floods its own TIEs
b. A node generates TIDEs as usual but when receiving TIREs with
requests for TIEs for a node for which it is not a flooding
leader or backup it ignores such TIDEs on first request only.
Normally, the flooding leader should satisfy the requestor and
with that no further TIREs for such TIEs will be generated.
Otherwise, the next set of TIDEs and TIREs will lead to flooding
independent of the flooding leader status.
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c. A node receiving a TIE originated by a node for which it is not a
flooding leader floods such TIEs only when receiving an out-of-
date TIDE for them, except for the first one.
The mechanism can be implemented optionally in each node. The
capability is carried in the node N-TIE.
Obviously flooding reduction does NOT apply to self originated TIEs.
Observe further that all policy-guided information consists of self-
originated TIEs.
4.2.4. Automatic Disaggregation on Link & Node Failures
Under normal circumstances, a node S-TIEs contain just its
adjacencies, a default route and policy-guided prefixes. However, if
a node detects that its default IP prefix covers one or more prefixes
that are reachable through it but not through one or more other nodes
at the same level, then it must explicitly advertise those prefixes
in an S-TIE. Otherwise, some percentage of the northbound traffic
for those prefixes would be sent to nodes without according
reachability, causing it to be blackholed. Even when not
blackholing, the resulting forwarding could 'backhaul' packets
through the higher level spines, clearly an undesirable condition
affecting the blocking probabilities of the fabric.
We refer to the process of advertising additional prefixes as 'de-
aggregation'.
A node determines the set of prefixes needing de-aggregation using
the following steps:
a. A DAG computation in the southern direction is performed first,
i.e. the N-TIEs are used to find all of prefixes it can reach and
the set of next-hops in the lower level for each. Such a
computation can be easily performed on a fat tree by e.g. setting
all link costs in the southern direction to 1 and all northern
directions to infinity. We term set of those prefixes |R, and
for each prefix, r, in |R, we define its set of next-hops to
be |H(r). Observe that policy-guided prefixes are NOT affected
since their scope is controlled by configuration. Overload bits
as introduced in Section 4.2.6.2.1 have to be respected during
the computation.
b. The node uses reflected S-TIEs to find all nodes at the same
level in the same PoD and the set of southbound adjacencies for
each. The set of nodes at the same level is termed |N and for
each node, n, in |N, we define its set of southbound adjacencies
to be |A(n).
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c. For a given r, if the intersection of |H(r) and |A(n), for any n,
is null then that prefix r must be explicitly advertised by the
node in an S-TIE.
d. Identical set of de-aggregated prefixes is flooded on each of the
node's southbound adjacencies. In accordance with the normal
flooding rules for an S-TIE, a node at the lower level that
receives this S-TIE will not propagate it south-bound. Neither
is it necessary for the receiving node to reflect the
disaggregated prefixes back over its adjacencies to nodes at the
level from which it was received.
To summarize the above in simplest terms: if a node detects that its
default route encompasses prefixes for which one of the other nodes
in its level has no possible next-hops in the level below, it has to
disaggregate it to prevent blackholing or suboptimal routing. Hence
a node X needs to determine if it can reach a different set of south
neighbors than other nodes at the same level, which are connected via
at least one south or east-west neighbor. If it can, then prefix
disaggregation may be required. If it can't, then no prefix
disaggregation is needed. An example of disaggregation is provided
in Section 5.3.
A possible algorithm is described last:
1. Create partial_neighbors = (empty), a set of neighbors with
partial connectivity to the node X's layer from X's perspective.
Each entry is a list of south neighbor of X and a list of nodes
of X.layer that can't reach that neighbor.
2. A node X determines its set of southbound neighbors
X.south_neighbors.
3. For each S-TIE originated from a node Y that X has which is at
X.layer, if Y.south_neighbors is not the same as
X.south_neighbors, for each neighbor N in X.south_neighbors but
not in Y.south_neighbors, add (N, (Y))to partial_neighbors if N
isn't there or add Y to the list for N.
4. If partial_neighbors is empty, then node X does not to
disaggregate any prefixes. If node X is advertising
disaggregated prefixes in its S-TIE, X SHOULD remove them and
readvertise its according S-TIEs.
A node X computes its SPF based upon the received N-TIEs. This
results in a set of routes, each categorized by (prefix,
path_distance, next-hop-set). Alternately, for clarity in the
following procedure, these can be organized by next-hop-set as (
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(next-hops), {(prefix, path_distance)}). If partial_neighbors isn't
empty, then the following procedure describes how to identify
prefixes to disaggregate.
disaggregated_prefixes = {empty }
nodes_same_layer = { empty }
for each S-TIE
if S-TIE.layer == X.layer
add S-TIE.originator to nodes_same_layer
end if
end for
for each next-hop-set NHS
isolated_nodes = nodes_same_layer
for each NH in NHS
if NH in partial_neighbors
isolated_nodes = intersection(isolated_nodes,
partial_neighbors[NH].nodes)
end if
end for
if isolated_nodes is not empty
for each prefix using NHS
add (prefix, distance) to disaggregated_prefixes
end for
end if
end for
copy disaggregated_prefixes to X's S-TIE
if X's S-TIE is different
schedule S-TIE for flooding
end if
Figure 3: Computation to Disaggregate Prefixes
Each disaggregated prefix is sent with the accurate path_distance.
This allows a node to send the same S-TIE to each south neighbor.
The south neighbor which is connected to that prefix will thus have a
shorter path.
Finally, to summarize the less obvious points:
a. all the lower level nodes are flooded the disaggregated prefixes
since we don't want to build an S-TIE per node to not complicate
things unnecessarily. The PoD containing the prefix will prefer
southbound anyway.
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b. disaggregated prefixes do NOT have to propagate to lower levels.
With that the disturbance in terms of new flooding is contained
to a single level experiencing failures only.
c. disaggregated S-TIEs are not "reflected" by the lower layer, i.e.
nodes within same level do NOT need to be aware which node
computed the need for disaggregation.
d. The fabric is still supporting maximum load balancing properties
while not trying to send traffic northbound unless necessary.
4.2.5. Policy-Guided Prefixes
In a fat tree, it can be sometimes desirable to guide traffic to
particular destinations or keep specific flows to certain paths. In
RIFT, this is done by using policy-guided prefixes with their
associated communities. Each community is an abstract value whose
meaning is determined by configuration. It is assumed that the
fabric is under a single administrative control so that the meaning
and intent of the communities is understood by all the nodes in the
fabric. Any node can originate a policy-guided prefix.
Since RIFT uses distance vector concepts in a southbound direction,
it is straightforward to add a policy-guided prefix to an S-TIE. For
easier troubleshooting, the approach taken in RIFT is that a node's
southbound policy-guided prefixes are sent in its S-TIE and the
receiver does inbound filtering based on the associated communities
(an egress policy is imaginable but would lead to different S-TIEs
per neighbor possibly which is not considered in RIFT protocol
procedures). A southbound policy-guided prefix can only use links in
the south direction. If an PGP S-TIE is received on an east-west or
northbound link, it must be discarded by ingress filtering.
Conceptually, a southbound policy-guided prefix guides traffic from
the leaves up to at most the northmost layer. It is also necessary
to to have northbound policy-guided prefixes to guide traffic from
the northmost layer down to the appropriate leaves. Therefore, RIFT
includes northbound policy-guided prefixes in its N PGP-TIE and the
receiver does inbound filtering based on the associated communities.
A northbound policy-guided prefix can only use links in the northern
direction. If an N PGP TIE is received on an east-west or southbound
link, it must be discarded by ingress filtering.
By separating southbound and northbound policy-guided prefixes and
requiring that the cost associated with a PGP is strictly
monotonically increasing at each hop, the path cannot loop. Because
the costs are strictly increasing, it is not possible to have a loop
between a northbound PGP and a southbound PGP. If east-west links
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were to be allowed, then looping could occur and issues such as
counting to infinity would become an issue to be solved. If complete
generality of path - such as including east-west links and using both
north and south links in arbitrary sequence - then a Path Vector
protocol or a similar solution must be considered.
If a node has received the same prefix, after ingress filtering, as a
PGP in an S-TIE and in an N-TIE, then the node determines which
policy-guided prefix to use based upon the advertised cost.
A policy-guided prefix is always preferred to a regular prefix, even
if the policy-guided prefix has a larger cost.
The set of policy-guided prefixes received in a TIE is subject to
ingress filtering and then regenerated to be sent out in the
receiver's appropriate TIE. Both the ingress filtering and the
regeneration use the communities associated with the policy-guided
prefixes to determine the correct behavior. The cost on re-
advertisement MUST increase in a strictly monotonic fashion.
4.2.5.1. Ingress Filtering
When a node X receives a PGP S-TIE or N-TIE that is originated from a
node Y which does not have an adjacency with X, such a TIE MUST be
discarded. Similarly, if node Y is at the same layer as node X, then
X MUST discard PGP S- and N-TIEs.
Next, policy can be applied to determine which policy-guided prefixes
to accept. Since ingress filtering is chosen rather than egress
filtering and per-neighbor PGPs, policy that applies to links is done
at the receiver. Because the RIFT adjacency is between nodes and
there may be parallel links between the two nodes, the policy-guided
prefix is considered to start with the next-hop set that has all
links to the originating node Y.
A policy-guided prefix has or is assigned the following attributes:
cost: This is initialized to the cost received
community_list: This is initialized to the list of the communities
received.
next_hop_set: This is initialized to the set of links to the
originating node Y.
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4.2.5.2. Applying Policy
The specific action to apply based upon a community is deployment
specific. Here are some examples of things that can be done with
communities. The length of a community is a 64 bits number and it
can be written as a single field M or as a multi-field (S = M[0-31],
T = M[32-63]) in these examples. For simplicity, the policy-guided
prefix is referred to as P, the processing node as X and the
originator as Y.
Prune Next-Hops: Community Required: For each next-hop in
P.next_hop_set, if the next-hop does not have the community, prune
that next-hop from P.next_hop_set.
Prune Next-Hops: Avoid Community: For each next-hop in
P.next_hop_set, if the next-hop has the community, prune that
next-hop from P.next_hop_set.
Drop if Community: If node X has community M, discard P.
Drop if not Community: If node X does not have the community M,
discard P.
Prune to ifIndex T: For each next-hop in P.next_hop_set, if the
next-hop's ifIndex is not the value T specified in the community
(S,T), then prune that next-hop from P.next_hop_set.
Add Cost T: For each appearance of community S in P.community_list,
if the node X has community S, then add T to P.cost.
Accumulate Min-BW T: Let bw be the sum of the bandwidth for
P.next_hop_set. If that sum is less than T, then replace (S,T)
with (S, bw).
Add Community T if Node matches S: If the node X has community S,
then add community T to P.community_list.
4.2.5.3. Store Policy-Guided Prefix for Route Computation and
Regeneration
Once a policy-guided prefix has completed ingress filtering and
policy, it is almost ready to store and use. It is still necessary
to adjust the cost of the prefix to account for the link from the
computing node X to the originating neighbor node Y.
There are three different policies that can be used:
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Minimum Equal-Cost: Find the lowest cost C next-hops in
P.next_hop_set and prune to those. Add C to P.cost.
Minimum Unequal-Cost: Find the lowest cost C next-hop in
P.next_hop_set. Add C to P.cost.
Maximum Unequal-Cost: Find the highest cost C next-hop in
P.next_hop_set. Add C to P.cost.
The default policy is Minimum Unequal-Cost but well-known communities
can be defined to get the other behaviors.
Regardless of the policy used, a node MUST store a PGP cost that is
at least 1 greater than the PGP cost received. This enforces the
strictly monotonically increasing condition that avoids loops.
Two databases of PGPs - from N-TIEs and from S-TIEs are stored. When
a PGP is inserted into the appropriate database, the usual
tiebreaking on cost is performed. Observe that the node retains all
PGP TIEs due to normal flooding behavior and hence loss of the best
prefix will lead to re-evaluation of TIEs present and readvertisement
of a new best PGP.
4.2.5.4. Regeneration
A node must regenerate policy-guided prefixes and retransmit them.
The node has its database of southbound policy-guided prefixes to
send in its S-TIE and its database of northbound policy-guided
prefixes to send in its N-TIE.
Of course, a leaf does not need to regenerate southbound policy-
guided prefixes.
4.2.5.5. Overlap with Disaggregated Prefixes
PGPs may overlap with prefixes introduced by automatic de-
aggregation. The topic is under further discussion. The break in
connectivity that leads to infeasiblity of a PGP is mirrored in
adjacency tear-down and according removal of such PGPs.
Nevertheless, the underlying link-state flooding will be likely
reacting significantly faster than a hop-by-hop redistribution and
with that the preference for PGPs may cause intermittant blackholes.
4.2.6. Reachability Computation
A node has three sources of relevant information. A node knows the
full topology south from the received N-TIEs. A node has the set of
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prefixes with associated distances and bandwidths from received
S-TIEs. A node can also have a set of PGPs.
4.2.6.1. Specification
A node uses the N-TIEs to build a network graph with unidirectional
links. As in IS-IS or OSPF, unidirectional links are associated
together to confirm bidirectional connectivity. Because of the
requirement that a packet traversing in a southbound direction must
not go take any northbound links, a node has topological visibility
only south of itself. There are no links at the computing node's
level that go to a northbound level. Therefore, all paths computed
must contain only east-west and southbound links. To enforce this,
the network graph MUST have either its northbound unidirectional
links removed or set to have a cost of COST_INFINITY.
A node runs a standard shortest path first (SPF) algorithm on this
network graph. If a node is minimized to have a cost of
COST_INFINITY, then it is not reachable.
4.2.6.1.1. Attaching Prefixes
After the SPF is run, it is necessary to attach prefixes. Prefixes
from an N-TIE are attached to the originating node with that node's
next-hop set and a distance equal to the prefix's cost plus the
node's minimized path distance. The RIFT route database, a set of
(prefix, type=spf, path_distance, next-hop set), accumulates these
results.
Prefixes from each S-TIE need to also be added to the RIFT route
database. There is no SPF to be run. Instead, the computing node
needs to determine, for each prefix in an S-TIE that originated from
adjacent node, what next-hops to use to reach that node. Since there
may be parallel links, the next-hops to use can be a set; presence of
the computing node in the associated Node S-TIE is sufficient to
verify that at least one link has bidirectional connectivity. The
set of minimum cost next-hops from the computing node X to the
originating adjacent node is determined.
Each prefix has its cost adjusted before being added into the RIFT
route database. The cost of the prefix is set to the cost received
plus the cost of the minimum cost next-hop to that neighbor. Then
each prefix can be added into the RIFT route database with the
next_hop_set; ties are broken based upon distance and type.
An exemplary implementation for node X follows:
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for each S-TIE
if S-TIE.layer > X.layer
next_hop_set = set of minimum cost links to the S-TIE.originator
next_hop_cost = minimum cost link to S-TIE.originator
end if
for each prefix P in the S-TIE
P.cost = P.cost + next_hop_cost
if P not in route_database:
add (P, type=DistVector, P.cost, next_hop_set) to route_database
end if
if (P in route_database) and
(route_database[P].type is not PolicyGuided):
if route_database[P].cost > P.cost):
update route_database[P] with (P, DistVector, P.cost, next_hop_set)
else if route_database[P].cost == P.cost
update route_database[P] with (P, DistVector, P.cost,
merge(next_hop_set, route_database[P].next_hop_set))
else
// Not prefered route so ignore
end if
end if
end for
end for
Figure 4: Adding Routes from S-TIE Prefixes
4.2.6.1.2. Attaching Policy-Guided Prefixes
Each policy-guided prefix P has its cost and next_hop_set already
stored in the associated database, as specified in Section 4.2.5.3;
the cost stored for the PGP is already updated to considering the
cost of the link to the advertising neighbor. By definition, a
policy-guided prefix is preferred to a regular prefix.
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for each policy-guided prefix P:
if P not in route_database:
add (P, type=PolicyGuided, P.cost, next_hop_set)
end if
if P in route_database :
if (route_database[P].type is not PolicyGuided) or
(route_database[P].cost > P.cost):
update route_database[P] with (P, PolicyGuided, P.cost, next_hop_set)
else if route_database[P].cost == P.cost
update route_database[P] with (P, PolicyGuided, P.cost,
merge(next_hop_set, route_database[P].next_hop_set))
else
// Not prefered route so ignore
end if
end if
end for
Figure 5: Adding Routes from Policy-Guided Prefixes
4.2.6.2. Further Mechanisms
4.2.6.2.1. Overload Bit
The leaf node SHOULD set the 'overload' bit on its N-TIE, since if
the spine nodes were to forward traffic not meant for the local node,
the leaf node does not have the topology information to prevent a
routing/forwarding loop.
Overload Bit MUST be respected in all according reachability
computations. A node with overload bit set MUST NOT advertise any
reachability prefixes southbound.
4.2.6.2.2. Optimized Route Computation on Leafs
Since the leafs do see only "one hop away" they do not need to run a
full SPF but can simply gather prefix candidates from their neighbors
and build the according routing table.
A leaf will have no N-TIEs except optionally from its east-west
neighbors. A leaf will have S-TIEs from its neighbors.
Instead of creating a network graph from its N-TIEs and running an
SPF, a leaf node can simply compute the minimum cost and next_hop_set
to each leaf neighbor by examining its local interfaces, determining
bi-directionality from the associated N-TIE, and specifying the
neighbor's next_hop_set set and cost from the minimum cost local
interfaces to that neighbor.
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Then a leaf attaches prefixes as in Section 4.2.6.1.1 as well as the
policy-guided prefixes as in Section 4.2.6.1.2.
4.2.7. Key/Value Store
The protocol supports a southbound distribution of key-value pairs
that can be used to e.g. distribute configuration information during
topology bringup. The KV TIEs (which are always S-TIEs) can arrive
from multiple nodes and need tie-breaking per key uses the following
rules
a. Only KV TIEs originated by a node to which the receiver has an
adjacency are considered.
b. Within all valid KV S-TIEs containing the key, the value of the
S-TIE with the highest level and within the same level highest
originator ID is prefered.
Observe that if a node goes down, the node south of it looses
adjacencies to it and with that the KVs will be disregarded and on
tie-break changes new KV readvertised to prevent stale information
being used by nodes further south. KV information is not result of
independent computation of every node but a diffused computation.
5. Examples
5.1. Normal Operation
This section describes RIFT deployment in the example topology
without any node or link failures. We disregard flooding reduction
for simplicity's sake.
As first step, the following bi-directional adjacencies will be
created (and any other links that do not fulfill LIE rules in
Section 4.2.2 disregarded):
o Spine 21 (PoD 0) to Node 111, Node 112, Node 121, and Node 122
o Spine 22 (PoD 0) to Node 111, Node 112, Node 121, and Node 122
o Node 111 to Leaf 111, Leaf 112
o Node 112 to Leaf 111, Leaf 112
o Node 121 to Leaf 121, Leaf 122
o Node 122 to Leaf 121, Leaf 122
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Consequently, N-TIEs would be originated by Node 111 and Node 112 and
each set would be sent to both Spine 21 and Spine 22. N-TIEs also
would be originated by Leaf 111 (w/ Prefix 111) and Leaf 112 (w/
Prefix 112 and the multihomed prefix) and each set would be sent to
Node 111 and Node 112. Node 111 and Node 112 would then flood these
N-TIEs to Spine 21 and Spine 22.
Similarly, N-TIEs would be originated by Node 121 and Node 122 and
each set would be sent to both Spine 21 and Spine 22. N-TIEs also
would be originated by Leaf 121 (w/ Prefix 121 and the multihomed
prefix) and Leaf 122 (w/ Prefix 122) and each set would be sent to
Node 121 and Node 122. Node 121 and Node 122 would then flood these
N-TIEs to Spine 21 and Spine 22.
At this point both Spine 21 and Spine 22, as well as any controller
to which they are connected, would have the complete network
topology. At the same time, Node 111/112/121/122 hold only the
N-ties of level 0 of their respective PoD. Leafs hold only their own
N-TIEs.
S-TIEs with adjacencies and a default IP prefix would then be
originated by Spine 21 and Spine 22 and each would be flooded to Node
111, Node 112, Node 121, and Node 122. Node 111, Node 112, Node 121,
and Node 122 would each send the S-TIE from Spine 21 to Spine 22 and
the S-TIE from Spine 22 to Spine 21. (S-TIEs are reflected up to
level from which they are received but they are NOT propagated
southbound.)
An S Tie with a default IP prefix would be originated by Node 111 and
Node 112 and each would be sent to Leaf 111 and Leaf 112. Leaf 111
and Leaf 112 would each send the S-TIE from Node 111 to Node 112 and
the S-TIE from Node 112 to Node 111.
Similarly, an S Tie with a default IP prefix would be originated by
Node 121 and Node 122 and each would be sent to Leaf 121 and Leaf
122. Leaf 121 and Leaf 122 would each send the S-TIE from Node 121
to Node 122 and the S-TIE from Node 122 to Node 121. At this point
IP connectivity with maximum possible ECMP has been established
between the Leafs while constraining the amount of information held
by each node to the minimum necessary for normal operation and
dealing with failures.
5.2. Leaf Link Failure
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. | | | |
.+-+---+-+ +-+---+-+
.| | | |
.|Node111| |Node112|
.+-+---+-+ ++----+-+
. | | | |
. | +---------------+ X
. | | | X Failure
. | +-------------+ | X
. | | | |
.+-+---+-+ +--+--+-+
.| | | |
.|Leaf111| |Leaf112|
.+-------+ +-------+
. + +
. Prefix111 Prefix112
Figure 6: Single Leaf link failure
In case of a failing leaf link between node 112 and leaf 112 the
link-state information will cause recomputation of the necessary SPF
and the higher levels will stop forwarding towards prefix 112 through
node 112. Only nodes 111 and 112, as well as both spines will see
control traffic. Leaf 111 will receive a new S-TIE from node 112 and
reflect back to node 111. Node 111 will deaggregate Prefix 111 and
Prefix 112 but we will not describe it further here since
deaggregation is emphasized in the next example. It is worth
observing however in this example that if Leaf111 would keep on
forwarding traffic towards Prefix112 using the advertised south-bound
default of Node112 the traffic would end up on Spine21 and Spine22
and cross back into Pod1 using Node111. This is arguably not as bad
as blackholing present in the next example but clearly undesirable.
Fortunately, deaggregation prevents this type of behavior except for
a transitory period of time.
5.3. Partitioned Fabric
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. +--------+ +--------+ S-TIE of Spine21
. | | | | received by
. |Spine 21| |Spine 22| reflection of
. ++-+--+-++ ++-+--+-++ Nodes 112 and 111
. | | | | | | | |
. | | | | | | | 0/0
. | | | | | | | |
. | | | | | | | |
. +--------------+ | +--- XXXXXX + | | | +---------------+
. | | | | | | | |
. | +-----------------------------+ | | |
. 0/0 | | | | | | |
. | 0/0 0/0 +- XXXXXXXXXXXXXXXXXXXXXXXXX -+ |
. | 1.1/16 | | | | | |
. | | +-+ +-0/0-----------+ | |
. | | | 1.1./16 | | | |
.+-+----++ +-+-----+ ++-----0/0 ++----0/0
.| | | | | 1.1/16 | 1.1/16
.|Node111| |Node112| |Node121| |Node122|
.+-+---+-+ ++----+-+ +-+---+-+ ++---+--+
. | | | | | | | |
. | +---------------+ | | +----------------+ |
. | | | | | | | |
. | +-------------+ | | | +--------------+ | |
. | | | | | | | |
.+-+---+-+ +--+--+-+ +-+---+-+ +---+-+-+
.| | | | | | | |
.|Leaf111| |Leaf112| |Leaf121| |Leaf122|
.+-+-----+ ++------+ +-----+-+ +-+-----+
. + + + +
. Prefix111 Prefix112 Prefix121 Prefix122
. 1.1/16
Figure 7: Fabric partition
Figure 7 shows the arguably most catastrophic but also the most
interesting case. Spine 21 is completely severed from access to
Prefix 121 (we use in the figure 1.1/16 as example) by double link
failure. However unlikely, if left unresolved, forwarding from leaf
111 and leaf 112 to P121 would suffer 50% blackholing based on pure
default route advertisements by spine 21 and spine 22.
The mechanism used to resolve this scenario is hinging on the
distribution of southbound representation by spine 21 that is
reflected by node 111 and node 112 to spine 22. Spine 22, having
computed reachability to all prefixes in the network, advertises with
the default route the ones that are reachable only via lower level
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neighbors that Spine 21 does not show an adjacency to. That results
in node 111 and node 112 obtaining a longest-prefix match to Prefix
121 which leads through Spine 22 and prevents blackholing through
Spine 21 still advertising the 0/0 aggregate only.
The Prefix 121 advertised by spine 22 does not have to be propagated
further towards leafs since they do no benefit from this information.
Hence the amount of flooding is restricted to spine 21 reissuing its
S-TIEs and reflection of those by node 111 and node 112. The
resulting SPF in Spine 22 issues the new S-TIEs containing 1.1/16 and
reflection of those by node 111 and node 112 again. None of the
leafs become aware of the changes and the failure is constrained
strictly to the level that became partitioned.
To finish with an example of the resulting sets computed using
notation introduced in Section 4.2.4, Spine 22 constructs the
following sets:
|R = Prefix 111, Prefix 112, Prefix 121, Prefix 122
|H (for r=Prefix 111) = Node 111, Node 112
|H (for r=Prefix 112) = Node 111, Node 112
|H (for r=Prefix 121) = Node 121, Node 122
|H (for r=Prefix 122) = Node 121, Node 122
|A (for Spine 21) = Node 111, Node 112
With that and |H (for r=Prefix 121) and |H (for r=Prefix 122) being
disjoint from |A (for Spine 21), Spine 22 will originate an S-TIE
with Prefix 121 and Prefix 122, that is flooded to Nodes 112, 112,
121 and 122.
6. Implementation and Operation: Further Details
6.1. Leaf to Leaf connection
[QUESTION] This would imply that the leaves have to understand the
N-TIE format and pull out the prefixes to figure out the next-hop...
Do we want this complexity?[/QUESTION]
6.2. Other End-to-End Services
Losing full, flat topology information at every node will have an
impact on some of the end-to-end network services. This is the price
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paid for minimal disturbance in case of failures and reduced flooding
and memory requirements on nodes lower south in the level hierarchy.
6.3. Address Family and Topology
Multi-Topology (MT)[RFC5120] and Multi-Instance (MI)[RFC6822] is used
today in link-state routing protocols to provide the option of
several instances on the same physical topology. RIFT supports this
capability by carrying transport ports in the LIE protocol exchanges.
Multiplexing of LIEs can be achieved by either choosing varying
multicast addresses or ports on the same address.
7. Information Elements Schema
This section introduces the schema for information elements.
On schema changes that
a. change field numbers or
b. add new required fields or
c. change lists into sets, unions into structures or
d. change multiplicity of fields or
e. change datatypes of any field or
f. changes default value of any field
major version of the schema MUST increase. All other changes MUST
increase minor version within the same major.
Thrift serializer/deserializer MUST not discard optional, unknown
fields but preserve and serialize them again when re-flooding.
//! Thrift file for RIFT, flooding for fat trees
//! @note: all numbers are implementation co'erced to unsigned versions using the highest bit
/// represents protocol major version
const i32 CURRENT_MAJOR_VERSION = 1
const i32 CURRENT_MINOR_VERSION = 0
typedef i64 SystemID
typedef i32 IPv4Address
/// this has to be of length long enough to accomodate prefix
typedef binary IPv6Address
typedef i16 UDPPortType
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typedef i16 TIENrType
typedef i16 MTUSizeType
typedef i32 SeqNrType
typedef i32 LifeTimeType
typedef i16 LevelType
typedef i16 PodType
typedef i16 VersionType
typedef i32 MetricType
typedef i64 KeyIDType
typedef i32 LinkIDType
typedef string KeyNameType
typedef bool TieDirectionType
const LevelType DEFAULT_LEVEL = 0
const PodType DEFAULT_POD = 0
const LinkIDType UNDEFINED_LINKID = 0
/// RIFT packet header
struct PacketHeader {
1: required VersionType major_version = CURRENT_MAJOR_VERSION;
2: required VersionType minor_version = CURRENT_MINOR_VERSION;
3: required SystemID sender;
4: optional LevelType level = DEFAULT_LEVEL;
}
struct ProtocolPacket {
1: required PacketHeader header;
2: required Content content;
}
union Content {
1: optional LIE hello;
2: optional TIDEPacket tide;
3: optional TIREPacket tire;
4: optional TIEPacket tie;
}
// serves as community for PGP
struct Community {
1: required i32 top;
2: required i32 bottom;
}
// content per N-S direction
union TIEElement {
1: optional NorthTIEElement north_element;
2: optional SouthTIEElement south_element;
}
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// @todo: flood header separately in UDP ?
// to allow caching to TIEs while changing lifetime?
struct TIEPacket {
1: required TIEHeader header;
// North and South TIEs need the correct union
// member to be sent, otherwise content is ignored
2: required TIEElement element;
}
enum TIETypeType {
Illegal = 0,
TIETypeMinValue = 1,
NodeTIEType = 2,
NorthPrefixTIEType = 3,
SouthPrefixTIEType = 4,
KeyValueTIEType = 5,
NorthPGPrefixTIEType = 6,
SouthPGPrefixTIEType = 7,
TIETypeMaxValue = 8,
}
/// RIFT LIE packet
struct LIE {
2: optional string name;
3: required SystemID originator;
// UDP port to which we can flood TIEs, same address
// as the hello TX this hello has been received on
4: required UDPPortType flood_port;
5: optional Neighbor neighbor;
6: optional PodType pod = DEFAULT_POD;
// level is already included on the packet header
}
struct LinkID {
1: required LinkIDType local_id;
2: required LinkIDType remote_id;
// more properties of the link can go in here
}
struct Neighbor {
1: required SystemID originator;
2: required UDPPortType flood_port;
// ignored on LIE
// can carry description of multiple
// parallel links in a TIE
3: optional set<LinkID> link_ids;
}
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/// ID of a TIE
/// @note: TIEID space is a total order achieved by comparing the elements in sequence defined
struct TIEID {
/// indicates whether N or S-TIE, True > False
1: required TieDirectionType northbound;
2: required SystemID originator;
3: required TIETypeType tietype;
4: required TIENrType tie_nr;
}
struct TIEHeader {
2: required TIEID tieid;
3: required SeqNrType seq_nr;
// in seconds
4: required LifeTimeType lifetime;
}
// sorted, otherwise protocol doesn't work properly
struct TIDEPacket {
/// all 00s marks starts
1: required TIEID start_range;
/// all FFs mark end
2: required TIEID end_range;
/// sorted list of headers
3: required list<TIEHeader> headers;
}
struct TIREPacket {
1: required set<TIEHeader> headers;
}
struct NodeNeighborsTIEElement {
/// if neighbor systemID repeats in set or TIEs
/// the behavior is undefined
1: required SystemID neighbor;
2: required LevelType level;
3: optional MetricType cost = 1;
}
/// capabilities the node supports
struct NodeCapabilities {
1: required bool flood_reduction = true;
}
/// flags the node sets
struct NodeFlags {
1: required bool overflow = false;
}
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struct NodeTIEElement {
1: required LevelType level;
2: optional NodeCapabilities capabilities;
3: optional NodeFlags flags;
4: required set<NodeNeighborsTIEElement> neighbors;
}
struct IPv4PrefixType {
1: required IPv4Address address;
2: required byte prefixlen;
}
struct IPv6PrefixType {
1: required IPv6Address address;
2: required byte prefixlen;
}
union IPPrefixType {
1: optional IPv4PrefixType ipv4prefix;
2: optional IPv6PrefixType ipv6prefix;
}
struct PrefixWithMetric {
1: required IPPrefixType prefix;
2: optional MetricType cost = 1;
}
struct PrefixTIEElement {
/// if the same prefix repeats in multiple TIEs
/// or with different metrics, behavior is unspecified
1: required set<PrefixWithMetric> prefixes;
}
struct KeyValue {
1: required KeyIDType keyid;
2: optional KeyNameType key;
3: optional string value = "";
}
struct KeyValueTIEElement {
1: required set<KeyValue> keyvalues;
}
/// single element in a N-TIE
union NorthTIEElement {
/// hinges of enum TIETypeType::NodeTIEType
1: optional NodeTIEElement node;
/// hinges of enum TIETypeType::NorthPrefixTIEType
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2: optional PrefixTIEElement prefixes;
/// @todo: policy guided prefixes
}
union SouthTIEElement {
/// hinges of enum TIETypeType::NodeTIEType
1: optional NodeTIEElement node;
2: optional KeyValueTIEElement keyvalues;
/// hinges of enum TIETypeType::SouthPrefixTIEType
3: optional PrefixTIEElement prefixes;
/// @todo: policy guided prefixes
}
8. IANA Considerations
9. Security Considerations
10. Acknowledgments
Many thanks to Naiming Shen for some of the early discussions around
the topic of using IGPs for routing in topologies related to Clos.
Adrian Farrel and Jeffrey Zhang provided thoughtful comments that
improved the readability of the document and found good amount of
corners where the light failed to shine.
11. References
11.1. Normative References
[ISO10589]
ISO "International Organization for Standardization",
"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, Second Edition.", Nov 2002.
[RFC1142] Oran, D., Ed., "OSI IS-IS Intra-domain Routing Protocol",
RFC 1142, DOI 10.17487/RFC1142, February 1990,
<http://www.rfc-editor.org/info/rfc1142>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<http://www.rfc-editor.org/info/rfc2328>.
[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,
<http://www.rfc-editor.org/info/rfc4271>.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<http://www.rfc-editor.org/info/rfc4655>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<http://www.rfc-editor.org/info/rfc5120>.
[RFC5303] Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way
Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303,
DOI 10.17487/RFC5303, October 2008,
<http://www.rfc-editor.org/info/rfc5303>.
[RFC5309] Shen, N., Ed. and A. Zinin, Ed., "Point-to-Point Operation
over LAN in Link State Routing Protocols", RFC 5309,
DOI 10.17487/RFC5309, October 2008,
<http://www.rfc-editor.org/info/rfc5309>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>.
[RFC6822] Previdi, S., Ed., Ginsberg, L., Shand, M., Roy, A., and D.
Ward, "IS-IS Multi-Instance", RFC 6822,
DOI 10.17487/RFC6822, December 2012,
<http://www.rfc-editor.org/info/rfc6822>.
[RFC7855] Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
Litkowski, S., Horneffer, M., and R. Shakir, "Source
Packet Routing in Networking (SPRING) Problem Statement
and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
2016, <http://www.rfc-editor.org/info/rfc7855>.
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[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<http://www.rfc-editor.org/info/rfc7938>.
11.2. Informative References
[CLOS] Yuan, X., "On Nonblocking Folded-Clos Networks in Computer
Communication Environments", IEEE International Parallel &
Distributed Processing Symposium, 2011.
[DIJKSTRA]
Dijkstra, E., "A Note on Two Problems in Connexion with
Graphs", Journal Numer. Math. , 1959.
[DYNAMO] De Candia et al., G., "Dynamo: amazon's highly available
key-value store", ACM SIGOPS symposium on Operating
systems principles (SOSP '07), 2007.
[FATTREE] Leiserson, C., "Fat-Trees: Universal Networks for
Hardware-Efficient Supercomputing", 1985.
[QUIC] Iyengar et al., J., "QUIC: A UDP-Based Multiplexed and
Secure Transport", 2016.
[VAHDAT08]
Al-Fares, M., Loukissas, A., and A. Vahdat, "A Scalable,
Commodity Data Center Network Architecture", SIGCOMM ,
2008.
Authors' Addresses
Tony Przygienda
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
US
Email: prz@juniper.net
John Drake
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
US
Email: jdrake@juniper.net
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Alia Atlas
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
US
Email: akatlas@juniper.net
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