Internet-Draft | RIFT | July 2023 |
Przygienda, et al. | Expires 11 January 2024 | [Page] |
This document defines a specialized, dynamic routing protocol for Clos and fat tree network topologies optimized towards minimization of control plane state as well as configuration and operational complexity.¶
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Clos [CLOS] topologies (called commonly a fat tree/network in modern IP fabric considerations [VAHDAT08] as homonym to the original definition of the term [FATTREE]) have gained prominence in today's networking, primarily as a result of the paradigm shift towards a centralized data-center architecture that is poised to deliver a majority of computation and storage services in the future. Many builders of such IP fabrics desire a protocol that auto-configures itself and deals with failures and mis-configurations with a minimum of human intervention. Such a solution would allow local IP fabric bandwidth to be consumed in a 'standard component' fashion, i.e. provision it much faster and operate it at much lower costs than today, much like compute or storage is consumed already.¶
In looking at the problem through the lens of such IP fabric requirements, RIFT (Routing in Fat Trees) addresses those challenges not through an incremental modification of either a link-state (distributed computation) or distance-vector (diffused computation) techniques but rather a mixture of both, colloquially best described as "link-state towards the spines" and "distance vector towards the leaves". In other words, "bottom" levels are flooding their link-state information in the "northern" direction while each node generates under normal conditions a "default route" and floods it in the "southern" direction. This type of protocol allows naturally for highly desirable aggregation. Alas, such aggregation could drop traffic in cases of misconfiguration or while failures are being resolved or even cause network partitioning and this has to be addressed by some adequate mechanism. The approach RIFT takes is described in Section 4.2.5 and is based on automatic, sufficient disaggregation of prefixes in case of link and node failures.¶
The protocol does further provide:¶
Figure 1 illustrates a simplified, conceptual view of a RIFT fabric and its routing and database information. The top of the fabric's link-state database holds information about the nodes below it and the routes to them. When referring to Figure 1, the /32 notation corresponds to each node's loopback address (e.g. A/32 is node A's loopback, etc.) and 0/0 indicates a default route. The first row of database information represents the nodes for which full topology information is available. The second row of database information indicates that partial information of other nodes in the same level is also available. Such information will be necessary to perform certain algorithms necessary for correct protocol operation. When the "bottom" of the fabric is considered, or in other words the leaves, the topology is basically empty and, under normal conditions, the leaves hold a load balanced default route to the next level.¶
The remainder of this document fills in the protocol specification details.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This section should serve as an initial guided tour through the document in order to convey the necessary information for any reader, depending on their level of interest. The authors recommend reading the HTML or PDF versions of this document.¶
The Terminology (Section 3.1) section should be used as a supporting reference as the document is read.¶
The indications of direction (i.e. "top", "bottom", etc.) referenced in Section 1 are of paramount importance. RIFT requires a topology with a sense of top and bottom in order to properly achieve a sorted topology. Clos, Fat Tree, and other similarly structured networks are conducive to such requirements. RIFT does allow for further relaxation of these constraints, they will be mentioned later in this section.¶
Operators and implementors alike must understand if multi-plane IP fabrics are of interest or not. Section 3.2 illustrates an example of both single-plane in Figure 2 and multi-plane fabric in Figure 3. Multi-plane fabrics require understanding of additional RIFT concepts (e.g. negative disaggregation in Section 4.2.5.2) that are otherwise unnecessary in context of strictly single-plane fabrics. The Overview (Section 4.1) and Section 4.1.2 aim to provide enough context to determine if multi-plane fabrics are of interest to the reader. The Fallen Leaf part (Section 4.1.3), and additionally Section 4.1.4 and Section 4.1.5 describe further considerations that are specific to multi-plane fabrics.¶
The fundamental protocol concepts are described starting in the specification part (Section 4.2), but some sub-sections are not quite as relevant unless dealing with implementation of the protocol. The protocol transport (Section 4.2.1) is of particular importance for two reasons. First, it introduces RIFT's packet formats in the form of a normative Thrift model given in Appendix B.3. Second, the Thrift model component is a prelude to understanding the RIFT's inherent security features as defined in both security models part (Section 4.4) and the security segment (Section 7). The normative schema defining the Thrift model can be found in both Appendix B.2 and Appendix B.3. Furthermore, while a detailed understanding of Thrift [thrift] and the models is not required unless implementing RIFT, they may provide additional useful information for other readers.¶
If implementing RIFT to support multi-plane topologies Section 4.2 should be reviewed in its entirety in conjunction with the previously mentioned Thrift schemas. Sections not relevant to single-plane implementations will be noted later in the section. Special attention should be paid to the Link Information Element (LIE) definitions part (Section 4.2.2) as it not only outlines basic neighbor discovery and adjacency formation, but also provides necessary context for RIFT's Zero Touch Provisioning (ZTP) (Section 4.2.7) and mis-cabling detection capabilities that allow it to automatically detect and build the underlay topology with negligible configuration. These specific capabilities are detailed in Section 4.2.7.¶
For other readers, the following sections provide a more detailed understanding of the fundamental properties and highlight some additional benefits of RIFT such as link state packet formats, efficient flooding, synchronization, loop-free path computation and link-state database maintenance - Section 4.2.3, Section 4.2.3.2, Section 4.2.3.3, Section 4.2.3.4, Section 4.2.3.6, Section 4.2.3.7, Section 4.2.3.8, Section 4.2.4, Section 4.2.4.1, Section 4.2.4.2, Section 4.2.4.3, Section 4.2.4.4. RIFT's ability to perform weighted unequal-cost load balancing of traffic across all available links is outlined in Section 4.3.7 with an accompanying example.¶
Section 4.2.5 is the place where the single-plane vs. multi-plane requirement is explained in more detail. For those interested in single-plane fabrics, only Section 4.2.5.1 is required. For the multi-plane interested reader Section 4.2.5.2, Section 4.2.5.2.1, Section 4.2.5.2.2, and Section 4.2.5.2.3 are also mandatory. Section 4.2.6 is especially important for any multi-plane interested reader as it outlines how the RIB (Routing Information Base) and FIB (Forwarding Information Base) are built via the disaggregation mechanisms, but also illustrates how they prevent defective routing decisions that cause traffic loss in both single or multi-plane topologies.¶
Section 5 contains a set of comprehensive examples that show how RIFT contains the impact of failures to only the required set of nodes. It should also help cement some of RIFT's core concepts in the reader's mind.¶
Last, but not least, RIFT has other optional capabilities. One example is the key-value data-store, which enables RIFT to advertise data post-convergence in order to bootstrap higher levels of functionality (e.g. operational telemetry). Those are covered in Section 4.3 and Section 6.¶
More information related to RIFT can be found in the "RIFT Applicability" [APPLICABILITY] document, which discusses alternate topologies upon which RIFT may be deployed, use cases where it is applicable, and presents operational considerations that complement this document.¶
This section presents the terminology used in this document.¶
Additionally, when the specification refers to elements of packet encoding or constants provided in the Appendix B grave accents are used, e.g. `invalid_distance`. Same convention is used when referring to finite state machine states or events outside the context of the machine itself, e.g. `OneWay`.¶
Topology in Figure 2 is refered to in all further considerations. This figure depicts a generic "single plane fat tree" and the concepts explained using three levels apply by induction to further levels and higher degrees of connectivity. Further, this document will deal also with designs that provide only sparser connectivity and "partitioned spines" as shown in Figure 3 and explained further in Section 4.1.2.¶
The remainder of this document presents the detailed specification of a protocol optimized for Routing in Fat Trees (RIFT) that in most abstract terms has many properties of a modified link-state protocol when distributing information northbound and a distance vector protocol when distributing information southbound. While this is an unusual combination, it does quite naturally exhibit the desirable properties desired.¶
The most singular property of RIFT is that it floods link-state information northbound only so that each level obtains the full topology of levels south of it. Link-State information is, with some exceptions, never flooded East-West or back South again. Exceptions like south reflection is explained in detail in Section 4.2.5.1 and east-west flooding at ToF level in multi-plane fabrics is outlined in Section 4.1.2. In the southbound direction, the necessary routing information required (normally just a default route as per Section 4.2.3.8) only propagates one hop south. Those nodes then generate their own routing information and flood it south to avoid the overhead of building an update per adjacency. For the moment describing the East-West direction is left out.¶
Those information flow constraints create not only an anisotropic protocol (i.e. the information is not distributed "evenly" or "clumped" but summarized along the N-S gradient) but also a "smooth" information propagation where nodes do not receive the same information from multiple directions at the same time. Normally, accepting the same reachability on any link, without understanding its topological significance, forces tie-breaking on some kind of distance metric. And such tie-breaking leads ultimately in hop-by-hop forwarding to shortest paths only. In contrast to that, RIFT, under normal conditions, does not need to tie-break the same reachability information from multiple directions. Its computation principles (south forwarding direction is always preferred) leads to valley-free [VFR] forwarding behavior. And since valley free routing is loop-free, it can use all feasible paths which is another highly desirable property if available bandwidth should be utilized to the maximum extent possible.¶
To account for the "northern" and the "southern" information split the link state database is partitioned accordingly into "north representation" and "south representation" TIEs. In simplest terms the North TIEs contain a link state topology description of lower levels and and South TIEs carry simply node description of the level above and default routes pointing north. This oversimplified view will be refined gradually in the following sections while introducing protocol procedures and state machines at the same time.¶
This section and resulting Section 4.2.5.2 are dedicated to multi-plane fabrics, in contrast with the single plane designs where all top-of-fabric nodes are topologically equal and initially connected to all the switches at the level below them.¶
It is quite difficult to visualize multi plane design, which are effectively multi-dimensional switching matrices. To cope with that, this document introduces a methodology allowing to depict the connectivity in two-dimensional pictures. Further, the fact can be leveraged that what is under consideration here are basically stacked crossbar fabrics where ports align "on top of each other" in a regular fashion.¶
A word of caution to the reader; at this point it should be observed that the language used to describe Clos variations, especially in multi-plane designs, varies widely between sources. This description follows the terminology introduced in Section 3.1. It is unavoidable to have it present to be able to follow the rest of this section correctly.¶
This section describes the terminology and acronyms used in the rest of the text. Though the glossary may not be comprehensible on a first read, the following sections will gradually introduce the terms in their proper context.¶
The typical topology for which RIFT is defined is built of P number of PoDs and connected together by S number of ToF nodes. A PoD node has K number of ports. From here on half of them (K=Radix/2) are assumed to connect host devices from the south, and the other half to connect to interleaved PoD Top-Level switches to the north. The K ratio can be chosen differently without loss of generality when port speeds differ or the fabric is oversubscribed but K=Radix/2 allows for more readable representation whereby there are as many ports facing north as south on any intermediate node. A node is hence represented in a schematic fashion with ports "sticking out" to its north and south rather than by the usual real-world front faceplate designs of the day.¶
Figure 4 provides a view of a leaf node as seen from the north, i.e. showing ports that connect northbound. For lack of a better symbol, the document chooses to use the "o" as ASCII visualisation of a single port. In this example, K_LEAF has 6 ports. Observe that the number of PoDs is not related to Radix unless the ToF Nodes are constrained to be the same as the PoD nodes in a particular deployment.¶
The Radix of a PoD's top node may be different than that of the leaf node. Though, more often than not, a same type of node is used for both, effectively forming a square (K*K). In the general case, switches at the top of the PoD with K_TOP southern ports not necessarily equal to K_LEAF could be considered . For instance, in the representations below, we pick a 6 port K_LEAF and a 8 port K_TOP. In order to form a crossbar, K_TOP Leaf Nodes are necessary as illustrated in Figure 5.¶
As further visualized in Figure 6 the K_TOP Leaf Nodes are fully interconnected with the K_LEAF ToP nodes, providing connectivity that can be represented as a crossbar when "looked at" from the north. The result is that, in the absence of a failure, a packet entering the PoD from the north on any port can be routed to any port in the south of the PoD and vice versa. And that is precisely why it makes sense to talk about a "switching matrix".¶
Side views of this PoD is illustrated in Figure 7 and Figure 8.¶
As next step, observe further that a resulting PoD can be abstracted as a bigger node with a number K of K_POD= K_TOP * K_LEAF, and the design can recurse.¶
It will be critical at this point that, before progressing further, the concept and the picture of "crossed crossbars" is clear. Else, the following considerations might be difficult to comprehend.¶
To continue, the PoDs are interconnected with each other through a Top-of-Fabric (ToF) node at the very top or the north edge of the fabric. The resulting ToF is *not* partitioned if, and only if (IIF), every PoD top level node (spine) is connected to every ToF Node. This topology is also referred to as a single plane configuration and is quite popular due to its simplicity. In order to reach a 1:1 connectivity ratio between the ToF and the leaves, it results that there are K_TOP ToF nodes, because each port of a ToP node connects to a different ToF node, and K_LEAF ToP nodes for the same reason. Consequently, it will take (P * K_LEAF) ports on a ToF node to connect to each of the K_LEAF ToP nodes of the P PoDs. Figure 9 illustrates this, looking at P=3 PoDs from above and 2 sides. The large view is the one from above, with the 8 ToF of 3*6 ports each interconnecting the PoDs, every ToP Node being connected to every ToF node.¶
The top view can be collapsed into a third dimension where the hidden depth index is representing the PoD number. One PoD can be shown then as a class of PoDs and hence save one dimension in the representation. The Spine Node expands in the depth and the vertical dimensions, whereas the PoD top level Nodes are constrained, in horizontal dimension. A port in the 2-D representation represents effectively the class of all the ports at the same position in all the PoDs that are projected in its position along the depth axis. This is shown in Figure 10.¶
As simple as single plane deployment is, it introduces a limit due to the bound on the available radix of the ToF nodes that has to be at least P * K_LEAF. Nevertheless, it will be come clear that a distinct advantage of a connected or non-partitioned Top-of-Fabric is that all failures can be resolved by simple, non-transitive, positive disaggregation (i.e. nodes advertising more specific prefixes with the default to the level below them that is however not propagated further down the fabric) as described in Section 4.2.5.1 . In other words; non-partitioned ToF nodes can always reach nodes below or withdraw the routes from PoDs they cannot reach unambiguously. And with this, positive disaggregation can heal all failures and still allow all the ToF nodes to be aware of each other via south reflection. Disaggregation will be explained in further detail in Section 4.2.5.¶
In order to scale beyond the "single plane limit", the Top-of-Fabric can be partitioned by an N number of identically wired planes where N is an integer divider of K_LEAF. The 1:1 ratio and the desired symmetry are still served, this time with (K_TOP * N) ToF nodes, each of (P * K_LEAF / N) ports. N=1 represents a non-partitioned Spine and N=K_LEAF is a maximally partitioned Spine. Further, if R is any integer divisor of K_LEAF, then N=K_LEAF/R is a feasible number of planes and R a redundancy factor that denotes the number of independent paths between 2 leaves within a plane. It proves convenient for deployments to use a radix for the leaf nodes that is a power of 2 so they can pick a number of planes that is a lower power of 2. The example in Figure 11 splits the Spine in 2 planes with a redundancy factor R=3, meaning that there are 3 non-intersecting paths between any leaf node and any ToF node. A ToF node must have, in this case, at least 3*P ports, and be directly connected to 3 of the 6 ToP nodes (spines) in each PoD. The ToP nodes are represented horizontally with K_TOP=8 ports northwards each.¶
At the extreme end of the spectrum it is even possible to fully partition the spine with N = K_LEAF and R=1, while maintaining connectivity between each leaf node and each Top-of-Fabric node. In that case the ToF node connects to a single Port per PoD, so it appears as a single port in the projected view represented in Figure 12. The number of ports required on the Spine Node is more than or equal to P, the number of PoDs.¶
As mentioned earlier, RIFT exhibits an anisotropic behavior tailored for fabrics with a North / South orientation and a high level of interleaving paths. A non-partitioned fabric makes a total loss of connectivity between a Top-of-Fabric node at the north and a leaf node at the south a very rare but yet possible occasion that is fully healed by positive disaggregation as described in Section 4.2.5.1. In large fabrics or fabrics built from switches with low radix, the ToF ends often being partitioned in planes which makes the occurrence of having a given leaf being only reachable from a subset of the ToF nodes more likely to happen. This makes some further considerations necessary.¶
A "Fallen Leaf" is a leaf that can be reached by only a subset, but not all, of Top-of-Fabric nodes due to missing connectivity. If R is the redundancy factor, then it takes at least R breakages to reach a "Fallen Leaf" situation.¶
In a maximally partitioned fabric, the redundancy factor is R=1, so any breakage in the fabric will cause one or more fallen leaves in the affected plane. R=2 guarantees that a single breakage will not cause a fallen leaf. However, not all cases require disaggregation. The following cases do not require particular action:¶
In a general manner, the mechanism of non-transitive positive disaggregation is sufficient when the disaggregating ToF nodes collectively connect to all the ToP nodes in the broken plane. This happens in the following case:¶
On the other hand, there is a need to disaggregate the routes to Fallen Leaves within the plane in a transitive fashion, that is, all the way to the other leaves, in the following cases:¶
For the sake of easy comprehension the abstractions are rolled back into a simple example that shows that in Figure 3 the loss of link between spine node 3 and leaf node 3 will make leaf node 3 a fallen leaf for Top-of-Fabric plane C. Worse, if the cabling was never present in the first place, plane C will not even be able to know that such a fallen leaf exists. Hence partitioning without further treatment results in two grave problems:¶
When aggregation is used, RIFT deals with fallen leaves by ensuring that all the ToF nodes share the same north topology database. This happens naturally in single plane design by the means of northbound flooding and south reflection but needs additional considerations in multi-plane fabrics. To enable routing to fallen leaves in multi-plane designs, RIFT requires additional interconnection across planes between the ToF nodes, e.g., using rings as illustrated in Figure 13. Other solutions are possible but they either need more cabling or end up having much longer flooding paths and/or single points of failure.¶
In detail, by reserving at least two ports on each Top-of-Fabric node it is possible to connect them together by interplane bi-directional rings as illustrated in Figure 13. The rings will be used to exchange full north topology information between planes. All ToFs having same north topology allows by the means of transitive, negative disaggregation described in Section 4.2.5.2 to efficiently fix any possible fallen leaf scenario. Somewhat as a side-effect, the exchange of information fulfills the requirement to have a full view of the fabric topology at the Top-of-Fabric level, without the need to collate it from multiple points.¶
One consequence of the "Fallen Leaf" problem is that some prefixes attached to the fallen leaf become unreachable from some of the ToF nodes. RIFT defines two methods to address this issue, the positive and the negative disaggregation. Both methods flood corresponding types of South TIEs to advertise the impacted prefix(es).¶
When used for the operation of disaggregation, a positive South TIE, as usual, indicates reachability to a prefix of given length and all addresses subsumed by it. In contrast, a negative route advertisement indicates that the origin cannot route to the advertised prefix.¶
The positive disaggregation is originated by a router that can still reach the advertised prefix, and the operation is not transitive. In other words, the receiver does *not* generate its own TIEs or flood them south as a consequence of receiving positive disaggregation advertisements from a higher level node. The effect of a positive disaggregation is that the traffic to the impacted prefix will follow the longest match and will be limited to the northbound routers that advertised the more specific route.¶
In contrast, the negative disaggregation can be transitive, and is propagated south when all the possible routes have been advertised as negative exceptions. A negative route advertisement is only actionable when the negative prefix is aggregated by a positive route advertisement for a shorter prefix. In such case, the negative advertisement "punches out a hole" in the positive route in the routing table, making the positive prefix reachable through the originator with the special consideration of the negative prefix removing certain next hop neighbors. The specific procedures will be explained in detail in Section 4.2.5.2.3.¶
When the top of fabric switches are not partitioned into multiple planes, the resulting southbound flooding of the positive disaggregation by the ToF nodes that can still reach the impacted prefix is in general enough to cover all the switches at the next level south, typically the ToP nodes. If all those switches are aware of the disaggregation, they collectively create a ceiling that intercepts all the traffic north and forwards it to the ToF nodes that advertised the more specific route. In that case, the positive disaggregation alone is sufficient to solve the fallen leaf problem.¶
On the other hand, when the fabric is partitioned in planes, the positive disaggregation from ToF nodes in different planes do not reach the ToP switches in the affected plane and cannot solve the fallen leaves problem. In other words, a breakage in a plane can only be solved in that plane. Also, the selection of the plane for a packet typically occurs at the leaf level and the disaggregation must be transitive and reach all the leaves. In that case, the negative disaggregation is necessary. The details on the RIFT approach to deal with fallen leaves in an optimal way are specified in Section 4.2.5.2.¶
This section specifies the protocol in a normative fashion by either prescriptive procedures or behavior defined by Finite State Machines (FSM).¶
The FSMs, as usual, are presented as states a neighbor can assume, events that it can be given and the corresponding actions performed when transitioning between states on event processing.¶
Actions are performed before the end state is assumed.¶
The FSMs can queue events against itself to chain actions or against other FSMs in the specification. Events are always processed in the sequence they have been queued.¶
Consequently, "On Entry" actions on FSM state are performed every time and right before the corresponding state is entered, i.e. after any transitions from previous state.¶
"On Exit" actions are performed every time and immediately when a state is exited, i.e. before any transitions towards target state are performed.¶
Any attempt to transition from a state towards another on reception of an event where no action is specified must be considered an unrecoverable error, i.e. the protocol MUST reset all adjacencies, discard all the state and may not start again.¶
The data structures and FSMs described in this document are conceptual and do not have to be implemented precisely as described here, as long as the implementations support the described functionality and exhibit the same externally visible behavior.¶
The machines can use conceptually "timers" for different situations. Those timers are started through actions and their expiration leads to queuing of corresponding events to be processed.¶
The term `holdtime` is used often as short-hand for `holddown timer` and signifies either the length of the holding down period or the timer used to expire after such period. Such timers are used to "hold down" state within an FSM that is cleaned if the machine triggers a `HoldtimeExpired` event.¶
All RIFT packet structures and their contents are defined in the Thrift [thrift] models in Appendix B. The packet structure itself is defined in `ProtocolPacket` which contains the packet header (`PacketHeader`) and the packet contents (`PacketContent`). `PacketContent` is a union of the LIE, TIE, TIDE, and TIRE packets and are defined in `LIEPacket`, `TIEPacket`, `TIDEPacket`, and `TIREPacket` respectively.¶
In terms of bits on the wire, it is the `ProtocolPacket` that is serialized and carried in an envelope defined in Section 4.4.3 within a UDP frame that provides security and allows validation/modification of several important fields without de-serialization for performance and security reasons. Security model and procedures are further explained in Section 7.¶
RIFT LIE exchange auto-discovers neighbors, negotiates ZTP parameters and discovers miscablings. The formation progresses under normal conditions from OneWay to TwoWay and then ThreeWay state at which point it is ready to exchange TIEs per Section 4.2.3. The adjacency exchanges ZTP information (Section 4.2.7) in any of the states, i.e. it is not necessary to reach ThreeWay for zero-touch provisioning to operate.¶
RIFT supports any combination of IPv4 and IPv6 addressing on the fabric with the additional capability for forwarding paths that are capable of forwarding IPv4 packets in presence of IPv6 addressing only.¶
For IPv4 LIE exchange happens over well-known administratively locally scoped and configured or otherwise well-known IPv4 multicast address [RFC2365]. For IPv6 [RFC8200] exchange is performed over link-local multicast scope [RFC4291] address which is configured or otherwise well-known. In both cases a destination UDP port defined in the schema Appendix B.2 is used unless configured otherwise. LIEs MUST be sent with an IPv4 Time to Live (TTL) or an IPv6 Hop Limit (HL) of either 1 or 255 to prevent RIFT information reaching beyond a single L3 next-hop in the topology. LIEs SHOULD be sent with network control precedence unless an implementation is prevented from doing so [RFC2474].¶
The originating port of the LIE has no further significance other than identifying the origination point. LIEs are exchanged over all links running RIFT.¶
An implementation may listen and send LIEs on IPv4 and/or IPv6 multicast addresses. A node MUST NOT originate LIEs on an address family if it does not process received LIEs on that family. LIEs on same link are considered part of the same LIE FSM independent of the address family they arrive on. Observe further that the LIE source address may not identify the peer uniquely in unnumbered or link-local address cases so the response transmission MUST occur over the same interface the LIEs have been received on. A node may use any of the adjacency's source addresses it saw in LIEs on the specific interface during adjacency formation to send TIEs (Section 4.2.3.3). That implies that an implementation MUST be ready to accept TIEs on all addresses it used as source of LIE frames.¶
A simplified version MAY be implemented on platforms with limited or no multicast support (e.g. IoT devices) by sending and receiving LIE frames on IPv4 subnet broadcast addresses or IPv6 all routers multicast address. However, this technique is less optimal and presents a wider attack surface from a security perspective.¶
A ThreeWay adjacency (as defined in the glossary) over any address family implies support for IPv4 forwarding if the `ipv4_forwarding_capable` flag in `LinkCapabilities` is set to true. In the absence of IPv4 LIEs with `ipv4_forwarding_capable` set to true, a node MUST forward IPv4 packets using gateways discovered on IPv6-only links advertising this capability. The mechanism to discover the corresponding IPv6 gateway is out of scope for this specification and may be implementation specific. It is expected that the whole fabric supports the same type of forwarding of address families on all the links, any other combination is outside the scope of this specification. If IPv4 forwarding is supported on an interface, `ipv4_forwarding_capable` MUST be set to true for all LIEs advertised from that interface. If IPv4 and IPv6 LIEs indicate contradicting information, protocol behavior is unspecified.¶
Operation of a fabric where only some of the links are supporting forwarding on an address family or have an address in a family and others do not is outside the scope of this specification.¶
Any attempt to construct IPv6 forwarding over IPv4 only adjacencies is outside this specification.¶
Table 1 outlines protocol behavior pertaining to LIE exchange over different address family combinations. Table 2 outlines the way in which neighbors forward traffic as it pertains to the `ipv4_forwarding_capable` flag setting across the same address family combinations.¶
The specific forwarding implementation to support the described behavior is out of scope for this document.¶
Local Neighbor AF | Remote Neighbor AF | LIE Exchange Behavior |
---|---|---|
IPv4 | IPv4 | LIEs and TIEs are exchanged over IPv4 only. The local neighbor receives TIEs from remote neighbors on any of the LIE source addresses. |
IPv6 | IPv6 | LIEs and TIEs are exchanged over IPv6 only. The local neighbor receives TIEs from remote neighbors on any of the LIE source addresses. |
IPv4, IPv6 | IPv6 | The local neighbor sends LIEs for both IPv4 and IPv6 while the remote neighbor only sends LIEs for IPv6. The resulting adjacency will exchange TIEs over IPv6 on any of the IPv6 LIE source addresses. |
IPv4, IPv6 | IPv4, IPv6 | LIEs and TIEs are exchanged over IPv6 and IPv4. TIEs are received on any of the IPv4 or IPv6 LIE source addresses. The local neighbor receives TIEs from the remote neighbors on any of the IPv4 or IPv6 LIE source addresses. |
Local Neighbor AF | Remote Neighbor AF | Forwarding Behavior |
---|---|---|
IPv4 | IPv4 | Both nodes are required to set the `ipv4_forwarding_capable` flag to true. Only IPv4 traffic can be forwarded. |
IPv6 | IPv6 | If either neighbor sets `ipv4_forwarding_capable` to false, only IPv6 traffic can be forwarded. If both neighbors set `ipv4_forwarding_capable` to true, IPv4 traffic is also forwarded via IPv6 gateways. |
IPv4, IPv6 | IPv6 | If the remote neighbor sets `ipv4_forwarding_capable` to false, only IPv6 traffic can be forwarded. If both neighbors set `ipv4_forwarding_capable` to true, IPv4 traffic is also forwarded via IPv6 gateways. |
IPv4, IPv6 | IPv4, IPv6 | IPv4 and IPv6 traffic can be forwarded. If IPv4 and IPv6 LIEs advertise conflicting `ipv4_forwarding_capable` flags, the behavior is unspecified. |
The protocol does *not* support selective disabling of address families after adjacency formation, disabling IPv4 forwarding capability or any local address changes in ThreeWay state, i.e. if a link has entered ThreeWay IPv4 and/or IPv6 with a neighbor on an adjacency and it wants to stop supporting one of the families or change any of its local addresses or stop IPv4 forwarding, it has to tear down and rebuild the adjacency. It also has to remove any state it stored about the remote side of the adjacency such as associated LIE source addresses.¶
Unless ZTP as described in Section 4.2.7 is used, each node is provisioned with the level at which it is operating and advertises it in the `level` of the `PacketHeader` schema element. It MAY be also provisioned with its PoD. If level is not provisioned it is not present in the optional `PacketHeader` schema element and established by ZTP procedures if feasible. If PoD is not provisioned it is as governed by the `LIEPacket` schema element assuming the `common.default_pod` value. This means that switches except top of fabric do not need to be configured at all. Necessary information to configure all values is exchanged in the `LIEPacket` and `PacketHeader` or derived by the node automatically.¶
Further definitions of leaf flags are found in Section 4.2.7 given they have implications in terms of level and adjacency forming here. Leaf flags are carried in `HierarchyIndications`.¶
A node MUST form a ThreeWay adjacency if at a minimum the following first order logic conditions are satisfied on a LIE packet as specified by the `LIEPacket` schema element and received on a link (such as LIE is considered a "minimally valid" LIE). Observe that depending on the FSM involved and its state further conditions may be checked and even a minimally valid LIE can be considered ultimately invalid if any of the additional conditions fail.¶
[¶
].¶
LIEs arriving with IPv4 Time to Live (TTL) or an IPv6 Hop Limit (HL) different than 1 or 255 MUST be ignored.¶
This section specifies the precise, normative LIE FSM. For easier reference the corresponding figure is given as well in Figure 14. Additionally, some sets of actions repeat often and are hence summarized into well-known procedures.¶
Events generated are fairly fine grained, especially when indicating problems in adjacency forming conditions. The intention of such differentiation is to simplify tracking of problems in deployment.¶
Initial state is `OneWay`.¶
The machine sends LIEs proactively on several transitions to accelerate adjacency bring-up without waiting for the corresponding timer tic.¶
The following words are used for well known procedures:¶
SEND_LIE: create and send a new LIE packet¶
PROCESS_LIE:¶
PUSH UpdateZTPOffer, construct temporary new neighbor structure with values from LIE, if no current neighbor exists then set current neighbor to new neighbor, PUSH NewNeighbor event, CHECK_THREE_WAY else¶
CHECK_THREE_WAY: if current state is OneWay do nothing else¶
States:¶
Events:¶
Actions:¶
Topology and reachability information in RIFT is conveyed by the means of TIEs.¶
The TIE exchange mechanism uses the port indicated by each node in the LIE exchange as `flood_port` in `LIEPacket` and the interface on which the adjacency has been formed as destination. TIEs MUST be sent with an IPv4 Time to Live (TTL) or an IPv6 Hop Limit (HL) of either 1 or 255 and also MUST be ignored if received with values different than 1 or 255. This prevents RIFT information from reaching beyond a single L3 next-hop in the topology. TIEs SHOULD be sent with network control precedence unless an implementation is prevented from doing so [RFC2474].¶
TIEs contain sequence numbers, lifetimes, and a type. Each type has ample identifying number space and information is spread across multiple TIEs with the same TIEElement type (this is true for all TIE types).¶
More information about the TIE structure can be found in the schema in Appendix B starting with `TIEPacket` root.¶
A central concept of RIFT is that each node represents itself differently depending on the direction in which it is advertising information. More precisely, a spine node represents two different databases over its adjacencies depending whether it advertises TIEs to the north or to the south/east-west. Those differing TIE databases are called either south- or northbound (South TIEs and North TIEs) depending on the direction of distribution.¶
The North TIEs hold all of the node's adjacencies and local prefixes while the South TIEs hold only all of the node's adjacencies, the default prefix with necessary disaggregated prefixes and local prefixes. Section 4.2.5 explains further details.¶
All TIE types are mostly symmetrical in both diredctions. The (Appendix B.3) defines the TIE types (i.e. the TIETypeType element) and their directionality (i.e. `direction` within the `TIEID` element).¶
As an example illustrating a databases holding both representations, the topology in Figure 2 with the optional link between spine 111 and spine 112 (so that the flooding on an East-West link can be shown) is considered. Unnumbered interfaces are implicitly assumed and for simplicity, the key value elements which may be included in their South TIEs or North TIEs are not shown. First, in Figure 15 are the TIEs generated by some nodes.¶
It may not be obvious here as to why the Node South TIEs contain all the adjacencies of the corresponding node. This will be necessary for algorithms further elaborated on in Section 4.2.3.9 and Section 4.3.7.¶
For Node TIEs to carry more adjacencies than fit into an MTU-sized packet, the element `neighbors` may contain a different set of neighbors in each TIE. Those disjoint sets of neighbors MUST be joined during corresponding computation. Nevertheless, in case across multiple Node TIEs¶
Since the receiving node cannot control the arrival order of TIEs, it is expected that an implementation will use the value of any of the valid TIEs it received.¶
The `miscabled_links` element SHOULD be included in every Node TIE, otherwise the behavior is undefined.¶
A ToF node MUST include all other ToFs it is aware of through reflection. The `same_plane_tofs` element is used to carry this information. To prevent MTU overrun problems, multiple Node TIEs can carry disjointed sets of ToFs which MUST be joined to form a single set.¶
Different TIE types are carried in `TIEElement`. Schema enum `common.TIETypeType` in `TIEID` indicates which elements MUST be present in the `TIEElement`. In case of mismatch the unexpected elements MUST be ignored. In case of lack of expected element in the TIE an error MUST be reported and the TIE MUST be ignored. The element `positive_disaggregation_prefixes` and `positive_external_disaggregation_prefixes` MUST be advertised southbound only and ignored in North TIEs. The element `negative_disaggregation_prefixes` MUST be propagated according to Section 4.2.5.2 southwards towards lower levels to heal pathological upper level partitioning, otherwise traffic loss may occur in multiplane fabrics. It MUST NOT be advertised within a North TIE and ignored otherwise.¶
As described before, TIEs themselves are transported over UDP with the ports indicated in the LIE exchanges and using the destination address on which the LIE adjacency has been formed.¶
TIEs are uniquely identified by the `TIEID` schema element. `TIEID` space is a total order achieved by comparing the elements in sequence defined in the element and comparing each value as an unsigned integer of corresponding length. The `TIEHeader` element contains a `seq_nr` element to distinguish newer versions of same TIE.¶
The TIEHEader can also carry an `origination_time` schema element (for fabrics that utilize precision timing) which contains the absolute timestamp of when the TIE was generated and an `origination_lifetime` to indicate the original lifetime when the TIE was generated. When carried, they can be used for debugging or security purposes (e.g. to prevent lifetime modification attacks).¶
`remaining_lifetime` counts down to 0 from `origination_lifetime`. TIEs with lifetimes differing by less than `lifetime_diff2ignore` MUST be considered EQUAL (if all other fields are equal). This constant MUST be larger than `purge_lifetime` to avoid retransmissions.¶
This normative ordering methodology is described in Figure 16 and MUST be used by all implementations.¶
All valid TIE types are defined in `TIETypeType`. This enum indicates what TIE type the TIE is carrying. In case the value is not known to the receiver, the TIE MUST be re-flooded. This allows for future extensions of the protocol within the same major schema with types opaque to some nodes with some restrictions.¶
On reception of a TIE with an undefined level value in the packet header the node MUST issue a warning and discard the packet.¶
This section specifies the precise, normative flooding mechanism and can be omitted unless the reader is pursuing an implementation of the protocol or looks for a deep understanding of underlying information distribution mechanism.¶
Flooding Procedures are described in terms of the flooding state of an adjacency and resulting operations on it driven by packet arrivals. Implementations MUST implement a behavior that is externally indistinguishable from a verbatim implementation of the FSMs and normative procedures given here.¶
RIFT does not specify any kind of flood rate limiting. To help with adjustement of flooding speeds the encoded packets provide hints to react accordingly to losses or overruns via `you_are_sending_too_quickly` in the `LIEPacket` and `Packet Number` in the security envelope described in Section 4.4.3. Flooding of all corresponding topology exchange elements SHOULD be performed at highest feasible rate whereas the rate of transmission MUST be throttled by reacting to packet elements and adequate features of the system such as e.g. queue lengths or congestion indications in the protocol packets.¶
A node SHOULD NOT send out any topology information elements if the adjacency is not in a "ThreeWay" state. No further tightening of this rule is possible. For example, link buffering may cause both LIEs and TIEs/TIDEs/TIREs to be re-ordered.¶
A node MUST drop any received TIEs/TIDEs/TIREs unless it is in ThreeWay state.¶
TIEs generated by other nodes MUST be re-flooded. TIDEs and TIREs MUST NOT be re-flooded.¶
The structure contains conceptually on each adjacency the following elements. The word collection or queue indicates a set of elements that can be iterated over:¶
Following words are used for well known elements and procedures operating on this structure:¶
The collection SHOULD be served with the following priorities if the system cannot process all the collections in real time:¶
`TIEID` and `TIEHeader` space forms a strict total order (modulo incomparable sequence numbers as explained in Appendix A in the very unlikely event that can occur if a TIE is "stuck" in a part of a network while the originator reboots and reissues TIEs many times to the point its sequence# rolls over and forms incomparable distance to the "stuck" copy) which implies that a comparison relation is possible between two elements. With that it is implicitly possible to compare TIEs, TIEHeaders and TIEIDs to each other whereas the shortest viable key is always implied.¶
When generating and sending TIDEs an implementation SHOULD ensure that enough bandwidth is left to send elements from other queues of `Floodstate` structure.¶
As given by timer constant, periodically generate TIDEs by:¶
while NEXT_TIDE_ID not equal to MAX_TIEID do¶
The constant `TIRDEs_PER_PKT` SHOULD be computed per interface and used by the implementation to limit the amount of TIE headers per TIDE so the sent TIDE PDU does not exceed interface MTU.¶
TIDE PDUs SHOULD be spaced on sending to prevent packet drops.¶
On reception of TIDEs the following processing is performed:¶
for every HEADER in TIDE do¶
if DBTIE not found then¶
if DBTIE.HEADER < HEADER then¶
if DBTIE.HEADER = HEADER then¶
Elements from both TIES_REQ and TIES_ACK MUST be collected and sent out as fast as feasible as TIREs. When sending TIREs with elements from TIES_REQ the `remaining_lifetime` field in `TIEHeaderWithLifeTime` MUST be set to 0 to force reflooding from the neighbor even if the TIEs seem to be same.¶
On reception of TIREs the following processing is performed:¶
On reception of TIEs the following processing is performed:¶
if DBTIE not found then¶
else¶
On a periodic basis all TIEs with lifetime left > 0 MUST be sent out on the adjacency, removed from TIES_TX list and requeued onto TIES_RTX list.¶
The Link State Database can be considered to be a switchboard that does not need any flooding procedures but can be given versions of TIEs by peers. Consecutively, after version tie-breaking by LSDB, a peer receives from the LSDB newest versions of TIEs received by other peers and processes them (without any filtering) just like receiving TIEs from its remote peer. Such a publisher model can be implemented in many ways, either in a single thread of execution of in parallel threads.¶
LSDB can be logically considered as the entity aging out TIEs, i.e. being responsible to discard TIEs that are stored longer than `remaining_lifetime` on their reception.¶
LSDB is also expected to periodically re-originate the node's own TIEs. It is recommended to originate at interval significantly shorter than `default_lifetime` to prevent TIE expiration by other nodes in the network which can lead to instabilities.¶
In a somewhat analogous fashion to link-local, area and domain flooding scopes, RIFT defines several complex "flooding scopes" depending on the direction and type of TIE propagated.¶
Every North TIE is flooded northbound, providing a node at a given level with the complete topology of the Clos or Fat Tree network that is reachable southwards of 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 will be routed directly towards the node advertising that prefix rather than sending the packet to a node at a higher level.¶
A node's Node South TIEs, consisting of all node's adjacencies and prefix South TIEs limited to those related to default IP prefix and disaggregated prefixes, are flooded southbound in order inform nodes one level down of connectivity of the higher level as well as reachability to the rest of the fabric. In order to allow an E-W disconnected node in a given level to receive the South TIEs of other nodes at its level, every *NODE* South TIE is "reflected" northbound to level from which it was received. It should be noted that East-West links are included in South TIE flooding (except at ToF level); those TIEs need to be flooded to satisfy algorithms in Section 4.2.4. In that way nodes at same level can learn about each other without a lower level except in case of leaf level. The precise, normative flooding scopes are given in Table 3. Those rules govern as well what SHOULD be included in TIDEs on the adjacency. Again, East-West flooding scopes are identical to South flooding scopes except in case of ToF East-West links (rings) which are basically performing northbound flooding.¶
Node South TIE "south reflection" allows to support positive disaggregation on failures as described in in Section 4.2.5 and flooding reduction in Section 4.2.3.9.¶
Type / Direction | South | North | East-West |
---|---|---|---|
Node South TIE | flood if level of originator is equal to this node | flood if level of originator is higher than this node | flood only if this node is not ToF |
non-Node South TIE | flood self-originated only | flood only if neighbor is originator of TIE | flood only if self-originated and this node is not ToF |
all North TIEs | never flood | flood always | flood only if this node is ToF |
TIDE | include at least all non-self originated North TIE headers and self-originated South TIE headers and Node South TIEs of nodes at same level | include at least all Node South TIEs and all South TIEs originated by peer and all North TIEs | if this node is ToF then include all North TIEs, otherwise only self-originated TIEs |
TIRE as Request | request all North TIEs and all peer's self-originated TIEs and all Node South TIEs | request all South TIEs | if this node is ToF then apply North scope rules, otherwise South scope rules |
TIRE as Ack | Ack all received TIEs | Ack all received TIEs | Ack all received TIEs |
If the TIDE includes additional TIE headers beside the ones specified, the receiving neighbor must apply the corresponding filter to the received TIDE strictly and MUST NOT request the extra TIE headers that were not allowed by the flooding scope rules in its direction.¶
As an example to illustrate these rules, consider using the topology in Figure 2, with the optional link between spine 111 and spine 112, and the associated TIEs given in Figure 15. The flooding from particular nodes of the TIEs is given in Table 4.¶
Local Node | Neighbor Node | TIEs Flooded from Local to Neighbor Node |
---|---|---|
Leaf111 | Spine 112 | Leaf111 North TIEs, Spine 111 Node South TIE |
Leaf111 | Spine 111 | Leaf111 North TIEs, Spine 112 Node South TIE |
... | ... | ... |
Spine 111 | Leaf111 | Spine 111 South TIEs |
Spine 111 | Leaf112 | Spine 111 South TIEs |
Spine 111 | Spine 112 | Spine 111 South TIEs |
Spine 111 | ToF 21 | Spine 111 North TIEs, Leaf111 North TIEs, Leaf112 North TIEs, ToF 22 Node South TIE |
Spine 111 | ToF 22 | Spine 111 North TIEs, Leaf111 North TIEs, Leaf112 North TIEs, ToF 21 Node South TIE |
... | ... | ... |
ToF 21 | Spine 111 | ToF 21 South TIEs |
ToF 21 | Spine 112 | ToF 21 South TIEs |
ToF 21 | Spine 121 | ToF 21 South TIEs |
ToF 21 | Spine 122 | ToF 21 South TIEs |
... | ... | ... |
RIFT includes an optional ECN (Explicit Congestion Notification) mechanism to prevent "flooding inrush" on restart or bring-up with many southbound neighbors. A node MAY set on its LIEs the corresponding `you_are_sending_too_quickly` flag to indicate to the neighbor that it should temporarily only flood Node TIEs to it and slow down the flooding of any other TIEs. It SHOULD only set it in the southbound direction. The receiving node should accommodate the request to lessen the flooding load on the affected node if south of the sender and should ignore the indication if north of the sender.¶
This mechanism is most useful in the southbound direction. The distribution of Node TIEs guarantees correct behavior of algorithms like disaggregation or default route origination. Furthermore though, the use of this bit presents an inherent trade-off between processing load and convergence speed since suppressing flooding of northbound prefixes from neighbors permanently will lead to traffic loss.¶
The initial exchange of RIFT includes periodic TIDE exchanges that contain description of the link state database and TIREs which perform the function of requesting unknown TIEs as well as confirming reception of flooded TIEs. The content of TIDEs and TIREs is governed by Table 3.¶
When a node exits the network, if "unpurged", residual stale TIEs may exist in the network until their lifetimes expire (which in case of RIFT is by default a rather long period to prevent ongoing re-origination of TIEs in very large topologies). RIFT does however not have a "purging mechanism" in the traditional sense based on sending specialized "purge" packets. In other routing protocols such mechanism has proven to be complex and fragile based on many years of experience. RIFT simply issues a new, i.e. higher sequence number, empty version of the TIE with a short lifetime given by `purge_lifetime` constant and relies on each node to age out and delete such TIE copy independently. Abundant amounts of memory are available today even on low-end platforms and hence keeping those relatively short-lived extra copies for a while is acceptable. The information will age out and in the meantime all computations will deliver correct results if a node leaves the network due to the new information distributed by its adjacent nodes breaking bi-directional connectivity checks in different computations.¶
Once a RIFT node issues a TIE with an ID, it SHOULD preserve the ID as long as feasible (also when the protocol restarts), even if the TIE looses all content. The re-advertisement of empty TIE fulfills the purpose of purging any information advertised in previous versions. The originator is free to not re-originate the corresponding 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 MUST timeout and clean up the corresponding empty TIEs independently.¶
Upon restart a node MUST be prepared to receive TIEs with its own system ID and supersede them with equivalent, newly generated, empty TIEs with a higher sequence number. As above, the lifetime can be relatively short since it only needs to exceed the necessary propagation and processing delay by all the nodes that are within the TIE's flooding scope.¶
TIE sequence numbers are rolled over using the method described in Appendix A. First sequence number of any spontaneously originated TIE (i.e. not originated to override a detected older copy in the network) MUST be a reasonably unpredictable random number in the interval [0, 2^30-1] which will prevent otherwise identical TIE headers to remain "stuck" in the network with content different from TIE originated after reboot. In traditional link-state protocols this is delegated to a 16-bit checksum on packet content. RIFT avoids this design due to the CPU burden presented by computation of such checksums and additional complications tied to the fact that the checksum must be "patched" into the packet after the generation of the content, a difficult proposition in binary hand-crafted formats already and highly incompatible with model-based, serialized formats. The sequence number space is hence consciously chosen to be 64-bits wide to make the occurrence of a TIE with same sequence number but different content as much or even more unlikely than the checksum method. To emulate the "checksum behavior" an implementation could e.g. choose to compute 64-bit checksum over the TIE content and use that as part of the first sequence number after reboot.¶
Under certain conditions nodes issue a default route in their South Prefix TIEs with costs as computed in Section 4.3.7.1.¶
A node X that¶
SHOULD originate in its south prefix TIE such a default route if and only if¶
The term "all other nodes at X's' level" describes obviously just the nodes at the same level in the PoD with a viable lower level (otherwise the Node South TIEs cannot be reflected and the nodes in e.g. PoD 1 and PoD 2 are "invisible" to each other).¶
A node originating a southbound default route SHOULD install a default discard route if it did not compute a default route during N-SPF. This basically means that the top of the fabric will drop traffic for unreachable addresses.¶
RIFT chooses only a subset of northbound nodes to propagate flooding and with that both balances it (to prevent 'hot' flooding links) across the fabric as well as reduces its volume. The solution is based on several principles:¶
In a fully connected Clos Network, this means that a node selects one arbitrary parent as FR and then a second one for redundancy. The computation can be kept relatively simple and completely distributed without any need for synchronization amongst nodes. In a "PoD" structure, where the Level L+2 is partitioned in silos of equivalent grandparents that are only reachable from respective parents, this means treating each silo as a fully connected Clos Network and solve the problem within the silo.¶
In terms of signaling, a node has enough information to select its set of FRs; this information is derived from the node's parents' Node South TIEs, which indicate the parent's reachable northbound adjacencies to its own parents, i.e. the node's grandparents. A node may send a LIE to a northbound neighbor with the optional boolean field `you_are_flood_repeater` set to false, to indicate that the northbound neighbor is not a flood repeater for the node that sent the LIE. In that case the northbound neighbor SHOULD NOT reflood northbound TIEs received from the node that sent the LIE. If the `you_are_flood_repeater` is absent or if `you_are_flood_repeater` is set to true, then the northbound neighbor is a flood repeater for the node that sent the LIE and MUST reflood northbound TIEs received from that node. The element `you_are_flood_repeater` MUST be ignored if received from a northbound adjacency.¶
This specification provides a simple default algorithm that SHOULD be implemented and used by default on every RIFT node.¶
The algorithm consists of the following steps:¶
Derive a 16-bits pseudo-random unsigned integer PR(N) from the resulting 64-bits number by splitting it in 16-bits-long words W1, W2, W3, W4 (where W1 are the least significant 16 bits of the 64-bits number, and W4 are the most significant 16 bits) and then XOR'ing the circularly shifted resulting words together:¶
Partition |A(N) in subarrays |A_k(N) of parents with equivalent cardinality of northbound adjacencies (in other words with equivalent number of grandparents they can reach):¶
/* At this point k is the total number of subarrays, initialized for the shuffling operation below */¶
shuffle individually each subarrays |A_k(N) of cardinality C_k(N) within |A(N) using the Durstenfeld variation of Fisher-Yates algorithm that depends on N's System ID:¶
For each grandparent G, initialize a counter c(G) with the number of its south-bound adjacencies to elected flood repeaters (which is initially zero):¶
Finally keep as FRs only parents that are needed to maintain the number of adjacencies between the FRs and any grandparent G equal or above the redundancy constant R:¶
Additional rules for flooding reduction:¶
First, due to the distributed, asynchronous nature of ZTP, it can create temporary convergence anomalies where nodes at higher levels of the fabric temporarily become lower than where they ultimately belong. Since flooding can begin before ZTP is "finished" and in fact must do so given there is no global termination criteria for the unsychronized ZTP algorithm, information may end up temporarily in wrong layers. A special clause when changing level takes care of that.¶
More difficult is a condition where a node (e.g. a leaf) floods a TIE north towards its grandparent, then its parent reboots, partitioning the grandparent from leaf directly and then the leaf itself reboots. That can leave the grandparent holding the "primary copy" of the leaf's TIE. Normally this condition is resolved easily by the leaf re-originating its TIE with a higher sequence number than it notices in the northbound TIEs, here however, when the parent comes back it won't be able to obtain leaf's North TIE from the grandparent easily and with that the leaf may not issue the TIE with a higher sequence number that can reach the grandparent for a long time. Flooding procedures are extended to deal with the problem by the means of special clauses that override the database of a lower level with headers of newer TIEs received in TIDEs coming from the north. Those headers are then propagated southbound towards the leaf nudging it to originate a higher sequence number of the TIE effectively refreshing it all the way up to ToF.¶
A node has three possible sources of relevant information for reachability computation. A node knows the full topology south of it from the received North Node TIEs or alternately north of it from the South Node TIEs. A node has the set of prefixes with their associated distances and bandwidths from corresponding prefix TIEs.¶
To compute prefix reachability, a node runs conceptually a northbound and a southbound SPF. N-SPF and S-SPF notation denotes here the direction in which the computation front is progressing.¶
Since neither computation can "loop", it is possible to compute non-equal-cost or even k-shortest paths [EPPSTEIN] and "saturate" the fabric to the extent desired. This specification however uses simple, familiar SPF algorithms and concepts as example due to their prevalence in today's routing.¶
For reachability computation purposes RIFT considers all parallel links between two nodes to be of the same cost advertised in `cost` element of `NodeNeighborsTIEElement`. In case the neighbor has multiple parallel links at different cost, the largest distance (highest numerical value) MUST be advertised. Given the range of thrift encodings, `infinite_distance` is defined as largest non-negative `MetricType`. Any link with metric larger than that (i.e. negative MetricType) MUST be ignored in computations. Any link with metric set to `invalid_distance` MUST be ignored in computation as well. In case of a negatively distributed prefix the metric attribute MUST be set to `infinite_distance` by the originator and it MUST be ignored by all nodes during computation except for the purpose of determining transitive propagation and building the corresponding routing table.¶
A prefix can carry the `directly_attached` attribute to indicate that the prefix is directly attached, i.e. should be routed to even if the node is in overload. In case of a negatively distributed prefix this attribute MUST not be included by the originator and it MUST be ignored by all nodes during SPF computation. If a prefix is locally originated the attribute `from_link` can indicate the interface to which the address belongs to. In case of a negatively distributed prefix this attribute MUST NOT be included by the originator and it MUST be ignored by all nodes during computation. A prefix can also carry the `loopback` attribute to indicate the said property.¶
Prefixes are carried in different type of TIEs indicating their type. For same prefix being included in different TIE types according to Section 4.3.1. In case the same prefix is included multiple times in multiple TIEs of same type originating at the same node the resulting behavior is unspecified.¶
N-SPF MUST use exclusively northbound and East-West adjacencies in the computing node's node North TIEs (since if the node is a leaf it may not have generated a Node South TIE) when starting SPF. Observe that N-SPF is really just a one hop variety since Node South TIEs are not re-flooded southbound beyond a single level (or East-West) and with that the computation cannot progress beyond adjacent nodes.¶
Once progressing, the computation uses the next higher level's Node South TIEs to find corresponding adjacencies to verify backlink connectivity. Two unidirectional links MUST be associated together to confirm bi