Kademlia-directed ID-based Routing Architecture (KIRA)
draft-bless-rtgwg-kira-01
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| Document | Type | Active Internet-Draft (individual) | |
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| Author | Roland Bless | ||
| Last updated | 2024-10-19 | ||
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draft-bless-rtgwg-kira-01
Internet Engineering Task Force R. Bless
Internet-Draft Karlsruhe Institute of Technology (KIT)
Intended status: Experimental 19 October 2024
Expires: 22 April 2025
Kademlia-directed ID-based Routing Architecture (KIRA)
draft-bless-rtgwg-kira-01
Abstract
This document describes the Kademlia-directed ID-based Routing
Architecture KIRA. KIRA offers highly scalable zero-touch IPv6
connectivity, i.e., it can connect hundred thousands of routers and
devices in a single network (without requiring any form of hierarchy
like areas). It is self-organizing to achieve a zero-touch solution
that provides resilient (control plane) IPv6 connectivity without
requiring any manual configuration by operators. It works well in
various topologies and is loop-free even during convergence. The
architecture consists of the ID-based network layer routing protocol
R²/Kad in its routing tier and a Path-ID-based forwarding tier. The
topological independent IDs can be embedded into IPv6 addresses, so
that KIRA provides zero-touch IPv6 connectivity between KIRA nodes.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 22 April 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Overview of KIRA . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 6
3.1. Addressing, NodeIDs and the XOR Metric . . . . . . . . . 7
3.2. NodeID Creation . . . . . . . . . . . . . . . . . . . . . 8
3.3. Routing Table . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Node Startup and Vicinity Discovery . . . . . . . . . . . 10
3.5. Join Procedure . . . . . . . . . . . . . . . . . . . . . 11
3.6. Path Discovery . . . . . . . . . . . . . . . . . . . . . 12
3.7. Overhearing of R²/Kad Messages . . . . . . . . . . . . . 13
3.8. Periodic Path Probing . . . . . . . . . . . . . . . . . . 14
3.9. Dynamics: Recovery from Failures . . . . . . . . . . . . 14
3.9.1. Path Rediscovery . . . . . . . . . . . . . . . . . . 15
3.9.2. Ensuring Routing Information Validity . . . . . . . . 17
3.10. Fast Forwarding of CP Traffic . . . . . . . . . . . . . . 18
3.11. Endsystem Mode . . . . . . . . . . . . . . . . . . . . . 21
4. Protocol Specification . . . . . . . . . . . . . . . . . . . 21
4.1. Protocol Message Transport . . . . . . . . . . . . . . . 21
4.2. Protocol Encoding . . . . . . . . . . . . . . . . . . . . 21
4.3. Protocol Message Notation . . . . . . . . . . . . . . . . 22
4.4. R²/Kad Message Format . . . . . . . . . . . . . . . . . . 22
4.4.1. Common Message Header . . . . . . . . . . . . . . . . 22
4.4.2. Protocol Objects . . . . . . . . . . . . . . . . . . 25
4.4.3. R²/Kad Messages . . . . . . . . . . . . . . . . . . . 29
5. Hash Function . . . . . . . . . . . . . . . . . . . . . . . . 43
6. Protocol Parameters . . . . . . . . . . . . . . . . . . . . . 43
6.1. Reserved NodeIDs . . . . . . . . . . . . . . . . . . . . 43
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43
8. Security Considerations . . . . . . . . . . . . . . . . . . . 43
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 43
9.1. Normative References . . . . . . . . . . . . . . . . . . 43
9.2. Informative References . . . . . . . . . . . . . . . . . 44
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 44
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 45
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1. Introduction
KIRA is a scalable zero-touch distributed routing solution that is
tailored to control planes, i.e., in contrast to commonly used
routing protocols like OSPF, ISIS, BGP etc., it prioritizes resilient
connectivity over route efficiency. It scales to 100,000s of nodes
in a single network, it uses ID-based addresses, is zero-touch (i.e.,
it requires no configuration for and after deployment) and is able to
work well in various network topologies. Moreover, it offers a
flexible memory/stretch trade-off per node, shows fast recovery from
link or node failures, and is loop-free, even during convergence.
Additionally, it includes a built-in Distributed Hash Table (DHT)
that can be used for simple name service registration and resolution,
thereby helping to realize autonomic network management and control
and zero-touch deployments.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Overview of KIRA
KIRA's main objective is to provide self-organized robust control
plane (CP) connectivity on top of a link-layer topology. The CP is
typically used to configure, monitor, manage, and control networked
resources (switches, routers, end-systems). The goal is to never
lose control over the resources as long as there exist paths leading
to them. KIRA is structured into a two-tier architecture consisting
of a Routing Tier and a Forwarding Tier (see Figure 1. KIRA runs the
zero-touch, distributed, highly scalable, ID-based routing protocol
R²/Kad in the Routing Tier to find viable paths to destinations. The
core concept of R²/Kad is that it discovers paths in the underlying
topology by using an ID-based overlay routing scheme combined with
source routing between overlay hops. KIRA nodes employ this
information to construct fast path forwarding tables in the
Forwarding Tier for CP data traffic (e.g., packets from control plane
applications). While source routing is robust (since it does not
require converged routes) it can cause significant overhead,
especially for small payloads. The Forwarding Tier avoids this
overhead for CP data traffic by using PathIDs as forwarding state
instead of the source routes and a scheme that is similar to label
switching. However, existing forwarding support for IPv6 in hardware
can be used, so KIRA does not require specialized new forwarding
mechanisms.
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R²/Kad employs a flat ID-based addressing scheme to easily support
zero-touch operation, self-organization as well as mobility and
multi-homing. ID-based routing (sometimes also denoted as name-
independent routing or routing with flat identifiers) has the
advantage of providing stable addresses (called NodeIDs) to upper
layers. Thus, in case (virtual) resources are moved within the
topology, any control connection to them stays alive. In contrast to
other ID-based addressing approaches, KIRA is a genuine ID-based
approach, because it does not use topological addresses at all and
thus does not require any additional identifier-locator mapping
(increased risk of non-consistency) and associated protocols
(additional overhead and convergence time).
As just motivated, R²/Kad uses topologically independent NodeIDs,
generated by the KIRA nodes themselves, so address assignment is
performed in a distributed manner by each node autonomously.
Typically, NodeIDs are taken from a 112 bit address space, but
depending on the network size, smaller NodeIDs are possible. KIRA
uses IPv6 packets [RFC8200] for its messages and CP data packets,
because NodeIDs can be easily embedded into an IPv6 address (e.g., 16
bit prefix + 112 bit NodeID) and existing hardware-based and
software-based forwarding mechanisms can be leveraged. Mainly very
basic IPv6 features like link-local addresses, the packet format,
fragmentation, and neighbor discovery are used, e.g., it does not
require any address configuration features or router discovery.
The _Forwarding Tier_ is able to forward IPv6 packets, so that
(control) applications can use IPv6 and all corresponding transport
protocols above. R²/Kad messages use source routing based on NodeIDs
whereas traffic in the Forwarding Tier uses NodeIDs and PathIDs for
its forwarding decision. PathIDs are conceptually a hash from a
sequence of NodeIDs that build a path segment. PathIDs are unique
(with high probability) for a path segment. To carry PathIDs in
addition to the final destination NodeID, the original IPv6 packet
becomes encapsulated, e.g., using a GRE header that contains the
PathID for the path segment that the packet should traverse next.
Intermediate nodes simply exchange the PathID at each hop with the
PathID of the remaining path segment (similar to label switching), or
they strip the outer header when the next node is the end of a path
segment. PathIDs are precomputed in a 2-hop vicinity of a KIRA node
and are installed by R²/Kad signaling in some intermediate nodes on
demand for paths longer than five hops. To forward CP packets KIRA
nodes only need to perform lookups in their NodeID forwarding table
and/or PathID forwarding table, perform encapsulation or
decapsulation and rewrite PathIDs.
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Routing Tier
┌──────────────────────────────────────────────────┐ ┌─────────────┐
│R²/Kad │ │ Control │
│ ┌───────────────────┐ ┌───────────┐ ┌──────────┐ │ │ Plane │
│ │- Path Discovery │ │ Routing │ │Path │ │ │ Applications│
│ │- Routing │ │ Table │ │Management│ │ │ │
│ │- Failure Recovery │ │ │ │ │ │ │ │
│ └───────────────────┘ └───────────┘ └────┬─────┘ │ │ │
│ │ │ │ │
└──────▲───────────────────────────────────┼───────┘ └─────▲───────┘
│ │ │Trans-
│ ┌────────────┬──────┘ │port
R²/Kad │ Forwarding │ │ │over
Msgs │ Tier │ │ │IPv6
│ ┌─┬─────────▼─┬──┬───────▼─────┬─┬─────────┬─┬──▼───────┬─┐
│ │ │NodeID │ │PathID │ │Encaps./ │ │Node Local│ │
│ │ │Forwarding │ │Forwarding │ │Decaps. │ │Forwarding│ │
│ │ │Table │ │Table │ │ │ │ │ │
│ │ └───────────┘ └─────────────┘ └─────────┘ └─▲────────┘ │
│ │ │ │
│ │ ┌─────────────────────────────────┴─┐ │
│ │ ┌─►│ Fast Forwarding ├──┐ │
│ └─────────┼──┴───────────────────────────────────┴──┼─────┘
▼ IPv6 ──┘ └─►IPv6
UDP+IPv6
Figure 1: KIRA-Architecture
CP applications (e.g., SDN controllers, Kubernetes Cluster
Controllers, Virtual Infrastructure Manager, traditional OAM
applications and so on) simply use NodeIDs as addresses for the
resources/devices they want to control or for other controllers they
want to exchange state with. Therefore, CP applications can
transparently use the connectivity established by KIRA. Since
NodeIDs are randomly generated, KIRA provides a simple built-in key/
value store (Distributed Hash Table – DHT) that can be used as name
service. KIRA nodes and services can dynamically register their
NodeID under a certain well-known name and other KIRA nodes can
lookup their corresponding NodeIDs. The DHT functionality will be
specified as a separate KIRA module and corresponding application
interfaces are out-of-scope for this specification.
KIRA nodes possess relatively small routing tables, that grow with
O(log(n)), where n is the number of KIRA nodes in the network (see
[KIRA-Networking-2022] for evaluation results). The advantage of
small routing tables is scalability, but comes at the cost of path
stretch. That is, packets to destinations that are not kept in the
routing table of a node take a longer path than the shortest possible
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path, because they are using the ID-based overlay routing strategy.
However, KIRA nodes will learn the shortest paths to all 'contacts'
in their routing tables and it is a node local decision how large the
routing table can be. For example, a controller node may add all
KIRA nodes that it controls as contacts to its routing table.
Because KIRA uses source routing in R²/Kad and PathID-based
forwarding in its forwarding tier, it can easily support multi-path
routing and keeping backup paths for fast failover reactions.
KIRA uses a mixture of reactive and periodic mechanisms to cope with
link and node failures. Error messages that indicate failed links
usually trigger routing updates and a path rediscovery procedure.
However, routing updates are not flooded to all KIRA nodes, so some
nodes may still have obsolete path information. These
inconsistencies will be detected either when using the obsolete path
to a contact (triggering an error message from the node before the
broken link) or by a maintenance procedure that is carried out
periodically. These periodic maintenance procedures test the
validity of the currently known paths and may also trigger a
rediscovery procedure to find alternative paths.
Moreover, KIRA also possesses a specific end-system mode, where KIRA
nodes are part of the KIRA network, but they are not exchanging
routing information and are not forwarding packets for other KIRA
nodes.
Finally, KIRA also supports a domain concept. A KIRA node may be
member of one or multiple domains. Unless configured otherwise, a
KIRA node is member of the global domain with DomainID=0 by default.
KIRA nodes keep their NodeID for all domains, the only difference is
that routes are guaranteed to run inside a domain D in case source
and destination node are both members of this domain D. This allows
for using domains for administrative purposes (e.g., all KIRA nodes
inside the same Autonomous System could be part of the same domain)
or to use domains to build clusters of KIRA nodes that are grouped by
closeness in the underlying network topology.
3. Protocol Operation
This section gives on overview of the main concepts with respect to
the R²/Kad protocol operation. First KIRA's ID-wise addressing
concept is introduced, then the routing table structure is presented.
After that several procedures are described, beginning with node
startup, vicinity discovery, and the join procedure to populate the
routing table, followed by path discovery, overhearing and
rediscovery mechanisms.
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3.1. Addressing, NodeIDs and the XOR Metric
Every KIRA instance uses a single NodeID as its address. The NodeID
is taken from a larger unstructured address space [0..2^B-1]
(typically B=112). KIRA uses the XOR (logical exclusive or) metric
in this address space to define the distance between two NodeIDs X
and Y (see also [Kademlia2002]). The distance function d(X,Y)= X XOR
Y is interpreted as integer and fulfills all properties of a metric
(d(X,Y)>=0, and d(X,Y)=0 <=> X=Y; d(X,Y)=d(X,Y); as well as the
triangle inequality d(X,Y)<=d(X,Z)+d(Z,Y)). This distance function
largely corresponds to a prefix bit distance metric d_p(X,Y) =
B-lcp(X,Y), where lcp(X,Y) denotes the length longest common prefix
in bits. The XOR metric is finer than the d_p(X,Y) metric though,
because when there are X,Y, and Z with d_p(X,Y)=d_p(Y,Z)=d_p(X,Z),
the XOR metric can uniquely determine whether Y or Z are closer to X.
More generally speaking, for a given distance d(X,Y)=d there exists
exactly one Y so that d(X,Y)=d. This property is important since it
allows to unambiguously determine which NodeID is ID-wise closer to a
given other NodeID and it provides the basis for KIRA's loop-freedom.
Note that the ID space with this metric is not cyclic (i.e., a node
with a very small NodeID is not close to a node with a very large
NodeID).
The XOR metric defines an overlay structure across all KIRA nodes in
the ID space: KIRA nodes establish logical connections with their ID-
wise closest overlay neighbors (which are typically different from
the underlay neighbors) with respect to the XOR metric as distance
metric, i.e., the ID-wise closer neighbors have a smaller distance
according to XOR. KIRA uses this metric to determine the next KIRA
node that a KIRA message is forwarded to in order to reach a certain
destination NodeID. R²/Kad messages are forwarded by using source
routing between overlay hops.
In addition to NodeIDs, KIRA can use any ID from the ID space as
destination address. Typically, names of objects can be hashed to
result in a key value, which is called Resource ID. In this case,
the ID-wise closest KIRA node will be found as responsible node for
storing a value (or a referral) for this key. This makes it possible
to provide an integrated DHT for name-to-NodeID registration and
lookup.
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3.2. NodeID Creation
NodeIDs are randomly generated and are taken from a 112 bit address
space. Reserved NodeIDs Section 6.1 MUST NOT be used as NodeID of a
KIRA node. Future versions of this specification will detail an
algorithm to create self-certifying NodeIDs as using certain hash
functions from a public key. NodeIDs are unique with high
probability. However, in case two nodes possess the same NodeID,
protocol mechanisms can be used to detect this situation and the
conflict can be resolved by letting one side generate a new NodeID.
Depending on a KIRA node's capabilities, NodeIDs (together with other
protocol parameters) may be stored in non-volatile memory so that
nodes keep their NodeID even after restart. Other KIRA nodes may
choose to generate a new NodeID on every restart.
3.3. Routing Table
The entries in the routing table (RT) are called 'contacts'. Contact
data contains the NodeID of the contact as well a set of discovered
paths that lead to this contact (besides other node state and
attributes). A path to a contact is stored as path vector that
contains a complete sequence of NodeIDs, which can be traversed to
reach the contact. Except for contacts in its routing table, a KIRA
node does not know paths to other destinations, but they can be
discovered by using a recursive overlay routing strategy: a KIRA node
source routes a packet to the contact (using the known path) that is
the ID-wise closest to the destination ID according to its routing
table. The next overlay hop performs the same action until the
destination node is reached. After the initial discovery phase, only
Underlay Neighbors (ULNs) and some contacts from the 3-hop underlay
neighborhood are stored in the routing table. Underlay Neighbors are
nodes attached to the links of the KIRA node (i.e., neighbors in the
sense of [RFC8200] that are directly reachable via link layer and the
Internet-layer or higher-layer tunnels).
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R²/Kad's efficiency and flexibility is closely related to its routing
table. It is structured as tree of k-buckets as in [Kademlia2002].
A k-bucket in the routing table contains a list of (at most) k
contacts in distance between 2^i and 2^{i+1} (i.e., the bucket's
range, where 0<=i<112) from this node. Usually, k>=20 is constant
and the same for all buckets and nodes, but it can also be varied per
node (k=40 is RECOMMENDED as default for R²/Kad). Buckets at deeper
levels share more prefix bits with the node's own ID, however,
buckets for small values of i are generally empty as no appropriate
nodes exist in this address space. Thus, the highest bucket (depth
1) contains contacts from half of the ID space whose highest NodeID
bit differs from the node's ID, whereas the deepest buckets contain
all nodes that are ID-wise closest to the node (i.e., the ID-wise
closest overlay neighbors).
If KIRA node X learns a new contact Y, it puts it into the
corresponding k-bucket b_l in case it still has capacity left. The
bucket index l is determined by calculating the common prefix length
between X and Y (number of high-order zero bits of d(X,Y)). If the
bucket contains k entries already, it is split into two new buckets
(and the contained entries moved to them accordingly) in case X falls
into the bucket's range. Otherwise, a selection algorithm determines
whether the new contact should replace an existing entry in this
bucket. In our case we use Proximity Neighbor Selection (PNS) so
that contacts with shorter path lengths are preferred. In case path
lengths are equal, nodes with a higher degree are preferred as this
results in shorter paths. An additional mechanism for path selection
improves path diversity and prevents route flapping: in case an
alternative path of equal length has been discovered for an already
known contact, this path replaces the previous path only if the hash
sum of the path elements is closer to the node's own NodeID. Due to
the uniqueness property of the XOR metric, the path selection will
always unambiguously converge to a unique path. Underlay neighbors
are kept in special buckets that have no capacity limit, i.e., they
will never be preempted. In general, routing also works without this
ULN buckets extension of [Kademlia2002], but the resulting stretch
will be slightly higher.
X identifies its closest known contact in its RT by locating the
k-bucket that corresponds to the longest matching prefix of
destination Z with its own NodeID X by using d(X,Z). It then selects
a contact with the shortest path vector from within the bucket; this
is called Proximity Routing (PR). The XOR metric is used to uniquely
select the ID-wise closest contact if all paths have equal length or
if destination Z falls into the node's deepest bucket.
Each contact typically contains the following information:
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* _NodeID_ of the contact.
* _State_: the state that the contact is considered to be in. It is
one of _undefined_, _valid_, _rediscovering_, _invalid_, _dead_.
* A set of path vectors, i.e., a complete sequence of NodeIDs that
lead to the contact when traversed. There is an _active_ path
(used as default), a _proposed_ path and possibly several backup
or alternative paths. Paths possess also timestamps for points in
time when they were last validated and when they were refreshed
(using PathSetupRsp or ProbeRsp, see also Section 3.10).
* _Sequence Number_: a sequence number that is determined by the
contact and that changes when the underlay connectivity state
changes at the contact, i.e., with newly detected or failed
underlay neighbors.
* _LastSeen_: time of last direct contact
* _Sync ULN Sequence Number_: a sequence number used for state
synchronization with ULNs.
* _UnderlayNeighbor_: a bool value indicating that this contact is
an underlay neighbor
* _NodeDegree_: the contact's number of underlay neighbors.
* _Rediscovery State_: temporary state keeping track of the
rediscovery process.
3.4. Node Startup and Vicinity Discovery
KIRA nodes generate their NodeID first (see Section 3.2). After that
they start an initial discovery phase to explore their physical
vicinity. After that, a join procedure and continuous discovery are
periodically repeated. To let the node stay connected to its overlay
and to improve the quality of discovered routes.
In its initial discovery phase, a KIRA node discovers its underlay
adjacencies, i.e., its underlay neighbors (ULNs) that can be directly
reached via link-local communication on one of their network
interfaces. KIRA nodes periodically send ULNHello messages to a
well-known link local multicast address ALL-KIRA-NODES, and receiving
nodes may reply with a ULNDiscoveryReq to set up an adjacency. This
ULNDiscoveryReq MUST be answered by a ULNDiscoveryRsp to establish an
adjacency. The protocol exchange ensures that bidirectional
communication is possible between directly adjacent nodes. ULNs may
be put into the _ULNTable_ either after receiving a ULNDiscoveryReq
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as answer to a transmitted ULNHello or after receiving a
ULNDiscoveryRsp as answer to a transmitted ULNDiscoveryReq. The
ULNTable contains a mapping from NodeID to the link local unicast
address of the ULN. This address is either taken from the source
address of the ULNDiscoveryReq or the ULNDiscoveryRsp respectively.
KIRA nodes also discover all nodes in their 3-hop physical
neighborhood to populate their routing table, but the 2-hop vicinity
is fully stored in a local graph structure ("vicinity graph"). The
latter is used to precompute PathIDs for the 2-hop vicinity.
ULNDiscoveryReq and ULNDiscoveryRsp messages contain a list of
underlay neighbors so that all ULNs of ULNs will be learned, i.e.,
the 2-hop vicinity. However, possibly not all nodes in the 2-hop
vicinity will be stored in the RT, therefore the vicinity graph
structure is used to keep track of the 2-hop vicinity (new nodes and
links). Especially, the state sequence numbers of all the nodes
should be stored in the vicinity graph as well. All nodes in the
2-hop vicinity are queried for their ULNs by using a QueryRouteReq.
QueryRouteReq/QueryRouteRsp messages are used to get ULNs or
RTable objects from nodes in the vicinity. Depending on their NodeID
and other criteria (e.g., NodeDegree), nodes from the 3-hop vicinity
will be stored as contacts into the routing table. The vicinity
graph structure is used to determine whether a new QueryRouteReq must
be initiated according to the State Sequence Number.
A KIRA node continues to populate its routing table by sending
FindNodeReq messages to certain nodes and also to randomly chosen
destinations (i.e., a randomly chosen ID from the NodeID space, a
KIRA node does not need to exist for the chosen ID, the FindNodeReq
will simply end at the KIRA node that is ID-wise closest to the
destination ID). FindNodeRsp messages return a set of contacts that
are ID-wise closest to the destination NodeID from the viewpoint of
the responding KIRA node. The returned information is analyzed
whether it can improve the local routing table, e.g., new and
'better' contacts or 'better' paths to already known contacts.
3.5. Join Procedure
As result of the vicinity discovery, all ULNs of X and some nodes
within its 3-hop radius will populate X’s RT. However, in order to
get network connectivity and to contribute to connectivity, the node
needs to find its ID-wise closest overlay neighbors and make itself
known to them. Thus, to join the network KIRA node X simply
"searches" for the k closest nodes to its own NodeID: X sends a
FindNodeReq for its own NodeID X and the closest neighbor replies
with FindNodeRsp. This is repeated with a limited exponential
backoff in order to detect or heal any network partitioning. In case
node X finds itself in situations where it needs to respond with
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"Dead End" Error messages to FindNodeReqs, it resets the backoff
timer again, because it may be a hint for network partitioning or
other inconsistencies. In order to let a joining node X quickly
learn all existing ID-wise closest overlay neighbors, X sends a
QueryRouteReq to every newly learned contact that enters X’s
currently deepest k-bucket. The queried contact replies with a
QueryRouteRsp returning its RT entries for the k closest contacts to
node X. The so returned contacts will very likely also fall into X’s
deepest bucket, possibly leading to a further split of its deepest
bucket. Therefore, X will quickly populate its set of ID-wise
closest overlay neighbors, which are needed for consistent overlay
connectivity.
3.6. Path Discovery
Consider the example topology in Figure 2. Assume node X needs to
send a message to node Z. In case Z is a known contact of X, a path
vector is stored already in the routing table that can be used for
strict source routing in order to reach Z. Otherwise, a path to Z
must be discovered using ID-based overlay routing. R²/Kad uses a
recursive version of Kademlia (hence its name Routing with Recursive
Kademlia – R²/Kad). The Path Discovery procedure uses a request/
response message pair, FindNodeReq/FindNodeRsp. In this example,
assume that X identifies its contact Y (learned from ULN A) as next
(ID-wise closest) overlay hop toward Z. In order to discover a path
to Z, X creates a FindNodeReq message that contains destination
NodeID Z and source route r=⟨X, A, Y⟩ using path vector ⟨A⟩ of
contact Y. The FindNodeReq is forwarded along r (strict source
route). Message forwarding between overlay neighbors requires source
routing, because the path in the underlay may lead via nodes that are
(ID-wise) further away from the destination (e.g., Y routes via ⟨A,
Q, M⟩ to Z in Figure 2): using the ID-based overlay routing scheme on
a hop-by-hop basis (i.e., between directly adjacent nodes in the
underlay) would inevitably lead to forwarding loops in most cases.
Y
/
X--A--Q--M--Z
\ /
B----/
Figure 2: An example topology. Letters resemble NodeIDs.
Letters closer in the alphabet have smaller distance in ID space.
When the FindNodeReq arrives at Y, the same procedure is repeated
(since it is a recursive variant of Kademlia). Node Y tries to find
a contact closer to Z than Y itself. If this contact exists, source
route r is appended by the corresponding path vector and the
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FindNodeReq is forwarded to this contact. In the given example of
Figure 2), we assume that Y knows Z as its contact with path vector
⟨A, Q, M⟩. It extends source route r of the FindNodeReq by ⟨A, Q, M,
Z⟩ and forwards it to A as next hop in the source route. If routing
information has been converged, this ID-based routing scheme
guarantees progress in the ID space [Kademlia2002] during forwarding
and eventually finds node Z.
The destination node Z responds with a FindNodeRsp message along the
reversed source path with any cycles removed (⟨Z, M, Q, A, X⟩). Due
to XOR’s symmetry, the responding node Z also learns the new contact
X as neighbor. The FindNodeRsp returned to X not only provides a
path to Z, but also a list of k closest contacts to Z together with
their path vectors. This list is used to improve X’s routing table.
In case that the FindNodeReq arrives at Y and it cannot find a
contact closer to Z, the FindNodeReq is terminated at Y and a
response is sent back depending on the "ExactFlag" Section 4.4.1 in
the FindNodeReq. If the ExactFlag was not set, Y sends a FindNodeRsp
back to originator X that contains an RT excerpt of Y’s (at most) k
closest contacts to Z in a so called RTable object. This enables
finding the responsible node for a destination ID Z if used as object
key. The latter allows for so called key-based routing that is used
to realize DHTs. If exact was set, X assumed that a node with ID Z
must exist, but the current node is the ID-wise closest node to Z and
does not know Z as contact. Consequently, the node cannot forward
the FindNodeReq closer to Z and returns a "Dead End" Error message
(which may happen occasionally during convergence). In case source
route r contains a broken link or unreachable node, a "Segment
Failure" Error message will be sent back to X along the reversed
source route.
3.7. Overhearing of R²/Kad Messages
KIRA nodes use overhearing mechanisms for R²/Kad messages. This is
an important mechanism for KIRA to learn new contacts and better or
improved paths.
Nodes that forward R²/Kad messages SHOULD use the contained source
route to improve their own routing information: they may learn new
contacts or shortcut routes to known contacts. However, only the so
far traversed path is considered as it can be assumed that all
traversed links worked recently. Additionally, NotViaList
information (see Section 4.4.2.3) is used to invalidate contacts that
have an active path vector containing a link from the NotViaList.
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The source routing path of incoming requests and responses is also
considered for improving the RT. Some messages like FindNodeRsp,
QueryRouteRsp or UpdateRouteReq contain RTable objects that are
evaluated likewise.
Bypassing QueryRouteRsp messages contain RTable objects (as requested
by QueryRouteReq) and are inspected for interesting contacts and
paths.
3.8. Periodic Path Probing
_Periodic Path Probing_ aims at reliably detecting any RT
inconsistencies (e.g., seemingly valid contacts with paths that
contain recently failed links). Each node periodically checks the
path validity for all of its contacts by sending a ProbeReq message
to them. ID-wise closest neighbors are probed more often than other
contacts and those recently contacted (≤ 2s) are not probed. In case
a path has a link or node failure, the ProbeReq will elicit a
"Segment Failure" Error message from an intermediate node along the
broken path, notifying about the failed link. The contact’s state
will be set to _invalid_ and a rediscovery process is scheduled (see
Section 3.9.1).
3.9. Dynamics: Recovery from Failures
In order to improve R²/Kad’s robustness against link or node failures
we introduce a recovery procedure that notifies about failures and
actively tries to find alternative paths that route around the
failure. This procedure is highly robust and achieves a fast
convergence. R²/Kad nodes detect link and node failures of ULNs by
link layer notifications, missing ULNHello or ULNDiscoveryRsp
messages as well as "Segment Failure" errors anytime during
forwarding along source routes. To recover from such failures, R²/
Kad’s recovery procedure uses the following mechanisms:
* Notify own nearest overlay neighbors about failed links or
unreachable nodes ("bad news") by sending UpdateRouteReqs via a
non-impacted physical link.
* Rediscover a feasible alternative route to the affected node using
FindNodeReqs. These carry NotViaList information about failed
links that must not be considered for routing. Rediscovery is not
performed for nodes that lost their only link, which can be
deduced by the node’s degree information that is conveyed in R²/
Kad messages.
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* Per contact _state sequence numbers_ avoid using obsolete
information for path rediscovery. Additionally, an _aging_
mechanism is used to avoid dissemination of obsolete routing
information. It uses time periods to assess the currentness of
the related path.
* Overhearing of NotViaList information and UpdateRouteReqs about
failed links during forwarding R²/Kad messages informs nodes about
failed links, which initiates a path rediscovery. Overhearing is
also used to update obsolete path information.
* When an alternative path has been found for a prior affected
contact or a link comes back up again, an UpdateRouteReq is sent
to own ID-wise closest overlay neighbors for disseminating the
"good news".
The ID-based overlay routing scheme is used for rediscovery of a
route, because NodeIDs are randomly distributed all over the
underlying topology. Therefore, a rediscovery uses different paths
that are likely not affected by the failure. However, if overlay
nodes still have obsolete routing information, i.e., they would
normally route via the failed link, they can detect the need to
update their routes as well by seeing the more current NotViaList
information.
3.9.1. Path Rediscovery
A node X that detects its ULN B (cf. Figure 2) or the corresponding
link ⟨X, B⟩ has failed, reacts as follows (unless isolated by that
failure):
1. Set the state of the corresponding contact to _invalid_ (in
Figure 2 contact B). Invalid contacts will temporarily not be
considered for routing.
2. Set the state of all contacts whose paths contain the failed link
⟨X, B⟩ to invalid (in Figure 2 contact Z with ⟨B, M, Z⟩ becomes
invalid).
3. Send UpdateRouteReq messages indicating the failure to four of
its ID-wise nearest neighbors (e.g., Y and Z in Figure 2) via
non-affected contacts. The UpdateRouteReq will also carry a
NotViaList that contains ⟨X, B⟩.
4. Trigger a rediscovery process (described below) for B (sets state
to rediscovery) and for other invalid contacts.
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5. If the rediscovery process is successful for a contact, its state
is set to _valid_ and UpdateRouteReq messages are sent to notify
ID-wise closest neighbors about the change.
Since UpdateRouteReqs have notification character only, they do not
create any responses (even no error messages if dropped). The
rediscovery process simply sends a FindNodeReq for all invalid
contacts (all invalid contacts will be ignored in finding the next
hop). This FindNodeReq for rediscovery (also denoted as rediscovery
message) contains a set ExactFlag and the failed link ⟨X, B⟩ as
additional NotViaList information. It is sent to X’s currently known
ID-wise closest neighbors of the invalid contact (e.g., A in the
example), which will then try to forward the FindNodeReq further
toward the failed contact. The NotViaList information avoids that
nodes use obsolete routing information when forwarding the
rediscovery message, i.e., paths that contain the failed link will
not be used for forwarding. Node A may not have heard yet about the
broken link and thus will invalidate contact B if its prior preferred
path is via ⟨X, B⟩. In order to ensure that only current NotViaList
information is considered, every link contained in the NotViaList is
also accompanied by a related age value ∆T, specifying in
milliseconds how long ago the sender heard about the failed link. In
case a FindNodeRsp is returned by B, a valid path has been discovered
and the contact’s state is set to valid (triggering subsequent
UpdateRouteReqs with the new path as mentioned before).
Nodes receiving UpdateRouteReqs or FindNodeReqs containing the failed
link also set their corresponding affected contacts to invalid and
trigger a rediscovery process of the routes (like Z in the example).
The actual rediscovery messages are sent after different randomly
chosen waiting times from an interval [0.5t_p, 1.5t_p]. The mean
value t_p is set as follows: for invalidated ULNs 100ms, affected ID-
wise near contacts (in the deepest buckets) 500ms, for contacts
affected by the failure of a link to a ULN 1s and for all other
affected contacts 2s. Rediscovery messages are sent simultaneously
to two different (overlay) neighbors of the affected contact at a
time, until k neighbors have been tried unsuccessfully to rediscover
a path to the currently invalid contact. In the latter case, a new
round of rediscovery attempts will be initiated with exponential
backoff until a certain limit of retry rounds (default: 6) have been
made without any success, after which the contact will be deleted.
Although there is no guarantee that a viable alternative route can be
found, our simulation results show that connectivity is very quickly
restored after a failure even in drastic failure scenarios (i.e.,
where a larger part of links, such as 15%, fail simultaneously and
randomly).
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Node B at the other end of the failed link ⟨X, B⟩ also tries to
rediscover X and thus sends an UpdateRouteReq to its ID-wise closest
overlay neighbors (e.g., A). Thereby, it may inform A as well as X
about a new alternative route via M.
3.9.2. Ensuring Routing Information Validity
R²/Kad uses state sequence numbers and aging to prevent obsolete
routing information from spreading or settling. Messages carry
routing information in an RTable object that contains a list of
contacts n_j , and for each contact n_j the corresponding path vector
p_j leading from the reporting node to the contact, its state
sequence number s_j and the age ∆T_j of this information. The
currentness of contact information can always be assessed by s_j.
However, s_j alone does not suffice to assess the currentness of the
associated path to this contact as intermediate links may have been
failed/repaired. Therefore, each reported path, as well as NotVia
links, carry an associated age value ∆T_j ≥ 0 that corresponds to the
time period when the path information was updated last at the
originating node. This avoids spreading and wrongfully accepting
obsolete routing information.
The age for physical links of a node is always 0ms, because related
information is always current at this node. A node simply sets a
"last modification" timestamp t_j for the contact n_j to t_j := t_now
− ∆T_j and reports n_j’s age as t_now-t_j in messages with
RTable objects (e.g., UpdateRouteReq, FindNodeRsp, QueryRouteRsp). A
contact’s timestamp t_j is also updated by messages that allow to
infer that the traversed path is current, e.g., incoming ProbeReq,
ProbeRsp , FindNodeRsp, QueryRouteReq, and QueryRouteRsp messages. A
path is updated only if the contact’s state sequence number is larger
than the prior known sequence number for this contact, or, in case of
equal sequence numbers, the received path information must be more
recent when comparing their age values. Since age values are
relative, they can be compared even if they stem from different
nodes, i.e., synchronized clocks are not required.
Finally, path information that originates from Source Route Objects
is considered to be validated, because the path was traversed by the
corresponding message recently. Path information taken from
RTable Objects or that was improved with local RT information is
considered to be not validated. A not validated path should not
overwrite an active valid path, because it may nevertheless include
broken links that the node is not aware of yet. Therefore, a not
validated path that is considered to be "better" (e.g., "shorter")
than the current active path is stored as proposed path needs to be
probed. A ProbeReq for the proposed path should be scheduled. Newly
arriving not validated path information for the same contact should
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be compared against this proposed path. In case the path probing of
the proposed path is successful, the Source Route Object of the
ProbeRsp will automatically update the active valid path and the
proposed path should be dismissed (unless newer than the ProbeReq,
but then another path probing is scheduled).
The previously described mechanisms cannot guarantee notification of
all affected nodes about link failures in their path vectors. In
order to reliably detect such inconsistencies, each node periodically
probes the paths to all its contacts as described in Section 3.8.
Nevertheless, if a node tries to use an obsolete path with a failed
link, a viable path will be rediscovered immediately after receipt of
the Error message from the node before the broken link.
3.10. Fast Forwarding of CP Traffic
A potential drawback of R²/Kad is its use of source routing to
forward between two overlay hops. Handling a (potentially long) list
of source routing hops is currently not as efficiently realized as
regular destination-based routing. Moreover, source routing
increases per-packet overhead. To forward data packets more
efficiently, the Forwarding Tier (see Figure 1) leverages an approach
similar to label switching, whereas the Routing Tier still uses
source routing for R²/Kad messages to remove cycles, detect
shortcuts, and so on. Every source routing path (that consists of
NodeIDs) to a contact is represented by up to two PathIDs that
correspond to _path segments_. A PathID is a hash value of all
NodeIDs along the corresponding path segment, e.g., PathID(⟨A, Q, M,
Z⟩)=H(A|Q|M|Z). It serves as unique label for the path segment. The
uniqueness is an important distinction from common label switching
approaches where nodes assign labels of node local scope. It enables
KIRA nodes to distributedly compute a set of PathIDs in advance.
This avoids explicit path setup signaling for PathID installation in
many cases. Only for paths longer than 5 hops PathID mappings have
to be installed in some intermediate nodes. Another feature of
PathIDs is their automatic aggregation toward a sink, i.e., paths
that merge in a certain node and use the same residual path to a
destination use the same PathID. The Forwarding Tier uses IPv6 GRE
[RFC7676] to carry PathIDs in addition to source and destination
NodeIDs (other encapsulation methods, e.g., using segment routing
[RFC8754] are possible and can be defined later).
In detail, KIRA implements fast forwarding as follows:
* All paths of length ≤2 for a node’s full 2-hop vicinity are
discovered (as described in Section 3.4) and are then used for the
precomputation of incoming and outgoing PathIDs, i.e.,
irrespective of their actual use. Longer paths to contacts are
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split into two path segments as follows: paths of lengths 4 and 5
hops have a second segment of 2 hops length and a first segment of
length 2 or 3 hops respectively. Paths of length L≥6 hops have a
second segment of 3 hops and a first segment of variable length
L-3.
* The node creates forwarding table entries in the form of Incoming
PathID -> (Outgoing PathID, Next Hop). The source route for the
incoming PathID includes the own NodeID, whereas it is stripped
off for computing the outgoing PathID. When forwarding to the
last node of a path segment, the outgoing PathID is omitted.
* A node that wants to send a data packet (see example in Figure 3),
sets the outgoing PathIDs of the source route path segments as
destination addresses of the outer encapsulation headers (X uses
H(A|Q|M|Z) as first segment and H(Z|C|E|T) as second segment in
Figure 3) and sends it to its ULN. Source routes of less than 4
hops in length require only one PathID, the other two PathIDs.
The source address of the outer header is set to the sender’s
NodeID so that errors can be reported back. The destination
address of the inner most header is the destination NodeID.
* A node that receives a packet containing an incoming PathID tries
to match it in its forwarding table. If it finds an entry, it
rewrites the PathID with an outgoing PathID or removes the
outermost PathID header in case the path segment ends at the next
node. Including the own NodeID into the incoming PathID has the
advantage of being more resilient against misrouted packets. If
no entry is found, a corresponding Error is sent back, indicating
a temporary inconsistency.
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1.[X,H(Q|M|Z)]
2.[X,H(Z|C|E|T)] 1.[X,H(Z|C|E|T)]
3.[X,S] 2.[X,S]
| | 1.[X,H(E|T)]
| | 2.[X,S]
| | |
X --> A --> Q --> M --> Z --> C --> E --> T
| | | |
| | | 1.[X,S]
| | 1.[X,H(C|E|T)]
| | 2.[X,S]
| 1.[X,H(M|Z)]
| 2.[X,H(Z|C|E|T)]
| 3.[X,S]
|
1.[X,H(A|Q|M|Z)]
2.[X,H(Z|C|E|T)]
3.[X,S]
Figure 3: Example for a path of seven hops between X and contact
T. X wants to send a packet to NodeID S and uses the path to its
closest known contact T . The annotations above and below the
arrows indicate up to three different packet headers with
[source, destination] pairs, where 1. indicates the topmost
header. X uses a PathID H(A|Q|M |Z) for the first segment and
H(Z|C|E|T ) for the second segment. Only node A must install a
mapping H(A|Q|M|Z)-> H(Q|M|Z) and node Z must install mapping
H(Z|C|E|T)-> H(C|E|T), all other mappings are precomputed.
Each node computes all PathIDs for its 2-hop vicinity to avoid path
setup signaling, because it allows all nodes to assume that PathIDs
exist for all source paths of length ≤3 hops. PathID precomputation
for the full 2-hop vicinity provides a good trade-off between the
number of a priori computed PathIDs and required path setup
signaling. Intermediate nodes along a source route may not have
computed the necessary PathIDs for others. Nodes explicitly setup
paths via PathSetupReq only for paths >5 hops. In the example of
Figure 3, only nodes A and Z must install additional forwarding
states when receiving a PathSetupReq, because all other nodes have
precomputed the PathIDs already. The PathSetupReq is answered with a
PathSetupRsp by the node that marks the beginning of the second path
segment. The forwarding states are implemented by soft states:
contact probing also refreshes any required PathIDs in the
intermediate nodes and so called "external" entries (i.e., those that
are neither locally used nor precomputed) are deleted after three
refresh intervals have passed without any refresh. Soft states are
necessary, because paths are aggregated toward the same destination
and usage relations are complex so it is nearly impossible to get a
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tracking completely right. For PathIDs that are used locally and not
precomputed, a counter is kept that reflects the number of contacts
using this PathID (note that different contacts may use the same
first path segment).
The routing information from the Routing Tier is used by the
Forwarding Tier to generate two forwarding tables inside each node:
one based on the calculated PathIDs and one based on NodeIDs
(generated from RT). One can employ common longest prefix matching
for both tables. For NodeIDs the matching prefix length corresponds
to the bucket depth. Thus, required prefix length is typically much
shorter than the full length of the NodeIDs. The PathID forwarding
table size comprises at least all stored contacts, but it is usually
larger due to the number of precomputed and external entries.
3.11. Endsystem Mode
This will be specified in future versions of this draft.
4. Protocol Specification
This section defines the message syntax and node behavior for the R²/
Kad protocol.
4.1. Protocol Message Transport
R²/Kad messages use the IPv6 packet format and are sent between KIRA
nodes by using link-local addresses of the respective interfaces as
source address and corresponding unicast or multicast addresses as
destination address.
4.2. Protocol Encoding
An R²/Kad message MUST be sent in the body of an IPv6 UDP datagram,
with network-layer hop count set to 1, destined to the well-known
KIRA multicast address or to an IPv6 link-local unicast address.
Both the source and destination UDP port are set to a well-known port
number. An R²/Kad packet MUST be silently ignored unless its source
port and destination is the well-known R²/Kad port (to be allocated
by IANA, use 19219 for experimentation). It MAY be silently ignored
if its destination address is a global IPv6 address.
R²/Kad messages consist of a common header and an optional sequence
of type-length-value (TLV) encoded protocol objects. A single R²/Kad
message is limited in its size by the maximum length of an IPv6
payload minus the UDP header size of 8 bytes, because IPv6
fragmentation can be used between two R²/Kad nodes. Larger message
payloads can be transferred by using R²/Kad fragmentation.
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R²/Kad messages use CBOR encoding [RFC8949] for the individual
message fields. The lengths in the message specifications do not
reflect the size after CBOR encoding on the wire. However, the final
message after CBOR encoding must fit into the UDP payload
(fragmentation of larger messages will be defined in later versions).
4.3. Protocol Message Notation
Messages consisting of multiple fields or objects are denoted by
their Message Name and the message content enclosed in { }. The
specification (L) after a field name indicates the length in bits of
the corresponding field, (..) denotes a variable length. Optional
fields are enclosed in [ ]. Optional repetitions of field or objects
are denoted by ... after the field. Specific types are separated by
a : after the message field name, e.g., enum is the typical
enumeration type, bitfield is a bitfield specification where the
assigned value denotes the number of the corresponding bit.
4.4. R²/Kad Message Format
The overall message format consists of a common header that MUST be
present in every message and a sequence of optional protocol objects
that immediately follow the common header.
R²/Kad Message {
Common Message Header,
[Protocol Object ...]
}
Figure 4: Basic R²/Kad Message Format
4.4.1. Common Message Header
The common message header has a fixed size and is present in every
R²/Kad message.
Common Message Header {
Version (8),
Message Type (8),
Flags (8),
Message Length (16),
Destination ID (112),
Source NodeID (112),
DomainID (64),
MessageID (64),
State Sequence Number (16),
Source Node Degree (16)
}
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Figure 5: Common Header Format
Version: Version is set to 0 for this specification.
Message Type: This field indicates the message type. Requests are
odd numbers, Responses or Indications are even numbers.
Message Type : enum {
ULNHello = 0x01,
ULNDiscoveryReq = 0x03,
ULNDiscoveryRsp = 0x04,
FindNodeReq = 0x09,
FindNodeRsp = 0x0a,
QueryRouteReq = 0x0b,
QueryRouteRsp = 0x0c,
UpdateRouteReq = 0x11,
ProbeReq = 0x21,
ProbeRsp = 0x22,
Error = 0x70,
PathSetupReq = 0x81,
PathSetupRsp = 0x82,
PathTearDownReq = 0x83
}
Flags:
Flags : bitfield {
ExactFlag = 0,
EndSystemFlag = 1,
Reserved = 2..13,
DiagnosticFlag = 14,
Reserved = 15
}
* _ExactFlag_ indicates whether the destination ID is a NodeID
and is assumed to exist. If set to 1, the NodeID should exist,
if set to 0, the node with the closest NodeID will process the
request.
* _DiagnosticFlag_ triggers explicit Error Messages instead of
dropping messages silently. This flag serves mainly debugging
purposes. Verbose Error Messages may be used for amplification
attacks. Therefore, a node may decide to ignore this flag in
case a sender sets it too often.
* The _EndSystemFlag_ indicates that the originating source node
is an endsystem that does not perform routing or forwarding.
Message Length: This field specifies the length of the R²/Kad
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Message including the Common Message Header.
Destination ID: The NodeID of the final destination or a Resource ID
for a lookup to find the responsible node for this ID. The
special value "AllNodes NodeID" (see Section 6.1) is only allowed
to be used ULNHello messages.
Source NodeID: The NodeID of the originating node that created the
message. Intermediate and receiving nodes do not modify the
Source NodeID.
DomainID: The DomainID 0x0 is the global domain of all KIRA nodes.
By default every KIRA node is part of this domain. Other domains
will have to be configured by an administrator or are constructed
by a distributed cluster algorithm. Messages with unknown
DomainIDs should be ignored. The DomainID is a selector for the
corresponding RT, i.e., every domain maintains conceptually its
own RT.
MessageID: The MessageID is chosen randomly for each request message
and the responding node MUST copy it to the corresponding response
message. It serves to uniquely map responses to related requests.
In case a response message is received without a corresponding
MessageID of the open request, it should be silently discarded
(unless the DiagnosticFlag is set, in this case an Error message
with MessageIDUnknown should be sent back to the Source NodeID);
the error may be logged depending on local policy.
State Sequence Number: This is the local state sequence number of
the source node that originated the message. Within the node the
sequence number is a global variable that changes with each
physical neighbor state change. That means, each time a new
physical neighbor is discovered or dismissed, the sequence number
will be increased. 0 is an invalid sequence number and all state
sequence numbers should start initially at 1. The sequence number
space is only monotonically increasing, i.e., comparisons should
be done without modulo wrap-around arithmetic. The value
0xffffffff signals a sequence number reset, i.e., a node receiving
a 0xffffffff must initiate a resynchronization with this node by
sending a ULNDiscoveryReq (for physical neighbors), QueryRouteReq
or ProbeReq to the corresponding node.
Source Node Degree: This number describes the number of active KIRA
interfaces where the KIRA instance sends ULNHello messages out and
has discovered other KIRA nodes. Nodes that have more than 65535
interfaces simply use 65535 as maximum number. The value 0 is not
allowed, since at least one interface must be present to sent this
message.
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4.4.2. Protocol Objects
Every Protocol Object starts with a common object header and has a
specific content.
Protocol Object {
Common Object Header (24),
Object Contents (..)
}
Figure 6: Protocol Object Format
4.4.2.1. Common Object Header
The common object header contains the type and the length of the
following object. The Object Length excludes the Common Object
Header, so a length of 0 indicates that no further object contents
follows.
Common Object Header {
Object Type (8),
Object Length (16)
}
Object Type : enum {
Source Route Object = 0x01,
NotViaList Object = 0x02,
ContactList Object = 0x03,
RTable Request Type Object = 0x04,
RTable Object = 0x05,
RTable Update Info Object = 0x06
}
Figure 7: Common Header Format
4.4.2.2. Source Route Object
This object contains a source route in form of a list of NodeIDs and
an index that points to the current NodeID when receiving and to the
next NodeID when sending a message. The first NodeID (at index 0) is
the Source NodeID of the originating node.
Source Route Object {
Common Object Header,
Index (10),
NodeID (112) ...
}
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Figure 8
In case Index points behind the end of the list of present NodeIDs, a
parameter problem error message MAY be sent back to the previous hop.
Typically, when forwarding an R²/Kad message, the Index pointer is
advanced to the next entry in the NodeID list. In case the last node
of the list received this object and the final destination has not
been reached, it will append a path to the existing list that leads
to the next overlay hop.
4.4.2.3. NotViaList Object
NotViaList Object {
Common Object Header,
Failed Link List : LinkListType ...
}
LinkListType = {
NodeID_1 (112),
NodeID_2 (112),
AgeInfo (32)
}
A LinkList is a sequence of NodeID pairs plus the AgeInfo value.
AgeInfo specifies the age of this information in milliseconds.
4.4.2.4. ContactList Object
ContactList Object {
Common Object Header,
Contact List : Contact Entry Type ...
}
Contact Entry Type = {
NodeID (112),
StateSequenceNumber (16),
AgeInfo (32),
NodeDegree (16)
}
The list of contacts contains for each entry a NodeID, the
corresponding known StateSequenceNumber, an AgeInfo that specifies
how old the contact info is (time since last updated) and the known
node degree.
4.4.2.5. RTable Request Type Object
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RTable Request Type Object {
Common Object Header,
RTable Request Type (8),
Radius (8)
}
RTable Request Type : enum {
None = 0x00,
ContactsOnly = 0x01,
OverlayNeighbors = 0x02,
OverlayNeighborsSource = 0x03,
ULNVicinity = 0x04
}
The following Request Types can be used: None will not return any
Routing Table information. This is useful in case a FindNodeRsp
should only report the source route back. ContactsOnly reports only
the ID-wise closest contacts to the destination ID of the FindNodeReq
without their paths. OverlayNeighbors reports the ID-wise closest
contacts to the destination ID of the FindNodeReq including their
path vectors. OverlayNeighborsSource reports the ID-wise closest
contacts to the source NodeID of the FindNodeReq. ULNVicinity
requests the underlay neighbors within the given radius. Radius
specifies the number of entries to be returned and SHOULD be set to
the bucket size k by default. A value of 0xff means to return the
full routing table (for any request type other than None).
4.4.2.6. RTable Object
RTable Object {
Common Object Header
RTableLength (16),
RTableEntry : RTableEntryType ...
}
RTableEntryType = {
NodeID (112),
Path : PathVectorType (..),
StateSequenceNumber (16),
AgeInfo (32),
NodeDegree (16),
[NodeAttributes Object],
[PathAttributes Object],
[LinkAttributes Object] ...
}
PathVectorType = { PathLength (16), NodeID (112) ... }
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The RTable contains a list of routing table entries. The list is
preceded by RTableLength that provides the number of the following
entries. For each entry the NodeID of the contact is given, the path
from the reporting node to the NodeID as sequence of NodeIDs as well
as the corresponding known StateSequenceNumber, an AgeInfo that
specifies how old the contact info is (time since last updated) and
the known node degree. Optional attributes for the node, the path or
individual links along the path follow. The PathVectorType is
basically a counter that specifies the number of the following node
IDs.
4.4.2.6.1. RTable Update Info Object
RTable Update Info Object {
Common Object Header
RTableLength (16),
RTableUpdateEntry : RTableUpdateEntryType ...
}
RTableUpdateEntryType = {
NodeID (112),
Path : PathVectorType (..),
StateSequenceNumber (16),
AgeInfo (32),
NodeDegree (16),
RouteUpdateAction (8),
[NodeAttributes Object],
[PathAttributes Object],
[LinkAttributes Object] ...
}
RouteUpdateAction : enum {
Announce = 0x00,
WithDraw = 0x01,
Change = 0x02,
Unreachable = 0x03
}
The RTable Update Info Object is similar to the RTable Object
(Section 4.4.2.6) contains a list of routing table entries with
associated an update action. _Announce_ means that the corresponding
contact is a new entry in the routing table. _WithDraw_ means that
the contact has been deleted from the routing table. _Change_ means
that the path to the contact has been changed. _Unreachable_ means
that the contact is not reachable.
Note: further objects will be detailed in future versions of this
draft
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4.4.3. R²/Kad Messages
This section describes the R²/Kad messages and what KIRA nodes do
when sending or receiving those messages.
4.4.3.1. General Node Behavior
There are some actions that are performed in the same way for all
messages.
* Most R²/Kad control messages are sent with a rate limit to avoid
overloading other nodes with high bursts of messages.
* In case of having problems to process a message (e.g., due to
malformed messages, unknown objects, and so on) the message should
be silently discarded. If the DiagnosticFlag is set, a
corresponding Error message should be sent back to the Source
NodeID. The error may be logged locally on the node depending on
its policy. An Error message MUST never be sent back for in
incoming Error message.
* In order to avoid undesired synchronization effects, many actions
are performed after a randomized waiting time. This specification
uses the notion RandTime(T_p) for clarifying that the actual
waiting time T_a is chosen randomly at every use from the Interval
[0.5*T_p,1.5*T_p], so the mean waiting time is T_p.
* Response messages MUST copy the MessageID of the related request
message into their header. In case a response message is received
without a corresponding MessageID of the corresponding request, it
should be silently discarded (unless the DiagnosticFlag is set, in
this case an Error message with MessageIDUnknown should be sent
back to the Source NodeID); the error may be logged depending on
local policy.
4.4.3.2. ULNHello Message
ULNHello messages are periodically sent (randomized) to each
interface to indicate presence of a KIRA node. Its format is shown
in Figure 9.
ULNHello {
Common Header
}
Figure 9: ULNHello Message
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4.4.3.2.1. Sending a ULNHello
The sender uses the Undefined NodeID as Destination ID and its own
NodeID as Source NodeID. Other nodes that want to establish an
adjacency SHOULD respond with a ULNDiscoveryReq message after a
randomized waiting time and if the trigger condition is met. The
trigger condition is calculated as follows: use the lowest 32 bit of
the other NodeID and the own NodeID and calculate delta = otherNodeID
mod 2^32 - ownNodeID mod 2^32. If (delta < 0x80000000 and delta != 0
) or ((delta == 0 or delta == 0x80000000UL) and ownNodeId <
otherNodeId the trigger a ULNDiscoveryReq, otherwise wait for a
ULNDiscoveryReq. This heuristic avoids initiating the ULN Discovery
handshake twice.
At node startup or when a new link comes up a ULNHelloMinInterval is
used per link. The sending interval for subsequent ULNHello messages
is doubled up to ULNHelloMaxInterval. The sending interval is reset
to ULNHelloMinInterval for a link that comes up after it failed.
Default values for fixed networks are: ULNHelloMinInterval = 200ms,
ULNHelloMaxInterval = 30s.
4.4.3.2.2. Receiving a ULNHello
On receipt of a ULNHello Message the receiving node should check
whether the originating node is already known as contact. If it is
already member of a ULN bucket and contained in the ULNTable, the
LastSeen timestamp is updated and the provided Sequence Number is
checked against the store Sync ULN Sequence Number of the contact.
If the provided sequence number is newer, a ULNDiscRequest should be
scheduled to discover recent changes in the underlay neighborhood.
If the source node is not known as contact yet, either a
ULNDiscoveryReq was triggered locally or will be triggered by the
source node after the node's own ULNHello was received.
4.4.3.3. ULNDiscovery Request Message (ULNDiscoveryReq)
A ULNDiscoveryReq is either sent as response to a ULNHello message or
sent to test liveliness and bidirectional connectivity of an already
known ULN or to resynchronize state with a ULN. Its format is shown
in Figure 10
ULNDiscovery Request Message {
Common Header,
[ ULNList : ContactList ]
}
Figure 10: ULNDiscovery Request Message
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4.4.3.3.1. Sending a ULNDiscoveryReq
The sending node fills its currently known ULNs into the ULNList on
first contact or each time its state sequence number has changed. It
expects a ULNDiscoveryRsp as immediate reply and should set a timer
as maximum waiting period (default 200ms). After the corresponding
timeout the ULNDiscoveryReq should be repeated. The timeout for an
answer should be doubled for each retry. The contact should be
considered dead after two unsuccessful retry attempts.
4.4.3.3.2. Receiving a ULNDiscoveryReq
On receipt of a ULNDiscoveryReq, the receiving node MUST reply with a
ULNDiscoveryRsp. If the contact was not known yet, it is put into
the ULNTable and into the corresponding ULN bucket. In case the
contact was already known, but not as direct underlay neighbor, the
contact should be moved to the ULN bucket. This can be the case if
the contact has been learned being an underlay neighbor of another
ULN. The UnderlayNeighbor flag of the contact must be set to true.
4.4.3.4. ULNDiscovery Response Message (ULNDiscoveryRsp)
A ULNDiscoveryRsp is sent as answer to a previous ULNDiscoveryReq.
Its format is shown in Figure 11.
ULNDiscovery Response Message {
Common Header,
[ ULNList : ContactList ]
}
Figure 11: ULNDiscovery Response Message
4.4.3.4.1. Sending a ULNDiscoveryRsp
The sending node must copy the MessageID from the ULNDiscoveryReq and
fills its currently known ULNs into the ULNList on first contact or
each time its state sequence number has changed.
4.4.3.4.2. Receiving a ULNDiscoveryRsp
Conceptually, on receipt of a ULNDiscovery Response bidirectional
connectivity to the underlay neighbor has been demonstrated. The
state of the contact should be changed to "valid", LastSeen, Sync ULN
Sequence Number and Sequence Number should be updated accordingly.
The ULNList should be parsed for new contacts or contact updates with
respect to newer StateSequenceNumber. In case of a new contact or
its newer StateSequenceNumber a QueryRouteReq SHOULD be sent to the
corresponding contact. The latter should request a ULNVicinity
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(RTable Request Type) of Radius 1 to learn the 2-hop underlay
vicinity.
4.4.3.5. FindNode Request Message (FindNodeReq)
FindNodeReq messages are used (together with the corresponding
FindNodeRsp) to find routes to nodes (Path Discovery, see
Section 3.6), to improve the own routing table, or to find
responsible nodes for a given key. Its message format is shown in
Figure 12.
FindNode Request Message {
Common Header,
RTable Request Type,
Source Route Object,
[NotViaList Object]
}
Figure 12: FindNode Request Message
4.4.3.5.1. Sending a FindNodeReq
Sending a FindNodeReq for an existing NodeID X (e.g., communicated by
other nodes as part of their RTable objects) MUST set the ExactFlag
in the Flags field of the Common Message Header. The RTable Request
Type is typically set to OverlayNeighbors (depending on the purpose
of the FindNodeReq). Destination ID is set to the NodeID X. In case
X is a known contact, a source route to X is known and filled into
the Source Route Object. In case X is not a known contact, the
routing table is used to look up the ID-wise closest known valid
contact Y (using proximity routing, see Section 3.3) and the node
fills in the corresponding path into the Source Route Object. The
own NodeID is used as first element of the source route and the
contact's NodeID Y is the last element of the source route (the path
vector is the sequence of NodeIDs in between both entries). The
index of the Source Route Object MUST be set to 1. In case of known
failed links (esp. during Path Rediscovery, Section 3.9.1), the
NotViaList Object is also filled correspondingly.
The sender of the FindNodeReq looks up the underlay address of the
next hop in the source route (using the ULNTable) and sends the
message to the underlay neighbor. For the Join procedure
(Section 3.5), the Destination ID is set to the Source ID of the
sending node.
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Depending on the context, the FindNodeReq may be repeated in case the
corresponding FindNodeRsp is not received within 500ms. Further
retries should double the timeout. A failure should be assumed after
2 unsuccessful retries.
4.4.3.5.2. Receiving a FindNodeReq
A node that receives a FindNodeReq checks at first whether it has the
NodeID at the current index of the source route. If the NodeID at
the current index is different from the node's NodeID, the message
has been misrouted. This is a severe error and should be logged at
least locally. In case of a set DiagnosticFlag an Error message with
a RouteFailureWrongHop may be sent back to the previous node.
An node that is an intermediate node on the source route should
evaluate the NotViaList object first and invalidate any affected
contacts (depending on the corresponding age values). Then the so
far traversed path (in reverse direction) should be analyzed for
interesting contacts or paths. Since the message just traveled along
this path, one can assume that the path information is current and
thus validated.
If the receiving node is neither the last node in the source route
nor the destination node according to the Destination ID, the
FindNodeReq message is forwarded along the source route: the index is
incremented by one and a lookup in the ULNTable is performed for this
next hop NodeID. In case the next hop is missing in the
ULNTable (e.g., failed link) there are two more options possible.
The first option is used in case the next hop is known as contact and
there is a valid route leading to it. The current source route is
then adapted by inserting the detour path to the former next hop and
continuing to forward to the new next hop. The second option is used
when the Destination ID is a known contact with a valid route. In
this case the rest of the source route is replaced by the known path
to the destination NodeID. If both options are not successful, an
Error message of type SegmentFailure is sent back to the source along
the reversed source route. The NodeID of the failed next hop node
and the Destination ID are provided as additional parameters in the
SegmentFailure Error message.
If the receiving node (e.g., having NodeID Y) is the last node in the
source route, it should look up the ID-wise closest contact (e.g.,
lets say NodeID Z) in its routing table. If the XOR distance d(Y,Z)
< d(Y,X), the next contact Z leads closer to the destination ID X and
the corresponding source route to Z is appended to the existing
source route. In case Z is not closer to destination ID X than the
current node Y, the behavior depends on the ExactFlag: if the
ExactFlag was set, an Error message is sent back indicating a
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RouteFailureDeadEnd, since there is no known overlay path leading to
X. If the ExactFlag is not set, a FindNodeRsp message is sent back,
containing the requested information according to the RTable Request
Type.
If the NodeID of the recipient corresponds to Destination ID, the
rest of the source route should be ignored and a corresponding
FindNodeRsp should be sent.
4.4.3.6. FindNode Response Message (FindNodeRsp)
A FindNodeRsp message is sent as response to a FindNodeReq either if
the node with the Destination ID has been reached or the FindNodeReq
cannot be forwarded ID-wise closer to the Destination ID when the
ExactFlag is set. Its message format is shown in Figure 13.
FindNode Response Message {
Common Header,
Source Route Object,
[NotViaList Object,]
[RTable Object]
}
Figure 13: FindNode Response Message
4.4.3.6.1. Sending a FindNodeRsp
The source route contained in the FindNodeReq is reversed and used as
new Source Route Object, any cycles MUST be removed. Since all links
of this possibly shortened path haven been traversed by the
FindNodeReq, the probability to have a working source route back to
the source of the FindNodeReq is very high. The Source NodeID is set
to the NodeID of the responding node, the Destination ID is set to
the last NodeID of the just generated Source Route. The NotViaList
from the FindNodeReq should be copied into the FindNodeRsp. The
RTable Object contains the requested information according to the
RTable Request Type of the FindNodeReq. In addition to the requested
contacts, additional gratuitous contacts SHOULD be provided as
follows: two randomly chosen contacts from every bucket that are not
already contained in the list of request contacts.
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4.4.3.6.2. Receiving a FindNodeRsp
The recipient of a FindNodeRsp Message processes the NotViaList
Object first if present, then analyzes the Source Route Object for
new contacts or path information. The receiving node processes the
RTable Object by analyzing each given contact and related path vector
information in order to improve its own routing table. Received path
vectors should be inspected for further improvement with the node's
valid paths. For example, if node X learns path ⟨A, Q, M⟩ to Z, it
can shorten the path by using the path vector ⟨B⟩ to contact M,
resulting in an improved path of ⟨B, M⟩ to Z. However, this path
needs to be validated by probing this proposed path before it can be
used as active and valid path.
4.4.3.7. QueryRoute Request Message (QueryRouteReq)
A QueryRouteReq message is used to receive requested routing
information from other nodes. This is especially used to discover
the 3-hop underlay neighborhood as described in Section 3.4. In this
case a QueryRouteReq using RTable Request Type ULNVicinity is sent to
2-hop underlay neighbors. However, QueryRouteReq may be sent to any
contact for improving a node's own RT. QueryRouteReq messages
require a known source route to the destination, i.e., in contrast to
FindNodeReq messages they are not forwarded closer to the Destination
ID. The message format is shown in Figure 14.
QueryRoute Request Message {
Common Header,
RTable Request Type,
Source Route Object,
[NotViaList Object]
}
Figure 14: QueryRoute Request Message
4.4.3.7.1. Sending a QueryRouteReq
The sender of a QueryRouteReq message MUST set the ExactFlag and use
the NodeID as Destination ID and fill in the source route
accordingly. QueryRouteReq messages can also be sent to nodes that
are not known as contacts (i.e., not part of the RT), however, a
valid path must be known to the destination node that can be used as
source route. The RTable Request Type should be set according to the
sending node's need. A NotViaList is optionally provided.
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4.4.3.7.2. Receiving a QueryRouteReq
The receiver of a QueryRouteReq message MUST inspect. QueryRouteReq
messages can also be sent to nodes that are not known as contacts
(i.e., not part of the RT), however, a valid path must be known to
the destination node that can be used as source route. The
RTable Request Type should be set according to the sending node's
need.
4.4.3.8. QueryRoute Response Message (QueryRouteRsp)
The QueryRouteRsp is a response to a QueryRouteReq and provides RT
information of the sending node. The message format is shown in
Figure 15.
QueryRoute Response Message {
Common Header,
Source Route Object,
[NotViaList Object,]
[RTable Object]
}
Figure 15: QueryRoute Response Message
4.4.3.8.1. Sending a QueryRouteRsp
The sender MUST respond to a received QueryRouteReq with a
corresponding QueryRouteRsp. The RTable Object is filled according
to the RTable Request Type of the QueryRouteReq. Gratuitous contacts
SHOULD be added, too.
4.4.3.8.2. Receiving a QueryRouteRsp
The processing of a QueryRouteRsp is analogous to processing a
FindNodeRsp.
4.4.3.9. UpdateRoute Request Message (UpdateRouteReq)
An UpdateRouteReq message provides information about changed routing
information, e.g., new or removed contacts as well as changed paths.
As the UpdateRouteReq message has informational character only, it is
transmitted unreliably, i.e., its receipt is not confirmed.
Consequently, there is no corresponding UpdateRouteRsp message. The
message format is shown in Figure 16.
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UpdateRoute Request Message {
Common Header,
Source Route Object,
[NotViaList Object,]
RTable Update Info Object
}
Figure 16: UpdateRoute Request Message
4.4.3.9.1. Sending an UpdateRouteReq
An UpdateRouteReq is sent for newly learned contacts (Announce),
removed (WithDraw) or failed direct underlay contacts (Unreachable)
as well as for changed paths (Change). UpdateRouteReq messages are
scheduled when the local RT of a node is changed. This is useful to
avoid sending too frequent update messages while the network is still
converging. A path to a contact may possibly change multiple times
in a short time frame or the modified contact may be preempted by
another contact later. Therefore, consistency has to be checked at
the time when the UpdateRouteReq should be sent, e.g., if a path
change occurred first, but afterwards the contact was deleted, then a
different type of UpdateRouteReq needs to be sent.
An UpdateRouteReq is sent to the four ID-wise closest contacts of the
sending node. These destination contacts do not have to be valid
contacts (i.e., possess a currently known valid active path) as
UpdateRouteReq messages are forwarded via the overlay routing as
close to the destination as possible. It is useful to create a list
of RT updates per destination contact so that multiple updates can be
aggregated into the same UpdateRouteReq message. Furthermore, path
changes may occur several time during convergence and only the latest
change should be reported. Moreover, updates are sent depending on
their criticality: urgent updates are sent for Unreachable contacts
after RandTime(250ms), other updates are sent after RandTime(500ms).
4.4.3.9.2. Receiving an UpdateRouteReq
A node that receives an UpdateRouteReq (it is at the end of the
current source route) and that is not the final destination (its
NodeID is equals to the Destination ID), tries to forward the
UpdateRouteReq closer toward the Destination ID. If this is not
possible, forwarding stops at this node (without triggering any Error
message, e.g., RouteFailureDeadEnd due to this fact; other errors
during message processing may trigger Error messages if the
DiagnosticFlag is set). Due to overhearing (see Section 3.7) the
RTable Update Info Object is processed nevertheless. A node the is
the final destination of the UpdateRouteReq simply processes the
information provided in the RTable Update Info Object. No response
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will be created.
4.4.3.10. Probe Request Message (ProbeReq)
ProbeReq messages are sent to test liveliness of a path and contact.
There are two processes that trigger sending of ProbeReq messages:
Periodic Path Probing (Section 3.8) and checking validity of proposed
paths (Section 3.9.2). Periodic Path Probing also refreshes any
related PathIDs for the particular path in the Forwarding Tier as
this information will otherwise be deleted after some time without
refreshes (see Section 3.10). Therefore ProbeReq messages need to
travel the complete path until the Destination ID (unless this is
impossible due to a failure and an Error message is sent back).
ProbeReq messages SHOULD never be sent to ULNs if their direct
attached link is working as there are periodic ULNHello messages used
to check the connectivity.
ProbeReq messages are strictly following the specified source route,
i.e., rerouting them is not allowed. A broken link will be reported
by an Error message indicating a SegmentFailure. If the path is
intact, a ProbeRsp will be sent back along the reverse source route
by the contact. The ProbeReq messages are sent via the Routing Tier,
i.e., they are not testing the actual data path in the Forwarding
Tier. The message format of the ProbeReq is shown in Figure 17.
Probe Request Message {
Common Header,
Source Route Object
}
Figure 17: Probe Request Message
4.4.3.10.1. Sending a ProbeReq
The sender uses the contact's NodeID as Destination ID, sets its own
NodeID as Source NodeID and fills in the source route. The ExactFlag
MUST be set and it is assumed that the Destination ID is a NodeID of
an existing contact. The last node of the source route must
correspond to the Destination ID as ProbeReq message are not
forwarded via overlay routing, but via strict source routing.
Therefore the source route must be complete that it leads to the
Destination ID. Retransmitting the ProbeReq in case of a missing
matching ProbeRsp is not necessary for Periodic Path Probing as this
will automatically send another ProbeReq message after a while.
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4.4.3.10.2. Receiving a ProbeReq
An intermediate node (i.e., neither the first nor the last node)
along the source route MUST check whether the corresponding PathID
information is present in the Forwarding Tier and either install it
or update its timestamp if already present. Then the ProbeReq MUST
be forwarded to the next hop in the source route if any is left. If
is not possible due to a failed link to the next hop or the next hop
node failed, a SegmentFailure Error message MUST be sent back to the
origin of the ProbeReq (along the reversed source route that was
already traversed). This error reporting must not be suppressed as
path liveliness check and connectivity detection of KIRA depends on
it. An intermediate node MUST NOT send back a ProbeRsp. In case the
node is the last node in the source route, but the Destination ID is
not the NodeID of the node, the source route is wrong or incomplete.
In this case an Error message RouteFailureWrongPath may be sent back
if the Diagnostic Flag is set.
The destination node of a ProbeReq updates its local contacts and
paths according to the source route. It sends back a ProbeRsp
message along the reversed source route.
4.4.3.11. Probe Response Message (ProbeRsp)
A ProbeRsp message MUST be sent as response to an incoming ProbeReq
if the KIRA NodeID is the Destination ID of the ProbeReq. The
message format is shown in Figure 18.
Probe Response Message {
Common Header,
Source Route Object
}
Figure 18: Probe Response Message
4.4.3.11.1. Sending a ProbeRsp
The node that was the destination of the ProbeReq sends back a
ProbeRsp message along the reversed source route and sets the
Destination ID to the Source NodeID of the ProbeReq.
4.4.3.11.2. Receiving a ProbeRsp
The node that originally sent the corresponding ProbeReq updates the
contact state information accordingly (State Sequence Number,
LastSeen, Validation and LastRefresh timestamps of the corresponding
path).
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4.4.3.12. Error Message (Error)
Error messages are sent back to indicate problems and for diagnostic
purposes. A conservative reaction to errors during message
processing is typically to drop the erroneous message silently and
not sent back any feedback. This also reduces possibilities for
potential amplification attacks. However, some Error messages must
be sent for proper protocol operation, e.g., returning a
SegmentFailure is essential for detecting broken paths. The
DiagnosticFlag in the Common Header solicits the transmission of
Error messages for debugging purposes. However, it is at a node's
discretion whether or how often this request is fulfilled (e.g., it
can apply rate limits). The format of an Error message is shown in
Figure 19.
Error Message {
Common Header,
Source Route Object,
Error Type (8),
OriginMessageID (64),
[Additional Error Information (..)}
}
Error Type : enum {
NoError = 0x00,
NodeUnreachable = 0x01,
MalformedMessage = 0x02,
ParameterProblem = 0x03,
HopLimitExceeded = 0x04,
SegmentFailure = 0x05,
PathIDUnknown = 0x06,
MessageIDUnknown = 0x07,
RouteFailureDeadEnd = 0x0a,
RouteFailureWrongHop = 0x0b,
RouteFailureWrongPath = 0x0c
}
Figure 19: Error Message
4.4.3.12.1. Sending an Error message
During processing protocol messages various error conditions may
occur that can produce an Error message. The Error Type must be set
according to the error condition that is given in this specification.
The OriginMessageID is set to the MessageID of the message that
caused the problem. Optional addition error information may be
given, too. (TBD in future versions of this specification).
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4.4.3.12.2. Receiving an Error message
Depending on the type of the Error message, the receiving node may
trigger certain protocol actions, e.g., a starting a Path Rediscovery
after receiving a SegmentFailure or silently discard the Error
message after optionally generating a log message (possibly rate
limited) with the information given in the Error message. An error
occurring during processing the Error message MUST never result in
sending an Error message back.
4.4.3.13. Path Setup Request Message (PathSetupReq)
A PathSetupReq is sent to install required PathID mappings in
intermediate nodes (see Section 3.10). This is only necessary for
paths to contacts that are at least 6 hops long. Its format is shown
in Figure 20.
Path Setup Request Message {
Common Header,
Source Route Object
}
Figure 20: Path Setup Request Message
4.4.3.13.1. Sending a PathSetupReq
The originating node simply constructs the source route according to
the path of the contact in question. The Destination ID is set to
the NodeID of the contact, the Source NodeID is set to the NodeID of
the sender. Typically, a PathSetupReq is scheduled when a new
contact is put into a bucket. When the time has come to send the
PathSetupReq a check should ensure that the current path of the
contact still requires the PathSetupReq (meanwhile a shorter path may
have been learned or the contact may have been preempted).
4.4.3.13.2. Receiving a PathSetupReq
A node that receives a PathSetupReq (also intermediate nodes) needs
to check whether it needs to install PathIDs. PathIDs that are
installed by a PathSetupReq get an "external entry" flag, indicating
that other nodes require this path. If the necessary PathID is
installed already, the "external entry" flag is set and a
PathSetupRsp MUST be sent back. Otherwise the corresponding PathID
mapping (incoming PathID to outgoing PathID and next hop) is
installed and the PathSetupReq is forwarded to the next node. If
there are only 3 hops left to the destination, a PathSetupRsp MUST be
sent back, because of path precomputation in the 2-hop vicinity the
next hop must have precomputed the PathID.
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4.4.3.14. Path Setup Response Message (PathSetupRsp)
A PathSetupRsp message confirms that the necessary PathIDs are
installed for the source route. The message format is shown in
Figure 21.
Path Setup Response Message {
Common Header,
Source Route Object
}
Figure 21: Path Setup Response Message
4.4.3.14.1. Sending a PathSetupRsp
A PathSetupRsp is sent back as response to a PathSetupReq if the
necessary PathIDs are already present in the node or have been
installed and PathIDs do not have to be installed in further nodes
along the path.
4.4.3.14.2. Receiving a PathSetupRsp
The node that receives a PathSetupRsp and that is not the Destination
ID of the PathSetupRsp simply forwards the PathSetupRsp forward to
the Destination ID.
4.4.3.15. Path TearDown Request Message (PathTearDownReq)
The PathTearDownReq initiates deletion of the PathID state
information along the specified source route. This mechanism
basically provides means a of optimization therefore a response
message is not required. The format of the PathTearDownReq is shown
in Figure 22.
Path TearDown Request Message {
Common Header,
Source Route Object
}
Figure 22: Path TearDown Request Message
4.4.3.15.1. Sending a PathTearDownReq
A PathTearDownReq SHOULD be initiated if a contact is removed that
had a path that required a PathSetupReq.
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4.4.3.15.2. Receiving a PathTearDownReq
An (intermediate) node that receives a PathTearDownReq needs to check
whether the corresponding PathIDs needs to be deleted. Normally,
this clears an "external entry" flag of the corresponding entry. The
flag will be set again in case another node requires this path by the
next periodical refresh. This depends also on whether the PathID is
required by local contacts or precomputed. Precomputed PathIDs will
never be deleted by a PathTearDownReq, locally required PathIDs can
be deleted if no contact uses this path anymore and it is neither
precomputed nor possesses a foreign flag. Similarly to the
PathSetupReq, a PathTearDownReq will not be forwarded if there are
only 3 hops left to the destination.
5. Hash Function
KIRA uses hash functions in various contexts. The used hash function
is SHAKE256 with 128bit length output.
6. Protocol Parameters
6.1. Reserved NodeIDs
Reserved NodeIDs:
Undefined NodeID = 0x00..00 (all zeros)
AllNodes NodeID = 0xff..ff (all ones)
7. IANA Considerations
This memo currently includes no request to IANA yet. This may change
in the future.
8. Security Considerations
There are various attacks that need to be considered. Future
versions of this draft will have more detailed security
considerations.
9. References
9.1. Normative References
[RFC7676] Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
for Generic Routing Encapsulation (GRE)", RFC 7676,
DOI 10.17487/RFC7676, October 2015,
<https://www.rfc-editor.org/info/rfc7676>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
9.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[Kademlia2002]
Maymounkov, P. and D. Mazières, "Kademlia: A Peer-to-Peer
Information System Based on the XOR Metric", 2002.
[KIRA-Networking-2022]
Bless, R., Zitterbart, M., Despotovic, Z., and A. Hecker,
"KIRA: Distributed Scalable ID-based Routing with Fast
Forwarding", June 2022, <https://doi.org/10.23919/
IFIPNetworking55013.2022.9829816>.
Acknowledgements
KIRA has been developed as joint work with Zoran Despotovic, Artur
Hecker, and Martina Zitterbart. Hendrik Mahrt and Paul Seehofer are
still contributing to KIRA's evolution.
Changes
changes in -01:
* few additions to the overview section
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* detailed contact data
* changed physical neighbor to underlay neighbor and PN to ULN as an
underlay neighbor can also be linked via L2 tunnel using IP etc.
* specified initial description of messages and their processing
when sending or receiving
* removed PathTeardownRsp as it is not required
* minor edits (wording)
Author's Address
Roland Bless
Karlsruhe Institute of Technology (KIT)
Kaiserstr. 12
76131 Karlsruhe
Germany
Phone: +4915201601400
Email: roland.bless@kit.edu
URI: https://tm.kit.edu/~bless
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