| Rfc | 7921 |
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| Title | An Architecture for the Interface to the Routing System |
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| Author | A. Atlas,
J. Halpern, S. Hares, D. Ward, T. Nadeau |
|---|
| Date | June 2016 |
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| Format: | TXT,
HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Internet Engineering Task Force (IETF) A. Atlas
Request for Comments: 7921 Juniper Networks
Category: Informational J. Halpern
ISSN: 2070-1721 Ericsson
S. Hares
Huawei
D. Ward
Cisco Systems
T. Nadeau
Brocade
June 2016
An Architecture for the Interface to the Routing System
Abstract
This document describes the IETF architecture for a standard,
programmatic interface for state transfer in and out of the Internet
routing system. It describes the high-level architecture, the
building blocks of this high-level architecture, and their
interfaces, with particular focus on those to be standardized as part
of the Interface to the Routing System (I2RS).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7921.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://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 extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................4
1.1. Drivers for the I2RS Architecture ..........................5
1.2. Architectural Overview .....................................6
2. Terminology ....................................................11
3. Key Architectural Properties ...................................13
3.1. Simplicity ................................................13
3.2. Extensibility .............................................14
3.3. Model-Driven Programmatic Interfaces ......................14
4. Security Considerations ........................................15
4.1. Identity and Authentication ...............................17
4.2. Authorization .............................................18
4.3. Client Redundancy .........................................19
4.4. I2RS in Personal Devices ..................................19
5. Network Applications and I2RS Client ...........................19
5.1. Example Network Application: Topology Manager .............20
6. I2RS Agent Role and Functionality ..............................20
6.1. Relationship to Its Routing Element .......................20
6.2. I2RS State Storage ........................................21
6.2.1. I2RS Agent Failure .................................21
6.2.2. Starting and Ending ................................22
6.2.3. Reversion ..........................................23
6.3. Interactions with Local Configuration .....................23
6.3.1. Examples of Local Configuration vs. I2RS
Ephemeral Configuration ............................24
6.4. Routing Components and Associated I2RS Services ...........26
6.4.1. Routing and Label Information Bases ................28
6.4.2. IGPs, BGP, and Multicast Protocols .................28
6.4.3. MPLS ...............................................29
6.4.4. Policy and QoS Mechanisms ..........................29
6.4.5. Information Modeling, Device Variation, and
Information Relationships ..........................29
6.4.5.1. Managing Variation: Object
Classes/Types and Inheritance .............29
6.4.5.2. Managing Variation: Optionality ...........30
6.4.5.3. Managing Variation: Templating ............31
6.4.5.4. Object Relationships ......................31
6.4.5.4.1. Initialization .................31
6.4.5.4.2. Correlation Identification .....32
6.4.5.4.3. Object References ..............32
6.4.5.4.4. Active References ..............32
7. I2RS Client Agent Interface ....................................32
7.1. One Control and Data Exchange Protocol ....................32
7.2. Communication Channels ....................................33
7.3. Capability Negotiation ....................................33
7.4. Scope Policy Specifications ...............................34
7.5. Connectivity ..............................................34
7.6. Notifications .............................................35
7.7. Information Collection ....................................35
7.8. Multi-headed Control ......................................36
7.9. Transactions ..............................................36
8. Operational and Manageability Considerations ...................37
9. References .....................................................38
9.1. Normative References ......................................38
9.2. Informative References ....................................38
Acknowledgements ..................................................39
Authors' Addresses ................................................40
1. Introduction
Routers that form the Internet routing infrastructure maintain state
at various layers of detail and function. For example, a typical
router maintains a Routing Information Base (RIB) and implements
routing protocols such as OSPF, IS-IS, and BGP to exchange
reachability information, topology information, protocol state, and
other information about the state of the network with other routers.
Routers convert all of this information into forwarding entries,
which are then used to forward packets and flows between network
elements. The forwarding plane and the specified forwarding entries
then contain active state information that describes the expected and
observed operational behavior of the router and that is also needed
by the network applications. Network-oriented applications require
easy access to this information to learn the network topology, to
verify that programmed state is installed in the forwarding plane, to
measure the behavior of various flows, routes or forwarding entries,
as well as to understand the configured and active states of the
router. Network-oriented applications also require easy access to an
interface, which will allow them to program and control state related
to forwarding.
This document sets out an architecture for a common, standards-based
interface to this information. This Interface to the Routing System
(I2RS) facilitates control and observation of the routing-related
state (for example, a Routing Element RIB manager's state), as well
as enabling network-oriented applications to be built on top of
today's routed networks. The I2RS is a programmatic asynchronous
interface for transferring state into and out of the Internet routing
system. This I2RS architecture recognizes that the routing system
and a router's Operating System (OS) provide useful mechanisms that
applications could harness to accomplish application-level goals.
These network-oriented applications can leverage the I2RS
programmatic interface to create new ways to combine retrieving
Internet routing data, analyzing this data, and setting state within
routers.
Fundamental to I2RS are clear data models that define the semantics
of the information that can be written and read. I2RS provides a way
for applications to customize network behavior while leveraging the
existing routing system as desired. I2RS provides a framework for
applications (including controller applications) to register and to
request the appropriate information for each particular application.
Although the I2RS architecture is general enough to support
information and data models for a variety of data, and aspects of the
I2RS solution may be useful in domains other than routing, I2RS and
this document are specifically focused on an interface for routing
data.
Security is a concern for any new I2RS. Section 4 provides an
overview of the security considerations for the I2RS architecture.
The detailed requirements for I2RS protocol security are contained in
[I2RS-PROT-SEC], and the detailed security requirements for
environment in which the I2RS protocol exists are contained in
[I2RS-ENV-SEC].
1.1. Drivers for the I2RS Architecture
There are four key drivers that shape the I2RS architecture. First
is the need for an interface that is programmatic and asynchronous
and that offers fast, interactive access for atomic operations.
Second is the access to structured information and state that is
frequently not directly configurable or modeled in existing
implementations or configuration protocols. Third is the ability to
subscribe to structured, filterable event notifications from the
router. Fourth, the operation of I2RS is to be data-model-driven to
facilitate extensibility and provide standard data models to be used
by network applications.
I2RS is described as an asynchronous programmatic interface, the key
properties of which are described in Section 5 of [RFC7920].
The I2RS architecture facilitates obtaining information from the
router. The I2RS architecture provides the ability to not only read
specific information, but also to subscribe to targeted information
streams, filtered events, and thresholded events.
Such an interface also facilitates the injection of ephemeral state
into the routing system. Ephemeral state on a router is the state
that does not survive the reboot of a routing device or the reboot of
the software handling the I2RS software on a routing device. A non-
routing protocol or application could inject state into a routing
element via the state-insertion functionality of I2RS and that state
could then be distributed in a routing or signaling protocol and/or
be used locally (e.g., to program the co-located forwarding plane).
I2RS will only permit modification of state that would be possible to
modify via Local Configuration; no direct manipulation of protocol-
internal, dynamically determined data is envisioned.
1.2. Architectural Overview
Figure 1 shows the basic architecture for I2RS between applications
using I2RS, their associated I2RS clients, and I2RS agents.
Applications access I2RS services through I2RS clients. A single
I2RS client can provide access to one or more applications. This
figure also shows the types of data models associated with the
routing system (dynamic configuration, static configuration, Local
Configuration, and routing and signaling configuration) that the I2RS
agent data models may access or augment.
Figure 1 is similar to Figure 1 in [RFC7920], but the figure in this
document shows additional detail on how the applications utilize I2RS
clients to interact with I2RS agents. It also shows a logical view
of the data models associated with the routing system rather than a
functional view (RIB, Forwarding Information Base (FIB), topology,
policy, routing/signaling protocols, etc.)
In Figure 1, Clients A and B each provide access to a single
application (Applications A and B, respectively), while Client P
provides access to multiple applications.
Applications can access I2RS services through local or remote
clients. A local client operates on the same physical box as the
routing system. In contrast, a remote client operates across the
network. In the figure, Applications A and B access I2RS services
through local clients, while Applications C, D, and E access I2RS
services through a remote client. The details of how applications
communicate with a remote client is out of scope for I2RS.
An I2RS client can access one or more I2RS agents. In Figure 1,
Clients B and P access I2RS agents 1 and 2. Likewise, an I2RS agent
can provide service to one or more clients. In this figure, I2RS
agent 1 provides services to Clients A, B, and P while Agent 2
provides services to only Clients B and P.
I2RS agents and clients communicate with one another using an
asynchronous protocol. Therefore, a single client can post multiple
simultaneous requests, either to a single agent or to multiple
agents. Furthermore, an agent can process multiple requests, either
from a single client or from multiple clients, simultaneously.
The I2RS agent provides read and write access to selected data on the
routing element that are organized into I2RS services. Section 4
describes how access is mediated by authentication and access control
mechanisms. Figure 1 shows I2RS agents being able to write ephemeral
static state (e.g., RIB entries) and to read from dynamic static
(e.g., MPLS Label Switched Path Identifier (LSP-ID) or number of
active BGP peers).
In addition to read and write access, the I2RS agent allows clients
to subscribe to different types of notifications about events
affecting different object instances. One example of a notification
of such an event (which is unrelated to an object creation,
modification or deletion) is when a next hop in the RIB is resolved
in a way that allows it to be used by a RIB manager for installation
in the forwarding plane as part of a particular route. Please see
Sections 7.6 and 7.7 for details.
The scope of I2RS is to define the interactions between the I2RS
agent and the I2RS client and the associated proper behavior of the
I2RS agent and I2RS client.
****************** ***************** *****************
* Application C * * Application D * * Application E *
****************** ***************** *****************
^ ^ ^
|--------------| | |--------------|
| | |
v v v
***************
* Client P *
***************
^ ^
| |-------------------------|
*********************** | *********************** |
* Application A * | * Application B * |
* * | * * |
* +----------------+ * | * +----------------+ * |
* | Client A | * | * | Client B | * |
* +----------------+ * | * +----------------+ * |
******* ^ ************* | ***** ^ ****** ^ ****** |
| | | | |
| |-------------| | | |-----|
| | -----------------------| | |
| | | | |
************ v * v * v ********* ***************** v * v ********
* +---------------------+ * * +---------------------+ *
* | Agent 1 | * * | Agent 2 | *
* +---------------------+ * * +---------------------+ *
* ^ ^ ^ ^ * * ^ ^ ^ ^ *
* | | | | * * | | | | *
* v | | v * * v | | v *
* +---------+ | | +--------+ * * +---------+ | | +--------+ *
* | Routing | | | | Local | * * | Routing | | | | Local | *
* | and | | | | Config | * * | and | | | | Config | *
* |Signaling| | | +--------+ * * |Signaling| | | +--------+ *
* +---------+ | | ^ * * +---------+ | | ^ *
* ^ | | | * * ^ | | | *
* | |----| | | * * | |----| | | *
* v | v v * * v | v v *
* +----------+ +------------+ * * +----------+ +------------+ *
* | Dynamic | | Static | * * | Dynamic | | Static | *
* | System | | System | * * | System | | System | *
* | State | | State | * * | State | | State | *
* +----------+ +------------+ * * +----------+ +------------+ *
* * * *
* Routing Element 1 * * Routing Element 2 *
******************************** ********************************
Figure 1: Architecture of I2RS Clients and Agents
Routing Element: A Routing Element implements some subset of the
routing system. It does not need to have a forwarding plane
associated with it. Examples of Routing Elements can include:
* A router with a forwarding plane and RIB Manager that runs
IS-IS, OSPF, BGP, PIM, etc.,
* A BGP speaker acting as a Route Reflector,
* A Label Switching Router (LSR) that implements RSVP-TE,
OSPF-TE, and the Path Computation Element (PCE) Communication
Protocol (PCEP) and has a forwarding plane and associated RIB
Manager, and
* A server that runs IS-IS, OSPF, and BGP and uses Forwarding and
Control Element Separation (ForCES) to control a remote
forwarding plane.
A Routing Element may be locally managed, whether via command-line
interface (CLI), SNMP, or the Network Configuration Protocol
(NETCONF).
Routing and Signaling: This block represents that portion of the
Routing Element that implements part of the Internet routing
system. It includes not merely standardized protocols (i.e.,
IS-IS, OSPF, BGP, PIM, RSVP-TE, LDP, etc.), but also the RIB
Manager layer.
Local Configuration: The black box behavior for interactions between
the ephemeral state that I2RS installs into the routing element;
Local Configuration is defined by this document and the behaviors
specified by the I2RS protocol.
Dynamic System State: An I2RS agent needs access to state on a
routing element beyond what is contained in the routing subsystem.
Such state may include various counters, statistics, flow data,
and local events. This is the subset of operational state that is
needed by network applications based on I2RS that is not contained
in the routing and signaling information. How this information is
provided to the I2RS agent is out of scope, but the standardized
information and data models for what is exposed are part of I2RS.
Static System State: An I2RS agent needs access to static state on a
routing element beyond what is contained in the routing subsystem.
An example of such state is specifying queueing behavior for an
interface or traffic. How the I2RS agent modifies or obtains this
information is out of scope, but the standardized information and
data models for what is exposed are part of I2RS.
I2RS agent: See the definition in Section 2.
Application: A network application that needs to observe the network
or manipulate the network to achieve its service requirements.
I2RS client: See the definition in Section 2.
As can be seen in Figure 1, an I2RS client can communicate with
multiple I2RS agents. Similarly, an I2RS agent may communicate with
multiple I2RS clients -- whether to respond to their requests, to
send notifications, etc. Timely notifications are critical so that
several simultaneously operating applications have up-to-date
information on the state of the network.
As can also be seen in Figure 1, an I2RS agent may communicate with
multiple clients. Each client may send the agent a variety of write
operations. In order to keep the protocol simple, two clients should
not attempt to write (modify) the same piece of information on an
I2RS agent. This is considered an error. However, such collisions
may happen and Section 7.8 ("Multi-headed Control") describes how the
I2RS agent resolves collision by first utilizing priority to resolve
collisions and second by servicing the requests in a first-in, first-
served basis. The I2RS architecture includes this definition of
behavior for this case simply for predictability, not because this is
an intended result. This predictability will simplify error handling
and suppress oscillations. If additional error cases beyond this
simple treatment are required, these error cases should be resolved
by the network applications and management systems.
In contrast, although multiple I2RS clients may need to supply data
into the same list (e.g., a prefix or filter list), this is not
considered an error and must be correctly handled. The nuances so
that writers do not normally collide should be handled in the
information models.
The architectural goal for I2RS is that such errors should produce
predictable behaviors and be reportable to interested clients. The
details of the associated policy is discussed in Section 7.8. The
same policy mechanism (simple priority per I2RS client) applies to
interactions between the I2RS agent and the CLI/SNMP/NETCONF as
described in Section 6.3.
In addition, it must be noted that there may be indirect interactions
between write operations. A basic example of this is when two
different but overlapping prefixes are written with different
forwarding behavior. Detection and avoidance of such interactions is
outside the scope of the I2RS work and is left to agent design and
implementation.
2. Terminology
The following terminology is used in this document.
agent or I2RS agent: An I2RS agent provides the supported I2RS
services from the local system's routing subsystems by interacting
with the routing element to provide specified behavior. The I2RS
agent understands the I2RS protocol and can be contacted by I2RS
clients.
client or I2RS client: A client implements the I2RS protocol, uses
it to communicate with I2RS agents, and uses the I2RS services to
accomplish a task. It interacts with other elements of the
policy, provisioning, and configuration system by means outside of
the scope of the I2RS effort. It interacts with the I2RS agents
to collect information from the routing and forwarding system.
Based on the information and the policy-oriented interactions, the
I2RS client may also interact with I2RS agents to modify the state
of their associated routing systems to achieve operational goals.
An I2RS client can be seen as the part of an application that uses
and supports I2RS and could be a software library.
service or I2RS service: For the purposes of I2RS, a service refers
to a set of related state access functions together with the
policies that control their usage. The expectation is that a
service will be represented by a data model. For instance, 'RIB
service' could be an example of a service that gives access to
state held in a device's RIB.
read scope: The read scope of an I2RS client within an I2RS agent
is the set of information that the I2RS client is authorized to
read within the I2RS agent. The read scope specifies the access
restrictions to both see the existence of data and read the value
of that data.
notification scope: The notification scope is the set of events and
associated information that the I2RS client can request be pushed
by the I2RS agent. I2RS clients have the ability to register for
specific events and information streams, but must be constrained
by the access restrictions associated with their notification
scope.
write scope: The write scope is the set of field values that the
I2RS client is authorized to write (i.e., add, modify or delete).
This access can restrict what data can be modified or created, and
what specific value sets and ranges can be installed.
scope: When unspecified as either read scope, write scope, or
notification scope, the term "scope" applies to the read scope,
write scope, and notification scope.
resources: A resource is an I2RS-specific use of memory, storage,
or execution that a client may consume due to its I2RS operations.
The amount of each such resource that a client may consume in the
context of a particular agent may be constrained based upon the
client's security role. An example of such a resource could
include the number of notifications registered for. These are not
protocol-specific resources or network-specific resources.
role or security role: A security role specifies the scope,
resources, priorities, etc., that a client or agent has. If an
identity has multiple roles in the security system, the identity
is permitted to perform any operations any of those roles permit.
Multiple identities may use the same security role.
identity: A client is associated with exactly one specific
identity. State can be attributed to a particular identity. It
is possible for multiple communication channels to use the same
identity; in that case, the assumption is that the associated
client is coordinating such communication.
identity and scope: A single identity can be associated with
multiple roles. Each role has its own scope, and an identity
associated with multiple roles can use the combined scope of all
its roles. More formally, each identity has:
* a read scope that is the logical OR of the read scopes
associated with its roles,
* a write scope that is the logical OR of the write scopes
associated with its roles, and
* a notification scope that is the logical OR of the notification
scopes associated with its roles.
secondary identity: An I2RS client may supply a secondary opaque
identifier for a secondary identity that is not interpreted by the
I2RS agent. An example of the use of the secondary opaque
identifier is when the I2RS client is a go-between for multiple
applications and it is necessary to track which application has
requested a particular operation.
ephemeral data: Ephemeral data is data that does not persist across
a reboot (software or hardware) or a power on/off condition.
Ephemeral data can be configured data or data recorded from
operations of the router. Ephemeral configuration data also has
the property that a system cannot roll back to a previous
ephemeral configuration state.
group: The NETCONF Access Control Model [RFC6536] uses the term
"group" in terms of an administrative group that supports the
well-established distinction between a root account and other
types of less-privileged conceptual user accounts. "Group" still
refers to a single identity (e.g., root) that is shared by a group
of users.
routing system/subsystem: A routing system or subsystem is a set of
software and/or hardware that determines where packets are
forwarded. The I2RS agent is a component of a routing system.
The term "packets" may be qualified to be layer 1 frames, layer 2
frames, or layer 3 packets. The phrase "Internet routing system"
implies the packets that have IP as layer 3. A routing
"subsystem" indicates that the routing software/hardware is only
the subsystem of another larger system.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Key Architectural Properties
Several key architectural properties for the I2RS protocol are
elucidated below (simplicity, extensibility, and model-driven
programmatic interfaces). However, some architectural properties
such as performance and scaling are not described below because they
are discussed in [RFC7920] and because they may vary based on the
particular use cases.
3.1. Simplicity
There have been many efforts over the years to improve access to the
information available to the routing and forwarding system. Making
such information visible and usable to network management and
applications has many well-understood benefits. There are two
related challenges in doing so. First, the quantity and diversity of
information potentially available is very large. Second, the
variation both in the structure of the data and in the kinds of
operations required tends to introduce protocol complexity.
While the types of operations contemplated here are complex in their
nature, it is critical that I2RS be easily deployable and robust.
Adding complexity beyond what is needed to satisfy well known and
understood requirements would hinder the ease of implementation, the
robustness of the protocol, and the deployability of the protocol.
Overly complex data models tend to ossify information sets by
attempting to describe and close off every possible option,
complicating extensibility.
Thus, one of the key aims for I2RS is to keep the protocol and
modeling architecture simple. So for each architectural component or
aspect, we ask ourselves, "Do we need this complexity, or is the
behavior merely nice to have?" If we need the complexity, we should
ask ourselves, "Is this the simplest way to provide this complexity
in the I2RS external interface?"
3.2. Extensibility
Extensibility of the protocol and data model is very important. In
particular, given the necessary scope limitations of the initial
work, it is critical that the initial design include strong support
for extensibility.
The scope of I2RS work is being designed in phases to provide
deliverable and deployable results at every phase. Each phase will
have a specific set of requirements, and the I2RS protocol and data
models will progress toward these requirements. Therefore, it is
clearly desirable for the I2RS data models to be easily and highly
extensible to represent additional aspects of the network elements or
network systems. It should be easy to integrate data models from
I2RS with other data. This reinforces the criticality of designing
the data models to be highly extensible, preferably in a regular and
simple fashion.
The I2RS Working Group is defining operations for the I2RS protocol.
It would be optimistic to assume that more and different ones may not
be needed when the scope of I2RS increases. Thus, it is important to
consider extensibility not only of the underlying services' data
models, but also of the primitives and protocol operations.
3.3. Model-Driven Programmatic Interfaces
A critical component of I2RS is the standard information and data
models with their associated semantics. While many components of the
routing system are standardized, associated data models for them are
not yet available. Instead, each router uses different information,
different mechanisms, and different CLI, which makes a standard
interface for use by applications extremely cumbersome to develop and
maintain. Well-known data modeling languages exist and may be used
for defining the data models for I2RS.
There are several key benefits for I2RS in using model-driven
architecture and protocol(s). First, it allows for data-model-
focused processing of management data that provides modular
implementation in I2RS clients and I2RS agents. The I2RS client only
needs to implement the models the I2RS client is able to access. The
I2RS agent only needs to implement the data models the I2RS agent
supports.
Second, tools can automate checking and manipulating data; this is
particularly valuable for both extensibility and for the ability to
easily manipulate and check proprietary data models.
The different services provided by I2RS can correspond to separate
data models. An I2RS agent may indicate which data models are
supported.
The purpose of the data model is to provide a definition of the
information regarding the routing system that can be used in
operational networks. If routing information is being modeled for
the first time, a logical information model may be standardized prior
to creating the data model.
4. Security Considerations
This I2RS architecture describes interfaces that clearly require
serious consideration of security. As an architecture, I2RS has been
designed to reuse existing protocols that carry network management
information. Two of the existing protocols that are being reused for
the I2RS protocol version 1 are NETCONF [RFC6241] and RESTCONF
[RESTCONF]. Additional protocols may be reused in future versions of
the I2RS protocol.
The I2RS protocol design process will be to specify additional
requirements (including security) for the existing protocols in order
in order to support the I2RS architecture. After an existing
protocol (e.g., NETCONF or RESTCONF) has been altered to fit the I2RS
requirements, then it will be reviewed to determine if it meets these
requirements. During this review of changes to existing protocols to
serve the I2RS architecture, an in-depth security review of the
revised protocol should be done.
Due to the reuse strategy of the I2RS architecture, this security
section describes the assumed security environment for I2RS with
additional details on a) identity and authentication, b)
authorization, and c) client redundancy. Each protocol proposed for
inclusion as an I2RS protocol will need to be evaluated for the
security constraints of the protocol. The detailed requirements for
the I2RS protocol and the I2RS security environment will be defined
within these global security environments.
The I2RS protocol security requirements for I2RS protocol version 1
are contained in [I2RS-PROT-SEC], and the global I2RS security
environment requirements are contained [I2RS-ENV-SEC].
First, here is a brief description of the assumed security
environment for I2RS. The I2RS agent associated with a Routing
Element is a trusted part of that Routing Element. For example, it
may be part of a vendor-distributed signed software image for the
entire Routing Element, or it may be a trusted signed application
that an operator has installed. The I2RS agent is assumed to have a
separate authentication and authorization channel by which it can
validate both the identity and permissions associated with an I2RS
client. To support numerous and speedy interactions between the I2RS
agent and I2RS client, it is assumed that the I2RS agent can also
cache that particular I2RS clients are trusted and their associated
authorized scope. This implies that the permission information may
be old either in a pull model until the I2RS agent re-requests it or
in a push model until the authentication and authorization channel
can notify the I2RS agent of changes.
Mutual authentication between the I2RS client and I2RS agent is
required. An I2RS client must be able to trust that the I2RS agent
is attached to the relevant Routing Element so that write/modify
operations are correctly applied and so that information received
from the I2RS agent can be trusted by the I2RS client.
An I2RS client is not automatically trustworthy. Each I2RS client is
associated with an identity with a set of scope limitations.
Applications using an I2RS client should be aware that the scope
limitations of an I2RS client are based on its identity (see
Section 4.1) and the assigned role that the identity has. A role
sets specific authorization limits on the actions that an I2RS client
can successfully request of an I2RS agent (see Section 4.2). For
example, one I2RS client may only be able to read a static route
table, but another client may be able add an ephemeral route to the
static route table.
If the I2RS client is acting as a broker for multiple applications,
then managing the security, authentication, and authorization for
that communication is out of scope; nothing prevents the broker from
using the I2RS protocol and a separate authentication and
authorization channel from being used. Regardless of the mechanism,
an I2RS client that is acting as a broker is responsible for
determining that applications using it are trusted and permitted to
make the particular requests.
Different levels of integrity, confidentiality, and replay protection
are relevant for different aspects of I2RS. The primary
communication channel that is used for client authentication and then
used by the client to write data requires integrity, confidentiality
and replay protection. Appropriate selection of a default required
transport protocol is the preferred way of meeting these
requirements.
Other communications via I2RS may not require integrity,
confidentiality, and replay protection. For instance, if an I2RS
client subscribes to an information stream of prefix announcements
from OSPF, those may require integrity but probably not
confidentiality or replay protection. Similarly, an information
stream of interface statistics may not even require guaranteed
delivery. In Section 7.2, additional logins regarding multiple
communication channels and their use is provided. From the security
perspective, it is critical to realize that an I2RS agent may open a
new communication channel based upon information provided by an I2RS
client (as described in Section 7.2). For example, an I2RS client
may request notifications of certain events, and the agent will open
a communication channel to report such events. Therefore, to avoid
an indirect attack, such a request must be done in the context of an
authenticated and authorized client whose communications cannot have
been altered.
4.1. Identity and Authentication
As discussed above, all control exchanges between the I2RS client and
agent should be authenticated and integrity-protected (such that the
contents cannot be changed without detection). Further, manipulation
of the system must be accurately attributable. In an ideal
architecture, even information collection and notification should be
protected; this may be subject to engineering trade-offs during the
design.
I2RS clients may be operating on behalf of other applications. While
those applications' identities are not needed for authentication or
authorization, each application should have a unique opaque
identifier that can be provided by the I2RS client to the I2RS agent
for purposes of tracking attribution of operations to an application
identifier (and from that to the application's identity). This
tracking of operations to an application supports I2RS functionality
for tracing actions (to aid troubleshooting in routers) and logging
of network changes.
4.2. Authorization
All operations using I2RS, both observation and manipulation, should
be subject to appropriate authorization controls. Such authorization
is based on the identity and assigned role of the I2RS client
performing the operations and the I2RS agent in the network element.
Multiple identities may use the same role(s). As noted in the
definitions of "identity" and "role" above, if multiple roles are
associated with an identity then the identity is authorized to
perform any operation authorized by any of its roles.
I2RS agents, in performing information collection and manipulation,
will be acting on behalf of the I2RS clients. As such, each
operation authorization will be based on the lower of the two
permissions of the agent itself and of the authenticated client. The
mechanism by which this authorization is applied within the device is
outside of the scope of I2RS.
The appropriate or necessary level of granularity for scope can
depend upon the particular I2RS service and the implementation's
granularity. An approach to a similar access control problem is
defined in the NETCONF Access Control Model (NACM) [RFC6536]; it
allows arbitrary access to be specified for a data node instance
identifier while defining meaningful manipulable defaults. The
identity within NACM [RFC6536] can be specified as either a user name
or a group user name (e.g., Root), and this name is linked a scope
policy that is contained in a set of access control rules.
Similarly, it is expected the I2RS identity links to one role that
has a scope policy specified by a set of access control rules. This
scope policy can be provided via Local Configuration, exposed as an
I2RS service for manipulation by authorized clients, or via some
other method (e.g., Authentication, Authorization, and Accounting
(AAA) service)
While the I2RS agent allows access based on the I2RS client's scope
policy, this does not mean the access is required to arrive on a
particular transport connection or from a particular I2RS client by
the I2RS architecture. The operator-applied scope policy may or may
not restrict the transport connection or the identities that can
access a local I2RS agent.
When an I2RS client is authenticated, its identity is provided to the
I2RS agent, and this identity links to a role that links to the scope
policy. Multiple identities may belong to the same role; for
example, such a role might be an Internal-Routes-Monitor that allows
reading of the portion of the I2RS RIB associated with IP prefixes
used for internal device addresses in the AS.
4.3. Client Redundancy
I2RS must support client redundancy. At the simplest, this can be
handled by having a primary and a backup network application that
both use the same client identity and can successfully authenticate
as such. Since I2RS does not require a continuous transport
connection and supports multiple transport sessions, this can provide
some basic redundancy. However, it does not address the need for
troubleshooting and logging of network changes to be informed about
which network application is actually active. At a minimum, basic
transport information about each connection and time can be logged
with the identity.
4.4. I2RS in Personal Devices
If an I2RS agent or I2RS client is tightly correlated with a person
(such as if an I2RS agent is running on someone's phone to control
tethering), then this usage can raise privacy issues, over and above
the security issues that normally need to be handled in I2RS. One
example of an I2RS interaction that could raise privacy issues is if
the I2RS interaction enabled easier location tracking of a person's
phone. The I2RS protocol and data models should consider if privacy
issues can arise when clients or agents are used for such use cases.
5. Network Applications and I2RS Client
I2RS is expected to be used by network-oriented applications in
different architectures. While the interface between a network-
oriented application and the I2RS client is outside the scope of
I2RS, considering the different architectures is important to
sufficiently specify I2RS.
In the simplest architecture of direct access, a network-oriented
application has an I2RS client as a library or driver for
communication with routing elements.
In the broker architecture, multiple network-oriented applications
communicate in an unspecified fashion to a broker application that
contains an I2RS client. That broker application requires additional
functionality for authentication and authorization of the network-
oriented applications; such functionality is out of scope for I2RS,
but similar considerations to those described in Section 4.2 do
apply. As discussed in Section 4.1, the broker I2RS client should
determine distinct opaque identifiers for each network-oriented
application that is using it. The broker I2RS client can pass along
the appropriate value as a secondary identifier, which can be used
for tracking attribution of operations.
In a third architecture, a routing element or network-oriented
application that uses an I2RS client to access services on a
different routing element may also contain an I2RS agent to provide
services to other network-oriented applications. However, where the
needed information and data models for those services differs from
that of a conventional routing element, those models are, at least
initially, out of scope for I2RS. The following section describes an
example of such a network application.
5.1. Example Network Application: Topology Manager
A Topology Manager includes an I2RS client that uses the I2RS data
models and protocol to collect information about the state of the
network by communicating directly with one or more I2RS agents. From
these I2RS agents, the Topology Manager collects routing
configuration and operational data, such as interface and Label
Switched Path (LSP) information. In addition, the Topology Manager
may collect link-state data in several ways -- via I2RS models, by
peering with BGP-LS [RFC7752], or by listening into the IGP.
The set of functionality and collected information that is the
Topology Manager may be embedded as a component of a larger
application, such as a path computation application. As a stand-
alone application, the Topology Manager could be useful to other
network applications by providing a coherent picture of the network
state accessible via another interface. That interface might use the
same I2RS protocol and could provide a topology service using
extensions to the I2RS data models.
6. I2RS Agent Role and Functionality
The I2RS agent is part of a routing element. As such, it has
relationships with that routing element as a whole and with various
components of that routing element.
6.1. Relationship to Its Routing Element
A Routing Element may be implemented with a wide variety of different
architectures: an integrated router, a split architecture,
distributed architecture, etc. The architecture does not need to
affect the general I2RS agent behavior.
For scalability and generality, the I2RS agent may be responsible for
collecting and delivering large amounts of data from various parts of
the routing element. Those parts may or may not actually be part of
a single physical device. Thus, for scalability and robustness, it
is important that the architecture allow for a distributed set of
reporting components providing collected data from the I2RS agent
back to the relevant I2RS clients. There may be multiple I2RS agents
within the same router. In such a case, they must have non-
overlapping sets of information that they manipulate.
To facilitate operations, deployment, and troubleshooting, it is
important that traceability of the requests received by I2RS agent's
and actions taken be supported via a common data model.
6.2. I2RS State Storage
State modification requests are sent to the I2RS agent in a routing
element by I2RS clients. The I2RS agent is responsible for applying
these changes to the system, subject to the authorization discussed
above. The I2RS agent will retain knowledge of the changes it has
applied, and the client on whose behalf it applied the changes. The
I2RS agent will also store active subscriptions. These sets of data
form the I2RS datastore. This data is retained by the agent until
the state is removed by the client, it is overridden by some other
operation such as CLI, or the device reboots. Meaningful logging of
the application and removal of changes are recommended. I2RS-applied
changes to the routing element state will not be retained across
routing element reboot. The I2RS datastore is not preserved across
routing element reboots; thus, the I2RS agent will not attempt to
reapply such changes after a reboot.
6.2.1. I2RS Agent Failure
It is expected that an I2RS agent may fail independently of the
associated routing element. This could happen because I2RS is
disabled on the routing element or because the I2RS agent, which may
be a separate process or even running on a separate processor,
experiences an unexpected failure. Just as routing state learned
from a failed source is removed, the ephemeral I2RS state will
usually be removed shortly after the failure is detected or as part
of a graceful shutdown process. To handle these two types of
failures, the I2RS agent MUST support two different notifications: a
notification for the I2RS agent terminating gracefully, and a
notification for the I2RS agent starting up after an unexpected
failure. The two notifications are described below followed by a
description of their use in unexpected failures and graceful
shutdowns.
NOTIFICATION_I2RS_AGENT_TERMINATING: This notification reports that
the associated I2RS agent is shutting down gracefully and that
I2RS ephemeral state will be removed. It can optionally include a
timestamp indicating when the I2RS agent will shut down. Use of
this timestamp assumes that time synchronization has been done,
and the timestamp should not have granularity finer than one
second because better accuracy of shutdown time is not guaranteed.
NOTIFICATION_I2RS_AGENT_STARTING: This notification signals to the
I2RS client(s) that the associated I2RS agent has started. It
includes an agent-boot-count that indicates how many times the
I2RS agent has restarted since the associated routing element
restarted. The agent-boot-count allows an I2RS client to
determine if the I2RS agent has restarted. (Note: This
notification will be sent by the I2RS agent to I2RS clients that
are known by the I2RS agent after a reboot. How the I2RS agent
retains the knowledge of these I2RS clients is out of scope of
this architecture.)
There are two different failure types that are possible, and each has
different behavior.
Unexpected failure: In this case, the I2RS agent has unexpectedly
crashed and thus cannot notify its clients of anything. Since
I2RS does not require a persistent connection between the I2RS
client and I2RS agent, it is necessary to have a mechanism for the
I2RS agent to notify I2RS clients that had subscriptions or
written ephemeral state; such I2RS clients should be cached by the
I2RS agent's system in persistent storage. When the I2RS agent
starts, it should send a NOTIFICATION_I2RS_AGENT_STARTING to each
cached I2RS client.
Graceful shutdowns: In this case, the I2RS agent can do specific
limited work as part of the process of being disabled. The I2RS
agent must send a NOTIFICATION_I2RS_AGENT_TERMINATING to all its
cached I2RS clients. If the I2RS agent restarts after a graceful
termination, it will send a NOTIFICATION_I2RS_AGENT_STARTING to
each cached I2RS client.
6.2.2. Starting and Ending
When an I2RS client applies changes via the I2RS protocol, those
changes are applied and left until removed or the routing element
reboots. The network application may make decisions about what to
request via I2RS based upon a variety of conditions that imply
different start times and stop times. That complexity is managed by
the network application and is not handled by I2RS.
6.2.3. Reversion
An I2RS agent may decide that some state should no longer be applied.
An I2RS client may instruct an agent to remove state it has applied.
In all such cases, the state will revert to what it would have been
without the I2RS client-agent interaction; that state is generally
whatever was specified via the CLI, NETCONF, SNMP, etc., I2RS agents
will not store multiple alternative states, nor try to determine
which one among such a plurality it should fall back to. Thus, the
model followed is not like the RIB, where multiple routes are stored
at different preferences. (For I2RS state in the presence of two
I2RS clients, please see Sections 1.2 and 7.8)
An I2RS client may register for notifications, subject to its
notification scope, regarding state modification or removal by a
particular I2RS client.
6.3. Interactions with Local Configuration
Changes may originate from either Local Configuration or from I2RS.
The modifications and data stored by I2RS are separate from the local
device configuration, but conflicts between the two must be resolved
in a deterministic manner that respects operator-applied policy. The
deterministic manner is the result of general I2RS rules, system
rules, knobs adjusted by operator-applied policy, and the rules
associated with the YANG data model (often in "MUST" and "WHEN"
clauses for dependencies).
The operator-applied policy knobs can determine whether the Local
Configuration overrides a particular I2RS client's request or vice
versa. Normally, most devices will have an operator-applied policy
that will prioritize the I2RS client's ephemeral configuration
changes so that ephemeral data overrides the Local Configuration.
These operator-applied policy knobs can be implemented in many ways.
One way is for the routing element to configure a priority on the
Local Configuration and a priority on the I2RS client's write of the
ephemeral configuration. The I2RS mechanism would compare the I2RS
client's priority to write with that priority assigned to the Local
Configuration in order to determine whether Local Configuration or
I2RS client's write of ephemeral data wins.
To make sure the I2RS client's requests are what the operator
desires, the I2RS data modules have a general rule that, by default,
the Local Configuration always wins over the I2RS ephemeral
configuration.
The reason for this general rule is if there is no operator-applied
policy to turn on I2RS ephemeral overwrites of Local Configuration,
then the I2RS overwrites should not occur. This general rule allows
the I2RS agents to be installed in routing systems and the
communication tested between I2RS clients and I2RS agents without the
I2RS agent overwriting configuration state. For more details, see
the examples below.
In the case when the I2RS ephemeral state always wins for a data
model, if there is an I2RS ephemeral state value, it is installed
instead of the Local Configuration state value. The Local
Configuration information is stored so that if/when an I2RS client
removes I2RS ephemeral state, the Local Configuration state can be
restored.
When the Local Configuration always wins, some communication between
that subsystem and the I2RS agent is still necessary. As an I2RS
agent connects to the routing subsystem, the I2RS agent must also
communicate with the Local Configuration to exchange model
information so the I2RS agent knows the details of each specific
device configuration change that the I2RS agent is permitted to
modify. In addition, when the system determines that a client's I2RS
state is preempted, the I2RS agent must notify the affected I2RS
clients; how the system determines this is implementation dependent.
It is critical that policy based upon the source is used because the
resolution cannot be time based. Simply allowing the most recent
state to prevail could cause race conditions where the final state is
not repeatably deterministic.
6.3.1. Examples of Local Configuration vs. I2RS Ephemeral Configuration
A set of examples is useful in order to illustrated these
architecture principles. Assume there are three routers: Router A,
Router B, and Router C. There are two operator-applied policy knobs
that these three routers must have regarding ephemeral state.
o Policy Knob 1: Ephemeral configuration overwrites Local
Configuration.
o Policy Knob 2: Update of Local Configuration value supersedes and
overwrites the ephemeral configuration.
For Policy Knob 1, the routers with an I2RS agent receiving a write
for an ephemeral entry in a data model must consider the following:
1. Does the operator policy allow the ephemeral configuration
changes to have priority over existing Local Configuration?
2. Does the YANG data model have any rules associated with the
ephemeral configuration (such as the "MUST" or "WHEN" rule)?
For this example, there is no "MUST" or "WHEN" rule in the data being
written.
The policy settings are:
Policy Knob 1 Policy Knob 2
=================== ==================
Router A ephemeral has ephemeral has
priority priority
Router B Local Configuration Local Configuration
has priority has priority
Router C ephemeral has Local Configuration
priority has priority
Router A has the normal operator policy in Policy Knob 1 and Policy
Knob 2 that prioritizes ephemeral configuration over Local
Configuration in the I2RS agent. An I2RS client sends a write to an
ephemeral configuration value via an I2RS agent in Router A. The
I2RS agent overwrites the configuration value in the intended
configuration, and the I2RS agent returns an acknowledgement of the
write. If the Local Configuration value changes, Router A stays with
the ephemeral configuration written by the I2RS client.
Router B's operator has no desire to allow ephemeral writes to
overwrite Local Configuration even though it has installed an I2RS
agent. Router B's policy prioritizes the Local Configuration over
the ephemeral write. When the I2RS agent on Router B receives a
write from an I2RS client, the I2RS agent will check the operator
Policy Knob 1 and return a response to the I2RS client indicating the
operator policy did not allow the overwriting of the Local
Configuration.
The Router B case demonstrates why the I2RS architecture sets the
default to the Local Configuration wins. Since I2RS functionality is
new, the operator must enable it. Otherwise, the I2RS ephemeral
functionality is off. Router B's operators can install the I2RS code
and test responses without engaging the I2RS overwrite function.
Router C's operator sets Policy Knob 1 for the I2RS clients to
overwrite existing Local Configuration and Policy Knob 2 for the
Local Configuration changes to update ephemeral state. To understand
why an operator might set the policy knobs this way, consider that
Router C is under the control of an operator that has a back-end
system that re-writes the Local Configuration of all systems at 11
p.m. each night. Any ephemeral change to the network is only
supposed to last until 11 p.m. when the next Local Configuration
changes are rolled out from the back-end system. The I2RS client
writes the ephemeral state during the day, and the I2RS agent on
Router C updates the value. At 11 p.m., the back-end configuration
system updates the Local Configuration via NETCONF, and the I2RS
agent is notified that the Local Configuration updated this value.
The I2RS agent notifies the I2RS client that the value has been
overwritten by the Local Configuration. The I2RS client in this use
case is a part of an application that tracks any ephemeral state
changes to make sure all ephemeral changes are included in the next
configuration run.
6.4. Routing Components and Associated I2RS Services
For simplicity, each logical protocol or set of functionality that
can be compactly described in a separable information and data model
is considered as a separate I2RS service. A routing element need not
implement all routing components described nor provide the associated
I2RS services. I2RS services should include a capability model so
that peers can determine which parts of the service are supported.
Each I2RS service requires an information model that describes at
least the following: data that can be read, data that can be written,
notifications that can be subscribed to, and the capability model
mentioned above.
The initial services included in the I2RS architecture are as
follows.
*************************** ************** *****************
* I2RS Protocol * * * * Dynamic *
* * * Interfaces * * Data & *
* +--------+ +-------+ * * * * Statistics *
* | Client | | Agent | * ************** *****************
* +--------+ +-------+ *
* * ************** *************
*************************** * * * *
* Policy * * Base QoS *
******************** ******** * Templates * * Templates *
* +--------+ * * * * * *************
* BGP | BGP-LS | * * PIM * **************
* +--------+ * * *
******************** ******** ****************************
* MPLS +---------+ +-----+ *
********************************** * | RSVP-TE | | LDP | *
* IGPs +------+ +------+ * * +---------+ +-----+ *
* +--------+ | OSPF | |IS-IS | * * +--------+ *
* | Common | +------+ +------+ * * | Common | *
* +--------+ * * +--------+ *
********************************** ****************************
**************************************************************
* RIB Manager *
* +-------------------+ +---------------+ +------------+ *
* | Unicast/multicast | | Policy-Based | | RIB Policy | *
* | RIBs & LIBs | | Routing | | Controls | *
* | route instances | | (ACLs, etc) | +------------+ *
* +-------------------+ +---------------+ *
**************************************************************
Figure 2: Anticipated I2RS Services
There are relationships between different I2RS services -- whether
those be the need for the RIB to refer to specific interfaces, the
desire to refer to common complex types (e.g., links, nodes, IP
addresses), or the ability to refer to implementation-specific
functionality (e.g., pre-defined templates to be applied to
interfaces or for QoS behaviors that traffic is directed into).
Section 6.4.5 discusses information modeling constructs and the range
of relationship types that are applicable.
6.4.1. Routing and Label Information Bases
Routing elements may maintain one or more information bases.
Examples include Routing Information Bases such as IPv4/IPv6 Unicast
or IPv4/IPv6 Multicast. Another such example includes the MPLS Label
Information Bases, per platform, per interface, or per context. This
functionality, exposed via an I2RS service, must interact smoothly
with the same mechanisms that the routing element already uses to
handle RIB input from multiple sources. Conceptually, this can be
handled by having the I2RS agent communicate with a RIB Manager as a
separate routing source.
The point-to-multipoint state added to the RIB does not need to match
to well-known multicast protocol installed state. The I2RS agent can
create arbitrary replication state in the RIB, subject to the
advertised capabilities of the routing element.
6.4.2. IGPs, BGP, and Multicast Protocols
A separate I2RS service can expose each routing protocol on the
device. Such I2RS services may include a number of different kinds
of operations:
o reading the various internal RIB(s) of the routing protocol is
often helpful for understanding the state of the network.
Directly writing to these protocol-specific RIBs or databases is
out of scope for I2RS.
o reading the various pieces of policy information the particular
protocol instance is using to drive its operations.
o writing policy information such as interface attributes that are
specific to the routing protocol or BGP policy that may indirectly
manipulate attributes of routes carried in BGP.
o writing routes or prefixes to be advertised via the protocol.
o joining/removing interfaces from the multicast trees.
o subscribing to an information stream of route changes.
o receiving notifications about peers coming up or going down.
For example, the interaction with OSPF might include modifying the
local routing element's link metrics, announcing a locally attached
prefix, or reading some of the OSPF link-state database. However,
direct modification of the link-state database must not be allowed in
order to preserve network state consistency.
6.4.3. MPLS
I2RS services will be needed to expose the protocols that create
transport LSPs (e.g., LDP and RSVP-TE) as well as protocols (e.g.,
BGP, LDP) that provide MPLS-based services (e.g., pseudowires,
L3VPNs, L2VPNs, etc). This should include all local information
about LSPs originating in, transiting, or terminating in this Routing
Element.
6.4.4. Policy and QoS Mechanisms
Many network elements have separate policy and QoS mechanisms,
including knobs that affect local path computation and queue control
capabilities. These capabilities vary widely across implementations,
and I2RS cannot model the full range of information collection or
manipulation of these attributes. A core set does need to be
included in the I2RS information models and supported in the expected
interfaces between the I2RS agent and the network element, in order
to provide basic capabilities and the hooks for future extensibility.
By taking advantage of extensibility and subclassing, information
models can specify use of a basic model that can be replaced by a
more detailed model.
6.4.5. Information Modeling, Device Variation, and Information
Relationships
I2RS depends heavily on information models of the relevant aspects of
the Routing Elements to be manipulated. These models drive the data
models and protocol operations for I2RS. It is important that these
information models deal well with a wide variety of actual
implementations of Routing Elements, as seen between different
products and different vendors. There are three ways that I2RS
information models can address these variations: class or type
inheritance, optional features, and templating.
6.4.5.1. Managing Variation: Object Classes/Types and Inheritance
Information modeled by I2RS from a Routing Element can be described
in terms of classes or types or object. Different valid inheritance
definitions can apply. What is appropriate for I2RS to use is not
determined in this architecture; for simplicity, "class" and
"subclass" will be used as the example terminology. This I2RS
architecture does require the ability to address variation in Routing
Elements by allowing information models to define parent or base
classes and subclasses.
The base or parent class defines the common aspects that all Routing
Elements are expected to support. Individual subclasses can
represent variations and additional capabilities. When applicable,
there may be several levels of refinement. The I2RS protocol can
then provide mechanisms to allow an I2RS client to determine which
classes a given I2RS agent has available. I2RS clients that only
want basic capabilities can operate purely in terms of base or parent
classes, while a client needing more details or features can work
with the supported subclass(es).
As part of I2RS information modeling, clear rules should be specified
for how the parent class and subclass can relate; for example, what
changes can a subclass make to its parent? The description of such
rules should be done so that it can apply across data modeling tools
until the I2RS data modeling language is selected.
6.4.5.2. Managing Variation: Optionality
I2RS information models must be clear about what aspects are
optional. For instance, must an instance of a class always contain a
particular data field X? If so, must the client provide a value for
X when creating the object or is there a well-defined default value?
From the Routing Element perspective, in the above example, each
information model should provide information regarding the following
questions:
o Is X required for the data field to be accepted and applied?
o If X is optional, then how does "X" as an optional portion of the
data field interact with the required aspects of the data field?
o Does the data field have defaults for the mandatory portion of the
field and the optional portions of the field?
o Is X required to be within a particular set of values (e.g.,
range, length of strings)?
The information model needs to be clear about what read or write
values are set by the client and what responses or actions are
required by the agent. It is important to indicate what is required
or optional in client values and agent responses/actions.
6.4.5.3. Managing Variation: Templating
A template is a collection of information to address a problem; it
cuts across the notions of class and object instances. A template
provides a set of defined values for a set of information fields and
can specify a set of values that must be provided to complete the
template. Further, a flexible template scheme may allow some of the
defined values to be overwritten.
For instance, assigning traffic to a particular service class might
be done by specifying a template queueing with a parameter to
indicate Gold, Silver, or Best Effort. The details of how that is
carried out are not modeled. This does assume that the necessary
templates are made available on the Routing Element via some
mechanism other than I2RS. The idea is that by providing suitable
templates for tasks that need to be accomplished, with templates
implemented differently for different kinds of Routing Elements, the
client can easily interact with the Routing Element without concern
for the variations that are handled by values included in the
template.
If implementation variation can be exposed in other ways, templates
may not be needed. However, templates themselves could be objects
referenced in the protocol messages, with Routing Elements being
configured with the proper templates to complete the operation. This
is a topic for further discussion.
6.4.5.4. Object Relationships
Objects (in a Routing Element or otherwise) do not exist in
isolation. They are related to each other. One of the important
things a class definition does is represent the relationships between
instances of different classes. These relationships can be very
simple or quite complicated. The following sections list the
information relationships that the information models need to
support.
6.4.5.4.1. Initialization
The simplest relationship is that one object instance is initialized
by copying another. For example, one may have an object instance
that represents the default setup for a tunnel, and all new tunnels
have fields copied from there if they are not set as part of
establishment. This is closely related to the templates discussed
above, but not identical. Since the relationship is only momentary,
it is often not formally represented in modeling but only captured in
the semantic description of the default object.
6.4.5.4.2. Correlation Identification
Often, it suffices to indicate in one object that it is related to a
second object, without having a strong binding between the two. So
an identifier is used to represent the relationship. This can be
used to allow for late binding or a weak binding that does not even
need to exist. A policy name in an object might indicate that if a
policy by that name exists, it is to be applied under some
circumstance. In modeling, this is often represented by the type of
the value.
6.4.5.4.3. Object References
Sometimes the relationship between objects is stronger. A valid ARP
entry has to point to the active interface over which it was derived.
This is the classic meaning of an object reference in programming.
It can be used for relationships like containment or dependence.
This is usually represented by an explicit modeling link.
6.4.5.4.4. Active References
There is an even stronger form of coupling between objects if changes
in one of the two objects are always to be reflected in the state of
the other. For example, if a tunnel has an MTU (maximum transmit
unit), and link MTU changes need to immediately propagate to the
tunnel MTU, then the tunnel is actively coupled to the link
interface. This kind of active state coupling implies some sort of
internal bookkeeping to ensure consistency, often conceptualized as a
subscription model across objects.
7. I2RS Client Agent Interface
7.1. One Control and Data Exchange Protocol
This I2RS architecture assumes a data-model-driven protocol where the
data models are defined in YANG 1.1 [YANG1.1] and associated YANG
based model documents [RFC6991], [RFC7223], [RFC7224], [RFC7277],
[RFC7317]. Two of the protocols to be expanded to support the I2RS
protocol are NETCONF [RFC6241] and RESTCONF [RESTCONF]. This helps
meet the goal of simplicity and thereby enhances deployability. The
I2RS protocol may need to use several underlying transports (TCP,
SCTP (Stream Control Transport Protocol), DCCP (Datagram Congestion
Control Protocol)), with suitable authentication and integrity-
protection mechanisms. These different transports can support
different types of communication (e.g., control, reading,
notifications, and information collection) and different sets of
data. Whatever transport is used for the data exchange, it must also
support suitable congestion-control mechanisms. The transports
chosen should be operator and implementor friendly to ease adoption.
Each version of the I2RS protocol will specify the following: a)
which transports may be used by the I2RS protocol, b) which
transports are mandatory to implement, and c) which transports are
optional to implement.
7.2. Communication Channels
Multiple communication channels and multiple types of communication
channels are required. There may be a range of requirements (e.g.,
confidentiality, reliability), and to support the scaling, there may
need to be channels originating from multiple subcomponents of a
routing element and/or to multiple parts of an I2RS client. All such
communication channels will use the same higher-layer I2RS protocol
(which combines secure transport and I2RS contextual information).
The use of additional channels for communication will be coordinated
between the I2RS client and the I2RS agent using this protocol.
I2RS protocol communication may be delivered in-band via the routing
system's data plane. I2RS protocol communication might be delivered
out-of-band via a management interface. Depending on what operations
are requested, it is possible for the I2RS protocol communication to
cause the in-band communication channels to stop working; this could
cause the I2RS agent to become unreachable across that communication
channel.
7.3. Capability Negotiation
The support for different protocol capabilities and I2RS services
will vary across I2RS clients and Routing Elements supporting I2RS
agents. Since each I2RS service is required to include a capability
model (see Section 6.4), negotiation at the protocol level can be
restricted to protocol specifics and which I2RS services are
supported.
Capability negotiation (such as which transports are supported beyond
the minimum required to implement) will clearly be necessary. It is
important that such negotiations be kept simple and robust, as such
mechanisms are often a source of difficulty in implementation and
deployment.
The protocol capability negotiation can be segmented into the basic
version negotiation (required to ensure basic communication), and the
more complex capability exchange that can take place within the base
protocol mechanisms. In particular, the more complex protocol and
mechanism negotiation can be addressed by defining information models
for both the I2RS agent and the I2RS client. These information
models can describe the various capability options. This can then
represent and be used to communicate important information about the
agent and the capabilities thereof.
7.4. Scope Policy Specifications
As Sections 4.1 and 4.2 describe, each I2RS client will have a unique
identity and may have a secondary identity (see Section 2) to aid in
troubleshooting. As Section 4 indicates, all authentication and
authorization mechanisms are based on the primary identity, which
links to a role with scope policy for reading data, for writing data,
and for limiting the resources that can be consumed. The
specifications for data scope policy (for read, write, or resources
consumption) need to specify the data being controlled by the policy,
and acceptable ranges of values for the data.
7.5. Connectivity
An I2RS client may or may not maintain an active communication
channel with an I2RS agent. Therefore, an I2RS agent may need to
open a communication channel to the client to communicate previously
requested information. The lack of an active communication channel
does not imply that the associated I2RS client is non-functional.
When communication is required, the I2RS agent or I2RS client can
open a new communication channel.
State held by an I2RS agent that is owned by an I2RS client should
not be removed or cleaned up when a client is no longer
communicating, even if the agent cannot successfully open a new
communication channel to the client.
For many applications, it may be desirable to clean up state if a
network application dies before removing the state it has created.
Typically, this is dealt with in terms of network application
redundancy. If stronger mechanisms are desired, mechanisms outside
of I2RS may allow a supervisory network application to monitor I2RS
clients and, based on policy known to the supervisor, clean up state
if applications die. More complex mechanisms instantiated in the
I2RS agent would add complications to the I2RS protocol and are thus
left for future work.
Some examples of such a mechanism include the following. In one
option, the client could request state cleanup if a particular
transport session is terminated. The second is to allow state
expiration, expressed as a policy associated with the I2RS client's
role. The state expiration could occur after there has been no
successful communication channel to or from the I2RS client for the
policy-specified duration.
7.6. Notifications
As with any policy system interacting with the network, the I2RS
client needs to be able to receive notifications of changes in
network state. Notifications here refer to changes that are
unanticipated, represent events outside the control of the systems
(such as interface failures on controlled devices), or are
sufficiently sparse as to be anomalous in some fashion. A
notification may also be due to a regular event.
Such events may be of interest to multiple I2RS clients controlling
data handled by an I2RS agent and to multiple other I2RS clients that
are collecting information without exerting control. The
architecture therefore requires that it be practical for I2RS clients
to register for a range of notifications and for the I2RS agents to
send notifications to a number of clients. The I2RS client should be
able to filter the specific notifications that will be received; the
specific types of events and filtering operations can vary by
information model and need to be specified as part of the information
model.
The I2RS information model needs to include representation of these
events. As discussed earlier, the capability information in the
model will allow I2RS clients to understand which events a given I2RS
agent is capable of generating.
For performance and scaling by the I2RS client and general
information confidentiality, an I2RS client needs to be able to
register for just the events it is interested in. It is also
possible that I2RS might provide a stream of notifications via a
publish/subscribe mechanism that is not amenable to having the I2RS
agent do the filtering.
7.7. Information Collection
One of the other important aspects of I2RS is that it is intended to
simplify collecting information about the state of network elements.
This includes both getting a snapshot of a large amount of data about
the current state of the network element and subscribing to a feed of
the ongoing changes to the set of data or a subset thereof. This is
considered architecturally separate from notifications due to the
differences in information rate and total volume.
7.8. Multi-headed Control
As described earlier, an I2RS agent interacts with multiple I2RS
clients who are actively controlling the network element. From an
architecture and design perspective, the assumption is that by means
outside of this system, the data to be manipulated within the network
element is appropriately partitioned so that any given piece of
information is only being manipulated by a single I2RS client.
Nonetheless, unexpected interactions happen, and two (or more) I2RS
clients may attempt to manipulate the same piece of data. This is
considered an error case. This architecture does not attempt to
determine what the right state of data should be when such a
collision happens. Rather, the architecture mandates that there be
decidable means by which I2RS agents handle the collisions. The
mechanism for ensuring predictability is to have a simple priority
associated with each I2RS client, and the highest priority change
remains in effect. In the case of priority ties, the first I2RS
client whose attribution is associated with the data will keep
control.
In order for this approach to multi-headed control to be useful for
I2RS clients, it is necessary that an I2RS client can register to
receive notifications about changes made to writeable data, whose
state is of specific interest to that I2RS client. This is included
in the I2RS event mechanisms. This also needs to apply to changes
made by CLI/NETCONF/SNMP within the write scope of the I2RS agent, as
the same priority mechanism (even if it is "CLI always wins") applies
there. The I2RS client may then respond to the situation as it sees
fit.
7.9. Transactions
In the interest of simplicity, the I2RS architecture does not include
multi-message atomicity and rollback mechanisms. Rather, it includes
a small range of error handling for a set of operations included in a
single message. An I2RS client may indicate one of the following
three methods of error handling for a given message with multiple
operations that it sends to an I2RS agent:
Perform all or none: This traditional SNMP semantic indicates that
the I2RS agent will keep enough state when handling a single
message to roll back the operations within that message. Either
all the operations will succeed, or none of them will be applied,
and an error message will report the single failure that caused
them not to be applied. This is useful when there are, for
example, mutual dependencies across operations in the message.
Perform until error: In this case, the operations in the message are
applied in the specified order. When an error occurs, no further
operations are applied, and an error is returned indicating the
failure. This is useful if there are dependencies among the
operations and they can be topologically sorted.
Perform all storing errors: In this case, the I2RS agent will
attempt to perform all the operations in the message and will
return error indications for each one that fails. This is useful
when there is no dependency across the operation or when the I2RS
client would prefer to sort out the effect of errors on its own.
In the interest of robustness and clarity of protocol state, the
protocol will include an explicit reply to modification or write
operations even when they fully succeed.
8. Operational and Manageability Considerations
In order to facilitate troubleshooting of routing elements
implementing I2RS agents, the routing elements should provide for a
mechanism to show actively provisioned I2RS state and other I2RS
agent internal information. Note that this information may contain
highly sensitive material subject to the security considerations of
any data models implemented by that agent and thus must be protected
according to those considerations. Preferably, this mechanism should
use a different privileged means other than simply connecting as an
I2RS client to learn the data. Using a different mechanism should
improve traceability and failure management.
Manageability plays a key aspect in I2RS. Some initial examples
include:
Resource Limitations: Using I2RS, applications can consume
resources, whether those be operations in a time frame, entries in
the RIB, stored operations to be triggered, etc. The ability to
set resource limits based upon authorization is important.
Configuration Interactions: The interaction of state installed via
I2RS and via a router's configuration needs to be clearly defined.
As described in this architecture, a simple priority that is
configured is used to provide sufficient policy flexibility.
Traceability of Interactions: The ability to trace the interactions
of the requests received by the I2RS agent's and actions taken by
the I2RS agents is needed so that operations can monitor I2RS
agents during deployment, and troubleshoot software or network
problems.
Notification Subscription Service: The ability for an I2RS client to
subscribe to a notification stream pushed from the I2RS agent
(rather than having I2RS client poll the I2RS agent) provides a
more scalable notification handling for the I2RS agent-client
interactions.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC7920] Atlas, A., Ed., Nadeau, T., Ed., and D. Ward, "Problem
Statement for the Interface to the Routing System",
RFC 7920, DOI 10.17487/RFC7920, June 2016,
<http://www.rfc-editor.org/info/rfc7920>.
9.2. Informative References
[I2RS-ENV-SEC]
Migault, D., Ed., Halpern, J., and S. Hares, "I2RS
Environment Security Requirements", Work in Progress,
draft-ietf-i2rs-security-environment-reqs-01, April 2016.
[I2RS-PROT-SEC]
Hares, S., Migault, D., and J. Halpern, "I2RS Security
Related Requirements", Work in Progress, draft-ietf-i2rs-
protocol-security-requirements-06, May 2016.
[RESTCONF] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", Work in Progress, draft-ietf-netconf-
restconf-14, June 2016.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<http://www.rfc-editor.org/info/rfc6241>.
[RFC6536] Bierman, A. and M. Bjorklund, "Network Configuration
Protocol (NETCONF) Access Control Model", RFC 6536,
DOI 10.17487/RFC6536, March 2012,
<http://www.rfc-editor.org/info/rfc6536>.
[RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC6991, July 2013,
<http://www.rfc-editor.org/info/rfc6991>.
[RFC7223] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
<http://www.rfc-editor.org/info/rfc7223>.
[RFC7224] Bjorklund, M., "IANA Interface Type YANG Module",
RFC 7224, DOI 10.17487/RFC7224, May 2014,
<http://www.rfc-editor.org/info/rfc7224>.
[RFC7277] Bjorklund, M., "A YANG Data Model for IP Management",
RFC 7277, DOI 10.17487/RFC7277, June 2014,
<http://www.rfc-editor.org/info/rfc7277>.
[RFC7317] Bierman, A. and M. Bjorklund, "A YANG Data Model for
System Management", RFC 7317, DOI 10.17487/RFC7317, August
2014, <http://www.rfc-editor.org/info/rfc7317>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<http://www.rfc-editor.org/info/rfc7752>.
[YANG1.1] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
Work in Progress, draft-ietf-netmod-rfc6020bis-14, June
2016.
Acknowledgements
Significant portions of this draft came from "Interface to the
Routing System Framework" (February 2013) and "A Policy Framework for
the Interface to the Routing System" (February 2013).
The authors would like to thank Nitin Bahadur, Shane Amante, Ed
Crabbe, Ken Gray, Carlos Pignataro, Wes George, Ron Bonica, Joe
Clarke, Juergen Schoenwalder, Jeff Haas, Jamal Hadi Salim, Scott
Brim, Thomas Narten, Dean Bogdanovic, Tom Petch, Robert Raszuk,
Sriganesh Kini, John Mattsson, Nancy Cam-Winget, DaCheng Zhang, Qin
Wu, Ahmed Abro, Salman Asadullah, Eric Yu, Deborah Brungard, Russ
Housley, Russ White, Charlie Kaufman, Benoit Claise, Spencer Dawkins,
and Stephen Farrell for their suggestions and review.
Authors' Addresses
Alia Atlas
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
United States
Email: akatlas@juniper.net
Joel Halpern
Ericsson
Email: Joel.Halpern@ericsson.com
Susan Hares
Huawei
7453 Hickory Hill
Saline, MI 48176
United States
Phone: +1 734-604-0332
Email: shares@ndzh.com
Dave Ward
Cisco Systems
Tasman Drive
San Jose, CA 95134
United States
Email: wardd@cisco.com
Thomas D. Nadeau
Brocade
Email: tnadeau@lucidvision.com