Internet DRAFT - draft-atlas-i2rs-policy-framework
draft-atlas-i2rs-policy-framework
Network Working Group A. Atlas
Internet-Draft Juniper Networks
Intended status: Informational S. Hares
Expires: August 29, 2013 Hickory Hill Consulting
J. Halpern
Ericsson
February 25, 2013
A Policy Framework for the Interface to the Routing System
draft-atlas-i2rs-policy-framework-01
Abstract
A key aspect of the Interface to the Routing System (I2RS) is what
mechanisms it includes to carry policy information and to enable
policy control. This applies both in the protocol itself and in the
services associated with the different components of the routing
system. Similarly, the data-models associated with the services must
be capable of expressing the appropriate granularity for access and
authorization-related policy. This document describes the policy
framework for I2RS.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 29, 2013.
Copyright Notice
Copyright (c) 2013 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. General I2RS Policy . . . . . . . . . . . . . . . . . . . . . 5
3.1. Use-Case of Overlapping Interactions . . . . . . . . . . . 6
3.2. Policy between client and agent . . . . . . . . . . . . . 6
3.2.1. Identity . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Security Role . . . . . . . . . . . . . . . . . . . . 7
3.2.3. Security Model . . . . . . . . . . . . . . . . . . . . 8
3.2.4. Scope . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.5. Resources . . . . . . . . . . . . . . . . . . . . . . 9
3.2.6. Connectivity . . . . . . . . . . . . . . . . . . . . . 10
3.2.7. Priority . . . . . . . . . . . . . . . . . . . . . . . 10
3.2.8. Precedence . . . . . . . . . . . . . . . . . . . . . . 11
3.3. Policy between Agent and Local System . . . . . . . . . . 14
3.3.1. Local Configuration . . . . . . . . . . . . . . . . . 15
3.3.2. Removal of I2RS-installed State . . . . . . . . . . . 16
3.3.3. On Reboot . . . . . . . . . . . . . . . . . . . . . . 16
4. Policy in an I2RS Service . . . . . . . . . . . . . . . . . . 17
4.1. Resource Reservation and Three-Phase Commit . . . . . . . 17
4.2. Defining I2RS Behavior Based on Implicit and Explicit
Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2.1. Example of Implicit Policy . . . . . . . . . . . . . . 18
4.2.2. Passing Explicit Policy . . . . . . . . . . . . . . . 19
4.2.2.1. Explicit policy on Data Forwarding, Resources,
and Policy passing . . . . . . . . . . . . . . . . 19
4.2.2.2. Example of Explicit Policy . . . . . . . . . . . . 19
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8. Informative References . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
The Interface to the Routing System (I2RS) provides read and write
access to the information and state that enable the routing
components of routing elements. The I2RS is introduced and described
in [I-D.atlas-irs-problem-statement] and [I-D.ward-irs-framework].
Policy helps provide filters and control on the access to information
and state that is enabled by individual protocol interactions. A
clear view of the policy features desirable at the I2RS is important
to shape the architecture and requirements for the protocols and
services of the I2RS. Policy can be explicitly defined or implicitly
assumed in a system, and can be enforced by that system's rules and
behavior. Since I2RS provides services to routing sub-systems that
already have policy defined (implicitly or explicitly), it is
important to consider the existing policy mechanisms and how an I2RS
services should interact with them.
I2RS policy has four different aspects that need to be considered.
1. Policy related to the I2RS protocol interactions between
different systems.
2. Policy related to the interaction between the I2RS Agent and the
local system to which the I2RS Agent is providing an interface.
3. Service policy to support scope and influence restrictions and to
preserve necessary policy associated with the related routing
sub-system.
4. Policy that can be installed or read via a service's data-model
that is associated with the related routing sub-system.
2. Terminology
The following terminology is used in this document.
agent or I2RS Agent: An I2RS agent provides the supported I2RS
services to the local system's routing sub-systems. The I2RS
agent understands the I2RS protocol and can be contacted by I2RS
clients.
client or I2RS Client: A client speaks the I2RS protocol to
communicate with I2RS Agents and uses the I2RS services to
accomplish a task as instructed by the client's local application.
An I2RS client can be seen as the part of an application that
supports I2RS and could be a software library.
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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 its usage. 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 set of information which the particular I2RS entity
(agent or client) is authorized to read. This access includes the
permission to see the existence of data and the ability to
retrieve the value of that data. In the context of an interaction
between a client and an agent, the effective read scope is
restricted to the intersection of the read scopes of the two
entities.
write scope: The set of field values which the particular I2RS
entity (agent or client) is authorized to write (i.e. add, modify
or delete). This access can restrict what fields can be modified
or created, and what specific value sets and ranges can be
installed. In the context of an interaction between a client and
an agent, the effective write scope is restricted to the
intersection of the write scopes of the two entities.
scope: When unspecified as either read scope or write scope, the
term scope applies to both the read scope and write 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 can be constrained. Examples of
such resources could include: the number of installed operations,
number of operations that haven't reached their start-time, etc.
These are not protocol-specific resources or network-specific
resources.
role or security role: A security role specifies the scope,
resources, precedences, etc. that a client or agent has.
identity: A client is associated with exactly one specific
identity. State installed by a particular identity is owned by
that identity; state ownership can not be transferred. 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. Similarly, an agent is
associated with a specific identity.
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3. General I2RS Policy
I2RS needs its own implicit and explicit policy. This section
articulates some of the those key concepts and policy decisions. The
I2RS policy applies to interactions between the agent and clients and
between the agent and the local system.
The agent's externally perceivable behavior and associated policy is
a key aspect of I2RS that must be described. The client's behavior
and functionality is specifically out-of-scope except where it needs
to be described with respect to the agent's behavior and the I2RS
protocol.
*********************** ***********************
* Application A * * Application B *
* * * *
* +----------------+ * * +----------------+ *
* | Client A | * * | Client B | *
* +----------------+ * * +----------------+ *
******* ^ ************* ***** ^ ****** ^ ******
| | |
| -----------------------| |
| | |
******* v ***** v ********* ************** v ********
* +----------------+ * * +----------------+ *
* | Agent 1 | * * | Agent 2 | *
* +----------------+ * * +----------------+ *
* ^ ^ * * ^ ^ *
* | | * * | | *
* v v * * v v *
* *********** ********** * * *********** ********* *
* * Routing * * Local * * * * Routing * * Local * *
* *********** * Config * * * *********** * Config* *
* ********** * * ********* *
* * * *
* Routing Element 1 * * Routing Element 2 *
*************************** *************************
Figure 1: Architecture of clients and agents
As can be seen in Figure 1, a client can communicate with multiple
agents. The application associated with a client may have multiple
tasks it is accomplishing (separate functions, short-term versus
longer-term, etc) and each such task may involve a set of agents
which may or may not differ.
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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. The set of write operations received by an agent may
overlap and conflict. No simple protocol or policy mechanisms by an
agent can completely avoid indirect interactions between different
install operations. The functional partitioning between the
different clients must be done to avoid undesirable indirect
interactions.
3.1. Use-Case of Overlapping Interactions
An I2RS Agent can receive overlapping operations from multiple
clients. An example is when there are two applications:
Client A: Special Flow Router: Client A is part of an application
that explicitly routes particular special flows using policy-based
routing (aka ACLs).
Client B: DDoS Detection and Mitigation: Client B is part of an
application that looks for flows that appear to be part of a DDoS
attack and explicitly routes them to mitigate the attack. Client
B also uses policy-based routing (aka ACLs).
If Client B is told to explicitly route prefix X, because it looks
suspicious, and Client A is also explicitly routing prefix X, then
the I2RS Agent must determine what to do based upon policy. Even
though intelligent functional partitioning has been done, this is an
example where the I2RS agent must still make an arbitration decision.
This document defines precedence as the policy mechanism by which the
I2RS agent can be instructed what to do in such cases.
3.2. Policy between client and agent
Multiple clients can communicate with the same agent. The agent must
have policies to manage the resulting complexity. Implicit policy
includes the assumptions about communication between the client and
agent. Explicit policy includes mechanisms to arbitrate between
different clients, between operations of the same client, and to
manage state owned by an client inside the agent.
Any easy way to look at the i2rs policies is that the policies answer
who, what, and how.
Who: The first type of policies concern identity, secure roles, and
security model for connecting. The same is true of any secured
connect between two hosts where each host has an identity, a secured
role in the communication and security model on who can connect.
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What: The second set of policies look at secure model on what data
can be examined, the scope of the read or writes, and the amount of
resources an active i2rs agent can consume. A security model defines
the barriers for the i2rs activity.
How: The third set of policies involve how the i2rs agent and i2rs
client communication, and how they mitigate the natural contention of
allowing an i2rs client to talk to multiple i2rs agents or an i2rs
agent to communicate with multiple clients. For example, a single
i2rs client connected to several i2rs agents (i2rs agent J and i2rs
agent K) may learn of an interface overload (on i2rs agent J), and
then want to reprioritize activities on i2rs agent K to find another
data path. This is priority policy. In the same way, the two i2rs
Clients (A and B) may try to install a RIB route on i2rs agent K. If
there are overlapping actions, policy needs to determine who wins.
3.2.1. Identity
By definition, a client is associated with exactly one identity. An
agent will store data that is owned by a particular client, based
upon that client's identity. Since a client can communicate via
multiple transport channels and no channel needs to be active for the
agent to have associated state, the client's identity is used to
identify the ownership of the data stored by the agent.
Similarly, by definition, an agent is associated with exactly one
identity. A client may also store local state associated with a
particular agent. The agent's identity can be used to identify
ownership of the data stored by the client.
The details of what constitutes an identity can be dependent upon the
specifics of the I2RS protocol and selected security mechanisms.
However, there are some critical considerations for identity that do
impose constraints.
An identity is not tied to a single communication channel. A client
may use multiple IP addresses; an identity should not be tied to a
specific IP address. If the client or agent is associated with a
system that may be mobile, that should be considered in its
identification. Finally, the syntax and semantics for identifiers
used for a client and for an agent may be different.
3.2.2. Security Role
In the context of an agent, each client will have a security role.
The client's identity and associated security role will have to be
verified via an acceptable security mechanism. A variety of such
mechanisms are anticipated to meet different security and operational
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objectives. Example mechanisms might include a role assertion from
the client to the agent that the agent can cryptographically verify
or having the agent to use an already trusted protocol to verify the
client's security role and identity.
An agent must know the scope and resources associated with each
particular security role. This information may vary across different
agents even in the same network or it may be consistent across
different agents in the same network. The latter can be enforced by
having a client that is authorized to influence the meta-data model
of security roles on the relevant set of agents.
A security role also defines what precedences (See Section 3.2.8) a
commissioner can use.
3.2.3. Security Model
As described above, roles identify the scope and resources allowed to
an I2RS Client. The policy model therefore needs to include these
roles. The question of the bindings of identities to roles, and the
selection of identities are protocol specific matters outside the
scope of this document.
The policy model for roles needs to address these two dimensions. It
needs to create the roles themselves. This should allow for use of
techniques like inheritance, presumably with some rules on how role
definitions can augment or restrict the inherited definitions.
The security model also needs to define, by reference to the policy
model itself, the scope of the role. The question of defining the
resources of a role is for further study. The role definition needs
to indicate what types and instances of data can be observed and what
information about those instances entities with that role can
observe. The security model also needs to define which data items
can be modified, and what modifications (ranges, specified values, or
other assertions that must be met) are permitted.
3.2.4. Scope
Scope is specified as part of a security role. A security role may
be defined and managed in an external repository, centralized within
an administration. The security role definitions must be accessible
to an agent.
In the context of an interaction between a client and an agent, the
effective scope is restricted to the intersection of the scopes of
the two entities.
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What information a particular client is authorized to read is known
as the clien's read scope. A read scope includes the ability to see
that particular data exists and to read the same data. The read
scope can have its constraints specified in terms of specific
portions of data models.
Similarly, what information a client can write (add/modify/delete)
may be contrained. This is known as its write scope. The write
scope is specific in both the parts of the data models and in the set
and range of data that can be written. For example, a client might
be able to write static routes in the RIB data-model for prefixes in
10.0/16.
While the client's behavior and functionality is specifically out-of-
scope, it is useful to describe the same scope concepts for an agent
operating in the context of a client.
An agent's read scope is the set of data that the agent can read or
have access to. An agent would generally learn such data because the
client has sent that data to the agent in an operation.
An agent's write scope is the set and range of data that the agent is
allowed to provide to the client and that will be accepted by the
client. For instance, client B may accept next-hop change
notifications for prefix 10.0/16 from agent 1 but not from agent 2.
3.2.5. Resources
When a client sends operations to an agent, those operations can
consume resources. Therefore, it is important that the agent have
policy to limit the resources available to a particular client. This
is based on the client's identity and security role. Such resource
policy specifications need to be provided in a data-model that can be
modified by appropriately authorized clients or local configuration.
Examples of such resource constraints include:
Number of installed operations owned,
Number of operations that haven't reached their start-time, and
Number of event notifications registered for.
As discussed in Section 3.2.7, a client can specify priorities for
the operations it sends.
If compute resources are considered, it is not the intent to try and
determine the computation associated with particular operations.
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Instead, the constraint could be on percentage of the I2RS agent's
compute-time given to a client every pre-defined period. This could
provide a mechanism for fair sharing of compute resources between
clients.
3.2.6. Connectivity
A client does not need to maintain an active communication channel
with an agent. Therefore, an 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 client is non-functional. When
communication is required, the agent or client can open a new
communication channel.
State held by an agent that is owned by a 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.
3.2.7. Priority
The motivating example for priority is when a single client is
sending operations to accomplish multiple tasks. For example, one
task might be long-term and another task might deal with unexpected
requests that are more important. In this case, the client may wish
to provide a hint to the relevant agents as to which operations
should be done first.
Communication from a client can come across multiple channels, so
simply specifying that operations be done in order is not sufficient.
Additionally, all operations may not be immediately carried out, due
to varying start-times or other constraints. With these factors and
this motivating example, it is useful to introduce the concept of
prioritization for operations sent from the same client.
By introducing the concept of priority for operations, a client can
accomplish multiple uncorrelated tasks that affect the same agent
with the specified prioritization.
A default priority can be specified for each particular communication
channel. In addition, an I2RS operation can specify a priority to
use instead. Priorities between operations from different clients
need not be compared.
The priority can be used by an agent to determine which operation
from a client to execute next.
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3.2.8. Precedence
A mechanism is needed for the agent to determine what state to
install when there are overlapping install operations. An install
operation may overlap with locally-installed configuration state or
with a previous install operation that was requested by a client.
The mechanism to resolve this is termed "precedence". No simple
mechanism can fully handle indirect interactions; considering such
interactions is out-of-scope. Indirect interactions must be
considered when different clients are given their tasks.
Precedence is a TLV; there is a precedence type and a precedence
value that can vary based upon the precedence type. The different
precedence types are ordered with regard to another so that, for
instance, a precedence type of "Simple Integer" is preferred to
precedence type of "mouse-type". If more than one operation has the
same precedence type, then the precedence values are compared based
upon the rules for the associated type. If multiple clients have
equivalent precedence (based both on type and compared values), then
preference is given to the newer operation that is being written.
This tie-breaking policy is equivalent to that used by CLI or
NetConf, where the new command or RPC gets to do its add/modify/
delete operation. The different precedence types are ordered with
regard to each other; the lowest precedence type will be preferred.
If there is a tie for precedence type, then the precedence values
will be compared and the preferred will be selected based on the
precedence type's policy.
Option A: Type 100 ("Simple Integer") Value 10.
Option B: Type 200 (mouse-type) Value "cheese"
Option C: Type 100 ("Simple Integer") Value 10
Option D: Type 100 ("Simple Integer") Value 8
If Operation A arrived first and installed state, then when the I2RS
agent decides whether to install Operation B, Operation B would be
rejected or stored because A's type 100 is better than B's type 200.
If Operation C were to arrive, however, then Operation C would be
installed and Operation A preempted because A and C have the same
type and value, so tie-breaking is done to prefer the new arrival.
If Operation D were to arrive with A installed, then D would be
rejected or stored because A and D share the same type but D's value
of 8 is less than A's value of 10.
Given that clients are dynamically sending write operations and the
associated arrival times can vary based on anything from program
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state to network conditions, predictability is much better provided
by using precedence instead of operation arrival time or start time
many operations may be immediate start).
Each write operation has a precedence associated with it. This
precedence may come from the default associated with the clientw,
with the specific communication channel, or with the specific
operation. The range of possible precedences that can be used is
known based on the client's security role. The determination of the
precedence associated with any operation is a policy decision at the
agent, but may utilize any or all of the information described above.
When a write operation is executed, the agent first determines if
there is overlapping existing I2RS-installed state. If not, the
agent must determine if it overlaps existing local-configuration
state. Local-configuration state will also have a precedence
associated with it so that the agent can make an appropriate
decision.
A client can specify whether a write operation should be store-if-
not-best. This allows a client to determine what happens when a
write operation doesn't win the precedence comparison. If store-if-
not-best is specified, then the write operation succeeds and the
associated installed state is stored but not actively installed by
the agent. If store-if-not-best is not specified, then the install
operation will fail.
The store-if-not-best flag is stored with the installed operation's
precedence. If the agent determines that an installed operation must
be preempted, then the agent consults the store-if-not-best flag. If
store-if-not-best is specified, then the agent stores the preempted
operation and does not notify the associated client. If store-if-
not-best is not specified, then the agent notifies the associated
client of the preemption and removes the previously installed state.
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/----------\ NO |------------|
/ Overlap? \________\| Install as |
\ / /| I2RS state |
\----------/ |------------|
|
| YES
V
/-----------------\ YES /------------\ YES |----------------|
/ New Precedence \_______\/ Old store-if \_____\| Store old I2RS |
\ better than Old? / /\ -not-best? / /|----------------|
\-----------------/ \------------/ |
| | |
| | NO |
| V V
| |------------------| |-------------|
| | Send Preempt |___\| Install new |
| NO | Notification to | /| I2RS state |
| | Old Client | |-------------|
| |------------------|
V
/-------------\ YES /----------\ NO /-----------\ NO
/ New precedence\____\ / same \___\ / new store- \___
\ equal to old / / \ Client / / \ if-best on? / |
\-------------/ \----------/ \-----------/ |
| NO |YES YES | V
| | | |---------------|
| | | | Send a reject |
| V | | to new Client |
| |-------------------| | |---------------|
| | Install new State | |
| |-------------------| V
| |----------------|
V | Save new State |
/-------------\ NO |------------------| |----------------|
/ New store-if \____\| Send Preempt |
\ -not-best? / /| Notification to |
\-------------/ | New Client and |
| | forget new I2RS |
| | state |
| Yes |------------------|
V
|----------------------|
| store new I2RS state |
|----------------------|
Figure 2: Precedence Decision-Making
If the overlapping new operation has a precedence that is better than
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the existing state, then the agent should preempt the existing state
and act according to the existing state's store-if-not-best flag. If
that store-if-not-best flag is set, the agent will store the old
state and install the new state. If the store-if-not-best flag is
clear, the agent will send a preemption notification to the old
client, install the new I2RS state, and forget the old.
If the overlapping existing state has the same precedence and the
same client associated, then the agent completes the write operation;
otherwise, the agent must reject or store the write operation, based
on the store-if-not-best flag.
If the new overlapping operation has a precedence that is worse than
the existing state, then the agent must reject or store the write
operation, based on the state of the new store-if-not-best flag. If
the store-if-not-best flag is set, then then the agent will store the
new I2RS state. If the store-if-not-best flag is clear, then the the
I2RS agent will send a preempt notification to the new client and
forget the new I2RS state.
This decision process is illustrated in Figure 2.
When a delete operation is done, the stored state with the next best
precedence should be selected and installed.
A consequence of the precedence policy mechanism is that a client
must be able to handle its installed operations being preempted at
any time, either explicitly or simply by having the active state
changed. Such preemption can be minimized by appropriate separation
of tasks, with their associated write operations, between the local
systems of the clients and by knowledgeable local system
configuration.
3.3. Policy between Agent and Local System
It is critical to understand and clearly specify how I2RS interacts
with local configuration. The key questions are:
1. What happens when Local Configuration overlaps with I2RS
installed state?
2. What happens when I2RS installed state is removed?
3. How is state recreated when a local system reboots?
A consequence of using I2RS is that the local system's state may not
be synchronized with the local configuration. Since this is a change
in understood behavior, any discrepancies should be clearly visible
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to the operator with an associated explanation.
Logically, the local configuration is essentially modeled as a local
client, with its own precedence, identity, and security role and
immediate permanent write operations. The key differences are both
that all relevant local configuration state need not be cached by the
agent and that reboot imposes the need to process local configuration
state before any other I2RS-installed state.
3.3.1. Local Configuration
The local system's local configuration may have overlapping write
scope with that of one or more clients using an agent. Therefore,
explicit and implicit policy interactions must be specified. The
mechanism that I2RS provides for deciding between overlapping install
operations is "precedence". This same mechanism can be used to
decide between local configuration and an I2RS operation. Local
configuration can specify the precedence to be used for the local
system.
A precedence that causes the desired behavior can be specified as
follows. (MAX is the highest precedence given to a client. MIN is
the lowest precedence given to a client.)
MAX+1 Precedence: If the local configuration has a precedence
higher than that given to any client, then state from the local
configuration will always be installed. If any I2RS-installed
state is therefore preempted, the agent will notify the associated
client.
MIN-1 Precedence: If the local configuration has a precedence
lower than that given to any client, then I2RS-installed state
will always override local configuration. That this preemption
has occurred should be reflected in how the local system displays
its state.
Other Precedence: The local configuration can have higher
precedence than that given to some clients, lower precedence than
that given to other clients, and equal precedence to that given to
other clients. Then some local configuration state may be
preempted by I2RS-installed state while some I2RS-installed state
can be preempted by local configuration.
Local-configuration wins all precedence ties.
Just as an agent must check to determine if a write operation
overlaps with existing installed state, the process of committing
local configuration must check to see if there is overlapping I2RS-
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installed state.
What the process of committing local configuration is can vary by
local system. Well known examples are when a return is sent to the
CLI and when an explicit commit command is specified. How the proper
checks for interaction between the agent and local configuration are
done is a local system matter.
Similarly, when an agent checks to see if an write operation overlaps
with existing installed state, the agent must determine if it
overlaps with existing local configuration.
If the precedence associated with local configuration is changed,
then it is retroactive. All local configuration state stored by the
agent must be updated with the new precedence and installation
decisions made for overlapping data. This change could be very
disruptive.
3.3.2. Removal of I2RS-installed State
When a piece of local configuration is removed, the local system goes
back to the appropriate system default. However, when an operation
deletes some I2RS-installed state, the correct behavior is not to
just go back to the system default. Instead, any stored state must
be considered - whether that comes from local configuration or stored
I2RS write operations that didn't have the highest precedence. If
there is any stored state, then the highest precedence of the options
is selected and installed. That existing overlapping state might
come from the local-configuration.
If I2RS's implicit policy were to just go to the system default, then
the local configuration and the local system state would not be
synchronized and there would be no remaining I2RS-state to explain
the discrepency. Since I2RS state can also be stored and not
installed, the same mechanism can be used for stored I2RS install
operations and for local configuration.
3.3.3. On Reboot
When the local system reboots, only persistent I2RS-installed state
is preserved by the agent. The implicit policy for I2RS is that the
local configuration is read and installed first. After the local
system has its local configuration installed, the persistent I2RS
write operations are executed to bring the system to the persistent
state.
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4. Policy in an I2RS Service
It is critical to consider how policy influences a service when
defining the service and its associated data-model(s). There are
several different aspects to consider.
How are scope and influence policy specified in the data model?
What granularity levels are necessary for the particular service?
How does the implicit policy in the associated routing sub-system
effect what I2RS can be allowed to influence?
Are the implicit policies of the associated routing sub-system
captured in the semantic content of the information model, data
model, and description?
What explicit policy communicated in the associated routing sub-
system needs to be included in the data-model? What indirection
and abstractions are needed?
4.1. Resource Reservation and Three-Phase Commit
Some agents and services may offer the ability to reserve resources
required by operations before the operation start time. There are
two aspects to how to support this.
First, if the agent can do time-aware resource reservation, then a
write operation can specify "reserve-only" to prompt an
acknowledgement or failure as to the ability of the agent to confirm
the reservation. Then the client can either send an operation to
commit the reservation, which causes the associated write operation,
or to remove the reservation. A "reserve-only" operation will have
its reservation expire at the end of its associated life-time.
Second, part of a service's data-model may be to request a
reservation with a known start-time and duration. An example might
be reserving a specific bandwidth on a path for an LSP between two
devices. It is important to consider whether a particular service
should offer a time-based reservation service as part of its data-
model.
4.2. Defining I2RS Behavior Based on Implicit and Explicit Policy
The semantics in a data-model must respect and describe the implicit
policy of the associated routing sub-system. This doesn't imply that
the data-model components should instantiate it or allow reading or
writing.
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Policy Routing systems must deal with the verification, reading and
installing of routes from sources such as EGP, IGP, and static
routes. Policy routing may also control forwarding and the
monitoring of data forwarding; and data resources. The explicit
policy examples are given for the routing framework. It is assumed
the reader can extend this framework to the data forwarding and data
resource arena.
4.2.1. Example of Implicit Policy
The ISIS protocol specification uses implicit policy to set
constraints on level 1 peers. Due to this fact, many ISIS
implementations only let one level 1 ISIS peer associate with one
Level 2 peer domain.
This policy is not encoded in any local configuration directly, but
is rather included as an implicit policy. When local configuration
policy is checked (prior to a configuration commit), this local
policy is checked. If the configuration input from a CLI is in
error, the input will be rejected, and the CLI will warn the user.
Similarily programmic interfaces for the local configuration cause
the implicit policy to be checked.
I2RS data models guide the client in an interoperable interaction
with the reading and installation of data at a particular agent. The
I2RS data models must contain both the implicit policy and the
explicit policy. Although an agent may not report the I2RS implicit
policy in the protocol, the client must know of the existence of the
implicit policy.
This knowledge allows the client to know the implicit policy
interactions on different systems in a heterogeneous network. For
example, assume a situation where a client is talking to two agents -
one on system A and one on system B. The routing process on system A
has has different implicit rules for the ISIS Level 1 peer to Level 2
peer connection than the routing process on system B. Routing process
A is built to allow one level 1 ISIS peer associated with 2 ISIS
Level 2 peers. Routing process B upholds the standard implicit
policy that 1 level ISIS peer can only be associated with 1 ISIS
Level 2 peer. The client setting up the ISIS peering in a network
containing system A and system B must know that System A will allow a
level 1 peer to connect to 2 ISIS Level 2 peers. When the client's
scope allows it to read data from system A and system B, it should
not flag the difference in ISIS level 1 peer connections as a
problem. Instead the client will need to determine if the use of the
different configurations can cause a network problem.
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4.2.2. Passing Explicit Policy
Routing systems' explicit policy controls protocols, associates/
deassociates interfaces, route verification policy, route forwarding
policy, route aggregation policy, and route deaggregation policy.
All of this policy can be found in the detailed configuration
specification of a routing process. However, even via CLI, it is
rarely possible to configure all the possible options. Other
configuration mechanisms do not have public models for all the
private router configuration. The developers of a routing system
often have a complete policy model either in formal modeling
languages or informal language.
Explicit policy contained in an I2RS data model is the detailed
configuration model at the deepest level that an agent can access.
This detailed configuration model may come from IETF Standards and/or
the vendor specific configurations. The public data models must
specify a vendor specific tree where the individual configuration is
plugged into.
4.2.2.1. Explicit policy on Data Forwarding, Resources, and Policy
passing
Forwarding policy has to do with the data flow may also be controlled
by an agent. If so, the explicit policy must be placed in a data
model along with the implicit policy.
Lastly, protocols have begun to pass explicit policy about passing
policy. Examples of this type of policy are BGP ORFs, BGP Flowspecs,
and ISIS policy passing. Clients must know the implicit policy and
explicit policy this policy impacts, and the precedence between these
policy. Due to the extensive use of BGP ORFs and the growing use in
BGP Flowspecs policy, early data models for BGP should describe the
implicit policy, explicit policy, policy precedence for the BGP ORFS
and BGP FlowSpecs, and how they interacts with other BGP, route
policy and preferences. This information should be placed inside an
I2RS data model for an agent supporting these features.
These explicit models for BGP policy are not trivial, but these
models exist today. Frequently, I2RS data models may be simply a
casting of existing implicit policy and explicit policy into a common
standard form so that programmic interfaces may interact with a
routing element.
4.2.2.2. Example of Explicit Policy
There are two clear explicit policy pieces for ISIS. First is the
peer level. Second is the policy of the external routes to be
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redistributed into and out of ISIS.
5. Acknowledgements
The authors would like to thank Ross Callon, Adrian Farrel, David
Meyer, David Ward, Rex Fernando, Russ White, Bruno Risjman, and
Thomas Nadeau for their suggestions and review.
6. IANA Considerations
This document includes no request to IANA.
7. Security Considerations
This is empty boilerplate for now.
8. Informative References
[I-D.atlas-irs-problem-statement]
Atlas, A., Nadeau, T., and D. Ward, "Interface to the
Routing System Problem Statement",
draft-atlas-irs-problem-statement-00 (work in progress),
July 2012.
[I-D.ward-irs-framework]
Atlas, A., Nadeau, T., and D. Ward, "Interface to the
Routing System Framework", draft-ward-irs-framework-00
(work in progress), July 2012.
Authors' Addresses
Alia Atlas
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: akatlas@juniper.net
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Susan Hares
Hickory Hill Consulting
Email: shares@ndzh.com
Joel Halpern
Ericsson
Email: Joel.Halpern@ericsson.com
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