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Keywords: NETCONF, network management, time, clock synchronization
Internet Engineering Task Force (IETF) T. Mizrahi
Request for Comments: 7758 Y. Moses
Category: Experimental Technion
ISSN: 2070-1721 February 2016
Time Capability in NETCONF
Abstract
This document defines a capability-based extension to the Network
Configuration Protocol (NETCONF) that allows time-triggered
configuration and management operations. This extension allows
NETCONF clients to invoke configuration updates according to
scheduled times and allows NETCONF servers to attach timestamps to
the data they send to NETCONF clients.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. 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 5741.
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/rfc7758.
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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.
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Table of Contents
1. Introduction ....................................................4
2. Conventions Used in This Document ...............................4
2.1. Key Words ..................................................4
2.2. Abbreviations ..............................................5
2.3. Terminology ................................................5
3. Using Time in NETCONF ...........................................5
3.1. The Time Capability in a Nutshell ..........................5
3.2. Notifications and Cancellation Messages ....................7
3.3. Synchronization Aspects ....................................9
3.4. Scheduled Time Format .....................................10
3.5. Scheduling Tolerance ......................................10
3.6. Scheduling the Near vs. Far Future ........................11
3.7. Time-Interval Format ......................................13
4. Time Capability ................................................14
4.1. Overview ..................................................14
4.2. Dependencies ..............................................14
4.3. Capability Identifier .....................................14
4.4. New Operations ............................................14
4.5. Modifications to Existing Operations ......................15
4.5.1. Affected Operations ................................15
4.5.2. Processing Scheduled Operations ....................16
4.6. Interactions with Other Capabilities ......................16
5. Examples .......................................................17
5.1. <scheduled-time> Example ..................................17
5.2. <get-time> Example ........................................18
5.3. Error Example .............................................19
6. Security Considerations ........................................19
6.1. General Security Considerations ...........................19
6.2. YANG Module Security Considerations .......................20
7. IANA Considerations ............................................21
8. References .....................................................22
8.1. Normative References ......................................22
8.2. Informative References ....................................22
Appendix A. YANG Module for the Time Capability ...................24
Acknowledgments ...................................................32
Authors' Addresses ................................................32
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1. Introduction
The Network Configuration Protocol (NETCONF), defined in [RFC6241],
provides mechanisms to install, manipulate, and delete the
configuration of network devices. NETCONF allows clients to
configure and monitor NETCONF servers using remote procedure calls
(RPCs).
NETCONF is asynchronous; when a client invokes an RPC, it has no
control over the time at which the RPC is executed, nor does it have
any feedback from the server about the execution time.
Time-based configuration ([OneClock] [Time4]) can be a useful tool
that enables an entire class of coordinated and scheduled
configuration procedures. Time-triggered configuration allows
coordinated network updates in multiple devices; a client can invoke
a coordinated configuration change by sending RPCs to multiple
servers with the same scheduled execution time. A client can also
invoke a time-based sequence of updates by sending n RPCs with n
different update times, T1, T2, ..., Tn, determining the order in
which the RPCs are executed.
This memo defines the :time capability in NETCONF. This extension
allows clients to determine the scheduled execution time of RPCs they
send. It also allows a server that receives an RPC to report its
actual execution time to the client.
The NETCONF time capability is intended for scheduling RPCs that
should be performed in the near future, allowing the coordination of
simultaneous configuration changes or specification of an order of
configuration updates. Time-of-day-based policies and far-future
scheduling, e.g., [Cond], are outside the scope of this memo.
This memo is defined for experimental purposes and will allow the
community to experiment with the NETCONF time capability. Based on
the lessons learned from this experience, it is expected that the
NETCONF working group will be able to consider whether to adopt the
time capability.
2. Conventions Used in This Document
2.1. Key Words
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].
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2.2. Abbreviations
NETCONF Network Configuration Protocol
RPC Remote Procedure Call
2.3. Terminology
o Capability [RFC6241]: A functionality that supplements the base
NETCONF specification.
o Client [RFC6241]: Invokes protocol operations on a server. In
addition, a client can subscribe to receive notifications from a
server.
o Execution time: The execution time of an RPC is defined as the
time at which a server completes the execution of an RPC, before
it sends the <rpc-reply> message.
o Scheduled RPC: an RPC that is scheduled to be performed at a
predetermined time, which is included in the <rpc> message.
o Scheduled time: The scheduled time of an RPC is the time at which
the RPC should be started, as determined by the client. It is the
server's role to enforce the execution of the scheduled time.
o Server [RFC6241]: Executes protocol operations invoked by a
client. In addition, a server can send notifications to a client.
3. Using Time in NETCONF
3.1. The Time Capability in a Nutshell
The :time capability provides two main functions:
o Scheduling:
When a client sends an RPC to a server, the <rpc> message MAY
include the scheduled-time element, denoted by Ts in Figure 1.
The server then executes the RPC at the scheduled time Ts; once
completed, the server can respond with an RPC reply message.
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o Reporting:
When a client sends an RPC to a server, the <rpc> message MAY
include a get-time element (see Figure 2), requesting the server
to return the execution time of the RPC. In this case, after the
server performs the RPC, it responds with an RPC reply that
includes the execution time, Te.
RPC _________
executed \
\/
Ts
server ---------------+------------- ----> time
/\ \
rpc / \ rpc-reply
(Ts)/ \
/ \/
client -----------------------------
Figure 1: Scheduled RPC
RPC _________
executed \
\/
Te
server ------------+---------------- ----> time
/\ \
rpc / \ rpc-reply
(get-time)/ \ (Te)
/ \/
client -----------------------------
Figure 2: Reporting the Execution Time of an RPC
Example 1. A client needs to trigger a commit at n servers, so that
the n servers perform the commit as close as possible to
simultaneously. Without the time capability, the client sends a
sequence of n commit messages; thus, each server performs the commit
at a different time. By using the time capability, the client can
send commit messages that are scheduled to take place at a chosen
time Ts, for example, 5 seconds in the future, causing the servers to
invoke the commit as close as possible to time Ts.
Example 2. In many applications, it is desirable to monitor events
or collect statistics regarding a common time reference. A client
can send a set of get-config messages that is scheduled to be
executed at multiple servers at the same time, providing a
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simultaneous system-wide view of the state of the servers. Moreover,
a client can use the get-time element in its get-config messages,
providing a time reference to the sampled element.
The scenarios of Figures 1 and 2 imply that a third scenario can also
be supported (Figure 3), where the client invokes an RPC that
includes a scheduled time, Ts, as well as the get-time element. This
allows the client to receive feedback about the actual execution
time, Te. Ideally, Ts=Te. However, the server may execute the RPC
at a slightly different time than Ts, for example, if the server is
tied up with other tasks at Ts.
RPC _________
executed \
\/
Ts Te
server -------------+-+------------- ----> time
/\ \
rpc / \ rpc-reply
(Ts + get-time)/ \ (Te)
/ \/
client -----------------------------
Figure 3: Scheduling and Reporting
3.2. Notifications and Cancellation Messages
Notifications
As illustrated in Figure 1, after a scheduled RPC is executed, the
server sends an <rpc-reply>. The <rpc-reply> may arrive a long
period of time after the RPC was sent by the client, leaving the
client without a clear indication of whether the RPC was received.
This document defines a new notification, the netconf-scheduled-
message notification, which provides an immediate acknowledgement
of the scheduled RPC.
The <netconf-scheduled-message> notification is sent to the client
if it is subscribed to the NETCONF notifications [RFC6470]; as
illustrated in Figure 4, when the server receives a scheduled RPC,
it sends a notification to the client.
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The <netconf-scheduled-message> notification includes a <schedule-
id> element. The <schedule-id> is a unique identifier that the
server assigns to every scheduled RPC it receives. Thus, a client
can keep track of all the pending scheduled RPCs; a client can
uniquely identify a scheduled RPC by the tuple {server, schedule-
id}.
RPC ____________
executed \
\/
Ts
server -------------------+--------- ----> time
/\ \ \
rpc / \notifi- \ rpc-reply
(Ts)/ \cation \
/ \/ \/
client -----------------------------
Figure 4: Scheduled RPC with Notification
Cancellation Messages
A client can cancel a scheduled RPC by sending a <cancel-schedule>
RPC. The <cancel-schedule> RPC includes the <schedule-id> of the
scheduled RPC that needs to be cancelled.
The <cancel-schedule> RPC, defined in this document, can be used
to perform a coordinated all-or-none procedure, where either all
the servers perform the operation on schedule or the operation is
aborted.
Example 3. A client sends scheduled <rpc> messages to server 1
and server 2, both scheduled to be performed at time Ts. Server 1
sends a notification indicating that it has successfully scheduled
the RPC, while server 2 replies with an unknown-element error
[RFC6241] that indicates that it does not support the time
capability. The client sends a <cancel-schedule> RPC to server 1
and receives an <rpc-reply>. The message exchange between the
client and server 1 in this example is illustrated in Figure 5.
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RPC not __________
executed \
\/
Ts
server --------------------------------+--- ----> time
/\ \ /\ \
rpc / \notifi- /cancel- \ rpc-reply
(Ts)/ \cation /schedule \
/ \/ / \/
client ------------------------------------
Figure 5: Cancellation Message
A <cancel-schedule> RPC MUST NOT include the scheduled-time
parameter. A server that receives a <cancel-schedule> RPC should try
to cancel the schedule as soon as possible. If the server is unable
to cancel the scheduled RPC, for example, because it has already been
executed, it should respond with an <rpc-error> [RFC6241], in which
the error-type is 'protocol', and the error-tag is 'operation-
failed'.
3.3. Synchronization Aspects
The time capability defined in this document requires clients and
servers to maintain clocks. It is assumed that clocks are
synchronized by a method that is outside the scope of this document,
e.g., [RFC5905] or [IEEE1588].
This document does not define any requirements pertaining to the
degree of accuracy of performing scheduled RPCs. Note that two
factors affect how accurately the server can perform a scheduled RPC:
one factor is the accuracy of the clock synchronization method used
to synchronize the clients and servers and the second factor is the
server's ability to execute real-time configuration changes, which
greatly depends on how it is implemented. Typical networking devices
are implemented by a combination of hardware and software. While the
execution time of a hardware module can typically be predicted with a
high level of accuracy, the execution time of a software module may
be variable and hard to predict. A configuration update would
typically require the server's software to be involved, thus
affecting how accurately the RPC can be scheduled.
Another important aspect of synchronization is monitoring; a client
should be able to check whether a server is synchronized to a
reference time source. Typical synchronization protocols, such as
the Network Time Protocol [RFC5905], provide the means ([RFC5907],
[RFC7317]) to verify that a clock is synchronized to a time reference
by querying its Management Information Base (MIB). The get-time
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feature defined in this document (see Figure 2) allows a client to
obtain a rough estimate of the time offset between the client's clock
and the server's clock.
Since servers do not perform configuration changes instantaneously,
the processing time of an RPC should not be overlooked. The
scheduled time always refers to the start time of the RPC, and the
execution time always refers to its completion time.
3.4. Scheduled Time Format
The scheduled time and execution time fields in <rpc> messages use a
common time format field.
The time format used in this document is the date-and-time format,
defined in Section 5.6 of [RFC3339] and Section 3 of [RFC6991].
leaf scheduled-time {
type yang:date-and-time;
description
"The time at which the RPC is scheduled to be performed.";
}
leaf execution-time {
type yang:date-and-time;
description
"The time at which the RPC was executed.";
}
3.5. Scheduling Tolerance
When a client sends an RPC that is scheduled to Ts, the server MUST
verify that the value Ts is not too far in the past or in the future.
As illustrated in Figure 6, the server verifies that Ts is within the
scheduling-tolerance range.
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RPC _________
received \
\/
Ts
-----+--------------+-----+------------+-------> time
<------------> <---------------->
sched-max-past sched-max-future
<------------------------------->
scheduling tolerance
Figure 6: Scheduling Tolerance
The scheduling tolerance is determined by two parameters: sched-max-
future and sched-max-past. These two parameters use the time-
interval format (Section 3.7.), and their default value is 15
seconds.
If the scheduled time, Ts, is within the scheduling-tolerance range,
the scheduled RPC is performed; if Ts occurs in the past and within
the scheduling tolerance, the server performs the RPC as soon as
possible; whereas if Ts is a future time, the server performs the RPC
at Ts.
If Ts is not within the scheduling-tolerance range, the scheduled RPC
is discarded, and the server responds with an error message [RFC6241]
including a bad-element error-tag. An example is provided in Section
5.3.
3.6. Scheduling the Near vs. Far Future
The scheduling bound defined by sched-max-future guarantees that
every scheduled RPC is restricted to a scheduling time in the near
future.
The scheduling mechanism defined in this document is intended for
near-future scheduling, on the order of seconds. Far-future
scheduling is outside the scope of this document.
Example 1 is a typical example of using near-future scheduling; the
goal in the example is to perform the RPC at multiple servers at the
same time; therefore, it is best to schedule the RPC to be performed
a few seconds in the future.
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The Challenges of Far-Future Scheduling
When an RPC is scheduled to be performed at a far-future time,
during the long period between the time at which the RPC is sent
and the time at which it is scheduled to be executed, the
following erroneous events may occur:
o The server may restart.
o The client's authorization level may be changed.
o The client may restart and send a conflicting RPC.
o A different client may send a conflicting RPC.
In these cases, if the server performs the scheduled operation, it
may perform an action that is inconsistent with the current
network policy or inconsistent with the currently active clients.
Near-future scheduling guarantees that external events, such as
the examples above, have a low probability of occurring during the
sched-max-future period, and even when they do, the period of
inconsistency is limited to sched-max-future, which is a short
period of time.
The Trade-off in Setting the sched-max-future Value
The sched-max-future parameter should be configured to a value
that is high enough to allow the client to:
1. Send the scheduled RPC, potentially to multiple servers.
2. Receive notifications or <rpc-error> messages from the
server(s) or wait for a timeout and decide that if no response
has arrived then something is wrong.
3. If necessary, send a cancellation message, potentially to
multiple servers.
On the other hand, sched-max-future should be configured to a
value that is low enough to allow a low probability of the
erroneous events above, typically on the order of a few seconds.
Note that, even if sched-max-future is configured to a low value,
it is still possible (with a low probability) that an erroneous
event will occur. However, this short, potentially hazardous
period is not significantly worse than in conventional
(unscheduled) RPCs, as even a conventional RPC may in some cases
be executed a few seconds after it was sent by the client.
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The Default Value of sched-max-future
The default value of sched-max-future is defined to be 15 seconds.
This duration is long enough to allow the scheduled RPC to be sent
by the client, potentially to multiple servers, and in some cases
to send a cancellation message, as described in Section 3.2. On
the other hand, the 15-second duration yields a very low
probability of a reboot or a permission change.
3.7. Time-Interval Format
The time-interval format is used for representing the length of a
time interval and is based on the date-and-time format. It is used
for representing the scheduling tolerance parameters, as described in
the previous section.
While the date-and-time type uniquely represents a specific point in
time, the time-interval type defined below can be used to represent
the length of a time interval without specifying a specific date.
The time-interval type is defined as follows:
typedef time-interval {
type string {
pattern '\d{2}:\d{2}:\d{2}(\.\d+)?';
}
description
"Defines a time interval, up to 24 hours.
The format is specified as HH:mm:ss.f,
consisting of two digits for hours,
two digits for minutes, two digits
for seconds, and zero or more digits
representing second fractions.";
}
Example
The sched-max-future parameter is defined (Appendix A) as a time-
interval, as follows:
leaf sched-max-future {
type time-interval;
default 00:00:15.0;
}
The default value specified for sched-max-future is 0 hours, 0
minutes, and 15 seconds.
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4. Time Capability
The structure of this section is as defined in Appendix D of
[RFC6241].
4.1. Overview
A server that supports the time capability can perform time-triggered
operations as defined in this document.
A server implementing the :time capability:
o MUST support the ability to receive <rpc> messages that include a
time element and perform a time-triggered operation accordingly.
o MUST support the ability to include a time element in the <rpc-
reply> messages that it transmits.
4.2. Dependencies
With-defaults Capability
The time-capability YANG module (Appendix A) uses default values;
thus, it is assumed that the with-defaults capability [RFC6243] is
supported.
4.3. Capability Identifier
The :time capability is identified by the following capability
string:
urn:ietf:params:netconf:capability:time:1.0
4.4. New Operations
<cancel-schedule>
The <cancel-schedule> RPC is used for cancelling an RPC that was
previously scheduled.
A <cancel-schedule> RPC MUST include the <cancelled-message-id>
element, which specifies the message ID of the scheduled RPC that
needs to be cancelled.
A <cancel-schedule> RPC MAY include the <get-time> element. In
this case, the <rpc-reply> includes the <execution-time> element,
specifying the time at which the scheduled RPC was cancelled.
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4.5. Modifications to Existing Operations
4.5.1. Affected Operations
The :time capability defined in this memo can be applied to any of
the following operations:
o get-config
o get
o copy-config
o edit-config
o delete-config
o lock
o unlock
o commit
Three new elements are added to each of these operations:
o <scheduled-time> This element is added to the input of each
operation, indicating the time at which the server is scheduled to
invoke the operation. Every <rpc> message MAY include the
<scheduled-time> element. A server that supports the :time
capability and receives an <rpc> message with a <scheduled-time>
element MUST perform the operation as close as possible to the
scheduled time.
The <scheduled-time> element uses the date-and-time format
(Section 3.4.).
o <get-time> This element is added to the input of each operation.
An <rpc> message MAY include a <get-time> element, indicating that
the server MUST include an <execution-time> element in its
corresponding <rpc-reply>.
o <execution-time> This element is added to the output of each
operation, indicating the time at which the server completed the
operation. An <rpc-reply> MAY include the <execution-time>
element. A server that supports the :time capability and receives
an operation with the <get-time> element MUST include the
execution time in its response.
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The <execution-time> element uses the date-and-time format
(Section 3.4.).
4.5.2. Processing Scheduled Operations
A server that receives a scheduled RPC MUST start executing the RPC
as close as possible to its scheduled execution time.
If a session between a client and a server is terminated, the server
MUST cancel all pending scheduled RPCs that were received in this
session.
Scheduled RPCs are processed serially, in an order that is defined by
their scheduled times. Thus, the server sends <rpc-reply> messages
to scheduled RPCs according to the order of their corresponding
schedules. Note that this is a modification to the behavior defined
in [RFC6241], which states that replies are sent in the order the
requests were received. Interoperability with [RFC6241] is
guaranteed by the NETCONF capability exchange; a server that does not
support the :time capability responds to RPCs in the order the
requests were received. A server that supports the :time capability
replies to conventional (non-scheduled) RPCs in the order they were
received and replies to scheduled RPCs in the order of their
scheduled times.
If a server receives two or more RPCs that are scheduled to be
performed at the same time, the server executes the RPCs serially in
an arbitrary order.
4.6. Interactions with Other Capabilities
Confirmed Commit Capability
The confirmed commit capability is defined in Section 8.4 of
[RFC6241]. According to that document, a confirmed <commit>
operation MUST be reverted if a confirming commit is not issued
within the timeout period (which is 600 seconds by default).
When the time capability is supported, and a confirmed <commit>
operation is used with the <scheduled-time> element, the
confirmation timeout MUST be counted from the scheduled time,
i.e., the client begins the timeout measurement starting at the
scheduled time.
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5. Examples
5.1. <scheduled-time> Example
The following example extends the example presented in Section 7.2 of
[RFC6241] by adding the time capability. In this example, the
<scheduled-time> element is used to specify the scheduled execution
time of the configuration update (as shown in Figure 1).
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<scheduled-time
xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-time">
2015-10-21T04:29:00.235Z
</scheduled-time>
<config>
<top xmlns="http://example.com/schema/1.2/config">
<interface>
<name>Ethernet0/0</name>
<mtu>1500</mtu>
</interface>
</top>
</config>
</edit-config>
</rpc>
<rpc-reply message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<ok/>
</rpc-reply>
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5.2. <get-time> Example
The following example is similar to the one presented in Section 5.1,
except that, in this example, the client includes a <get-time>
element in its RPC and the server consequently responds with an
<execution-time> element (as shown in Figure 2).
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<get-time
xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-time">
</get-time>
<config>
<top xmlns="http://example.com/schema/1.2/config">
<interface>
<name>Ethernet0/0</name>
<mtu>1500</mtu>
</interface>
</top>
</config>
</edit-config>
</rpc>
<rpc-reply message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<ok/>
<execution-time>
2015-10-21T04:29:00.235Z
</execution-time>
</rpc-reply>
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5.3. Error Example
The following example presents a scenario in which the scheduled-time
is not within the scheduling tolerance, i.e., it is too far in the
past; therefore, an <rpc-error> is returned.
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<scheduled-time
xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-time">
2010-10-21T04:29:00.235Z
</scheduled-time>
<config>
<top xmlns="http://example.com/schema/1.2/config">
<interface>
<name>Ethernet0/0</name>
<mtu>1500</mtu>
</interface>
</top>
</config>
</edit-config>
</rpc>
<rpc-reply message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<rpc-error>
<error-type>application</error-type>
<error-tag>bad-element</error-tag>
<error-severity>error</error-severity>
<error-info>
<bad-element>scheduled-time</bad-element>
</error-info>
</rpc-error>
</rpc-reply>
6. Security Considerations
6.1. General Security Considerations
The security considerations of the NETCONF protocol in general are
discussed in [RFC6241].
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The usage of the time capability defined in this document can assist
an attacker in gathering information about the system, such as the
exact time of future configuration changes. Moreover, the time
elements can potentially allow an attacker to learn information about
the system's performance. Furthermore, an attacker that sends
malicious <rpc> messages can use the time capability to amplify her
attack; for example, by sending multiple <rpc> messages with the same
scheduled time. It is important to note that the security measures
described in [RFC6241] can prevent these vulnerabilities.
The time capability relies on an underlying time synchronization
protocol. Thus, by attacking the time protocol, an attack can
potentially compromise NETCONF when using the time capability. A
detailed discussion about the threats against time protocols and how
to mitigate them is presented in [RFC7384].
The time capability can allow an attacker to attack a NETCONF server
by sending malicious RPCs that are scheduled to take place in the
future. For example, an attacker can send multiple scheduled RPCs
that are scheduled to be performed at the same time. Another
possible attack is to send a large number of scheduled RPCs to a
NETCONF server, potentially causing the server's buffers to overflow.
These attacks can be mitigated by a carefully designed NETCONF
server; when a server receives a scheduled RPC that exceeds its
currently available resources, it should reply with an <rpc-error>
and discard the scheduled RPC.
Note that if an attacker has been detected and revoked, its future
scheduled RPCs are not executed; as defined in Section 4.5.2, once
the session with the attacker has been terminated, the corresponding
scheduled RPCs are discarded.
6.2. YANG Module Security Considerations
This memo defines a new YANG module, as specified in Appendix A.
The YANG module defined in this memo is designed to be accessed via
the NETCONF protocol [RFC6241]. The lowest NETCONF layer is the
secure transport layer and the mandatory-to-implement secure
transport is Secure SHell (SSH) [RFC6242]. The NETCONF access
control model [RFC6536] provides the means to restrict access for
particular NETCONF users to a preconfigured subset of all available
NETCONF protocol operations and content.
This YANG module defines <sched-max-future> and <sched-max-past>,
which are writable/creatable/deletable. These data nodes may be
considered sensitive or vulnerable in some network environments. An
attacker may attempt to maliciously configure these parameters to a
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low value, thereby causing all scheduled RPCs to be discarded. For
instance, if a client expects <sched-max-future> to be 15 seconds,
but in practice it is maliciously configured to 1 second, then a
legitimate scheduled RPC that is scheduled to be performed 5 seconds
in the future will be discarded by the server.
This YANG module defines the <cancel-schedule> RPC. This RPC may be
considered sensitive or vulnerable in some network environments.
Since the value of the <schedule-id> is known to all the clients that
are subscribed to notifications from the server, the <cancel-
schedule> RPC may be used maliciously to attack servers by cancelling
their pending RPCs. This attack is addressed in two layers: (i)
security at the transport layer, limiting the attack only to clients
that have successfully initiated a secure session with the server,
and (ii) the authorization level required to cancel an RPC should be
the same as the level required to schedule it, limiting the attack
only to attackers with an authorization level that is equal to or
higher than that of the client that initiated the scheduled RPC.
7. IANA Considerations
The following capability identifier URN has been registered in the
"Network Configuration Protocol (NETCONF) Capability URNs" registry:
urn:ietf:params:netconf:capability:time:1.0
The following XML namespace URN has been registered in the "IETF XML
Registry", following the format defined in [RFC3688]:
URI: urn:ietf:params:xml:ns:yang:ietf-netconf-time
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.
The following module name has been registered in the "YANG Module
Names" registry, defined in [RFC6020].
name: ietf-netconf-time
prefix: nct
namespace: urn:ietf:params:xml:ns:yang:ietf-netconf-time
RFC: 7758
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8. References
8.1. 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>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<http://www.rfc-editor.org/info/rfc3339>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81,
RFC 3688, DOI 10.17487/RFC3688, January 2004,
<http://www.rfc-editor.org/info/rfc3688>.
[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>.
[RFC6470] Bierman, A., "Network Configuration Protocol (NETCONF)
Base Notifications", RFC 6470, DOI 10.17487/RFC6470,
February 2012,
<http://www.rfc-editor.org/info/rfc6470>.
[RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC6991, July 2013,
<http://www.rfc-editor.org/info/rfc6991>.
8.2. Informative References
[Cond] Watsen, K., "Conditional Enablement of Configuration
Nodes", draft-kwatsen-conditional-enablement-00, Work in
Progress, February 2013.
[IEEE1588] IEEE, "IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems Version 2", IEEE Standard 1588.
[OneClock] Mizrahi, T. and Y. Moses, "OneClock to Rule Them All:
Using Time in Networked Applications", IEEE/IFIP Network
Operations and Management Symposium (NOMS), 2016.
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RFC 7758 Time Capability in NETCONF February 2016
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905,
DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.
[RFC5907] Gerstung, H., Elliott, C., and B. Haberman, Ed.,
"Definitions of Managed Objects for Network Time Protocol
Version 4 (NTPv4)", RFC 5907,
DOI 10.17487/RFC5907, June 2010,
<http://www.rfc-editor.org/info/rfc5907>.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)",
RFC 6020, DOI 10.17487/RFC6020, October 2010,
<http://www.rfc-editor.org/info/rfc6020>.
[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
<http://www.rfc-editor.org/info/rfc6242>.
[RFC6243] Bierman, A. and B. Lengyel, "With-defaults Capability for
NETCONF", RFC 6243, DOI 10.17487/RFC6243, June 2011,
<http://www.rfc-editor.org/info/rfc6243>.
[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>.
[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>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384,
DOI 10.17487/RFC7384, October 2014,
<http://www.rfc-editor.org/info/rfc7384>.
[Time4] Mizrahi, T. and Y. Moses, "Software Defined Networks:
It's About Time", IEEE INFOCOM, 2016.
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Appendix A. YANG Module for the Time Capability
This section is normative.
<CODE BEGINS> file "ietf-netconf-time@2016-01-26.yang"
module ietf-netconf-time {
namespace "urn:ietf:params:xml:ns:yang:ietf-netconf-time";
prefix nct;
import ietf-netconf { prefix nc; }
import ietf-yang-types { prefix yang; }
import ietf-netconf-monitoring { prefix ncm; }
organization
"IETF";
contact
"Editor: Tal Mizrahi
<dew@tx.technion.ac.il>
Editor: Yoram Moses
<moses@ee.technion.ac.il>";
description
"This module defines a capability-based extension to the
Network Configuration Protocol (NETCONF) that allows
time-triggered configuration and management operations.
This extension allows NETCONF clients to invoke configuration
updates according to scheduled times and allows NETCONF
servers to attach timestamps to the data they send to NETCONF
clients.
Copyright (c) 2016 IETF Trust and the persons identified as
the authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(http://trustee.ietf.org/license-info).";
revision 2016-01-26 {
description
"Initial version.";
Mizrahi & Moses Experimental [Page 24]
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reference
"RFC 7758:
Time Capability in NETCONF";
}
typedef time-interval {
type string {
pattern '\d{2}:\d{2}:\d{2}(\.\d+)?';
}
description
"Defines a time interval, up to 24 hours.
The format is specified as HH:mm:ss.f,
consisting of two digits for hours,
two digits for minutes, two digits
for seconds, and zero or more digits
representing second fractions.";
}
grouping scheduling-tolerance-parameters {
leaf sched-max-future {
type time-interval;
default 00:00:15.0;
description
"When the scheduled time is in the future, i.e., greater
than the present time, this leaf defines the maximal
difference between the scheduled time
and the present time that the server is willing to
accept. If the difference exceeds this number, the
server responds with an error.";
}
leaf sched-max-past {
type time-interval;
default 00:00:15.0;
description
"When the scheduled time is in the past, i.e., less
than the present time, this leaf defines the maximal
difference between the present time
and the scheduled time that the server is willing to
accept. If the difference exceeds this number, the
server responds with an error.";
}
description
"Contains the parameters of the scheduling tolerance.";
}
// extending the get-config operation
augment /nc:get-config/nc:input {
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leaf scheduled-time {
type yang:date-and-time;
description
"The time at which the RPC is scheduled to be performed.";
}
leaf get-time {
type empty;
description
"Indicates that the rpc-reply should include the
execution-time.";
}
description
"Adds the time element to <get-config>.";
}
augment /nc:get-config/nc:output {
leaf execution-time {
type yang:date-and-time;
description
"The time at which the RPC was executed.";
}
description
"Adds the time element to <get-config>.";
}
augment /nc:get/nc:input {
leaf scheduled-time {
type yang:date-and-time;
description
"The time at which the RPC is scheduled to be performed.";
}
leaf get-time {
type empty;
description
"Indicates that the rpc-reply should include the
execution-time.";
}
description
"Adds the time element to <get>.";
}
augment /nc:get/nc:output {
leaf execution-time {
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type yang:date-and-time;
description
"The time at which the RPC was executed.";
}
description
"Adds the time element to <get>.";
}
augment /nc:copy-config/nc:input {
leaf scheduled-time {
type yang:date-and-time;
description
"The time at which the RPC is scheduled to be performed.";
}
leaf get-time {
type empty;
description
"Indicates that the rpc-reply should include the
execution-time.";
}
description
"Adds the time element to <copy-config>.";
}
augment /nc:copy-config/nc:output {
leaf execution-time {
type yang:date-and-time;
description
"The time at which the RPC was executed.";
}
description
"Adds the time element to <copy-config>.";
}
augment /nc:edit-config/nc:input {
leaf scheduled-time {
type yang:date-and-time;
description
"The time at which the RPC is scheduled to be performed.";
}
leaf get-time {
type empty;
description
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"Indicates that the rpc-reply should include the
execution-time.";
}
description
"Adds the time element to <edit-config>.";
}
augment /nc:edit-config/nc:output {
leaf execution-time {
type yang:date-and-time;
description
"The time at which the RPC was executed.";
}
description
"Adds the time element to <edit-config>.";
}
augment /nc:delete-config/nc:input {
leaf scheduled-time {
type yang:date-and-time;
description
"The time at which the RPC is scheduled to be performed.";
}
leaf get-time {
type empty;
description
"Indicates that the rpc-reply should include the
execution-time.";
}
description
"Adds the time element to <delete-config>.";
}
augment /nc:delete-config/nc:output {
leaf execution-time {
type yang:date-and-time;
description
"The time at which the RPC was executed.";
}
description
"Adds the time element to <delete-config>.";
}
augment /nc:lock/nc:input {
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leaf scheduled-time {
type yang:date-and-time;
description
"The time at which the RPC is scheduled to be performed.";
}
leaf get-time {
type empty;
description
"Indicates that the rpc-reply should include the
execution-time.";
}
description
"Adds the time element to <lock>.";
}
augment /nc:lock/nc:output {
leaf execution-time {
type yang:date-and-time;
description
"The time at which the RPC was executed.";
}
description
"Adds the time element to <lock>.";
}
augment /nc:unlock/nc:input {
leaf scheduled-time {
type yang:date-and-time;
description
"The time at which the RPC is scheduled to be performed.";
}
leaf get-time {
type empty;
description
"Indicates that the rpc-reply should include the
execution-time.";
}
description
"Adds the time element to <unlock>.";
}
augment /nc:unlock/nc:output {
leaf execution-time {
type yang:date-and-time;
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description
"The time at which the RPC was executed.";
}
description
"Adds the time element to <unlock>.";
}
augment /nc:commit/nc:input {
leaf scheduled-time {
type yang:date-and-time;
description
"The time at which the RPC is scheduled to be performed.";
}
leaf get-time {
type empty;
description
"Indicates that the rpc-reply should include the
execution-time.";
}
description
"Adds the time element to <commit>.";
}
augment /nc:commit/nc:output {
leaf execution-time {
type yang:date-and-time;
description
"The time at which the RPC was executed.";
}
description
"Adds the time element to <commit>.";
}
augment /ncm:netconf-state {
container scheduling-tolerance {
uses scheduling-tolerance-parameters;
description
"The scheduling tolerance when the time capability
is enabled.";
}
description
"The scheduling tolerance of the server.";
}
rpc cancel-schedule {
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description
"Cancels a scheduled message.";
reference
"RFC 7758:
Time Capability in NETCONF";
input {
leaf cancelled-message-id {
type string;
description
"The ID of the message to be cancelled.";
}
leaf get-time {
type empty;
description
"Indicates that the rpc-reply should include
the execution-time.";
}
}
output {
leaf execution-time {
type yang:date-and-time;
description
"The time at which the RPC was executed.";
}
}
}
notification netconf-scheduled-message {
leaf schedule-id {
type string;
description
"The ID of the scheduled message.";
}
leaf scheduled-time {
type yang:date-and-time;
description
"The time at which the RPC is scheduled to be performed.";
}
description
"Indicates that a scheduled message was received.";
reference
"RFC 7758:
Time Capability in NETCONF";
}
}
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<CODE ENDS>
Acknowledgments
The authors gratefully acknowledge Joe Marcus Clarke, Andy Bierman,
Balazs Lengyel, Jonathan Hansford, John Heasley, Robert Sparks, Al
Morton, Olafur Gudmundsson, Juergen Schoenwaelder, Joel Jaeggli, Alon
Schneider, and Eylon Egozi for their insightful comments.
This work was supported in part by Israel Science Foundation grant
ISF 1520/11.
Authors' Addresses
Tal Mizrahi
Department of Electrical Engineering
Technion - Israel Institute of Technology
Technion City, Haifa, 32000
Israel
Email: dew@tx.technion.ac.il
Yoram Moses
Department of Electrical Engineering
Technion - Israel Institute of Technology
Technion City, Haifa, 32000
Israel
Email: moses@ee.technion.ac.il
Mizrahi & Moses Experimental [Page 32]