NETCONF E. Voit
Internet-Draft A. Clemm
Intended status: Informational A. Tripathy
Expires: September 18, 2016 E. Nilsen-Nygaard
A. Gonzalez Prieto
Cisco Systems
March 17, 2016
Restconf, HTTP, and HTTP2 Transport for YANG Push
draft-voit-netconf-restconf-yang-push-02
Abstract
This document defines YANG Subscription and Push mechanisms for
Restconf, HTTP, and HTTP2 transports.
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
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This Internet-Draft will expire on September 18, 2016.
Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Subscription Model . . . . . . . . . . . . . . . . . . . 3
3.2. Mechanisms for Subscription Establishment and Maintenance 3
3.3. Subscription Multiplexing . . . . . . . . . . . . . . . . 6
3.4. Push Data Stream and Transport Mapping . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 12
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Normative References . . . . . . . . . . . . . . . . . . 13
5.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Dynamic YANG Subscription when the Subscriber and
Receiver are different . . . . . . . . . . . . . . . 14
Appendix B. End-to-End Deployment Guidance . . . . . . . . . . . 15
B.1. Call Home . . . . . . . . . . . . . . . . . . . . . . . . 16
B.2. TLS Heartbeat . . . . . . . . . . . . . . . . . . . . . . 16
B.3. Putting it together . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
Requirements for subscriptions to YANG datastores are defined in
[pub-sub-reqs]. Mechanisms to support YANG subscriptions and
datastore object push over a NETCONF are defined in [yang-push].
Restconf support is also needed by the market. This document
provides such a specification for Restconf by reusing the YANG data
model, and expanding the transport requirements of [yang-push].
These extensions are not limited to just Restconf. There are
benefits which can be realized when transporting push updates
directly over HTTP such as simplified support for static
subscriptions. Additionally if HTTP/2 [RFC7540] transport is used,
HTTP/2 capabilities which can be applied include:
o Subscription multiplexing over independent HTTP/2 streams
o Stream prioritization and stream dependencies
o Flow control on independent streams
2. Terminology
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 RFC 2119 [RFC2119].
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Datastore: a conceptual store of instantiated management information,
with individual data items represented by data nodes which are
arranged in hierarchical manner.
Dynamic Subscription: a Subscription negotiated between Subscriber
and Publisher via create, establish, modify, and delete RPC control
plane signaling messages.
Publisher: an entity responsible for distributing subscribed YANG
object data per the terms of a Subscription. In general, a Publisher
is the owner of the YANG datastore that is subjected to the
Subscription.
Receiver: a target to which Publisher pushes updates. In many
deployments, the Receiver and Subscriber will be the same entity.
Static Subscription: A Subscription installed via a configuration
interface.
Subscriber: an entity able to request and negotiate a contract for
push updates from a Publisher.
Subscription: a contract between a Subscriber and a Publisher,
stipulating which information the Receiver wishes to have pushed from
the Publisher without the need for further solicitation.
Subscription Update: a set of data nodes and object values pushed
together as a unit and intended to meet the obligations of a single
subscription at a snapshot in time.
3. Solution
This document specifies transport mechanisms that allow subscribed
information updates to be pushed from a YANG datastore.
3.1. Subscription Model
Subscriptions use the base data model and subscription state machine
from [yang-push].
3.2. Mechanisms for Subscription Establishment and Maintenance
On a Publisher, it must be possible to instantiate a Subscription via
dynamic Subscriber signaling, as well as via Static configuration.
Dynamic Subscriptions are signaled Subscriptions aimed at the running
datastore and are unable to impact the startup configuration. They
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should always terminate when there is loss of transport session
connectivity between the Publisher and Receiver.
Static Subscriptions are applied via an operations interface to the
startup and running configurations. Loss or non-availability of
transport session connectivity will place the Subscription into the
suspended state. Logic beyond the scope of this specification will
dictate when any particular Subscription should be reactivated.
There are three models for Subscription establishment and
maintenance:
1. Dynamic Subscription: Subscriber and Receiver are the same
2. Static Subscription
3. Dynamic Subscription: Subscriber and Receiver are different
The first two are described in this section. The third is described
in Appendix A. This third option can be moved into the body of this
specification should the IETF community desire. In theory, all three
models may be intermixed in a single deployment.
.---------------.
| Publisher |
'---------------'
^ ^ | ^
| | | |
.-----Restconf----' | | '-----Restconf----.
| .-----' '-HTTP-. |
V | V |
.-------------. .------------. .----------. .------------.
| Subscriber+ | | Operations | | Receiver | | Subscriber |
| Receiver | | /Config | '----------' '------------'
'-------------' '------------' ^ ^ ^
^ (out of scope) : : :
: ^ : :....Model 3....:
Model 1 :...Model 2...: (out of scope)
3.2.1. Dynamic YANG Subscription: Subscriber and Receiver are the same
With all Dynamic Subscriptions, as with [yang-push] it must be
possible to configure and manage Subscriptions via signaling. This
signaling is transported over [restconf]. Once established,
streaming Subscription Updates are then delivered via Restconf SSE.
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3.2.2. Static Subscription
With a Static Subscription, all information needed to establish a
secure object push relationship with that Receiver must be configured
via a configuration interface on the Publisher. This information
includes all the signaled information identified in section 3.2.1.
This information also include the Receiver address, egress interface
instructions, and security credentials required to establish TLS
between the Publisher and Receiver. Mechanisms for locally
configuring these parameters are outside the scope of this document.
With this information, the Publisher will establish a secure
transport connection with the Receiver and then begin pushing the
streaming updates to the Receiver. Since Restconf might not exist on
the Receiver, it is not desirable to require that updates be pushed
via Restconf. In place of Restconf, a TLS secured HTTP Client
connection must be established with an HTTP Server located on the
Receiver. Subscription Updates will then be sent via HTTP Post
messages to the Receiver.
Post messages will be addressed to HTTP augmentation code on the
Receiver capable accepting and responding to Subscription Updates.
At least the initial Post message must include the URI for the
subscribed resource. This URI can be retained for future use by the
Receiver.
After successful receipt of an initial Subscription Update for a
particular Subscription, this augmentation should reply back with an
HTTP status code of 201 (Created). Further successful receipts
should result in the return of code of 202 (Accepted). At any point,
receipt of any status codes from 300-510 with the exception of 408
(Request Timeout) should result in the movement of the Subscription
to the suspended state. A sequential series of multiple 408
exceptions should also drive the Subscription to a suspended state.
Security on an HTTP client/Publisher can be strengthened by only
accepting Response code feedback for recently initiated HTTP POSTs.
Figure 3 depicts this message flow.
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+-----------+ +----------+
| Publisher | | Receiver |
+-----------+ +----------+
|<--------------TLS------------>|
| |
|HTTP POST (Sub ID, URI, data1) |
|------------------------------>|
| HTTP 201 (Created)|
|<------------------------------|
|HTTP POST (Sub ID, data2) |
|------------------------------>|
| HTTP 200 or 202 (Accepted)|
|<------------------------------|
| data3 |
|<----------------------------->|
If HTTP/2 transport is available to a Receiver, the Publisher should
also:
o point individual Subscription Updates to a unique HTTP/2 stream
for that Subscription,
o take any subscription-priority and provision it into the HTTP/2
stream priority, and
o take any subscription-dependency and provision it into the HTTP/2
stream dependency.
3.3. Subscription Multiplexing
When pushed directly over HTTP/2, it is expected that each
Subscription Update will be allocated a separate Stream. The will
enable multiplexing, and address issues of Head-of-line blocking with
different priority Subscriptions.
When pushed via Restconf over HTTP/2, different Subscriptions will
not be mapped to independent HTTP/2 streams. When Restconf specifies
this mapping, it should be integrated into this specification.
Even without HTTP/2 multiplexing, it is possible that updates might
be delivered in a different sequence than generated. Reasons for
this might include (but are not limited to):
o different durations needed to create various Subscription Updates,
o marshalling and bundling of multiple Subscription Updates for
transport, and
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o parallel HTTP1.1 sessions
Therefore each Subscription Update will include a microsecond level
timestamp to ensure that a receiver understands the time when a that
update was generated. Use of this timestamp can give an indication
of the state of objects at a Publisher when state-entangled
information is received across different subscriptions. The use of
the latest Subscription Update timestamp for a particular object
update can introduce errors. So when state-entangled updates have
inconsistent object values and temporally close timestamps, a
Receiver might consider performing a 'get' to validate the current
state of objects.
3.4. Push Data Stream and Transport Mapping
Transported updates will contain data for one or more Subscription
Updates. Each transported Subscription Update notification contains
several parameters:
o A global subscription ID correlator, referencing the name of the
Subscription on whose behalf the notification is sent.
o Data nodes containing a representation of the datastore subtree
containing the updates. The set of data nodes must be filtered
per access control rules to contain only data that the subscriber
is authorized to see.
o An event time which contains the time stamp at publisher when the
event is generated.
3.4.1. Pushing Subscription Updates via Restconf
Subscribers can dynamically learn whether a RESTCONF server supports
yang-push. This is done by issuing an HTTP request OPTIONS, HEAD, or
GET on the stream push-update. E.g.:
GET /restconf/data/ietf-restconf-monitoring:restconf-state/
streams/stream=yang-push HTTP/1.1
Host: example.com
Accept: application/yang.data+xml
If the server supports it, it may respond
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HTTP/1.1 200 OK
Content-Type: application/yang.api+xml
<stream xmlns="urn:ietf:params:xml:ns:yang:ietf-restconf-monitoring">
<name>yang-push</name>
<description>Yang push stream</description>
<access>
<encoding>xml</encoding>
<location>https://example.com/streams/yang-push-xml
</location>
</access>
<access>
<encoding>json</encoding>
<location>https://example.com/streams/yang-push-json
</location>
</access>
</stream>
If the server does not support yang push, it may respond
HTTP/1.1 404 Not Found
Date: Mon, 25 Apr 2012 11:10:30 GMT
Server: example-server
Subscribers can determine the URL to receive updates by sending an
HTTP GET request for the "location" leaf with the stream list entry.
The stream to use for yang push is the push-update stream. The
location returned by the publisher can be used for the actual
notification subscription. Note that different encodings are
supporting using different locations. For example, he subscriber
might send the following request:
GET /restconf/data/ietf-restconf-monitoring:restconf-state/
streams/stream=yang-push/access=xml/location HTTP/1.1
Host: example.com
Accept: application/yang.data+xml
The publisher might send the following response:
HTTP/1.1 200 OK
Content-Type: application/yang.api+xml
<location
xmlns="urn:ietf:params:xml:ns:yang:ietf-restconf-monitoring">
https://example.com/streams/yang-push-xml
</location>
To subscribe and start receiving updates, the subscriber can then
send an HTTP GET request for the URL returned by the publisher in the
request above. The accept header must be "text/event -stream". The
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publisher handles the connection as an event stream, using the Server
Sent Events[W3C-20121211] transport strategy.
The publisher MUST support as query parameters for a GET method on
this resource all the parameters of a subscription. The only
exception is the encoding, which is embedded in the URI. An example
of this is:
// subtree filter = /foo
// periodic updates, every 5 seconds
GET /mystreams/yang-push?subscription-id=my-sub&period=5&
xpath-filter=%2Fex:foo[starts-with("bar"."some"]
Should the publisher not support the requested subscription, it may
reply:
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HTTP/1.1 501 Not Implemented
Date: Mon, 23 Apr 2012 17:11:00 GMT
Server: example-server
Content-Type: application/yang.errors+xml
<errors xmlns="urn:ietf:params:xml:ns:yang:ietf-restconf">
<error>
<error-type>application</error-type>
<error-tag>operation-not-supported</error-tag>
<error-severity>error</error-severity>
<error-message>Xpath filters not supported</error-message>
<error-info>
<supported-subscription xmlns="urn:ietf:params:xml:ns:
netconf:datastore-push:1.0">
<subtree-filter/>
</supported-subscription>
</error-info>
</error>
</errors>
with an equivalent JSON encoding representation of:
HTTP/1.1 501 Not Implemented
Date: Mon, 23 Apr 2012 17:11:00 GMT
Server: example-server
Content-Type: application/yang.errors+json
{
"ietf-restconf:errors": {
"error": {
"error-type": "protocol",
"error-tag": "operation-not-supported",
"error-message": "Xpath filters not supported."
"error-info": {
"datastore-push:supported-subscription": {
"subtree-filter": [null]
}
}
}
}
}
The following is an example of a push Subscription Update data for
the subscription above. It contains a subtree with root foo that
contains a leaf called bar:
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XML encoding representation:
<?xml version="1.0" encoding="UTF-8"?>
<notification xmlns="urn:ietf:params:xml:ns:yang:ietf-restconf">
<subscription-id xmlns="urn:ietf:params:xml:ns:restconf:
datastore-push:1.0">
my-sub
</subscription-id>
<eventTime>2015-03-09T19:14:56Z</eventTime>
<datastore-contents xmlns="urn:ietf:params:xml:ns:restconf:
datastore-push:1.0">
<foo xmlns="http://example.com/yang-push/1.0">
<bar>some_string</bar>
</foo>
</datastore-contents>
</notification>
Or with the equivalent YANG over JSON encoding representation as
defined in[yang-json] :
{
"ietf-restconf:notification": {
"datastore-push:subscription-id": "my-sub",
"eventTime": "2015-03-09T19:14:56Z",
"datastore-push:datastore-contents": {
"example-mod:foo": { "bar": "some_string" }
}
}
}
To modify a subscription, the subscriber issues another GET request
on the provided URI using the same subscription-id as in the original
request. For example, to modify the update period to 10 seconds, the
subscriber may send:
GET /mystreams/yang-push?subscription-id=my-sub&period=10&
subtree-filter=%2Ffoo'
To delete a subscription, the subscriber issues a DELETE request on
the provided URI using the same subscription-id as in the original
request
DELETE /mystreams/yang-push?subscription-id=my-sub
3.4.2. Pushing Subscription Updates directly via HTTP
For any version of HTTP, the basic encoding will look as below is the
above JSON representation wrapped in an HTTP header. Mechanism will
be
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POST (IP+Port) HTTP/1.1
From: (Identifier for Network Element)
User-Agent: (CiscoYANGPubSub/1.0)
Content-Type: multipart/form-data
Content-Length: (determined runtime)
{
"ietf-yangpush:notification": {
"datastore-push:subscription-id": "my-sub",
"eventTime": "2015-03-09T19:14:56Z",
"datastore-push:datastore-contents": {
"foo": { "bar": "some_string" }
}
}
}
4. Security Considerations
Subscriptions could be used to intentionally or accidentally overload
resources of a Publisher. For this reason, it is important that the
Publisher has the ability to prioritize the establishment and push of
updates where there might be resource exhaust potential. In
addition, a server needs to be able to suspend existing subscriptions
when needed. When this occurs, the subscription status must be
updated accordingly and the clients are notified.
A Subscription could be used to retrieve data in subtrees that a
client has not authorized access to. Therefore it is important that
data pushed via a Subscription is authorized equivalently with
regular data retrieval operations. Data being pushed to a client
needs therefore to be filtered accordingly, just like if the data
were being retrieved on-demand. The Netconf Authorization Control
Model [RFC6536] applies even though the transport is not NETCONF.
One or more Publishers could be used to overwhelm a Receiver which
doesn't even support subscriptions. Therefore Updates MUST only be
transmittable over Encrypted transports. Clients which do not want
pushed data need only terminate or refuse any transport sessions from
the Publisher.
One or more Publishers could overwhelm a Receiver which is unable to
control or handle the volume of Updates received. In deployments
where this might be a concern, transports supporting per-subscription
Flow Control and Prioritization (such as HTTP/2) should be selected.
Another benefit is that a well-behaved Publisher implementation is
that it is difficult to a Publisher to perform a DoS attack on a
Receiver. DoS attack protection comes from:
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o the requirement for trust of a TLS session before publication,
o the need for an HTTP transport augmentation on the Receiver, and
o that the Publication process is suspended when the Receiver
doesn't respond.
5. References
5.1. 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>.
[RFC6520] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) Heartbeat Extension", RFC 6520,
DOI 10.17487/RFC6520, February 2012,
<http://www.rfc-editor.org/info/rfc6520>.
[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>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
5.2. Informative References
[call-home]
Watsen, K., "NETCONF Call Home and RESTCONF Call Home",
December 2015, <https://tools.ietf.org/html/draft-ietf-
netconf-call-home-17>.
[pub-sub-reqs]
Voit, Eric., Clemm, Alexander., and Alberto. Gonzalez
Prieto, "Subscribing to datastore push updates", February
2016, <https://datatracker.ietf.org/doc/draft-ietf-i2rs-
pub-sub-requirements/>.
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[restconf]
Bierman, Andy., Bjorklund, Martin., and Kent. Watsen,
"RESTCONF Protocol", March 2016,
<https://datatracker.ietf.org/doc/draft-ietf-netconf-
restconf/>.
[W3C-20121211]
"Server-Sent Events, World Wide Web Consortium CR CR-
eventsource-20121211", December 2012,
<http://www.w3.org/TR/2012/CR-eventsource-20121211>.
[yang-json]
Lhotka, Ladislav., "JSON Encoding of Data Modeled with
YANG", March 2016, <https://datatracker.ietf.org/doc/
draft-ietf-netmod-yang-json/>.
[yang-push]
Clemm, Alexander., Gonzalez Prieto, Alberto., Voit, Eric.,
Prasad Tripathy, Ambika., and Einar. Nilsen-Nygaard,
"Subscribing to YANG datastore push updates", February
2016, <https://datatracker.ietf.org/doc/draft-ietf-
netconf-yang-push/>.
Appendix A. Dynamic YANG Subscription when the Subscriber and Receiver
are different
The methods of Sections 3.2.1 and 3.2.2 can be combined to enable
deployment models where the Subscriber and Receiver are different.
Such separation can be useful with some combination of:
o An operator wants any Subscriptions immediately deleted should TLS
connectivity be lost. (I.e., Subscriptions don't default into a
'Suspended' state on the Publisher.)
o An operator wants the Publisher to include highly restrictive
capacity management and security mechanisms outside of domain of
existing operational or programmatic interfaces.
o Restconf is not desired on the Receiver.
o The Publisher doesn't want to maintain Restconf subscriptions with
many Receivers.
To do this, first the necessary information must be signaled as part
of the <create-subscription>. This includes all the information
described in section 3.3.2, with the exception of the security
credentials. (It is assumed that any security credentials required
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for establishing any transport connections are pre-provisioned on all
devices.)
Using this set of Subscriber provided information, the same process
described within section 3.3.2 will be followed. There is one
exception. When an HTTP status code is 201 is received by the
Publisher, it will inform the Subscriber of Subscription
establishment success via its Restconf connection.
After successful establishment, if the Subscriber wishes to maintain
the state of Receiver subscriptions, it can simply place a separate
on-change Subscription into the "Subscriptions" subtree of the YANG
Datastore on the Publisher.
Putting it all together, the message flow is:
+------------+ +-----------+ +----------+
| Subscriber | | Publisher | | Receiver |
+------------+ +-----------+ +----------+
| Restconf PUT: | |
| <create-subscription>| |
|--------------------->| |
| | |
| |<-----------TLS------------>|
| | |
| |HTTP POST (Sub ID, data1, |
| |(stream ID, URI?)) |
| |--------------------------->|
| | HTTP 201 (Created)|
| |<---------------------------|
| Success: HTTP 204| |
|<---------------------| |
| |HTTP POST (Sub ID, data2) |
| |--------------------------->|
| | HTTP 200 or 202 (Accepted)|
| |<---------------------------|
| | data3 |
| |<-------------------------->|
| | |
Appendix B. End-to-End Deployment Guidance
Several technologies are expected to be seen within a deployment to
achieve security and ease-of-use requirements. These are not
necessary for an implementation of this specification, but will be
useful to consider when considering the operational context.
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B.1. Call Home
Pub/Sub implementations should have the ability to transparently
incorporate lower layer technologies such as Call Home so that secure
TLS connections are always originated from the Publisher. There is a
Restconf Call home function in [call-home]. For security reasons,
this should be implemented when applicable.
B.2. TLS Heartbeat
Unlike NETCONF, HTTP sessions might not quickly allow a Subscriber to
recognize when the communication path has been lost from the
Publisher. To recognize this, it is possible for a Receiver (usually
the subscriber) to establish a TLS heartbeat [RFC6520]. In the case
where a TLS heartbeat is included, it should be sent just from
Receiver to Publisher. Loss of the heartbeat should result in the
Subscription being terminated with the Subscriber (even when the
Subscriber and Receiver are different). The Subscriber can then
attempt to re-establish the subscription if desired. If the
Subscription remains active on the Publisher, future receipt of
objects associated with that (or any other unknown) subscription ID
should result in a <delete-subscription> being returned to the
Publisher from the Receiver.
B.3. Putting it together
If Subscriber and receiver are same entity then subscriber can direct
send create_subscription message to publisher. Once the subscription
moved to accepted state, the receiver can use Server Sent Events
[W3C-20121211] transport strategy to subscriber event notifications
for the data as defined in [restconf].
Authors' Addresses
Eric Voit
Cisco Systems
Email: evoit@cisco.com
Alexander Clemm
Cisco Systems
Email: alex@cisco.com
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Ambika Prasad Tripathy
Cisco Systems
Email: ambtripa@cisco.com
Einar Nilsen-Nygaard
Cisco Systems
Email: einarnn@cisco.com
Alberto Gonzalez Prieto
Cisco Systems
Email: albertgo@cisco.com
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