netconf B. Liu
Internet-Draft G. Zheng
Intended status: Standards Track Huawei Technologies
Expires: September 23, 2016 M. Jethanandani
Cisco Systems
K. Watsen
Juniper Networks
March 22, 2016

Processing Multiple Replies for One Request in NETCONF


This document discusses several scenarios that multiple replies for a single request are needed, with the ability to terminate the replies at any time. Such scenarios are not well supported by current NETCONF (Network Configuration) protocol. An extention at the NETCONF messaging layer is needed to fulfill the requirement.

Status of This Memo

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Table of Contents

1. Introduction

The message procedures of NETCONF [RFC6241] are based on RPC (Remote Procedure Call) interactions. A NETCONF client/server sends a <rpc> message to the counterpart and then receives a replying <rpc-reply> message.

In some situations, it might need multiple <rpc-reply> messages for a <rpc> request. For example, the the <rpc-reply> message might be very large that it needs to be fragmented into multiple small ones; or some operations (e.g. ping) need persistent replies till such time that a terminaing condition is encountered or when the operation is cancelled.

This document discusses such kind of multiple replies scenarios, analyzes the issues of current NETCONF protocol in supporting these scenarios, and proposes several candidate solutions for discussion.

2. Requirements Language and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] when they appear in ALL CAPS. When these words are not in ALL CAPS (such as "should" or "Should"), they have their usual English meanings, and are not to be interpreted as [RFC2119] key words.


CLI: Command Line interface

DOM: Document Object Model, which is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents. Objects in the DOM tree may be addressed and manipulated by using methods on the objects.

RPC: Remote Procedure Call

SAX: Simple API for XML, which is an event sequential access parser API developed by the XML-DEV mailing list for XML documents. SAX provides a mechanism for reading data from an XML document that is an alternative to that provided by the DOM. Where the DOM operates on the document as a whole, SAX parsers operate on each piece of the XML document sequentially.

libxml: a software library for parsing XML documents.

<get-block>: a capability and operation defined in this document to handle large size <rpe-reply> messages.

3. Scenarios and Problems

This section discusses several scenarios where multiple replies might be needed, and anylizes the problems of current NETCONF protocol in supporting these scenarios.

3.1. Bulk <rpc-reply>

The <rpc-reply> message might be very large in following situations:

Current there are already some methods of processing bulk replies as the following. However, there are some issues as analyzed below.

1) Stream-Oriented Handling

Stream-Oriented handling mainly includes the following two aspects:

The problems are:

2) Requesting a Portion of Data

The clients actively limit the search range of the data so that the servers only need to reply with a part of the large size data. Thus the clients could control the replies in a reasonable size. One example is that the clients get a list of the content, and provide a start offset and a max-count, to get a portion at a time.

The problems are:

3.2. Persistent <rpc-reply>

One of the operations that CLI offers today is the ability to issue an operation that might result in multiple responses being returned, till such time that a terminaing condition is encountered or when the operation is cancelled. An example of such an operation is when the ping or a traceroute command is issued. In the former case, the operation can continue sending responses back till it is cancelled, while in the latter case there is usually a terminating condition that stops the responses.

NETCONF protocol as defined today sends a single RPC request and expects a single reply to that request. The "persistent" operation defined above expects multiple responses for a single request, till such time a terminating condition is encountered.

3.3. Long time <rpc-reply>

Some operations might take a long time to perform (e.g. network link performance validation), so there could be multiple responses for the request. For example, initial responses returns handle which the client uses to monitor progress till the final result. The client should be able to cancel the request at any time.

3.4. Datastore push updates

In [I-D.clemm-netconf-yang-push] and [I-D.ietf-i2rs-pub-sub-requirements], it describes a scenario where client applications need to request updates from a YANG datastore, without requiring additional client requests. [I-D.clemm-netconf-yang-push] proposes to extend notification messaging to fulfill the requirement that a comprehensive subsription/publication model could be well supported.

The datastore sub/pub might need specific data modeling and operation extention. However, at the NETCONF message layer, this draft considers multiple replies could be an alternative solution for sub/pub comparing with notification messaging extention.

[Open Question] Need more in-depth analysis and comparison of the two alternatives.

4. Requriments for NETCONF

Given above mentioned multiple-replies scenarios and problems, the requriments for NETCONF protocol could be summarized as a mechanism with the following abilities:

5. Candidate Solutions

(Editor notes: this section discusses possible solutions. The fragmentation mechanism is from the draft [I-D.liu-netconf-fragmentation]. The other one was proposed during mailing list discussion by Juergen Schoenwaelder. We include both of them for discussion and solution selection.)

5.1. Get-block Mechanism

This section proposes a method of extending the NETCONF protocol to allow handling the replying data as multiple <rpc-reply> blocks. It allows the NETCONF client to terminate the block data processing momentarily by protocol interactions; and also allows the blocked messages to be instantly parsed piece by piece. Specifically, the mechanism is achieved by a newly defined <get-block> capability and relevant operations.

5.1.1. Design Requirements

Two essential requirements of the Get-block mechanism are:

(Editor notes: this solution was originally designed for large size fragmentation processing. However, the rationale of this solution has the potential to fulfill the other scenarios.)

5.1.2. <get-block> extention

o Function

o Parameters

o Successful Operation Reply

o Exception Handling

Please refer to Appendix A.1 for an example.

5.2. Linked Replies

Another solution is to change or augment NETCONF at some point in time such that an <rpc> can lead to a sequence of <rpc-reply> with a suitable cancel mechanism. A simple approach is to add a linked-replies capability. If a server announces "linked-replies" capablility and the client supports it as well, the client can add an additional parameter to an rpc to indicate the possible use of linked-replies.

Please refer to Appendix A.2 for an example.

This would address the concern of large data retrievals but would also allow long running asynchronous rpcs (the ping or traceroute example). This approach may lead to better support for asynchronous rpcs and rpcs that potentially return very large chunks of data than trying to solve this problem without enhancements of the rpc layer. Design details concerning data merging, error handling, how to send a cancel for a given link-id (e.g., by sending a new <rpc-cancel> message with a matching link-id) and whether it is necessary to negotiate linked rpc-reply sizes or whether it is good enough for the server to decide freely as it likes etc. need further study.

6. Security Considerations


7. IANA Considerations

This draft does not request any IANA action.

8. Acknowledgements

Gang Yan and Shouchuan Yang made significant contribution to form the draft. Valuable comments were received from Andy Bierman, Juergen Schoenwaelder, Balazs Lengyel, Martin Bjorklund, Chong Feng and some other people in Netconf working group.

This document was produced using the xml2rfc tool [RFC2629]. (initiallly prepared using )

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, DOI 10.17487/RFC2629, June 1999.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J. and A. Bierman, "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011.

9.2. Informative References

[I-D.clemm-netconf-yang-push] Clemm, A., Prieto, A. and E. Voit, "Subscribing to YANG datastore push updates", Internet-Draft draft-clemm-netconf-yang-push-02, October 2015.
[I-D.ietf-i2rs-pub-sub-requirements] Voit, E., Clemm, A. and A. Prieto, "Requirements for Subscription to YANG Datastores", Internet-Draft draft-ietf-i2rs-pub-sub-requirements-05, February 2016.
[I-D.liu-netconf-fragmentation] Liu, B. and G. Zheng, "A NETCONF Extension for Data Fragmentation", Internet-Draft draft-liu-netconf-fragmentation-01, October 2014.

Appendix A. Examples of the Candidate Solutions

A.1. <get-block> Example

Example 1: Get the next block
     <rpc message-id="101"
         <filter type="subtree">
           <top xmlns="">

     <rpc-reply message-id="101"
         <top xmlns="">
               <full-name>Charlie Root</full-name>
             <!-- additional <user> elements appear here... -->

<rpc message-id="102"
       <get-block xmlns=""
     <rpc-reply message-id="102"
         <top xmlns="">
               <full-name>Jim Green</full-name>
             <!-- additional <user> elements appear here... -->

Example 2: Abandon the remaining blocks
     <rpc message-id="103"
       <get-block xmlns=
        /1.0 set-id="1">

     <rpc-reply message-id="103"

Example 3: Following is an example of the rule f in Section 4.1.2. 
The child eTable is in a different message with the parents 
aTable->bTable->cTable->dTable. Then the path and index information 
of all the ancestors MUST included in the search data. 


A.2. Linked-replies Example

Here is what a new client might do if it wants to use linked replies:

<rpc message-id="101" link-id="123" xmlns="...">

The server can either simply send an rpc-reply or it starts sending
linked replies, e.g.:

<rpc-reply message-id="101" next-message-id="101" link-id="123" xmlns="...">

<rpc-reply message-id="101" next-message-id="101" link-id="124" xmlns="...">

<rpc-reply message-id="101" link-id="125" xmlns="...">

Authors' Addresses

Bing Liu Huawei Technologies Q14, Huawei Campus, No.156 Beiqing Road Hai-Dian District, Beijing, 100095, P.R. China EMail:
Guangying Zheng Huawei Technologies Huawei Nanjing R&D Center 101 Software Avenue, Yuhua District, Nanjing, Jiangsu, 210012, P.R. China EMail:
Mahesh Jethanandani Cisco Systems USA EMail:
Kent Watsen Juniper Networks EMail: