Network Working Group T. Goddard
Internet-Draft ICEsoft Technologies Inc.
Expires: August 13, 2004 February 13, 2004
NETCONF Over SOAP
draft-ietf-netconf-soap-01
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The device management protocol NETCONF is applicable to a wide range
of devices in a variety of environments. The emergence of Web
Services gives one such environment, and is presently characterized
by the use of SOAP over HTTP. NETCONF finds many benefits in this
environment: from the re-use of existing standards, to ease of
software development, to integration with deployed systems. Herein,
we describe a SOAP over HTTP binding that, when used with persistent
HTTP connections, yields an application protocol sufficient for
NETCONF.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. SOAP Background for NETCONF . . . . . . . . . . . . . . . . 4
2.1 Use and Storage of WSDL and XSD . . . . . . . . . . . . . . 4
2.2 SOAP over HTTP . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 HTTP Drawbacks . . . . . . . . . . . . . . . . . . . . . . . 5
2.4 BCP56: On the Use of HTTP as a Substrate . . . . . . . . . . 6
2.5 Important HTTP 1.1 Features . . . . . . . . . . . . . . . . 6
2.6 SOAP Implementation Considerations . . . . . . . . . . . . . 7
2.6.1 SOAP Feature Exploitation . . . . . . . . . . . . . . . . . 7
2.6.2 SOAP Headers . . . . . . . . . . . . . . . . . . . . . . . . 7
3. A SOAP Web Service for NETCONF . . . . . . . . . . . . . . . 8
3.1 Fundamental Use Case . . . . . . . . . . . . . . . . . . . . 8
3.2 NETCONF Sessions . . . . . . . . . . . . . . . . . . . . . . 8
3.3 Capabilities Exchange . . . . . . . . . . . . . . . . . . . 8
3.4 A NETCONF/SOAP example . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . 11
4.1 Integrity, Privacy, and Authentication . . . . . . . . . . . 11
4.2 Vulnerabilities . . . . . . . . . . . . . . . . . . . . . . 11
4.3 Environmental Specifics . . . . . . . . . . . . . . . . . . 12
Normative References . . . . . . . . . . . . . . . . . . . . 13
Informative References . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . 15
A. WSDL Definitions . . . . . . . . . . . . . . . . . . . . . . 16
A.1 NETCONF SOAP Binding . . . . . . . . . . . . . . . . . . . . 16
A.2 Sample Service Definition . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . 18
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1. Introduction
Given the use of XML [2] and the remote procedure call
characteristics, it is natural to consider a binding of the NETCONF
[1] operations to a SOAP [3] application protocol. This document
proposes a binding of this form.
In general, SOAP over HTTP is a natural application protocol for
NETCONF, essentially because of the remote procedure call character
of both, but care must be taken in some cases as HTTP is inherently
synchronous and client-driven.
Four basic topics are presented: SOAP specifics of interest to
NETCONF, specifics on implementing NETCONF as a SOAP-based web
service, security considerations, and an appendix with functional
WSDL. In some sense, the most important part of the document is the
brief WSDL document presented in the Appendix. With the right tools,
the WSDL combined with the base NETCONF XML Schemas provide machine
readable descriptions sufficient for the development of software
applications using NETCONF.
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2. SOAP Background for NETCONF
Why introduce SOAP as yet another wrapper around what is already a
remote procedure call message? There are, in fact, both technical
and practical reasons. The technical reasons are perhaps less
compelling, but let's examine them first.
The use of SOAP does offer a few technical advantages. SOAP is
fundamentally an XML messaging scheme (which is capable of supporting
remote procedure call) and it defines a simple message format
composed of a "header" and a "body" contained within an "envelope".
The "header" contains meta-information relating to the message, and
can be used to indicate such things as store-and-forward behaviour or
transactional characteristics. In addition, SOAP specifies an
optional encoding for the "body" of the message. However, this
encoding is not applicable to NETCONF as one of the goals is to have
highly readable XML, and SOAP-encoding is optimized instead for ease
of automated deserialization. These benefits of the SOAP message
structure are simple, but worthwhile due to the fact that they are
already standardized.
It is the practical reasons that truly make SOAP over HTTP an
interesting choice for device management. It is not difficult to
invent a mechanism for exchanging XML messages over TCP, but what is
difficult is getting that mechanism supported in a wide variety of
tools and operating systems and having that mechanism understood by a
great many developers. SOAP over HTTP (with WSDL) is seeing good
success at this, and this means that a device management protocol
making use of these technologies has advantages in being implemented
and adopted. Admittedly, there are interoperability problems with
SOAP and WSDL, but such problems have wide attention and can be
expected to be resolved.
2.1 Use and Storage of WSDL and XSD
One of the advantages of using machine readable formats such as Web
Services Description Language (WSDL) [4] and XML Schemas [5] is that
they can be used automatically in the software development process.
With appropriate tools, WSDL and XSD can be used to generate classes
that act as remote interfaces or application specific data
structures. Other uses, such as document generation and service
location, are also common. A great innovation found with many
XML-based definition languages is the use of hyperlinks for referring
to documents containing supporting definitions. For instance, in
WSDL, the import statement
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imports the definitions of XML types and elements from the base
NETCONF schema. Ideally, the file containing that schema is hosted
on a web server under the authority of the standards body that
defined the schema. In this way, dependent standards can be built up
over time and all are accessible to automated software tools that
ensure adherence to the standards. Thus, it will gradually become as
important for iana.org to host documents like
http://iana.org/netconf/1.0/base/base.xsd
as the IETF now hosts documents such as
http://www.ietf.org/rfc/rfc2616.txt
2.2 SOAP over HTTP
While it is true that SOAP focuses on messages and can be bound to
different underlying protocols such as HTTP, SMTP, or BEEP, most
existing SOAP implementations support only HTTP or HTTP/TLS. For
this discussion we will assume SOAP over HTTP or HTTP/TLS unless
otherwise specified. (This also includes applications of IPSec to
SOAP over HTTP.)
Note that there are a number of advantages to considering SOAP over
protocols other than HTTP, as HTTP assigns the very distinct client
and server roles by connection initiation. This would cause
difficulties in supporting asynchronous notification and could be
relieved in many ways by replacing HTTP with BEEP.
2.3 HTTP Drawbacks
HTTP is not the ideal transport for messaging, but it is adequate for
the most basic interpretation of "remote procedure call". HTTP is
based on a communication pattern whereby the client (which initiates
the TCP connection) makes a "request" to the server. The server
returns a "response" and this process is continued (possibly over a
persistent connection, as described below). This matches the basic
idea of a remote procedure call where the caller invokes a procedure
on a remote server and waits for the return value.
Potential criticisms of HTTP could include the following:
o server-initiated data flow is awkward
o headers are verbose and text-based
o idle connections may be closed by intermediate proxies
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o data encapsulation must adhere to MIME
o bulk transfer relies on stream-based ordering
In many ways these criticisms are directed at particular compromises
in the design of HTTP. As such, they are important to consider, but
it is not clear that they result in fatal drawbacks for a device
management protocol.
2.4 BCP56: On the Use of HTTP as a Substrate
Best Current Practice 56 [9] presents a number of important
considerations on the use of HTTP in application protocols. In
particular, it raises the following concerns:
o HTTP may be more complex than is necessary for the application
o The use of HTTP may mask the application from some firewalls
o A substantially new service should not re-use port 80 as assigned
to HTTP
Fundamentally, these concerns lie directly with SOAP over HTTP,
rather than the application of SOAP over HTTP to NETCONF. As BCP 56
indicates, it is debatable whether HTTP is an appropriate protocol
for SOAP at all, and it is likely that BEEP would be a superior
protocol for most SOAP applications. Unfortunately, SOAP over HTTP is
in common use and must be supported if the practical benefits of SOAP
are to be realized.
It is possible, however, to respond to the concern on the re-use of
port 80. A NETCONF SOAP service can be offered on any desired port,
and it is recommended that a new standard port for SOAP over HTTP, or
a new standard port for NETCONF over SOAP (over HTTP) be defined.
2.5 Important HTTP 1.1 Features
HTTP 1.1 [8] includes two important features that provide for
relatively efficient transport of SOAP messages. These features are
"persistent connections" and "chunked transfer-coding".
Persistent connections allow a single TCP connection to be used
across multiple HTTP requests. This permits multiple SOAP request/
response message pairs to be exchanged without the overhead of
creating a new TCP connection for each request. Given that a single
stream is used for both requests and responses, it is clear that some
form of framing is necessary. For messages whose length is known in
advance, this is handled by the HTTP header "Content-length". For
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messages of dynamic length, "Chunking" is required.
HTTP "Chunking" or "chunked transfer-coding" allows the sender to
send an indefinite amount of binary data. This is accomplished by
informing the receiver of the size of each "chunk" (substring of the
data) before the chunk is transmitted. The last chunk is indicated
by a chunk of zero length. Chunking can be effectively used to
transfer a large XML document where the document is generated on-line
from a non-XML form in memory.
In terms of application to SOAP message exchanges, persistent
connections are clearly important for performance reasons, and are
particularly important when it is the persistence of authenticated
connections that is at stake. When one considers that messages of
dynamic length are the rule rather than the exception for SOAP
messages, it is also clear that Chunking is very useful. In some
cases it is possible to buffer a SOAP response and determine its
length before sending, but the storage requirements for this are
prohibitive for many devices. Together, these two features provide a
good foundation for device management using SOAP over HTTP.
2.6 SOAP Implementation Considerations
It is not the goal of this document to cover the SOAP [3]
specification in detail. Instead, we provide a few comments that may
be of interest to an implementor of NETCONF over SOAP.
2.6.1 SOAP Feature Exploitation
NETCONF over SOAP does not make extensive use of SOAP features. For
instance, NETCONF operations are not broken into SOAP message parts,
and the SOAP header is not used to convey metadata. This is a
deliberate design decision as it allows the implementor to easily
provide NETCONF over multiple substrates while handling the messages
over those different substrates in a common way.
2.6.2 SOAP Headers
Implementors of NETCONF over SOAP should be aware of the following
characteristic of SOAP headers: a SOAP header may have the attribute
"mustUnderstand" and, if so, the recipient must either process the
header block or not process the SOAP message at all, and instead
generate a fault. A "mustUnderstand" header must not be silently
discarded.
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3. A SOAP Web Service for NETCONF
3.1 Fundamental Use Case
The fundamental use case for NETCONF over SOAP (NETCONF/SOAP) over
HTTP is that of a management console ("manager" role) managing one or
more devices running NETCONF agents ("agent" role). The manager
initiates an HTTP connection to an agent and drives the NETCONF
session via a sequence of SOAP messages over HTTP requests. When the
manager closes the HTTP connection, the NETCONF session is also
closed.
3.2 NETCONF Sessions
NETCONF sessions are persistent for both performance and semantic
reasons. NETCONF session state contains the following:
1. Authentication Information
2. Capability Information
3. Locks
4. Pending Operations
5. Operation Sequence Numbers
Authentication must be maintained throughout a session due to the
fact that it is expensive to establish. Capability Information is
maintained so that appropriate operations can be applied during a
session. Locks are released upon termination of a session as this
makes the protocol more robust. Pending operations come and go from
existence during the normal course of RPC operations. Operation
sequence numbers provide the small but necessary state information to
refer to operations during the session.
In the case of SOAP over HTTP, a NETCONF "session" is supported by an
HTTP connection with an authenticated user. To support automated
cleanup, a NETCONF over SOAP session is closed when the HTTP
connection is closed.
3.3 Capabilities Exchange
Capabilities exchange, if defined through a NETCONF RPC operation,
can easily be accommodated in the SOAP binding.
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3.4 A NETCONF/SOAP example
Since the proposed WSDL (in Appendix A.1) uses document/literal
encoding, the use of a SOAP header and body has little impact on the
representation of a NETCONF operation. This example shows HTTP/1.0
for simplicity.
POST /netconf HTTP/1.0
Content-Type: text/xml; charset=utf-8
Accept: application/soap+xml, text/*
Cache-Control: no-cache
Pragma: no-cache
Content-Length: 470
xml
The HTTP/1.0 response is also straightforward:
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HTTP/1.0 200 OK
Content-Type: text/xml; charset=utf-8
rootsuperuserfredadminbarneyadmin
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4. Security Considerations
NETCONF is used to access and modify configuration information, so
the ability to access this protocol should be limited to users and
systems that are authorized to view or modify the agent's
configuration data.
Because configuration information is sent in both directions, it is
not sufficient for just the client or user to be authenticated with
the server. The identity of the server should also be authenticated
with the client.
Configuration data may include sensitive information, such as user
names or security keys. So, NETCONF should only be used over
communications channels that provide strong encryption for data
privacy.
If the NETCONF server provides remote access through insecure
protocols, such as HTTP, care should be taken to prevent execution of
the NETCONF program when strong user authentication or data privacy
is not available.
4.1 Integrity, Privacy, and Authentication
The NETCONF SOAP binding relies on an underlying secure transport for
integrity and privacy. Such transports are expected to include TLS
[12] and IPSec. There are a number of options for authentication
(some of which are deployment-specific):
o within the transport (such as with TLS client certificates)
o within HTTP (such as Digest Access Authentication [10])
o within SOAP (such as a digital signature in the header [16])
HTTP and SOAP level authentication can be integrated with RADIUS [13]
to support remote authentication databases.
4.2 Vulnerabilities
The above protocols may have various vulnerabilities, and these may
be inherited by NETCONF/SOAP.
NETCONF itself may have vulnerabilities due to the fact that an
authorization model is not currently specified.
It is important that device capabilities and authorization remain
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constant for the duration of any outstanding NETCONF session. In the
case of NETCONF, it is important to consider that device management
may be taking place over multiple substrates (in addition to SOAP)
and it is important that the different substrates have a common
authentication model.
4.3 Environmental Specifics
Some deployments of NETCONF/SOAP may choose to use HTTP without
encryption. This presents vulnerabilities but may be selected for
deployments involving closed networks or debugging scenarios.
A device managed by NETCONF may interact (over protocols other than
NETCONF) with devices managed by other protocols, all of differing
security. Each point of entry brings with it a potential
vulnerability.
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Normative References
[1] Enns, R., "XMLCONF Configuration Protocol",
draft-enns-xmlconf-spec-00 (work in progress), Feb 2003,
.
[2] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
"Extensible Markup Language (XML) 1.0 (Second Edition)", W3C
REC REC-xml-20001006, October 2000, .
[3] Box, D., Ehnebuske, D., Kakivaya, G., Layman, A., Mendelsohn,
N., Nielsen, H., Thatte, S. and D. Winer, "Simple Object Access
Protocol (SOAP) 1.1", W3C Note NOTE-SOAP-20000508, May 2000,
.
[4] Christensen, E., Curbera, F., Meredith, G. and S. Weerawarana,
"Web Services Description Language (WSDL) 1.1", W3C Note
NOTE-wsdl-20010315, March 2001, .
[5] Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn, "XML
Schema Part 1: Structures", W3C Recommendation
REC-xmlschema-1-20010502, May 2001, .
[6] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996, .
[7] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996, .
[8] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999, .
[9] Moore, K., "On the use of HTTP as a Substrate", RFC 3205,
February 2002, .
[10] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
Luotonen, A., Sink, E. and L. Stewart, "An Extension to HTTP:
Digest Access Authentication", RFC 2069, January 1997, .
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[11] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997, .
[12] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
1999, .
[13] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000, .
[14] Rose, M. and D. New, "Reliable Delivery for syslog", RFC 3195,
November 2001, .
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Informative References
[15] Barton, J., Nielsen, H. and S. Thatte, "SOAP Messages with
Attachments", W3C Note NOTE-SOAP-attachments-20001211, Dec
2000, .
[16] Brown, A., Fox, B., Hada, S., LaMacchia, B. and H. Maruyama,
"SOAP Security Extensions: Digital Signature", W3C Note
NOTE-SOAP-dsig-20010206, Feb 2001, .
[17] Rose, M., "The Blocks Extensible Exchange Protocol Core", RFC
3080, March 2001, .
[18] O'Tuathail, E. and M. Rose, "Using the Simple Object Access
Protocol (SOAP) in Blocks Extensible Exchange Protocol (BEEP)",
RFC 3288, June 2002, .
Author's Address
Ted Goddard
ICEsoft Technologies Inc.
Suite 300, 1717 10th St. NW
Calgary, AB T2M 4S2
Canada
Phone: (403) 663-3322
EMail: ted.goddard@icesoft.com
URI: http://www.icesoft.com
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Appendix A. WSDL Definitions
A.1 NETCONF SOAP Binding
The following WSDL document assumes a hypothetical location for the
NETCONF schema.
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A.2 Sample Service Definition
The following WSDL document assumes a hypothetical location for the
NETCONF/SOAP WSDL definitions. A typical deployment of a device
manageable via NETCONF/SOAP would provide a service definition
similar to the following to identify the address of the device.
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