Yunling Wang yw2291@columbia.edu VoIP Security COMS 4995 Nov 24, 2008 XCAP The Extensible Markup...

Yunling Wang

yw2291@columbia.edu

VoIP Security COMS 4995

Nov 24, 2008

XCAPThe Extensible Markup Language (XML)Configuration Access Protocol (XCAP)

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Contents Motivation

Resource list, presence state, presence authentication

Application Usage URL Construction

XCAP root, document/node selector Operation

Create/fetch/delete Security Concerns

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Motivation

Presentity

Watcher

Notify

Subscribe

Presence System

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Motivation-Resource list

Presentity

Presence list

server

Watcher

Notify

Subscribe

Resource list

XCAP Client

XCAP Server

•Presence list server: manages multiple subscriptions to different presentities

•Resource list: keeps a list of presentities for presence list server

•XCAP: directly manipulates resource lists stored on presence list server

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Presentity

Event Sate

Composer

PresenceAgent

Watcher

Event State

Notify

Subscribe

Publish

•Composer: compose event state according to the presence information in PUBLISH

•Event state: keeps current presence state of the presentity

•XCAP: directly manipulates event state without constant refreshing publicationsXCAP

Client

XCAP Server

Motivation-Presence State

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Motivation-Presence Authentication

Presentity

Event Sate

Composer

PresenceAgent

Watcher

Presence Authentication Documentation

Notify

Subscribe

Publish

XCAP Client

XCAP Server

•Presence Agent: manages multiple subscriptions from different watchers

•Presence Authentication Document: the authentication information for all the subscribers

•XCAP: directly manipulates event state without constant refreshing publications

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What is XCAP? XCAP allows a client to read, write, and

modify application configuration data stored in XML format on a server

Normal HTTP primitives can be used to manipulate the data

<?xml version="1.0" encoding="UTF-8"?> <resource-lists"> <list name="friends"> <entry uri="sip:bob@example.com"> <name>Bob Jones</name> </entry> </list> </resource-lists>

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Application Usage

Each application has its own Application Usage

Define how the XCAP server can manipulate corresponding application documents

Key components: AUID, XML Schema, data validation, resource interdependency, authorization policies

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AUID Application Unique ID

Unique Identifier for each application Two sub-namespaces

IETF tree: IANA Registry Example: “resource-lists”, “pidf-manipulation” , “pres-rules”

Vendor tree: prefixed with the reverse domain name of the

organization meant to be used in lab environments where no

central registry is needed Example: “com.example.customer-list”

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Application Usage

Protection Concerns Authorization policies Default: User can read & write their own data; User

can only read global data Resource interdependency

Operation of one element may affect other elements; especially cross-document affection

Data Validation

Uniqueness constraints, referential integrity

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Background: XML Basics

XML Document: to structure, store, and transport information

XML element: everything from (including) the element's start tag to (including) the element's end tag.

XML Attributes: specified in XML elements’ tags; provide additional information about elements.

<?xml version="1.0" encoding="UTF-8"?> <resource-lists"> <list name="friends"> <entry uri="sip:bob@example.com"> <name>Bob Jones</name> </entry> </list> </resource-lists></xml>

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Background: XPath

XPath = XML Addressing How to point to specific pieces of an

XML document

resource-lists/list/entry/name

<?xml version="1.0" encoding="UTF-8"?> <resource-lists"> <list name="friends"> <entry uri="sip:bob@example.com"> <name>Bob Jones</name> </entry> </list> </resource-lists>

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URI Construction

Based on the Concept of XPath

Example: XCAP root / Document Selector / Node Selector XCAP rootContext in which all other resources exist

"http://xcap.example.com" for domain "example.com " Document Selector"/resource-lists/users/sip:joe@example.com/index" Node Selector~~/resource-lists/list%5b@name=%22l1%22%5d

The HierarchyGEThttp://xcap.example.com/resource-lists/users/hiroshi/doc1/ ~~/resource-lists/list/list/entry/name

<?xml version="1.0" encoding="UTF-8"?> <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"> <list name="friends"> <entry uri="sip:bob@example.com"> <name>Bob Jones</name> </entry> <list name="close-friends"> <entry uri="sip:hiroshi@example.com"> <name>Hiroshi Aukia</name> </entry> </list> </list> </resource-lists>

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Operations

HTTP PUT

Create or Replace a Document/Element/Attribute

HTTP DELETE

Delete a Document/Element/Attribute HTTP GET

Fetch a Document/Element/Attribute

Example: Modify an Element

PUT /pidf-manipulation/users /sip:someone@example.com/index/ ~~/presence/tuple%5b@id='x8eg92n'%5d/note HTTP/1.1

<note>I'm sleeping</note>

HTTP/1.1 200 OK

<?xml version="1.0" encoding="UTF-8"?> <presence xmlns="urn:ietf:params:xml:ns:pidf"> <tuple id="x8eg92n"> <status> <basic>open</basic> </status> <rp:class>auth-1</rp:class> <contact priority="1.0"> mailto:someone@example.com </contact> <note>I'm reading mail </note> </tuple> </presence>

<?xml version="1.0" encoding="UTF-8"?> <presence xmlns="urn:ietf:params:xml:ns:pidf"> <tuple id="x8eg92n"> <status> <basic>open</basic> </status> <rp:class>auth-1</rp:class> <contact priority="1.0"> mailto:someone@example.com </contact> <note>I'm sleeping </note> </tuple> </presence>

index

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Conditional Operation

Conflicts occur with simultaneous multiple modifications

Use etag: A version control

When one resource changes, all resources in the same documents get the same new etag

Client: contain previously known etag in the If-Match header field of request

Server: return new etag in response on success

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Security Considerations

Data manipulated by XCAP often contains sensitive information

Using HTTP port: 80

– Hard to apply port-based filtering

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Solutions

Connection over TLS HTTP Digest Authentication URL-analysis-based traffic filtering

The presence of the double tilde (~~) is a strong hint that the URL points to an XML element or attribute

Authorization policies in Application Usage

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Summary

Access configuration documents on server: Presence system

Maps XML documents and document components into HTTP URIs

HTTP primitives can be used to directly manipulate the data

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References

http://tools.ietf.org/html/rfc4825 http://tools.ietf.org/html/rfc5025 http://tools.ietf.org/html/rfc4827 http://tools.ietf.org/html/rfc4826 http://www.jdrosen.net/simple_acap.html http://tools.ietf.org/html/rfc3903 http://tools.ietf.org/html/rfc2778 http://openxcap.org/ http://www.w3schools.com/Xml/xml_attributes.asp