XCAP

1
XCAP

• Jonathan Rosenberg
• Cisco Systems

2
History

• -07 was IESG approved and now in RFC-ED queue in
  normative hold on xcap-diff
• Several small bugs reported against XCAP and a
  major issue raised on namespace fragments in PUT
• How to know namespace prefixes in content of
  body?
• Proposal on list to
• Yank from queue (not done yet)
• Revise and republish to
• Fix minor bugs reported
• Address extensibility issue
• Fix namespace problem

3
Changes in -08

• Added URI extensibility
• There in principle previously unknown URI
  would be rejected with 404
• Still true, but URI grammar now has hooks
• Negotiation of extensions through existing
  xcap-caps mechanism
• Added namespace selector from xpath for
  learning namespaces

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Example
Client
Server
lthello xmlnsNS1gt ltthis xmlnsbarNS2gt
ltbaris xmlnstestNS3/gt lt/thisgt lt/hellogt
GET http//doc//hello/this/bar/namespace
ltbaris xmlnsNS1 xmlnsbarNS2
xmlnstestNS3/gt
Only GET Allowed!
5
Additional Changes

• Clarified 200 vs. 201
• 201 on Creation
• 200 on Insertion
• URI reference to 3986
• Clarified SHOULD NOT escape code , but
  normalization happens for comparison (ref 3986)
• Removed schema-instance declarations
• Clarified insertion rules relative to other
  elements, comments and whitespace earliest last
• After the last element with the same name, but
  before any other elements, but after any
  whitespace/comments

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Example
ltrootgt ltel1 att"first"/gt ltel1 att"second"/gt
lt!--comment --gt ltel2 att"first/gt lt/rootgt
ltrootgt ltel1 att"first"/gt ltel1 att"second"/gt
lt!--comment --gt ltel1 att"third"gtltel2
att"first/gt lt/rootgt
PUT root/el1_at_attthird ltel1 att"third"/gt
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Changes

• Document selector configuration simplified
• XUI SIP AOR for SIP applications
• Single document usages, doc is called index
• Sub-directories not recommended
• Cannot have document and subdirectory with same
  name

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Pros/Cons of Namespace Proposal

• Client needs to do a GET to obtain namespaces in
  scope at insertion point and THEN do a PUT with
  new content
• Need to worry about namespace bindings changing
  between GET and PUT
• Can cache bindings, though
• Cannot construct static PUTs
• i.e., map a UI command add buddy to an HTTP PUT
  with a URI that is a function of the usage,
  filled in with template data from UI

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Alternative Proposals for GET

• Use Canonical XML
• Instead of getting namespace bindings, those
  bindings are returned as a result of a normal GET
  anyway

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Example
lt?xml version"1.0" encoding"UTF-8"?gt ltfooroot
xmlnsfoo"ns1" xmlnsbar"ns2"gt ltfooelementgt
ltbarelementgt ltbarchild attr"value"
battr"value" xmlnsb"ns3"/gt lt/barelementgt
lt/fooelementgt lt/foorootgt
GET ..//root/element/
ltfooelement xmlnsbar"ns2" xmlnsfoo"ns1"gt
ltbarelement gt ltbarchild xmlnsb"ns3"
attr"value" battr"value"gt lt/barchildgt
lt/barelementgt lt/fooelementgt
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Canonical Example Pros/Cons

• Unfortunately changes nothing for PUT still
  need to do GET first, still issues with
  conditional PUT
• A little cleaner
• Getting an element and its namespaces happens in
  one shot, BUT no way to get JUST namespaces
  problem for elements with many children
• Does GET(PUT(X)) X?
• Depends on equality from XML perspective I
  believe so
• Requires XCAP server to do canonicalization in
  all GET responses

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Making the PUT Easier

• Proposal 1 An app usage can require namespace
  prefixes to be hashes of namespace URI
• Makes all prefixes well-known
• No referential integrity problem, unlike
  namespace prefix munging
• But kind of kludgey
• Proposal 2
• PUT bodies contain namespace declarations dont
  depend on in-scope bindings
• Will pollute document with lots of redundant
  binding declarations, but otherwise works with
  existing spec

13
Recommendation

• Reject canonical XML approach
• Requires getting all elements to just get
  namespace bindings
• Go for proposal 2 for blind insertions
• Document this use case
• But, we still have a problem with blind
  insertions on simultaneous writers

14
The Insertion Problem in XCAP

• XCAP is fundamentally about mapping XML
  components to resources
• In current spec, if an element or attribute
  doesnt exist in the document, neither does its
  corresponding HTTP resource
• This is the natural definition
• Big drawback an insertion is a resource creation
  and thus resource had no etag
• Makes it impossible to do conditional insertions
• This was a conscious group choice, but it is
  proving to be a huge practical mess

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Implications of Unconditional Insertions

• XCAP server needs to return what document etag
  was prior to insertion
• Requires normative reference to xcap-diff
• If this doesnt match client copy, client needs
  to
• Undo its change
• Do a PUT to repair document
• Deal with possibility that this PUT might fail
• Criticisms on this problem continue

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Proposed Change

• An HTTP resource if its parent exists in the
  document
• If the parent exists in the document but the
  component doesnt, the resource is considered
  empty
• A GET of it would return a document with no
  content
• Kind of hokey
• But, all element insertions end up being HTTP
  modifications, not creations, so etags can be
  used
• Eliminates the need for xcap-diff at all as part
  of xcap core