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Network Working Group                                           J. BoyerRequest for Comments: 3653                            PureEdge SolutionsCategory: Informational                                        M. Hughes                                                         Betrusted, Inc.                                                               J. Reagle                                                                     W3C                                                           December 2003XML-Signature XPath Filter 2.0Status of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2003).  All Rights Reserved.Abstract   XML Signature recommends a standard means for specifying information   content to be digitally signed and for representing the resulting   digital signatures in XML.  Some applications require the ability to   specify a subset of a given XML document as the information content   to be signed.  The XML Signature specification meets this requirement   with the XPath transform.  However, this transform can be difficult   to implement efficiently with existing technologies.  This   specification defines a new XML Signature transform to facilitate the   development of efficient document subsetting implementations that   interoperate under similar performance profiles.   This document is the W3C XML Signature XPath-Filter 2.0   Recommendation.  This document has been reviewed by W3C Members and   other interested parties and has been endorsed by the Director as a   W3C Recommendation.  It is a stable document and may be used as   reference material or cited as a normative reference from another   document.  W3C's role in making the Recommendation is to draw   attention to the specification and to promote its widespread   deployment.  This enhances the functionality and interoperability of   the Web.Boyer, et al.                Informational                      [Page 1]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Acknowledgements (Informative) . . . . . . . . . . . .41.2.  W3C Status . . . . . . . . . . . . . . . . . . . . . .42.  Terminology. . . . . . . . . . . . . . . . . . . . . . . . .43.  Specification of Signature Filter Transform. . . . . . . . .53.1.  Algorithm Identifier . . . . . . . . . . . . . . . . .53.2.  Syntax of Signature Filter Transform . . . . . . . . .5       3.3.  Input and Evaluation Context of Signature Filter             Transform. . . . . . . . . . . . . . . . . . . . . . .73.4.  Processing Model of Signature Filter Transform . . . .74.  Examples of Signature Filter Transform . . . . . . . . . . .95.  Normative References . . . . . . . . . . . . . . . . . . . .136.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . .147.  Full Copyright Statement . . . . . . . . . . . . . . . . . .151.  Introduction   The XML Recommendation [XML] specifies the syntax of a class of   objects called XML documents.  The Namespaces in XML Recommendation   [XML-NS] specifies additional syntax and semantics for XML documents.   The XML Signature Recommendation [XML-DSig] defines standard means   for specifying information content to be digitally signed, including   the ability to select a portion of an XML document to be signed using   an XPath transform.   This specification describes a new signature filter transform that,   like the XPath transform [XML-DSig,section 6.6.3], provides a method   for computing a portion of a document to be signed.  In the interest   of simplifying the creation of efficient implementations, the   architecture of this transform is not based on evaluating an [XPath]   expression for every node of the XML parse tree (as defined by the   [XPath] data model).  Instead, a sequence of XPath expressions is   used to select the roots of document subtrees -- location sets, in   the language of [XPointer] -- which are combined using set   intersection, subtraction and union, and then used to filter the   input node-set.  The principal differences from the XPath transform   are:      *  A sequence of XPath operations can be executed in a single         transform, allowing complex filters to be more easily expressed         and optimized.      *  The XPath expressions are evaluated against the input document         resulting in a set of nodes, instead of being used as a boolean         test against each node of the input node-set.Boyer, et al.                Informational                      [Page 2]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003      *  To increase efficiency, the expansion of a given node to         include all nodes having the given node as an ancestor is now         implicit so it can be performed by faster means than the         evaluation of an XPath expression for each document node.      *  The resulting node-sets can be combined using the three         fundamental set operations (intersection, subtraction, and         union), and then applied as a filter against the input node-         set, allowing operations such as signing an entire document         except for a specified subset, to be expressed more clearly and         efficiently.   As with the original XPath transform, the primary purpose of this   transform is to ensure that only specifically defined changes to the   input XML document are permitted after the signature is affixed.   This can be done by excluding precisely those nodes that are allowed   to change once the signature is affixed, and including all other   input nodes in the output.  It is the responsibility of the signature   filter transform author to ensure that nodes are not excluded which   could affect the interpretation of the transform output in the   application context.   Consider the motivating scenario where an application wishes to affix   two enveloped signatures to the document; any other change to the   document must cause the signatures to be invalid.  When the   application creates the first signature that signature is   automatically omitted from its own digest calculations.  However, it   will also be necessary to exclude the subsequent (second) signature   element from the digest calculations of the first signature.  This   specification can be used to efficiently satisfy this requirement   using the set subtraction operation.   This transform also supports the ability to specify a set of nodes   that will be included in a signature, with all non-specified nodes   being excluded.  This formulation is useful for isolating a portion   of a document, such as a chapter of a document, or a payload in a   protocol message, and can be expressed using the set intersection   operation.   Complete familiarity with the first XML Signature XPath Transform   [XML-DSig,section 6.6.3] is required.   NOTE: Since XPath Filter 2.0 depends on details of XPath, be sure to   take into account the XPath Errata at   <http://www.w3.org/1999/11/REC-xpath-19991116-errata>.Boyer, et al.                Informational                      [Page 3]

RFC 3653             XML-Signature XPath Filter 2.0        December 20031.1.  Acknowledgements (Informative)   The following people provided valuable feedback that improved the   quality of this specification:      *  Christian Geuer-Pollmann, Universitat Siegen      *  Donald Eastlake, 3rd, Motorola      *  Gregor Karlinger, IAK TU Graz      *  Aleksey Sanin1.2.  W3C Status   The World Wide Web Consortium Recommendation corresponding to this   RFC is at:http://www.w3.org/TR/xmldsig-filter2/2.  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 inBCP 14,RFC 2119   [Keywords].   The XPath 1.0 Recommendation [XPath] defines the term node-set as   "(an unordered collection of nodes without duplicates)" and specifies   a data model for representing an input XML document as a set of nodes   of various types (element, attribute, namespace, text, comment,   processing instruction, and root).   An input document is the document that contains all the nodes   available to processing by this transform.  A document subset is a   portion of an XML document indicated by an XPath node-set, which may   not include all of the nodes in the document.  For example, the input   node-set is a collection of XPath nodes from the input document that   is passed as a parameter to this transform.  A subtree rooted by a   given node is a document subset containing the given node and every   node having the given node as an ancestor.  Subtree expansion is the   process of expanding a node-set to include all subtrees rooted at any   node in the node-set.  For example, the subtree expansion of a node-   set consisting of just a single element node would be a node-set   containing that element, its attribute nodes, namespace nodes, and   all its descendants including their attribute nodes and namespaces   nodes.   The XML Signature Recommendation [XML-DSig] defines a reference as a   sequence of steps performed to obtain an octet stream to be digitally   signed.  A transform is an identified algorithm to be used as a stepBoyer, et al.                Informational                      [Page 4]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003   in the reference processing model.  A transform takes an octet stream   or XPath node-set as input, and it produces an octet stream or XPath   node-set as output (the reference processing model automatically   converts the final output to an octet stream if it is an XPath node-   set).3.  Specification of Signature Filter Transform   The transform operates by computing a node-set that is used to filter   the input node-set: The output node-set consists of only those nodes   in both the input node-set and the filter node-set.  In other words,   the output node-set is the intersection of the input node-set and the   computed filter node-set.   The filter node-set is computed by evaluating a sequence of XPath   expressions and combining their results.  A node-set is initially   computed containing the entire input document.  In sequence, each   XPath expression is then evaluated, subtree-expanded, and then used   to transform the filter node-set according to a specified set   operation; intersection, subtraction, or union.  After all XPaths   have been applied, the resulting node-set is used as the filter   node-set.3.1.  Algorithm Identifier   The XML Signature Recommendation [XML-DSig] uses a [URI] to identify   each algorithm to be performed when creating or validating a   signature.  The signature filter transform is identified as follows:   Algorithm Identifierhttp://www.w3.org/2002/06/xmldsig-filter23.2.  Syntax of Signature Filter Transform   The signature filter transform shall be represented by a sequence of   one or more elements named XPath.  The content of XPath is character   data containing an XPath expression.  The XPath has an attribute   named Filter whose possible values are intersect, subtract, and   union.  The Filter attribute indicates the set operation that is   performed with the resulting node-set when computing the filter   node-set.  The following is an example of markup for a signature   filter that signs the entire input node-set except for elements with   identifier foo and bar (and all nodes with one of those elements as   an ancestor): <XPath Filter="subtract"  xmlns="http://www.w3.org/2002/06/xmldsig-filter2">    id("foo bar")Boyer, et al.                Informational                      [Page 5]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003 </XPath> Schema Definition: <?xml version="1.0" encoding="utf-8"?> <!DOCTYPE schema  PUBLIC "-//W3C//DTD XMLSchema 200102//EN"         "http://www.w3.org/2001/XMLSchema.dtd" [   <!ATTLIST schema     xmlns:xf CDATA #FIXED 'http://www.w3.org/2002/06/xmldsig-filter2'>   <!ENTITY xf 'http://www.w3.org/2002/06/xmldsig-filter2'>   <!ENTITY % p ''>   <!ENTITY % s ''>  ]> <schema xmlns="http://www.w3.org/2001/XMLSchema"         xmlns:xf="http://www.w3.org/2002/06/xmldsig-filter2"         targetNamespace="http://www.w3.org/2002/06/xmldsig-filter2"         version="0.1" elementFormDefault="qualified"> <element name="XPath"          type="xf:XPathType"/> <complexType name="XPathType">  <simpleContent>    <extension base="string">      <attribute name="Filter">         <simpleType>           <restriction base="string">             <enumeration value="intersect"/>             <enumeration value="subtract"/>             <enumeration value="union"/>           </restriction>         </simpleType>      </attribute>    </extension>  </simpleContent> </complexType> </schema> DTD: <!ELEMENT XPath    (#PCDATA) > <!ATTLIST XPath    Filter         (intersect|subtract|union) #REQUIRED >Boyer, et al.                Informational                      [Page 6]

RFC 3653             XML-Signature XPath Filter 2.0        December 20033.3.  Input and Evaluation Context of Signature Filter Transform   The input required by this transform is an XPath node-set over the   input document.  If the input document is an octet stream, then the   application MUST convert the octet stream to an XPath node-set that   contains all of the document nodes (including comment nodes).  The   evaluation context for the XPath expressions in the filter transform   will be:      *  A context node equal to the root node of the document whose         node-set was provided as input to this transform.  The root         node is the parent of the document element and any comment and         processing instruction nodes outside of the document element.      *  A context position, initialized to 1.      *  A context size, initialized to 1.      *  A library of functions equal to the function set defined in         [XPath] plus a function named here().      *  A set of variable bindings.  No means for initializing these is         defined.  Thus, the set of variable bindings used when         evaluating the XPath expression is empty, and use of a variable         reference in the XPath expression results in an error.      *  The set of namespace declarations in scope for the XPath         element.   The function here() is defined as follows:   Function: node-set here()      The here() function returns a node-set containing the attribute or      processing instruction node or the parent element of the text node      that directly bears the XPath expression.  In this transform, this      will be the XPath element.  This expression results in an error if      the containing XPath expression does not appear in the same XML      document against which the XPath expression is being evaluated.3.4.  Processing Model of Signature Filter Transform   Using the aforementioned evaluation context, the signature filter   transform evaluates the XPath expressions appearing in the character   content of the XPath elements and uses these to compute a filter   node-set F, which is then used to filter the input node-set I   resulting in an output node-set O:      *  Initialize the filter node-set F to consist of all nodes in the         input document.      *  Iterate through each XPath expression, X, in sequence, and         update the filter node-set F as follows:Boyer, et al.                Informational                      [Page 7]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003         o  Evaluate the XPath expression X.  The result is a node-set            S.         o  Compute the set S' consisting of all nodes in the input            document that are either present in S or that have an            ancestor in S.  This is equal to the union of all the            document subtrees rooted by a node in S.         o  If the Filter attribute value is intersect, then compute the            intersection of the selected subtrees, S', with the filter            node-set F.  The result will include only those nodes that            are in both the filter node-set and the selected subtrees:            F' = F INTERSECT S'.         o  If the Filter attribute value is subtract, then compute the            subtraction of the selected subtrees, S', from the filter            node-set F.  The result will include only those nodes that            are in the filter node-set, but not the selected subtrees:            F' = F - S'.         o  Otherwise, if the Filter attribute value is union, then            compute the union the selected subtrees, S', with the filter            node-set F.  The result will include all those nodes that            are in either the filter node-set, the selected subtrees, or            both: F' = F UNION S'.         o  Update the filter node-set F to be the new node-set F'.      *  Finally, after applying all the XPath expressions, compute the         output node-set O to be the intersection of the computed filter         node-set, F, with the input node-set, I.  The result will         include all nodes from the input node-set that are also in the         filter node-set: O = I INTERSECT F.      *  An empty input node-set will always result in an empty output         node-set.   In this processing model, the conversion from a subtree   interpretation of the XPath expressions to a node-set containing all   nodes that must be used during the set operation, along with actual   performance of the set operation, is described explicitly.   Implementors SHOULD observe that it is possible to compute the   effective result of this operation in a single pass through the input   document without performing subtree expansion or any set operations:      *  For each XPath expression X, in sequence, evaluate the         expression and store the resulting node-set, S, along with the         associated set operation.      *  Prepend a node-set consisting of just the document node, along         with the operation union.      *  Create a new, empty filter node-set.      *  Process each node in the input node-set document, adding each         node to the output node-set F if a flag Z is true.  The flag is         computed as follows:Boyer, et al.                Informational                      [Page 8]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003         o  Z is true if and only if the node is present in any            subtree-expanded union node-set and all subsequent subtree-            expanded intersect node-sets but no subsequent subtree-            expanded subtract node-sets, or false otherwise.  If there            are no subsequent intersect or subtract node-sets, then that            part of the test is automatically passed.         o  Presence in a subtree-expanded node-set can be efficiently            determined without actually expanding the node-set, by            simply maintaining a stack or count that identifies whether            any nodes from that node-set are an ancestor of the node            being processed.   Implementers MAY further observe that, if this transform is followed   by a canonicalization operation (e.g., [XML-C14N]), the described   filter computation can be efficiently commingled with the document-   order canonicalization processing.4.  Examples of Signature Filter Transform   The example below illustrates one way to create an enveloped   signature with the signature filter transform.  The function here()   identifies the XPath element, and the subsequent location path   obtains the nearest ancestor Signature element.  Due to the subtract   value of the Filter attribute, the output of the signature filter   transform is a node-set containing every node from the input node-set   except the nodes in the subtree rooted by the Signature element   containing the example signature filter transform below.   <XPath Filter="subtract"    xmlns="http://www.w3.org/2002/06/xmldsig-filter2"    xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">      here()/ancestor::dsig:Signature[1] </XPath>   A suitable signature reference URI to use with this subtract filter   would be URI="" (the entire signature document, without comments),   URI="#xpointer(/)" (the entire signature document, with comments) or   any same-document reference that includes the signature itself.   An example of an intersect filter is a signature that co-signs   another signature.  In this example, a Signature element identified   by PrimaryBorrowSig must be signed.  The XPath expression obtains the   element node, and the transform expands the output node-set to   contain all nodes from the input node-set that are also in the   subtree rooted by the element node.   <XPath Filter="intersect"    xmlns="http://www.w3.org/2002/06/xmldsig-filter2">      id("PrimaryBorrowerSig") </XPath>Boyer, et al.                Informational                      [Page 9]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003   This type of intersect filter is useful for efficiently signing   subsets of a document, whether this is the same document as the   signature or an external document.  For example, if the signature   reference URI is URI="document.xml", then this document will be   automatically parsed and just the identified element and its   descendants will be signed.   Union filters, by themselves are of no particular use: The initial   filter node-set consists of the entire input document; any union with   this will have no effect, so the output of the transform will be   identical to the input.  The union operation is intended to follow a   subtract operation, to allow a subtree to be removed, with the   exception of a lower subtree which is still included in the output.   Consider the following document which contains a same-document   enveloped signature reference with an XPath filter containing three   XPath operations:   <Document>     <ToBeSigned>       <!-- comment -->       <Data />       <NotToBeSigned>         <ReallyToBeSigned>           <!-- comment -->           <Data />         </ReallyToBeSigned>       </NotToBeSigned>     </ToBeSigned>     <ToBeSigned>       <Data />       <NotToBeSigned>         <Data />       </NotToBeSigned>     </ToBeSigned>     <dsig:Signature      xmlns:dsig="http://www.w3.org/2000/09/xmldsig#"      xmlns:dsig-xpath="http://www.w3.org/2002/06/xmldsig-filter2">       <dsig:SignedInfo>         ...         <dsig:Reference URI="">           <dsig:Transforms>             <dsig:Transform              Algorithm="http://www.w3.org/2002/06/xmldsig-filter2">               <dsig-xpath:XPath                Filter="intersect"> //ToBeSigned </dsig-xpath:XPath>               <dsig-xpath:XPathBoyer, et al.                Informational                     [Page 10]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003                Filter="subtract"> //NotToBeSigned </dsig-xpath:XPath>               <dsig-xpath:XPath                Filter="union"> //ReallyToBeSigned </dsig-xpath:XPath>             </dsig:Transform>           </dsig:Transforms>           ...         </dsig:Reference>       </dsig:SignedInfo>       ...     </dsig:Signature> </Document>   The intersect operation computes the intersection of the XPath-   selected subtrees with the filter node-set.  In this case, the filter   node-set initially contains the entire input document, and the XPath   expression evaluates to the two ToBeSigned elements; these are   expanded to include all their descendents and intersected with the   filter node-set, resulting in the following:     <ToBeSigned>       <!-- comment -->       <Data />       <NotToBeSigned>         <ReallyToBeSigned>           <!-- comment -->           <Data />         </ReallyToBeSigned>       </NotToBeSigned>     </ToBeSigned><ToBeSigned>       <Data />       <NotToBeSigned>         <Data />       </NotToBeSigned>     </ToBeSigned>   The subtract filter computes the subtraction of the XPath-selected   subtrees from the filter node-set.  In this case, the XPath   expression evaluates to the two NotToBeSigned elements; these are   expanded to include all their descendents and subtracted from the   filter node-set:   <ToBeSigned>       <!-- comment -->       <Data />     </ToBeSigned><ToBeSigned>       <Data />   </ToBeSigned>Boyer, et al.                Informational                     [Page 11]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003   Next, the union filter computes the union of the XPath-selected   subtrees with the filter node-set.  In this case, the XPath   expression evaluates to the ReallyToBeSigned element; this is   expanded to include all its descendents and added to the filter   node-set:   <ToBeSigned>       <!-- comment -->       <Data />       <ReallyToBeSigned>           <!-- comment -->           <Data />         </ReallyToBeSigned>     </ToBeSigned><ToBeSigned>       <Data />   </ToBeSigned>   Finally, this resulting filter node-set is used to transform the   input node-set.  In this example, the input node-set is the entire   document, with comments removed.  The transformed node-set will thus   be all those nodes from the input document, less comments, that are   also in the filter node-set:   <ToBeSigned>       <Data />       <ReallyToBeSigned>           <Data />         </ReallyToBeSigned>     </ToBeSigned><ToBeSigned>       <Data />   </ToBeSigned>   Note that the result contains no nodes that were not in the input   node-set.  Although the filter node-set included comments, these were   not present in the input node-set so they are not present in the   output node-set.   This signature filter does not provide any increased capability over   the original XPath transform.  For example, this reference could be   replicated using the XPath transform as follows.   <dsig:Reference URI="">     <dsig:Transforms>       <dsig:TransformBoyer, et al.                Informational                     [Page 12]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003        Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">         <dsig:XPath>           (ancestor-or-self::ToBeSigned and            not (ancestor-or-self::NotToBeSigned))           or ancestor-or-self::ReallyToBeSigned         </dsig:XPath>       </dsig:Transform>     </dsig:Transforms>     ...  </dsig:Reference>   The advantage of the signature filter transform over the XPath   transform is that the latter requires evaluation of a potentially-   complex expression against every node in the input set, which has   proved costly in practice for many useful operations.  This   specification's filter requires evaluation of simple XPath   expressions and then the execution of some basic set operations or   their equivalent, which can be implemented significantly more   efficiently.5.  Normative References   [Keywords]  Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.   [URI]       Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform               Resource Identifiers (URI): Generic Syntax",RFC 2396,               August 1998.   [XML]       "Extensible Markup Language (XML) 1.0 (Second Edition)",               T.  Bray, E.  Maler, J. Paoli, and C. M. Sperberg-               McQueen.  W3C Recommendation, October 2000.  Available at               <http://www.w3.org/TR/2000/REC-xml-20001006>.   [XML-C14N]  Boyer, J., "Canonical XML",RFC 3076, March 2001.  Also a               W3C Recommendation available at               <http://www.w3.org/TR/2001/REC-xml-c14n-20010315>.   [XML-DSig]  Eastlake, J., Reagle, J. and D. Solo, "XML-Signature               Syntax and Processing",RFC 3275, March 2002.  Also a W3C               Recommendation available at               <http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/>.   [XML-NS]    "Namespaces in XML", T. Bray, D. Hollander, and A.               Layman.  W3C Recommendation, January 1999.  Available at               <http://www.w3.org/TR/1999/REC-xml-names-19990114/>.Boyer, et al.                Informational                     [Page 13]

RFC 3653             XML-Signature XPath Filter 2.0        December 2003   [XPath]     "XML Path Language (XPath) Version 1.0", J. Clark and S.               DeRose.  W3C Recommendation, November 1999.  Available at               <http://www.w3.org/TR/1999/REC-xpath-19991116>. (Note               also XPath Errata at <http://www.w3.org/1999/11/REC-xpath-19991116-errata>.)   [XPointer]  "XML Pointer Language (XPointer)", S. DeRose, R. Daniel,               and E. Maler.  W3C Candidate Recommendation, January               2001.  Available at <http://www.w3.org/TR/2001/CR-xptr-20010911/>.6.  Authors' Addresses   John Boyer   PureEdge Solutions Inc.   4396 West Saanich Rd.   Victoria, BC, Canada V8Z 3E9   Phone: +1-888-517-2675   EMail: jboyer@PureEdge.com   Merlin Hughes   Betrusted, Inc.   11000 Broken Land Parkway Suite 900   Columbia, MD 21044   Phone: +1-443-367-7000   EMail: Merlin.Hughes@betrusted.com   Joseph M. Reagle Jr., W3C   Massachusetts Institute of Technology   Laboratory for Computer Science   NE43-350, 545 Technology Square   Cambridge, MA 02139   Phone: +1.617.258.7621   EMail: reagle@mit.eduBoyer, et al.                Informational                     [Page 14]

RFC 3653             XML-Signature XPath Filter 2.0        December 20037.  Full Copyright Statement   Copyright (C) The Internet Society (2003).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assignees.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Boyer, et al.                Informational                     [Page 15]