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Table of Contents

1 Introduction. 6

1.1 Terminology. 6

1.2 Normative References. 7

1.3 Non-Normative References. 7

1.4 Namespace. 8

1.5 Conformance. 9

2 Reliable Messaging Model 10

2.1 Glossary. 11

2.2 Protocol Preconditions. 12

2.3 Protocol Invariants. 12

2.4 Delivery Assurances. 12

2.5 Example Message Exchange. 13

3 RM Protocol Elements. 16

3.1 Considerations on the Use of Extensibility Points. 16

3.2 Considerations on the Use of "Piggy-Backing" 16

3.3 Composition with WS-Addressing. 16

3.4 Sequence Creation. 17

3.5 Closing A Sequence. 21

3.6 Sequence Termination. 23

3.7 Sequences. 25

3.8 Request Acknowledgement 26

3.9 Sequence Acknowledgement 27

4 Faults. 30

4.1 SequenceFault Element 31

4.2 Sequence Terminated. 32

4.3 Unknown Sequence. 32

4.4 Invalid Acknowledgement 33

4.5 Message Number Rollover 33

4.6 Create Sequence Refused. 34

4.7 Sequence Closed. 34

4.8 WSRM Required. 35

5 Security Threats and Countermeasures. 36

5.1 Threats and Countermeasures. 36

5.2 Security Solutions and Technologies. 38

6 Securing Sequences. 41

6.1 Securing Sequences Using WS-Security. 41

6.2 Securing Sequences Using SSL/TLS. 42

Appendix A. Schema. 44

Appendix B. WSDL. 49

Appendix C. Message Examples. 51

Appendix C.1 Create Sequence. 51

Appendix C.2 Initial Transmission. 51

Appendix C.3 First Acknowledgement 53

Appendix C.4 Retransmission. 53

Appendix C.5 Termination. 54

Appendix D. State Tables. 56

Appendix E. Acknowledgments. 61

1 Introduction

It is often a requirement for two Web services that wish to communicate to do so reliably in the presence of software component, system, or network failures. The primary goal of this specification is to create a modular mechanism for reliable transfer of messages. It defines a messaging protocol to identify, track, and manage the reliable transfer of messages between a source and a destination. It also defines a SOAP binding that is required for interoperability. Additional bindings can be defined.

This mechanism is extensible allowing additional functionality, such as security, to be tightly integrated. This specification integrates with and complements the WS-Security [WS-Security], WS-Policy [WS-Policy], and other Web services specifications. Combined, these allow for a broad range of reliable, secure messaging options.

1.1 Terminology

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [KEYWORDS].

This specification uses the following syntax to define normative outlines for messages:

· The syntax appears as an XML instance, but values in italics indicate data types instead of values.

· Characters are appended to elements and attributes to indicate cardinality:

o "?" (0 or 1)

o "*" (0 or more)

o "+" (1 or more)

· The character "|" is used to indicate a choice between alternatives.

· The characters "[" and "]" are used to indicate that contained items are to be treated as a group with respect to cardinality or choice.

· An ellipsis (i.e. "...") indicates a point of extensibility that allows other child or attribute content specified in this document. Additional children elements and/or attributes MAY be added at the indicated extension points but they MUST NOT contradict the semantics of the parent and/or owner, respectively. If an extension is not recognized it SHOULD be ignored.

· XML namespace prefixes (see section 1.4) are used to indicate the namespace of the element being defined.

Elements and Attributes defined by this specification are referred to in the text of this document using XPath 1.0 [XPath_10] expressions. Extensibility points are referred to using an extended version of this syntax:

· An element extensibility point is referred to using {any} in place of the element name. This indicates that any element name can be used, from any namespace other than the wsrm: namespace.

· An attribute extensibility point is referred to using @{any} in place of the attribute name. This indicates that any attribute name can be used, from any namespace other than the wsrm: namespace.

1.2 Normative References

[KEYWORDS] S. Bradner, “Key words for use in RFCs to Indicate Requirement Levels,” RFC 2119, Harvard University, March 1997
http://www.ietf.org/rfc/rfc2119.txt

[WS-RM Policy] OASIS WS-RX Technical Committee Specification, "Web Services Reliable Messaging Policy Assertion( WS-RM Policy)," April 2007
http://docs.oasis-open.org/ws-rx/wsrmp/v1.1/wsrmp.pdf

[SOAP 1.1] W3C Note, "SOAP: Simple Object Access Protocol 1.1," 08 May 2000.
http://www.w3.org/TR/2000/NOTE-SOAP-20000508/

[SOAP 1.2] W3C Recommendation, "SOAP Version 1.2 Part 1: Messaging Framework" June 2003.
http://www.w3.org/TR/2003/REC-soap12-part1-20030624/

[URI] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax," RFC 3986, MIT/LCS, U.C. Irvine, Xerox Corporation, January 2005.
http://ietf.org/rfc/rfc3986

[UUID] P. Leach, M. Mealling, R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace," RFC 4122, Microsoft, Refactored Networks - LLC, DataPower Technology Inc, July 2005
http://www.ietf.org/rfc/rfc4122.txt

[XML] W3C Recommendation, "Extensible Markup Language (XML) 1.0 (Fourth Edition)", September 2006.
http://www.w3.org/TR/REC-xml/

[XML-ns] W3C Recommendation, "Namespaces in XML," 14 January 1999.
http://www.w3.org/TR/1999/REC-xml-names-19990114/

[XML-Schema Part1] W3C Recommendation, "XML Schema Part 1: Structures," October 2004.
http://www.w3.org/TR/xmlschema-1/

[XML-Schema Part2] W3C Recommendation, "XML Schema Part 2: Datatypes," October 2004.
http://www.w3.org/TR/xmlschema-2/

[XPATH 1.0] W3C Recommendation, "XML Path Language (XPath) Version 1.0," 16 November 1999.
http://www.w3.org/TR/xpath

[WSDL 1.1] W3C Note, "Web Services Description Language (WSDL 1.1)," 15 March 2001.
http://www.w3.org/TR/2001/NOTE-wsdl-20010315

[WS-Addressing] W3C Recommendation, “Web Services Addressing 1.0 – Core,” May 2006.
http://www.w3.org/TR/2006/REC-ws-addr-core-20060509/
W3C Recommendation, “Web Services Addressing 1.0 – SOAP Binding,” May 2006
http://www.w3.org/TR/2006/REC-ws-addr-soap-20060509/

1.3 Non-Normative References

[BSP 1.0] WS-I Working Group Draft. "Basic Security Profile Version 1.0," August 2006
http://www.ws-i.org/Profiles/BasicSecurityProfile-1.0.html

[RDDL 2.0] Jonathan Borden, Tim Bray, eds. “Resource Directory Description Language (RDDL) 2.0,” January 2004
http://www.openhealth.org/RDDL/20040118/rddl-20040118.html

[RFC 2617] J. Franks, P. Hallam-Baker, J. Hostetler, S. Lawrence, P. Leach, A. Loutonen, L. Stewart, "HTTP Authentication: Basic and Digest Access Authentication," June 1999.
http://www.ietf.org/rfc/rfc2617.txt

[RFC 4346] T. Dierks, E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1," April 2006.
http://www.ietf.org/rfc/rfc4346.txt

[WS-Policy] W3C Member Submission "Web Services Policy 1.2 - Framework", April 2006
http://www.w3.org/Submission/2006/SUBM-WS-Policy-20060425/

W3C Candidate Recommendation, "Web Services Policy 1.5 - Framework," February 2007.
http://www.w3.org/TR/2007/CR-ws-policy-20070228

[WS-PolicyAttachment] W3C Member Submission "Web Services Policy 1.2 - Attachment", April 2006
http://www.w3.org/Submission/2006/SUBM-WS-PolicyAttachment-20060425/

W3C Candidate Recommendation, "Web Services Policy 1.5 - Attachment," February 2007.
http://www.w3.org/TR/2007/CR-ws-policy-attach-20070228

[WS-Security] Anthony Nadalin, Chris Kaler, Phillip Hallam-Baker, Ronald Monzillo, eds. "OASIS Web Services Security: SOAP Message Security 1.0 (WS-Security 2004)", OASIS Standard 200401, March 2004.
http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0.pdf

Anthony Nadalin, Chris Kaler, Phillip Hallam-Baker, Ronald Monzillo, eds. "OASIS Web Services Se-curity: SOAP Message Security 1.1 (WS-Security 2004)", OASIS Standard 200602, February 2006.
http://docs.oasis-open.org/wss/v1.1/wss-v1.1-spec-os-SOAPMessageSecurity.pdf

[RTTM] V. Jacobson, R. Braden, D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992.
http://www.rfc-editor.org/rfc/rfc1323.txt

[SecurityPolicy] G. Della-Libra, et. al. "Web Services Security Policy Language (WS-SecurityPolicy)", July 2005
http://specs.xmlsoap.org/ws/2005/07/securitypolicy/ws-securitypolicy.pdf

[SecureConversation] S. Anderson, et al, "Web Services Secure Conversation Language (WS-SecureConversation)," February 2005.
http://schemas.xmlsoap.org/ws/2004/04/sc/

[Trust] S. Anderson, et al, "Web Services Trust Language (WS-Trust)," February 2005.
http://schemas.xmlsoap.org/ws/2005/02/trust

1.4 Namespace

The XML namespace [XML-ns] URI that MUST be used by implementations of this specification is:

http://docs.oasis-open.org/ws-rx/wsrm/200702

Dereferencing the above URI will produce the Resource Directory Description Language [RDDL 2.0] document that describes this namespace.

Table 1 lists the XML namespaces that are used in this specification. The choice of any namespace prefix is arbitrary and not semantically significant.

Table 1

Prefix	Namespace
S	(Either SOAP 1.1 or 1.2)
S11	http://schemas.xmlsoap.org/soap/envelope/
S12	http://www.w3.org/2003/05/soap-envelope
wsrm	http://docs.oasis-open.org/ws-rx/wsrm/200702
wsa	http://www.w3.org/2005/08/addressing
wsam	http://www.w3.org/2007/02/addressing/metadata
wsse	http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd
xs	http://www.w3.org/2001/XMLSchema

The normative schema for WS-ReliableMessaging can be found linked from the namespace document that is located at the namespace URI specified above.

All sections explicitly noted as examples are informational and are not to be considered normative.

1.5 Conformance

An implementation is not conformant with this specification if it fails to satisfy one or more of the MUST or REQUIRED level requirements defined herein. A SOAP Node MUST NOT use the XML namespace identifier for this specification (listed in section 1.4) within SOAP Envelopes unless it is conformant with this specification.

Normative text within this specification takes precedence over normative outlines, which in turn take precedence over the XML Schema [XML Schema Part 1, Part 2] descriptions.

2 Reliable Messaging Model

Many errors can interrupt a conversation. Messages can be lost, duplicated or reordered. Further the host systems can experience failures and lose volatile state.

The WS-ReliableMessaging specification defines an interoperable protocol that enables a Reliable Messaging (RM) Source to accurately determine the disposition of each message it Transmits as perceived by the RM Destination, so as to allow it to resolve any in-doubt status regarding receipt of the message Transmitted. The protocol also enables an RM Destination to efficiently determine which of those messages it Receives have been previously Received, enabling it to filter out duplicate message transmissions caused by the retransmission, by the RM Source, of an unacknowledged message. It also enables an RM Destination to Deliver the messages it Receives to the Application Destination in the order in which they were sent by an Application Source, in the event that they are Received out of order. Note that this specification places no restriction on the scope of the RM Source or RM Destination entities. For example, either can span multiple WSDL Ports or Endpoints.

The protocol enables the implementation of a broad range of reliability features which include ordered Delivery, duplicate elimination, and guaranteed receipt. The protocol can also be implemented with a range of robustness characteristics ranging from in-memory persistence that is scoped to a single process lifetime, to replicated durable storage that is recoverable in all but the most extreme circumstances. It is expected that the Endpoints will implement as many or as few of these reliability characteristics as necessary for the correct operation of the application using the protocol. Regardless of which of the reliability features is enabled, the wire protocol does not change.

Figure 1 below illustrates the entities and events in a simple reliable exchange of messages. First, the Application Source Sends a message for reliable transfer. The Reliable Messaging Source accepts the message and Transmits it one or more times. After accepting the message, the RM Destination Acknowledges it. Finally, the RM Destination Delivers the message to the Application Destination. The exact roles the entities play and the complete meaning of the events will be defined throughout this specification.

Figure 1: Reliable Messaging Model

2.1 Glossary

The following definitions are used throughout this specification:

Accept: The act of qualifying a message by the RM Destination such that it becomes eligible for Delivery and acknowledgement.

Acknowledgement: The communication from the RM Destination to the RM Source indicating the successful receipt of a message.

Acknowledgement Message: A message containing a SequenceAcknowledgement header block. Acknowledgement Messages may or may not contain a SOAP body.

Acknowledgement Request: A message containing an AckRequested header. Acknowledgement Requests may or may not contain a SOAP body.

Application Destination: The Endpoint to which a message is Delivered.

Application Source: The Endpoint that Sends a message.

Back-channel: When the underlying transport provides a mechanism to return a transport-protocol specific response, capable of carrying a SOAP message, without initiating a new connection, this specification refers to this mechanism as a back-channel.

Deliver: The act of transferring responsibility for a message from the RM Destination to the Application Destination.

Endpoint: As defined in the WS-Addressing specification [WS-Addressing]; a Web service Endpoint is a (referenceable) entity, processor, or resource to which Web service messages can be addressed. Endpoint references (EPRs) convey the information needed to address a Web service Endpoint.

Receive: The act of reading a message from a network connection and accepting it.

RM Destination: The Endpoint that Receives messages Transmitted reliably from an RM Source.

RM Protocol Header Block: One of Sequence, SequenceAcknowledgement, or AckRequested.

RM Source: The Endpoint that Transmits messages reliably to an RM Destination.

Send: The act of transferring a message from the Application Source to the RM Source for reliable transfer.

Sequence Lifecycle Message: A message that contains one of: CreateSequence, CreateSequenceResponse, CloseSequence, CloseSequenceResponse, TerminateSequence, TerminateSequenceResponse as the child element of the SOAP body element.

Sequence Traffic Message: A message containing a Sequence header block.

Transmit: The act of writing a message to a network connection.

2.2 Protocol Preconditions

The correct operation of the protocol requires that a number of preconditions MUST be established prior to the processing of the initial sequenced message:

· For any single message exchange the RM Source MUST have an endpoint reference that uniquely identifies the RM Destination Endpoint.

· The RM Source MUST have successfully created a Sequence with the RM Destination.

· The RM Source MUST be capable of formulating messages that adhere to the RM Destination's policies.

· If a secure exchange of messages is REQUIRED, then the RM Source and RM Destination MUST have a security context.

2.3 Protocol Invariants

During the lifetime of a Sequence, the following invariants are REQUIRED for correctness:

· The RM Source MUST assign each message within a Sequence a message number (defined below) beginning at 1 and increasing by exactly 1 for each subsequent message. These numbers MUST be assigned in the same order in which messages are sent by the Application Source.

· Within every Acknowledgement Message it issues, the RM Destination MUST include one or more AcknowledgementRange child elements that contain, in their collective ranges, the message number of every message accepted by the RM Destination. The RM Destination MUST exclude, in the AcknowledgementRange elements, the message numbers of any messages it has not accepted. If no messages have been received the RM Destination MUST return None instead of an AcknowledgementRange(s). The RM Destination MAY transmit a Nack for a specific message or messages instead of an AcknowledgementRange(s).

· While the Sequence is not closed or terminated, the RM Source SHOULD retransmit unacknowledged messages.

2.4 Delivery Assurances

This section defines a number of Delivery Assurance assertions, which can be supported by RM Sources and RM Destinations. These assertions can be specified as policy assertions using the WS-Policy framework [WS-Policy]. For details on this see the WSRM Policy specification [WS-RM Policy].

AtLeastOnce

Each message is to be delivered at least once, or else an error MUST be raised by the RM Source and/or RM Destination. The requirement on an RM Source is that it SHOULD retry transmission of every message sent by the Application Source until it receives an acknowledgement from the RM Destination. The requirement on the RM Destination is that it SHOULD retry the transfer to the Application Destination of any message that it accepts from the RM Source, until that message has been successfully delivered. There is no requirement for the RM Destination to apply duplicate message filtering.

AtMostOnce

Each message is to be delivered at most once. The RM Source MAY retry transmission of unacknowledged messages, but is NOT REQUIRED to do so. The requirement on the RM Destination is that it MUST filter out duplicate messages, i.e. that it MUST NOT deliver a duplicate of a message that has already been delivered.

ExactlyOnce

Each message is to be delivered exactly once; if a message cannot be delivered then an error MUST be raised by the RM Source and/or RM Destination. The requirement on an RM Source is that it SHOULD retry transmission of every message sent by the Application Source until it receives an acknowledgement from the RM Destination. The requirement on the RM Destination is that it SHOULD retry the transfer to the Application Destination of any message that it accepts from the RM Source until that message has been successfully delivered, and that it MUST NOT deliver a duplicate of a message that has already been delivered.

InOrder

Messages from each individual sequence are to be delivered in the same order they have been sent by the Application Source. The requirement on an RM Source is that it MUST ensure that the ordinal position of each message in the sequence (as indicated by a message sequence number) is consistent with the order in which the messages have been sent from the Application Source. The requirement on the RM Destination is that it MUST deliver received messages for each sequence in the order indicated by the message numbering. This DeliveryAssurance can be used in combination with any of the AtLeastOnce, AtMostOnce or ExactlyOnce assertions, and the requirements of those assertions MUST also be met. In particular if the AtLeastOnce or ExactlyOnce assertion applies and the RM Destination detects a gap in the sequence then the RM Destination MUST NOT deliver any subsequent messages from that sequence until the missing messages are received or until the sequence is closed.

2.5 Example Message Exchange

Figure 2 illustrates a possible message exchange between two reliable messaging Endpoints A and B.

Figure 2: The WS-ReliableMessaging Protocol

1. The protocol preconditions are established. These include policy exchange, endpoint resolution, and establishing trust.

2. The RM Source requests creation of a new Sequence.

3. The RM Destination creates a new Sequence and returns its unique identifier.

4. The RM Source begins Transmitting messages in the Sequence beginning with MessageNumber 1. In the figure above, the RM Source sends 3 messages in the Sequence.

5. The 2^nd message in the Sequence is lost in transit.

6. The 3^rd message is the last in this Sequence and the RM Source includes an AckRequested header to ensure that it gets a timely SequenceAcknowledgement for the Sequence.

7. The RM Destination acknowledges receipt of message numbers 1 and 3 as a result of receiving the RM Source's AckRequested header.

8. The RM Source retransmits the unacknowledged message with MessageNumber 2. This is a new message from the perspective of the underlying transport, but it has the same Sequence Identifier and MessageNumber so the RM Destination can recognize it as a duplicate of the earlier message, in case the original and retransmitted messages are both Received. The RM Source includes an AckRequested header in the retransmitted message so the RM Destination will expedite an acknowledgement.

9. The RM Destination Receives the second transmission of the message with MessageNumber 2 and acknowledges receipt of message numbers 1, 2, and 3.

10. The RM Source Receives this Acknowledgement and sends a TerminateSequence message to the RM Destination indicating that the Sequence is completed. The TerminateSequence message indicates that message number 3 was the last message in the Sequence. The RM Destination then reclaims any resources associated with the Sequence.

11. The RM Destination Receives the TerminateSequence message indicating that the RM Source will not be sending any more messages. The RM Destination sends a TerminateSequenceResponse message to the RM Source and reclaims any resources associated with the Sequence.

The RM Source will expect to Receive Acknowledgements from the RM Destination during the course of a message exchange at occasions described in section 3 below. Should an Acknowledgement not be Received in a timely fashion, the RM Source MUST re-transmit the message since either the message or the associated Acknowledgement might have been lost. Since the nature and dynamic characteristics of the underlying transport and potential intermediaries are unknown in the general case, the timing of re-transmissions cannot be specified. Additionally, over-aggressive re-transmissions have been demonstrated to cause transport or intermediary flooding which are counterproductive to the intention of providing a reliable exchange of messages. Consequently, implementers are encouraged to utilize adaptive mechanisms that dynamically adjust re-transmission time and the back-off intervals that are appropriate to the nature of the transports and intermediaries envisioned. For the case of TCP/IP transports, a mechanism similar to that described as RTTM in RFC 1323 [RTTM] SHOULD be considered.

Now that the basic model has been outlined, the details of the elements used in this protocol are now provided in section 3.

3 RM Protocol Elements

The following sub-sections define the various RM protocol elements, and prescribe their usage by a conformant implementations.

3.1 Considerations on the Use of Extensibility Points

The following protocol elements define extensibility points at various places. Implementations MAY add child elements and/or attributes at the indicated extension points but MUST NOT contradict the semantics of the parent and/or owner, respectively. If a receiver does not recognize an extension, the receiver SHOULD ignore the extension.

3.2 Considerations on the Use of "Piggy-Backing"

Some RM Protocol Header Blocks may be added to messages that are targeted to the same Endpoint to which those headers are to be sent (a concept often referred to as "piggy-backing"), thus saving the overhead of an additional message exchange. Reference parameters MUST be considered when determining whether two EPRs are targeted to the same Endpoint. The determination of if and when a Header Block will be piggy-backed onto another message is made by the entity (RM Source or RM Destination) that is sending the header. In order to ensure optimal and successful processing of RM Sequences, endpoints that receive RM-related messages SHOULD be prepared to process RM Protocol Header Blocks that are included in any message it receives. See the sections that define each RM Protocol Header Block to know which ones may be considered for piggy-backing.

3.3 Composition with WS-Addressing

When the RM protocol, defined in this specification, is composed with the WS-Addressing specification, the following rules prescribe the constraints on the value of the wsa:Action header:

1. When an Endpoint generates a message that carries an RM protocol element, that is defined in the following sections, in the body of a SOAP envelope that Endpoint MUST include in that envelope a wsa:Action SOAP header block whose value is an IRI that is a concatenation of the WS-RM namespace URI, followed by a "/", followed by the value of the local name of the child element of the SOAP body . For example, for a Sequence creation request message as described in section 3.4 below, the value of the wsa:Action IRI would be:

http://docs.oasis-open.org/ws-rx/wsrm/200702/CreateSequence

2. When an Endpoint generates an Acknowledgement Message that has no element content in the SOAP body, then the value of the wsa:Action IRI MUST be:

http://docs.oasis-open.org/ws-rx/wsrm/200702/SequenceAcknowledgement

3. When an Endpoint generates an Acknowledgement Request that has no element content in the SOAP body, then the value of the wsa:Action IRI MUST be:

http://docs.oasis-open.org/ws-rx/wsrm/200702/AckRequested

4. When an Endpoint generates an RM fault as defined in section 4 below, the value of the wsa:Action IRI MUST be as defined in section 4 below.