Using the AMQP Anonymous Terminus for Message Routing. Version 1.0

The Advanced Message Queuing Protocol (AMQP) is an open internet protocol for business messaging. AMQP defines links as a unidirectional transport for messages between a source and a target. The target of a link identifies the node to which messages are to be sent to. If a large number of distinct destinations are in use, or if the destinations to be sent to are not known ahead of time (for example, they are provided as a reply-to in incoming messages) then creating a link per destination can be burdensome. This document defines a mechanism whereby a single outgoing link can be used to transfer messages which are then routed using the address carried in their "to" field.

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←1 Introduction

←1.1 IPR Policy

←1.2 Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in [RFC2119] and [RFC8174] when, and only when, they appear in all capitals, as shown here.

The authoritative form of this specification consists of a set of XML source documents. These documents are transformed into PDF and HTML representations for readability. The machine readable version of the AMQP DTD describes the XML used for the authoritative source documents. This DTD includes the definition of the syntax used any excerpts of XML presented in the PDF and HTML representations.

←1.3 Normative References

[AMQP]
Godfrey, Robert; Ingham, David; Schloming, Rafael, Advanced Message Queuing Protocol (AMQP) Version 1.0. October 2012, OASIS Standard.
<http://docs.oasis-open.org/amqp/core/v1.0/os/amqp-core-overview-v1.0-os.html>

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP14, RFC2119, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>

[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC2119 Key Words", BCP14, RFC8174, DOI 10.17487/RFC8174, May 2017,
<http://www.rfc-editor.org/info/rfc8174>

←1.4 Non Normative References

[AMQPCONNCAP]
AMQP Capabilities Registry: Connection Capabilities
http://www.amqp.org/specification/1.0/connection-capabilities

[AMQPCONNPROP]
AMQP Capabilities Registry: Connection Properties
http://www.amqp.org/specification/1.0/connection-properties

←2 Message Routing

←2.1 Sending A Message

When creating and sending a message AMQP defines two fields which are to be used to determine how the message should be routed:

the to field of properties as part of the message
the address field of target as part of the link attach

The to field specifies the intended ultimate recipient of the message. The address field determines the node to which the messages will be transferred to on a given link. The behavior of the node identified by this address determines how the message will be processed. A class of nodes which stores messages for onward distribution between consumers is identified in 3.3 Distribution Nodes of [AMQP], here we define a second class of nodes which do not store messages, but simply route incoming messages to other nodes based on the to field of the message.

←2.2 Routing Nodes

A routing node acts as a proxy for links to other nodes. Messages sent over links into a routing node will be forwarded to the node referenced in the to field of properties of the message just as if a direct link has been established to that node.

Figure 2.1: Router Example
Container A Container B +---------------+ +---------------+ \| \| src=ADDR1 tgt=ROUTER \| \| \| \|------------------------------------->\| ROUTER \| \| \| M1 {to=ADDR1}, M2 {to=ADDR2} \| \| \| \| \| ADDR1 \| \| \| \| \| \| \| \| ADDR2 \| \| \| \| \| +---------------+ +---------------+ Messages M1 and M2 travel along a single link to the ROUTER node. Message M1 is delivered to address ADDR1, Message M2 is delivered to address ADDR2.

←2.2.1 Link Properties And Target Capabilities

When creating a sending link, the properties field of attach and the capabilities field of target can be used to influence the behavior of the receiving node, or other aspects of the interaction between the source and target. The receiving peer will then respond by setting the properties field of attach and the capabilities field of target to reflect values that the target node supports.

Since a routing node acts only as a proxy between the source and intended target nodes, it itself does not directly support the properties and capabilities but rather the intended target nodes do. Unless every target node that the routing node can route to supports the same set of properties and capabilities, it is impossible for it to accurately reply with the correct set of supported behaviors.

If a routing node receives an attach which carries one or more properties or for which the capabilities field of target has one or more entries then the corresponding attach SHOULD contain the those capabilities and properties which are supported by at least some subset of the nodes the routing node can route to. If a message is subsequently sent along the link with an address in the to field of properties which resolves to a node that does not support these behaviors then the routing node MUST signal an error, either by rejecting the message (if the source supports the rejected outcome and the incoming message was unsettled) or by detaching the link with a not-implemented error.

←2.2.2 Routing Errors

It is possible that a message sent to a routing node has an address in the to field of properties which, if used in the address field of target of an attach, would result in an unsuccessful link establishment (for example, if the address cannot be resolved to a node). In this case the routing node MUST communicate the error back to the sender of the message.

If the source of the link supports the rejected outcome, and the message has not already been settled by the sender, then the routing node MUST reject the message. In this case the error field of rejected MUST contain the error which would have been communicated in the detach which would have be sent if a link to the same address had been attempted.

If the source of the link does not support the rejected outcome, or the message has already been settled by the sender, then an error MUST be reported. If the message was sent as part of a transaction then the rules defined in 4.4.4 Interaction Of Settlement With Transactions of [AMQP] for Transactional Posting apply - that is the error can be reported by immediately destroying the controlling link on which the transaction was declared, or by rejecting any attempt to discharge the transaction where the fail flag is not set to true. If the message was not transactionally posted, then the routing node MUST detach the link over which the message was sent with an error. The error sent by the routing node MUST contain the error which would have been communicated in the detach sent on attempting to link directly to the address in the message's to field. Additionally the info field of error MUST contain an entry with symbolic key delivery-tag and binary value of the delivery-tag of the message which caused the failure.

←2.3 Anonymous Terminus

A target with a null address is referred to as the anonymous terminus. The anonymous terminus is reserved for use as a routing node by containers that support such capabilities. Support for the anonymous terminus by a container is determined by the use of a connection capability.

Name Description

ANONYMOUS-RELAY If present in the offered-capabilities field of open then the sender of open performative supports anonymous terminus with the semantics defined in this document. If not present then the receiver of open MUST assume that its partner does not support the anonymous terminus.

If present in the desired-capabilities field of open then the sender of open MAY use the anonymous if its partner offers its support. If it is not present then the sender of open MUST NOT attempt to use the anonymous terminus supports even if its partner offered the capability in the open performative which it sent.

Name	Description
ANONYMOUS-RELAY	If present in the `offered-capabilities` field of `open` then the sender of `open` performative supports anonymous terminus with the semantics defined in this document. If not present then the receiver of `open` MUST assume that its partner does not support the anonymous terminus. If present in the `desired-capabilities` field of `open` then the sender of `open` MAY use the anonymous if its partner offers its support. If it is not present then the sender of `open` MUST NOT attempt to use the anonymous terminus supports even if its partner offered the capability in the `open` performative which it sent.

Revision	Date	Editor	Changes Made
WD01	23-Mar-2017	Rob Godfrey	Initial Revision
WD02	13-Apr-2017	Rob Godfrey	AMQP-111 : Typos and repeated words AMQP-112 : Detail behaviour when addressed node doesn't exist
WD03	6-Oct-2017	Rob Godfrey	AMQP-124 : Add a (minimal) conformance section
WD04	23-Mar-2018	Rob Godfrey	AMQP-140 : Make the behaviour of the anonymous terminus clear when faced with transactional pre-settled messages
WD05	13-Jun-2018	Rob Godfrey	AMQP-124 : Update conformance section to define roles
WD06	15-Jun-2018	Rob Godfrey	Fix typos

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