Network Working Group M. Rose
Request for Comments: 3340 Dover Beach Consulting, Inc.
Category: Standards Track G. Klyne
Clearswift Corporation
D. Crocker
Brandenburg InternetWorking
July 2002
The Application Exchange Core
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This memo describes Application Exchange (APEX) Core, an extensible,
asynchronous message relaying service for application layer programs.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Architecture at a Glance . . . . . . . . . . . . . . . . . 4
2. Service Principles . . . . . . . . . . . . . . . . . . . . 5
2.1 Modes of Operation . . . . . . . . . . . . . . . . . . . . 5
2.2 Naming of Entities . . . . . . . . . . . . . . . . . . . . 6
2.2.1 Comparing Endpoints . . . . . . . . . . . . . . . . . . . 7
3. Service Provisioning . . . . . . . . . . . . . . . . . . . 7
3.1 Connection Establishment . . . . . . . . . . . . . . . . . 7
3.2 Authentication . . . . . . . . . . . . . . . . . . . . . . 8
3.3 Authorization . . . . . . . . . . . . . . . . . . . . . . 8
3.4 Confidentiality . . . . . . . . . . . . . . . . . . . . . 8
3.5 Relaying Integrity . . . . . . . . . . . . . . . . . . . . 8
3.6 Traffic Analysis . . . . . . . . . . . . . . . . . . . . . 9
4. The APEX . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1 Use of XML and MIME . . . . . . . . . . . . . . . . . . . 9
4.2 Profile Identification and Initialization . . . . . . . . 10
4.3 Message Syntax . . . . . . . . . . . . . . . . . . . . . . 11
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4.4 Message Semantics . . . . . . . . . . . . . . . . . . . . 11
4.4.1 The Attach Operation . . . . . . . . . . . . . . . . . . . 11
4.4.2 The Bind Operation . . . . . . . . . . . . . . . . . . . . 13
4.4.3 The Terminate Operation . . . . . . . . . . . . . . . . . 14
4.4.4 The Data Operation . . . . . . . . . . . . . . . . . . . . 15
4.4.4.1 Relay Processing of Data . . . . . . . . . . . . . . . . . 17
4.4.4.2 Application Processing of Data . . . . . . . . . . . . . . 18
4.5 APEX Access Policies . . . . . . . . . . . . . . . . . . . 19
4.5.1 Access Policies in the Endpoint-Relay Mode . . . . . . . . 19
4.5.2 Access Policies in the Relay-Relay Mode . . . . . . . . . 20
5. APEX Options . . . . . . . . . . . . . . . . . . . . . . . 20
5.1 The statusRequest Option . . . . . . . . . . . . . . . . . 22
6. APEX Services . . . . . . . . . . . . . . . . . . . . . . 26
6.1 Use of the APEX Core DTD . . . . . . . . . . . . . . . . . 27
6.1.1 Transaction-Identifiers . . . . . . . . . . . . . . . . . 27
6.1.2 The Reply Element . . . . . . . . . . . . . . . . . . . . 28
6.2 The Report Service . . . . . . . . . . . . . . . . . . . . 28
7. Registration Templates . . . . . . . . . . . . . . . . . . 29
7.1 APEX Option Registration Template . . . . . . . . . . . . 29
7.2 APEX Service Registration Template . . . . . . . . . . . . 29
7.3 APEX Endpoint Application Registration Template . . . . . 30
8. Initial Registrations . . . . . . . . . . . . . . . . . . 30
8.1 Registration: The APEX Profile . . . . . . . . . . . . . . 30
8.2 Registration: The System (Well-Known) TCP port number for
apex-mesh . . . . . . . . . . . . . . . . . . . . . . . . 31
8.3 Registration: The System (Well-Known) TCP port number for
apex-edge . . . . . . . . . . . . . . . . . . . . . . . . 31
8.4 Registration: The statusRequest Option . . . . . . . . . . 31
8.5 Registration: The Report Service . . . . . . . . . . . . . 32
9. DTDs . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1 The APEX Core DTD . . . . . . . . . . . . . . . . . . . . 32
9.2 The Report Service DTD . . . . . . . . . . . . . . . . . . 34
10. Reply Codes . . . . . . . . . . . . . . . . . . . . . . . 35
11. Security Considerations . . . . . . . . . . . . . . . . . 36
References . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . 38
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 39
B. IANA Considerations . . . . . . . . . . . . . . . . . . . 39
Full Copyright Statement . . . . . . . . . . . . . . . . . 40
1. Introduction
Network applications can be broadly distinguished by five operational
characteristics:
o server push or client pull;
o synchronous (interactive) or asynchronous (batch);
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o time-assured or time-insensitive;
o best-effort or reliable; and,
o stateful or stateless.
For example:
o the world-wide web is a pull, synchronous, time-insensitive,
reliable, stateless service; whilst
o Internet mail is a push, asynchronous, time-insensitive, best-
effort (without DSN), stateless service.
Messaging applications vary considerably in their operational
requirements. For example, some messaging applications require
assurance of timeliness and reliability, whilst others do not.
These features come at a cost, in terms of both infrastructural and
configuration complexity. Accordingly, the underlying service must
be extensible to support different requirements in a consistent
manner.
This memo defines a core messaging service that supports a range of
operational characteristics. The core service supports a variety of
tailored services for both user-based and programmatic exchanges.
1.1 Overview
APEX provides an extensible, asynchronous message relaying service
for application layer programs.
APEX, at its core, provides a best-effort datagram service. Each
datagram, simply termed "data", is originated and received by APEX
"endpoints" -- applications that dynamically attach to the APEX
"relaying mesh".
The data transmitted specifies:
o an originating endpoint;
o an opaque content (via a URI-reference);
o one or more recipient endpoints; and,
o zero or more options.
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Options are used to alter the semantics of the service, which may
occur on a per-recipient or per-data basis, and may be processed by
either a single or multiple relays.
Additional APEX services are provided on top of the relaying mesh;
e.g., access control and presence information.
APEX is specified, in part, as a BEEP [1] "profile". Accordingly,
many aspects of APEX (e.g., authentication) are provided within the
BEEP core. Throughout this memo, the terms "peer", "initiator",
"listener", "client", and "server" are used in the context of BEEP.
In particular, Section 2.1 of the BEEP core memo discusses the roles
that a BEEP peer may perform.
When reading this memo, note that the terms "endpoint" and "relay"
are specific to APEX, they do not exist in the context of BEEP.
1.2 Architecture at a Glance
The APEX stack:
+-------------+
| APEX | an APEX process is either:
| process |
+-------------+ - an application attached as an APEX
| | endpoint; or,
| APEX |
| | - an APEX relay
+-------------+
| | APEX services are realized as applications
| BEEP | having a special relationship with the APEX
| | relays in their administrative domain
+-------------+
| TCP |
+-------------+
| ... |
+-------------+
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The APEX entities:
administrative domain #1 administrative domain #2
+----------------------------+ +----------------------------+
| +------+ | | +------+ |
| | | | | | | |
| | appl | | | | appl | |
| | | | | | | |
| +......+ +------+ | | +------+ +......+ |
| | | | | | | | | | | |
| |end- | |relay | | | |relay | |end- | |
| | point| | | | | | | | point| |
| +------+ +------+ | | +------+ +------+ |
| | | | | | | | | | | |
| | APEX | | APEX | | | | APEX | | APEX | |
| | | | | | | | | | | |
| +------+ +------+ | | +------+ +------+ |
| || || || | | || || || |
| ============= ================ ============= |
+----------------------------+ +----------------------------+
| <---- APEX relaying mesh ----> |
Note: relaying between administrative domains is configured
using SRV RRs. Accordingly, the actual number of
relays between two endpoints is not fixed.
2. Service Principles
2.1 Modes of Operation
APEX is used in two modes:
endpoint-relay: in which the endpoint is always the BEEP initiator of
the service, whilst relays are always the BEEP listeners. In this
context, applications attach as endpoints, and then the
transmission of data occurs.
relay-relay: in which relays typically, though not necessarily,
reside in different administrative domains. In this context,
applications bind as relays, and then the transmission of data
occurs.
In the endpoint-relay mode, an endpoint (BEEP initiator) may:
o attach as one or more endpoints;
o send data to other endpoints;
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o receive data from other endpoints; and,
o terminate any of its attachments.
A relay (BEEP listener), in addition to servicing requests from a
BEEP initiator, may:
o terminate any of the endpoint's attachments;
o deliver data from other endpoints; and,
o indicate the delivery status of data sent earlier by the endpoint.
In the relay-relay mode, a relay (BEEP listener or initiator) may:
o bind as one or more administrative domains;
o send data;
o receive data; and,
o terminate any bindings.
2.2 Naming of Entities
Endpoints are named using the following ABNF [2] syntax:
;; Domain is defined in [3], either a FQDN or a literal
entity = local "@" Domain
local = address [ "/" subaddress ]
address = token
subaddress = token
;; all non-control characters, excluding "/" and "@" delimiters
token = 1*(%x20-2E / %x30-3F / %x41-7E / UTF-8) ;; [4]
Two further conventions are applied when using this syntax:
the "apex=" convention: All endpoint identities having a local-part
starting with "apex=" are reserved for use by APEX services
registered with the IANA; and,
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the "subaddress" convention: If the solidus character ("/", decimal
code 47) occurs in the local-part, this identifies a subaddress of
an endpoint identity (e.g., "fred/appl=wb@example.com" is a
subaddress of the APEX endpoint "fred@example.com").
All subaddresses starting with "appl=" are reserved for use by
APEX endpoint applications registered with the IANA.
Relays, although not named, serve of behalf of administrative
domains, as identified by a FQDN or a domain-literal, e.g.,
"example.com" or "[10.0.0.1]".
In APEX, "endpoints" and "relays" are the fundamental entities. APEX
is carried over BEEP, which has the "peer" as its fundamental entity.
The relationship between BEEP peer entities and APEX endpoint and
relay entities are defined by APEX's Access Policies (Section 4.5).
2.2.1 Comparing Endpoints
Note that since the "local" part of an entity is a string of UTF-8
[4] octets, comparison operations on the "local" part use exact
matching (i.e., are case-sensitive).
Accordingly, "fred@example.com" and "Fred@example.com" refer to
different endpoints. Of course, relays serving the "example.com"
administrative domain may choose to treat the two endpoints
identically for the purposes of routing and delivery.
Finally, note that if an APEX endpoint is represented using a
transmission encoding, then, prior to comparison, the encoding is
reversed. For example, if the URL encoding is used, then
"apex:fred@example.com" is identical to "apex:f%72ed@example.com".
3. Service Provisioning
3.1 Connection Establishment
The SRV algorithm [5] is used to determine the IP/TCP addressing
information assigned to the relays for an administrative domain
identified by a FQDN:
service: "apex-edge" (for the endpoint-relay mode), or "apex-mesh"
(for the relay-relay mode);
protocol: "tcp"; and,
domain: the administrative domain.
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If the administrative domain is identified by a domain-literal, then
the IP address information is taken directly from the literal and the
TCP port number used is assigned by the IANA for the registration in
Section 8.2.
3.2 Authentication
Authentication is a matter of provisioning for each BEEP peer (c.f.,
Section 4.5).
An APEX relay might be provisioned to allow a BEEP peer identity to
coincide with a given endpoint identity. For example, a relay in the
"example.com" administrative domain may be configured to allow a BEEP
peer identified as "fred@example.com" to be authorized to attach as
the APEX endpoint "fred@example.com".
3.3 Authorization
Authorization is a matter of provisioning for each BEEP peer (c.f.,
Section 4.5).
Typically, a relay requires that its BEEP peer authenticate as a
prelude to authorization, but an endpoint usually does not require
the same of its BEEP peer.
3.4 Confidentiality
Confidentiality is a matter of provisioning for each BEEP peer.
Typically, any data considered sensitive by an originating endpoint
will have its content encrypted for the intended recipient
endpoint(s), rather than relying on hop-by-hop encryption.
Similarly, an originating endpoint will sign the content if end-to-
end authentication is desired.
3.5 Relaying Integrity
Data are relayed according to SRV entries in the DNS. Accordingly,
relaying integrity is a function of the DNS and the applications
making use of the DNS. Additional assurance is provided if the BEEP
initiator requires that the BEEP listener authenticate itself.
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3.6 Traffic Analysis
Hop-by-hop protection of data transmitted through the relaying mesh
(endpoint identities and content) is afforded at the BEEP level
through the use of a transport security profile. Other traffic
characteristics, e.g., volume and timing of transmissions, are not
protected from third-party analysis.
4. The APEX
Section 8.1 contains the BEEP profile registration for APEX.
4.1 Use of XML and MIME
Each BEEP payload exchanged via APEX consists of an XML document and
possibly an arbitrary MIME content.
If only an XML document is sent in the BEEP payload, then the mapping
to a BEEP payload is straight-forward, e.g.,
C: MSG 1 2 . 111 39
C: Content-Type: application/beep+xml
C:
C:
C:
C:
C:
C:
C:
C:
C: END
4.2 Profile Identification and Initialization
The APEX is identified as
http://iana.org/beep/APEX
in the BEEP "profile" element during channel creation.
No elements are required to be exchanged during channel creation;
however, in the endpoint-relay mode, the BEEP initiator will
typically include an "attach" element during channel creation, e.g.,
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]]>
Similarly, in the relay-relay mode, the BEEP initiator will typically
include an "bind" element during channel creation, e.g.,
]]>
4.3 Message Syntax
Section 9.1 defines the BEEP payloads that are used in the APEX.
4.4 Message Semantics
4.4.1 The Attach Operation
When an application wants to attach to the relaying mesh as a given
endpoint, it sends an "attach" element to a relay, e.g.,
+-------+ +-------+
| | -- attach -----> | |
| appl. | | relay |
| | <--------- ok -- | |
+-------+ +-------+
C: access denied
The "attach" element has an "endpoint" attribute, a "transID"
attribute, and contains zero or more "option" elements:
o the "endpoint" attribute specifies the endpoint that the
application wants to attach as;
o the "transID" attribute specifies the transaction-identifier
associated with this operation; and,
o the "option" elements, if any, specify additional processing
options (Section 5).
When a relay receives an "attach" element, it performs these steps:
1. If the transaction-identifier refers to a previous, non-terminated
operation on this BEEP channel, an "error" element having code 555
is returned.
2. If the relay is in a different administrative domain than this
endpoint, an "error" element having code 553 is returned.
3. If the application is not authorized to attach as this endpoint
(c.f., Section 4.5.1), an "error" element having code 537 is
returned.
4. If any options are present, they are processed.
5. If another application has already attached as this endpoint, an
"error" element having code 554 is returned.
6. Otherwise, the application is bound as this endpoint, and an "ok"
element is returned.
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4.4.2 The Bind Operation
When an application wants to identify itself as a relay, it sends a
"bind" element to another relay, e.g.,
+-------+ +-------+
| | -- bind -------> | |
| relay | | relay |
| #1 | <--------- ok -- | #2 |
+-------+ +-------+
C: access denied
The "bind" element has a "relay" attribute, a "transID" attribute,
and contains zero or more "option" elements:
o the "relay" attribute specifies the administrative domain on whose
behalf the application wants to serve;
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o the "transID" attribute specifies the transaction-identifier
associated with this operation; and,
o the "option" elements, if any, specify additional processing
options (Section 5).
When a relay receives an "bind" element, it performs these steps:
1. If the transaction-identifier refers to a previous, non-terminated
operation on this BEEP channel, an "error" element having code 555
is returned.
2. If the application is not authorized to bind on behalf of this
administrative domain (c.f., Section 4.5.2), an "error" element
having code 537 is returned.
3. If any options are present, they are processed.
4. Otherwise, the application is accepted as serving this
administrative domain, and an "ok" element is returned.
4.4.3 The Terminate Operation
When an application or relay wants to release an attachment or
binding, it sends a "terminate" element, e.g.,
+-------+ +-------+
| | -- terminate --> | |
| appl. | | relay |
| | <--------- ok -- | |
+-------+ +-------+
C: unknown transaction-identifier
or
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+-------+ +-------+
| | <-- terminate -- | |
| appl. | | relay |
| | -- ok ---------> | |
+-------+ +-------+
C: access denied
or
+-------+ +-------+
| | -- data -------> | |
| relay | | appl. |
| | <--------- ok -- | #2 |
+-------+ +-------+
C:
S:
unknown endpoint
identity
S:
S:
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S:
RFC, FYI, BCP