Network Working Group M.T. Rose
Internet-Draft Invisible Worlds, Inc.
Expires: December 10, 2000 G. Klyne
Content Technologies Limited
D.H. Crocker
Brandenburg Consulting
June 11, 2000
The IMXP
draft-mrose-imxp-core-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This memo describes IMXP, an extensible, asynchronous message
relaying service for application layer programs.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Architecture at a Glance . . . . . . . . . . . . . . . . . 5
2. Service Principles . . . . . . . . . . . . . . . . . . . . 7
2.1 Modes of Operation . . . . . . . . . . . . . . . . . . . . 7
2.2 Naming of Entities . . . . . . . . . . . . . . . . . . . . 8
3. Service Provisioning . . . . . . . . . . . . . . . . . . . 9
3.1 Connection Establishment . . . . . . . . . . . . . . . . . 9
3.2 Authentication . . . . . . . . . . . . . . . . . . . . . . 9
3.3 Authorization . . . . . . . . . . . . . . . . . . . . . . 9
3.4 Confidentiality . . . . . . . . . . . . . . . . . . . . . 9
3.5 Routing Integrity . . . . . . . . . . . . . . . . . . . . 10
3.6 Traffic Analysis . . . . . . . . . . . . . . . . . . . . . 10
4. The IMXP . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1 Use of XML and MIME . . . . . . . . . . . . . . . . . . . 11
4.2 Profile Identification and Initialization . . . . . . . . 13
4.3 Request and Response Messages . . . . . . . . . . . . . . 13
4.4 Message Semantics . . . . . . . . . . . . . . . . . . . . 14
4.4.1 The Attach Operation . . . . . . . . . . . . . . . . . . . 14
4.4.2 The Terminate Operation . . . . . . . . . . . . . . . . . 16
4.4.3 The Data Operation . . . . . . . . . . . . . . . . . . . . 17
4.4.3.1 Relay Processing of Data . . . . . . . . . . . . . . . . . 19
4.4.3.2 Application Processing of Data . . . . . . . . . . . . . . 20
4.5 IMXP Access Policies . . . . . . . . . . . . . . . . . . . 21
5. IMXP Options . . . . . . . . . . . . . . . . . . . . . . . 23
5.1 The statusRequest Option . . . . . . . . . . . . . . . . . 25
6. IMXP Services . . . . . . . . . . . . . . . . . . . . . . 30
6.1 Use of the IMXP Core DTD . . . . . . . . . . . . . . . . . 31
6.1.1 Transaction-Identifiers . . . . . . . . . . . . . . . . . 31
6.1.2 The Reply Operation . . . . . . . . . . . . . . . . . . . 32
6.2 The Report Service . . . . . . . . . . . . . . . . . . . . 33
7. IMXP Reply Codes . . . . . . . . . . . . . . . . . . . . . 34
8. IMXP Option Registration Template . . . . . . . . . . . . 35
9. IMXP Service Registration Template . . . . . . . . . . . . 36
10. Registration: The IMXP Profile . . . . . . . . . . . . . . 37
11. Registration: The statusRequest Option . . . . . . . . . . 38
12. Registration: The Report Service . . . . . . . . . . . . . 39
13. The IMXP Core DTD . . . . . . . . . . . . . . . . . . . . 40
14. The Report Service DTD . . . . . . . . . . . . . . . . . . 43
15. Security Considerations . . . . . . . . . . . . . . . . . 44
16. IANA Considerations . . . . . . . . . . . . . . . . . . . 45
References . . . . . . . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . 46
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 48
Full Copyright Statement . . . . . . . . . . . . . . . . . 49
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1. Introduction
Network applications can be broadly distinguished by five
operational characteristics:
o server push or client pull;
o synchronous (interactive) or asynchronous (batch);
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.
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1.1 Overview
IMXP provides an extensible, asynchronous message relaying service
for application layer programs.
IMXP, at its core, provides a best-effort datagram service. Each
datagram, simply termed "data", is originated and received by IMXP
"endpoints" -- applications that dynamically attach to the IMXP
"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.
Options are used to alter the semantics of the the service, may
occur on a per-recipient or per-data basis, and may be processed by
either a single or multiple relays.
Additional IMXP services are provided on top of the relaying mesh;
e.g., access control and presence information. Typically these
services are provided by servers that are co-resident with one or
more relays within an administrative domain.
IMXP is specified as a BXXP[1] "profile". Accordingly, many aspects
of IMXP (e.g., authentication) are provided within the BXXP
framework. Throughout this memo, the terms "peer", "initiator",
"listener", "client" (or "requestor"), and "server" (or "responder")
are used in the context of BXXP. In particular, Section 2.1 of the
BXXP framework memo discusses the roles that a BXXP peer may perform.
When reading this memo, note that the terms "endpoint" and "relay"
are specific to IMXP, they do not exist in the context of BXXP.
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1.2 Architecture at a Glance
The IMXP stack:
+-------------+
| IMXP | an IMXP process is either:
| process |
+-------------+ - an application attached as an IMXP
| | endpoint; or,
| IMXP |
| | - an IMXP relay
+-------------+
| | IMXP services are realized as applications
| BXXP | having a special relationship with the IMXP
| | relays in their administrative domain
+-------------+
| TCP/IP |
+-------------+
| ... |
+-------------+
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The IMXP entities:
administrative domain #1 administrative domain #2
+----------------------------+ +----------------------------+
| +------+ | | +------+ |
| | | | | | | |
| | appl | | | | appl | |
| | | | | | | |
| +......+ +------+ | | +------+ +......+ |
| | | | | | | | | | | |
| |end- | |relay | | | |relay | |end- | |
| | point| | | | | | | | point| |
| +------+ +------+ | | +------+ +------+ |
| | | | | | | | | | | |
| | IMXP | | IMXP | | | | IMXP | | IMXP | |
| | | | | | | | | | | |
| +------+ +------+ | | +------+ +------+ |
| | | | | | | | | | | |
| | BXXP | | BXXP | | | | BXXP | | BXXP | |
| | | | | | | | | | | |
| +------+ +------+ | | +------+ +------+ |
| || || || | | || || || |
| ============= ================ ============= |
+----------------------------+ +----------------------------+
| <---- IMXP relaying mesh ----> |
Note: routing between administrative domains is configured
using SRV RRs. Accordingly, the actual number of
relays between two endpoints is not fixed.
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2. Service Principles
2.1 Modes of Operation
IMXP is used in two modes:
endpoint-relay: in which the endpoint is always the BXXP initiator
of the service, whilst relays are always the BXXP 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, only
the transmission of data occurs.
In the endpoint-relay mode, an endpoint (BXXP initiator) may:
o attach as one or more endpoints;
o send data to other endpoints;
o receive data from other endpoints; and,
o terminate any of its attachments.
A relay (BXXP listener), in addition to servicing requests from a
BXXP 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, the BXXP initiators and listeners transmit
data on behalf of endpoints.
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2.2 Naming of Entities
Endpoints are named using the "addr-spec" syntax of RFC 822[2],
i.e., "local@domain".
Using the conventions of RFC 2303[3], all endpoint identities having
a local-part starting with "imxp=" are reserved for use by IMXP
services registered with the IANA.
The URL scheme "im" is used to identify endpoints when written as
URIs, e.g., "im:fred@example.com".
Relays, although not named, belong to an administrative domain, as
identified by a FQDN, e.g., "example.com".
In IMXP, the "endpoint" is the fundamental entity. IMXP is carried
over BXXP, which has the "peer" as its fundamental entity. The
relationship between BXXP peer entities and IMXP endpoint entities
are defined by IMXP's Access Policies (Section 4.5).
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3. Service Provisioning
3.1 Connection Establishment
The SRV algorithm[4] is used to determine the IP address and TCP
port number assigned to a relay for an administrative domain:
service: "imxp-edge" (for the endpoint-relay mode), or "imxp-mesh"
(for the relay-relay mode);
protocol: "tcp"; and,
domain: the administrative domain.
3.2 Authentication
Authentication is a matter of provisioning for each BXXP peer (c.f.,
Section 4.5).
An IMXP relay might be provisioned to allow a BXXP 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
BXXP peer identified as "fred@example.com" to be authorized to
attach as the IMXP endpoint "fred@example.com".
3.3 Authorization
Authorization is a matter of provisioning for each BXXP peer (c.f.,
Section 4.5).
Typically, a relay requires that its BXXP peer authenticate as a
prelude to authorization, but an endpoint usually does not require
the same of its BXXP peer.
3.4 Confidentiality
Confidentiality is a matter of provisioning for each BXXP 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.
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3.5 Routing Integrity
Data are routed according to SRV entries in the DNS. Accordingly,
routing integrity is a function of the DNS and the applications
making use of the DNS. Additional assurance is provided if the BXXP
initiator requires that the BXXP listener authenticate itself.
3.6 Traffic Analysis
Hop-by-hop protection of data transmitted through the relaying mesh
(endpoint identities and content) is afforded at the BXXP level
through the use of a transport security profile. Other traffic
characteristics, e.g., volume and timing of transmissions, is not
protected from third-party analysis.
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4. The IMXP
Section 10 contains the BXXP profile registration for IMXP.
4.1 Use of XML and MIME
Each BXXP payload exchanged via IMXP consists of an XML document and
possibly an arbitrary MIME content.
If only an XML document is sent, then the BXXP payload defaults
("Content-Type: text/xml" and "Content-Transfer-Encoding: binary")
are used, e.g.,
C: REQ . 1 0 27 1
C:
C: <terminate transID='1' />
C: END
Otherwise, if an arbitrary MIME content is present, it is indicated
by a URI-reference[5] in the XML control document. The URI-reference
may contain an absolute-URI (and possibly a fragment-identifier), or
it may be a relative-URI consisting only of a fragment-identifier.
Arbitrary MIME content is included in the BXXP payload by using a
"multipart/related"[6], identified using a "cid" URL[7], and the XML
control document occurs as the start of the "multipart/related",
e.g.,
C: REQ . 1 0 1234 1
C: Content-Type: multipart/related; boundary="boundary";
C: start="<1@example.com>";
C: type="text/xml"
C:
C: --boundary
C: Content-Type: text/xml
C: Content-ID: <1@example.com>
C:
C: <data originator='fred@example.com'
C: content='cid:2@example.com'>
C: <recipient identity='barney@example.com' />
C: </data>
C: --boundary
C: Content-Type: image/gif
C: Content-Transfer-Encoding: binary
C: Content-ID: <2@example.com>
C:
C: ...
C: --boundary--
C: END
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Because BXXP provides an 8bit-wide path, a "transformative"
Content-Transfer-Encoding (e.g., "base64" or "quoted-printable")
should not be used. Further, note that MIME[8] requires that the
value of the "Content-ID" header be globally unique.
If the arbitrary MIME content is itself an XML document, it may be
contained with the control document directly, and identified using a
URI-reference consisting of only a fragment-identifier, e.g.,
C: REQ . 1 0 259 1
C:
C: <data originator='fred@example.com' content='#Content'>
C: <recipient identity='barney@example.com' />
C: <data-content Name='Content'>
C: <statusResponse transID='86'>
C: <destination identity='barney@example.com'>
C: <reply code='250' />
C: </destination>
C: <statusResponse>
C: </data-content>
C: </data>
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4.2 Profile Identification and Initialization
The IMXP is identified as
http://xml.resource.org/profiles/IMXP
in the BXXP "profile" element during channel creation.
No elements are required to be exchanged during channel creation;
however, in the endpoint-relay mode, the BXXP initiator will
typically include an "attach" element during channel creation, e.g.,
<start number='1'>
<profile uri='http://xml.resource.org/profiles/IMXP'>
<attach transID='1'>
<recipient identity='fred@example.com' />
</attach>
</profile>
</start>
4.3 Request and Response Messages
Section 13 defines the BXXP payloads that are used in the IMXP:
o "REQ" messages carry either the "attach", "terminate", or "data"
element as BXXP payload;
o positive "RSP" messages carry the "ok" element as BXXP payload;
and,
o negative "RSP" messages carry the "error" element as BXXP payload.
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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: <attach transID='1'>
C: <recipient identity='fred@example.com' />
C: </attach>
S: <ok />
or
+-------+ +-------+
| | -- attach -----> | |
| | | |
| | <--------- ok -- | |
| appl. | | relay |
| | -- attach -----> | |
| | | |
| | <--------- ok -- | |
+-------+ +-------+
C: <attach transID='1'>
C: <recipient identity='fred@example.com' />
C: </attach>
S: <ok />
C: <attach transID='2'>
C: <recipient identity='wilma@example.com' />
C: </attach>
S: <ok />
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or
+-------+ +-------+
| | -- attach -----> | |
| appl. | | relay |
| | <------ error -- | |
+-------+ +-------+
C: <attach transID='1'>
C: <recipient identity='fred@example.com' />
C: </attach>
S: <error code='537'>access denied</error>
The "attach" element has a "transID" attribute, and contains a
"recipient" element, which, in turn, has an "identity" attribute and
contains zero or more "option" elements:
o the "transID" attribute specifies the transaction-identifier
associated with this operation;
o the "identity" attribute specifies the desired endpoint; 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 attach
operation on this BXXP channel (that has not yet been
terminated), 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,
an "error" element having code 537 is returned.
4. If any options are present, they are examined.
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 Terminate Operation
When an application or relay wants to detach an endpoint, it sends a
"terminate" element, e.g.,
+-------+ +-------+
| | -- terminate --> | |
| appl. | | relay |
| | <--------- ok -- | |
+-------+ +-------+
C: <terminate transID='1' />
S: <ok />
or
+-------+ +-------+
| | -- terminate --> | |
| appl. | | relay |
| | <------ error -- | |
+-------+ +-------+
C: <terminate transID='13' />
S: <error code='550'>unknown transaction-identifier</error>
or
+-------+ +-------+
| | <-- terminate -- | |
| appl. | | relay |
| | -- ok ---------> | |
+-------+ +-------+
C: <terminate transID='1' />
S: <ok />
The "terminate" element has a "transID" attribute that specifies the
transaction-identifier associated a previous attach operation, and
has no content.
When an application or relay receives a "terminate" element, it
performs these steps:
1. If the transaction-identifier does not refer to a previous
attach operation on this BXXP channel (that has not yet been
terminated), an "error" element having code 550 is returned.
2. Otherwise, the application is no longer bound as the
corresponding endpoint, and an "ok" element is returned.
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4.4.3 The Data Operation
When an application or relay wants to transmit data over the
relaying mesh, it sends a "data" element, e.g.,
+-------+ +-------+
| | -- data -------> | |
| appl. | | relay |
| #1 | <--------- ok -- | |
+-------+ +-------+
C: <data originator='fred@example.com' content='cid:1@example.com'>
<recipient identity='barney@example.com' />
</data>
S: <ok />
or
+-------+ +-------+
| | -- data -------> | |
| appl. | | relay |
| #1 | <------ error -- | |
+-------+ +-------+
C: <data originator='fred@example.com' content='cid:1@example.com'>
<recipient identity='barney@example.com' />
</data>
S: <error code='450'>barney@example.com not attached</error>
or
+-------+ +-------+
| | -- data -------> | |
| relay | | appl. |
| | <--------- ok -- | #2 |
+-------+ +-------+
C: <data originator='fred@example.com' content='cid:1@example.com'>
<recipient identity='barney@example.com' />
</data>
S: <ok />
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The "data" element has an "originator" and a "content" attribute,
and contains one or more "recipient" elements, zero or more "option"
elements, and, optionally, a "data-content" element:
o the "originator" attribute specifies the endpoint sending the
data;
o the "content" attribute is a URI-reference that specifies the
contents of the data (c.f., Section 4.1);
o each "recipient" element refers to an endpoint destination for
the data;
o the "option" elements, if any, specify additional processing
options (Section 5), termed per-data options; and,
o the "data-content" element, if present, specifies a nested XML
entity using a URI fragment-identifier as the value of the
"content" attribute.
Each "recipient" element has an "identity" attribute, and contains
zero or more option elements:
o the "identity" attribute specifies the destination endpoint; and
o the "option" elements, if any, specify additional processing
options for this recipient, termed per-recipient options.
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4.4.3.1 Relay Processing of Data
When a relay receives a "data" element, it performs these steps:
1. If the BXXP client is not authorized to originate or relay data
on behalf of the "originator" endpoint, an "error" element
having code 537 is returned.
2. If any per-data options are present, they are examined.
3. For each recipient:
1. If any per-recipient options are present, they are examined.
2. If the recipient endpoint is not in the administrative
domain associated with the relay, then an IMXP session is
established to a relay that accepts data for the recipient's
administrative domain, and a new "data" element, containing
only that "recipient" element (and all applicable options),
is sent to that relay.
If no errors (e.g., an IMXP session can not be established)
occur during processing, and if the recipient's relay
returns an "ok" element, then the recipient is considered to
be successfully processed.
3. Otherwise, the IMXP access service must allow the originator
endpoint to communicate with the recipient endpoint (the
recipient's access entry[9] must contain a "core:data" token
for the originator), and the recipient endpoint must be
currently attached.
If so, a new "data" element, containing only that
"recipient" element (and no options), is sent to the
corresponding application, and the recipient is considered
to be successfully processed.
4. If no recipients could be successfully processed, an "error"
element is returned; otherwise, an "ok" element is returned.
Note that an implementation may choose to optimize its behavior by
grouping multiple recipients in a single "data" element that is
subsequently transmitted. It may do so providing that the
optimization retains these semantics and any other semantics related
to per-data and per-recipient options.
Finally, note that a relay receiving a "data" element from an
application may be configured to add domain-specific options.
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4.4.3.2 Application Processing of Data
When an application receives a "data" element, it performs these
steps:
1. For each recipient:
1. If the application is not attached as the recipient
endpoint, then an error in processing has occurred.
2. Otherwise, the "data" element is further processed in an
application-specific manner, and the recipient is considered
to be successfully processed.
2. If no recipients could be successfully processed, an "error"
element is returned; otherwise, an "ok" element is returned.
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4.5 IMXP Access Policies
Access to IMXP is provided by the juxtaposition of:
o authenticating as a BXXP peer;
o attaching as an IMXP endpoint; and,
o being listed as an actor by the IMXP access service (c.f., [9]).
Each of these activities occurs according to the policies of the
relevant administrative domain:
o each administrative domain is responsible for keeping its own
house in order through "local provisioning"; and,
o each administrative domain decides the level of trust to
associate with other administrative domains.
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IMXP access policies are used in these contexts:
o In the endpoint-relay mode, when an application wishes to attach
to the relaying mesh, local provisioning maps BXXP peer
identities to allowed IMXP endpoints (c.f., Step 3 of Section
4.4.1).
Typically, the identity function is used, e.g., if an application
authenticates itself as the BXXP peer named as
"fred@example.com", it is allowed to attach as the IMXP endpoint
named as "fred@example.com".
o In the endpoint-relay mode, when an application wishes to send
data, local provisioning maps attached endpoints to allowed
originators (c.f., Step 1 of Section 4.4.3.1).
Typically, the identity function is used, e.g., if an application
attaches as the IMXP endpoint named as "fred@example.com", it is
allowed to send data originating from the same IMXP endpoint.
However, other policies are permissible, for example, the
administrative domain may allow the application attached as the
IMXP endpoint named as "wilma@example.com" to send data
originating as either "wilma@example.com" or "fred@example.com".
o In the relay-relay mode, when a relay is sending data, no access
policies, per se, are applied.
o In the relay-relay mode, when a relay is receiving data, local
provisioning maps BXXP peer identities to allowed originators
(c.f., Step 1 of Section 4.4.3.1).
Typically, if a BXXP peer authenticates itself as being from the
same administrative domain as the originator of the data, or if
the BXXP peer authenticates itself as being from a "trusted"
intermediate relay, then the data is accepted.
o Finally, when a relay is delivering to an endpoint within its own
administrative domain, it consults the recipient's access entry
looking for an entry having the originator as an actor (c.f.,
Step 3.3 of Section 4.4.3.1).
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5. IMXP Options
IMXP, at its core, provides a best-effort datagram service. Options
are used to alter the semantics of the core service.
The semantics of the IMXP "option" element are context-specific.
Accordingly, the specification of an IMXP option must define:
o the identity of the option;
o the context in which the option may appear;
o what content, if any, is contained within the option; and,
o the processing rules for the option.
An option registration template (Section 8) organizes this
information.
An "option" element is contained within either a "data" element or a
"recipient" element, either of which is termed the "containing
element". The "option" element has several attributes and contains
arbitrary content:
o the "internal" and the "external" attributes, exactly one of
which is present, uniquely identify the option;
o the "targetHop" attribute specifies which relays should process
the option;
o the "seeNoEvil" attribute specifies whether the option, if
unrecognized, may be safely ignored;
o the "transID" attribute specifies a transaction-identifier for
the option; and,
o the "localize" attribute, if present, specifies one or more
language tokens, each identifying a desirable language tag to be
used if textual diagnostics are returned to the originator.
The value of the "internal" attribute is the IANA-registered name
for the option. If the "internal" attribute is not present, then the
value of the "external" attribute is a URI or URI with a
fragment-identifier. Note that a relative-URI value is not allowed.
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The "targetHop" attribute specifies which relay(s) should process
the option:
this: the option applies to this relay, and must be removed prior
to transmitting the containing element.
final: the option applies to this relay, only if the relay will
not transmit the containing element to another relay.
all: the option applies to this relay and is retained for the
next.
Note that in all cases, prior to transmitting the containing element
to an endpoint, all options are removed.
The "seeNoEvil" attribute specifies whether the relay may ignore the
option if it is unrecognized, and is consulted only if the
"targetHop" attribute indicates that the option applies to that
relay. If the option applies, and if the value of the "seeNoEvil"
attribute is "false", and if the relay does not "understand" the
option, then this is considered a processing error.
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5.1 The statusRequest Option
Section 11 contains the IMXP option registration for the
"statusRequest" option.
If this option is present, then each applicable relay sends a
"statusResponse" message to the originator. This is done by issuing
a data operation whose originator is the report service associated
with the issuing relay, whose recipient is the endpoint address of
the "statusRequest" originator, and whose content is a
"statusResponse" element.
A "statusRequest" option MUST NOT be present in any data operation
containing a "statusResponse" element.
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Consider these examples:
+-------+ +-------+
| | -- data -------> | |
| appl. | | relay |
| #1 | <--------- ok -- | |
+-------+ +-------+
C: <data originator='fred@example.com' content='cid:1@example.com'>
<recipient identity='barney@example.com' />
<option internal='statusRequest' targetHop='final'
seeNoEvil='false' transID='86' />
</data>
S: <ok />
+-------+ +-------+
| | -- data -------> | |
| relay | | appl. |
| | <--------- ok -- | #2 |
+-------+ +-------+
C: <data originator='fred@example.com' content='cid:1@example.com'>
<recipient identity='barney@example.com' />
</data>
S: <ok />
+-------+ +-------+
| | <------- data -- | |
| appl. | | relay |
| #1 | -- ok ---------> | |
+-------+ +-------+
C: <data originator='imxp=report@example.com' content='#Content'>
<recipient identity='fred@example.com' />
<data-content Name='Content'>
<statusResponse transID='86'>
<destination identity='barney@example.com'>
<reply code='250' />
</destination>
<statusResponse>
</data-content>
</data>
S: <ok />
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or
+-------+ +-------+
| | -- data -------> | |
| appl. | | relay |
| | <--------- ok -- | #1 |
+-------+ +-------+
C: <data originator='fred@example.com' content='cid:1@example.com'>
<recipient identity='barney@rubble.com' />
<option internal='statusRequest' targetHop='final'
seeNoEvil='false' transID='86' />
</data>
S: <ok />
+-------+ +-------+
| | -- data -------> | |
| relay | | relay |
| #1 | <------ error -- | #2 |
+-------+ +-------+
C: <data originator='fred@example.com' content='cid:1@example.com'>
<recipient identity='barney@rubble.com' />
<option internal='statusRequest' targetHop='final'
seeNoEvil='false' transID='86' />
</data>
S: <error code='550'>unknown endpoint identity</error>
+-------+ +-------+
| | <------- data -- | |
| appl. | | relay |
| | -- ok ---------> | #1 |
+-------+ +-------+
C: <data originator='imxp=report@example.com' content='#Content'>
<recipient identity='fred@example.com' />
<data-content Name='Content'>
<statusResponse transID='86'>
<destination identity='barney@example.com'>
<reply code='550'>unknown endpoint
identity</reply>
</destination>
<statusResponse>
</data-content>
</data>
S: <ok />
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or
+-------+ +-------+
| | -- data -------> | |
| appl. | | relay |
| | <--------- ok -- | #1 |
+-------+ +-------+
C: <data originator='fred@example.com' content='cid:1@example.com'>
<recipient identity='barney@rubble.com' />
<option internal='statusRequest' targetHop='final'
seeNoEvil='false' transID='86' />
</data>
S: <ok />
+-------+ +-------+
| | -- data -------> | |
| relay | | relay |
| #1 | <--------- ok -- | #2 |
+-------+ +-------+
C: <data originator='fred@example.com' content='cid:1@example.com'>
<recipient identity='barney@rubble.com' />
<option internal='statusRequest' targetHop='final'
seeNoEvil='false' transID='86' />
</data>
S: <ok />
+-------+ +-------+
| | <------- data -- | |
| relay | | relay |
| #1 | -- ok ---------> | #2 |
+-------+ +-------+
C: <data originator='imxp=report@rubble.com' content='#Content'>
<recipient identity='fred@example.com' />
<data-content Name='Content'>
<statusResponse transID='86'>
<destination identity='barney@rubble.com'>
<reply code='250' />
</destination>
<statusResponse>
</data-content>
</data>
S: <ok />
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+-------+ +-------+
| | -- data -------> | |
| appl. | | relay |
| | <--------- ok -- | #1 |
+-------+ +-------+
C: <data originator='imxp=report@rubble.com' content='#Content'>
<recipient identity='fred@example.com' />
<data-content Name='Content'>
<statusResponse transID='86'>
<destination identity='barney@rubble.com'>
<reply code='250' />
</destination>
<statusResponse>
</data-content>
</data>
S: <ok />
Note that a trace of a data's passage through the relaying mesh can
be achieved by setting the "targetHop" attribute to "all".
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6. IMXP Services
IMXP, at its core, provides a best-effort datagram service. Errors
are reported through the use of a co-resident IMXP report service.
All other IMXP services are provided on top of the relaying mesh,
e.g.,
+----------+ +----------+ +----------+
| IMXP | | IMXP | | |
| access | | presence | | ... |
| service | | service | | |
+----------+ +----------+ +----------+
| | |
| | |
+------------------------------------------------+---------+
| | IMXP |
| IMXP core | report |
| | service |
+------------------------------------------------+---------+
Applications communicate with IMXP services by sending data to a
"well-known endpoint" (WKE).
The specification of an IMXP service must define:
o the WKE of the service;
o the syntax of messages exchanged with the service;
o the sequence of messages exchanged with the service; and,
o what access control tokens are consulted by the service.
A service registration template (Section 9) organizes this
information.
Note that both the IMXP access[9] and presence[10] services are
logically layered above the IMXP core; however, implementers may
choose to physically co-reside these services with IMXP relay
software.
Finally, note that within a single administrative domain, the
relaying mesh makes use of the IMXP access service in order to
determine if an originator is allowed to transmit data to a
recipient (c.f., Step 3.3 of Section 4.4.3.1)
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6.1 Use of the IMXP Core DTD
The specification of an IMXP service may use definitions found in
the IMXP core DTD (Section 13). For example, the reply operation
(Section 6.1.2) is defined to provide a common format for responses.
6.1.1 Transaction-Identifiers
In using IMXP's transaction-identifiers, note the following:
o In the endpoint-relay mode, transaction-identifiers are
meaningful only during the lifetime of a BXXP channel.
For example, when an application issues the attach operation, the
associated transaction-identifier has meaning only within the
context of the BXXP channel used for the attach operation. When
the BXXP connection is released, the channel no longer exists and
the application is no longer attached to the relaying mesh.
o In contrast, when an application communicates with an IMXP
service, transaction-identifiers are often embedded in the data
that is sent. This means that transaction-identifiers are
potentially long-lived.
For example, an application may attach as an endpoint, send data
(containing an embedded transaction-identifier) to a service,
and, some time later, detach from the relaying mesh. Later on, a
second application may attach as the same endpoint, and send data
of its own (also containing embedded transaction-identifiers).
Subsequently, the second application may receive data from the
service responding to the first application's request and
containing the transaction-identifier used by the first
application.
To minimize the likelihood of ambiguities with long-lived
transaction-identifiers, the values of transaction-identifiers
generated by applications should appear to be unpredictable.
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6.1.2 The Reply Operation
Many IMXP services make use of a reply operation. Accordingly,
Section 13 contains a definition of a "reply" element that can be
used for this purpose.
The "reply" element has a "code" attribute, a "transID" attribute,
an optional "xml:lang" attribute, and may contain arbitrary textual
content:
o the "code" element specifies a three-digit reply code (c.f.,
Section 7);
o the "transID" attribute specifies the transaction-identifier
corresponding to this reply;
o the "xml:lang" attribute, if present, specifies the language that
the element's content is written in; and,
o the textual content is a diagnostic (possibly multiline) which is
meaningful to implementers, perhaps administrators, and possibly
even users.
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6.2 The Report Service
Section 12 contains the IMXP service registration for the report
service:
o Within an administrative domain, the service is addressed using
the well-known endpoint of "imxp=report".
o Section 14 defines the syntax of the operations exchanged with
the service.
o A consumer of the service does not initiate communications with
the service.
o The service initiates communications by sending data containing
the "statusResponse" operation.
Unlike other IMXP services, the report service is co-resident with
the IMXP core -- the report service is provided by each and every
relay.
If a relay processes a "statusRequest" option (Section 5.1), then it
sends data to the originator containing a "statusResponse" element
(Section 14).
The "statusResponse" element has a "transID" attribute and contains
one or more "destination" elements:
o the "transID" attribute specifies the value contained in the
"statusRequest" option; and,
o each "destination" element has an "identity" attribute and
contains a "reply" element:
* the "identity" attribute specifies the recipient endpoint that
is being reported on; and,
* the "reply" element (Section 6.1.2) specifies the delivery
status of that recipient.
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7. IMXP Reply Codes
code meaning
==== =======
250 transaction successful
421 service not available
450 requested action not taken
451 requested action aborted
454 temporary authentication failure
500 general syntax error (e.g., poorly-formed XML)
501 syntax error in parameters (e.g., non-valid XML)
504 parameter not implemented
530 authentication required
534 authentication mechanism insufficient
535 authentication failure
537 action not authorized for user
538 authentication mechanism requires encryption
550 requested action not taken
553 parameter invalid
554 transaction failed (e.g., policy violation)
555 transaction already in progress
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8. IMXP Option Registration Template
When an IMXP option is registered, the following information is
supplied:
Option Identification: specify the NMTOKEN or the URI that
authoritatively identifies this option.
Present in: specify the IMXP elements in which the option may appear.
Contains: specify the XML content that is contained within the
"option" element.
Processing Rules: specify the processing rules associated with the
option.
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9. IMXP Service Registration Template
When an IMXP service is registered, the following information is
supplied:
Well-Known Endpoint: specify the local-part of an endpoint identity,
starting with "imxp=".
Syntax of Messages Exchanged: specify the elements exchanged with
the service.
Sequence of Messages Exchanged: specify the order in which data is
exchanged with the service.
Access Control Tokens: specify the token(s) used to control access
to the service (c.f., [9]).
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10. Registration: The IMXP Profile
Profile Identification: http://xml.resource.org/profiles/IMXP
Elements Exchanged during Channel Creation: none required
Messages in "REQ" frames: "attach", "terminate", "data"
Messages in positive "RSP" frames: "ok"
Messages in negative "RSP" frames: "error"
Message Syntax: c.f., Section 13
Message Semantics: c.f., Section 4.4
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11. Registration: The statusRequest Option
Option Identification: statusRequest
Present in: IMXP's "data" and "recipient" elements
Contains: nothing
Processing Rules: c.f., Section 5.1
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12. Registration: The Report Service
Well-Known Endpoint: imxp=report
Syntax of Messages Exchanged: c.f., Section 14
Sequence of Messages Exchanged: c.f., Section 6.2
Access Control Tokens: none
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13. The IMXP Core DTD
<!--
DTD for the IMXP core, as of 2000-06-11
Refer to this DTD as:
<!ENTITY % IMXPCORE PUBLIC "-//Blocks//DTD IMXP CORE//EN"
"http://xml.resource.org/profiles/IMXP/imxp-core.dtd">
%IMXPCORE;
-->
<!ENTITY % BXXP PUBLIC "-//Blocks//DTD BXXP//EN"
"http://xml.resource.org/profiles/BXXP/bxxp.dtd">
%BXXP;
<!--
DTD data types:
entity syntax/reference example
====== ================ =======
IMXP endpoint
ENDPOINT addr-spec, fred@example.com
c.f., [RFC-0822]
seconds
SECONDS 0..2147483647 600
timestamp
TIMESTAMP date-time, 14 May 2000 13:02:00 -0800
c.f., [RFC-1123]
transaction-identifier
TRANSID 1..2147483647 42
-->
<!ENTITY % ENDPOINT "CDATA">
<!ENTITY % SECONDS "CDATA">
<!ENTITY % TIMESTAMP "CDATA">
<!ENTITY % TRANSID "CDATA">
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<!--
BXXP profile http://xml.resource.org/profiles/IMXP
role REQ RSP
======= === ===
I attach +: ok
-: error
I or L terminate +: ok
-: error
I or L data +: ok
-: error
-->
<!ELEMENT attach (recipient)>
<!ATTLIST attach
transID %TRANSID; #REQUIRED>
<!ELEMENT terminate EMPTY>
<!ATTLIST terminate
transID %TRANSID; #REQUIRED>
<!ELEMENT data (recipient+,option*,data-content?)>
<!ATTLIST data
originator %ENDPOINT; #REQUIRED
content %URI; #REQUIRED>
<!ELEMENT recipient (option*)>
<!ATTLIST recipient
identity %ENDPOINT; #REQUIRED>
<!ELEMENT data-content
ANY>
<!ATTLIST Name ID #REQUIRED>
<!ELEMENT ok EMPTY>
<!ELEMENT reply (#PCDATA)*>
<!ATTLIST reply
code %XYZ; #REQUIRED
transID %TRANSID; #REQUIRED
xml:lang %LANG; #IMPLIED>
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<!-- either the "internal" or the "external" attribute is present in
an option -->
<!ELEMENT option ANY>
<!ATTLIST option
internal NMTOKEN ""
external %URI; ""
targetHop (this|final|all) "final"
seeNoEvil (true|false) "true"
transID %TRANSID; #REQUIRED
localize %LOCS; "i-default">
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14. The Report Service DTD
<!--
DTD for the IMXP report service, as of 2000-06-11
Refer to this DTD as:
<!ENTITY % IMXPREPORT PUBLIC "-//Blocks//DTD IMXP REPORT//EN"
"http://xml.resource.org/profiles/IMXP/imxp-report.dtd">
%IMXPREPORT;
-->
<!ENTITY % IMXPCORE PUBLIC "-//Blocks//DTD IMXP CORE//EN"
"http://xml.resource.org/profiles/IMXP/imxp-core.dtd">
%IMXPCORE;
<!--
Synopsis of the IMXP report service
service WKE: imxp=report
message exchanges:
service initiates consumer replies
================= ================
statusResponse (nothing)
access control tokens: none
-->
<!ELEMENT statusResponse
(destination+)>
<!ATTLIST statusResponse
transID %TRANSID; #REQUIRED>
<!ELEMENT destination (reply)>
<!ATTLIST destination
identity %ENDPOINT; #REQUIRED>
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15. Security Considerations
Consult Section 3 and Section 4.5 for a discussion of security
issues, e.g., routing integrity. In addition, since IMXP is a
profile of the BXXP, consult [1]'s Section 8 for a discussion of
BXXP-specific security issues.
In addition, the statusRequest option (Section 5.1) may be used to
expose private network topology. Accordingly, administrators may
wish to choose to disable this option except at the ingress/egress
points for their domain.
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16. IANA Considerations
The IANA assigns the "im" URL scheme to identify IMXP endpoints.
The IANA maintains a list of:
o IMXP options, c.f., Section 5 and Section 8; and,
o IMXP services, c.f., Section 6 and Section 9.
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References
[1] Rose, M.T., "The Blocks eXtensible eXchange Protocol
Framework", draft-mrose-blocks-protocol-03 (work in progress),
May 2000.
[2] Crocker, D., "Standard for the format of ARPA Internet text
messages", RFC 822, STD 11, Aug 1982.
[3] Allocchio, C., "Minimal PSTN address format in Internet Mail",
RFC 2303, March 1998.
[4] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[5] Berners-Lee, T., Fielding, R.T. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998.
[6] Levinson, E., "The MIME Multipart/Related Content-type", RFC
2387, August 1998.
[7] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[8] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[9] Rose, M.T., Klyne, G. and D.H. Crocker, "The IMXP Access
Service", draft-mrose-imxp-access-00 (work in progress), June
2000.
[10] Rose, M.T., Klyne, G. and D.H. Crocker, "The IMXP Presence
Service", draft-mrose-imxp-presence-00 (work in progress),
June 2000.
[11] mailto:dnew@san.rr.com
[12] mailto:spead@novell.com
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Authors' Addresses
Marshall T. Rose
Invisible Worlds, Inc.
1179 North McDowell Boulevard
Petaluma, CA 94954-6559
US
Phone: +1 707 789 3700
EMail: mrose@invisible.net
URI: http://invisible.net/
Graham Klyne
Content Technologies Limited
1220 Parkview
Arlington Business Park
Theale, Reading RG7 4SA
UK
Phone: +44 118 930 1300
EMail: gk@acm.org
David H. Crocker
Brandenburg Consulting
675 Spruce Drive
Sunnyvale, CA 94086
US
Phone: +1 408 246 8253
EMail: dcrocker@brandenburg.com
URI: http://www.brandenburg.com/
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Appendix A. Acknowledgements
The authors gratefully acknowledge the contributions of: Darren
New[11] and Scott Pead[12].
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Full Copyright Statement
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This document and translations of it may be copied and furnished to
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Acknowledgement
Funding for the RFC editor function is currently provided by the
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Rose, et. al. Expires December 10, 2000 [Page 49]