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Keywords: MIME, multipurpose, Internet, mail extensions, service, users
Network Working Group M. Day
Request for Comments: 2779 Lotus
Category: Informational S. Aggarwal
Microsoft
G. Mohr
Activerse
J. Vincent
Into Networks
February 2000
Instant Messaging / Presence Protocol Requirements
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
Presence and Instant Messaging have recently emerged as a new medium
of communications over the Internet. Presence is a means for
finding, retrieving, and subscribing to changes in the presence
information (e.g. "online" or "offline") of other users. Instant
messaging is a means for sending small, simple messages that are
delivered immediately to online users.
Applications of presence and instant messaging currently use
independent, non-standard and non-interoperable protocols developed
by various vendors. The goal of the Instant Messaging and Presence
Protocol (IMPP) Working Group is to define a standard protocol so
that independently developed applications of instant messaging and/or
presence can interoperate across the Internet. This document defines
a minimal set of requirements that IMPP must meet.
Day, et al. Informational [Page 1]
RFC 2779 Instant Messaging/Presence Protocol February 2000
Table of Contents
1. Terminology................................................... 3
2. Shared Requirements........................................... 4
2.1. Namespace and Administration............................... 5
2.2. Scalability................................................ 5
2.3. Access Control............................................. 6
2.4. Network Topology........................................... 6
2.5. Message Encryption and Authentication...................... 7
3. Additional Requirements for PRESENCE INFORMATION.............. 7
3.1. Common Presence Format..................................... 7
3.2. Presence Lookup and Notification........................... 8
3.3. Presence Caching and Replication........................... 8
3.4. Performance................................................ 9
4. Additional Requirements for INSTANT MESSAGES.................. 9
4.1. Common Message Format...................................... 9
4.2. Reliability................................................ 10
4.3. Performance................................................ 10
4.4. Presence Format............................................ 10
5. Security Considerations....................................... 11
5.1. Requirements related to SUBSCRIPTIONS...................... 11
5.2. Requirements related to NOTIFICATION....................... 12
5.3. Requirements related to receiving a NOTIFICATION........... 13
5.4. Requirements related to INSTANT MESSAGES................... 13
6. References.................................................... 14
7. Authors' Addresses............................................ 15
8. Appendix: Security Expectations and Deriving Requirements..... 16
8.1. Presence Information....................................... 16
8.1.1. Subscription............................................ 16
8.1.2. Publication............................................. 19
8.1.3. Publication for Notification............................ 19
8.1.4. Receiving a Notification................................ 20
8.2. Instant Messaging.......................................... 21
8.2.1. Named Instant Messaging................................. 21
8.2.2. Anonymous Instant Messaging............................. 23
8.2.3. Administrator Expectations.............................. 24
Full Copyright Statement......................................... 26
Day, et al. Informational [Page 2]
RFC 2779 Instant Messaging/Presence Protocol February 2000
1. Terminology
The following terms are defined in [RFC 2778] and are used with those
definitions in this document:
ACCESS RULES
CLOSED
FETCHER
INSTANT INBOX
INSTANT MESSAGE
NOTIFICATION
OPEN
POLLER
PRESENCE INFORMATION
PRESENCE SERVICE
PRESENTITY
PRINCIPAL
PROXY
SERVER
STATUS
SUBSCRIBER
SUBSCRIPTION
WATCHER
The terms MUST and SHOULD are used in the following sense while
specifying requirements:
MUST: A proposed solution will have to meet this requirement.
SHOULD: A proposed solution may choose not to meet this requirement.
Note that this usage of MUST and SHOULD differs from that of RFC
2119.
Additionally, the following terms are used in this document and
defined here:
ADMINISTRATOR: A PRINCIPAL with authority over local computer and
network resources, who manages local DOMAINS or FIREWALLS. For
security and other purposes, an ADMINISTRATOR often needs or wants to
impose restrictions on network usage based on traffic type, content,
volume, or endpoints. A PRINCIPAL's ADMINISTRATOR has authority over
some or all of that PRINCIPAL's computer and network resources.
DOMAIN: A portion of a NAMESPACE.
ENTITY: Any of PRESENTITY, SUBSCRIBER, FETCHER, POLLER, or WATCHER
(all defined in [RFC 2778]).
Day, et al. Informational [Page 3]
RFC 2779 Instant Messaging/Presence Protocol February 2000
FIREWALL: A point of administrative control over connectivity.
Depending on the policies being enforced, parties may need to take
unusual measures to establish communications through the FIREWALL.
IDENTIFIER: A means of indicating a point of contact, intended for
public use such as on a business card. Telephone numbers, email
addresses, and typical home page URLs are all examples of IDENTIFIERS
in other systems. Numeric IP addresses like 10.0.0.26 are not, and
neither are URLs containing numerous CGI parameters or long arbitrary
identifiers.
INTENDED RECIPIENT: The PRINCIPAL to whom the sender of an INSTANT
MESSAGE is sending it.
NAMESPACE: The system that maps from a name of an ENTITY to the
concrete implementation of that ENTITY. A NAMESPACE may be composed
of a number of distinct DOMAINS.
OUT OF CONTACT: A situation in which some ENTITY and the PRESENCE
SERVICE cannot communicate.
SUCCESSFUL DELIVERY: A situation in which an INSTANT MESSAGE was
transmitted to an INSTANT INBOX for the INTENDED RECIPIENT, and the
INSTANT INBOX acknowledged its receipt. SUCCESSFUL DELIVERY usually
also implies that an INBOX USER AGENT has handled the message in a
way chosen by the PRINCIPAL. However, SUCCESSFUL DELIVERY does not
imply that the message was actually seen by that PRINCIPAL.
2. Shared Requirements
This section describes non-security requirements that are common to
both an PRESENCE SERVICE and an INSTANT MESSAGE SERVICE. Section 6
describes requirements specific to a PRESENCE SERVICE, while Section
7 describes requirements specific to an INSTANT MESSAGE SERVICE.
Section 8 describes security considerations. The reader should note
that Section 11 is an appendix that provides historical context and
aids in tracing the origins of requirements in Section 8. Section 11
is not, however, a statement of current IMPP requirements.
It is expected that Presence and Instant Messaging services will be
particularly valuable to users over mobile IP wireless access
devices. Indeed the number of devices connected to the Internet via
wireless means is expected to grow substantially in the coming years.
It is not reasonable to assume that separate protocols will be
available for the wireless portions of the Internet. In addition, we
note that wireless infrastructure is maturing rapidly; the work
undertaken by this group should take into account the expected state
of the maturity of the technology in the time-frame in which the
Day, et al. Informational [Page 4]
RFC 2779 Instant Messaging/Presence Protocol February 2000
Presence and Instant Messaging protocols are expected to be deployed.
To this end, the protocols designed by this Working Group must be
suitable for operation in a context typically associated with mobile
wireless access devices, viz. high latency, low bandwidth and
possibly intermittent connectivity (which lead to a desire to
minimize round-trip delays), modest computing power, battery
constraints, small displays, etc. In particular, the protocols must
be designed to be reasonably efficient for small payloads.
2.1. Namespace and Administration
2.1.1. The protocols MUST allow a PRESENCE SERVICE to be available
independent of whether an INSTANT MESSAGE SERVICE is available, and
vice-versa.
2.1.2. The protocols must not assume that an INSTANT INBOX is
necessarily reached by the same IDENTIFIER as that of a PRESENTITY.
Specifically, the protocols must assume that some INSTANT INBOXes may
have no associated PRESENTITIES, and vice versa.
2.1.3. The protocols MUST also allow an INSTANT INBOX to be reached
via the same IDENTIFIER as the IDENTIFIER of some PRESENTITY.
2.1.4. The administration and naming of ENTITIES within a given
DOMAIN MUST be able to operate independently of actions in any other
DOMAIN.
2.1.5. The protocol MUST allow for an arbitrary number of DOMAINS
within the NAMESPACE.
2.2. Scalability
2.2.1. It MUST be possible for ENTITIES in one DOMAIN to interoperate
with ENTITIES in another DOMAIN, without the DOMAINS having
previously been aware of each other.
The protocol MUST be capable of meeting its other functional and
performance requirements even when
-- (2.2.2) there are millions of ENTITIES within a single DOMAIN.
-- (2.2.3) there are millions of DOMAINS within the single
NAMESPACE.
Day, et al. Informational [Page 5]
RFC 2779 Instant Messaging/Presence Protocol February 2000
-- (2.2.4) every single SUBSCRIBER has SUBSCRIPTIONS to hundreds
of PRESENTITIES.
-- (2.2.5) hundreds of distinct SUBSCRIBERS have SUBSCRIPTIONS to
a single PRESENTITY.
-- (2.2.6) every single SUBSCRIBER has SUBSCRIPTIONS to
PRESENTITIES in hundreds of distinct DOMAINS.
These are protocol design goals; implementations may choose to place
lower limits.
2.3. Access Control
The PRINCIPAL controlling a PRESENTITY MUST be able to control
-- (2.3.1) which WATCHERS can observe that PRESENTITY's PRESENCE
INFORMATION.
-- (2.3.2) which WATCHERS can have SUBSCRIPTIONS to that
PRESENTITY's PRESENCE INFORMATION.
-- (2.3.3) what PRESENCE INFORMATION a particular WATCHER will see
for that PRESENTITY, regardless of whether the WATCHER gets it
by fetching or NOTIFICATION.
-- (2.3.4) which other PRINCIPALS, if any, can update the PRESENCE
INFORMATION of that PRESENTITY.
The PRINCIPAL controlling an INSTANT INBOX MUST be able to control
-- (2.3.5) which other PRINCIPALS, if any, can send INSTANT
MESSAGES to that INSTANT INBOX.
-- (2.3.6) which other PRINCIPALS, if any, can read INSTANT
MESSAGES from that INSTANT INBOX.
2.3.7. Access control MUST be independent of presence: the PRESENCE
SERVICE MUST be able to make access control decisions even when the
PRESENTITY is OUT OF CONTACT.
2.4. Network Topology
Note that intermediaries such as PROXIES may be necessitated between
IP and non-IP networks, and by an end-user's desire to provide
anonymity and hide their IP address.
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RFC 2779 Instant Messaging/Presence Protocol February 2000
2.4.1. The protocol MUST allow the creation of a SUBSCRIPTION both
directly and via intermediaries, such as PROXIES.
2.4.2. The protocol MUST allow the sending of a NOTIFICATION both
directly and via intermediaries, such as PROXIES.
2.4.3. The protocol MUST allow the sending of an INSTANT MESSAGE both
directly and via intermediaries, such as PROXIES.
2.4.4. The protocol proxying facilities and transport practices MUST
allow ADMINISTRATORS ways to enable and disable protocol activity
through existing and commonly-deployed FIREWALLS. The protocol MUST
specify how it can be effectively filtered by such FIREWALLS.
2.5. Message Encryption and Authentication
2.5.1. The protocol MUST provide means to ensure confidence that a
received message (NOTIFICATION or INSTANT MESSAGE) has not been
corrupted or tampered with.
2.5.2. The protocol MUST provide means to ensure confidence that a
received message (NOTIFICATION or INSTANT MESSAGE) has not been
recorded and played back by an adversary.
2.5.3. The protocol MUST provide means to ensure that a sent message
(NOTIFICATION or INSTANT MESSAGE) is only readable by ENTITIES that
the sender allows.
2.5.4. The protocol MUST allow any client to use the means to ensure
non-corruption, non-playback, and privacy, but the protocol MUST NOT
require that all clients use these means at all times.
3. Additional Requirements for PRESENCE INFORMATION
The requirements in section 6 are applicable only to PRESENCE
INFORMATION and not to INSTANT MESSAGES. Additional constraints on
PRESENCE INFORMATION in a system supporting INSTANT MESSAGES appear
in Section 7.4.
3.1. Common Presence Format
3.1.1. All ENTITIES MUST produce and consume at least a common base
format for PRESENCE INFORMATION.
3.1.2. The common presence format MUST include a means to uniquely
identify the PRESENTITY whose PRESENCE INFORMATION is reported.
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3.1.3. The common presence format MUST include a means to encapsulate
contact information for the PRESENTITY's PRINCIPAL (if applicable),
such as email address, telephone number, postal address, or the like.
3.1.4. There MUST be a means of extending the common presence format
to represent additional information not included in the common
format, without undermining or rendering invalid the fields of the
common format.
3.1.5. The working group must define the extension and registration
mechanisms for presence information schema, including new STATUS
conditions and new forms for OTHER PRESENCE MARKUP.
3.1.6. The presence format SHOULD be based on IETF standards such as
vCard [RFC 2426] if possible.
3.2. Presence Lookup and Notification
3.2.1. A FETCHER MUST be able to fetch a PRESENTITY's PRESENCE
INFORMATION even when the PRESENTITY is OUT OF CONTACT.
3.2.2. A SUBSCRIBER MUST be able to request a SUBSCRIPTION to a
PRESENTITY's PRESENCE INFORMATION, even when the PRESENTITY is OUT OF
CONTACT.
3.2.3. If the PRESENCE SERVICE has SUBSCRIPTIONS for a PRESENTITY's
PRESENCE INFORMATION, and that PRESENCE INFORMATION changes, the
PRESENCE SERVICE MUST deliver a NOTIFICATION to each SUBSCRIBER,
unless prevented by the PRESENTITY's ACCESS RULES.
3.2.4. The protocol MUST provide a mechanism for detecting when a
PRESENTITY or SUBSCRIBER has gone OUT OF CONTACT.
3.2.5. The protocol MUST NOT depend on a PRESENTITY or SUBSCRIBER
gracefully telling the service that it will no longer be in
communication, since a PRESENTITY or SUBSCRIBER may go OUT OF CONTACT
due to unanticipated failures.
3.3. Presence Caching and Replication
3.3.1. The protocol MUST include mechanisms to allow PRESENCE
INFORMATION to be cached.
3.3.2. The protocol MUST include mechanisms to allow cached PRESENCE
INFORMATION to be updated when the master copy changes.
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RFC 2779 Instant Messaging/Presence Protocol February 2000
3.3.3 The protocol caching facilities MUST NOT circumvent established
ACCESS RULES or restrict choice of authentication/encryption
mechanisms.
3.4 Performance
3.4.1 When a PRESENTITY changes its PRESENCE INFORMATION, any
SUBSCRIBER to that information MUST be notified of the changed
information rapidly, except when such notification is entirely
prevented by ACCESS RULES. This requirement is met if each
SUBSCRIBER's NOTIFICATION is transported as rapidly as an INSTANT
MESSAGE would be transported to an INSTANT INBOX.
4. Additional Requirements for INSTANT MESSAGES
The requirements in section 4 are applicable only to INSTANT MESSAGES
and not to PRESENCE INFORMATION, with the exception of Section 4.4.
Section 4.4 describes constraints on PRESENCE INFORMATION that are
relevant only to systems that support both INSTANT MESSAGES and
PRESENCE INFORMATION.
4.1. Common Message Format
4.1.1. All ENTITIES sending and receiving INSTANT MESSAGES MUST
implement at least a common base format for INSTANT MESSAGES.
4.1.2. The common base format for an INSTANT MESSAGE MUST identify
the sender and intended recipient.
4.1.3. The common message format MUST include a return address for
the receiver to reply to the sender with another INSTANT MESSAGE.
4.1.4. The common message format SHOULD include standard forms of
addresses or contact means for media other than INSTANT MESSAGES,
such as telephone numbers or email addresses.
4.1.5. The common message format MUST permit the encoding and
identification of the message payload to allow for non-ASCII or
encrypted content.
4.1.6. The protocol must reflect best current practices related to
internationalization.
4.1.7. The protocol must reflect best current practices related to
accessibility.
Day, et al. Informational [Page 9]
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4.1.8. The working group MUST define the extension and registration
mechanisms for the message format, including new fields and new
schemes for INSTANT INBOX ADDRESSES.
4.1.9. The working group MUST determine whether the common message
format includes fields for numbering or identifying messages. If
there are such fields, the working group MUST define the scope within
which such identifiers are unique and the acceptable means of
generating such identifiers.
4.1.10. The common message format SHOULD be based on IETF-standard
MIME [RFC 2045].
4.2. Reliability
4.2.1. The protocol MUST include mechanisms so that a sender can be
informed of the SUCCESSFUL DELIVERY of an INSTANT MESSAGE or reasons
for failure. The working group must determine what mechanisms apply
when final delivery status is unknown, such as when a message is
relayed to non-IMPP systems.
4.3 Performance
4.3.1. The transport of INSTANT MESSAGES MUST be sufficiently rapid
to allow for comfortable conversational exchanges of short messages.
4.4 Presence Format
4.4.1. The common presence format MUST define a minimum standard
presence schema suitable for INSTANT MESSAGE SERVICES.
4.4.2. When used in a system supporting INSTANT MESSAGES, the common
presence format MUST include a means to represent the STATUS
conditions OPEN and CLOSED.
4.4.3. The STATUS conditions OPEN and CLOSED may also be applied to
messaging or communication modes other than INSTANT MESSAGE SERVICES.
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RFC 2779 Instant Messaging/Presence Protocol February 2000
5. Security Considerations
Security considerations are addressed in section 2.3, Access Control,
and section 2.5, Message authentication and encryption.
This section describes further security-related requirements that the
protocol must meet.
The security requirements were derived from a set of all-encompassing
"security expectations" that were then evaluated for practicality and
implementability and translated into requirements. In the appendix,
we describe the expectations and the process used to transform them
into requirements. In this section, we simply list the consolidated
set of derived requirements.
Note that in the requirements, ADMINISTRATORs may have privileges
beyond those allowed to PRINCIPALs referred to in the requirements.
(Unless otherwise noted, the individual expectations specifically
refer to PRINCIPALs.) It is up to individual implementations to
control administrative access and implement the security privileges
of ADMINISTRATORs without compromising the requirements made on
PRINCIPALs.
Unless noted otherwise, A,B,C are all names of non-ADMINISTRATOR
PRINCIPALS.
5.1. Requirements related to SUBSCRIPTIONS
When A establishes a SUBSCRIPTION to B's PRESENCE INFORMATION:
5.1.1. The protocol MUST provide A means of identifying and
authenticating that the PRESENTITY subscribed to is controlled by B.
5.1.2. If A so chooses, the protocol SHOULD NOT make A's SUBSCRIPTION
to B obvious to a third party C.
5.1.3. The protocol MUST provide B with means of allowing an
unauthenticated subscription by A.
5.1.4. The protocol MUST provide A means of verifying the accurate
receipt of the content B chooses to disclose to A.
5.1.5. B MUST inform A if B refuses A's SUBSCRIPTION. Note that B may
choose to accept A's SUBSCRIPTION, but fail to deliver any
information to it (so-called "polite blocking"). See 5.1.15.
5.1.6. The protocol MUST NOT let any third party C force A to
subscribe to B's PRESENCE INFORMATION without A's consent.
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5.1.7. A MUST be able to cancel her SUBSCRIPTION to B's PRESENCE
INFORMATION at any time and for any reason. When A does so, the
PRESENCE SERVICE stops informing A of changes to B's PRESENCE
INFORMATION.
5.1.8. The protocol MUST NOT let an unauthorized party C cancel A's
SUBSCRIPTION to B.
5.1.9. If A's SUBSCRIPTION to B is cancelled, the service SHOULD
inform A of the cancellation.
5.1.10. A SHOULD be able to determine the status of A's SUBSCRIPTION
to B, at any time.
5.1.11. The protocol MUST provide B means of learning about A's
SUBSCRIPTION to B, both at the time of establishing the SUBSCRIPTION
and afterwards.
5.1.12. The protocol MUST provide B means of identifying and
authenticating the SUBSCRIBER's PRINCIPAL, A.
5.1.13. It MUST be possible for B to prevent any particular PRINCIPAL
from subscribing.
5.1.14. It MUST be possible for B to prevent anonymous PRINCIPALS
from subscribing.
5.1.15. It MUST be possible for B to configure the PRESENCE SERVICE
to deny A's subscription while appearing to A as if the subscription
has been granted (this is sometimes called "polite blocking"). The
protocol MUST NOT mandate the PRESENCE SERVICE to service
subscriptions that are treated in this manner.
5.1.16. B MUST be able to cancel A's subscription at will.
5.1.17. The protocol MUST NOT require A to reveal A's IP address to
B.
5.1.18 The protocol MUST NOT require B to reveal B's IP address to A.
5.2. Requirements related to NOTIFICATION
When a PRINCIPAL B publishes PRESENCE INFORMATION for NOTIFICATION to
another PRINCIPAL A:
5.2.1. The protocol MUST provide means of ensuring that only the
PRINCIPAL A being sent the NOTIFICATION by B can read the
NOTIFICATION.
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5.2.2. A should receive all NOTIFICATIONS intended for her.
5.2.3. It MUST be possible for B to prevent A from receiving
notifications, even if A is ordinarily permitted to see such
notifications. It MUST be possible for B to, at its choosing, notify
different subscribers differently, through different notification
mechanisms or through publishing different content. This is a
variation on "polite blocking".
5.2.4. The protocol MUST provide means of protecting B from another
PRINCIPAL C "spoofing" notification messages about B.
5.2.5. The protocol MUST NOT require that A reveal A's IP address to
B.
5.2.6. The protocol MUST NOT require that B reveal B's IP address to
A.
5.3. Requirements related to receiving a NOTIFICATION
When a PRINCIPAL A receives a notification message from another
principal B, conveying PRESENCE INFORMATION,
5.3.1. The protocol MUST provide A means of verifying that the
presence information is accurate, as sent by B.
5.3.2. The protocol MUST ensure that A is only sent NOTIFICATIONS
from entities she has subscribed to.
5.3.3. The protocol MUST provide A means of verifying that the
notification was sent by B.
5.4. Requirements related to INSTANT MESSAGES
When a user A sends an INSTANT MESSAGE M to another user B,
5.4.1. A MUST receive confirmation of non-delivery.
5.4.2. If M is delivered, B MUST receive the message only once.
5.4.3. The protocol MUST provide B means of verifying that A sent the
message.
5.4.4. B MUST be able to reply to the message via another instant
message.
5.4.5. The protocol MUST NOT always require A to reveal A's IP
address, for A to send an instant message.
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5.4.6. The protocol MUST provide A means of ensuring that no other
PRINCIPAL C can see the content of M.
5.4.7. The protocol MUST provide A means of ensuring that no other
PRINCIPAL C can tamper with M, and B means to verify that no
tampering has occurred.
5.4.8. B must be able to read M.
5.4.9. The protocol MUST allow A to sign the message, using existing
standards for digital signatures.
5.4.10. B MUST be able to prevent A from sending him messages
6. References
[RFC 2778] Day, M., Rosenberg, J. and H. Sagano, "A Model for
Presence and Instant Messaging", RFC 2778, February 2000.
[RFC 2426] Dawson, F. and T. Howes, "vCard MIME Directory Profile",
RFC 2426, September 1998.
[RFC 2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) - Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC 2119] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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7. Authors' Addresses
Mark Day
SightPath, Inc.
135 Beaver Street
Waltham, MA 02452
USA
EMail: mday@alum.mit.edu
(Formerly Mark_Day@lotus.com)
Sonu Aggarwal
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
EMail: sonuag@microsoft.com
Gordon Mohr
EMail: gojomo@usa.net
(Formerly gojomo@activerse.com)
Jesse Vincent
Into Networks, Inc.
150 Cambridgepark Drive
Cambridge, MA 02140
USA
EMail: jesse@intonet.com
(Formerly jvincent@microsoft.com)
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8. Appendix: Security Expectations and Deriving Requirements
This appendix is based on the security expectations discussed on the
impp mailing list and assembled by Jesse Vincent. The original form
of numbering has been preserved in this appendix (so there are
several different items labeled B1, for example). The derived
requirements have new numbers that are consistent with the main body
of the document. This appendix is included to provide a connection
from discussions on the list to the requirements of Section 8, but it
is not intended to introduce any new requirements beyond those
presented in Sections 5 through 8.
8.1. PRESENCE INFORMATION
In the case of PRESENCE INFORMATION, the controlling PRINCIPAL's
privacy interests are paramount; we agreed that "polite blocking"
(denying without saying that the subscription is denied, or providing
false information) should be possible.
8.1.1. Subscription
When a user Alice subscribes to another person, Bob's presence info,
Alice expects:
A1. the PRESENTITY's PRINCIPAL, B, is identifiable and authenticated
Discussion: Stands as a requirement. Note that the protocol
should provide Alice the capability of authenticating, without
requiring that Alice authenticate every SUBSCRIPTION. This
caveat is made necessary by performance concerns, among others,
and applies to many of the other requirements derived below.
[Requirement 5.1.1]
A2. no third party will know that A has subscribed to B.
Discussion: This is somewhat unreasonable to enforce as is. For
example, in some topologies, nothing can prevent someone doing
traffic analysis to deduce that A has subscribed to B. We should
merely require that the protocol not expose subscription
information in any obvious manner. [Requirement 5.1.2]
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A3. A has the capability to subscribe to B's presence without B's
knowledge, if B permits anonymous subscriptions.
Discussion: An "anonymous subscription" above can have two
implications - (i) B may allow an unauthenticated subscription by
A, and (ii) B may be unaware of A's stated identity. Requirement
(i) is reasonable [Requirement 8.1.3], but (ii) doesn't appear to
be a core requirement -- it can be adequately simulated via a
subscription pseudonym.
A4. A will accurately receive what B chooses to disclose to A
regarding B's presence.
Discussion: Stands as a requirement, with the "optional"
caveat. [Requirement 8.1.4]
A5. B will inform A if B refuses A's subscription
Discussion: Stands as a requirement. [Requirement 5.1.5]
A6. No third party, C can force A to subscribe to B's presence
without A's consent.
Discussion: Stands as a requirement. [Requirement 5.1.6]
A7. A can cancel her subscription to B's presence at any time and for
any reason. When A does so, she will receive no further information
about B's presence information.
Discussion: This essentially stands. However, implementations
may have to contend with a timing window where A receives, after
sending her cancellation request, a notification sent by B before
B received the cancellation request. Therefore, the requirement
should focus on B's ceasing to send presence information, rather
than A's ceasing to receive it. [Requirement 5.1.7]
A8. no third party, C, can cancel A's subscription to B.
Discussion: Stands, although the administrative exception does
apply. [Requirement 5.1.8]
A9. A is notified if her subscription to B is cancelled for any
reason.
Discussion: Although the intent is reasonable, there are a number
of scenarios (e.g. overburdened server, clogged network, server
crash) where delivering a notification to A of the cancellation
is undesirable or impossible. Therefore, the service should make
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an attempt to inform, but this is not required. [Requirement
5.1.9]
Bob expects:
B1. B will be informed that A subscribed to B's presence information,
as long as A has not subscribed anonymously.
Discussion: This essentially stands. However, B can also choose
to determine A's subscription after the fact. [Requirement
5.1.10]
B2. A is identifiable and authenticated.
Discussion: This stands as a requirement. [Requirement 5.1.11]
B3. B can prevent a particular user, D, from subscribing.
Discussion: This stands as a requirement. [Requirement 5.1.12]
B4. B can prevent anonymous users from subscribing.
Discussion: This stands as a requirement. [Requirement 5.1.13]
B5. B's presence information is not republished by A to a third
party, E, who does not.
Discussion: This is practically impossible to enforce, so it is
omitted from the requirement set.
B6. B can deny A's subscription without letting A know that she's
been blocked.
Discussion: This "polite blocking" capability essentially stands;
accepting a "denied" subscription should bear no implication on
servicing it for status notifications. [Requirement 5.1.14]
B7. B can cancel A's subscription at will.
Discussion: Stands as a requirement. [Requirement 5.1.15]
Charlie, bob's network administrator expects:
C1. C knows who is subscribed to B at all times.
Discussion: Administrators should be able to determine who is
subscribed, but needn't be continuously informed of the list of
subscribers. Also, in some cases user agents (e.g. proxies) may
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have subscribed on behalf of users, and in these cases the
administrator can only determine the identity of these agents,
not their users. [Requirement 5.1.16]
C2. C can manage all aspects of A's presence information.
Discussion: This stands as a requirement. [Requirement 5.1.17]
C3. C can control who can access A's presence information and
exchange instant messages with A.
Discussion: This stands in principle, but C should be able to
waive these capabilities if C desires. [Requirement 5.1.18]
8.1.2. Publication
The publisher of status information, Bob, expects:
B1. That information about B is not provided to any entity without
B's knowledge and consent.
Discussion: This is nearly impossible to accomplish, so it is
omitted from the requirements.
8.1.3. Publication for Notification
When information is published for notification, B expects:
B1. only a person being sent a notification, A, can read the
notification.
Discussion: Stands as a requirement. [Requirement 5.2.1]
B2. A reliably receives all notifications intended for her.
Discussion: This stands, although "Reliably" is a little strong
(e.g. network outages, etc.). [Requirement 5.2.2]
B3. B can prevent A from receiving notifications, even if A is
ordinarily permitted to see such notifications. This is a variation
on "polite blocking."
Discussion: This stands as a requirement. Also incorporated into
this requirement is the notifications equivalent of the next
expectation, B4. [Requirement 5.2.3]
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B4. B can provide two interested parties A and E with different
status information at the same time. (B could represent the same
event differently to different people.)
Discussion: This stands as a requirement; it has been
incorporated into the corresponding requirement for B3 above.
B5. B expects that malicious C cannot spoof notification messages
about B.
Discussion: Stands in principle, but it should be optional for B.
[Requirement 5.2.4]
8.1.4. Receiving a Notification
When Alice receives a notification, the recipient, Alice, expects:
A1. That the notification information is accurate, truthful.
Discussion: Stands in principle, although being "truthful" can't
be a requirement, and the verification is optional for Alice.
[Requirement 5.3.1]
A2. That information about subscriptions remains private; people do
not learn that A's subscription to B's information exists by watching
notifications occur.
Discussion: This is omitted from the requirements, as traffic
analysis, even of encrypted traffic, can convey this information
in some situations.
A3. That she only receives notifications of things she's subscribed
to.
Discussion: Stands as a requirement. [Requirement 5.3.2]
A4. Notifications come from the apparent sender, B.
Discussion: Stands in principle, although the verification should
be optional for A. [Requirement 5.3.3]
A5. A can tell the difference between a message generated by the
user, and a message legitimately generated by the agent on behalf of
the user.
Discussion: This could be quite difficult to enforce and could
unduly restrict usage scenarios; this is omitted from the
requirements.
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A6. That information given by agents on behalf of users can also be
expected to be truthful, complete, and legitimately offered; the user
permitted the agent to publish these notifications.
Discussion: This is difficult to enforce and is omitted from the
requirements.
A7. A can prove that a notification from B was delivered in a timely
fashion and can prove exactly how long the message took to be
delivered.
Discussion: This is difficult to enforce and is omitted from the
requirements. For example, such proof may entail global time
synchronization mechanisms (since any system clocks have
associated unreliability), which is outside the scope of this
effort.
A8. A can prove that B was indeed the sender of a given message.
Discussion: This is a duplication of expectation A4 above and is
reflected in the corresponding requirement 5.3.3.
8.2. INSTANT MESSAGEs
8.2.1. Named Instant Messaging
When a user Alice sends an instant message M to another user Bob:
Alice expects that she:
A1. will receive notification of non-delivery
Discussion: Stands as a requirement. [Requirement 5.4.1]
Alice expects that Bob:
B1. will receive the message
Discussion: covered by A1 and is reflected in the corresponding
requirement 5.4.1.
B2. will receive the message quickly
Discussion: Stands as a requirement, although this is also
covered elsewhere (in the non-security requirements), so this is
omitted from the security requirements.
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B3. will receive the message only once
Discussion: Stands as a requirement. [Requirement 5.4.2]
B4. will be able to verify that Alice sent the message
Discussion: Stands as a requirement. [Requirement 5.4.3]
B5. will not know whether there were BCCs
Discussion: Emulating e-mail conventions and social protocols is
not a core goal of this effort, and therefore references to
standard mail fields are omitted from the requirements.
B6. will be able to reply to the message
Discussion: Stands in principle; the recipient should be able to
reply via an instant message. [Requirement 5.4.4]
B7. will know if he was a bcc recipient
Discussion: Omitted, as noted above.
B8. will not be able to determine any information about A (such as
her location or IP address) without A's knowledge and consent.
Discussion: "Any information about A" is too general; the
requirement should focus on IP address. Further, "without A's
knowledge and consent" may be overkill. [Requirement 5.4.5]
Alice expects that no other user Charlie will be able to:
C1. see the content of M
Discussion: Stands in principle, although this should not be
mandated for all IM communication. [Requirement 5.4.6]
C2. tamper with M
Discussion: Stands, with the same caveat as above.
[Requirement 5.4.7]
C3. know that M was sent
Discussion: It is impossible to prevent traffic analysis, and
this is therefore omitted from the requirements.
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When a user Bob receives an instant message M from another user
Alice:
Bob expects that Bob:
D1. will be able to read M
Discussion: Stands as a requirement. [Requirement 5.4.8]
D2. will be able to verify M's authenticity (both Temporal and the
sender's identity)
Discussion: As noted earlier, it is not reasonable to directly
require temporal checks. The protocol should, however, allow
signing messages using existing standards for signing.
[Requirement 5.4.9]
D3. will be able to verify M's integrity
Discussion: Stands as a requirement. [Requirement 5.4.10]
D4. will be able to prevent A from sending him future messages
Discussion: Stands as a requirement. [Requirement 5.4.11]
Bob expects that Alice:
E1. intended to send the message to Bob
Discussion: This is covered by the corresponding requirement
5.4.6 for C1 above.
E2. informed Bob of all CCs.
Discussion: As noted earlier, references to cc:'s are omitted
from the requirements.
8.2.2. Anonymous Instant Messaging
Discussion: Anonymous instant messaging, as in "hiding the
identity of the sender", is not deemed to be a core requirement
of the protocol and references to it are therefore omitted from
the requirements. Implementations may provide facilities for
anonymous messaging if they wish, in ways that are consistent
with the other requirements.
When a user Alice sends an anonymous instant message to another user
Bob:
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Alice expects that Bob:
B1. will receive the message
B2. will receive the message quickly
B3. will receive the message only once
AB4.1. cannot know Alice sent it
AB4.2. will know that the IM is anonymous, and not from a specific
named user
AB4.3 may not allow anonymous IMs
B5. will not know whether there were BCCs
B6. will be able to reply to the message
Alice expects that she:
C1. will receive notification of non-delivery
AC2. will receive an error if the IM was refused
Bob expects that he:
D1. will be able to read M
D2. will be able to verify M's authenticity (both temporal and the
sender's identity)
D3. will be able to verify M's integrity
AD4. will know if an IM was sent anonymously
AD5. will be able to automatically discard anonymous IM if desired
AD6. will be able to control whether an error is sent to Alice if M
is discarded.
8.2.3. Administrator Expectations
Charlie, Alice's network administrator expects:
C1. that C will be able to send A instant messages at any time.
C2. that A will receive any message he sends while A is online.
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C3. that A will not be able to refuse delivery of any instant
messages sent by C.
Discussion for C1-C3: It is not clear this needs to be specially
handled at the protocol level; Administrators may accomplish the
above objectives through other means. For example, an
administrator may send a message to a user through the normal
mechanisms. This is therefore omitted from the requirements.
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Full Copyright Statement
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