INTERNET-DRAFT Mark Day
Expires: November 17, 1999 Lotus
Sonu Aggarwal
Microsoft
Gordon Mohr
Activerse
Jesse Vincent
Instant Messaging / Presence Protocol Requirements
draft-ietf-impp-reqts-00.txt
[IMPP LIST DISCUSSION DRAFT 01, 1999-05-17]
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
2. 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.
3. Contents
1. Status of this Memo
2. Abstract
3. Contents
4. Terminology
5. Shared Requirements
5.1. Namespace and Administration
5.2. Scalability
5.3. Access Control
5.4. Network Topology
5.5. Message Encryption and Authentication
6. Additional Requirements for PRESENCE INFORMATION
6.1. Common Presence Format
6.2. Presence Lookup and Notification
6.3. Presence Caching and Replication
7. Additional Requirements for INSTANT MESSAGES
7.1. Common Message Format
7.2. Reliability
8. Open Issues and ToDos for this Draft
9. Security Considerations
9.1. Requirements related to SUBSCRIPTIONS
9.2. Requirements related to NOTIFICATION
9.3. Requirements related to receiving a NOTIFICATION
9.4. Requirements related to INSTANT MESSAGES
10. References
11. Authors' Addresses
12. Appendix: Security Expectations and Deriving Requirements
12.1. Presence Information
12.1.1. Subscription
12.1.2. Publication
12.1.3. Publication for Notification
12.1.4. Receiving a Notification
12.2. Instant Messaging
12.2.1. Named Instant Messaging
12.2.2 Anonymous Instant Messaging
12.2.3 Administrator Expectations
4. Terminology
The following terms are defined in [Model] and are used with those
definitions in this document:
ACCESS RULES
FETCHER
INSTANT INBOX
INSTANT MESSAGE
NOTIFICATION
PRESENCE INFORMATION
PRESENCE SERVICE
PRESENTITY
PRINCIPAL
PROXY
SERVER
STATUS
SUBSCRIBER
SUBSCRIPTION
WATCHER
The terms MUST, SHOULD, and MAY are used with the meaning defined in
[RFC 2119].
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, or WATCHER (all
defined in [Model]).
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.
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.
OFFLINE: A STATUS indicating that any associated INSTANT INBOX cannot
serve as a means to convey an INSTANT MESSAGE to its PRINCIPAL.
ONLINE: A STATUS indicating that any associated INSTANT INBOX may be
able to serve as a means to convey an INSTANT MESSAGE to its
PRINCIPAL. Note that an ONLINE status may be further refined to
indicate "busy," "do not disturb," and the like.
OUT OF CONTACT: A situation in which som 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.
5. Shared Requirements
5.1. Namespace and Administration
5.1.1. The protocol(s) MUST define a single NAMESPACE. In general, an
ENTITY that makes PRESENCE INFORMATION available can receive INSTANT
MESSAGES, and vice versa.
5.1.2. The administration and naming of ENTITIES within a given DOMAIN
MUST be able to operate independently of actions in any other DOMAIN.
5.1.3. The protocol MUST allow for an arbitrary number of DOMAINS
within the NAMESPACE.
5.2. Scalability
5.2.1. 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.
5.2.2. The protocol MUST continue to meet its other functional and
performance requirements even when there are millions of ENTITIES
within a single DOMAIN.
5.2.3. The protocol MUST continue to meet its other functional and
performance requirements even when there are millions of DOMAINS
within the single NAMESPACE.
5.2.4. The protocol MUST continue to meet its other functional and
performance requirements even when every single SUBSCRIBER has
SUBSCRIPTIONS to hundreds of PRESENTITIES.
5.2.5. The protocol MUST continue to meet its other functional and
performance requirements even when thousands of distinct SUBSCRIBERS
have SUBSCRIPTIONS to a single PRESENTITY.
5.2.6. The protocol MUST continue to meet its other functional and
performance requirements even when every single SUBSCRIBER has
SUBSCIPTIONS to PRESENTITIES in hundreds of distinct DOMAINS.
5.2.7. The transport of INSTANT MESSAGES SHOULD be sufficiently rapid
to allow for comfortable conversational exchanges of short messages.
5.2.8. When a PRESENTITY changes its PRESENCE INFORMATION, any
SUBSCRIBER to that information MUST be notified of the changed
information, except when prevented by ACCESS RULES, as rapidly as an
INSTANT MESSAGE would be delivered.
5.3. Access Control
5.3.1. The PRINCIPAL controlling a PRESENTITY MUST be able to control
which FETCHERS can observe its PRESENCE INFORMATION.
5.3.2. The PRINCIPAL controlling a PRESENTITY MUST be able to control
which SUBSCRIBERS can have SUBSCRIPTIONS to its PRESENCE INFORMATION.
5.3.3. The PRINCIPAL controlling a PRESENTITY MUST be able to control
what PRESENCE INFORMATION a particular WATCHER will see, either by
fetching or NOTIFICATION.
5.3.4. The PRINCIPAL controlling a PRESENTITY MUST be able to control
which other PRINCIPALS, if any, can update the PRESENCE INFORMATION of
that PRESENTITY.
5.3.5. The PRINCIPAL controlling an INSTANT INBOX MUST be able to
control which other PRINCIPALS, if any, can send INSTANT MESSAGES to
that INSTANT INBOX.
5.3.6. The PRINCIPAL controlling an INSTANT INBOX MUST be able to
control which other PRINCIPALS, if any, can read INSTANT MESSAGES from
that INSTANT INBOX.
5.3.7. 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.
5.4. Network Topology
5.4.1. The protocol MUST allow the creation of a SUBSCRIPTION both
directly and via intermediaries, such as PROXIES.
5.4.2. The protocol MUST allow the sending of a NOTIFICATION both
directly and via intermediaries, such as PROXIES.
5.4.3. The protocol MUST allow the sending of an INSTANT MESSAGE both
directly and via intermediaries, such as PROXIES.
5.4.4 The protocol's network traffic MUST be sufficiently well-
characterized that ADMINISTRATORS can apply local policies via
FIREWALLS.
5.4.5 The protocol proxying facilities and transport practices MUST
allow ADMINISTRATORS ways to enable protocol activity through existing
and commonly-deployed FIREWALLS.
5.5. Message Encryption and Authentication
5.5.1. The protocol MUST provide means to ensure confidence that a
received message (NOTIFICATION or INSTANT MESSAGE) has not been
corrupted or tampered with.
5.5.2. 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.
5.5.3. The protocol MUST provide means to ensure that a sent message
(NOTIFICATION or INSTANT MESSAGE) is only readable by ENTITIES that
the sender allows.
5.5.4. 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.
6. Additional Requirements for PRESENCE INFORMATION
The requirements in section 6 are applicable only to PRESENCE
INFORMATION and not to INSTANT MESSAGES.
6.1. Common Presence Format
6.1.1. All ENTITIES MUST produce and consume at least a common base
format for PRESENCE INFORMATION.
6.1.2. The common presence format MUST include a means to uniquely
identify the PRESENTITY whose PRESENCE INFORMATION is reported.
6.1.3. The common presence format MUST include a means to represent at
least the following STATUS conditions: online/offline, and
available/busy/idle.
6.1.4. 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.
6.1.5. 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.
6.1.6. The working group MUST define the extension and registration
mechanisms for presence information schema.
6.1.7. The presence format SHOULD be based on IETF-standards such as
vCard [RFC 2426] if possible.
6.2. Presence Lookup and Notification
6.2.1. A FETCHER MUST be able to fetch a PRESENTITY's PRESENCE
INFORMATION even when the PRESENTITY is not in communication with the
PRESENCE SERVICE.
6.2.2. A SUBSCRIBER MUST be able to request a SUBSCRIPTION to a
PRESENTITY's PRESENCE INFORMATION, even when the PRESENTITY is not in
communication with the PRESENCE SERVICE.
6.2.3. 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.
6.2.4. The protocol MUST provide a mechanism for detecting when a
PRESENTITY or SUBSCRIBER goes OUT OF CONTACT.
6.2.5. 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.
6.3. Presence Caching and Replication
6.3.1. The protocol MUST include mechanisms to allow PRESENCE
INFORMATION to be cached.
6.3.2. The protocol MUST include mechanisms to allow cached PRESENCE
INFORMATION to be updated when the master copy changes.
6.3.3 The protocol caching facilities MUST NOT circumvent established
ACCESS RULES or restrict choice of authentication/encryption
mechanisms.
7. Additional Requirements for INSTANT MESSAGES
The requirements in section 7 are applicable only to INSTANT MESSAGES
and not to PRESENCE INFORMATION.
7.1. Common Message Format
7.1.1. All ENTITIES sending and receiving INSTANT MESSAGES MUST
implement at least a common base format for INSTANT MESSAGES.
7.1.2. The common base format for an INSTANT MESSAGE MUST identify the
sender and intended recipient.
7.1.3. The common message format MUST include a return address for the
receiver to reply to the sender with another INSTANT MESSAGE.
7.1.4. The common message format SHOULD include standard forms of
return addresses or contact means for media other than INSTANT
MESSAGES, such as telephone numbers or email addresses.
7.1.5. The common message format MUST permit the encoding and
identification of the message payload to allow for non-ASCII or
encrypted content.
7.1.6. The common message format MUST provide for the association of a
reply with the message that prompted that reply.
7.1.7. This reply-association mechanism MUST allow a user agent to
organize instant message exchanges into conversational threads.
7.1.8. The common message format SHOULD be based on IETF-standard MIME
[RFC 2045].
7.2. Reliability
7.2.1. The protocol MUST inform the sender of the INSTANT MESSAGE's
SUCCESSFUL DELIVERY or reasons for failure.
8. Open Issues and ToDos for this Draft
Is it required that there be a one-to-one mapping of the existing
email namespace onto the IMPP NAMESPACE?
9. Security Considerations
Security considerations are addressed in section 5.3, Access Control,
and section 5.5, 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.
The following are the requirements derived from the expectations
above. 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.
9.1. Requirements related to SUBSCRIPTIONS
When A establishes a SUBSCRIPTION to B's PRESENCE INFORMATION:
9.1.1. The protocol MUST provide A means of identifying and
authenticating that the PRESENTITY subscribed to is controlled by B.
9.1.2. If A so chooses, the protocol SHOULD NOT make A's SUBSCRIPTION
to B obvious to a third party C.
9.1.3. The protocol MUST provide B with means of allowing an
unauthenticated subscription by A.
9.1.4. The protocol MUST provide A means of verifying the accurate
receipt of the content B chooses to disclose to A.
9.1.5. B MUST inform A if B refuses A's SUBSCRIPTION.
9.1.6. The protocol MUST NOT let any third party C force A to subscribe
to B's PRESENCE INFORMATION without A's consent.
9.1.7. A MUST be able to cancel her SUBSCRIPTION to B's PRESENCE
INFORMATION at any time and for any reason. When A does so, B stops
informing A of changes to its PRESENCE INFORMATION.
9.1.8. The protocol MUST NOT let a third party C cancel A's
subscription to B.
9.1.9. If A's subscription to B is cancelled, the service SHOULD inform
A of the cancellation.
9.1.10. A SHOULD be able to determine the status of A's subscription to
B, at any time.
9.1.11. The protocol MUST provide B means of learning about A's
SUBSCRIPTION to B, both at the time of establishing the SUBSCRIPTION
and afterwards.
9.1.12. The protocol MUST provide B means of identifying and
authenticating the SUBSCRIBER's PRINCIPAL, A.
9.1.13. It MUST be possible for B to prevent any particular PRINCIPAL
from subscribing.
9.1.14. It MUST be possible for B to prevent anonymous PRINCIPALS from
subscribing.
9.1.15. It MUST be possible for B to deny A's subscription while
appearing to A as if the subscription has been granted ("polite
blocking"). The protocol MUST NOT mandate B to service subscriptions
that it denies in this manner.
9.1.16. B MUST be able to cancel A's subscription at will.
9.1.17. B's ADMINISTRATOR MUST have the option to determine the set of
PRINCIPALS or their PROXIES subscribed to B at any time.
9.1.18. B's ADMINISTRATOR MUST have the option to manage all aspects of
B's presence information.
9.1.19. B's ADMINISTRATOR MUST have the option to control who can
access B's presence information and exchange instant messages with B.
9.2. Requirements related to NOTIFICATION
When a PRINCIPAL B publishes PRESENCE INFORMATION for NOTIFICATION to
another PRINCIPAL A:
9.2.1. The protocol MUST provide means of ensuring that only the
PRINCIPAL A being sent the NOTIFICATION by B can read the
NOTIFICATION.
9.2.2. A SHOULD receive all NOTIFICATIONS intended for her.
9.2.3. 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".
9.2.4. The protocol MUST provide means of protecting B from another
PRINCIPAL C "spoofing" notification messages about B.
9.3. Requirements related to receiving a NOTIFICATION
When a PRINCIPAL A receives a notification message from another
principal B, conveying PRESENCE INFORMATION,
9.3.1. The protocol MUST provide A means of verifying that the
presence information is accurate, as sent by B.
9.3.2. The protocol MUST ensure that A only receives NOTIFICATIONS
from entities she's subscribed to.
9.3.3. The protocol MUST provide A means of verifying that the
notification was sent by B.
9.4. Requirements related to INSTANT MESSAGES
When a user A sends an INSTANT MESSAGE M to another user B,
9.4.1. A MUST receive confirmation of non-delivery.
9.4.2. If M is delivered, B MUST receive the message only once.
9.4.3. The protocol MUST provide B means of verifying that A sent the message.
9.4.4. B MUST be able to reply to the message via another instant message.
9.4.5. The protocol MUST NOT always require A to reveal A's IP
address, for A to send an instant message.
9.4.6. The protocol MUST provide A means of ensuring that no other
PRINCIPAL C can see the content of M.
9.4.7. The protocol MUST provide A means of ensuring that no other
PRINCIPAL C can tamper with M.
9.4.8. B MUST be able to read M.
9.4.9. The protocol MUST allow A to sign the message, using existing
standards for digital signatures.
9.4.10. The protocol MUST provide B means of verifying M's integrity.
9.4.11. B MUST be able to prevent A from sending him future messages
10. References
[Aggarwal et al., 1998]
S. Aggarwal, M. Day, G. Mohr, "Presence Information Protocol
Requirements", draft-aggarwal-pip-reqts-00.txt
[Day, 1998]
M. Day, "Requirements for Presence and Instant Messaging",
draft-day-rpim-00.txt
[Model]
M. Day and J. Rosenberg. "A Model for Presence."
draft-ietf-impp-model-00.txt.
[Calsyn & Dusseault, 1998]
M. Calsyn and L. Dusseault. "Presence Information Protocol
Requirements", draft-dusseault-pipr-00.txt
[RFC 2426]
F. Dawson and T. Howes. "vCard MIME Directory Profile." RFC
2426, September 1998.
[RFC 2045]
N. Freed and N. Borenstein. "Multipurpose Internet Mail Extensions
(MIME) - Part One: Format of Internet Message Bodies." RFC 2045,
November 1996.
[RFC 2119]
S. Bradner. "Key Words for Use in RFCs to Indicate Requirement
Levels." RFC 2119, March 1997.
11. Authors' Addresses
Mark Day
<Mark_Day@lotus.com>
Lotus Development Corporation
55 Cambridge Parkway
Cambridge, MA 02142
USA
Sonu Aggarwal
<sonuag@microsoft.com>
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Gordon Mohr
<gojomo@activerse.com>
Activerse, Inc.
1301 W. 25th St Suite 500
Austin, TX
USA
Jesse Vincent
<jesse@fsck.com>
22 Kirkstall Rd
Newton, MA 02160
USA
12. 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.
12.1. 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.
12.1.1. 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 9.1.1]
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 9.1.2]
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 9.1.3], 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 9.1.4]
A5. B will inform A if B refuses A's subscription
Discussion: Stands as a requirement. [Requirement 9.1.5]
A6. No third party, C can force A to subscribe to B's presence without
A's consent.
Discussion: Stands as a requirement. [Requirement 9.1.6]
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 9.1.7]
A8. no third party, C, can cancel A's subscription to B.
Discussion: Stands, although the administrative exception does
apply. [Requirement 9.1.8]
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
an attempt to inform, but this is not required. [Requirement 9.1.9]
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 9.1.10]
B2. A is identifiable and authenticated.
Discussion: This stands as a requirement. [Requirement 9.1.11]
B3. B can prevent a particular user, D, from subscribing.
Discussion: This stands as a requirement. [Requirement 9.1.12]
B4. B can prevent anonymous users from subscribing.
Discussion: This stands as a requirement. [Requirement 9.1.13]
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 9.1.14]
B7. B can cancel A's subscription at will.
Discussion: Stands as a requirement. [Requirement 9.1.15]
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
have subscribed on behalf of users, and in these cases the
administrator can only determine the identity of these agents, not
their users. [Requirement 9.1.16]
C2. C can manage all aspects of A's presence information.
Discussion: This stands as a requirement. [Requirement 9.1.17]
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 9.1.18]
12.1.2. 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.
12.1.3. 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 9.2.1]
B2. A reliably receives all notifications intended for her.
Discussion: This stands, although "Reliably" is a little strong
(e.g. network outages, etc.). [Requirement 9.2.2]
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 9.2.3]
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 9.2.4]
12.1.4. 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 9.3.1]
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 9.3.2]
A4. Notifications come from the apparent sender, B.
Discussion: Stands in principle, although the verification should
be optional for A. [Requirement 9.3.3]
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.
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 9.3.3.
12.2. INSTANT MESSAGEs
12.2.1. 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 9.4.1]
Alice expects that Bob:
B1. will receive the message
Discussion: covered by A1 and is reflected in the corresponding
requirement 9.4.1.
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.
B3. will receive the message only once
Discussion: Stands as a requirement. [Requirement 9.4.2]
B4. will be able to verify that Alice sent the message
Discussion: Stands as a requirement. [Requirement 9.4.3]
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 9.4.4]
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 9.4.5]
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 9.4.6]
C2. tamper with M
Discussion: Stands, with the same caveat as above.
[Requirement 9.4.7]
C3. know that M was sent
Discussion: It is impossible to prevent traffic analysis, and this
is therefore omitted from the requirements.
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 9.4.8]
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 9.4.9]
D3. will be able to verify M's integrity
Discussion: Stands as a requirement. [Requirement 9.4.10]
D4. will be able to prevent A from sending him future messages
Discussion: Stands as a requirement. [Requirement 9.4.11]
Bob expects that Alice:
E1. intended to send the message to Bob
Discussion: This is covered by the corresponding requirement 9.4.6
for C1 above.
E2. informed Bob of all CCs.
Discussion: As noted earlier, references to cc:'s are omitted from
the requirements.
12.2.2. 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:
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.
12.2.3. 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.
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.