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RFC 4417 - Report of the 2004 IAB Messaging Workshop


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Network Working Group                                    P. Resnick, Ed.
Request for Comments: 4417                                           IAB
Category: Informational                              P. Saint-Andre, Ed.
                                                                     JSF
                                                           February 2006

               Report of the 2004 IAB Messaging Workshop

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 (2006).

Abstract

   This document reports the outcome of a workshop held by the Internet
   Architecture Board (IAB) on the future of Internet messaging.  The
   workshop was held on 6 and 7 October 2004 in Burlingame, CA, USA.
   The goal of the workshop was to examine the current state of
   different messaging technologies on the Internet (including, but not
   limited to, electronic mail, instant messaging, and voice messaging),
   to look at their commonalities and differences, and to find
   engineering, research, and architectural topics on which future work
   could be done.  This report summarizes the discussions and
   conclusions of the workshop and of the IAB.

Table of Contents

   1. Introduction ....................................................3
   2. Methodology .....................................................4
   3. Issues ..........................................................5
      3.1. Authorization ..............................................5
      3.2. Multiple Communication Channels ............................6
      3.3. Negotiation ................................................8
      3.4. User Control ...............................................9
      3.5. Message Transport ..........................................9
      3.6. Identity Hints and Key Distribution .......................10
   4. Recommendations ................................................11
      4.1. Authorization .............................................11
      4.2. Multiple Communication Channels ...........................12
      4.3. Negotiation ...............................................13
      4.4. User Control ..............................................13
      4.5. Message Transport .........................................14
      4.6. Identity Hints and Key Distribution .......................16
   5. Security Considerations ........................................16
   6. Acknowledgements ...............................................16
   Appendix A.  Participants .........................................17
   Appendix B.  Pre-Workshop Papers ..................................18

1.  Introduction

   Current email infrastructure is a mixture of facilities to accomplish
   its task of end-to-end communications through a relay mesh.  That
   mixture has come about as requirements have changed over the years.
   Discussions recur over the years, often including complaints that
   some desired features of email (such as internationalization,
   efficient encoding of structured data, trusted communication) are
   ill-served by the current infrastructure, or that some of the current
   infrastructure seems to be adversely affected by current problems on
   the Internet (most recently including problems such as spam, viruses,
   and lack of security infrastructure).  For many years, the daunting
   task of revamping email infrastructure has been considered, with
   justifiably little enthusiasm for taking on such a task.  However,
   there has been some recent informal discussion on the kinds of things
   that would be desirable in a "next generation" email.

   At the same time, other messaging infrastructures (including those
   associated with "instant messaging" and "web logging") are currently
   being deployed that appear to address many of the above desired
   features and outstanding problems, while adding many features not
   currently considered part of traditional email (like prior-consent-
   based acceptance of messages).  However, each of these technologies
   (at least in their current deployment) seem to lack some of the
   features commonly associated with email (such as selective and
   partial message delivery, queued multi-hop relaying, offline message
   management, and efficient non-textual content delivery).

   The Internet Architecture Board (IAB) believed that the time was ripe
   to examine the current state of messaging technologies on the
   Internet and to see if there are areas of work that can be taken on
   to advance these technologies.  Therefore, the IAB held a workshop on
   Internet messaging, taking some of the above issues as input, in
   order to formulate some direction for future study of the area of
   messaging.

   The topic of messaging is broad, and the boundaries of what counts as
   messaging are not always well-defined.  Rather than limit themselves
   to a philosophical discussion of the nature of messages, the workshop
   participants adopted the attitude of "we know it when we see it" and
   used as their primary examples such well-established types of
   messaging as email and instant messaging (IM), while also discussing
   more "peripheral" types of messaging such as voice messaging and
   event notifications.  (Message queuing systems with guaranteed
   delivery and transactional integrity, such as those used in
   enterprise workflow engines and some "web services" architectures,
   were operationally if not intentionally out of scope.)  The
   participants worked to discover common themes that apply to all the

   types of messaging under consideration.  Among the themes identified
   were the following:

   o  Authorization of senders and recipients
   o  Negotiation of messaging parameters
   o  Consent models and privacy
   o  Identity hints, reputation, and key distribution
   o  Cross-protocol unification of messaging models
   o  Enabling greater user control over messaging
   o  Transport issues (unreliable links, push/pull, etc.)
   o  Message organization (e.g., conversations and threading)

   Purposely missing from the foregoing list is the topic of unsolicited
   commercial email or unsolicited bulk email (UCE or UBE, colloquially
   known as "spam") and analogous communications in other messaging
   environments such as instant messaging ("spim") and Internet
   telephony ("spit").  While this topic was an impetus for the IAB's
   holding the workshop, it was kept off the workshop agenda due to
   concerns that it would crowd out discussion of other messaging-
   related issues.  The more general topics of authorization and
   identity were thought to be broad enough to cover the architectural
   issues involved with spam without devolving into more unproductive
   discussions.

   This document is structured so as to provide an overview of the
   discussion flow as well as proposed recommendations of the workshop.
   Section 3 summarizes the discussions that occurred during the
   workshop on various topics or themes, while Section 4 provides an
   overview of recommended research topics and protocol definition
   efforts that resulted from the workshop.  Section 5 provides some
   perspective on the security-related aspects of the topics discussed
   during the workshop.  Appendix B lists the pre-workshop topic papers
   submitted by workshop participants as background for the workshop
   discussions.

2.  Methodology

   Prior to the workshop, brief topic papers were submitted to set the
   context for the discussions to follow; a list of the papers and their
   authors is provided in Appendix B of this document.

   During the workshop itself, discussion centered on several topics or
   themes, as summarized in the following sections.  Naturally, it was
   not possible in a two-day workshop to treat these topics in depth;
   however, rough consensus was reached on the importance of these
   topics, if not always on the details of potential research programs
   and protocol standardization efforts that might address the issues

   raised.  It is hoped that these summaries will inspire work by
   additional investigators.

   The in-workshop discussions quite naturally fell into three kinds of
   "tracks": (1) possible engineering tasks to recommend to the IESG and
   other standardization groups, (2) "blue sky" research topics to
   recommend to the IRTF and other researchers, and (3) general
   architectural or "framework" issues for consideration by both
   engineers and researchers alike.  After a full-group discussion each
   morning to identify possible topics for more in-depth investigation,
   participants self-selected for involvement in one of three "break-
   out" sessions.  Toward the end of each day, the full groups
   reconvened, gathered reports from the break-out discussion leaders,
   and attempted to come to consensus regarding lessons learned and
   recommendations for further research.  The results of the two-day
   workshop therefore consist of discussion issues and research/
   engineering recommendations related to the six topics described in
   this report.

3.  Issues

3.1.  Authorization

   It is one thing for a sender to send a message, and another thing for
   the intended recipient to accept it.  The factors that lead a
   recipient to accept a message include the identity of the sender,
   previous experience with the sender, the existence of an ongoing
   conversation between the parties, meta-data about the message (e.g.,
   its subject or size), the message medium (e.g., email vs. IM), and
   temporal or psychological factors.  Authorization or acceptance
   applies most commonly at the level of the message or the level of the
   sender, and occasionally also at other levels (conversation thread,
   medium, sender domain).

   Traditionally, sender authorization has been handled by recipient-
   defined block and allow lists (also called "blacklists" and
   "whitelists").  Block lists are of limited value, given the ease of
   gaining or creating new messaging identities (e.g., an email address
   or IM address).  Allow lists are much more effective (since the list
   of people you like or want to communicate with is smaller than the
   large universe of people you don't), but they make it difficult for a
   sender to initiate communication with a new or previously unknown
   recipient.  The workshop participants discussed several ways around
   this problem, including reputation systems and better ways for one
   person to introduce another person to a third party (e.g., through
   signed invitations).

   Reputation systems may be especially worthy of future research, since
   they emulate a pattern that is familiar from real life.  (It may also
   be valuable to distinguish between (1) reputation as the reactive
   assessment of a sender created by one or more recipients based on
   message history and (2) accreditation as a proactive assessment
   provided by trusted third parties.)  Reputation might be based on
   summing an individual's "scores" provided by recipients on the
   network.  (Naturally, the more important reputation becomes, the more
   bad actors might attempt to sabotage any given reputation system, so
   that a distributed as opposed to centralized system might be more
   desirable.)  The actions taken by any given recipient based on the
   sender's reputation would not necessarily be limited to a simple
   allow/deny decision; more subtle actions might include placing
   messages from individuals with lower reputation scores into separate
   inboxes or redirecting them to other media (e.g., from IM to email).

3.2.  Multiple Communication Channels

   It is a fact of life that many people use multiple forms of messaging
   channels: phone, email, IM, pager, and so on.  Unfortunately, this
   can make it difficult for a sender or initiator to know the best way
   to contact a recipient at any given time.  One model is for the
   initiator to guess, for example, by first sending an email message
   and then escalating to pager or telephone if necessary; this may
   result in delivery of redundant messages to the recipient.  A second
   model is for the recipient to publish updated contact information on
   a regular basis, perhaps as one aspect of his or her presence; this
   might enable the initiator to determine beforehand which contact
   medium is most appropriate.  A third model is for the recipient to
   use some kind of "unifier" service that enables intelligent routing
   of messages or notifications to the recipient based on a set of
   delivery rules (e.g., "notify me via pager if I receive a voicemail
   message from my boss after 17:00").

   The workshop participants did not think it necessary to choose
   between these models, but did identify several issues that are
   relevant in unifying or at least coordinating communication across
   multiple messaging channels:

   o  While suppression of duplicate messages could be enabled by
      setting something like a "seen" flag on copies received via
      different messaging media, in general the correlation of multi-
      channel, multi-message exchanges is not well supported by existing
      standards.
   o  A recipient could communicate his or her best contact mechanism to
      the initiator by explicitly granting permission to the initiator,
      perhaps by means of a kind of "authorization token".

   o  It may be worthwhile to define frameworks or protocols for
      recipient-defined delivery rules.  Currently, routing decisions
      tend to be made mostly by the sender through the choice of a
      messaging channel, but in the future the recipient may play a
      larger role in such decisions.
   o  The logic behind contact publication needs to be explored, for
      example, whether it is an aspect of or extension to presence and
      whether contact addresses for one medium are best obtained by
      communicating in a different medium ("email me to get my mobile
      number").

   A multiplicity of delivery channels also makes it more complex for a
   senders to establish a "reliable" relationship with a recipient.
   From the sender's point of view, it is not obvious that a recipient
   on one channel is the same recipient on another channel.  How these
   recipient "identities" are tied together is an open question.

   Another area for investigation is that of recipient capabilities.
   When the sender does not have capability information, the most common
   result is downgrading to a lowest common denominator of
   communication, which seriously underutilizes the capabilities of the
   entire system.  Previous standards efforts (e.g., LDAP, Rescap,
   vCard, Conneg) have attempted to address parts of the capability
   puzzle, but without great success.

   The existing deployment model uses several out-of-band mechanisms for
   establishing communications in the absence of programmatic
   capabilities information.  Many of these mechanisms are based on
   direct human interaction and social policies, which in many cases are
   quite efficient and more appropriate than any protocol-based means.
   However, a programmatic means for establishing communications between
   "arms length" parties (e.g., business-to-business and business-to-
   customer relationships) might be very beneficial.

   Any discussion of relationships inevitably leads to a discussion of
   trust (e.g., "from what kinds of entities do I want to receive
   messages?").  While this is a large topic, the group did discuss
   several ideas that might make it easier to broker communications
   within different relationships, including:

   o  Whitelisting is the explicit definition of a relationship from the
      recipient's point of view, consisting of a list of senders with
      whom a recipient is willing to engage in conversation.  While
      allow lists can be a workable solution, they are a relatively
      static authorization scheme.

   o  Token-based authorization enables the recipient to define a one-
      time or limited-time relationship with a sender.  The issuer
      possesses a token that grants a limited-time right to communicate
      with the recipient.  This is a more dynamic authorization scheme.
   o  Rule-based authorization involves an algorithmic assessment of the
      viability of a relationship based on a wide set of criteria.  This
      is a more general authorization scheme that can incorporate both
      allow lists and tokens, plus additional evaluation criteria such
      as message characterization and issuer characterization.

3.3.  Negotiation

   In the area of negotiation, the workshop participants focused mainly
   on the process by which a set of participants agree on the media and
   parameters by which they will communicate.  (One example of the end
   result of such a "rendezvous" negotiation is a group of colleagues
   who agree to hold a voice conference, with a textual "groupchat" as a
   secondary communications channel.)  In order to enable cross-media
   negotiation, it may be necessary to establish a bridge between
   various identities.  For example, the negotiation may occur via
   email, but the communication may occur via phone, and in order to
   authorize participants the conference software needs to know their
   phone numbers, not their email addresses.  Furthermore, the
   parameters to be negotiated may include a wide variety of aspects,
   including:

   o  Prerequisites for the communication (e.g., distribution of a
      "backgrounder" document).
   o  Who will initiate the communication.
   o  Who will participate in the communication.
   o  The primary "venue" (e.g., a telephone number that all
      participants will call).
   o  One or more secondary venues (e.g., a chatroom address).
   o  Backup plans if the primary or secondary venue is not available.
   o  The topic or topics for the discussion.
   o  The identities of administrators or moderators.
   o  Whether or not the discussion will be logged or recorded.
   o  Scheduling of the event, including recurrence (e.g., different
      instances may have different venues or other details).

   Indeed, in some contexts it might even be desirable to negotiate or
   re-negotiate parameters after communication has already begun (e.g.,
   to invite new participants or change key parameters such as logging).
   While the workshop participants recognized that in-depth negotiation
   of a full set of parameters is likely to be unnecessary in many
   classes of communication, parts of a generalized framework or
   protocol for the negotiation of multiparty communication might prove
   useful in a wide range of applications and contexts.

3.4.  User Control

   A common perception among "power users" (and, increasingly, average
   users) on the Internet is that messaging is not sufficiently under
   their control.  This is not merely a matter of unsolicited
   communications, but also of managing multiple messaging media and
   handling the sheer volume of messages received from familiar and
   unfamiliar senders alike.  Currently, individuals attempt to cope
   using various personal techniques and ad hoc software tools, but
   there may be an opportunity to provide more programmatic support
   within Internet protocols and technologies.

   One area of investigation is message filtering.  Based on certain
   information -- the identity of the sender and/or recipient(s), the
   sender's reputation, the message thread or conversational context,
   message headers, message content (e.g., the presence of attachments),
   and environmental factors such as time of day or personal mood -- a
   user or agent may decide to take one of a wide variety actions with
   regard to a message (bounce, ignore, forward, file, replicate,
   archive, accept, notify, etc.).  While it is an open question how
   much formalization would be necessary or even helpful in this
   process, the workgroup participants identified several areas of
   possible investigation:

   o  Cross-media threads and conversations -- it may be helpful to
      determine ways to tag messages as belonging to a particular thread
      or conversation across media (e.g., a forum discussion that
      migrates to email or IM), either during or after a message
      exchange.
   o  Communication hierarchies -- while much of the focus is on
      messages, often a message does not stand alone but exists in the
      context of higher-level constructs such as a thread (i.e., a
      coherent or ordered set of messages within a medium), a
      conversation (i.e., a set of threads that may cross media), or an
      activity (a set of conversations and related resources, such as
      documents).
   o  Control protocols -- the workgroup participants left as an open
      question whether there may be a need for a cross-service control
      protocol for use in managing communications across messaging
      media.

3.5.  Message Transport

   Different messaging media use different underlying transports.  For
   instance, some messaging systems are more tolerant of slow links or
   lossy links, while others may depend on less loss-tolerant transport
   mechanisms.  Integrating media that have different transport profiles
   can be difficult.  For one, assuming that the same addressing

   endpoint represents the same entity over time may not be warranted
   (it is possible that further work in identifying, addressing, and
   discovering endpoints may be appropriate, even at the URI level).  It
   is also possible that the same endpoint or entity could be available
   via different transport mechanisms at different times, or even
   available via multiple transports at the same time.  The process of
   choosing an appropriate transport mechanism when there are multiple
   paths introduces addressing issues that have not yet been dealt with
   in Internet protocol development (possible heuristics might include
   predictive routing, opportunistic routing, and scheduled routing).
   For links that can be unreliable, there may be value in being able to
   gracefully restart the link after any given failure, possibly by
   switching to a different transport mechanism.

   Another issue that arises in cross-media and cross-transport
   integration is synchronization of references.  This applies to
   particular messages but might also apply to message fragments.  It
   may be desirable for some message fragments, such as large ancillary
   data, to be transported separately from others, for example small
   essential text data.  Message fragments might also be forwarded,
   replicated, archived, etc., separately from other parts of a message.
   One factor relevant to synchronization across transports is that some
   messaging media are push-oriented (e.g., IM) whereas others are
   generally pull-oriented (e.g., email); when content is pushed to a
   recipient in one medium before it has been pulled by the recipient in
   another medium, it is possible for content references to get out of
   sync.

   If message fragments can be transported over different media,
   possibly arriving at separate times or through separate paths, the
   issue of package security becomes a serious one.  Traditionally,
   messages are secured by encrypting the entire package at the head end
   and then decrypting it on the receiving end.  However, if we want to
   allow transports to fragment messages based upon the media types of
   the parts, that approach will not be feasible.

3.6.  Identity Hints and Key Distribution

   While it is widely recognized that both message encryption and
   authentication of conversation partners are highly desirable, the
   consensus of the workshop participants was that current business and
   implementation models in part discourage deployment of existing
   solutions.  For example, it is often hard to get new root
   certificates installed in clients, certificates are (or are perceived
   to be) difficult or expensive to obtain, one-click or zero-click
   service enrollment is a worthy but seemingly unreachable goal, and

   once one has created a public/private key pair and certified the
   public key, it is less than obvious how to distribute that
   certificate or discover other people's certificates.

   One factor that may make widespread message encryption more feasible
   is that email, instant messaging, and Internet telephony have quite
   similar trust models.  Yet the definition of communication differs
   quite a bit between these technologies: in email "the message is the
   thing", and it is a discrete object in its own right; in telephony
   the focus is on the real-time flow of a conversation or session
   rather than discrete messages; and IM seems to hold a mediate
   position since it is centered on the rapid, back-and-forth exchange
   of text messages (which can be seen as messaging sessions).

   Another complicating factor is the wide range of contexts in which
   messaging technologies are used: everything from casual conversations
   in public chatrooms and social networking applications, through
   communications between businesses and customers, to mission-critical
   business-to-business applications such as supply chain management.
   Different audiences may have different needs with regard to messaging
   security and identity verification, resulting in varying demand for
   services such as trusted third parties and webs of trust.

   In the context of communication technologies, identity hints --
   shared knowledge, conversational styles, voice tone, messaging
   patterns, vocabulary, and the like -- can often provide more useful
   information than key fingerprints, digital signatures, and other
   electronic artifacts, which are distant from the experience of most
   end users.  To date, the checking of such identity hints is intuitive
   rather than programmatic.

4.  Recommendations

4.1.  Authorization

   The one clearly desired engineering project that came out of the
   authorization discussion was a distributed reputation service.  It
   was agreed that whatever else needed to be done in regard to
   authorization of messages, at some point the recipient of the message
   would want to be able to check the reputation of the sender of the
   message.  This is especially useful in the case of senders with whom
   the recipient has no prior experience; i.e., using a reputation
   service as a way to get an "introduction to a stranger".  There was
   clearly a need for this reputation service to be decentralized;
   though a single centralized reputation service can be useful in some
   contexts, it does not scale to an Internet-wide service.

   Two potential research topics in authorization were discussed.
   First, a good deal of discussion centered around the use of
   whitelists and blacklists in authorization decision, but it was
   thought that research was necessary to examine the relative
   usefulness of each of the approaches fully.  It was clear to the
   participants that blacklists can weed out known non-authorized
   senders, but do not stop "aggressive" unwanted senders because of the
   ease of simply obtaining a new identity.  Whitelists can be useful
   for limiting messages to only those known to the recipient, but would
   require the use of some sort of introduction service in order to
   allow for messages from unknown parties.  Participants also thought
   that there might be useful architectural work done in this area.

   The other potential research area was in recipient responses to
   authorization decisions.  Upon making an authorization decision,
   recipients have to do two things: First, obviously the recipient must
   dispatch the message in some way either to deliver it or to deny it.
   But that decision will also have side effects back into the next set
   of authorization decisions the recipient may make.  The decision may
   feed back into the reputation system, either "lauding" or "censuring"
   the sender of the message.

4.2.  Multiple Communication Channels

   Several interesting and potentially useful ideas were discussed
   during the session, which the participants worked to transform into
   research or engineering tasks, as appropriate.

   In the area of contact information management, the workshop
   participants identified a possible engineering task to define a
   service that publishes contact information such as availability,
   capabilities, channel addresses (routing information), preferences,
   and policies.  While aspects of this work have been attempted
   previously within the IETF (with varying degrees of success), there
   remain many potential benefits with regard to managing business-to-
   business and business-to-customer relationships.

   The problem of suppressing redundant messages is becoming more
   important as the use of multiple messaging channels becomes the rule
   for most Internet users, and as users become accustomed to receiving
   notifications in one channel of communications received in another
   channel.  Unfortunately, there are essentially no standards for
   cross-referencing and linking of messages across channels; standards
   work in this area may be appropriate.

   Another possible engineering task is defining a standardized
   representation for the definition and application of recipient
   message processing rules.  Such an effort would extend existing work
   on the Sieve language within the IETF to incorporate some of the
   concepts discussed above.

   Discussion of token-based authorization focused on the concept of
   defining a means for establishing time-limited or usage-limited
   relationships for exchanging messages.  The work would attempt to
   define the identity, generation, and use of tokens for authorization
   purposes.  Most likely this is more of a research topic than an
   engineering topic.

   Work on recipient rules processing and token-based authentication may
   be related at a higher level of abstraction (we can call it
   "recipient authorization processing").  When combined with insights
   into authorization (see Sections 3.1 and 4.1), this may be an
   appropriate topic for further research.

4.3.  Negotiation

   Discussion in the area of negotiation resulted mostly in research-
   oriented output.  The session felt that participants in a
   conversation would require some sort of rendezvous mechanism during
   which the parameters of the conversation would be negotiated.  To
   facilitate this, a "conversation identifier" would be needed so that
   participants could identify the conversation that they wished to
   participate in.  In addition, there are at least five dimensions
   along which a conversation negotiation may occur:

   o  The participants in the conversation
   o  The topic for the conversation
   o  The scheduling and priority parameters
   o  The mechanism used for the conversation
   o  The capabilities of the participants
   o  The logistical details of the conversation

   Research into how to communicate these different parameters may prove
   useful, as may research into the relationship between the concepts of
   negotiation, rendezvous, and conversation.

4.4.  User Control

   A clear architectural topic to come out of the user control
   discussion was work on activities, conversations, and threads.  In
   the course of the discussion, the user's ability to organize messages
   into threads became a focus.  The participants got some start on
   defining threads as a semi-ordered set of messages, a conversation as

   a set of threads, and an activity as a collection of conversations
   and related resources.  The discussion expanded the traditional
   notion of a thread as an ordered tree of messages.  Conversations can
   collect together threads and have them be cross-media.  Messages can
   potentially belong to more than one thread.  Threads themselves might
   have subthreads.  All of these topics require an architectural
   overview to be brought into focus.

   There is also engineering work that is already at a sufficient level
   of maturity to be undertaken on threads.  Though there is certainly
   some simple threading work being done now with messaging, it is
   pretty much useful only for a unidirectional tree of messages in a
   single context.  Engineering work needs to be done on identifiers
   that could used in threads that cross media.  Additionally, there is
   likely work to be done for messages that may not be strictly ordered
   in a thread.

   The topics of "control panels" and automated introductions were
   deemed appropriate for further research.

4.5.  Message Transport

   A central research topic that came out of the transport session was
   that of multiple transports.  It was felt that much research could be
   done on the idea of transporting pieces of messages over separate
   transport media in order to get the message to its final destination.
   Especially in some high-latency, low-bandwidth environments, the
   ability to run parallel transports with different parts of messages
   could be extremely advantageous.  The hard work in this area is
   re-associating all of the pieces in a timely manner, and identifying
   the single destination of the message when addressing will involve
   multiple media.

   A common theme that arose in several of the discussions (including
   user control and message unification), but that figured prominently
   in the transport discussion, was a need for some sort of identifier.
   In the transport case, identifiers are necessary on two levels.
   Identifiers are needed to mark the endpoints in message transport.
   As described in the discussion, there are many cases where a message
   could reasonably be delivered to different entities that might all
   correspond to a single person.  Some sort of identifier to indicate
   the target person of the message, as well as identifiers for the
   different endpoints, are all required in order to get any traction in
   this area.  In addition, identifiers are also required for the
   messages being transported, as well as their component parts.
   Certainly, the idea of transporting different parts of a message over
   different mechanisms requires the identification of the containing
   message so that re-assembly can occur at the receiving end.  However,

   identifying the entire package is also necessary for those cases
   where duplicate copies of a message might be sent using two different
   mechanisms: The receiving end needs to find out that it has already
   received a copy of the message through one mechanism and identify
   that another copy of the message is simply a duplicate.

   Workshop participants felt that, at the very least, a standard
   identifier syntax was a reasonable engineering work item that could
   be tackled.  Though there exist some identifier mechanisms in current
   messaging protocols, none were designed to be used reliably across
   different transport environments or in multiple contexts.  There is
   already a reasonable amount of engineering work done in the area of
   uniform resource identifiers (URI) that participants felt could be
   leveraged.  Syntax would be required for identifiers of messages and
   their components as well as for identifiers of endpoint entities.

   Work on the general problem of identifier use might have some
   tractable engineering aspects, especially in the area of message part
   identifiers, but workshop participants felt that more of the work was
   ripe for research.  The ability to identify endpoints as belonging to
   a single recipient, and to be able to distribute identifiers of those
   endpoints with information about delivery preferences, is certainly
   an area where research could be fruitful.  Additionally, it would be
   worthwhile to explore the collection of identified message components
   transported through different media, while delivering to the correct
   end-recipient with duplicate removal and re-assembly.

   Package security was seen as an area for research.  As described in
   Section 3.5, the possibility that different components of messages
   might travel over different media and need to be re-assembled at the
   recipient end breaks certain end-to-end security assumptions that are
   currently made.  Participants felt that a worthwhile research goal
   would be to examine security mechanisms that could be used for such
   multi-component messages without sacrificing desirable security
   features.

   Finally, a more architectural topic was that of restartability.  Most
   current message transports, in the face of links with reliability
   problems, will cancel and restart the transport of a message from the
   beginning.  Though some mechanisms do exist for restart mid-session,
   they are not widely implemented, and they certainly can rarely be
   used across protocol boundaries.  Some architectural guidance on
   restart mechanisms would be a useful addition.

4.6.  Identity Hints and Key Distribution

   It would be helpful to develop Internet-wide services to publish and
   retrieve keying material.  One possible solution is to build such a
   service into Secure DNS, perhaps as an engineering item in an
   existing working group.  However, care is needed since that would
   significantly increase the size and scope of DNS.  A more research-
   oriented approach would be to investigate the feasibility of building
   Internet-wide key distribution services outside of DNS.  In doing so,
   it is important to keep in mind that the problem of distribution is
   separate from the problem of enrollment, and that name subordination
   (control over what entities are allowed to create sub-domains)
   remains necessary.

   Research may be needed to define the different audiences for message
   security.  For example, users of consumer-oriented messaging services
   on the open Internet may not generally be willing or able to install
   new trusted roots in messaging client software, which may hamper the
   use of security technologies between businesses and customers.  By
   contrast, within a single organization it may be possible to deploy
   new trusted roots more widely, since (theoretically) all of the
   organization's computing infrastructure is under the centralized
   control.

   In defining security frameworks for messaging, it would be helpful to
   specify more clearly the similarities and differences among various
   messaging technologies with regard to trust models and messaging
   metaphors (e.g., stand-alone messages in email, discrete
   conversations in telephony, messaging sessions in instant messaging).
   The implications of these trust models and messaging metaphors for
   communications security have not been widely explored.

5.  Security Considerations

   Security is discussed in several sections of this document,
   especially Sections 3.5, 3.6, 4.5, and 4.6.

6.  Acknowledgements

   The IAB would like to thank QUALCOMM Incorporated for their
   sponsorship of the meeting rooms and refreshments.

   The editors would like to thank all of the workshop participants.
   Eric Allman, Ted Hardie, and Cullen Jennings took helpful notes,
   which eased the task of writing this document.

Appendix A.  Participants

   Eric Allman
   Nathaniel Borenstein
   Ben Campbell
   Dave Crocker
   Leslie Daigle
   Mark Day
   Mark Crispin
   Steve Dorner
   Lisa Dusseault
   Kevin Fall
   Ned Freed
   Randy Gellens
   Larry Greenfield
   Ted Hardie
   Joe Hildebrand
   Paul Hoffman
   Steve Hole
   Scott Hollenbeck
   Russ Housley
   Cullen Jennings
   Hisham Khartabil
   John Klensin
   John Levine
   Rohan Mahy
   Alexey Melnikov
   Jon Peterson
   Blake Ramsdell
   Pete Resnick
   Jonathan Rosenberg
   Peter Saint-Andre
   Greg Vaudreuil

Appendix B.  Pre-Workshop Papers

   The topic papers circulated before the workshop were as follows:

   Calendaring Integration (Nathaniel Borenstein)
   Channel Security (Russ Housley)
   Collaborative Authoring (Lisa Dusseault)
   Consent-Based Messaging (John Klensin)
   Content Security (Blake Ramsdell)
   Event Notifications (Joe Hildebrand)
   Extended Messaging Services (Dave Crocker)
   Group Messaging (Peter Saint-Andre)
   Identity and Reputation (John Levine)
   Instant Messaging and Presence Issues in Messaging (Ben Campbell)
   Large Email Environments (Eric Allman)
   Mail/News/Blog Convergence (Larry Greenfield)
   Messaging and Spam (Cullen Jennings)
   Messaging Metaphors (Ted Hardie)
   MUA/MDA, MUA/MSA, and MUA/Message-Store Interaction (Mark Crispin)
   Presence for Consent-Based Messaging (Jon Peterson)
   Rich Payloads (Steve Hole)
   Session-Oriented Messaging (Rohan Mahy)
   Spam Expectations for Mobile Devices (Greg Vaudreuil)
   Communication in Difficult-to-Reach Networks (Kevin Fall)
   Store-and-Forward Needs for IM (Hisham Khartabil)
   Syndication (Paul Hoffman)
   Transport Security (Alexey Melnikov)
   VoIP Peering and Messaging (Jonathan Rosenberg)
   Webmail, MMS, and Mobile Email (Randy Gellens)

Authors' Addresses

   Peter W. Resnick (Editor)
   Internet Architecture Board
   QUALCOMM Incorporated
   5775 Morehouse Drive
   San Diego, CA  92121-1714
   US

   Phone: +1 858 651 4478
   EMail: presnick@qualcomm.com
   URI:   http://www.qualcomm.com/~presnick/

   Peter Saint-Andre (Editor)
   Jabber Software Foundation
   P.O.  Box 1641
   Denver, CO  80201-1641
   US

   Phone: +1 303 308 3282
   EMail: stpeter@jabber.org
   URI:   http://www.jabber.org/people/stpeter.shtml

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