[Search] [txt|pdf|bibtex] [Tracker] [WG] [Email] [Nits]

Versions: 00 01 02 03 04 rfc2542                                        
Internet Fax Working Group                                Larry Masinter
INTERNET-DRAFT                                         Xerox Corporation
December 12, 1997                                    Expires in 6 months

            Terminology and Goals for Internet Fax

Status of this memo

This document is an Internet-Draft.  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.''

To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).

Table of contents:
         1. Introduction
         2. Definitions
          2.1 User model of fax
          2.2 Definition of Internet Fax
          2.3 Internet Fax Service roles
          2.4 Internet Fax Devices
          2.5 Operational modes
         3. Goals for Internet Fax
         4. Operational Requirements for Internet Fax
          4.1 Functionality
          4.2 Interoperability
          4.3 Confirmation
          4.4 Quick Delivery
          4.5 Capabilities
          4.6 Simplicity
          4.7 Security
          4.8 Reliability
          4.9 Fax-like use
          4.10 Legal
         5. Functional Requirements for Internet Fax
          5.1 Requirements for image data representation
          5.2 Requirements for transmission
          5.3 Requirements for addressing
          5.4 Requirements for security
          5.5 Requirements for capability exchange
         6. Security Considerations
         7. Acknowledgements
         8. Copyright
         9. Author's address
         10. References

[[notes: there are still several terms that need clearer definitions,
e.g., "image" and "data" and "format", since these are used differently
in ITU and Internet contexts.]]

1. Introduction

Facsimile (Fax) has a long tradition as a telephony application for
sending a document from one terminal device to another, where the
terminal device consists of a paper input device (scanner), a paper
output device (printer), with (a limited amount of) processing power.
Communication over the telephone network is accomplished using modems.
The transmission of data end-to-end is accompanied by negotiation (to
ensure that the scanned data can be rendered at the recipient) and
confirmation of delivery (to give the sender assurance that the final
data has been received and processed.)  Over time, facsimile has been
extended to allow for PCs using fax modems to send and receive fax, to
send data other than scanned facsimile images. In addition, there have
been many extensions to the basic image model, to allow for additional
compression methods and for representation of images with grey-scale
and color. Other delivery extensions have included sub-addressing
(additional signals after the call is established to facilitate
automated routing of faxes to desktops or mailboxes), and enhanced
features such as fax-back, polling, and even the transfer of binary

Many mechanisms for sending fax documents over the Internet have been
demonstrated and deployed and are currently in use. The general
application of using the Internet for the application of facsimile is
Internet Fax.

This document defines a number of terms useful for the discussion of
Internet Fax, as discussed in the IETF Internet Fax working group.  In
addition, it summarizes the goals for Internet Fax, and establishes a
baseline of desired functionality against which any proposal for
Internet Fax can be judged. It encompasses the goals for _all_ modes
of facsimile delivery, including 'real-time', 'session', and 'store
and forward' (terms defined in Section 2 of this document).

To establish different levels of desirability, language similar to
that in [RFC2119] is used, although the language is intended to apply
to the protocol specification itself rather than conformance of a
particular implementation. The phrases "REQUIRED", "MUST", "SHOULD",
"MAY" and "DESIRABLE" are used to indicate different (decreasing)
levels of desirability of attributes of an Internet Fax specification;
for example, the terms "MUST" and "REQUIRED" are for those
characteristics which are universally acceptable; the term "SHOULD" is
used to indicate a characteristic considered essential by most of the
working group, while the terms "MAY" and "DESIRABLE" indicate an
attribute which is deemed desirable by most, but for which other
factors might override.

2. Definitions and Operation Modes

This section defines some of the basic terms for Internet Fax.

2.1 User model of fax and basic operations

Facsimile involves sending a document from one system to another.
Traditional facsimile has a simple user operational model; the user

        1) inserts paper into a device
        2) dials a number corresponding to the destination
        3) presses the 'start' button on the device
        4) the sending device connects to the receiving device
           using the telephone network
        5) the sending device scans the paper and transmits the
           image of the paper
        6) simultaneously, the remote device prints the image on paper
        7) upon completion of transmission and successful processing
           by the recipient, the sending user is notified of success

The technical operation of facsimile has several components, then: the
"sending terminal" ("sender") and the "receiving terminal"

Although not usually visible to the user, the operation (5) of
transmission consists of

   5a) negotiation: the capabilities of the sender and recipient
     are exchanged, and suitable mutually acceptable parameters
     for the communication are selected
   5b) scanning: creating digitized images of pages of a document
   5c) compression: the image data is encoded using a data compression method
   5d) transmission: the data is sent from one terminal to the other

>From a protocol perspective, the information conveyed in the
transmission consists of both "protocol" (control information,
capabilities, identification) and also "document content".

The document content consists primarily of the "document image" plus
additional metadata accompanying the image. The means by which an
image of a document is encoded within the fax content is the "image
data representation".

When the fax has been sucessfully transmitted, the sender receives a
"confirmation": an indication that the fax content was delivered,
received, and processed.  This "confirmation" is an internal signal
and is not normally visible to the sending user, although some error
messages are visible, to allow a page to be retransmitted.

2.2 Definition of Internet Fax

"Internet Fax" is the service of providing a related (but not exactly
duplicated) user model, but where the Internet is used instead of the
telephone network (at least for some part of the transmission.)

2.3 Internet Fax Service Roles

Internet Fax can be thought of as involving communication among
possible service roles. These roles include standard Internet hosts
(PC, workstation, etc.), but also new kinds of service elements:

  * Network scanner
    A device that can scan a paper document and transmit the scanned
    image via the Internet

  * Network printer
    A device that can accept an image transmission via the Internet
    and print the received document automatically

  * Fax onramp gateway
    A device that can accept a facsimile telephone call and
    automatically forward it via the Internet

  * Fax offramp gateway
    A device that can accept a transmission from the Internet and
    forward it to a traditional fax terminal

2.4 Internet Fax Devices

The Internet Fax service roles can be embedded in any a variety
of combinations and configurations within devices and larger services.
Many different kinds of devices and services SHOULD be able to
participate in the transmission of Internet Fax.

The phrase "IFax device" is used to indicate any combination of these
roles embodied in a single device which is engaged in Internet Fax

2.4.1 Gateway devices

A traditional fax terminal has a telephone line connection (PSTN) with
a fax modem used to connect over the telephone network. To connect a
fax terminal to the Internet requires a _gateway_: a service which
offers connections on one side to the PSTN using standard fax signals,
and on the other side to the Internet.

The same device MAY function as an onramp and offramp. With these
services, the role of Internet Fax is to transport the fax content
across the Internet, e.g., with

[fax-term]-PSTNfax->[onramp]-Internet Fax->[recipient]
                    [sender]-Internet Fax->[offramp]-PSTNFax->[fax-term]

A onramp and/or offramp service MAY be local to a single fax terminal.
For example, the gateway service might exist within a small device
which has a telephone interface on one side and a network connection
on the other. To the fax machine, it looks like a telephone
connection, although it MAY shunt some or all connections to Internet
Fax instead (Such devices are called "Bump-in-cord.")

An onramp or offramp service MAY be a local facility serving many fax
terminals. For example, outgoing telephone fax calls through a company
telephone PBX could be rerouted through a local onramp. An internet to
telephone outbound connection could be part of a "LAN Fax" package.

Onramp or offramp services MAY serve a wider area or broader
collection of users, e.g., services run by service bureaus, offering
subscription services; the telephone sender or the recipient might
subscribe to the service.

The target of an offramp may be a "hunt group": a set of telephone
numbers, each of which have a possibly different fax terminal

2.4.2 New "IFax" terminals

Manufacturers of traditional facsimile devices may offer new devices
built out of similar components (scanner, processor, and printer),
which offer a similar functionality to a fax device, but which
connects to the Internet. These devices might also offer a traditional
fax modem capability, or might send documents exclusively through the
Internet.  Such devices might have a permanent Internet connection
(through a LAN connection) or might have occasional connectivity
through a (data) modem to an Internet Service Provider via PPP.

2.4.3 Internet hosts

Internet users using Internet hosts with standard application suites
should be able to exchange faxes with other participants in Internet
Fax, with a minimum required enhancements to their operating

Internet users might receive faxes over the Internet and display them
on their screens, or have them automatically printed when received.
Similarly, the Internet Fax messages originating from the user might
be the output of a software application which would normally print, or
specially constructed fax-sending software, or MAY be input directly
from a scanner attached to the user's terminal.

The Internet Fax capability might be integrated into existing
fax/network fax software or email software, e.g., by the addition of
"Ifax Printer Drivers" that would render the document to the
appropriate content-type and cause it to be delivered using the
Internet Fax protocol.

In some cases, the user might have a multi-function peripheral which
integrated scanner and printer, and which gave operability similar to
that of the stand-alone fax terminal.

2.4.4 Universal messaging and Internet messaging

Many software vendors are now promoting software packages that support
"universal messaging": a combined communication package that combines
electronic mail, voice mail, and fax. Interoperability with these
environments is desirable for Internet Fax.

In Internet mail, there are a number of components that operate in the
infrastructure to perform additional services beyond mail
store-and-forward. Interoperability with these components is a
consideration for the store and forward profile of Internet Fax.  For
example, mailing list software accepts mail to a single address and
forwards it to a distribution list of many users. Mail archive
software creates repositories of searchable messages. Mail firewalls
operate at organizational boundaries and scan incoming messages for
malicious or harmful mail attachments. Vacation programs send return
messages to the senders of messages when the recipient is on vacation
and not available to respond.

2.5 Operational Modes for Internet Fax

Facsimile over the Internet can occur in several modes.

"Store and forward" Internet Fax is defined as the kind of operation
where an intermediate agent or sequence of agents are involved in the
communication: the sender connects to an agent, and engages in a
communication to send the to the agent. The agent then
subsequently contacts the destination (or attempts to, until the
destination is available), and delivers the message.

"Session" Internet Fax is defined such that delivery notification is
provided to the transmitting terminal prior to disconnection. Unlike
"store and forward", there is some expection that direct
communication, negotiation, and retransmission can take place between
the two endpoints.

"Real-time" Internet Fax allows for two (ITU-T based) standard
facsimile terminals to engage in a document transmission in a way that
all (or as much as practical) of the ITU-T communication protocol is

These modes are different in the end-user expectation of immediacy,
reliability, and in the ease of total compatibility with legacy or
traditional facsimile terminals; the modes differ in the requirements
on the operational infrastructure connecting sender and recipient.

3. Goals for Internet Fax

Facsimile over the Internet must consist of using Internet protocol(s)
to transmit the document from a sender to a recipient.  The recipient
SHOULD be identified by an address.  The capabilities of the sender to
generate different kinds of image data representations MAY be known to
the recipient, and the capabilities, preferences, and characteristics
of the recipient MAY be known to the sender. Faxes MAY be
authenticated as to their origin, or secured to protect the privacy of
the message. In general, an Internet Fax protocol SHOULD specify
functional elements for the following; specific requirements for these
elements are described in section 5:

 Data formats: what image data representation(s) are used, appropriate,
  required? What other data representations must be supported?
 Transmission protocol: what Internet protocol(s) and extensions are used
  to transmit the fax content?  What options are available in that
 Addressing: How are Internet Fax recipients identified? How is that
   identification represented in user directories? How are traditional
   fax terminals addressed?
 Security: How may the authenticity of a fax be determined by
   the recipient?  How may the privacy of a message be guaranteed?
 Capabilities: How are the capabilities, preferences, and
   characteristics of senders and recipients expressed, and
   communicated to each other?

4. Operational Requirements for Internet Fax

This section lists the required and desirable traits of an Internet
Fax protocol. The basic operational requirements are based on the
requirements originally identified in [F.Ifax].

4.1 Functionality

Traditionally, images sent between fax machines are transmitted over
the global switched telephone network. An Internet Fax protocol should
provide for a method to accomplish the features of traditional fax,
but using Internet protocols.

4.2 Interoperability

It is desirable that a standard for Internet Fax allow for the
interoperability between all of the several devices and services
listed in section 2. This means that any and all of the devices or
systems might send a document, using the protocol specified, to any of
the other kinds of devices or systems, and expect the document to
arrive and be processed successfully, with high reliability. Overall
interoperability requires interoperability for all of the protocol
elements: the image data representations must be understood, the
transport protocol must function, it must be possible to address all
manner of terminals, the security mechanism must not require manual
operations in devices that are intended for unattended operation, etc.

Interoperability with Internet mail agents is a consideration only for
store and forward facsimile, since such agents would not be applicable
to the real-time delivery of Internet Fax.

The requirement for interoperability has strong implications for the
protocol design. Interoperability doesn't depend on having the same
kind of networking equipment at each end, or on the kind of
intervening Internet Protocol network: interoperability SHOULD be
independent of the nature of the networking link, whether a simple
IP-based LAN, an internal private IP networks, or the public
Internet. The standard for Internet Fax must be global and have no
special features for local operations. (This is normally the case with
Internet protocol standards.)

If Internet Fax is to use the Internet mail transport mechanisms, it
is REQUIRED that it interoperate consistently with the current
Internet mail environment, and, in particular, with the non-terminal
devices listed in section 3d.  If Internet Fax messages might arrive
in user's mailboxes, it is REQUIRED that the protocol interoperate
successfully with common user practices for mail messages: storing
them in databases, retransmission, forwarding, creation of mail
digests, replay of old messages at times long after the original
receipt, and replying to messages using non-fax equipment.

If Internet Fax requires additions to the operational environment
(services, firewall support, gateways, quality of service, protocol
extensions), then it is preferable if those additions are useful for
other applications than Fax. Features shared with other messaging
applications (voice mail, short message service, paging, etc.) are
DESIRABLE, so as not to require different operational changes for
other applications.

Many vendors are attempting to build a system of 'universal
messaging', in which a user's email, voice mail, facsimile, and other
communication mechanisms are integrated, from the user interface point
of view, into a single application suite. It is DESIRABLE for the
Internet Fax standard to support such applications.

4.3 Confirmation

Traditional fax applications are often used for important business
correspondence, where it is important to get assurance that the
transmitted data was actually received by a terminal at the address
dialed by the user.

In Internet Fax, it SHOULD be possible for a sender to request
notification of the completion of transmission of the message, and to
receive a determinate response as to whether the message was
delivered, not delivered, or that no confirmation of receipt is

Traditionally, fax 'confirmation' has indicated that the message was
'received', e.g., delivered to the output paper tray of the recipient
fax device. This is not the same as a confirmation that the message
was 'read': that a human had confirmed that the message was
received. Confirmation that the message was read (above and beyond the
notification that the message was delivered) is NOT REQUIRED.

4.4 Quick Delivery

In many (if not most) cases, fax transmission is used for urgent
delivery of documents, with some guarantees that if transmission
begins at all, it will complete quickly. Email doesn't normally have
this characteristic.

Internet Fax SHOULD allow the sender of a document to request
immediate delivery, if such delivery is possible.

It is convenient if the protocol to request quick delivery is the same
as, or similar to, the protocol for delayed delivery, so that two
separate mechanisms are NOT REQUIRED.

It SHOULD be possible for the sender of a message to avoid sending the
message at all if quick delivery is not available.

For real-time fax delivery, immediate delivery is the norm, since the
protocol must guarantee that when the session connecting sender to
recipient has terminated, the message has been delivered to the
ultimate recipient.

4.5 Capabilities: reliable, upgrade possible

Traditionally, facsimile has guaranteed interworking between senders
and recipients by having a strict method of negotiation of the
capabilities between the two devices. The image representation of
facsimile originally was a relatively low resolution, but has
increasingly offered additional capabilities (higher resolution,
color) as options.

The use of fax has grown in an evolving world (from 'Group 1' and
'Group 2', to 'Group 3' facsimile) because of two elements: (a) a
useful baseline of capabilities that all terminals implemented, and
(b) the use of capabilities exchange to go beyond that.

To accommodate current use as well as future growth, Internet Fax
SHOULD have a simple minimum set of required features that will
guarantee interoperability, as well as a mechanism by which higher
capability devices can be deployed into a network of lower capability
devices while ensuring interoperability.  If recipients with minimum
capabilities were, for example, to merely drop non-minimum messages
without warning, the result would be that no non-minimum message could
be sent reliably. This situation can be avoided in a variety of ways,
e.g., through communication of recipient capabilities or by sending
multiple renditions. Even minimum-capability recipients of messages
SHOULD be required to provide a capability indication in some reliable
way, e.g., through a directory entry, determine the capabilities of
the recipient with reasonable reliability in advance of transmission,
in a reply to a message with multiple renditions, or as an addition to
a negative acknowledgement requiring retransmission.

On the other hand, for reliability, senders cannot rely on capability
information of recipients before transmission. That is, for
reliability, senders SHOULD have an operational mode which can
function when capabilities are not present, even when recipients must
always provide capabilities.

4.6 Simplicity

Internet Fax SHOULD NOT require terminals to possess a large amount of
processing power, and a base level implementation SHOULD interoperate,
even if it does not offer complex processing.

Internet Fax SHOULD allow interoperability with fax terminal devices
which have limited buffering capabilities and cannot buffer an entire
fax message prior to printing, or cannot buffer an entire set of fax
pages before beginning transmission of scanned pages.

It is possible that different operational modes (real-time, session,
store and forward) might use different protocols, in order to preserve
the simplicity of each.

It is preferable to make as few restrictions and additions to existing
protocols as possible while satisfying the other requirements.  It is
important that it be possible to use Internet Fax end-to-end in the
current Internet environment without any changes to the existing
infrastucture, although some features MAY require adoption of existing

4.7 Security: Cause No Harm, Allow for privacy

The widespread introduction of Internet Fax must not cause harm,
either to its users or to others. It is important, for example, that
no automatic mechanism for returning notification of delivery or
capabilities of fax recipients by email SHOULD expose the users or
others to mail loops, bombs, or replicated delivery. Automatic
capability exchange based on email might not be sufficiently robust
and, without sufficient precautions, might expose users to denial of
service attacks, or merely the bad effects of errors on the part of
system administrators.  Similar considerations apply in these areas to
those that have been addressed by work on electronic mail receipt
acknowledgements [MDN].

Internet Fax SHOULD NOT, by default, release information that the
users consider private, e.g., as might be forthcoming in response to a
broadcast requests for capabilities to a company's Internet fax
devices. Public recipients of Internet Fax (e.g., public agencies
which accept facsimile messages) SHOULD NOT be required to broadcast
messages with capability statements to all potential senders in order
to receive facsimile messages appropriate for the capabilities of
their device.

The possibility for "causing harm" might be created by a combination
of facilities and other features which individually may be viewed as
harmless. Thus, the overall operation of a network full of Internet
Fax devices MUST be considered.

Interoperation with ITU defined T.30 fax security methods, as well as
standard Internet e-mail security methods is DESIRABLE.

4.8 Reliability: Avoid inconsistent operations

Insofar as there is information about the capabilities of recipients
in a store-and-forward message environment, the capabilities and
preferences of the recipient must be known by the sender prior to the
construction and transmission of the message. Because this information
must be accessible by the sender even when the recipient cannot be
contacted directly, the sender must access capabilities in some kind
of storage mechanism. Most commonly these stored capabilities will be
in a directory of some kind.  This directory of capabilities is, in
fact, a distributed database, and is subject to all of the well-known
failure modes of distributed databases. For example, update messages
with capability descriptions might be delivered out of order, from old
archives, might be lost, non-authenticated capability statements might
be spoofed or widely distributed by malicious senders.

Unfortunately, the mechanisms by which a distributed database of
directory information can be maintained and updated reliably are not
yet widely deployed in the Internet environment. Establishing a robust
protocol for capability information with asynchronous information
requires considerable care.

4.9 User Experience

The primary user experience with fax is:

        immediate delivery
        delivery confirmation
        ease of use

The primary user experience with email is:

        delayed delivery
        no delivery confirmation
        ability to reply to sender
        easy to send to multiple recipients

An Internet Fax standard SHOULD attempt to reconcile the differences
between the two environments.

4.10 Legal

An Internet Fax standard SHOULD accomodate the legal requirements for
facsimile, and attempt to support functionality similar to that
legally required even for devices that do not operate over the public
switched telephone network.

The United States Federal Communication Commission regulations
(applicable only within the USA) state:

   "Identification Required on Fax Messages

    The FCC's rules require that any message sent to a fax machine
    must clearly mark on the first page or on each page of the message:
        *     the date and time the transmission is sent;
        *     the identity of the sender; and
        *     the telephone number of the sender or of the sending fax
      All fax machines manufactured on or after December 20, 1992 and
      all facsimile modem boards manufactured on or after December 13,
      1995 must have the capability to clearly mark such identifying
      information on the first page or on each page of the transmission."

5. Functional Requirements for Internet Fax

These requirements for specific elements of Internet Fax follow
from the operational requirements described in section 4.

5.1 Requirements for image and other data representations

Interoperability with Internet Mail or other transmission mechanisms
that cause data files to appear in Internet terminal environments
implies that Internet Fax should use a format for images that is in
wide use.

Interoperability with Internet Mail would require that Internet Fax
recipients handle those message types that are common in the email
environment, including a minimum set of MIME mail formats.

Interoperability with traditional fax terminals requires that the data
format be capable of representing all of the standard compression and
image representations that are defined for traditional facsimile. In
addition, interoperability with 'private use' facsimile messages
requires that the standard accommodate arbitrary bit sequences.

5.2 Requirements for transmission

It is useful for Internet Fax to work in the context of the current
Internet, Intranet, and the combination across firewalls.

A single protocol with various extensions is simpler than multiple
separate protocols, if there are devices that might require, at
different times and for different recipients, different protocols.

5.3 Requirements for addressing

Complete interoperability between all of the terminal types in section
2 requires that all of the different kinds of terminals can be
addressed. The address of a recipient must give sufficient information
to allow the sender to initiate communication.

Interoperability with offramps to legacy fax terminals requires that
the message contain some way of addressing the final destination of
facsimile messages, including telephone numbers, various ISDN
addressing modes, and facsimile sub-addresses.

Interoperability with Internet Mail would require that it be possible
to address Internet Fax to any email address. For interworking with
the rest of the mail transportation network (section 3d), addressing
SHOULD be handled in the envelope of the email layer rather than in
the headers or body, so that normal mail processing methods can be
used for Internet Fax.

Sending devices might not have local storage for directories of
addresses, and addresses might be cumbersome for users to type
in. Internet Fax devices might require configuration to locate
directories of recipients and their capabilities.

The source of a fax message SHOULD be clearly identified. The address
of the appropriate return message (whether via fax or via email)
SHOULD be clearly identified in a way that is visible to all manner of
recipients.  In the case of Internet Fax delivered by email, it SHOULD
be possible to use the normal 'reply' functions for email to return a
message to the sender.

Traditionally, it is common for the first page of a fax message sent
to a facsimile terminal to contain an (image) representation of the
name, address, return number, etc. of the sender of the document.
Some legal jurisdictions for facsimile require an identification of the
sender on every page. The standard for Internet Fax SHOULD cover the
issues of sender and recipient identification in the cases where fax
messages are re-routed, forwarded, sent through gateways.

5.4 Requirements for Security

In order to give Internet Fax users the same assurance of privacy and
integrity that is common with telephone-based fax, the Internet Fax
standard must specify how secure messages can be sent, in an
interoperable fashion. The Internet Fax protocol SHOULD encourage
the introduction of security features, e.g., by requiring that minimum
capability devices still accept signed messages (even if ignoring the

In the case where the sender is responsible for payment for offramp
services in a remote location, it may be necessary to provide for
authentication of the sender and billing information from the offramp
to be negotiated securely.

5.5 Requirements for capabilities exchange

Traditional fax supports a wide range of devices, including high
resolution ("Superfine"); recent enhancements include methods for
color. Fax messaging includes the capability for "non-standard frames",
which allow vendors to introduce proprietary data formats. In addition,
facsimile supports "binary file transfer": a method of sending
arbitrary binary data in a fax message.

To support interoperability with these mechanisms, it SHOULD be possible
to express a wide variety of fax capabilities.

Capability support has three elements: expression of the capabilities
of the sender (as far as a particular message is concerned), expressing
the capabilities of a recipient (in advance of the transmission of the
message), and then the protocol by which capabilities are exchanged.

The Internet Fax standard must specify a uniform mechanism for
capabilities expression. If capabilities are being sent at times
other than the time of message transmission, then capabilities
SHOULD include sufficient information to allow it to be validated,
authenticated, etc.

The Internet Fax standard MAY include one or several methods for
transmission, storage, or distribution of capabilities.

A request for capability information, if sent to a recipient at any
time other than the immediate time of delivery of the message, SHOULD
clearly identify the sender, the recipient whose capabilities
are being requested, and the time of the request. Som kind of
signature would be useful, too.

A capability assertion (sent from recipient to sender) SHOULD clearly
identify the recipient and some indication of the date/time or range
of validity of the information inside. To be secure, capability
assertions SHOULD be protected against interception and the
substitution of valid data by invalid data.

6. Security Considerations

This document lays out several security considerations for Internet

7. Acknowledgements

The author gratefully acknowledges the contributions of Graham Klyne,
Vivian Cancio, Dan Wing, Jim Dahmen, Neil Joffe, Mike Lake, Lloyd
McIntyre, Richard Shockey, Herman Silbiger, Nadesan Narenthiran, and
George Pajari for their valuable comments on this document.

8. Copyright

Copyright (C) The Internet Society, 1997. All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works.  However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of developing
Internet standards in which case the procedures for copyrights defined
in the Internet Standards process must be followed, or as required to
translate it into languages other than English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

9. Author's address

Larry Masinter
Xerox Corporation
3333 Coyote Hill Road
Palo Alto, CA 94304
Fax: (650) 812-4333

10. References

  [T.30] ITU-T, Recommendation T.4, Standardization of Group 3
  facsimile terminals for document transmission.

  [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
  Requirement Levels", RFC 2119, Harvard University, March 1997.