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Network Working Group                                        P. Riikonen
Internet-Draft
draft-riikonen-flags-payloads-00.txt                         15 May 2002
Expires: 15 November 2002

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SILC Message Flag Payloads
<draft-riikonen-flags-payloads-00.txt>

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Status of this Memo

This document is an Internet-Draft and is in full conformance with   
all provisions of Section 10 of RFC 2026.  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   

The distribution of this memo is unlimited.  


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Abstract

This memo describes the data payloads associated with the SILC Message
Flags, as defined in the SILC Packet Protocol Internet Draft [SILC2].  The 
purpose of the Message Flags is to augment the function of the Private 
Message Payload and Channel Message Payload by allowing the sender to 
tell the receiver what type of data the payload includes, and how the 
data should be processed.  Some of the Message Flags may define additional 
payloads to be associated with the flag, and this memo describes these 
payloads.








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Table of Contents

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1 Introduction ..................................................  2
  1.1 Requirements Terminology ..................................  2
2 SILC Message Flags ............................................  2
3 SILC Message Flag Payloads ....................................  3
  3.1 SILC_MESSAGE_FLAG_REQUEST .................................  3
  3.2 SILC_MESSAGE_FLAG_REPLY ...................................  3
  3.3 SILC_MESSAGE_FLAG_SIGNED ..................................  4
  3.4 SILC_MESSAGE_FLAG_DATA ....................................  4
4 Security Considerations .......................................  5
5 References ....................................................  5
6 Author's Address ..............................................  6


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1. Introduction

The Secure Internet Live Conferencing [SILC1] supports sending binary
messages between users in the network.  To make the data sending, and
processing at the receiver's end as simple as possible the SILC defines
Message Flags to the Private Message Payload and Channel Message Payload
[SILC2], which can help the receiver to decide how the data is encoded,
and how it should be interpreted.  Some of the Message Flags may define 
additional payloads to be associated with the flag, but the [SILC2] does 
not define them.  This memo defines the payloads for those Message Flags 
that was marked to include additional payloads in [SILC2].

By defining the payloads for the Message Flags the SILC message payloads 
can be augmented to support any kind of data, which can be easily 
interpreted at the receiver end.  For example, it would be possible to 
send audio stream, video stream, image files and HTML pages as messages, 
and the receiver can either choose to ignore the message or to process 
it, or to perhaps pass the message to some application for processing.
Without specific payloads for Message Flags it is almost impossible for 
the receiver to interpret binary data from the payload.


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1.1 Requirements Terminology
  
The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
MAY, and OPTIONAL, when they appear in this document, are to be
interpreted as described in [RFC2119].


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2 SILC Message Flags

The Message Flags was added to the SILC protocol for the reason that SILC
provides sending binary data as messages between users, and entities in
the network, and interpreting pure binary data is almost impossible.  
With the flags the purpose, the reason, and the way the message is
supposed to be interpreted can be told to the recipient.  Other
conferencing protocols which are usually ASCII based protocols do not have
such problems since they do not generally support sending of binary data
at all, or require encoding of the data before it can be sent over the
network.

SILC Private Message Payload and Channel Message Payload can have flags
that can augment the function of the payload.  The flags can tell for
example that the message is a request, or a reply to an earlier received
request.  They can tell that the message is some action that the sender 
is performing, or they can tell that the message is an auto reply, or 
that it is explicitly digitally signed by the sender.

The problem of Message Flags is that the space for flags mask is only 16
bits, so there is a limited number of flags available.  For this reason a
flag that defines some generic way of sending any kind of data as a
message, and that it can be easily interpreted at the receiver's end is
important.  For this reason the flag SILC_MESSAGE_FLAG_DATA was added to
the protocol which can represent any data.  This memo describe how this 
flag is used and how the associated payload is constructed and processed.  
This memo also describes payloads for all the other flags that can have 
associated payloads.


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3 SILC Message Flag Payloads

The [SILC2] defines the flags which may have associated payloads.  This 
section will list these flags and define the payloads.


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3.1 SILC_MESSAGE_FLAG_REQUEST

Currently this flag can be used in the context of application specific, 
service specific or vendor specific requests, and the data payload type is 
dependent of this context.  Therefore, payload is not defined for this 
flag in this memo.  This flag may also be masked with some other flag in 
the message payload, including with some other flag that defines 
additional payload.


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3.2 SILC_MESSAGE_FLAG_REPLY

Currently this flag can be used in the context of application specific, 
service specific or vendor specific replies, and the data payload type is 
dependent of this context.  Therefore, payload is not defined for this 
flag in this memo.  This flag may also be masked with some other flag in  
the message payload, including with some other flag that defines 
additional payload.


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3.3 SILC_MESSAGE_FLAG_SIGNED

Not defined yet.


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3.4 SILC_MESSAGE_FLAG_DATA

This flag is used to represent any data as a message in the way that it
can be easily interpreted by the recipient.  This flag is used to send
MIME objects as messages from the sender to the receiver.  The MIME as
defined in [RFC2045], [RFC2046], [RFC2047], [RFC2048] and [RFC2049] is
well established protocol for sending different kind of data with many
applications and protocols.  It support dozens of different media types
and encodings, and for this reason is ideal for sending data in SILC
message payloads as well.

When the receiver has checked that the message payload includes the 
SILC_MESSAGE_FLAG_DATA flag, it may then start parsing the MIME header.  
It would also be possible to pass the message to some application which 
can already interpret MIME objects.  If the receiver does not support the 
media type received in the MIME header, it SHOULD be treated as
"application/octet-stream".  The receiver MAY also ignore and discard 
messages that it does not support.

The MIME header MUST be at the start of the data area of the Private 
Message Payload or Channel Message Payload.  The MIME header received in 
the data area of the payload SHOULD have the MIME-Version field at first 
and then Content-Type field.  The MIME-Version field is not required to be 
present in each body part of multipart entity.  Additionally the header
MAY also include any other MIME compliant headers.  The character encoding 
for the MIME Header strings inside the message payload is US-ASCII, as 
defined in [RFC2045].  The actual MIME object may define additional 
character sets or encodings for the data it delivers.

Hence, the MIME Header in the message payload may be as follows:

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MIME-Version: 1.0\\r\\n
Content-Type: discrete/composite\\r\\n
Content-Transfer-Encoding: binary\\r\\n
\\r\\n
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The Content-Transfer-Encoding field behaves as defined in [RFC2045] and 
defines the encoding of the data in the MIME object.  The preferred data 
encoding with SILC is "binary".  However, many MIME media types defines 
their preferred encoding and they may be used if binary encoding is not 
suitable.

When sending large amounts of traffic or large files as MIME objects the 
limits of the SILC Packet needs to be taken into consideration.  The 
maximum length of SILC Packet is 2^16 bytes, and larger messages would 
need to be fragmented.  MIME provides way of fragmenting and reassembling
messages, and it is to be done with SILC as defined in [RFC2046].  The 
MIME fragmentation is defined for gateway usage, but in case of SILC the 
sender may also start sending fragmented MIME objects.

This flag SHOULD NOT be masked with some other Message Flag that defines 
payloads.  Generally this sort of setting would be impossible for the 
receiver to interpret.  However, flags that does not define any specific 
payloads MAY be masked with this flag as well.  For example, this flag
could be masked also with SILC_MESSAGE_FLAG_REQUEST flag.


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4 Security Considerations

In case of SILC_MESSAGE_FLAG_DATA the implementors should pay special
attention to the security implications of any media type that can cause
the remote execution of any actions in the receiver's environment.  The
[RFC2046] and [RFC2048] discusses more MIME specific security
considerations.  Even though SILC provides secured messages, in case of
MIME which can be used to transfer files and documents which are stored in
the receiver's local environment, securing separately the MIME object may
be desired.  For example, augmenting the MIME support in SILC messages to
support S/MIME may be desired in some implementations.



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5 References 

[SILC1]      Riikonen, P., "Secure Internet Live Conferencing (SILC),
             Protocol Specification", Internet Draft, May 2002.

[SILC2]      Riikonen, P., "SILC Packet Protocol", Internet Draft,
             May 2002.

[RFC2045]    Freed, N., et al., "Multipurpose Internet Mail Extensions 
             (MIME) Part One: Format of Internet Message Bodies",
             Standards Track, RFC 2045, November 1996.

[RFC2046]    Freed, N., et al., "Multipurpose Internet Mail Extensions 
             (MIME) Part Two: Media Types", Standards Track, RFC 2045, 
             November 1996.

[RFC2047]    Moore K., "MIME (Multipurpose Internet Mail Extensions) 
             Part Three: Message Header Extensions for Non-ASCII Text"
             Standards Track, RFC 2047, November 1996.

[RFC2048]    Freed, N., et al., "Multipurpose Internet Mail Extensions
             (MIME) Part Four: Registration Procedures", Standards 
             Track, RFC 2048, November 1996.

[RFC2049]    Freed, N., et al., "Multipurpose Internet Mail Extensions
             (MIME) Part Five: Conformance Criteria and Examples", 
             Standards Track, RFC 2049, November 1996.

[RFC2119]    Bradner, S., "Key Words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.



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6 Author's Address

Pekka Riikonen
Snellmaninkatu 34 A 15
70100 Kuopio
Finland

EMail: priikone@iki.fi

This Internet-Draft expires 15 November 2002
