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Encapsulation of Email over Delay-Tolerant Networks(DTN) using the Bundle Protocol

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Author Marc Blanchet
Last updated 2024-03-04
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Internet Engineering Task Force                              M. Blanchet
Internet-Draft                                              4 March 2024
Intended status: Standards Track                                        
Expires: 5 September 2024

   Encapsulation of Email over Delay-Tolerant Networks(DTN) using the
                            Bundle Protocol


   This document describes the encapsulation of emails using RFC2442
   format in the payload of bundles of the Bundle Protocol for the use
   case of Delay-Tolerant Networks(DTN) such as in space communications.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 5 September 2024.

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   Copyright (c) 2024 IETF Trust and the persons identified as the
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
     1.2.  Vocabulary  . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Description . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Encapsulation . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Use case where the endpoint is only a BP agent  . . . . . . .   4
   6.  Alternatives  . . . . . . . . . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   An important use case of Delay-Tolerant Networks(DTN) using the
   Bundle Protocol[RFC9171] is in space communications.  Current
   scenarios by space agencies[ioag] involves the use of an IP network
   on the planetary body and the use of the Bundle Protocol between
   planetary bodies, including Earth.  Therefore, there are IP endpoints
   at both ends, and then bundles could be used as a transport of
   Internet related application payload.  This document describes the
   encapsulation of emails over bundles so that end-users on the remote
   end (aka on a planetary body such as Moon or Mars) or processes can
   use typical Internet Email software and tools to read/write emails,
   while the emails when transiting in space are encapsulated into
   bundles of the Bundle Protocol.

   It should be noted that in DTNs, delays may be very large compared to
   normal delays on (Earth) Internet.  Therefore, the SMTP [RFC5321]
   "conversation" between the two SMTP peers should be avoided since it
   will take many round-trips over long delays networks to achieve the
   delivery.  Therefore, this document proposes to encapsulate the whole
   Email, including the envelope, using the Batch SMTP media-type
   [RFC2442] as a single file into bundles of the Bundle Protocol.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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1.2.  Vocabulary

   *  Internet: identifies the global IP network on Earth as we know it

   *  Planetary body: describes Moon, Mars and others.  In this
      document, we only care about ones that would have some IP
      networking installed

2.  Description

   In a typical scenario, the email would be created on (Earth)
   Internet, sent using regular delivery (DNS MX records, SMTP, ...) to
   a destination address that points to a location on a planetary body.
   That email would arrive to an SMTP server which is connected to the
   Bundle agent[RFC9171] capable of routing bundles to the final Bundle
   agent on the other planetary body, who has also a connection to an
   SMTP server.  That SMTP server on the other planetary body is
   responsible for final delivery on that planetary body network.  The
   target bundle protocol service number contained in the bundle is the
   one allocated by IANA per this document.

   TBD: artwork representation

   This document assumes that there is a close interaction between a
   Mail Transfer Agent(MTA) and a Bundle protocol agent, in, for
   example, the form of interprocess communication.  However, the
   specific interaction is outside the scope of this document and is
   left to the implementation.

3.  Encapsulation

   The payload of the bundle [RFC9171] is a Batch SMTP media-type
   content [RFC2442] that includes both the email itself and the
   envelope.  A bundle can only contain a single email.

   If the email is too large to fit in a single bundle, then the bundle
   agent uses bundle fragmentation as described in section 5.8 of
   [RFC9171] to slice the email into multiple bundles.  It is the
   responsability of the receiving bundle agent to properly reassemble
   the multiple bundle payloads into the source email.

   The receiving bundle agent will receive the email-containing
   bundle(s) on this document specifically assigned IANA service number.
   The agent transfers the email to a Mail Transfer Agent(MTA) that will
   deliver to the appropriate location as normal practice on Internet.

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4.  Considerations

   Configuring and deploying an isolated IP network on a planetary body
   with local mail servers, DNS servers and email client needs careful
   consideration.  For example, emails sent between two end-users on the
   same planetary body should not go through space links down to Earth
   and back to the planetary body.  This operational consideration is
   not described here and is outside the scope of this document.

   By using the encapsulation of emails using the [RFC2442] format,
   there is no negotiation and no declaration of capabilities as it is
   done in normal SMTP [RFC5321].  Therefore, the source endpoint has no
   way by this solution to know the capabilities of the other endpoint.
   Therefore, on the target planetary bodies MTAs should be properly
   configured to receive the appropriate kind of emails sent from
   another planetary body.

   As with typical SMTP on Internet, it is very possible that either
   improper configuration or other reasons cause the destination MTA to
   reject the email.  In this case, it should send an error using the
   same technique on the reverse path, if at least the From address is
   parsable.  If the email is not parsable on the destination MTA, then
   normal operational logging shall be used.  Similarly to the previous
   paragraph, this consideration of non-negotiation of capabilities is
   not described here and is outside the scope of this document.  It is
   however expected that this environment will be highly configured and
   managed, so that such issues shall not happen often.

   It should be noted that attachments to emails will be part of this
   encapsulation mechanism by default.

5.  Use case where the endpoint is only a BP agent

   There are cases such as a spacecraft currently moving in space where
   there might be no IP network attached to it and has only a BP agent.
   This specification works also as is in this use case, if that device
   is augmented by a local IP stack, with an SMTP listener, where the
   final source or destination of the SMTP packet is within the

6.  Alternatives

   There are other ways to send email for this use case.

   *  JMAP (RFC8620) could be used but would require http encapsulation
      over bundle protocol.

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   *  JMAP (RFC8620) JSON encoding of its data model could be
      encapsulated with a media-type similarly to Batch SMTP.

   *  Batch SMTP could be also encapsulated into a file transfer
      protocol such as CFDP and then MTA on both sides would have to
      watch the directory for new uploaded files and act upon those new

   *  A file synchronization mechanism such as rsync over some transport
      could also be used to synchronize user's mail stores.  This method
      could work for some use cases, but have some issues.  First, if
      the delay between two synchronisation events is sufficiently long,
      the origin mail store may contain a large amount of emails and be
      very big in size, therefore the synchronisation over the DTN
      network will be not only a big burden, but might contain non
      useful data, such as big files that are not relevant to read on
      the other side of the DTN network.  Moreover, it does not create a
      way to exchange emails between the two end of the DTN network.  It
      assumes that the same users are on both sides of the DTN network,
      which may well not be the case in many use cases.

   *  Inventing a new mail protocol native over bundle protocol

   There are probably other ways to acheive the same goal.  However, we
   believe this specification is the most simple and effective way.  An
   implementation is pretty straightforward and could leverage all
   software and experience of Internet mail.  For example, with this
   solution, it would be possible for an astronaut to use his mobile
   phone mail application to read his email while not knowing it has
   been carried over bundle protocol for some portions of the path.

7.  IANA Considerations

   This document requests IANA to allocate a new Bundle Protocol service
   number under the current CBHE Service Numbers and assign it to this
   document.  Description should be: "RFC5322 content (aka Email)".  If
   the registry is updated to indicate the Bundle Protocol version, this
   specification do apply for both BPv6 and BPv7, as it is agnostic of
   the BP version.

   Note to IANA (to be removed by the RFC editor): prefer 25 to relate
   to the Internet email service, but not a big deal if not.

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8.  Security Considerations

   Sending any payload with bad data over a space link is a somewhat DOS
   attack.  It is expected that this environment will be highly managed
   and controlled, therefore, before a bundle is sent, the payload is
   properly verified and access control to the space network will be
   tightly controlled.

   BPSEC[RFC9172] can be used to provide authentication and encryption
   at the Bundle layer.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2442]  Freed, N., Newman, D., Belissent, J., and M. Hoy, "The
              Batch SMTP Media Type", RFC 2442, DOI 10.17487/RFC2442,
              November 1998, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

9.2.  Informative References

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,

   [RFC9171]  Burleigh, S., Fall, K., and E. Birrane, III, "Bundle
              Protocol Version 7", RFC 9171, DOI 10.17487/RFC9171,
              January 2022, <>.

   [RFC9172]  Birrane, III, E. and K. McKeever, "Bundle Protocol
              Security (BPSec)", RFC 9172, DOI 10.17487/RFC9172, January
              2022, <>.

   [ioag]     Lunar Communications Architecture Working Group,
              Interagency Operations Advisory Group, "The Future Lunar
              Communications Architecture, Report of the Interagency
              Operations Advisory Group", January 2022.

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   The following people have reviewed and provided comments to improve
   this document (in no particular order): John Levine, Stephen Farrell,
   Stuart Card, Jorge Amodio, Ed Birrane, Pete Resnick.

Author's Address

   Marc Blanchet

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