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Network Working Group                                            L. Wood
Internet-Draft                                             Cisco Systems
Intended status: Experimental                                   J. McKim
Expires: May 1, 2008                                                RSIS
                                                                 W. Eddy
                                                              W. Ivancic
                                                              C. Jackson
                                                        October 29, 2007

                   Using Saratoga with a Bundle Agent
          as a Convergence Layer for Delay-Tolerant Networking

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   Saratoga is a simple, lightweight, UDP-based transfer protocol.  This
   is a companion document to the Saratoga specification, given in
   draft-wood-tsvwg-saratoga.  This document describes how to use
   Saratoga as a Delay-Tolerant Networking (DTN) "convergence layer"
   with the DTN Bundle Protocol, and with DTN Bundle Agents.

Table of Contents

   1.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  3
   3.  Using Saratoga with a DTN Bundle Agent . . . . . . . . . . . .  4
   4.  Reactive Fragmentation . . . . . . . . . . . . . . . . . . . .  6
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   7.  A Note on Naming . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Informative References . . . . . . . . . . . . . . . . . . . .  7
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  8
   Intellectual Property and Copyright Statements . . . . . . . . . . 10

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1.  Background

   The Saratoga protocol is specified in [SaratogaProtocol].  Saratoga
   was originally designed prior to the DTNRG's work on the Bundle
   Protocol [I-D.irtf-dtnrg-bundle-spec].  It was later recognized that
   Saratoga could be used to reliably exchange bundles between DTN
   Bundle Agents by using a logical mapping between DTN bundles and
   Saratoga files and back.  Saratoga can be used in Delay/
   Disruption-Tolerant Networking (DTN) [RFC4838], as a "convergence
   layer" to exchange DTN bundles [I-D.irtf-dtnrg-bundle-spec] between
   peer nodes.  The DTN concept is applicable to networks where ad-hoc,
   intermittent connectivity is expected, connections may be
   infrequently established or short-lived, and end-to-end paths are not
   present.  This, coincidentally, is Saratoga's operating environment,
   making the Saratoga transfer protocol a natural fit as a convergence
   layer for DTN.

   This document contains notes on use of Saratoga for the bundle
   transfer procedure.

2.  Applicability Statement

   Why use Saratoga as a DTN convergence layer?  The DTN architecture
   already has a number of choices of convergence layer.  Convergence
   layers have been proposed for various link types, e.g.  Ethernet or
   Bluetooth.  As IP already runs over many link types, a convergence
   layer that can run over many links using IP is likely to take
   advantage of TCP or UDP.

   For traversing the terrestrial Internet while supporting congestion
   control, a simple TCP convergence layer has been implemented in the
   DTN software reference implementation.  A simple UDP convergence
   layer, able to be used over dedicated private links where congestion
   control is not required, is also present.  However, this simple UDP
   convergence layer presumes that a bundle will always fit into a
   single UDP packet, and does not support segmentation of bundles
   across multiple UDP packets.

   Two protocols capable of supporting segmentation of large bundles
   across multiple UDP packets, with ARQ-based flexible delivery robust
   to packet loss, are Saratoga [SaratogaProtocol] and the Licklider
   Transmission Protocol (LTP) [I-D.irtf-dtnrg-ltp-motivation].

   Both Saratoga and LTP were designed based on experience gained with
   using the CCSDS File Delivery Protocol (CFDP), which was developed
   for the Consultative Committee for Space Data Systems (CCSDS).  The
   main design difference between LTP and Saratoga is that LTP transfers

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   arbitrary un-named data blobs (binary large objects), requiring a
   higher layer (normally delay-tolerant-networking bundling) to handle
   naming, while Saratoga transfers named files including file metadata,
   and can be independent of higher layers.  Both protocols can run over
   the User Datagram Protocol, UDP [RFC0768], though LTP also is
   intended to run at other layers in the stack (including directly over
   the link), while Saratoga is only intended to run above the UDP or
   UDP-Lite transport protocols.  If errors in delivered content can be
   tolerated (perhaps because the data being transferred has its own
   integrity checks), Saratoga can also be used to transfer an entire
   file or stream without error checking, using UDP-Lite [RFC3828],
   which can protect only header content from errors.

   Saratoga includes a file checksum mechanism to detect transfer errors
   and to provide an overall degree of reliability.  Licklider has no
   similar reliability mechanism, although Licklider's optional security
   mechanism can be implemented to give some error detection.

   Saratoga can also be used for delivery over unidirectional broadcast
   links.  Another UDP-based convergence layer proposed for
   unidirectional links is Uni-DTN [I-D.kutscher-dtnrg-uni-clayer].
   Uni-DTN is based on FLUTE forward layered coding for multicast
   delivery.  Saratoga presumes that the forward error coding needed to
   prevent errors in transmission is present at another layer in the
   stack, usually near the physical layer.

3.  Using Saratoga with a DTN Bundle Agent

   While Saratoga was first developed for efficient file transfer, the
   similarity between bundle payloads and files, in that both are
   arbitrary blobs of some number of octets, allows Saratoga to be used
   as a convergence layer for exchanging bundles between DTN bundle
   agents.  This section explains the basic concepts involved in mapping
   bundle exchange onto the file transfer mechanism.

   Routing of bundles is outside the scope of Saratoga and of this
   document.  Once a complete bundle file has been transferred between
   peers using Saratoga, that bundle can be forwarded onwards along a
   next available hop in any way.  Saratoga provides a mechanism for
   forwarding, but does provide input to routing or forwarding

   A DTN bundle agent can work alongside a Saratoga peer to move
   bundles.  One simple method of communicating bundles between the
   bundle agent and the Saratoga peer is to have a shared directory that
   is accessible to both the bundle and Saratoga processes.  To send a
   bundle, the bundle agent can place the complete bundle (the

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   concatenated set of Bundle Protocol blocks) into a file in this
   shared directory.  The local Saratoga instance is then able to _put_
   this bundle to peers or allow them to _get_ it.  A flag bit in the
   Saratoga METADATA and DATA packets indicates whether a particular
   file is a bundle or not.  This enables the receiving Saratoga peer to
   know whether to handle the file itself, or to pass it to the local
   bundle agent.

   When using Saratoga as a convergence layer to transfer bundles, the
   local bundle agent will either place bundles as files for Saratoga to
   transfer from its directory, or otherwise use interprocess
   communication to notify Saratoga of and provide a bundle to be

   Key to the use of Saratoga for bundle transfer are:

   - indicating the capability to interoperate with a local bundle
   agent.  This involves advertising the capability to handle bundles
   via setting Flag bit 10 in Saratoga BEACON packets, and indicating
   when a bundle is being transferred by setting Flag bits 10 and 11 in
   the METADATA and DATA packets.

   - identifying the Bundle Agent in use, by providing an Endpoint
   Identifier (EID) in the Saratoga BEACON packet.

   Note that the name of a file holding a bundle is actually
   unimportant, as long as it can be determined that it does hold a
   bundle.  The filename becomes temporary, and local only to the
   transfer.  One implementation strategy is to name each bundle file
   with a file name constructed from two fields of the Primary Bundle
   Header: the DTN Endpoint Identifier (EID) of the destination node and
   the bundle's creation time field.  In the rare case of filename
   collisions in using this scheme, additional octets can be appended to
   the filename following some arbitrary local scheme.  Bundle files
   might be placed in different directories with different Saratoga-peer
   access controls depending on the intended next-hop, if this
   information is known ahead of time.  In any case, Saratoga only
   provides the transfer mechanism, and any forwarding decisions based
   on routing intelligence would be made within the DTN bundle agents.
   All of this detail is considered a matter of implementation for the
   bundle agent, and is not specified here.

   The identity field in the Saratoga BEACON packet allows a local DTN
   bundle agent to advertise its administrative EID via Saratoga.  Other
   Saratoga peers that hear that BEACON can then notify their local DTN
   bundle agents of the contact.  These notifications might be used to
   integrate contact information into a routing information base, as
   they are similar to the "hello" packets used in several routing

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   protocols.  However, this is outside the scope of this document.

   The "epoch" format used in Saratoga timestamps in file object records
   is the number of seconds since January 1, 2000 in UTC, which is the
   same epoch used in the DTN Bundle Protocol for timestamps.  This
   should include all leapseconds.

   We expect that Saratoga instances will often work in conjunction with
   DTN bundle agents to fill the role of a convergence-layer adapter
   between bundle agents connected via point-to-point links.  Saratoga
   implementations designed to work this way should have a way of
   notifying bundle agents when they receive BEACONs from other nodes,
   and when transfers have completed.  In order for custody transfer to
   function properly, notifications between the Saratoga instances and
   bundle agents on both sides of a fully-successful bundle file
   transfer is required.

   When Saratoga is used as a convergence-layer adapter, it is desirable
   to turn on Saratoga's end-to-end checksum facilty to provide an
   indication of correct bundle transfer.  This is necessary due to the
   bundle protocol design not including reliability checks or internal
   robustness.  See [I-D.irtf-dtnrg-bundle-checksum].

4.  Reactive Fragmentation

   For bundle file transfers, the local bundle agent could interact with
   Saratoga in order to perform a reactive fragmentation of the bundle
   whose transfer was interrupted by expiration of the inactivity timer.
   For DTN custody transfer, we expect complications to be encountered
   in making this reactive fragmentation work properly, and the details
   required to implement this functionality are left out of this
   specification until more experience has been obtained with reactive
   fragmentation in general.

   This document does not specify the functionality required for
   reactive fragmentation of bundles as described in [RFC4838], other
   than what is needed to support disrupted delivery and hop-by-hop
   custody transfer of complete bundles.  Reactive fragmentation of
   bundles lies outside the scope of custody transfer of complete
   bundles, and of this document.

   However, the status of a transfer that Saratoga provides to a bundle
   agent could be used to trigger the reactive fragmentation of bundles
   if a bundle file transfer is interrupted part-way through (assuming
   at least the bundle protocol headers and some portion of the data was
   successfully transferred first).  This would allow for efficient
   recovery when unplanned interruptions occur.  This requires some

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   coordination between the Saratoga node and the local bundle agent at
   each end.  The local API or coupling between the Saratoga peer and
   its bundle agent does not affect the interoperability between either
   the Saratoga peers or the DTN bundle agents, assuming that both sides
   agree that fragmentation will occur at the lowest un-acknowledged
   octet of the bundle file after the disruption.  Reactive
   fragmentation and any forwarding of the fragments onwards for
   reassembly at some downstream node is solely a bundle-agent problem.

5.  IANA Considerations

   This document has no actions for IANA.

6.  Security Considerations

   When Saratoga is used with a bundle agent, the security
   considerations outlined in [I-D.irtf-dtnrg-bundle-checksum] should be
   borne in mind.  Security in DTNs is in general considered an open
   issue.  If a framework of techniques for handling security in DTN
   scenarios emerges, Saratoga might be adapted to conform to this.

7.  A Note on Naming

   Saratoga is named for the USS Saratoga (CV-3), the aircraft carrier
   sunk at Bikini Atoll and now a popular diving site.  The philosophy
   behind the protocol and its use described here can be summarized as
   Saratoga Carries Upper Bundles Adequately, or SCUBA.

8.  Informative References

              Eddy, W., "Checksum Ciphersuites for the Bundle Protocol",
              draft-irtf-dtnrg-bundle-checksum-00 (work in progress),
              September 2007.

              Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", draft-irtf-dtnrg-bundle-spec-10 (work in
              progress), July 2007.

              Burleigh, S., Ramadas, M., and S. Farrell, "Licklider
              Transmission Protocol - Motivation",
              draft-irtf-dtnrg-ltp-motivation-05 (work in progress),

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              October 2007.

              Kutscher, D., "Uni-DTN: A DTN Convergence Layer Protocol
              for Unidirectional Transport",
              draft-kutscher-dtnrg-uni-clayer-00 (work in progress),
              April 2007.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
              G. Fairhurst, "The Lightweight User Datagram Protocol
              (UDP-Lite)", RFC 3828, July 2004.

   [RFC4838]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Networking Architecture", RFC 4838, April 2007.

              Wood, L., McKim, J., Eddy, W., Ivancic, W., and C.
              Jackson, "Saratoga: A Scalable File Transfer Protocol",
              draft-wood-tsvwg-saratoga, work in progress as an
              internet-draft, October 2007.

Authors' Addresses

   Lloyd Wood
   Cisco Systems
   11 New Square Park, Bedfont Lakes
   Feltham, Middlesex  TW14 8HA
   United Kingdom

   Phone: +44-20-8824-4236
   Email: lwood@cisco.com

   Jim McKim
   RS Information Systems
   NASA Glenn Research Center
   21000 Brookpark Road, MS 142-1
   Cleveland, OH  44135

   Phone: +1-216-433-6536
   Email: James.H.McKim@grc.nasa.gov

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   Wesley M. Eddy
   Verizon Federal Network Systems
   NASA Glenn Research Center
   21000 Brookpark Road, MS 54-5
   Cleveland, OH  44135

   Phone: +1-216-433-6682
   Email: weddy@grc.nasa.gov

   Will Ivancic
   NASA Glenn Research Center
   21000 Brookpark Road, MS 54-5
   Cleveland, OH  44135

   Phone: +1-216-433-3494
   Email: William.D.Ivancic@grc.nasa.gov

   Chris Jackson
   Surrey Satellite Technology Ltd
   Tycho House
   Surrey Space Centre
   20 Stephenson Road
   Guildford, Surrey  GU2 7YE
   United Kingdom

   Phone: +44-1483-803-803
   Email: C.Jackson@sstl.co.uk

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