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Network Working Group                                            L. Wood
Internet-Draft                                             Cisco Systems
Intended status: Experimental                                   J. McKim
Expires: January 7, 2008                                            RSIS
                                                                 W. Eddy
                                                                 Verizon
                                                              W. Ivancic
                                                                    NASA
                                                              C. Jackson
                                                                    SSTL
                                                            July 6, 2007


      Saratoga: A Convergence Layer for Delay-Tolerant Networking
                      draft-wood-dtnrg-saratoga-01

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 7, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Saratoga is a simple, lightweight UDP-based transport protocol



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   intended for use in moving files between immediately neighbouring
   peers which have sporadic, intermittent connectivity using dedicated
   IP links.  Saratoga focuses on high link utilization for fast file
   transfers.  Loss recovery is implemented via a simple ARQ mechanism.
   Saratoga is proposed for use as a convergence layer by Delay-Tolerant
   Networking for deep-space and ad-hoc networking scenarios.

   Saratoga is not intended for use over shared paths, so transport
   congestion control is unnecessary, although it could be optionally
   implemented at the sender to provide rate control of packet output.

   Saratoga is in continuous operational use by the IP-based Disaster
   Monitoring Constellation satellites in Low Earth Orbit (LEO) for
   mission-critical transfer of remote-sensing imagery.


Table of Contents

   1.  Background and Introduction  . . . . . . . . . . . . . . . . .  3
   2.  Overview of Saratoga File/Bundle Transfer  . . . . . . . . . .  5
     2.1.  Example File Transfers . . . . . . . . . . . . . . . . . .  5
     2.2.  Using Saratoga with a DTN Bundle Agent . . . . . . . . . .  8
     2.3.  Optional Parts of Saratoga . . . . . . . . . . . . . . . . 11
   3.  Packet Types . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.1.  BEACON . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     3.2.  REQUEST  . . . . . . . . . . . . . . . . . . . . . . . . . 17
     3.3.  METADATA . . . . . . . . . . . . . . . . . . . . . . . . . 20
     3.4.  DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     3.5.  HOLESTOFILL  . . . . . . . . . . . . . . . . . . . . . . . 27
   4.  Directory Entry  . . . . . . . . . . . . . . . . . . . . . . . 31
   5.  Behavior of a Saratoga Peer  . . . . . . . . . . . . . . . . . 33
     5.1.  Saratoga Transactions  . . . . . . . . . . . . . . . . . . 33
     5.2.  Beacons  . . . . . . . . . . . . . . . . . . . . . . . . . 36
     5.3.  Upper-Layer Interface  . . . . . . . . . . . . . . . . . . 37
     5.4.  Inactivity Timer . . . . . . . . . . . . . . . . . . . . . 37
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 38
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 39
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39
   9.  A Note on Naming . . . . . . . . . . . . . . . . . . . . . . . 39
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 40
     10.2. Informative References . . . . . . . . . . . . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
   Intellectual Property and Copyright Statements . . . . . . . . . . 43







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1.  Background and Introduction

   Saratoga is a rate-based UDP file transfer protocol capable of
   efficiently transferring both small and very large files.  Saratoga
   is also capable of streaming continuous content.  Saratoga was
   originally designed for the purpose of file transfer from LEO imaging
   satellites to the ground, and has been implemented and used since
   2004 to move mission imaging data from the Disaster Monitoring
   Constellation (DMC) remote-sensing satellites to ground stations.
   These satellites, built at the University of Surrey by Surrey
   Satellite Technology Ltd (SSTL), all use IP for payload
   communications and delivery of Earth imagery.  Five DMC satellites
   have been launched into orbit; four are currently operational; three
   further DMC satellites are under construction.  The DMC satellites
   use Saratoga to provide imagery under the aegis of the International
   Charter on Space and Major Disasters.

   Further details on how these DMC satellites use IP to communicate
   with the ground and the terrestrial Internet are discussed in other
   documents [Hogie05][Wood07a].

   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 the norm, connections between peers are
   intermittent and infrequent, and end-to-end paths are not present.
   Saratoga's design presumes that links are not shared, but rather that
   the links are dedicated to a pair of peer DTN nodes that can exchange
   store-and-forward bundle messages.  Each node runs a single instance
   of Saratoga; any multiplexing of simultaneous data transfers takes
   place in sessions between the peering instances of Saratoga.

   Use of store-and-forward delivery is typical of DTN scenarios in
   space exploration for both near-Earth and deep-space missions, and
   useful for other DTN scenarios, such as underwater networking, ad-hoc
   sensor networks, and some message-ferrying relay scenarios.

   High link utilization while a link is established is important in
   order to get the most utility out of the available, but limited,
   connectivity.  Recovery from packets lost due to channel errors is
   important.  Congestion control, to arbitrate fairly between competing
   traffic, is not considered important, as nodes peer across dedicated
   links without competition from other nodes, and loss is never due to



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   congestion.  Saratoga's design is intended for dedicated point-to-
   point links between peers.  In a shared wireless ad-hoc environment,
   allocation of channel resources to nodes to establish these links
   becomes a MAC-layer function.  Forward Error Coding (FEC) to get the
   most reliable transmission through a channel is best left near the
   physical layer so that it can be tailored for the channel.  Use of
   FEC complements the transport-level negative-acknowledgement approach
   to provide a reliable ARQ mechanism [RFC3366] that is presented here.

   Saratoga is capable of transferring small or large files, by choosing
   a width of file offset descriptor appropriate for the filesize, and
   advertising accepted offset descriptor sizes. 16-bit, 32-bit, 64-bit
   and 128-bit descriptors can be selected, for maximum file sizes of
   64KiB-1, 4GiB-1, 2^64-1 and 2^128-1 octets.  Earth imaging files
   currently transferred by Saratoga are mostly up to a few gigabytes in
   size.  Some implementations do transfer more than 4GiB in size, and
   so require offset descriptors larger than 32 bits.  We expect that a
   128-bit descriptor will satisfy all future needs, but we expect
   current implementations to only support up to 32-bit or 64-bit
   descriptors, depending on their application needs.  The 16-bit
   descriptor is useful for small messages, including messages from
   8-bit devices, and is always supported.  The 128-bit descriptor is
   useful for moving very large files stored on a 128-bit filesystem,
   such as on OpenSolaris ZFS.

   Saratoga can be used with either IPv4 or IPv6.  By using an IP-based
   convergence layer, compatibility between Saratoga and the wide
   variety of links that can already carry IP traffic is assured.  Due
   to widespread compatibility with a vast variety of existing link
   layers, IP is the logical choice of network layer to support DTN
   convergence layers, even though the scheduled or intermittent and
   point-to-point peering operating paradigm behind this use of IP here
   differs from the traditional congestion and competition-oriented end-
   to-end model that has become established in the fixed terrestrial
   Internet.

   Some of the motivations in the design of Saratoga are similar to
   those for the Licklider Transmission Protocol (LTP)
   [I-D.irtf-dtnrg-ltp-motivation].  Both of these protocols were
   designed based on experience gained with using the CCSDS File
   Delivery Protocol (CFDP), which was developed for the Consulative
   Committee for Space Data Systems (CCSDS).  The main design difference
   between LTP and Saratoga is that LTP transfers arbitrary un-named
   data blobs (binary large objects), while Saratoga transfers named
   files including file metadata.  Both run over the User Datagram
   Protocol, UDP [RFC0768].  LTP can also permit some portion of
   trailing bytes of a blob to be transferred unreliably, while Saratoga
   can be used to transfer an entire file or bundle unreliably using



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   UDP-Lite [RFC3828].

   Saratoga was originally implemented as outlined in [Jackson04], but
   the specification given here differs substantially, as we have added
   a number of features to support DTN bundling, while cleaning up the
   initial Saratoga specification.  The original Saratoga code uses a
   version number of 0, while code that implements this version of the
   protocol advertises a version number of 1.  Further discussion of the
   history and development of Saratoga is given in [Wood07b].

   This document contains an overview of the file or bundle transfer
   procedure and transactions using Saratoga in Section 2, followed by a
   formal definition of the packet types used by Saratoga in Section 3,
   and the details of the various protocol mechanisms in Section 5.

   Here, Saratoga transaction types are labelled with underscores around
   lowercase names (such as a "_get_" transaction), while Saratoga
   packet types are labelled in all capitals (such as a "REQUEST"
   packet) in order to distinguish between the two.

   This specification uses 'file' as a shorthand for 'file or bundle'.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.  [RFC2119]


2.  Overview of Saratoga File/Bundle Transfer

   In this section we first describe a typical file transfer process
   using Saratoga and then describe the way that Saratoga has been used
   to reliably and efficiently transfer bundles.

   Saratoga is a peer-to-peer protocol in the sense that between two
   Saratoga peers files may be transferred in both directions
   simultaneously.  Two separate unidirectional file transfers would
   need to be established to do simultaneous transfers.  The
   implementations of Saratoga at each end can be identical equivalent
   peers.

2.1.  Example File Transfers

   Saratoga nodes are simple file servers.  Saratoga supports several
   types of operations on files including upload, download, directory
   listing, and deletion requests.  Each request begins a "transaction".

   Saratoga nodes MAY advertise their presence, capabilities and desires
   by periodically sending BEACON packets.  These BEACONs are sent to



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   either a dedicated all-hosts multicast address when using IPv4, or a
   link-local all-Saratoga-peers multicast address when using IPv6.
   Saratoga nodes may discover other Saratoga nodes either through
   listening for BEACONs, through pre-configuration, or via some other
   trigger from a user, lower-layer protocol, or another process.  The
   BEACON is simply useful in low-delay ad-hoc networking or as explicit
   confirmation that another node is present; it is not required in
   order to begin a Saratoga transaction.

   There is an assumption that if multiple nodes do hear a BEACON on a
   link shared by more than two peers and respond with transaction
   requests, the node that sent the BEACON will favour one request and
   complete that before turning to other requests.  In such cases, a
   cooperative coarse-grained scheduling model, where transactions
   between different nodes are completed consecutively rather than
   simultaneously, would be desirable.  Currently, no protocol mechanism
   enforces this.  There is no required response to received BEACONs.
   Since request packets are generally small, and the number of nodes
   sharing a link should be small, the number of simultaneous
   transaction requests for a node to deal with, and decide between,
   will also be small.

   A Saratoga transaction begins with either a _get_, _put_, _getdir_,
   or _delete_ transaction request, corresponding to a desired download,
   upload, directory listing, or deletion operation.  The most common
   envisioned transaction is the _get_, which begins with a single
   Saratoga REQUEST packet sent from the peer wishing to receive the
   file, to the peer who currently has the file.  If the transaction is
   rejected, then a brief METADATA packet that conveys rejection is
   generated.  If the file-serving peer accepts the transaction, it
   generates and sends a more useful descriptive METADATA packet,
   follwed by some number of DATA packets constituting the directory
   listing's contents or the requested file.

   These DATA packets are finished by (and can intermittently include) a
   DATA packet with a flag bit set that demands the file-receiver send a
   reception report in the form of a HOLESTOFILL packet.  The
   HOLESTOFILL packet lists which octets of the file have not yet been
   received and whether or not the METADATA packet was received.  From
   this HOLESTOFILL packet, the file-sender begins a cycle of selective
   retransmission of DATA packets, until it sees a HOLESTOFILL packet
   that acknowledges total reception of all file data.

   In the example scenario in Figure 1, a_get_ request is granted and
   experiences loss of a single DATA packet due to channel-induced
   errors.





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        File-Receiver               File-Sender

           REQUEST --------------------->
      (transfer accepted) <--------- METADATA
           HOLESTOFILL -----------------> (voluntarily sent at start)
              <------------------------- DATA #1
                       (lost) <------ DATA #2
              <------------------------- DATA #3 (bit set
                                       requesting HOLESTOFILL)
           HOLESTOFILL ----------------->
      (indicating that range in DATA #2 was lost)
              <------------------------- DATA #2 (bit set
                                       requesting HOLESTOFILL)
           HOLESTOFILL ----------------->
      (complete file and METADATA received)

               Figure 1: Example _get_ transaction sequence

   A _getdir_ request proceeds similarly.

   The HOLESTOFILL and DATA packets are allowed to be sent at any time
   within the scope of a transaction in order for the file-sending node
   to optimize buffer management and transmission order.  For example,
   if the file-receiver already has the first half of a file from a
   previous disrupted transfer, it may send a HOLESTOFILL at the
   beginning of the transaction indicating that it has the first half of
   the file, and so only needs the last half of the file.  Thus,
   efficient recovery from interrupted sessions between peers becomes
   possible.

   In deep-space scenarios, the large propagation delays and round-trip
   times involved prohibit ping-pong packet exchanges for starting
   transactions.  The Saratoga _put_ transaction is useful in such
   cases.  A _put_ is initiated by the file-sender sending a METADATA
   packet followed by immediate DATA packets.  This is highly desirable
   in long-propagation deep-space (and similar) scenarios, without first
   waiting for a HOLESTOFILL.  This can be considered an "optimistic"
   mode of protocol operation, as it assumes the transaction request
   will be granted.  If the sender of a PUT request sees a METADATA
   packet indicating that the request was declined, it MUST stop sending
   any DATA packets within that transaction immediately.

   Figure 2 illustrates the sequence of packets in an example _put_
   transaction where the second DATA packet is lost.







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                      File-Sender           File-Receiver

                       METADATA ---------------->
                 (transfer accepted) <---------- HOLESTOFILL
                       DATA  #1 ---------------->
                       DATA  #2 ---> (lost)
                  DATA  #3 (bit set ------------>
                requesting HOLESTOFILL)
                   (DATA #2 lost) <------------- HOLESTOFILL
                  DATA  #2 (bit set  ----------->
                requesting HOLESTOFILL)
                 (transfer complete) <---------- HOLESTOFILL

                Figure 2: Example PUT transaction sequence

   The _delete_ transactions are simple single packet requests that
   trigger a HOLESTOFILL packet with a status code that indicates
   whether the file was deleted or not.  If the file is not able to be
   deleted for some reason, this reason can be conveyed in the Status
   field of the HOLESTOFILL packet.

2.2.  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, it can be forwarded onwards along a next
   available hop in any way.

   A DTN bundle agent can work alongside a Saragota peer to move bundles
   if there is a directory accessible to both the DTN and Saratoga
   processes.  To send a bundle, the bundle agent places the complete
   bundle (the concatenated set of Bundle Protocol blocks) into a file
   in the 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 packet indicates whether a particular file is a
   bundle or not, so the receiving Saratoga peer knows whether to write
   the received data to the local filesystem as a simple file, or to
   pass it to a local bundle agent that it is working with.

   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



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   bundle.  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
   handles reliable transfers, and any forwarding decisions based on
   routing intelligence are 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.

   A _get_ REQUEST packet that does not specify a filename (i.e. the
   request contains a zero-length File Path field) is specially defined
   to be a request for any chosen file that the peer wishes to send it.
   This allows a bundle agent coordinating with one Saratoga peer to
   blindly request any bundle files that the other Saratoga peer's
   bundle agent has ready for it, without prior knowledge of the
   directory listing, and without requiring the ability to determine
   whether files are bundle files or decode remote file names/paths for
   meaningful information such as final destination.

   A field in the Saratoga BEACON packet allows a local DTN bundle agent
   to advertise its administrative EID via Saratoga, so that other
   Saratoga peers that hear that BEACON can 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
   protocols.  However, this is outside the scope of this document.

   If a bundle is larger than Saratoga can be expected to transfer
   during a time-limited contact, there are at least two feasible
   options:

   (1) A bundle agent can use proactive fragmentation to create multiple
   smaller-sized bundle files.  Saratoga can transfer some number of
   these bundles fully during a contact.

   (2) To avoid bundle fragmentation, a Saratoga file-receiver can
   retain a partially-transferred file and request transfer of the
   unreceived bytes during a later contact.  This uses a HOLESTOFILL
   packet to make clear how much of the file has been successfully
   received and where transfer should be resumed from.  On resumption,
   the new METADATA (including file length, timestamps, and possibly MD5
   sum) MUST match that of the previous METADATA in order to re-
   establish the transfer.  Otherwise, the file-receiver MUST assume



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   that the file has changed and purge the DATA received during the
   first contact.

   If a bundle contains separate parts that require unreliable and
   reliable transmission, proactive fragmentation can be used to split
   the bundle into separate reliable and unreliable bundles that can be
   transferred separately, using UDP or UDP-Lite.  (If parts of a file
   require reliability but the rest can be sent by unreliable transfer,
   the file-sender can use its knowledge of the internal file structure
   and vary DATA packet size so that the reliable parts always start
   after the offset field and are covered by the UDP-Lite checksum.)  A
   file that permits unreliable delivery may be transferred onwards
   using UDP, if the current sender does not understand the internal
   file format to be able to decide what parts must be protected, the
   current sender or receiver does not support UDP-Lite, or the current
   protocol stack only implements error-free frame delivery below the
   UDP layer.

   Like the BEACON packets, a _put_ or a response to a _get_ may be sent
   to a dedicated IPv4 all-hosts multicast address (the experimental
   224.0.0.254 is used, pending an IANA assignment) or a dedicated IPv6
   link-local multicast address (FF02:0:0:0:0:0:0:FE is used, pending an
   IANA assignment) for multiple file-receivers on the link to hear.
   This is at the discretion of the file-sender, if it believes there is
   interest from multiple receivers.  In-progress DATA transfers may
   also be moved seamlessly from unicast to multicast if the file-sender
   learns during a transfer, from receipt of further unicast _get_
   REQUEST packets, that multiple nodes are interested in the file.  The
   associated METADATA packet is multicast when this transition takes
   place, and is then repeated periodically while the DATA stream is
   being sent, to inform newly-arrived listeners about the file being
   multicast.  Acknowledgements MUST NOT be demanded by multicast DATA
   packets, to prevent ack implosion at the file-sender, and instead
   holestofill information is aggregated and sent voluntarily by all
   file-receivers.  File-receivers respond to multicast DATA with
   multicast HOLESTOFILL packets.  File-receivers should introduce a
   short random delay before sending a HOLESTOFILL packet, to prevent
   ack implosion after a channel-induced loss, and must listen for
   HOLESTOFILL packets from others, to avoid duplicating fill requests.
   The file-sender SHOULD repeat any initial unicast portion of the
   transfer as multicast last of all, and may repeat and cycle through
   multicast of the file several times while file-receivers express
   interest via HOLESTOFILL or _get_ packets.  (Once in multicast and
   with METADATA being repeated periodically, new file-receivers do not
   need to send individual REQUEST packets.)  If a transfer has been
   started using UDP-Lite and new receivers indicate UDP-only
   capability, multicast transfers MUST switch to using UDP to
   accommodate them.



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   This version of the Saratoga specification 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.

   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.  On each end, this
   requires some coordination between the Saratoga node and the local
   bundle agent, but the local API or coupling between these 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.  This reactive fragmentation and any forwarding
   of the fragments onwards for reassembly at some downstream node is a
   bundle problem.  Reactive fragmentation lies outside the scope of
   custody transfer, of Saratoga and of this document.

2.3.  Optional Parts of Saratoga

   Implementing support for some parts of Saratoga is optional.  These
   parts are:

   - bundle support.  Use of a filesystem is expected.

   - transfers permitting some errors in content delivered, using UDP-
   Lite.  These can be useful for decreasing delivery time over
   unreliable channels, especially for unidirectional links - but
   requires that lower-layer frames permit delivery of unreliable data.

   - real-time streaming of content of unknown length, which requires
   some form of rate control at the sender.

   - including and recognising packet timestamps in DATA and
   HOLESTOFILL.  These timestamps are useful for streaming and for
   giving a file-sender an indication of path latency for rate control.
   There is no need for a file-receiver to understand the format used
   for these timestamps for it to be able to receive and reflect them.

   - any congestion control at the sender, based on feedback from
   acknowledgement HOLESTOFILL packets, or rate control limiting sender
   output.

   - multicast DATA transfers, if judged useful for the environment in
   which Saratoga is deployed.



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3.  Packet Types

   Saratoga is defined for use with UDP over either IPv4 or IPv6
   [RFC0768].  UDP checksums, which are mandatory with IPv6, MUST be
   used with IPv4.  Within either version of IP datagram, a Saratoga
   packet appears as a typical UDP header followed by an octet
   indicating how the remainder of the packet is to be interpreted:

                       1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       UDP source port         |     UDP destination port      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          UDP length           |         UDP checksum          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Ver|Packet Type| other Saratoga fields ... //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+//

   Saratoga data transfers can also be carried out using UDP-Lite
   [RFC3828].  If Saratoga can be carried over UDP-Lite, the
   implementation MUST also support UDP.  All packet types except DATA
   MUST be sent using UDP with checksums turned on.  For reliable
   transfers, DATA packets are sent using UDP with checksums turned on.
   For files or bundles where unreliable transfer has been indicated as
   desired and possible, the sender MAY send DATA packets unreliably
   over UDP-Lite, where UDP-Lite protects only the Saratoga headers and
   parts of the file that must be transmitted reliably.

   The two-bit Saratoga version field ("Ver") identifies the version of
   the Saratoga protocol that the packet conforms to.  The value 01
   should be used in this field for implementations conforming to the
   specification in this document, which specifies version 1 of
   Saratoga.  The value 00 was used in earlier implementations, prior to
   the formal specification and public submission of the protocol
   design, and is incompatible with version 01 in several respects.

   The six-bit Saratoga "Packet Type" field indicates how the remainder
   of the packet is intended to be decoded and processed:

      +---+-------------+-------------------------------------------+
      | # | Type        | Use                                       |
      +---+-------------+-------------------------------------------+
      | 0 | BEACON      | Beacon packet indicating peer status      |
      | 1 | REQUEST     | Commands peer to start a transfer         |
      | 2 | METADATA    | Carries file transfer metadata            |
      | 3 | DATA        | Carries octets of file data               |
      | 4 | HOLESTOFILL | Signals list of unreceived data to sender |
      +---+-------------+-------------------------------------------+



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   Several of these packet types include a Flags field, for which only
   some of the bits have defined meanings and usages in this document.
   Other, undefined, bits may be reserved for future use.  Following the
   principle of being conservative in what you send and liberal in what
   you accept, a packet sender MUST set any undefined bits to zero, and
   a packet recipient MUST NOT rely on these undefined bits being zero
   on reception.

   The specific formats for the different types of packets are given in
   this section.  Some packet types contain file offset descriptor
   fields, which contain unsigned integers.  The lengths of the offset
   descriptors are fixed within a transfer, but vary between file
   transfers.  The size is set for each particular transfer, depending
   on the choice of offset descriptor width made in the METADATA packet,
   which in turn depends on the size of file being transferred.

   In this document, all of the packet structure figures illustrating a
   packet format assume 32-bit lengths for these offset descriptor
   fields, and indicate the transfer-dependent length of the fields by
   using a "(descriptor)" designation within the [field] in all packet
   diagrams.  That is:

   The example 32-bit descriptors shown in all diagrams here

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                          (descriptor)                         ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   are suitable for files of up to 4GiB - 1 octets in length, and may be
   replaced in a file transfer by descriptors using a different length,
   depending on the size of file to be transferred:

   128-bit descriptor for very long files (optional)

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                          (descriptor)                         /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                     (descriptor, continued)                   /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                     (descriptor, continued)                   /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                     (descriptor, continued)                   ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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   64-bit descriptor for longer files (optional)

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                          (descriptor)                         /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                     (descriptor, continued)                   ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   16-bit descriptor for short files (MUST be supported)

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [         (descriptor)          ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Saratoga packets are intended to fit within link MTUs to avoid the
   inefficiencies and overheads of lower-layer fragmentation.  A
   Saratoga implementation itself does not perform any form of MTU
   discovery, but is assumed to be configured with knowledge of usable
   maximum IP MTUs for the link interfaces it uses.

3.1.  BEACON

   BEACON packets may be multicast periodically by nodes willing to act
   as Saratoga peers.  Some implementations have done so every 100
   milliseconds, but this rate is arbitrary, and should be chosen to be
   appropriate for the environment and implementation.

   The main purpose for sending BEACONs is to announce the presence of
   the node to potential peers (e.g. satellites, ground stations) to
   provide automatic service discovery, and also to confirm the liveness
   or presence of the peer.

   Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1|    Type   |                      Flags                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Bundle agent EID...                                  //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+//

   where








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   +--------+----------------------------------------------------------+
   | Field  | Description                                              |
   +--------+----------------------------------------------------------+
   | Type   | 0                                                        |
   | Flags  | convey whether or not the peer is ready to send/receive  |
   |        | and what the maximum supported file size range and       |
   |        | descriptor is.                                           |
   | Bundle | advertises the EID of a bundle agent administrative node |
   | Agent  | that the BEACON-sender is associated with, or some other |
   | EID    | peer-identifying string if no bundle agent is present.   |
   +--------+----------------------------------------------------------+

   The Flags field is used to provide some additional information about
   the peer.  The first octet of the Flags field is currently in use.
   The later two octets are for future use, and MUST be set to zero.

   The two highest-order bits (bits 8 and 9 above) indicate the maximum
   supported file size parameters that the peer's Saratoga
   implementation permits.  Other Saratoga packet types contain
   variable-length fields that convey file sizes or offsets into a file
   -- the file offset descriptors.  These descriptors may be 16-bit, 32-
   bit, 64-bit, or 128-bit in length, depending on the size of the file
   being transferred and/or the integer types supported by the sending
   peer.  The indicated bounds for the possible values of these bits are
   summarized below:

      +-------+-------+-------------------------+-------------------+
      | Bit 8 | Bit 9 | Supported Field Sizes   | Maximum File Size |
      +-------+-------+-------------------------+-------------------+
      | 0     | 0     | 16 bits                 | 2^16 - 1 octets.  |
      | 0     | 1     | 16 or 32 bits           | 2^32 - 1 octets.  |
      | 1     | 0     | 16, 32, or 64 bits      | 2^64 - 1 octets.  |
      | 1     | 1     | 16, 32, 64, or 128 bits | 2^128 - 1 octets. |
      +-------+-------+-------------------------+-------------------+

   If a Saratoga peer advertises it is capable of receiving a certain
   size of file, then it MUST also be capable of receiving files sent
   using smaller descriptor values.  This avoids overhead on small
   files, while increasing interoperability between peers.

   It is likely when sending unbounded streams that a larger descriptor
   size will be preferred to minimise problems with offset sequences
   wrapping.








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   +-----+-------+-----------------------------------------------------+
   | Bit | Value | Meaning                                             |
   +-----+-------+-----------------------------------------------------+
   | 10  | 0     | not able to pass bundles to a local bundle agent;   |
   |     |       | handles files.                                      |
   | 10  | 1     | can pass marked bundles to a local bundle agent.    |
   +-----+-------+-----------------------------------------------------+

   Bit 10 is used to indicate whether the sender is capable of handling
   bundles via a local bundle agent.

   Although a Saratoga implementation does not need to be working in
   conjunction with a DTN bundle agent, the combination will frequently
   be the case.  Saratoga BEACON packets include the administrative
   Endpoint ID of a DTN bundle agent that it is working in conjunction
   with.  This is not a prerequisite for bundle file transfers, or any
   other functionality, but is merely a convenient way to advertise the
   presence of a bundle agent along with the advertisement of Saratoga
   services.  For Saratoga implementations or deployments that do not
   involve bundle agent cooperation, some other type of host-identifier
   can be used in this field, as long as it is a reasonably unique
   string within the range of operational deployment.  This field
   encompasses the remainder of the packet, and might contain non-UTF-8
   and/or null characters.

          +-----+-------+--------------------------------------+
          | Bit | Value | Meaning                              |
          +-----+-------+--------------------------------------+
          | 11  | 0     | not capable of supporting streaming. |
          | 11  | 1     | capable of supporting streaming.     |
          +-----+-------+--------------------------------------+

   Bit 11 is used to indicate whether the sender is capable of sending
   and receiving continous streams.

   +--------+--------+-------------------------------------------------+
   | Bit 12 | Bit 13 | Capability and willingness to send              |
   |        |        | files/bundles                                   |
   +--------+--------+-------------------------------------------------+
   | 0      | 0      | cannot send files at all.                       |
   | 0      | 1      | invalid.                                        |
   | 1      | 0      | capable of sending, but not willing right now.  |
   | 1      | 1      | capable of and willing to send files.           |
   +--------+--------+-------------------------------------------------+







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   +--------+--------+-------------------------------------------------+
   | Bit 14 | Bit 15 | Capability and willingness to receive           |
   |        |        | files/bundles                                   |
   +--------+--------+-------------------------------------------------+
   | 0      | 0      | cannot receive files at all.                    |
   | 0      | 1      | invalid.                                        |
   | 1      | 0      | capable of receiving, but will reject METADATA. |
   | 1      | 1      | capable of and willing to receive files.        |
   +--------+--------+-------------------------------------------------+

   Also in the Flags field, bits 12 and 14 act as capability bits, while
   bits 13 and 15 augment these to with bits indicating willingess.
   Bits 12 and 13 deal with sending, while bits 14 and 15 deal with
   receiving.  If bit 12 is set, then the peer has the capability to
   send files.  If bit 14 is set, then the peer has the capability to
   receive files.  Bits 13 and 15 indicate willingness to send and
   receive files, respectively.

   A peer that is able to act as a file-sender MUST set the capability
   bit 12 in all BEACONs that it sends, regardless of whether it is
   willing to send any particular files to a particular peer at a
   particular time.  Bit 13 indicates the current presence of data to
   send and a willingness to send it in general, in order to augment the
   capability advertised by bit 12.

   If bit 14 is set, then the peer is capable of acting as a receiver,
   although it still might not currently be ready or willing to receive
   files (for instance, it may be low on free storage).  This bit MUST
   be set in any BEACON packets sent by nodes capable of acting as file-
   receivers.  Bit 15 augments this by expresses a current general
   willingness to receive and accept files.

   +-----+-------+-----------------------------------------------------+
   | Bit | Value | Meaning                                             |
   +-----+-------+-----------------------------------------------------+
   | 16  | 0     | supports DATA transfers over UDP only.              |
   | 16  | 1     | supports DATA transfers over both UDP and UDP-Lite. |
   +-----+-------+-----------------------------------------------------+

   Bit 16 is used to indicate whether the sender is capable of sending
   and receiving unreliable transfers via UDP-Lite.

3.2.  REQUEST

   A REQUEST packet is a command to perform either a _get_, _getdir_, or
   _delete_ transaction.





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   Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1|   Type    |                     Flags                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               Id                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                        File Path ...                          /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                           |   Directory Entry (optional)... //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+//

   where

   +------------+------------------------------------------------------+
   | Field      | Description                                          |
   +------------+------------------------------------------------------+
   | Type       | 1                                                    |
   | Flags      | provide additional information about the requested   |
   |            | file/operation; see table below for definition.      |
   | Id         | uniquely identifies the transaction between two      |
   |            | peers.                                               |
   | File Path  | the path of the requested file/directory following   |
   |            | the rules described below.                           |
   | Directory  | if the receiver already has the file at the expected |
   | Entry      | file path and is requesting an update to that file,  |
   | (optional) | this Directory Entry record information includes     |
   |            | known information about the file - length,           |
   |            | timestamp, etc. - to allow the file-sender to decide |
   |            | whether an updated file needs to be sent.            |
   +------------+------------------------------------------------------+

   The Id that is used during transactions serves to uniquely associate
   a given packet with a particular transaction.  This enables multiple
   simultaneous data transfer transactions between two peers.  The Id
   for a transaction is selected by the initiator so as to not conflict
   with any other in-progress or recent transactions with the same host.
   This Id should be unique and generated using properties of the file,
   which will remain constant across a host reboot.  The 3-tuple of both
   host identifiers and a carefully-generated transaction Id field can
   be used to uniquely index a particular transaction's state.

   In the Flags field, the bits labelled 8 and 9 in the figure above
   indicate the maximum supported file length fields that the peer can
   handle, and are interpreted exactly as the bits 8 and 9 in the BEACON
   packet described above.  The remaining defined bits are:



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   +-----+-------+-----------------------------------------------------+
   | Bit | Value | Meaning                                             |
   +-----+-------+-----------------------------------------------------+
   | 10  | 0     | The requester cannot handle bundles locally.        |
   | 10  | 1     | The requester can handle bundles.                   |
   | 11  | 0     | The requester cannot receive streams.               |
   | 11  | 1     | The requester is also able to receive streams.      |
   | 14  | 0     | a _get_ or _getdir_ transaction is requested        |
   | 14  | 1     | a _delete_ transaction is requested                 |
   | 15  | 0     | the File Path field holds a file for a _get_ or     |
   |     |       | _delete_                                            |
   | 15  | 1     | the File Path field specifies a directory name for  |
   |     |       | a _getdir_ or _delete_                              |
   | 16  | 0     | The requester is able to receive DATA over UDP      |
   |     |       | only.                                               |
   | 16  | 1     | The requester is also able to receive DATA over     |
   |     |       | UDP-Lite.                                           |
   +-----+-------+-----------------------------------------------------+

   The File Path portion of a _get_ packet is a null-terminated UTF-8
   encoded string [RFC3629] that represents the path and base file name
   on the file-sender of the file (or directory) that the file-receiver
   wishes to perform the _get_, _getdir_, or _delete_ operation on.
   Implementations SHOULD only send as many octets of File Path as are
   needed for carrying this string, although some implementations MAY
   choose to send a fixed-size File Path field in all REQUEST packets
   that is filled with null octets after the last UTF-8 encoded octet of
   the path.  A maximum of 1024 octets for this field, and for the File
   Path fields in other Saratoga packet types, is used to limit the
   total packet size to within a single IPv6 minimum MTU (minus some
   padding for network layer headers), and thus avoid the need for
   fragmentation.  The 1024-octet maximum applies after UTF-8 encoding
   and null termination.

   As in the standard Internet File Transfer Protocol (FTP) [RFC0959],
   for path separators, Saratoga allows whatever the local conventions
   on the peers are to be used.  There are security implications to
   processing these strings without some intelligent filtering and
   checking on the filesystem items they refer to, as discussed in the
   Security Considerations section later within this document.

   If the length of the REQUEST packet indicates that the File Path and
   File Record fields are empty (zero-length), then this indicates that
   the file-receiver is ready to receive any file that the file-sender
   would like to send it, rather than requesting a particular file.
   This is useful in bundle transfer, as it permits the file-sender to
   determine the order and selection of bundle files that it would like
   to forward through the file-receiving host's bundle agent in more of



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   a "push" manner.  Of course, bundle file retrieval could also follow
   a "pull" manner, with the file-receiving host requesting specific
   bundle files from the file-sender.  This may be desirable at times if
   the file-receiver is low on storage space, or other resources.  The
   file-receiver could also use the Saratoga _getdir_ transaction
   results in order to select small-sized bundles, or make other
   optimizations, such as using its local knowledge of contacts to pick
   small bundles that it has the best odds of being able to deliver
   completely.  Bundle file transfer through pushing sender-selected
   files implements delivery prioritization decisions made solely at the
   Saratoga file-sending node.  Bundle file transfer through pulling
   specific receiver-selected files implements prioritization involving
   more participation from the Saratoga file-receiver.  This is how
   Saratoga implements Quality of Service (QoS).

   REQUEST packets may be sent multicast, to learn about all listening
   nodes.  A multicast _get_ request for a file that elicits multiple
   unicast METADATA responses should be followed by unicast HOLESTOFILL
   packets with status errors cancelling all but one of the proposed
   transfers.  Timestamps in the Directory Entry can be used to select
   the most recent version of an offered file, and the host to fetch it
   from.

   Delete requests are ignored for files currently being transferred.

3.3.  METADATA

   METADATA packets are sent as part of a data transfer transaction
   (_get_, _getfile_, and _put_).  A METADATA packet says how large the
   file is and what its name is, as well as what size of file offset
   descriptor is chosen for the session.  METADATA packets are normally
   sent at the start of a data transfer, but may be repeated if
   requested.


















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   Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1|    Type   |                      Flags                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               Id                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               /
   /                                                               /
   /             MD5 integrity checksum (128 bits)                 /
   /                                                               /
   /                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               /
   /            single Directory Entry describing file             /
   /                      (variable length)                        /
   /                                                              //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-//

   where

   +-----------+-------------------------------------------------------+
   | Field     | Description                                           |
   +-----------+-------------------------------------------------------+
   | Type      | 2                                                     |
   | Flags     | indicate additional boolean metadata about a file     |
   | Id        | identifies the transaction that this packet describes |
   | MD5 Sum   | full 128-bit MD5 checksum over file contents          |
   | Directory | describes file system information about the file,     |
   | Entry     | including length, timestamps, etc.; the format is     |
   |           | specified in Section 4                                |
   +-----------+-------------------------------------------------------+

   The first octet of the Flags field is fully specified for use.  The
   later two octets are reserved for future use, and MUST be set to
   zero.

   In the Flags field, the bits labelled 8 and 9 in the figure above
   indicate the exact size of the offset descriptor fields used in this
   particular packet and are interpreted exactly as the bits 8 and 9 in
   the BEACON packet described above.  The value of these bits
   determines the size of the File Length field in the current packet,
   as well as indicating the size of the offset fields used in DATA and
   HOLESTOFILL packets within the session that will follow this packet.





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   +-----+-------+-----------------------------------------------------+
   | Bit | Value | Meaning                                             |
   +-----+-------+-----------------------------------------------------+
   | 10  | 0     | This is a file.                                     |
   | 10  | 1     | This file can be passed to a local bundle agent to  |
   |     |       | be handled as a bundle.                             |
   +-----+-------+-----------------------------------------------------+

   Bit 10 indicates whether a file or bundle is being transferred.

                  +-----+-------+-----------------------+
                  | Bit | Value | Meaning               |
                  +-----+-------+-----------------------+
                  | 11  | 0     | This is not a stream. |
                  | 11  | 1     | This is a stream.     |
                  +-----+-------+-----------------------+

   Bit 11 indicates whether an indefinite-length stream is being
   transferred.

   +-----+-------+-----------------------------------------------------+
   | Bit | Value | Meaning                                             |
   +-----+-------+-----------------------------------------------------+
   | 12  | 0     | This transfer is in progress.                       |
   | 12  | 1     | This transfer is no longer in progress, and has     |
   |     |       | been terminated.                                    |
   +-----+-------+-----------------------------------------------------+

   Bit 12 indicates whether the transfer is in progress, or has been
   terminated by the sender.  It is normally set to 1 only when METADATA
   is resent to indicate that a stream transfer has been ended.

               +-----+-------+-----------------------------+
               | Bit | Value | Meaning                     |
               +-----+-------+-----------------------------+
               | 13  | 0     | no MD5 checksum is present. |
               | 13  | 1     | MD5 checksum is present.    |
               +-----+-------+-----------------------------+

   Also inside the Flags field, bit 13 indicates the integrity checksum
   for the file to be transferred.  If 0, no MD5 checksum is present,
   and higher-level end-to-end checks within the bundle must be relied
   upon.  If 1, an MD5 checksum of the file to be transferred is carried
   as shown, with a fixed 128-bit field before the varying-length File
   Length and File Name information fields.  It is expected that the MD5
   checksum will be used, unless the Saratoga implementation is used
   exclusively for small transfers at the low end of the 16-bit file
   descriptor range, such as on low-performing hardware, where a weaker



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   checksum can suffice.

   The MD5 Sum field is generated via the MD5 algorithm [RFC1321],
   computed over the entire contents of the file being transferred.  The
   file-receiver can compute the MD5 result over the reassembled
   Saratoga DATA packet contents, and compare this to the METADATA's MD5
   Sum field in order to gain confidence that there were no undetected
   protocol errors or UDP checksum weaknesses encountered during the
   transfer.  Although MD5 is known to be less than optimal for security
   uses, it remains excellent for non-security use in integrity checking
   (as is done here in Saratoga), and has better performance
   implications than cryptographically-stronger alternatives given the
   limited available processing of many DTN use cases.

   +--------+--------+-------------------------------------------------+
   | Bit 14 | Bit 15 | Bundles and records                             |
   +--------+--------+-------------------------------------------------+
   | 0      | 0      | a file is being sent.                           |
   | 0      | 1      | the file being sent should be interpreted as a  |
   |        |        | directory record.                               |
   | 1      | 0      | a bundle is being sent.                         |
   | 1      | 1      | a stream is being sent.                         |
   +--------+--------+-------------------------------------------------+

   Also inside the Flags field, bits 14 and 15 indicate what is being
   transferred - a file, special file that contains directory records,
   bundle, or stream.  The value 01 indicates that the METADATA and DATA
   packets are being generated in response to a _getdir_ REQUEST, and
   that the assembled DATA contents should be interpreted as a sequence
   of Directory Records, as defined in Section 4.

   If a stream is being transferred, its length is unknown -- otherwise
   it would be a file.  The length property of a Directory Entry for a
   stream is expected to be zero.

   +--------+----------------------------------------------------------+
   | Bit 16 | Use                                                      |
   +--------+----------------------------------------------------------+
   | 0      | This file MUST be delivered reliably using UDP.          |
   | 1      | This file MAY be delivered unreliably or partly          |
   |        | unreliably using UDP-Lite.                               |
   +--------+----------------------------------------------------------+

   Bit 16 indicates whether the file must be sent reliably or can be
   sent at least partly unreliably, using UDP-Lite.  This flag can only
   be set if the originator of the file knows that at least some of the
   file content is suitable for sending unreliably and is robust to
   errors.  This flag reflects a property of the file itself.  This flag



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   may still be set if the immediate file-receiver is only capable of
   UDP delivery, on the assumption that this preference will be
   preserved for later transfers where UDP-Lite transfers may be taken
   advantage of by senders with knowledge of the internal file
   structure.  The file-sender may know that the receiver is capable of
   handling UDP-Lite, either from a _get_ REQUEST, from exchange of
   BEACONs, or a-priori.

   If the METADATA is in response to a GET request including a file
   record, and the record information for the held file matches what the
   requester has, then only the METADATA is sent repeating this
   information and verifying that the file is up to date.  If the record
   information does not match, the METADATA is used to initiate a
   transfer with following DATA packets to update the file.

3.4.  DATA

   A series of DATA packets form the main part of a data transfer
   transaction (_get_, _put_, or _getdir_).  The payloads constitute the
   actual file/bundle data being transferred.

   Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1|    Type   |                      Flags                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                               Id                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                Timestamp (optional, descriptor-size)          ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                      Offset (descriptor)                      ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where

   +---------+---------------------------------------------------------+
   | Field   | Description                                             |
   +---------+---------------------------------------------------------+
   | Type    | 3                                                       |
   | Flags 8 | bit 8 and 9 specify the size of offset descriptor, as   |
   | and 9   | elsewhere.                                              |
   | Flag 11 | bit 11 indicates that a stream is being carried.        |
   |         | Normally this bit will be zero for files/bundles.       |
   | Flag 12 | bit 12 indicates that an optional timestamp is included |
   |         | in the DATA header before the offset descriptor.        |




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   | Flag 15 | bit 15 requests an immediate HOLESTOFILL ack to be      |
   |         | generated in response to receiving this packet.         |
   | Id      | identifies the transaction that this packet belongs to  |
   | Offset  | the offset in octets to the location where the first    |
   |         | byte of this packet's payload is to be written          |
   +---------+---------------------------------------------------------+

   The DATA packet has a minimum size of ten octets, using sixteen-bit
   descriptors and no timestamps.

   DATA packets are normally sent error-free using UDP for reliable
   transfer (of both content and delivery).  However, for transfers that
   can tolerate content errors, DATA packets MAY be sent using UDP-Lite.
   If UDP-Lite is used, the file-sender must know that the file-receiver
   is capable of handling UDP-Lite, and the file or bundle contents to
   be transferred should be resilient to errors.  The UDP-Lite checksum
   MUST protect the Saratoga headers, up to and including the offset
   descriptor, and MAY protect more of each packet's payload, depending
   on the file-sender's knowledge of the internal structure of the file
   and the file's reliability requirements.

   Flag bits 8 and 9 are set to indicate the size of the offset
   descriptor as described for BEACON and METADATA packets, so that each
   DATA packet is self-describing.  This allows the DATA packet to be
   used to construct a file even when the initial METADATA is lost and
   must be resent.  The flag values here MUST be the same as indicated
   in the initial METADATA packet.

               +-----+-------+----------------------------+
               | Bit | Value | Meaning                    |
               +-----+-------+----------------------------+
               | 10  | 0     | A file is being carried.   |
               | 10  | 1     | A bundle is being carried. |
               +-----+-------+----------------------------+

   If bit 10 is set, a bundle is being carried for the local bundle
   agent.

             +-----+-------+---------------------------------+
             | Bit | Value | Meaning                         |
             +-----+-------+---------------------------------+
             | 11  | 0     | A file/bundle is being carried. |
             | 11  | 1     | A stream is being carried.      |
             +-----+-------+---------------------------------+

   Flag bit 11 indicates whether an open-ended indefinite-length stream
   is being transferred.




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   +-----+-------+-----------------------------------------------------+
   | Bit | Value | Meaning                                             |
   +-----+-------+-----------------------------------------------------+
   | 12  | 0     | This packet does not include an optional timestamp  |
   |     |       | field.                                              |
   | 12  | 1     | This packet includes an optional timestamp field.   |
   +-----+-------+-----------------------------------------------------+

   Flag bit 12 indicates that an optional timestamp is carried in the
   packet before the offset field.  The length of this timestamp is the
   length of the offset descriptor field.  Timestamps are particularly
   useful when streaming.  The timestamp format is implementation-
   dependent.

           +-----+-------+------------------------------------+
           | Bit | Value | Meaning                            |
           +-----+-------+------------------------------------+
           | 15  | 0     | No response is requested.          |
           | 15  | 1     | A HOLESTOFILL packet is requested. |
           +-----+-------+------------------------------------+

   Within the Flags field, if bit 15 of the packet is set, the file-
   receiver is to immediately generate a HOLESTOFILL packet to provide
   the file-sender with up-to-date information regarding the status of
   the file transfer.  This flag is set carefully and rarely.  It may be
   set periodically, but infrequently.  Asymmetric links with
   constrained backchannels can only carry a limited amount of
   HOLESTOFILL packets before ack congestion becomes a problem.  This
   flag is never set if a stream is being transferred, or if multicast
   is in use.

   Immediately following the DATA header is the payload, which consumes
   the remainder of the packet and whose length is implicitly defined by
   the end of the packet.  The payload octets are directly formed from
   the continuous octets starting at the specified Offset in the file
   being transferred.  No special coding is performed.  A zero-octet
   payload length is allowable.

   The length of the Offset fields used within all DATA packets for a
   given transaction MUST be consistent with the length indicated by
   bits 8 and 9 of the transactions METADATA packet.  If the METADATA
   packet has not yet been received, a file-receiver SHOULD request it
   via a HOLESTOFILL packet, and MAY choose to enqueue received DATA
   packets for later processing after the METADATA arrives.







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3.5.  HOLESTOFILL

   The HOLESTOFILL packet type is used for feedback from a Saratoga
   file-receiver to a Saratoga file-sender to indicate transaction
   progress and request transmission (usually re-transmission) of
   specific sets of octets within the current transaction (called
   "holes").  This can be used to clean up losses (or indicate no
   losses) at the end of, or during, a transaction, or to efficiently
   resume a transfer that was interrupted in a previous transaction.

   Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1 0|    Type   |               Flags           |     Status    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                Id                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [             Cumulative Acknowledgement (descriptor)           ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                 In-Response-To (timestamp, optional)          ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                 In-Response-To (descriptor)                   ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               (possibly, several Hole fields)                 /
   /                              ...                              /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where

   +-----------------+-------------------------------------------------+
   | Field           | Description                                     |
   +-----------------+-------------------------------------------------+
   | Type            | 4                                               |
   | Flags           | are defined below.                              |
   | Id              | identifies the transaction that this packet     |
   |                 | belongs to.                                     |
   | Status          | a value of 0x00 indicates the transfer is       |
   |                 | sucessfully proceeding.  All other values are   |
   |                 | errors terminating the transfer, explained      |
   |                 | below.                                          |
   | Zero-Pad        | an octet fixed at 0x00 to allow later fields to |
   |                 | be conveniently aligned for processing.         |
   | Cumulative      | the offset of the lowest-numbered octet of the  |
   | Acknowledgement | file not yet received.                          |





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   | In-Response-To  | This is only present if the timestamp flag is   |
   | (optional       | set.  If the HOLESTOFILL packet is voluntary    |
   | timestamp)      | and the voluntary flag is set, this will repeat |
   |                 | the timestamp of the DATA packet containing the |
   |                 | highest offset seen.  If the HOLESTOFILL packet |
   |                 | is in response to a mandatory request, this     |
   |                 | will repeat the timestamp of the requesting     |
   |                 | DATA packet.  The file-sender may use these     |
   |                 | timestamps to estimate latency.                 |
   | In-Response-To  | the offset of the highest-numbered octet within |
   | (descriptor)    | a DATA packet that generated this HOLESTOFILL   |
   |                 | packet, or the offset of the highest-numbered   |
   |                 | octet seen if this HOLESTOFILL is generated     |
   |                 | voluntarily and the voluntary flag is set.      |
   | Holes           | indications of offset ranges of missing data,   |
   |                 | defined below.                                  |
   +-----------------+-------------------------------------------------+

   The HOLESTOFILL packet has a minimum size of twelve octets, using
   sixteen-bit descriptors and no timestamps.

   The Id field is needed to associate the packet with the transaction
   that it refers to.  Using the Id as a key, the receiver of a packet
   can determine the lengths of the Cumulative Acknowledgement, In-
   Response-To, and Hole offsets used within the HOLESTOFILL packet, as
   this file offset descriptor size was set in the initial METADATA
   packet that established the Id.

   Flags bits 8 and 9 are set to indicate the size of the offset
   descriptor as described for BEACON and METADATA packets, so that each
   HOLESTOFILL packet is self-describing.  The flag values here MUST be
   the same as indicated in the initial METADATA and DATA packets.

   Other bits in the Flags field are defined as:

    +-----+-------+---------------------------------------------------+
    | Bit | Value | Meaning                                           |
    +-----+-------+---------------------------------------------------+
    | 12  | 0     | This packet does not include a timestamp field.   |
    | 12  | 1     | This packet includes an optional timestamp field. |
    +-----+-------+---------------------------------------------------+

   Flag bit 12 indicates that an optional timestamp is carried in the
   packet before the In-Response-To descriptor.  The length of this
   timestamp is the length of the descriptor field.






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         +-----+-------+----------------------------------------+
         | Bit | Value | Meaning                                |
         +-----+-------+----------------------------------------+
         | 13  | 0     | file's METADATA has been received.     |
         | 13  | 1     | file's METADATA has not been received. |
         +-----+-------+----------------------------------------+

   If bit 13 of a HOLESTOFILL packet has been set to indicate that the
   METADATA has not yet been received, then the METADATA should be
   resent.  This flag should normally be clear.

   +-----+-------+-----------------------------------------------------+
   | Bit | Value | Meaning                                             |
   +-----+-------+-----------------------------------------------------+
   | 14  | 0     | this packet contains the complete current set of    |
   |     |       | holes at the file-receiver.                         |
   | 14  | 1     | this packet contains incomplete hole-state; holes   |
   |     |       | shown in this packet should supplement other        |
   |     |       | incomplete hole-state known to the file-sender.     |
   +-----+-------+-----------------------------------------------------+

   Bit 14 of the HOLESTOFILL packet is only set when there are too many
   holes to fit within a single HOLESTOFILL packet due to MTU
   limitations.  This causes the hole list to be spread out over
   multiple HOLESTOFILL packets, each of which conveys distinct sets of
   holes.  This could occur, for instance, in a large file _put_
   scenario with a long-delay feedback loop and poor physical layer
   conditions.  When losses are light and/or hole reporting and repair
   is relatively frequent, all holes should easily fit within a single
   HOLESTOFILL packet, and this flag will be clear.  Bit 14 should
   normally be clear.

   In some rare cases of high loss, there may be too many holes in the
   received data to convey within a single HOLESTOFILL's size, which is
   limited by the link MTU size.  In this case, multiple HOLESTOFILL
   packets may be generated, and Flags bit 14 should be set on each
   HOLESTOFILL packet accordingly, to indicate that each packet holds
   incomplete results.  The complete group of HOLESTOFILL packets, each
   containing incomplete information, will share common In-Response-To
   information to distinguish them from any earlier groups.

   +-----+-------+----------------------------------------------------+
   | Bit | Value | Meaning                                            |
   +-----+-------+----------------------------------------------------+
   | 15  | 0     | This HOLESTOFILL was requested by the file-sender. |
   | 15  | 1     | This HOLESTOFILL is sent voluntarily.              |
   +-----+-------+----------------------------------------------------+




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   Flag bit 15 indicates whether the HOLESTOFILL is sent voluntarily or
   due to a request by the sender.  It affects content of the In-
   Response-To timestamp and descriptor fields.

   In the case of a transfer proceeding normally, immediately following
   the HOLESTOFILL packet header shown above, is a set of "Hole"
   definitions.  Each Hole definition is a pair of unsigned integers.
   For a 32-bit offset descriptor, each Hole definition consists of two
   four-octet unsigned integers:

   Hole Definition Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [             offset to start of hole (descriptor)              ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [              offset to end of hole (descriptor)               ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The start of the hole means the offset of the first unreceived byte
   in that hole.  The end of the hole means the last unreceived byte in
   that hole.

   For 16-bit descriptors, each Hole definition holds two two-octet
   unsigned integers, while Hole definitions for 64- and 128-bit
   descriptors require two eight- and two sixteen-octet unsigned
   integers respectively.

   Since each Hole definition takes up eight octets when 32-bit offset
   lengths are used, we expect that well over 100 such definitions can
   fit in a single HOLESTOFILL packet, given the IPv6 minimum MTU.

   A 'voluntary' HOLESTOFILL is sent at the start of each transaction,
   once METADATA information has been received.  This indicates that the
   receiver is ready to receive the file, or indicates an error or
   rejection code, described below.  A HOLESTOFILL indicating a
   successfully established transfer has a Cumulative Acknowledgement of
   zero and an In-Response-To field of zero.

   In the case of an error causing a transfer to be aborted, the Status
   field holds a code that can be used to explain the cause of the error
   to the other peer.  A zero value indicates that there have been no
   significant errors (this is called a "success HOLESTOFILL" within
   this document), while any non-zero value means the transaction should
   be aborted (this is called a "failure HOLESTOFILL").





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   +------------+------------------------------------------------------+
   | Status     | Meaning                                              |
   | Value      |                                                      |
   +------------+------------------------------------------------------+
   | 0x00       | Success, No Errors.                                  |
   | 0x01       | Unspecified Error.                                   |
   | 0x02       | Unable to send file due to resource constraints.     |
   | 0x03       | Unable to receive file due to resource constraints.  |
   | 0x04       | File not found.                                      |
   | 0x05       | Access Denied.                                       |
   | 0x06       | Unknown Id field for transaction.                    |
   | 0x07       | Did not delete file.                                 |
   | 0x08       | File length is longer than REQUEST indicates support |
   |            | for.                                                 |
   | 0x09       | File offset descriptors do not match expected use or |
   |            | file length.                                         |
   +------------+------------------------------------------------------+

   The recipient of a failure HOLESTOFILL MUST NOT try to process the
   Cumulative Acknowledgement, In-Response-To, or Hole offsets, because,
   in some types of error conditions, the packet's sender may not have
   any way of setting them to the right length for the transaction.


4.  Directory Entry

   Directory Entries have two uses within Saratoga:

   1.  Within a METADATA packet, a Directory Entry is used to give
       information about the file being transferred, in order to
       facilitate proper reassembly of the file and to help the file-
       receiver understand how recently the file may have been created
       or modified.

   2.  When a peer requests a directory listing via a _getdir_ REQUEST,
       the other peer generates a file containing a series of one or
       more concatenated Directory Entry records, and transfers this
       file as it would transfer the response to a normal _get_ REQUEST,
       sending the records together within DATA packets.  This file may
       be either temporary or within-memory and not actually a part of
       the host's file system itself.










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   Directory Entry Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                    Size  (descriptor)                         ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Mtime                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Ctime                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Properties   |                                               /
   +-+-+-+-+-+-+-+-+                                               /
   /                                                               /
   /           File Path (max 1024 octets,variable length)         /
   /                                                          ... //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-//

   where

   +------------+------------------------------------------------------+
   | field      | description                                          |
   +------------+------------------------------------------------------+
   | Size       | the size of the file or directory in octets.         |
   | Mtime      | a timestamp showing when the file or directory was   |
   |            | modified.                                            |
   | Ctime      | the timestamp of the last status change for this     |
   |            | file or directory.                                   |
   | Properties | if set, bit 7 of this field indicates that the entry |
   |            | corresponds to a directory.  Bit 6, if set,          |
   |            | indicates that the file is "special".  A special     |
   |            | file may not be directly transferable as it          |
   |            | corresponds to a symbolic link, a named pipe, a      |
   |            | device node, or some other "special" filesystem      |
   |            | object.  A file-sender may simply choose not to      |
   |            | include these types of files in the results of a     |
   |            | _getdir_ request.                                    |
   | File Path  | contains the file's name relative within the         |
   |            | requested path of the _getdir_ transaction, a        |
   |            | maximum of 1024-octet UTF-8 string, that is          |
   |            | null-terminated to indicate the beginning of the     |
   |            | next directory entry in _getdir_ results             |
   +------------+------------------------------------------------------+








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                  +-------+-------+---------------------+
                  | Bit 6 | Bit 7 | Properties conveyed |
                  +-------+-------+---------------------+
                  | 0     | 0     | normal file.        |
                  | 0     | 1     | normal directory.   |
                  | 1     | 0     | special file.       |
                  | 1     | 1     | special directory.  |
                  +-------+-------+---------------------+

   Whether a particular Directory Entry is being interpreted as the
   contents of a METADATA packet, or as the result of a _getdir_
   transaction, the width of the Size field is the same as that used for
   all lengths and offsets within the transfer, given by the METADATA
   and DATA Flags bits 8 and 9.

   Streams listed in a directory should be marked as special.

   It is expected that files are only listed in Directory Entries if
   they can be transferred to the requester.  An implementation only
   capable of receiving small files using 16-bit descriptors will only
   see small files capable of being transferred to it when browsing the
   filesystem of an implementation capable of larger sizes.  Directory
   sizes are not sent, and a Size of 0 is given instead for directories.

   The "epoch" format used in the timestamps for Mtime and Ctime 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.

   A file-receiver should preserve the timestamp information received in
   the METADATA for its own copy of the file, to allow newer versions of
   files to propagate and supercede older versions.


5.  Behavior of a Saratoga Peer

   This section describes some details of Saratoga implementations and
   uses the RFC 2119 standards language to describe which portions are
   needed for interoperability.

5.1.  Saratoga Transactions

   Following are descriptions of the packet exchanges between two peers
   for each type of transaction.







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5.1.1.  The _get_ Transaction

   1.  A peer (the file-receiver) sends a REQUEST packet to its peer
       (the file-sender).  The Flags bits are set to indicate that this
       is not a _delete_ request, nor does the File Path indicate a
       directory.  Each _get_ transaction corresponds to a single file,
       and fetching multiple files requires sending multiple REQUEST
       packets and using multiple transaction Ids. If a specific file is
       being requested, then its name is filled into the File Path
       field, otherwise it is left null and the file-sender will send a
       file of its choice.

   2.  If the request is rejected, then a HOLESTOFILL packet containing
       an error code in the Status field is sent and the transaction is
       terminated.  This HOLESTOFILL packet MUST be sent to reject and
       terminate the transaction.  The error code MAY make use of the
       "Unspecified Error" value for security reasons.  Some REQUESTs
       might also be rejected for specifying files that are too large to
       have their lengths encoded within the maximum integer field width
       advertised by bits 8 and 9 of the REQUEST.

   3.  If the request is accepted, then a HOLESTOFILL packet MUST be
       sent with an error code of 0x00 and an In-Response-To field of
       zero.

   4.  Otherwise, if the request is granted, then the file-sender
       generates and sends a METADATA packet along with the contents of
       the file as a series of DATA packets.  In the absence of
       HOLESTOFILL packets, if the file-sender believes it has finished
       sending the file, it MUST send the last DATA packet with the
       Flags bit set requesting a HOLESTOFILL response from the file-
       receiver.  This can be followed by empty DATA packets with the
       Flags bit set requesting a HOLESTOFILL until either a HOLESTOFILL
       packet is received, or the inactivity timer expires.  All of the
       DATA packets MUST use field widths for the file offset descriptor
       fields that match what the Flags of the METADATA packet
       specified.  Some arbitrarily selected DATA packets may have the
       Flags bit set that requests a HOLESTOFILL packet.  The file-
       receiver MAY voluntarily send HOLESTOFILL packets at other times,
       where the In-Response-To field MUST set to zero.  The file-
       receiver SHOULD voluntarily send a HOLESTOFILL packet in response
       to the first DATA packet.

   5.  As the file-receiver takes in the DATA packets, it writes them
       into the file locally.  The file-receiver keeps track of missing
       data in a hole list.  Periodically the file sender will set the
       ack flag bit in a DATA packet and request a HOLESTOFILL packet
       from the file-receiver, with a copy of this hole list.  File-



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       receivers MUST send a HOLESTOFILL packet immediately in response
       to receiving a DATA packet with the Flags bit set requesting a
       HOLESTOFILL.

   6.  If the file-sender receives a HOLESTOFILL packet with a non-zero
       number of holes, it re-fetches the file data at the specified
       offsets and re-transmits it.  If the METADATA packet requires
       retransmission, this is indicated by a bit in the HOLESTOFILL
       packet, and the METADATA packet is retransmitted.  The file-
       sender MUST retransmit data from any holes reported by the file-
       receiver before proceeding further with new DATA packets.

   7.  When the file-receiver has fully received the file data and the
       METADATA packet, then it sends a HOLESTOFILL packet indicating
       that the transaction is complete, and it terminates the
       transaction locally, although it MUST persist in responding to
       DATA packets requesting HOLESTOFILLs from the file-sender for
       some reasonable amount of time.

   Given that there may be a high degree of asymmetry in link bandwidth
   between the file-sender and file-receiver, the HOLESTOFILL packets
   should be carefully generated so as to not congest the feedback path.
   This means that both a file-sender should be cautious in setting the
   DATA Flags bit requesting HOLESTOFILLs, and also that a file-receiver
   should be cautious in gratuitously generating HOLESTOFILL packets of
   its own volition.  On unidirectional links, a file-sender cannot
   reasonably expect to receive HOLESTOFILL packets, so should never
   request them.

5.1.2.  The _getdir_ Transaction

   A _getdir_ transaction proceeds through the same states as the _get_
   transaction.  The two differences are that the REQUEST has the
   directory bit set in its Flags field, and that, rather than
   transferring the contents of a file from the file-receiver to the
   file-sender, a set of records representing the contents of a
   directory are transferred.  These can be parsed and dealt with by the
   file-receiver as desired.  There is no requirement that a Saratoga
   peer send the full contents of a directory listing; a peer may filter
   the results to only those entries that are actually accessible to the
   requesting peer.

   For _getdir_ transactions, the METADATA's bits 8 and 9 in the Flags
   field specify both the width of the offset and length fields used
   within the transfers DATA and HOLESTOFILL packets, and also the width
   of file Size fields within Directory Entries in the interpreted
   _getdir_ results.  These Flags bits are set to the minimum of the
   file-sender's locally-supported maximum width and the advertised



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   maximum width within the REQUEST packet, and any file system entries
   that would normally be contained in the results, but that have sizes
   greater than this width can convey, MUST be filtered out.

5.1.3.  The _delete_ Transaction

   1.  A peer sends a REQUEST packet with the bit set indicating that it
       is a deletion request and the path to be deleted is filled into
       the File Path field.  The File Path MUST be filled in for
       _delete_ transactions, unlike for _get_ transactions.

   2.  The other peer replies with a feedback HOLESTOFILL packet having
       a Status code that indicates whether the deletion was granted and
       occurred successfully (indicated by the 0x00 Status field in a
       success HOLESTOFILL), or whether some error occurred (indicated
       by the non-zero Status field in a failure HOLESTOFILL).  This
       HOLESTOFILL packet MUST have no Holes and 16-bit width zero-
       valued Cumulative Acknowledgement and In-Response-To fields.

5.1.4.  The _put_ Transaction

   A _put_ transaction proceeds exactly as a _get_, except the file-
   sender and file-receiver roles are exchanged between peers, and no
   REQUEST packet is ever sent.  The file-sending end senses that the
   transaction is in progress when it receives METADATA or DATA packets
   for which it has no knowledge of the Id field.  If the file-receiver
   decides that it will store and handle this request (at least
   provisionally), then it MUST send a voluntary (ie, not requested)
   success HOLESTOFILL packet to the file-sender.  Otherwise, it sends a
   failure HOLESTOFILL packet.  After sending a failure HOLESTOFILL
   packet, it may ignore future packets with the same Id field from the
   file-sender, but it should, at a low rate, periodically regenerate
   the failure HOLESTOFILL packet if the flow of packets does not stop.

5.2.  Beacons

   Sending BEACON packets is not needed in any of the transactions
   discussed in this specification, but optional BEACONs can provide
   useful information in many situations.  If a node periodically
   generates BEACON packets, then it should do so at a low rate which
   does not significantly affect in-progress data transfers.

   A node that supports multiple versions of Saratoga (e.g. version 1
   from this specification along with the older version 0), MAY send
   multiple BEACON packets showing different version numbers.  The
   version number in a single BEACON should not be used to infer the
   larger set of protocol versions that a peer is compatible with.




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   If a node receives BEACONs from a peer, then it SHOULD NOT attempt to
   start any _get_, _getdir_, or _delete_ transactions with that peer if
   bit 14 is not set in the latest received BEACONs.  Likewise, if
   received BEACONs from a peer do not have bit 15 set, then _put_
   transactions SHOULD NOT be attempted to that peer.  Unlike the
   capabilities bits which prevent certain types of transactions from
   being attempted, the willingness bits are advisory, and transactions
   MAY be attempted even if the node is not advertising a willingness,
   as long as it advertises a capability.  This avoids waiting for a
   willingness indication across long-delay links.

5.3.  Upper-Layer Interface

   No particular interface functionality is required in implementations
   of this specification.  The means and degree of access to Saratoga
   configuration settings and transaction control that is offered to
   upper layers is completely implementation-dependent.  In general, it
   is expected that upper layers (or users) can set timeout values for
   transaction requests and for inactivity periods during the
   transaction, on a per-peer or per-transaction basis, but in some
   implementations where the Saratoga code is restricted to run only
   over certain interfaces with well-understood operational latency
   bounds, then these timers MAY be hard-coded.

   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.

5.4.  Inactivity Timer

   In order to determine the liveliness of a transaction, Saratoga nodes
   may implement an inactivity timer for each peer they are expecting to
   see packets from.  For each packet received from a peer, its
   associated inactivity timer is reset.  If no packets are received for
   some amount of time, and the inactivity timer expires, this serves as
   a signal to the node that it should abort (and optionally retry) any
   sessions that were in progress with the peer.  Information from the
   link interface (i.e. link down) can override this timer for point-to-
   point links.

   The actual length of time that the inactivity timer runs for is a
   matter of both implementation and deployment situation.  Relatively



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   short timers (on the order of several round-trip times) allow nodes
   to quickly react to loss of contact, while longer timers allow for
   transaction robustness in the presence of transient link problems.
   This document deliberately does not specify a particular inactivity
   timer value nor any rules for setting the inactivity timer, because
   the protocol is intended to be used in both long- and short-delay
   regimes.

   Specifically, the inactivity timer is started on sending REQUEST or
   HOLESTOFILL packets.  When sending packets not expected to elicit
   responses (BEACON, METADATA, or DATA without acknowledgement
   requests), there is no point to starting the local inactivity timer.

   For normal file (non-bundle file) transfers, there are simple rules
   for handling expiration of the inactivity timer during a _get_ or
   _put_ transaction.  The file-sender should terminate the transaction
   state and cease to send DATA or METADATA packets.  The file-receiver
   should stop sending HOLESTOFILL packets, and MAY choose to store the
   file in some cache location so that the transfer can be recovered.
   This is possible by waiting for an opportunity to re-attempt the
   transaction and immediately sending a HOLESTOFILL that only lists the
   parts of the file not yet received if the transaction is granted.  In
   any case, a partially-received file MUST NOT be handled in any way
   that would allow another application to think it is complete.

   For bundle file transfers, the local bundle agent might be interacted
   with in order to perform a reactive fragmentation of the bundle whose
   transfer was interrupted by expiration of the inactivity timer.
   (Reactive fragmentation is discussed in [RFC4838].)  For custody
   transfer, there are some complications to making this reactive
   fragmentation work properly, and the details required to implement
   this functionality are left out of this specification until more
   experience with reactive fragmentation in general is obtained.

   The file-sender may implement more complex timers to allow rate-based
   pacing or simple congestion control using information provided in
   HOLESTOFILL packets, but such possible timers and their effects are
   deliberately not specified here.


6.  Security Considerations

   Saratoga does not in itself provide any services for authentication
   of session requests or data, nor does it in itself provide any
   privacy or access control for data files transferred.  These issues
   may be addressed within an implementation or deployment in several
   ways that do not affect the file transfer protocol itself.  For
   instance, IPsec may be used to protect Saratoga implementations from



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   forged packets, to provide privacy, or to authenticate the identity
   of a peer, for instance.  Other implementation-specific or
   configuration-specific mechanisms and policies might also be employed
   for authentication and authorization of requests.  Protection of file
   data and meta-data can also be provided by a higher level file
   encryption facility.  Basic security practices like not accepting
   paths with "..", not following symbolic links, and using a chroot()
   system call, among others, should also be considered within an
   implementation.

   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.  IANA Considerations

   IANA has allocated port 7542 (tcp/udp) for use by Saratoga.

   saratoga        7542/tcp    Saratoga Transfer Protocol
   saratoga        7542/udp    Saratoga Transfer Protocol

   A dedicated IPv4 all-hosts multicast address and a dedicated IPv6
   link-local multicast address will be requested from IANA.


8.  Acknowledgements

   Developing and deploying the on-orbit IP infrastructure of the
   Disaster Monitoring Constellation, in which Saratoga has proven
   useful, has taken the efforts of hundreds of people over more than a
   decade.  We thank them all.

   Work on this document at NASA's Glenn Research Center was funded by
   NASA's Earth Science Technology Office (ESTO).

   We thank Stewart Bryant for his review comments.


9.  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.


10.  References



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10.1.  Normative References

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

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

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

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [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.

10.2.  Informative References

   [Hogie05]  Hogie, K., Criscuolo, E., and R. Parise, "Using Standard
              Internet Protocols and Applications in Space", Computer
              Networks Special Issue on Interplanetary Internet, vol.
              47, no. 5, pp. 603-650, April 2005.

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

   [I-D.irtf-dtnrg-ltp-motivation]
              Burleigh, S., "Licklider Transmission Protocol -
              Motivation", draft-irtf-dtnrg-ltp-motivation-04 (work in
              progress), April 2007.

   [Jackson04]
              Jackson, C., "Saratoga File Transfer Protocol", SSTL
              Internal Document , 2004.

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, October 1985.

   [RFC3366]  Fairhurst, G. and L. Wood, "Advice to link designers on
              link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
              August 2002.

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



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              Networking Architecture", RFC 4838, April 2007.

   [Wood07a]  Wood, L., Ivancic, W., Hodgson, D., Miller, E., Conner,
              B., Lynch, S., Jackson, C., da Silva Curiel, A., Cooke,
              D., Shell, D., Walke, J., and D. Stewart, "Using Internet
              Nodes and Routers Onboard Satellites", International
              Journal of Satellite Communications and Networking Special
              Issue on Space Networks, vol. 25, no. 2, pp. 195-216,
              March/April 2007.

   [Wood07b]  Wood, L., Eddy, W., Ivancic, W., Miller, E., McKim, J.,
              and C. Jackson, "Saratoga: a Delay-Tolerant Networking
              convergence layer with efficient link utilization", to
              appear in the International Workshop on Satellite and
              Space Communications (IWSSC '07) Salzburg, September 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
   USA

   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
   USA

   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
   USA

   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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