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Network Working Group                                            L. Wood
Internet-Draft                                      University of Surrey
Intended status: Experimental                                    W. Eddy
Expires: June 26, 2011                                       MTI Systems
                                                                C. Smith
                                                                   CSIRO
                                                              W. Ivancic
                                                                    NASA
                                                              C. Jackson
                                                                    SSTL
                                                       December 23, 2010


              Saratoga: A Scalable File Transfer Protocol
                      draft-wood-tsvwg-saratoga-08

Abstract

   This document specifies the Saratoga transfer protocol.  Saratoga was
   originally developed to efficiently transfer remote-sensing imagery
   from a low-Earth-orbiting satellite constellation, but is useful for
   many other scenarios, including ad-hoc peer-to-peer communications,
   delay-tolerant networking, and grid computing.  Saratoga is a simple,
   lightweight, content dissemination protocol that builds on UDP, and
   optionally uses UDP-Lite.  Saratoga is intended for use when moving
   files or streaming data between peers which may have permanent,
   sporadic or intermittent connectivity, and is capable of transferring
   very large amounts of data reliably under adverse conditions.  The
   Saratoga protocol is designed to cope with highly asymmetric link or
   path capacity between peers, and can support fully-unidirectional
   data transfer if required.  In scenarios with dedicated links,
   Saratoga focuses on high link utilization to make the most of limited
   connectivity times, while standard congestion control mechanisms can
   be implemented for operation over shared links.  Loss recovery is
   implemented via a simple negative-ack ARQ mechanism.  The protocol
   specified in this document is considered to be appropriate for
   experimental use on private IP networks.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may not be modified,
   and derivative works of it may not be created, except to format it
   for publication as an RFC and to translate it into languages other
   than English.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute



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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 26, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



























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Table of Contents

   1.  Background and Introduction  . . . . . . . . . . . . . . . . .  4
   2.  Overview of Saratoga File Transfer . . . . . . . . . . . . . .  6
   3.  Optional Parts of Saratoga . . . . . . . . . . . . . . . . . . 11
   4.  Packet Types . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  BEACON . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.2.  REQUEST  . . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.3.  METADATA . . . . . . . . . . . . . . . . . . . . . . . . . 22
     4.4.  DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     4.5.  STATUS . . . . . . . . . . . . . . . . . . . . . . . . . . 30
   5.  The Directory Entry  . . . . . . . . . . . . . . . . . . . . . 36
   6.  Behaviour of a Saratoga Peer . . . . . . . . . . . . . . . . . 39
     6.1.  Saratoga Transactions  . . . . . . . . . . . . . . . . . . 39
     6.2.  Beacons  . . . . . . . . . . . . . . . . . . . . . . . . . 43
     6.3.  Upper-Layer Interface  . . . . . . . . . . . . . . . . . . 43
     6.4.  Inactivity Timer . . . . . . . . . . . . . . . . . . . . . 43
   7.  Mailing list . . . . . . . . . . . . . . . . . . . . . . . . . 44
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 44
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 45
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 45
   11. A Note on Naming . . . . . . . . . . . . . . . . . . . . . . . 46
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 46
     12.2. Informative References . . . . . . . . . . . . . . . . . . 46
   Appendix A.  Timestamp/Nonce field considerations  . . . . . . . . 48
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
























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1.  Background and Introduction

   Saratoga is a file transfer and content dissemination protocol
   capable of efficiently sending both small and very large files as
   well as streaming continuous content.  Saratoga was originally
   designed for the purpose of large file transfer from small low-Earth-
   orbit satellites.  It has been used in daily operations since 2004 to
   move mission imaging data files of the order of several hundred
   megabytes each from the Disaster Monitoring Constellation (DMC)
   remote-sensing satellites to ground stations.

   The DMC satellites, built at the University of Surrey by Surrey
   Satellite Technology Ltd (SSTL), all use IP for payload
   communications and delivery of Earth imagery.  At the time of this
   writing, in early December 2010, seven DMC satellites have been
   launched into orbit, five of those are currently operational in
   orbit, and two further DMC satellites are being readied for launch.
   The DMC satellites use Saratoga to provide Earth imagery under the
   aegis of the International Charter on Space and Major Disasters.  A
   pass of connectivity between a satellite and ground station offers an
   8-12 minute time window in which to transfer imagery files using a
   minimum of an 8.1 Mbps downlink and a 9.6 kbps uplink.  The latest
   operational DMC satellites have faster downlinks, capable of 20, 40
   or 80 Mbps.  Newer satellites are expected to provide 200 Mbps or
   more, without significant increases in uplink rates.  This high
   degree of asymmetry, with the need to fully utilize the available
   downlink capacity to move the volume of data required within the
   limited time available, motivates much of Saratoga's design.

   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][Ivancic10].

   Store-and-forward delivery relies on reliable hop-by-hop transfers of
   files, removing the need for the final receiver to talk to the
   original sender across long delays and allowing for the possibility
   that an end-to-end path may never exist between sender and receiver
   at any given time.  Use of store-and-forward hop-by-hop delivery is
   typical of scenarios in space exploration for both near-Earth and
   deep-space missions, and useful for other scenarios, such as
   underwater networking, ad-hoc sensor networks, and some message-
   ferrying relay scenarios.  Saratoga is intended to be useful for
   relaying data in these scenarios, and can optionally also be used to
   carry the Bundle Protocol "bundles" that is proposed for use in Delay
   and Disruption-Tolerant Networking (DTN) by the IRTF DTN Research
   Group [RFC5050].  This has been tested from orbit using the UK-DMC
   satellite [Ivancic10].  How Saratoga can optionally function as a
   "bundle convergence layer" alongside a DTN bundle agent is specified



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   in a companion document [I-D.wood-dtnrg-saratoga].

   High link utilization is important during periods of limited
   connectivity.  Given that Saratoga was originally developed for
   scheduled peer-to-peer communications over dedicated links in private
   networks, where each application has the entire link for the duration
   of its transfer, early Saratoga implementations deliberately lack any
   form of congestion control and send at line rate to maximise
   throughput and link utilisation.  Newer implementations may perform
   TCP-Friendly Rate Control (TFRC) [RFC5348] or other congestion
   control mechanisms such as LEDBAT [I-D.ietf-ledbat-congestion], if
   appropriate for the environment, and where simultaneous sharing of
   capacity with other traffic and applications is required.

   Saratoga contains a Selective Negative Acknowledgement (SNACK)
   'holestofill' mechanism to provide reliable retransmission of data.
   This is intended to correct losses of corrupted link-layer frames due
   to channel noise over a space link.  Packet losses in the DMC are due
   to corruption introducing non-recoverable errors in the frame.  The
   DMC design uses point-to-point links and scheduling of applications
   in order, so that the link is dedicated to one application transfer
   at a time, meaning that packet loss cannot be due to congestion when
   applications compete for link capacity simultaneously.  In other
   wireless environments that may be shared by many nodes and
   applications, allocation of channel resources to nodes 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
   Saratoga's transport-level negative-acknowledgement approach to
   provide a reliable ARQ mechanism [RFC3366].

   Saratoga is scalable in that it is capable of efficiently
   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 4 GiB 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.




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   As a UDP-based protocol, Saratoga can be used with either IPv4 or
   IPv6.  Compatibilit between Saratoga and the wide variety of links
   that can already carry IP traffic is assured.

   Saratoga was originally implemented as outlined in [Jackson04], but
   the specification given here differs substantially, as we have added
   a number of features, 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 transfer process and
   transactions using Saratoga in Section 2, followed by a formal
   definition of the packet types used by Saratoga in Section 4, and the
   details of the various protocol mechanisms in Section 6.

   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.

   The remainder of this specification uses 'file' as a shorthand for
   'binary object', which may be a DTN bundle, or other type of data.

   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 Transfer

   Saratoga is a peer-to-peer protocol in the sense that multiple files
   may be transferred in both directions simultaneously between two
   communicating Saratoga peers, and there is not intended to be a
   strict client-to-server relationship.

   Saratoga nodes can act as simple file servers.  Saratoga supports
   several types of operations on files including "pull" downloads,
   "push" uploads, directory listing, and deletion requests.  Each
   operation is handled as a distinct "transaction" between the peers.

   Saratoga nodes MAY advertise their presence, capabilities, and
   desires by sending BEACON packets.  These BEACONs are sent to either
   a reserved, unforwardable, multicast address when using IPv4, or a
   link-local all-Saratoga-peers multicast address when using IPv6.  A
   BEACON might also be unicast to another known node as a sort of
   "keepalive".  Saratoga nodes may dynamically discover other Saratoga



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   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.  BEACONs have been used by the DMC satellites to
   indicate to ground stations that a link has become functional, a
   solid-state data recorder is online, and the software is ready to
   transfer any requested files.

   A Saratoga transaction begins with either a _get_, _put_, _getdir_,
   or _delete_ transaction REQUEST packet corresponding to a desired
   download, upload, directory listing, or deletion operation. _put_
   transactions can begin directly with METADATA and DATA.  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 STATUS packet that conveys rejection is
   generated.  If the file-serving peer accepts the transaction, it
   generates and can send a more useful descriptive METADATA packet,
   along with some number of DATA packets constituting 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 STATUS packet.  The STATUS packet
   can include 'holestofill' Selective Negative Acknowledgement (SNACK)
   information listing spans of octets within the file that have not yet
   been received, as well as whether or not the METADATA packet was
   received.  Based on the information in this STATUS packet, the file-
   sender can begin a cycle of selective retransmissions of missing DATA
   packets, until it sees a STATUS packet that acknowledges total
   reception of all file data.

   In the example scenario in Figure 1, a _get_ request is granted.  The
   reliable file delivery experiences loss of a single DATA packet due
   to channel-induced errors.














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           File-Receiver               File-Sender

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

               Figure 1: Example _get_ transaction sequence

   A _getdir_ request proceeds similarly, though the DATA packets
   contain the contents of a directory listing, described later, rather
   than a given file's bytes. _getdir_ is the only request to apply to
   directories.

   The STATUS 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 STATUS 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,
   similar to ranged FTP and HTTP requests.

   The Saratoga _put_ transaction is initiated by the file-sender
   sending an optional METADATA packet followed by immediate DATA
   packets, without waiting for a STATUS response.  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 STATUS packet indicating that the request was
   declined, it MUST stop sending any DATA packets within that
   transaction immediately.  Since this type of _put_ is open-loop for
   some period of time, it should not be used in scenarios where
   congestion is a valid concern; in these cases, the file-sender should
   wait on its METADATA to be acknowledged by a STATUS before sending
   DATA packets within the transaction.

   Figure 2 illustrates the sequence of packets in an example _put_
   transaction, beginning directly with METADATA and DATA, where the



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   second DATA packet is lost.  Other than the way that it is initiated,
   a _put_ transaction is identical to a _get_ transaction.

                         File-Sender           File-Receiver

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

                Figure 2: Example PUT transaction sequence

   In deep-space scenarios, the large propagation delays and round-trip
   times involved discourage use of ping-pong packet exchanges (such as
   TCP's SYN/ACK) for starting transactions, and unidirectional
   transfers via these optimistic 'blind _put_s' are desirable.  Blind
   _puts_ are the only mode of transfer suitable for unidirectional
   links.  Senders sending on unidirectional links SHOULD send a copy of
   the METADATA in advance of DATA packets, and MAY resend METADATA at
   intervals.

   The _delete_ transactions are simple single packet requests that
   trigger a STATUS 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
   STATUS packet.

   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 'blind _get_' allows a Saratoga peer to request any files that
   the other Saratoga peer has ready for it, without prior knowledge of
   the directory listing, and without requiring the ability to examine
   files or decode remote file names/paths for meaningful information
   such as final destination.

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

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



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   (2) To avoid file 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 STATUS packet to make
   clear how much of the file has been successfully received and where
   transfer should be resumed from, and relies on use of METADATA to
   identify the file.  On resumption of a transfer, the new METADATA
   (including file length, file 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 that the file has
   changed and purge the DATA payload received during previous contacts.

   If a file contains separate parts that require reliable transmission
   without errors or that can tolerate errors in delivered content,
   proactive fragmentation can be used to split the file into separate
   reliable and unreliable files 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, although the METADATA flag indicating that unreliable
   transmission is permitted is retained for later hops, which may
   revert to using UDP-Lite.  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, then the file MAY be delivered using
   UDP.

   Like the BEACON packets, a _put_ or a response to a _get_ MAY be sent
   to the dedicated IPv4 Saratoga multicast address (allocated to
   224.0.0.108) or the dedicated IPv6 link-local multicast address
   (allocated to FF02:0:0:0:0:0:0:6C) for multiple file-receivers on the
   link to hear.  This is at the discretion of the file-sender, if it
   believes that 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 status SNACK information is aggregated and sent



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   voluntarily by all file-receivers.  File-receivers respond to
   multicast DATA with multicast STATUS packets.  File-receivers SHOULD
   introduce a short random delay before sending a multicast STATUS
   packet, to prevent ack implosion after a channel-induced loss, and
   MUST listen for STATUS 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 STATUS 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.


3.  Optional Parts of Saratoga

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

   - sending and parsing BEACONs.

   - sending and parsing METADATA.  However, sending and receiving
   METADATA is considered useful, and this SHOULD be done.

   - support for working with DTN bundles and a bundle agent as an
   application driving Saratoga.  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, or in
   decreasing computational overheard for the UDP Lite checksum.  Error
   tolerance requires that lower-layer frames permit delivery of
   unreliable data to be really useful.

   - streaming data, including real-time streaming of content of unknown
   length.  This streaming can be unreliable (without resend requests)
   or reliable (with resend requests).  Session protocols such as http
   expect reliable streaming, and can be used in delay-tolerant networks
   [I-D.wood-dtnrg-http-dtn-delivery].  Although Saratoga data delivery
   is inherently one-way, where a stream of DATA packets elicits a
   stream of STATUS packets, bidirectional duplex communication can be
   established by using two Saratoga transfers flowing in opposite
   directions.

   - sending and responding to packet timestamps in DATA and STATUS
   packets.  These timestamps are useful for streaming and for giving a



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   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.

   - performing congestion control at the sender, based on feedback from
   acknowledgement STATUS packets, or having the sender configured to
   use simple open-loop rate control to only use a fixed amount of link
   capacity.  Congestion control is expected to be undesirable for
   Saratoga's use cases and expected environmental conditions, while
   simple rate control is considered useful.

   - multicast DATA transfers, if judged useful for the environment in
   which Saratoga is deployed, when multiple receivers are participating
   and are receiving the same file or stream.

   - sending and parsing STATUS messages, which are expected for
   bidirectional communication, but cannot be sent on and are not
   required for sending over unidirectional links.


4.  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 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.




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   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 | STATUS   | responds to REQUEST or DATA.  Can signal list of   |
   |   |          | unreceived data to sender during a transfer.       |
   +---+----------+----------------------------------------------------+

   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:








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   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:

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

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

   64-bit descriptor for longer files (optional)

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

   128-bit descriptor for very long files (optional)

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

   For offset descriptors and types of content being transferred, the
   related flag bits in BEACON and REQUEST indicate capabilities, while
   in METADATA and DATA those flag bits are used slightly differently,
   to indicate the content being transferred.

   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.





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4.1.  BEACON

   BEACON packets may be multicast periodically by nodes willing to act
   as Saratoga peers, or unicast to individual peers to indicate
   properties for that peer.  Some implementations have sent BEACONS
   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 activity
   or presence of the peer.

   The Endpoint Identifier (EID) in the BEACON serves to uniquely
   identify the Saratoga peer.  Whenever the Saratoga peer begins using
   a new IP address, it SHOULD issue a BEACON on it and repeat the
   BEACON periodically, to enable listeners to associate the IP address
   with the EID and 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1|    Type   |                      Flags                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [[               Available free space (optional)               ]]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Endpoint identifier...                               //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+//

   where

   +------------+------------------------------------------------------+
   | Field      | Description                                          |
   +------------+------------------------------------------------------+
   | Type       | 0                                                    |
   | Flags      | convey whether or not the peer is ready to           |
   |            | send/receive, what the maximum supported file size   |
   |            | range and descriptor is, and whether and how free    |
   |            | space is indicated.                                  |
   | Available  | This optional field can be used to indicate the      |
   | free space | current free space available for storage.            |








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   | Endpoint   | This can be used to uniquely identify the sending    |
   | identifier | Saratoga peer, or the administrative node that the   |
   |            | BEACON-sender is associated with.  If Saratoga is    |
   |            | being used with a bundle agent, a bundle endpoint ID |
   |            | (EID) can be used here.                              |
   +------------+------------------------------------------------------+

   The Flags field is used to provide some additional information about
   the peer.  The first two octets of the Flags field is currently in
   use.  The later octet is 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 offset
   descriptor field size will be preferred to minimise problems with
   offset sequences wrapping.  Protecting against sequence wrapping is
   discussed in the STATUS section.

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



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   Bit 10 is reserved for DTN bundle agent use, indicating whether the
   sender is capable of handling bundles via a local bundle agent.  This
   is described in [I-D.wood-dtnrg-saratoga].

          +-----+-------+--------------------------------------+
          | 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 continuous streams.

   +--------+--------+------------------------------------------------+
   | Bit 12 | Bit 13 | Capability and willingness to send files       |
   +--------+--------+------------------------------------------------+
   | 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.          |
   +--------+--------+------------------------------------------------+

   +-------+-------+---------------------------------------------------+
   | Bit   | Bit   | Capability and willingness to receive files       |
   | 14    | 15    |                                                   |
   +-------+-------+---------------------------------------------------+
   | 0     | 0     | cannot receive files at all.                      |
   | 0     | 1     | invalid.                                          |
   | 1     | 0     | capable of receiving, but unwilling.  Will reject |
   |       |       | METADATA or DATA packets.                         |
   | 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 those flags with bits indicating current
   willingness to use the capability.

   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



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   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.

   +-----+-------+-----------------------------------------------------+
   | Bit | Value | Meaning                                             |
   +-----+-------+-----------------------------------------------------+
   | 17  | 0     | available free space is not advertised in this      |
   |     |       | BEACON.                                             |
   | 17  | 1     | available free space is advertised in this BEACON.  |
   +-----+-------+-----------------------------------------------------+

   Bit 17 is used to indicate whether the sender is indicating how much
   free space is indicated in an optional field in this BEACON packet.
   If bit 17 is set, then bits 18 and 19 are used to indicate the size
   in bits of the optional free-space-size field.  If bit 17 is not set,
   then bits 18 and 19 are zero.

              +--------+--------+--------------------------+
              | Bit 18 | Bit 19 | Size of free space field |
              +--------+--------+--------------------------+
              | 0      | 0      | 16 bits.                 |
              | 0      | 1      | 32 bits.                 |
              | 1      | 0      | 64 bits.                 |
              | 1      | 1      | 128 bits.                |
              +--------+--------+--------------------------+

   The free space field size can vary as indicated by a varying-size
   field indicated in bits 18 and 19 of the flags field.  Unlike other
   offset descriptor use where the value in the descriptor indicates a
   byte or octet position for retransmission, or gives a file size in
   bytes, this particular field indicates the available free space in



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   KILOBYTES (KiB, 1024 byte multiples), rather than octets.  (Kilobytes
   are used as storage can be in local memory.)  Available free space is
   rounded down to the nearest KiB, so advertising zero means that less
   than 1KiB is free and available.  Advertising the maximum size
   possible in the field means that more free space than that is
   available.  While this field is intended to be scalable, it is
   expected that 32 bits (up to 4TiB) will be most common in use.

   A BEACON unicast to an individual peer MAY choose to indicate the
   free space available for use by that particular peer, and MAY
   indicate capabilities only available to that particular peer,
   overriding or supplementing the properties advertised to all local
   peers by multicast BEACONs.

   Any type of host identifier can be used in the endpoint identifier
   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.

4.2.  REQUEST

   A REQUEST packet is an explicit command to perform either a _get_,
   _getdir_, or _delete_ 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 1|   Type    |                     Flags                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               Id                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 variable-length File Path ...                 /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /               |    null byte    |                             /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /     variable-length Authentication Field (optional)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   where







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   +--------+----------------------------------------------------------+
   | 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   | the path of the requested file/directory following the   |
   | Path   | rules described below.                                   |
   +--------+----------------------------------------------------------+

   The Id that is used during transactions serves to uniquely associate
   a given packet with a particular transaction.  This enables multiple
   simultaneous data transfer or request/status transactions between two
   peers, with each peer deciding how to multiplex and prioritise the
   parallel flows it sends.  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:

   +-----+-------+-----------------------------------------------------+
   | 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



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   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 the local naming convention on
   the peers 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 File Path field is empty, i.e. is a null-terminated zero-
   length string one octet long, 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 allows
   the file-sender to determine the order and selection of files that it
   would like to forward to the receiver in more of a "push" manner.  Of
   course, file retrieval could also follow a "pull" manner, with the
   file-receiving host requesting specific 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 files,
   or make other optimizations, such as using its local knowledge of
   contact times to pick files of a size likely to be able to be
   delivered completely.  File transfer through pushing sender-selected
   files implements delivery prioritization decisions made solely at the
   Saratoga file-sending node.  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).

   The null-terminated File Path string MAY be followed by an optional
   Authentication Field that can be used to validate the REQUEST packet.
   Any value in the Authentication Field is the result of a computation
   of packet contents that SHOULD include, at a minimum, source and
   destination IP addresses and port numbers and packet length in a
   'pseudo-header', as well as the content of all Saratoga fields from
   Version to File Path, excluding the predictable null-termination



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   octet.  This Authentication Field can be used to allow the REQUEST
   receiver to discriminate between other peers, and permit and deny
   various REQUEST actions as appropriate.  The format of this field is
   unspecified for local use.

   REQUEST packets may be sent multicast, to learn about all listening
   nodes.  A multicast _get_ request for a file that elicits multiple
   METADATA or DATA responses should be followed by unicast STATUS
   packets with status errors cancelling all but one of the proposed
   transfers.  File 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.

   If the receiver already has the file at the expected file path and is
   requesting an update to that file, REQUEST can be sent after a
   METADATA advertising that file, to allow the sender to determine
   whether a replacement for the file should be sent.

   Delete requests are ignored for files currently being transferred.

4.3.  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 optional.
   They 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                 |Sumtype|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               Id                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               /
   /                                                               /
   /     example error-detection checksum (128-bit MD5 shown)      /
   /                                                               /
   /                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               /
   /            single Directory Entry describing file             /
   /                      (variable length)                        /
   /                                                              //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-//

   where

   +-----------+-------------------------------------------------------+
   | Field     | Description                                           |
   +-----------+-------------------------------------------------------+
   | Type      | 2                                                     |
   | Flags     | indicate additional boolean metadata about a file.    |
   | Sumtype   | indicates whether a checksum is present after the Id, |
   |           | and what type it is.                                  |
   | Id        | identifies the transaction that this packet           |
   |           | describes.                                            |
   | Checksum  | an example included checksum covering file contents.  |
   | Directory | describes file system information about the file,     |
   | Entry     | including file length, file timestamps, etc.; the     |
   |           | format is specified in Section 5.                     |
   +-----------+-------------------------------------------------------+

   The first octet of the Flags field is currently 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



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   STATUS packets within the session that will follow this packet.

   +--------+--------+-------------------------------------------------+
   | Bit 10 | Bit 11 | Type of transfer                                |
   +--------+--------+-------------------------------------------------+
   | 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      | an indefinite-length stream is being sent.      |
   +--------+--------+-------------------------------------------------+

   Also inside the Flags field, bits 10 and 11 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 5.

   +-----+-------+-----------------------------------------------------+
   | 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 13 | Use                                                      |
   +--------+----------------------------------------------------------+
   | 0      | This file's content MUST be delivered reliably without   |
   |        | errors using UDP.                                        |
   | 1      | This file's content MAY be delivered unreliably, or      |
   |        | partly unreliably, where errors are tolerated, using     |
   |        | UDP-Lite.                                                |
   +--------+----------------------------------------------------------+

   Bit 13 indicates whether the file must be sent reliably or can be
   sent at least partly unreliably, using UDP-Lite.  This flag SHOULD
   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
   may still be set if the immediate file-receiver is only capable of
   UDP delivery, on the assumption that this preference will be



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   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.

   The high four bits of the Flags field, bits 28-31, are used to
   indicate if an error-detection checksum has been included in the
   METADATA for the file to be transferred.  Here, bits 0000 indicate
   that no checksum is present, with the implicit assumption that the
   application will do its own end-to-end check.  Other values indicate
   the type of checksum to use.  The choice of checksum depends on the
   available computing power and the length of the file to be
   checksummed.  Longer files require stronger checksums to ensure
   error-free delivery.  The checksum of the file to be transferred is
   carried as shown, with a fixed-length field before the varying-length
   File Length and File Name information fields.

   Assigned values for the checksum type field are:

   +-----------+-------------------------------------------------------+
   | Value     | Use                                                   |
   | (0-15)    |                                                       |
   +-----------+-------------------------------------------------------+
   | 0         | No checksum is provided.                              |
   | 1         | 32-bit CRC32 checksum, suitable for small files.      |
   | 2         | 128-bit MD5 checksum, suitable for larger files.      |
   | 3         | 160-bit SHA-1 checksum, suitable for larger files but |
   |           | slower to process than MD5.                           |
   +-----------+-------------------------------------------------------+

   It is expected that higher values will be allocated to new and
   stronger checksums able to better protect larger files.  A checksum
   SHOULD be included for files being transferred.  The checksum SHOULD
   be as strong as possible.  Streaming of an indefinite-length stream
   MUST set the checksum type field to zero.

   It is expected that a minimum of 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 the weaker CRC-32c checksum can
   suffice.

   The CRC32 checksum is computed as described for the CRC-32c algorithm
   given in [RFC3309].

   The MD5 Sum field is generated via the MD5 algorithm [RFC1321],
   computed over the entire contents of the file being transferred.  The



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   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 error detection
   (as is done here in Saratoga), and has better performance
   implications than cryptographically-stronger alternatives given the
   limited available processing of many use cases.

   Checksums may be privately keyed for local use, to allow transmission
   of authenticated or encrypted files delivered in DATA packets.  This
   has limitations, discussed further in Section 8 at end.

   Use of the checksum to ensure that a file has been correctly relayed
   to the receiving node is important.  A provided checksum MUST be
   checked against the received data file.  If checksum verification
   fails, either due to corruption or due to the receiving node not
   having the right key for a keyed checksum), the file MUST be
   discarded.  If the file is to be relayed onwards later to another
   Saratoga peer, the metadata, including the checksum, MUST be retained
   with the file and SHOULD be retransmitted onwards unchanged with the
   file for end-to-end coverage.  If it is necessary to recompute the
   checksum or encrypted data for the new peer, either because a
   different key is in use or the existing checksum algorithm is not
   supported, the new checksum MUST be computed before the old checksum
   is verified, to ensure overlapping checksum coverage and detect
   errors introduced in file storage.

   METADATA can be used as an indication to update copies of files.  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 already has, as has been indicated by a previously-received
   METADATA advertisement from the requester, 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 and a newer file can
   be supplied, the METADATA begins a transfer with following DATA
   packets to update the file.

4.4.  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 data being transferred.







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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                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               /
   /            Timestamp/nonce information (optional)             /
   /                                                               /
   /                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                      Offset (descriptor)                      ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where

   +-----------------+-------------------------------------------------+
   | Field           | Description                                     |
   +-----------------+-------------------------------------------------+
   | Type            | 3                                               |
   | Flags 8 and 9   | bit 8 and 9 specify the size of offset          |
   |                 | descriptor, as elsewhere.                       |
   | Flag 10         | bit 10, with bit 11, indicates whether a file,  |
   |                 | bundle, stream or directory entry is being      |
   |                 | carried.  This bit will normally be zero for    |
   |                 | files.                                          |
   | Flag 11         | bit 11 is used with bit 10.  Normally this bit  |
   |                 | will be zero for files.                         |
   | Flag 12         | bit 12 indicates that an optional timestamp or  |
   |                 | nonce is included in the DATA header before the |
   |                 | offset descriptor.                              |
   | Flag 15         | bit 15 requests an immediate STATUS ack to be   |
   |                 | generated in response to receiving this packet. |
   | Id              | identifies the transaction to which this packet |
   |                 | belongs.                                        |
   | Timestamp/nonce | is an optional 128-bit field providing timing   |
   |                 | or identification information unique to this    |
   |                 | packet.  See Appendix A for details.            |
   | 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.



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   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 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 for bits 8, 9, 10 and 11 MUST be the
   same as indicated in the initial METADATA packet.

   +--------+--------+-------------------------------------------------+
   | Bit 10 | Bit 11 | Type of transfer                                |
   +--------+--------+-------------------------------------------------+
   | 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      | an indefinite-length stream is being sent.      |
   +--------+--------+-------------------------------------------------+

   Also inside the Flags field, bits 10 and 11 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 5.

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

   Flag bit 12 indicates that an optional packet timestamp/nonce is
   carried in the packet before the offset field.  This packet
   timestamp/nonce field is always sixteen octets (128 bits) long.
   Timestamps can be useful to the sender even when the receiver does



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   not understand them, as the receiver can simply echo any provided
   timestamps back, as specified for STATUS packets, to allow the sender
   to monitor flow conditions.  Packet timestamps are particularly
   useful when streaming.  Packet timestamps are discussed further in
   Appendix A.

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

   Within the Flags field, if bit 15 of the packet is set, the file-
   receiver is to immediately generate a STATUS 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.  This flag may
   be set periodically, but infrequently.  Asymmetric links with
   constrained backchannels can only carry a limited amount of STATUS
   packets before ack congestion becomes a problem.  This flag SHOULD
   NOT be set if an unreliable stream is being transferred, or if
   multicast is in use.  This flag SHOULD be set periodically for
   reliable file transfers, or reliable streaming.

            +-----+-------+----------------------------------+
            | Bit | Value | Meaning                          |
            +-----+-------+----------------------------------+
            | 16  | 0     | Normal use.                      |
            | 16  | 1     | The EOD End of Data flag is set. |
            +-----+-------+----------------------------------+

   The End of Data flag is set in DATA packets carrying the last byte of
   a transfer.  This is useful for streams and for Saratoga
   implementations that do not support METADATA.

   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 STATUS packet, and MAY choose to enqueue received DATA packets
   for later processing after the METADATA arrives.



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4.5.  STATUS

   The STATUS packet type is the single acknowledgement method that is
   used for feedback from a Saratoga receiver to a Saratoga sender to
   indicate transaction progress, both as a response to a REQUEST, and
   as a response to a DATA packet when demanded or volunteered.

   When responding to a DATA packet, the STATUS packet MAY, as needed,
   include selective acknowledgement (SNACK) 'hole' information to
   enable transmission (usually re-transmission) of specific sets of
   octets within the current transaction (called "holes").  This
   'holestofill' information 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                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               /
   /             Timestamp/nonce information (optional)            /
   /                                                               /
   /                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                Progress Indicator (descriptor)                ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   [                In-Response-To (descriptor)                    ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               (possibly, several Hole fields)                 /
   /                              ...                              /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where













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   +----------------+--------------------------------------------------+
   | 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.          |
   | Timestamp      | an optional fixed 128-bit field, that is only    |
   | (optional)     | present and used to return a packet timestamp if |
   |                | the timestamp flag is set.  If the STATUS packet |
   |                | is voluntary and the voluntary flag is set, this |
   |                | should repeat the timestamp of the DATA packet   |
   |                | containing the highest offset seen.  If the      |
   |                | STATUS 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.Packet      |
   |                | timestamps are particularly useful when          |
   |                | streaming.  There are special considerations for |
   |                | streaming, to protect against the ambiguity of   |
   |                | wrapped offset descriptor sequence numbers,      |
   |                | discussed below.  Packet timestamps are          |
   |                | discussed further in Appendix A.                 |
   | Progress       | the offset of the lowest-numbered octet of the   |
   | Indicator      | file not yet received.                           |
   | In-Response-To | the offset of the highest-numbered octet within  |
   | (descriptor)   | a DATA packet that generated this STATUS packet, |
   |                | or the offset of the highest-numbered octet seen |
   |                | if this STATUS is generated voluntarily and the  |
   |                | voluntary flag is set.                           |
   | Holes          | indications of offset ranges of missing data,    |
   |                | defined below.                                   |
   +----------------+--------------------------------------------------+

   The STATUS packet has a minimum size of twelve octets, using sixteen-
   bit descriptors, a progress indicator but no Hole fields, and no
   timestamps.  The progress indicator is always zero when responding to
   requests that may initiate a transfer.

   The Id field is needed to associate the STATUS packet with the
   transaction that it refers to.




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   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
   STATUS 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 sixteen-byte packet timestamp/
   nonce field is carried in the packet before the Progress Indicator
   descriptor, as discussed for the DATA packet format.  Packet
   timestamps are discussed further in Appendix A.

         +-----+-------+----------------------------------------+
         | 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 STATUS 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.

   A receiver SHOULD tolerate lost METADATA that is later resent, but
   MAY insist on receiving METADATA at the start of a transfer.  This is
   done by responding to early DATA packets with a voluntary STATUS
   packet that sets this flag bit, reports a status error code 0x10,
   sets the Progress Indicator field to zero, and does not include
   HOLESTOFILL information.

   +-----+-------+-----------------------------------------------------+
   | 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 a 'holestofill' STATUS packet is only set when there are



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   too many holes to fit within a single STATUS packet due to MTU
   limitations.  This causes the hole list to be spread out over
   multiple STATUS 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.  These multiple STATUS packets will share In-Response-To
   information.  When losses are light and/or hole reporting and repair
   is relatively frequent, all holes should easily fit within a single
   STATUS 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 STATUS's size, which is
   limited by the link MTU size.  In this case, multiple STATUS packets
   may be generated, and Flags bit 14 should be set on each STATUS
   packet accordingly, to indicate that each packet holds incomplete
   results.  The complete group of STATUS 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 STATUS was requested by the file-sender. |
      | 15  | 1     | This STATUS is sent voluntarily.              |
      +-----+-------+-----------------------------------------------+

   Flag bit 15 indicates whether the STATUS 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 STATUS packet header shown above, is a set of "Hole" definitions
   indicating any lost packets.  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



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   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 STATUS packet, given the IPv6 minimum MTU.  (There
   may be cases where there is a very constrained backchannel compared
   to the forward channel streaming DATA packets.  For these cases,
   implementations might deliberately request large holes that span a
   number of smaller holes and intermediate areas where DATA has already
   been received, so that previously-received DATA is deliberately
   resent.  This aggrgation of separate holes keeps the backchannel
   STATUS packet size down to avoid backchannel congestion.)

   A 'voluntary' STATUS can be 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 STATUS indicating a successfully
   established transfer has a Progress Indicator of zero and an In-
   Response-To field of zero.

   On receiving a STATUS packet, the sender SHOULD prioritize sending
   the necessary data to fill those holes, in order to advance the
   Progress Indicator at the receiver.

   The sender infers a completely-received transfer from the reported
   receiver window position.  In the final STATUS packet sent by the
   receiver once the file to be transferred has been completely
   received, bit 14 MUST be 0 (indicating a complete set of holes in
   this packet), there MUST NOT be any holestofill offset pairs
   indicating holes, the In-Response-To field points to the last byte of
   the file, and the voluntary flag MUST be set.  This 'completed'
   STATUS may be repeated, depending on subsequent sender behaviour,
   while internal state about the transfer remains available to the
   receiver.

   Because METADATA is optional in implementations, the file receiver
   may not know the length of a file if METADATA is never sent.  The
   sender MUST set the EOD End of Data flag in each DATA packet that
   sends the last byte of the file, and SHOULD request a STATUS
   acknowledgement when the EOD flag is set.  If METADATA has been sent
   and the EOD comes earlier than a previously reported length of a
   file, an unspecified error 0x01, as described below, is returned in



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   the STATUS message responding to that DATA packet and EOD flag.  If a
   stream is being marked EOD, the receiver acknowledges this with a
   Success 0x00 code.

   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 STATUS" within this
   document), while any non-zero value means the transaction should be
   aborted (this is called a "failure STATUS").

   +-------------------+-----------------------------------------------+
   | Error Code 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.                           |
   | 0x0A              | Unsupported packet type received.             |
   | 0x0B              | DATA flag bits describing transfer have       |
   |                   | changed unexpectedly.                         |
   | 0x0C              | Receiver is no longer interested in receiving |
   |                   | this file.                                    |
   | 0x0D              | Receiver wants sender to pause its transfer.  |
   | 0x0E              | Receiver wants sender to resume a             |
   |                   | previously-paused transfer.                   |
   | 0x0F              | File is in use.                               |
   | 0x10              | METADATA required before transfer can be      |
   |                   | accepted.                                     |
   +-------------------+-----------------------------------------------+

   The recipient of a failure STATUS MUST NOT try to process the
   Progress Indicator, 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.

   When sending an indefinite-length stream, the possibility of offset



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   sequence numbers wrapping back to zero must be considered.  This can
   be protected against by using large offsets, and by the stream
   receiver.  The receiver MUST separate out holes before the offset
   wraps to zero from holes after the wrap, and send Hole definitions in
   different STATUS packets, with Flag 14 set to mark them as
   incomplete.  Any Hole straddling a sequence wrap MUST be broken into
   two separate Holes, with the second Hole starting at zero.  The
   timestamps in STATUS packets carrying any pre-wrap holes should be
   earlier than the timestamp in later packets, and should repeat the
   timestamp of the last DATA packet seen for that offset sequence
   before the following wrap to zero occurred.  Receivers indicate that
   they no longer wish to receive streams by sending Status Code 0x0C.


5.  The 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Properties           [        Size (descriptor)      ]
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Mtime                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Ctime                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               /
   +                                                               /
   /                                                               /
   /           File Path (max 1024 octets,variable length)         /
   /                                                          ... //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-//

   where

   +------------+------------------------------------------------------+
   | field      | description                                          |
   +------------+------------------------------------------------------+
   | 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.  Bits 8 and 9 are flags that       |
   |            | indicate the width of the following descriptor field |
   |            | that gives file size.  Bit 10 indicates that the     |
   |            | file is to be handled by Saratoga as a bundle, and   |
   |            | passed to a bundle agent.                            |
   | Size       | the size of each file or directory in octets.  This  |
   |            | is a descriptor, varying as needed in each entry for |
   |            | the size of the file.  For convenience in the        |
   |            | figure, it is shown here as a 16-bit descriptor for  |
   |            | a small file.                                        |
   | Mtime      | a timestamp showing when the file or directory was   |
   |            | modified.                                            |
   | Ctime      | a timestamp of the last status change for this file  |
   |            | or directory.                                        |






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   | 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.            |
   +------------+------------------------------------------------------+

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

   Streams listed in a directory should be marked as special.  If a
   stream is being transferred, its size is unknown -- otherwise it
   would be a file.  The size property of a Directory Entry for a stream
   is therefore expected to be zero.

    +-------+-------+-------------------------------------------------+
    | Bit 8 | Bit 9 | Properties conveyed                             |
    +-------+-------+-------------------------------------------------+
    | 0     | 0     | File size is indicated in a 16-bit descriptor.  |
    | 0     | 1     | File size is indicated in a 32-bit descriptor.  |
    | 1     | 0     | File size is indicated in a 64-bit descriptor.  |
    | 1     | 1     | File size is indicated in a 128-bit descriptor. |
    +-------+-------+-------------------------------------------------+

   Flag bits 8 and 9 of Properties are descriptor size flags, with
   similar meaning as elsewhere, describing the size of the File Size
   descriptor that follows the Properties field.  When a single
   Directory Entry appears in the METADATA packet, these flags SHOULD
   match flag bits 8 and 9 in the METADATA header.  (A smaller
   descriptor size may be indicated in the Directory Entry when doing
   test transfers of small files using large descriptors.)

              +--------+------------------------------------+
              | Bit 10 | Properties conveyed                |
              +--------+------------------------------------+
              | 0      | File really is a file.             |
              | 1      | File is to be treated as a bundle. |
              +--------+------------------------------------+

   Bit 10 of Directory Entry Properties is a bundle flag, as indicated
   in and matching the METADATA header.  Use of Saratoga with bundles is
   discussed further in [I-D.wood-dtnrg-saratoga].



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   +--------+----------------------------------------------------------+
   | Bit 13 | Use                                                      |
   +--------+----------------------------------------------------------+
   | 0      | This file's content MUST be delivered reliably without   |
   |        | errors using UDP.                                        |
   | 1      | This file's content MAY be delivered unreliably, or      |
   |        | partly unreliably, where errors are tolerated, using     |
   |        | UDP-Lite.                                                |
   +--------+----------------------------------------------------------+

   Bit 13 indicates whether the file must be sent reliably or can be
   sent at least partly unreliably, using UDP-Lite.  This matches
   METADATA flag use.

   Undefined or unused flag bits of the Properties field default to
   zero.  In general, bits 0-7 of Properties are for matters related to
   the sender's filesystem, while bits 8-15 are for matters related to
   transport over Saratoga.

   It may be reasonable that files are visible in Directory Entries only
   when they can be transferred to the requester - this may depend on
   e.g. having appropriate access permissions or being able to handle
   large filesizes.  But requesters only capable of handling small files
   MUST be able to skip through large descriptors for large file sizes.
   Directory sizes are not calculated or sent, and a Size of 0 is given
   instead for directories, which are considered zero-length files.

   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 and postpones wrapping for 30 years beyond typical 1970-
   based 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.


6.  Behaviour 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.

6.1.  Saratoga Transactions

   Following are descriptions of the packet exchanges between two peers
   for each type of transaction.  Exchanges rely on use of the Id field



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   to match responses to requests, as described earlier in Section 4.2.

6.1.1.  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 different transaction Ids so that
       responses can be differentiated and matched to REQUESTs based on
       the Id field.  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 STATUS packet containing an
       error code in the Status field is sent and the transaction is
       terminated.  This STATUS 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 STATUS 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 STATUS
       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 STATUS response from the file-receiver, and with the
       End of Data (EOD) bit set.  This can be followed by empty DATA
       packets with the Flags bits set with EOD and requesting a STATUS
       until either a STATUS 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 STATUS packet.
       The file-receiver MAY voluntarily send STATUS packets at other
       times, where the In-Response-To field MUST set to zero.  The
       file-receiver SHOULD voluntarily send a STATUS 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



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       ack flag bit in a DATA packet and request a STATUS packet from
       the file-receiver.  The STATUS packet can include a copy of this
       hole list if there are holes.  File-receivers MUST send a STATUS
       packet immediately in response to receiving a DATA packet with
       the Flags bit set requesting a STATUS.

   6.  If the file-sender receives a STATUS 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 STATUS 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 STATUS packet indicating that
       the transaction is complete, and it terminates the transaction
       locally, although it MUST persist in responding to any further
       DATA packets received from the file-sender with 'completed'
       STATUSes, as described in Section 4.5, for some reasonable amount
       of time.  Starting a timer on sending a completed STATUS and
       resetting it whenever a received DATA/sent 'completed' STATUS
       transaction takes place, then removing all session state on timer
       expiry, is one approach to this.

   Given that there may be a high degree of asymmetry in link bandwidth
   between the file-sender and file-receiver, the STATUS 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 STATUSes, and also that a file-receiver should
   be cautious in gratuitously generating STATUS packets of its own
   volition.  When sending on known unidirectional links, a file-sender
   cannot reasonably expect to receive STATUS packets, so should never
   request them.

6.1.2.  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.




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   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 STATUS 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 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.

6.1.3.  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 STATUS packet whose Id
       matches the Id field of the _delete_ REQUEST.  This STATUS has a
       Status code that indicates whether the deletion was granted and
       occurred successfully (indicated by the 0x00 Status field in a
       success STATUS), or whether some error occurred (indicated by the
       non-zero Status field in a failure STATUS).  This STATUS packet
       MUST have no Holes and 16-bit width zero-valued Progress
       Indicator and In-Response-To fields.

   If a request is received to delete a file that is already deleted, a
   STATUS with Status code 0x00 and other fields as described above is
   sent back in acknowledgement.  This ensures that loss of STATUS
   acknowledgements and repeated _delete_ requests are handled properly.

6.1.4.  The _put_ Transaction

   A _put_ transaction proceeds as a _get_ does, 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 STATUS packet to the file-sender.  Otherwise, it sends a
   failure STATUS packet.  After sending a failure STATUS 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
   STATUS packet if the flow of packets does not stop.






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6.2.  Beacons

   Sending BEACON packets is not required 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.
   Similarly, a node capable of communicating via IPv4 and IPv6 MAY send
   separate BEACONs via both protocols, or MAY only send BEACONs on its
   preferred protocol.

   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.

6.3.  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 and applications 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.

6.4.  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



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   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
   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
   STATUS 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 transfers, there are simple rules for handling
   expiration of the inactivity timer during a _get_ or _put_
   transaction.  Once the timer expires, the file-sender SHOULD
   terminate the transaction state and cease to send DATA or METADATA
   packets.  The file-receiver SHOULD stop sending STATUS 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
   STATUS 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.

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


7.  Mailing list

   There is a mailing list for discussion of Saratoga and its
   implementations.  Contact Lloyd Wood for details.


8.  Security Considerations

   The design of Saratoga provides limited, deliberately lightweight,



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   services for authentication of session requests, and for
   authentication or encryption of data files via keyed metadata
   checksums.  This document does not specify privacy or access control
   for data files transferred.  Privacy, access, authentication and
   encryption issues may be addressed within an implementation or
   deployment in several ways that do not affect the file transfer
   protocol itself.  As examples, IPSec may be used to protect Saratoga
   implementations from forged packets, to provide privacy, or to
   authenticate the identity of a peer.  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.  If IPsec is not required, use
   of encryption before the file is given to Saratoga is preferable.
   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.

   Note that Saratoga is intended for single-hop transfers between
   peers.  A METADATA checksum using a previously shared key can be used
   to decrypt or authenticate delivered DATA files.  Saratoga can only
   provide payload encryption across a single Saratoga transfer, not
   end-to-end across concatenated separate hop-by-hop transfers through
   untrusted peers, as checksum verification of file integrity is
   required at each node.  End-to-end data encryption, if required, MUST
   be implemented by the application using Saratoga.


9.  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

   IANA has allocated a dedicated IPv4 all-hosts multicast address
   (224.0.0.108) and a dedicated IPv6 link-local multicast addresses
   (FF02:0:0:0:0:0:0:6c) for use by Saratoga.


10.  Acknowledgements

   Developing and deploying the on-orbit IP-based 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.

   We thank James H. McKim as an early contributor to Saratoga



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   implementations and specifications, while working for RSIS
   Information Systems at NASA Glenn.  We regard Jim as an author of
   this document, but are prevented by the boilerplate five-author limit
   from naming him earlier.

   We thank Stewart Bryant, Cathryn Peoples, Abu Zafar Shahriar and Dave
   Stewart for their review comments.

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


11.  A Note on Naming

   Saratoga is named for the USS Saratoga (CV-3), the aircraft carrier
   sunk at Bikini Atoll that is now a popular diving site.


12.  References

12.1.  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.

   [RFC3309]  Stone, J., Stewart, R., and D. Otis, "Stream Control
              Transmission Protocol (SCTP) Checksum Change", RFC 3309,
              September 2002.

   [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.

12.2.  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.



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   [I-D.ietf-ledbat-congestion]
              Shalunov, S., Hazel, G., and J. Iyengar, "Low Extra Delay
              Background Transport (LEDBAT)",
              draft-ietf-ledbat-congestion-03 (work in progress),
              October 2010.

   [I-D.wood-dtnrg-http-dtn-delivery]
              Wood, L. and P. Holliday, "Using HTTP for delivery in
              Delay/Disruption-Tolerant Networks",
              draft-wood-dtnrg-http-dtn-delivery-06 (work in progress) ,
              November 2010.

   [I-D.wood-dtnrg-saratoga]
              Wood, L., McKim, J., Eddy, W., Ivancic, W., and C.
              Jackson, "Using Saratoga with a Bundle Agent as a
              Convergence Layer for Delay-Tolerant Networking",
              draft-wood-dtnrg-saratoga-08 (work in progress) ,
              November 2010.

   [Ivancic10]
              Ivancic, W., Eddy, W., Stewart, D., Wood, L., Northam, J.,
              and C. Jackson, "Experience with delay-tolerant networking
              from orbit", International Journal of Satellite
              Communications and Networking, Special Issue on best
              papers of the Fourth Advanced Satellite Mobile Systems
              Conference (ASMS 2008), vol. 28, issues 5-6, pp. 335-351,
              September-December 2010.

   [Jackson04]
              Jackson, C., "Saratoga File Transfer Protocol", Surrey
              Satellite Technology Ltd internal technical 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.

   [RFC5050]  Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, November 2007.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, September 2008.

   [Wood07a]  Wood, L., Ivancic, W., Hodgson, D., Miller, E., Conner,



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              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",
              International Workshop on Satellite and Space
              Communications (IWSSC '07) Salzburg, September 2007.


Appendix A.  Timestamp/Nonce field considerations

   Timestamps are useful in DATA packets when the time that the packet
   or its payload was generated is of importance; this can be necessary
   when streaming sensor data recorded and packetized in real time.  The
   format of the optional timestamp, whose presence is indicated by a
   flag bit, is implementation-dependent within the available fixed-
   length 128-bit field.  How the contents of this timestamp field are
   used and interpreted depends on local needs and conventions and the
   local implementation.  However, one simple suggested format for
   timestamps is to begin with a POSIX time_t representation of time, in
   network byte order.  This is either a 32-bit or 64-bit signed integer
   representing the number of seconds since 1970.  The remainder of this
   field can be used either for a representation of elapsed time within
   the current second, if that level of accuracy is required, or as a
   nonce field uniquely identifying the packet or including other
   information.  Any locally-meaningful flags identifying a type of
   timestamp or timebase can be included before the end of the field.
   Unused parts of this field MUST be set to zero.

   There are many different representations of timestamps and timebases,
   and this draft is too short to cover them in detail.  One suggested
   flag representation of different timestamp fields is to use the least
   significant bits at the end of the timestamp/nonce field as:

   +---------+---------------------------------------------------------+
   | Status  | Meaning                                                 |
   | Value   |                                                         |
   +---------+---------------------------------------------------------+
   | 0x00    | No flags set, local interpretation of field.            |
   | 0x01    | 32-bit timestamp at start of field indicating whole     |
   |         | seconds from epoch.                                     |
   | 0x02    | 64-bit timestamp at start of field indicating whole     |
   |         | seconds elapsed from epoch.                             |



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   | 0x03    | 32-bit timestamp, as in 0x01, followed by 32-bit        |
   |         | timestamp indicating fraction of the second elapsed.    |
   | 0x04    | 64-bit timestamp, as in 0x02, followed by 32-bit        |
   |         | timestamp indicating fraction of the second elapsed.    |
   +---------+---------------------------------------------------------+

   Other values may indicate specific epochs or timebases, as local
   requirements dictate.  There are many ways to define and use time
   usefully.

   Echoing timestamps back to the receiver is also useful for tracking
   flow conditions, for e.g. optional congestion control.  Timestamp
   values provide a useful mechanism for Saratoga peers to measure path
   and round-trip latency.  The use of timestamp values may assist in
   developing algorithms for flow control (including TCP-Friendly Rate
   Control) or other purposes.


Authors' Addresses

   Lloyd Wood
   Centre for Communication Systems Research, University of Surrey
   Guildford, Surrey  GU2 7XH
   United Kingdom

   Phone: +44-1483-689123
   Email: L.Wood@surrey.ac.uk


   Wesley M. Eddy
   MTI Systems
   MS 500-ASRC
   NASA Glenn Research Center
   21000 Brookpark Road
   Cleveland, OH  44135
   USA

   Phone: +1-216-433-6682
   Email: wes@mti-systems.com












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   Charles Smith
   Commonwealth Scientific and Industrial Research Organisation
   Cnr Vimiera and Pembroke Roads
   Marsfield, New South Wales  2122
   Australia

   Phone: +61-404-058974
   Email: charles.smith@csiro.au


   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: +44-1483-803803
   Email: C.Jackson@sstl.co.uk





















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