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Delay Tolerant Networking Research Group                     S. Burleigh
Internet Draft                            NASA/Jet Propulsion Laboratory
Intended Status: Informational                                M. Ramadas
<draft-irtf-dtnrg-ltp-motivation-04.txt>                 Ohio University
April 2007                                                    S. Farrell
Expires October 2007                              Trinity College Dublin


              Licklider Transmission Protocol - Motivation


Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).





Abstract

   This document describes the Licklider Transmission Protocol (LTP)
   designed to provide retransmission-based reliability over links



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   characterized by extremely long message round-trip times (RTTs)
   and/or frequent interruptions in connectivity.  Since communication
   across interplanetary space is the most prominent example of this
   sort of environment, LTP is principally aimed at supporting "long-
   haul" reliable transmission in interplanetary space, but has
   applications in other environments as well.

   In an Interplanetary Internet setting deploying the Bundle protocol,
   LTP is intended to serve as a reliable convergence layer over single
   hop deep-space RF links. LTP does ARQ of data transmissions by
   soliciting selective-acknowledgment reception reports.  It is
   stateful and has no negotiation or handshakes.

   This document is a product of the Delay Tolerant Networking Research
   Group and has been reviewed by that group. No objections to its
   publication as an RFC were raised.

Table of Contents

    1. Introduction .................................................  3
    2. Motivation ...................................................  4
       2.1 IPN Operating Environment ................................  4
       2.2 Why not Standard Internet Protocols? .....................  6
    3. Features .....................................................  7
       3.1 Massive State Retention ..................................  8
          3.1.1 Multiplicity of Protocol State Machines .............  8
          3.1.2 Session IDs .........................................  9
       3.2 Absence of Negotiation ...................................  9
       3.3 Partial Reliability ...................................... 10
       3.4 Laconic Acknowledgment ................................... 11
       3.5 Adjacency ................................................ 12
       3.6 Optimistic and Dynamic Timeout Interval Computation ...... 13
       3.7 Deferred Transmission .................................... 14
    4. Overall Operation ............................................ 14
       4.1 Nominal Operation ........................................ 14
       4.2 Retransmission ........................................... 16
       4.3 Accelerated Retransmission ............................... 18
       4.4 Session Cancellation ..................................... 19
    5. Functional Model ............................................. 20
       5.1 Deferred Transmission .................................... 20
       5.2 Timers ................................................... 20
    6. Security Considerations ...................................... 25
    7. IANA Considerations .......................................... 25
    8. Acknowledgments .............................................. 25
    9. References ................................................... 25
       9.1 Normative References ..................................... 25
       9.2 Informative References ................................... 26
   10. Author's Addresses ........................................... 26



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1.  Introduction

   The Licklider Transmission Protocol (LTP) described in this memo is
   designed to provide retransmission-based reliability over links
   characterized by extremely long message round-trip times and/or
   frequent interruptions in connectivity.  Communication in
   interplanetary space is the most prominent example of this sort of
   environment, and LTP is principally aimed at supporting "long-haul"
   reliable transmission over deep-space RF links.

   Since 1982, the principal source of standards for space
   communications has been the Consultative Committee for Space Data
   Systems (CCSDS) [CCSDS].  Engineers of CCSDS member agencies have
   developed communication protocols that function within the
   constraints imposed by operations in deep space.  Among the most
   prominent of these constraints are:


      o Extremely long signal propagation delays, on the order of
        seconds, minutes, or hours rather than milliseconds.

      o Frequent and lengthy interruptions in connectivity.

      o Low levels of traffic coupled with high rates of
        transmission error.

      o Meager bandwidth and highly asymmetrical data rates.


   The CCSDS File Delivery Protocol (CFDP) [CFDP], in particular,
   automates reliable file transfer across interplanetary distances by
   detecting data loss and initiating the requisite retransmission
   without mission operator intervention.

   CFDP by itself is sufficient for operating individual missions, but
   its built-in networking capabilities are rudimentary.  In order to
   operate within the IPN environment it must rely on the routing and
   incremental retransmission capabilities of the Bundle protocol [BP]
   defined for Delay-Tolerant Networks [DTN].  LTP is intended to serve
   as a reliable "convergence layer" protocol, underlying the Bundle
   protocol, in DTN deployments where datalinks are characterized by
   very long round-trip times.  Its core design notions are directly
   descended from the retransmission procedures defined for CFDP.

   This document describes the motivation for LTP, its features,
   functions, and overall design.  It is part of a series of documents
   describing LTP.  Other documents in the series include the main
   protocol specification document [LTPSPEC] and the protocol extensions



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   document [LTPEXT] respectively.

   The protocol is named in honor of ARPA/Internet pioneer JCR
   Licklider.

2.  Motivation

2.1  IPN Operating Environment

   There are a number of fundamental differences between the environment
   for terrestrial communications (such as seen in the Internet) and the
   operating environments envisioned for the Interplanetary Internet
   (IPN).

   The most challenging difference between communication among points on
   Earth and communication among planets is round-trip delay, of which
   there are two main sources, both relatively intractable: natural law
   and economics.

   The more obvious type of delay imposed by nature is signal
   propagation time.  Our inability to transmit data at speeds higher
   than the speed of light means that while round-trip times in the
   terrestrial Internet range from milliseconds to a few seconds,
   minimum round-trip times to Mars range from 8 to 40 minutes,
   depending on the planet's position.  Round-trip times between Earth
   and Jupiter's moon Europa run between 66 and 100 minutes.

   Less obvious and more dynamic is the delay imposed by occultation.
   Communication between planets must be by radiant transmission, which
   is usually possible only when the communicating entities are in line
   of sight of each other.  An entity residing on a planetary surface
   will be periodically taken out of sight by the planet's rotation (it
   will be "on the other side of" the planet); an entity that orbits a
   planet will be periodically taken out of sight by orbital motion (it
   will again be on the other side of the planet); and planets
   themselves lose mutual visibility when their trajectories take them
   to opposite sides of the Sun.  During the time that communication is
   impossible, delivery is impaired and messages must wait in a queue
   for later transmission.

   Round-trip times and occultations can at least be readily computed
   given the ephemerides of the communicating entities.  Additional
   delay that is less easily predictable is introduced by discontinuous
   transmission support, which is rooted in economics.

   Communicating over interplanetary distances requires expensive
   special equipment: large antennas, high-performance receivers, etc.
   For most deep-space missions, even non-NASA ones, these are currently



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   provided by NASA's Deep Space Network (DSN) [DSN].  The communication
   resources of the DSN are currently oversubscribed and will probably
   remain so for the foreseeable future.  While studies have been done
   as to the feasibility of upgrading or replacing the current DSN, the
   number of deep space missions will probably continue to grow faster
   than the terrestrial infrastructure that supports them, making over-
   subscription a persistent problem.  Radio contact via the DSN must
   therefore be carefully scheduled and is often severely limited.

   This over-subscription means that the round-trip times experienced by
   packets will be affected not only by the signal propagation delay and
   occultation, but also by the scheduling and queuing delays imposed by
   the management of Earth-based resources: packets to be sent to a
   given destination may have to be queued until the next scheduled
   contact period, which may be hours, days, or even weeks away.  While
   queuing and scheduling delays are generally known well in advance
   except when missions need emergency service (such as during landings
   and maneuvers), the long and highly variable delays make the design
   of timers, retransmission timers in particular, quite difficult.

   Another significant difference between deep space and terrestrial
   communication is bandwidth availability.  The combined effects of
   large distances (resulting in signal attenuation), the expense and
   difficulty of deploying large antennas to distant planets, and the
   difficulty of generating electric power in space all mean that the
   available bandwidth for communication in the IPN will likely remain
   modest compared to terrestrial systems.  Maximum data rates on the
   order of a few tens of megabits per second will probably be the norm
   for the next few decades.

   Moreover, the available bandwidths are highly asymmetrical: data are
   typically transmitted at different rates in different directions on
   the same link.  Current missions are usually designed with a much
   higher data "return" rate (from spacecraft to Earth) than "command"
   rate (from Earth to spacecraft).  The reason for the asymmetry is
   simple: nobody ever wanted a high-rate command channel, and, all else
   being equal, it was deemed better to have a more reliable command
   channel than a faster one.  This design choice has led to data rate
   asymmetries in excess of 100:1, sometimes approaching 1000:1.  A
   strong desire for a very robust command channel will probably remain,
   so any transport protocol designed for use in the IPN will need to
   function with a relatively low-bandwidth outbound channel to
   spacecraft and landers.

   The difficulty of generating power on and around other planets will
   also result in relatively low signal-to-noise ratios and thus high
   bit error rates.  Current deep-space missions operate with raw bit
   error rates on the order of 10^(-1) to 10^(-3).  While heavy coding



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   is used to reduce error rates, the coding overhead further reduces
   the residual bandwidth available for data transmission.

   Signal propagation delay is the only truly immutable characteristic
   that distinguishes the IPN from terrestrial communications.  Queuing
   and scheduling delays, low data rates, intermittent connectivity, and
   high bit error rates can all be mitigated or eliminated by adding
   more infrastructure.  But this additional infrastructure is likely to
   be provided (if at all) only in the more highly developed core areas
   of the IPN.  We see the IPN growing outwards from Earth as we explore
   more and more planets, moons, asteroids, and possibly other stars.
   This suggests that there will always be a "fringe" to the fabric of
   the IPN, an area without a rich communications infrastructure.  The
   delay, data rate, connectivity, and error characteristics mentioned
   above will probably always be an issue somewhere in the IPN.

2.2  Why not Standard Internet Protocols?

   These environmental characteristics - long delays, low and asymmetric
   bandwidth, intermittent connectivity, and relatively high error rates
   - make using unmodified TCP for end to end communications in the IPN
   infeasible.  Using the TCP throughput equation from [TFRC] we can
   calculate the loss event rate (p) required to achieve a given steady-
   state throughput.  Assuming the minimum RTT to Mars from planet Earth
   is 8 minutes (one-way speed of light delay to Mars at its closest
   approach to Earth is 4 minutes), assuming a packet size of 1500
   bytes, assuming that the receiver acknowledges every other packet,
   and ignoring negligible higher order terms in p (i.e., ignoring the
   second additive term in the denominator of the TCP throughput
   equation), we obtain the following table of loss event rates required
   to achieve various throughput values.

              Throughput              Loss event rate (p)
              ----------              -------------------
                10 Mbps                  4.68 * 10^(-12)
                 1 Mbps                  4.68 * 10^(-10)
               100 Kbps                  4.68 * 10^(-8)
                10 Kbps                  4.68 * 10^(-6)

   Note that although multiple losses encountered in a single RTT are
   treated as a single loss event in the TCP throughput equation [TFRC],
   such loss event rates are still unrealistic on deep space links.

   The above values are upper bounds on steady-state throughput.  Since
   the number of packets in an episode of connectivity will generally be
   under 10,000 due to the low available bandwidth, TCP performance
   would be dominated by its behavior during slow-start.  This means
   that even when Mars is at its closest approach to Earth it would take



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   a TCP session nearly 100 minutes to ramp up to an Earth-Mars
   transmission rate of 20kbps.

   Note: Lab experiments using a channel emulator and standard
   applications show that even if TCP could be pushed to work
   efficiently at such distances, many applications either rely on
   several rounds of handshaking or have built-in timers that render
   them non-functional when the round-trip-time is over a couple of
   minutes.  For example, it typically takes eight round trips for FTP
   to get to a state where data can begin flowing.  Since an FTP server
   may time out and reset the connection after 5 minutes of inactivity,
   a conformant standard FTP server could be unusable for communicating
   even with the closest planets.

   The TCP characteristic of establishing a new connection between a
   pair of peer entities for transferring every new unit of application
   data is a further obstacle, because the initial three-way handshake
   procedure of each connection (not to mention the connection slow-
   start overhead) could in itself be exorbitant in a long delay
   environment. The SCTP [SCTP] protocol can multiplex "chunks" (units
   of application data) for multiple sessions over a single layer
   connection (called an 'association' in SCTP terminology) as LTP does,
   but it still requires multiple round trips prior to transmitting
   application data for session setup and so clearly does not suit the
   needs of the IPN operating environment.

3. Features

   The design of LTP differs from that of TCP in several significant
   ways.  The common themes running through these differences are two
   central design assumptions, both of which amount to making virtues of
   necessity.

   First: given the severe innate constraints on interplanetary
   communication discussed above, we assume that the computational
   resources available to LTP engines will always be ample compared to
   the communication resources available on the link between them.

   Certainly, in many cases the computational resources available to a
   given LTP engine - such as one on board a small robotic spacecraft -
   will not be ample by the standards of the Internet.  But in those
   cases we expect that the associated communication resources
   (transmitter power, antenna size) will be even less ample, preserving
   the expected disproportion between the two.

   Second, we note that establishing a communication link across
   interplanetary distance entails enacting several hardware
   configuration measures based on the presumed operational state of the



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   remote communicating entity like:

      o orienting a directional antenna correctly;

      o tuning a transponder to pre-selected transmission and/or
        reception frequencies;

      o diverting precious electrical power to the transponder at the
        last possible moment, and for the minimum necessary length of
        time.

   We therefore assume that the operating environment in which LTP
   functions is able to pass information on the link status (termed
   "link state cues" in this document) to LTP, telling it which remote
   LTP engine(s) should currently be transmitting to the local LTP
   engine and vice versa.  The operating environment itself must have
   this information in order to configure communication link hardware
   correctly.

3.1  Massive State Retention

   Like any reliable transport service employing ARQ (Automatic Repeat
   reQuests), LTP is "stateful".  In order to assure the reception of a
   block of data it has sent, LTP must retain for possible
   retransmission all portions of that block which might not have been
   received yet.  In order to do so, it must keep track of which
   portions of the block are known to have been received so far and
   which are not, together with any additional information needed for
   purposes of retransmitting part or all of that block.

   Long round-trip times mean substantial delay between the transmission
   of a block of data and the reception of an acknowledgment from the
   block's destination, signaling arrival of the block.  If LTP
   postponed transmission of additional blocks of data until it received
   acknowledgment of the arrival of all prior blocks, valuable
   opportunities to utilize what little deep space transmission
   bandwidth is available would be forever lost.

   For this reason, LTP is based in part on a notion of massive state
   retention.  Any number of requested transmissions may be concurrently
   "in flight" at various displacements along the link between two LTP
   engines, and the LTP engines must necessarily retain transmission
   status and retransmission resources for all of them.  Moreover, if
   any of the data of a given block are lost en route, it will be
   necessary to retain the state of that transmission during an
   additional round trip while the lost data are retransmitted; even
   multiple retransmission cycles may be necessary.




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   In sum, LTP transmission state information persists for a long time
   because a long time must pass before LTP can be assured of
   transmission success - so LTP must retain a great deal of state
   information.  Since the alternatives are non-reliability on the one
   hand and severe under-utilization of transmission opportunities on
   the other, we believe such massive statefulness is cost-justified
   (though probably not for all LTP applications).

3.1.1  Multiplicity of Protocol State Machines

   This design decision is reflected in a significant structural
   difference between LTP and TCP.

   Both TCP and LTP provide mechanisms for multiplexing access by a
   variety of higher-layer services or applications: LTP's "client
   service IDs" correspond to TCP's port numbers.  Also, both TCP and
   LTP implement devices for encapsulating threads of retransmission
   protocol (protocol state machines): LTP's "sessions" functionally
   correspond to TCP connections.  At any moment each such thread of
   retransmission protocol is engaged in conveying a single block of
   application data from one protocol end-point to another.

   However, a single TCP connection (local host address, local port
   number, foreign host address, foreign port number) can accommodate at
   most one such thread of retransmission protocol at any one time.  In
   contrast, a single LTP association (local engine ID, local client
   service ID, foreign engine ID, foreign client service ID) can
   accommodate multiple concurrent sessions, one for each block of data
   in transit on the association.

3.1.2   Session IDs

   In TCP, the fact that a single connection maps one-on-one to a single
   protocol state machine enables the protocol to use host addresses and
   port numbers to demultiplex arriving data to the appropriate protocol
   state machine.  LTP's possible multiplicity of sessions per
   association makes it necessary for each segment of application data
   to include an additional demultiplexing token: a "session ID", that
   uniquely identifies the session in which the segment was issued and,
   implicitly, the block of data being conveyed by this session.

3.2  Absence of Negotiation

   In the IPN, round-trip times may be so long and communication
   opportunities so brief that a negotiation exchange, such as an
   adjustment of transmission rate, might not be completed before
   connectivity is lost.  Even if connectivity is uninterrupted, waiting
   for negotiation to complete before revising data transmission



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   parameters might well result in costly under-utilization of link
   resources.

   For this reason, LTP communication session parameters are asserted
   unilaterally, subject to application-level network management
   activity that may not take effect for hours, days, or weeks.

3.3  Partial Reliability

   For environments where application data is not critical, overall link
   bandwidth utilization may be improved if the data is transmitted on a
   "best efforts" basis, i.e., without being subject to acknowledgment
   and retransmission.  However, we believe that for many applications,
   unreliable transmission of data is likely to be useful only if any
   application headers/meta-data describing the actual data are received
   reliably.  For example, suppose a block transmission involves a high-
   definition photograph (which can afford to be sent on "best
   efforts"): the first 40 bytes of the block might be a prologue
   containing information such as camera settings and time of exposure,
   without which the photograph data is useless, while the actual
   photograph data is an array of fixed-length scan lines.  In this case
   the assured delivery of the first 40 bytes of the block is critical
   for interpreting the data, but the loss of a few individual scan
   lines may not be important enough to justify the cost of
   retransmission. A more typical example would be when the bundle
   protocol [BP] is the upper-layer protocol operating over LTP: even if
   a bundle is to be transmitted on "best efforts", it would at least be
   expected that the bundle protocol headers be received reliably;
   otherwise the bundle itself would be meaningless to the receiving BP
   node.

   The motivation for "partially reliable" transmission, as opposed to
   an alternative unreliable mode, is therefore to provide a mechanism
   for upper layer protocols to get their header and meta-data
   transmitted reliably (as necessary) but have the actual data
   transmitted unreliably.

   LTP regards each block of data as comprising two parts: a "red-part",
   whose delivery must be assured by acknowledgment and retransmission
   as necessary, and a "green-part" whose delivery is attempted, but not
   assured.  The length of either part may be zero; that is, any given
   block may be designated entirely red (retransmission continues until
   reception of the entire block has been asserted by the receiver) or
   entirely green (no part of the block is acknowledged or
   retransmitted).  Thus LTP can provide both TCP-like and UDP-like
   functionality concurrently on a single session.

   Note that in a red-green block transmission, the red-part data does



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   NOT have any urgency or higher-priority semantics relative to the
   block's green-part data; the red-part data is merely intended as
   imperative meta-data without which green-part data are likely to be
   futile.

3.4  Laconic Acknowledgment

   Another respect in which LTP differs from TCP is that, while TCP
   connections are bidirectional (blocks of application data may be
   flowing in both directions on any single connection), LTP sessions
   are unidirectional.  This design decision derives from the fact that
   the flow of data in deep space flight missions is usually
   unidirectional.  (Long round trip times make interactive spacecraft
   operation infeasible, so spacecraft are largely autonomous and
   command traffic is very light.)

   One could imagine an LTP instance, upon being asked to transmit a
   block of data, searching through all existing sessions in hopes of
   finding one that was established upon reception of data from the new
   block's destination; transmission of the new block could be
   piggybacked on the acknowledgment traffic for that session.  But the
   prevailing unidirectionality of space data communications means that
   such a search would frequently fail and a new unidirectional session
   would have to be established anyway.  Session bidirectionality
   therefore seemed to entail somewhat greater complexity unmitigated by
   any clear performance advantage, so we abandoned it.  Bidirectional
   data transfer is instead accomplished by opening two individual LTP
   sessions.

   Since they are not piggybacked on data segments, LTP data
   acknowledgments - "reception reports" - are carried in a separate
   segment type.  To minimize consumption of low and asymmetric
   transmission bandwidth in the IPN, these report segments are sent
   infrequently; each one contains a comprehensive report of all data
   received within some specified range of offsets from the start of the
   transmitted block.  The expectation is that most data segments will
   arrive safely, so individual acknowledgment of each one would be
   expensive in information-theoretical terms: the real information
   provided per byte of acknowledgment data transmitted would be very
   small.  Instead, report segments are normally sent only upon
   encountering explicit solicitations for reception reports -
   "checkpoints" - in the sequence of incoming data segments.

   The aggregate nature of reception reports gives LTP transmission an
   episodic character:

      o "Original transmissions" are sequences of data segments issued
        in response to transmission requests from client services.



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      o "Retransmissions" are sequences of data segments issued in
        response to the arrival of report segments that indicate
        incomplete reception.

   Checkpoints are mandatory only at the end of the red-part of each
   original transmission and at the end of each retransmission.  For
   applications that require accelerated retransmission (and can afford
   the additional bandwidth consumption entailed), reception reporting
   can be more aggressive.  Additional checkpoints may optionally be
   inserted at other points in the red-part of an original transmission,
   and additional reception reports may optionally be sent on an
   asynchronous basis during reception of the red-part of an original
   transmission.

3.5  Adjacency

   TCP reliability is "end to end": traffic between two TCP endpoints
   may traverse any number of intermediate network nodes, and two
   successively transmitted segments may travel by entirely different
   routes to reach the same destination.  The underlying IP network-
   layer protocol accomplishes this routing.  Although TCP always
   delivers data segments to any single port in order and without gaps,
   the IP datagrams delivered to TCP itself may not arrive in the order
   in which they were transmitted.

   In contrast, LTP is a protocol for "point to point" reliability on a
   single link: traffic between two LTP engines is expected not to
   traverse any intermediate relays.  Point-to-point topology is innate
   in the nature of deep space communication, which is simply the
   exchange of radiation between two mutually visible antennae.  No
   underlying network infrastructure is presumed, so no underlying
   network-layer protocol activity is expected; the underlying
   communication service is assumed to be a point-to-point link-layer
   protocol such as CCSDS Telemetry/Telecommand [TM][TC] (or, for
   terrestrial applications, PPP).  The contents of link-layer frames
   delivered to LTP are always expected to arrive in the order in which
   they were transmitted, though possibly with any number of gaps due to
   data loss or corruption.

   Note that building an interplanetary network infrastructure - the
   DTN-based architecture of the IPN - *on top of* LTP does not conflict
   with LTP design principles.  Bundle protocol functions as an overlay
   network protocol, and LTP bears essentially the same relationship to
   it as a reliable link protocol (for example, the ARQ capabilities of
   LLC) bears to IP.

   The design of LTP relies heavily on this topological premise, in at
   least two ways:



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      If two successively transmitted segments could travel by
      materially different routes to reach the same destination, then
      the assumption of rough determinism in timeout interval
      computation discussed below would not hold.  Our inability to
      estimate timeout intervals with any accuracy would severely
      compromise performance; while spurious timeouts cause redundant
      retransmissions wasting precious bandwidth, overly conservative
      timeout intervals delay loss recovery.

      If data arrived at an LTP engine out of transmission order, then
      the assumptions based on which the rules for reception reporting
      are designed would no longer hold.  A more complex and/or less
      efficient retransmission mechanism would be needed.

3.6  Optimistic and Dynamic Timeout Interval Computation

   TCP determines timeout intervals by measuring and recording actual
   round trip times, then applying statistical techniques to recent RTT
   history to compute a predicted round trip time for each transmitted
   segment.

   The problem is at once both simpler and more difficult for LTP:

      Since multiple sessions can be conducted on any single
      association, retardation of transmission on any single session
      while awaiting a timeout need not degrade communication
      performance on the association as a whole.  Timeout intervals that
      would be intolerably optimistic in TCP don't necessarily degrade
      LTP's bandwidth utilization.

      But the reciprocal half-duplex nature of LTP communication makes
      it infeasible to use statistical analysis of round-trip history as
      a means of predicting round-trip time.  The round-trip time for
      transmitted segment N could easily be orders of magnitude greater
      than that for segment N-1 if there happened to be a transient loss
      of connectivity between the segment transmissions.

   Since statistics derived from round-trip history cannot safely be
   used as a predictor of LTP round-trip times, we have to assume that
   round-trip timing is at least roughly deterministic - i.e., that
   sufficiently accurate RTT estimates can be computed individually in
   real time from available information.

   This computation is performed in two stages:

      We calculate a first approximation of RTT by simply doubling the
      known one-way light time to the destination and adding an
      arbitrary margin for any additional anticipated latency, such as



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      queuing and processing delay at both ends of the transmission.
      For deep space operations, the margin value is typically a small
      number of whole seconds.  Although such a margin is enormous by
      Internet standards, it is insignificant compared to the two-way
      light time component of round-trip time in deep space.  We choose
      to risk tardy retransmission, which will postpone delivery of one
      block by a relatively tiny increment, in preference to premature
      retransmission, which will unnecessarily consume precious
      bandwidth and thereby degrade overall performance.

      Then, to account for the additional delay imposed by interrupted
      connectivity, we dynamically suspend timers during periods when
      the relevant remote LTP engines are known to be unable to transmit
      responses.  This knowledge of the operational state of remote
      entities is assumed to be provided by link state cues from the
      operating environment.

3.7  Deferred Transmission

   Link state cues also notify LTP when it is and isn't possible to
   transmit segments.

   Continuous duplex communication is the norm for TCP operations in the
   Internet; when communication links are not available, TCP simply does
   not operate.  In deep space communications, however, at no moment can
   there ever be any expectation of two-way connectivity.  It is always
   possible for LTP to be generating outbound segments - in response to
   received segments, timeouts, or requests from client services - that
   cannot immediately be transmitted.  These segments must be queued for
   transmission at a later time when a link has been established, as
   signaled by a link state cue.

4. Overall Operation

4.1  Nominal Operation

   The nominal sequence of events in an LTP transmission session is as
   follows.

   Operation begins when a client service instance asks an LTP engine to
   transmit a block to a remote client service instance.  The sending
   engine opens a Sending State Record (SSR) for a new session, thereby
   starting the session, and notifies the client service instance that
   the session has been started.  The sending engine then initiates the
   original transmission: it queues for transmission as many data
   segments as are necessary to transmit the entire block, within the
   constraints on maximum segment size imposed by the underlying
   communication service.  The last segment of the red-part of the block



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   is marked as the End of Red-Part (EORP) indicating the end of red-
   part data for the block, and as a checkpoint (identified by a unique
   checkpoint serial number) indicating that the receiving engine must
   issue a reception report upon receiving the segment.  The last
   segment of the block overall is marked End of Block (EOB) indicating
   that the receiving engine can calculate the size of the block by
   summing the offset and length of the data in the segment.

   At the next opportunity, subject to allocation of bandwidth to the
   queue into which the block data segments were written, the enqueued
   segments are transmitted to the LTP engine serving the remote client
   service instance.  A timer is started for the EORP, so that it can be
   retransmitted automatically if no response is received.

   On reception of the first data segment for the block, the receiving
   engine opens a Receiving State Record (RSR) for the new session and
   notifies the local instance of the relevant client service that the
   session has been started.  In the nominal case it receives all
   segments of the original transmission without error.  Therefore on
   reception of the EORP data segment it responds by (a) queuing for
   transmission to the sending engine a report segment indicating
   complete reception and (b) delivering the received red-part of the
   block to the local instance of the client service; on reception of
   each data segment of the green-part, it responds by immediately
   delivering the received data to the local instance of the client
   service.

   At the next opportunity, the enqueued report segment is immediately
   transmitted to the sending engine and a timer is started so that the
   report segment can be retransmitted automatically if no response is
   received.

   The sending engine receives the report segment, turns off the timer
   for the EORP, enqueues for transmission to the receiving engine a
   report-acknowledgment segment, notifies the local client service
   instance that the red-part of the block has been successfully
   transmitted, and closes the SSR for the session.

   At the next opportunity, the enqueued report-acknowledgment segment
   is immediately transmitted to the receiving engine.

   The receiving engine receives the report-acknowledgment segment,
   turns off the timer for the report segment, and closes the RSR for
   the session.

   Closing both the SSR and RSR for a session terminates the session.

4.2  Retransmission



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   Loss or corruption of transmitted segments may cause the operation of
   LTP to deviate from the nominal sequence of events described above.

   Loss of one or more red-part data segments other than the EORP
   segment triggers data retransmission: the receiving engine returns a
   reception report detailing all the contiguous ranges of red-part data
   received (assuming no discretionary checkpoints were received, which
   are described below).  The Reception Report is normally sent in a
   single Report segment which carries a unique report serial number and
   the scope of red-part data covered.  For example, if the red-part
   data covered block offsets [0:1000] and all but the segment in range
   [500:600] were received, the report segment with a unique serial
   number (say 100) and scope [0:1000] would carry two report entries:
   (0:500) and (600:1000).  The maximum size of a report segment, like
   all LTP segments, is constrained by the datalink MTU; if many non-
   contiguous segments were lost in a large block transmission and/or
   the datalink MTU was relatively small, multiple report segments need
   to be generated. In this case, LTP generates as many report segments
   as are necessary and splits the scope of red-part data covered across
   multiple report segments so that each of them may stand on their own.
   For example, if three report segments are to be generated as part of
   a reception report covering red-part data in range [0:1,000,000],
   they could look like this: RS 19, scope [0:300,000], RS 20, scope
   [300,000:950,000], and RS 21, scope [950,000:1,000,000].  In all
   cases, a timer is started upon transmission of each report segment of
   the reception report.

   On reception of each report segment the sending engine responds as
   follows:

      It turns off the timer for the checkpoint referenced by the report
      segment, if any.

      It enqueues a reception-acknowledgment segment acknowledging the
      report segment (to turn off the report retransmission timer at the
      receiving engine).  This segment is sent immediately at the next
      transmission opportunity.

      If the reception claims in the report segment indicate that not
      all data within the scope have been received, it normally
      initiates a retransmission by enqueuing all data segments not yet
      received.  The last such segment is marked as a checkpoint and
      contains the report serial number of the report segment to which
      the retransmission is a response.  These segments are likewise
      sent at the next transmission opportunity, but only after all data
      segments previously queued for transmission to the receiving
      engine have been sent.  A timer is started for the checkpoint, so
      that it can be retransmitted automatically if no responsive report



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      segment is received.

      On the other hand, if the reception claims in the report segment
      indicate that all data within the scope of the report segment have
      been received, and the union of all reception claims received so
      far in this session indicates that all data in the red-part of the
      block have been received, then the sending engine notifies the
      local client service instance that the red-part of the block has
      been successfully transmitted and closes the SSR for the session.

   On reception of a report-acknowledgment segment, the receiver turns
   off the timer for the referenced report segment.  On reception of a
   checkpoint segment with a non-zero report serial number, the
   receiving engine responds as follows :

      It returns a reception report comprising as many report segments
      as are needed in order to report in detail on all data reception
      within the scope of the referenced report segment, and a timer is
      started for each report segment.

      If at this point all data in the red-part of the block have been
      received, the receiving engine delivers the received block's red-
      part to the local instance of the client service and, upon
      reception of reception-acknowledgment segments acknowledging all
      report segments, closes the RSR for the session.  Otherwise the
      data retransmission cycle continues.

   Loss of a checkpoint segment or the report segment generated in
   response causes timer expiry; when this occurs, the sending engine
   normally retransmits the checkpoint segment.  Similarly, the loss of
   a report segment or the corresponding report-acknowledgment segment
   causes the report segment's timer to expire; when this occurs, the
   receiving engine normally retransmits the report segment.

   Note that the redundant reception of a report segment (i.e., one that
   was already received and processed by the sender), retransmitted due
   to loss of the corresponding report-acknowledgment segment for
   example, causes another report-acknowledgment segment to be
   transmitted in response but is otherwise ignored; if any of the data
   segments retransmitted in response to the original reception of the
   report segment were lost, further retransmission of those data
   segments will be requested by the reception report generated in
   response to the last retransmitted data segment marked as a
   checkpoint.  Thus unnecessary retransmission is suppressed.

   Note also that the responsibility for responding to segment loss in
   LTP is shared between the sender and receiver of a block: the sender
   retransmits checkpoint segments in response to checkpoint timeouts,



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   and retransmits missing data in response to reception reports
   indicating incomplete reception, while the receiver retransmits
   report segments in response to timeouts.  An alternative design would
   have been to make the sender responsible for all retransmission, in
   which case the receiver would not expect report-acknowledgment
   segments and would not retransmit report segments.  There are two
   disadvantages to this approach:

      First, because of constraints on segment size that might be
      imposed by the underlying communication service, it is at least
      remotely possible that the response to any single checkpoint might
      be multiple report segments.  An additional sender-side mechanism
      for detecting and appropriately responding to the loss of some
      proper subset of those reception reports would be needed.  We
      believe that the current design is simpler.

      Second, an engine that receives a block needs a way to determine
      when the session can be closed.  In the absence of explicit final
      report acknowledgment (which entails retransmission of the report
      in case of the loss of the report acknowledgment), the
      alternatives are (a) to close the session immediately on
      transmission of the report segment that signifies complete
      reception and (b) to close the session on receipt of an explicit
      authorization from the sender.  In case (a), loss of the final
      report segment would cause retransmission of a checkpoint by the
      sender, but the session would no longer exist at the time the
      retransmitted checkpoint arrived; the checkpoint could reasonably
      be interpreted as the first data segment of a new block, most of
      which was lost in transit, and the result would be redundant
      retransmission of the entire block.  In case (b), the explicit
      session termination segment and the responsive acknowledgment by
      the receiver (needed in order to turn off the timer for the
      termination segment, which in turn would be needed in case of in-
      transit loss or corruption of the termination segment) would
      somewhat complicate the protocol, increase bandwidth consumption,
      and retard the release of session state resources at the sender.
      Here again we believe that the current design is simpler and more
      efficient.

4.3  Accelerated Retransmission

   Data segment retransmission occurs only on receipt of a report
   segment indicating incomplete reception; report segments are normally
   transmitted only at the end of original transmission of the red-part
   of a block or at the end of a retransmission.  For some applications
   it may be desirable to trigger data segment retransmission
   incrementally during the course of red-part original transmission so
   that the missing segments are recovered sooner.  This can be



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   accomplished in two ways:

      Any red-part data segment prior to the EORP can additionally be
      flagged as a checkpoint.  Reception of each such "discretionary"
      checkpoint causes the receiving engine to issue a reception
      report.

      At any time during the original transmission of a block's red-part
      (that is, prior to reception of any data segment of the block's
      green-part), the receiving engine can unilaterally issue
      additional asynchronous reception reports. Note that the CFDP
      protocol's "Immediate" mode is an example of this sort of
      asynchronous reception reporting.  The reception reports generated
      for accelerated retransmission are processed in exactly the same
      way as the standard reception reports.

4.4  Session Cancellation

   A transmission session may be canceled by either the sending or the
   receiving engine in response either to a request from the local
   client service instance or to an LTP operational failure as noted
   earlier.  Session cancellation is accomplished as follows.

   The canceling engine deletes all currently queued segments for the
   session and notifies the local instance of the affected client
   service that the session is canceled.  If no segments for this
   session have yet been sent to or received from the corresponding LTP
   engine, then at this point the canceling engine simply closes its
   state record for the session and cancellation is complete.

   Otherwise, a session cancellation segment is queued for transmission.
   At the next opportunity, the enqueued cancellation segment is
   immediately transmitted to the LTP engine serving the remote client
   service instance.  A timer is started for the segment, so that it can
   be retransmitted automatically if no response is received.

   The corresponding engine receives the cancellation segment, enqueues
   for transmission to the canceling engine a cancellation-
   acknowledgment segment, deletes all other currently queued segments
   for the indicated session, notifies the local client service instance
   that the block has been canceled, and closes its state record for the
   session.

   At the next opportunity, the enqueued cancellation-acknowledgment
   segment is immediately transmitted to the canceling engine.

   The canceling engine receives the cancellation-acknowledgment, turns
   off the timer for the cancellation segment, and closes its state



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   record for the session.

   Loss of a cancellation segment or of the responsive cancellation-
   acknowledgment causes the cancellation segment timer to expire.  When
   this occurs, the canceling engine normally retransmits the
   cancellation segment.

5.  Functional Model

   The functional model underlying the specification of LTP is one of
   deferred, opportunistic transmission, with access to the active
   transmission link apportioned between two (conceptual) outbound
   traffic queues.  The accuracy of LTP retransmission timers depend in
   large part on a faithful adherence to this model.

5.1  Deferred Transmission

   In concept, every outbound LTP segment is appended to one of two
   queues -- forming a queue-set -- of traffic bound for the LTP engine
   that is that segment's destination.  One such traffic queue is the
   "internal operations queue" of that queue set; the other is the
   application data queue for the queue set.  The de-queuing of a
   segment always implies delivering it to the underlying communication
   system for immediate transmission.  Whenever the internal operations
   queue is non-empty, the oldest segment in that queue is the next
   segment de-queued for transmission to the destination; at all other
   times, the oldest segment in the application data queue is the next
   segment de-queued for transmission to the destination.

   The production and enqueuing of a segment and the subsequent actual
   transmission of that segment are in principle wholly asynchronous.

   In the event that (a) a transmission link to the destination is
   currently active and (b) the queue to which a given outbound segment
   is appended is otherwise empty and (c) either this queue is the
   internal operations queue or else the internal operations queue is
   empty, the segment will be transmitted immediately upon production.
   Transmission of a newly queued segment is necessarily deferred in all
   other circumstances.

   Conceptually, the de-queuing of segments from traffic queues bound
   for a given destination is initiated upon reception of a link state
   cue indicating that the underlying communication system is now
   transmitting to that destination, i.e., the link to that destination
   is now active.  It ceases upon reception of a link state cue
   indicating that the underlying communication system is no longer
   transmitting to that destination, i.e., the link to that destination
   is no longer active.



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5.2  Timers

   LTP relies on accurate calculation of expected arrival times for
   report and acknowledgment segments in order to know when proactive
   retransmission is required.  If a calculated time were even slightly
   early, the result would be costly unnecessary retransmission.  On the
   other hand, calculated arrival times may safely be several seconds
   late: the only penalties for late timeout and retransmission are
   slightly delayed data delivery and slightly delayed release of
   session resources.

   The following discussion is the basis for LTP's expected arrival time
   calculations.

   The total time consumed in a single "round trip" (transmission and
   reception of the original segment, followed by transmission and
   reception of the acknowledging segment) has the following components:

      Protocol processing time: The time consumed in issuing the
      original segment, receiving the original segment, generating and
      issuing the acknowledging segment, and receiving the acknowledging
      segment.

      Outbound queuing delay: The delay at the sender of the original
      segment while that segment is in a queue waiting for transmission,
      and delay at the sender of the acknowledging segment while that
      segment is in a queue waiting for transmission.

      Radiation time: The time that passes while all bits of the
      original segment are being radiated, and the time that passes
      while all bits of the acknowledging segment are being radiated.
      (This is significant only at extremely low data transmission
      rates.)

      Round-trip light time: The signal propagation delay at the speed
      of light, in both directions.

      Inbound queuing delay: delay at the receiver of the original
      segment while that segment is in a reception queue, and delay at
      the receiver of the acknowledging segment while that segment is in
      a reception queue.

      Delay in transmission of the original segment or the acknowledging
      segment due to loss of connectivity - that is, interruption in
      outbound link activity at the sender of either segment due to
      occultation, scheduled end of tracking pass, etc.

   In this context, where errors on the order of seconds or even minutes



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   may be tolerated, protocol processing time at each end of the session
   is assumed to be negligible.

   Inbound queuing delay is also assumed to be negligible because, even
   on small spacecraft, LTP processing speeds are high compared to data
   transmission rates.

   Two mechanisms are used to make outbound queuing delay negligible:

      The expected arrival time of an acknowledging segment is not
      calculated until the moment the underlying communication system
      notifies LTP that radiation of the original segment has begun.
      All outbound queuing delay for the original segment has already
      been incurred at that point.

      LTP's deferred transmission model [Sec 5.1] minimizes latency in
      the delivery of acknowledging segments (reports and
      acknowledgments) to the underlying communication system; that is,
      acknowledging segments are (in concept) appended to the internal
      operations queue rather than the application data queue, so they
      have higher transmission priority than any other outbound
      segments, i.e., they should always be de-queued for transmission
      first.  This limits outbound queuing delay for a given
      acknowledging segment to the time needed to de-queue and radiate
      all previously generated acknowledging segments that have not yet
      been de-queued for transmission.  Since acknowledging segments are
      sent infrequently and are normally very small, outbound queuing
      delay for a given acknowledging segment is likely to be minimal.

   Deferring calculation of the expected arrival time of the
   acknowledging segment until the moment at which the original segment
   is radiated has the additional effect of removing from consideration
   any original segment transmission delay due to loss of connectivity
   at the original segment sender.

   Radiation delay at each end of the session is simply segment size
   divided by transmission data rate.  It is insignificant except when
   data rate is extremely low (for example, 10 bps), in which case the
   use of LTP may well be inadvisable for other reasons (LTP header
   overhead for example, could be too much under such data rates).
   Therefore radiation delay is normally assumed to be negligible.

   We assume that one-way light time to the nearest second can always be
   known (for example, provided by the operating environment).

   So the initial expected arrival time for each acknowledging segment
   is typically computed as simply the current time at the moment that
   radiation of the original segment begins, plus twice the one-way



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   light time, plus 2*N seconds of margin to account for processing and
   queuing delays and for radiation time at both ends. N is a parameter
   set by network management for which 2 seconds seem to be a reasonable
   default value.

   This leaves only one unknown, the additional round trip time
   introduced by loss of connectivity at the sender of the acknowledging
   segment.  To account for this, we again rely on external link state
   cues.  Whenever interruption of transmission at a remote LTP engine
   is signaled by a link state cue, we suspend the countdown timers for
   all acknowledging segments expected from that engine.  Upon a signal
   that transmission has resumed at that engine, we resume those timers
   after (in effect) adding to each expected arrival time the length of
   the timer suspension interval.

6. Security Considerations

   There is a clear risk that unintended receivers can listen in on LTP
   transmissions over satellite and other radio broadcast datalinks.
   Such unintended recipients of LTP transmissions may also be able to
   manipulate LTP segments at will.

   Hence there is a potential requirement for confidentiality, integrity
   and anti-DoS (Denial of Service) security services and mechanisms.

   In particular, DoS problems are more severe for LTP compared to
   typical internet protocols because LTP inherently retains state for
   long periods and has very high time-out values.  Further, it could be
   difficult to reset LTP nodes to recover from an attack.  Thus any
   adversary who can actively attack an LTP transmission has the
   potential to create severe DoS conditions for the LTP receiver.

   To give a terrestrial example: were LTP to be used in a sparse sensor
   network, DoS attacks could be mounted resulting in nodes missing
   critical information, such as communications schedule updates.  In
   such cases, a single successful DoS attack could take a node entirely
   off the network until the node was physically visited and reset.

   Even for deep space applications of LTP we need to consider certain
   terrestrial attacks, in particular those involving insertion of
   messages into an on-going session (usually without having seen the
   exact bytes of the previous messages in the session).  Such attacks
   are likely in the presence of firewall failures at various nodes in
   the network, or due to Trojan software running on an authorized host.
   Many message insertion attacks will depend on the attacker correctly
   "guessing" something about the state of the LTP peers, but experience
   shows that successful guesses are easier than might be thought [DDJ].




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   We now consider the appropriate layer(s) at which security mechanisms
   can be deployed to increase the security properties of LTP, and the
   trade-offs entailed in doing so.

   The Application layer (above-LTP)

      Higher layer security mechanisms clearly protect LTP payload, but
      leave LTP headers open.  Such mechanisms provide little or no
      protection against DoS type attacks against LTP, but may well
      provide sufficient data integrity and ought to be able to provide
      data confidentiality.

   The LTP layer

      An authentication header (similar to IPSEC [AH]) can help protect
      against replay attacks and other bogus packets.  However, an
      adversary may still see the LTP header of segments passing by in
      the ether.  This approach also requires some key management
      infrastructure to be in place in order to provide strong
      authentication, which may not always be an acceptable overhead.
      Such an authentication header could mitigate many DoS attacks.

      Similarly, a confidentiality service could be defined for LTP
      payload and (some) header fields. However, this seems less
      attractive since (a) confidentiality is arguably better provided
      either above or below the LTP layer, (b) key management for such a
      service is harder (in a high-delay context) than for an integrity
      service, and (c) forcing LTP engines to attempt decryption of
      incoming segments can in itself provide a DoS opportunity.

      Further, within the LTP layer we can make various design decisions
      to reduce the probability of successful DoS attacks.  In
      particular, we can mandate that values for certain fields in the
      header (session numbers, for example) be chosen randomly.

   The Datalink layer (below-LTP)

      The lower layers can clearly provide confidentiality and integrity
      services, although such security may result in unnecessary
      overhead (if a service provided is not required for all LTP
      sessions, for example) and loss of flexibility. However, the lower
      layers may well be the optimal place to do compression and
      encryption.


7.  IANA Considerations

   Not relevant for this document. Please follow the IANA Considerations



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   sections of the internet-drafts on the series [LTPSPEC, LTPEXT].

8.  Acknowledgments

   Many thanks to Tim Ray, Vint Cerf, Bob Durst, Kevin Fall, Adrian
   Hooke, Keith Scott, Leigh Torgerson, Eric Travis, and Howie Weiss for
   their thoughts on this protocol and its role in Delay-Tolerant
   Networking architecture.

   Part of the research described in this document was carried out at
   the Jet Propulsion laboratory, California Institute of Technology,
   under a contract with the National Aeronautics and Space
   Administration. This work was performed under DOD Contract DAA-B07-
   00-CC201, DARPA AO H912; JPL Task Plan No. 80-5045, DARPA AO H870;
   and NASA Contract NAS7-1407.

   Thanks are also due to Shawn Ostermann, Hans Kruse, and Dovel Myers
   at Ohio University for their suggestions and advice in making various
   design decisions.

   Part of this work was carried out at Trinity College Dublin as part
   of the SeNDT contract funded by Enterprise Ireland's research
   innovation fund.

9.  References

9.1 Normative References

   [B97] S. Bradner, "Key words for use in RFCs to Indicate Requirement
   Levels", BCP 14, RFC 2119, March 1997.

   [LTPSPEC] Ramadas, M., Burleigh, S., and Farrell, S., "Licklider
   Transmission Protocol - Specification", draft-irtf-dtnrg-ltp-06.txt
   (Work in Progress), April 2007.

   [LTPEXT] Farrell, S., Ramadas, M., and Burleigh, S., "Licklider
   Transmission Protocol - Extensions", draft-irtf-dtnrg-ltp-
   extensions-05x.txt (Work in Progress), April 2007.

9.2 Informative References

   [AH] Kent, S., and R. Atkinson, "IP Authentication Header", RFC 2402,
   November 1998.

   [BP] K. Scott, and S. Burleigh, "Bundle Protocol Specification",
   draft-irtf-dtnrg-bundle-spec-08.txt (Work in Progress), December
   2006.




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   [CCSDS] Consultative Committee for Space Data Systems web page,
   "http://www.ccsds.org".

   [CFDP] CCSDS File Delivery Protocol (CFDP). Recommendation for Space
   Data System Standards, CCSDS 727.0-B-2 BLUE BOOK Issue 1, October
   2002.

   [DDJ]  I. Goldberg and E. Wagner, "Randomness and the Netscape
   Browser", Dr. Dobb's Journal, 1996, (pages 66-70).

   [DSN] Deep Space Mission Systems Telecommunications Link Design
   Handbook (810-005) web-page,
   "http://eis.jpl.nasa.gov/deepspace/dsndocs/810-005/"

   [DTN] K. Fall, "A Delay-Tolerant Network Architecture for Challenged
   Internets", In Proceedings of ACM SIGCOMM 2003, Karlsruhe, Germany,
   Aug 2003.

   [IPN] InterPlanetary Internet Special Interest Group web page,
   "http://www.ipnsig.org".

   [TFRC] M. Handley, S. Floyd, J. Padhye, and J. Widmer, "TCP Friendly
   Rate Control (TFRC): Protocol Specification", RFC 3448, January 2003.

   [TM] Packet Telemetry Specification. Recommendation for Space Data
   System Standards, CCSDS 103.0-B-2 BLUE BOOK Issue 2, June 2001.

   [TC] Telecommand Part 2 - Data Routing Service. Recommendation for
   Space Data System Standards, CCSDS 202.0-B-3 BLUE BOOK Issue 3, June
   2001.

   [ECS94] D. Eastlake, S. Crocker, and J. Schiller, "Randomness
   Recommendations for Security", RFC 1750, December 1994.

   [SCTP] R. Stewart et al, "Stream Control Transmission Protocol", RFC
   2960, October 2000.

10.  Author's Addresses

      Scott C. Burleigh
      Jet Propulsion Laboratory
      4800 Oak Grove Drive
      M/S: 301-485B
      Pasadena, CA 91109-8099
      Telephone +1 (818) 393-3353
      FAX +1 (818) 354-1075
      Email Scott.Burleigh@jpl.nasa.gov




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      Manikantan Ramadas
      Internetworking Research Group
      301 Stocker Center
      Ohio University
      Athens, OH 45701
      Telephone +1 (740) 593-1562
      Email mramadas@irg.cs.ohiou.edu

      Stephen Farrell
      Distributed Systems Group
      Computer Science Department
      Trinity College Dublin
      Ireland
      Telephone +353-1-896-1761
      Email stephen.farrell@cs.tcd.ie


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