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RFC 5325 - Licklider Transmission Protocol - Motivation


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Network Working Group                                        S. Burleigh
Request for Comments: 5325                NASA/Jet Propulsion Laboratory
Category: Informational                                       M. Ramadas
                                                            ISTRAC, ISRO
                                                              S. Farrell
                                                  Trinity College Dublin
                                                          September 2008

              Licklider Transmission Protocol - Motivation

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

IESG Note

   This RFC is not a candidate for any level of Internet Standard.  It
   represents the consensus of the Delay Tolerant Networking (DTN)
   Research Group of the Internet Research Task Force (IRTF).  See RFC
   3932 for more information.

Abstract

   This document describes the motivation for the development of the
   Licklider Transmission Protocol (LTP) designed to provide
   retransmission-based reliability over links 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 it 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 radio frequency (RF) links.  LTP does Automatic
   Repeat reQuest (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 ....................................................2
   2. Problem .........................................................3
      2.1. IPN Operating Environment ..................................3
      2.2. Why Not TCP or SCTP? .......................................5
   3. Protocol Overview ...............................................6
      3.1. Nominal Operation ..........................................6
           3.1.1. Link State Cues .....................................9
           3.1.2. Deferred Transmission ...............................9
           3.1.3. Timers .............................................10
      3.2. Retransmission ............................................13
      3.3. Accelerated Retransmission ................................16
      3.4. Session Cancellation ......................................17
   4. Security Considerations ........................................17
   5. IANA Considerations ............................................20
   6. Acknowledgments ................................................20
   7. References .....................................................20
      7.1. Informative References ....................................20

1.  Introduction

   The Licklider Transmission Protocol (LTP) 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.  Specifically, LTP is intended to serve as a reliable
   "convergence layer" protocol, underlying the Delay-Tolerant
   Networking (DTN) [DTN] Bundle protocol [BP], in DTN deployments where
   data links are characterized by very long round-trip times.

   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
   document [LTPEXT].

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

2.  Problem

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) [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: physics and
   economics.

   The more obvious type of delay imposed by nature is signal
   propagation time.  Round-trip times between Earth and Jupiter's moon
   Europa, for example, 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.  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
   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.  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.

   These operating conditions imply a number of additional constraints
   on any protocol designed to ensure reliable communication over deep-
   space links.

   - 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.
     Multiple parallel data block transmission "sessions" must be in
     progress concurrently in order to avoid under-utilization of the
     links.

   - Like any reliable transport service employing ARQ, LTP is
     "stateful".  In order to ensure the reception of a block of data it
     has sent, LTP must retain for possible retransmission all portions
     of that block that 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.

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

   - 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.)  Bidirectional data flow, where
     possible, is performed using two unidirectional links in opposite
     directions and at different data rates.

   - Finally, the problem of timeout interval computation in the
     environment for which LTP is mainly intended is different from the
     analogous problem in the Internet.  Since multiple sessions can be
     conducted in parallel, 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.  A different
     mechanism for timeout interval computation is needed.

2.2.  Why Not TCP or SCTP?

   These environmental characteristics -- long and highly variable
   delays, 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.

   For the purposes of this discussion, we are not considering the more
   aggressive TCP throughput equation that characterizes HighSpeed TCP
   [HSTCP].

   The TCP characteristic of an initial three-way handshake for each new
   connection, followed by slow-start, is a further obstacle, because
   the delay of the three-way handshake and the additional delay of
   slow-start could be exorbitant in a long-delay environment.

   The Stream Control Transmission Protocol (SCTP) [SCTP] 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. Protocol Overview

3.1.  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 of data to a remote client service instance.

   LTP regards each block of data as comprising two parts: a "red-part",
   whose delivery must be assured by acknowledgment and retransmission
   as necessary, followed by 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
   NOT have any urgency or higher-priority semantics relative to the
   block's green-part data.  The red-part data is merely data for which
   the user has requested reliable transmission, possibly (though not
   necessarily) data without which the green-part data may be useless,
   such as an application-layer header or other metadata.

   The client service instance uses the LTP implementation's application
   programming interface to specify to LTP the identity of the remote
   client service instance to which the data must be transmitted, the
   location of the data to be transmitted, the total length of data to
   be transmitted, and the number of leading data bytes that are to be
   transmitted reliably as "red" data.  The sending engine starts a
   transmission session for this block and notifies the client service
   instance that the session has been started.  Note that

   LTP communication session parameters are not negotiated but are
   instead asserted unilaterally, subject to application-level network
   management activity; the sending engine does not negotiate the start
   of the session with the remote client service instance's engine.

   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 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.

   LTP is designed to run directly over a data-link layer protocol, but
   it may instead be deployed directly over UDP in some cases (i.e., for
   software development or in private local area networks).  In either
   case, the protocol layer immediately underlying LTP is here referred
   to as the "local data-link layer".

   At the next opportunity, subject to allocation of bandwidth to the
   queue into which the block data segments were written, the enqueued
   segments and their lengths are passed to the local data-link layer
   protocol (which might be UDP/IP) via the API supported by that
   protocol, for transmission 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.

   The content of each local data-link layer protocol data unit (link-
   layer frame or UDP datagram) is required to be an integral number of
   LTP segments, and the local data-link layer protocol is required
   never to deliver incomplete LTP segments to the receiving LTP engine.
   When the local data-link layer protocol is UDP, the LTP
   authentication [LTPEXT] extension should be used to ensure data
   integrity unless the end-to-end path is one in which either the
   likelihood of datagram content corruption is negligible (as in some
   private local area networks) or the consequences of receiving and
   processing corrupt LTP segments are insignificant (as perhaps during
   software development).  When the LTP authentication extension is not

   used, LTP requires the local data-link layer protocol to perform
   integrity checking of all segments received; specifically, the local
   data-link layer protocol is required to detect any corrupted segments
   that are received and to discard them silently.

   Received segments that are not discarded are passed up to the
   receiving LTP engine via the API supported by the local data-link
   layer protocol.

   On reception of the first data segment for the block, the receiving
   engine starts a reception session for this block 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.

   All delivery of data and protocol event notices to the local client
   service instance is performed using the LTP implementation's
   application programming interface.

   Note that since LTP data flows are unidirectional, LTP's data
   acknowledgments -- "reception reports" -- can't be piggybacked on
   data segments as in TCP.  They are instead carried in a separate
   segment type.

   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, and notifies the local client service
   instance that the red-part of the block has been successfully
   transmitted.  The session's red-part transmission has now ended.

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

   The receiving engine receives the report-acknowledgment segment and
   turns off the timer for the report segment.  The session's red-part
   reception has now ended and transmission of the block is complete.

3.1.1.  Link State Cues

   Establishing a communication link across interplanetary distances may
   entail hardware configuration changes based on the presumed
   operational state of the remote communicating entity, for example:

      o orienting a directional antenna correctly;

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

      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.

3.1.2.  Deferred Transmission

   Link state cues also notify LTP when it is and isn't possible to
   transmit segments.  In deep-space communications, 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.

   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.

3.1.3.  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.

   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
        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.

   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
   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 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
   the 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
   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.

3.2.  Retransmission

   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 that 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 data-link MTU; if many non-
   contiguous segments were lost in a large block transmission and/or
   the data-link 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 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; the
        session's red-part transmission has ended.

   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, the session's red-part reception ends and
        transmission of the block is complete.  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,
   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.

3.3.  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
   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 [CFDP].  The reception reports
        generated for accelerated retransmission are processed in
        exactly the same way as the standard reception reports.

3.4.  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
   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 retransmits the cancellation
   segment.

4. Security Considerations

   There is a clear risk that unintended receivers can listen in on LTP
   transmissions over satellite and other radio broadcast data links.
   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 long 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 ongoing 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].

   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 Data-link layer (below-LTP)

      The lower layers can clearly provide confidentiality and integrity
      services, although such security may result in unnecessary
      overhead if the cryptographic service provided is not required for
      all data.  For example, it can be harder to manage lower layers so
      that only the data that requires encryption is in fact encrypted.
      Encrypting all data could represent a significant overhead for
      some LTP use cases.  However, the lower layers are often the place
      where compression and error-correction is done, and so may well
      also be the optimal place to do encryption since the same issues
      with applying or not applying the service apply to both encryption
      and compression.

   In light of these considerations, LTP includes the following security
   mechanisms:

      The optional LTP Authentication mechanism is an LTP segment
      extension comprising a ciphersuite identifier and optional key
      identifier that precede the segment's content, plus an
      authentication value (either a message authentication code or a
      digital signature) that follows the segment's content; the
      ciphersuite ID is used to indicate the length and format of the
      authentication value.  The authentication mechanism serves to
      assure the segment's integrity and, depending on the ciphersuite
      selected and the key management regime, its authenticity.

      The optional LTP cookie mechanism is an LTP segment extension
      comprising a "cookie" -- a randomly chosen numeric value -- that
      precedes the segment's content.  By increasing the number of bytes
      in a segment that cannot be easily predicted by an inauthentic
      data source, and by requiring that segments lacking the correct
      values of these bytes be silently discarded, the cookie mechanism
      increases the difficulty of mounting a successful denial-of-
      service attack on an LTP engine.

      The above mechanisms are defined in detail in the LTP extensions
      document [LTPEXT].

      In addition, the serial numbers of LTP checkpoints and reports are
      required to be randomly chosen integers, and implementers are
      encouraged to choose session numbers randomly as well.  This
      randomness adds a further increment of protection against DoS
      attacks.  See [PRNG] for recommendations related to randomness.

5.  IANA Considerations

   Please see the IANA Considerations sections of [LTPSPEC] and
   [LTPEXT].

6.  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.  This work was done when Manikantan Ramadas was a
   graduate student at the EECS Dept., Ohio University, in the
   Internetworking Research Group Laboratory.

   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.

7.  References

7.1.  Informative References

   [LTPSPEC] Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
             Transmission Protocol - Specification", RFC 5326, September
             2008.

   [LTPEXT]  Farrell, S., Ramadas, M., and S. Burleigh, "Licklider
             Transmission Protocol - Security Extensions", RFC 5327,
             September 2008.

   [AH]      Kent, S., "IP Authentication Header", RFC 4302, December
             2005.

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

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

   [HSTCP]   Floyd, S., "HighSpeed TCP for Large Congestion Windows",
             RFC 3649, December 2003.

   [SCTP]    Stewart, R., Ed., "Stream Control Transmission Protocol",
             RFC 4960, September 2007.

   [PRNG]    Eastlake, D., 3rd, Schiller, J., and S. Crocker,
             "Randomness Requirements for Security", BCP 106, RFC 4086,
             June 2005.

Authors' 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

   Manikantan Ramadas
   ISRO Telemetry Tracking and Command Network (ISTRAC)
   Indian Space Research Organization (ISRO)
   Plot # 12 & 13, 3rd Main, 2nd Phase
   Peenya Industrial Area
   Bangalore 560097
   India
   Telephone: +91 80 2364 2602
   EMail: mramadas@gmail.com

   Stephen Farrell
   Computer Science Department
   Trinity College Dublin
   Ireland
   Telephone: +353-1-896-1761
   EMail: stephen.farrell@cs.tcd.ie

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