[Docs] [txt|pdf] [Tracker] [Email] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 5326

Delay Tolerant Networking Research Group                     S. Burleigh
Internet Draft                            NASA/Jet Propulsion Laboratory
<draft-irtf-dtnrg-ltp-00.txt>                                 M. Ramadas
May 2004                                                 Ohio University
Expires November 2004                                         S. Farrell
                                                  Trinity College Dublin


                    Licklider Transmission Protocol


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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

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

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   Discussions on this internet-draft are being made in the Delay
   Tolerant Networking Research Group (DTNRG) mailing list. More
   information can be found in the DTNRG web-site at
   http://www.dtnrg.org

Abstract

   This document describes the Licklider Transmission Protocol (LTP)
   designed to provide retransmission-based reliability over links
   characterized by extremely long message round-trip times (RTTs).
   These long round-trip times may result from the use of half-duplex
   channels or from data propagation delays that are so lengthy as to
   simulate half-duplex transmission.  Such environments are not well
   served by TCP, which depends on relatively short round-trip times for
   retransmission buffer management, timely flow control, and
   negotiation of other connection parameters.



Burleigh et al.          Expires - November 2004                [Page 1]

Internet Draft       Licklider Transmission Protocol            May 2004


   Communication across interplanetary space is the most prominent
   example of this sort of environment, and LTP is in fact adapted from
   existing communication technologies designed to support deep space
   flight missions.  It is principally aimed at supporting "long-haul"
   reliable transmission in the interplanetary space but might have
   applications in other long-RTT environments as well.

Table of Contents
    1. Introduction .................................................  3
    2. Motivation ...................................................  5
       2.1 IPN Operating Environment ................................  5
       2.2 Why not TCP? .............................................  7
    3. Features .....................................................  7
       3.1 Massively Parallel State Retention .......................  8
          3.1.1 Out-of-order Delivery ...............................  9
          3.1.2 Session IDs ......................................... 10
          3.1.3 Use of Non-volatile Storage ......................... 11
       3.2 Absence of Negotiation ................................... 11
       3.3 Laconic Acknowledgment ................................... 12
       3.4 Adjacency ................................................ 13
       3.5 Optimistic and Dynamic Timeout Interval Computation ...... 14
       3.6 Deferred Transmission .................................... 15
    4. Terminology .................................................. 15
    5. Overall Operation ............................................ 19
       5.1 Nominal Operation ........................................ 19
       5.2 Retransmission ........................................... 20
          5.2.1 Reception Reporting Rules ........................... 22
          5.2.2 Design Rationale .................................... 22
       5.3 Accelerated Delivery ..................................... 24
       5.4 Accelerated Retransmission ............................... 24
       5.5 Session Cancellation ..................................... 25
       5.6 Unreliable Transmission .................................. 26
    6. Functional Model ............................................. 26
       6.1 Deferred Transmission .................................... 27
       6.2 Bandwidth Management ..................................... 27
       6.3 Timers ................................................... 28
    7. Segment Structure ............................................ 30
       7.1 Segment Header ........................................... 30
          7.1.1 Segment Type Flags .................................. 31
          7.1.2 Segment Type Codes .................................. 32
          7.1.3 Segment Class Masks ................................. 32
       7.2 Segment Content .......................................... 33
          7.2.1 Data Segment ........................................ 33
          7.2.2 Report Segment ...................................... 34
          7.2.3 Report Acknowledgment Segment ....................... 36
          7.2.4 Session Management Segments ......................... 36
    8. Requests from Client Service ................................. 37
       8.1 Transmission Request ..................................... 37



Burleigh et al.          Expires - November 2004                [Page 2]

Internet Draft       Licklider Transmission Protocol            May 2004


       8.2 Cancellation Request ..................................... 38
    9. Internal Procedures .......................................... 39
       9.1 Start Transmission ....................................... 39
       9.2 Start Checkpoint Timer ................................... 40
       9.3 Start RS Timer ........................................... 40
       9.4 Stop Transmission ........................................ 40
       9.5 Suspend Timers ........................................... 40
       9.6 Resume Timers ............................................ 41
       9.7 Retransmit Checkpoint .................................... 42
       9.8 Retransmit RS ............................................ 42
       9.9 Signify Segment Arrival .................................. 42
       9.10 Signify Block Reception ................................. 43
       9.11 Send Reception Report ................................... 43
       9.12 Signify Transmission Completion ......................... 44
       9.13 Retransmit Data ......................................... 44
       9.14 Stop RS Timer ........................................... 45
       9.15 Start Cancel Timer ...................................... 45
       9.16 Retransmit Cancellation Segment ......................... 45
       9.17 Acknowledge Cancellation ................................ 46
       9.18 Stop Cancellation Timer ................................. 46
       9.19 Cancel Session .......................................... 47
       9.20 Close Session ........................................... 47
   10.  Notices to Client Service ................................... 47
      10.1 Session Start ............................................ 47
      10.2 Data Segment Arrival ..................................... 47
      10.3 Block Reception .......................................... 48
      10.4 Transmission Completion .................................. 48
      10.5 Transmission Cancellation ................................ 48
      10.6 Reception Cancellation ................................... 49
   11. Requirements from the Operating Environment .................. 49
   12. Security Considerations ...................................... 49
      12.1 Mechanisms and Layering Considerations ................... 51
      12.2 Denial of Service Considerations ......................... 52
      12.3 Authentication header .................................... 53
      12.4 Implementation Considerations ............................ 53
      12.5 Miscellaneous ............................................ 54
   13. Tracing LTP back to CFDP ..................................... 54
   14. IANA Considerations .......................................... 56
   15. Acknowledgments .............................................. 56
   16. References ................................................... 57
   17. Author's Addresses ........................................... 57

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.  These long
   round-trip times may result from the use of half-duplex channels or



Burleigh et al.          Expires - November 2004                [Page 3]

Internet Draft       Licklider Transmission Protocol            May 2004


   from data propagation delays that are so lengthy as to simulate half-
   duplex transmission.  Such environments are not well served by TCP,
   which depends on relatively short round-trip times for retransmission
   buffer management, timely flow control, and negotiation of other
   connection parameters.

   As 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 the Interplanetary
   Internet (IPN) [IPN].

   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.  These constraints differ sharply from
   those within which the Internet typically functions:


      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 communication 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 Bundling protocol
   defined for Delay-Tolerant Networking [BP][DTNRG].  LTP is intended
   to serve as a reliable "convergence layer" protocol underlying
   Bundling in DTN regions whose links are characterized by very long
   round-trip times.  Its design notions are directly descended from the
   retransmission procedures defined for CFDP.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [B97].



Burleigh et al.          Expires - November 2004                [Page 4]

Internet Draft       Licklider Transmission Protocol            May 2004


2.  Motivation

2.1  IPN Operating Environment

   There are a number of fundamental differences between the environment
   for terrestrial communications and the operating environments
   envisioned for the 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 be "behind" 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, messages must
   wait in a queue for later transmission.  Delivery is necessarily
   retarded.

   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).  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-



Burleigh et al.          Expires - November 2004                [Page 5]

Internet Draft       Licklider Transmission Protocol            May 2004


   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 propagation delay and
   occultation, but also by the scheduling and queuing delays imposed by
   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, and 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, or one error in ten bits; heavy
   coding is used to reduce these rates, and of course this coding
   further reduces the residual bandwidth available for data
   transmission.

   Propagation delay is the only truly immutable characteristic that
   distinguishes the IPN from terrestrial communications (unless and



Burleigh et al.          Expires - November 2004                [Page 6]

Internet Draft       Licklider Transmission Protocol            May 2004


   until we find a way to transmit information faster than the speed of
   light).  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 TCP?

   These environmental characteristics - long delays, low and asymmetric
   bandwidth, intermittent connectivity, and relatively high error rate
   - make using unmodified TCP for end to end communications in the IPN
   infeasible.  Using the equations from Mathis, et al., [MSM97], we can
   calculate an upper bound on the sustainable throughput of a TCP
   connection, taking into account TCP's congestion avoidance
   mechanisms.  Even if only 1 in 100 million packets are lost, a TCP
   connection to Mars is limited to just under 250kbps.  If we assume
   that 1 in 5000 packets is lost (this figure was reported by Paxson as
   the packet corruption rate in the Internet [P97] then that number
   falls to around 1,600bps.

   These 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
   a TCP 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.

3. Features




Burleigh et al.          Expires - November 2004                [Page 7]

Internet Draft       Licklider Transmission Protocol            May 2004


   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
   remote communicating entity:

      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 "link state cues" 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  Massively Parallel State Retention

   Like any reliable transport service, 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 yet have been received.  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.




Burleigh et al.          Expires - November 2004                [Page 8]

Internet Draft       Licklider Transmission Protocol            May 2004


   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 massively
   parallel transmission.

   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.

   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 in all applications).

3.1.1  Out-of-order Delivery

   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: LTP's "sessions" functionally correspond to TCP
   connections.  At any moment each such thread of retransmission
   protocol is engaged in conveying some single block of data from one
   protocol end point to another.

   But while TCP permits only a single connection on a given port at any
   time, LTP permits an unlimited number of concurrent sessions for each
   client service.  And just as in TCP the vagaries of retransmission
   may cause data transmitted on one connection (on one port) to be



Burleigh et al.          Expires - November 2004                [Page 9]

Internet Draft       Licklider Transmission Protocol            May 2004


   delivered after data that were subsequently transmitted on another
   connection (another port), so too in LTP is it possible for data
   transmitted in one session (for one client service) to be delivered
   after data that were subsequently transmitted in another session (for
   another - or possibly the same - client service).

   In short, while TCP always delivers data in transmission order on a
   single port, LTP may well deliver data out of transmission order to a
   single client service.  The contrasts may be summarized as follows:

                             ------TCP------   -------LTP-------

   number of "ports"             65,535      unlimited; normally 1

   "connections" per "port"         1             unlimited

   maximum number of             65,535           unlimited
   concurrent connection
   state machines

   blocks transmitted per       unlimited             1
   "connection"              (one at a time)

   Out-of-transmission-order delivery of transmitted blocks to client
   services averts two serious problems that could be raised by a single
   "bit hit" - the unrecoverable corruption of a single segment of one
   block - followed by the successful reception of some number of
   subsequently transmitted blocks while retransmission of the lost
   segment is requested and accomplished.

   First, it ensures that delivery of the successfully received data is
   not unnecessarily postponed.  LTP leaves up to the client service all
   decisions on what can and cannot be done with this data pending
   delivery of the undelivered block.  [Note that this places on the
   client services all responsibility for establishing sequence
   relationships among transmitted blocks, e.g., embedding timestamps
   and/or sequence numbers within the blocks.]

   Second, it enables LTP to release resources allocated to the
   completed sessions' state information as rapidly as possible.  This
   somewhat mitigates the burden of statefulness at the receiving
   engine.

3.1.2   Session IDs

   In TCP, the delivery of data in transmission order on any single
   port, without gaps, enables the application that is receiving data on
   that port to use delivery order as the basis for reconstituting the



Burleigh et al.          Expires - November 2004               [Page 10]

Internet Draft       Licklider Transmission Protocol            May 2004


   originally transmitted data items.  LTP client services count on LTP
   to accomplish this reconstitution at the block level, but LTP itself
   cannot rely on data delivery order being similarly useful for this
   purpose.  LTP instead attaches to each segment of application data
   the "session ID" that uniquely identifies the original transmission
   in which the data were issued.  Session ID and block offset enable
   LTP to reassemble the originally transmitted data block from segments
   of data received out of offset order.

   Note that, even so, the order of delivery of completed blocks may
   differ from the order in which the blocks were originally
   transmitted.  Again, attaching the appropriate service-specific
   semantic significance to each delivered block is a client service
   responsibility.

3.1.3  Use of Non-volatile Storage

   Another important implication of massive statefulness is that
   implementations of LTP should consider retaining transmission state
   information in non-volatile storage of some kind, such as magnetic
   disk or flash memory.  Transport protocols such as TCP typically
   retain transmission state in dynamic RAM; if the computer on which
   the software resides is rebooted or powered down, then all
   transmissions currently in progress are aborted but the resulting
   degree of communication disruption is limited, because the number of
   concurrent transmissions is limited.  Rebooting or power-cycling a
   computer on which an LTP engine resides could abort a much larger
   number of transmission sessions in various stages of completion, at a
   much larger total cost.

3.2  Absence of Negotiation

   Implicit in the design of TCP is the assumption that the parameters
   of communication over a given connection can be bilaterally
   negotiated and renegotiated in a timely manner. Adjustment of
   transmission rate in particular is accomplished by the exchange of
   information in real time.

   In the IPN, however, round-trip times may be so long and
   communication opportunities so brief that a negotiation exchange
   might not be completed before connectivity was lost.  Even if
   connectivity were uninterrupted, waiting for negotiation to complete
   before revising data transmission parameters might well result in
   costly under-utilization of link resources.

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



Burleigh et al.          Expires - November 2004               [Page 11]

Internet Draft       Licklider Transmission Protocol            May 2004


   weeks.

3.3  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 possible
   multiplicity of parallel sessions for any single client service,
   together with 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 supported, but it requires opening two individual
   LTP sessions.

   Because 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.

      o "Retransmissions" are sequences of data segments issued in



Burleigh et al.          Expires - November 2004               [Page 12]

Internet Draft       Licklider Transmission Protocol            May 2004


        response to the arrival of report segments that indicate
        incomplete reception.

   Checkpoints are mandatory only at the end of each original
   transmission or 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
   an original transmission, and additional reception reports may
   optionally be sent on an asynchronous basis.

3.4  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 reliability is "point to point": 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.  The Bundling protocol functions at a new
   hyper-network level, and LTP bears essentially the same relationship
   to Bundling that a reliable link protocol would bear to IP.  The
   design of LTP relies heavily on this topological premise, in at least
   two ways:

   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



Burleigh et al.          Expires - November 2004               [Page 13]

Internet Draft       Licklider Transmission Protocol            May 2004


   accuracy would severely compromise performance.

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

3.5  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:

      o The massively parallel nature of LTP changes the granularity at
        which timer accuracy is required.  Confirmation of the reception
        of one block, or one segment, does not retard transmission of
        the next, so timeout values that would be intolerably optimistic
        in TCP don't constrain LTP's bandwidth utilization.

      o 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.

   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 possible processing delay at both ends of the
   transmission.  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.




Burleigh et al.          Expires - November 2004               [Page 14]

Internet Draft       Licklider Transmission Protocol            May 2004


   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, as discussed earlier.

3.6  Deferred Transmission

   Link state cues also notify LTP when it is and isn't possible to
   transmit segments by passing them to the underlying communication
   service.

   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. Terminology

(1) Engine ID

   A number that uniquely identifies a given LTP engine, within some
   closed set of communicating LTP engines.  Note that, when LTP is
   operating underneath the DTN Bundling protocol, the convergence layer
   adapter mediating between the two will be responsible for translating
   between DTN endpoint IDs and LTP engine IDs in an implementation-
   specific manner.

(2) Block

   An array of contiguous octets of application data handed down by the
   upper layer (typically the bundling layer) to be transmitted via LTP
   from one client service instance to another.

(3) Block Prefix

   Any subset of a block that begins at the start of the block.

(4) Session

   A thread of LTP protocol activity conducted for the purpose of
   transmitting a block.



Burleigh et al.          Expires - November 2004               [Page 15]

Internet Draft       Licklider Transmission Protocol            May 2004


(5) Segment

   The unit of LTP data transmission activity. It is the data structure
   transmitted from one LTP engine to another in the course of a
   session. An LTP segment is either a data segment, a report segment, a
   report-acknowledgment segment, a cancel segment, or a cancel-
   acknowledgment segment.

(6) Scope

   A subset of a block.  Scope comprises two numbers, upper bound and
   lower bound.

   For a data segment, lower bound is the offset of the segment's client
   service data from the start of the block, while upper bound is the
   sum of the offset and length of the segment's client service data.
   For example, a segment with block offset 1000 and length 500 would
   have a lower bound 1000 and upper bound 1500.

   For a report segment, upper bound is the end of the block prefix to
   which the reception claims in the report apply, while lower bound is
   the end of the (smaller) interior block prefix to which the reception
   claims in the report do *not* apply.  That is, data at any offset
   equal to or greater than the report's lower bound but less than its
   upper bound, and not designated as "received" by any of the report's
   reception claims, must be assumed not yet received and therefore
   eligible for retransmission. For example, if a report segment carried
   a lower bound of 1000 and an upper bound of 5000, and the reception
   claims indicated reception of data within offsets 1000-1999 and
   3000-4999, data within the block offsets 2000-2999 can be considered
   eligible for retransmission.

   Reception reports (which may comprise multiple report segments) also
   have scope, as defined in Section 5.2.1 below.

(7) End of Block (EOB)

   The last data segment transmitted as part of the original
   transmission of a block. This data segment also indicates that the
   segment's upper bound is the total length of the block.

(8) Checkpoint

   A data segment soliciting a reception report from the receiving LTP
   engine.  All checkpoints other than the EOB segment that are NOT
   themselves issued in response to a reception report, are
   discretionary checkpoints, sent unreliably.  The EOB segment and all
   checkpoints issued in response to reception reports are mandatory



Burleigh et al.          Expires - November 2004               [Page 16]

Internet Draft       Licklider Transmission Protocol            May 2004


   checkpoints, sent reliably.

(9) Reception Report

   A sequence of one or more report segments reporting on all block data
   reception (within some scope) since the start of the block's
   transmission session.

(10) Synchronous Reception Report

   A reception report that is issued in response to a checkpoint.

(11) Asynchronous Reception Report

   A reception report that is issued in response to some implementation
   defined event other than the arrival of a checkpoint.

(12) Primary Reception Report

   A reception report that is issued in response to some event other
   than the arrival of a checkpoint segment that was itself issued in
   response to a reception report.  Primary reception reports include
   all asynchronous reception reports and all synchronous reception
   reports that are sent in response to discretionary checkpoints or to
   the EOB for a session.

(13) Secondary Reception Report

   A reception report that is issued in response to the arrival of a
   checkpoint segment that was itself issued in response to a reception
   report.

(14) Self-Delimiting Numeric Value (SDNV)

   The design of LTP attempts to reconcile minimal consumption of
   transmission bandwidth with

      (a) extensibility to satisfy requirements not yet identified and
      (b) scalability across a very wide range of network sizes and
          transmission payload sizes.

   A key strategic element in the design is the use of self-delimiting
   numeric values (SDNVs) that are similar in design to the Abstract
   Syntax Notation One [ASN1] encoding.  An SDNV can be used to encode a
   variable length number from 1 to 128 bytes long, and is of two basic
   types, SDNV-8 and SDNV-16.

   The first octet of an SDNV - the "discriminant" - fully characterizes



Burleigh et al.          Expires - November 2004               [Page 17]

Internet Draft       Licklider Transmission Protocol            May 2004


   the SDNV's value.

   SDNV-8

      If the first bit of the discriminant is zero, the length of the
      SDNV-8 is 1 octet and the contents of the remaining 7 bits of the
      discriminant viewed as an unsigned integer is the value encoded.
      An integer in the range of 0 to 127 can be encoded this way.

      Otherwise (if the first bit of the discriminant is 1), the
      remaining 7 bits encode the length of the encoded number. If the
      content of the remaining 7 bits viewed as an unsigned integer has
      the value N, the encoded number is N+1 octets long and has the
      value found by concatenating octets 2 through N+2 of the SDNV-8,
      viewed as an unsigned integer. For example, if N were 0, the
      following single octet would have the value of the SDNV-8; if N
      were 127, the following 128 octets would have the encoded unsigned
      number.

   SDNV-16

      If the first bit of the discriminant is zero, then the length of
      the SDNV-16 is 2 octets and the contents of the remaining 7 bits
      of the discriminant concatenated with the following octet, viewed
      as a 15-bit unsigned integer, is the value encoded. An integer in
      the range of 0 to 32767 can be encoded this way.

      Otherwise (if the first bit of the discriminant is 1), the
      encoding is similar to SDNV-8. If the content of the remaining 7
      bits viewed as an unsigned integer has the value N, the encoded
      number is N+1 octets long, and has the value found by
      concatenating octets 2 through N+2 of the SDNV-16, viewed as an
      unsigned integer.

   An SDNV can therefore be used to represent both very large and very
   small integer values.  For example, the maximum size of an SDNV - a
   1024-bit number - is large enough to contain a fairly safe encryption
   key, while any whole-degree Celsius temperature in the range that
   humans tolerate can be represented in a single-octet SDNV-8.

   In the LTP header specification that follows, various fields in the
   header are defined to be of types SDNV-8 or SDNV-16 depending on the
   typical range of values expected for the field. If a field in the
   header carries a number that typically requires 16 bits to encode,
   but under certain infrequent conditions may grow longer and require,
   say 32 bits to encode, it might be optimal to specify it as an
   SDNV-16 instead of specifying the field as a fixed 32 bit integer.




Burleigh et al.          Expires - November 2004               [Page 18]

Internet Draft       Licklider Transmission Protocol            May 2004


   However, SDNV is clearly not the best way to represent every numeric
   value.  When the maximum possible value of a number is known without
   question, the cost of an additional 8 bits of discriminant may not be
   justified.  For example, an SDNV-8 is a poor way to represent an
   integer whose value typically falls in the range 128 to 255.

   In general, though, we believe that SDNV representation of selected
   numeric values in LTP segments yields the smallest segment sizes
   without sacrificing scalability.


5. Overall Operation

5.1  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 it notifies the client service instance
   that the session has been started.  The sending engine then initiates
   an 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 such segment is marked both as a
   checkpoint indicating that the receiving engine must issue a
   reception report upon receving the segment, and as an 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 EOB, so that it can
   automatically be retransmitted 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
   it 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 EOB data segment it responds by (a) queuing for
   transmission to the sending engine a report segment indicating
   complete reception and (b) delivering the received block to the local
   instance of the client service.




Burleigh et al.          Expires - November 2004               [Page 19]

Internet Draft       Licklider Transmission Protocol            May 2004


   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 EOB, enqueues for transmission to the receiving engine a
   report-acknowledgment segment, notifies the local client service
   instance that 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.

5.2  Retransmission

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

   Loss of one or more data segments other than the EOB triggers data
   retransmission:

   Rather than returning a single reception report indicating complete
   reception, the receiving engine returns a reception report comprising
   as many report segments as are needed in order to report in detail on
   all data reception for this session (other than data reception that
   was previously reported in response to any discretionary
   checkpoints), within the constraints on maximum segment size imposed
   by the underlying communication service.  [Still, only one report
   segment is normally returned; multiple report segments are needed
   only when a large number of segments comprising non-contiguous
   intervals of block data are lost.]  A timer is started for *each*
   report segment.

   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



Burleigh et al.          Expires - November 2004               [Page 20]

Internet Draft       Licklider Transmission Protocol            May 2004


      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 re-enqueuing all data segments not
      yet received.  The last such segment is marked 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 subject to
      allocation of bandwidth to the queue into which the retransmission
      data segments were written.  A timer is started for the
      checkpoint, so that it can automatically be retransmitted if no
      responsive report segment is received.

         However, if the number of checkpoints issued for this session
         has reached a predefined limit (established by network
         management), then the receiving engine instead cancels the
         session as described later.

      If, on the other hand, the reception claims in the RS indicate
      that all data within the scope of the RS have been received, and
      moreover the union of all reception claims received so far in this
      session indicate that all data in the block have been received,
      then the sending engine notifies the local client service instance
      that the block has been successfully transmitted and closes the
      SSR for the session.

   On reception of a checkpoint segment with a non-zero report serial
   number, the receiving engine first turns off the timer for the
   referenced report segment. Then 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, within the constraints on maximum segment size
   imposed by the underlying communication service; a timer is started
   for each report segment.  If at this point all data in the block have
   been received, the receiving engine delivers the received block to
   the local instance of the client service and closes the RSR for the
   session; otherwise the data retransmission cycle continues.

      However, if the number of reception reports issued for this
      session has reached a predefined limit (established by network
      management), then the receiving engine instead cancels the session
      as described later.

   The detailed rules under which reception reports are produced are
   defined in Section 5.2.1 below.




Burleigh et al.          Expires - November 2004               [Page 21]

Internet Draft       Licklider Transmission Protocol            May 2004


   Loss of an EOB or other checkpoint segment, or of the responsive
   report segment causes the checkpoint timer to expire.  When this
   occurs, the sending engine normally retransmits the checkpoint
   segment.  However, if the number of times this checkpoint has been
   retransmitted has reached a predefined limit (established by network
   management), then the sending agent instead cancels the session.

   Similarly, loss of a report segment or of the responsive report-
   acknowledgment segment causes the report segment's timer to expire.
   When this occurs, the receiving engine normally retransmits the
   report segment.  However, if the number of times this report segment
   has been retransmitted has reached a predefined limit (established by
   network management), then the receiving agent instead cancels the
   session.

   Reception of a previously received report segment that was
   retransmitted due to loss of an report-acknowledgment segment causes
   another responsive report-acknowledgment segment to be transmitted,
   but is otherwise ignored; if any of the data retransmitted in
   response to the previously received report segment were lost, further
   retransmission of those data will be requested by one or more new
   report segments issued in response to that earlier retransmission's
   checkpoint.  Thus unnecessary retransmission is suppressed.

5.2.1  Reception Reporting Rules

   The upper bound of a synchronous reception report is the upper bound
   of the checkpoint segment to which it responds.

   The upper bound of an asynchronous reception report is the maximum
   upper bound value among all data segments received so far in the
   affected session.

   The lower bound of a primary reception report is the upper bound of
   the previously issued primary reception report for the same session,
   if any; otherwise it is zero.

   The lower bound of a secondary reception report is the lower bound of
   the report segment to which the triggering checkpoint was itself a
   response.

   Asynchronous reception reports are never issued after the arrival of
   the EOB segment for a session.

   A reception report comprises as many reception segments as are
   necessary to report on all data reception in the scope of the
   reception report, within the constraints on maximum segment size
   imposed by the underlying communication service.  [Again, a reception



Burleigh et al.          Expires - November 2004               [Page 22]

Internet Draft       Licklider Transmission Protocol            May 2004


   report normally comprises only a single reception segment; multiple
   report segments are needed only when a large number of segments for
   non-contiguous intervals of block data are lost.]  The lower bound of
   the first report segment of a reception report is the reception
   report's lower bound; the upper bound of the last report segment of a
   reception report is the reception report's upper bound.

5.2.2  Design Rationale

   Note 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 it retransmits non-checkpoint data segments in response to
   reception reports indicating incomplete reception, while the receiver
   additionally 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 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 that segment) would somewhat complicate the protocol,
   increase bandwidth consumption, and retard the release of session
   state resources at the sender.  Again we believe the current design
   is simpler and more efficient.



Burleigh et al.          Expires - November 2004               [Page 23]

Internet Draft       Licklider Transmission Protocol            May 2004


5.3  Accelerated Delivery

   The receiving engine normally delivers block data content to the
   client service only at the moment when reception of the block is
   completed - that is, on reception of the last not-yet-received
   segment of the block.  For some applications, however, it may be
   desirable to deliver block data content incrementally, upon arrival,
   because portions of the block may be individually useful to the
   client service.

   When the client service requests accelerated delivery of a block, the
   arrival of each new data segment causes the receiving engine to
   deliver to the client service the data content of the segment
   together with the segment's offset within the block.  The client
   service assumes all responsibility for reassembling the block; upon
   completion of reception, the receiving engine just delivers the final
   data segment's content and offset to the client service as usual but
   additionally indicates that reception is now complete.

5.4  Accelerated Retransmission

   Data segment retransmission occurs only on receipt of a report
   segment indicating incomplete reception; report segments in turn, are
   normally transmitted only at the end of an original transmission or
   retransmission.  For some applications it may be desirable to trigger
   data segment retransmission incrementally during the course of an
   original transmission so that the retransmitted segments arrive
   sooner.  This can be accomplished in two ways:

      Any data segment prior to the last one in the transmission can
      additionally be flagged as a checkpoint.  Reception of each
      checkpoint causes the receiving engine to issue a reception
      report.

      At any time during the original transmission of a session (that
      is, prior to reception of the EOB), 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; see Section 12.]
      The reception reports generated for accelerated retransmission are
      processed in exactly the same way as the standard reception
      reports.

5.5  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



Burleigh et al.          Expires - November 2004               [Page 24]

Internet Draft       Licklider Transmission Protocol            May 2004


   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, the canceling engine queues for transmission to the
   corresponding engine a session cancellation segment.

   At the next opportunity, subject to allocation of bandwidth to the
   queue into which the cancellation segment was written, the enqueued
   cancellation segment is transmitted to the LTP engine serving the
   remote client service instance.  A timer is started for the segment,
   so that it can automatically be retransmitted 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 normally retransmits the
   cancellation segment.  However, if the number of times this
   cancellation segment has been retransmitted has reached a predefined
   limit (established by network management), then the canceling agent
   instead simply closes its state record for the session.

5.6  Unreliable Transmission

   For operational conditions in which the massive statefulness of LTP
   reliability is unsupportable or unnecessary, LTP can perform
   unreliable transmission.  In unreliable mode all retransmission and
   session cancellation capabilities are disabled, but LTP's block
   segmentation, bandwidth management, interface to the underlying
   communication service, and incremental data delivery may still make



Burleigh et al.          Expires - November 2004               [Page 25]

Internet Draft       Licklider Transmission Protocol            May 2004


   it useful to client services.

   The nominal sequence of events in an unreliable transmission session
   is much simplified.

   Operation begins when a client service instance asks an LTP engine to
   transmit a block unreliably to a remote client service instance.  The
   sending engine 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 such segment is marked an EOB signifying that the receiving
   engine can calculate the size of the block by summing the offset and
   length of the data in this segment. Note that this segment is an EOB
   but not a checkpoint.

   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.

   The arrival of each data segment causes the receiving engine to
   deliver to the client service the data content of the segment
   together with the segment's offset within the block.  The client
   service assumes all responsibility for reassembling the block.

   Upon arrival of the EOB segment, the receiving engine just delivers
   that final data segment's content and offset to the client service as
   usual but additionally indicates that reception of the block is now
   complete.

   Loss or corruption of transmitted data segments is not recoverable in
   this mode.  Loss of the EOB is particularly troublesome: the
   receiving client service instance cannot readily distinguish between
   actual data loss and very severe queuing delay in this case, and the
   total size of the block can never be known.  But for some
   applications (e.g., continuous telemetry streaming), or in deployment
   over a reliable link-layer protocol, this deficiency may be
   unimportant.

6.  Functional Model

   The functional model underlying the specification of LTP is one of
   deferred, opportunistic transmission, with access to the active
   transmission link apportioned among multiple outbound traffic queues.
   The accuracy of LTP retransmission timers depends in large part on a
   faithful adherence to this model.

6.1  Deferred Transmission



Burleigh et al.          Expires - November 2004               [Page 26]

Internet Draft       Licklider Transmission Protocol            May 2004


   Every outbound LTP segment is appended to one of several (conceptual)
   queues of traffic bound for that segment's destination.  Production
   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) this queue is determined to
   have the highest transmission priority among all outbound traffic
   queues associated with that destination, the segment will be
   transmitted immediately upon production.  Transmission of a newly
   appended 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.

   Note: in the following discussion, the de-queuing of a segment always
   implies delivering it to the underlying communication system for
   immediate transmission.

6.2  Bandwidth Management

   We believe it is necessary for LTP to provide a mechanism for
   apportioning access to the active transmission link, possibly
   unevenly, among multiple classes of client service data traffic, and
   at the same time to provide a minimum-latency control channel for
   acknowledgment traffic.  To accomplish these ends, the LTP functional
   model is based on the use of N outbound traffic queues, N > 1, for
   each destination with which the LTP engine can communicate.

   One such queue is strictly reserved for LTP internal operations: it
   contains only report and acknowledgment segments (collectively,
   "acknowledging segments"), which must be transmitted promptly to
   protect timer accuracy.  A second queue is reserved for segments
   produced in sessions designated as "priority" sessions.  Any other
   queues supported by a given LTP engine are for segments produced in
   non-priority sessions, typically of varying levels of urgency.  The
   client service specifies the queue to be used for transmitting a
   given block - either the priority session queue or one of the non-
   priority session queues - at the time transmission of the block is
   requested.




Burleigh et al.          Expires - November 2004               [Page 27]

Internet Draft       Licklider Transmission Protocol            May 2004


   While the link to a given destination is active, continuous iteration
   of the following algorithm governs the de-queuing of segments from
   the N traffic queues bound for that destination:

   If any segments are currently in the internal operations queue, then
   de-queue the oldest such segment.

   Otherwise, if any segments are currently in the priority session
   queue, then de-queue the oldest such segment.

   Otherwise, if there are any other non-empty queues, invoke an
   implementation-specific algorithm to select the next queue to
   transmit from and then de-queue the oldest segment in that queue.

6.3  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.

   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 consumed in issuing the original segment,
   receiving the original segment, generating and issuing the
   acknowledging segment, and receiving the acknowledging segment.

   Outbound queuing delay: 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: propagation delay at the speed of light, in
   both directions.



Burleigh et al.          Expires - November 2004               [Page 28]

Internet Draft       Licklider Transmission Protocol            May 2004


   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 acknowledging segment due to loss of
   connectivity - that is, interruption in outbound link activity at the
   sender of the acknowledging 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, 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.

   Acknowledging segments (reports and acknowledgments) are always
   appended to the internal operations queue.  This limits outbound
   queuing delay for an acknowledging segment to the time needed to de-
   queue and radiate all other acknowledging segments that are currently
   in that queue.  Since acknowledging segments are sent infrequently
   and are normally very small, outbound queuing delay for a given
   acknowledging segment is likely to be minimal.

   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 (e.g., 10 bps), in which case the use of
   LTP may well be inadvisable for other reasons.  Therefore radiation
   delay is normally assumed to be negligible.

   And we assume that one-way light time to the nearest second can
   always be known (e.g., provided by the operating environment).

   So the initial expected arrival time for each acknowledging segment
   is 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



Burleigh et al.          Expires - November 2004               [Page 29]

Internet Draft       Licklider Transmission Protocol            May 2004


   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.  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.

7.  Segment Structure

   Each LTP segment comprises (a) a "header" in a standard format and
   (b) zero or more octets of "content".  LTP segments are of four
   general types, depending on the nature of the content carried.

      Data segments carry client service (application) data, together
      with metadata enabling the receiving client service instance to
      receive and make use of that data.

      Report segments carry data reception claims together with the
      upper and lower bounds of the data block scope to which the claims
      pertain.

      Report acknowledgment segments carry only the serial number of the
      report being acknowledged.

      Session management segments are of two general subtypes:
      Cancellation and Cancellation acknowledgment. The Cancellation
      segments carry a single byte reason-code to indicate the reason
      for the cancellation. Cancellation acknowledgment segments have no
      content.

7.1  Segment Header

   << Recommendations of SDNV-8 / SDNV-16 for fields in the segment
   header as recommended in this section, are under discussion.  Future
   versions of the draft may recommend fields to be of one SDNV type
   instead of the other (SDNV-8 in place of SDNV-16, for example), if
   found to be more appropriate. >>

   An LTP segment header comprises three data items: a single-octet
   control byte, a session ID, and an expansion zone.

   Control byte comprises the following:



Burleigh et al.          Expires - November 2004               [Page 30]

Internet Draft       Licklider Transmission Protocol            May 2004


      Version number (4 bits): MUST be set to the binary value 0000 for
      this version of the protocol.

      Segment type flags (4 bits): described below.

   Session ID uniquely identifies, among all transmissions between the
   segment's sender and receiver, the session of which the segment is
   one token.  It comprises the following:

      Session originator: the engine ID of the LTP engine that initiated
      the session, in SDNV-8 representation.

      Session number: a number in SDNV-16 representation, typically a
      timestamp, generated by the LTP engine identified as the session
      originator.

      The format and resolution of session number are matters that are
      private to the session-originating engine; the only requirement
      imposed by LTP is that every session initiated by an LTP engine
      MUST be uniquely identified by the session ID.  For example, if
      timestamp resolution is not sufficient, an LTP implementation may
      choose to append an 8 or 16 bit sequence number to the timestamp
      to guard against the possibility of multiple sessions starting at
      the same system time.

   Expansion zone is a numeric value in SDNV-8 representation intended
   for future expansion of LTP capabilities.  In the absence of any
   expansion features, it MUST be a single octet whose binary value is
   zero.

7.1.1  Segment Type Flags

   The last four bits of the control byte in the segment header are
   flags that indicate the nature of the segment.  In order (most
   significant bit first), these flags are as follows.

   Control flag (CTRL)

      A value of 0 indicates that the segment carries data and is a data
      segment, while a value of 1 indicates that the segment carries
      control information for the protocol (and not data).

   Exception flag (EXC)

      A value of 1 in a data segment indicates that the segment is being
      transmitted unreliably. In a control segment (CTRL flag set), this
      indicates that the segment pertains to session cancellation
      activity.



Burleigh et al.          Expires - November 2004               [Page 31]

Internet Draft       Licklider Transmission Protocol            May 2004


   Request flag (REQ)

      If set, this flag signifies a request for some specific response
      from the receiver.  The nature of that response depends on the
      values of the other flags as described below.

   Closure flag (CLOS)

      When set, this flag signifies the termination of some element of
      protocol activity.  The nature of the activity being terminated
      again depends on the values of the other flags as described below.

7.1.2  Segment Type Codes

      Combinations of the settings of the segment type flags CTRL, EXC,
      REQ and CLOS constitute segment type codes which serve as concise
      representations of detailed segment nature.

      CTRL  EXC  REQ CLOS  Code  Nature of segment
      ---- ---- ---- ----  ----  ---------------------------------------
        0    0    0    0     0   Data, NOT a Checkpoint, NOT EOB
        0    0    0    1     1   Undefined
        0    0    1    0     2   Data, Checkpoint, NOT EOB
        0    0    1    1     3   Data, Checkpoint, EOB

        0    1    0    0     4   Data [unreliable transmission], not EOB
        0    1    0    1     5   Data [unreliable transmission], EOB
        0    1    1    0     6   Undefined
        0    1    1    1     7   Undefined

        1    0    0    0     8   Report segment
        1    0    0    1     9   Report acknowledgment
        1    0    1    0    10   Undefined
        1    0    1    1    11   Undefined

        1    1    0    0    12   Cancel segment
        1    1    0    1    13   Cancel acknowledgment
        1    1    1    0    14   Undefined
        1    1    1    1    15   Undefined


7.1.3  Segment Class Masks

      For the purposes of this specification, some bit patterns in the
      segment type flags field correspond to "segment classes" that are
      designated by mnemonics.  The mnemonics are intended to evoke the
      characteristics shared by all types of segments characterized by
      these flag bit patterns.



Burleigh et al.          Expires - November 2004               [Page 32]

Internet Draft       Licklider Transmission Protocol            May 2004


      CTRL   EXC   REQ  CLOS  Mnemonic  Description
      ----  ----  ----  ----  --------  ---------------------------
        0     0     1     -     CP      Checkpoint

        0     0     1     1    EOB      End of block;
        0     1     0     1             block size = offset + length

        1     0     0     0      R      Report segment;
                                        carries reception claims

        1     0     0     1     RA      Report acknowledgment

        1     1     0     0      C      Cancellation

        1     1     0     1     CA      Cancellation acknowledgment

7.2  Segment Content

7.2.1  Data Segment

   The content of a data segment includes client service data and
   metadata enabling the receiving client service instance to receive
   and make use of that data.

   If the data segment is a checkpoint, the segment MUST additionally
   include the following two serial numbers (Checkpoint serial number,
   Report serial number) to support efficient retransmission. All non-
   checkpoint data segments MUST NOT have these two fields and MUST
   begin with the Client service ID field defined below as the first
   element of the data segment.

   Checkpoint serial number [SDNV-8]

      The checkpoint serial number uniquely identifies the checkpoint
      among all checkpoints issued by the block sender in a session.
      The first checkpoint issued by the sender MUST have this serial
      number chosen randomly for security reasons, and it is RECOMMENDED
      that the sender use the guidelines in [ECS94] for this. Any
      subsequent checkpoints issued by the sender MUST have the serial
      number value one more than the last checkpoint serial number
      issued. Any retransmission of the checkpoint segment MUST have the
      same serial number as the original transmission.

   Report serial number [SDNV-8]

      If the checkpoint was queued for transmission in response to the
      reception of an R segment [Sec 9.13], then its value MUST be the
      report serial number value of the R segment that caused the data



Burleigh et al.          Expires - November 2004               [Page 33]

Internet Draft       Licklider Transmission Protocol            May 2004


      segment to be queued for transmission.

      Otherwise, the value of report serial number MUST be zero.

   Client service ID [SDNV-8]

      The client service ID number identifies the upper-level service to
      which the segment is to be delivered by the destination LTP
      engine.  It is functionally analogous to a well-known TCP port
      number.  If multiple instances of the client service are present
      at the destination, multiplexing must be done by the client
      service itself on the basis of information encoded within the
      transmitted block.

      At this time the only LTP client service we envision in the IPN is
      the DTN Bundling protocol. The client service ID value assigned to
      this client service is the number 1.


   Offset [SDNV-16]

      Offset indicates the location of the segment's client service data
      within the session's transmitted block.  It is the number of bytes
      in the block prior to the byte from which the first octet of the
      segment's client service data was copied.

   Length [SDNV-16]

      The length of the following client service data, in octets.

   Client service data [array of octets]

      The client service data carried in the segment is a copy of a
      subset of the bytes in the original client service data block,
      starting at the indicated offset.

7.2.2  Report Segment

   The content of an R segment comprises one or more data reception
   claims, together with the upper and lower bounds of the scope within
   the data block to which the claims pertain.  It also includes two
   serial numbers to support efficient retransmission.

   Report serial number [SDNV-8]

      The report serial number uniquely identifies the report among all
      reports issued by the block receiver in a session.  The first
      report issued by the receiver MUST have this serial number chosen



Burleigh et al.          Expires - November 2004               [Page 34]

Internet Draft       Licklider Transmission Protocol            May 2004


      randomly for security reasons, and it is RECOMMENDED that the
      receiver use the guidelines in [ECS94] for this. Any subsequent
      reports issued by the receiver MUST have the serial number value
      one more than the last report serial number issued. Any
      retransmission of the R segment MUST have the same serial number
      as the original transmission.

   Checkpoint serial number [SDNV-8]

      The value of checkpoint serial number MUST be zero if the report
      segment is NOT a response to reception of a checkpoint, i.e., the
      reception report is asynchronous; otherwise it is the checkpoint
      serial number of the checkpoint that caused the R segment to be
      issued.

   Upper bound [SDNV-16]

      The upper bound of a report segment is the size of the block
      prefix to which the segment's reception claims pertain.

   Lower bound [SDNV-16]

      The lower bound of a report segment is the size of the (interior)
      block prefix to which the segment's reception claims do NOT
      pertain.

   Reception claim count [SDNV-8]

      The number of data reception claims in this report segment.

   Data reception claims

      Each reception claim comprises two elements: offset and length.

      Offset [SDNV-16]

         The offset indicates the successful reception of data beginning
         at the indicated offset from the lower bound of the report. The
         offset within the entire block can be calculated by summing
         this offset with the lower bound of the report.

      Length [SDNV-16]

         The length of a reception claim indicates the number of
         contiguous octets of block data starting at the indicated
         offset (within the scope of the report) that have been
         successfully received so far.




Burleigh et al.          Expires - November 2004               [Page 35]

Internet Draft       Licklider Transmission Protocol            May 2004


      Reception claims MUST conform to the following rules:

         A reception claim's offset shall never be less than zero and
         its length shall never be less than 1.

         The offset of a reception claim shall always be greater than
         the sum of the offset and length of the prior claim, if any.

         The sum of a reception claim's offset and length shall never
         exceed the difference between the upper and lower bounds of the
         report segment.

   Implied requests for retransmission of client service data can be
   inferred from an R segment's data reception claims.  However,
   *nothing* can be inferred regarding reception of block data at any
   offset equal to or greater than the segment's upper bound or at any
   offset less than the segment's lower bound.

   For example, if the scope of a report segment has lower bound 0 and
   upper bound 6000, and the report contains a single data reception
   claim with offset 0 and length 6000, then the report signifies
   successful reception of the first 6000 bytes of the block.  If the
   total length of the block is 6000, then the report additionally
   signifies successful reception of the entire block.

   If on the other hand, the scope of a report segment has lower bound
   1000 and upper bound 6000, and the report contains two data reception
   claims, one with offset 0 and length 2000 and the other with offset
   3000 and length 500, then the report signifies successful reception
   only of bytes 1000-2999 and  4000-4499 of the block.  From this we
   can infer that bytes 3000-3999 and 4500-5999 of the block need to be
   retransmitted, but we cannot infer anything about reception of the
   first 1000 bytes.

7.2.3  Report Acknowledgment Segment

   The content of an RA segment is simply the report serial number of
   the R segment in response to which the segment was generated.

   Report serial number [SDNV-8]

      This field returns the report serial number of the R segment being
      acknowledged.

7.2.4  Session Management Segments

   The C segment carries a single byte reason-code with the following
   semantics.



Burleigh et al.          Expires - November 2004               [Page 36]

Internet Draft       Licklider Transmission Protocol            May 2004


   Reason-Code    Semantics
   -----------    --------------------------------
       00         Client Service canceled session
       01         Unreachable Client Service
       02         Retransmission limit exceeded
      03-FF       Undefined

   The CA segments have no content.

8.  Requests from Client Service

   In all cases the representation of request parameters is a local
   implementation matter, as are validation of parameter values and
   notification of the client service in the event that a request is
   found to be invalid.

8.1  Transmission Request

   In order to request transmission of a block of client service data,
   the client service MUST provide the following parameters to LTP:

      Client service ID

      Destination LTP engine ID

      Data to send, as an array of bytes.

      Length of the data to be sent.

      Quality of service required: reliable or unreliable transmission.

      Flow-label, used to choose the queue within the queue-set for the
      LTP destination.

   On reception of a valid transmission request from a client service,
   LTP proceeds as follows.

   First the array of data to be sent is subdivided as necessary, with
   each subdivision serving as the client service data of a single new
   LTP data segment.  The algorithm used for subdividing the data is a
   local implementation matter; it is expected that data size
   constraints imposed by the underlying communication service, if any,
   will be accommodated in this algorithm.

   The last (and only the last) of the resulting data segments MUST be
   marked as an EOB, with appropriate EOB segment flag bits set
   depending on reliable / unreliable transmission [Sec 7.1.2].




Burleigh et al.          Expires - November 2004               [Page 37]

Internet Draft       Licklider Transmission Protocol            May 2004


   If the requested quality of service is reliable transmission, then
   all data segments resulting from subdivision of the data MUST have
   the EXC flag cleared.  Moreover, at least the EOB segment MUST also
   be marked a checkpoint by having the REQ flag set; zero or more other
   data segments (selected according to an algorithm that is a local
   implementation matter) MAY additionally have the REQ flag set to act
   as checkpoints.

   On the other hand if the requested quality of service is unreliable
   transmission, then all data segments resulting from subdivision of
   the data MUST have the EXC flag set and REQ flag cleared.

   All data segments are appended to the appropriate (conceptual)
   transmission queue as specified in the transmission request.

   Finally, a session start notice [Sec 10.1] is sent back to the client
   service that requested the transmission.

8.2  Cancellation Request

   In order to request cancellation of a session, either as sender or as
   receiver of the associated data block, the client service must
   provide to LTP the session ID of the session to be cancelled.

   On reception of a valid cancellation request from a client service,
   LTP proceeds as follows.

   First the internal "Cancel session" procedure [Sec 9.19] is invoked.

   Next, if the session is being canceled by the block sender, i.e., the
   session originator part of the session ID supplied in the
   cancellation request is the local LTP engine ID:

      If none of the data segments previously queued for transmission as
      part of this session have yet been de-queued and radiated, i.e.,
      if the destination engine cannot possibly be aware of this session
      - then the session is simply closed; the "Close session" procedure
      [Sec 9.20] is invoked.

      Otherwise, a C segment with reason-code value 00 [Sec 7.2.4] MUST
      be appended to the internal operations queue of the queue-set
      bound for the destination LTP engine specified in the transmission
      request that started this session.

   Otherwise, (i.e., the session is being canceled by the block
   receiver):

      If there is no transmission queue-set bound for the block sender



Burleigh et al.          Expires - November 2004               [Page 38]

Internet Draft       Licklider Transmission Protocol            May 2004


      (possibly because the local LTP engine is running on a receive-
      only device), then the session is simply closed; the "Close
      session" procedure [Sec 9.20] is invoked.

      Otherwise, a C segment with reason-code value 00 [Sec 7.2.4] MUST
      be appended to the internal operations queue of the queue-set
      bound for the block sender.

9.  Internal Procedures

   This section describes the internal procedures that are triggered by
   the occurrence of various events during the life-time of the LTP
   session.

   Whenever the content of any of the fields of the header of any
   received LTP segment does not conform to this specification document,
   the segment is assumed to be corrupt and MUST be discarded
   immediately and processed no further.  This procedure supersedes all
   other procedures described below.

   The data segments transmitted in the course of any single LTP session
   MUST either all have segment type code less than 4 (reliable
   transmission) or else all have segment type code greater than 3
   (unreliable transmission).

   All internal procedures described below that are triggered by the
   arrival of a data segment are superseded by the following procedure
   in the event that the client service identified by the data segment
   does not exist at the local LTP engine:

   If there is no transmission queue-set bound for the block sender
   (possibly because the local LTP engine is running on a receive-only
   device), then the data segment is simply discarded.  Otherwise, if
   the data segment was transmitted reliably (segment type code less
   than 4), a C segment with reason-code value 01 [Sec 7.2.4] MUST be
   added to the internal operations queue of the queue-set bound for the
   block sender; a C segment with reason-code value 01 SHOULD be
   similarly added to the internal operations queue bound for the data
   sender if the data segment was transmitted unreliably (segment type
   code greater than 3) [For example, in the case where the block
   receiver knows that the sender is functioning in a "beacon"
   (transmit-only) fashion, a C segment need not be sent].  In either
   case the received data segment is discarded.

9.1  Start Transmission

   This procedure is triggered by arrival of a link state cue indicating
   the start of transmission to a specified remote LTP engine.



Burleigh et al.          Expires - November 2004               [Page 39]

Internet Draft       Licklider Transmission Protocol            May 2004


   Response: the de-queuing and delivery to the underlying communication
   system of segments from traffic queues bound for the LTP engine
   specified in the link state cue begins.

9.2  Start Checkpoint Timer

   This procedure is triggered by arrival of a link state cue indicating
   the de-queuing for transmission of a CP segment, provided that it is
   also the EOB for a session or if it was issued in response to an R
   segment i.e., the segment's report serial number is non-zero.

   Response: the expected arrival time of the R segment that will be
   produced on reception of this CP segment is computed, and a countdown
   timer for this arrival time is started.  However, if it is known that
   the remote LTP engine has ceased transmission [Sec 9.5], then this
   timer is immediately suspended, because the computed expected arrival
   time may require an adjustment that cannot yet be computed.

9.3  Start RS Timer

   This procedure is triggered by arrival of a link state cue indicating
   the de-queuing (for transmission) of an R segment.

   Response: the expected arrival time of the RA segment that will be
   produced on reception of this R segment is computed, and a countdown
   timer for this arrival time is started.  However, if it is known that
   the remote LTP engine has ceased transmission [Sec 9.5], then this
   timer is immediately suspended, because the computed expected arrival
   time may require an adjustment that cannot yet be computed.

9.4  Stop Transmission

   This procedure is triggered by arrival of a link state cue indicating
   the cessation of transmission to a specified remote LTP engine.

   Response: the de-queuing and delivery to the underlying communication
   system of segments from traffic queues bound for the LTP engine
   specified in the link state cue ceases.

9.5  Suspend Timers

   This procedure is triggered by arrival of a link state cue indicating
   the cessation of transmission from a specified remote LTP engine to
   the local LTP engine.  Normally, this event is inferred from advance
   knowledge of the remote engine's planned transmission schedule.

   Response: countdown timers for the acknowledging segments that the
   remote engine is expected to return are suspended as necessary based



Burleigh et al.          Expires - November 2004               [Page 40]

Internet Draft       Licklider Transmission Protocol            May 2004


   on the following procedure:

   The nominal acknowledge transmission time is computed as the sum of
   the transmission time of the original segment (to which the
   acknowledging segment will respond) and the one-way light time to the
   remote engine, plus N seconds of margin to account for processing and
   queuing delay at the remote engine; N should be a network management
   parameter, for which 2 seconds seems to be a reasonable default
   value.

   If the nominal acknowledge transmission time is greater than or equal
   to the current time (i.e., the acknowledging segment may be presented
   for transmission during the time that transmission at the remote
   engine is suspended), then the countdown timer for this acknowledging
   segment is suspended.

9.6  Resume Timers

   This procedure is triggered by arrival of a link state cue indicating
   the start of transmission from a specified remote LTP engine to the
   local LTP engine.  Normally, this event is inferred from advance
   knowledge of the remote engine's planned transmission schedule.

   Response: expected arrival time is adjusted for every acknowledging
   segment that the remote engine is expected to return, for which the
   countdown timer has been suspended.  In each case, expected arrival
   time is increased by a transmission delay interval that is calculated
   as follows:

      The nominal acknowledge transmission time is computed as the sum
      of the transmission time of the original segment (to which the
      acknowledging segment will respond) and the one-way light time to
      the remote engine, plus N seconds of margin to account for
      processing and queuing delay at the remote engine; again, N should
      be a network management parameter, for which 2 seconds seems to be
      a reasonable default value.

      If the nominal acknowledge transmission time is greater than the
      current time i.e., the remote engine resumed transmission prior to
      presentation of the acknowledging segment for transmission, then
      the transmission delay interval is zero.

      Otherwise, the transmission delay interval is computed as the
      current time less the nominal acknowledge transmission time.

   After adjustment of expected arrival time, each of the suspended
   countdown timers is resumed.




Burleigh et al.          Expires - November 2004               [Page 41]

Internet Draft       Licklider Transmission Protocol            May 2004


9.7  Retransmit Checkpoint

   This procedure is triggered by the expiration of a countdown timer
   associated with a CP segment.

   Response: if the number of times this CP segment has been queued for
   transmission exceeds the checkpoint retransmission limit established
   for the local LTP engine by network management, then the session of
   which the segment is one token is canceled: the "Cancel session"
   procedure [Sec 9.19] is invoked, a C segment with reason-code value
   02 [Sec 7.2.4] is appended to the transmission queue specified in the
   transmission request that started this session, and a transmission
   cancellation notice [Sec 10.5] is sent back to the client service
   that requested the transmission.

   Otherwise, a new copy of the CP segment is appended to the
   transmission queue specified in the transmission request that started
   this session.

9.8  Retransmit RS

   This procedure is triggered by either (a) expiration of a countdown
   timer associated with an R segment or (b) reception of a CP segment
   whose checkpoint serial number is equal to that of one or more
   previously issued R segments for the same session -- an unnecessarily
   retransmitted checkpoint.

   Response: if the number of times any affected R segment has been
   queued for transmission exceeds the report retransmission limit
   established for the local LTP engine by network management, then the
   session of which the segment is one token is canceled: the "Cancel
   session" procedure [Sec 9.19] is invoked, a C segment with reason-
   code value 02 [Sec 7.2.4] is appended to the queue of internal
   operations traffic bound for the LTP engine that originated the
   session, and a reception cancellation notice [Sec 10.6] is sent to
   the client service identified in each of the data segments received
   in this session.

   Otherwise, a new copy of each affected R segment is appended to the
   queue of internal operations traffic bound for the LTP engine that
   originated the session.

9.9  Signify Segment Arrival

   This procedure is triggered by the arrival of a data segment, but
   only when either (a) the data segment is being transmitted unreliably
   or (b) segment arrival notification has been authorized for the local
   LTP engine by client service or network management.



Burleigh et al.          Expires - November 2004               [Page 42]

Internet Draft       Licklider Transmission Protocol            May 2004


   Response: a segment arrival notice [Sec 10.2] is sent to the
   specified client service.

9.10  Signify Block Reception

   This procedure is triggered by the arrival of a data segment, but
   only when either (a) the segment is also the EOB segment for a block
   being transmitted unreliably or (b) the segment is also a CP segment
   for a reliably transmitted block, and the EOB for this session has
   been received (including the case where this segment itself is the
   EOB segment) and the data block's size is known, and all data in the
   block being transmitted in this session have been received.

   Response: a block reception notice [Sec 10.3] is sent to the
   specified client service.

9.11  Send Reception Report

   This procedure is triggered by either (a) reception of a CP segment
   whose checkpoint serial number is not equal to that of any previously
   issued R segment or (b) an implementation-specific circumstance
   pertaining to a particular block reception session for which no EOB
   has yet been received ("asynchronous" reception reporting).  The
   response in either case is the same.

   Response: if the number of reception reports issued for this session
   exceeds the limit established for the local LTP engine by network
   management, then the affected session is canceled: the "Cancel
   session" procedure [Sec 9.19] is invoked, a C segment with reason-
   code value 02 [Sec 7.2.4] is appended to the queue of internal
   operations traffic bound for the LTP engine that originated the
   session, and a reception cancellation notice [Sec 10.6] is sent to
   the client service identified in each of the data segments received
   in this session.  Otherwise, a reception report is issued as follows.

   As many R segments are produced as are needed in order to report on
   all data reception within the scope of the report, given whatever
   data size constraints are imposed by the underlying communication
   service.  They are appended to the queue of internal operations
   traffic bound for the LTP engine that originated the indicated
   session.  If production of the reception report was triggered by
   reception of a checkpoint:

      The upper bound of the report is the upper bound (the sum of the
      offset and length) of the checkpoint data segment.

      If the checkpoint was itself issued in response to a report
      segment, then this report is a "secondary" reception report and



Burleigh et al.          Expires - November 2004               [Page 43]

Internet Draft       Licklider Transmission Protocol            May 2004


      the lower bound of the report is that earlier report segment's
      lower bound.  Otherwise, this report is a "primary" reception
      report and the lower bound of the report is the upper bound of the
      prior primary reception report issued for this session.

   Otherwise, i.e., the reception report is asynchronous:

      The upper bound of the report is the maximum upper bound among all
      data segments received so far for this session.

      The lower bound of the report is the upper bound of the prior
      primary reception report issued for this session.

9.12  Signify Transmission Completion

   This procedure is triggered by either (a) reception of an R segment
   whose reception claims taken together with the reception claims of
   all other report segments previously received in the course of this
   session indicate complete reception of an entire data block, or (b)
   arrival of a link state cue indicating the de-queuing (for
   transmission) of an EOB segment for a block transmitted unreliably.

   Response: a transmission completion notice [Sec 10.4] is sent to the
   client service that requested the transmission identified in the
   segment header and the session is closed: the "Close session"
   procedure [Sec 9.20] is invoked.

9.13  Retransmit Data

   This procedure is triggered by reception of an R segment.

   Response: first, an RA segment with the same report serial number as
   the R segment is appended to the queue of internal operations traffic
   bound for the LTP engine that originated the indicated session.  If
   the R segment is redundant i.e., either the indicated session is
   unknown (if for example, the R segment is received after the session
   has been completed or canceled), or the R segment's report serial
   number is equal to that of a previously received report segment for
   this session -- then no further action is taken.  Otherwise the
   procedure below is followed.

   If the report's checkpoint serial number is not zero, then the
   countdown timer associated with the indicated checkpoint segment is
   deleted.

   All retransmission buffer space occupied by data whose reception is
   claimed in the report segment can be released.




Burleigh et al.          Expires - November 2004               [Page 44]

Internet Draft       Licklider Transmission Protocol            May 2004


   If the segment's reception claims indicate incomplete data reception
   within the scope of the report segment:

      If the number of CP segments issued for this session exceeds the
      limit established for the local LTP engine by network management,
      then the session of which the segment is one token is canceled:
      the "Cancel session" procedure [Sec 9.19] is invoked, a C segment
      with reason-code value 02 [Sec 7.2.4] is appended to the
      transmission queue specified in the transmission request that
      started this session, and a transmission cancellation notice [Sec
      10.5] is sent back to the client service that requested the
      transmission.

      Otherwise, new data segments encapsulating all block data whose
      non-reception is implied by the reception claims are appended to
      the transmission queue specified in the transmission request that
      started this session.  The last - and only the last - such segment
      must me marked as a CP segment and must contain the report serial
      number of the received R segment.

9.14  Stop RS Timer

      This procedure is triggered by reception of an RA segment.

      Response: the countdown timer associated with the original R
      segment (identified by the report serial number of the RA segment)
      is deleted.  If no other countdown timers associated with R
      segments existed for this session, then the session is closed: the
      "Close session" procedure [Sec 9.20] is invoked.

9.15  Start Cancel Timer

      This procedure is triggered by arrival of a link state cue
      indicating the de-queuing (for transmission) of a C segment.

      Response: the expected arrival time of the CA segment that will be
      produced on reception of this C segment is computed and a
      countdown timer for this arrival time is started.  However, if it
      is known that the remote LTP engine has ceased transmission [Sec
      9.5] then this timer is immediately suspended, because the
      computed expected arrival time may require an adjustment that
      cannot yet be computed.


9.16  Retransmit Cancellation Segment

      This procedure is triggered by the expiration of a countdown timer
      associated with a C segment.



Burleigh et al.          Expires - November 2004               [Page 45]

Internet Draft       Licklider Transmission Protocol            May 2004


      Response: if the number of times this C segment has been queued
      for transmission exceeds the cancellation retransmission limit
      established for the local LTP engine by network management, then
      the session of which the segment is one token is simply closed:
      the "Close session" procedure [Sec 9.20] is invoked.

      Otherwise, a new copy of the C segment (retaining the same reason-
      code value) is appended to the appropriate transmission queue.  If
      the segment is being sent by the block sender, then the
      appropriate transmission queue is the one specified in the
      transmission request that started this session; otherwise, the
      appropriate transmission queue is the queue of internal operations
      traffic bound for the LTP engine that originated the session.

9.17  Acknowledge Cancellation

      This procedure is triggered by the reception of a C segment.

      Response: if the local segment is not the block sender for the
      block identified in the C segment and there is no transmission
      queue-set bound for the block sender (possibly because the local
      LTP engine is running on a receive-only device), then no action is
      taken.  Otherwise, a CA segment is appended to the queue of
      internal operations traffic bound for the LTP engine that sent the
      C segment.

      It is possible that the C segment is being retransmitted because a
      previous CA was lost, in which case there will no longer be any
      record of the session of which the segment is one token. If so, no
      further action is taken.

      Otherwise that session is locally canceled: the "Cancel session"
      procedure [Sec 9.19] is invoked and a reception cancellation
      notice [Sec 10.6] is sent to the client service identified in each
      of the data segments received in this session.  Finally, the
      session is closed: the "Close session" procedure [Sec 9.20] is
      invoked.

9.18  Stop Cancellation Timer

      This procedure is triggered by reception of a CA segment.

      Response: the session of which the segment is one token is closed,
      i.e., the "Close session" procedure [Sec 9.20] is invoked.

9.19  Cancel Session

      This procedure is triggered internally by one of the other



Burleigh et al.          Expires - November 2004               [Page 46]

Internet Draft       Licklider Transmission Protocol            May 2004


      procedures described above.

      Response: all segments of the affected session that are currently
      queued for transmission can be deleted from the outbound traffic
      queues.  If the local LTP engine is the originator of the session,
      then all remaining data retransmission buffer space allocated to
      the session can be released.  All countdown timers currently
      associated with the session are deleted.

9.20  Close Session

      This procedure is triggered internally by one of the other
      procedures described above.

      Response: any remaining countdown timers associated with the
      session (such as the timer associated with a C segment) are
      deleted.  All other state information regarding the session is
      deleted; existence of the session is no longer recognized.

10.  Notices to Client Service

      In all cases the representation of notice parameters is a local
      implementation matter.

10.1  Session Start

      The LTP engine returns the Session ID of the new transmission
      session when a session start notice is delivered.

      A session start notice informs the client service of the
      initiation of a transmission session in response to a transmission
      request from that client service.  On receiving this notice the
      client service may, for example, release resources of its own that
      are allocated to the block being transmitted, or remember the
      Session ID so that the session can be canceled in the future.

10.2  Data Segment Arrival

      The parameters provided by the LTP engine when a data segment
      arrival notice is delivered are:

         Session ID of the transmission session

         Array of client service data bytes contained in the data
         segment

         Offset of the data segment's content from the start of the
         block



Burleigh et al.          Expires - November 2004               [Page 47]

Internet Draft       Licklider Transmission Protocol            May 2004


         Length of the data segment's content

         Source LTP engine ID

      Data segment arrival notices deliver block data incrementally, as
      it is received.  This enables the receiving client service
      instance to make use of partial data immediately, rather than
      potentially waiting hours or days for the retransmission of
      missing segments and the ultimate delivery of the completed block.
      Incremental block data delivery is mandatory for unreliable
      transmission, because there's never any guarantee that the EOB
      segment- which is required in order to deliver a complete block -
      will ever be received at all.  Incremental delivery also enables
      the client service to cancel reception of a block, if necessary.

10.3  Block Reception

      The parameters provided by the LTP engine when a block reception
      notice is delivered are:

         Session ID of the transmission session.

         Array of client service data bytes that constitutes the block

         Length of the block.

         Source LTP engine ID.

      A block reception notice delivers a complete data block to the
      client service.

10.4  Transmission Completion

      The sole parameter provided by the LTP engine when a transmission
      completion notice is delivered is the Session ID of the
      transmission session.

      A transmission completion notice informs the client service that
      the indicated session has successfully completed; the destination
      LTP engine has received the entire data block.

10.5  Transmission Cancellation

      The sole parameter provided by the LTP engine when a transmission
      cancellation notice is delivered is the Session ID of the
      transmission session.

      A transmission cancellation notice informs the client service that



Burleigh et al.          Expires - November 2004               [Page 48]

Internet Draft       Licklider Transmission Protocol            May 2004


      the indicated session was terminated, either by decision of the
      destination client service instance or due to violation of a
      retransmission limit in the local LTP engine.  There is no
      assurance that the destination client service instance received
      the data block.

10.6  Reception Cancellation

      The sole parameter provided by the LTP engine when a reception
      cancellation notice is delivered is the Session ID of the
      transmission session.

      A reception cancellation notice informs the client service that
      the indicated session was terminated, either by decision of the
      source client service instance or due to violation of a
      retransmission limit in the local LTP engine.  The complete data
      block will not be delivered.

11.  Requirements from the Operating Environment

      LTP requires support from its operating environment (which
      includes network management activities) and link-state cues from
      the data-link layer for its operations.

      The local data-link layer needs to inform LTP whenever the link to
      a specific LTP destination is brought up or torn down.  Similarly,
      the operating environment needs to inform the local LTP engine
      whenever it is known that a remote LTP engine is set to begin
      communication with the local engine from the operating schedules.
      LTP also needs to be able to query the current speed-of-light to
      its peer engine from the operating environment to calculate its
      timers.

      A MIB (Management Information Base) with the above parameters
      filled in by the local data-link layer and the operating
      environment periodically, should be made available for the LTP
      engine for its operations. The exact details of the MIB are beyond
      the scope of this document.

      LTP also requires the underlying data-link layer to perform data
      integrity check of the segments received. Specifically, the data-
      link layer is expected to detect any segments received corrupted,
      and to silently discard them.

12.  Security Considerations

      <<This section is really only an initial set of notes resulting
      from discussions on and off the DTNRG list. Further analysis will



Burleigh et al.          Expires - November 2004               [Page 49]

Internet Draft       Licklider Transmission Protocol            May 2004


      be required as LTP itself develops and is implemented.>>

      LTP is designed as a hyper-datalink layer to primarily do ARQ of
      data transmissions over a point-to-point link exhibiting some or
      all of the following characteristics: high delays, high BERs and
      intermittent, possibly strictly scheduled connectivity.

      However, how "point-to-point" the link is physically, depends on
      the underlying datalink. For a deep-space link using radio
      datalink, say we are talking from a Deep-Space-Network giant dish
      antenna to an orbiter on Mars, the signal could be received and
      processed by any other orbiter passing at the same time in the
      Mars orbiter's reception area, which could potentially cover
      hundreds or thousands of square miles. Similarly a link
      transmitting data from an orbiter to a ground-rover could have a
      reception-coverage area of tens of square miles. So there is an
      inherent risk that of unintended receivers listening in on LTP
      transmissions over such datalinks.

      Such promiscuous recipients of our LTP transmissions could
      potentially replay the transmissions sent, twiddle with control
      bits in the LTP header before they do so (more dangerous is the
      case when they make the bits claim the LTP segment to be a Cancel
      segment closing the session). Such problems are more severe for
      LTP compared to other terrestrial Internet protocols because LTP
      inherently does a lot of state retention (to handle the high
      delays and intermittent connectivity of its links), has very high
      time-out values and LTP nodes may be quite difficult to reset. In
      other words, by design, there is a long delay before LTP gives up
      on a session. Thus any such adversary listening in on the LTP
      transmissions has the potential to create severe DoS conditions
      for an LTP receiver.

      To give a terrestrial example - were LTP to be used in a sparse
      sensor network, then denial of service attacks could be mounted
      which would result in nodes missing critical information, e.g.
      communications schedule updates. In such cases, a single DoS
      attack success could take a node entirely off the network until
      the node is physically visited and reset.

      Even in the deep space cases, we do need to consider some
      terrestrial based attacks, in particular those involving insertion
      of a message into an on-going session (usually without having seen
      the exact bytes of the previous messages in the session). This
      could arise due to firewall failures at various points or due to
      Trojan software running on an authorized host.

      Such attacks may depend on the attacker correctly "guessing" about



Burleigh et al.          Expires - November 2004               [Page 50]

Internet Draft       Licklider Transmission Protocol            May 2004


      the state of LTP nodes, but it is worth noting that patterns of
      deep space communication may well be considered guessable from the
      Earth (e.g. a session to a Mars rover is only going to happen
      while that rover is visible from the Earthstation -- all public
      information) and the delay-tolerance inherent in LTP may decrease
      the required accuracy of such guesses. Most of the attacks
      concerned here are DoS attacks.

12.1  Mechanisms and Layering Considerations

      There is thus a real need to consider security of LTP
      transmissions, so we need to consider (to the extent possible) the
      appropriate layer(s) at which security mechanisms can best be
      deployed.

      The Application layer (above-LTP)

         This seems like a reasonable approach to protect LTP data, but
         would leave the LTP headers open. The headers carry information
         on where the transmission is coming from, which could be
         valuable in itself. Further, this approach provides little or
         no protection against DoS type attacks against LTP.

      The LTP layer

         Security at this layer offers nice authentication
         possibilities.  For example, an authentication header (like
         that in IPSEC [AH]) can help to protect against replay attacks
         and bogus packets.  However, an adversary can still see the LTP
         header of segments passing by in the ether. This approach also
         requires some key management infrastructure be in place in
         order to provide strong authentication.

      The Datalink layer (below-LTP)

         Providing encryption/authentication in the layer(s) below LTP
         has some nice properties, like being able to do encryption on-
         chip in hardware, making it fast. However, depending on the
         datalink we may be forced to use encryption / authentication on
         all LTP sessions which may not be required. A more flexible
         scheme might enable us to do encryption / authentication on
         only critical information sessions. For example we might want
         it only for commands that ask a rover to reinstall a new OS
         image and reboot; and may be not so much when we are
         transmitting a picture (though this can be hard to achieve
         without layering violations).  Security provided by the
         datalink may result in unnecessary overhead and lessens
         flexibility, but may well be the optimal place to include



Burleigh et al.          Expires - November 2004               [Page 51]

Internet Draft       Licklider Transmission Protocol            May 2004


         compression and encryption.

12.2  Denial of Service Considerations

      Potential DoS attack points

      Implementers SHOULD consider the following DoS attacks :

         A fake C segment could be inserted thus bringing down a
         session.

         Various ACK messages may be deleted causing timers to expire
         which could deny service if done with knowledge of the
         communications schedule. One possible way to achieve this would
         be to mount a denial of service attack on a lower layer device
         in order to prevent it sending an ACK, or perhaps to simply jam
         the transmission (all of which are more likely for terrestrial
         applications of LTP).  An attacker might also flip some bits,
         which is (hopefully!)  tantamount to deleting that message.

         An attacker may flood a node with "internal" messages (using
         the terminology of section 6.2) which require processing, thus
         preventing "real" data segments from being transmitted.

         An attacker could attempt to fill up the storage in some node
         by sending many large messages to it. In terrestrial LTP
         applications this may be much more serious since spotting the
         additional traffic may not be possible from any network
         management point.

         - <<More TBD>>

      Anti-DoS measures

      <<We are considering including some or all (or none!) of the
      following anti-DoS measures in future versions of this
      specification:

         A value in C segments that increases the likelihood that the
         message is genuine -- possibly using a hash over some previous
         data that is assumed to have gotten through.

         Providing a mechanism whereby a node which considers that it is
         under DoS attack can use frequent checkpoints hopefully
         eliciting an earlier response from the real receiver or else
         timing out earlier due to the failure of the attacker to
         respond.




Burleigh et al.          Expires - November 2004               [Page 52]

Internet Draft       Licklider Transmission Protocol            May 2004


         Mandating that serial numbers (checkpoint serial numbers,
         report serial numbers) begin each session anew using random
         numbers rather than from 0. Maintaining serial number ordering
         across multiple sessions as a further option (two sides both
         knowing this trick can get the benefit, without hurting a node
         that doesn't maintain this cross-session state.)

         Add a Destination Engine ID field in segments for to provide
         some simple protection against replay attacks (the
         effectiveness of this countermeasure will depend upon lower
         layer data integrity.)

         A node might provide an interface to its higher layers which
         allows the higher layers to indicate which traffic has highest
         priority. In the event of a resource scarcity (e.g. nearly full
         storage) a node could drop all other traffic. (This would
         depend for effectiveness upon some lower layer authentication
         and/or integrity mechanisms.)
      >>

12.3  Authentication header

      An LTP Authentication Header (LTP-AH) MAY be used to provide
      cryptographic authentication of the segment.

      Implementations MAY support this header. If they do not support
      this header then they MUST ignore it. <<Check that ignoring is ok
      for all cases of response generation.>>

      The LTP-AH uses the LTP expansion field <<Exactly how is TBD>> and
      contains the following fields <<Only abstract definitions for
      now.>>

       - Ciphersuite: an eight bit integer value <<probably want two
       initially -- an AES-MAC and RSA-SIG>>
       - KeyID: An SDNV <<with some mapping to relevant ID formats,
       e.g. OCTET STRING for AES-MAC, IssuerAndSerial for RSA-SIG>>
       - AuthVal: An SDNV-16 value containing the authentication
       value <<Either a MAC or signature -- check if SDNV-16 allows
       long enough signatures for futureproofing>>

      <<Have to say something about key management!!!>>

12.4  Implementation Considerations

      SDNV

         Implementations SHOULD make sanity checks on SDNV length fields



Burleigh et al.          Expires - November 2004               [Page 53]

Internet Draft       Licklider Transmission Protocol            May 2004


         and SHOULD check that no SDNV field is too long when compared
         with overall segment length.

         Implementations SHOULD check that SDNV values are within
         suitable ranges where possible, e.g. <<TBD>>

      Byte ranges

         Various report and other segments contain offset and length
         fields. Implementations MUST ensure that these are consistent
         and sane.

      Randomness

         Various fields in LTP (e.g. serial numbers) should be
         initialised using random values. Good sources of randomness
         which are not easily guessable SHOULD be used [ECS94].

      <<More TBD>>

12.5  Miscellaneous

         An attacker could modify a message or insert a new message with
         the aim of making a session which should use reliable transport
         into one which uses unreliable transport and could also switch
         from normal to accelerated delivery. The end result could be
         that real data arrives out of order. Higher layer processing
         SHOULD take this into account if the LTP or lower layers do not
         preclude such attacks.

         <<Other stuff that crops up.>>

13.  Tracing LTP back to CFDP

      LTP in effect implements most of the "core procedures" of the
      CCSDS File Delivery Protocol (CFDP) specification, minus all
      features that are specific to operations on files and filestores
      (file systems); in the IPN architecture we expect that file and
      filestore operations will be conducted by file transfer
      application protocols -- notably CFDP itself -- operating on top
      of the DTN Bundling protocol.  (Bundling in effect implements the
      CFDP "extended procedures" in a more robust and scalable manner
      than is prescribed by the CFDP standard.)  The fundamental
      difference is that, while CFDP delivers named files end-to-end,
      LTP is used to transmit arbitrary, unnamed blocks of data point-
      to-point.

      Some differences between LTP and CFDP are simply matters of



Burleigh et al.          Expires - November 2004               [Page 54]

Internet Draft       Licklider Transmission Protocol            May 2004


      terminology; the following table summarizes the correspondences in
      language between the two.

      --------------LTP-------------     ------------CFDP-------------

         LTP engine                      CFDP entity

         Segment                         Protocol Data Unit (PDU)

         Reception Report                NAK

         Client service request          Service request

         Client service notice           Service indication

      CFDP specifies four mechanisms for initiating data retransmission,
      called "lost segment detection modes".  LTP effectively supports
      all four:

         "Deferred" mode is implemented in LTP by the Request flag in
         the EOB data segment, which triggers reception reporting upon
         receipt of the EOB.

         "Prompted" mode is implemented in LTP by turning on Request
         flags in data segments that precede the EOB; these additional
         checkpoints trigger interim reception reporting under the
         control of the source LTP engine.

         "Asynchronous" mode is implemented in LTP by the autonomous
         production, under locally specified conditions, of additional
         reception reports prior to arrival of the EOB.

         "Immediate" mode is simply a special case of asynchronous mode,
         where the condition that triggers autonomous reception
         reporting is detection of a gap in the incoming data.

      CFDP uses a cyclic timer to iterate reception reporting until
      reception is complete.  Because choosing a suitable interval for
      such a timer is potentially quite difficult, LTP instead flags the
      last data segment of each retransmission as a checkpoint, sent
      reliably; the cascading reliable transmission of checkpoint and RS
      segments assures the continuous progress of the transmission
      session.

      CFDP's Suspend and Resume PDUs are functionally displaced in LTP
      by deferred transmission and automated bandwidth management.

      As the following table indicates, most of the functions of CFDP



Burleigh et al.          Expires - November 2004               [Page 55]

Internet Draft       Licklider Transmission Protocol            May 2004


      PDUs are accomplished in some way by LTP segments.

      --------------LTP-------------     -------------CFDP------------

      Data segments                   File data and metadata PDUs

      Closure flag on data segment    EOF (Complete)

      Request flag on data segment    EOF (Complete), Prompt (NAK),
                                      Prompt (Keep Alive)

      Report segment                  ACK (EOF Complete), NAK,
                                      Keep Alive, Finished (Complete)

      Report acknowledgment           ACK (Finished Complete)

      Cancel segment                  EOF (Cancel, Protocol Error)
                                      Finished (Cancel, Protocol Error)

      Cancellation Acknowledgment     ACK (EOF (Cancel, Protocol Error),
                                      Finished (Cancel, Protocol Error))

      But some CFDP PDUs have no LTP equivalent because in an IPN
      architecture they will likely be implemented elsewhere.  CFDP's
      EOF (Filestore error) and Finished (Filestore error) PDUs would be
      implemented in an IPN application-layer file transfer protocol,
      e.g., CFDP itself.  CFDP's Finished [End System] PDU is a feature
      of the Extended Procedures, which would in effect be implemented
      by the Bundling protocol.

14. IANA Considerations

      At present there are none known. However if someone wanted to run
      LTP over IP (or even TCP or UDP), then we would want to allocate a
      port number. <<Considering this is TBD>>

15.  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



Burleigh et al.          Expires - November 2004               [Page 56]

Internet Draft       Licklider Transmission Protocol            May 2004


      H870; and NASA Contract NAS7-1407. 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.

      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.

16.  References

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

      [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.

      [BP] K. Scott, and S. Burleigh, "Bundle Protocol Specification",
      Work in Progress, October 2003.

      [DTNRG] Delay Tolerant Networking Research Group web page,
      "http://www.dtnrg.org".

      [MSM97] M. Mathis, J. Semke, and J. Mahdavi, "The Macroscopic
      Behavior of the TCP Congestion Avoidance Algorithm", Computer
      Communications Review Vol.27(3), 1997.

      [P97] V. Paxson, "Measurements and Analysis of End-to-End Internet
      Dynamics", Ph.D. Thesis., Computer Science Divisions, University
      of California, Berkeley, 1997.

      [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.

      [ASN1] Abstract Syntax Notation One (ASN.1). Specification of
      Basic Notation. ITU-T Rec. X.680 (2002) | ISO/IEC 8824-1:2002.

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

      [ECS94] D. Eastlake, S. Crocker, and J. Schiller, "Randomness



Burleigh et al.          Expires - November 2004               [Page 57]

Internet Draft       Licklider Transmission Protocol            May 2004


      Recommendations for Security", RFC 1750, December 1994.

17.  Author's Addresses

         Scott C. Burleigh
         Jet Propulsion Laboratory
         4800 Oak Grove Drive
         M/S: 179-206
         Pasadena, CA 91109-8099
         Telephone +1 (818) 393-3353
         FAX +1 (818) 354-1075
         Email Scott.Burleigh@jpl.nasa.gov

         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 +353-1-608-3070
         Email stephen.farrell@cs.tcd.ie























Burleigh et al.          Expires - November 2004               [Page 58]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/