Licklider Transmission Protocol - Motivation
RFC 5325

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

              Licklider Transmission Protocol - Motivation

Status of This Memo

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

IESG Note

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

Abstract

   This document describes the motivation for the development of the
   Licklider Transmission Protocol (LTP) designed to provide
   retransmission-based reliability over links characterized by
   extremely long message round-trip times (RTTs) and/or frequent
   interruptions in connectivity.  Since communication across
   interplanetary space is the most prominent example of this sort of
   environment, LTP is principally aimed at supporting "long-haul"
   reliable transmission in interplanetary space, but it has
   applications in other environments as well.

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

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

Burleigh, et al.              Experimental                      [Page 1]
RFC 5325                    LTP - Motivation              September 2008

Table of Contents

   1. Introduction ....................................................2
   2. Problem .........................................................3
      2.1. IPN Operating Environment ..................................3
      2.2. Why Not TCP or SCTP? .......................................5
   3. Protocol Overview ...............................................6
      3.1. Nominal Operation ..........................................6
           3.1.1. Link State Cues .....................................9
           3.1.2. Deferred Transmission ...............................9
           3.1.3. Timers .............................................10
      3.2. Retransmission ............................................13
      3.3. Accelerated Retransmission ................................16
      3.4. Session Cancellation ......................................17
   4. Security Considerations ........................................17
   5. IANA Considerations ............................................20
   6. Acknowledgments ................................................20
   7. References .....................................................20
      7.1. Informative References ....................................20

1.  Introduction

   The Licklider Transmission Protocol (LTP) is designed to provide
   retransmission-based reliability over links characterized by
   extremely long message round-trip times and/or frequent interruptions
   in connectivity.  Communication in interplanetary space is the most
   prominent example of this sort of environment, and LTP is principally
   aimed at supporting "long-haul" reliable transmission over deep-space
   RF links.  Specifically, LTP is intended to serve as a reliable
   "convergence layer" protocol, underlying the Delay-Tolerant
   Networking (DTN) [DTN] Bundle protocol [BP], in DTN deployments where
   data links are characterized by very long round-trip times.

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

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

Burleigh, et al.              Experimental                      [Page 2]
RFC 5325                    LTP - Motivation              September 2008