LTP Fragmentation
draft-templin-dtn-ltpfrag-03

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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                         December 14, 2020
Expires: June 17, 2021

                           LTP Fragmentation
                      draft-templin-dtn-ltpfrag-03

Abstract

   The Licklider Transmission Protocol (LTP) provides a reliable
   datagram convergence layer for the Delay/Disruption Tolerant
   Networking (DTN) Bundle Protocol.  In common practice, LTP is often
   configured over UDP/IP sockets and inherits its maximum segment size
   from the maximum-sized UDP datagram, however when this size exceeds
   the maximum IP packet size for the path a service known as IP
   fragmentation must be employed.  This document discusses LTP
   interactions with IP fragmentation and mitigations for managing the
   amount of IP fragmentation employed.

Status of This Memo

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   This Internet-Draft will expire on June 17, 2021.

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   Copyright (c) 2020 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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Templin                   Expires June 17, 2021                 [Page 1]
Internet-Draft              LTP Fragmentation              December 2020

   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  IP Fragmentation Issues . . . . . . . . . . . . . . . . . . .   3
   4.  LTP Fragmentation . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Beyond "sendmmsg()" . . . . . . . . . . . . . . . . . . . . .   6
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Licklider Transmission Protocol (LTP) [RFC5326] provides a
   reliable datagram convergence layer for the Delay/Disruption Tolerant
   Networking (DTN) Bundle Protocol (BP) [I-D.ietf-dtn-bpbis].  In
   common practice, LTP is often configured over the User Datagram
   Protocol (UDP) [RFC0768] and Internet Protocol (IP) [RFC0791] using
   the "socket" abstraction.  LTP inherits its maximum segment size from
   the maximum-sized UDP datagram (i.e. 2^16 bytes minus header sizes),
   however when the UDP datagram size exceeds the maximum IP packet size
   for the path a service known as IP fragmentation must be employed.

   LTP breaks BP bundles into "blocks", then further breaks these blocks
   into "segments".  The segment size is a configurable option and
   represents the largest atomic block of data that LTP will require
   underlying layers to deliver as a single unit.  The segment size is
   therefore also known as the "retransmission unit", since each lost
   segment must be retransmitted in its entirety.  Experimental and
   operational evidence has shown that on robust networks increasing the
   LTP segment size (up to the maximum UDP datagram size of slightly
   less than 64KB) can result in substantial performance increases over
   smaller segment sizes.  However, the performance increases must be
   tempered with the amount of IP fragmentation invoked as discussed
   below.

   When LTP presents a segment to the operating system kernel (e.g., via
   a sendmsg() system call), the UDP layer prepends a UDP header to