LTP Fragmentation
draft-templin-dtn-ltpfrag-00

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Last updated 2020-04-10
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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                            April 10, 2020
Expires: October 12, 2020

                           LTP Fragmentation
                      draft-templin-dtn-ltpfrag-00

Abstract

   The Licklider Transmission Protocol (LTP) provides a reliable
   datagram "covergence 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.  This document discussses LTP
   interactions with IP fragmentation and mitigations for managing the
   amount of IP fragmentation employed.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 12, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  LTP Fragmentation . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   The Licklider Transmission Protocol (LTP) [RFC5326] provides a
   reliable datagram "covergence layer" for the Delay/Disruption
   Tolerant Networking (DTN) Bundle Protocol (BP) [I-D.ietf-dtn-bpbis].
   In common practice, LTP is often configured over UDP/IP sockets and
   inherits its maximum segment size from the maximum-sized UDP datagram
   (i.e. 2^16 bytes minus header sizes).

   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 entireity.

   When LTP presents a segment to the operating system kernel (e.g., via
   a sendmsg() system call), the UDP layer frames the segment in a UDP
   header.  The UDP layer then presents the resulting datagram to the IP
   layer for packet framing and transmission over a networked path.  The
   path is further characterized by the path Maximum Transmission Unit
   (Path-MTU) which is a measure of the smallest link MTU (Link-MTU)
   among all links in the path.

   When LTP presents a segment to the kernel that is larger than the
   Path-MTU, the IP layer performs IP fragmentation to break the
   datagram into fragments that are no larger than the Path-MTU.  For
   example, if the LTP segment size is 64000 bytes and the Path-MTU is
   1280 bytes IP fragmentation results in 50+ fragments that are
   transmitted as individual IP packets.

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