LTP Fragmentation
draft-templin-dtn-ltpfrag-03
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Fred Templin
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2020-12-14
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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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on June 17, 2021.
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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
Templin Expires June 17, 2021 [Page 1]
Internet-Draft LTP Fragmentation December 2020
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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. 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
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