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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


   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
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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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   Each IP fragment is subject to the same best-effort delivery service
   offered by the network according to current congestion and/or link
   signal quality conditions; therefore, the IP fragment size becomes
   known as the "loss unit".  Especially when the packet loss rate is
   considerable, however, performance can suffer dramatically when the
   loss unit is significantly smaller than the retransmission unit.  In
   particular, if even a single IP fragment of a fragmented LTP segment
   is lost then the entire LTP segment is deemed lost and must be

   This document discussses LTP interactions with IP fragmentation and
   mitigations for managing the amount of IP fragmentation employed.

2.  Terminology

   IETF keywords per [RFC2119] are not applicable within the scope of
   this document.

3.  LTP Fragmentation

   In common LTP implementations over UDP/IP (e.g., the Interplanetary
   Overlay Network (ION)), performance is greatly dependent on the LTP
   segment size.  This is due to the fact that a larger segment
   presented to UDP/IP as a single uint incurs only a single system call
   and a single data copy from application to kernel space via the
   sendmsg() system call.  Once inside the kernel, the segment incurs
   UDP/IP encapsulation and IP fragmentation which again results in a
   loss unit smaller than the retransmission unit.  However, during
   fragmentation, each fragment is transmitted immediately following the
   previous without delay so that the fragments appear as a "burst" of
   consecutive packets over the network path resulting in high network

   In order to avoid retransmission congestion (i.e., especially when
   the loss probability is non-negligible), the natural choice would be
   to set the LTP segment size to a size that is no larger than the
   Path-MTU.  However, transmission of 64KB of data using a 1280B
   segment size would require 50+ independent sendmsg() system calls and
   data copies as opposed to just one when the largest segment size is
   used.  This greatly reduces the bandwidth advantage offered by IP
   fragmentation bursts.  Therefore, a means for providing the best
   aspects of both large segment fragment bursting and small segment
   retransmission efficiency is needed.

   Fortunately, common operating systems such as linux provide a
   facility such as the sendmmsg() ("send multiple message") system call
   that allows the LTP application to present the kernel with a vector
   of segments instead of just a single segment.  This affords the

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   bursting behavior of IP fragmentation coupled with the retransmission
   efficiency of employing small segment sizes.

   This work therefore recommends implementations of LTP to employ a
   large block size, a conservative segment size and a new configuration
   option known as the "Burst-Limit" which determines the number of
   segments that can be presented in a single sendmmsg() system call.
   When the implementation receives an LTP block, it carves Burst-Limit-
   many segments from the block and presents the vector of segments to
   sendmmsg().  The kernel will prepare each segment as an independent
   UDP/IP packet and transmit them into the network as a burst in a
   fashion that parallels IP fragmentation.  The loss unit and
   retransmission unit will be the same, therefore loss of a single
   segment does not result in a retransmission congestion event.

   It should be noted that the Burst-Limit is bounded only by the LTP
   block size and not by the maximum UDP datagram size.  Therefore,
   bursts can in practice convey much more data than a singe IP
   fragmentation event.  It should also be noted that the segment size
   can still be made larger than the Path-MTU in low-loss environments
   without danger of triggering retransmission storms.  This would
   result in combined UDP message and IP fragment bursting for high
   network utilization in more robust environments.  Finally, Burst-
   Limit need not be a static value and can adaptively increase or
   decrease according to time varying network conditions.

4.  Implementation Status

   A prototype implementation has been developed, and early testing is

5.  IANA Considerations

   This document introduces no IANA considerations.

6.  Security Considerations

   Communications networking security is necessary to preserve the
   confidentiality, integrity and availability.

7.  Acknowledgements

   The NASA Space Communications and Networks (SCaN) directorate
   coordinates DTN activities for the International Space Station (ISS)
   and other space exploration initiatives.

   Keith Philpott, Bill Pohlchuck and Eric Yeh are acknowledged for
   their significant contributions.

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8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5326]  Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
              Transmission Protocol - Specification", RFC 5326,
              DOI 10.17487/RFC5326, September 2008,

8.2.  Informative References

              Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol
              Version 7", draft-ietf-dtn-bpbis-24 (work in progress),
              March 2020.

Author's Address

   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA  98124

   Email: fltemplin@acm.org

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