Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Informational December 4, 2020
Expires: June 7, 2021
LTP Fragmentation
draft-templin-dtn-ltpfrag-01
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. This document discusses LTP
interactions with IP fragmentation and mitigations for managing the
amount of IP fragmentation employed.
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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 . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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 User Datagram Protocol
(UDP) / Internet Protocol (IP) [RFC0768][RFC0791] 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 entirety.
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. (Note that for IPv4 [RFC0791],
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fragmentation may occur either in the source host or in a router in
the network path, while for IPv6 [RFC8200] only the source host may
perform fragmentation.)
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
retransmitted.
This document discusses LTP interactions with IP fragmentation and
mitigations for managing the amount of IP fragmentation employed.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
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 unit 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
utilization during the burst period.
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. Assuming the minimum IPv4 MTU of 576 bytes, however,
transmission of 64KB of data using a 576B segment size would require
over 100 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
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bursts. Therefore, a means for providing the best aspects of both
large segment fragment bursting and small segment retransmission
efficiency is needed.
Common operating systems such as linux provide facilities 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 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 single 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 due to loss of IP
fragments. 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
An implementation is included in the official ION source code
distribution, beginning with release ion-4.0.1.
5. IANA Considerations
This document introduces no IANA considerations.
6. Security Considerations
Communications networking security is necessary to preserve the
confidentiality, integrity and availability.
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7. Acknowledgements
The NASA Space Communications and Networks (SCaN) directorate
coordinates DTN activities for the International Space Station (ISS)
and other space exploration initiatives.
Madhuri Madhava Badgandi, Keith Philpott, Bill Pohlchuck,
Vijayasarathy Rajagopalan and Eric Yeh are acknowledged for their
significant contributions. Tyler Doubrava was the first to mention
the "sendmmsg()" facility.
8. References
8.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5326] Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
Transmission Protocol - Specification", RFC 5326,
DOI 10.17487/RFC5326, September 2008,
<https://www.rfc-editor.org/info/rfc5326>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
8.2. Informative References
[I-D.ietf-dtn-bpbis]
Burleigh, S., Fall, K., and E. Birrane, "Bundle Protocol
Version 7", draft-ietf-dtn-bpbis-29 (work in progress),
November 2020.
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Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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