LTP Fragmentation
draft-templin-dtn-ltpfrag-08
| Document | Type | Active Internet-Draft (individual) | |
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| Author | Fred Templin | ||
| Last updated | 2022-02-01 | ||
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draft-templin-dtn-ltpfrag-08
Network Working Group F. L. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Informational 1 February 2022
Expires: 5 August 2022
LTP Fragmentation
draft-templin-dtn-ltpfrag-08
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/IP 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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 August 2022.
Copyright Notice
Copyright (c) 2022 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 to this document. Code Components
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extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. IP Fragmentation Issues . . . . . . . . . . . . . . . . . . . 4
4. LTP Fragmentation . . . . . . . . . . . . . . . . . . . . . . 5
5. Beyond "sendmmsg()" . . . . . . . . . . . . . . . . . . . . . 6
6. LTP Performance Enhancement Using GSO/GRO . . . . . . . . . . 7
6.1. LTP and GSO . . . . . . . . . . . . . . . . . . . . . . . 7
6.2. LTP and GRO . . . . . . . . . . . . . . . . . . . . . . . 8
6.3. LTP GSO/GRO Over OMNI Interfaces . . . . . . . . . . . . 9
6.4. IP Parcels . . . . . . . . . . . . . . . . . . . . . . . 11
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. IPv4/IPv6 Protocol Considerations . . . . . . . . . 14
Appendix B. The Intergalactic Jigsaw Puzzle Builders Club . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
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/IP datagram (i.e. 64KB minus header sizes),
however when that 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 portion 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/IP datagram size of slightly
less than 64KB) can result in substantial performance increases over
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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
create a UDP datagram. 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 resulting UDP datagram is presented to the IP layer
which in turn 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 64KB 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], 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
non-negligible, 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. Since LTP does not support flow control or congestion
control, this can result in a cascading flood of redundant
information when fragments are systematically lost in transit.
This document discusses LTP interactions with IP fragmentation and
mitigations for managing the amount of IP fragmentation employed. It
further discusses methods for increasing LTP performance both with
and without the aid of IP fragmentation.
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.
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3. IP Fragmentation Issues
IP fragmentation is a fundamental service of the Internet Protocol,
yet it has long been understood that its use can be problematic in
some environments. Beginning as early as 1987, "Fragmentation
Considered Harmful" [FRAG] outlined multiple issues with the service
including a performance-crippling condition that can occur at high
data rates when the loss unit is considerably smaller than the
retransmission unit during intermittent and/or steady-state loss
conditions.
Later investigations also identified the possibility for undetected
corruption at high data rates due to a condition known as "ID
wraparound" when the 16-bit IP identification field (aka the "IP ID")
increments such that new fragments overlap with existing fragments
still alive in the network and with identical ID values
[RFC4963][RFC6864]. Although this issue occurs only in the IPv4
protocol (and not in IPv6 where the IP ID is 32-bits in length), the
IPv4 concerns along with the fact that IPv6 does not permit routers
to perform "network fragmentation" have led many to discourage the
use of fragmentation whenever possible.
Even in the modern era, investigators have seen fit to declare "IP
Fragmentation Considered Fragile" in an Internet Engineering Task
Force (IETF) Best Current Practice (BCP) reference [RFC8900].
Indeed, the BCP recommendations cite the Bundle Protocol LTP
convergence layer as a user of IP fragmentation that depends on some
of its properties to realize greater performance. However, the BCP
summarizes by saying:
"Rather than deprecating IP fragmentation, this document
recommends that upper-layer protocols address the problem of
fragmentation at their layer, reducing their reliance on IP
fragmentation to the greatest degree possible."
While the performance implications are considerable and have serious
implications for real-world applications, our goal in this document
is neither to condemn nor embrace IP fragmentation as it pertains to
the Bundle Protocol LTP convergence layer operating over UDP/IP
sockets. Instead, we examine ways in which the benefits of IP
fragmentation can be realized while avoiding the pitfalls. We
therefore next discuss our systematic approach to LTP fragmentation.
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4. 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. Additionally, the use of IP
fragmentation with a larger segment size conserves header framing
bytes since the upper layer headers only appear in the first IP
fragment as opposed to appearing in all fragments.
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
well 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
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 the sendmmsg() ("send
multiple messages") system call that allows the LTP application to
present the kernel with a vector of up to 1024 segments instead of
just a single segment. This theoretically affords the bursting
behavior of IP fragmentation coupled with the retransmission
efficiency of employing small segment sizes. (Note that LTP
receivers can also use the recvmmsg() ("receive multiple messages")
system call to receive a vector of segments from the kernel in case
multiple recent packet arrivals can be combined.)
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
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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/IP datagram size. Therefore,
each burst can in practice convey significantly 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 large UDP/IP
message transmission and IP fragmentation bursting for increased
network utilization in more robust environments. Finally, both the
Burst-Limit and UDP/IP message sizes need not be static values, and
can be tuned to adaptively increase or decrease according to time
varying network conditions.
5. Beyond "sendmmsg()"
Implementation experience with the ION-DTN distribution along with
two recent studies have demonstrated limited performance increases
for employing sendmmsg() for transmission over UDP/IP sockets. A
first study used sendmmsg() as part of an integrated solution to
produce 1M packets per second assuming only raw data transmission
conditions [MPPS], while a second study focused on performance
improvements for the QUIC reliable transport service [QUIC]. In both
studies, the use of sendmmsg() alone produced observable increases
but complimentary enhancements were identified that (when combined
with sendmmsg()) produced considerable additional increases.
In [MPPS], additional enhancements such as using recvmmsg() and
configuring multiple receive queues at the receiver were introduced
in an attempt to achieve greater parallelism and engage multiple
processors and threads. However, the system was still limited to a
single thread until multiple receiving processes were introduced
using the "SO_REUSEPORT" socket option. By having multiple receiving
processes (each with its own socket buffer), the performance
advantages of parallel processing were employed to achieve the 1M
packets per second goal.
In [QUIC], a new feature available in recent linux kernel versions
was employed. The feature, known as "Generic Segmentation Offload
(GSO) / Generic Receive Offload (GRO)" allows an application to
provide the kernel with a "super-buffer" containing up to 64 separate
upper layer protocol segments. When the application presents the
super-buffer to the kernel, GSO segmentation then sends up to 64
separate UDP/IP packets in a burst. (Note that GSO requires each
UDP/IP packet to be no larger than the path MTU so that receivers can
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invoke GRO without interactions with IP reassembly.) The GSO
facility can be invoked by either sendmsg() (i.e., a single super-
buffer) or sendmmsg() (i.e., multiple super-buffers), and the study
showed a substantial performance increase over using just sendmsg()
and sendmmsg() alone.
For LTP fragmentation, our ongoing efforts explore using these
techniques in a manner that parallels the effort undertaken for QUIC.
Using these higher-layer segmentation management facilities is
consistent with the guidance in "IP Fragmentation Considered Fragile"
that states:
"Rather than deprecating IP fragmentation, this document
recommends that upper-layer protocols address the problem of
fragmentation at their layer, reducing their reliance on IP
fragmentation to the greatest degree possible."
By addressing fragmentation at their layer, the LTP/UDP functions can
then be tuned to minimize IP fragmentation in environments where it
may be problematic or to adaptively engage IP fragmentation in
environments where performance gains can be realized without risking
sustained loss and/or data corruption.
6. LTP Performance Enhancement Using GSO/GRO
Some modern operating systems include Generic Segment Offload (GSO)
and Generic Receive Offload (GRO) services. For example, GSO/GRO
support has been included in linux beginning with kernel version
4.18. Some network drivers and network hardware also support GSO/GRO
at or below the operating system network device driver interface
layer to provide benefits of delayed segmentation and/or early
reassembly. The following sections discuss LTP interactions with GSO
and GRO.
6.1. LTP and GSO
GSO allows LTP implementations to present the sendmsg() or sendmmsg()
system calls with "super-buffers" that include up to 64 LTP segments
which the kernel will subdivide into individual UDP/IP datagrams.
LTP implementations enable GSO either on a per-socket basis using the
"setsockopt()" system call or on a per-message basis for
sendmsg()/sendmmsg() as follows:
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/* Set LTP segment size */
unsigned integer gso_size = SEGSIZE;
...
/* Enable GSO for all messages sent on the socket */
setsockopt(fd, SOL_UDP, UDP_SEGMENT, &gso_size, sizeof(gso_size)));
...
/* Alternatively, set per-message GSO control */
cm = CMSG_FIRSTHDR(&msg);
cm->cmsg_level = SOL_UDP;
cm->cmsg_type = UDP_SEGMENT;
cm->cmsg_len = CMSG_LEN(sizeof(uint16_t));
*((uint16_t *) CMSG_DATA(cm)) = gso_size;
Implementations must set SEGSIZE to a value no larger than the path
MTU via the underlying network interface, minus the header sizes
(see: Appendix A); this ensures that UDP/IP datagrams generated
during GSO segmentation will not incur local IP fragmentation prior
to transmission (NB: the linux kernel returns EINVAL if SEGSIZE is
set to a value that would exceed the path MTU.)
Implementations should therefore dynamically determine SEGSIZE for
paths that traverse multiple links through Packetization Layer Path
MTU Discovery for Datagram Transports [RFC8899] (DPMTUD).
Implementations should set an initial SEGSIZE to either a known
minimum MTU for the path or to the protocol-defined minimum path MTU
(i.e., 576 for IPv4 or 1280 for IPv6). Implementations may then
dynamically increase SEGSIZE without service interruption if the
discovered path MTU is larger.
6.2. LTP and GRO
GRO allows the kernel to return "super-buffers" that contain multiple
concatenated received segments to the LTP implementation in recvmsg()
or recvmmsg() system calls, where each concatenated segment is
distinguished by an LTP segment header per [RFC5326]. LTP
implementations enable GRO on a per-socket basis using the
"setsockopt()" system call, then optionally set up per receive
message ancillary data to receive the segment length for each message
as follows:
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/* Enable GRO */
unsigned integer use_gro = 1; /* boolean */
setsockopt(fd, SOL_UDP, UDP_GRO, &use_gro, sizeof(use_gro)));
...
/* Set per-message GRO control */
cmsg->cmsg_len = CMSG_LEN(sizeof(int));
*((int *)CMSG_DATA(cmsg)) = 0;
cmsg->cmsg_level = SOL_UDP;
cmsg->cmsg_type = UDP_GRO;
...
/* Receive per-message GRO segment length */
if ((segmentLength = *((int *)CMSG_DATA(cmsg))) <= 0)
segmentLength = messageLength;
Implementations include a pointer to a "use_gro" boolean indication
to the kernel to enable GRO; the only interoperability requirement
therefore is that each UDP/IP packet includes an integral number of
properly-formed LTP segments. The kernel and/or underlying network
hardware will first coalesce multiple received segments into a larger
single segment whenever possible and/or return multiple coalesced or
singular segments to the LTP implementation so as to maximize the
amount of data returned in a single system call. The "super-buffer"
thus prepared MUST contain at most 64 segments where each non-final
segment MUST be equal in length and the final segment MUST NOT be
longer than the non-final segment length.
Implementations that invoke recvmsg( ) and/or recvmmsg() will
therefore receive "super-buffers" that include one or more
concatenated received LTP segments. The LTP implementation accepts
all received LTP segments and identifies any segments that may be
missing. The LTP protocol then engages segment report procedures if
necessary to request retransmission of any missing segments.
6.3. LTP GSO/GRO Over OMNI Interfaces
LTP engines produce UDP/IP packets that can be forwarded over an
underlying network interface as the head-end of a "link-layer service
that transits IP packets". UDP/IP packets that enter the link near-
end are deterministically delivered to the link-far end modulo loss
due to corruption, congestion or disruption. The link-layer service
is associated with an MTU that deterministically establishes the
maximum packet size that can transit the link. The link-layer
service may further support a segmentation and reassembly function
with fragment retransmissions at a layer below IP; in many cases,
these timely link-layer retransmissions can reduce dependency on
(slow) end-to-end retransmissions.
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LTP engines that connect to networks traversed by paths consisting of
multiple concatenated links must be prepared to adapt their segment
sizes to match the minimum MTU of all links in the path. This could
result in a small SEGSIZE that would interfere with the benefits of
GSO/GRO layering. However, nodes that configure LTP engines can also
establish an Overlay Multilink Network Interface (OMNI)
[I-D.templin-6man-omni] that spans the multiple concatenated links
while presenting an assured (64KB-1) MTU to the LTP engine.
The OMNI interface internally uses IPv6 fragmentation as an OMNI
Adaptation Layer (OAL) service not visible to the LTP engine to allow
timely link-layer retransmissions of lost fragments where the
retransmission unit matches the loss unit. The LTP engine can then
dynamically vary its SEGSIZE (up to a maximum value of (64KB-1) minus
headers) to determine the size that produces the best performance at
the current time by engaging the combined operational factors at all
layers of the multi-layer architecture. This dynamic factoring
coupled with the ideal link properties provided by the OMNI interface
support an effective layering solution for many DTN networks.
When an LTP/UDP/IP packet is transmitted over an OMNI interface, the
OAL inserts an IPv6 header and performs IPv6 fragmentation to produce
fragments small enough to fit within the path MTU. The OAL then
replaces the IPv6 encapsulation headers with OMNI Compressed Headers
(OCHs) which are significantly smaller that their uncompressed IPv6
header counterparts and even smaller than the IPv4 headers would have
been had the packet been sent directly over a physical interface such
as Ethernet using IPv4 fragmentation.
The end result is that the first fragment produced by the OAL will
include a small amount of additional overhead to accommodate the OCH
encapsulation header while all additional fragments will include only
an OCH header which is significantly smaller than even an IPv4
header. The act of forwarding the large LTP/UDP/IP packet over the
OMNI interface will therefore produce a considerable overhead savings
in comparison with direct Ethernet transmission.
Using the OMNI interface with its OAL service in addition to the GSO/
GRO mechanism, an LTP engine can therefore theoretically present
concatenated LTP segments in a "super-buffer" of up to (64 * ((64KB-
1) minus headers)) octets for transmission in a single sendmsg()
system call, and may present multiple such "super-buffers" in a
single system call when sendmmsg() is used. (Note however that
existing implementations limit the maximum-sized "super-buffer" to
only 64KB total.) In the future, this service may realize even
greater benefits through the use of IP Jumbograms [RFC2675] over
paths that support them.
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6.4. IP Parcels
The so-called "super-buffers" discussed in the previous sessions can
be applied for GSO/GRO only when the LTP application endpoints are
co-resident with the OAL source and destination, respectively.
However, it may be desirable for the future architecture to support
network forwarding for these "super-buffers" in case the LTP source
and/or destination are located one or more IP networking hops away
from nodes that configure their respective source and destination
OMNI interfaces. Moreover, if the OMNI virtual link spans multiple
OMNI intermediate nodes on the path from the OAL source to the OAL
destination it may be desirable to keep the "super-buffers" together
as much as possible as they traverse the intermediate hops. For this
reason, a new construct known as the "IP Parcel" has been specified
[I-D.templin-intarea-parcels].
An IP parcel is a special form of an IP Jumbogram that includes a
non-zero value in the IP {Total, Payload} Length field. The value in
that field sets the segment size for the first segment included in
the parcel, while the value coded in the Jumbo Payload header
determines the number of segments included. Each segment "shares"
the same IP header, and the parcel can be broken down into sub-
parcels if necessary to traverse paths with length restrictions. A
full discussion of IP parcels is found in
[I-D.templin-intarea-parcels].
7. Implementation Status
Supporting code for invoking the sendmmsg() facility is included in
the official ION source code distribution, beginning with release
ion-4.0.1.
Working code for GSO/GRO has been incorporated into a pre-release of
ION and scheduled for integration following the next major release.
8. IANA Considerations
This document introduces no IANA considerations.
9. Security Considerations
Communications networking security is necessary to preserve
confidentiality, integrity and availability.
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10. Acknowledgements
The NASA Space Communications and Networks (SCaN) directorate
coordinates DTN activities for the International Space Station (ISS)
and other space exploration initiatives.
Akash Agarwal, Madhuri Madhava Badgandi, Keith Philpott, Bill
Pohlchuck, Vijayasarathy Rajagopalan, Bhargava Raman Sai Prakash and
Eric Yeh are acknowledged for their significant contributions. Tyler
Doubrava was the first to mention the "sendmmsg()" facility. Scott
Burleigh provided review input, and David Zoller provided useful
perspective.
11. References
11.1. 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>.
11.2. Informative References
[FRAG] Mogul, J. and C. Kent, "Fragmentation Considered Harmful,
ACM Sigcomm 1987", August 1987.
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[I-D.ietf-dtn-bpbis]
Burleigh, S., Fall, K., and E. J. Birrane, "Bundle
Protocol Version 7", Work in Progress, Internet-Draft,
draft-ietf-dtn-bpbis-31, 25 January 2021,
<https://www.ietf.org/archive/id/draft-ietf-dtn-bpbis-
31.txt>.
[I-D.templin-6man-omni]
Templin, F. L. and T. Whyman, "Transmission of IP Packets
over Overlay Multilink Network (OMNI) Interfaces", Work in
Progress, Internet-Draft, draft-templin-6man-omni-52, 31
December 2021, <https://www.ietf.org/archive/id/draft-
templin-6man-omni-52.txt>.
[I-D.templin-intarea-parcels]
Templin, F. L., "IP Parcels", Work in Progress, Internet-
Draft, draft-templin-intarea-parcels-06, 22 December 2021,
<https://www.ietf.org/archive/id/draft-templin-intarea-
parcels-06.txt>.
[MPPS] Majkowski, M., "How to Receive a Million Packets Per
Second, https://blog.cloudflare.com/how-to-receive-a-
million-packets/", June 2015.
[QUIC] Ghedini, A., "Accelerating UDP Packet Transmission for
QUIC, https://calendar.perfplanet.com/2019/accelerating-
udp-packet-transmission-for-quic/", December 2019.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, DOI 10.17487/RFC2675, August 1999,
<https://www.rfc-editor.org/info/rfc2675>.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<https://www.rfc-editor.org/info/rfc4963>.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, DOI 10.17487/RFC6864, February 2013,
<https://www.rfc-editor.org/info/rfc6864>.
[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.
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[RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile",
BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
<https://www.rfc-editor.org/info/rfc8900>.
Appendix A. IPv4/IPv6 Protocol Considerations
LTP/UDP/IP peers can communicate either via IPv4 or IPv6 addressing
when both peers configure a unique address of the same protocol
version on the OMNI interface. The IPv4 Total Length field includes
the length of both the UDP header and base IPv4 header, while the
IPv6 Payload Length field includes the length of the UDP header but
not the base IPv6 header.
Therefore, unless header extensions are included, each maximum-sized
LTP/UDP/IPv6 packet would contain 20 octets more actual LTP data than
a maximum-sized LTP/UDP/IPv4 packet can contain for the price of
including only 20 additional header octets for IPv6. The overhead
percentage for carrying this additional 20 header octets in maximum-
sized packets is therefore insignificant and becomes smaller still
when IPv6 header compression is used.
Appendix B. The Intergalactic Jigsaw Puzzle Builders Club
The process we are optimizing is like an imaginary Intergalactic
Jigsaw Puzzle Builders Club. A first builder starts with an original
image, admires it momentarily then breaks it up into like-sized
puzzle pieces with unique serial numbers. The first builder then
delivers each piece to their local post office which has an
Intergalactic Puzzle Piece Transporter.
The transporter can instantly deliver each puzzle piece to a remote
post office which could be nearby or in a far-off distant galaxy.
The remote post office then delivers each piece to the next builder
who very quickly puts it in the correct place based on the serial
number. This builder eventually reconstructs the entire original
image, then admires it and forwards it on to the next builder in the
same fashion.
All original images are the same dimensions, but each consecutive
builder can choose to break them into fewer and larger pieces or more
and smaller pieces - for example 100, 250, 500, 1000 or even more
pieces. The local post office transporter can send smaller pieces
intact, but must cut larger pieces into fragments that the remote
post office will paste back together. This process is both fast and
invisible to the builders who only see whole puzzle pieces and not
fragments.
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For ION-DTN LTP performance, we believed that performance could
increase if builders could exchange MULTIPLE puzzle pieces in
packages called PARCELs with their local post offices instead of just
one piece at a time, and we have shown that this is true to a limited
extent for small- to medium-sized pieces. But, we see that overall
system performance is dominated by the time needed for the receiver
to install a SINGLE puzzle piece, and we see that builders can
reassemble puzzles with fewer and larger pieces MUCH faster than for
ones with more and smaller pieces.
So, why not just use larger puzzle pieces all the time? The problem
is the transporter is imperfect and can lose, damage and/or reorder
pieces. And, if even a single bit is lost or damaged the sender must
retransmit the entire large piece all over again. This is not only
expensive (since the post office charges for transporter use by
weight) but the whole service degrades because the loss unit is
smaller than the retransmission unit resulting in a cascading flood
of redundant information.
The system is a multi-variable optimization problem, and there are
many knobs to turn. Tuning characteristics can also vary over time
due to fluctuations in transporter performance. We also believe that
if the transporter can be made to quickly retransmit lost fragments,
it can often salvage partial puzzle pieces that otherwise would have
been discarded. This would allow builders to use larger pieces to
increase performance.
Studies of the QUIC protocol have shown that PARCELs can result in
major performance increases by making the builder to post office
interface exchanges more efficient. For LTP, we have seen limited
increases (less than factor-2) using smaller segment sizes. While
any increase is good, we believe that either increasing the single
puzzle piece placement speed or supporting placement of multiple
pieces simultaneously will be gating factors for increased
performance. This may shift the performance bottleneck back to the
builder to post office interface, and PARCELS may help achieve
greater increases even for larger puzzle pieces.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
United States of America
Email: fltemplin@acm.org
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