Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                          October 11, 2021
Expires: April 14, 2022


                           LTP Fragmentation
                      draft-templin-dtn-ltpfrag-05

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 April 14, 2022.

Copyright Notice

   Copyright (c) 2021 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 April 14, 2022                 [Page 1]


Internet-Draft              LTP Fragmentation               October 2021


   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 . . . . . . . . . . . . . . . . . . .   4
   4.  LTP Fragmentation . . . . . . . . . . . . . . . . . . . . . .   4
   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  . . . . . . . . . . . .   8
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     11.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

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 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 datagram size of slightly
   less than 64KB) can result in substantial performance increases over
   smaller segment sizes.  However, the performance increases must be




Templin                  Expires April 14, 2022                 [Page 2]


Internet-Draft              LTP Fragmentation               October 2021


   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 catastrophic communication failure 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.





Templin                  Expires April 14, 2022                 [Page 3]


Internet-Draft              LTP Fragmentation               October 2021


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
   data 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 its
   use.

   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.

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



Templin                  Expires April 14, 2022                 [Page 4]


Internet-Draft              LTP Fragmentation               October 2021


   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 LTP layer headers only appear in the first IP
   fragment as opposed to appearing in all IP packets.

   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 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
   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.





Templin                  Expires April 14, 2022                 [Page 5]


Internet-Draft              LTP Fragmentation               October 2021


   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, 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 UDP message and IP fragment
   bursting for increased network utilization in more robust
   environments.  Finally, both the Burst-Limit and UDP 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 modest 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 64 separate
   UDP/IP packets in a burst.  If each packet is larger than the Path-
   MTU, then IP fragmentation will be invoked for each packet leading to
   high network utilization (at the risk of IP fragment loss and
   retransmission storms).  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.



Templin                  Expires April 14, 2022                 [Page 6]


Internet-Draft              LTP Fragmentation               October 2021


   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
   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-to-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 datagrams.  LTP
   implementations enable GSO on a per-socket basis using the
   "setsockopt()" system call as follows:

     unsigned integer gso_size = SEGSIZE;
     setsockopt(fd, SOL_UDP, UDP_SEGMENT, &gso_size, sizeof(gso_size)));

   Implementations must set SEGSIZE to an initial value no larger than
   the MTU of the underlying network interface minus the UDP and IP
   header sizes; this ensures that UDP 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 MTU of the underlying interface).
   For paths that traverse multiple links, implementations should also
   dynamically adjust SEGSIZE according to the per-destination path MTU
   to avoid sustained in-the-network fragmentation resulting in a loss
   unit smaller than the retransmission unit.



Templin                  Expires April 14, 2022                 [Page 7]


Internet-Draft              LTP Fragmentation               October 2021


   Implementations should therefore dynamically determine SEGSIZE for
   paths that traverse multiple links through Packetization Layer Path
   MTU Discovery for Datagram Transports [RFC8899] (DPMTUD).  For IPv4
   paths, implementations may initially set SEGSIZE according to the MTU
   of the underlying interface and invoke DPMTUD while initial packets
   are flowing, then should dynamically reduce SEGSIZE without service
   interruption if the discovered path MTU is smaller.  For IPv6 paths,
   implementations should initially set SEGSIZE according to the minimum
   IPv6 Path MTU (i.e., 1280) then may 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 as follows:

     unsigned integer gro_size = 0;
     setsockopt(fd, SOL_UDP, UDP_GRO, &gro_size, sizeof(gro_size)));

   Implementations include a pointer to a gro_size variable as a boolean
   indication to the kernel using any arbitrary initialization value
   (e.g., '0'), as GRO will accept received segments of any size; the
   only interoperability requirement therefore is that each UDP packet
   includes one or more 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.

   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



Templin                  Expires April 14, 2022                 [Page 8]


Internet-Draft              LTP Fragmentation               October 2021


   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.

   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
   establish an Overlay Multilink Network Interface (OMNI)
   [I-D.templin-6man-omni] that spans the multiple concatenated links
   while presenting an assured 9180 byte MTU to the LTP engine.

   The OMNI interface internally uses IP fragmentation as a link-layer
   adaptation service not visible to the LTP engine, including 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 9180 bytes) to determine the
   size that produces the best performance given 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.

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.

8.  IANA Considerations

   This document introduces no IANA considerations.

9.  Security Considerations

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

10.  Acknowledgements

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





Templin                  Expires April 14, 2022                 [Page 9]


Internet-Draft              LTP Fragmentation               October 2021


   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.  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.

   [I-D.ietf-dtn-bpbis]
              Burleigh, S., Fall, K., and E. J. Birrane, "Bundle
              Protocol Version 7", draft-ietf-dtn-bpbis-31 (work in
              progress), January 2021.





Templin                  Expires April 14, 2022                [Page 10]


Internet-Draft              LTP Fragmentation               October 2021


   [I-D.templin-6man-omni]
              Templin, F. L. and T. Whyman, "Transmission of IP Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-6man-omni-47 (work in progress), September 2021.

   [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.

   [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., Tuexen, M., Ruengeler, I., and
              T. Voelker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/info/rfc8899>.

   [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>.

Author's Address

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

   Email: fltemplin@acm.org










Templin                  Expires April 14, 2022                [Page 11]