Controlling IP Fragmentation on Common Platforms
draft-seemann-tsvwg-udp-fragmentation-00

Transport and Services Working Group                          M. Seemann
Internet-Draft                                          15 November 2024
Intended status: Informational                                          
Expires: 19 May 2025

            Controlling IP Fragmentation on Common Platforms
                draft-seemann-tsvwg-udp-fragmentation-00

Abstract

   When performing Path MTU Discovery (PMTUD) over UDP, applications
   must prevent fragmentation of UDP datagrams both by the sender's
   kernel and during network transit.  This document provides guidance
   for implementers on configuring socket options to prevent
   fragmentation of IPv4 and IPv6 packets across commonly used
   platforms.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Transport and Services
   Working Group Working Group mailing list (tsvwg@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/tsvwg/.

   Source for this draft and an issue tracker can be found at
   https://github.com/marten-seemann/draft-seemann-tsvwg-udp-
   fragmentation.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 19 May 2025.

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Setting the DF bit  . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Linux . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Apple . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Windows . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   [RFC0791] defines the Don't Fragment (DF) bit in the IPv4 header.
   When set, this bit prevents routers from fragmenting IP packets.  If
   a router needs to fragment a packet with the DF bit set, it will
   instead drop the packet and send an ICMP "fragmentation needed"
   message back to the sender.

   The DF bit has historically been most relevant to TCP ([RFC9293]),
   where the kernel handles Path MTU Discovery (PMTUD) internally.
   Applications using TCP sockets do not need to interact with the DF
   bit directly.

   In IPv6 ([RFC8200]), fragmentation by intermediate nodes is not
   permitted.  All IPv6 packets effectively have the DF bit set,
   however, the endpoint's kernel might still break up UDP datagrams
   that are too large to fit the MTU of the interface before sending a
   packet into the network.

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   [RFC8899] defines Datagram Packetization Layer Path MTU Discovery
   (DPLPMTUD), a mechanism that allows protocols running over UDP to
   determine the maximum packet size they can send.  Setting the DF bit
   is crucial for DPLPMTUD, as it ensures that packets larger than the
   Path MTU are dropped, allowing the endpoint to detect the MTU
   limitation.

   QUIC [RFC9000] is one such protocol that runs over UDP and make use
   of DPLPMTUD.  As QUIC implementations typically run in user space,
   they need to configure the DF bit on their UDP sockets to perform
   DPLPMTUD correctly.

   This document provides guidance for implementers on how to set the DF
   bit on UDP sockets across different platforms.

2.  Conventions and Definitions

   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.  Setting the DF bit

   While routers don't fragment IPv6 packets in transit, the sender's
   kernel will still fragment UDP datagrams that are too large to fit
   the MTU of the interface before sending a packet into the network.
   Therefore, operating systems offer socket options to control the
   fragmentation behavior for IPv6 packets.

   For user-space implementations of DPLPMTUD, applications need to set
   the DF bit on IPv4 sockets and prevent fragmentation on IPv6 sockets.

3.1.  Linux

   Linux uses the socket option of level IPPROTO_IP with name
   IP_MTU_DISCOVER with value IP_PMTUDISC_DO for IPv4.

   For IPv6, IPV6_MTU_DISCOVER with a value of IPV6_PMTUDISC_DO is used
   for the IPPROTO_IPV6 level.

   For dual-stack sockets, both socket options can be set independently.

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3.2.  Apple

   For IPv4, Apple platforms use the socket option of level IPPROTO_IP
   with name IP_DONTFRAG with value 1.  For IPv6, IPV6_DONTFRAG with
   value 1 is used for the IPPROTO_IPV6 level.

   However, dual-stack sockets are handled differently: To open a dual-
   stack socket, an IPv6 socket needs to be opened and the IPV6_V6ONLY
   option needs to be set to 0.  This enables the socket to send both
   IPv4 and IPv6 packets.  IPv4 packets must be sent using an
   IPv4-mapped IPv6 address.

   When using a dual-stack socket, it is only necessary (and possible)
   to set the IPV6_DONTFRAG socket option.  This results in the DF bit
   being set when sending IPv4 packets.  It is not possible to control
   the fragmentation behavior of IPv4 and IPv6 separately.

3.3.  Windows

   For IPv4, Windows uses the socket option of level IPPROTO_IP with
   name IP_DONTFRAGMENT with value 1.  For IPv6, IPV6_DONTFRAG with
   value 1 is used for the IPPROTO_IPV6 level.

   Similar to the Apple platforms, dual-stack sockets are IPv6 sockets
   with the IPV6_V6ONLY option set to 0.  IPv4 packets must be sent
   using an IPv4-mapped IPv6 address.  However, contractary to Apple
   platforms, the DF bit on IPv4 packets is controlled by the
   IP_DONTFRAGMENT socket option.

4.  Security Considerations

   TODO Security

5.  IANA Considerations

   This document has no IANA actions.

6.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc2119>.

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   [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/rfc/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/rfc/rfc8200>.

   [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/rfc/rfc8899>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [RFC9293]  Eddy, W., Ed., "Transmission Control Protocol (TCP)",
              STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9293>.

Acknowledgments

   TODO acknowledge.

Author's Address

   Marten Seemann
   Email: martenseemann@gmail.com

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