IP Fragmentation Considered Fragile
RFC 8900

Document Type RFC - Best Current Practice (September 2020; No errata)
Also known as BCP 230
Authors Ron Bonica  , Fred Baker  , Geoff Huston  , Bob Hinden  , Ole Trøan  , Fernando Gont 
Last updated 2020-09-11
Replaces draft-bonica-intarea-frag-fragile
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Stream WG state Submitted to IESG for Publication
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Shepherd write-up Show (last changed 2019-07-09)
IESG IESG state RFC 8900 (Best Current Practice)
Consensus Boilerplate Yes
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Responsible AD Suresh Krishnan
Send notices to Joel Halpern <joel.halpern@ericsson.com>, Joel Halpern <jmh@joelhalpern.com>
IANA IANA review state Version Changed - Review Needed
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Internet Engineering Task Force (IETF)                         R. Bonica
Request for Comments: 8900                              Juniper Networks
BCP: 230                                                        F. Baker
Category: Best Current Practice                             Unaffiliated
ISSN: 2070-1721                                                G. Huston
                                                                   APNIC
                                                               R. Hinden
                                                    Check Point Software
                                                                O. Troan
                                                                   Cisco
                                                                 F. Gont
                                                            SI6 Networks
                                                          September 2020

                  IP Fragmentation Considered Fragile

Abstract

   This document describes IP fragmentation and explains how it
   introduces fragility to Internet communication.

   This document also proposes alternatives to IP fragmentation and
   provides recommendations for developers and network operators.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8900.

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
   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 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
     1.1.  Requirements Language
   2.  IP Fragmentation
     2.1.  Links, Paths, MTU, and PMTU
     2.2.  Fragmentation Procedures
     2.3.  Upper-Layer Reliance on IP Fragmentation
   3.  Increased Fragility
     3.1.  Virtual Reassembly
     3.2.  Policy-Based Routing
     3.3.  Network Address Translation (NAT)
     3.4.  Stateless Firewalls
     3.5.  Equal-Cost Multipath, Link Aggregate Groups, and Stateless
           Load Balancers
     3.6.  IPv4 Reassembly Errors at High Data Rates
     3.7.  Security Vulnerabilities
     3.8.  PMTU Black-Holing Due to ICMP Loss
       3.8.1.  Transient Loss
       3.8.2.  Incorrect Implementation of Security Policy
       3.8.3.  Persistent Loss Caused by Anycast
       3.8.4.  Persistent Loss Caused by Unidirectional Routing
     3.9.  Black-Holing Due to Filtering or Loss
   4.  Alternatives to IP Fragmentation
     4.1.  Transport-Layer Solutions
     4.2.  Application-Layer Solutions
   5.  Applications That Rely on IPv6 Fragmentation
     5.1.  Domain Name Service (DNS)
     5.2.  Open Shortest Path First (OSPF)
     5.3.  Packet-in-Packet Encapsulations
     5.4.  UDP Applications Enhancing Performance
   6.  Recommendations
     6.1.  For Application and Protocol Developers
     6.2.  For System Developers
     6.3.  For Middlebox Developers
     6.4.  For ECMP, LAG, and Load-Balancer Developers And Operators
     6.5.  For Network Operators
   7.  IANA Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Operational experience [Kent] [Huston] [RFC7872] reveals that IP
   fragmentation introduces fragility to Internet communication.  This
   document describes IP fragmentation and explains the fragility it
   introduces.  It also proposes alternatives to IP fragmentation and
   provides recommendations for developers and network operators.

   While this document identifies issues associated with IP
   fragmentation, it does not recommend deprecation.  Legacy protocols
   that depend upon IP fragmentation would do well to be updated to
   remove that dependency.  However, some applications and environments
   (see Section 5) require IP fragmentation.  In these cases, the
   protocol will continue to rely on IP fragmentation, but the designer
   should be aware that fragmented packets may result in black holes.  A
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