Close Encounters of the ICMP Type 2 Kind (Near Misses with ICMPv6 Packet Too Big (PTB))
RFC 7690

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Internet Engineering Task Force (IETF)                         M. Byerly
Request for Comments: 7690                                        Fastly
Category: Informational                                          M. Hite
ISSN: 2070-1721                                                 Evernote
                                                              J. Jaeggli
                                                                  Fastly
                                                            January 2016

                Close Encounters of the ICMP Type 2 Kind
             (Near Misses with ICMPv6 Packet Too Big (PTB))

Abstract

   This document calls attention to the problem of delivering ICMPv6
   type 2 "Packet Too Big" (PTB) messages to the intended destination
   (typically the server) in ECMP load-balanced or anycast network
   architectures.  It discusses operational mitigations that can be
   employed to address this class of failures.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7690.

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RFC 7690                 Misses with ICMPv6 PTB             January 2016

Copyright Notice

   Copyright (c) 2016 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
   (http://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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Mitigation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Alternative Mitigations . . . . . . . . . . . . . . . . .   5
     3.2.  Implementation  . . . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Alternative Implementation  . . . . . . . . . . . . .   6
   4.  Improvements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Operators of popular Internet services face complex challenges
   associated with scaling their infrastructure.  One scaling approach
   is to utilize equal-cost multipath (ECMP) routing to perform
   stateless distribution of incoming TCP or UDP sessions to multiple
   servers or to middle boxes such as load balancers.  Distribution of
   traffic in this manner presents a problem when dealing with ICMP
   signaling.  Specifically, an ICMP error is not guaranteed to hash via
   ECMP to the same destination as its corresponding TCP or UDP session.
   A case where this is particularly problematic operationally is path
   MTU discovery (PMTUD) [RFC1981].

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RFC 7690                 Misses with ICMPv6 PTB             January 2016

2.  Problem

   A common application for stateless load balancing of TCP or UDP flows
   is to perform an initial subdivision of flows in front of a stateful
   load-balancer tier or multiple servers so that the workload becomes
   divided into manageable fractions of the total number of flows.  The
   flow division is performed using ECMP forwarding and a stateless but
   sticky algorithm for hashing across the available paths (see
   [RFC2991] for background on ECMP routing).  For the purposes of flow
   distribution, this next-hop selection is a constrained form of
   anycast topology, where all anycast destinations are equidistant from
   the upstream router responsible for making the last next-hop
   forwarding decision before the flow arrives on the destination
   device.  In this approach, the hash is performed across some set of