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

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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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
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   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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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
   available protocol headers.  Typically, these headers may include all
   or a subset of (IPv6) Flow-Label, IP-source, IP-destination,
   protocol, source-port, destination-port, and potentially others such
   as ingress interface.

   A problem common to this approach of distribution through hashing is
   impact on path MTU discovery.  An ICMPv6 type 2 PTB message generated
   on an intermediate device for a packet sent from a server that is
   part of an ECMP load-balanced service to a client will have the load-
   balanced anycast address as the destination and hence will be
   statelessly load balanced to one of the servers.  While the ICMPv6
   PTB message contains as much of the packet that could not be
   forwarded as possible, the payload headers are not considered in the
   forwarding decision and are ignored.  Because the PTB message is not
   identifiable as part of the original flow by the IP or upper-layer
   packet headers, the results of the ICMPv6 ECMP hash calculation are
   unlikely to be hashed to the same next hop as packets matching the
   TCP or UDP ECMP hash of the flow.

   An example packet flow and topology follow.  The packet for which the
   PTB message was generated was intended for the client.

   ptb -> router ecmp -> next hop L4/L7 load balancer -> destination

     router --> load balancer 1 --->
          \\--> load balancer 2 ---> load-balanced service
           \--> load balancer N --->

                                 Figure 1

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   The router ECMP decision is used because it is part of the forwarding
   architecture, can be performed at line rate, and does not depend on
   shared state or coordination across a distributed forwarding system
   that may include multiple linecards or routers.  The ECMP routing
   decision is deterministic with respect to packets having the same
   computed hash.

   A typical case in which ICMPv6 PTB messages are received at the load
   balancer is where the path MTU from the client to the load balancer
   is limited by a tunnel of which the client itself is not aware.

   Direct experience says that the frequency of PTB messages is small
   compared to total flows.  One possible conclusion is that tunneled
   IPv6 deployments that cannot carry 1500 MTU packets are relatively
   rare.  Techniques employed by clients (e.g., Happy Eyeballs
   [RFC6555]) may actually contribute some amelioration to the IPv6
   client experience by preferring IPv4 in cases that might be
   identified as failures.  Still, the expectation of operators is that
   PMTUD should work and that unnecessary breakage of client traffic
   should be avoided.

   A final observation regarding server tuning is that it is not always
   possible, even if it is potentially desirable to be able to
   independently set the TCP MSS (Maximum Segment Size) for different
   address families on some end systems.  On Linux platforms, advmss
   (advertised mss) may be set on a per-route basis for selected
   destinations in cases where discrimination by route is possible.

   The problem as described does also impact IPv4; however,
   implementation of RFC 4821 [RFC4821] TCP MTU probing, the ability to
   fragment on the wire at tunnel ingress points, and the relative
   rarity of sub-1500-byte MTUs that are not coupled to changes in
   client behavior (for example, endpoint VPN clients set the tunnel
   interface MTU accordingly to avoid fragmentation for performance
   reasons) makes the problem sufficiently rare that some existing
   deployments have chosen to ignore it.

3.  Mitigation

   Mitigation of the potential for PTB messages to be misdelivered
   involves ensuring that an ICMPv6 error message is distributed to the
   same anycast server responsible for the flow for which the error is
   generated.  With appropriate hardware support, flows could be
   identified using the same technique as hosts by inspecting the
   payload of the ICMPv6 message.  The ECMP hash calculation can then be
   performed using values identified from the inner TCP flow parameters
   of the ICMPv6 message.  Because the encapsulated IP header occurs at
   a fixed offset in the ICMP message, it is not outside the realm of

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   possibility that routers with sufficient header processing capability
   could parse that far into the payload.  Employing a mediation device
   that handles the parsing and distribution of PTB messages after
   policy routing or on each load balancer / server is a possibility.

   Another mitigation approach is predicated upon distributing the PTB
   message to all anycast servers under the assumption that the one for
   which the message was intended will be able to match it to the flow
   and update the route cache with the new MTU and that devices not able
   to match the flow will discard these packets.  Such distribution has
   potentially significant implications for resource consumption and for
   self-inflicted denial of service (DOS) if not carefully employed.
   Fortunately, we have observed that the number of flows for which this
   problem occurs is relatively small in real-world deployments (for
   example, 10 or fewer pps on 1 Gbit/s or more worth of HTTPS);
   sensible ingress rate limiters that will discard excessive message
   volume can be applied to protect even very large anycast server tiers
   with the potential for fallout limited to circumstances of deliberate
   duress.

3.1.  Alternative Mitigations

   As an alternative, it may be appropriate to lower the TCP MSS to 1220
   in order to accommodate 1280-byte MTU.  We consider this undesirable,
   as hosts may not be able to independently set TCP MSS by address
   family thereby impacting IPv4, or alternatively that middle-boxes
   need to be employed to clamp the MSS independently from the end
   systems.  Potentially, extension headers might further alter the
   lower bound that the MSS would have to be set to, making clamping
   even more undesirable.

3.2.  Implementation

   1.  Filter-based forwarding matches next-header ICMPv6 type 2 and
       matches a next hop on a particular subnet directly attached to
       one or more routers.  The filter is policed to reasonable limits
       (we chose 1000 pps; more conservative rates might be required in
       other implementations).

   2.  The filter is applied on the input side of all external
       (Internet- or customer-facing) interfaces.

   3.  A proxy located at the next hop forwards ICMPv6 type 2 packets it
       receives to an Ethernet broadcast address (example
       ff:ff:ff:ff:ff:ff) on all specified subnets.  This was
       necessitated by router inability (in IPv6) to forward the same
       packet to multiple unicast next hops.

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   4.  Anycasted servers receive the PTB error and process the packet as
       needed.

   A simple Python scapy [SCAPY] script that can perform the ICMPv6
   proxy reflection is included.

         #!/usr/bin/python

         from scapy.all import *

         IFACE_OUT = ["p2p1", "p2p2"]

         def icmp6_callback(pkt):
             if pkt.haslayer(IPv6) and (ICMPv6PacketTooBig in pkt) \
             and pkt[Ether].dst != 'ff:ff:ff:ff:ff:ff':
                 del(pkt[Ether].src)
                 pkt[Ether].dst = 'ff:ff:ff:ff:ff:ff'
                 pkt.show()
                 for iface in IFACE_OUT:
                     sendp(pkt, iface=iface)

         def main():
             sniff(prn=icmp6_callback, filter="icmp6 \
             and (ip6[40+0] == 2)", store=0)

         if __name__ == '__main__':
             main()

   This example script listens on all interfaces for IPv6 PTB errors
   being forwarded using filter-based forwarding.  It removes the
   existing Ethernet source and rewrites a new Ethernet destination of
   the Ethernet broadcast address.  It then sends the resulting frame
   out the p2p1 and p2p2 interfaces that are attached to VLANs where our
   anycast servers reside.

3.2.1.  Alternative Implementation

   Alternatively, network designs in which a common layer 2 network
   exists on the ECMP hop could distribute the proxy onto the end
   systems, eliminating the need for policy routing.  They could then
   rewrite the destination -- for example, using iptables before
   forwarding the packet back to the network containing all of the
   server or load-balancer interfaces.  This implementation can be done
   entirely within the Linux iptables firewall.  Because of the
   distributed nature of the filter, more conservative rate limits are
   required than when a global rate limit can be employed.

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   An example ip6tables/nftables rule to match icmp6 traffic, not match
   broadcast traffic, impose a rate limit of 10 pps, and pass to a
   target destination would resemble:

       ip6tables -I INPUT -i lo -p icmpv6 -m icmpv6 --icmpv6-type 2/0 \
       -m pkttype ! --pkt-type broadcast -m limit --limit 10/second \
       -j TEE 2001:DB8::1

   As with the scapy example, once the destination has been rewritten
   from a hardcoded ND entry to an Ethernet broadcast address -- in this
   case to an IPv6 documentation address -- the traffic will be
   reflected to all the hosts on the subnet.

4.  Improvements

   There are several ways that improvements could be made to improve
   handling ECMP load balancing of ICMPv6 PTB messages.  Little in the
   way of change to the Internet protocol specification is required;
   rather, we foresee practical implementation change, which, insofar as
   we are aware, does not exist in current router, switch, or layer 3/4
   load balancers.  Alternatively, improved behavior on the part of
   client/server detection of path MTU in band could render the behavior
   of devices in the path irrelevant.

   1.  Routers with sufficient capacity within the lookup process could
       parse all the way through the L3 or L4 header in the ICMPv6
       payload beginning at bit offset 32 of the ICMP header.  By
       reordering the elements of the hash to match the inward direction
       of the flow, the PTB error could be directed to the same next hop
       as the incoming packets in the flow.

   2.  The FIB (Forwarding Information Base) on the router could be
       programmed with a multicast distribution tree that includes all
       of the necessary next hops, and unicast ICMPv6 packets could be
       policy routed to these destinations.

   3.  Ubiquitous implementation of RFC 4821 [RFC4821] Packetization
       Layer Path MTU Discovery would probably go a long way towards
       reducing dependence on ICMPv6 PTB by end systems.

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5.  Security Considerations

   The employed mitigation has the potential to greatly amplify the
   impact of a deliberately malicious sending of ICMPv6 PTB messages.
   Sensible ingress rate limiting can reduce the potential for impact;
   legitimate PMTUD messages may be lost once the rate limit is reached.
   The scenario where drops of legitimate traffic occur is analogous to
   other cases where DOS traffic can crowd out legitimate traffic,
   however only a limited subset of overall traffic is impacted.

   The proxy replication results in all devices on the subnet receiving
   ICMPv6 PTB errors, even those not associated with the flow.  This
   could arguably result in information disclosure due to the wide
   replication of the ICMPv6 PTB error on the subnet and the large
   fragment of the offending IP packet embedded in the ICMPv6 error.
   Because of this, recipient machines should be in a common
   administrative domain.

6.  Informative References

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
              1996, <http://www.rfc-editor.org/info/rfc1981>.

   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              Multicast Next-Hop Selection", RFC 2991,
              DOI 10.17487/RFC2991, November 2000,
              <http://www.rfc-editor.org/info/rfc2991>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <http://www.rfc-editor.org/info/rfc4821>.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <http://www.rfc-editor.org/info/rfc6555>.

   [SCAPY]    Scapy, <http://www.secdev.org/projects/scapy/>.

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Acknowledgements

   The authors thank Marak Majkowsiki for contributing text, examples,
   and a very thorough review.  The authors would like to thank Mark
   Andrews, Brian Carpenter, Nick Hilliard, and Ray Hunter, for review.

Authors' Addresses

   Matt Byerly
   Fastly
   Kapolei, HI
   United States

   Email: suckawha@gmail.com

   Matt Hite
   Evernote
   Redwood City, CA
   United States

   Email: mhite@hotmail.com

   Joel Jaeggli
   Fastly
   Mountain View, CA
   United States

   Email: joelja@gmail.com

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