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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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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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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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