Internet DRAFT - draft-v6ops-pmtud-ecmp-problem
draft-v6ops-pmtud-ecmp-problem
v6ops M. Byerly
Internet-Draft Fastly
Intended status: Informational M. Hite
Expires: February 25, 2015 Evernote
J. Jaeggli
Fastly
August 24, 2014
Close encounters of the ICMP type 2 kind (near misses with ICMPv6 PTB)
draft-v6ops-pmtud-ecmp-problem-00
Abstract
This document calls attention to the problem of delivering ICMPv6
type 2 "Packet Too Big" (PTB) messages to intended destinations in
ECMP load balanced, anycast network architectures. It discusses
operational mitigations that can address this class of failure.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 25, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Mitigation . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Alternatives . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Implementation . . . . . . . . . . . . . . . . . . . . . 5
4. Improvements . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Informative References . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Operators of popular Internet services face unique challenges
associated with scaling their infrastructure. One approach is to
utilize equal-cost multi-path (ECMP) routing to perform stateless
distribution of incoming TCP or UDP sessions to multiple servers or
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).
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. This
nexthop selection for the purposes of flow distribution is a
constrained form of anycast d where all anycast destinations are
equidistant topologically 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 (IPv6)Flow-Label, IP-source, IP-destination,
protocol, source-port, destination-port and potentially others such
as ingress interface.
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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 an ECMP load
balanced server to a client, will have the load-balanced anycast
address as the destination and will be statelessly load balanced to
one of the anycast servers. While the ICMPv6 PTB message contains as
much of the packet that could not be forwarded as possible, the
payload headers do not factor into the forwarding decision and are
ignored. Because the PTB message is not identifiable as part of the
original flow by the packet header the results of the ICMPv6 ECMP
hash are unlikely to be hashed to the same nexthop as packets
matching TCP or UDP ECMP hash.
An example packet flow and topology follow.
ptb -> router ecmp -> nexthop L4/L7 load balancer -> destination
router --> load balancer 1 --->
\\--> load balancer 2 ---> load-balanced service
\--> load balancer N --->
Figure 1
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
architecture which may include multiple linecards or routers. The
ECMP routing decision is deterministic with respect to packets having
the same computed hash.
The typical case where ICMPv6 PTB messages are received at the load
balancer is a case where the path MTU from the client to the load
balancer is limited by a tunnel in which the client itself is not
aware of. In the common case of a TCP flow where TLS is employed,
the first packet that is likely to exceed a tunnel MTU lower than
that specified by the MSS on the client and the load balancer/server
is the TLS ServerHello and certificate.
Direct experience says that the frequency of PTB messages is small
compared to total flows. One possible conclusion being that tunneled
IPv6 deployments that cannot carry 1500 mtu packets are relatively
rare. Techniques employed by clients such as happy-eyeballs may
actually contribute some amelioration to the IPv6 client experience
by preferring IPv4 in cases that might be identified as slow or
failed. Still, the expectation of operators is that PMTUD should
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work and that unnecessary breakage of client traffic should be
avoided.
A final observation regarding server tuning is that it is typically
not possible even if it is potentially desirable to be able to
independently set the TCP MSS for different address families on end-
systems.
The problem as described does also impact IPv4; however, the ability
to fragment on 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 for performance reasons) makes the problem
sufficiently rare that some deployments simply choose to ignore it.
3. Mitigation
Mitigation of the potential for PTB messages to be mis-delivered
involves ensuring that an ICMPv6 error message is distributed to the
same anycast server responsible for the flow for which the error is
generated. Ideally Mitigation could be done by the mechanism hosts
use to identify the flow, by looking into the payload of the ICMPv6
message (to determine which TCP flow it was associated with) before
making a forwarding decision. Because the encapsulated IP header
occurs at a fixed offset in the icmp message it is not outside the
realm of 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, devices not able to
match the flow will discard these packets. Such distribution has
potentially significant implications for resource consumption and the
potential for self-inflicted denial-of-service if not carefully
employed. Fortunately, in real-world-deployment we have observed
that, the number of flows for which this problem occurs is relatively
small (example, 10 or fewer pps on 1Gb/s or more worth of https
traffic) and sensible ingress rate limiters which will discard
excessive message volume can be applied to protect even very large
anycast server tiers with the potential for fallout only under
circumstances of deliberate duress.
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3.1. Alternatives
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 it relies on a
middle-box to clamp the MSS independently from the end-systems.
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
both border routers. The filter is policed to reasonable limits
(we chose 1000pps).
2. Filter is applied on input side of all external interfaces
3. A proxy located at the next-hop forwards ICMPv6 type-2 packets
received at the next-hop 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.
4. Anycast servers receive the PTB error and process packet as
needed.
A simple Python scapy script that can perform the ICMPv6 proxy
reflection is included.
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#!/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 where our anycast servers reside.
4. Improvements
There are several ways that improvements could be made to the
situation with respect to ECMP load balancing of ICMPv6 PTB.
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 could be programmed with a multicast distribution tree
that included all of the necessary next-hops.
3. Ubiquitous implementation of RFC 4821 [RFC4821] Packetization
Layer Path MTU Discovery would probably go a long way towards
reducing dependence on ICMPv6 PTB.
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5. Acknowledgements
The authors would like to thank Mark Andrews, Brian Carpenter, Nick
Hilliard and Ray Hunter, for review.
6. IANA Considerations
This memo includes no request to IANA.
7. 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;
however, legitimate traffic may be lost once the rate limit is
reached.
The proxy replication results in devices not associated with the flow
that generated the PTB being recipients of an ICMPv6 message which
contains a fragment of a packet. This could arguably result in
information disclosure. Recipient machines should be in a common
administrative domain.
8. Informative References
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
Authors' Addresses
Matt Byerly
Fastly
Kapolei, HI
US
Email: mbyerly@zynga.com
Matt Hite
Evernote
Redwood City, CA
US
Email: mhite@hotmail.com
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Joel Jaeggli
Fastly
Mountain View, CA
US
Email: joelja@gmail.com
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