draft-ietf-v6ops-pmtud-ecmp-problem-02.txt   draft-ietf-v6ops-pmtud-ecmp-problem-03.txt 
v6ops M. Byerly v6ops M. Byerly
Internet-Draft Fastly Internet-Draft Fastly
Intended status: Informational M. Hite Intended status: Informational M. Hite
Expires: December 19, 2015 Evernote Expires: December 30, 2015 Evernote
J. Jaeggli J. Jaeggli
Fastly Fastly
June 17, 2015 June 28, 2015
Close encounters of the ICMP type 2 kind (near misses with ICMPv6 PTB) Close encounters of the ICMP type 2 kind (near misses with ICMPv6 PTB)
draft-ietf-v6ops-pmtud-ecmp-problem-02 draft-ietf-v6ops-pmtud-ecmp-problem-03
Abstract Abstract
This document calls attention to the problem of delivering ICMPv6 This document calls attention to the problem of delivering ICMPv6
type 2 "Packet Too Big" (PTB) messages to the intended destination in type 2 "Packet Too Big" (PTB) messages to the intended destination in
ECMP load balanced or anycast network architectures. It discusses ECMP load balanced or anycast network architectures. It discusses
operational mitigations that can be employed to address this class of operational mitigations that can be employed to address this class of
failure. failures.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 19, 2015. This Internet-Draft will expire on December 30, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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2. Problem 2. Problem
A common application for stateless load balancing of TCP or UDP flows 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 is to perform an initial subdivision of flows in front of a stateful
load balancer tier or multiple servers so that the workload becomes load balancer tier or multiple servers so that the workload becomes
divided into manageable fractions of the total number of flows. The divided into manageable fractions of the total number of flows. The
flow division is performed using ECMP forwarding and a stateless but flow division is performed using ECMP forwarding and a stateless but
sticky algorithm for hashing across the available paths. This sticky algorithm for hashing across the available paths. This
nexthop selection for the purposes of flow distribution is a nexthop selection for the purposes of flow distribution is a
constrained form of anycast topology where all anycast destinations constrained form of anycast topology, where all anycast destinations
are equidistant from the upstream router responsible for making the are equidistant from the upstream router responsible for making the
last next-hop forwarding decision before the flow arrives on the last next-hop forwarding decision before the flow arrives on the
destination device. In this approach, the hash is performed across destination device. In this approach, the hash is performed across
some set of available protocol headers. Typically, these headers may some set of available protocol headers. Typically, these headers may
include all or a subset of (IPv6) Flow-Label, IP-source, IP- include all or a subset of (IPv6) Flow-Label, IP-source, IP-
destination, protocol, source-port, destination-port and potentially destination, protocol, source-port, destination-port and potentially
others such as ingress interface. others such as ingress interface.
A problem common to this approach of distribution through hashing is A problem common to this approach of distribution through hashing is
impact on path MTU discovery. An ICMPv6 type 2 PTB message generated 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 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 part of an ECMP load balanced service to a client will have the load
balanced anycast address as the destination and hence will be balanced anycast address as the destination and hence will be
statelessly load balanced to one of the servers. While the ICMPv6 statelessly load balanced to one of the servers. While the ICMPv6
PTB message contains as much of the packet that could not be PTB message contains as much of the packet that could not be
forwarded as possible, the payload headers are not considered in the forwarded as possible, the payload headers are not considered in the
forwarding decision and are ignored. Because the PTB message is not forwarding decision and are ignored. Because the PTB message is not
identifiable as part of the original flow by the IP or upper layer identifiable as part of the original flow by the IP or upper layer
packet headers, the results of the ICMPv6 ECMP hash are unlikely to packet headers, the results of the ICMPv6 ECMP hash calculation are
be hashed to the same nexthop as packets matching TCP or UDP ECMP unlikely to be hashed to the same nexthop as packets matching the TCP
hash. or UDP ECMP hash of the flow.
An example packet flow and topology follow. An example packet flow and topology follow.
ptb -> router ecmp -> nexthop L4/L7 load balancer -> destination ptb -> router ecmp -> nexthop L4/L7 load balancer -> destination
router --> load balancer 1 ---> router --> load balancer 1 --->
\\--> load balancer 2 ---> load-balanced service \\--> load balancer 2 ---> load-balanced service
\--> load balancer N ---> \--> load balancer N --->
Figure 1 Figure 1
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independently set the TCP MSS for different address families on some independently set the TCP MSS for different address families on some
end-systems. On Linux platforms, advmss may be set on a per route end-systems. On Linux platforms, advmss may be set on a per route
basis for selected destinations in cases where discrimination by basis for selected destinations in cases where discrimination by
route is possible. route is possible.
The problem as described does also impact IPv4; however The problem as described does also impact IPv4; however
implementation of RFC 4821 [RFC4821] TCP MTU probing, the ability to implementation of RFC 4821 [RFC4821] TCP MTU probing, the ability to
fragment on wire at tunnel ingress points and the relative rarity of 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 sub-1500 byte MTUs that are not coupled to changes in client behavior
(for example, endpoint VPN clients set the tunnel interface MTU (for example, endpoint VPN clients set the tunnel interface MTU
accordingly for performance reasons) makes the problem sufficiently accordingly to avoid fragmentation for performance reasons) makes the
rare that some existing deployments have choosen to ignore it. problem sufficiently rare that some existing deployments have choosen
to ignore it.
3. Mitigation 3. Mitigation
Mitigation of the potential for PTB messages to be mis-delivered Mitigation of the potential for PTB messages to be mis-delivered
involves ensuring that an ICMPv6 error message is distributed to the involves ensuring that an ICMPv6 error message is distributed to the
same anycast server responsible for the flow for which the error is same anycast server responsible for the flow for which the error is
generated. Ideally, mitigation could be done by the mechanism hosts generated. Ideally, mitigation could be done by the mechanism hosts
use to identify the flow, by looking into the payload of the ICMPv6 use to identify the flow, by looking into the payload of the ICMPv6
message (to determine which TCP flow it was associated with) before message (to determine which TCP flow it was associated with) before
making a forwarding decision. Because the encapsulated IP header making a forwarding decision. Because the encapsulated IP header
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capability could parse that far into the payload. Employing a capability could parse that far into the payload. Employing a
mediation device that handles the parsing and distribution of PTB mediation device that handles the parsing and distribution of PTB
messages after policy routing or on each load-balancer/server is a messages after policy routing or on each load-balancer/server is a
possibility. possibility.
Another mitigation approach is predicated upon distributing the PTB Another mitigation approach is predicated upon distributing the PTB
message to all anycast servers under the assumption that the one for 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 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 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 to match the flow will discard these packets. Such distribution has
potentially significant implications for resource consumption and the potentially significant implications for resource consumption and for
potential for self-inflicted denial-of-service if not carefully self-inflicted denial-of-service if not carefully employed.
employed. Fortunately, in real-world deployments we have observed Fortunately, in real-world deployments we have observed that the
that the number of flows for which this problem occurs is relatively number of flows for which this problem occurs is relatively small
small (example, 10 or fewer pps on 1Gb/s or more worth of https (example, 10 or fewer pps on 1Gb/s or more worth of https traffic in
traffic) and sensible ingress rate limiters which will discard a real world deployment); sensible ingress rate limiters which will
excessive message volume can be applied to protect even very large discard excessive message volume can be applied to protect even very
anycast server tiers with the potential for fallout only under large anycast server tiers with the potential for fallout limited to
circumstances of deliberate duress. circumstances of deliberate duress.
3.1. Alternatives 3.1. Alternatives
As an alternative, it may be appropriate to lower the TCP MSS to 1220 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 in order to accommodate 1280 byte MTU. We consider this undesirable
as hosts may not be able to independently set TCP MSS by address- as hosts may not be able to independently set TCP MSS by address-
family thereby impacting IPv4, or alternatively that middle-boxes family thereby impacting IPv4, or alternatively that middle-boxes
need to be employed to clamp the MSS independently from the end- need to be employed to clamp the MSS independently from the end-
systems. Potentialy, extension might further alter the lower bound systems. Potentially, extension headers might further alter the
that the mss would have to be set to making clamping still more lower bound that the MSS would have to be set to, making clamping
undesirable. still more undesirable.
3.2. Implementation 3.2. Implementation
1. Filter-based-forwarding matches next-header ICMPv6 type-2 and 1. Filter-based-forwarding matches next-header ICMPv6 type-2 and
matches a next-hop on a particular subnet directly attached to matches a next-hop on a particular subnet directly attached to
both border routers. The filter is policed to reasonable limits both border routers. The filter is policed to reasonable limits
(we chose 1000pps more conservative rates might be required in (we chose 1000pps, more conservative rates might be required in
other imlementations). other implementations).
2. Filter is applied on input side of all external interfaces 2. Filter is applied on input side of all external interfaces
3. A proxy located at the next-hop forwards ICMPv6 type-2 packets 3. A proxy located at the next-hop forwards ICMPv6 type-2 packets
received at the next-hop to an Ethernet broadcast address received at the next-hop to an Ethernet broadcast address
(example ff:ff:ff:ff:ff:ff) on all specified subnets. This was (example ff:ff:ff:ff:ff:ff) on all specified subnets. This was
necessitated by router inability (in IPv6) to forward the same necessitated by router inability (in IPv6) to forward the same
packet to multiple unicast next-hops. packet to multiple unicast next-hops.
4. Anycast servers receive the PTB error and process packet as 4. Anycast servers receive the PTB error and process packet as
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sniff(prn=icmp6_callback, filter="icmp6 \ sniff(prn=icmp6_callback, filter="icmp6 \
and (ip6[40+0] == 2)", store=0) and (ip6[40+0] == 2)", store=0)
if __name__ == '__main__': if __name__ == '__main__':
main() main()
This example script listens on all interfaces for IPv6 PTB errors This example script listens on all interfaces for IPv6 PTB errors
being forwarded using filter-based-forwarding. It removes the being forwarded using filter-based-forwarding. It removes the
existing Ethernet source and rewrites a new Ethernet destination of existing Ethernet source and rewrites a new Ethernet destination of
the Ethernet broadcast address. It then sends the resulting frame the Ethernet broadcast address. It then sends the resulting frame
out the p2p1 and p2p2 interfaces where our anycast servers reside. out the p2p1 and p2p2 interfaces which attached to vlans where our
anycast servers reside.
3.2.1. Alternatives 3.2.1. Alternatives
Alternatively, network designs in which a common layer 2 network Alternatively, network designs in which a common layer 2 network
exists on the ECMP hop could distribute the proxy onto the end exists on the ECMP hop could distribute the proxy onto the end
systems, eleminating the need for policy routing. They could then systems, eliminating the need for policy routing. They could then
rewrite the destination -- for example, using iptables before rewrite the destination -- for example, using iptables before
forwarding the packet back to the network containing all of the forwarding the packet back to the network containing all of the
server or load balancer interfaces. This implmentation can be done server or load balancer interfaces. This implmentation can be done
entirely within the Linux iptables firewall. Because of the entirely within the Linux iptables firewall. Because of the
distributed nature of the filter, more conservative rate limits are distributed nature of the filter, more conservative rate limits are
required than when a global rate limit can be employed. required than when a global rate limit can be employed.
An example ip6tables / nftables rule to match icmp6 traffic, not An example ip6tables / nftables rule to match icmp6 traffic, not
match broadcast traffic, impose a rate limit of 10 pps, and pass to a match broadcast traffic, impose a rate limit of 10 pps, and pass to a
target destination would resemble: target destination would resemble:
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1. Routers with sufficient capacity within the lookup process could 1. Routers with sufficient capacity within the lookup process could
parse all the way through the L3 or L4 header in the ICMPv6 parse all the way through the L3 or L4 header in the ICMPv6
payload beginning at bit offset 32 of the ICMP header. By payload beginning at bit offset 32 of the ICMP header. By
reordering the elements of the hash to match the inward direction 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 of the flow, the PTB error could be directed to the same next-hop
as the incoming packets in the flow. as the incoming packets in the flow.
2. The FIB (Forwarding Information Base) on the router could be 2. The FIB (Forwarding Information Base) on the router could be
programmed with a multicast distribution tree that included all programmed with a multicast distribution tree that included all
of the necessary next-hops. of the necessary next-hops, and ICMPv6 packets could be policy
routed to this destination.
3. Ubiquitous implementation of RFC 4821 [RFC4821] Packetization 3. Ubiquitous implementation of RFC 4821 [RFC4821] Packetization
Layer Path MTU Discovery would probably go a long way towards Layer Path MTU Discovery would probably go a long way towards
reducing dependence on ICMPv6 PTB. reducing dependence on ICMPv6 PTB by end systems.
5. Acknowledgements 5. Acknowledgements
The authors would like to thank Marak Majkowsiki for contributing The authors would like to thank Marak Majkowsiki for contributing
text, examples, and a very close review. The authors would like to text, examples, and a very close review. The authors would like to
thank Mark Andrews, Brian Carpenter, Nick Hilliard and Ray Hunter, thank Mark Andrews, Brian Carpenter, Nick Hilliard and Ray Hunter,
for review. for review.
6. IANA Considerations 6. IANA Considerations
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