Network Working Group Jari Arkko
INTERNET-DRAFT Ericsson
Category: Standards Track
<draft-arkko-icmpv6-ike-effects-00.txt>
9 February 2001
Effects of ICMPv6 on IKE and IPsec Policies
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026. Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and
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ing documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or made obsolete by other documents at
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material or to cite them other than as work in progress.
The list of current Internet-Drafts may be found at
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The list of Internet-Draft Shadow Directories may be found at
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The distribution of this memo is unlimited. It is filed as <draft-
arkko-icmpv6-ike-effects-00.txt>, and expires July 9, 2001. Please
send comments to the author or to IPsec and/or IPNG working group
mailing lists.
2. Abstract
The ICMPv6 protocol provides many functions which in IPv4 were either
non-existent or provided by lower layers. IPv6 architecture also makes
it possible to secure all IP packets using IPsec, even ICMPv6 mes-
sages. IPsec architecture has a Security Policy Database that speci-
fies which traffic is protected, and how. It turns out that the speci-
fication of policies in the presence of ICMPv6 traffic is hard. Sound
looking policies may easily lead to loops: The establishment of secu-
rity requires ICMPv6 messages which can't be sent since security
hasn't been established yet. The purpose of this draft is to inform
system administrators and IPsec implementors in which manner they can
handle the ICMPv6 messages. Common understanding of the way that these
messages are handled is also necessary for interoperability, in case
vendors hardcode such rules in to products.
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3. Introduction
The ICMPv6 protocol [3] provides many functions which in IPv4 were
either non-existent or provided by lower layers. For instance, IPv6
implements address resolution using an IP packet, ICMPv6 Neighbour
Solicitation message [1]. In contrast, IPv4 uses an ARP message at a
lower layer.
IPv6 architecture makes it possible to secure all IP packets using
IPsec [6], even ICMPv6 messages and even to multicast addresses.
IPsec architecture has a Security Policy Database that specifies which
traffic is protected, and how. It turns out that the specification of
policies in the presence of ICMPv6 traffic is not easy. For instance,
a simple policy of protecting all traffic between two hosts on the
same network would trap even address resolution messages, leading to a
situation where IKE can't establish a Security Association since in
order to send the IKE UDP packets one would have had to send the
Neighbour Solicitation Message, which would have required an SA.
The purpose of this draft is to inform system administrators and IPsec
implementors in which manner they can handle the ICMPv6 messages. Sys-
tem administrators do not want to study the IPv6 specifications in
order to understand how they shall configure their routers. IPsec
implementors want to understand what kind of policies they can offer
with respect to the ICMPv6 messages.
Common understanding of the way that these messages are handled is
also very much necessary for interoperability, as some vendors may be
hardcoding some of the low-level policy operations in their products.
If the rules between two vendors' products are incompatible for a par-
ticular message we may end with the sender sending cleartext and the
receiver requiring IPsec, causing the packet to be dropped and possi-
bly all connectivity between the two nodes lost.
4. ICMPv6 Tasks
In IPv6, ICMP has several tasks, and many of these tasks are over-
loaded on a few central message types such as the Neighbour Discovery
message. In this chapter we explain the tasks and their effects in
order to understand better how the messages should be treated.
4.1. Path MTU Discovery
Path MTUs are dynamically determined by IPv6 in order to optimize the
size of the packets sent to a particular destination [5].
The ICMPv6 Packet Too Big messages are used as a part of the Path MTU
Discovery procedure [3].
4.2. Error Notification
ICMPv6 handles basic error situations of the IP layer, such as finding
out that a particular destination isn't available.
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The Destination Unreachable, Packet Too Big, Parameter Problem, and
Time Exceeded messages are a part of the error handling procedure [3].
Note that the Packet Too Big message also plays a role in the Path MTU
Discovery procedure.
4.3. Informational Notifications
For debugging and network analysis purposes, ICMPv6 includes informa-
tional messages [3]. These message are necessary also in IPsec con-
texts and over IPsec tunnels due to the complex nature of some tunnel
setups.
The Echo Request and Echo Reply messages are used solely for this pur-
pose.
4.4. Router and Prefix Discovery
Router and prefix discovery is a part of the Neighbour Discovery pro-
tocol [1], which in turn is a part of the ICMPv6. The main purpose of
the router discovery is to find neighboring routers that are willing
to forward packets on the behalf of hosts. Prefix discovery involves
determining which destinations are local for an attached link. This
information is used both by the address autoconfiguration process, and
routing. Typically, address autoconfiguration and other tasks can't
proceed at all until the router discovery process has run.
The Router Solicitation and Router Advertisement messages are used for
this and only this purpose.
4.5. Address Autoconfiguration
Address autoconfiguration is another part of the Neighbour Discovery
protocol [1]. It's purpose is to automatically assign addresses to
interfaces. It comes in two variants, stateless and statefull. In this
document we consider only the stateless autoconfiguration aspects.
Obviously, no higher layer traffic can be sent until all participating
nodes have addresses. This includes also IKE UDP traffic.
The Neighbour Solicitation and Advertisement messages are used for
this purpose, among other things. Furthermore, Router and Prefix Dis-
covery and Duplicate Address Detection have an effect to the Address
Autoconfiguration tasks.
4.6. Duplicate Address Detection
As a part of the stateless address autoconfiguration procedure, nodes
check for duplicate addresses prior to assigning an address to an
interface [2]. This procedure uses the same messages as the Neighbour
Discovery protocol [1]. Since the rules outlined in [2] forbid the use
of an address for both sending and receiving packets until it has been
found unique, no higher layer traffic is possible until this procedure
has completed.
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The Neighbour Solicitation and Advertisement messages are used also
for this purpose.
4.7. Address Resolution
In address resolution, nodes determine the link-layer address of a
local destination given only the destination's IP address [1]. Again,
no higher level traffic can proceed until the sender knows the hard-
ware address of the destination or the next hop router.
The Neighbour Solicitation and Advertisement messages are used also
for this purpose.
4.8. Neighbor Reachability Detection
Hosts monitor the reachability of local destinations and routers in
the Neighbour Unreachability procedure, which is a part of the Neigh-
bour Discovery protocol [1]. No higher level traffic can proceed if
this procedure flushes out neighbour cache entries after (perhaps
incorrectly) determining that the peer is not reachable.
The Neighbour Solicitation and Advertisement messages are used also
for this purpose.
4.9. Redirect
In the Redirect procedure, a router informs a host of a better first-
hop node to reach a particular destination [1]. It is a part of the
Neighbour Discovery protocol. As routers forward packets regardless of
them being sent first to the wrong place, communications can still be
established without the ability to process Redirect messages.
The Redirect message is used solely for the Redirect procedure.
4.10. Router Renumbering
This procedure [4] allows address prefixes on routers to be configured
and reconfigured in the similar manner as Neighbor Discovery and
Address Autoconfiguration works for hosts. Incorrect processing or
blocking of messages related to this procedure may render a node's
address sets invalid, thereby preventing further communications.
The Router Renumbering message is used solely for the Router Renumber-
ing procedure.
5. Factors Affecting the Policy Rules
5.1. Nature of the Addresses
ICMPv6 messages are sent using various kinds of source and destination
address types. The source address is usually a unicast address, but
during address autoconfiguration message exchanges, the unspecified
address :: is also used as a source address [2]. The destination
address can be either a well known multicast address, a generated
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multicast address such as the solicited-node multicast address, or a
unicast address. While many ICMPv6 messages use multicast addresses
most of the time, some also use unicast addresses sometimes. For
instance, the Neighbour Solicitation messages are usually sent to mul-
ticast addresses, but the Neighbour Advertisement messages are also
sent to unicast addresses when sent as a response to a node that has
an address.
IPsec [6] can be used for the protection of both unicast and multicast
traffic. However, in order to automatically negotiate mutually accept-
able security associations and to refresh keys, IKE [7] needs to be
used. IKE is only capable of negotiating SAs for unicast communica-
tions.
Obviously, policies MUST be configured so that multicast traffic
doesn't require dynamic SAs. However, while this is a necessary condi-
tion it is not sufficient to make sure that that IKE works. The poli-
cies MUST also exclude unicast traffic which is contains ICMPv6 mes-
sages required before UDP can work between the two nodes.
5.2. Network Topology
ICMP traffic has different implications for hosts and security gate-
ways. In general, security gateways SHOULD carry all ICMP traffic
related to the protected traffic in the same tunnel as the traffic
itself. For instance, when an ICMPv6 Packet Too Big message is gener-
ated on the unprotected segment of a packet's path, that message
should relayed through the tunnel to ensure that the sender recognizes
the MTU problem.
Between hosts similar rules apply. However, messages related to the
establishment of communication between the hosts - such as for address
resolution - MUST NOT be passed through the tunnel at least when the
tunnel does not exist yet and IKE would be needed to establish it.
Note that the distinctions in network topology are more due to the
actual network architecture than the selected IPsec mode, be it tunnel
or transport.
ICMPv6 messages can be classified according to whether they are meant
for end-to-end communications or communications within a link. There
are also messages that we classify as 'any-to-end', which can be sent
from any point within a path back to the source, typically to announce
an error in processing the original packet. For instance, the address
resolution messages are solely for local communications [1], whereas
the Destination Unreachable messages are any-to-end in nature. End-to-
end and any-to-end messages MUST always be passed through tunnels.
Local messages may be passed through IPsec process under certain con-
ditions.
5.3. Role in Estaliblishing Communications
ICMPv6 messages can also be classified according to their role for
establishing communications between two nodes. For the purposes of
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this discussion, the relevant issue is whether or not the messages
must be passed through before IKE can use UDP packets to negotiate
SAs. For instance, address autoconfiguration, duplicate address detec-
tion, and address resolution obviously MUST be completed before UDP
packets can be passed.
Neighbour reachability detection is also capable of disrupting IKE
communications. The reference [1] states the following:
In some cases (e.g., UDP-based protocols and routers
forwarding packets to hosts) such reachability information
may not be readily available from upper-layer protocols.
When no hints are available and a node is sending packets
to a neighbor, the node actively probes the neighbor using
unicast Neighbor Solicitation messages to verify that the
forward path is still working.
This means that unless the IKE implementation explicitly handles for-
ward progress notifications towards the IPv6 stack, the stack can not
know about the reachability towards the other host. Since the hosts
may be using tunnel mode and other address in the inner packets than
the regular addresses on the hosts, the stack can not learn of forward
progress through regular IPsec AH or ESP packets.
Therefore, neighbour reachability MUST also be allowed to work inde-
pendent of IKE SA establishment.
As IKE messages may contain certificates, it is quite possible that an
MTU limit may be exceeded somewhere within the network. If this is
possible in a given network, the policies MUST allow ICMP Packet Too
Big messages to be received. Note that these messages may well be
received either in the clear, on manually configured SAs, or on
dynamic SAs. If the router generating the Packet Too Big message does
not yet have an SA with the original host, it can initiate IKE negoti-
ations to create one. In case that this new negotiation fails due to
reaching another MTU limit, other routers may be involved along the
way. But ultimately the process reaches the closest router to which
the MTU is known and will not cause any ICMP error messages.
5.4. Protecting the Infrastructure versus Communications
IPsec can be used to protect the end-to-end communications or the
underlying control messages (such as ICMPv6). It can even be used to
protect both. Since many of the control messages are sent to multicast
addresses, if IPsec is used then manual SA configuration MUST be per-
formed instead of IKE-based SA negotiation.
As we have talked about some messages in some situations having to be
independent of IKE, it does not necessarily imply that they have to
passed through in the clear. Instead, systems MAY use manually config-
ured IPsec SAs to protect e.g. all ICMPv6 communications within one
network. (Note that setting these manual SAs up requires some care as
discussed in [8].)
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A plausible security policy configuration could therefore be one where
all ICMPv6 messages within the local network must be protected by man-
ual SAs, and all other communications must be protected by IKE-negoti-
ated SAs.
6. Analysis of the ICMPv6 Messages
6.1. Destination Unreachable
The ICMPv6 type of this message is 1.
This message is always sent between unicast addresses [3]. It is an
end-to-end message Destination Unreachable is never a relevant mes-
sage for establishing dynamic SAs, unless advanced failover schemes
rely on the knowledge to quickly determine unreachable IKE peers.
6.2. Packet Too Big
The ICMPv6 type of this message is 2.
This message is also always sent between unicast addresses [3] even if
might be sent as a response to a multicast message. It is an end-to-
end message.
Packet Too Big has, however, a role in establishing communications.
End-to-end communications, that is. In order to pass through long IKE
packets, Packet Too Big responses from the network MUST be considered.
Therefore, it MUST be possible for policies to be configured so that
such messages can be received. Note that as dicussed previously, the
Packet Too Big messages themselves can be protected in various ways.
6.3. Time Exceeded
The ICMPv6 type of this message is 3.
This message is also always sent between unicast addresses [3] and is
an end-to-end message. Like Packet Too Big, it too has a role in
establishing end-to-end communications under certain special situa-
tions.
6.4. Parameter Problem
The ICMPv6 type of this message is 4.
This message is similar to Packet Too Big in the sense that it uses
only unicast messages even if it could be sent as a response to a mul-
ticast packet. It's role is also end-to-end. While in theory its role
in establishing communications is similar to Packet Too Big and Time
Exceeded, in practise it is hard to see the kind of IKE and IPv6 stack
version problem that could result in this message being sent.
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6.5. Echo Request
The ICMPv6 type of this message is 128.
Echo Request uses unicast addresses as source addresses, but may be
sent to any legal IPv6 address, even multicast and anycast addresses
[3]. Echo Requests run end-to-end but never have a role in establish-
ing communications.
6.6. Echo Reply
The ICMPv6 type of this message is 129.
Echo Reply is similar to Echo Request in other respects, but uses only
unicast addresses.
6.7. Redirect
The ICMPv6 type of this message is 137.
The Redirect message is always sent between unicast addresses [1]. It
is only used for local purposes, not for end-to-end communications. It
isn't strictly necessary in order to establish communications. Never-
theless, it can be viewed as a logical add-on to the Neighbour Discov-
ery messages such as Router Advertisement, and as such SHOULD be
treated in a similar manner.
6.8. Router Solicitation
The ICMPv6 type of this message is 133.
This message uses either the unspecified address or an unicast address
as a source address. The destination address is typically a multicast
address. This message is always used only local. Since address auto-
configuration and routing depend on the ability of the routers and
address prefixes to be found, this message is required before any com-
munications can be established. Therefore, this message MUST be
allowed to work independent of IKE SA establishment.
6.9. Router Advertisement
The ICMPv6 type of this message is 134.
This message has always a unicast source address, but the destination
address can be either a unicast or a multicast address. Like the
solicitation message, the advertisement is also link local only and
required for establishing any communications. Therefore, this message
MUST be allowed to work independent of IKE SA establishment.
6.10. Neighbour Solicitation
The ICMPv6 type of this message is 135.
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The source address of this message is either a unicast address or (if
Duplicate Address Detection is in progress) the unspecified address
[1, 3]. The destination is either a multicast address, unicast
address, or an anycast address. Neighbour Solicitation and Advertise-
ment messages are used for multiple purposes: address autoconfigura-
tion, duplicate address detection, and reachability detection. In all
these roles they act only locally on the link, and getting them
through is required before any communications can be established.
Therefore, this message MUST be allowed to work independent of IKE SA
establishment.
6.11. Neighbour Advertisement
The ICMPv6 type of this message is 136.
The source address of this message is a unicast address, and the des-
tination is either a unicast or a multicast address. Like the solica-
tion message, this message is link local only and is required before
any communications can be established. Therefore, this message MUST
be allowed to work independent of IKE SA establishment.
6.12. Router Renumbering
The ICMPv6 type of this message is 138. The code is 0 for a Router
Renumbering Command, 1 for a Router Renumbering Result, and 255 for a
Sequence Number Reset [4].
These messages are sent from a unicast address to either a multicast
or a unicast address. The message are not solely link local, they are
used for end-to-end purposes such as having a central management sta-
tion renumber all routers in a corporate network. As a result of the
RR procedure, automatically configured addresses and prefixes may be
changed. However, it is expected that a transition period exists where
both addresses are still acceptable, making it possible to still pro-
ceed with IKE negotiations to create SAs for the RR procedure. We can
therefore assume that the procedure MAY use manual or dynamic SAs as
desired by the system administrators.
7. Summary
Based on the above, the ICMPv6 messages can be classified as follows:
+-------------------+------------+-----------------+
| MESSAGE | ROLE | USE IKE? |
+-------------------+------------+-----------------+
| Dest Unreachable | Any-to-End | MAY(1,2) |
+-------------------+------------+-----------------+
| Packet Too Big | Any-to-End | MAY(1,3) |
+-------------------+------------+-----------------+
| Time Exceeded | Any-to-End | MAY(1,3) |
+-------------------+------------+-----------------+
| Parameter Problem | End-to-End | MAY(4) |
+-------------------+------------+-----------------+
| Echo Request | End-to-End | MAY(4) |
+-------------------+------------+-----------------+
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| Echo Reply | End-to-End | MAY(4) |
+-------------------+------------+-----------------+
| Redirect | Link Local | SHOULD NOT(5) |
+-------------------+------------+-----------------+
| Router Solicit | Link Local | MUST NOT(6) |
+-------------------+------------+-----------------+
| Router Advert | Link Local | MUST NOT(6) |
+-------------------+------------+-----------------+
| Neighbour Solicit | Link Local | MUST NOT(6) |
+-------------------+------------+-----------------+
| Neighbour Advert | Link Local | MUST NOT(6) |
+-------------------+------------+-----------------+
| Router Renumbering| End-to-End | MAY(4) |
+-------------------+------------+-----------------+
Explanations:
(1) These error messages have an end-to-end nature but may be gener-
ated by intermediate routers as well.
(2) This MAY have to be considered by implementations that wish to
base failover decisions on the Unreachable message.
(3) These messages have an impact on the success of IKE messages e.g.
when certificates are passed in IKE packets. It MUST be possible for
policies to be configured so that these messages can be received while
the IKE negotiations are still ongoing. Different security policy con-
figurations MUST be supported, including trusting cleartext messages
or protecting the messages from intermediate nodes using other, new
dynamic SA negotiations.
(4) These messages MAY be treated using regular IPsec and/or IKE pro-
cessing.
(5) This message SHOULD NOT use IKE in order to make their treatment
equal with the rest of the link local messages, but in theory Redirect
MAY be handled differently, e.g. using dynamic SAs.
(6) These messages MUST NOT use dynamic SAs.
These policy rules may be expressed in various ways on a particular
host or a router. It is necessary to use the ICMPv6 type in making the
policy decisions. As [4] states, "This is consistent with, although
not mentioned by, the Security Architecture specification". Only the
following requirement for all implementations is stated here. Products
that provide hardcoded security policies for ICMPv6 messages SHOULD
enable user specified policies to be expressed at a higher priority
level so that a possibility is still retained for modifying the rules
due to e.g. interoperability problems.
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8. Further Work
This draft discusses the use of IPsec on ICMPv6 messages on a princi-
ple level. It does not take a stand on how the policies are expressed,
for instance whether IPsec products need to have hardcoded rules for
handling these messages, or whether the Security Policy Databases
should be general enough to make it possible to express the policies
in them even for the ICMPv6 messages.
This draft does not address stateful address autoconfiguration aspects
of IPv6.
This draft does not address the use of dynamic security associations
in the context of multicast traffic. Now that the multicast key man-
agement working group has been founded in the IETF, a question eventu-
ally arises whether or not the results of that work can be used to
protect the infrastructure multicast messages.
9. Acknowledgements
The author would like to thank Pekka Nikander, Markku Rossi, Tero
Kivinen, and Michael Richardson for interesting discussions in this
problem space.
10. References
[1] T. Narten, E. Nordmark, W. Simpson "Neighbor Discovery for IP Ver-
sion 6 (IPv6)" RFC 2461, IBM, Sun Microsystems, Daydreamer, December
1998.
[2] S. Thomson, T. Narten "IPv6 Stateless Address Autoconfiguration"
RFC 2462, Bellcore, IBM, December 1998.
[3] A. Conta, S. Deering "Internet Control Message Protocol (ICMPv6)
for the Internet Protocol Version 6 (IPv6) Specification" RFC 2463,
Lucent, Cisco Systems, December 1998.
[4] M. Crawford "Router Renumbering for IPv6" RFC 2894, Fermilab,
August 2000.
[5] J. McCann, S. Deering, J. Mogul "Path MTU Discovery for IP version
6" RFC 1981, Digital Equipment Corporation, Xerox PARC, August 1996.
[6] S. Kent, R. Atkinson "Security Architecture for the Internet Pro-
tocol" RFC 2401, BBN Corp, @Home Network, November 1998.
[7] D. Harkins and D. Carrel "The Internet Key Exchange", RFC 2409,
Cisco Systems, November 1998.
[8] J. Arkko, P. Nikander, T. Kivinen, M. Rossi "Manual SA Configura-
tion for IPv6 Link Local Messages", draft-arkko-manual-
icmpv6-sas-00.txt, Work In Progress, IETF, February 2001.
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11. Author's Address
Jari Arkko
Oy LM Ericsson Ab
02420 Jorvas
Finland
Phone: +358 40 5079256 (hand)
+358 9 2992480 (desk)
EMail: Jari.Arkko@ericsson.com
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