Network Working Group S. Hartman
Internet-Draft Painless Security
Intended status: Informational D. Zhang
Expires: January 12, 2012 Huawei Technologies co. ltd
G. Lebovitz
Juniper Networks, Inc.
July 11, 2011
Multicast Router Key Management Protocol (MaRK)
draft-hartman-karp-mrkmp-02
Abstract
Several routing protocols engage in one-to-many communication. In
order to authenticate these communications using symmetric
cryptography, a group key needs to be established. This
specification defines a group protocol for establishing and managing
such keys.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 12, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Relationship to IKEv2 . . . . . . . . . . . . . . . . . . 4
1.3. Relationship to GDOI . . . . . . . . . . . . . . . . . . . 5
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Types of Keys . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Key Encryption Key . . . . . . . . . . . . . . . . . . 6
2.1.2. Protocol Master Keys . . . . . . . . . . . . . . . . . 7
2.2. GCKS Election . . . . . . . . . . . . . . . . . . . . . . 8
2.3. Initial Exchange . . . . . . . . . . . . . . . . . . . . . 9
2.4. Group Join Exchange . . . . . . . . . . . . . . . . . . . 10
2.5. Group Key Management . . . . . . . . . . . . . . . . . . . 10
3. GKCS Election . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. A new GCKS is Elected . . . . . . . . . . . . . . . . . . 12
3.1.1. Parameters, Timers, and Events . . . . . . . . . . . . 12
3.1.2. Initial . . . . . . . . . . . . . . . . . . . . . . . 14
3.1.3. Validate . . . . . . . . . . . . . . . . . . . . . . . 15
3.1.4. GCKS2 . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1.5. GCKS . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.6. Member . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1.7. Follower . . . . . . . . . . . . . . . . . . . . . . . 18
3.2. Merging Partitioned Networks . . . . . . . . . . . . . . . 19
3.3. Operations on Receiving a Packet . . . . . . . . . . . . . 20
4. Key Download Payload . . . . . . . . . . . . . . . . . . . . . 21
5. Initial Exchange Details . . . . . . . . . . . . . . . . . . . 21
6. Group Management Unicast Exchanges . . . . . . . . . . . . . . 21
6.1. Group Join Exchange . . . . . . . . . . . . . . . . . . . 21
7. Group Key Management Operation . . . . . . . . . . . . . . . . 22
7.1. General operation . . . . . . . . . . . . . . . . . . . . 22
7.2. Out of Sequence Space . . . . . . . . . . . . . . . . . . 22
7.3. Changing the Active GCKS . . . . . . . . . . . . . . . . . 22
7.4. Reboot Cases . . . . . . . . . . . . . . . . . . . . . . . 22
8. Interface to Routing Protocol . . . . . . . . . . . . . . . . 23
8.1. Joining a Group . . . . . . . . . . . . . . . . . . . . . 23
8.2. Priority Adjustment . . . . . . . . . . . . . . . . . . . 23
8.3. Leaving a Group . . . . . . . . . . . . . . . . . . . . . 24
8.4. Out of Sequence Space . . . . . . . . . . . . . . . . . . 24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 24
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
11.1. Normative References . . . . . . . . . . . . . . . . . . . 25
11.2. Informative References . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
Many routing protocols such as OSPF [RFC2328] and IS-IS [RFC1142] use
a one-to-many or multicast model of communications. The same message
is sent to a number of recipients.
These protocols have cryptographic authentication mechanisms that use
a key shared among all members of a communicating group in order to
protect messages sent within that group. From a security standpoint,
all routers in a group are considered equal. Protecting against a
misbehaving router that is part of the group is out of scope for this
protocol.
Routers need to be provisioned with some credentials for a one-to-one
authentication protocol. Preshared keys or asymmetric keys and an
authorization list are expected to be common deployments.
The members of a group elect a Group Controller/Key Server (GCKS).
Potentially any member of the group may act as a GCKS. Since
protecting against misbehaving routers is out of scope, there is no
need to protect against an entity that is not currently the GCKS
impersonating the GCKS.
To prove membership in the group, a router authenticates using its
provisioned credentials to the current GCKS. If successful, the
router is given the current key material for the group. Group size
is relatively small and need for forced eviction of members is rare.
If a GCKS needs to evict a member, then it can simply re-authenticate
with the existing members and provide them new key material.
1.1. Terminology
GCKS (Group Controller/Key Server): a GCKS is a particular group
memeber which establishes security associations among other
authorized group members which it serves.
group: a group specified in this document is a set of routers, called
group members, which are located on a single broadcast domain/ link/
NBMA segment and use a one-to-many or multicast model of
communication.
1.2. Relationship to IKEv2
IKEv2 [RFC4306] provides a protocol for authenticating IPsec security
associations between two peers. It currently provides no group
keying. IKEv2 is attractive as a basis for this protocol because
while it is much simpler than IKE [RFC2409], it provides all the
needed flexibility in one-to-one authentication.
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Unlike IKE, IKEv2 is explicitly designed for IPsec. The document
does not separate handling of aspects of the protocol that would be
needed for IPsec from those that apply to general key management.
IPsec specific rules are combined with more general requirements.
While concepts and protocol payloads can be used in a different key
management protocol, the current structure of IKEv2 does not provide
a mechanism for applying IKEv2 to a domain of interpretation other
than IPsec. In addition, the complexity required in the IKE
specification when compared to IKEv2 suggests that the generality of
IKE may not be worth the complexity cost.
So this protocol borrows concepts and payloads from IKEv2 but does
not normatively depend on the IKEv2 specification.
1.3. Relationship to GDOI
[RFC3547] provides a protocol that is structurally very similar to
this one. As specified, IKE can be used to provide phase 1
authentication to a GCKS. After that, GDOI provides phase 2 messages
to establish key-encryption keys and traffic keys. Key management
operations can be accomplished via GDOI messages sent to the group
after the phase 2 exchange.
GDOI is defined for IKE not for IKEv2. In addition, GDOI's phase 2
uses its own hashing mechanism and nonce mechanism to provide
integrity protection and replay protection. Like IKE, GDOI has
significant complexity to support phase 2 identities that are
different than the phase 1 identity. GDOI requires a GCKS to have a
signature key used to sign GDOI messages. Since attacks caused by
members of the group masquerading as the GCKS are out of scope, this
is significant unnecessary complexity in the protocol.
So, this protocol can be thought of as a simplified GDOI based on
IKEv2 rather than IKE. However, integrity and replay mechanisms are
taken from IKEv2. Support for phase 2 identities is removed as
unneeded complexity. Security for the group key management messages
is provided using symmetric primitives rather than asymmetric
signatures. Phase 1 authentication will often still involve
asymmetric signatures.
2. Overview
2.1. Types of Keys
MaRK manipulates several different types of symmetric keys:
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PSK (Pre-Shared Key) : PSKs are pair-wise unique keys used for
authenticating one router to another during the initial exchange.
These keys are configured by some mechanism such as manual
configuration or a management application outside of the scope of
MaRK.
Peer key management key: Routers share a key with the GCKS that is
a result of the mark_init exchange.
KEK (Key Encryption Key): A KEK is a key used to encrypt group key
management messages to the current members of a group. A KEK is
learned as the product of establishing an MaRK association or
through a group key management message encrypted in a previous
KEK. A KEK has an explicit expiration but may also be retired by
a message encrypted in the KEK sent by the GCKS.
Protocol master key: A protocol master key is the key exported by
MaRK for use by a routing protocol such as OSPF or IS-IS. The
Protocol master key is the key that would be manually configured
if a routing protocol is used without key management.This key is
distinguished from the 'transport key' (see next) in that this
Protocol Master Key may be used in a cryptographic operation in
order to derive a specific transport key.
Transport key: A transport key is the key used to integrity
protect routing messages in a protocol such as IS-IS or OSPF. In
today's routing protocol cryptographic authentication mechanisms
the transport key is the same as the protocol master key. A
disadvantage of this approach is that replay prevention is
challenging with this design. Ideally some key derivation step
would be used to establish a fresh transport key among all the
participants in the group.
2.1.1. Key Encryption Key
When a router wishes to join a group, the router performs the
mark_init and mark_auth exchange with a GCKS. If the exchanges are
successful, the router can establish an association with a specific
group. Part of that association will be delivery of a KEK and
associated parameters.
Group key management messages are sent to a group address rather than
unicast to an individual peer. The authenticity, integrity and
confidentiality of group key management messages need to be protected
with the KEK.
As part of establishing the association, the router joining the group
is given an valid period( which is identified by a start time point
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and an expire time point) for the KEK. A group key management
message may establish a new KEK with new parameters.
From time to time, a GCKS may wish to either force early expiration
of a KEK or allow a KEK to expire. Protocol master keys are
permitted to be valid for somewhat longer than the KEK that created
them so as to avoid disrupting routing when this happens. When a KEK
is retired or expires without being replaced by a new KEK announced
in the old KEK, the group members delete that KEK. Unless local
policy configuration dictates otherwise, the group member will
perform a new initial exchange to the GCKS in order to establish a
new KEK. This solution is useful for enforcing "forward security" in
the cases where a router is no longer authorized to be part of the
group. That is, only valid group members can obtain the new KEK
while the ones which have leavn the group will be rejected.
Other mechanisms such as LKH (section 5.4 [RFC2627]) could be used to
permit removal of a group member while avoiding new initial
authentications. However these mechanisms come at a complexity cost
that is not justified for a small number of routers participating in
a single multicast link.
2.1.2. Protocol Master Keys
Current routing protocols directly use the protocol master key to
protect the integrity of messages. One advantage for this approach
is that the initial hello messages used for discovery and capability
exchange can be protected using the same mechanism as other messages.
Typically a sequence number is used for replay detection. Without
changing the key, the existing protocols are vulnerable to a number
of serious denial of service attacks from replays.
The MaRK can solve this replay problem by changing the protocol
master key whenever a peer is about to exhaust its sequence number
space or whenever a peer loses information about what sequence
numbers it used. This could potentially involve changing the
protocol master key whenever a router reboots that was part of the
group using the current protocol master key. Since key changes will
not disrupt active adjacencies and can be accomplished relatively
quickly, this is not expected to be a huge problem. Note that after
one key change, others routers can boot without causing additional
key changes; a flurry of key changes would not be required if several
routers reboot near each other.
Another approach would be to separate the protocol master key from
the transport keys. For example the transport key used by a given
peer could be a fresh key derived from the protocol master key and
nonces announced by that peer. Some secure mechanism would be
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provisioned to enable one to confirm that the peer's announcement of
its nonce was fresh and authentic; this mechanism would almost
certainly involve some form of interaction with the router wishing to
guarantee freshness in order to resistant, e.g., replay attacks.
There are two key advantages of this separation between transport
keys and protocol master keys. The first is that the interaction
between the MaRK and routing protocol can be simplified
significantly. The second is that even when manually configured
protocol master keys are used, replay and adequate DOS protection can
be achieved.
A simple compare between the keys described in this section is
provided in the following table.
+-----------+------------+-------------+---------+----------------------+
| Keys |KMP usage |Bootstrapping| Group vs| Other |
| |vs. RP usage|vs. Traffic | Pair-Wis| |
+-----------+------------+-------------+---------+----------------------+
|Pre-Shared |KMP usage |Bootstrapping|Pair-Wise|Distributed in an out-|
|Keys | | | |of-band way |
+-----------+------------+-------------+---------+----------------------+
|Key |KMP usage |Bootstrapping|Group |For GCKS to |
|Encryption | | | |distribute protocol |
|Key | | | |master keys |
+-----------+------------+-------------+---------+----------------------+
|Protocol |KMP usage |Bootstrapping|Group |Used by group |
|Master Key |or Both |or Both | |members to secure |
| | | | |routng packets or |
| | | | |generate traffic keys |
+-----------+------------+-------------+---------+----------------------+
|Transport |RP usage |Traffic |Group |Used by group |
|Key | | | |members to secure |
| | | | |routing packets |
+-----------+------------+-------------+---------+----------------------+
2.2. GCKS Election
Before a MaRK system actually starts working, the routers in the
multicast group need to select a GCKS so that they can obtain
cryptographic keys to secure subsequent exchanges of routing
information. MaRK specifies an election protocol that dynamically
assigns the responsibility of key management to one of the group
members. Note that there are already announcer-electing mechanisms
provided in some routing protocols (e.g., OSPF and IS-IS). However,
much involvement between a MaRK system and a routing protocol
implementation will be introduced if the MaRK system reuses the
announcer-electing mechanism for the election of the GCKS. The state
machine of the routing protocol also has to be modified. For
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instance, in OSPF, after a DR has been elected, routers need to halt
their OSPF executions, and carry out the initial exchange to
authenticate the DR and collect the keys for subsequent
communications. After this step, the routers need to re-start their
OSPF state machines so as to exchange routing information. As a
consequence of such cases, an individual GCKS electing solution
within MaRK is preferable.
Each router has a GCKS priority. Higher priorities are more
preferred GCKSes. As discussed in Section 8, the routing protocol
can influence the GCKS election protocol by manipulating the priority
so that it is likely that the same router will be the announcer for
the routing protocol and the GCKS. Even if two different routers are
elected as the announcer and GCKS, then the routing protocol and MaRK
will function correctly.
A key design goal of the election protocol is to maximize the chance
that some router permitted to take on the role of GCKS will be
elected to that role even when attackers are injecting messages into
the election process. The election process can be attacked to cause
a router other than the most preferred router to be elected.
2.3. Initial Exchange
The initial exchange is based on IKEv2's IKE_SA_INIT and IKE_SA_AUTH
exchanges. During this exchange, an initiating router attempts to
authenticate to the router it believes is a GCKS for a group that the
initiating router wants to join. Messages are unicast from the
initiator to the responding GCKS. Unicast MaRK messages form a
request/response protocol; the party sending the messages is
responsible for retransmissions.
The initial exchange provides capability negotiation, specifically
including supported cryptographic suites for the key management
protocol. Identification of the initiator and responder is also
exchanged. A symmetric key is established to protect integrity,
confidenality and authenticity of key management messages. While
routing security does not typically require confidentiality, the key
management protocol does because keys are exchanged and these must be
protected.
Then the identities of each party are cryptographically verified.
This can be done using, e.g., a preshared key, asymmetric keys or
self-signing certificates. Other mechanisms may be added as a future
extension.
The authentication exchange also provides an opportunity to join a
group as part of the initial exchange. In the typical case, a router
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can obtain the needed key material for a group in two round-trips.
2.4. Group Join Exchange
The primary purpose of the unicast MaRK messages is to get an
initiator the information it needs to join a group and participate in
a routing protocol. The initiator can contact a GCKS to apply to
join a group that the GCKS manages. In the case a GCKS manages
multiple groups concurrently, the initiator can additionally provide
a group identifier to indicate which particular group it intends to
join.
The responder performs several checks. First, the responder confirms
that the responder is currently acting as GCKS for the group in
question. Then, the responder confirms that the initiator is
permitted to join the group. If these checks pass, then the
responder provides a key download payload to the initiator encrypted
in the peer key management key. As discussed in Section 2.1.2, the
GCKS MUST change the protocol master key if a router was part of the
group under the current protocol master key and reboots. In this
case, the GCKS SHOULD provide the new and old protocol master key to
the initiator, setting the validity times for the old key to permit
reception but not transmission. The GCKS MUST use the mechanism in
the next section to flood the new key to the rest of the group.
A group association created by this exchange may last beyond the
unicast MaRK association used to create it. Once membership in a
group is established, resources are not required to maintain the
unicast association with the GCKS.
2.5. Group Key Management
The GCKS shares a KEK with all members of a group. The GCKS can send
a multicast message to the group to update the set of protocol master
keys, update the KEK, or retire the KEK and request new group join
exchanges.
Typically the protocol master key is changed only when needed to
provide replay protection or when the KEK changes. The KEK changes
whenever a new GCKS is elected or whenever it is administratively
desirable to change the keys. For example if an employee leaves an
organization it might be desirable to change the KEKs. A KEK is
retired whenever forward security is desired: whenever the
authorization of who is permitted to be in a group changes and the
GCKS needs to make sure that the router is no longer participating.
Most authorization changes such as removing a router from service do
not require forward security in practical deployments.
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3. GKCS Election
The GCKS election process selects a single router to act as GCKS for
a group.Similar with other popular announcer electing mechanisms
(e.g., VRRP, HSRP), in MaRK, only GCKSes use multicast to
periodically send Advertisement messages. Such advertisements can be
used as heart beat packets to indicate the aliveness of GCKSes. In
addition, a state machine with six states (Initial, Validate, GCKS,
GCKS2, Follower, and Member) is specified for GCKS election. When a
router is initially connected to a multicast network, its state is
set as Initial. The router then sends a multicast initial
advertisement. If a GCKS is working on the network, it will reply to
the router with an advertisement. After receiving the advertisement
from the GCKS, the router will try to register with the GCKS using
the initial exchange. Typically this registration will succeed, and
the state of the router is transferred to Member. After a certain
period, if the router still does not receive any advertisement from a
GCKS or other group members, the router then believes there is no
other group member on the network and sets its state as GCKS. If
during the period the router does not receive any advertisement from
a GCKS but receives advertisements from other more preferred routers
on the network, the router believes that the group is involved in a
GCKS election process. The router then puts these routers into its
candidate list. When the timer to end the Initial state expires, the
router tries to authenticate the most preferred router in the
candidate list and validate whether it can be a GCKS. If the
validation result is possitive, the router then transfer its state to
Member, and the router being validated transfers its state to GCKS.
In the absence of attacks, this process functions similar to
designated router election protocols in existing routing protocols.
Because the election process happens before group keys are
established, the initial election process is not integrity-protected.
An attacker can inject fake GCKS announcements or initial
announcements from fake routers that are more preferred than any
router actually in the group. Such attacks can create a denial of
service situation. If the election process does not converge within
the expected time, or if an authentication attempt fails, then the
group is probably under attack. A new state called GCKS2 is
introduced. A router permitted to be the GCKS can enter the GCKS2
state after failing to validate a received announcement in the
expected time. GCKS2 is used to increase the convergence speed while
the system is under attack. If an initial router receives a GCKS2
announcement, the initial router can authenticate and validate the
sender, and transfer its own state to Follower, similar to how it
would respond to a GCKS announcement. GCKS2 routers attempt to
validate each other and to use the resulting security keys to
establish a router to act as GCKS. The GCKS2 state does not generate
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protocol master keys: until the election result in a GCKS only keying
material needed for the election is produced. In the subsequent
election, the router will wait for the election results from its
GCKS2 router until its GCKS2 end timer expires. In this way, the
authenticated entities generate a tree structure and avoid generating
large amount of keks and protocol master keys when a adversary keeps
sending fake GCKS announcements to distrupt election.
Apart from the initialization of a multicast network, the fail-over
of a GCKS can also trigger an election process. For instance, if a
router does not receive the heart beat advertisement for a certain
period, it will transfer its state to Initial and try to elect a new
one. In a GCKS electing process, a router has to stay in the Initial
state until a new GCKS is allocated. Particularly, the router first
sends its initial advertisement with its priority and waits for a
certain period. During the period, if a router receives an initial
advertisement which consists of a lower priority, the router then
sends the advertisement again with a limited rate. After period, if
the router does not find any router with a higher priority, it
announces itself as the GCKS. If two routers have the same priority,
the one with the lowest IP source address used for messages on the
link will be the GCKS. After a router transfers its state to GCKS,
it will reply to the initial advertisements from other routers with
GCKS advertisements, even when the initial advertisements consist of
higher priorities than its priority. This approach guarantees that a
GCKS will not be changed frequently after it has been elected. After
receiving the GCKS advertisement of the new elected GCKS, other
routers transfer their states to Member. However, if a GCKS G1
receives a GCKS advertisement from another router G2 and G2 is a more
preferred GCKS, G1 follows the procedure in Section 3.2.
If a node in state member fails to perform an initial exchange with
the router it believes to be GCKS, it resets its state to initial but
ignores advertisements from that router. This way an attacker cannot
disrupt communications indefinitely by masquerading as a GCKS.
3.1. A new GCKS is Elected
This section is a detailed description of the election process.
In the following discussion, the packets are identified by all upper
case characters.
3.1.1. Parameters, Timers, and Events
Before going into detailed discussion, several parameters are
introduced:
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o Initial_Anno_Interval, which is the time interval between
INITIAL_ANNOUNCEMENTS ).
o Initial_End_Interval, which is the time interval to transfer the
state of a router from Initial to GCKS/Validate if it does not
receive any GCKS or GCKS2 announcement on the link ).
o Validate_End_Interval, which is the time interval for a router to
transfer its state from Validate to GCKS2 if it does not find any
other more preferred router ).
o GCKS_Down_Interval, which is the time interval for a Member router
to declare a GCKS router is down ).
o GCKS2_Down_Interval, which is the time interval for a Follower
router to declare a GCKS2 router is down ).
o GCKS2_End_Interval, which is the time interval for a router to
transfer its state from GCKS2 to GCKS if it does not find any
other more preferred router ).
o GCKS_Anno_Interval, which is the time interval between
GCKS_ANNOUNCEMENTS ).
o GCKS2_Anno_Interval, which is the time interval between
GCKS2_ANNOUNCEMENTS ).
Correspondingly, each router in MaRK has several timers,
Initial_Anno_Timer, Initial_End_Timer, Validate_End_Timer,
GCKS_Down_Timer, GCKS2_Down_Timer, GCKS2_End_Timer, GCKS_Anno_Timer,
GCKS2_Anno_Timer. Initial_Anno_Timer fires to trigger sending of an
INITIAL_ANNOUNCEMENT based on Initial_Announcement_Interval.
Initial_End_Timer fires to trigger the transition of a router state
from Initial to some other state. Validate_End_Timer fires to
trigger the transition of a router state from Validate to GCKS2.
GCKS_Down_Timer fires when no GCKS_ANNOUNCEMENT has been heard for
GCKS_Down_Interval. GCKS2_Down_Timer fires when no
GCKS2_ANNOUNCEMENT has not been heard for GCKS2_Down_Interval.
GCKS2_End_Timer fires to trigger the transition of the state of a
router from GCKS2 to GCKS. GCKS_Anno_Timer fires to trigger sending
of a GCKS_ANNOUNCEMENT based on GCKS_Announcement_Interval.
GCKS2_Anno_Timer fires to trigger sending of a GCKS2_ANNOUNCEMENT
based on GCKS2_Anno_Interval.
During an election process, a MaRK router may have to deal with
following types of events:
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o X_Anno_Received: an X_ANNOUNCEMENT is received.
o Requester_Validated: have authenticated and validated against a
some router who believes we should be a GCKS or GCKS2.
o GCKS_Validated: a remote entity has been authenticated and
validated to be a GCKS router.
o GCKS2_Validated: a remote entity has been authenticated and
validated to be a GCKS2 router.
o Referral_Validated: have authenticated and validated against a
candidate who is not a GCKS router but knows one is .
o Referral2_Validated: have authenticated and validated against a
candidate who knows a GCKS2 router.
o Authentication/Validation_Failed: the remote entity fails in the
authentication or cannot be either a GCKS/GCKS2 or a referral.
o X_Timer_Expired: the timer of type X expired.
o KEK_Expired: we have no valid KEK.
3.1.2. Initial
The timers utilized in this state are Initial_Anno_Timer and
Initial_End_Timer.
On entry:
o Send an INITIAL_ANNOUNCEMENT.
o Set the Initial_Anno_Timer with Initial_Anno_Interval.
o Set the Initial_End_Timer with Initial_End_Interval.
Events:
o Initial_Anno_Timer_Expired: send an INITIAL_ANNOUNCEMENT and reset
the Initial_Anno_Timer.
o Initial_Anno_Received: if the sender of the announcement is more
preferred, add the entity into the candidate list; if less
preferred, send an INITIAL_ANNOUNCEMENT with a limited rate.
o GCKS_Anno_Received: add the sender of the announcement to the
candidate list; set the the Validate_End_Timer with the remaining
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period of Initial_End_Interval; transfer to validate.
o GCKS2_Anno_Received: add the sender of the announcement to
candidate list; set the Validate_End_Timer with the remaining
period of Initial_End_Interval; transfer to validate.
o Requester_Validated: If the requester is looking for a GCKS router
and the local policy permits, transfer the state to GCKS2 setting
GCKS2_End_Interval to time remaining on Initial_End_timer.
o Initial_End_Timer_Expired: if there are candidates, transfer the
state to Validate. If there is no entry in the candidate list,
transfer to GCKS.
3.1.3. Validate
The timer utilized in this state is Validate_End_Timer.
Entering this state means that we have a router we believe should be
GCKS. The purpose of this state is to confirm that e can establish a
security association with that router and that router's policy
permits it to be a GCKS for this group. The two normal paths through
the state machine are Initial leading to GCKS for the most preferred
router and Initial leading to Validate leading to Member for other
routers.
On entry:
o Authenticate and validate the most preferred entry in the
candidate list.
o If Validate_End_timer has more time than Validate_end_Interval,
set Validate_End_timer to Validate_End_interval.
Events:
o GCKS_Validated: transfer the state to Member.
o GCKS2_Validated: Transfer the state to Follower.
o Referral_Validated: perform the authentication/validation on the
recommended node; move the referring from the candidate list to
the black list for Blacklist_Interval.
o Referral2_Validated: perform the authentication/validation on the
recommended node; move the referring node from the candidate list
to the black list for Blacklist_Interval.
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o Requester_Validated: If the requester is looking for a GCKS/GCKS2
router and the local policy permits, transfer the state to GCKS2.
o Validation_Failed: move the router being validated from the
candidate list to black list for Blacklist_interval.
o Initial_Anno_Received: if the sender of the announcement is more
preferred, add the router into the candidate list; if less
preferred, send an INITIAL_ANNOUNCEMENT with a limited rate.
o GCKS_Anno_Received: add the router sending the announcement into
the candidate list and perform authentication against that entity.
o GCKS2_Anno_Received: add the router sending the announcement into
the candidate list and start the authentication/validation against
that entity.
o Validate_End_Timer_Expired: transfer the state to GCKS2.
3.1.4. GCKS2
The timers utilized in this state include GCKS2_Anno_Timer and
GCKS2_End_Timer.
This state is not expected to be used in normal operation. This
state indicates there has been some authentication/validation problem
or another node is behaving in a manner inconsistent with the
election state. The purpose of this state is to establish sufficient
security keys to integrity protect the election process. It is
possible during normal operation to send a brief time in this state
if the router being elected GCKS gets an authentication request
before Initial_End_timer expires.
On entry:
o Send an GCSK2_ANNOUNCEMENT.
o Set the GCKS2_Anno_Timer with GCKS2_Anno_Interval.
o Set the the GCKS2_End_Timer with GCKS2_End_Interval unless it was
set on entry transferring from Initial.
Events:
o GCKS_Anno_Received: add to candidate list; start authentication/
validation.
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o GCKS2_Anno_Received: if more preferred, add to candidate list,
start authentication/validation. If less preferred, send
GCKS2_ANNOUNCEMENT if rate limiting is permitted.
o GCKS_Validated: Transfer to member state; flood KEK to the
associated followers.
o GCKS2_Validated: Transfer the state to Follower; flood KEK to the
associated followers.
o Referral_Validated: Perform authentication and validation on the
recommended node; move the referring node from the candidate list
to the black list for Blacklist_Interval.
o Referral2_Validated: if the recommended GCKS2 is more preferred,
perform authentication and validation on the recommended node;
move the referring from the candidate list to the black list for
Blacklist_Interval.
o Requester_Validated: if the requester is looking for a GCKS2,
distribute kek.
o Validation_Failed: move the router being validated from the
candidate list to black list for Blacklist_interval.
o GCKS2_End_Timer_Expired: transition the state to GCKS.
o GCKS2_Anno_Timer_Expired: send a GCKS2_ANNOUNCEMENT.
3.1.5. GCKS
The timer utilized in this state is GCKS_Anno_Timer.
On entry:
o Senda GCKS_ANNOUNCEMENT.
o Set the GCKS_Anno_Timer with GCKS_Anno_Interval.
o Generate protocol keys; if needed, generate KEK.
Events:
o GCKS_Anno_Timer_Expired: send a GCKS_ANNOUNCEMENT.
o Initial_Anno_Received: send an GCKS_ANNOUNCEMENT immediately if
the rate limiting is permitted.
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o GCKS2_Anno_Received: send an GCKS_ANNOUNCEMENT immediately if the
rate limiting is permitted.
o GCKS_Anno_Received: if the sender is more preferred, add to
candidate list and start authentication/validation; Otherwise,
send an GCKS_ANNOUNCEMENT immediately if the rate limiting is
permitted.
o GCKS_Validated: start network merging operations as what is
illustrated in Section 3.2.
o Requester_Validated: If the requester is looking for a GCKS
router, distribute KEK and protocol master keys; if the requester
is another GCKS, start network merging operations as what is
illustrated in Section 3.2.
3.1.6. Member
The timer utilized in this state is GCKS_Down_Timer.
On entry:
o Set the GCKS_Down_Timer with GCKS_Down_Interval.
Events:
o GCKS_Down_Timer_Expired: Transfer the state into Initial.
o GCKS_Anno_Received: reset GCKS_Down_Timer.
o Requester_Validated: if the requester is legal, recommend the GCKS
router to it.
3.1.7. Follower
The timer utilized in this state is GCKS2_Down_Timer.
On entry:
o Set the GCKS2_Down_Timer with GCKS2_Down_Interval.
Events:
o GCKS2_Down_Timer_Expired: Transfer the state into Initial.
o GCKS2_Anno_Received: reset GCKS2_Down_Timer.
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o GCKS_Anno_Received: Add the announcer to the candidate list and
start validation.
o Requester_Validated: if the requester is legal, recommend the
GCKS2 router to it.
o GCKS_Validated: Transfer the state to member.
The following diagram illustrates the rules of transiting the states
introduced this section.
+---------------------------------------------+
| +-----------+ |
| +---->| | |
| | | Follower |--+ |
| | +--| | | |
| | | +-----------+ | |
+----------+ | | +-----------+ | +----------+ |
| |-+ +->| | +->| |<-+
| Validate |<----->| Initial |<----| Member |
| | +->| |<-+ | |<-+
+----------+ | +-----------+ | +----------+ |
| | | +----------+ |
| | +->| | |
| | +-----------+ | GCKS | |
| +->| |---->| | |
| | GCKS2 | +----------+ |
+------------>| |-------------------+
+-----------+
3.2. Merging Partitioned Networks
Whenever a GCKS finds that a more preferred router is also acting as
a GCKS for the same group, then the group is partitioned. Typically
if there is already an active GCKS for a group, even if a more
preferred GCKS joins, the GCKS will not change. Two situations can
result in multiple GCKSes active for a group. The first is that
members of the group do not share common authentication credentials.
The second is that the group was previously partitioned so that some
nodes could not see election messages from other nodes. After the
problem resulting in the partition is fixed, then both active GCKSes
will see each others election announcements. The group needs to
merge.
The less preferred GCKS performs a unicast mark_merge_sa unicast key
management message to the more preferred GCKS. In this message the
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less preferred GCKS includes its key download payload, so the more
preferred GCKS learns the protocol master keys of the less preferred
GCKS.
The more preferred GCKS generates a new key download payload
including a KEK and the union of all the protocol master keys. The
GCKS SHOULD mark the existing protocol master keys as expiring for
usage in transmitted packets in a relatively short time. The GCKS
SHOULD introduce a new protocol master key. This key download
payload is returned to the less preferred GCKS and is sent out in the
current KEK using a group key management message.
The less preferred GCKS sends the received key download payload
encrypted in its existing KEK. XXX how many retransmits. After all
retransmissions of this payload the less preferred GCKS sets its
state to member.
As a result of this procedure, members learn the protocol master keys
of both GCKSes and converge on a single KEK and GCKS. Changing the
protocol master keys during a merge is important for protocols that
use the protocol master key as a transport key. The new GCKS does
not know which routers have joined the group with the other GCKS.
Therefore, it could not correctly detect one of these routers
rebooting and change the protocol master key at that point. If the
key is changed as part of the merge, replays are handled.
3.3. Operations on Receiving a Packet
When a router attempts to join an election process, it may have a
valid kek. For instance, when a GCKS cannot work properly, the
routers on the link need to transfer their state to Initial and raise
an election to find a new valid GCKS. If there is Still a valid KEK
shared by the router, they can use the KEK to secure the packets
transmitted during the election until a new KEK is distributed by the
new GCKS. A router holding the valid KEK is regarded to be more
preferred than a router which doesn't have the key. By using the
kek, it is able to prevent an attacker from disturbing the election
process by broadcasting fake announcements. Therefore, after an
initial router does not find any more preferred router holding the
valid key, it then can transfer its state to GCKS directly.
Therefore, the operations on receiving a packet are as follows:
o Check the blacklist. If the sender of the packet is on the
blacklist, discard the packet.
o If the state is GCKS, accept the packet and generate an event.
GCKS announcements need to be excepted in GCKS state for merges to
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work.
o If there is a KEK that is not expired, check the packet integrity
against any matching KEK.
o If no KEK matches or if the integrity fails to validate, discard
the packet.
o If there is no KEK at all or the KEK integrity check passed,
process the packet and generate an event.
It is notable this approach limits the scope of the election within
the routers managed by the failed GCKS. If there are routers newly
accessing the link during the election, no router with a KEK will
process their packets. However these routers can process packets
from routers with the KEK. In many cases one of the routers with a
KEK will be elected GCKS and the other routers can authenticate and
join. In the worst case, two independent GCKSes will be elected and
then merge.
4. Key Download Payload
What all is actually in the message you get at the end of phase 2
exchange (the mark_auth Exchange) and that is sent out periodically
during group key management.
For the KEK, this needs to include the key itself, the algorithm
(presumably drawn from the IKEv2 symmetric algorithms), key ID, group
ID transmit start time, receive start time, and expire time.
The protocol master keys include the key, an algorithm ID, the key ID
and thelifetimes.
5. Initial Exchange Details
6. Group Management Unicast Exchanges
6.1. Group Join Exchange
If a router receives a group join exchange for a group for which it
is not the GCKS, it MUST return a notification. If it knows the GCKS
for the group then it returns MaRK_WRONG_GCKS including the address
of the GCKS or GCKS2 in the notification payload along with an
indication of whether the router is a GCKS or GCKS2. The initiator
tries the group join exchange (probably with a new initial exchange)
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with the indicated router. If the responder does not know the GCKS
for the group, either because it is not a member of the group or
because its GCKS election state is initial, it returns the
MaRK_GCKS_UNKNOWN notification.
7. Group Key Management Operation
7.1. General operation
Periodically the GCKS will send out an update message encrypted in
the current KEK including the current group key download payload and
parameters. If a new KEK is about to be valid for receiving
messages, this is included. Any protocol master keys that are valid
for sending or receiving SHOULD be included.
If a previous KEK is still valid for sending, then an update message
is sent encrypted in the old KEK. This message MUST include the new
KEK. This message SHOULD include the protocol master keys.
7.2. Out of Sequence Space
A member of a group can also use the unicast exchange to request a
GCKS to change a protocol master key, on the occassions, for example,
where the member is going to exhaust its sequence space of the
associated routing protocol. For protocols where the protocol master
key is the same as the transport key, it is critical that no two
messages be sent by the same router with the same sequence number and
protocol master key. The sequence number space is finite. So if a
router is running low on available sequence space it needs to request
a new protocol master key be generated.
7.3. Changing the Active GCKS
When a GCKS finds a more preferred router accouncing itself as a
GCKS, it will forward its privilege to another one in the following
conditions. The operations are introduced in Section 3.2.
When a GCKS cannot work properly, it will just stop sending the
GCKS_ANNOUNCEMENT. Then after a certain time period, a new GCKS
election process will be raised.
7.4. Reboot Cases
After a reboot, a router in a group will lost the state information
about the group (e.g., protocol master keys, traffic keys, the
sequence numbers used by GCKS). Therefore, the router needs to find
and authenticate the GCKS, and apply to join the group. If the GCKS
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finds that the router is already a group member, the GCKS will update
the transport keys (and the protocol master keys if necessary) used
in the group first in order to avoid inter-session replay attacks.
8. Interface to Routing Protocol
This section describes signaling between MaRK and the routing
protocol. The primary communication between these protocols is that
MaRK populates rows in the key table making protocol master keys
available to the routing protocol. However additional signaling is
also required from the routing protocol to MaRK. This section
discusses that signaling. All required communication from MaRK to
the routing protocol can be accomplished by manipulating the key
table. However an implementation MAY wish to signal MaRK failures to
the routing protocol in order to provide consistent management
feedback.
8.1. Joining a Group
When a routing protocol instance wishes to begin communicating on a
multicast group, it signals a group join event to MaRK. This event
includes the identity of the group as well as this router's priority
for being a GCKS for the group. When MaRK receives this event, it
starts MaRK for this group and attempts to find a GCKS.
8.2. Priority Adjustment
It is desirable that the GCKS function track the functions within a
routing protocol. For example for protocols such as OSPF that
designate a router on a link to manage adjacencies for that link, it
would be desirable for the GCKS role to be assigned to that router.
The routing protocol provides a priority input to the GCKS election
process. Initially the routing protocol should map any priority
mechanism within the routing protocol to the GCKS election procedure
so that routers favored as announcer for a link will also be favored
as a GCKS.
However, the routing protocol SHOULD also dynamically manipulate the
GCKS election priority based on what happens within the routing
protocol. The router actually elected as the announcer SHOULD have a
GCKS election priority higher than any other group member.
Typically, by the time the routing protocol is able to elect an
announcer, a GCKS will already be chosen. However, if a GCKS
election is triggered when the routing protocol is already
operational, then the election can choose the routing protocol's
announcer.
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8.3. Leaving a Group
If a routing protocol terminates on an interface, MaRK implementation
on the router needs to be notified that group is no longer joined.
MaRK MUST stop participating in the GCKS election process, stop
monitoring for key management messages and if the current router is a
GCKS, stop acting in that role.
8.4. Out of Sequence Space
If a routing protocol is running out its sequence space, the MaRK
implementation on the router needs to be notified. The MaRK
implementation then needs to contact the GCKS to request the update
of the transport keys (and the protocol master keys if necessary).
9. Security Considerations
An attacker who can suppress packets sent to the group can create a
denial of service condition. One attack is to suppress GCKS election
packets and cause two routers to believe they are both the GCKS for
the group. If the least preferred router never hears the GCKS
advertisement from the more preferred router, then the group will
remain partitioned. Such an attacker is likely to be able to mount
more direct denial of service, for example suppressing the actual
routing protocol packets.
The election protocol has been designed to try and resist denial of
service conditions. However, the election protocol maintains state
in the form of a candidate list and black list. An attacker can
consume state by generating fake election announcements. An
implementation can discard state if it has insufficient resources.
However, if legitimate routers are discarded from the candidate list,
the protocol may take longer to converge or may not converge. If
entries are removed from the black list, then more resources may be
spent on attackers. So the solution has some residual denial of
service possibilities. The election protocol requires significant
analysis to confirm it meets its design goals.
The security of the election protocol depends on the denial of
service resistance of the authentication protocol. It is important
that an attacker not be able to cause an authentication to fail by
injecting a packet. So, rather than failing an authentication if a
bad packet is received, an implementation needs to wait and see if a
good packet appears in some timeout.
The security of the system as a whole depends on the pair-wise
security between the router currently in the GCKS role and the other
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routers in the group. Since any router can potentially act as GCKS,
the pair-wise security between all members of the group is critical
to the security of the system. In practical deployments, information
used by the router acting as GCKS to authorize a member joining the
group will be configured by some management application. In these
deployments, the security of the system depends on the management
application correctly maintaining this information on all routers
potentially in the group.
10. Acknowledgements
The funding for Sam Hartman's work on this document is provided by
Huawei.
XXX add the list of people in the lunch time group unless they are
willing to be listed as authors.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
11.2. Informative References
[RFC1142] Oran, D., "OSI IS-IS Intra-domain Routing Protocol",
RFC 1142, February 1990.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC2627] Wallner, D., Harder, E., and R. Agee, "Key Management for
Multicast: Issues and Architectures", RFC 2627, June 1999.
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Authors' Addresses
Sam Hartman
Painless Security
Email: hartmans-ietf@mit.edu
Dacheng Zhang
Huawei Technologies co. ltd
Huawei Building No.3 Xinxi Rd., Shang-Di Information Industrial Base Hai-Dian District, Beijing
China
Email: zhangdacheng@huawei.com
Gregory Lebovitz
Juniper Networks, Inc.
1194 North Mathilda Ave.
Sunnyvale, California 94089-1206
USA
Email: gregory.ietf@gmail.com
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