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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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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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