Internet DRAFT - draft-atwood-karp-akam-rp
draft-atwood-karp-akam-rp
KARP W. Atwood
Internet-Draft R. Bangalore Somanatha
Intended status: Standards Track Concordia University/CSE
Expires: August 29, 2013 February 25, 2013
Automatic Key and Adjacency Management for Routing Protocols
draft-atwood-karp-akam-rp-03
Abstract
When tightening the security of the core routing infrastructure, two
steps are necessary. The first is to secure the routing protocols'
packets on the wire. The second is to ensure that the keying
material for the routing protocol exchanges is distributed only to
the appropriate routers. This document specifies requirements on
that distribution and proposes the use of a set of protocols to
achieve those requirements.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 29, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Keying Groups (Key Scopes) . . . . . . . . . . . . . . . . . . 4
2.1. Keying Groups . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Key Scopes . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Security Goals . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Non-security Goals . . . . . . . . . . . . . . . . . . . . 6
4. High Level Design . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Global View . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Entities in the system . . . . . . . . . . . . . . . . . . 7
4.3. Protocol Operations . . . . . . . . . . . . . . . . . . . 9
5. Detailed Design . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. System Design . . . . . . . . . . . . . . . . . . . . . . 11
5.1.1. Communication among the Entities . . . . . . . . . . . 11
5.1.2. Inner View of a GM . . . . . . . . . . . . . . . . . . 13
5.1.3. Hierarchical Design . . . . . . . . . . . . . . . . . 14
5.2. Protocol Design . . . . . . . . . . . . . . . . . . . . . 14
5.2.1. Step 1 - Initial Exchanges: GCKS, GM mutual
authentication . . . . . . . . . . . . . . . . . . . . 15
5.2.2. Step 2 - Key Management Message Exchanges between
GCKS, GM . . . . . . . . . . . . . . . . . . . . . . . 16
5.2.3. Step 3 - GM-GM mutual authentication . . . . . . . . . 19
5.2.4. Step 4 - Key Management Message Exchanges between
GMs . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2.5. Variations for handling other Keying Groups . . . . . 22
6. Other Aspects of the Key Management Problem . . . . . . . . . 24
6.1. Key Updates . . . . . . . . . . . . . . . . . . . . . . . 24
6.2. Regular Key Updates . . . . . . . . . . . . . . . . . . . 26
6.2.1. Same key for the entire AD . . . . . . . . . . . . . . 26
6.2.2. Key per link . . . . . . . . . . . . . . . . . . . . . 26
6.2.3. Key per sending router . . . . . . . . . . . . . . . . 27
6.2.4. Key per sending router per interface . . . . . . . . . 27
6.2.5. Key per peer . . . . . . . . . . . . . . . . . . . . . 27
6.3. Router Installation/ Uninstallation . . . . . . . . . . . 27
6.3.1. Same key for the entire AD . . . . . . . . . . . . . . 28
6.3.2. Key per link . . . . . . . . . . . . . . . . . . . . . 28
6.3.3. Key per sending router . . . . . . . . . . . . . . . . 29
6.3.4. Key per sending router per interface . . . . . . . . . 29
6.3.5. Key per peer . . . . . . . . . . . . . . . . . . . . . 29
6.4. Router Reboots . . . . . . . . . . . . . . . . . . . . . . 29
6.5. Scalability . . . . . . . . . . . . . . . . . . . . . . . 32
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6.6. Option to Turn Off Adjacency Management . . . . . . . . . 33
6.7. Incremental Deployment . . . . . . . . . . . . . . . . . . 34
6.8. Smooth Key Rollover . . . . . . . . . . . . . . . . . . . 34
6.9. Eliminating Single Point of Failure . . . . . . . . . . . 35
7. An Alternate Mechanism for Transporting the Messages . . . . . 35
8. Detailed Packet Formats . . . . . . . . . . . . . . . . . . . 35
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
11. Change History (RFC Editor: Delete Before Publishing) . . . . 36
12. Needs Work in Next Draft (RFC Editor: Delete Before
Publishing) . . . . . . . . . . . . . . . . . . . . . . . . . 36
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
13.1. Normative References . . . . . . . . . . . . . . . . . . . 37
13.2. Informative References . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
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1. Introduction
Within the Keying and Authentication for Routing Protocols working
group, there are several goals:
o Determining how to update the security of existing routing
protocols, and guiding this work;
o Development of automated mechanisms for management of the keying
material.
Within the second goal, protocols and procedures for creating shared
keys for specific environments have been developed
[I-D.hartman-karp-mrkmp][I-D.mahesh-karp-rkmp][I-D.tran-karp-mrmp],
under the assumption that the end points of the exchanges (the
routers) are entitled to enter into the conversation, i.e., that they
can prove that they are who they say they are. However, these
documents provide no mechanism to assess or ensure that the end
points are entitled to be neighbors.
In addition, requirements for an operations and management model are
specified in [I-D.ietf-karp-ops-model].
This document addresses this issue of policy distribution for
automatic key management and adjacency management in secure routing
protocols. In particular, it addresses the need to ensure that
keying material is distributed only to routers that legitimately form
part of the "neighbor set" of a particular speaking router.
1.1. Terminology
Autonomous System ...
Administrative Domain ...
Traffic Encryption Key (TEK) ...
2. Keying Groups (Key Scopes)
2.1. Keying Groups
In an AD, all routers having the same TEK can be referred to as
forming a 'keying group'. We can have routers forming a 'keying
group' as follows:
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A group per AD - This is the most coarsely grained category of
keying group where all routers in an AD share the same traffic
key. Hence the incoming and outgoing keys for protecting
control traffic on all routers are the same. This is the case
typically in usage today with manual keying.
A group per link - Here, all routers sharing a link share the key
for that link. The routers could have different keys on their
different interfaces, and share them with the other routers
connected to those respective links.
A group per sending router - This category is more finely grained
compared to the previous two cases; each router uses a
different key to secure its outgoing control traffic.
A group per sending router per interface - This is the most finely
grained category wherein each router has a different key for
each of its interfaces, which in turn is different from the
keys used by other routers to secure their outgoing traffic.
A group per peer router - This category is strictly for unicast
communication wherein peer routers share keys for their
interaction. There is one outgoing key corresponding to each
router in every pair of routers. These keys can be
established through a unicast key management protocol such as
IKE [RFC2409] or IKEv2 [RFC5996].
2.2. Key Scopes
Alternatively, keying groups can be viewed from another perspective.
Instead of looking at the granularity of keying from the point of
view of the members, we can look at it from the point of view of the
keys. This can be referred to as 'key scope'.
The key scopes corresponding to the above categories of keying groups
in the same order could be defined as follows:
Same key for the entire AD - all routers in the domain share the
same key.
Key per link - all routers on a link share the same key.
Key per sending router - each router has a different key to secure
its outgoing control traffic.
Key per sending router per interface - each router uses different
keys for each of its interfaces, which in turn are different
from the keys used by the other routers for securing their
outgoing traffic.
Key per peer router - there exist two keys corresponding to every
pair of routers.
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3. Problem Statement
The overall aim of this document is to specify an overall system for
automated key management, which will eliminate the disadvantages of
the manual method of key updating. The basic function of this
automated system is to distribute and enforce the key management
policies of the administrative domain. In accordance with these
policies, secure generation and distribution of keys will be
effected. The system will also enable key updates at regular
intervals so as to protect against both active intruders and passive
intruders who could be eavesdropping the traffic after having gained
access to the keys secretly.
Along with these basic goals, a key management system should satisfy
an additional set of requirements. These requirements ensure among
other things, security, easy deployment, robustness and scalability.
We have compiled this set after referring to the KARP Design Guide
[RFC6518], the KARP Threats and Requirements Guide
[I-D.ietf-karp-threats-reqs] and the PIM-SM "security on the wire"
specification [RFC5796].
3.1. Security Goals
1. Peer authentication for unicast and authentication of all
members of the group for multicast protocols.
2. Message authentication, which includes data origin
authentication and message integrity.
3. Protection of the system from replay attacks.
4. Peer liveness.
5. Secrecy of key management messages.
6. Authorization to ensure that only authorized routers get the
keys.
7. Adjacency management, which implies ensuring the legitimacy of
neighbor relationships of each router. Also providing an option
to turn off adjacency management if required.
8. Ensuring Perfect Forward Security (PFS) and Perfect Backward
Security (PBS).
9. Resistance to man-in-the-middle attacks.
10. Resistance to DoS attacks.
11. Usage of strong keys; those that are unpredictable and are of
sufficient length.
3.2. Non-security Goals
1. Ability to handle various categories of keying groups depending
on the security level required.
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2. Possibility for easy and incremental deployment.
3. Smooth key rollover.
4. Robustness across router reboots.
5. Scalable design.
6. Single key management architecture accommodating both unicast and
multicast systems.
4. High Level Design
In this section, we propose an architecture for an automated key
management and adjacency management system. In order to build this
framework, we have reused parts of some existing proposals and fitted
them into their correct places in the overall architecture. We have
then extended/ modified them so as to handle the key management
issues that the previous proposals have assumed to be in place.
Our design deals with securing the control traffic of routers within
an AD.
4.1. Global View
The main entities in our system are the following:
1. Administrator
2. Policy Server
3. GCKS
4. Standby GCKS
5. GMs
These entities and their functions are explained in the next section.
4.2. Entities in the system
The entities are based on those in GSAKMP. The difference is that
the Group Owner in GSAKMP has been replaced by a Policy Server, and
the Subordinate GC/KS has been replaced by a Standby GCKS in our
design. We have chosen the term 'Policy Server' in order to be
consistent with RFC 3740 [RFC3740], and the term 'Standby GCKS' since
it is not a subordinate in our design and is a standby that is
capable of performing all operations performed by the active GCKS.
Our design conforms to the Multicast Group Security Architecture
[RFC3740].
The network administrator makes configurations for the Policy Server
and the GCKS. Security policies go to the policy server, and
configurations related to the AD go to the GCKS.
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Policy Server is the entity that manages security policies for the
AD. The behavior of the policy server we describe here draws
contents from and is very similar to the 'Group Owner' in GSAKMP.
The security policies include general policies such as authorization
details for the GCKS, access control for the GMs, rekey intervals, as
well as other specific policies that may be necessary for the group.
These policies are put together into a 'Policy Token' [RFC4535] and
sent to the GCKS.
The GCKS is either a router or a server chosen by the administrator
as the group controller. It is the entity whose major function is
key management and adjacency management. The GCKS should also ensure
that the security policies in the policy token are enforced. This
implies that whenever a GM requests keys from the GCKS, the GCKS
should enforce access control for the GM according to the terms
specified in the policy token. The administrator configures the GCKS
with information such as the type of keying group to be enforced for
the AD and the adjacencies for each router in the AD corresponding to
a particular routing protocol (or a set of similar routing
protocols). This last point is due to our proposal that there could
be one instance of a GCKS per routing protocol or a set of similar
routing protocols. This is in fact necessary because GCKS is the
entity that should ensure adjacency management, and adjacencies may
be defined differently for different routing protocols. Also,
according to [I-D.ietf-karp-ops-model] , "KARP must not permit
configuration of an inappropriate key scope". This means that each
routing protocol could have a different requirement of key scope and
that needs to be satisfied. The GCKS may also generate, distribute
and update keys, depending on the type of keying group to be enforced
in the AD.
The standby GCKS is an entity that is always kept in sync with the
active GCKS, ready to take over at any time should the active fail.
This design eliminates the possibility of a single point of failure
in a centralized system.
GMs are the group member routers that communicate with each other as
well as with the GCKS. When they request keys from the GCKS, they
are given the keys along with the policy token. GMs are required to
check the rules specified in the policy token to determine if the
GCKS is authorized to act in that role. Each GM has a Local Key
Server (LKS) [atwo2009:AKM]. It is a key generation and storage
entity within the GM. A GM may sometimes be required to generate
keys itself depending on the category of keying group being enforced.
This kind of design ensures that the architecture is distributed in
the sense that key management responsibility is divided between the
GCKS and the LKSes.
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From the description above, it can be seen that the architecture we
propose is a balance between a completely centralized model and a
completely distributed one, developed by picking the plus points of
both types. It defines the concept of a GCKS, which is a centralized
entity, as well as the concept of a LKS, which is distributed as
being one entity per router. The design tries to bring in the
advantages of both models. A centralized entity is considered
necessary mainly to make adjacency management possible. In the
absence of a central controller that has information about the
adjacencies of each router in the AD, individual routers will not be
able to establish the legitimacy of their neighbors. Adjacency
management is especially important since we are dealing with control
packets, which are usually exchanged with immediate neighbors. At
the same time, loading the centralized entity with multiple
responsibilities may lead to its failure. Hence we have a localized
entity that can take up some of the functions of the central
controller as and when the need arises. This enhances scalability,
which is so important in a key management system. Another factor
leading to scalability is the presence of the standby GCKS. A
centralized system could have the disadvantage of having a single
point of failure. Our design tries to eliminate this by defining a
standby for the central controller that is always kept in sync with
it, ready to take over at any time.
4.3. Protocol Operations
The operations of key management and adjacency management occur at
two different levels. To ensure scalability of the system, as many
operations as possible need to take place among adjacent routers.
However, to ensure overall control, policies needs to be set
centrally for the entire AD.
We recognize two types of groups, which represent the two levels of
operation:
o a group consisting of the GCKS and all the routers (called group
members or GMs);
o many small groups, each consisting of a set of adjacent routers.
The overall operation proceeds in four steps:
1. Establishment of a secure path between each GM and the GCKS.
2. Exchange of policy information between each GM and the GCKS.
This policy information defines the key management approach and
parameters and the adjacency management approach and parameters.
3. Establishment of a secure path between pairs of adjacent GMs,
where the legitimacy of the adjacency was established in step 2;
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4. (if required) Exchange or generation of the shared key (and other
security parameters) that will be used to protect the routing
protocol packets.
If the key scope corresponds to "same key for the entire AD", then
the key management policy in step 2 could be "use this key", where
"this key" is the same for all GMs, and is sent as a parameter along
with the policy. In this case, the key generation in step 4 is not
necessary.
If the key scope corresponds to "key per link", then the key may be
mutually determined by the routers on that link, or a "local" GCKS
may be elected and assume the task of generating the key, which will
then be distributed on the secure paths established in step 3.
If the key scope corresponds to "key per sending router" or "key per
sending router per interface", then the sending router assumes the
responsibility for generating and distributing the key(s) that it
will use to send its routing protocol traffic. In the first case,
each router maintains (n+1) keys, one for each neighbor, for incoming
traffic from that neighbor, and one key for outgoing traffic. In the
second case, each router maintains (n+k) keys, where "k" is the
number of interfaces.
Similarly, if the key scope coresponds to "same key for the entire
AD", then the adjacency management policy is probably "accept any
router that claims to be your neighbor" or "accept any router that
presents a valid router identification string".
For other key scopes, the authentication part of step 3 will have to
confirm that a match exists between what is presented by the neighbor
router and what is specified in the adjacency management policy
information.
If IPsec is to be used to protect the routing protocol packets,
negotiation of the Security Parameter Index (SPI) to be used will be
done as part of step 4. This has to be mutually negotiated among the
users of a particular key, because it cannot be arbitrarily set by
any particular member of the group of adjacent routers. (This is in
contrast with a two-party Security Association, where the SPI can be
safely set by the (single) receiver of the incoming packets.)
However, in the case where a single key is being used for the entire
AD, the SPI may be dictated by the GCKS
5. Detailed Design
This section provides a detailed description of the automated key and
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adjacency management system. This is followed by the details of the
communication among the various entities of the system.
5.1. System Design
This section provides a detailed description of the architecture,
showing also the communication among the different entities.
5.1.1. Communication among the Entities
Figure 1 gives a closer view of the entities in our design as
described previously and shows the interactions among them.
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-----------------
| Policy Server |
-----------------
^ |
Security / |
Policies / |
/ |
/ |
----------------- |
| Administrator | | Security
----------------- | Policies
\ |
Config- \ |
urations \ |
\ |
v v
----------------- ----------------
| GCKS (Group | Synchronization | Standby GCKS |
| Controller |<--------------->| |
| Key Server | | |
----------------- ----------------
| |
Step 1 | | Step 1
followed by | | followed by
Step 2 | | Step 2
----------- ----------------
| |
| |
--------------- ---------------
| GM 1 (Group | | GM 2 |
| Member) | Step 3 | |
| | followed by | |
| also hosts | Step 4 | also hosts |
| an LKS |<---------------->| an LKS |
| (Local Key | | |
| Server) | | |
--------------- ---------------
Figure 1: Communication between the entities
Basically there is a centralized GCKS in the system and localized
LKS, local to each GM router. The GCKS and the LKS have the ability
to generate SA parameters through a KMP, and to store them in a key
store. The different scenarios to be considered and the steps of
communication are described in this section and the next.
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5.1.2. Inner View of a GM
Figure 2 shows an inner view of a GM with interactions among the KMP,
a routing protocol and the LKS.
-----------------------
| KMP (Key Management |
| Protocol) |
-----------------------
^ | \ - SA parameters related to TEK
- request for an | | -------- (Traffic Encryption Key)
initial key | | \
- request to change | | v
the key (if | | --------------------------
required) | | | LKS (Local Key Server) |
| | | |
| | | -------------- |
| | | | Key Store | |
| | - notification | -------------- |
| | of new keys --------------------------
| | /
| | / - SA parameters related
| | ----------- to TEK
| | /
| v v
-------------------
| RP (Routing |
| Protocol) |
-------------------
Figure 2: Inside view of a GM
Initially the routing protocol requests keys from the KMP to secure
its control traffic. This starts the communication between the GM
and the GCKS through the KMP, as shown by the numbered steps in
Figure 1. The key generation policy specified by the GCKS is
transferred to the GM. Then the keys are generated by the LKS of the
GM, and stored into a key store hosted by the LKS. The KMP notifies
the routing protocol that new keys are available for its use as shown
in Figure 2. The routing protocol then retrieves the keys from the
key store. For some categories of keying groups, the LKS is given
the keys directly by the GCKS. For others, it may negotiate the keys
with its neighbors. These cases are explored in detail in the
sections that follow.
The proposed KMP runs between the GCKS and the GMs, and among the GMs
themselves. The KMP messages need to be protected, and this can be
achieved by running a protocol prior to it to derive keys to protect
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it. This is similar to the manner in which GDOI messages are
protected by keys generated by a phase 1 protocol such as IKE.
5.1.3. Hierarchical Design
The design we propose is a hierarchical one. There are two kinds of
groups that can be formed here (not to be confused with keying
groups). The first kind is the one formed by the GCKS with each GM
in the AD. The second kind is the one formed among the GMs. The
design can be seen as comprised of 5 main steps. The steps together
help ensure key and adjacency management in a secure manner.
Step 1 - Mutual authentication between the GCKS and each GM in the
AD.
Step 2 - Communication between the GCKS and each GM in the AD for
secure distribution of policies and keys.
Step 3 - Inter-GM authentication.
Step 4 - Communication among the GMs themselves for key
distribution.
Step 5 - The actual transfer of routing protocol control packets
using the keys derived through the previous four steps.
Each step is dependant on the previous ones leading to a hierarchy
and ensuring modularity of design. Our design concentrates on steps
1 through 4 in order to enable a secure step 5.
The details of each of these steps are explained in the next section.
5.2. Protocol Design
In this section, we give a detailed description of our proposal for a
protocol that serves as a solution to the key management problem
outlined in Section 3. To summarise, the intention is to develop a
protocol for an automated key management system such that all the
requirements listed in Section 3 are satisfied.
We have seen the set of entities in the proposed design in Section 4.
Now we shall see the exact messages exchanged among them so that the
keys required for securing routing protocol control traffic can be
generated and distributed to the appropriate routers.
Initially the administrator configures security rules on the Policy
Server, and configuration parameters on the GCKS. The security rules
have among other things, access control rules related to GMs, and
authorization rules related to the GCKS. The configuration
parameters include among other things, the key scope information
pertaining to the AD and adjacency information corresponding to each
router in the AD. If required, the Policy Server generates other
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security policies relevant to the group and puts them together into a
policy token. This policy token is sent to the GCKS.
Once this is done, steps 1, 2, 3 and 4 as outlined in Section 5.1.3
follow. Step 1 is for GCKS-GM authentication, step 2 is for key and/
or policy transfer from the GCKS to each GM, step 3 is for GM-GM
authentication, and step 4 is for key exchange between GMs that need
to communicate with each other. Steps 2 and 4 have small variations
depending on the key scope being enforced for the AD.
Steps 1 and 2 are based on the GDOI GROUPKEY-PULL protocol [RFC6407].
However, step 2 in our case is an extension of GROUPKEY-PULL in the
sense that it accommodates various cases of keying groups and
adjacency management as well. Steps 3 and 4 have been designed such
that GROUPKEY-PULL has been extended to inter-GM communication.
Now we shall look at each of these steps in detail.
5.2.1. Step 1 - Initial Exchanges: GCKS, GM mutual authentication
Initially, when a routing protocol instance wishes to start
communication, be it unicast or multicast communication, it informs
the same to the KMP instance on the router. This information is
communicated by the KMP instance from that router to the KMP instance
on the router or server it believes to be the GCKS. At this point,
the GCKS needs the identity of the requesting router in order to
authenticate it. The requesting router also has to authenticate the
GCKS. Any of the ISAKMP group of unicast protocols could be used for
step 1 communication between the GCKS and each router that requests
keys from it. IKE/ IKEv2 is an example of such a protocol. This
protocol provides peer authentication, and parameters for an SA
including a key to help provide confidentiality and message integrity
for the next step where the actual traffic keys would be generated.
We call the key derived in this phase as SKEYID_a (term taken from
GDOI). It is assumed that the routers have agreed upon a way to
establish their identity during authentication, either through pre-
shared keys, asymmetric keys or certificates. If peer authentication
is successful, the router becomes a GM.
As already mentioned, GM stands for 'Group Member'. When talking
about the GCKS-GM interactions, 'group' typically means the entire
set of GMs in the AD. When talking about the GM-GM interactions,
'group' typically means the sending router and some set of its
neighbors. This set may include all of its neighbors or only a
subset, depending on the key scope in use. For example, when the key
scope is per link, a 'group' may refer to all routers sharing a link.
This will become evident as we see the GM-GM interactions shortly.
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5.2.1.1. Message Exchanges for Step 1
The protocol message exchanges for this step are the standard IKE
exchanges since we propose using IKE for this step. We would like to
mention at this point that whenever we say IKE, we intend to refer to
IKE or IKEv2, unless explicitly stated otherwise.
5.2.2. Step 2 - Key Management Message Exchanges between GCKS, GM
This is the step where the KMP takes over. The goal of the KMP is to
provide parameters for an SA to be eventually used by a routing
protocol to secure its control traffic.
Messages in this step are secured by the key generated by the step 1
protocol, that is, SKEYID_a. This key helps achieve authentication
and confidentiality for step 2. For step 2, we have taken most of
the messages from GROUPKEY-PULL protocol of GDOI. However, there are
some modifications and important addition of functionality in our
case, with the GCKS passing additional information to the GMs. We
shall see this in this section.
We shall initially look at the KMP details for one of the finely
grained cases of keying groups, namely, the group per sending router.
This is a flavor of multicast communication. Soon after this we will
see the small variations necessary in order to handle the other
categories of keying groups.
In step 2, the (each) GM makes requests from the GCKS through the KMP
for SA parameters required to secure its control traffic. In the
request to the GCKS, the GM specifies the identity of the routing
protocol for which it needs the keys. Although the GCKS
corresponding to the routing protocol would have already been
selected in step 1, specifying the routing protocol id again here
helps to handle the case where the same GCKS may be used for a
category of similar routing protocols.
When the GCKS receives this request from the GM, it checks to verify
if the GM can be given access to key related information according to
the rules in the policy token. If the checks fail, the communication
with the GM should not be continued. The exact behavior can be
determined from the rules in the policy token. If the checks
succeed, the GCKS delivers to the GM the following information:
o SA policy corresponding to the TEK. This could include the actual
SA parameters as well depending on the category of keying group
being enforced. The TEK is the traffic key whose scope could be
anything among those described under key scopes in Section 2. The
SA policy includes policy information about SA parameters. This
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could include information pertaining to the algorithms, the TEK,
the SPI and other parameters. For the category of keying group
being discussed now, that is, the key per sending router, the
exact TEK and SA parameters are not delivered by the GCKS to the
GM. Only rules pertaining to their generation are handed down.
The actual SA parameters are generated by the GM itself soon after
step 2 so that the GCKS is not overloaded.
o A certificate signed with the private key of the GCKS. This is to
be used by the GM for authentication purposes when it communicates
with neighboring GMs and with the GCKS for any SA updates in
future.
o The policy token information received by the GCKS from the Policy
Server. As already mentioned, this includes authorization and
access control related information. This is read by the GM in
order to authorize the GCKS and verify if it is entitled to
perform the role of GCKS.
o The key scope being enforced in the AD. This configuration is
made by the administrator on the GCKS and is pushed to the GM.
This is necessary so that the GM knows whether to expect the
traffic keys from the GCKS, or whether it needs to generate them
itself.
o The adjacency information, which includes details of all
legitimate neighbors on all interfaces of the GM and not only the
neighbors online at that point of time. This is in order to avoid
a DoS attack on the GCKS that could result if the GMs started
querying the GCKS for every router coming up, especially during
the boot up sequence, to know if it is a legitimate neighbor.
Also, this ensures completeness of information. It even helps
eliminate spoofing attacks where a legitimate neighbor may appear
on an interface other than the one it was supposed to appear on.
The adjacency information is used by the GM to know the set of
authorized neighbors with which it should communicate during steps
3 and 4.
5.2.2.1. Message Exchanges for Step 2
The protocol message exchanges for step 2 are shown in Figure 3.
GM->GCKS: HDR*, HASH(1), Ni, RP_ID (1)
GCKS->GM: HDR*, HASH(2), Nr, SA, CERT, K_SCOPE, PT, ADJ (2)
GM->GCKS: HDR*, HASH(3) (3)
Figure 3: Message exchanges for Step 2
In the message exchanges, HDR is an ISAKMP header payload. It has a
message id M-ID. The '*' indicates that the message contents
following the header are encrypted. The encryption is done with
SKEYID_a. This ensures authentication (since the key is a secret
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generated in step 1 and can be possessed only by the GCKS and the GM
with which the step 1 has been carried out) as well as secrecy (due
to the encryption). Hashes are used for ensuring message integrity
and data origin authentication; this will be explained shortly.
In exchange (1), the GM requests SA information from the GCKS to
protect its control traffic corresponding to the routing protocol
whose id is given by RP_ID. Ni is a nonce used to protect against
replay attacks as well as to ensure liveness of the GM.
In exchange (2), the GCKS initially confirms from the rules in the
policy token that the GM can be given SA information. It also
verifies the freshness of the nonce Ni. If this is successful, the
GCKS proceeds to deliver to the GM the following information:
o SA policy corresponding to the TEK - through the parameter SA
o A signed certificate - CERT
o Key Scope - K_SCOPE
o Policy token - PT
o Adjacency information - ADJ
The details of these pieces of information have already been
explained. Nr is a nonce used for replay protection and to ensure
liveness of the GCKS.
In exchange (3), the GM initially verifies freshness of the nonce Nr
so as to detect a replay attack. It then proceeds to confirm the
authorization of the GCKS by referring to the policy token. If the
GCKS is an authorized entity, the GM uses the key scope information
to know how to proceed with respect to key generation. The adjacency
list is used to note the list of legitimate neighbors and the allowed
interfaces on which they can appear online. Once this is done, the
GM sends an acknowledgement. This acknowledgement includes a hash
for integrity purposes. If the GCKS is not authorized, the GM needs
to end the communication with the GCKS. The behavior in such cases
can be determined by the policies specified in the policy token.
The hashes are pseudorandom functions (prf) computed as shown in
Figure 4.
HASH(1) = prf(SKEYID_a, M-ID | Ni | RP_ID)
HASH(2) = prf(SKEYID_a, M-ID | Ni_b | Nr | SA | CERT | K_SCOPE |
PT | ADJ)
HASH(3) = prf(SKEYID_a, M-ID | Ni_b | Nr_b)
Figure 4: Hashes used in Step 2
According to [RFC6407], "Each HASH calculation is a pseudo-random
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function ("prf") over the message ID (M-ID) from the ISAKMP header
concatenated with the entire message that follows the hash including
all payload headers, but excluding any padding added for encryption."
SKEYID_a is included in the hashes to ensure that both parties have
the step 1 key. The hashes include the nonces from previous messages
to ensure that both the parties have the exchanged nonces. This is
used for data origin authentication purposes. Hence Ni_b and Nr_b
refer to Ni and Nr from exchanges (1) and (2) respectively.
An important function of hashes is to provide message integrity. The
receiver computes the hash of the received message and compares it
with the hash value received to determine whether the message has
been tampered with or not.
Once the GM has received this information, it generates the TEK and
determines the parameters to be used for its outgoing SA. Here the
functionality of the LKS of the GM as a generator of keys comes into
play. Since the key scope being discussed now is one key per sending
router, the LKS of each GM generates one TEK. The key generation is
to be followed by key information exchange with legitimate neighbors
so that the incoming SAs can be determined. It is to be noted that
this key generation can even be done at the beginning of step 4 once
the inter-GM mutual authentication has happened in step 3.
5.2.3. Step 3 - GM-GM mutual authentication
After the GM generates TEK based information, before exchanging it
with its neighbors, it needs to ensure that a secure TEK exchange can
take place. This is done in step 3 by each GM engaging in a unicast
communication with each of its legitimate neighbors through any of
the ISAKMP group of unicast key management protocols, such as IKE.
This protocol provides peer authentication as well as a secret key to
provide confidentiality, authentication and message integrity for
step 4, which is the actual TEK exchange step. We call this secret
key as SKEYID_b. The legitimate neighbors are determined by
referring to the adjacency information given by the GCKS to the GM in
step 2. During peer authentication in step 3, the certificate given
to the GM by the GCKS could be used.
5.2.3.1. Message Exchanges for Step 3
The protocol message exchanges for this step are the standard IKE
exchanges since we propose using IKE for this step.
5.2.4. Step 4 - Key Management Message Exchanges between GMs
This is the step where the TEK information is exchanged between GMs
that need to communicate with each other. Unicast communication is
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anyway between two peers. For multicast communication, since we are
dealing with control traffic only, and control traffic is typically
link-local, each router on a link needs to be aware of the TEK of all
other routers on the same link. These legitimate neighbors are
determined from the adjacency information received from the GCKS.
The LKS of the corresponding GMs communicate to exchange their TEK
information in order to help them populate their incoming and
outgoing SAs.
Messages in this step are secured by the key generated by the step 3
protocol, that is, SKEYID_b. This key helps provide authentication
as well as confidentiality.
In step 4, the LKS of the GM pushes the SA information corresponding
to its TEK to each of its neighbors. The LKS also requests TEK
information from its neighbors. Each of the neighbors then sends its
outgoing TEK information and this is maintained as an incoming key on
the querying LKS. As a result of step 4, all GMs have the TEK
information corresponding to all their neighbors so that a secure
control traffic exchange can start.
5.2.4.1. Message Exchanges for Step 4
The message exchanges for Step 4 are shown in Figure 5.
GMi->GMr: HDR*, HASH(4), N1, CERT1 (4)
GMr->GMi: HDR*, HASH(5), N2, CERT2 (5)
GMi->GMr: HDR*, HASH(6), SA1, KD1, KREQ (6)
GMr->GMi: HDR*, HASH(7), SA2, KD2 (7)
Figure 5: Message exchanges for Step 4
GMi and GMr depict the initiator and the responder GMs respectively.
The message exchanges in this step are similar to those in step 2 in
that the HDR is an ISAKMP header payload with a message id M-ID. The
'*' indicates that the message contents following the header are
encrypted. The encryption is now done with the key SKEYID_b derived
in step 3. This ensures both authentication and secrecy. Hashes are
used for ensuring message integrity and data origin authentication.
Nonces are used to resist replay attacks and to ensure peer liveness.
In exchanges (4) and (5), we show mutual authentication between GMs
through the certificates received from the GCKS in step 2. CERT1 is
the certificate received by GMi and CERT2 is the one received by GMr
from the GCKS. Authentication would have happened in step 3 so
exchanges (4) and (5) can be eliminated. They have been shown here
for the sake of completeness.
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In exchange (6), the initiator GM communicates to its neighbor its
outgoing SA parameters in SA1 as well as the outgoing TEK information
explicitly in KD1. This is the TEK that it will be using henceforth
to secure its control packets. It also requests the outgoing SA
information from the neighboring GM so that it can be installed as
incoming SA information on the querying GM. This request is
represented by KREQ, which stands for Key Request.
In exchange (7), the neighboring GM responds with its outgoing SA
information in SA2 as well as the TEK in KD2. This will be the TEK
the neighboring GM will use henceforth to secure its control packets.
As already mentioned, the nonces N1 and N2 help provide replay
protection and a confirmation that the peer is alive.
The hashes are pseudorandom functions computed as shown in Figure 6.
HASH(4) = prf(SKEYID_b, M-ID | N1 | CERT1)
HASH(5) = prf(SKEYID_b, M-ID | N1_b | N2 | CERT2)
HASH(6) = prf(SKEYID_b, M-ID | N1_b | N2_b | SA1 | KD1 | KREQ)
HASH(7) = prf(SKEYID_b, M-ID | N1_b | N2_b | SA2 | KD2)
Figure 6: Hashes used in Step 4
Hash computation is similar to that explained in step 2. In step 4
hashes are computed by applying a pseudorandom function to the key
SKEYID_b, along with the message id concatenated with the message
contents following the hash. Also, nonces from a message exchange
are included in the hash computation of the subsequent exchanges in
order to ensure that both parties have the nonces just exchanged.
This helps in data origin authentication. Hence N1_b and N2_b refer
to N1 and N2 in exchanges (4) and (5) respectively. Hashes are very
essential to ensure message integrity and to confirm that the
messages have not been modified (possibly by an intruder) during
transit.
All information received by the LKS of a GM from the GCKS as well as
from neighboring LKSes is written to stable storage persistent across
reboots. This can be effectively used to avoid flooding the GCKS
with requests on a router reboot. This is one of the advantages of
the proposed design over GDOI [RFC6407], where, when routers reboot
they come back up with no information and the GCKS is flooded with
requests. The routing protocol is notified by the KMP about the new
SA being available in the key table for it to protect its control
traffic.
The routing protocol security mechanism would store the incoming and
outgoing SA information, and the adjacency information into the
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relevant databases.
As we can see, confidentiality and authentication has been ensured
for all steps by means of secret keys and certificates.
In the following section, we shall see the small variations required
in the basic protocol design proposed above, in order to handle the
various categories of keying groups.
5.2.5. Variations for handling other Keying Groups
We have seen the different granularities possible for a keying group,
that is, the different key scopes, in Section 2. We have also seen
that the design proposed in Section 5.2 is able to handle the keying
group where there is a separate key per sending router. This has
been achieved by each router generating its own key, which would be
the same for all its interfaces. Hence each router has a different
SA for outgoing traffic and multiple SAs for incoming traffic, one
corresponding to each neighbor. It is to be noted here that the key
generation being done locally could have a small possibility of two
routers ending up with the same key when they generate it randomly.
However, if a good random number generator is used for key
generation, the probability of ending up with the same key is
drastically reduced. This extremely small possibility can be ignored
since the method more importantly has the advantages that it reduces
the load on the GCKS. Also the GCKS does not have the need to be
aware of the individual keys of each router. This could be
considered as a case of tradeoff.
In this section, we shall see how the remaining cases of keying
groups can be handled. They can actually be handled by minor
variations to the basic design. In essence, these variations can be
implemented by the GM interpreting the key scope information given to
it by the GCKS in step 2, and thereby knowing whether to expect keys
from the GCKS or to derive them itself. This also makes the GM aware
of the path to be followed. As we shall see, in a majority of cases
it is step 4 that gets slightly altered.
Same key for the entire AD - Let us take the most coarsely grained
case, namely, a keying group per AD. Since all routers have
to share the same key (TEK), the centralized GCKS is the one
that should generate it. Every GM gets the TEK and other SA
parameters directly from the GCKS in step 2. The TEK
information received from the GCKS can be stored as both the
outgoing as well as the incoming key since all GMs share the
same key. Therefore, step 4 can be eliminated. However, step
3, which involves GMs authenticating neighboring GMs is
necessary before the GMs can start exchanging control packets.
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In essence, this variation of key scope can be implemented by
the GM interpreting the key scope information given to it from
the GCKS in step 2, and thereby knowing that it should expect
the TEK from the GCKS (TEK is also received in the same step).
Key per link - This is another flavor of keying groups wherein
there exists a TEK per link, that is, a key is shared by all
routers sharing a link. This can be handled in a manner
similar to the single key per router case described as far as
steps 1, 2 and 3 are concerned. However, there is a slight
variation required in step 4. Previously, the LKS of each GM
generated a single key to be used on all interfaces of the GM.
However in this case, an LKS needs to generate as many TEKs as
the number of its interfaces by interacting with the neighbors
on the respective links. This is done by GMs on a link
interacting to derive a TEK and other SA parameters through
any of the mutual key agreement protocols. Some examples of
protocols that could be used for this purpose are MRKMP
[I-D.hartman-karp-mrkmp], group Diffie-Hellman, and the STS
protocol. Since MRKMP specifies how keys can be generated and
distributed on a LAN by electing a GCKS, it can be used for
TEK generation for the case where the key scope is per link.
The TEK and the other SA parameters generated are stored by
all LKSes sharing the link as the outgoing and incoming
parameters on that particular link. This procedure is
repeated by all GMs for all their links in turn.
Key per sending router per interface - The only difference here
when compared to the separate key per router case is that in
that case, each GM generates a single TEK to be used on all of
its interfaces, whereas, here each GM generates a different
TEK for each of its interfaces. In step 4, it gives each
neighbor the TEK that it plans to use on the connecting link
between them.
Key per peer - This is the last category of keying groups. This
refers to unicast communication where peer routers exchange
control packets. Here the SA parameters corresponding to the
traffic key TEK and the TEK itself can be generated using a
unicast key management protocol such as IKE or even KMPRP.
However, an important point to note here is that adjacency
management is necessary even for this case since routers
should exchange keys only with legitimate neighbors. This can
be achieved only by having a central authority that is aware
of all valid adjacencies. Our design handles this. Steps 1,
2 and 3 of the design are sufficient. The key derived in step
3, namely, SKEYID_b serves as the TEK.
We have mentioned that the SA parameters along with the TEK are
either delivered to the GMs by the GCKS (for the single key per AD
case) or generated by the GMs themselves, possibly through
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interactions with other GMs (for the other keying groups, depending
on the particular category). A parameter that could have a slightly
different behavior is the SPI. This is also one of the parameters of
an SA. However the range of SPIs to be used in an AD could be
decided by the administrator. Whatever be the category of keying
group, it could so happen that the administrator chooses to have the
same SPI for all GMs. In this case, the GCKS could deliver the SPI
to the GMs along with the policy for the remaining parameters of the
SA. It could also be that the administrator wants each GM to use a
different SPI for its outgoing traffic. In this case, the GCKS
should not be overloaded with the task of generating a different SPI
for each GM. GMs should generate the SPI themselves, possibly with
communication with other GMs. If that happens, even for the single
key per AD category of keying groups, the SPI is generated by the
GMs, although the TEK may be obtained from the GCKS (since the TEK is
to be the same for all GMs for this category of key scope). In other
words, the key scope may be different from the scope of the SPI used
in the AD. Our design is flexible enough to handle this since the SA
policy handed down by the GCKS to the GMs would indicate to the GM
the exact steps to be followed.
In all cases of keying groups, the LKS stores SA information to
persistent storage to be used across reboots. Keys are stored into
the key table [I-D.ietf-karp-crypto-key-table] and the KMP informs
the same to the routing protocol, which would start using the keys to
secure its control traffic. This is the step 5 mentioned in the
explanation of the concept of hierarchical design in Section 5.1.3.
6. Other Aspects of the Key Management Problem
In this section, we address some of the other important aspects of
the key management problem. Firstly we show how this automated
system allows key updates to be done as frequently as desired. Soon
after that, we show how various good-to-have features have been
incorporated in the proposed design. Some of these features are
scalability, incremental deployment ability, effective handling of
router reboots and smooth key rollover. Addition of these features
would help in achieving the requirements stated in Section 3.
6.1. Key Updates
Keys used by the routing protocols to secure their traffic need to be
updated at regular intervals. They may have to be updated at other
non-specific times as well depending on the requirement. There are a
couple of reasons why key updates are required:
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o As a good practice in order to protect against passive intruders
who could have obtained access to the keys and could be
eavesdropping the traffic.
o Whenever a new member comes up on a link, in order to ensure PBS.
This means that the new member should not be able to get access to
keys currently being used on the link since that could mean that
the member can comprehend old messages exchanged on the link when
it was not part of it.
o Whenever a member leaves, in order to ensure PFS. This means that
going forward, even if the old member manages to get hold of
messages exchanged among the remaining members on the same link,
it should not be able to comprehend them.
One of the important points to be noted here is that PFS and PBS can
be achieved very easily and in a straight forward way for unicast
communication. Unicast communication involves a pair of routers that
share keys for securing their traffic. Every pair of routers derives
its own set of keys and those keys are known only to that particular
pair of routers. Hence a change in any one of the members of the
pair of routers would mean that the old keys are no longer valid and
new keys are derived for communication. This automatically takes
care of PFS and PBS. When a router, say R1, is uninstalled, the keys
used by the other routers for pairwise (unicast) communication with
R1 are no longer used. This ensures PFS. When a new router, say R2,
is installed, all routers engaging in a unicast communication with it
derive new pairwise keys with it. This ensures PBS.
For multicast communication, key updates are essential on a router
uninstallation or an installation to ensure PFS and PBS respectively.
This is because in multicast communication, multiple routers share
the same key and a key remains valid even if one of the routers
involved in the communication is changed. To achieve PFS and PBS,
keys have to be updated so that the leaving or entering routers do
not have access to information they are not entitled to.
We now have to determine what are the keys that need to be updated.
For regular updates, it is quite obvious that the traffic keys of all
the routers would have to be changed. The other case to consider is
when the routers in an AD change, either due to an installation or an
uninstallation. It is interesting to note that when the same traffic
key is used for the entire AD, that key should be changed, leading to
the effect of changing the keys for all the routers. However, for
all other key scopes, only the keys corresponding to the neighbors of
the leaving/ entering router need to be changed. This is because as
far as control traffic is concerned, routers have knowledge of the
keys of their neighbors only. Of course the adjacencies and hence
the neighbors, may be defined differently for the various routing
protocols.
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One of the major problems with the manual method of key management is
that keys cannot be updated as frequently as desired. This is due to
the lack of authorized people to carry out the task. This issue can
be easily overcome by an automated key management system. Let us see
how these two cases of regular rekey and a rekey on a router
installation/ uninstallation can be handled by the automated key
management system we propose.
6.2. Regular Key Updates
In this section, we discuss how our design for automated key
management aids key updates at regular intervals. The interval at
which key updates are to be done is determined from the policies
handed down by the Policy Server entity described in Section 4.2.
These policies are handed down by the Policy Server to the GCKS in
the form of a policy token, which in turn is handed down by the GCKS
to the GMs in Step 2 of the protocol as explained in Section 5.2. We
now need to see how key updates for all variations of keying groups
can be addressed. As we shall see, when all routers in the AD share
the same traffic key, the centralized GCKS is the generator of the
new key, whereas in all other cases, the GMs generate the new keys
appropriately. This is in fact similar to the process of initial key
generation described in Section 5.2.
6.2.1. Same key for the entire AD
First, let us take the case of having a single key for the entire AD.
Here, when a rekey is required, the GCKS generates the new traffic
key and unicasts it to each individual GM. This ensures that all GMs
share the same new TEK after the rekey. As an alternative to
transferring the new TEK through unicast communication, the GCKS and
all GMs in the AD could share a key called a 'TEK Encryption Key'.
This key could be used by the GCKS for encrypting the new TEK
derived, and multicasting to all GMs. The advantage of this approach
over the unicast method is that it eliminates the need to have
multiple key update messages sent out by the GCKS, one corresponding
to each GM. This in turn reduces the network traffic. However, the
downside to the multicast approach is the overhead of maintaining a
group key (and appropriately updating it) just for the rekey
purposes. This is a case of tradeoff.
6.2.2. Key per link
In this category of keying group, routers sharing a link also share
the traffic key for that link. Here when a TEK update is required,
GMs on a link execute one of the key agreement protocols such as
MRKMP, group Diffie-Hellman or the STS protocol to derive a new TEK.
This is similar to the manner in which they interact to derive the
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initial TEK for the link. The interval after which the TEK should be
changed is of course determined from the policy token.
6.2.3. Key per sending router
In this case, every router has a different TEK that it uses for
securing its control traffic. When a rekey is required, each GM
generates a new TEK individually and then communicates the same to
all its neighbors. The neighbors update the incoming TEK information
corresponding to that router in their databases.
6.2.4. Key per sending router per interface
This case is very similar to the previous one. The only difference
is that here, each GM generates as many new TEKs as the number of its
interfaces, one per interface. The GM then communicates to each of
its neighbors the TEK it plans to use on the interface corresponding
to that particular neighbor.
6.2.5. Key per peer
This is the unicast case. Keys can be updated just by every pair of
routers executing a unicast key management protocol such as IKE.
In all the above cases, the LKS updates the key store as well as its
persistent storage with the updated key information. The KMP
notifies the routing protocol of a change in the keys used to secure
the control traffic.
6.3. Router Installation/ Uninstallation
Along with the regular key updates, keys need to be updated even when
an existing router is uninstalled or a new router is installed.
These are for PFS and PBS purposes respectively as already explained
in Section 6.1. There are a couple of differences between key
updates in these cases when compared with the regular key updates.
o Regular traffic key updates require that the traffic keys
corresponding to all routers in the AD be updated. However, key
updates on a router removal or addition require only the keys
corresponding to the neighbors of the leaving or entering router
to be changed. This is because routers have knowledge of the keys
corresponding to their neighbors only as far as control traffic is
concerned. But if it so happens that the same traffic key is
being used for all routers in the AD, then a change in the key
automatically implies that the key gets changed for all the
routers.
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o Regular key updates are done at intervals determined from the
policy token given by the Policy Server. However, key updates on
a router removal or addition are done based on instructions given
by the GCKS in such a situation. This is because routers in the
AD (other than the GCKS) would not be aware of the fact that a
particular router is either installed or uninstalled.
Apart from these differences, the process of key updates during a
router change is very similar to the regular key updates. We shall
now discuss briefly how key updates on a router change can be handled
for each of the categories of keying groups.
6.3.1. Same key for the entire AD
For this category of key scope, the same traffic key is shared by all
routers in the AD. When a router is removed or a new router is
installed, the GCKS derives a new TEK and unicasts it to each of the
routers in the AD.
As an alternative to transferring the new key through unicast method,
the GCKS and all GMs could share a key called the 'TEK Encryption
Key'. If this option is followed, first of all, the TEK Encryption
Key would have to be changed on a router change. Then for the case
of router installation, the GCKS multicasts the new TEK Encryption
Key, encrypted in the old key to all existing routers. It then
unicasts the new TEK Encryption Key to the newly installed router.
After this, the GCKS derives a new TEK and multicasts it to all the
routers after encrypting it in the new TEK Encryption Key. This can
be decoded by the new router as well since it now possesses the
latest TEK Encryption Key. For the case of router uninstallation, the
GCKS changes the TEK Encryption Key and unicasts it to all the
remaining routers. The new TEK Encryption Key cannot be multicast in
this case since the old router would also be able to decrypt it.
Changing of the TEK would be the same as for router installation.
The new TEK is sent in a multicast message to all routers encrypted
in the new TEK Encryption Key.
When compared with the unicast method of key updates, this multicast
method has the advantage of low bandwidth consumption. However the
disadvantage of the multicast method is that an extra key, the TEK
Encryption Key, now needs to be maintained and updated accurately.
So the exact method chosen depends on the administrator.
6.3.2. Key per link
For this case, on a router installation or an uninstallation, the
GCKS informs the neighbors of that router. These routers interact
with each other (and with the new router if it is a case of router
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installation) and derive a new traffic key for that particular link
where the neighbor change has occurred. Any of the mutual key
agreement protocols such as MRKMP, group Diffie-Hellman or the STS
protocol can be used.
6.3.3. Key per sending router
Here again the GCKS appropriately informs the neighbors of the
affected router. Each such neighbor runs a randomized key generation
algorithm to derive a new traffic key and communicates the key to its
neighbors. This is very similar to the case of regular key updates.
6.3.4. Key per sending router per interface
This category of keying group can also be handled in an easy manner.
The GCKS informs the neighbors of the affected router. Each such
router derives a new traffic key for that interface on which the
neighbor change has occurred. The router then communicates the new
key to its new set of neighbors on that particular interface.
6.3.5. Key per peer
As already explained, key updates on a router change are not valid
for unicast communication. This is because in unicast communication,
a key is shared by only two routers. A router addition or a removal
results in a change in a particular pair (or pairs) of routers.
Hence new keys are anyway derived to be shared by the new pair. Thus
this can be considered as an automatic update of keys without any
explicit processing.
6.4. Router Reboots
Router reboots form a very important case to be considered in any
design pertaining to networks. Especially in a centralized
architecture, care should be taken to prevent the central entity from
being stormed with requests when multiple routers happen to reboot
almost simultaneously. In our architecture, it is the persistent
storage of the distributed LKS that plays a major role on a router
reboot. As already seen the LKS of each GM writes to persistent
storage some configuration and policy information such as the key
scope, adjacencies, SAs, the traffic keys corresponding to itself and
its neighbors, certificate received from the GCKS, and the policy
token. Hence on a GM reboot, the LKS retrieves information from the
persistent storage. This is an extremely important feature since it
avoids the GCKS being flooded with requests for information when
multiple routers in the AD happen to reboot.
However, information retrieval from the persistent storage may not
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always be sufficient. Occasionally a rekey could have happened when
a router was down. This could have been either a regular rekey or a
rekey due to a router installation or removal. These cases should be
dealt with in an appropriate manner so as to ensure that the rebooted
router gets the latest SA and adjacency information.
In order to handle these cases, a router needs to query its neighbors
on a reboot. This is done as soon as the router has rebooted and
read the relevant information from its persistent store. The
neighbors communicate their traffic key and SA information to the
rebooted router. Depending on this information as well as the key
scope information retrieved from the persistent storage, the rebooted
router can handle a rekey appropriately. This interaction with the
neighbors for the different cases of key scopes is explained below:
Same key for the entire AD - To handle this case, a router gets the
TEK related information initially from one of its neighbors.
It compares this key with the key corresponding to that
neighbor (which is the same as its own key since the same key
is shared by all routers in the AD) as retrieved from the
persistent storage. If the two keys match, then it is evident
that no rekey has happened on the neighbor. Since the key
scope is such that the same key is used for the entire AD, it
can be concluded that there has been no rekey in the AD.
Hence the rebooted router need not do anything else. If the
keys are in mismatch, the rebooted router concludes that a
rekey has happened in the AD, either due to a regular key
update or due to a key update based on a router change. In
either case, the router changes its outgoing traffic key to be
the same as the new one got from its neighbor. This helps
maintain consistency of all traffic keys across the AD.
Key per link - For this case, the rebooted router queries its
neighbors in turn, one neighbor on each of its links. Again
it compares the traffic key received from its neighbor with
the corresponding information retrieved from its persistent
store. If the two keys match, it means that there has been no
rekey on that link. If the keys are in mismatch, it means
that a rekey has happened on the link. The rebooted router
then changes its own outgoing traffic key on that link to be
the same as the new key got from the neighbor. In either
case, the router proceeds with querying its neighbors on its
remaining links. This is different from the previous case
where a single key was used by all routers in the AD. This is
because in the key per link case, determining whether a rekey
has happened on a particular link does not help determine the
status on other links. Hence at least one neighbor on each
link has to be queried.
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Key per sending router - For this case, the rebooted router starts
by querying one neighbor on each of its interfaces. If the
traffic keys of all the queried neighbors are the same as the
corresponding keys retrieved from the persistent storage of
the rebooted router, there is nothing to be done. If there is
at least one neighbor whose key has changed, the rebooted
router changes its own key and communicates it to its
neighbors. The rebooted router can stop querying its
neighbors at this point. An interesting observation here is
that a neighbor's key could have changed either due to a
regular rekey or due to an installation/ uninstallation of its
neighboring router. This neighboring router may or may not be
a common neighbor to the rebooted router. Since the exact
situation cannot be determined, the rebooted router just goes
ahead with its key change once it sees that the key of its
neighbor has changed. This should be fine since an extra key
update is not harmful.
Key per sending router per interface - This case is similar to the
key per link case. The rebooted router queries one neighbor
per interface and compares the traffic key information
received with the corresponding information from the
persistent key store. If the keys match, there has been
neither a regular update nor a router change on that
interface. If the keys do not match, it means that there has
been a key update either as part of a regular rekey or due to
a neighbor change on that interface. Hence the rebooted
router derives a new traffic key for that interface and
communicates the same to its neighbors on that interface. The
router then proceeds with querying its neighbors on the
remaining interfaces to determine whether the keys used on its
remaining interfaces are required to be changed or not.
Key per peer - This category of keying group represents unicast
communication. Here when a router comes back up after a
reboot, it queries its counterpart for the traffic keys
corresponding to this pair of routers. Since for unicast
communication, a pair of routers together derives traffic
keys, new keys for this pair would not be available as yet
even though a regular rekey interval may have passed when the
router was down. Therefore the two routers could engage in a
unicast key management protocol such as IKE to derive new
traffic keys or could decide to proceed with using the old
keys itself till the next rekey interval has passed.
The method described above helps ensure that in a majority of cases,
rekeys that could have happened when a router was down are handled.
There are a couple of cases to be considered as yet.
Firstly, the rebooted router should verify whether the adjacencies as
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retrieved from its persistent storage are accurate still. They could
now be stale due to the fact that a router could have been installed/
uninstalled when it was rebooting.
Secondly, in the discussion above regarding the ways in which reboots
can be handled for the different categories of keying groups, we have
mentioned that a router queries only one neighbor in some cases and
one neighbor per link or interface in other cases. A situation could
arise wherein the queried neighbor itself had gone through a reboot
resulting in its own key being stale. This in turn would mean that
the querying router cannot rely on the information got from this
single neighbor.
One way in which both of these issues could be addressed is for the
rebooted router to query the GCKS to get the updated information.
However we do not want the GCKS to be flooded with requests from the
various routers in the AD. Hence there are two layers of protection
designed as follows:
o As already explained, the rebooted router retrieves information
from its persistent store. It then queries its neighbors and
appropriately changes its keys or realises that a key update is
not required.
o Once this is done, in order to query the GCKS, the rebooted router
chooses a random time interval so as to avoid clashes with other
routers querying the GCKS.
Due to the randomness introduced, chances of the GCKS being flooded
with requests are reduced. The GCKS when queried, could give the
router information corresponding to its new adjacencies, probably the
time of change of its adjacencies and any other relevant rekey
information. This enables the rebooted router to know whether its
traffic keys are stale or not.
Another fine point here is that very rarely the rekey process could
be in progress when the router comes up. This is a corner case and
is being left for future work.
6.5. Scalability
Any system that has widespread deployment should be designed keeping
the scalability feature in mind. If scalability is overlooked during
the design phase, the system would fail on high loads when actually
deployed.
We have designed the automated key management system so as to make it
scalable. We have already mentioned that we are limiting the scope
of our problem to key and adjacency management within an AD. Even
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within an AD since the number of routers is not fixed, the system
should be able to handle a variable/ large number of routers. The
proposed protocol involves a set of GCKS-GM interactions and a set of
GM-GM interactions. The GM-GM communication is only among
neighboring GMs and hence scalability is not an issue for that. Even
for the GCKS-GM communication in the normal case, there should not be
any issue since all GMs are not installed or turned on at the same
time. However, a situation to be considered is when the GMs reboot.
It could so happen that due to a power outage, all GMs in the AD go
down and come back up at approximately the same time. It is
extremely important to ensure that the GCKS is not stormed with
requests at this point.
Our proposal handles this case in a couple of ways. Firstly we have
seen that the LKS of each GM maintains a stable storage. All
important pieces of information, such as the ones got from the GCKS
and from the neighboring GMs are written to this storage, which is
persistent across reboots. Hence a GM after a reboot, reads
information directly from its persistent storage thereby preventing
the GCKS from being flooded with requests. Secondly after retrieving
information from the local storage, when the GMs need to query the
GCKS itself, they do so by starting a timer and querying at a random
time interval. This plays a major role in preventing the GCKS from
being overloaded thereby leading to scalability.
Another factor that enables partial distribution of functionality
thereby enhancing scalability is the presence of the Standby GCKS.
If a situation arises such that the active GCKS fails (which could be
due to an overload), the Standby GCKS would immediately take over the
functionality of the active one. This eliminates a single point of
failure and hence allows the system to withstand higher loads, or
more number of GMs in the AD.
6.6. Option to Turn Off Adjacency Management
We have already discussed why it is important for an automated key
management system to manage adjacencies well. In fact, this is
because routing protocol updates are usually exchanged with
neighbors, which in turn leads to the requirement that communicating
routers should be legitimate neighbors. It is a good practice to
have adjacency management turned on in a network so that for any
router, only its legitimate neighbors and all of its legitimate
neighbors get to know the keys it uses for securing its control
traffic.
However, sometimes an administrator may decide to turn off adjacency
checks because his network of routers is probably too small and the
extra overhead is not required. This would mean that any router is
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then allowed to query for and receive the traffic keys of any other
router in the network even though the routers may not be neighbors.
If adjacency management is turned off, even routing protocols would
respond to all control packets without performing adjacency checks.
This definitely reduces security in the network.
If the key scope is such that the same traffic key is used throughout
the AD, not much harm is caused if a router gives its key information
to any other router in the AD since all routers share the same key.
Of course mutual authentication of the routers should happen in order
to know if the routers are valid members of the AD. However, an
administrator could use the key per sender model, for example, and
turn off adjacency management. The administrator then relies on the
physical adjacency to ensure that a router far away from another
router does not query it for keys.
6.7. Incremental Deployment
Whenever a new system is to be deployed in the real world, the ease
with which that can be done is of utmost importance. Network
operators may not be ready to switch over to a new system if it is
not easy to deploy it. Also, operators using a certain setup, when
switching over to a new one would usually want to deploy the new
system on an incremental basis. This would help them detect problems
in the new system, if any, and then decide whether to completely move
to the new model or not. We have designed our automated key
management system keeping this requirement in mind. The model we
have proposed can be deployed on a per interface basis. This means
that initially GMs could be manually configured with the TEKs for
some of their interfaces, and made to run the key management protocol
to derive TEKs corresponding to the other interfaces. This is for
the case of separate key per interface of each router. The other
cases of keying groups can be handled in a similar manner. Secondly,
the new system can be used to provide TEKs for one routing protocol
at a time. This again makes the transition from the manual method of
configuration to the automated method smooth.
6.8. Smooth Key Rollover
Whenever the TEK is changed, smooth key rollover should be ensured so
that no packets are dropped during the process of key transitions.
In order to achieve this, while transitioning from the old key to the
new one, for a short duration routers have to accept messages secured
using either key. This allows for the time delay involved in the new
keys being received by all routers participating in that particular
communication. After a certain time period as determined by a timer,
the old key information could be cleared. For smooth key rollover in
multicast communication, these points have been explained in more
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detail in [RFC5374]. For unicast communication, either this method
could be followed or the two participating routers could exchange new
keys and acknowledge the receipt of the keys just before beginning to
use them.
6.9. Eliminating Single Point of Failure
The proposed design for key management describes the use of a
centralized GCKS as the controller and co-ordinator for the entire
AD. In any centralized system, there is a possibility of having a
single point of failure. In such a system, if the central entity
goes down, it could so happen that the entire system stops
functioning due to loss of important data. This can be avoided by
having a backup entity to take over when the primary controller goes
down. This is precisely what is proposed in our design in
Section 4.2. We propose maintaining a Standby GCKS, which is always
kept in sync with the primary GCKS. This can be done by correctly
syncing all data from the active to the standby at regular intervals.
The appropriate interval could be determined by the policies handed
down by the Policy Server to the GCKS. Whenever the active goes
down, the standby can immediately take over its responsibility
thereby preventing any interruption in the functioning of the system.
This introduces a certain degree of distribution of functionality and
hence can successfully eliminate a single point of failure.
7. An Alternate Mechanism for Transporting the Messages
It is possible that TCP-AO could provide a suitable vehicle for the
necessary message exchanges. This will be explored in detail in the
next revision of this document.
8. Detailed Packet Formats
TBD
9. IANA Considerations
This document has no actions for IANA.
10. Acknowledgements
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11. Change History (RFC Editor: Delete Before Publishing)
[NOTE TO RFC EDITOR: this section for use during I-D stage only.
Please remove before publishing as RFC.]
atwood-karp-akam-rp-03
o changed focus to be on policy for key and adjacency management,
instead of on the key and adjacency management itself
o Proposed that TCP-AO might serve as a suitable vehicle for the
exchanges
o
atwood-karp-akam-rp-02
o Inserted ASCII art for figures and hashes
o Resolved internal cross-references
o Resolved external citations
atwood-karp-akam-rp-01
o copied in the rest of the relevant material from Revathi's thesis
o added overview material on protocol operations
atwood-karp-akam-rp-00 (original submission, based on Revathi's
thesis)
o copied in some sections of the thesis that are relevant to the
specification.
12. Needs Work in Next Draft (RFC Editor: Delete Before Publishing)
[NOTE TO RFC EDITOR: this section for use during I-D stage only.
Please remove before publishing as RFC.]
List of stuff that still needs work
o
o Determine if TCP-AO is a viable platform for this work
o Create the section on packet formats
o
o
13. References
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13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
13.2. Informative References
[I-D.hartman-karp-mrkmp]
Hartman, S., Zhang, D., and G. Lebovitz, "Multicast Router
Key Management Protocol (MaRK)",
draft-hartman-karp-mrkmp-05 (work in progress),
September 2012.
[I-D.ietf-karp-crypto-key-table]
Housley, R., Polk, T., Hartman, S., and D. Zhang,
"Database of Long-Lived Symmetric Cryptographic Keys",
draft-ietf-karp-crypto-key-table-06 (work in progress),
February 2013.
[I-D.ietf-karp-ops-model]
Hartman, S. and D. Zhang, "Operations Model for Router
Keying", draft-ietf-karp-ops-model-05 (work in progress),
February 2013.
[I-D.ietf-karp-threats-reqs]
Lebovitz, G., Bhatia, M., and B. Weis, "Keying and
Authentication for Routing Protocols (KARP) Overview,
Threats, and Requirements",
draft-ietf-karp-threats-reqs-07 (work in progress),
December 2012.
[I-D.mahesh-karp-rkmp]
Jethanandani, M., Weis, B., Patel, K., Zhang, D., Hartman,
S., Chunduri, U., Tian, A., and J. Touch, "Negotiation for
Keying Pairwise Routing Protocols in IKEv2",
draft-mahesh-karp-rkmp-04 (work in progress),
February 2013.
[I-D.tran-karp-mrmp]
Tran, P. and B. Weis, "The Use of G-IKEv2 for Multicast
Router Key Management", draft-tran-karp-mrmp-02 (work in
progress), October 2012.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security
Architecture", RFC 3740, March 2004.
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[RFC4535] Harney, H., Meth, U., Colegrove, A., and G. Gross,
"GSAKMP: Group Secure Association Key Management
Protocol", RFC 4535, June 2006.
[RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, November 2008.
[RFC5796] Atwood, W., Islam, S., and M. Siami, "Authentication and
Confidentiality in Protocol Independent Multicast Sparse
Mode (PIM-SM) Link-Local Messages", RFC 5796, March 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
[RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
of Interpretation", RFC 6407, October 2011.
[RFC6518] Lebovitz, G. and M. Bhatia, "Keying and Authentication for
Routing Protocols (KARP) Design Guidelines", RFC 6518,
February 2012.
[atwo2009:AKM]
Atwood, J., "Automated Key Management for Router Updates",
October 2009.
Authors' Addresses
William Atwood
Concordia University/CSE
1455 de Maisonneuve Blvd, West
Montreal, QC H3G 1M8
Canada
Phone: +1(514)848-2424 ext3046
Email: william.atwood@concordia.ca
URI: http://users.encs.concordia.ca/~bill
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Revathi Bangalore Somanatha
Concordia University/CSE
1455 de Maisonneuve Blvd, West
Montreal, QC H3G 1M8
Canada
Email: revathi.bs@gmail.com
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