Network Working Group M. Behringer
Internet-Draft F. Le Faucheur
Intended status: Informational Cisco Systems Inc
Expires: December 31, 2007 June 29, 2007
A Framework for RSVP Security Using Dynamic Group Keying
draft-behringer-tsvwg-rsvp-security-groupkeying-00.txt
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Abstract
The Resource reSerVation Protocol (RSVP) allows hop-by-hop
authentication of RSVP neighbors. This requires messages to be
cryptographically signed using a shared secret between participating
nodes. This document compares group keying for RSVP with per
neighbor or per interface keying, and discusses the applicability and
limitations of these approaches. Draft-weis-gdoi-for-rsvp describes
how the Group Domain of Interpretation (GDOI) can be used to
distribute group keys to RSVP nodes. The document also discusses
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applicability of group keying to RSVP encryption.
Table of Contents
1. Introduction and Problem Statement . . . . . . . . . . . . . . 3
2. The RSVP Trust Model . . . . . . . . . . . . . . . . . . . . . 3
3. Key types for RSVP . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Interface based keys . . . . . . . . . . . . . . . . . . . 4
3.2. Neighbor based keys . . . . . . . . . . . . . . . . . . . 4
3.3. Group keys . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Key Provisioning Methods for RSVP . . . . . . . . . . . . . . 4
4.1. Static Key Provisioning . . . . . . . . . . . . . . . . . 4
4.2. Per Neighbor Key Negotiation . . . . . . . . . . . . . . . 5
4.3. Dynamic Key Distribution using GDOI . . . . . . . . . . . 5
5. Applicability of Various Keying Methods for RSVP . . . . . . . 5
5.1. Per Neighbor or Per Interface Keys for Authentication . . 5
5.2. Group Keys for Authentication . . . . . . . . . . . . . . 6
5.3. Non-RSVP Hops . . . . . . . . . . . . . . . . . . . . . . 7
5.4. Subverted RSVP Nodes . . . . . . . . . . . . . . . . . . . 8
5.5. RSVP Encryption . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Informative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction and Problem Statement
The Resource reSerVation Protocol [RFC2205] allows hop-by-hop
authentication of RSVP neighbors, as specified in [RFC2747]. In this
mode, an integrity object is attached to each RSVP message to
transmit a keyed message digest. This message digest allows the
recipient to verify the authenticity of the sender and validate
integrity of the message. Through the inclusion of a sequence number
in the scope of the digest, the digest also offers replay protection.
[RFC2747] does not dictate how the key for the integrity operation is
derived. Currently, most implementations of RSVP use a statically
configured key, per interface or per neighbor. However, to manually
configure key per router pair across an entire network is
operationally hard, especially for key changes. Effectively, many
users of RSVP therefore resort to the same key throughout their
network, and change it rarely if ever, because of the operational
burden. [RFC 3562] however recommends regular key changes, at least
every 90 days.
[I-D.weis-gdoi-for-rsvp] provides an alternative solution, using GDOI
([RFC3547]) for key distribution. This allows dynamic key updates,
valid for the entire group of RSVP speakers.
The present document describes the various keying methods and their
applicability to different RSVP deployment environments, for both
message integrity and encryption. It does not mandate any particular
method, but is meant as a comparative guideline to understand where
each RSVP keying method is best deployed, and where it cannot be
deployed. Furthermore, it discusses the impact on RSVP hop by hop
authentication of non-RSVP nodes, as well as subverted nodes, in the
reservation path.
2. The RSVP Trust Model
Many protocol security mechanisms used in networks require and use
per peer authentication. Each hop authenticates its neighbor with a
shared key or certificate. This is also the model used for RSVP.
Trust in this model is transitive. Each RSVP node trusts explicitely
only its RSVP next hop peers, through the message digest contained in
the INTEGRITY object. The next hop RSVP speaker in turn trusts its
own peers and so on.
The RSVP protocol can operate in the presence of a non-RSVP router in
the path from the sender to the receiver. The non-RSVP hop will
ignore the RSVP message and just pass it along. The next RSVP node
can then process the RSVP message. For RSVP authentication to work
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in this case, the key used for computing the RSVP message digest
needs to be shared by the two RSVP neighbors, even if they are not IP
neighbors. However, in the presence of non-RSVP hops, while an RSVP
node always know the next IP hop before forwarding an RSVP Message,
it does not always know the RSVP next hop. Thus, the presence of
non-RSVP hops impacts operation of RSVP authentication and may
influence the keying approaches. This is further discussed in
Section 5.3.
3. Key types for RSVP
3.1. Interface based keys
Most current implementations support interface based RSVP keys. All
RSVP speakers on a given subnet have to share the same key in this
model, which makes it unsuitable for deployment scenarios where
different trust groups share a subnet, for example Internet exchange
points. In such a case, neighbor based keys are required.
3.2. Neighbor based keys
In this model, an RSVP key is bound to an interface plus a neighbor
on that interface. It allows the distinction of different trust
groups on a single subnet. (Assuming that layer-2 security is
correctly implemented to prevent layer-2 attacks.)
3.3. Group keys
Here, all members of a group of RSVP nodes share the same key. This
implies that a node uses the same key regardless of the next RSVP hop
that will process the message (within the group of nodes sharing the
particular key). It also implies that a node will use the same key
on the receiving as on the sending side (when exchanging RSVP
messages withn the group).
4. Key Provisioning Methods for RSVP
4.1. Static Key Provisioning
The simplest way to implement RSVP authentication is to use static,
preconfigured keys. However, on the operational side key management
is heavy, since no secure automated mechanism can be used. This
method is therefore mostly useful for small deployments, where key
changes can be carried out manually, or for deployments with
automated configuration tools which support key changes.
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Static key provisioning is therefore not an ideal model in a large
network.
Often, the number of interconnection points across two domains where
RSVP is allowed to transit is relatively small and well controlled.
Also, the different domains may not be in a position to use an
infrastructure trusted by both domains to update keys on both sides.
Thus, manually configured keys may be applicable to inter-domain RSVP
authentication.
Since it is not practical to carry out the key change at the exact
same time on both sides, some grace period nees to be implemented
during which an RSVP node will accept both the old and the new key.
Otherwise, RSVP operation would suffer interruptions.
4.2. Per Neighbor Key Negotiation
To avoid the problem of key rollover in static key deployments, per
neighbor key negotiation could be used. However, existing key
distribution protocols may not be appropriate in all environments
because of the complexity or operational burden they involve.
4.3. Dynamic Key Distribution using GDOI
[I-D.weis-gdoi-for-rsvp] describes a mechanism to distribute group
keys to a group of RSVP speakers, using GDOI [RFC3547]. In this
model, a key server authenticates each of the RSVP nodes
independently, and then distributes a group key to the entire group.
5. Applicability of Various Keying Methods for RSVP
5.1. Per Neighbor or Per Interface Keys for Authentication
Per interface and per peer keys can be used within a single security
domain. As mentioned above, per interface keys are only applicable
when all the hosts reachable on the specific interface belong to the
same security domain.
These key types can also be used between security domains, since they
are specific to a particular interface or neighbor. Again, interface
level keys can only be deployed safely when all the reachable
neighbors on the interface belong to the same security domain.
As discussed in Section 5.3, per neighbor and per interface keys can
not be used in the presence of non-RSVP hops.
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5.2. Group Keys for Authentication
Group keys apply naturally to intra-domain RSVP authentication, since
all RSVP nodes trust each other, and trust the group key server in
this model. This is presented Figure 1.
......GKS1.............
: : : : :
: : : : :
source--R1--R2--R3-----destination
| |
|<-----domain 1----------------->|
Figure 1: Group Key Server within a single security domain
A single group key cannot normally be used to cover multiple security
domains however, because by definition the different domains do not
trust each other and would not be willing to trust the same group key
server. For a single group key to be used in several security
domains, there is a need for a single group key server, which is
trusted by both sides. While this is theoretically possible, in
practice it is unlikely that there is a single such trusted entity.
Figure 2 illustrates this setup.
...............GKS1....................
: : : : : : : :
: : : : : : : :
source--R1--R2--R3--------R4--R5--R6--destination
| | | |
|<-----domain 1--->| |<-------domain 2----->|
Figure 2: A Single Group Key Server across security domains
A more practical approach for RSVP operation across security domains,
to use a separate group key server for each security domain, and to
use per interface or per peer authentication between the two domains.
Figure 3 shows this set-up.
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....GKS1...... ....GKS2.........
: : : : : : : :
: : : : : : : :
source--R1--R2--R3--------R4--R5--R6--destination
| | | |
|<-----domain 1--->| |<-------domain 2----->|
Figure 3: A group Key Server per security domain
5.3. Non-RSVP Hops
In the presence of a non-RSVP router in the path from the sender to
the receiver, regular RSVP keeps working. The non-RSVP node ignores
the RSVP message, and passes it on transparently to the next node.
Figure 4 illustrates this scenario. R2 in this picture does not
participate in RSVP, the other nodes do. In this case, R2 will pass
on any RSVP messages unchanged, and will ignore them.
sender----R1---R2(*)---R3---R4----receiver
\ /
\ ------
\ /
R5
(*) Non-RSVP hop
Figure 4: A non-RSVP Node in the path
However, this creates an additional challenge for RSVP
authentication. In the presence of a non-RSVP hop, with some RSVP
messages such as a Path message, an RSVP router does not know the
RSVP next hop for that message at the time of forwarding it. In
fact, part of the role of a Path message is precisely to discover the
RSVP next hop (and to dynamically re-discover it when it changes, say
because of a routing change). For example, in Figure 4, R1 knows
that the next IP hop for a Path message addresed to the receiver is
R2, but it does necessarily not know if the RSVP next hop is R3 or
R5.
This means that per interface and per neighbor keys cannot easily be
used in the presence of non-RSVP routers on the path between senders
and receivers.
By contrast, group keying will naturally work in the presence of non-
RSVP routers. Referring back to Figure 4, with group keying, R1
would use the group key to sign a Path message addressed to the
receiver and forwards it to R2. Being a non-RSVP node, R2 and will
ignore and forward the Path message to R3 or R5 depending on the
current shortest path as determined by routing. Whether it is R3 or
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R5, the RSVP router that receives the Path message will be able to
authenticate it successfully with the group key.
5.4. Subverted RSVP Nodes
A subverted node is defined here as an untrusted node, for example
because an intruder has gained control over it. Since RSVP
authentication is hop-by-hop and not end-to-end, a subverted node in
the path breaks the chain of trust. This is to a large extent
independent of the type of keying used.
For interface or per-peer keying, the subverted node can now
introduce fake messages to its neighbors. This can be used in a
variety of ways, for example by changing the receiver address in the
Path message, or by generating fake Path messages. This allows path
states to be created on every RSVP router along any arbitrary path
through the RSVP domain. That in itself could result in a form of
Denial of Service by allowing exhaustion of some router resources
(e.g. memory). The subverted node could also generate fake Resv
messages upstream corresponding to valid Path states. In doing so,
the subverted node can reserve excessive amounts of bandwidth thereby
possibly performing a denial of service attack.
It has to be noted specifically that even though the per interface or
per neighbor keys have only local significance, the messages
themselves can be created arbitrarily so that they are then
authenticated and forwarded by the RSVP neighbor of the subverted
node, eventually potentially affecting the entire RSVP domain.
For group keying the impact of subverted nodes on the path is
comparable. Group keying allows the additional abuse of sending fake
messages to any node in the RSVP domain, however, in practice this
can be achieved to a large extend also with per neighbor keys, as
discussed above.
5.5. RSVP Encryption
The keying material can also be used to encrypt the RSVP messages,
instead of, or in addition to authenticating them. The same
considerations apply for this case as discussed above for the
authentication case. Group keys are applicable only within a trusted
domain, but they have the potential of passing a non-RSVP speaker
without further configuration. Per interface or per nighbor keys
work also inter-domain, but do not operate in the presence of a non-
RSVP router.
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6. Security Considerations
This entire document discusses security of the RSVP authentication
and encryption mechanisms, depending on the key scheme used.
7. Informative References
[I-D.weis-gdoi-for-rsvp]
Weis, B., "Group Domain of Interpretation (GDOI) support
for RSVP", July 2007.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.
[RFC3097] Braden, R. and L. Zhang, "RSVP Cryptographic
Authentication -- Updated Message Type Value", RFC 3097,
April 2001.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
Authors' Addresses
Michael H. Behringer
Cisco Systems Inc
Village d'Entreprises Green Side
400, Avenue Roumanille, Batiment T 3
Biot - Sophia Antipolis 06410
France
Email: mbehring@cisco.com
URI: http://www.cisco.com
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Francois Le Faucheur
Cisco Systems Inc
Village d'Entreprises Green Side
400, Avenue Roumanille, Batiment T 3
Biot - Sophia Antipolis 06410
France
Email: flefauch@cisco.com
URI: http://www.cisco.com
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