RFC 4230
RSVP Security Properties
4. Detailed Security Property Discussion
This section describes the protection of the RSVP-provided mechanisms for authentication, authorization, integrity and replay protection individually, user identity confidentiality, and confidentiality of the signaling messages,4.1. Network Topology
This paragraph shows the basic interfaces in a simple RSVP network architecture. The architecture below assumes that there is only a single domain and that the two routers are RSVP- and policy-aware. These assumptions are relaxed in the individual paragraphs, as necessary. Layer 2 devices between the clients and their corresponding first-hop routers are not shown. Other network elements like a Kerberos Key Distribution Center and, for example, an LDAP server from which the PDP retrieves its policies are also omitted. The security of various interfaces to the individual servers (KDC, PDP, etc.) depends very much on the security policy of a specific network service provider. +--------+ | Policy | +----|Decision| | | Point +---+ | +--------+ | | | | | +------+ +-+----+ +---+--+ +------+ |Client| |Router| |Router| |Client| | A +-------+ 1 +--------+ 2 +----------+ B | +------+ +------+ +------+ +------+ Figure 4: Simple RSVP Architecture.4.2. Host/Router
When considering authentication in RSVP, it is important to make a distinction between user and host authentication of the signaling messages. The host is authenticated using the RSVP INTEGRITY object, whereas credentials inside the AUTH_DATA object can be used to authenticate the user. In this section, the focus is on host authentication, whereas the next section covers user authentication. (1) Authentication The term "host authentication" is used above, because the selection of the security association is bound to the host's IP
address, as mentioned in Section 3.1 and Section 3.2. Depending
on the key management protocol used to create this security
association and the identity used, it is also possible to bind a
user identity to this security association. Because the key
management protocol is not specified, it is difficult to evaluate
this part, and hence we speak about data-origin authentication
based on the host's identity for RSVP INTEGRITY objects. The
fact that the host identity is used for selecting the security
association has already been described in Section 3.1.
Data-origin authentication is provided with a keyed hash value
computed over the entire RSVP message, excluding the keyed
message digest field itself. The security association used
between the user's host and the first-hop router is, as
previously mentioned, not established by RSVP, and it must
therefore be available before signaling is started.
* Kerberos for the RSVP INTEGRITY object
As described in Section 7 of [1], Kerberos may be used to
create the key for the RSVP INTEGRITY object. How to learn
the principal name (and realm information) of the other node
is outside the scope of [1]. [20] describes a way to
distribute principal and realm information via DNS, which can
be used for this purpose (assuming that the FQDN or the IP
address of the other node for which this information is
desired is known). All that is required is to encapsulate the
Kerberos ticket inside the policy element. It is furthermore
mentioned that Kerberos tickets with expired lifetime must not
be used, and the initiator is responsible for requesting and
exchanging a new service ticket before expiration.
RSVP multicast processing in combination with Kerberos
involves additional considerations. Section 7 of [1] states
that in the multicast case all receivers must share a single
key with the Kerberos Authentication Server (i.e., a single
principal used for all receivers). From a personal discussion
with Rodney Hess, it seems that there is currently no other
solution available in the context of Kerberos. Multicast
handling therefore leaves some open questions in this context.
In the case where one entity crashed, the established security
association is lost and therefore the other node must
retransmit the service ticket. The crashed entity can use an
Integrity Challenge message to request a new Kerberos ticket
to be retransmitted by the other node. If a node receives
such a request, then a reply message must be returned.
(2) Integrity protection
Integrity protection between the user's host and the first-hop
router is based on the RSVP INTEGRITY object. HMAC-MD5 is
preferred, although other keyed hash functions may also be used
within the RSVP INTEGRITY object. In any case, both
communicating entities must have a security association that
indicates the algorithm to use. This may, however, be difficult,
because no negotiation protocol is defined to agree on a specific
algorithm. Hence, if RSVP is used in a mobile environment, it is
likely that HMAC-MD5 is the only usable algorithm for the RSVP
INTEGRITY object. Only in local environments may it be useful to
switch to a different keyed hash algorithm. The other possible
alternative is that every implementation support the most
important keyed hash algorithms. e.g., MD5, SHA-1, RIPEMD-160,
etc. HMAC-MD5 was chosen mainly because of its performance
characteristics. The weaknesses of MD5 [21] are known and were
initially described in [22]. Other algorithms like SHA-1 [15]
and RIPEMD-160 [21] have stronger security properties.
(3) Replay Protection
The main mechanism used for replay protection in RSVP is based on
sequence numbers, whereby the sequence number is included in the
RSVP INTEGRITY object. The properties of this sequence number
mechanism are described in Section 3.1 of [1]. The fact that the
receiver stores a list of sequence numbers is an indicator for a
window mechanism. This somehow conflicts with the requirement
that the receiver only has to store the highest number given in
Section 3 of [1]. We assume that this is an oversight. Section
4.2 of [1] gives a few comments about the out-of-order delivery
and the ability of an implementation to specify the replay
window. Appendix C of [3] describes a window mechanism for
handling out-of-sequence delivery.
(4) Integrity Handshake
The mechanism of the Integrity Handshake is explained in Section
3.5. The Cookie value is suggested to be a hash of a local
secret and a timestamp. The Cookie value is not verified by the
receiver. The mechanism used by the Integrity Handshake is a
simple Challenge/Response message, which assumes that the key
shared between the two hosts survives the crash. If, however,
the security association is dynamically created, then this
assumption may not be true.
In Section 10 of [1], the authors note that an adversary can
create a faked Integrity Handshake message that includes
challenge cookies. Subsequently, it could store the received
response and later try to replay these responses while a
responder recovers from a crash or restart. If this replayed
Integrity Response value is valid and has a lower sequence number
than actually used, then this value is stored at the recovering
host. In order for this attack to be successful, the adversary
must either have collected a large number of challenge/response
value pairs or have "discovered" the cookie generation mechanism
(for example by knowing the local secret). The collection of
Challenge/Response pairs is even more difficult, because they
depend on the Cookie value, the sequence number included in the
response message, and the shared key used by the INTEGRITY
object.
(5) Confidentiality
Confidentiality is not considered to be a security requirement
for RSVP. Hence, it is not supported by RSVP, except as
described in paragraph d) of Section 4.3. This assumption may
not hold, however, for enterprises or carriers who want to
protect billing data, network usage patterns, or network
configurations, in addition to users' identities, from
eavesdropping and traffic analysis. Confidentiality may also
help make certain other attacks more difficult. For example, the
PathErr attack described in Section 5.2 is harder to carry out if
the attacker cannot observe the Path message to which the PathErr
corresponds.
(6) Authorization
The task of authorization consists of two subcategories: network
access authorization and RSVP request authorization. Access
authorization is provided when a node is authenticated to the
network, e.g., using EAP [23] in combination with AAA protocols
(for example, RADIUS [24] or DIAMETER [9]). Issues related to
network access authentication and authorization are outside the
scope of RSVP.
The second authorization refers to RSVP itself. Depending on the
network configuration:
* the router either forwards the received RSVP request to the
policy decision point (e.g., using COPS [10] and [11]) to
request that an admission control procedure be executed, or
* the router supports the functionality of a PDP and, therefore,
there is no need to forward the request, or
* the router may already be configured with the appropriate
policy information to decide locally whether to grant this
request.
Based on the result of the admission control, the request may be
granted or rejected. Information about the resource-requesting
entity must be available to provide policy-based admission
control.
(7) Performance
The computation of the keyed message digest for an RSVP INTEGRITY
object does not represent a performance problem. The protection
of signaling messages is usually not a problem, because these
messages are transmitted at a low rate. Even a high volume of
messages does not cause performance problems for an RSVP router
due to the efficiency of the keyed message digest routine.
Dynamic key management, which is computationally more demanding,
is more important for scalability. Because RSVP does not specify
a particular key exchange protocol, it is difficult to estimate
the effort needed to create the required security associations.
Furthermore, the number of key exchanges to be triggered depends
on security policy issues like lifetime of a security
association, required security properties of the key exchange
protocol, authentication mode used by the key exchange protocol,
etc. In a stationary environment with a single administrative
domain, manual security association establishment may be
acceptable and may provide the best performance characteristics.
In a mobile environment, asymmetric authentication methods are
likely to be used with a key exchange protocol, and some sort of
public key or certificate verification needs to be supported.
4.3. User to PEP/PDP
As noted in the previous section, RSVP supports both user-based and
host-based authentication. Using RSVP, a user may authenticate to
the first hop router or to the PDP as specified in [1], depending on
the infrastructure provided by the network domain or the architecture
used (e.g., the integration of RSVP and Kerberos V5 into the Windows
2000 Operating System [25]). Another architecture in which RSVP is
tightly integrated is the one specified by the PacketCable
organization. The interested reader is referred to [26] for a
discussion of their security architecture.
(1) Authentication
When a user sends an RSVP PATH or RESV message, this message may
include some information to authenticate the user. [7] describes
how user and application information is embedded into the RSVP
message (AUTH_DATA object) and how to protect it. A router
receiving such a message can use this information to authenticate
the client and forward the user or application information to the
policy decision point (PDP). Optionally, the PDP itself can
authenticate the user, which is described in the next section.
To be able to authenticate the user, to verify the integrity, and
to check for replays, the entire POLICY_DATA element has to be
forwarded from the router to the PDP (e.g., by including the
element into a COPS message). It is assumed, although not
clearly specified in [7], that the INTEGRITY object within the
POLICY_DATA element is sent to the PDP along with all other
attributes.
* Certificate Verification
Using the policy element as described in [7], it is not
possible to provide a certificate revocation list or other
information to prove the validity of the certificate inside
the policy element. A specific mechanism for certificate
verification is not discussed in [7] and hence a number of
them can be used for this purpose. For certificate
verification, the network element (a router or the policy
decision point) that has to authenticate the user could
frequently download certificate revocation lists or use a
protocol like the Online Certificate Status Protocol (OCSP)
[27] and the Simple Certificate Validation Protocol (SCVP)
[28] to determine the current status of a digital certificate.
* User Authentication to the PDP
This alternative authentication procedure uses the PDP to
authenticate the user instead of the first-hop router. In
Section 4.2.1 of [7], the choice is given for the user to
obtain a session ticket either for the next hop router or for
the PDP. As noted in the same section, the identity of the
PDP or the next hop router is statically configured or
dynamically retrieved. Subsequently, user authentication to
the PDP is considered.
* Kerberos-based Authentication to the PDP
If Kerberos is used to authenticate the user, then a session
ticket for the PDP must be requested first. A user who roams
between different routers in the same administrative domain
does not need to request a new service ticket, because the
same PDP is likely to be used by most or all first-hop routers
within the same administrative domain. This is different from
the case in which a session ticket for a router has to be
obtained and authentication to a router is required. The
router therefore plays a passive role of simply forwarding the
request to the PDP and executing the policy decision returned
by the PDP. Appendix B describes one example of user-to-PDP
authentication.
User authentication with the policy element provides only
unilateral authentication, whereby the client authenticates to
the router or to the PDP. If an RSVP message is sent to the
user's host and public-key-based authentication is not used,
then the message does not contain a certificate and digital
signature. Hence, no mutual authentication can be assumed.
In case of Kerberos, mutual authentication may be accomplished
if the PDP or the router transmits a policy element with an
INTEGRITY object computed with the session key retrieved from
the Kerberos ticket, or if the Kerberos ticket included in the
policy element is also used for the RSVP INTEGRITY object as
described in Section 4.2. This procedure only works if a
previous message was transmitted from the end host to the
network and such key is already established. Reference [7]
does not discuss this issue, and therefore there is no
particular requirement for transmitting network-specific
credentials back to the end-user's host.
(2) Integrity Protection
Integrity protection is applied separately to the RSVP message
and the POLICY_DATA element, as shown in Figure 1. In case of
a policy-ignorant node along the path, the RSVP INTEGRITY
object and the INTEGRITY object inside the policy element
terminate at different nodes. Basically, the same is true for
the user credentials if they are verified at the policy
decision point instead of the first hop router.
* Kerberos
If Kerberos is used to authenticate the user to the first hop
router, then the session key included in the Kerberos ticket
may be used to compute the INTEGRITY object of the policy
element. It is the keyed message digest that provides the
authentication. The existence of the Kerberos service ticket
inside the AUTH_DATA object does not provide authentication or
a guarantee of freshness for the receiving host.
Authentication and guarantee of freshness are provided by the
keyed hash value of the INTEGRITY object inside the
POLICY_DATA element. This shows that the user actively
participated in the Kerberos protocol and was able to obtain
the session key to compute the keyed message digest. The
Authenticator used in the Kerberos V5 protocol provides
similar functionality, but replay protection is based on
timestamps (or on a sequence number if the optional seq-number
field inside the Authenticator is used for KRB_PRIV/KRB_SAFE
messages as described in Section 5.3.2 of [8]).
* Digital Signature
If public-key-based authentication is provided, then user
authentication is accomplished with a digital signature. As
explained in Section 3.3.3 of [7], the DIGITAL_SIGNATURE
attribute must be the last attribute in the AUTH_DATA object,
and the digital signature covers the entire AUTH_DATA object.
In the case of PGP, which hash algorithm and public key
algorithm are used for the digital signature computation is
described in [19]. In the case of X.509 credentials, the
situation is more complex because different mechanisms like
CMS [29] or PKCS#7 [30] may be used for digitally signing the
message element. X.509 only provides the standard for the
certificate layout, which seems to provide insufficient
information for this purpose. Therefore, X.509 certificates
are supported, for example, by CMS or PKCS#7. [7], however,
does not make any statements about the usage of CMS or PKCS#7.
Currently, there is no support for CMS or for PKCS#7 [7],
which provides more than just public-key-based authentication
(e.g., CRL distribution, key transport, key agreement, etc.).
Furthermore, the use of PGP in RSVP is vaguely defined,
because there are different versions of PGP (including OpenPGP
[19]), and no indication is given as to which should be used.
Supporting public-key-based mechanisms in RSVP might increase
the risks of denial-of-service attacks. The large processing,
memory, and bandwidth requirements should also be considered.
Fragmentation might also be an issue here.
If the INTEGRITY object is not included in the POLICY_DATA
element or not sent to the PDP, then we have to make the
following observations:
For the digital signature case, only the replay protection
provided by the digital signature algorithm can be used.
It is not clear, however, whether this usage was
anticipated or not. Hence, we might assume that replay
protection is based on the availability of the RSVP
INTEGRITY object used with a security association that is
established by other means.
Including only the Kerberos session ticket is insufficient,
because freshness is not provided (because the Kerberos
Authenticator is missing). Obviously there is no guarantee
that the user actually followed the Kerberos protocol and
was able to decrypt the received TGS_REP (or, in rare
cases, the AS_REP if a session ticket is requested with the
initial AS_REQ).
(3) Replay Protection
Figure 5 shows the interfaces relevant for replay protection of
signaling messages in a more complicated architecture. In this
case, the client uses the policy data element with PEP2, because
PEP1 is not policy-aware. The interfaces between the client and
PEP1 and between PEP1 and PEP2 are protected with the RSVP
INTEGRITY object. The link between the PEP2 and the PDP is
protected, for example, by using the COPS built-in INTEGRITY
object. The dotted line between the Client and the PDP indicates
the protection provided by the AUTH_DATA element, which has no
RSVP INTEGRITY object included.
AUTH_DATA +----+
+---------------------------------------------------+PDP +-+
| +----+ |
| |
| |
| COPS |
| INTEGRITY|
| |
| |
| |
+--+---+ RSVP INTEGRITY +----+ RSVP INTEGRITY +----+ |
|Client+-------------------+PEP1+----------------------+PEP2+-+
+--+---+ +----+ +-+--+
| |
+-----------------------------------------------------+
POLICY_DATA INTEGRITY
Figure 5: Replay Protection.
Host authentication with the RSVP INTEGRITY object and user
authentication with the INTEGRITY object inside the POLICY_DATA
element both use the same anti-replay mechanism. The length of
the Sequence Number field, sequence number rollover, and the
Integrity Handshake have already been explained in Section 3.1.
Section 9 of [7] states: "RSVP INTEGRITY object is used to
protect the policy object containing user identity information
from security (replay) attacks." When using public-key-based
authentication, RSVP-based replay protection is not supported,
because the digital signature does not cover the POLICY_DATA
INTEGRITY object with its Sequence Number field. The digital
signature covers only the entire AUTH_DATA object.
The use of public key cryptography within the AUTH_DATA object
complicates replay protection. Digital signature computation
with PGP is described in [31] and in [19]. The data structure
preceding the signed message digest includes information about
the message digest algorithm used and a 32-bit timestamp of when
the signature was created ("Signature creation time"). The
timestamp is included in the computation of the message digest.
The IETF standardized version of OpenPGP [19] contains more
information and describes the different hash algorithms (MD2,
MD5, SHA-1, RIPEMD-160) supported. [7] does not make any
statements as to whether the "Signature creation time" field is
used for replay protection. Using timestamps for replay
protection requires different synchronization mechanisms in the
case of clock-skew. Traditionally, these cases assume "loosely
synchronized" clocks but also require specifying a replay window.
If the "Signature creation time" is not used for replay
protection, then a malicious, policy-ignorant node can use this
weakness to replace the AUTH_DATA object without destroying the
digital signature. If this was not simply an oversight, it is
therefore assumed that replay protection of the user credentials
was not considered an important security requirement, because the
hop-by-hop processing of the RSVP message protects the message
against modification by an adversary between two communicating
nodes.
The lifetime of the Kerberos ticket is based on the fields
starttime and endtime of the EncTicketPart structure in the
ticket, as described in Section 5.3.1 of [8]. Because the ticket
is created by the KDC located at the network of the verifying
entity, it is not difficult to have the clocks roughly
synchronized for the purpose of lifetime verification.
Additional information about clock-synchronization and Kerberos
can be found in [32].
If the lifetime of the Kerberos ticket expires, then a new ticket
must be requested and used. Rekeying is implemented with this
procedure.
(4) (User Identity) Confidentiality
This section discusses privacy protection of identity information
transmitted inside the policy element. User identity
confidentiality is of particular interest because there is no
built-in RSVP mechanism for encrypting the POLICY_DATA object or
the AUTH_DATA elements. Encryption of one of the attributes
inside the AUTH_DATA element, the POLICY_LOCATOR attribute, is
discussed.
To protect the user's privacy, it is important not to reveal the
user's identity to an adversary located between the user's host
and the first-hop router (e.g., on a wireless link).
Furthermore, user identities should not be transmitted outside
the domain of the visited network provider. That is, the user
identity information inside the policy data element should be
removed or modified by the PDP to prevent revealing its contents
to other (unauthorized) entities along the signaling path. It is
not possible (with the offered mechanisms) to hide the user's
identity in such a way that it is not visible to the first
policy-aware RSVP node (or to the attached network in general).
The ASCII or Unicode distinguished name of the user or
application inside the POLICY_LOCATOR attribute of the AUTH_DATA
element may be encrypted as specified in Section 3.3.1 of [7].
The user (or application) identity is then encrypted with either
the Kerberos session key or with the private key in case of
public-key-based authentication. When the private key is used,
we usually speak of a digital signature that can be verified by
everyone possessing the public key. Because the certificate with
the public key is included in the message itself, decryption is
no obstacle. Furthermore, the included certificate together with
the additional (unencrypted) information in the RSVP message
provides enough identity information for an eavesdropper. Hence,
the possibility of encrypting the policy locator in case of
public-key-based authentication is problematic. To encrypt the
identities using asymmetric cryptography, the user's host must be
able somehow to retrieve the public key of the entity verifying
the policy element (i.e., the first policy-aware router or the
PDP). Then, this public key could be used to encrypt a symmetric
key, which in turn encrypts the user's identity and certificate,
as is done, e.g., by PGP. Currently, no such mechanism is
defined in [7].
The algorithm used to encrypt the POLICY_LOCATOR with the
Kerberos session key is assumed to be the same as the one used
for encrypting the service ticket. The information about the
algorithm used is available in the etype field of the
EncryptedData ASN.1 encoded message part. Section 6.3 of [8]
lists the supported algorithms. [33] defines newer encryption
algorithms (Rijndael, Serpent, and Twofish).
Evaluating user identity confidentiality also requires looking at
protocols executed outside of RSVP (for example, the Kerberos
protocol). The ticket included in the CREDENTIAL attribute may
provide user identity protection by not including the optional
cname attribute inside the unencrypted part of the Ticket.
Because the Authenticator is not transmitted with the RSVP
message, the cname and the crealm of the unencrypted part of the
Authenticator are not revealed. In order for the user to request
the Kerberos session ticket for inclusion in the CREDENTIAL
attribute, the Kerberos protocol exchange must be executed. Then
the Authenticator sent with the TGS_REQ reveals the identity of
the user. The AS_REQ must also include the user's identity to
allow the Kerberos Authentication Server to respond with an
AS_REP message that is encrypted with the user's secret key.
Using Kerberos, it is therefore only possible to hide the content
of the encrypted policy locator, which is only useful if this
value differs from the Kerberos principal name. Hence, using
Kerberos it is not "entirely" possible to provide user identity
confidentiality.
It is important to note that information stored in the policy
element may be changed by a policy-aware router or by the policy
decision point. Which parts are changed depends upon whether
multicast or unicast is used, how the policy server reacts, where
the user is authenticated, whether the user needs to be re-
authenticated in other network nodes, etc. Hence, user-specific
and application-specific information can leak after the messages
leave the first hop within the network where the user's host is
attached. As mentioned at the beginning of this section, this
information leakage is assumed to be intentional.
(5) Authorization
In addition to the description of the authorization steps of the
Host-to-Router interface, user-based authorization is performed
with the policy element providing user credentials. The
inclusion of user and application specific information enables
policy-based admission control with special user policies that
are likely to be stored at a dedicated server. Hence, a Policy
Decision Point can query, for example, an LDAP server for a
service level agreement that states the amount of resources a
certain user is allowed to request. In addition to the user
identity information, group membership and other non-security-
related information may contribute to the evaluation of the final
policy decision. If the user is not registered to the currently
attached domain, then there is the question of how much
information the home domain of the user is willing to exchange.
This also impacts the user's privacy policy.
In general, the user may not want to distribute much of this
policy information. Furthermore, the lack of a standardized
authorization data format may create interoperability problems
when exchanging policy information. Hence, we can assume that
the policy decision point may use information from an initial
authentication and key agreement protocol (which may have already
required cross-realm communication with the user's home domain,
if only to show that the home domain knows the user and that the
user is entitled to roam), to forward accounting messages to this
domain. This represents the traditional subscriber-based
accounting scenario. Non-traditional or alternative means of
access might be deployed in the near future that do not require
any type of inter-domain communication.
Additional discussions are required to determine the expected
authorization procedures. [34] and [35] discuss authorization
issues for QoS signaling protocols. Furthermore, a number of
mobility implications for policy handling in RSVP are described
in [36].
(6) Performance
If Kerberos is used for user authentication, then a Kerberos
ticket must be included in the CREDENTIAL Section of the
AUTH_DATA element. The Kerberos ticket has a size larger than
500 bytes, but it only needs to be sent once because a
performance optimization allows the session key to be cached as
noted in Section 7.1 of [1]. It is assumed that subsequent RSVP
messages only include the POLICY_DATA INTEGRITY object with a
keyed message digest that uses the Kerberos session key.
However, this assumes that the security association required for
the POLICY_DATA INTEGRITY object is created (or modified) to
allow the selection of the correct key. Otherwise, it difficult
to say which identifier is used to index the security
association.
If Kerberos is used as an authentication system then, from a
performance perspective, the message exchange to obtain the
session key needs to be considered, although the exchange only
needs to be done once in the lifetime of the session ticket.
This is particularly true in a mobile environment with a fast
roaming user's host.
Public-key-based authentication usually provides the best
scalability characteristics for key distribution, but the
protocols are performance demanding. A major disadvantage of the
public-key-based user authentication in RSVP is the lack of a
method to derive a session key. Hence, every RSVP PATH or RESV
message includes the certificate and a digital signature, which
is a huge performance and bandwidth penalty. For a mobile
environment with low power devices, high latency, channel noise,
and low-bandwidth links, this seems to be less encouraging. Note
that a public key infrastructure is required to allow the PDP (or
the first-hop router) to verify the digital signature and the
certificate. To check for revoked certificates, certificate
revocation lists or protocols like the Online Certificate Status
Protocol [27] and the Simple Certificate Validation Protocol [28]
are needed. Then the integrity of the AUTH_DATA object can be
verified via the digital signature.
4.4. Communication between RSVP-Aware Routers
(1) Authentication
RSVP signaling messages have data origin authentication and are
protected against modification and replay with the RSVP INTEGRITY
object. The RSVP message flow between routers is protected based
on the chain of trust, and hence each router needs only a
security association with its neighboring routers. This
assumption was made because of performance advantages and because
of special security characteristics of the core network to which
no user hosts are directly attached. In the core network the
network structure does not change frequently and the manual
distribution of shared secrets for the RSVP INTEGRITY object may
be acceptable. The shared secrets may be either manually
configured or distributed by using appropriately secured network
management protocols like SNMPv3.
Independent of the key distribution mechanism, host
authentication with built-in RSVP mechanisms is accomplished
using the keyed message digest in the RSVP INTEGRITY object,
computed using the previously exchanged symmetric key.
(2) Integrity Protection
Integrity protection is accomplished with the RSVP INTEGRITY
object with the variable length Keyed Message Digest field.
(3) Replay Protection
Replay protection with the RSVP INTEGRITY object is extensively
described in previous sections. To enable crashed hosts to learn
the latest sequence number used, the Integrity Handshake
mechanism is provided in RSVP.
(4) Confidentiality
Confidentiality is not provided by RSVP.
(5) Authorization
Depending on the RSVP network, QoS resource authorization at
different routers may need to contact the PDP again. Because the
PDP is allowed to modify the policy element, a token may be added
to the policy element to increase the efficiency of the re-
authorization procedure. This token is used to refer to an
already computed policy decision. The communications interface
from the PEP to the PDP must be properly secured.
(6) Performance
The performance characteristics for the protection of the RSVP
signaling messages is largely determined by the key exchange
protocol, because the RSVP INTEGRITY object is only used to
compute a keyed message digest of the transmitted signaling
messages.
The security associations within the core network, that is,
between individual routers (in comparison with the security
association between the user's host and the first-hop router or
with the attached network in general), can be established more
easily because of the normally strong trust assumptions.
Furthermore, it is possible to use security associations with an
increased lifetime to avoid frequent rekeying. Hence, there is
less impact on the performance compared with the user-to-network
interface. The security association storage requirements are
also less problematic.
5. Miscellaneous Issues
This section describes a number of issues that illustrate some of the
shortcomings of RSVP with respect to security.
5.1. First-Hop Issue
In case of end-to-end signaling, an end host starts signaling to its attached network. The first-hop communication is often more difficult to secure because of the different requirements and a missing trust relationship. An end host must therefore obtain some information to start RSVP signaling: o Does this network support RSVP signaling? o Which node supports RSVP signaling? o To which node is authentication required? o Which security mechanisms are used for authentication? o Which algorithms are required? o Where should the keys and security associations come from? o Should a security association be established? RSVP, as specified today, is used as a building block. Hence, these questions have to be answered as part of overall architectural considerations. Without answers to these questions, ad hoc RSVP communication by an end host roaming to an unknown network is not possible. A negotiation of security mechanisms and algorithms is not supported for RSVP.
(page 30 continued on part 3)
