NSIS H. Tschofenig
Internet-Draft Siemens
Expires: August 21, 2005 R. Graveman
RFG Security
February 20, 2005
RSVP Security Properties
draft-ietf-nsis-rsvp-sec-properties-06.txt
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Copyright (C) The Internet Society (2005).
Abstract
This document summarizes the security properties of RSVP. The goal
of this analysis is to benefit from previous work done on RSVP and to
capture knowledge about past activities.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Architectural Assumptions . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 The RSVP INTEGRITY Object . . . . . . . . . . . . . . . . 6
3.2 Security Associations . . . . . . . . . . . . . . . . . . 8
3.3 RSVP Key Management Assumptions . . . . . . . . . . . . . 9
3.4 Identity Representation . . . . . . . . . . . . . . . . . 9
3.5 RSVP Integrity Handshake . . . . . . . . . . . . . . . . . 13
4. Detailed Security Property Discussion . . . . . . . . . . . . 15
4.1 Network Topology . . . . . . . . . . . . . . . . . . . . . 15
4.2 Host/Router . . . . . . . . . . . . . . . . . . . . . . . 15
4.3 User to PEP/PDP . . . . . . . . . . . . . . . . . . . . . 19
4.4 Communication between RSVP-Aware Routers . . . . . . . . . 26
5. Miscellaneous Issues . . . . . . . . . . . . . . . . . . . . . 29
5.1 First Hop Issue . . . . . . . . . . . . . . . . . . . . . 29
5.2 Next-Hop Problem . . . . . . . . . . . . . . . . . . . . . 29
5.3 Last-Hop Issue . . . . . . . . . . . . . . . . . . . . . . 32
5.4 RSVP and IPsec protected data traffic . . . . . . . . . . 33
5.5 End-to-End Security Issues and RSVP . . . . . . . . . . . 35
5.6 IPsec protection of RSVP signaling messages . . . . . . . 35
5.7 Authorization . . . . . . . . . . . . . . . . . . . . . . 36
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 37
7. Security Considerations . . . . . . . . . . . . . . . . . . . 39
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 40
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 41
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 42
10.1 Normative References . . . . . . . . . . . . . . . . . . . . 42
10.2 Informative References . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 45
A. Dictionary Attacks and Kerberos . . . . . . . . . . . . . . . 47
B. Example of User-to-PDP Authentication . . . . . . . . . . . . 48
C. Literature on RSVP Security . . . . . . . . . . . . . . . . . 49
Intellectual Property and Copyright Statements . . . . . . . . 50
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1. Introduction
As the work of the NSIS working group has begun, there are also
concerns about security and its implications for the design of a
signaling protocol. In order to understand the security properties
and available options of RSVP a number of documents have to be read.
This document summarizes the security properties of RSVP and is part
of the overall process of analyzing other signaling protocols and
learning from their design considerations. This document should also
provide a starting point for further discussions.
The content of this document is organized as follows:
Section 3 provides an overview of the security mechanisms provided by
RSVP including the INTEGRITY object, a description of the identity
representation within the POLICY_DATA object (i.e., user
authentication), and the RSVP Integrity Handshake mechanism. Section
4 provides a more detailed discussion of the mechanisms used and
tries to describe in detail the mechanisms provided.
RSVP also supports multicast but this document does not address
security aspects for supporting multicast QoS signaling. Multicast
is currently outside the scope of the NSIS working group.
Although a variation of RSVP, namely RSVP-TE, is used in the context
of MPLS to distribute labels for a label switched path its usage is
different than the usage scenarios envisioned for NSIS. Hence, this
document does not address RSVP-TE and the security properties of it.
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2. Terminology and Architectural Assumptions
This section describes some important terms and explains some
architectural assumptions:
Chain-of-Trust:
The security mechanisms supported by RSVP [1] heavily rely on
optional hop-by-hop protection using the built-in INTEGRITY
object. Hop-by-hop security with the INTEGRITY object inside the
RSVP message thereby refers to the protection between
RSVP-supporting network elements. Additionally, there is the
notion of policy-aware network elements that understand the
POLICY_DATA element within the RSVP message. Because this element
also includes an INTEGRITY object, there is an additional
hop-by-hop security mechanism that provides security between
policy-aware nodes. Policy-ignorant nodes are not affected by the
inclusion of this object in the POLICY_DATA element, because they
do not try to interpret it.
To protect signaling messages that are possibly modified by each
RSVP router along the path, it must be assumed that each incoming
request is authenticated, integrity protected, and replay
protected. This provides protection against unauthorized nodes'
injecting bogus messages. Furthermore, each RSVP-aware router is
assumed to behave in the expected manner. Outgoing messages
transmitted to the next hop network element receive protection
according RSVP security processing.
Using the above described mechanisms, a chain-of-trust is created
whereby a signaling message transmitted by router A via router B
and received by router C is supposed to be secure if routers A and
B and routers B and C share security associations and all routers
behave as expected. Hence router C trusts router A although
router C does not have a direct security association with router
A. We can therefore conclude that the protection achieved with
this hop-by-hop security for the chain-of-trust is no better than
the weakest link in the chain.
If one router is malicious (for example because an adversary has
control over this router), then it can arbitrarily modify
messages, cause unexpected behavior, and mount a number of attacks
not limited only to QoS signaling. Additionally, it must be
mentioned that some protocols demand more protection than others
(which depends in part on which nodes are executing these
protocols). For example, edge devices, where end-users are
attached, may more likely be attacked in comparison with the more
secure core network of a service provider. In some cases a
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network service provider may choose not to use the RSVP-provided
security mechanisms inside the core network because a different
security protection is deployed.
Section 6 of [2] mentions the term chain-of-trust in the context
of RSVP integrity protection. In Section 6 of [18] the same term
is used in the context of user authentication with the INTEGRITY
object inside the POLICY_DATA element . Unfortunately the term is
not explained in detail and the assumptions behind it are not
clearly specified.
Host and User Authentication:
The presence of RSVP protection and a separate user identity
representation leads to the fact that both user-identity and
host-identity are used for RSVP protection. Therefore, user-based
security and host-based security are covered separately, because
of the different authentication mechanisms provided. To avoid
confusion about the different concepts, Section 3.4 describes the
concept of user authentication in more detail.
Key Management:
It is assumed that most of the security associations required for
the protection of RSVP signaling messages are already available,
and hence key management was done in advance. There is, however,
an exception with respect to support for Kerberos. Using
Kerberos, an entity is able to distribute a session key used for
RSVP signaling protection.
RSVP INTEGRITY and POLICY_DATA INTEGRITY Objects:
RSVP uses an INTEGRITY object in two places in a message. The
first is in the RSVP message itself and covers the entire RSVP
message as defined in [1]. The second is included in the
POLICY_DATA object and defined in [2]. To differentiate the two
objects regarding their scope of protection, the two terms RSVP
INTEGRITY and POLICY_DATA INTEGRITY object are used, respectively.
The data structure of the two objects, however, is the same.
Hop versus Peer:
In the past, the terminology for nodes addressed by RSVP has been
discussed considerably. In particular, two favorite terms have
been used: hop and peer. This document uses the term hop, which
is different from an IP hop. Two neighboring RSVP nodes
communicating with each other are not necessarily neighboring IP
nodes (i.e., they may be more than one IP hop away).
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3. Overview
This section describes the security mechanisms provided by RSVP.
Although use of IPsec is mentioned in Section 10 of [1], the security
mechanisms primarily envisioned for RSVP are described.
3.1 The RSVP INTEGRITY Object
The RSVP INTEGRITY object is the major component of RSVP security
protection. This object is used to provide integrity and replay
protection for the content of the signaling message between two RSVP
participating routers or between an RSVP router and host.
Furthermore, the RSVP INTEGRITY object provides data origin
authentication. The attributes of the object are briefly described:
Flags field:
The Handshake Flag is the only defined flag. It is used to
synchronize sequence numbers if the communication gets out of sync
(e.g., it allows a restarting host to recover the most recent
sequence number). Setting this flag to one indicates that the
sender is willing to respond to an Integrity Challenge message.
This flag can therefore be seen as a negotiation capability
transmitted within each INTEGRITY object.
Key Identifier:
The Key Identifier selects the key used for verification of the
Keyed Message Digest field and, hence, must be unique for the
sender. It has a fixed 48-bit length. The generation of this Key
Identifier field is mostly a decision of the local host. [1]
describes this field as a combination of an address, sending
interface, and key number. We assume that the Key Identifier is
simply a (keyed) hash value computed over a number of fields with
the requirement to be unique if more than one security association
is used in parallel between two hosts (e.g., as is the case with
security associations having overlapping lifetimes). A receiving
system uniquely identifies a security association based on the Key
Identifier and the sender's IP address. The sender's IP address
may be obtained from the RSVP_HOP object or from the source IP
address of the packet if the RSVP_HOP object is not present. The
sender uses the outgoing interface to determine which security
association to use. The term outgoing interface may be confusing.
The sender selects the security association based on the
receiver's IP address (i.e., the address of the next RSVP-capable
router). The process of determining which node is the next
RSVP-capable router is not further specified and is likely to be
statically configured.
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Sequence Number:
The sequence number used by the INTEGRITY object is 64 bits in
length, and the starting value can be selected arbitrarily. The
length of the sequence number field was chosen to avoid exhaustion
during the lifetime of a security association as stated in Section
3 of [1]. In order for the receiver to distinguish between a new
and a replayed message, the sequence number must be monotonically
incremented modulo 2^64 for each message. We assume that the
first sequence number seen (i.e., the starting sequence number) is
stored somewhere. The modulo-operation is required because the
starting sequence number may be an arbitrary number. The receiver
therefore only accepts packets with a sequence number larger
(modulo 2^64) than the previous packet. As explained in [1] this
process is started by handshaking and agreeing on an initial
sequence number. If no such handshaking is available then the
initial sequence number must be part of the establishment of the
security association.
The generation and storage of sequence numbers is an important
step in preventing replay attacks and is largely determined by the
capabilities of the system in presence of system crashes, failures
and restarts. Section 3 of [1] explains some of the most
important considerations. However, the description of how the
receiver distinguishes proper from improper sequence numbers is
incomplete--it implicitly assumes that gaps large enough to cause
the sequence number to wrap around cannot occur.
If delivery in order were guaranteed, the following procedure
would work: The receiver keeps track of the first sequence number
received, INIT-SEQ, and most recent sequence number received,
LAST-SEQ, for each key identifier in a security association. When
the first message is received, set INIT-SEQ = LAST-SEQ = value
received and accept. When a subsequent message is received, if
its sequence number is strictly between LAST-SEQ and INIT-SEQ,
modulo 2^64, accept and update LAST-SEQ with the value just
received. If it is between INIT-SEQ and LAST-SEQ, inclusive,
modulo 2^64, reject and leave the value of LAST-SEQ unchanged.
Because delivery in order is not guaranteed, the above rules need
to be combined with a method of allowing a fixed sized window in
the neighborhood of LAST-SEQ for out-of-order delivery, for
example, as described in Appendix C of [3].
Keyed Message Digest:
The Keyed Message Digest is a security mechanism built into RSVP
and used to provide integrity protection of a signaling message
(including its sequence number). Prior to computing the value for
the Keyed Message Digest field, the Keyed Message Digest field
itself must be set to zero and a keyed hash computed over the
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entire RSVP packet. The Keyed Message Digest field is variable in
length but must be a multiple of four octets. If HMAC-MD5 is
used, then the output value is 16 bytes long. The keyed hash
function HMAC-MD5 [4] is required for a RSVP implementation as
noted in Section 1 of [1]. Hash algorithms other than MD5 [5]
like SHA-1 [19] may also be supported.
The key used for computing this Keyed Message Digest may be
obtained from the pre-shared secret, which is either manually
distributed or the result of a key management protocol. No key
management protocol, however, is specified to create the desired
security associations. Also, no guidelines for key length are
given. It should be recommended that HMAC-MD5 keys be 128 bits
and SHA-1 key 160 bits, as in IPsec AH [20] and ESP [21].
3.2 Security Associations
Different attributes are stored for security associations of sending
and receiving systems (i.e., unidirectional security associations).
The sending system needs to maintain the following attributes in such
a security association [1]:
o Authentication algorithm and algorithm mode
o Key
o Key Lifetime
o Sending Interface
o Latest sequence number (received with this key identifier)
The receiving system has to store the following fields:
o Authentication algorithm and algorithm mode
o Key
o Key Lifetime
o Source address of the sending system
o List of last n sequence numbers (received with this key
identifier)
Note that the security associations need to have additional fields to
indicate their state. It is necessary to have an overlapping
lifetime of security associations to avoid interrupting an ongoing
communication because of expired security associations. During such
a period of overlapping lifetime it is necessary to authenticate
either one or both active keys. As mentioned in [1], a sender and a
receiver may have multiple active keys simultaneously.If more than
one algorithm is supported then the algorithm used must be specified
for a security association.
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3.3 RSVP Key Management Assumptions
[6] assumes that security associations are already available. An
implementation must support manual key distribution as noted in
Section 5.2 of [1]. Manual key distribution, however, has different
requirements for key storage - a simple plaintext ASCII file may be
sufficient in some cases. If multiple security associations with
different lifetimes need to be supported at the same time, then a key
engine would be more appropriate. Further security requirements
listed in Section 5.2 of [1] are the following:
o The manual deletion of security associations must be supported.
o The key storage should persist a system restart.
o Each key must be assigned a specific lifetime and a specific Key
Identifier.
3.4 Identity Representation
In addition to host-based authentication with the INTEGRITY object
inside the RSVP message, user-based authentication is available as
introduced in [2]. Section 2 of [7] states that "Providing policy
based admission control mechanism based on user identities or
application is one of the prime requirements." To identify the user
or the application, a policy element called AUTH_DATA, which is
contained in the POLICY_DATA object, is created by the RSVP daemon at
the user's host and transmitted inside the RSVP message. The
structure of the POLICY_DATA element is described in [2]. Network
nodes like the policy decision point (PDP) then use the information
contained in the AUTH_DATA element to authenticate the user and to
allow policy-based admission control to be executed. As mentioned in
[7], the policy element is processed and the PDP replaces the old
element with a new one for forwarding to the next hop router.
A detailed description of the POLICY_DATA element can be found in
[2]. The attributes contained in the authentication data policy
element AUTH_DATA, which is defined in [7], are briefly explained in
this Section. Figure 1 shows the abstract structure of the RSVP
message with its security-relevant objects and the scope of
protection. The RSVP INTEGRITY object (outer object) covers the
entire RSVP message, whereas the POLICY_DATA INTEGRITY object only
covers objects within the POLICY_DATA element.
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+--------------------------------------------------------+
| RSVP Message |
+--------------------------------------------------------+
| Object |POLICY_DATA Object ||
| +-------------------------------------------+|
| | INTEGRITY +------------------------------+||
| | Object | AUTH_DATA Object |||
| | +------------------------------+||
| | | Various Authentication |||
| | | Attributes |||
| | +------------------------------+||
| +-------------------------------------------+|
+--------------------------------------------------------+
Figure 1: Security Relevant Objects and Elements within the RSVP
Message
The AUTH_DATA object contains information for identifying users and
applications together with credentials for those identities. The
main purpose of these identities seems to be usage for policy-based
admission control and not authentication and key management. As
noted in Section 6.1 of [7], an RSVP message may contain more than
one POLICY_DATA object and each of them may contain more than one
AUTH_DATA object. As indicated in Figure 1 and in [7], one AUTH_DATA
object may contain more than one authentication attribute. A typical
configuration for Kerberos-based user authentication includes at
least the Policy Locator and an attribute containing the Kerberos
session ticket.
Successful user authentication is the basis for executing
policy-based admission control. Additionally, other information such
as time-of-day , application type, location information, group
membership, etc. may be relevant to implement an access control
policy.
The following attributes are defined for the usage in the AUTH_DATA
object:
1. Policy Locator
* ASCII_DN
* UNICODE_DN
* ASCII_DN_ENCRYPT
* UNICODE_DN_ENCRYPT
The policy locator string that is an X.500 distinguished name
(DN) used to locate user or application specific policy
information. The following types of X.500 DNs are listed:
The first two types are the ASCII and the Unicode representation
of the user or application DN identity. The two "encrypted"
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distinguished name types are either encrypted with the Kerberos
session key or with the private key of the user's digital
certificate (i.e., digitally signed). The term encrypted
together with a digital signature is easy to misconceive. If
user identity confidentiality is provided, then the policy
locator has to be encrypted with the public key of the recipient.
How to obtain this public key is not described in the document.
Such an issue may be specified in a concrete architecture where
RSVP is used.
2. Credentials
Two cryptographic credentials are currently defined for a user:
Authentication with Kerberos V5 [8], and authentication with the
help of digital signatures based on X.509 [22] and PGP [23]. The
following list contains all defined credential types currently
available and defined in [7]:
+--------------+--------------------------------+
| Credential | Description |
| Type | |
+===============================================|
| ASCII_ID | User or application identity |
| | encoded as an ASCII string |
+--------------+--------------------------------+
| UNICODE_ID | User or application identity |
| | encoded as a Unicode string |
+--------------+--------------------------------+
| KERBEROS_TKT | Kerberos V5 session ticket |
+--------------+--------------------------------+
| X509_V3_CERT | X.509 V3 certificate |
+--------------+--------------------------------+
| PGP_CERT | PGP certificate |
+--------------+--------------------------------+
Figure 2: Credentials Supported in RSVP
The first two credentials contain only a plaintext string, and
therefore they do not provide cryptographic user authentication.
These plaintext strings may be used to identify applications,
which are included for policy-based admission control. Note that
these plain-text identifiers may, however, be protected if either
the RSVP INTEGRITY or the INTEGRITY object of the POLICY_DATA
element is present. Note that the two INTEGRITY objects can
terminate at different entities depending on the network
structure. The digital signature may also provide protection of
application identifiers. A protected application identity (and
the entire content of the POLICY_DATA element) cannot be modified
as long as no policy ignorant nodes are encountered in between.
A Kerberos session ticket, as previously mentioned, is the ticket
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of a Kerberos AP_REQ message [8] without the Authenticator.
Normally, the AP_REQ message is used by a client to authenticate
to a server. The INTEGRITY object (e.g., of the POLICY_DATA
element) provides the functionality of the Kerberos
Authenticator, namely protecting against replay and showing that
the user was able to retrieve the session key following the
Kerberos protocol. This is, however, only the case if the
Kerberos session was used for the keyed message digest field of
the INTEGRITY object. Section 7 of [1] discusses some issues for
establishment of keys for the INTEGRITY object. The
establishment of the security association for the RSVP INTEGRITY
object with the inclusion of the Kerberos Ticket within the
AUTH_DATA element may be complicated by the fact that the ticket
can be decrypted by node B whereas the RSVP INTEGRITY object
terminates at a different host C. The Kerberos session ticket
contains, among many other fields, the session key. The Policy
Locator may also be encrypted with the same session key. The
protocol steps that need to be executed to obtain such a Kerberos
service ticket are not described in [7] and may involve several
roundtrips depending on many Kerberos-related factors. The
Kerberos ticket does not need to be included in every RSVP
message as an optimization, as described in Section 7.1 of [1].
Thus the receiver must store the received service ticket. If the
lifetime of the ticket has expired, then a new service ticket
must be sent. If the receiver lost its state information
(because of a crash or restart) then it may transmit an Integrity
Challenge message to force the sender to re-transmit a new
service ticket.
If either the X.509 V3 or the PGP certificate is included in the
policy element, then a digital signature must be added. The
digital signature computed over the entire AUTH_DATA object
provides authentication and integrity protection. The SubType of
the digital signature authentication attribute is set to zero
before computing the digital signature. Whether or not a
guarantee of freshness with replay protection (either timestamps
or sequence numbers) is provided by the digital signature is an
open issue as discussed in Section 4.3
3. Digital Signature
The digital signature computed over the data of the AUTH_DATA
object must be the last attribute. The algorithm used to compute
the digital signature depends on the authentication mode listed
in the credential. This is only partially true, because, for
example, PGP again allows different algorithms to be used for
computing a digital signature. The algorithm identifier used for
computing the digital signature is not included in the
certificate itself. The algorithm identifier included in the
certificate only serves the purpose of allowing the verification
of the signature computed by the certificate authority (except
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for the case of self-signed certificates).
4. Policy Error Object
The Policy Error Object is used in the case of a failure of
policy-based admission control or other credential verification.
Currently available error messages allow notification if the
credentials are expired (EXPIRED_CREDENTIALS), if the
authorization process disallowed the resource request
(INSUFFICIENT_PRIVILEGES), or if the given set of credentials is
not supported (UNSUPPORTED_CREDENTIAL_TYPE). The last error
message returned by the network allows the user's host to
discover the type of credentials supported. Particularly for
mobile environments this might be quite inefficient.
Furthermore, it is unlikely that a user supports different types
of credentials. The purpose of the error message
IDENTITY_CHANGED is unclear. Also, the protection of the error
message is not discussed in [7].
3.5 RSVP Integrity Handshake
The Integrity Handshake protocol was designed to allow a crashed or
restarted host to obtain the latest valid challenge value stored at
the receiving host. Due to the absence of key management, it must be
guaranteed that two messages do not use the same sequence number with
the same key. A host stores the latest sequence number of a
cryptographically verified message. An adversary can replay
eavesdropped packets if the crashed host has lost its sequence
numbers. A signaling message from the real sender with a new
sequence number would therefore allow the crashed host to update the
sequence number field and prevent further replays. Hence, if there
is a steady flow of RSVP protected messages between the two hosts, an
attacker may find it difficult to inject old messages, because new,
authenticated messages with higher sequence numbers arrive and get
stored immediately.
The following description explains the details of a RSVP Integrity
Handshake that is started by Node A after recovering from a
synchronization failure:
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Integrity Challenge
(1) Message (including
+----------+ a Cookie) +----------+
| |-------------------------->| |
| Node A | | Node B |
| |<--------------------------| |
+----------+ Integrity Response +----------+
(2) Message (including
the Cookie and the
INTEGRITY object)
Figure 3: RSVP Integrity Handshake
The details of the messages are as follows:
CHALLENGE:=(Key Identifier, Challenge Cookie)
Integrity Challenge Message:=(Common Header, CHALLENGE)
Integrity Response Message:=(Common Header, INTEGRITY, CHALLENGE)
The "Challenge Cookie" is suggested to be a MD5 hash of a local
secret and a timestamp [1].
The Integrity Challenge message is not protected with an INTEGRITY
object as shown in the protocol flow above. As explained in Section
10 of [1] this was done to avoid problems in situations where both
communicating parties do not have a valid starting sequence number.
Using the RSVP Integrity Handshake protocol is recommended although
it is not mandatory (since it may not be needed in all network
environments).
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4. Detailed Security Property Discussion
The purpose of this section is to describe the protection of the
RSVP-provided mechanisms individually for authentication,
authorization, integrity and replay protection, user identity
confidentiality, and confidentiality of the signaling messages.
4.1 Network Topology
The main purpose of this paragraph is to show the basic interfaces in
a simple RSVP network architecture. The architecture below assumes
that there is only a single domain and that 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 a
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 . By using the RSVP INTEGRITY object the host is
authenticated while 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
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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 the 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 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]. [24] 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 requires
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
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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 must support the most
important keyed hash algorithms for example MD5, SHA-1,
RIPEMD-160, etc. HMAC-MD5 was mainly chosen because of its
performance characteristics. The weaknesses of MD5 [25] are
known and described in [26]. Other algorithms like SHA-1 [19]
and RIPEMD-160 [25] 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. 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 a typo. 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
Section 3.5. The Cookie value is suggested to be 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 including 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
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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, in
addition to users' identities, also billing data, network usage
patterns, or network configurations 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 [27] in combination with AAA protocols
(for example using RADIUS [28] 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., by 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 or not
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 a 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 a RSVP routers
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 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
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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, both user-based and host-based
authentication are supported by RSVP. 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 [29]. Another architecture
in which RSVP is tightly integrated is the one specified by the
PacketCable organization. The interested reader is referred to [30]
for a discussion of their security architecture.
1. Authentication
When a user sends a 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), which has to authenticate
the user, could frequently download certificate revocation lists
or use a protocol like the Online Certificate Status Protocol
(OCSP) [31] and the Simple Certificate Validation Protocol (SCVP)
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[32] 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 needs to 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 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 forwarding the request only 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 only provides
unilateral authentication whereby the client authenticates to the
router or to the PDP. If a RSVP message is sent to the user's
host and public key based authentication is 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. [7] does not discuss this issue and therefore there
is no particular requirement dealing with 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
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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 and 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. Which
hash algorithm and public key algorithm are used for the digital
signature computation is described in [23] in the case of PGP.
In the case of X.509 credentials the situation is more complex,
because different mechanisms like CMS [33] or PKCS#7 [34] 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 and PKCS#7.
[7], however, does not make any statements about the usage of CMS
and PKCS#7. Currently there is no support for CMS or PKCS#7
described in [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 [23]), 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. Additionally, the large
processing, memory, and bandwidth utilization should 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:
3. 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
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security association that is established by other means.
4. Including only the Kerberos session ticket is insufficient,
because freshness is not provided (since 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).
5. 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
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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 [35] and in [23]. 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
OpenPGP version [23] 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 [36].
If the lifetime of the Kerberos ticket expires, then a new
ticket must be requested and used. Rekeying is implemented
with this procedure.
3. (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
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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). User
identities should furthermore not be transmitted outside the
domain of the visited network provider, i.e., the user identity
information inside the policy data element should be removed or
modified by the PDP to prevent revealing its contents to other
(non-authorized) 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 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. [12] defines new encryption
algorithms (Rijndael, Serpent, and Twofish).
Evaluating user identity confidentiality requires also 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
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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 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.
4. 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, a LDAP server for a
service level agreement stating 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 assume that
the home domain knows the user and that the user is entitled to
roam and to be able to forward accounting messages to this
domain. This represents the traditional subscriber-based
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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. [37] and [38] discuss authorization
issues for QoS signaling protocols. Furthermore, a number of
mobililty implications for policy handling in RSVP are described
in [39]
5. 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 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. This,
however, 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.
When 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 [31] and the Simple Certificate Validation Protocol [32]
are needed. Then the integrity of the AUTH_DATA object via the
digital signature can be verified.
4.4 Communication between RSVP-Aware Routers
1. Authentication
RSVP signaling messages are data origin authenticated and
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protected against modification and replay using the RSVP
INTEGRITY object. The RSVP message flow between routers is
protected based on the chain of trust and hence each router only
needs to have 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 where 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 RSVP built-in mechanisms is accomplished with
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, i.e., 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.
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The security association storage requirements are also less
problematic.
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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 have to be used?
o Where should the keys and security association 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 giving an answer to this question, 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.
5.2 Next-Hop Problem
Throughout the document it was assumed that the next RSVP node along
the path is always known. Knowing your next hop is important to be
able to select the correct key for the RSVP Integrity object and to
apply the proper protection. In case in which an RSVP node assumes
it knows which node is the next hop the following protocol exchange
can occur:
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Integrity
(A<->C) +------+
(3) | RSVP |
+------------->+ Node |
| | B |
Integrity | +--+---+
(A<->C) | |
+------+ (2) +--+----+ |
(1) | RSVP +----------->+Router | | Error
----->| Node | | or +<-----------+ (I am B)
| A +<-----------+Network| (4)
+------+ (5) +--+----+
Error .
(I am B) . +------+
. | RSVP |
...............+ Node |
| C |
+------+
Figure 6: Next-Hop Issue
When RSVP node A in Figure 6 receives an incoming RSVP Path message,
standard RSVP message processing takes place. Node A then has to
decide which key to select to protect the signaling message. We
assume that some unspecified mechanism is used to make this decision.
In this example node A assumes that the message will travel to RSVP
node C. However, because of some reasons (e.g. a route change,
inability to learn the next RSVP hop along the path, etc.) the
message travels to node B via a non-RSVP supporting router that
cannot verify the integrity of the message (or cannot decrypt the
Kerberos service ticket). The processing failure causes a PathErr
message to be returned to the originating sender of the Path message.
This error message also contains information about the node
recognizing the error. In many cases a security association might
not be available. Node A receiving the PathErr message might use the
information returned with the PathErr message to select a different
security association (or to establish one).
Figure 6 describes a behavior that might help node A learn that an
error occurred. However, the description of Section 4.2 of [1]
describes in step (5) that a signaling message is silently discarded
if the receiving host cannot properly verify the message: "If the
calculated digest does not match the received digest, the message is
discarded without further processing." For RSVP Path and similar
messages this functionality is not really helpful.
The RSVP Path message therefore provides a number of functions: path
discovery, detecting route changes, learning of QoS capabilities
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along the path using the Adspec object, (with some interpretation)
next-hop discovery, and possibly security association establishment
(for example, in the case of Kerberos).
From a security point of view there is a conflict between
o Idempotent message delivery and efficiency
The RSVP Path message especially performs a number of functions.
Supporting idempotent message delivery somehow contradicts with
security association establishment, efficient message delivery,
and message size. For example, a "real" idempotent signaling
message would contain enough information to perform security
processing without depending on a previously executed message
exchange. Adding a Kerberos ticket with every signaling message
is, however, inefficient. Using public key based mechanisms is
even more inefficient when included in every signaling message.
With public key based protection for idempotent messages, there is
additionally a risk of introducing denial of service attacks.
o RSVP Path message functionality and next-hop discovery
To protect an RSVP signaling message (and a RSVP Path message in
particular) it is necessary to know the identity of the next
RSVP-aware node (and some other parameters). Without a mechanism
for next-hop discovery, an RSVP Path message is also responsible
for this task. Without knowing the identity of the next hop, the
Kerberos principal name is also unknown. The so-called Kerberos
user-to-user authentication mechanism, which would allow the
receiver to trigger the process of establishing Kerberos
authentication, is not supported. This issue will again be
discussed in relationship with the last-hop problem.
It is fair to assume that a RSVP-supporting node might not have
security associations with all immediately neighboring RSVP nodes.
Especially for inter-domain signaling, IntServ over DiffServ, or
some new applications such as firewall signaling, the next
RSVP-aware node might not be known in advance. The number of next
RSVP nodes might be considerably large if they are separated by a
large number of non-RSVP aware nodes. Hence, a node transmitting
a RSVP Path message might experience difficulties in properly
protecting the message if it serves as a mechanism to detect both
the next RSVP node (i.e., Router Alert Option added to the
signaling message and addressed to the destination address) and to
detect route changes. It is fair to note that in an intra-domain
case with a dense distribution of RSVP nodes this might be
possible with manual configuration.
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Nothing prevents an adversary from continuously flooding an RSVP
node with bogus PathErr messages, although it might be possible to
protect the PathErr message with an existing, available security
association. A legitimate RSVP node would believe that a change
in the path took place. Hence, this node might try to select a
different security association or try to create one with the
indicated node. If an adversary is located somewhere along the
path and either authentication or authorization is not performed
with the necessary strength and accuracy, then it might also be
possible to act as a man-in-the-middle. One method of reducing
susceptibility to this attack is as follows: when a PathErr
message is received from a node with which no security association
exists, attempt to establish a security association and then
repeat the action that led to the PathErr message.
5.3 Last-Hop Issue
This section tries to address practical difficulties when
authentication and key establishment are accomplished with a
two-party protocol that shows some asymmetry in message processing.
Kerberos is such a protocol and also the only supported protocol that
provides dynamic session key establishment for RSVP. For first-hop
communication, authentication is typically done between a user and
some router (for example the access router). Especially in a mobile
environment, it is not feasible to authenticate end hosts based on
their IP or MAC address. To illustrate this problem, the typical
processing steps for Kerberos are shown for first-hop communication:
1. The end host A learns the identity (i.e., Kerberos principal
name) of some entity B. This entity B is either the next RSVP
node, a PDP, or the next policy-aware RSVP node.
2. Entity A then requests a ticket granting ticket for the network
domain. This assumes that the identity of the network domain is
known.
3. Entity A then requests a service ticket for entity B, whose name
was learned in step (a).
4. Entity A includes the service ticket with the RSVP signaling
message (inside the policy object). The Kerberos session key is
used to protect the integrity of the entire RSVP signaling
message.
For last-hop communication this processing step theoretically has to
be reversed; entity A is then a node in the network (for example the
access router) and entity B is the other end host (under the
assumption that RSVP signaling is accomplished between two end hosts
and not between an end host and a application server). The access
router might, however, in step (a) not be able to learn the user's
principal name, because this information might not be available.
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Entity A could reverse the process by triggering an IAKERB exchange.
This would cause entity B to request a service ticket for A as
described above. IAKERB is however not supported in RSVP.
5.4 RSVP and IPsec protected data traffic
QoS signaling requires flow information to be established at routers
along a path. This flow identifier installed at each device tells
the router which data packets should receive QoS treatment. RSVP
typically establishes a flow identifier based on the 5-tuple (source
IP address, destination IP address, transport protocol type, source
port, and destination port). If this 5-tuple information is not
available, then other identifiers have to be used. IPsec-protected
data traffic is such an example where the transport protocol and the
port numbers are not accessible. Hence the IPsec SPI is used as a
substitute for them. [13] considers these IPsec implications for
RSVP and is based on three assumptions:
1. An end host, which initiates the RSVP signaling message exchange,
has to be able to retrieve the SPI for given flow. This requires
some interaction with the IPsec security association database
(SAD) and security policy database (SPD) [3]. An application
usually does not know the SPI of the protected flow and cannot
provide the desired values. It can provide the signaling
protocol daemon with flow identifiers. The signaling daemon
would then need to query the SAD by providing the flow
identifiers as input parameters and the SPI as an output
parameter.
2. [13] assumes end-to-end IPsec protection of the data traffic. If
IPsec is applied in a nested fashion, then parts of the path do
not experience QoS treatment. This can be treated as a tunneling
problem, but it is initiated by the end host. A figure better
illustrates the problem in the case of enforcing secure network
access:
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+------+ +---------------+ +--------+ +-----+
| Host | | Security | | Router | | Host|
| A | | Gateway (SGW) | | Rx | | B |
+--+---+ +-------+-------+ +----+---+ +--+--+
| | | |
|IPsec-Data( | | |
| OuterSrc=A, | | |
| OuterDst=SGW, | | |
| SPI=SPI1, | | |
| InnerSrc=A, | | |
| OuterDst=B, | | |
| Protocol=X, |IPsec-Data( | |
| SrcPort=Y, | SrcIP=A, | |
| DstPort=Z) | DstIP=B, | |
|=====================>| Protocol=X, |IPsec-Data( |
| | SrcPort=Y, | SrcIP=A, |
| --IPsec protected-> | DstPort=Z) | DstIP=B, |
| data traffic |------------------>| Protocol=X, |
| | | SrcPort=Y, |
| | | DstPort=Z) |
| | |---------------->|
| | | |
| | --Unprotected data traffic-> |
| | | |
Figure 7: RSVP and IPsec protected data traffic
Host A transmitting data traffic would either indicate a 3-tuple
<A, SGW, SPI1> or a 5-tuple <A, B, X, Y, Z>. In any case it is
not possible to make a QoS reservation for the entire path. Two
similar examples are remote access using a VPN and protection of
data traffic between a home agent (or a security gateway in the
home network) and a mobile node. With a nested application of
IPsec (for example, IPsec between A and SGW and between A and B)
the same problem occurs.
One possible solution to this problem is to change the flow
identifier along the path to capture the new flow identifier
after an IPsec endpoint.
IPsec tunnels that neither start nor terminate at one of the
signaling end points (for example between two networks) should be
addressed differently by recursively applying an RSVP signaling
exchange for the IPsec tunnel. RSVP signaling within tunnels is
addressed in [14].
3. It is assumed that SPIs do not change during the lifetime of the
established QoS reservation. If a new IPsec SA is created, then
a new SPI is allocated for the security association. To reflect
this change, either a new reservation has to be established or
the flow identifier of the existing reservation has to be
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updated. Because IPsec SAs usually have a longer lifetime, this
does not seem to be a major issue. IPsec protection of SCTP data
traffic might more often require an IPsec SA (and an SPI) change
to reflect added and removed IP addresses from an SCTP
association.
5.5 End-to-End Security Issues and RSVP
End-to-end security for RSVP has not been discussed throughout the
document. In this context end-to-end security refers to credentials
transmitted between the two end hosts using RSVP. It is obvious that
care must be taken to ensure that routers along the path are able to
process and modify the signaling messages according to prescribed
processing procedures. Some objects or mechanisms, however, could be
used for end-to-end protection. The main question however is what
the benefit of such an end-to-end security is. First, there is the
question of how to establish the required security association.
Between two arbitrary hosts on the Internet this might turn out to be
quite difficult. Furthermore, te usefulness of end-to-end security
depends on the architecture in which RSVP is deployed. If RSVP is
only used to signal QoS information into the network, and other
protocols have to be executed beforehand to negotiate the parameters
and to decide which entity is charged for the QoS reservation, then
no end-to-end security is likely to be required. Introducing
end-to-end security to RSVP would then cause problems with extensions
like RSVP proxy [40], Localized RSVP [41], and others that terminate
RSVP signaling somewhere along the path without reaching the
destination end host. Such a behavior could then be interpreted as a
man-in-the-middle attack.
5.6 IPsec protection of RSVP signaling messages
It is assumed throughout that RSVP signaling messages can also be
protected by IPsec [3] in a hop-by-hop fashion between two adjacent
RSVP nodes. RSVP, however, uses special processing of signaling
messages, which complicates IPsec protection. As explained in this
section, IPsec should only be used for protection of RSVP signaling
messages in a point-to-point communication environment (i.e., a RSVP
message can only reach one RSVP router and not possibly more than
one). This restriction is caused by the combination of signaling
message delivery and discovery into a single message. Furthermore,
end-to-end addressing complicates IPsec handling considerably. This
section describes at least some of these complications.
RSVP messages are transmitted as raw IP packets with protocol number
46. It might be possible to encapsulate them in UDP as described in
Appendix C of [6]. Some RSVP messages (Path, PathTear, and ResvConf)
must have the Router Alert IP Option set in the IP header. These
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messages are addressed to the (unicast or multicast) destination
address and not to the next RSVP node along the path. Hence an IPsec
traffic selector can only use these fields for IPsec SA selection.
If there is only a single path (and possibly all traffic along it is
protected) then there is no problem for IPsec protection of signaling
messages. This type of protection is not common and might only be
used to secure network access between an end host and its first-hop
router. Because the described RSVP messages are addressed to the
destination address instead of the next RSVP node, it is not possible
to use IPsec ESP [21] or AH [20] in transport mode--only IPsec in
tunnel mode is possible.
5.7 Authorization
[37] describes two trust models (NJ Turnpike and NJ Parkway) and two
authorization models (per-session and per-channel financial
settlement). The NJ Turnpike model gives a justification for
hop-by-hop security protection. RSVP focuses on the NJ Turnpike
model although the different trust models are not described in
detail. RSVP supports the NJ Parkway model and per-channel financial
settlement only to a certain extent. Authentication of the user (or
end host) can be provided with the user identity representation
mechanism but authentication might in many cases be insufficient for
authorization. The communication procedures defined for policy
objects [42] can be improved to support the more efficient
per-channel financial settlement model by avoiding policy handling
between inter-domain networks at a signaling message granularity.
Additional information about expected behavior of policy handling in
RSVP can also be obtained from [43].
[38] and [39] provide additional information on authorization. No
good and agreed mechanism for dealing with authorization of QoS
reservations in roaming environments is provided. Price distribution
mechanisms are only described in papers and never made their way
through standardization. RSVP focuses on receiver-initiated
reservations with authorization for the QoS reservation by the data
receiver which introduces a fair number of complexity for mobility
handling as described, for example, in [39].
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6. Conclusions
RSVP was the first QoS signaling protocol that provided some security
protection. Whether RSVP provides enough security protection heavily
depends on the environment where it is deployed. RSVP as specified
today should be seen as a building block that has to be adapted to a
given architecture.
This document aims to provide more insights into the security of
RSVP. It cannot not be interpreted as a pass or fail evaluation of
the security provided by RSVP.
Certainly this document is not a complete description of all security
issues related to RSVP. Some issues that require further
consideration are RSVP extensions (for example [13]), multicast
issues, and other security properties like traffic analysis.
Additionally, the interaction with mobility protocols (micro- and
macro-mobility) from a security point of view demands further
investigation.
What can be learned from practical protocol experience and from the
increased awareness regarding security is that some of the available
credential types have received more acceptance than others. Kerberos
is a system that is integrated into many IETF protocols today.
Public key based authentication techniques are however still
considered to be too heavy-weight (computationally and from a
bandwidth perspective) to be used for per-flow signaling. The
increased focus on denial of service attacks put additional demands
on the design of public key based authentication.
The following list briefly summarizes a few security or architectural
issues that deserve improvement:
o Discovery and signaling message delivery should be separated.
o For some applications and scenarios it cannot be assumed that
neighboring RSVP-aware nodes know each other. Hence some in-path
discovery mechanism should be provided.
o Addressing for signaling messages should be done in a hop-by-hop
fashion.
o Standard security protocols (IPsec, TLS or CMS) should be used
whenever possible. Authentication and key exchange should be
separated from signaling message protection. In general, it is
necessary to provide key management to establish security
associations dynamically for signaling message protection.
Relying on manually configured keys between neighboring RSVP nodes
is insufficient. A separate, less frequently executed key
management and security association establishment protocol is a
good place to perform entity authentication, security service
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negotiation and selection, and agreement on mechanisms,
transforms, and options.
o The use of public key cryptography in authorization tokens,
identity representations, selective object protection, etc. is
likely to cause fragmentation, the need to protect against denial
of service attacks, and other problems.
o Public key authentication and user identity confidentiality
provided with RSVP require some improvement.
o Public key based user authentication only provides entity
authentication. An additional security association is required to
protect signaling messages.
o Data origin authentication should not be provided by non-RSVP
nodes (such as the PDP). Such a procedure could be accomplished
by entity authentication during the authentication and key
exchange phase.
o Authorization and charging should be better integrated into the
base protocol.
o Selective message protection should be provided. A protected
message should be recognizable from a flag in the header.
o Confidentiality protection is missing and should therefore be
added to the protocol. The general principle is that protocol
designers can seldom foresee all of the environments in which
protocols will be run, so they should allow users to select from a
full range of security services, as the needs of different user
communities vary.
o Parameter and mechanism negotiation should be provided.
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7. Security Considerations
This document discusses security properties of RSVP and, as such, it
is concerned entirely with security.
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8. IANA considerations
This document does not address any IANA considerations.
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9. Acknowledgments
We would like to thank Jorge Cuellar, Robert Hancock, Xiaoming Fu,
Guenther Schaefer, Marc De Vuyst, Bob Grillo and Jukka Manner for
their valuable comments. Additionally, we would like to thank Robert
and Jorge for their time to discuss various issues with me.
Finally we would Allison Mankin and John Loughney for their comments.
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10. References
10.1 Normative References
[1] Baker, F., Lindell, B. and M. Talwar, "Identity Representation
for RSVP", January 2000.
[2] Herzog, S., "RSVP Extensions for Policy Control", January 2000.
[3] Kent, S., Atkinson, R. and M. Talwar, "Security Architecture
for the Internet Protocol", November 1998.
[4] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", February 1997.
[5] Rivest, R., "The MD5 Message-Digest Algorithm", April 1992.
[6] Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
"Resource ReSerVation Protocol (RSVP) - Version 1 Functional
Specification", September 1997.
[7] Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore, T.,
Herzog, S. and R. Hess, "Identity Representation for RSVP",
October 2001.
[8] Kohl, J. and C. Neuman, "The Kerberos Network Authentication
Service (V5)", September 1993.
[9] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J. Arkko,
"Diameter Base Protocol", RFC 3588, September 2003.
[10] Boyle, J., Cohen, R., Durham, D., Herzog, S., Rajan, R. and A.
Sastry, "The COPS(Common Open Policy Service) Protocol",
January 2000.
[11] Boyle, J., Cohen, R., Durham, D., Herzog, S., Rajan, R. and A.
Sastry, "COPS usage for RSVP", January 2000.
[12] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", draft-ietf-krb-wg-crypto-07 (work in progress),
February 2004.
[13] Berger, L. and T. O'Malley, "RSVP Extensions for IPSEC Data
Flows", September 1997.
[14] Terzis, A., Krawczyk, J., Wroclawski, J. and L. Zhang, "RSVP
Operation Over IP Tunnels", January 2000.
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[15] Tung, B. and L. Zhu, "Public Key Cryptography for Initial
Authentication in Kerberos", draft-ietf-cat-kerberos-pk-init-24
(work in progress), February 2005.
[16] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-17 (work in progress), October 2004.
[17] Thomas, M. and J. Vilhuber, "Kerberized Internet Negotiation of
Keys (KINK)", draft-ietf-kink-kink-06 (work in progress), July
2004.
10.2 Informative References
[18] Hess, R. and S. Herzog, "RSVP Extensions for Policy Control",
Internet-Draft(Expired) draft-ietf-rap-new-rsvp-ext-00.txt,
June 2001.
[19] "Secure Hash Standard,NIST, FIPS PUB 180-1", April 1995.
[20] Kent, S. and R. Atkinson, "IP Authentication Header", November
1998.
[21] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", November 1998.
[22] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and CRL Profile", January
1999.
[23] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
Message Format", November 1998.
[24] Hornstein, K. and J. Altman, "Distributing Kerberos KDC and
Realm Information with DNS", Internet-Draft(Expired)
draft-ietf-krb-wg-krb-dns-locate-03.txt, July 2002.
[25] Dobbertin, H., Bosselaers, A. and B. Preneel, "RIPEMD-160: A
strengthened version of RIPEMD in Fast Software Encryption,
LNCS Vol 1039, pp. 71-82", 1996.
[26] Dobbertin, H., "The Status of Md5 After a Recent Attack, RSA
Laboratories CryptoBytes, Volume 2, Number 2", 1996.
[27] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication
Protocol (EAP)", March 1998.
[28] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", June 2000.
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[29] ""Microsoft Authorization Data Specification v. 1.0 for
Microsoft Windows 2000 Operating Systems", April 2000.
[30] Cable Television Laboratories, Inc.,, "PacketCable Security
Specification,PKT-SP-SEC-I01-991201", website
http://www.PacketCable.com/ , June 2003.
[31] Myers, M., Ankney, R., Malpani, A., Galperin, S. and C. Adams,
"X.509 Internet Public Key Infrastructure Online Certificate
Status Protocol - OCSP", June 1999.
[32] Malpani, A., Hoffman, P., Housley, R. and T. Freeman, "Simple
Certificate Validation Protocol (SCVP)", Internet-Draft(Work in
progress) draft-ietf-pkix-scvp-11.txt, December 2002.
[33] Housley, R., "Cryptographic Message Syntax", June 1999.
[34] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version
1.5", March 1998.
[35] "Specifications and standard documents", website
http://www.PacketCable.com/ , March 2002.
[36] Davis, D. and D. Geer, "Kerberos With Clocks Adrift: History,
Protocols and Implementation in "USENIX Computing Systems
Volume 9 no. 1, Winter", 1996.
[37] Tschofenig, H., Buechli, M., Van den Bosch, S. and H.
Schulzrinne, "NSIS Authentication, Authorization and Accounting
Issues", Internet-Draft(Work in progress)
draft-tschofenig-nsis-aaa-issues-01.txt, March 2003.
[38] Tschofenig, H., Buechli, M., Van den Bosch, S., Schulzrinne, H.
and T. Chen, "QoS NSLP Authorization Issues",
Internet-Draft(Work in progress)
draft-tschofenig-nsis-qos-authz-issues-00.txt, June 2003.
[39] Thomas, M., "Analysis of Mobile IP and RSVP Interactions",
Internet-Draft(Work in progress)
draft-thomas-nsis-rsvp-analysis-00.txt, October 2002.
[40] Gai, S., Dutt, D., Elfassy, N. and Y. Bernet, "RSVP Proxy",
Internet-Draft(Expired) draft-ietf-rsvp-proxy-03.txt, March
2002.
[41] Manner, J., Suihko, T., Kojo, M., Liljeberg, M. and K.
Raatikainen, "Localized RSVP", Internet-Draft(Expired)
draft-manner-lrsvp-00.txt, May 2002.
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[42] Herzog, S., "Accounting and Access Control in RSVP,", PhD
Dissertation,", Internet-Draft(Expired)
draft-ietf-rsvp-lpm-arch-00.txt, November 1995.
[43] Herzog, S., "Accounting and Access Control for Multicast
Distributions: Models and Mechanisms", June 1996.
[44] Pato, J., "Using Pre-Authentication to Avoid Password Guessing
Attacks ,Open Software Foundation DCE Request for Comments",
December 1992.
[45] Wu, T., "A Real-World Analysis of Kerberos Password Security",
February 1999.
[46] Wu, T., Wu, F. and F. Gong, "Securing QoS: Threats to RSVP
Messages and Their Countermeasures in "IEEE IWQoS, pp. 62-64",
1999.
[47] Talwar, V., Nahrstedt, K. and F. Gong, "Securing RSVP For
Multimedia Applications in "Proceedings of ACM Multimedia
(Multimedia Security Workshop)"", November 2000.
[48] Talwar, V., Nahrstedt, K. and S. Nath, "RSVP-SQoS : A Secure
RSVP Protocol in "International Conference on Multimedia and
Exposition", Tokyo , Japan", August 2001.
[49] Jablon, D., "Strong password-only authenticated key exchange
Computer Communication Review, 26(5), pp. 5-26",
Internet-Draft(Expired) draft-ietf-rap-new-rsvp-ext-00.txt,
October 1996.
[50] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
November 1998.
Authors' Addresses
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
EMail: Hannes.Tschofenig@siemens.com
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Richard Graveman
RFG Security
15 Park Avenue
Morristown, NJ 07960
USA
EMail: rfg@acm.org
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Appendix A. Dictionary Attacks and Kerberos
Kerberos might be used with RSVP as described in this document.
Because dictionary attacks are often mentioned in relationship with
Kerberos, a few issues are addressed here.
The initial Kerberos AS_REQ request (without pre-authentication,
without various extensions, and without PKINIT) is unprotected. The
response message AS_REP is encrypted with the client's long-term key.
An adversary can take advantage of this fact by requesting AS_REP
messages to mount an off-line dictionary attack. Pre-authentication
([44]) can be used to reduce this problem. However,
pre-authentication does not entirely prevent dictionary attacks by an
adversary who can still eavesdrop on Kerberos messages along the path
between a mobile node and a KDC. With mandatory pre-authentication
for the initial request, an adversary cannot request a Ticket
Granting Ticket for an arbitrary user. On-line password guessing
attacks are still possible by choosing a password (e.g., from a
dictionary) and then transmitting an initial request including a
pre-authentication data field. An unsuccessful authentication by the
KDC results in an error message and the gives the adversary a hint to
restart the protocol and try a new password.
There are, however, some proposals that prevent dictionary attacks.
The use of Public Key Cryptography for initial authentication [15]
(PKINIT) is one such solution. Other proposals use
strong-password-based authenticated key agreement protocols to
protect the user's password during the initial Kerberos exchange.
[45] discusses the security of Kerberos and also discusses mechanisms
to prevent dictionary attacks.
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Appendix B. Example of User-to-PDP Authentication
The following Section describes an example of user-to-PDP
authentication. Note that the description below is not fully covered
by the RSVP specification and hence it should only be seen as an
example.
Windows 2000, which integrates Kerberos into RSVP, uses a
configuration with the user authentication to the PDP as described in
[29]. The steps for authenticating the user to the PDP in an
intra-realm scenario are the following:
o Windows 2000 requires the user to contact the KDC and to request a
Kerberos service ticket for the PDP account AcsService in the
local realm .
o This ticket is then embedded into the AUTH_DATA element and
included in either the PATH or the RESV message. In case of
Microsoft's implementation, the user identity encoded as a
distinguished name is encrypted with the session key provided with
the Kerberos ticket. The Kerberos ticket is sent without the
Kerberos authdata element that contains authorization information,
as explained in [29].
o The RSVP message is then intercepted by the PEP, which forwards it
to the PDP. [29] does not state which protocol is used to forward
the RSVP message to the PDP.
o The PDP that finally receives the message decrypts the received
service ticket. The ticket contains the session key used by the
user's host to
* Encrypt the principal name inside the policy locator field of
the AUTH_DATA object and to
* Create the integrity-protected Keyed Message Digest field in
the INTEGRITY object of the POLICY_DATA element. The
protection described here is between the user's host and the
PDP. The RSVP INTEGRITY object on the other hand is used to
protect the path between the user's host and the first-hop
router, because the two message parts terminate at different
nodes and different security associations must be used. The
interface between the message-intercepting, first-hop router
and the PDP must be protected as well.
* The PDP does not maintain a user database, and [29] describes
how the PDP may query the Active Directory (a LDAP based
directory service) for user policy information.
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Appendix C. Literature on RSVP Security
Few documents address the security of RSVP signaling. This section
briefly describes some important documents.
Improvements to RSVP are proposed in [46] to deal with insider
attacks. Insider attacks are caused by malicious RSVP routers that
modify RSVP signaling messages in such a way that they cause harm to
the nodes participating in the signaling message exchange.
As a solution, non-mutable RSVP objects are digitally signed by the
sender. This digital signature is added to the RSVP PATH message.
Additionally, the receiver attaches an object to the RSVP RESV
message containing a "signed" history. This value allows
intermediate RSVP routers (by examining the previously signed value)
to detect a malicious RSVP node.
A few issues are, however, left open in the document. Replay attacks
are not covered, and it is therefore assumed that timestamp-based
replay protection is used. To detect a malicious node, it is
necessary that all routers along the path are able to verify the
digital signature. This may require a global public key
infrastructure and also client-side certificates. Furthermore the
bandwidth and computational requirements to compute, transmit, and
verify digital signatures for each signaling message might place a
burden on a real-world deployment.
Authorization is not considered in the document, which might have an
influence on the implications of signaling message modification.
Hence, the chain-of-trust relationship (or this step in a different
direction) should be considered in relationship with authorization.
In [47], the above-described idea of detecting malicious RSVP nodes
is improved by addressing performance aspects. The proposed solution
is somewhere between hop-by-hop security and the approach in [46],
insofar as it separates the end-to-end path into individual networks.
Furthermore, some additional RSVP messages (e.g., feedback messages)
are introduced to implement a mechanism called "delayed integrity
checking." In [48], the approach presented in [47] is enhanced.
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