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Keywords: resource reservation protocol
Network Working Group H. Tschofenig
Request for Comments: 4230 Siemens
Category: Informational R. Graveman
RFG Security
December 2005
RSVP Security Properties
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
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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RFC 4230 RSVP Security Properties December 2005
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Architectural Assumptions . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. The RSVP INTEGRITY Object . . . . . . . . . . . . . . 5
3.2. Security Associations . . . . . . . . . . . . . . . . 8
3.3. RSVP Key Management Assumptions . . . . . . . . . . . 8
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 . . . . . . . 28
5. Miscellaneous Issues . . . . . . . . . . . . . . . . . . . . 29
5.1. First-Hop Issue . . . . . . . . . . . . . . . . . . . 30
5.2. Next-Hop Problem . . . . . . . . . . . . . . . . . . . 30
5.3. Last-Hop Issue . . . . . . . . . . . . . . . . . . . 33
5.4. RSVP- and IPsec-protected data traffic . . . . . . . . 34
5.5. End-to-End Security Issues and RSVP . . . . . . . . . 36
5.6. IPsec protection of RSVP signaling messages . . . . . 36
5.7. Authorization . . . . . . . . . . . . . . . . . . . . 37
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 38
7. Security Considerations . . . . . . . . . . . . . . . . . . 40
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 40
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
9.1. Normative References . . . . . . . . . . . . . . . . . 40
9.2. Informative References . . . . . . . . . . . . . . . . 41
A. Dictionary Attacks and Kerberos . . . . . . . . . . . . . . 45
B. Example of User-to-PDP Authentication . . . . . . . . . . . 45
C. Literature on RSVP Security . . . . . . . . . . . . . . . . 46
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1. Introduction
As the work of the NSIS working group began, concerns about security
and its implications for the design of a signaling protocol were
raised. 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 2
introduces the terminology used throughout the document. 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. Several
miscellaneous issues are covered in Section 5.
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 from the usage scenarios envisioned for NSIS. Hence, this
document does not address RSVP-TE or its security properties.
2. Terminology and Architectural Assumptions
This section describes some important terms and explains some
architectural assumptions.
o 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 nodes 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.
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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 bogus messages
injected by unauthorized nodes. Furthermore, each RSVP-aware
router is assumed to behave in the expected manner. Outgoing
messages transmitted to the next-hop network element receive new
protection according to RSVP security processing.
Using the mechanisms described above, a chain-of-trust is created
whereby a signaling message that is 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
that are not limited 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 be more likely to be attacked in comparison with the
more secure core network of a service provider. In some cases, a
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 [14] 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.
o 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.
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o 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.
o 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 by 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.
o 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).
3. Overview
This section describes the security mechanisms provided by RSVP.
Although use of IPsec is mentioned in Section 10 of [1], the other
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:
o 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
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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.
o 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.
o 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.
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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 the 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 the 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].
o Keyed Message Digest:
The Keyed Message Digest is a security mechanism built into RSVP
that 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 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 an RSVP
implementation, as noted in Section 1 of [1]. Hash algorithms
other than MD5 [5], like SHA-1 [15], 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 keys 160 bits, as in IPsec AH [16] and ESP [17].
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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 overlapping lifetimes
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 with
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.
3.3. RSVP Key Management Assumptions
RFC 2205 [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
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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 during 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 acting as policy decision points (PDPs) 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 the implementation of an access
control policy.
The following attributes are defined for use in the AUTH_DATA object:
o Policy Locator
* ASCII_DN
* UNICODE_DN
* ASCII_DN_ENCRYPT
* UNICODE_DN_ENCRYPT
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The policy locator string is an X.500 distinguished name (DN)
used to locate user or application-specific policy information.
The four types of X.500 DNs are listed above. The first two
types are the ASCII and the Unicode representation of the user
or application DN identity. The two "encrypted" 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. This detail
may be specified in a concrete architecture in which RSVP is
used.
o 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 [18] and PGP
[19]. 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, that are included for policy-based
admission control. Note that these plain-text identifiers may,
however, be protected if either the RSVP INTEGRITY or the
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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 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. As an optimization, the
Kerberos ticket does not need to be included in every RSVP
message, 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
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timestamps or sequence numbers) is provided by the digital
signature is an open issue as discussed in Section 4.3.
o Digital Signature
The digital signature computed over the contents 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 for the
case of self-signed certificates).
o 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
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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 an RSVP Integrity
Handshake that is started by Node A after recovering from a
synchronization failure:
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 (because it may not be needed in all network
environments).
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4. Detailed Security Property Discussion
This section describes the protection of the RSVP-provided mechanisms
for authentication, authorization, integrity and replay protection
individually, user identity confidentiality, and confidentiality of
the signaling messages,
4.1. Network Topology
This paragraph shows the basic interfaces in a simple RSVP network
architecture. The architecture below assumes that there is only a
single domain and that the two routers are RSVP- and policy-aware.
These assumptions are relaxed in the individual paragraphs, as
necessary. Layer 2 devices between the clients and their
corresponding first-hop routers are not shown. Other network
elements like a Kerberos Key Distribution Center and, for example, an
LDAP server from which the PDP retrieves its policies are also
omitted. The security of various interfaces to the individual
servers (KDC, PDP, etc.) depends very much on the security policy of
a specific network service provider.
+--------+
| Policy |
+----|Decision|
| | Point +---+
| +--------+ |
| |
| |
+------+ +-+----+ +---+--+ +------+
|Client| |Router| |Router| |Client|
| A +-------+ 1 +--------+ 2 +----------+ B |
+------+ +------+ +------+ +------+
Figure 4: Simple RSVP Architecture.
4.2. Host/Router
When considering authentication in RSVP, it is important to make a
distinction between user and host authentication of the signaling
messages. The host is authenticated using the RSVP INTEGRITY object,
whereas credentials inside the AUTH_DATA object can be used to
authenticate the user. In this section, the focus is on host
authentication, whereas the next section covers user authentication.
(1) Authentication
The term "host authentication" is used above, because the
selection of the security association is bound to the host's IP
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address, as mentioned in Section 3.1 and Section 3.2. Depending
on the key management protocol used to create this security
association and the identity used, it is also possible to bind a
user identity to this security association. Because the key
management protocol is not specified, it is difficult to evaluate
this part, and hence we speak about data-origin authentication
based on the host's identity for RSVP INTEGRITY objects. The
fact that the host identity is used for selecting the security
association has already been described in Section 3.1.
Data-origin authentication is provided with a keyed hash value
computed over the entire RSVP message, excluding the keyed
message digest field itself. The security association used
between the user's host and the first-hop router is, as
previously mentioned, not established by RSVP, and it must
therefore be available before signaling is started.
* Kerberos for the RSVP INTEGRITY object
As described in Section 7 of [1], Kerberos may be used to
create the key for the RSVP INTEGRITY object. How to learn
the principal name (and realm information) of the other node
is outside the scope of [1]. [20] describes a way to
distribute principal and realm information via DNS, which can
be used for this purpose (assuming that the FQDN or the IP
address of the other node for which this information is
desired is known). All that is required is to encapsulate the
Kerberos ticket inside the policy element. It is furthermore
mentioned that Kerberos tickets with expired lifetime must not
be used, and the initiator is responsible for requesting and
exchanging a new service ticket before expiration.
RSVP multicast processing in combination with Kerberos
involves additional considerations. Section 7 of [1] states
that in the multicast case all receivers must share a single
key with the Kerberos Authentication Server (i.e., a single
principal used for all receivers). From a personal discussion
with Rodney Hess, it seems that there is currently no other
solution available in the context of Kerberos. Multicast
handling therefore leaves some open questions in this context.
In the case where one entity crashed, the established security
association is lost and therefore the other node must
retransmit the service ticket. The crashed entity can use an
Integrity Challenge message to request a new Kerberos ticket
to be retransmitted by the other node. If a node receives
such a request, then a reply message must be returned.
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(2) Integrity protection
Integrity protection between the user's host and the first-hop
router is based on the RSVP INTEGRITY object. HMAC-MD5 is
preferred, although other keyed hash functions may also be used
within the RSVP INTEGRITY object. In any case, both
communicating entities must have a security association that
indicates the algorithm to use. This may, however, be difficult,
because no negotiation protocol is defined to agree on a specific
algorithm. Hence, if RSVP is used in a mobile environment, it is
likely that HMAC-MD5 is the only usable algorithm for the RSVP
INTEGRITY object. Only in local environments may it be useful to
switch to a different keyed hash algorithm. The other possible
alternative is that every implementation support the most
important keyed hash algorithms. e.g., MD5, SHA-1, RIPEMD-160,
etc. HMAC-MD5 was chosen mainly because of its performance
characteristics. The weaknesses of MD5 [21] are known and were
initially described in [22]. Other algorithms like SHA-1 [15]
and RIPEMD-160 [21] have stronger security properties.
(3) Replay Protection
The main mechanism used for replay protection in RSVP is based on
sequence numbers, whereby the sequence number is included in the
RSVP INTEGRITY object. The properties of this sequence number
mechanism are described in Section 3.1 of [1]. The fact that the
receiver stores a list of sequence numbers is an indicator for a
window mechanism. This somehow conflicts with the requirement
that the receiver only has to store the highest number given in
Section 3 of [1]. We assume that this is an oversight. Section
4.2 of [1] gives a few comments about the out-of-order delivery
and the ability of an implementation to specify the replay
window. Appendix C of [3] describes a window mechanism for
handling out-of-sequence delivery.
(4) Integrity Handshake
The mechanism of the Integrity Handshake is explained in Section
3.5. The Cookie value is suggested to be a hash of a local
secret and a timestamp. The Cookie value is not verified by the
receiver. The mechanism used by the Integrity Handshake is a
simple Challenge/Response message, which assumes that the key
shared between the two hosts survives the crash. If, however,
the security association is dynamically created, then this
assumption may not be true.
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In Section 10 of [1], the authors note that an adversary can
create a faked Integrity Handshake message that includes
challenge cookies. Subsequently, it could store the received
response and later try to replay these responses while a
responder recovers from a crash or restart. If this replayed
Integrity Response value is valid and has a lower sequence number
than actually used, then this value is stored at the recovering
host. In order for this attack to be successful, the adversary
must either have collected a large number of challenge/response
value pairs or have "discovered" the cookie generation mechanism
(for example by knowing the local secret). The collection of
Challenge/Response pairs is even more difficult, because they
depend on the Cookie value, the sequence number included in the
response message, and the shared key used by the INTEGRITY
object.
(5) Confidentiality
Confidentiality is not considered to be a security requirement
for RSVP. Hence, it is not supported by RSVP, except as
described in paragraph d) of Section 4.3. This assumption may
not hold, however, for enterprises or carriers who want to
protect billing data, network usage patterns, or network
configurations, in addition to users' identities, from
eavesdropping and traffic analysis. Confidentiality may also
help make certain other attacks more difficult. For example, the
PathErr attack described in Section 5.2 is harder to carry out if
the attacker cannot observe the Path message to which the PathErr
corresponds.
(6) Authorization
The task of authorization consists of two subcategories: network
access authorization and RSVP request authorization. Access
authorization is provided when a node is authenticated to the
network, e.g., using EAP [23] in combination with AAA protocols
(for example, RADIUS [24] or DIAMETER [9]). Issues related to
network access authentication and authorization are outside the
scope of RSVP.
The second authorization refers to RSVP itself. Depending on the
network configuration:
* the router either forwards the received RSVP request to the
policy decision point (e.g., using COPS [10] and [11]) to
request that an admission control procedure be executed, or
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* the router supports the functionality of a PDP and, therefore,
there is no need to forward the request, or
* the router may already be configured with the appropriate
policy information to decide locally whether to grant this
request.
Based on the result of the admission control, the request may be
granted or rejected. Information about the resource-requesting
entity must be available to provide policy-based admission
control.
(7) Performance
The computation of the keyed message digest for an RSVP INTEGRITY
object does not represent a performance problem. The protection
of signaling messages is usually not a problem, because these
messages are transmitted at a low rate. Even a high volume of
messages does not cause performance problems for an RSVP router
due to the efficiency of the keyed message digest routine.
Dynamic key management, which is computationally more demanding,
is more important for scalability. Because RSVP does not specify
a particular key exchange protocol, it is difficult to estimate
the effort needed to create the required security associations.
Furthermore, the number of key exchanges to be triggered depends
on security policy issues like lifetime of a security
association, required security properties of the key exchange
protocol, authentication mode used by the key exchange protocol,
etc. In a stationary environment with a single administrative
domain, manual security association establishment may be
acceptable and may provide the best performance characteristics.
In a mobile environment, asymmetric authentication methods are
likely to be used with a key exchange protocol, and some sort of
public key or certificate verification needs to be supported.
4.3. User to PEP/PDP
As noted in the previous section, RSVP supports both user-based and
host-based authentication. Using RSVP, a user may authenticate to
the first hop router or to the PDP as specified in [1], depending on
the infrastructure provided by the network domain or the architecture
used (e.g., the integration of RSVP and Kerberos V5 into the Windows
2000 Operating System [25]). Another architecture in which RSVP is
tightly integrated is the one specified by the PacketCable
organization. The interested reader is referred to [26] for a
discussion of their security architecture.
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(1) Authentication
When a user sends an RSVP PATH or RESV message, this message may
include some information to authenticate the user. [7] describes
how user and application information is embedded into the RSVP
message (AUTH_DATA object) and how to protect it. A router
receiving such a message can use this information to authenticate
the client and forward the user or application information to the
policy decision point (PDP). Optionally, the PDP itself can
authenticate the user, which is described in the next section.
To be able to authenticate the user, to verify the integrity, and
to check for replays, the entire POLICY_DATA element has to be
forwarded from the router to the PDP (e.g., by including the
element into a COPS message). It is assumed, although not
clearly specified in [7], that the INTEGRITY object within the
POLICY_DATA element is sent to the PDP along with all other
attributes.
* Certificate Verification
Using the policy element as described in [7], it is not
possible to provide a certificate revocation list or other
information to prove the validity of the certificate inside
the policy element. A specific mechanism for certificate
verification is not discussed in [7] and hence a number of
them can be used for this purpose. For certificate
verification, the network element (a router or the policy
decision point) that has to authenticate the user could
frequently download certificate revocation lists or use a
protocol like the Online Certificate Status Protocol (OCSP)
[27] and the Simple Certificate Validation Protocol (SCVP)
[28] to determine the current status of a digital certificate.
* User Authentication to the PDP
This alternative authentication procedure uses the PDP to
authenticate the user instead of the first-hop router. In
Section 4.2.1 of [7], the choice is given for the user to
obtain a session ticket either for the next hop router or for
the PDP. As noted in the same section, the identity of the
PDP or the next hop router is statically configured or
dynamically retrieved. Subsequently, user authentication to
the PDP is considered.
* Kerberos-based Authentication to the PDP
If Kerberos is used to authenticate the user, then a session
ticket for the PDP must be requested first. A user who roams
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between different routers in the same administrative domain
does not need to request a new service ticket, because the
same PDP is likely to be used by most or all first-hop routers
within the same administrative domain. This is different from
the case in which a session ticket for a router has to be
obtained and authentication to a router is required. The
router therefore plays a passive role of simply forwarding the
request to the PDP and executing the policy decision returned
by the PDP. Appendix B describes one example of user-to-PDP
authentication.
User authentication with the policy element provides only
unilateral authentication, whereby the client authenticates to
the router or to the PDP. If an RSVP message is sent to the
user's host and public-key-based authentication is not used,
then the message does not contain a certificate and digital
signature. Hence, no mutual authentication can be assumed.
In case of Kerberos, mutual authentication may be accomplished
if the PDP or the router transmits a policy element with an
INTEGRITY object computed with the session key retrieved from
the Kerberos ticket, or if the Kerberos ticket included in the
policy element is also used for the RSVP INTEGRITY object as
described in Section 4.2. This procedure only works if a
previous message was transmitted from the end host to the
network and such key is already established. Reference [7]
does not discuss this issue, and therefore there is no
particular requirement for transmitting network-specific
credentials back to the end-user's host.
(2) Integrity Protection
Integrity protection is applied separately to the RSVP message
and the POLICY_DATA element, as shown in Figure 1. In case of
a policy-ignorant node along the path, the RSVP INTEGRITY
object and the INTEGRITY object inside the policy element
terminate at different nodes. Basically, the same is true for
the user credentials if they are verified at the policy
decision point instead of the first hop router.
* Kerberos
If Kerberos is used to authenticate the user to the first hop
router, then the session key included in the Kerberos ticket
may be used to compute the INTEGRITY object of the policy
element. It is the keyed message digest that provides the
authentication. The existence of the Kerberos service ticket
inside the AUTH_DATA object does not provide authentication or
a guarantee of freshness for the receiving host.
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Authentication and guarantee of freshness are provided by the
keyed hash value of the INTEGRITY object inside the
POLICY_DATA element. This shows that the user actively
participated in the Kerberos protocol and was able to obtain
the session key to compute the keyed message digest. The
Authenticator used in the Kerberos V5 protocol provides
similar functionality, but replay protection is based on
timestamps (or on a sequence number if the optional seq-number
field inside the Authenticator is used for KRB_PRIV/KRB_SAFE
messages as described in Section 5.3.2 of [8]).
* Digital Signature
If public-key-based authentication is provided, then user
authentication is accomplished with a digital signature. As
explained in Section 3.3.3 of [7], the DIGITAL_SIGNATURE
attribute must be the last attribute in the AUTH_DATA object,
and the digital signature covers the entire AUTH_DATA object.
In the case of PGP, which hash algorithm and public key
algorithm are used for the digital signature computation is
described in [19]. In the case of X.509 credentials, the
situation is more complex because different mechanisms like
CMS [29] or PKCS#7 [30] may be used for digitally signing the
message element. X.509 only provides the standard for the
certificate layout, which seems to provide insufficient
information for this purpose. Therefore, X.509 certificates
are supported, for example, by CMS or PKCS#7. [7], however,
does not make any statements about the usage of CMS or PKCS#7.
Currently, there is no support for CMS or for PKCS#7 [7],
which provides more than just public-key-based authentication
(e.g., CRL distribution, key transport, key agreement, etc.).
Furthermore, the use of PGP in RSVP is vaguely defined,
because there are different versions of PGP (including OpenPGP
[19]), and no indication is given as to which should be used.
Supporting public-key-based mechanisms in RSVP might increase
the risks of denial-of-service attacks. The large processing,
memory, and bandwidth requirements should also be considered.
Fragmentation might also be an issue here.
If the INTEGRITY object is not included in the POLICY_DATA
element or not sent to the PDP, then we have to make the
following observations:
For the digital signature case, only the replay protection
provided by the digital signature algorithm can be used.
It is not clear, however, whether this usage was
anticipated or not. Hence, we might assume that replay
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protection is based on the availability of the RSVP
INTEGRITY object used with a security association that is
established by other means.
Including only the Kerberos session ticket is insufficient,
because freshness is not provided (because the Kerberos
Authenticator is missing). Obviously there is no guarantee
that the user actually followed the Kerberos protocol and
was able to decrypt the received TGS_REP (or, in rare
cases, the AS_REP if a session ticket is requested with the
initial AS_REQ).
(3) Replay Protection
Figure 5 shows the interfaces relevant for replay protection of
signaling messages in a more complicated architecture. In this
case, the client uses the policy data element with PEP2, because
PEP1 is not policy-aware. The interfaces between the client and
PEP1 and between PEP1 and PEP2 are protected with the RSVP
INTEGRITY object. The link between the PEP2 and the PDP is
protected, for example, by using the COPS built-in INTEGRITY
object. The dotted line between the Client and the PDP indicates
the protection provided by the AUTH_DATA element, which has no
RSVP INTEGRITY object included.
AUTH_DATA +----+
+---------------------------------------------------+PDP +-+
| +----+ |
| |
| |
| COPS |
| INTEGRITY|
| |
| |
| |
+--+---+ RSVP INTEGRITY +----+ RSVP INTEGRITY +----+ |
|Client+-------------------+PEP1+----------------------+PEP2+-+
+--+---+ +----+ +-+--+
| |
+-----------------------------------------------------+
POLICY_DATA INTEGRITY
Figure 5: Replay Protection.
Host authentication with the RSVP INTEGRITY object and user
authentication with the INTEGRITY object inside the POLICY_DATA
element both use the same anti-replay mechanism. The length of
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the Sequence Number field, sequence number rollover, and the
Integrity Handshake have already been explained in Section 3.1.
Section 9 of [7] states: "RSVP INTEGRITY object is used to
protect the policy object containing user identity information
from security (replay) attacks." When using public-key-based
authentication, RSVP-based replay protection is not supported,
because the digital signature does not cover the POLICY_DATA
INTEGRITY object with its Sequence Number field. The digital
signature covers only the entire AUTH_DATA object.
The use of public key cryptography within the AUTH_DATA object
complicates replay protection. Digital signature computation
with PGP is described in [31] and in [19]. The data structure
preceding the signed message digest includes information about
the message digest algorithm used and a 32-bit timestamp of when
the signature was created ("Signature creation time"). The
timestamp is included in the computation of the message digest.
The IETF standardized version of OpenPGP [19] contains more
information and describes the different hash algorithms (MD2,
MD5, SHA-1, RIPEMD-160) supported. [7] does not make any
statements as to whether the "Signature creation time" field is
used for replay protection. Using timestamps for replay
protection requires different synchronization mechanisms in the
case of clock-skew. Traditionally, these cases assume "loosely
synchronized" clocks but also require specifying a replay window.
If the "Signature creation time" is not used for replay
protection, then a malicious, policy-ignorant node can use this
weakness to replace the AUTH_DATA object without destroying the
digital signature. If this was not simply an oversight, it is
therefore assumed that replay protection of the user credentials
was not considered an important security requirement, because the
hop-by-hop processing of the RSVP message protects the message
against modification by an adversary between two communicating
nodes.
The lifetime of the Kerberos ticket is based on the fields
starttime and endtime of the EncTicketPart structure in the
ticket, as described in Section 5.3.1 of [8]. Because the ticket
is created by the KDC located at the network of the verifying
entity, it is not difficult to have the clocks roughly
synchronized for the purpose of lifetime verification.
Additional information about clock-synchronization and Kerberos
can be found in [32].
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If the lifetime of the Kerberos ticket expires, then a new ticket
must be requested and used. Rekeying is implemented with this
procedure.
(4) (User Identity) Confidentiality
This section discusses privacy protection of identity information
transmitted inside the policy element. User identity
confidentiality is of particular interest because there is no
built-in RSVP mechanism for encrypting the POLICY_DATA object or
the AUTH_DATA elements. Encryption of one of the attributes
inside the AUTH_DATA element, the POLICY_LOCATOR attribute, is
discussed.
To protect the user's privacy, it is important not to reveal the
user's identity to an adversary located between the user's host
and the first-hop router (e.g., on a wireless link).
Furthermore, user identities should not be transmitted outside
the domain of the visited network provider. That is, the user
identity information inside the policy data element should be
removed or modified by the PDP to prevent revealing its contents
to other (unauthorized) entities along the signaling path. It is
not possible (with the offered mechanisms) to hide the user's
identity in such a way that it is not visible to the first
policy-aware RSVP node (or to the attached network in general).
The ASCII or Unicode distinguished name of the user or
application inside the POLICY_LOCATOR attribute of the AUTH_DATA
element may be encrypted as specified in Section 3.3.1 of [7].
The user (or application) identity is then encrypted with either
the Kerberos session key or with the private key in case of
public-key-based authentication. When the private key is used,
we usually speak of a digital signature that can be verified by
everyone possessing the public key. Because the certificate with
the public key is included in the message itself, decryption is
no obstacle. Furthermore, the included certificate together with
the additional (unencrypted) information in the RSVP message
provides enough identity information for an eavesdropper. Hence,
the possibility of encrypting the policy locator in case of
public-key-based authentication is problematic. To encrypt the
identities using asymmetric cryptography, the user's host must be
able somehow to retrieve the public key of the entity verifying
the policy element (i.e., the first policy-aware router or the
PDP). Then, this public key could be used to encrypt a symmetric
key, which in turn encrypts the user's identity and certificate,
as is done, e.g., by PGP. Currently, no such mechanism is
defined in [7].
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The algorithm used to encrypt the POLICY_LOCATOR with the
Kerberos session key is assumed to be the same as the one used
for encrypting the service ticket. The information about the
algorithm used is available in the etype field of the
EncryptedData ASN.1 encoded message part. Section 6.3 of [8]
lists the supported algorithms. [33] defines newer encryption
algorithms (Rijndael, Serpent, and Twofish).
Evaluating user identity confidentiality also requires looking at
protocols executed outside of RSVP (for example, the Kerberos
protocol). The ticket included in the CREDENTIAL attribute may
provide user identity protection by not including the optional
cname attribute inside the unencrypted part of the Ticket.
Because the Authenticator is not transmitted with the RSVP
message, the cname and the crealm of the unencrypted part of the
Authenticator are not revealed. In order for the user to request
the Kerberos session ticket for inclusion in the CREDENTIAL
attribute, the Kerberos protocol exchange must be executed. Then
the Authenticator sent with the TGS_REQ reveals the identity of
the user. The AS_REQ must also include the user's identity to
allow the Kerberos Authentication Server to respond with an
AS_REP message that is encrypted with the user's secret key.
Using Kerberos, it is therefore only possible to hide the content
of the encrypted policy locator, which is only useful if this
value differs from the Kerberos principal name. Hence, using
Kerberos it is not "entirely" possible to provide user identity
confidentiality.
It is important to note that information stored in the policy
element may be changed by a policy-aware router or by the policy
decision point. Which parts are changed depends upon whether
multicast or unicast is used, how the policy server reacts, where
the user is authenticated, whether the user needs to be re-
authenticated in other network nodes, etc. Hence, user-specific
and application-specific information can leak after the messages
leave the first hop within the network where the user's host is
attached. As mentioned at the beginning of this section, this
information leakage is assumed to be intentional.
(5) Authorization
In addition to the description of the authorization steps of the
Host-to-Router interface, user-based authorization is performed
with the policy element providing user credentials. The
inclusion of user and application specific information enables
policy-based admission control with special user policies that
are likely to be stored at a dedicated server. Hence, a Policy
Decision Point can query, for example, an LDAP server for a
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service level agreement that states the amount of resources a
certain user is allowed to request. In addition to the user
identity information, group membership and other non-security-
related information may contribute to the evaluation of the final
policy decision. If the user is not registered to the currently
attached domain, then there is the question of how much
information the home domain of the user is willing to exchange.
This also impacts the user's privacy policy.
In general, the user may not want to distribute much of this
policy information. Furthermore, the lack of a standardized
authorization data format may create interoperability problems
when exchanging policy information. Hence, we can assume that
the policy decision point may use information from an initial
authentication and key agreement protocol (which may have already
required cross-realm communication with the user's home domain,
if only to show that the home domain knows the user and that the
user is entitled to roam), to forward accounting messages to this
domain. This represents the traditional subscriber-based
accounting scenario. Non-traditional or alternative means of
access might be deployed in the near future that do not require
any type of inter-domain communication.
Additional discussions are required to determine the expected
authorization procedures. [34] and [35] discuss authorization
issues for QoS signaling protocols. Furthermore, a number of
mobility implications for policy handling in RSVP are described
in [36].
(6) Performance
If Kerberos is used for user authentication, then a Kerberos
ticket must be included in the CREDENTIAL Section of the
AUTH_DATA element. The Kerberos ticket has a size larger than
500 bytes, but it only needs to be sent once because a
performance optimization allows the session key to be cached as
noted in Section 7.1 of [1]. It is assumed that subsequent RSVP
messages only include the POLICY_DATA INTEGRITY object with a
keyed message digest that uses the Kerberos session key.
However, this assumes that the security association required for
the POLICY_DATA INTEGRITY object is created (or modified) to
allow the selection of the correct key. Otherwise, it difficult
to say which identifier is used to index the security
association.
If Kerberos is used as an authentication system then, from a
performance perspective, the message exchange to obtain the
session key needs to be considered, although the exchange only
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needs to be done once in the lifetime of the session ticket.
This is particularly true in a mobile environment with a fast
roaming user's host.
Public-key-based authentication usually provides the best
scalability characteristics for key distribution, but the
protocols are performance demanding. A major disadvantage of the
public-key-based user authentication in RSVP is the lack of a
method to derive a session key. Hence, every RSVP PATH or RESV
message includes the certificate and a digital signature, which
is a huge performance and bandwidth penalty. For a mobile
environment with low power devices, high latency, channel noise,
and low-bandwidth links, this seems to be less encouraging. Note
that a public key infrastructure is required to allow the PDP (or
the first-hop router) to verify the digital signature and the
certificate. To check for revoked certificates, certificate
revocation lists or protocols like the Online Certificate Status
Protocol [27] and the Simple Certificate Validation Protocol [28]
are needed. Then the integrity of the AUTH_DATA object can be
verified via the digital signature.
4.4. Communication between RSVP-Aware Routers
(1) Authentication
RSVP signaling messages have data origin authentication and are
protected against modification and replay with the RSVP INTEGRITY
object. The RSVP message flow between routers is protected based
on the chain of trust, and hence each router needs only a
security association with its neighboring routers. This
assumption was made because of performance advantages and because
of special security characteristics of the core network to which
no user hosts are directly attached. In the core network the
network structure does not change frequently and the manual
distribution of shared secrets for the RSVP INTEGRITY object may
be acceptable. The shared secrets may be either manually
configured or distributed by using appropriately secured network
management protocols like SNMPv3.
Independent of the key distribution mechanism, host
authentication with built-in RSVP mechanisms is accomplished
using the keyed message digest in the RSVP INTEGRITY object,
computed using the previously exchanged symmetric key.
(2) Integrity Protection
Integrity protection is accomplished with the RSVP INTEGRITY
object with the variable length Keyed Message Digest field.
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(3) Replay Protection
Replay protection with the RSVP INTEGRITY object is extensively
described in previous sections. To enable crashed hosts to learn
the latest sequence number used, the Integrity Handshake
mechanism is provided in RSVP.
(4) Confidentiality
Confidentiality is not provided by RSVP.
(5) Authorization
Depending on the RSVP network, QoS resource authorization at
different routers may need to contact the PDP again. Because the
PDP is allowed to modify the policy element, a token may be added
to the policy element to increase the efficiency of the re-
authorization procedure. This token is used to refer to an
already computed policy decision. The communications interface
from the PEP to the PDP must be properly secured.
(6) Performance
The performance characteristics for the protection of the RSVP
signaling messages is largely determined by the key exchange
protocol, because the RSVP INTEGRITY object is only used to
compute a keyed message digest of the transmitted signaling
messages.
The security associations within the core network, that is,
between individual routers (in comparison with the security
association between the user's host and the first-hop router or
with the attached network in general), can be established more
easily because of the normally strong trust assumptions.
Furthermore, it is possible to use security associations with an
increased lifetime to avoid frequent rekeying. Hence, there is
less impact on the performance compared with the user-to-network
interface. The security association storage requirements are
also less problematic.
5. Miscellaneous Issues
This section describes a number of issues that illustrate some of the
shortcomings of RSVP with respect to security.
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5.1. First-Hop Issue
In case of end-to-end signaling, an end host starts signaling to its
attached network. The first-hop communication is often more
difficult to secure because of the different requirements and a
missing trust relationship. An end host must therefore obtain some
information to start RSVP signaling:
o Does this network support RSVP signaling?
o Which node supports RSVP signaling?
o To which node is authentication required?
o Which security mechanisms are used for authentication?
o Which algorithms are required?
o Where should the keys and security associations come from?
o Should a security association be established?
RSVP, as specified today, is used as a building block. Hence, these
questions have to be answered as part of overall architectural
considerations. Without answers to these questions, ad hoc RSVP
communication by an end host roaming to an unknown network is not
possible. A negotiation of security mechanisms and algorithms is not
supported for RSVP.
5.2. Next-Hop Problem
Throughout the document it was assumed that the next RSVP node along
the path is always known. Knowing the next hop is important to be
able to select the correct key for the RSVP Integrity object and to
apply the proper protection. In the 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, for 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 that recognized 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 in Section 4.2 of [1]
states 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.
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The RSVP Path message therefore provides a number of functions: path
discovery, detecting route changes, discovery of QoS capabilities
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 are conflicts 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
the additional risk of introducing denial-of-service attacks.
o RSVP Path message functionality and next-hop discovery
To protect an RSVP signaling message (and an 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 an 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 an
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 the intra-
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domain case with a dense distribution of RSVP nodes, protection
might be possible with manual configuration.
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 (1).
(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.
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For last-hop communication, this processing 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 an application server). However, the
access router in step (1) might not be able to learn the user's
principal name because this information might not be available.
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. However, IAKERB is 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. ESP-encrypted
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. [12] considers these IPsec implications for RSVP
and is based on three assumptions:
(1) An end host that initiates the RSVP signaling message exchange
has to be able to retrieve the SPI for a 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 receiving the SPI as an
output parameter.
(2) [12] 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 problem
of tunneling that is initiated by the end host. The following
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, | | |
| InnerDst=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. The same problem occurs
with a nested application of IPsec (for example, IPsec between A
and SGW and between A and B).
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 [13].
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(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
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 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. However, some objects or mechanisms could be
used for end-to-end protection. The main question, however, is the
benefit of such end-to-end security. 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. Second, the usefulness of end-to-end security depends on
the architecture in which RSVP is deployed. If RSVP is used only 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
[37], Localized RSVP [38], 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., an 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.
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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
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 [17] or AH [16] in transport
mode--only IPsec in tunnel mode is possible.
If an RSVP message can taket more than one possible path, then the
IPsec engine will experience difficulties protecting the message.
Even if the RSVP daemon installs a traffic selector with the
destination IP address, still, no distinguishing element allows
selection of the correct security association for one of the possible
RSVP nodes along the path. Even if it possible to apply IPsec
protection (in tunnel mode) for RSVP signaling messages by
incorporating some additional information, there is still the
possibility that the tunneled messages do not recognize a path change
in a non-RSVP router. In this case the signaling messages would
simply follow a different path than the data.
RSVP messages like RESV can be protected by IPsec, because they
contain enough information to create IPsec traffic selectors that
allow differentiation between various next RSVP nodes. The traffic
selector would then contain the protocol number and the source and
destination address pair of the two communicating RSVP nodes.
One benefit of using IPsec is the availability of key management
using either IKE [39], KINK [40] or IKEv2 [41].
5.7. Authorization
[34] 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
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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].
[35] and [36] 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 amount of complexity for mobility
handling as described, for example, in [36].
6. Conclusions
RSVP was the first QoS signaling protocol that provided some security
protection. Whether RSVP provides appropriate security protection
heavily depends on the environment where it is deployed. RSVP as
specified today should be viewed as a building block that has to be
adapted to a given architecture.
This document aims to provide more insight into the security of RSVP.
It cannot 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 [12]), multicast
issues, and other security properties like traffic analysis.
Additionally, the interaction with mobility protocols (micro- and
macro-mobility) demands further investigation from a security point
of view.
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 puts additional demands
on the design of public-key-based authentication.
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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
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.
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RFC 4230 RSVP Security Properties December 2005
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.
7. Security Considerations
This document discusses security properties of RSVP and, as such, it
is concerned entirely with security.
8. Acknowledgements
We would like to thank Jorge Cuellar, Robert Hancock, Xiaoming Fu,
Guenther Schaefer, Marc De Vuyst, Bob Grillo, and Jukka Manner for
their comments. Additionally, Hannes would like to thank Robert and
Jorge for their time discussing various issues.
Finally, we would like to thank Allison Mankin and John Loughney for
their guidance and input.
9. References
9.1. Normative References
[1] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.
[2] Herzog, S., "RSVP Extensions for Policy Control", RFC 2750,
January 2000.
[3] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[4] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
1992.
[6] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
Tschofenig & Graveman Informational [Page 40]
RFC 4230 RSVP Security Properties December 2005
[7] Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore, T.,
Herzog, S., and R. Hess, "Identity Representation for RSVP",
RFC 3182, October 2001.
[8] Kohl, J. and C. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993. Obsoleted by RFC
4120.
[9] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko,
"Diameter Base Protocol", RFC 3588, September 2003.
[10] Durham, D., Boyle, J., Cohen, R., Herzog, S., Rajan, R., and A.
Sastry, "The COPS (Common Open Policy Service) Protocol", RFC
2748, January 2000.
[11] Herzog, S., Boyle, J., Cohen, R., Durham, D., Rajan, R., and A.
Sastry, "COPS usage for RSVP", RFC 2749, January 2000.
[12] Berger, L. and T. O'Malley, "RSVP Extensions for IPSEC Data
Flows", RFC 2207, September 1997.
[13] Terzis, A., Krawczyk, J., Wroclawski, J., and L. Zhang, "RSVP
Operation Over IP Tunnels", RFC 2746, January 2000.
9.2. Informative References
[14] Hess, R. and S. Herzog, "RSVP Extensions for Policy Control",
Work in Progress, June 2001.
[15] "Secure Hash Standard, NIST, FIPS PUB 180-1", Federal
Information Processing Society, April 1995.
[16] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[17] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[18] Fowler, D., "Definitions of Managed Objects for the DS1, E1,
DS2 and E2 Interface Types", RFC 2495, January 1999.
[19] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[20] Hornstein, K. and J. Altman, "Distributing Kerberos KDC and
Realm Information with DNS", Work in Progress, July 2002.
Tschofenig & Graveman Informational [Page 41]
RFC 4230 RSVP Security Properties December 2005
[21] 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.
[22] Dobbertin, H., "The Status of MD5 After a Recent Attack", RSA
Laboratories CryptoBytes, vol. 2, no. 2, 1996.
[23] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[24] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[25] "Microsoft Authorization Data Specification v. 1.0 for
Microsoft Windows 2000 Operating Systems", April 2000.
[26] Cable Television Laboratories, Inc., "PacketCable Security
Specification, PKT-SP-SEC-I01-991201", website:
http://www.PacketCable.com/, June 2003.
[27] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams,
"X.509 Internet Public Key Infrastructure Online Certificate
Status Protocol - OCSP", RFC 2560, June 1999.
[28] Malpani, A., Housley, R., and T. Freeman, "Simple Certificate
Validation Protocol (SCVP)", Work in Progress, October 2005.
[29] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
August 2002.
[30] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version
1.5", RFC 2315, March 1998.
[31] "Specifications and standard documents", website:
http://www.PacketCable.com/, March 2002.
[32] Davis, D. and D. Geer, "Kerberos With Clocks Adrift: History,
Protocols and Implementation", USENIX Computing Systems, vol 9
no. 1, Winter 1996.
[33] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[34] Tschofenig, H., Buechli, M., Van den Bosch, S., and H.
Schulzrinne, "NSIS Authentication, Authorization and Accounting
Issues", Work in Progress, March 2003.
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RFC 4230 RSVP Security Properties December 2005
[35] Tschofenig, H., Buechli, M., Van den Bosch, S., Schulzrinne,
H., and T. Chen, "QoS NSLP Authorization Issues", Work in
Progress, June 2003.
[36] Thomas, M., "Analysis of Mobile IP and RSVP Interactions", Work
in Progress, October 2002.
[37] Gai, S., Gaitonde, S., Elfassy, N., and Y. Bernet, "RSVP
Proxy", Work in Progress, March 2002.
[38] Manner, J., Suihko, T., Kojo, M., Liljeberg, M., and K.
Raatikainen, "Localized RSVP", Work in Progress, September
2004.
[39] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[40] Thomas, M., "Kerberized Internet Negotiation of Keys (KINK)",
Work in Progress, October 2005.
[41] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
4306, November 2005.
[42] Herzog, S., "Accounting and Access Control in RSVP", PhD
Dissertation, USC, Work in Progress, 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] Tung, B. and L. Zhu, "Public Key Cryptography for Initial
Authentication in Kerberos", Work in Progress, November 2005.
[46] Wu, T., "A Real-World Analysis of Kerberos Password Security",
in Proceedings of the 1999 Internet Society Network and
Distributed System Security Symposium, San Diego, February
1999.
[47] Wu, T., Wu, F., and F. Gong, "Securing QoS: Threats to RSVP
Messages and Their Countermeasures", IEEE IWQoS, pp. 62-64,
1999.
[48] Talwar, V., Nahrstedt, K., and F. Gong, "Securing RSVP For
Multimedia Applications", Proc ACM Multimedia 2000 (Multimedia
Security Workshop), November 2000.
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[49] Talwar, V., Nahrstedt, K., and S. Nath, "RSVP-SQoS: A Secure
RSVP Protocol", International Conf on Multimedia and
Exposition, Tokyo, Japan, August 2001.
[50] Jablon, D., "Strong Password-only Authenticated Key Exchange",
ACM Computer Communication Review, 26(5), pp. 5-26, October
1996.
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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 that includes a
pre-authentication data field. An unsuccessful authentication by the
KDC results in an error message and thus 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 [45]
(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. [46] discusses the
security of Kerberos and also discusses mechanisms to prevent
dictionary attacks.
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 viewed as an
example.
Windows 2000, which integrates Kerberos into RSVP, uses a
configuration with the user authentication to the PDP as described in
[25]. 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.
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o This ticket is then embedded into the AUTH_DATA element and
included in either the PATH or the RESV message. In the 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 [25].
o The RSVP message is then intercepted by the PEP, which forwards it
to the PDP. [25] does not state which protocol is used to forward
the RSVP message to the PDP.
o The PDP that finally receives the message and 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 [25] describes
how the PDP may query the Active Directory (a LDAP based
directory service) for user policy information.
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 [47] 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
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intermediate RSVP routers (by examining the previously signed value)
to detect a malicious RSVP node.
A few issues are, however, left open in this document. Replay
attacks are not covered, and it is therefore assumed that timestamp-
based replay protection is used. To identify 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 [48], 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 [47],
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 [49], the approach presented in [48] is enhanced.
Authors' Addresses
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
EMail: Hannes.Tschofenig@siemens.com
Richard Graveman
RFG Security
15 Park Avenue
Morristown, NJ 07960
USA
EMail: rfg@acm.org
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