Hannes Tschofenig
   Internet Draft                                               Siemens
   Expires: September, 2003
                                                            March, 2003
                         RSVP Security Properties
  Status of this Memo
  This document is an Internet-Draft and is in full conformance
  with all provisions of Section 10 of RFC2026.
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                       RSVP Security Properties             March 2003
  As the work of the NSIS working group has begun there are also
  concerns about security and its implication for the design of a
  signaling protocol. In order to understand the security properties
  and available options of RSVP a number of documents have to be read.
  This document tries to summarize the security properties of RSVP and
  to view them from a different point of view. This work in NSIS is
  part of the overall process of analyzing other signaling protocols
  and to learn from their design considerations. This document should
  also provide a starting point for security discussions.
  Table of Contents
   1  Introduction...................................................3
   2  Terminology....................................................3
   3  Overview.......................................................5
   3.1  The RSVP INTEGRITY Object....................................5
   3.2  Security Associations........................................6
   3.3  RSVP Key Management Assumptions..............................7
   3.4  Identity Representation......................................7
   3.5  RSVP Integrity Handshake....................................11
   4  Detailed Security Property Discussion.........................12
   4.1  Discussed Network Topology..................................12
   4.2  Host/Router.................................................13
   4.3  User to PEP/PDP.............................................17
   4.4  Communication between RSVP aware routers....................25
   5  Miscellaneous Issues..........................................28
   5.1  First Hop Issue.............................................28
   5.2  Next-Hop Problem............................................28
   5.3  Last-Hop Issue..............................................30
   5.4  RSVP and IPsec..............................................31
   5.5  End-to-End Security Issues and RSVP.........................33
   5.6  IPsec protection of RSVP signaling messages.................33
   5.7  Accounting/Charging Framework...............................34
   6  Conclusions...................................................34
   7  Security Considerations.......................................36
   8  IANA considerations...........................................36
   9  Open Issues...................................................36
   10 Acknowledgments...............................................36
   Appendix A. Dictionary Attacks and Kerberos......................36
   Appendix B. Example of User-to-PDP Authentication................38
   11 References....................................................39
   12 Author's Contact Information..................................42
   13 Full Copyright Statement......................................43
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  1  Introduction
  As the work of the NSIS working group has begun there are also
  concerns about security and its implication for the design of a
  signaling protocol. In order to understand the security properties
  and available options of RSVP a number of documents have to be read.
  This document tries to summarize the security properties of RSVP and
  to view them from a different point of view. This work in NSIS is
  part of the overall process of analyzing other signaling protocols
  and to learn from their design considerations. This document should
  also provide a starting point for further discussions.
  The content of this document is organized as follows:
  Section 3 provides an overview of the security mechanisms provided by
  RSVP including the INTEGRITY object, a description of the identity
  representation within the POLICY_DATA object (i.e. user
  authentication) and the RSVP Integrity Handshake mechanism.
  Section 4 provides a more detailed discussion of the used mechanism
  and tries to describe the mechanisms provided in detail.
  Finally a number of miscellaneous issues are described which address
  first-hop, next-hop and last-hop issues. Furthermore the problem of
  IPsec security protection of data traffic and RSVP signaling message
  is discussed.
  2  Terminology
  To begin with the description of the security properties of RSVP it
  is natural to explain some terms used throughout the document.
  - Chain-of-Trust
  The security mechanisms supported by RSVP [RFC2747] heavily relies on
  optional hop-by-hop protection using the built-in INTEGRITY object.
  Hop-by-hop security with the INTEGRITY object inside the RSVP message
  thereby refers to the protection between RSVP supporting network
  elements. Additionally there is the notion of policy aware network
  elements that additionally understand the POLICY_DATA element within
  the RSVP message. Since 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 since they do not try to interpret it.
  To protect signaling messages that are possibly modified by each RSVP
  router along the path it must be assumed that each incoming request
  is authenticated, integrity and replay protected. This provides
  protection against unauthorized nodes injecting bogus messages.
  Furthermore each RSVP-router is assumed to behave in the expected
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  manner. Outgoing messages transmitted to the next hop network element
  experience protection according RSVP security processing.
  Using the above described mechanisms a chain-of-trust is created
  whereby a signaling message transmitted by router A via router B and
  received by router C is supposed to be secure if router A and B and
  router B and C share a security association and all routers behave
  expectedly. 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 as good as 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 and
  cause unexpected behavior and mount a number of attacks not only
  restricted to QoS signaling. Additionally it must be mentioned that
  some protocols demand more protection than others (this depends
  between which nodes these protocols are executed). For example edge
  devices, where end-users are attached, may more likely be attacked in
  comparison to 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 [RFC2750] mentions the term chain-of-trust in the
  context of RSVP integrity protection. In Section 6 of [HH01] 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 assumption is not clearly specified.
  - Host and User Authentication
  The presence of the RSVP protection and a separate user identity
  representation leads to the fact that both user- and the host-
  identities are used for RSVP protection. Therefore user and host
  based security is investigated separately because of the different
  authentication mechanisms provided. To avoid confusion about the
  different concepts Section 3.4 will describe the concept of user
  authentication in more detail.
  - Key Management
  For most of the security associations required for the protection of
  RSVP signaling messages it is assumed that they are already available
  and hence key management was done in advance. There is however an
  exception with the support for Kerberos. Using Kerberos an entity is
  able to distribute a session key used for RSVP signaling protection.
  RSVP uses the INTEGRITY object in two places of the message. The
  first usage is in the RSVP message itself and covers the entire RSVP
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  message as defined in [RFC2747] whereas the latter is included in the
  POLICY_DATA object and defined in [RFC2750]. In order to
  differentiate the two objects regarding their scope of protection the
  two terms RSVP INTEGRITY and POLICY_DATA INTEGRITY object are used.
  The data structure of the two objects however is the same.
  - Hop vs. Peer
  In the past there was considerable discussion about the terminology
  of a nodes that are addressed by RSVP. In particular two favorites
  have used: hop and peer. This document uses the term hop which is
  different to an IP hop. Two neighboring RSVP nodes communicating with
  each other are not necessarily neighboring IP nodes (i.e. one IP hop
  3  Overview
  3.1  The RSVP INTEGRITY Object
  The RSVP INTEGRITY object is the major component of the RSVP security
  protection. This object is used to provide integrity and replay
  protect the content of the signaling message between two RSVP
  participating router. Furthermore the RSVP INTEGRITY object provides
  data origin authentication. The attributes of the object are briefly
  - Flags field
  The Handshake Flag is the only defined flag and is used to
  synchronize sequence numbers if the communication gets out-of-sync
  (i.e. for a restarting host to recover the most recent sequence
  number). Setting this flag to one indicates that the sender is
  willing to respond to an Integrity Challenge message. This flag can
  therefore be seen as a capability negotiation transmitted within each
  INTEGRITY object.
  - Key Identifier
  The Key Identifier selects the key used for verification of the Keyed
  Message Digest field and hence must be unique for the sender. Its
  length is fixed with 48-bit. The generation of this Key Identifier
  field is mostly a decision of the local host. [RFC2747] describes
  this field as a combination of an address, the sending interface and
  a 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 (i.e. as it is the case with security association
  that have 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
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  interface to determine which security association to use. The term
  outgoing interface might be confusing. The sender selects the
  security association based on the receiver's IP address (of the next
  RSVP capable router). To determine which node is the next capable
  RSVP router is not further specified and is likely to be statically
  - 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
  [RFC2747]. In order for the receiver to distinguish between a new and
  a replayed sequence number each value must be monotonically
  increasing modulo 2^64. 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 [RFC2747] this process is started by handshaking and
  agreeing on an initial sequence number. If no such handshaking is
  available then the initial sequence number must be part of the
  establishment of the security association.
  The generation and storage of sequence numbers is an important step
  in preventing replay attacks and is largely determined by the
  capabilities of the system in presence of system crashes, failures
  and restarts. Section 3 of [RFC2747] explains some of the most
  important considerations.
  - Keyed Message Digest
  The Keyed Message Digest is an RSVP built-in security mechanism used
  to provide integrity protection of the signaling messages. 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 [RFC2104] is required for a RSVP implementation as
  noted in Section 1 of [RFC2747]. Hash algorithms other than MD5
  [RFC1321] like SHA [SHA] 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.
  3.2  Security Associations
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  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 [RFC2747]:
  - Authentication algorithm and algorithm mode
  - Key
  - Key Lifetime
  - Sending Interface
  - Latest sequence number (sent with this key identifier)
  The receiving system has to store the following fields:
  - Authentication algorithm and algorithm mode
  - Key
  - Key Lifetime
  - Source address of the sending system
  - List of last n sequence numbers (received with this key identifier)
  Note that the security associations need to have additional fields to
  indicate their state. It is necessary to have an overlapping lifetime
  of security associations to avoid interrupting an ongoing
  communication because of expired security associations. During such a
  period of overlapping lifetime it is necessary to authenticate either
  one or both active keys. As mentioned in [RFC2747] a sender and a
  receiver might 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
  [RFC2205] assumes that security associations are already available.
  Manual key distribution must be provided by an implementation as
  noted in Section 5.2 of [RFC2747]. Manual key distribution however
  has different requirements to a key storage – a simple plaintext
  ASCII file may be sufficient in some cases. If multiple security
  associations with different lifetimes should be supported at the same
  time then a key engine, for example PF_KEY [RFC2367], would be more
  appropriate. Further security requirements listed in Section 5.2 of
  [RFC2747] are the following:
  - The manual deletion of security associations must be supported.
  - The key storage should persist a system restart.
  - Each key must be assigned a specific lifetime and a specific Key
  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 with [RFC2750]. Section 2 of [RFC3182] stated that
  "Providing policy based admission control mechanism based on user
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  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
  [RFC2750]. Network nodes like the PDP then use the information
  contained in the AUTH_DATA element to authenticate the user and to
  allow policy-based admission control to be executed. As mentioned in
  [RFC3182] the policy element is processed and the policy decision
  point 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
  [RFC2750]. The attributes contained in the authentication data policy
  element AUTH_DATA, which is defined in [RFC3182], 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.
   | RSVP Message                                           |
   | INTEGRITY +-------------------------------------------+|
   | Object    |POLICY_DATA Object                         ||
   |           +-------------------------------------------+|
   |           | INTEGRITY +------------------------------+||
   |           | Object    | AUTH_DATA Object             |||
   |           |           +------------------------------+||
   |           |           | Various Authentication       |||
   |           |           | Attributes                   |||
   |           |           +------------------------------+||
   |           +-------------------------------------------+|
     Figure 1: Security relevant Objects and Elements within the RSVP
  The AUTH_DATA object contains information for identifying users and
  applications together with credentials for those identities. The main
  purpose of those identities seems to be the usage for policy based
  admission control and not for authentication and key management. As
  noted in Section 6.1 of [RFC3182] an RSVP may contain more than one
  POLICY_DATA object and each of them may contain more than one
  AUTH_DATA object. As indicated in the Figure above and in [RFC3182]
  one AUTH_DATA object contains more than one authentication attribute.
  A typical configuration for a Kerberos-based user authentication
  includes at least the Policy Locator and an attribute containing the
  Kerberos session ticket.
  A successful user authentication is the basis for doing policy-based
  admission control. Additionally other information such as time-of-
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  day, application type, location information, group membership etc.
  may be relevant for a policy.
  The following attributes are defined for the usage in the AUTH_DATA
  a) Policy Locator
  The policy locator string that is a X.500 distinguished name (DN)
  used to locate the user and/or application specific policy
  information. The following types of X.500 DNs are listed:
  The first two types are the ASCII and the Unicode representation of
  the user or application DN identity. The two "encrypted"
  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
  shall be provided then the policy locator has to be encrypted with
  the public key of the recipient. How to obtain this public key is not
  described in the document. Such an issue may be specified in a
  concrete architecture where RSVP is used.
  b) Credentials
  Two cryptographic credentials are currently defined for a user:
  Authentication with Kerberos V5 [RFC1510], and authentication with
  the help of digital signatures based on X.509 [RFC2495] and PGP
  [RFC2440]. The following list contains all defined credential types
  currently available and defined in [RFC3182]:
  | Credential   |  Description                   |
  |    Type      |                                |
  | ASCII_ID     |  User or application identity  |
  |              |  encoded as an ASCII string    |
  | UNICODE_ID   |  User or application identity  |
  |              |  encoded as an Unicode string  |
  | KERBEROS_TKT |  Kerberos V5 session ticket    |
  | X509_V3_CERT |  X.509 V3 certificate          |
  | PGP_CERT     |  PGP certificate               |
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                  Table 1: Credentials Supported in RSVP
  The first two credentials only contain a plaintext string and
  therefore they do not provide cryptographic user authentication.
  These plaintext strings may be used to identify applications, which
  are included for policy-based admission control. Note that these
  plain-text identifiers may, however, be protected if either the RSVP
  INTEGRITY and/or the INTEGRITY object of the POLICY_DATA element is
  present. Note that the two INTEGRITY objects can terminate at
  different entities depending on the network structure. The digital
  signature may also provide protection of application identifiers. A
  protected application identity (and the entire content of the
  POLICY_DATA element) cannot be modified as long as no policy ignorant
  nodes are used in between.
  A Kerberos session ticket, as previously mentioned, is the ticket of
  a Kerberos AP_REQ message [RFC1510] 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
  replay protection and shows 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 [RFC2747]
  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 [RFC3182] and may involve several roundtrips depending
  on many Kerberos related factors. The Kerberos ticket does not need
  to be included in every RSVP message as an optimisation as described
  in Section 7.1 of [RFC2747]. Thus the receiver must store the
  received service ticket. If the lifetime of the ticket is expired
  then a new service ticket must be sent. If the receiver lost his
  state information (because of a crash or restart) then he 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
  the replay protection (either timestamps or sequence numbers) is
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  provided by the digital signature is an open issue as discussed in
  Section 4.3.
  c) Digital Signature
  The digital signature computed over the data of the AUTH_DATA object
  must be the last attribute. The algorithm used to compute the digital
  signature depends on the authentication mode listed in the
  credential. This is only partially true since for example PGP again
  allows different algorithms to be used for computing a digital
  signature. The algorithm used for computing the digital signature is
  not included in the certificate itself. The algorithm identifier
  included in the certificate only serves the purpose to allow the
  verification of the signature computed by the certificate authority
  (except for the case of self-signed certificates).
  d) Policy Error Object
  The Policy Error Object is used in the case of a failure of the
  policy based admission control or other credential verification.
  Currently available error messages allow to notify if the credentials
  are expired (EXPIRED_CREDENTIALS), if the authorization process
  disallowed the resource request (INSUFFICIENT_PRIVILEGES) and if the
  given set of credentials is not supported
  (UNSUPPORTED_CREDENTIAL_TYPE). The last error message allows the
  user's host to discover the type of credentials supported although by
  very inefficient means. Furthermore it is unlikely that a user
  supports different types of credentials. The purpose of the error
  message IDENTITY_CHANGED is unclear. The protection of the error
  message is not discussed in [RFC3182].
  3.5  RSVP Integrity Handshake
  The Integrity Handshake is a protocol that was designed to allow a
  crashed or restarted host to obtain the latest valid challenge value
  stored at the receiving host. A host stores the latest sequence
  number of a fresh and correctly authenticated packet. An adversary
  can replay eavesdropped packets if the crashed host has lost its
  sequence numbers. A signaling message from the real sender with a new
  sequence number would therefore allow the crashed host to update the
  sequence number field and prevent further replays. Hence if there is
  a steady flow of RSVP protected messages between the two hosts an
  attacker may find it difficult to inject old messages since new
  authenticated packets with high sequence numbers arrive and get
  stored immediately.
  The following description explains the details of the RSVP Integrity
  Handshake that is started by Node A after recovering from a
  synchronization failure:
                     Integrity Challenge
                 (1) Message (including
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   +----------+      a Cookie)            +----------+
   |          |-------------------------->|          |
   |  Node A  |                           |  Node B  |
   |          |<--------------------------|          |
   +----------+      Integrity Response   +----------+
                 (2) Message (including
                     the Cookie and the
                     INTEGRITY object)
                    Figure 2: RSVP Integrity Handshake
  The details of the messages are described below:
  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 [RFC2747].
  The Integrity Challenge message is not protected with an INTEGRITY
  object as show in the protocol flow above. As explained in Section 10
  of [RFC2747] this was done to avoid problems in situations where both
  communication parties do not have a valid starting sequence number.
  Whether or not to use the RSVP Integrity Challenge/Response mechanism
  is a site-local decision since it may not be needed in all network
  environments. It is however recommended to use the RSVP Integrity
  Handshake protocol.
  4  Detailed Security Property Discussion
  The purpose of this section is to describe the security protection of
  the RSVP provided mechanisms individually for authentication,
  authorization, integrity and replay protection, user identity
  confidentiality, confidentiality of the signaling messages.
  4.1  Discussed Network Topology
  The main purpose of this paragraph is to show the basic interface of
  a simple RSVP network architecture. The architecture below assumes
  that there is only a very single domain and that two routers are RSVP
  and policy aware. These assumptions are relaxed in the individual
  paragraphs as necessary. Layer 2 devices between the clients and
  their corresponding first hop routers are not shown. Other network
  elements like a Kerberos Key Distribution Center and for example an
  LDAP server where the PDP retrieves his 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.
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                          |Policy  |
                     +----+Point   +---+
                     |    +--------+   |
                     |                 |
                     |                 |
                     |                 |
    +------+       +-+----+        +---+--+          +------+
    |Client|       |Router|        |Router|          |Client|
    |  A   +-------+  1   +--------+  2   +----------+  B   |
    +------+       +------+        +------+          +------+
                    Figure 3: Simple RSVP Architecture
  4.2  Host/Router
  When talking about authentication in RSVP it is very important to
  make a distinction between user and host authentication of the
  signaling messages. By using the RSVP INTEGRITY object the host is
  authenticated while credentials inside the AUTH_DATA object can be
  used to authenticate the user. In this Section the focus is on host
  authentication whereas the next Section covers user authentication.
  a) Authentication
  We use the term host authentication above since the selection of the
  security association is bound to the host’s IP address as mentioned
  in Section 3.1 and 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. Since
  the key management protocol is not specified it is difficult to
  evaluate this part and hence we speak about data origin
  authentication based on the host’s identity for RSVP INTEGRITY
  objects. The fact that the host identity is used for selecting the
  security association has already been described in Section 3.1.
  Data origin authentication is provided with the keyed hash value
  computed over the entire RSVP message excluding the keyed message
  digest field itself. The security association used between the user’s
  host and the first-hop router is, as previously mentioned, not
  established by RSVP and must therefore be available before the
  signaling is started.
  Although not mentioned in [RFC2747] it is also possible to use IPsec
  [RFC2401] to protect the RSVP signaling traffic from the client to
  the first-hop router. Note that IPsec usage for RSVP signaling
  protocol requires preconditions which are described in Section 5.6.
  If we use IPsec to protect the interface between the user’s host and
  the first hop router then the optional RSVP INTEGRITY object may not
  be required. It may also be possible (which requires a further
  investigation) whether an existing IPsec security association may
  also be (re-)used for RSVP. IPsec allows the key exchange protocol
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  IKE [RFC2409] to be used to dynamically negotiate IPsec security
  associations. Note that KINK [FH+01] and other protocols are
  available that are also able to establish an IPsec security
  association. This text mainly refers to IKE since it is the most
  frequently used protocol for this purpose. A detailed description of
  IPsec and IKE is outside the scope of this document. Since IKE is
  computationally expensive it might create a computational burden to
  re-establish a new IPsec SA based of the movement of a mobile user
  host. Work at the SEAMOBY group tries to tackle this problem by using
  IPsec Context Transfer protocols. Hence in this case we would avoid
  triggering a separate key exchange protocol run for RSVP to protect
  messages at each layer if they terminate at the same node.
  It is an open issue whether it is enough to provide IPsec protection
  of messages between the user’s host and the first-hop router although
  different protocols (i.e. protocols executed at different protocol
  layers) (possibly) terminate at different endpoints.
  - Kerberos for the RSVP INTEGRITY object
  As described in Section 7 of [RFC2747] 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 [RFC2747]. Section 4.2.1 of [RFC2747] states that the required
  identities can be obtained statically or dynamically via a directory
  service or DHCP. [HA01] 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 is known
  for which this information is desired). It is only required to
  encapsulate the Kerberos ticket inside the policy element. It is
  furthermore mentioned that Kerberos tickets with expired lifetime
  must not be used and the initiator is responsible for requesting and
  exchanging a new service ticket before expiration.
  RSVP multicast processing in combination with Kerberos requires
  additional thoughts:
  Section 7 of [RFC2747] 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.
  An additional protocol needs to be executed after each user is
  authenticated via Kerberos to establish a session key and to allow
  multicast specific functionality like entering a group, leaving a
  group to be executed securely. This would additionally allow
  accounting and billing to be used efficiently and on a per-user
  basis. This session key is then used to protect RSVP signaling
  messages. These issues definitely need further investigation and are
  not fully described in this version of the document.
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                       RSVP Security Properties             March 2003
  In case that 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
  b) Integrity Protection
  Integrity protection between the user’s host and the first hop router
  is based on the RSVP INTEGRITY object. Since the RSVP Integrity
  object is an optional element of the RSVP message IPsec protection of
  the signaling message to the router may also provide integrity
  protection either with IPsec AH [RFC2402] or IPsec ESP [RFC2406] as
  mentioned already in the previous paragraph.
  Furthermore it is stated that other keyed hash functions apart from
  HMAC-MD5 may be used within the RSVP INTEGRITY object and it is
  obvious that both communicating entities must have security
  associations indicating the algorithm used. This may be however
  difficult since there is no negotiation protocol defined to agree on
  a specific algorithm. Hence it is very likely that HMAC-MD5 is the
  only usable algorithm for the RSVP INTEGRITY object if RSVP is used
  in a mobile environment and only in local environments it may be
  useful to switch to a different keyed hash algorithm. The other
  possible alternative is that every implementation must support the
  most important keyed hash algorithms for example MD5, SHA-1, RIPEMD-
  160 etc. HMAC-MD5 was mainly chosen because of the performance
  characteristics. The weaknesses of MD5 [DBP96] are known and
  described in [Dob96]. Other algorithms like SHA-1 [SHA] and RIPEMD-
  160 [DBP96] instead are known to provide better security properties.
  c) Replay Protection
  The main mechanism used for replay protection in RSVP are sequence
  numbers whereby the sequence number is included in the RSVP INTEGRITY
  object. The properties of this sequence number mechanisms are
  described in Section 3.1. The fact that the receiver stores a list of
  sequence numbers is an indicator for a window mechanism. This somehow
  conflicts with the requirement that the receiver only has to store
  the highest number given in Section 3 of [RFC2747]. We assume that
  this is a typo. Section 4.1 of [RFC2747] gives a few comments about
  the out-of-order delivery and the ability of an implementation to
  specify the replay window.
  If IPsec is used to protect RSVP messages then the optional IPsec
  replay protection mechanism may be used which is also based on
  sequence numbers with a window mechanism. This window mechanism may
  (theoretically) also cause problems whereby an adversary reorders
  messages. This is however very difficult to exploit since the
  signaling messages are exchanged at a relatively low rate compared to
  regular data traffic that may also be protected with IPsec.
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                       RSVP Security Properties             March 2003
  - Integrity Handshake
  The mechanism of the Integrity Handshake is explained in Section 3.5.
  The Cookie value is suggested to be hash of a local secret and a
  timestamp. The Cookie value is not verified by the receiver. The
  mechanism used by the Integrity Handshake is a simple
  Challenge/Response message which assumes that the key shared between
  the two hosts survives the crash. If the security association is
  however dynamically created then this assumption may not be true.
  In Section 10 of [RFC2747] the authors note that an adversary can
  create faked Integrity Handshake message including challenge cookies.
  Subsequently he would store the received response. Later he tries 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 the adversary "discovered" the
  cookie generation mechanism (for example by knowing the local
  secret). The collection of Challenge/Response pairs is even more
  difficult since they depend on the Cookie value, on sequence number
  included in the response message and on the shared key which is used
  by the INTEGRITY object.
  d) Confidentiality
  Confidentiality is not considered to be a security requirement for
  RSVP. Hence it is not directly supported by RSVP. However, IPsec can
  provide confidentiality by encrypting the transmitted signaling
  traffic with IPsec ESP.
  e) 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. via AAA protocols (for example using RADIUS [RFC2865] or
  DIAMETER [CA+02]) and authorization information is downloaded to one
  or more network elements for example to the access router/first hop
  router to modify filter rules to enable the IP traffic forwarding.
  The access router is therefore acting as a firewall with dynamically
  created filter rules based on a successful host or user
  authentication. Issues related to network access authorization are
  outside the scope of RSVP.
  The second authorization refers to RSVP itself. Depending on the
  network configuration
  - the router either forwards the received RSVP request to the policy
  decision point e.g. by using COPS (see [RFC2748] and [RFC2749]) and
  to request admission control procedure to be executed or
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                       RSVP Security Properties             March 2003
  - the router supports the functionality of a PDP and therefore there
  is no need to forward the request or
  - the router may already be configured with the appropriate policy
  information to decide locally whether to grant this request or not.
  Based on the result of the admission control the request may be
  granted or rejected. Without a policy element being embedded inside
  the RSVP message no policy-based admission control can be done.
  The interaction between the two access authorization procedures (and
  the filter-installation at the various network devices) will likely
  be investigated in more detail in the MIDCOM working group.
  f) Performance
  The computation of the keyed message digest for a RSVP INTEGRITY
  object does not represent a performance problem. The same is true for
  IPsec AH (or IPsec ESP). The protection of signaling messages is
  usually not a problem since these messages are transmitted at a low
  rate. Even a high number of messages does not cause performance
  problems for a RSVP routers because of the efficiency of the keyed
  message digest routine.
  The key management which is computationally more demanding is more
  important for scalability. Since RSVP does not specify a particular
  key exchange protocol to be used it is difficult to estimate the
  effort to create the required security associations. Furthermore the
  number of key exchanges to be triggered depends on security policy
  issues like lifetime of a security 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 the manual security association
  distribution may be acceptable and provides 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 certificate verification needs to be supported.
  4.3  User to PEP/PDP
  As noted in the previous section both user and host based
  authentication is supported by RSVP. Using RSVP, a user may
  authenticate to the first hop router or to the PDP as specified in
  [RFC2747] depending on the infrastructure provided by the network
  domain or on the architecture used (e.g. the integration of RSVP and
  Kerberos V5 into the Windows 2000 Operating System [MADS01]). Another
  architecture where RSVP is tightly integrated is the one specified by
  the PacketCable organization. The interested reader is referred to
  [PKTSEC] for a discussion of the security architecture.
  a) Authentication
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                       RSVP Security Properties             March 2003
  When a user sends a RSVP PATH or RESV message then this message may
  include some information to authenticate the user. [RFC3182]
  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/application information to the policy
  decision point (PDP). Optionally the PDP itself can authenticate the
  user, which is described in the next section. In order 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 that the INTEGRITY object within the POLICY_DATA
  element is sent to the PDP along with all other attributes although
  not clearly specified in [RFC3182].
  Certificate Verification
  Using the policy element as described in [RFC3182] it is not possible
  to provide a certificate revocation list or other information to
  proof the validity of the certificate inside the policy element. A
  specific mechanism for certificate verification is not discussed in
  [RFC3182] and hence a number of them can be used for this purpose.
  For certificate verification the network element (a router or the
  policy decision point), which has to authenticate the user, could
  frequently download certificate revocation lists or should use a
  protocol like the Online Certificate Status Protocol (OCSP) [RFC2560]
  and the Simple Certificate Validation Protocol (SCVP) [MHHF01] 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 in [RFC3182] the choice is given for the user to either obtain
  a session ticket 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 first a session
  ticket for the PDP needs to be requested. If the user roams between
  different routers in the same administrative domain then he does not
  need to request a new service ticket since the PDP is likely to be
  used by most or all first-hop routers within the same administrative
  domain. This is different if a session ticket for a router has to be
  obtained and authentication to a router is required. The router
  therefore plays a passive role of forwarding the request only to the
  PDP and executing the policy decision returned by the PDP.
  Appendix B describes one example of user-to-PDP authentication.
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                       RSVP Security Properties             March 2003
  User authentication with the policy element only provides unilateral
  authentication where the client authenticates to the router or to the
  PDP. If a RSVP message is sent to the user’s host and public keyed
  based authentication is used then the message does not contain a
  certificate and digital signature. Hence no mutual authentication can
  be assumed. In case of Kerberos mutual authentication may be
  accomplished if the PDP or the router transmits a policy element with
  an INTEGRITY object computed with the session key retrieved from the
  Kerberos ticket or if the Kerberos ticket included in the policy
  element is also used for the RSVP INTEGRITY object as described in
  Section 4.2. This procedure only works if a previous message was
  transmitted from the end host to the network and such key is already
  established. [RFC3182] does not discuss this issue and therefore
  there is no particular requirement dealing with transmitting network
  specific credentials back to the end-user's host.
  b) Integrity Protection
  The integrity protection of the RSVP message and the POLICY_DATA
  element are protected separately 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 credentials of the user 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 and a guarantee of freshness for the receiving host.
  Authentication and guarantee of freshness is provided by the keyed
  hash value of the INTEGRITY object inside the POLICY_DATA element.
  The user thereby shows that he actively participated in the Kerberos
  protocol and that he 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 based on 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 [RFC1510]).
  - Digital Signature
  If public key based authentication is provided then user
  authentication is accomplished with the digital signature. As
  explained in Section 3.3.3 of [RFC3182] the DIGITAL_SIGNATURE
  attribute must be the last attribute in the AUTH_DATA object and the
  digital signature covers the entire AUTH_DATA object. Which hash
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                       RSVP Security Properties             March 2003
  algorithm and public key algorithm is used for the digital signature
  computation is described in [RFC2440] in case that PGP is used. In
  case of X.509 credentials the situation is more complex since
  different mechanisms like CMS [RFC2630] or PKCS#7 [RFC2315] may be
  used for the digitally signing the message element. X.509 only
  provides the standard for the certificate layout which seems to
  provide insufficient information for this purpose. Therefore X.509
  certificates are supported for example by CMS and PKCS#7. [RFC3182],
  however, does not make any statements about the usage of CMS and
  PKCS#7. Currently there is no support for CMS or PKCS#7 described in
  [RFC3182], which provides more than only public key based
  authentication (e.g. CRL distribution, key transport, key agreement,
  etc.). Furthermore the usage of PGP in RSVP is vague since there are
  different versions of PGP (including a OpenPGP [RFC2440]) and there
  has been no indication which version should be used. When thinking
  about CMS support for RSVP the main question that has to be answered
  is whether a public key based authentication (and key agreement
  mechanism) should be supported for a QoS signaling protocol.
  Especially the risks of denial of service attacks, large processing,
  memory and bandwidth utilization should be considered.
  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 observation:
  a) For the digital signature case only the replay protection provided
  by the digital signature algorithm can be used. It is however not
  clear whether this usage was anticipated or not. Hence we might
  assume that the replay protection is based on the availability of
  RSVP INTEGRITY object used with a security association that is
  established by other means.
  b) If a Kerberos session ticket is included but without using the
  Kerberos session key then the analogon of 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).
  c) Replay Protection
  Figure 4 below shows the interfaces relevant for replay protection of
  signaling messages in a more complicated architecture. The client
  therefore uses the policy data element with PEP2 since PEP1 is not
  policy aware. The interfaces between the client and the PEP1 and
  between the 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 +-+
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                       RSVP Security Properties             March 2003
                                                         +----+ |
     |                                                          |
     |                                                 COPS     |
     |                                                          |
     |                                                          |
  +--+---+   RSVP INTEGRITY  +----+    RSVP INTEGRITY    +----+ |
  +--+---+                   +----+                      +-+--+
     |                                                     |
                      POLICY_DATA INTEGRITY
                       Figure 4: 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 replay mechanism. The length of the
  Sequence Number field, sequence number rollover and the Integrity
  Handshake is already explained in Section 3.1.
  Section 9 in [RFC3182] states "RSVP INTEGRITY object is used to
  protect the policy object containing user identity information from
  security (replay) attacks.". Hence the public key based
  authentication does not support the RSVP based replay protection
  since the digital signature does not cover the POLICY_DATA INTEGRITY
  object with its Sequence Number field. The digital signature covers
  the entire AUTH_DATA object.
  The use of public key systems within the AUTH_DATA object complicates
  replay protection. Digital signature computation with PGP is
  described in [PGP] and in [RFC2440]. The data structure preceding the
  signed message digest includes information about the message digest
  algorithm used and a 32-bit timestamp when the signature was created
  ("Signature creation time"). The timestamp is included in the
  computation of the message digest. The IETF standardized OpenPGP
  version [RFC2440] contains more information and describes the
  different hash algorithms (MD2, MD5, SHA-1, RIPEMD-160) provided.
  [RFC3182] does not make any statements whether the "Signature
  creation time" field is used for replay protection. Using timestamps
  for replay protection requires different synchronization mechanisms
  in case of clock-screws. Traditionally "loosely" synchronized clocks
  are assumed in those cases but also requires specifying a replay-
  If the "Signature creation time" is not used for replay protection
  then a malicious policy ignorant node can use this weakness to
  replace the user's credentials without destroying the digital
  signature. Additionally the RSVP initiating host, where multiple
  users may have access, must be trustworthy even if a smartcard is
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                       RSVP Security Properties             March 2003
  used since otherwise, replay attacks with a recorded AUTH_DATA object
  are possible. Note that this however violates the hop-by-hop security
  assumption. It is therefore assumed that replay protection of the
  user credentials is not considered as an important security
  requirement since the hop-by-hop processing of the RSVP message
  protects the message against modification by an adversary between two
  communicating nodes.
  There are two additional issues related to a Kerberos based user
  authentication in the context of replay protection. The lifetime of
  the Kerberos ticket is based on the fields starttime and endtime of
  the EncTicketPart structure of the ticket as described in Section
  5.3.1 of [RFC1510]. Since 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 at [DG96].
  If we assume that the Kerberos session key is used for RSVP then
  there may be a need for rekeying. If we assume that a policy at the
  user's host indicates when to rekey then the next RSVP message
  includes a new Kerberos session ticket that is then used by the
  verifying entity. If the lifetime of the Kerberos ticket or other
  policies do not affect rekeying then an RSVP security association may
  never require rekeying at all because of the large sequence number
  d) (User Identity) Confidentiality
  This Section discusses the privacy protection of the identity
  information transmitted inside the policy element. Especially the
  user identity confidentiality is of interest because there is no
  built-in RSVP mechanism for encryption of the POLICY_DATA or the
  AUTH_DATA elements.  The encryption of one of the attributes inside
  the AUTH_DATA element - of the POLICY_LOCATOR attribute is discussed
  in the next section.
  There has often been the discussion whether the effort for protecting
  user identity is worth the additional complexity. With the increasing
  privacy awareness there must be at least a discussion on the
  mechanisms provided by the given protocol. The main question in this
  context is about the threat model i.e. against which entity the user
  identity should be protected. Since RSVP does not make any
  assumptions about the underlying key management protocol for most
  parts it is difficult to make a judgment. However for the identity
  representation part of the protocol it is possible to make some
  observations. We assume that the most important threat for a user is
  to reveal his identity to an adversary located between the user’s
  host and the first-hop router. Identities should furthermore not be
  transmitted outside the domain of the visited network provider i.e.
  the user identity information inside the policy data element should
  be removed or modified by the PDP to prevent revealing information to
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                       RSVP Security Properties             March 2003
  other (non-authorized) entities along the signaling path. We cannot
  however provide user identity confidentiality against the network
  provider to which the user is attached. Different mechanisms must be
  deployed to disallow the network provider to create a profile of the
  user. These mechanisms are outside the scope of this document since
  there is a strong involvement with the initial authentication and key
  agreement protocol executed between the user and the visited network.
  If the link between the user’s host and the first hop router is
  protected with IPsec ESP then confidentiality of the entire signaling
  messages is provided. Note however that the IPsec protection may
  terminate at the different node than the RSVP policy aware signaling
  does. The focus of this Section is, however, the functionality
  provided by RSVP.
  The ASCII or Unicode distinguished name of user or application inside
  the POLICY_LOCATOR attribute of the AUTH_DATA element may be
  encrypted as specified in Section 3.3.1 of [RFC3182].  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. Since the private key is used we usually speak of a
  digital signature which can be verified by everyone possessing the
  public key. Since the certificate with the public key is included in
  the message itself this is no obstacle. Furthermore the included
  certificate provides enough identity information for an eavesdropper
  together with the additional (unencrypted) information provided in
  the RSVP message. Hence the possibility of encrypting the policy
  locator in case of public key based authentication is less obvious.
  To encrypt the identities using asymmetric cryptography the user’s
  host must be able to somehow retrieve the public key of the entity
  verifying the policy element (i.e. the first policy aware router or
  the PDP). Currently no such mechanism is defined in [RFC3182].
  There is no option to encrypt the user or application identity
  without Kerberos or public key mechanisms are used since the
  selection of an appropriate security association is not possible.
  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 used algorithm is
  available in the etype field of the EncryptedData ASN.1 encoded
  message part. Section 6.3 of [RFC1510] lists the supported
  algorithms. [Rae01] defines new encryption algorithms (Rijndael,
  Serpent, and Twofish) that were published in the context of the AES
  The task of evaluating the confidentiality provided for the user
  requires to look at protocols executed outside of RSVP (for example
  to look at 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. Since the Authenticator is not transmitted with the RSVP
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                       RSVP Security Properties             March 2003
  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 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 not to reveal content of the encrypted policy locator,
  which is only useful if this value differs from the user identity
  used with Kerberos. 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 and whether he needs to be re-authenticated in other
  network nodes etc. Hence user and application specific information
  can leak after the messages leave the first hop within the network
  where the user's host is attached. As mentioned at the beginning of
  this Section this information leakage is assumed to be intentional.
  e) Authorization
  Additional to the description of the authorization steps of the
  Host/Router interface, user based authorization is added with the
  policy element providing user credentials. The inclusion of user and
  application specific information enables policy-based admission
  control with special user policies that are likely to be stored at a
  dedicated server. Hence a Policy Decision Point can query for example
  a LDAP server for a service level agreement stating the amount of
  resources a certain user is allowed to request. Additional 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 users privacy policy. In general the user may not want to
  distribute much of his policy information. Furthermore the missing
  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 already required
  cross-realm communication with the user's home domain to only assume
  that the home domain knows the user and that the user is entitled to
  roam and to be able to forward accounting messages to this domain.
  This represents the traditional subscriber based accounting scenario.
  Non-traditional or alternative means of accounting might be deployed
  in the near future that do not require the any type of inter-domain
  communication. Obviously there is a strong interrelationship between
  the authorization and the process of accounting. Note that the term
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                       RSVP Security Properties             March 2003
  accounting in this context is not only related to process of
  metering. Metering is only the process of measuring and collecting
  resource usage information. Instead the term unites metering,
  pricing, charging and billing.
  f) Performance
  If Kerberos is used for user authentication then a Kerberos ticket
  must be included in the CREDENTIAL Section of the AUTH_DATA element.
  The Kerberos ticket has a size larger than 500 bytes but only needs
  to be sent once since a performance optimization allows the session
  key to be cached as noted in Section 7.1 of [RFC2747]. It is assumed
  that subsequent RSVP messages only include the POLICY_DATA INTEGRITY
  object with a keyed message digest that uses the Kerberos session
  key. This however assumes that the security association required for
  the POLICY_DATA INTEGRITY object is created after (or modified) to
  allow the selection of the correct key. Otherwise it difficult to say
  which identifier is used to index the security association.
  When Kerberos is used as an authentication system then, from a
  performance perspective, then the message exchange to obtain the
  session key needs to be considered although the exchange only needs
  to be done once in a long time frame depending on 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 non-existing possibility 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 performance
  devices, high latency 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 [RFC2560] and the Simple Certificate Validation
  Protocol [MHHF01]. Then the integrity of the AUTH_DATA object via the
  digital signature is verified.
  4.4 Communication between RSVP aware routers
  a) Authentication
  RSVP signaling messages are data origin authenticated and protected
  against modification and replay using the RSVP INTEGRITY object.
  IPsec may also provide RSVP signaling message protection is it,
  however, not suggested because of the problems described in Section
  5.6. Only in certain environments IPsec protection might not cause
  problems. The RSVP message flow between routers is protected based on
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                       RSVP Security Properties             March 2003
  the chain of trust and hence each router only needs to have a
  security association with its neighboring routers. This assumption
  was made because of performance advantages and because of special
  security characteristics of the core network where no user hosts are
  directly attached. In the core network the network structure does not
  change frequently and the manual distribution of shared secrets for
  the RSVP INTEGRITY object may be acceptable. The shared secrets may
  be either manually configured or distributed by using network
  management protocols like SNMP.
  If IPsec is used in a hop-by-hop fashion then the required security
  associations may be manually created or dynamically distributed with
  IKE by either using symmetric or asymmetric authentication modes. A
  description of the existing IKE authentication modes and IKE security
  properties is outside the scope of this document. The reader is
  referred to the relevant documents at the IPsec working group.
  Independent of the key distribution mechanism host authentication
  with RSVP built-in mechanisms is accomplished with the keyed message
  digest in the RSVP INTEGRITY object computed using the previously
  exchanged symmetric key. In case of IPsec host authentication is
  accomplished with the keyed message digest included in the
  Authentication Data field of the IPsec Authentication Header included
  in every IP packet.
  b) Integrity Protection
  Integrity protection is either accomplished with the RSVP INTEGRITY
  object with the variable length Keyed Message Digest field or with
  the IPsec Authentication Header. A description of the IPsec AH is
  found in [RFC2402] and IPsec ESP [RFC2406] with null encryption is
  found in [RFC2410]. The main difference between IPsec and RSVP
  protection is the layer at which the security is applied.
  c) Replay Protection
  Replay protection with the RSVP INTEGRITY object is extensively
  described in previous Sections. IPsec provides an optional window-
  based replay protection, which may cause problems if a strict message
  ordering of RSVP messages is required. This problem was already
  discussed in a previous Section and a possible solution is to include
  the RSVP INTEGRITY object without a key, which reduces the RSVP
  integrity protection to a simple MD5 hash. This modification must
  however be integrated into an existing implementation and it is not
  clear whether the RSVP standard allows this modification. If the RSVP
  implementation is able to access the IPsec Security Association
  Database and retrieve the required security association then no such
  modification to RSVP is required and IKE is only used to distribute
  the security associations. This however requires the RSVP
  implementation to trigger the IKE exchange.
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                       RSVP Security Properties             March 2003
  To enable crashed hosts to learn the latest sequence number used the
  Integrity Handshake mechanism is used in RSVP as explained in a
  Section above. IPsec does not provide such a mechanism since a
  crashed host looses its negotiated security associations and
  therefore has to re-negotiate them using IKE. Note that manually
  configured IPsec security associations do not provide replay
  protection because a sequence number rollover would require the
  establishment of a new SA. This is obviously not possible when using
  manually configured IPsec SAs. Using IKE with pre-shared secrets is
  therefore a simple solution.
  d) Confidentiality
  Confidentiality is not provided by RSVP but using IPsec ESP in a hop-
  by-hop mode can provide it. The usage of IPsec ESP for RSVP is not
  recommended because of the additional overhead for little additional
  security benefit if we think of the underlying assumed trust model of
  chain of trust. Hence there must be a good reason why to require
  confidentiality in a hop-by-hop fashion in the core network of the
  same administrative domain. If the RSVP network spawns different
  provider networks then it is possible to encapsulate RSVP messages
  between RSVP networks over a non-RSVP cloud similar to a VPN. Such a
  configuration is mainly determined by the network structure of a
  e) Authorization
  Depending on the RSVP network QoS resource authorization at different
  routers may need to contact the PDP again. Since 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
  f) Performance
  The performance characteristics the protection of the RSVP signaling
  messages is largely determined by the key exchange protocol since the
  RSVP INTEGRITY object or IPsec AH are only used to compute a keyed
  message digest of the transmitted messages. Furthermore only RSVP
  signaling messages are protected and the protection of the
  application data stream is outside the scope of RSVP. IPsec ESP
  provides a performance penalty but may only be rarely used. A network
  administrator may however use IPsec ESP in transport mode with NULL
  encryption to provide the same functionality as IPsec AH but with the
  chance of better hardware support.
  The security associations within the core network i.e. between
  individual routers (in comparison to 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 strong
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                       RSVP Security Properties             March 2003
  trust assumptions. Furthermore it is possible to use security
  associations with an increased lifetime to avoid too frequent
  rekeying. Hence there is less impact for the performance compared to
  the user to network interface. The security association storage
  requirements are also less problematic.
 5  Miscellaneous Issues
   This section describes a number of issues which illustrate some of
   the short-comings of RSVP with respect to security.
 5.1  First Hop Issue
   In case of end-to-end signaling an end host starts signaling to its
   attached network. The first-hop communication is often more difficult
   because of the different requirements and a missing trust
   relationship. An end host must therefore obtain some information to
   start RSVP signaling:
   - Does this network support RSVP signaling?
   - Which node supports RSVP signaling?
   - To which node is authentication required?
   - Which identity is used for authentication?
   - Which security mechanisms are used for authentication?
   - Which algorithms have to be used?
   - Where should the keys/security association come from?
   - Should a security association be established?
   RSVP, as specified today, is used as a building block. Hence these
   questions have to be answered as part of overall architectural
   considerations. Without giving an answer to this question "ad-hoc"
   RSVP communication by an end host roaming to an unknown network is
   not possible. A negotiation of security mechanisms and algorithms is
   not supported for RSVP.
 5.2  Next-Hop Problem
  Throughout the document it was always assumed that the next RSVP node
  along the path is always known. Knowing your next hop is important to
  be able to select the correct key for the RSVP Integrity object to
  provide proper protection. In case that an RSVP node assumes to know
  which node is the next hop then the following protocol exchange can
                                    (A<->C)    +------+
                                     (3)       | RSVP |
                                +------------->+ Node |
                                |              |  B   |
                   Integrity    |              +--+---+
                    (A<->C)     |                 |
         +------+    (2)     +--+----+            |
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                       RSVP Security Properties             March 2003
    (1)  | RSVP +----------->+Router |            |  Error
   ----->| Node |            | or    +<-----------+ (I am B)
         |  A   +<-----------+Network|       (4)
         +------+    (5)     +--+----+
                    Error       .
                   (I am B)     .              +------+
                                .              | RSVP |
                                ...............+ Node |
                                               |  C   |
                         Figure 5: Next-Hop Issue
  When RSVP node A in Figure x receives an incoming RSVP Path message
  then 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 mechanism which is not further specified is used to
  make this decision. In this example node A assumes that the message
  will travel to RSVP node C. However because of some reasons (e.g. a
  route change, inability to learn the next RSVP hop along the path,
  etc.) the message travels to node B via a non-RSVP supporting router
  which cannot verify the integrity of the message (or cannot decrypt
  the Kerberos service ticket). The processing failure causes a PathErr
  message to be returned to the originating sender of the Path message.
  This error message also contains information about the node
  recognizing the error. In many cases a security association might not
  be available. Node A receiving the PathErr message might use the
  information returned with the PathErr message to select a different
  security association (or to establish one). The RSVP Path message
  therefore provides a number of functions: path discovery, detecting
  route changes, learning of QoS capabilities along the path using the
  Adspec object, (with some interpretation) next-hop discovery and
  possibly security association establishment (for example in case of
  From a security point of view there is a conflict between
  - Idempotent messages delivery and efficiency
  Especially the RSVP Path message performs a number of functions.
  Supporting idempotent message delivery somehow contradicts with
  security association establishment and efficient message delivery and
  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, very inefficient. Using
  public key based mechanisms is even more inefficient when included in
  every signaling message. With public key based protection for
  idempotent messages there is additionally a risk of introducing
  denial of service attacks.
  - RSVP Path message functionality and next-hop discovery
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                       RSVP Security Properties             March 2003
  To protect an RSVP signaling message (and a RSVP Path message in
  particular) it is necessary to know the identity of the next RSVP
  aware node (and some other parameters). Without a mechanism for next-
  hop discovery an RSVP Path message is also responsible for this task.
  Without knowing the identity of the next hop the Kerberos principal
  name is also unknown. The so-called Kerberos user-to-user
  authentication mechanism is not supported which would allow the
  receiver to trigger the process of establishing Kerberos
  authentication is not supported. This issue will again be discussed
  in relationship with the last-hop problem.
  It is fair to assume that a RSVP supporting node might not have a
  security association with all immediately neighboring RSVP nodes.
  Especially for inter-domain signaling, IntServ over DiffServ or for
  some new applications such as firewall signaling the next RSVP aware
  node might not be known in advance. The number of next RSVP nodes
  might be considerably large if they are separated by a large number
  of non-RSVP aware nodes. Hence a node transmitting a RSVP Path
  message might experience difficulties to properly protect the message
  if it serves as a mechanism to detect both the next RSVP node (i.e.
  Router Alert Option added to the signaling message and addressed to
  the destination address) and to detect route changes. It is fair to
  note that in an intra-domain case this might be possible due to
  manual configuration in case of a dense distribution of RSVP nodes.
  There is nothing which prevents an adversary from continuously
  flooding an RSVP node with bogus PathErr messages. It might be
  possible to protect the PathErr message with an existing security
  association if available. A legitimate RSVP node would believe that a
  change in the path took place. Hence this node would try to select a
  different security association or try to create one with the
  indicated node. Hence an adversary can send a PathErr message at any
  time to confuse an RSVP node. If an adversary is located along
  somewhere along the path then it might also be possible to act as a
  man-in-the-middle adversary if either authentication and/or
  authorization is not performed with the necessary accuracy.
 5.3  Last-Hop Issue
  This section tries to address practical difficulties when
  authentication and key establishment is accomplished with a protocol
  which shows some asymmetry in message processing when executed
  between two nodes. Kerberos is such a protocol and also the only
  supported protocol which provides dynamic session key establishment
  for RSVP. For first-hop communication authentication is typically
  done between a user and some network in the network (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 show
  the problem the typical processing steps for Kerberos are shown for
  first-hop communication:
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                       RSVP Security Properties             March 2003
  a) 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 or a PDP
  or the next policy aware RSVP node.
  b) Entity A then requests a ticket granting ticket for the network
  domain. This assumes that the identity of the network domain is
  c) Entity A then requests a service ticket for entity B which was
  learned in step (a).
  d) Entity A includes the service ticket to the RSVP signaling message
  (inside the policy object). The Kerberos session key is used to
  protect the entire RSVP signaling message.
  For last-hop communication this processing step theoretically has to
  be reversed; entity A is then a node in the network (for example the
  access router) and entity B is the other end host. This assumes that
  RSVP signaling is accomplished between two end hosts and not between
  an end host and a application server. The access router might however
  in step (a) not be able to learn the identity of the user's principal
  name since 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.
  IAKERB is however not supported.
 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 devices tells
   the router which data packets should experience QoS treatment. RSVP
   typically establishes a flow identifier based on the 5-tuple (source
   IP address, destination IP address, transport protocol type, source
   port and destination port). If this 5-tuple information is not
   available then other identifiers have to be used. IPsec protected
   data traffic is such an example where the transport protocol and the
   port numbers are not accessible. Hence the IPsec SPI is used as a
   substitute for them. RFC 2207 considers these IPsec implications for
   RSVP and is based on three assumptions:
   a) An end host, which initiates the RSVP signaling message exchange,
   has to be able to retrieve the SPI for given flow. This requires some
   interaction with the IPsec SADB and SPD. 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 IPsec
   security association database by providing the flow identifiers as
   input parameters and the SPI as an output parameter.
   b) RFC 2207 assumes an end-to-end IPsec protection of the data
   traffic. In IPsec is applied in a nested fashion then parts of the
   path do not experience QoS treatment. This problem can be treated as
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                       RSVP Security Properties             March 2003
   a tunneling problem but is initiated by the end host. A figure better
   illustrates the problem in case of enforcing secure network access:
   +------+          +---------------+      +--------+          +------+
   | Host |          | Security      |      | Router |          | Host |
   |  A   |          | Gateway (SGW) |      |   Rx   |          |  B   |
   +--+---+          +-------+-------+      +----+---+          +--+---+
      |                      |                   |                 |
      |IPsec-Data(           |                   |                 |
      | OuterSrc=A,          |                   |                 |
      | OuterDst=SGW,        |                   |                 |
      | SPI=SPI1,            |                   |                 |
      | InnerSrc=A,          |                   |                 |
      | OuterDst=B,          |                   |                 |
      | Protocol=X,          |IPsec-Data(        |                 |
      | SrcPort=Y,           | SrcIP=A,          |                 |
      | DstPort=Z)           | DstIP=B,          |                 |
      |=====================>| Protocol=X,       |IPsec-Data(      |
      |                      | SrcPort=Y,        | SrcIP=A,        |
      | --IPsec protected->  | DstPort=Z)        | DstIP=B,        |
      |    data traffic      |------------------>| Protocol=X,     |
      |                      |                   | SrcPort=Y,      |
      |                      |                   | DstPort=Z)      |
      |                      |                   |---------------->|
      |                      |                   |                 |
      |                      |     --Unprotected data traffic->    |
      |                      |                   |                 |
              Figure 6: 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. Similar
   examples are remote access using a VPN, protection of data traffic
   between the home agent (or a security gateway in the home network)
   and the mobile node and other. With a nested application of IPsec
   (for example IPsec between A and SGW and between A and B) the same
   problem occurs.
   One possible solution to this problem is to change the flow
   identifier along the path to capture the new flow identifier after an
   IPsec endpoint.
   IPsec tunnels which 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 [RFC2746].
   c) 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
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                       RSVP Security Properties             March 2003
   of the existing reservation has to be updated. Since IPsec SAs have a
   longer lifetime this issue does not seem to be a major issue. IPsec
   protection of SCTP data traffic might more often require an IPsec SA
   (and an SPI) change to reflect added and removed IP addresses from an
   SCTP association.
 5.5  End-to-End Security Issues and RSVP
  End-to-end security for RSVP has not been discussed throughout the
  document. In this context end-to-end security refers to credentials
  transmitted between the two end hosts using RSVP. It is obvious that
  care must be taken to ensure that routers along the path are able to
  process and modify the signaling messages according to the processing
  procedure. Some objects however could be used for end-to-end
  protection. The main question however is what the benefit of such an
  end-to-end security is. First there is the question how to establish
  the required security association. Between two arbitrary hosts on the
  Internet this might turn out to be quite difficult. Furthermore it
  depends on an architecture where RSVP is deployed whether it is
  useful to provide end-to-end security. If RSVP is only used to signal
  QoS information into the network and other protocols have to be
  executed beforehand to negotiate the parameters and to decide which
  entity is charged for the QoS reservation then no end-to-end security
  is likely to be required. Introducing end-to-end security to RSVP
  would then cause problems with extensions like RSVP proxy [GD+02],
  Localized RSVP [MS+02] and others which 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
 5.6  IPsec protection of RSVP signaling messages
   In this document it was assumed that RSVP signaling messages can also
   be protected by IPsec in a hop-by-hop fashion between two adjacent
   RSVP nodes. RSVP uses a special processing of signaling messages
   which complicates IPsec protection. As we explain in this section
   IPsec should only be used for protection of RSVP signaling messages
   in a point-to-point communication environment (i.e. a RSVP message
   can only reach one RSVP router and not possibly more than one). This
   circumstance is caused by the combination of signaling message
   delivery and discovery into a single message. Furthermore the end-to-
   end addressing complicates IPsec handling considerably. This section
   tries to describe these complications.
   RSVP messages are transmitted as raw IP packets with protocol number
   46. It might be possible to encapsulate them in UDP as described in
   Appendix C of [RFC2205]. 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 every
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                       RSVP Security Properties             March 2003
   traffic 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. Since the described RSVP messages are addressed
   to the destination address instead of the next RSVP node it is not
   possible to use IPsec in transport mode - only IPsec in tunnel mode
   is possible.
   If there is more than one possible path which an RSVP message can
   take then the IPsec engine will experience difficulties to protect
   the message. Even if the RSVP daemon installs a traffic selector with
   the destination IP address then still there is no distinguishing
   element which allows to select the correct security association of
   one of the possible RSVP nodes along. Even if it possible to apply
   IPsec protection (in tunnel mode) for RSVP signaling messages by
   incorporating some additional information then there is still the
   possibility that the tunneled messages do not recognize a path change
   in a non-RSVP router. Then the signaling messages would simply follow
   different path than the data.
   RSVP messages like RESV can be protected by IPsec since they are
   contain enough information to create IPsec traffic selectors which
   allow a differentiation between different next RSVP nodes. A traffic
   selector would then contain the protocol number and the source /
   destination address pair.
 5.7  Accounting/Charging Framework
  In [TB+03] two trust models (NJ Turnpike and NJ Parkway model) and
  two authorization models (per-session and per-channel financial
  settlement). The NJ Turnpike model gives a justification for the hop-
  by-hop security protection. RSVP supports the NJ Parkway model and
  per-channel financial settlement to some extend only. The
  communication procedures defined for policy object [Her95] can be
  improved to support the more efficient per-channel financial
  settlement by avoiding policy handling between inter-domain networks
  at a signaling message granularity.
  6  Conclusions
  RSVP was the first QoS signaling protocol which provided some
  security protection. Whether RSVP provides enough security protection
  heavily depends on the environment where it is deployed. As RSVP is
  specified today should be seen as a building block that has to be
  adapted to a given architecture.
  This document aims to provide more insights into the security of
  RSVP. It cannot not be interpreted as a pass or fail evaluation of
  the security provided by RSVP.
  Certainly this document is not complete to describe all security
  issues related to RSVP. Some issues that require further
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                       RSVP Security Properties             March 2003
  considerations are RSVP extensions (for example [RFC2207]), multicast
  issues and other security properties like traffic analysis etc.
  Additionally the interaction with mobility protocols (micro- and
  macro-mobility) from a security point of view demands further
  What can be learned from a practical protocol experience and from the
  increased awareness regarding security is that some of the available
  credential types have received more acceptance. Kerberos is such a
  system which is integrated in 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 a per-flow signaling. The
  increased focus on denial of service attacks additionally demands a
  closer look on public key based authentication.
  The following list briefly summarizes a few security or architectural
  issues which desire improvement:
  * Discovery and signaling message delivery should be separated.
  * 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.
  * Addressing for signaling messages should be done in a hop-by-hop
  * Standard security protocols (IPsec, TLS or CMS) should be used
  whenever possible. Authentication and key exchange should separated
  from signaling message protection. In general it is necessary to
  provide key management to dynamically establish a security
  association for signaling message protection. Relying on manually
  configured keys between neighboring RSVP nodes is insufficient.
  * The usage of public key cryptography for authorization tokens,
  identity representation, selective object protection, etc. is likely
  to cause fragmentation and problems.
  * Public key authentication and user identity confidentiality
  provided with RSVP require some improvement.
  * Public key based user authentication only provides entity
  authentication. An additional security association is required to
  protect the signaling message.
  * 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.
  * Authorization and charging should be better integrated in the base
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                       RSVP Security Properties             March 2003
  * Selective message protection should be provided. A protected
  message should be recognizable from a flag in the header.
  * Confidentiality protection is missing and should therefore be added
  to the protocol.
  * 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  IANA considerations
  This document does not address any IANA considerations.
  9  Open Issues
  A future version of this draft will restructure and shorten the
  document and include references to other RSVP security related
  activities and papers.
  10   Acknowledgments
  I would like to thank Jorge Cuellar, Robert Hancock, Xiaoming Fu and
  Guenther Schaefer for their valuable comments. Additionally I would
  like to thank Robert and Jorge for their time to discuss various
  issues with me. Furthermore I would like to thank Marc De Vuyst for
  his comments to the draft.
  Appendix A.  Dictionary Attacks and Kerberos
  This section addresses issues related to Kerberos and its
  vulnerability against dictionary attacks since there often seems to
  be a misunderstanding. The reason for including this discussion in
  this document is that Kerberos seems to be one of the most widely
  supported authentication and key distribution systems available.
  The initial Kerberos AS_REQ request (without pre-authentication,
  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. Using pre-
  authentication ([Pat92]) can be used to reduce this problem.
  However pre-authentication does not entirely prevent dictionary
  attacks by an adversary since he can still eavesdrop Kerberos
  messages if being located at the path between the mobile node and the
  KDC. With mandatory pre-authentication for the initial request an
  adversary cannot request a Ticket Granting Ticket for an arbitrary
   Tschofenig     Informational - Expires September 2003            36
                       RSVP Security Properties             March 2003
  user. On-line password guessing attacks are still possible by
  choosing a password (e.g. from a dictionary) and then transmitting an
  initial request including pre-authentication data field. An
  unsuccessful authentication by the KDC results in an error message
  and the gives the adversary a hint to try a new password and restart
  the protocol again.
  There are however some proposals that prevent dictionary attacks from
  happening. The use of Public Key Cryptography for initial
  authentication [TN+01] (PKINIT) is one such solution. Other proposals
  use strong-password based authenticated key agreement protocols like
  the Encrypted Key Exchange protocol (EKE) to avoid leaking of user
  password information. B. Jaspan investigated the use of EKE for
  Kerberos V5 called "Dual-workfactor Encrypted Key Exchange" [Jas96]
  which is described below.
  With the PA-ENC-DH pre-authentication Jaspan included the Diffie-
  Hellman "public key" of the client encrypted with the user password
  in the initial AS_REQ to the Authentication Server. Additionally the
  modulus m is included since the client can choose this value
  It is interesting to note that pre-authentication was orginally
  introduced to allow the user to authenticate to the AS with the
  inital AS_REQ message . The use of the Encrypted Key Exchange
  protocol [BM92] as a pre-authentication mechanism does not allow the
  Authentication Server to authenticate the client since this would
  require the client to include verifiable data (e.g. a keyed message
  digest for data origin authentication) but this destroys the
  properties of EKE. EKE was designed to create a strong-password based
  authentication protocol that is resistant against dictionary attacks.
  Hence after the second message the Authentication Server is
  authenticated to the client by showing that he was able to compute
  the shared key k(a,as) used to encrypt the first part of message (2).
  The client is not authenticated to the Authentication Server.
  It is obvious that both the client and the Authentication Server must
  be able to provide good random numbers for the creation of the
  Diffie-Hellman key pair. Jaspan additionally noted that the timestamp
  in the response from the Authentication Server (AS_REP message) can
  be used to eliminate the dependency on time synchronization of the
  Kerberos protocol.  The client can use this value to adjust his clock
  after successful authentication of the Authentication Server.
  The vulnerability against denial of service attacks is a disadvantage
  common to many strong-password based authenticated key agreement
  protocols. Nothing prevents an adversary from flooding the
  Authentication Server with bogus AS_REQ messages using the pre-
  authentication method PA-ENC-DH. This forces the Authentication
  Server to create a Diffie-Hellman public/private key pair, to decrypt
  the received response and to compute the session key k(a,as) and to
  return a message to the source IP address of the previously received
   Tschofenig     Informational - Expires September 2003            37
                       RSVP Security Properties             March 2003
  message. Even if the Authentication Server does not re-create a new
  public/private key pair with every session he still has to compute
  the session key which requires multiprecision operations and this is
  time consuming.
  Jaspan furthermore noted that the missing client authentication can
  be used by an undetectable on-line password guessing attack as
  described in [DH95]. An adversary sends an AS_REQ for a user B
  encrypted with a password k(b’). The Authentication Server decrypts
  the value of the pre-authentication field with the real user password
  k(b) and encrypts his response to the adversary. If the adversary
  correctly guessed the password of user B then the receive response
  verifies correctly. Jaspan proposed to modify the KDC to allow only a
  certain number of requests per day but this can be used by an
  attacker to mount a denial of service attack against such users to
  lock their accounts by sending a number of incorrect requests to the
  KDC. The KDC would then reject Ticket Granting Ticket or even a
  service ticket  from legitimate users.
  Tom Wu mentioned in [Wu99] the use of a variant of SRP [Wu98] and the
  use of SPEKE [Jab96] to be used in the pre-authentication process as
  possible candidates to prevent dictionary attacks. Unfortunately Wu
  does not explain the proposals in detail.
  Currently only PKINIT is available for preventing off-line dictionary
  attacks. Other proposals described above like SPEKE, SRP etc. are not
  included in the current Kerberos version. IPR issues may be one of
  the reasons.
  Appendix B.  Example of User-to-PDP Authentication
  The following Section describes an example of user-to-PDP
  authentication. Note that the description below is not fully covered
  by the RSVP specification and hence it should only be seen as an
  Windows 2000, which integrates Kerberos into RSVP, uses a
  configuration with the user authentication to the PDP as described in
  [MADS01]. The steps for authenticating the user to the PDP in an
  intra-realm scenario are the following:
  - Windows 2000 requires the user to contact the KDC and to request a
  Kerberos service ticket for the PDP account AcsService in the local
  - This ticket is then embedded in the AUTH_DATA element and included
  in either the PATH or the RESV message. In case of Microsoft’s
  implementation the user identity encoded as a distinguished name is
  encrypted with the session key provided with the Kerberos ticket. The
  Kerberos ticket is sent without the Kerberos authdata element that
  contains authorization information as explained in [MADS01].
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                       RSVP Security Properties             March 2003
  - The RSVP message is then intercepted by the PEP who forwards it to
  the PDP. [MADS01] does not state which protocol is used to forward
  the RSVP message to the PDP.
  - The PDP who finally receives the message decrypts the received
  service ticket. The ticket contains the session key which was used by
  the user's host to
  a) Encrypt the principal name inside the policy locator field of the
  AUTH_DATA object and to
  b) 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
  users host and the first-hop router since the two message parts
  terminate at a different node and a different security association
  must be used. The interface between the message intercepting first-
  hop router and the PDP must be protected as well.
  c) The PDP does not maintain a user database and [MADS01] describes
  that the PDP may query the Active Directory (a LDAP based directory
  service) for user policy information.
  11   References
  [BM92]       Bellovin, B., Merrit, M.: "Encrypted Key Exchange:
  Password-based protocols secure against dictionary attacks", in
  "Proceedings of the IEEE Symposium on Research in Security and
  Privacy", May, 1992.
  [CA+02]      Calhoun, P., Arkko, J., Guttman, E., Zorn, G.,
  Loughney, J.: "DIAMETER Base Protocol", <draft-ietf-aaa-diameter-
  17.txt>, (work in progress), December, 2002.
  [DBP96]      Dobbertin, H., Bosselaers, A., Preneel, B.: "RIPEMD-
  160: A strengthened version of RIPEMD", in "Fast Software Encryption,
  LNCS Vol 1039, pp. 71-82", 1996.
  [DG96]       Davis, D., Geer, D.: "Kerberos With Clocks Adrift:
  History, Protocols and Implementation", in "USENIX Computing Systems
  Volume 9 no. 1, Winter", 1996.
  [DH95]       Ding, Y., Horster, P.: "Undetectable On-line Password
  Guessing Attacks", Operating Systems Review, 29(No. 4), pp. 77-86,
  [Dob96]      Dobbertin, H.: "The Status of Md5 After a Recent
  Attack," RSA Laboratories' CryptoBytes, Volume 2, Number 2, 1996.
  [FH+01]      Thomas, M., Vilhuber, J.: "Kerberized Internet
  Negotiation of Keys (KINK)", <draft-ietf-kink-kink-05.txt>, (work in
  progress), January, 2003.
   Tschofenig     Informational - Expires September 2003            39
                       RSVP Security Properties             March 2003
  [GD+02]      Gai, S., Dutt, D., Elfassy, N., Bernet, Y.: "RSVP
  Proxy", <draft-ietf-rsvp-proxy-03.txt>, (expired), March, 2002.
  [HA01]       Hornstein, K., Altman, J.: "Distributing Kerberos KDC
  and Realm Information with DNS", <draft-ietf-krb-wg-krb-dns-locate-
  03.txt>, (work in progress), July, 2002.
  [HH01]       Hess, R., Herzog, S.: "RSVP Extensions for Policy
  Control", <draft-ietf-rap-new-rsvp-ext-00.txt>, (expired), June,
  [Jab96]      Jablon, D.: "Strong password-only authenticated key
  exchange", Computer Communication Review, 26(5), pp. 5-26, October,
  [Jas96]      Jaspan, B.: "Dual-workfactor Encrypted Key Exchange:
  Efficiently Preventing Password Chaining and Dictionary Attacks", in
  "Proceedings of the Sixth Annual USENIX Security Conference", pp. 43-
  50, July, 1996.
  [MADS01]     "Microsoft Authorization Data Specification v. 1.0 for
  Microsoft Windows 2000 Operating Systems", April, 2000, available at:
  February, 2001.
  [MHHF01]     Malpani, A., Hoffman, P., Housley, R., Freeman, T.:
  "Simple Certificate Validation Protocol (SCVP)", <draft-ietf-pkix-
  scvp-11.txt>, (work in progress), December, 2002.
  [MS+02]              Manner, J., Suihko, T., Kojo, M., Liljeberg,
  M., Raatikainen, K.: "Localized RSVP", <draft-manner-lrsvp-00.txt>,
  (expired), May, 2002.
  [Pat92]      Pato, J., "Using Pre-Authentication to Avoid Password
  Guessing Attacks", Open Software Foundation DCE Request for Comments
  26, December, 1992.
        "Specifications and standard documents",
  http://www.pgpi.org/doc/specs/, March, 2002.
  [PKTSEC]     PacketCable Security Specification, PKT-SP-SEC-I01-
  991201, Cable Television Laboratories, Inc., December 1, 1999,
  [Rae01]      Raeburn, K.: "Rijndael, Serpent, and Twofish
  Cryptosystems for Kerberos 5", <draft-raeburn-krb-rijndael-krb-
  01.txt>, (expired), July, 2001.
  [RF2367]     McDonald, D., Metz, C., Phan, B.: "PF_KEY Key
  Management API, Version 2", RFC 2367, July, 1998.
   Tschofenig     Informational - Expires August 2002               40
                       RSVP Security Properties             March 2003
  [RFC1321]    Rivest, R.: "The MD5 Message-Digest Algorithm", RFC
  1321, April, 1992.
  [RFC1510]    Kohl, J., Neuman, C.: "The Kerberos Network
  Authentication Service (V5)", RFC 1510, September 1993.
  [RFC2104]    Krawczyk, H., Bellare, M., Canetti, R.: "HMAC: Keyed-
  Hashing for Message Authentication", RFC 2104, February, 1997.
  [RFC2205]    Braden, R., Zhang, L., Berson, S., Herzog, S., Jamin,
  S.: „Resource ReSerVation Protocol (RSVP) – Version 1 Functional
  Specification", RFC 2205, September 1997.
  [RFC2207]    Berger, L., O’Malley, T.: „RSVP Extensions for IPSEC
  Data Flows", RFC 2207, September 1997.
  [RFC2315]    Kaliski, B.: " PKCS #7: Cryptographic Message Syntax
  Version 1.5", RFC 2315, March, 1998.
  [RFC2367]    McDonald, D., Metz, C., Phan, B.: "PF_KEY Key
  Management API, Version 2", RFC 2367, July, 1998.
  [RFC2401]    Kent, S., Atkinson, R.: "Security Architecture for the
  Internet Protocol", RFC 2401, November, 1998.
  [RFC2402]    Kent, S., Atkinson, R.: "IP Authentication Header", RFC
  2402, November, 1998.
  [RFC2406]    Kent, S., Atkinson, R.: "IP Encapsulating Security
  Payload (ESP)", RFC 2406, November, 1998.
  [RFC2409]    Harkins, D., Carrel, D.: "The Internet Key Exchange
  (IKE)", RFC 2409, November, 1998.
  [RFC2410]    Glenn, R., Kent, S.: "The NULL Encryption Algorithm and
  Its Use With IPsec", RFC 2410, November, 1998.
  [RFC2440]    Callas, J.,  Donnerhacke, L., Finney, H., Thayer, R.:
  "OpenPGP Message Format", RFC 2440, November, 1998.
  [RFC2495]    Housley, R., Ford, W., Polk, W., Solo, D.: "Internet
  X.509 Public Key Infrastructure Certificate and CRL Profile", RFC
  2459, January, 1999.
  [RFC2560]    Myers, M., Ankney, R., Malpani, A., Galperin, S.,
  Adams, C.: "X.509 Internet Public Key Infrastructure Online
  Certificate Status Protocol – OCSP", RFC 2560, June, 1999.
  [RFC2630]    Housley, R.: "Cryptographic Message Syntax", RFC 2630,
  June, 1999.
   Tschofenig     Informational - Expires August 2002               41
                       RSVP Security Properties             March 2003
  [RFC2747]    Baker, F., Lindell, B., Talwar, M.: "RSVP Cryptographic
  Authentication", RC 2747, January, 2000.
  [RFC2748]    Boyle, J., Cohen, R., Durham, D., Herzog, S., Rajan,
  R., Sastry, A.: "The COPS(Common Open Policy Service) Protocol", RFC
  2748, January, 2000.
  [RFC2749]    Boyle, J., Cohen, R., Durham, D., Herzog, S., Rajan,
  R., Sastry, A.: "COPS usage for RSVP", RFC 2749, January, 2000.
  [RFC2750]    Herzog, S.: "RSVP Extensions for Policy Control", RFC
  2750, January, 2000.
  [RFC2865]    Rigney, C., Willens, S., Rubens, A., Simpson, W.:
  "Remote Authentication Dial In User Service (RADIUS)", RFC 2865,
  June, 2000.
  [RFC3182]    Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore,
  T., Herzog, S., Hess, R.: "Identity Representation for RSVP", RFC
  3182, October, 2001.
        NIST, FIPS PUB 180-1, "Secure Hash Standard", April, 1995.
  [TN+01]      Tung, B., Neuman, C., Hur, M., Medvinsky, A.,
  Medvinsky, S., Wray, J., Trostle, J.: "Public Key Cryptography for
  Initial Authentication in Kerberos", <draft-ietf-cat-kerberos-pk-
  init-16.txt>, (work in progress), October, 2001.
  [Wu98]       Wu, T.: "The Secure Remote Password Protocol", in
  "Proceedings of the Internet Society Network and Distributed System
  Security Symposium", pp. 97-111, March, 1998.
  [Wu99]       Wu, T.: "A Real-World Analysis of Kerberos Password
  Security", in "Proceedings of the 1999 Network and Distributed System
  Security", February, 1999.
  [TB+03]      H. Tschofenig, M. Buechli, S. Van den Bosch, H.
  Schulzrinne: "NSIS Authentication, Authorization and Accounting
  Issues", <draft-tschofenig-nsis-aaa-issues-01.txt>, (work in
  progress), March, 2003.
  [Her95]      Herzog, S.: " Accounting and Access Control in RSVP",
  <draft-ietf-rsvp-lpm-arch-00.txt>, (expired), November, 1995.
  12   Author's Contact Information
  Hannes Tschofenig
  Siemens AG
  Otto-Hahn-Ring 6
  81739 Munich
  Email: Hannes.Tschofenig@siemens.com
   Tschofenig     Informational - Expires August 2002               42
                       RSVP Security Properties             March 2003
  13   Full Copyright Statement
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  This document and the information contained herein is provided on an
     Funding for the RFC Editor function is currently provided by the
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   Tschofenig     Informational - Expires August 2002               43