Network Working Group F. Baker
Request for Comments: 2747 Cisco
Category: Standards Track B. Lindell
USC/ISI
M. Talwar
Microsoft
January 2000
RSVP Cryptographic Authentication
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes the format and use of RSVP's INTEGRITY object
to provide hop-by-hop integrity and authentication of RSVP messages.
1. IntrodUCtion
The Resource ReSerVation Protocol RSVP [1] is a protocol for setting
up distributed state in routers and hosts, and in particular for
reserving resources to implement integrated service. RSVP allows
particular users to oBTain preferential Access to network resources,
under the control of an admission control mechanism. Permission to
make a reservation will depend both upon the availability of the
requested resources along the path of the data, and upon satisfaction
of policy rules.
To ensure the integrity of this admission control mechanism, RSVP
requires the ability to protect its messages against corruption and
spoofing. This document defines a mechanism to protect RSVP message
integrity hop-by-hop. The proposed scheme transmits an
authenticating digest of the message, computed using a secret
Authentication Key and a keyed-hash algorithm. This scheme provides
protection against forgery or message modification. The INTEGRITY
object of each RSVP message is tagged with a one-time-use sequence
number. This allows the message receiver to identify playbacks and
hence to thwart replay attacks. The proposed mechanism does not
afford confidentiality, since messages stay in the clear; however,
the mechanism is also eXPortable from most countries, which would be
impossible were a privacy algorithm to be used. Note: this document
uses the terms "sender" and "receiver" differently from [1]. They
are used here to refer to systems that face each other across an RSVP
hop, the "sender" being the system generating RSVP messages.
The message replay prevention algorithm is quite simple. The sender
generates packets with monotonically increasing sequence numbers. In
turn, the receiver only accepts packets that have a larger sequence
number than the previous packet. To start this process, a receiver
handshakes with the sender to get an initial sequence number. This
memo discusses ways to relax the strictness of the in-order delivery
of messages as well as techniques to generate monotonically
increasing sequence numbers that are robust across sender failures
and restarts.
The proposed mechanism is independent of a specific cryptographic
algorithm, but the document describes the use of Keyed-Hashing for
Message Authentication using HMAC-MD5 [7]. As noted in [7], there
exist stronger hashes, such as HMAC-SHA1; where warranted,
implementations will do well to make them available. However, in the
general case, [7] suggests that HMAC-MD5 is adequate to the purpose
at hand and has preferable performance characteristics. [7] also
offers source code and test vectors for this algorithm, a boon to
those who would test for interoperability. HMAC-MD5 is required as a
baseline to be universally included in RSVP implementations providing
cryptographic authentication, with other proposals optional (see
Section 6 on Conformance Requirements).
The RSVP checksum MAY be disabled (set to zero) when the INTEGRITY
object is included in the message, as the message digest is a much
stronger integrity check.
1.1. Conventions used in this document
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [8].
1.2. Why not use the Standard IPSEC Authentication Header?
One obvious question is why, since there exists a standard
authentication mechanism, IPSEC [3,5], we would choose not to use it.
This was discussed at length in the working group, and the use of
IPSEC was rejected for the following reasons.
The security associations in IPSEC are based on destination address.
It is not clear that RSVP messages are well defined for either source
or destination based security associations, as a router must forward
PATH and PATH TEAR messages using the same source address as the
sender listed in the SENDER TEMPLATE. RSVP traffic may otherwise not
follow exactly the same path as data traffic. Using either source or
destination based associations would require opening a new security
association among the routers for which a reservation traverses.
In addition, it was noted that neighbor relationships between RSVP
systems are not limited to those that face one another across a
communication channel. RSVP relationships across non-RSVP clouds,
such as those described in Section 2.9 of [1], are not necessarily
visible to the sending system. These arguments suggest the use of a
key management strategy based on RSVP router to RSVP router
associations instead of IPSEC.
2. Data Structures
2.1. INTEGRITY Object Format
An RSVP message consists of a sequence of "objects," which are type-
length-value encoded fields having specific purposes. The
information required for hop-by-hop integrity checking is carried in
an INTEGRITY object. The same INTEGRITY object type is used for both
IPv4 and IPv6.
The INTEGRITY object has the following format:
Keyed Message Digest INTEGRITY Object: Class = 4, C-Type = 1
+-------------+-------------+-------------+-------------+
Flags 0 (Reserved)
+-------------+-------------+ +
Key Identifier
+-------------+-------------+-------------+-------------+
Sequence Number
+-------------+-------------+-------------+-------------+
+ +
+ Keyed Message Digest
+ +
+-------------+-------------+-------------+-------------+
o Flags: An 8-bit field with the following format:
Flags
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
H
F 0
+---+---+---+---+---+---+---+---+
Currently only one flag (HF) is defined. The remaining flags
are reserved for future use and MUST be set to 0.
o Bit 0: Handshake Flag (HF) concerns the integrity
handshake mechanism (Section 4.3). Message senders
willing to respond to integrity handshake messages SHOULD
set this flag to 1 whereas those that will reject
integrity handshake messages SHOULD set this to 0.
o Key Identifier: An unsigned 48-bit number that MUST be unique
for a given sender. Locally unique Key Identifiers can be
generated using some combination of the address (IP or MAC or
LIH) of the sending interface and the key number. The
combination of the Key Identifier and the sending system's IP
address uniquely identifies the security association (Section
2.2).
o Sequence Number: An unsigned 64-bit monotonically increasing,
unique sequence number.
Sequence Number values may be any monotonically increasing
sequence that provides the INTEGRITY object [of each RSVP
message] with a tag that is unique for the associated key's
lifetime. Details on sequence number generation are presented
in Section 3.
o Keyed Message Digest: The digest MUST be a multiple of 4
octets long. For HMAC-MD5, it will be 16 bytes long.
2.2. Security Association
The sending and receiving systems maintain a security association for
each authentication key that they share. This security association
includes the following parameters:
o Authentication algorithm and algorithm mode being used.
o Key used with the authentication algorithm.
o Lifetime of the key.
o Associated sending interface and other security association
selection criteria [REQUIRED at Sending System].
o Source Address of the sending system [REQUIRED at Receiving
System].
o Latest sending sequence number used with this key identifier
[REQUIRED at Sending System].
o List of last N sequence numbers received with this key
identifier [REQUIRED at Receiving System].
3. Generating Sequence Numbers
In this section we describe methods that could be chosen to generate
the sequence numbers used in the INTEGRITY object of an RSVP message.
As previous stated, there are two important properties that MUST be
satisfied by the generation procedure. The first property is that
the sequence numbers are unique, or one-time, for the lifetime of the
integrity key that is in current use. A receiver can use this
property to unambiguously distinguish between a new or a replayed
message. The second property is that the sequence numbers are
generated in monotonically increasing order, modulo 2^64. This is
required to greatly reduce the amount of saved state, since a
receiver only needs to save the value of the highest sequence number
seen to avoid a replay attack. Since the starting sequence number
might be arbitrarily large, the modulo operation is required to
accommodate sequence number roll-over within some key's lifetime.
This solution draws from TCP's approach [9].
The sequence number field is chosen to be a 64-bit unsigned quantity.
This is large enough to avoid exhaustion over the key lifetime. For
example, if a key lifetime was conservatively defined as one year,
there would be enough sequence number values to send RSVP messages at
an average rate of about 585 gigaMessages per second. A 32-bit
sequence number would limit this average rate to about 136 messages
per second.
The ability to generate unique monotonically increasing sequence
numbers across a failure and restart implies some form of stable
storage, either local to the device or remotely over the network.
Three sequence number generation procedures are described below.
3.1. Simple Sequence Numbers
The most straightforward approach is to generate a unique sequence
number using a message counter. Each time a message is transmitted
for a given key, the sequence number counter is incremented. The
current value of this counter is continually or periodically saved to
stable storage. After a restart, the counter is recovered using this
stable storage. If the counter was saved periodically to stable
storage, the count should be recovered by increasing the saved value
to be larger than any possible value of the counter at the time of
the failure. This can be computed, knowing the interval at which the
counter was saved to stable storage and incrementing the stored value
by that amount.
3.2. Sequence Numbers Based on a Real Time Clock
Most devices will probably not have the capability to save sequence
number counters to stable storage for each key. A more universal
solution is to base sequence numbers on the stable storage of a real
time clock. Many computing devices have a real time clock module
that includes stable storage of the clock. These modules generally
include some form of nonvolatile memory to retain clock information
in the event of a power failure.
In this approach, we could use an NTP based timestamp value as the
sequence number. The roll-over period of an NTP timestamp is about
136 years, much longer than any reasonable lifetime of a key. In
addition, the granularity of the NTP timestamp is fine enough to
allow the generation of an RSVP message every 200 picoseconds for a
given key. Many real time clock modules do not have the resolution
of an NTP timestamp. In these cases, the least significant bits of
the timestamp can be generated using a message counter, which is
reset every clock tick. For example, when the real time clock
provides a resolution of 1 second, the 32 least significant bits of
the sequence number can be generated using a message counter. The
remaining 32 bits are filled with the 32 least significant bits of
the timestamp. Assuming that the recovery time after failure takes
longer than one tick of the real time clock, the message counter for
the low order bits can be safely reset to zero after a restart.
3.3. Sequence Numbers Based on a Network Recovered Clock
If the device does not contain any stable storage of sequence number
counters or of a real time clock, it could recover the real time
clock from the network using NTP. Once the clock has been recovered
following a restart, the sequence number generation procedure would
be identical to the procedure described above.
4. Message Processing
Implementations SHOULD allow specification of interfaces that are to
be secured, for either sending messages, or receiving them, or both.
The sender must ensure that all RSVP messages sent on secured sending
interfaces include an INTEGRITY object, generated using the
appropriate Key. Receivers verify whether RSVP messages, except of
the type "Integrity Challenge" (Section 4.3), arriving on a secured
receiving interface contain the INTEGRITY object. If the INTEGRITY
object is absent, the receiver discards the message.
Security associations are simplex - the keys that a sending system
uses to sign its messages may be different from the keys that its
receivers use to sign theirs. Hence, each association is associated
with a unique sending system and (possibly) multiple receiving
systems.
Each sender SHOULD have distinct security associations (and keys) per
secured sending interface (or LIH). While administrators may
configure all the routers and hosts on a subnet (or for that matter,
in their network) using a single security association,
implementations MUST assume that each sender may send using a
distinct security association on each secured interface. At the
sender, security association selection is based on the interface
through which the message is sent. This selection MAY include
additional criteria, such as the destination address (when sending
the message unicast, over a broadcast LAN with a large number of
hosts) or user identities at the sender or receivers [2]. Finally,
all intended message recipients should participate in this security
association. Route flaps in a non RSVP cloud might cause messages
for the same receiver to be sent on different interfaces at different
times. In such cases, the receivers should participate in all
possible security associations that may be selected for the
interfaces through which the message might be sent.
Receivers select keys based on the Key Identifier and the sending
system's IP address. The Key Identifier is included in the INTEGRITY
object. The sending system's address can be obtained either from the
RSVP_HOP object, or if that's not present (as is the case with
PathErr and ResvConf messages) from the IP source address. Since the
Key Identifier is unique for a sender, this method uniquely
identifies the key.
The integrity mechanism slightly modifies the processing rules for
RSVP messages, both when including the INTEGRITY object in a message
sent over a secured sending interface and when accepting a message
received on a secured receiving interface. These modifications are
detailed below.
4.1. Message Generation
For an RSVP message sent over a secured sending interface, the
message is created as described in [1], with these exceptions:
(1) The RSVP checksum field is set to zero. If required, an RSVP
checksum can be calculated when the processing of the
INTEGRITY object is complete.
(2) The INTEGRITY object is inserted in the appropriate place, and
its location in the message is remembered for later use.
(3) The sending interface and other appropriate criteria (as
mentioned above) are used to determine the Authentication Key
and the hash algorithm to be used.
(4) The unused flags and the reserved field in the INTEGRITY
object MUST be set to 0. The Handshake Flag (HF) should be
set according to rules specified in Section 2.1.
(5) The sending sequence number MUST be updated to ensure a
unique, monotonically increasing number. It is then placed in
the Sequence Number field of the INTEGRITY object.
(6) The Keyed Message Digest field is set to zero.
(7) The Key Identifier is placed into the INTEGRITY object.
(8) An authenticating digest of the message is computed using the
Authentication Key in conjunction with the keyed-hash
algorithm. When the HMAC-MD5 algorithm is used, the hash
calculation is described in [7].
(9) The digest is written into the Cryptographic Digest field of
the INTEGRITY object.
4.2. Message Reception
When the message is received on a secured receiving interface, and is
not of the type "Integrity Challenge", it is processed in the
following manner:
(1) The RSVP checksum field is saved and the field is subsequently
set to zero.
(2) The Cryptographic Digest field of the INTEGRITY object is
saved and the field is subsequently set to zero.
(3) The Key Identifier field and the sending system address are
used to uniquely determine the Authentication Key and the hash
algorithm to be used. Processing of this packet might be
delayed when the Key Management System (Appendix 1) is queried
for this information.
(4) A new keyed-digest is calculated using the indicated algorithm
and the Authentication Key.
(5) If the calculated digest does not match the received digest,
the message is discarded without further processing.
(6) If the message is of type "Integrity Response", verify that
the CHALLENGE object identically matches the originated
challenge. If it matches, save the sequence number in the
INTEGRITY object as the largest sequence number received to
date.
Otherwise, for all other RSVP Messages, the sequence number is
validated to prevent replay attacks, and messages with invalid
sequence numbers are ignored by the receiver.
When a message is accepted, the sequence number of that
message could update a stored value corresponding to the
largest sequence number received to date. Each subsequent
message must then have a larger (modulo 2^64) sequence number
to be accepted. This simple processing rule prevents message
replay attacks, but it must be modified to tolerate limited
out-of-order message delivery. For example, if several
messages were sent in a burst (in a periodic refresh generated
by a router, or as a result of a tear down function), they
might get reordered and then the sequence numbers would not be
received in an increasing order.
An implementation SHOULD allow administrative configuration
that sets the receiver's tolerance to out-of-order message
delivery. A simple approach would allow administrators to
specify a message window corresponding to the worst case
reordering behavior. For example, one might specify that
packets reordered within a 32 message window would be
accepted. If no reordering can occur, the window is set to
one.
The receiver must store a list of all sequence numbers seen
within the reordering window. A received sequence number is
valid if (a) it is greater than the maximum sequence number
received or (b) it is a past sequence number lying within the
reordering window and not recorded in the list. Acceptance of
a sequence number implies adding it to the list and removing a
number from the lower end of the list. Messages received with
sequence numbers lying below the lower end of the list or
marked seen in the list are discarded.
When an "Integrity Challenge" message is received on a secured
sending interface it is processed in the following manner:
(1) An "Integrity Response" message is formed using the Challenge
object received in the challenge message.
(2) The message is sent back to the receiver, based on the source
IP address of the challenge message, using the "Message
Generation" steps outlined above. The selection of the
Authentication Key and the hash algorithm to be used is
determined by the key identifier supplied in the challenge
message.
4.3. Integrity Handshake at Restart or Initialization of the Receiver
To obtain the starting sequence number for a live Authentication Key,
the receiver MAY initiate an integrity handshake with the sender.
This handshake consists of a receiver's Challenge and the sender's
Response, and may be either initiated during restart or postponed
until a message signed with that key arrives.
Once the receiver has decided to initiate an integrity handshake for
a particular Authentication Key, it identifies the sender using the
sending system's address configured in the corresponding security
association. The receiver then sends an RSVP Integrity Challenge
message to the sender. This message contains the Key Identifier to
identify the sender's key and MUST have a unique challenge cookie
that is based on a local secret to prevent guessing. see Section
2.5.3 of [4]). It is suggested that the cookie be an MD5 hash of a
local secret and a timestamp to provide uniqueness (see Section 9).
An RSVP Integrity Challenge message will carry a message type of 11.
The message format is as follows:
<Integrity Challenge message> ::= <Common Header> <CHALLENGE>
he CHALLENGE object has the following format:
CHALLENGE Object: Class = 64, C-Type = 1
+-------------+-------------+-------------+-------------+
0 (Reserved)
+-------------+-------------+ +
Key Identifier
+-------------+-------------+-------------+-------------+
Challenge Cookie
+-------------+-------------+-------------+-------------+
The sender accepts the "Integrity Challenge" without doing an
integrity check. It returns an RSVP "Integrity Response" message
that contains the original CHALLENGE object. It also includes an
INTEGRITY object, signed with the key specified by the Key Identifier
included in the "Integrity Challenge".
An RSVP Integrity Response message will carry a message type of 12.
The message format is as follows:
<Integrity Response message> ::= <Common Header> <INTEGRITY>
<CHALLENGE>
The "Integrity Response" message is accepted by the receiver
(challenger) only if the returned CHALLENGE object matches the one
sent in the "Integrity Challenge" message. This prevents replay of
old "Integrity Response" messages. If the match is successful, the
receiver saves the Sequence Number from the INTEGRITY object as the
latest sequence number received with the key identifier included in
the CHALLENGE.
If a response is not received within a given period of time, the
challenge is repeated. When the integrity handshake successfully
completes, the receiver begins accepting normal RSVP signaling
messages from that sender and ignores any other "Integrity Response"
messages.
The Handshake Flag (HF) is used to allow implementations the
flexibility of not including the integrity handshake mechanism. By
setting this flag to 1, message senders that implement the integrity
handshake distinguish themselves from those that do not. Receivers
SHOULD NOT attempt to handshake with senders whose INTEGRITY object
has HF = 0.
An integrity handshake may not be necessary in all environments. A
common use of RSVP integrity will be between peering domain routers,
which are likely to be processing a steady stream of RSVP messages
due to aggregation effects. When a router restarts after a crash,
valid RSVP messages from peering senders will probably arrive within
a short time. Assuming that replay messages are injected into the
stream of valid RSVP messages, there may be only a small window of
opportunity for a replay attack before a valid message is processed.
This valid message will set the largest sequence number seen to a
value greater than any number that had been stored prior to the
crash, preventing any further replays.
On the other hand, not using an integrity handshake could allow
exposure to replay attacks if there is a long period of silence from
a given sender following a restart of a receiver. Hence, it SHOULD
be an administrative decision whether or not the receiver performs an
integrity handshake with senders that are willing to respond to
"Integrity Challenge" messages, and whether it accepts any messages
from senders that refuse to do so. These decisions will be based on
assumptions related to a particular network environment.
5. Key Management
It is likely that the IETF will define a standard key management
protocol. It is strongly desirable to use that key management
protocol to distribute RSVP Authentication Keys among communicating
RSVP implementations. Such a protocol would provide scalability and
significantly reduce the human administrative burden. The Key
Identifier can be used as a hook between RSVP and such a future
protocol. Key management protocols have a long history of subtle
flaws that are often discovered long after the protocol was first
described in public. To avoid having to change all RSVP
implementations should such a flaw be discovered, integrated key
management protocol techniques were deliberately omitted from this
specification.
5.1. Key Management Procedures
Each key has a lifetime associated with it that is recorded in all
systems (sender and receivers) configured with that key. The concept
of a "key lifetime" merely requires that the earliest (KeyStartValid)
and latest (KeyEndValid) times that the key is valid be programmable
in a way the system understands. Certain key generation mechanisms,
such as Kerberos or some public key schemes, may directly produce
ephemeral keys. In this case, the lifetime of the key is implicitly
defined as part of the key.
In general, no key is ever used outside its lifetime (but see Section
5.3). Possible mechanisms for managing key lifetime include the
Network Time Protocol and hardware time-of-day clocks.
To maintain security, it is advisable to change the RSVP
Authentication Key on a regular basis. It should be possible to
switch the RSVP Authentication Key without loss of RSVP state or
denial of reservation service, and without requiring people to change
all the keys at once. This requires an RSVP implementation to
support the storage and use of more than one active RSVP
Authentication Key at the same time. Hence both the sender and
receivers might have multiple active keys for a given security
association.
Since keys are shared between a sender and (possibly) multiple
receivers, there is a region of uncertainty around the time of key
switch-over during which some systems may still be using the old key
and others might have switched to the new key. The size of this
uncertainty region is related to clock synchrony of the systems.
Administrators should configure the overlap between the expiration
time of the old key (KeyEndValid) and the validity of the new key
(KeyStartValid) to be at least twice the size of this uncertainty
interval. This will allow the sender to make the key switch-over at
the midpoint of this interval and be confident that all receivers are
now accepting the new key. For the duration of the overlap in key
lifetimes, a receiver must be prepared to authenticate messages using
either key.
During a key switch-over, it will be necessary for each receiver to
handshake with the sender using the new key. As stated before, a
receiver has the choice of initiating a handshake during the
switchover or postponing the handshake until the receipt of a message
using that key.
5.2. Key Management Requirements
Requirements on an implementation are as follows:
o It is strongly desirable that a hypothetical security breach
in one Internet protocol not automatically compromise other
Internet protocols. The Authentication Key of this
specification SHOULD NOT be stored using protocols or
algorithms that have known flaws.
o An implementation MUST support the storage and use of more
than one key at the same time, for both sending and receiving
systems.
o An implementation MUST associate a specific lifetime (i.e.,
KeyStartValid and KeyEndValid) with each key and the
corresponding Key Identifier.
o An implementation MUST support manual key distribution (e.g.,
the privileged user manually typing in the key, key lifetime,
and key identifier on the console). The lifetime may be
infinite.
o If more than one algorithm is supported, then the
implementation MUST require that the algorithm be specified
for each key at the time the other key information is entered.
o Keys that are out of date MAY be automatically deleted by the
implementation.
o Manual deletion of active keys MUST also be supported.
o Key storage SHOULD persist across a system restart, warm or
cold, to ease operational usage.
5.3. Pathological Case
It is possible that the last key for a given security association has
expired. When this happens, it is unacceptable to revert to an
unauthenticated condition, and not advisable to disrupt current
reservations. Therefore, the system should send a "last
authentication key expiration" notification to the network manager
and treat the key as having an infinite lifetime until the lifetime
is extended, the key is deleted by network management, or a new key
is configured.
6. Conformance Requirements
To conform to this specification, an implementation MUST support all
of its ASPects. The HMAC-MD5 authentication algorithm defined in [7]
MUST be implemented by all conforming implementations. A conforming
implementation MAY also support other authentication algorithms such
as NIST's Secure Hash Algorithm (SHA). Manual key distribution as
described above MUST be supported by all conforming implementations.
All implementations MUST support the smooth key roll over described
under "Key Management Procedures."
Implementations SHOULD support a standard key management protocol for
secure distribution of RSVP Authentication Keys once such a key
management protocol is standardized by the IETF.
7. Kerberos generation of RSVP Authentication Keys
Kerberos[10] MAY be used to generate the RSVP Authentication key used
in generating a signature in the Integrity Object sent from a RSVP
sender to a receiver. Kerberos key generation avoids the use of
shared keys between RSVP senders and receivers such as hosts and
routers. Kerberos allows for the use of trusted third party keying
relationships between security principals (RSVP sender and receivers)
where the Kerberos key distribution center(KDC) establishes an
ephemeral session key that is subsequently shared between RSVP sender
and receivers. In the multicast case all receivers of a multicast
RSVP message MUST share a single key with the KDC (e.g. the receivers
are in effect the same security principal with respect to Kerberos).
The Key information determined by the sender MAY specify the use of
Kerberos in place of configured shared keys as the mechanism for
establishing a key between the sender and receiver. The Kerberos
identity of the receiver is established as part of the sender's
interface configuration or it can be established through other
mechanisms. When generating the first RSVP message for a specific
key identifier the sender requests a Kerberos service ticket and gets
back an ephemeral session key and a Kerberos ticket from the KDC.
The sender encapsulates the ticket and the identity of the sender in
an Identity Policy Object[2]. The sender includes the Policy Object
in the RSVP message. The session key is then used by the sender as
the RSVP Authentication key in section 4.1 step (3) and is stored as
Key information associated with the key identifier.
Upon RSVP Message reception, the receiver retrieves the Kerberos
Ticket from the Identity Policy Object, decrypts the ticket and
retrieves the session key from the ticket. The session key is the
same key as used by the sender and is used as the key in section 4.2
step (3). The receiver stores the key for use in processing
subsequent RSVP messages.
Kerberos tickets have lifetimes and the sender MUST NOT use tickets
that have expired. A new ticket MUST be requested and used by the
sender for the receiver prior to the ticket expiring.
7.1. Optimization when using Kerberos Based Authentication
Kerberos tickets are relatively long (> 500 bytes) and it is not
necessary to send a ticket in every RSVP message. The ephemeral
session key can be cached by the sender and receiver and can be used
for the lifetime of the Kerberos ticket. In this case, the sender
only needs to include the Kerberos ticket in the first Message
generated. Subsequent RSVP messages use the key identifier to
retrieve the cached key (and optionally other identity information)
instead of passing tickets from sender to receiver in each RSVP
message.
A receiver may not have cached key state with an associated Key
Identifier due to reboot or route changes. If the receiver's policy
indicates the use of Kerberos keys for integrity checking, the
receiver can send an integrity Challenge message back to the sender.
Upon receiving an integrity Challenge message a sender MUST send an
Identity object that includes the Kerberos ticket in the integrity
Response message, thereby allowing the receiver to retrieve and store
the session key from the Kerberos ticket for subsequent Integrity
checking.
8. Acknowledgments
This document is derived directly from similar work done for OSPF and
RIP Version II, jointly by Ran Atkinson and Fred Baker. Significant
editing was done by Bob Braden, resulting in increased clarity.
Significant comments were submitted by Steve Bellovin, who actually
understands this stuff. Matt Crawford and Dan Harkins helped revise
the document.
9. References
[1] Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC2205, September 1997.
[2] Yadav, S., et al., "Identity Representation for RSVP", RFC2752,
January 2000.
[3] Atkinson, R. and S. Kent, "Security Architecture for the
Internet Protocol", RFC2401, November 1998.
[4] Maughan, D., Schertler, M., Schneider, M. and J. Turner,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC2408, November 1998.
[5] Kent, S. and R. Atkinson, "IP Authentication Header", RFC2402,
November 1998.
[6] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC2406, November 1998.
[7] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC2104, March 1996.
[8] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[9] Postel, J., "Transmission Control Protocol", STD 7, RFC793,
September 1981.
[10] Kohl, J. and C. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC1510, September 1993.
10. Security Considerations
This entire memo describes and specifies an authentication mechanism
for RSVP that is believed to be secure against active and passive
attacks.
The quality of the security provided by this mechanism depends on the
strength of the implemented authentication algorithms, the strength
of the key being used, and the correct implementation of the security
mechanism in all communicating RSVP implementations. This mechanism
also depends on the RSVP Authentication Keys being kept confidential
by all parties. If any of these assumptions are incorrect or
procedures are insufficiently secure, then no real security will be
provided to the users of this mechanism.
While the handshake "Integrity Response" message is integrity-
checked, the handshake "Integrity Challenge" message is not. This
was done intentionally to avoid the case when both peering routers do
not have a starting sequence number for each other's key.
Consequently, they will each keep sending handshake "Integrity
Challenge" messages that will be dropped by the other end. Moreover,
requiring only the response to be integrity-checked eliminates a
dependency on an security association in the opposite direction.
This, however, lets an intruder generate fake handshaking challenges
with a certain challenge cookie. It could then save the response and
attempt to play it against a receiver that is in recovery. If it was
lucky enough to have guessed the challenge cookie used by the
receiver at recovery time it could use the saved response. This
response would be accepted, since it is properly signed, and would
have a smaller sequence number for the sender because it was an old
message. This opens the receiver up to replays. Still, it seems very
difficult to exploit. It requires not only guessing the challenge
cookie (which is based on a locally known secret) in advance, but
also being able to masquerade as the receiver to generate a handshake
"Integrity Challenge" with the proper IP address and not being
caught.
Confidentiality is not provided by this mechanism. If
confidentiality is required, IPSEC ESP [6] may be the best approach,
although it is subject to the same criticisms as IPSEC
Authentication, and therefore would be applicable only in specific
environments. Protection against traffic analysis is also not
provided. Mechanisms such as bulk link encryption might be used when
protection against traffic analysis is required.
11. Authors' Addresses
Fred Baker
Cisco Systems
519 Lado Drive
Santa Barbara, CA 93111
Phone: (408) 526-4257
EMail: fred@cisco.com
Bob Lindell
USC Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90292
Phone: (310) 822-1511
EMail: lindell@ISI.EDU
Mohit Talwar
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 705 3131
EMail: mohitt@microsoft.com
12. Appendix 1: Key Management Interface
This appendix describes a generic interface to Key Management. This
description is at an abstract level realizing that implementations
may need to introduce small variations to the actual interface.
At the start of execution, RSVP would use this interface to obtain
the current set of relevant keys for sending and receiving messages.
During execution, RSVP can query for specific keys given a Key
Identifier and Source Address, discover newly created keys, and be
informed of those keys that have been deleted. The interface
provides both a polling and asynchronous upcall style for wider
applicability.
12.1. Data Structures
Information about keys is returned using the following KeyInfo data
structure:
KeyInfo {
Key Type (Send or Receive)
KeyIdentifier
Key
Authentication Algorithm Type and Mode
KeyStartValid
KeyEndValid
Status (Active or Deleted)
Outgoing Interface (for Send only)
Other Outgoing Security Association Selection Criteria
(for Send only, optional)
Sending System Address (for Receive Only)
}
12.2. Default Key Table
This function returns a list of KeyInfo data structures corresponding
to all of the keys that are configured for sending and receiving RSVP
messages and have an Active Status. This function is usually called
at the start of execution but there is no limit on the number of
times that it may be called.
KM_DefaultKeyTable() -> KeyInfoList
12.3. Querying for Unknown Receive Keys
When a message arrives with an unknown Key Identifier and Sending
System Address pair, RSVP can use this function to query the Key
Management System for the appropriate key. The status of the element
returned, if any, must be Active.
KM_GetRecvKey( INTEGRITY Object, SrcAddress ) -> KeyInfo
12.4. Polling for Updates
This function returns a list of KeyInfo data structures corresponding
to any incremental changes that have been made to the default key
table or requested keys since the last call to either
KM_KeyTablePoll, KM_DefaultKeyTable, or KM_GetRecvKey. The status of
some elements in the returned list may be set to Deleted.
KM_KeyTablePoll() -> KeyInfoList
12.5. Asynchronous Upcall Interface
Rather than repeatedly calling the KM_KeyTablePoll(), an
implementation may choose to use an asynchronous event model. This
function registers interest to key changes for a given Key Identifier
or for all keys if no Key Identifier is specified. The upcall
function is called each time a change is made to a key.
KM_KeyUpdate ( Function [, KeyIdentifier ] )
where the upcall function is parameterized as follows:
Function ( KeyInfo )
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