Network Working Group Tony Li
INTERNET DRAFT Procket Networks
draft-ietf-isis-hmac-03.txt RJ Atkinson
Extreme Networks
4 July 2001
IS-IS Cryptographic Authentication
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026 except that the right to produce
derivative works is not granted.
Internet-Drafts are working documents of the Internet Engineering Task
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is inappropriate to use Internet-Drafts as reference material or to cite
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
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ABSTRACT
This document describes the authentication of IS-IS PDUs using the
HMAC-MD5 algorithm [1]. IS-IS is specified in [2], with extensions
to support IPv4 described in [3]. The base specification includes an
authentication mechanism that allows for multiple authentication
algorithms. The base specification only specifies the algorithm for
cleartext passwords.
This document proposes an extension to that specification that
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allows the use of the HMAC-MD5 authentication algorithm to be used in
conjunction with the existing authentication mechanisms.
1. Introduction
The IS-IS protocol, as specified in ISO 10589, provides for the
authentication of Link State PDUs (LSPs) through the inclusion of
authentication information as part of the LSP. This authentication
information is encoded as a Type-Length-Value (TLV) tuple.
The type of the TLV is specified as 10. The length of the TLV
is variable. The value of the TLV depends on the authentication
algorithm and related secrets being used. The first octet of the
value is used to specify the authentication type. Type 0 is
reserved, type 1 indicates a cleartext password, and type 255 is used
for routing domain private authentication methods. The remainder of
the TLV value is known as the Authentication Value.
This document extends the above situation by allocating a new
authentication type for HMAC-MD5 and specifying the algorithms for
the computation of the Authentication Value. This document also
describes modifications to the base protocol to insure that the
authentication mechanisms described in this document are effective.
This document is a publication of the IS-IS Working Group within
the IETF, and is a contribution to ISO IEC JTC1/SC6, for eventual
inclusion with ISO 10589.
2. Authentication Procedures
The authentication type used for HMAC-MD5 is 54 (0x36). The
length of the Authentication Value for HMAC-MD5 is 16, and the length
field in the TLV is 17.
The HMAC-MD5 algorithm requires a key K and text T as input.
The key K is the password for the PDU type, as specified in ISO
10589. The text T is the IS-IS PDU to be authenticated with the
Authentication Value field inside of the Authentication Information
TLV set to zero. Note that the Authentication Type is set to 54 and
the length of the TLV is set to 17 before authentication is computed.
When LSPs are authenticated, the Checksum and Remaining Lifetime
fields are set to zero (0) before authentication is computed. The
result of the algorithm is placed in the Authentication Value field.
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When calculating the HMAC-MD5 result for Sequence Number PDUs
and IS-IS HELLO PDUs, Level 1 Sequence Number PDUs SHALL use the Area
Authentication string as in Level 1 Link State PDUs. Level 2
Sequence Number PDUs shall use the domain authentication string as in
Level 2 Link State PDUs. IS-IS HELLO PDUs SHALL use the Link Level
Authentication String, which MAY be different from that of Link State
PDUs. The HMAC-MD5 result for the IS-IS HELLO PDUs SHALL be
calculated after the Packet is padded to the MTU size, if padding is
not disabled. Implementations that support the optional checksum for
the Sequence Number PDUs and IS-IS HELLO PDUs MUST NOT include the
Checksum TLV.
An implementation that implements HMAC-MD5 authentication and
receives HMAC-MD5 Authentication Information MUST discard the PDU if
the Authentication Value is incorrect.
An implementation MAY have a transition mode where it includes
HMAC-MD5 Authentication Information in PDUs but does not verify the
HMAC-MD5 authentication information. This is a transition aid for
networks in the process of deploying authentication.
An implementation MAY check a set of passwords when verifying the
Authentication Value. This provides a mechanism for incrementally
changing passwords in a network.
An implementation that does not implement HMAC-MD5 authentication
MAY accept a PDU that contains the HMAC-MD5 Authentication Type.
ISes (routers) that implement HMAC-MD5 authentication and initiate
LSP purges MUST remove the body of the LSP and add the authentication
TLV. ISes implementing HMAC-MD5 authentication MUST NOT accept
unauthenticated purges. ISes MUST NOT accept purges that contain
TLVs other than the authentication TLV. These restrictions are
necessary to prevent a hostile system from receiving an LSP, setting
the Remaining Lifetime field to zero, and flooding it, thereby
initiating a purge without knowing the authentication password.
2.1 Implementation Considerations
There is an implementation issue just after password rollover on
an IS-IS router that might benefit from additional commentary.
Immediately after password rollover on the router, the router or IS-
IS process may restart. If this happens, this causes the LSP
Sequence Number restarts from the value 1 using the new password.
However, neighbors will reject those new LSPs because the Sequence
Number is smaller. The router can not increase its own LSP Sequence
Number because it fails to authenticate its own old LSP that
neighbors keep sending to it. So the router can not update its LSP
Sequence Number to its neighbors until all the neighbors time out all
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of the original LSPs. One possible solution to this problem is for
the IS-IS process to detect if any inbound LSP with an authentication
failure has the local System ID and also has a higher Sequence Number
than the IS-IS process has. In this event, the IS-IS process SHOULD
increase its own LSP Sequence Number accordingly and re-flood the
LSPs. However, as this scenario could also be triggered by an active
attack by an adversary, it is recommended that a counter also be kept
on this case to mitigate the risk from such an active attack.
3. Security Considerations
This document enhances the security of the IS-IS routing
protocol. Because a routing protocol contains information that need
not be kept secret, privacy is not a requirement. However,
authentication of the messages within the protocol is of interest, to
reduce the risk of an adversary compromising the routing system by
deliberately injecting false information into the routing system.
The technology in this document provides an authentication
mechanism for IS-IS. The mechanism described here is not perfect and
does not need to be perfect. Instead, this mechanism represents a
significant increase in the work function of an adversary attacking
the IS-IS protocol, while not causing undue implementation,
deployment, or operational complexity.
This mechanism does not prevent replay attacks, however such
attacks would trigger existing mechanisms in the IS-IS protocol that
would effectively reject old information. Denial of service attacks
are not generally preventable in a useful networking protocol [4].
Changes to the authentication mechanism described here
(primarily: to add a Key-ID field such as OSPFv2 and RIPv2 have) were
considered at some length, but ultimately were rejected. The
mechanism here was already widely implemented in 1999. As of this
writing, this mechanism is fairly widely deployed within the users
interested in cryptographic authentication of IS-IS. The improvement
provided by the proposed revised mechanism was not large enough to
justify the change, given the installed base and lack of operator
interest in deploying the proposed revised mechanism.
If and when a key management protocol appears that is both
widely implemented and easily deployed to secure routing protocols
such as IS-IS, a different authentication mechanism that is designed
for use with that key management schema could be added if desired.
If a stronger authentication were believed to be required, then
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the use of a full digital signature [5] would be an approach that
should be seriously considered. It was rejected for this purpose at
this time because the computational burden of full digital signatures
is believed to be much higher than is reasonable given the current
threat environment in operational commercial networks.
ACKNOWLEDGMENTS
The author would like to thank (in alphabetical order) Ran
Atkinson, Dave Katz, Steven Luong, Tony Przygienda, Nai-Ming Shen,
and Henk Smit for their comments and suggestions on this document.
REFERENCES
[1] H. Krawczyk, M. Bellare, & R. Canetti, "HMAC: Keyed-Hashing for Message
Authentication", RFC-2104, February 1997
[2] ISO, "Intermediate System to Intermediate System Intra- Domain Routing
Exchange Protocol for use in Conjunction with the Protocol for Providing
the Connectionless-mode Network Service (ISO 8473)", International Standard
10589 [Also republished as RFC 1142].
[3] R. Callon, "Use of OSI IS-IS for Routing in TCP/IP and Dual
environments", RFC-1195, December 1990.
[4] V.L. Voydock & S. T. Kent, "Security Mechanisms in High-level
Networks", ACM Computing Surveys, Vol. 15, No. 2, June 1983.
[5] S. Murphy, M. Badger, & B. Wellington, "OSPF with Digital Signatures",
RFC-2154, June 1997.
Author's Address
Tony Li
Procket Networks
1100 Cadillac Ct.
Milpitas, California
95035 USA
Email: tli@procket.net
Phone: +1 (408) 635-7903
RJ Atkinson
Extreme Networks
3585 Monroe Street
Santa Clara, CA
95051 USA
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Email: rja@extremenetworks.com
Phone: +1 (408) 579-2800
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