Network Working Group T. Li
Request for Comments: 5304 Redback Networks, Inc.
Obsoletes: 3567 R. Atkinson
Updates: 1195 Extreme Networks, Inc.
Category: Standards Track October 2008 IS-IS 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.
This document describes the authentication of Intermediate System to
Intermediate System (IS-IS) Protocol Data Units (PDUs) using the
Hashed Message Authentication Codes - Message Digest 5 (HMAC-MD5)
algorithm as found in RFC 2104. IS-IS is specified in International
Standards Organization (ISO) 10589, with extensions to support
Internet Protocol version 4 (IPv4) described in RFC 1195. 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
replaces RFC 3567.
This document proposes an extension to that specification that allows
the use of the HMAC-MD5 authentication algorithm to be used in
conjunction with the existing authentication mechanisms.
The IS-IS protocol, as specified in [ISO-10589], provides for the
authentication of Link State Protocol Data Units (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 use of IS-IS for IPv4 networks is described in [RFC1195].
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 ensure that the
authentication mechanisms described in this document are effective.
This document is a publication of the IS-IS Working Group within the
IETF. This document replaces [RFC3567], which is an Informational
RFC. This document is on the Standards Track. This document has
revised Section 3, with the significant addition of a discussion of
recent attacks on MD5 in Section 3.2. This document has also added a
substantive "IANA Considerations" section to create a missing
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 [RFC2119].
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
[RFC2104]. 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.
When calculating the HMAC-MD5 result for Sequence Number 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.
To authenticate an incoming PDU, a system should save the values of
the Authentication Value field, the Checksum field, and the Remaining
Lifetime field, set these fields to zero, compute authentication, and
then restore the values of these fields.
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 that occurs just after password
rollover on an IS-IS router and 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 to restart from the value 1 using the new
password. However, neighbors will reject those new LSPs because the
Sequence Number is smaller. The router cannot increase its own LSP
Sequence Number because it fails to authenticate its own old LSP that
neighbors keep sending to it. So the router cannot update its LSP
Sequence Number to its neighbors until all the neighbors time out all
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 be kept on
this case to mitigate the risk from such an 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 in order to reduce the
risk of an adversary compromising the routing system by deliberately
injecting false information into that system.
3.1. Security Limitations
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. It provides improved security against
passive attacks, as defined in [RFC1704], when compared to cleartext
This mechanism does not prevent replay attacks; however, in most
cases, 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
The mechanisms in this document do not provide protection against
compromised, malfunctioning, or misconfigured routers. Such routers
can, either accidentally or deliberately, cause malfunctions that
affect the whole routing domain. The reader is encouraged to consult
[RFC4593] for a more comprehensive description of threats to routing
Users need to understand that the quality of the security provided by
this mechanism depends completely 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 IS-IS implementations. This mechanism also depends on
the IS-IS Authentication Key being kept confidential by all parties.
If any of these are incorrect or insufficiently secure, then no real
security will be provided to the users of this mechanism.
Since Dobbertin's attacks on MD5 [Dobb96a] [Dobb96b] [Dobb98] were
first published a dozen years ago, there have been growing concerns
about the effectiveness of the compression function within MD5. More
recent work by Wang and Yu [WY05] accentuates these concerns.
However, despite these research results, there are no published
attacks at present on either Keyed-MD5 or HMAC-MD5. A recent paper
by Bellare [Bell06a] [Bell06b] provides new proofs for the security
of HMAC that require fewer assumptions than previous published proofs
for HMAC. Those proofs indicate that the published issues with MD5
(and separately with SHA-1) do not create an attack on HMAC-MD5 (or
HMAC SHA-1). Most recently, Fouque and others [FLN07] have published
new attacks on NMAC-MD4, HMAC-MD4, and NMAC-MD5. However, their
attacks are non-trivial computationally, and they have not found an
equivalent attack on HMAC-MD5. So, despite the published issues with
the MD5 algorithm, there is currently no published attack that
applies to HMAC-MD5 as used in this IS-IS specification. As with any
cryptographic technique, there is the possibility of the discovery of
future attacks against this mechanism.
3.3. Key Configuration
It should be noted that the key configuration mechanism of routers
may restrict the possible keys that may be used between peers. It is
strongly recommended that an implementation be able to support, at
minimum, a key composed of a string of printable ASCII of 80 bytes or
less, as this is current practice.
3.4. Other Considerations
Changes to the authentication mechanism described here (primarily: to
add a Key-ID field such as that of OSPFv2 and RIPv2) 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 a 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.
3.5. Future Directions
If a stronger authentication were believed to be required, then the
use of a full digital signature [RFC2154] 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.
If and when additional authentication mechanisms are defined (for
example, to provide a cryptographically stronger hash function), it
will also be necessary to define mechanisms that allow graceful
transition from the existing mechanisms (as defined in this document)
to any future mechanism.
4. IANA Considerations
IANA has created a new codepoint registry to administer the
Authentication Type codepoints for TLV 10. This registry is part of
the existing IS-IS codepoints registry as established by [RFC3563]
and [RFC3359]. This registry is managed using the Designated Expert
policy as described in [RFC5226] and is called "IS-IS Authentication
Type Codes for TLV 10".
The values in the "IS-IS Authentication Type Codes for TLV 10"
registry should be recorded in decimal and should only be approved
after a designated expert, appointed by the IESG area director, has
been consulted. The intention is that any allocation will be
accompanied by a published RFC. However, the designated expert can
approve allocations once it seems clear that an RFC will be
published, allowing for the allocation of values prior to the
document being approved for publication as an RFC. New items should
be documented in a publicly and freely available specification. We
should also allow external specifications to allocate and use the
IS-IS Authentication Type Codes maintained by this registry.
Initial values for the "IS-IS Authentication Type Codes for TLV 10"
registry are given below; future assignments are to be made through
Expert Review. Assignments consist of an authentication type name
and its associated value.
| Authentication Type Code | Value | Reference |
| Reserved | 0 | [ISO-10589] |
| Cleartext Password | 1 | [ISO-10589] |
| ISO 10589 Reserved | 2 | [ISO-10589] |
| HMAC-MD5 Authentication | 54 | RFC 5304 |
| Routeing Domain private authentication | 255 | [ISO-10589] |
| method | | |
The authors would like to thank (in alphabetical order) Stephen
Farrell, Dave Katz, Steven Luong, Tony Przygienda, Nai-Ming Shen, and
Henk Smit for their comments and suggestions on this document.
6.1. Normative References
[ISO-10589] ISO, "Intermediate System to Intermediate System intra-
domain routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode network service (ISO 8473)",
International Standard 10589:2002, Second Edition, 2002.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2. Informative References
[Bell06a] Bellare, M., "New Proofs for NMAC and HMAC: Security
without Collision-Resistance", Preliminary Version, in
Proceedings of Crypto 2006, Lecture Notes in Computer
Science, Vol. 4117, August 2006.
[Bell06b] Bellare, M., "New Proofs for NMAC and HMAC: Security
without Collision-Resistance", August 2006, <http://
[DoS] Voydock, V. and S. Kent, "Security Mechanisms in High-
level Networks", ACM Computing Surveys Vol. 15, No. 2,
[Dobb96a] Dobbertin, H., "Cryptanalysis of MD5 Compress",
EuroCrypt Rump Session 1996, May 1996.
[Dobb96b] Dobbertin, H., "The Status of MD5 After a Recent
Attack", CryptoBytes, Vol. 2, No. 2, 1996.
[Dobb98] Dobbertin, H., "Cryptanalysis of MD4", Journal of
Cryptology, Vol. 11, No. 4, 1998.
[FLN07] Fouque, P., Leurent, G., and P. Nguyen, "Full Key-
Recovery Attacks on HMAC/NMAC-MD5 and NMAC-MD5",
Proceedings of Crypto 2007, August 2007.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC1704] Haller, N. and R. Atkinson, "On Internet
Authentication", RFC 1704, October 1994.
[RFC2154] Murphy, S., Badger, M., and B. Wellington, "OSPF with
Digital Signatures", RFC 2154, June 1997.
[RFC3359] Przygienda, T., "Reserved Type, Length and Value (TLV)
Codepoints in Intermediate System to Intermediate
System", RFC 3359, August 2002.
[RFC3563] Zinin, A., "Cooperative Agreement Between the ISOC/IETF
and ISO/IEC Joint Technical Committee 1/Sub Committee 6
(JTC1/SC6) on IS-IS Routing Protocol Development",
RFC 3563, July 2003.
[RFC3567] Li, T. and R. Atkinson, "Intermediate System to
Intermediate System (IS-IS) Cryptographic
Authentication", RFC 3567, July 2003.
[RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", RFC 4593, October 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
[WY05] Wang, X. and H. Yu, "How to Break MD5 and Other Hash
Functions", Proceedings of EuroCrypt 2005, Lecture Notes
in Computer Science, Vol. 3494, 2005.
Redback Networks, Inc.
300 Holger Way
San Jose, CA 95134
Phone: +1 408 750 5160
Extreme Networks, Inc.
3585 Monroe St.
Santa Clara, CA 95051
Phone: +1 408 579 2800
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