Request for Comments: 7349
Category: Standards Track
LDP Hello Cryptographic Authentication
This document introduces a new optional Cryptographic Authentication TLV that LDP can use to secure its Hello messages. It secures the Hello messages against spoofing attacks and some well-known attacks against the IP header. This document describes a mechanism to secure the LDP Hello messages using Hashed Message Authentication Code (HMAC) with the National Institute of Standards and Technology (NIST) Secure Hash Standard family of algorithms.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7349.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 2. Cryptographic Authentication TLV . . . . . . . . . . . . . . 4 2.1. Optional Parameter for Hello Message . . . . . . . . . . 4 2.2. LDP Security Association . . . . . . . . . . . . . . . . 4 2.3. Cryptographic Authentication TLV Encoding . . . . . . . . 6 2.4. Sequence Number Wrap . . . . . . . . . . . . . . . . . . 7 3. Cryptographic Authentication Procedure . . . . . . . . . . . 8 4. Cross-Protocol Attack Mitigation . . . . . . . . . . . . . . 8 5. Cryptographic Aspects . . . . . . . . . . . . . . . . . . . . 8 5.1. Preparing the Cryptographic Key . . . . . . . . . . . . . 9 5.2. Computing the Hash . . . . . . . . . . . . . . . . . . . 9 5.3. Result . . . . . . . . . . . . . . . . . . . . . . . . . 10 6. Processing Hello Message Using Cryptographic Authentication . 10 6.1. Transmission Using Cryptographic Authentication . . . . . 10 6.2. Receipt Using Cryptographic Authentication . . . . . . . 10 7. Operational Considerations . . . . . . . . . . . . . . . . . 11 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 11.1. Normative References . . . . . . . . . . . . . . . . . . 13 11.2. Informative References . . . . . . . . . . . . . . . . . 14
The Label Distribution Protocol (LDP) [RFC5036] sets up LDP sessions that run between LDP peers. The peers could either be directly connected at the link level or be multiple hops away. An LDP Label Switching Router (LSR) could either be configured with the identity of its peers or could discover them using LDP Hello messages. These messages are sent encapsulated in UDP addressed to "all routers on this subnet" or to a specific IP address. Periodic Hello messages are also used to maintain the relationship between LDP peers necessary to keep the LDP session active.
Since the Hello messages are sent using UDP and not TCP, these messages cannot use the security mechanisms defined for TCP [RFC5926]. While some configuration guidance is given in [RFC5036] to help protect against false discovery messages, it does not provide an explicit security mechanism to protect the Hello messages.
Spoofing a Hello message for an existing adjacency can cause the valid adjacency to time out and in turn can result in termination of the associated session. This can occur when the spoofed Hello specifies a smaller Hold Time, causing the receiver to expect Hellos within this smaller interval, while the true neighbor continues sending Hellos at the previously agreed lower frequency. Spoofing a Hello message can also cause the LDP session to be terminated directly, which can occur when the spoofed Hello specifies a different Transport Address, other than the previously agreed one between neighbors. Spoofed Hello messages have been observed and reported as a real problem in production networks [RFC6952].
For Link Hello, [RFC5036] states that the threat of spoofed Hellos can be reduced by accepting Hellos only on interfaces to which LSRs that can be trusted are directly connected and ignoring Hellos not addressed to the "all routers on this subnet" multicast group. The Generalized TTL Security Mechanism (GTSM) provides a simple and reasonably robust defense mechanism for Link Hello [RFC6720], but it does not secure against packet spoofing attacks or replay attacks [RFC5082].
Spoofing attacks via Targeted Hellos are a potentially more serious threat. [RFC5036] states that an LSR can reduce the threat of spoofed Targeted Hellos by filtering them and accepting only those originating at sources permitted by an access list. However, filtering using access lists requires LSR resources and does not prevent IP-address spoofing.
This document introduces a new Cryptographic Authentication TLV that is used in LDP Hello messages as an optional parameter. It enhances the authentication mechanism for LDP by securing the Hello message against spoofing attacks. It also introduces a cryptographic sequence number carried in the Hello messages that can be used to protect against replay attacks.
Using this Cryptographic Authentication TLV, one or more secret keys (with corresponding Security Association (SA) IDs) are configured in each system. For each LDP Hello message, the key is used to generate and verify an HMAC Hash that is stored in the LDP Hello message. For the cryptographic hash function, this document proposes to use SHA-1, SHA-256, SHA-384, and SHA-512 defined in US NIST Secure Hash Standard (SHS) [FIPS-180-4]. The HMAC authentication mode defined in [RFC2104] is used. Of the above, implementations MUST include support for at least HMAC-SHA-256, SHOULD include support for HMAC- SHA-1, and MAY include support for HMAC-SHA-384 and HMAC-SHA-512.
1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. Cryptographic Authentication TLV
2.1. Optional Parameter for Hello Message
[RFC5036] defines the encoding for the Hello message. Each Hello message contains zero or more Optional Parameters, each encoded as a TLV. Three Optional Parameters are defined by [RFC5036]. This document defines a new Optional Parameter: the Cryptographic Authentication parameter.
Optional Parameter Type -------------------------------- -------- IPv4 Transport Address 0x0401 (RFC 5036) Configuration Sequence Number 0x0402 (RFC 5036) IPv6 Transport Address 0x0403 (RFC 5036) Cryptographic Authentication TLV 0x0405 (this document)
The encoding for the Cryptographic Authentication TLV is described in Section 2.3.
2.2. LDP Security Association
An LDP Security Association (SA) contains a set of parameters shared between any two legitimate LDP speakers.
Parameters associated with an LDP SA are as follows:
- Security Association Identifier (SA ID)
This is a 32-bit unsigned integer used to uniquely identify an LDP SA between two LDP peers, as manually configured by the network operator (or, possibly by some key management protocol specified by the IETF in the future).
The receiver determines the active SA by looking at the SA ID field in the incoming Hello message.
The sender, based on the active configuration, selects an SA to use and puts the correct SA ID value associated with the SA in the LDP Hello message. If multiple valid and active LDP SAs exist for a given interface, the sender may use any of those SAs to protect the packet.
Using SA IDs makes changing keys while maintaining protocol operation convenient. Each SA ID specifies two independent parts, the authentication algorithm and the authentication key, as explained below.
Normally, an implementation would allow the network operator to configure a set of keys in a key chain, with each key in the chain having a fixed lifetime. The actual operation of these mechanisms is outside the scope of this document.
Note that each SA ID can indicate a key with a different authentication algorithm. This allows the introduction of new authentication mechanisms without disrupting existing LDP sessions.
- Authentication Algorithm
This signifies the authentication algorithm to be used with the LDP SA. This information is never sent in clear text over the wire. Because this information is not sent on the wire, the implementer chooses an implementation-specific representation for this information.
Currently, the following algorithms are supported:
HMAC-SHA-1, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512.
- Authentication Key
This value denotes the cryptographic authentication key associated with the LDP SA. The length of this key is variable and depends upon the authentication algorithm specified by the LDP SA.
The time that this LDP router will accept packets that have been created with this LDP Security Association.
The time that this LDP router will begin using this LDP Security Association for LDP Hello message generation.
The time that this LDP router will stop using this LDP Security Association for LDP Hello message generation.
The time that this LDP router will stop accepting packets generated with this LDP Security Association.
In order to achieve smooth key transition, KeyStartAccept SHOULD be less than KeyStartGenerate, and KeyStopGenerate SHOULD be less than KeyStopAccept. If KeyStartGenerate or KeyStartAccept are left unspecified, the time will default to 0, and the key will be used immediately. If KeyStopGenerate or KeyStopAccept are left unspecified, the time will default to infinity, and the key's lifetime will be infinite. When a new key replaces an old, the KeyStartGenerate time for the new key MUST be less than or equal to the KeyStopGenerate time of the old key. Any unspecified values are encoded as zero.
Key storage SHOULD persist across a system restart, warm or cold, to avoid operational issues. In the event that the last key associated with an interface expires, it is unacceptable to revert to an unauthenticated condition and not advisable to disrupt routing. Therefore, the router 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.
2.3. Cryptographic Authentication TLV Encoding
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0| Auth (0x0405) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Security Association ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cryptographic Sequence Number (High-Order 32 Bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cryptographic Sequence Number (Low-Order 32 Bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Authentication Data (Variable) | ~ ~ | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ o Type: 0x0405, Cryptographic Authentication
- Length: The length in octets of the value field, including the Security Association ID and Cryptographic Sequence Number fields.
- Security Association ID: The 32-bit field that maps to the authentication algorithm and the secret key used to create the message digest carried in LDP payload.
Though the SA ID implies the algorithm, the HMAC output size should not be used by implementers as an implicit hint because additional algorithms may be defined in the future that have the same output size.
- Cryptographic Sequence Number: The 64-bit, strictly increasing sequence number that is used to guard against replay attacks. The 64-bit sequence number MUST be incremented for every LDP Hello message sent by the LDP router. Upon reception, the sequence number MUST be greater than the sequence number in the last LDP Hello message accepted from the sending LDP neighbor. Otherwise, the LDP message is considered a replayed packet and is dropped. The Cryptographic Sequence Number is a single space per LDP router.
LDP routers implementing this specification MUST use existing mechanisms to preserve the sequence number's strictly increasing property for the deployed life of the LDP router (including cold restarts). One mechanism for accomplishing this could be to use the high-order 32 bits of the sequence number as a boot count that is incremented anytime the LDP router loses its sequence number state. Techniques such as sequence number space partitioning described above or non-volatile storage preservation can be used but are beyond the scope of this specification. Sequence number wrap is described in Section 2.4.
- Authentication Data: This field carries the digest computed by the Cryptographic Authentication algorithm in use. The length of the Authentication Data varies based on the cryptographic algorithm in use, which is shown below:
Auth type Length --------------- ---------- HMAC-SHA1 20 bytes HMAC-SHA-256 32 bytes HMAC-SHA-384 48 bytes HMAC-SHA-512 64 bytes
2.4. Sequence Number Wrap
When incrementing the sequence number for each transmitted LDP message, the sequence number should be treated as an unsigned 64-bit value. If the lower-order 32-bit value wraps, the higher-order 32-bit value should be incremented and saved in non-volatile storage. If the LDP router is deployed long enough that the 64-bit sequence number wraps, all keys, independent of the key distribution mechanism, MUST be reset. This is done to avoid the possibility of replay attacks. Once the keys have been changed, the higher-order sequence number can be reset to 0 and saved to non-volatile storage.
3. Cryptographic Authentication Procedure
As noted earlier, the Security Association ID maps to the authentication algorithm and the secret key used to generate and verify the message digest. This specification discusses the computation of LDP Cryptographic Authentication data when any of the NIST SHS family of algorithms is used in the Hashed Message Authentication Code (HMAC) mode.
The currently valid algorithms (including mode) for LDP Cryptographic Authentication include:
HMAC-SHA-1, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512
Of the above, implementations of this specification MUST include support for at least HMAC-SHA-256, SHOULD include support for HMAC- SHA-1, and MAY also include support for HMAC-SHA-384 and HMAC-SHA- 512.
Implementations of this standard MUST use HMAC-SHA-256 as the default authentication algorithm.
4. Cross-Protocol Attack Mitigation
In order to prevent cross-protocol replay attacks for protocols sharing common keys, the 2-octet LDP Cryptographic Protocol ID is appended to the authentication key prior to use (refer to Section 8). Other protocols using the common key similarly append their own Cryptographic Protocol IDs to their keys prior to use, thus ensuring that a different key value is used for each protocol.
5. Cryptographic Aspects
In the algorithm description below, the following nomenclature is used:
- H is the specific hashing algorithm (e.g., SHA-256).
- K is the Authentication Key from the LDP Security Association.
- Ks is a Protocol-Specific Authentication Key obtained by appending Authentication Key (K) with the 2-octet LDP Cryptographic Protocol ID.
- Ko is the cryptographic key used with the hash algorithm.
- L is the length of the hash, measured in octets rather than bits.
- AuthTag is a value that is the same length as the hash output. In the case of IPv4, the first 4 octets contain the IPv4 source address followed by the hexadecimal value 0x878FE1F3 repeated (L-4)/4 times. In the case of IPv6, the first 16 octets contain the IPv6 source address followed by the hexadecimal value 0x878FE1F3 repeated (L-16)/4 times. This implies that hash output is always a length of at least 16 octets.
5.1. Preparing the Cryptographic Key
The LDP Cryptographic Protocol ID is appended to the Authentication Key (K) yielding a Protocol-Specific Authentication Key (Ks). In this application, Ko is always L octets long. Keys that are longer than the bit length of the hash function are hashed to force them to this length, as we describe below. Ks is computed as follows.
If the Protocol-Specific Authentication Key (Ks) is L octets long, then Ko is equal to Ks. If the Protocol-Specific Authentication Key (Ks) is more than L octets long, then Ko is set to H(Ks). If the Protocol-Specific Authentication Key (Ks) is less than L octets long, then Ko is set to the Protocol-Specific Authentication Key (Ks) with zeros appended to the end of the Protocol-Specific Authentication Key (Ks) such that Ko is L octets long.
For higher entropy, it is RECOMMENDED that Key Ks should be at least L octets long.
5.2. Computing the Hash
First, the Authentication Data field in the Cryptographic Authentication TLV is filled with the value AuthTag. Then, to compute HMAC over the Hello message it performs:
AuthData = HMAC(Ko, Hello Message)
Hello Message refers to the LDP Hello message excluding the IP and the UDP headers.
The resultant Hash becomes the Authentication Data that is sent in the Authentication Data field of the Cryptographic Authentication TLV. The length of the Authentication Data field is always identical to the message digest size of the specific hash function H that is being used.
This also means that the use of hash functions with larger output sizes will also increase the size of the LDP message as transmitted on the wire.
6. Processing Hello Message Using Cryptographic Authentication
6.1. Transmission Using Cryptographic Authentication
Prior to transmitting the LDP Hello message, the Length in the Cryptographic Authentication TLV header is set as per the authentication algorithm that is being used. It is set to 24 for HMAC-SHA-1, 36 for HMAC-SHA-256, 52 for HMAC-SHA-384, and 68 for HMAC-SHA-512.
The Security Association ID field is set to the ID of the current authentication key. The HMAC Hash is computed as explained in Section 5. The resulting Hash is stored in the Authentication Data field prior to transmission. The authentication key MUST NOT be carried in the packet.
6.2. Receipt Using Cryptographic Authentication
The receiving LSR applies acceptability criteria for received Hellos using cryptographic authentication. If the Cryptographic Authentication TLV is unknown to the receiving LSR, the received packet MUST be discarded according to Section 18.104.22.168.2 of [RFC5036].
The receiving router MUST determine whether or not to accept a Hello message from a particular source IP address as follows. First, if the router has, for that source IP address, a stored LDP Hello cryptographic sequence number or is configured to require LDP Hello authentication, then the router MUST discard any unauthenticated Hello packets. As specified later in this section, a cryptographic sequence number is only stored for a source IP address as a result of receiving a valid authenticated Hello.
The receiving LSR locates the LDP SA using the Security Association ID field carried in the message. If the SA is not found or if the SA is not valid for reception (i.e., current time < KeyStartAccept or current time >= KeyStopAccept), the LDP Hello message MUST be discarded, and an error event SHOULD be logged.
If the cryptographic sequence number in the LDP Hello message is less than or equal to the last sequence number received from the same neighbor, the LDP Hello message MUST be discarded, and an error event SHOULD be logged.
Before the receiving LSR performs any processing, it needs to save the values of the Authentication Data field. The receiving LSR then replaces the contents of the Authentication Data field with AuthTag and computes the Hash using the authentication key specified by the received Security Association ID field, as explained in Section 3. If the locally computed Hash is equal to the received value of the Authentication Data field, the received packet is accepted for other normal checks and processing as described in [RFC5036]. Otherwise, if the locally computed Hash is not equal to the received value of the Authentication Data field, the received LDP Hello message MUST be discarded, and an error event SHOULD be logged. The aforesaid logging needs to be carefully rate limited, because while a LDP router is under attack by a storm of spoofed Hellos, the resources required for logging could be overwhelming.
After the LDP Hello message has been successfully authenticated, implementations MUST store the 64-bit cryptographic sequence number for the LDP Hello message received from the source IP address. The saved cryptographic sequence numbers will be used for replay checking for subsequent packets received from the source IP address.
7. Operational Considerations
Careful consideration must be given to when and how to enable and disable authentication on LDP Hellos. On the one hand, it is critical that an attack cannot cause the authentication to be disabled. On the other hand, it is equally important that an operator can change the hardware and/or software associated with a neighbor's IP address and successfully bring up an LDP adjacency with the desired level of authentication, which may be with different or no authentication due to software restrictions.
LDP Hello authentication information (e.g., whether authentication is enabled and what the last cryptographic sequence number is) associated with an IP address is learned via a set of interfaces. If an interface is administratively disabled, the LDP Hello authentication information learned via that interface MAY be forgotten. This enables an operator that is not specifically manipulating LDP Hello authentication configurations to easily bring up an LDP adjacency. An implementation of this standard SHOULD provide a configuration mechanism by which the LDP Hello authentication information associated with an IP address can be shown and can be forgotten; configuration mechanisms are assumed to be accessed via an authenticated channel.
8. Security Considerations
Section 1 of this document describes the security issues arising from the use of unauthenticated LDP Hello messages. In order to address those issues, it is RECOMMENDED that all deployments use the Cryptographic Authentication TLV to authenticate the Hello messages.
The quality of the security provided by the Cryptographic Authentication TLV depends completely on the strength of the cryptographic algorithm in use, the strength of the key being used, and the correct implementation of the security mechanism in communicating LDP implementations. Also, the level of security provided by the Cryptographic Authentication TLV varies based on the authentication type used.
It should be noted that the authentication method described in this document is not being used to authenticate the specific originator of a packet but is rather being used to confirm that the packet has indeed been issued by a router that has access to the authentication key.
Deployments SHOULD use sufficiently long and random values for the authentication key so that guessing and other cryptographic attacks on the key are not feasible in their environments. In support of these recommendations, management systems SHOULD support hexadecimal input of authentication keys.
The mechanism described herein is not perfect. However, this mechanism introduces a significant increase in the effort required for an adversary to successfully attack the LDP Hello protocol while not causing undue implementation, deployment, or operational complexity.
9. IANA Considerations
The IANA has assigned a new TLV from the "Label Distribution Protocol (LDP) Parameters" registry, "TLV Type Name Space".
Value Description Reference ------ -------------------------------- --------------------------- 0x0405 Cryptographic Authentication TLV this document (Section 2.3)
The IANA has also assigned a value from the "Authentication Cryptographic Protocol ID" registry under the "Keying and Authentication for Routing Protocols (KARP) Parameters" category.
Value Description Reference ----- -------------------------------- ------------------------- 2 LDP Cryptographic Protocol ID this document (Section 4)
We are indebted to Yaron Sheffer who helped us enormously in rewriting the document to get rid of the redundant crypto mathematics that we had added here.
We would also like to thank Liu Xuehu for his work on background and motivation for LDP Hello authentication. Last but not least, we would also thank Adrian Farrel, Eric Rosen, Sam Hartman, Stephen Farrell, Eric Gray, Kamran Raza, and Acee Lindem for their valuable comments.
11.1. Normative References
National Institute of Standards and Technology (NIST), "Secure Hash Standard (SHS)", FIPS 180-4, March 2012.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP Specification", RFC 5036, October 2007.
11.2. Informative References
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C. Pignataro, "The Generalized TTL Security Mechanism (GTSM)", RFC 5082, October 2007. [RFC5926] Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms for the TCP Authentication Option (TCP-AO)", RFC 5926, June 2010. [RFC6720] Pignataro, C. and R. Asati, "The Generalized TTL Security Mechanism (GTSM) for the Label Distribution Protocol (LDP)", RFC 6720, August 2012. [RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of BGP, LDP, PCEP, and MSDP Issues According to the Keying and Authentication for Routing Protocols (KARP) Design Guide", RFC 6952, May 2013.