| Path Computation Element | D. Lopez |
| Internet-Draft | O. Gonzalez de Dios |
| Intended status: Experimental | Telefonica I+D |
| Expires: September 5, 2015 | Q. Wu |
| D. Dhody | |
| Huawei | |
| March 4, 2015 |
Secure Transport for PCEP
draft-ietf-pce-pceps-03
The Path Computation Element Communication Protocol (PCEP) defines the mechanisms for the communication between a Path Computation Client (PCC) and a Path Computation Element (PCE), or among PCEs. This document describe the usage of Transport Layer Security (TLS) to enhance PCEP security, hence the PCEPS acronym proposed for it. The additional security mechanisms are provided by the transport protocol supporting PCEP, and therefore they do not affect the flexibility and extensibility of PCEP.
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PCEP [RFC5440] defines the mechanisms for the communication between a Path Computation Client (PCC) and a Path Computation Element (PCE), or between two PCEs. These interactions include requests and replies that can be critical for a sustainable network operation and adequate resource allocation, and therefore appropriate security becomes a key element in the PCE infrastructure. As the applications of the PCE framework evolves, and more complex service patterns emerge, the definition of a secure mode of operation becomes more relevant.
[RFC5440] analyzes in its section on security considerations the potential threats to PCEP and their consequences, and discusses several mechanisms for protecting PCEP against security attacks, without making a specific recommendation on a particular one or defining their application in depth. Moreover, [RFC6952] remarks the importance of ensuring PCEP communication privacy, especially when PCEP communication endpoints do not reside in the same Autonomous System (AS), as the interception of PCEP messages could leak sensitive information related to computed paths and resources.
Among the possible solutions mentioned in these documents, Transport Layer Security (TLS) [RFC5246] provides support for peer authentication, and message encryption and integrity. TLS supports the usage of well-know mechanisms to support key configuration and exchange, and means to perform security checks on the results of PCE discovery procedures via Interior Gateway Protocol (IGP) ([RFC5088] and [RFC5089]).
This document describes a security container for the transport of PCEP requests and replies, and therefore they do not affect the flexibility and extensibility of PCEP.
This document describes how to apply TLS in securing PCE interactions, including initiation of the TLS procedures, the TLS handshake mechanisms, the TLS methods for peer authentication, the applicable TLS ciphersuites for data exchange, and the handling of errors in the security checks. In the rest of the document we will refer to this usage of TLS to provide a secure transport for PCEP as "PCEPS".
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].
The steps involved in the PCEPS establishment consists of following successive steps:
It should be noted that this procedure update what is defined in section 6.7 of [RFC5440] regarding the processing of messages prior to the Open message. The details of processing including backward comaptibility is discussed in the following sections.
Since PCEP can operate either with or without TLS, it is necessary for the PCEP speaker to indicate whether it wants to set up a TLS connection or not. For this purpose, this document proposes a new PCEP message called StartTLS. This message MUST be issued by the party willing to use TLS, prior to any other PCEP message. PCEP speaker MAY discover that the PCEP peer supports PCEPS or can be preconfigured to use PCEPS for a given peer (see Section 4 for more details). Thus the PCEP session is secured via TLS from the start before exchange of any other PCEP message including the open message. Securing via TLS, of an existing PCEP session is not permitted, session must be closed and restablished with TLS as per the procedure described in this document.
The StartTLS message is a PCEP message sent by a PCC to a PCE and by a PCE to a PCC in order to initiate the TLS procedure for PCEP. The Message-Type field of the PCEP common header for the StartTLS message is set to [TBA].
Once the TCP connection has been successfully established, the first message sent by the PCC to the PCE or by the PCE to the PCC MUST be a StartTLS message for the PCEPS. Note this is a significant change from [RFC5440] where the first PCEP message is Open.
A PCEP speaker receiving a StartTLS message after any other PCEP exchange has taken place (by receiving or sending any other messages from either side) MUST treat it as an unexpected message and reply with a PCErr message with Error-Type set to xx (TBA by IANA)(PCEP StartTLS failure) and Error-value set to 1 (reception of StartTLS after any PCEP exchange). A PCEP speaker receives any other message apart from StartTLS or PCErr MUST treat it as an unexpected message and reply with a PCErr message with Error-Type set to xx (TBA by IANA)(PCEP StartTLS failure) and Error-value set to 2 (reception of non-StartTLS or non-PCErr message).
If the PCEP speaker that does not support PCEPS, receives a StartTLS message, it MUST behave according to the existing error mechanism described in section 6.2 of [RFC5440] (in case message is received prior to an Open message) or section 6.9 of [RFC5440] (for the case of reception of unknown message).
If the PCEP speaker supports PCEPS but cannot establish a TLS connection for some reason (e.g. the certificate server is not responding) it MUST return a PCErr message with Error-Type set to xx (TBA by IANA) (PCEP StartTLS failure) and Error-value set to:
If the PCEP speaker supports PCEPS and can establish a TLS connection it MUST start the TLS connection establishment steps described in Section 3.4 before the PCEP initialization procedure (section 4.2.1 of [RFC5440]).
These procedures minimize the impact of PCEPS support in PCEP implementations without requiring additional dedicated ports for running PCEP with TLS.
The StartTLS message is used to initiate the TLS procedure for a PCEP session between the PCEP peers. A PCEP speaker sends the StartTLS message to request negotiation and establishment of TLS connection for PCEP. On receiving a StartTLS message form the PCEP peer (i.e. when PCEP speaker has sent and received StartTLS message) it is ready to start TLS negotiation and establishment and move to steps described in Section 3.4.
The format of a StartTLS message is as follows:
<StartTLS Message>::= <Common Header>
The StartTLS message MUST contain only the PCEP common header with Message-Type field set to [TBA].
Once the TCP connection has been successfully established, the sender MUST start a timer called StartTLSWait timer, after the expiration of which, if no StartTLS message has been received, it sends a PCErr message and releases the TCP connection with Error-Type set to xx (TBA by IANA) and Error-value set to 5 (no StartTLS message received before the expiration of the StartTLSWait timer).
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| |
| StartTLS |
| msg |
|------- |
| \ StartTLS |
| \ msg |
| \ ---------|
| \/ |
| /\ |
| / -------->|
| / |
|<------ |
|:::::::::TLS:::::::::|
|:::::Establishment:::|
| |
| |
|:::::::PCEP::::::::::|
| |
Figure 1: Both PCEP Speaker supports PCEPS
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| | Does not send
| StartTLS | StartTLS as
|-------------------->| cannot establish
| | TLS
| |
|<--------------------| Send Error
| PCErr | Error-Value 3/4
| |
Figure 2: Both PCEP Speaker supports PCEPS, But cannot establish TLS
+-+-+ +-+-+
|PCC| |PCE|
+-+-+ +-+-+
| | Does not support
| StartTLS | PCEPS and thus
| msg | sends open
|------- |
| \ open |
| \ msg |
| \ ---------|
| \/ |
| /\ |
| / -------->|
| / |
|<------ |
| |
|<--------------------| Send Error
| PCErr | (non-open message
| | received)
Figure 3: One PCEP Speaker does not support PCEPS
Once the establishment of TLS has been agreed by the PCEP peers, the connection establishment SHALL follow the following steps:
To support TLS re-negotiation both peers MUST support the mechanism described in [RFC5746]. Any attempt of initiate a TLS handshake to establish new cryptographic parameters not aligned with [RFC5746] SHALL be considered a TLS negotiation failure.
Depending on the peer authentication method in use, PCEPS supports different operation modes to establish peer's identity and whether it is entitled to perform requests or can be considered authoritative in its replies. PCEPS implementations SHOULD provide mechanisms for associating peer identities with different levels of access and/or authoritativeness, and they MUST provide a mechanism for establish a default level for properly identified peers. Any connection established with a peer that cannot be properly identified SHALL be terminated before any PCEP exchange takes place.
In TLS-X.509 mode using fingerprints, a peer is uniquely identified by the fingerprint of the presented client certificate.
There are numerous trust models in PKIX environments, and it is beyond the scope of this document to define how a particular deployment determines whether a client is trustworthy. Implementations that want to support a wide variety of trust models should expose as many details of the presented certificate to the administrator as possible so that the trust model can be implemented by the administrator. As a suggestion, at least the following parameters of the X.509 client certificate should be exposed:
In addition, a PCC MAY apply the procedures described in [RFC6698] DNS-Based Authentication of Named Entities (DANE) to verify its peer identity when using DNS discovery. See section Section 4.1 for further details.
In case the initial TLS negotiation or the peer identity check fail according to the procedures listed in this document, the peer MUST immediately terminate the session. It SHOULD follow the procedure listed in [RFC5440] to retry session setup along with an exponential back-off session establishment retry procedure.
A PCE can advertise its capability to support PCEPS using the IGP advertisement and discovery mechanism. The PCE-CAP-FLAGS sub-TLV is an optional sub-TLV used to advertise PCE capabilities. It MAY be present within the PCE Discovery (PCED) sub-TLV carried by OSPF or IS-IS. [RFC5088] and [RFC5089] provide the description and processing rules for this sub-TLV when carried within OSPF and IS-IS, respectively. PCE capability bits are defined in [RFC5088]. A new capability flag bit for the PCE-CAP-FLAGS sub-TLV that can be announced as attribute to distribute PCEP security support information is proposed in [I-D.wu-pce-discovery-pceps-support]
When DNS is used by a PCC (or a PCE acting as a client, for the rest of the section, PCC refers to both) willing to use PCEPS to locate an appropriate PCE [I-D.wu-pce-dns-pce-discovery], the PCC as an initiating entity, chooses at least one of the returned FQDNs to resolve, which it does by performing DNS "A" or "AAAA" lookups on the FDQN. This will eventually result in an IPv4 or IPv6 address. The PCC SHALL use the IP address(es) from the successfully resolved FDQN (with the corresponding port number returned by the DNS Service Record (SRV) lookup) as the connection address(es) for the receiving entity.
If the PCC fails to connect using an IP address but the "A" or "AAAA" lookups returned more than one IP address, then the PCC SHOULD use the next resolved IP address for that FDQN as the connection address. If the PCC fails to connect using all resolved IP addresses for a given FDQN, then it SHOULD repeat the process of resolution and connection for the next FQDN returned by the SRV lookup based on the priority and weight.
If the PCC receives a response to its SRV query but it is not able to establish a PCEPS connection using the data received in the response, as initiating entity it MAY fall back to lookup a PCE that uses TCP as transport.
DANE [RFC6698] defines a secure method to associate the certificate that is obtained from a TLS server with a domain name using DNS, i.e., using the TLSA DNS resource record (RR) to associate a TLS server certificate or public key with the domain name where the record is found, thus forming a "TLSA certificate association". The DNS information needs to be protected by DNS Security (DNSSEC). A PCC willing to apply DANE to verify server identity MUST conform to the rules defined in section 4 of [RFC6698].
The procedures described in this document define a security container for the transport of PCEP requests and replies carried by a TLS connection initiated by means of a specific extended message (StartTLS) that does not interfere with PCEP speaker implementations not supporting it.
If a PCEP implementation that does not support PCEPS receives a StartTLS message it MUST behave according to the existing error mechanism of [RFC5440].
Each PCEP message has a message type value.
One new PCEP messages is defined in this document:
Value Description Reference
TBA The Start TLS Message (StartTLS) This document
A registry was created for the Error-type and Error-value of the PCEP Error Object. Following new Error-Types and Error-Values are defined:
Error-
Type Meaning Reference
TBA StartTLS Failure This document
Error-value=1:
Reception of StartTLS after
any PCEP exchange This document
Error-value=2:
Reception of non-StartTLS
or non-PCErr message This document
Error-value=3:
Failure, connection without
TLS not possible This document
Error-value=4:
Failure, connection without
TLS possible This document
Error-value=5:
No StartTLS message before
StartTLSWait timer expiry This document
While the application of TLS satisfies the requirement on privacy as well as fine-grained, policy-based peer authentication, there are security threats that it cannot address. It is advisable to apply additional protection measures, in particular in what relates to attacks specifically addressed to forging the TCP connection underpinning TLS. TCP-AO (TCP Authentication Option [RFC5925]) is fully compatible with and deemed as complementary to TLS, so its usage is to be considered as a security enhancement whenever any of the PCEPS peers require it, especially in the case of long-lived connections. The mechanisms to configure the requirements to use TCP-AO and other lower-layer protection measures, as well as the association of the required crypto material (MKT in the case of TCP-AO) with a particular peer are outside the scope of this document. [I-D.chunduri-karp-using-ikev2-with-tcp-ao] defines a method to perform such association.
Since computational resources required by TLS handshake and ciphersuite are higher than unencrypted TCP, clients connecting to a PCEPS server can more easily create high load conditions and a malicious client might create a Denial-of-Service attack more easily.
Some TLS ciphersuites only provide integrity validation of their payload, and provide no encryption. This specification does not forbid the use of such ciphersuites, but administrators must weight carefully the risk of relevant internal data leakage that can occur in such a case, as explicitly stated by [RFC6952].
When using certificate fingerprints to identify PCEPS peers, any two certificates that produce the same hash value will be considered the same peer. Therefore, it is important to make sure that the hash function used is cryptographically uncompromised so that attackers are very unlikely to be able to produce a hash collision with a certificate of their choice. This document mandates support for SHA-256, but a later revision may demand support for stronger functions if suitable attacks on it are known.
This specification relies on the analysis and profiling of TLS included in [RFC6614] and the procedures described for the STARTTLS command in [RFC2830].
We would like to thank Joe Touch for his suggestions and support regarding the TLS start mechanisms.