Security Requirements for BGP Path Validation

Origin validation based on Resource Public Key Infrastructure (RPKI) provides a measure of resilience to accidental mis-origination of prefixes; however, it provides neither cryptographic assurance (announcements are not signed) nor assurance of the AS Path of the announcement. This document describes requirements to be placed on a BGP security protocol, herein termed "BGPsec", intended to rectify these gaps. The threat model assumed here is documented in and . As noted in the threat model , this work is limited to threats to the BGP protocol. Issues of business relationship conformance, while quite important to operators, are not security issues per se and are outside the scope of this document. It is hoped that these issues will be better understood in the future.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 only when they appear in all upper case. They may also appear in lower or mixed case, without normative meaning.

This document assumes knowledge of the RPKI and the RPKI Repository Structure . This document assumes ongoing incremental deployment of Route Origin Authorizations (ROAs) , the RPKI to the Router Protocol , and RPKI-based Prefix Validation . And, of course, a knowledge of BGP is required.

The following are general requirements for a BGPsec protocol: A BGPsec design MUST allow the receiver of a BGP announcement to determine, to a strong level of certainty, that the originating AS in the received PATH attribute possessed the authority to announce the prefix. A BGPsec design MUST allow the receiver of a BGP announcement to determine, to a strong level of certainty, that the received PATH attribute accurately represents the sequence of External BGP (eBGP) exchanges that propagated the prefix from the origin AS to the receiver, particularly if an AS has added or deleted any AS number other than its own in the PATH attribute. This includes modification to the number of AS prepends. BGP attributes other than the AS_PATH are used only locally, or have meaning only between immediate neighbors, may be modified by intermediate systems and figure less prominently in the decision process. Consequently, it is not appropriate to try to protect such attributes in a BGPsec design. A BGPsec design MUST be amenable to incremental deployment. This implies that incompatible protocol capabilities MUST be negotiated. A BGPsec design MUST provide analysis of the operational considerations for deployment and particularly of incremental deployment, e.g., contiguous islands, non-contiguous islands, universal deployment, etc. As proofs of possession and authentication may require cryptographic payloads and/or storage and computation, likely increasing processing and memory requirements on routers, a BGPsec design MAY require use of new hardware. That is, compatibility with current hardware abilities is not a requirement that this document imposes on a solution. A BGPsec design need not prevent attacks on data-plane traffic. It need not provide assurance that the data plane even follows the control plane. A BGPsec design MUST resist attacks by an enemy who has access to the inter-router link layer, per Section 3.1.1.2 of . In particular, such a design MUST provide mechanisms for authentication of all data, including protecting against message insertion, deletion, modification, or replay. Mechanisms that suffice include TCP sessions authenticated with the TCP Authentication Option (TCP-AO) , IPsec , or Transport Layer Security (TLS) . It is assumed that a BGPsec design will require information about holdings of address space and Autonomous System Numbers (ASNs), and assertions about binding of address space to ASNs. A BGPsec design MAY make use of a security infrastructure (e.g., a PKI) to distribute such authenticated data. It is entirely OPTIONAL to secure AS SETs and prefix aggregation. The long-range solution to this is the deprecation of AS_SETs; see . If a BGPsec design uses signed prefixes, given the difficulty of splitting a signed message while preserving the signature, it need not handle multiple prefixes in a single UPDATE PDU. A BGPsec design MUST enable each BGPsec speaker to configure use of the security mechanism on a per-peer basis. A BGPsec design MUST provide backward compatibility in the message formatting, transmission, and processing of routing information carried through a mixed security environment. Message formatting in a fully secured environment MAY be handled in a non-backward compatible manner. While the formal validity of a routing announcement should be determined by the BGPsec protocol, local routing policy MUST be the final arbiter of the best path and other routing decisions. A BGPsec design MUST support 'transparent' route servers, meaning that the AS of the route server is not counted in downstream BGP AS-path-length tie-breaking decisions. A BGPsec design MUST support AS aliasing. This technique is not well defined or universally implemented but is being documented in . A BGPsec design SHOULD accommodate AS 'migration' techniques such as common proprietary and non-standard methods that allow a router to have two AS identities, without lengthening the effective AS Path. If a BGPsec design makes use of a security infrastructure, that infrastructure SHOULD enable each network operator to select the entities it will trust when authenticating data in the security infrastructure. See, for example, . A BGPsec design MUST NOT require operators to reveal more than is currently revealed in the operational inter-domain routing environment, other than the inclusion of necessary security credentials to allow others to ascertain for themselves the necessary degree of assurance regarding the validity of Network Layer Reachability Information (NLRI) received via BGPsec. This includes peering, customer/provider relationships, an ISP's internal infrastructure, etc. It is understood that some data are revealed to the savvy seeker by BGP, traceroute, etc., today. A BGPsec design MUST signal (e.g., via logging or SNMP) security exceptions that are significant to the operator. The specific data to be signaled are an implementation matter. Any routing information database MUST be re-authenticated periodically or in an event-driven manner, especially in response to events such as, for example, PKI updates. Any inter-AS use of cryptographic hashes or signatures MUST provide mechanisms for algorithm agility. For a discussion, see . A BGPsec design SHOULD NOT presume to know the intent of the originator of a NLRI, nor that of any AS on the AS Path, other than that they intend to pass it to the next AS in the path. A BGPsec listener SHOULD NOT trust non-BGPsec markings, such as communities, across trust boundaries.

The following requirements MUST be met in the processing of BGP UPDATE messages: A BGPsec design MUST enable each recipient of an UPDATE to formally validate that the origin AS in the message is authorized to originate a route to the prefix(es) in the message. A BGPsec design MUST enable the recipient of an UPDATE to formally determine that the NLRI has traversed the AS Path indicated in the UPDATE. Note that this is more stringent than showing that the path is merely not impossible. Replay of BGP UPDATE messages need not be completely prevented, but a BGPsec design SHOULD provide a mechanism to control the window of exposure to replay attacks. A BGPsec design SHOULD provide some level of assurance that the origin of a prefix is still 'alive', i.e., that a monkey in the middle has not withheld a WITHDRAW message or the effects thereof. The AS Path of an UPDATE message SHOULD be able to be authenticated as the message is processed. Normal sanity checks of received announcements MUST be done, e.g., verification that the first element of the AS_PATH list corresponds to the locally configured AS of the peer from which the UPDATE was received. The output of a router applying BGPsec validation to a received UPDATE MUST be unequivocal and conform to a fully specified state in the design.

If an external "security infrastructure" is used, as mentioned in Section 3, paragraphs and above, the authenticity and integrity of the data of such an infrastructure MUST be assured. In addition, the integrity of those data MUST be assured when they are used by BGPsec, e.g., in transport. The requirement of backward compatibility to BGP4 may open an avenue to downgrade attacks. The data plane might not follow the path signaled by the control plane. Security for subscriber traffic is outside the scope of this document and of BGP security in general. IETF standards for payload data security should be employed. While adoption of BGP security measures may ameliorate some classes of attacks on traffic, these measures are not a substitute for use of subscriber-based security.

The authors wish to thank the authors of from whom we liberally stole, Roque Gagliano, Russ Housley, Geoff Huston, Steve Kent, Sandy Murphy, Eric Osterweil, John Scudder, Kotikalapudi Sriram, Sam Weiler, and a number of others.