| Internet-Draft | Route Leak Prevention | December 2021 |
| Azimov, et al. | Expires 7 June 2022 | [Page] |
Route leaks are the propagation of BGP prefixes that violate assumptions of BGP topology relationships, e.g., announcing a route learned from one transit provider to another transit provider or a lateral (i.e., non-transit) peer or announcing a route learned from one lateral peer to another lateral peer or a transit provider. These are usually the result of misconfigured or absent BGP route filtering or lack of coordination between autonomous systems (ASes). Existing approaches to leak prevention rely on marking routes by operator configuration, with no check that the configuration corresponds to that of the eBGP neighbor, or enforcement that the two eBGP speakers agree on the relationship. This document enhances the BGP OPEN message to establish an agreement of the relationship on each eBGP session between autonomous systems in order to enforce appropriate configuration on both sides. Propagated routes are then marked according to the agreed relationship, allowing both prevention and detection of route leaks.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
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Route leaks are the propagation of BGP prefixes that violate assumptions of BGP topology relationships, e.g., announcing a route learned from one transit provider to another transit provider or a lateral (i.e., non-transit) peer or announcing a route learned from one lateral peer to another lateral peer or a transit provider [RFC7908]. These are usually the result of misconfigured or absent BGP route filtering or lack of coordination between autonomous systems (ASes).¶
Existing approaches to leak prevention rely on marking routes by operator configuration, with no check that the configuration corresponds to that of the eBGP neighbor, or enforcement that the two eBGP speakers agree on the relationship. This document enhances the BGP OPEN message to establish an agreement of the relationship on each eBGP session between autonomous systems in order to enforce appropriate configuration on both sides. Propagated routes are then marked according to the agreed relationship, allowing both prevention and detection of route leaks.¶
This document provides configuration automation using BGP Roles, which are negotiated using a BGP Role Capability in the OPEN message [RFC5492]. An eBGP speaker may require the use of this capability and confirmation of BGP Role with a neighbor for the BGP OPEN to succeed.¶
An optional, transitive BGP Path Attribute, called Only to Customer (OTC), is specified in Section 4. It prevents ASes from creating leaks and detects leaks created by the ASes in the middle of an AS path. The main focus/applicability is the Internet (IPv4 and IPv6 unicast route advertisements).¶
In the rest of this document, the term "Peer" is used to refer to a "lateral (i.e., non-transit) peer" for simplicity. Also, the terms Provider and Customer are used to refer to a transit provider and a transit customer, respectively. Further, the terms RS and RS-Client are used to refer to a Route Server and its client, respectively.¶
The terms "local AS" and "remote AS" are used to refer to the two ends of an eBGP session. The "local AS" is the AS where the protocol action being described is to be performed, and "remote AS" is the AS at the other end of the eBGP session in consideration.¶
The use of the term "route is ineligible" in this document has the same meaning as in [RFC4271], i.e., "route is ineligible to be installed in Loc-RIB and will be excluded from the next phase of route selection."¶
The terms defined and used in this document (see below) do not necessarily represent business relationships based on payment agreements. These terms are used to represent restrictions on BGP route propagation, sometimes known as the Gao-Rexford model [Gao]. The following is a list of BGP Roles for eBGP peering and the corresponding rules for route propagation:¶
A BGP speaker may apply policy to reduce what is announced, and a recipient may apply policy to reduce the set of routes they accept. Violation of the above rules may result in route leaks. Automatic enforcement of these rules should significantly reduce route leaks that may otherwise occur due to manual configuration mistakes.¶
As specified in Section 4, the Only to Customer (OTC) Attribute is used to identify all the routes in the AS that have been received from a Peer, Provider, or RS.¶
The BGP Role characterizes the relationship between the eBGP speakers forming a session. One of the Roles described below SHOULD be configured at the local AS for each eBGP session (see definitions in Section 1.1) based on the local AS's knowledge of its Role. The only exception is when the eBGP connection is 'complex' (see Section 5). BGP Roles are mutually confirmed using the BGP Role Capability (described in Section 3.1) on each eBGP session.¶
Allowed Roles for eBGP sessions are:¶
The BGP Role Capability is defined as follows:¶
| Value | Role name (for the local AS) |
|---|---|
| 0 | Provider |
| 1 | RS |
| 2 | RS-Client |
| 3 | Customer |
| 4 | Peer (Lateral Peer) |
| 5-255 | Unassigned |
If BGP Role is locally configured, the eBGP speaker MUST advertise BGP Role Capability in the BGP OPEN message. An eBGP speaker MUST NOT advertise multiple versions of the BGP Role Capability.¶
Section 3.1 described how BGP Role encodes the relationship on each eBGP session between autonomous systems (ASes).¶
The mere receipt of BGP Role Capability does not automatically guarantee the Role agreement between two eBGP neighbors. If the BGP Role Capability is advertised, and one is also received from the peer, the roles MUST correspond to the relationships in Table 2. If the roles do not correspond, the BGP speaker MUST reject the connection using the Role Mismatch Notification (code 2, subcode 8).¶
| Local AS Role | Remote AS Role |
|---|---|
| Provider | Customer |
| Customer | Provider |
| RS | RS-Client |
| RS-Client | RS |
| Peer | Peer |
For backward compatibility, if the BGP Role Capability is sent but one is not received, the BGP Speaker SHOULD ignore the absence of the BGP Role Capability and proceed with session establishment. The locally configured BGP Role is used for the procedures described in Section 4.¶
An operator may choose to apply a "strict mode" in which the receipt of a BGP Role Capability from the remote AS is required. When operating in the "strict mode", if the BGP Role Capability is sent, but one is not received, then the connection is rejected using the Role Mismatch Notification (code 2, subcode 8). See comments in Section 7.¶
If an eBGP speaker receives multiple but identical BGP Role Capabilities with the same value in each, then the speaker must consider it to be a single BGP Role Capability and proceed [RFC5492]. If multiple BGP Role Capabilities are received and not all of them have the same value, then the BGP speaker MUST reject the connection using the Role Mismatch Notification (code 2, subcode 8).¶
The BGP Role value for the local AS is used in the route leak prevention and detection procedures described in Section 4.¶
The Only to Customer (OTC) Attribute is an optional transitive path attribute with Attribute Type Code 35 and a length of 4 octets. The purpose of this attribute is to guarantee that once a route is sent to a Customer, Peer, or RS-Client, it will subsequently go only to Customers. The attribute value is an AS number (ASN) determined by the policy described below.¶
The following ingress policy applies to the processing of the OTC Attribute:¶
The following egress policy applies to the processing of the OTC Attribute:¶
The described policies provide both leak prevention for the local AS and leak detection and mitigation multiple hops away. In the case of prevention at the local AS, the presence of an OTC Attribute indicates to the egress router that the route was learned from a Peer, Provider, or RS, and it can be advertised only to the customers. The same OTC Attribute which is set locally also provides a way to detect route leaks by an AS multiple hops away if a route is received from a Customer, Peer, or RS-Client.¶
The OTC Attribute may be set by the egress policy of the remote AS or by the ingress policy of the local AS. In both scenarios, the OTC value will be the same. This makes the scheme more robust and benefits early adopters.¶
If an eBGP speaker receives an UPDATE with an OTC Attribute with a length different from 4 octets, then the UPDATE SHALL be considered malformed. If malformed, the UPDATE message SHALL be handled using the approach of "treat-as-withdraw" [RFC7606].¶
The procedures specified in this document are NOT RECOMMENDED to be used between autonomous systems in an AS Confederation [RFC5065]. If an OTC Attribute is added on egress from the AS Confederation, its value MUST equal the AS Confederation Identifier. Also, on egress from the AS Confederation, an UPDATE MUST NOT contain an OTC Attribute with a value corresponding to any Member-AS Number other than the AS Confederation Identifier.¶
The procedures specified in this document in scenarios that use private AS numbers behind an Internet-facing ASN (e.g., a data center network [RFC7938] or stub customer) may be used, but any details are outside the scope of this document. On egress from the Internet-facing AS, the OTC Attribute MUST NOT contain a value other than the Internet-facing ASN.¶
Once the OTC Attribute has been set, it MUST be preserved unchanged (this also applies to an AS Confederation).¶
Correct implementation of the procedures specified in this document is not expected to result in the presence of multiple OTC Attributes in an UPDATE. However, if an eBGP speaker receives multiple OTC Attributes with a route, then the only difference in the processing is in Step 2 of the ingress policy.¶
The described ingress and egress policies are applicable only for unicast IPv4 and IPv6 address families and MUST NOT be applied to other address families by default. The operator MUST NOT have the ability to modify the policies defined in this section.¶
There are peering relationships that are 'complex', i.e., both parties intentionally advertise prefixes received from each other to their Peers and/or transit Providers. If multiple eBGP sessions can segregate the 'complex' parts of the relationship, then the complex peering roles can be segregated into different normal eBGP sessions, and BGP Roles MUST be used on each of the resulting normal (non-complex) eBGP sessions.¶
No Roles SHOULD be configured on a 'complex' eBGP session (assuming it is not segregated). An operator may want to achieve an equivalent outcome by configuring policies on a per-prefix basis to follow the definitions of peering relations as described in Section 2. However, in this case, there are no built-in measures to check the correctness of the per-prefix peering configuration.¶
The incorrect setting of BGP Roles and/or OTC Attributes may affect prefix propagation. Further, this document does not specify any special handling of incorrect AS numbers in the OTC Attribute.¶
IANA has registered a new BGP Capability (Section 3.1) in the "Capability Codes" registry's "IETF Review" range [RFC5492]. The description for the new capability is "BGP Role". IANA has assigned the value 9 [to be removed upon publication: https://www.iana.org/assignments/capability-codes/capability-codes.xhtml]. This document is the reference for the new capability.¶
The BGP Role capability includes a Value field, for which IANA is requested to create and maintain a new sub-registry called "BGP Role Value" in the Capability Codes registry. Assignments consist of a Value and a corresponding Role name. Initially, this registry is to be populated with the data contained in Table 1 found in Section 3.1. Future assignments may be made by the "IETF Review" policy as defined in [RFC8126]. The registry is as shown in Table 3.¶
| Value | Role name (for the local AS) | Reference |
|---|---|---|
| 0 | Provider | This document |
| 1 | RS | This document |
| 2 | RS-Client | This document |
| 3 | Customer | This document |
| 4 | Peer (i.e., Lateral Peer) | This document |
| 5-255 | To be assigned by IETF Review |
IANA has registered a new OPEN Message Error subcode named the "Role Mismatch" (see Section 3.2) in the OPEN Message Error subcodes registry. IANA has assigned the value 8 [to be removed upon publication: https://www.iana.org/assignments/bgp-parameters/bgp-parameters.xhtml#bgp-parameters-6]. This document is the reference for the new subcode.¶
IANA has also registered a new path attribute named "Only to Customer (OTC)" (see Section 4) in the "BGP Path Attributes" registry. IANA has assigned code value 35 [To be removed upon publication: http://www.iana.org/assignments/bgp-parameters/bgp-parameters.xhtml#bgp-parameters-2]. This document is the reference for the new attribute.¶
The security considerations of BGP (as specified in [RFC4271] and [RFC4272]) apply.¶
This document proposes a mechanism using BGP Role for the prevention and detection of route leaks that are the result of BGP policy misconfiguration. A misconfiguration of the BGP Role may affect prefix propagation. For example, if a downstream (i.e., towards a Customer) peering link were misconfigured with a Provider or Peer role, this will limit the number of prefixes that can be advertised in this direction. On the other hand, if an upstream provider were misconfigured (by a local AS) with the Customer role, this may result in propagating routes that are received from other Providers or Peers. But the BGP Role negotiation and the resulting confirmation of Roles make such misconfigurations unlikely.¶
Setting the strict mode of operation for BGP Role negotiation as the default may result in a situation where the eBGP session will not come up after a software update. Implementations with such default behavior are strongly discouraged.¶
Removing the OTC Attribute or changing its value can limit the opportunity of route leak detection. Such activity can be done on purpose as part of an on-path attack. For example, an AS can remove the OTC Attribute on a received route and then leak the route to its transit provider. This kind of threat is not new in BGP and it may affect any Attribute (Note: BGPsec [RFC8205] offers protection only for the AS_PATH Attribute).¶
Adding an OTC Attribute when the route is advertised from Customer to Provider will limit the propagation of the route. Such a route may be considered as ineligible by the immediate Provider or its Peers or upper layer Providers. This kind of OTC Attribute addition is unlikely to happen on the Provider side because it will limit the traffic volume towards its Customer. On the Customer side, adding an OTC Attribute for traffic engineering purposes is also discouraged because it will limit route propagation in an unpredictable way.¶
The authors wish to thank Alvaro Retana, Andrei Robachevsky, Daniel Ginsburg, Jeff Haas, Ruediger Volk, Pavel Lunin, Gyan Mishra, Ignas Bagdonas, Sue Hares, and John Scudder for comments, suggestions, and critique.¶
Brian Dickson Independent Email: brian.peter.dickson@gmail.com Doug Montgomery USA National Institute of Standards and Technology Email: dougm@nist.gov¶