Internet Engineering Task Force (IETF) A. Azimov

Internet Engineering Task Force (IETF) A. Azimov
Request for Comments: 9234 Qrator Labs & Yandex
Category: Standards Track E. Bogomazov
ISSN: 2070-1721 Qrator Labs
R. Bush
IIJ & Arrcus
K. Patel
Arrcus
K. Sriram
USA NIST
May 2022

Route Leak Prevention and Detection Using Roles in UPDATE and OPEN
Messages

Abstract

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 External BGP (eBGP) neighbor, or
enforcement of the two eBGP speakers agreeing on the peering
relationship. This document enhances the BGP OPEN message to
establish an agreement of the peering 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.

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 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9234.

Copyright Notice

Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
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Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. Requirements Language
3. Terminology
3.1. Peering Relationships
4. BGP Role
4.1. BGP Role Capability
4.2. Role Correctness
5. BGP Only to Customer (OTC) Attribute
6. Additional Considerations
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

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 of the two
eBGP speakers agreeing 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 specifies a means of replacing the operator-driven
configuration-based method of route leak prevention, described above,
with an in-band method for route leak prevention and detection.

This method uses a new configuration parameter, BGP Role, which is
negotiated using a BGP Role Capability in the OPEN message [RFC5492].
An eBGP speaker may require the use of this capability and
confirmation of the 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 5. 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).

2. Requirements Language

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.

3. Terminology

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."

3.1. Peering Relationships

The terms for peering relationships 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-REXFORD]. The terms "Provider", "Customer", and
"Peer" used here are synonymous to the terms "transit provider",
"customer", and "lateral (i.e., non-transit) peer", respectively,
used in [RFC7908].

The following is a list of BGP Roles for eBGP peering and the
corresponding rules for route propagation:

Provider: MAY propagate any available route to a Customer.

Customer: MAY propagate any route learned from a Customer, or that
is locally originated, to a Provider. All other routes MUST NOT
be propagated.

Route Server (RS): MAY propagate any available route to a Route
Server Client (RS-Client).

Route Server Client (RS-Client): MAY propagate any route learned
from a Customer, or that is locally originated, to an RS. All
other routes MUST NOT be propagated.

Peer: MAY propagate any route learned from a Customer, or that is
locally originated, to a Peer. All other routes MUST NOT be
propagated.

If the local AS has one of the above Roles (in the order shown), then
the corresponding peering relationship with the remote AS is
Provider-to-Customer, Customer-to-Provider, RS-to-RS-Client, RS-
Client-to-RS, or Peer-to-Peer (i.e., lateral peers), respectively.
These are called normal peering relationships.

If the local AS has more than one peering Role with the remote AS,
such a peering relation is called "Complex". An example is when the
peering relationship is Provider-to-Customer for some prefixes while
it is Peer-to-Peer for other prefixes [GAO-REXFORD].

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 route propagation rules listed above may result in
route leaks [RFC7908]. Automatic enforcement of these rules should
significantly reduce route leaks that may otherwise occur due to
manual configuration mistakes.

As specified in Section 5, the OTC Attribute is used to identify all
the routes in the AS that have been received from a Peer, a Provider,
or an RS.

4. BGP Role

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 3) based on the local AS's knowledge of its Role. The only
exception is when the eBGP connection is Complex (see Section 6).
BGP Roles are mutually confirmed using the BGP Role Capability
(described in Section 4.1) on each eBGP session.

Allowed Roles for eBGP sessions are:

Provider: the local AS is a transit provider of the remote AS;

Customer: the local AS is a transit customer of the remote AS;

RS: the local AS is a Route Server (usually at an Internet exchange
point), and the remote AS is its RS-Client;

RS-Client: the local AS is a client of an RS and the RS is the
remote AS; and

Peer: the local and remote ASes are Peers (i.e., have a lateral
peering relationship).

4.1. BGP Role Capability

The BGP Role Capability is defined as follows:

Code: 9

Length: 1 (octet)

Value: integer corresponding to the speaker's BGP Role (see Table 1)

+=======+==============================+
| Value | Role name (for the local AS) |
+=======+==============================+
| 0 | Provider |
+-------+------------------------------+
| 1 | RS |
+-------+------------------------------+
| 2 | RS-Client |
+-------+------------------------------+
| 3 | Customer |
+-------+------------------------------+
| 4 | Peer (i.e., Lateral Peer) |
+-------+------------------------------+
| 5-255 | Unassigned |
+-------+------------------------------+

Table 1: Predefined BGP Role Values

If the BGP Role is locally configured, the eBGP speaker MUST
advertise the BGP Role Capability in the BGP OPEN message. An eBGP
speaker MUST NOT advertise multiple versions of the BGP Role
Capability. The error handling when multiple BGP Role Capabilities
are received is described in Section 4.2.

4.2. Role Correctness

Section 4.1 describes how the BGP Role encodes the relationship on
each eBGP session between ASes.

The mere receipt of the 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 11).

+===============+================+
| Local AS Role | Remote AS Role |
+===============+================+
| Provider | Customer |
+---------------+----------------+
| Customer | Provider |
+---------------+----------------+
| RS | RS-Client |
+---------------+----------------+
| RS-Client | RS |
+---------------+----------------+
| Peer | Peer |
+---------------+----------------+

Table 2: Allowed Pairs of Role
Capabilities

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 5.

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, the connection is rejected using the Role
Mismatch Notification (code 2, subcode 11). See comments in
Section 8.

If an eBGP speaker receives multiple but identical BGP Role
Capabilities with the same value in each, then the speaker considers
them to be a single BGP Role Capability and proceeds [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 11).

The BGP Role value for the local AS (in conjunction with the OTC
Attribute in the received UPDATE message) is used in the route leak
prevention and detection procedures described in Section 5.

5. BGP Only to Customer (OTC) Attribute

The OTC Attribute is an optional transitive Path Attribute of the
UPDATE message with Attribute Type Code 35 and a length of 4 octets.
The purpose of this attribute is to enforce that once a route is sent
to a Customer, a Peer, or an RS-Client (see definitions in
Section 3.1), it will subsequently go only to the Customers. The
attribute value is an AS number (ASN) determined by the procedures
described below.

The following ingress procedure applies to the processing of the OTC
Attribute on route receipt:

1. If a route with the OTC Attribute is received from a Customer or
an RS-Client, then it is a route leak and MUST be considered
ineligible (see Section 3).

2. If a route with the OTC Attribute is received from a Peer (i.e.,
remote AS with a Peer Role) and the Attribute has a value that is
not equal to the remote (i.e., Peer's) AS number, then it is a
route leak and MUST be considered ineligible.

3. If a route is received from a Provider, a Peer, or an RS and the
OTC Attribute is not present, then it MUST be added with a value
equal to the AS number of the remote AS.

The following egress procedure applies to the processing of the OTC
Attribute on route advertisement:

1. If a route is to be advertised to a Customer, a Peer, or an RS-
Client (when the sender is an RS), and the OTC Attribute is not
present, then when advertising the route, an OTC Attribute MUST
be added with a value equal to the AS number of the local AS.

2. If a route already contains the OTC Attribute, it MUST NOT be
propagated to Providers, Peers, or RSes.

The above-described procedures 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, a Provider, or an RS, and it can be advertised only to
the Customers. The same OTC Attribute that 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, a Peer, or an RS-Client. For
example, if an AS sets the OTC Attribute on a route sent to a Peer
and the route is subsequently received by a compliant AS from a
Customer, then the receiving AS detects (based on the presence of the
OTC Attribute) that the route is a leak.

The OTC Attribute might be set at the egress of the remote AS or at
the ingress of the local AS, i.e., if the remote AS is non-compliant
with this specification, then the local AS will have to set the OTC
Attribute if it is absent. In both scenarios, the OTC value will be
the same. This makes the scheme more robust and benefits early
adopters.

The OTC Attribute is considered malformed if the length value is not
4. An UPDATE message with a malformed OTC Attribute SHALL be handled
using the approach of "treat-as-withdraw" [RFC7606].

The BGP Role negotiation and OTC-Attribute-based 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).

The described ingress and egress procedures are applicable only for
the address families AFI 1 (IPv4) and AFI 2 (IPv6) with SAFI 1
(unicast) in both cases and MUST NOT be applied to other address
families by default. The operator MUST NOT have the ability to
modify the procedures defined in this section.

6. Additional Considerations

Roles MUST NOT be configured on an eBGP session with a Complex
peering relationship. If multiple eBGP sessions can segregate the
Complex peering relationship into eBGP sessions with normal peering
relationships, BGP Roles SHOULD be used on each of the resulting eBGP
sessions.

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 3.1. However, in this case, there
are no in-band 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 an incorrect AS number in the OTC Attribute.

In AS migration scenarios [RFC7705], a given router may represent
itself as any one of several different ASes. This should not be a
problem since the egress procedures in Section 5 specify that the OTC
Attribute is to be attached as part of route transmission.
Therefore, a router is expected to set the OTC value equal to the ASN
it is currently representing itself as.

Section 6 of [RFC7606] documents possible negative impacts of "treat-
as-withdraw" behavior. Such negative impacts may include forwarding
loops or dropped traffic. It also discusses debugging considerations
related to this behavior.

7. IANA Considerations

IANA has registered a new BGP Capability (Section 4.1) in the
"Capability Codes" registry within the "IETF Review" range [RFC5492].
The description for the new capability is "BGP Role". IANA has
assigned the value 9. This document is the reference for the new
capability.

IANA has created and now maintains a new subregistry called "BGP Role
Value" within the "Capability Codes" registry. Registrations should
include a value, a role name, and a reference to the defining
document. IANA has registered the values in Table 3. Future
assignments may be made by the "IETF Review" policy as defined in
[RFC8126].

+=======+===============================+===============+
| 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 | |
+-------+-------------------------------+---------------+

Table 3: IANA Registry for BGP Role

IANA has registered a new OPEN Message Error subcode named "Role
Mismatch" (see Section 4.2) in the "OPEN Message Error subcodes"
registry. IANA has assigned the value 11. This document is the
reference for the new subcode.

Due to improper use of the values 8, 9, and 10, IANA has marked
values 8-10 as "Deprecated" in the "OPEN Message Error subcodes"
registry. This document is listed as the reference.

IANA has also registered a new Path Attribute named "Only to Customer
(OTC)" (see Section 5) in the "BGP Path Attributes" registry. IANA
has assigned code value 35. This document is the reference for the
new attribute.

8. Security Considerations

The security considerations of BGP (as specified in [RFC4271] and
[RFC4272]) apply.

This document proposes a mechanism that uses the 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, it would 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,
it 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 for 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 that 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.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.

[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065,
DOI 10.17487/RFC5065, August 2007,
<https://www.rfc-editor.org/info/rfc5065>.

[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>.

[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.

[RFC7908] Sriram, K., Montgomery, D., McPherson, D., Osterweil, E.,
and B. Dickson, "Problem Definition and Classification of
BGP Route Leaks", RFC 7908, DOI 10.17487/RFC7908, June
2016, <https://www.rfc-editor.org/info/rfc7908>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2. Informative References

[GAO-REXFORD]
Gao, L. and J. Rexford, "Stable Internet routing without
global coordination", IEEE/ACM Transactions on Networking,
Volume 9, Issue 6, pp. 689-692, DOI 10.1109/90.974523,
December 2001,
<https://ieeexplore.ieee.org/document/974523>.

[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.

[RFC7705] George, W. and S. Amante, "Autonomous System Migration
Mechanisms and Their Effects on the BGP AS_PATH
Attribute", RFC 7705, DOI 10.17487/RFC7705, November 2015,
<https://www.rfc-editor.org/info/rfc7705>.

[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.

[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.

Acknowledgments

The authors wish to thank Alvaro Retana, Bruno Decraene, Jeff Haas,
John Scudder, Sue Hares, Ben Maddison, Andrei Robachevsky, Daniel
Ginsburg, Ruediger Volk, Pavel Lunin, Gyan Mishra, and Ignas Bagdonas
for their reviews, comments, and suggestions during the course of
this work. Thanks are also due to many IESG reviewers whose comments
greatly helped improve the clarity, accuracy, and presentation in the
document.

Contributors

Brian Dickson
Independent
Email: [email protected]

Doug Montgomery
USA National Institute of Standards and Technology
Email: [email protected]

Authors' Addresses

Alexander Azimov
Qrator Labs & Yandex
Ulitsa Lva Tolstogo 16
Moscow
119021
Russian Federation
Email: [email protected]

Eugene Bogomazov
Qrator Labs
1-y Magistralnyy tupik 5A
Moscow
123290
Russian Federation
Email: [email protected]

Randy Bush
Internet Initiative Japan & Arrcus, Inc.
5147 Crystal Springs
Bainbridge Island, Washington 98110
United States of America
Email: [email protected]

Keyur Patel
Arrcus
2077 Gateway Place
Suite #400
San Jose, CA 95119
United States of America
Email: [email protected]

Kotikalapudi Sriram
USA National Institute of Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899
United States of America
Email: [email protected]