Route Leak Prevention and Detection using Roles in UPDATE and OPEN Messages
draft-ietf-idr-bgp-open-policy-17
Network Working Group A. Azimov
Internet-Draft Qrator Labs & Yandex
Intended status: Standards Track E. Bogomazov
Expires: April 16, 2022 Qrator Labs
R. Bush
Internet Initiative Japan & Arrcus, Inc.
K. Patel
Arrcus
K. Sriram
USA NIST
October 13, 2021
Route Leak Prevention and Detection using Roles in UPDATE and OPEN
Messages
draft-ietf-idr-bgp-open-policy-17
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 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.
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.
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Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on April 16, 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Peering Relationships . . . . . . . . . . . . . . . . . . . . 4
3. BGP Role . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. BGP Role Capability . . . . . . . . . . . . . . . . . . . 5
3.2. Role Correctness . . . . . . . . . . . . . . . . . . . . 6
4. BGP Only to Customer (OTC) Attribute . . . . . . . . . . . . 7
5. Additional Considerations . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
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Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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 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).
1.1. Terminology
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.
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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."
2. Peering Relationships
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:
Provider: MAY propagate any available route to a Customer.
Customer: MAY propagate any route learned from a Customer, or
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).
RS-Client: MAY propagate any route learned from a Customer, or
locally originated, to an RS. All other routes MUST NOT be
propagated.
Peer: MAY propagate any route learned from a Customer, or locally
originated, to a Peer. All other routes MUST NOT be propagated.
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.
3. 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 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).
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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:
o Provider - the local AS is a transit Provider of the remote AS;
o Customer - the local AS is a transit Customer of the remote AS;
o RS - the local AS is a Route Server (usually at an Internet
exchange point) and the remote AS is its RS-Client;
o RS-Client - the local AS is a client of an RS and the RS is the
remote AS;
o Peer - the local and remote ASes are Peers (i.e., have a lateral
peering relationship).
3.1. BGP Role Capability
The BGP Role Capability is defined as follows:
o Code - 9
o Length - 1 (octet)
o Value - integer corresponding to 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 (Lateral Peer) |
| 5-255 | Unassigned |
+-------+------------------------------+
Table 1: Predefined BGP Role Values
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.
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3.2. Role Correctness
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 |
+---------------+----------------+
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 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.
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4. BGP Only to Customer (OTC) Attribute
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:
1. If a route with the OTC Attribute is received from a Customer or
RS-Client, then it is a route leak and MUST be considered
ineligible (see Section 1.1).
2. If a route with the OTC Attribute is received from a Peer and at
least one of the OTC Attributes 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, Peer, or 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 policy applies to the processing of the OTC
Attribute:
1. If a route is to be advertised to a Customer, Peer, or RS-Client
(when the sender is an RS), and the OTC Attribute is not present,
then 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 RS(s).
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
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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.
5. Additional Considerations
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.
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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.
6. IANA Considerations
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 |
+-------+--------------------------------+---------------+
Table 3: IANA Registry for BGP Role
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.
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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.
7. Security Considerations
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.
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8. References
8.1. 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>.
[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>.
[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>.
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8.2. Informative References
[Gao] 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>.
[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, Andrei Robachevsky, Daniel
Ginsburg, Jeff Haas, Ruediger Volk, Pavel Lunin, Gyan Mishra, Ignas
Bagdonas, Sue Hares, and John Scudder for comments, suggestions, and
critique.
Contributors
Brian Dickson
Independent
Email: brian.peter.dickson@gmail.com
Doug Montgomery
USA National Institute of Standards and Technology
Email: dougm@nist.gov
Authors' Addresses
Alexander Azimov
Qrator Labs & Yandex
Ulitsa Lva Tolstogo 16
Moscow 119021
Russian Federation
Email: a.e.azimov@gmail.com
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Eugene Bogomazov
Qrator Labs
1-y Magistralnyy tupik 5A
Moscow 123290
Russian Federation
Email: eb@qrator.net
Randy Bush
Internet Initiative Japan & Arrcus, Inc.
5147 Crystal Springs
Bainbridge Island, Washington 98110
United States of America
Email: randy@psg.com
Keyur Patel
Arrcus
2077 Gateway Place, Suite #400
San Jose, CA 95119
US
Email: keyur@arrcus.com
Kotikalapudi Sriram
USA National Institute of Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899
United States of America
Email: ksriram@nist.gov
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