Network Working Group A. Azimov
Internet-Draft E. Bogomazov
Intended status: Standards Track Qrator Labs
Expires: July 12, 2020 R. Bush
Internet Initiative Japan & Arrcus
K. Patel
Arrcus, Inc.
K. Sriram
US NIST
January 9, 2020
Route Leak Prevention using Roles in Update and Open messages
draft-ietf-idr-bgp-open-policy-07
Abstract
Route leaks are the propagation of BGP prefixes which violate
assumptions of BGP topology relationships; e.g. passing a route
learned from one peer to another peer or to a transit provider,
passing a route learned from one transit provider to another transit
provider or to a peer. Today, approaches to leak prevention rely on
marking routes by operator configuration, with no check that the
configuration corresponds to that of the BGP neighbor, or enforcement
that the two BGP speakers agree on the relationship. This document
enhances BGP OPEN to establish agreement of the (peer, customer,
provider, Route Server, Route Server client) relationship of two
neighboring BGP speakers to enforce appropriate configuration on both
sides. Propagated routes are then marked with an OTC attribute
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.
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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working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 12, 2020.
Copyright Notice
Copyright (c) 2020 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Peering Relationships . . . . . . . . . . . . . . . . . . . . 3
3. BGP Role . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. BGP Role Capability . . . . . . . . . . . . . . . . . . . . . 4
5. Role correctness . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Strict mode . . . . . . . . . . . . . . . . . . . . . . . 6
6. BGP Only to Customer (OTC) Attribute . . . . . . . . . . . . 6
7. Enforcement . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Additional Considerations . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
12.1. Normative References . . . . . . . . . . . . . . . . . . 8
12.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
BGP route leak occurs when a route is learned from a transit provider
or peer and then announced to another provider or peer. See
[RFC7908]. These are usually the result of misconfigured or absent
BGP route filtering or lack of coordination between two BGP speakers.
The mechanism proposed in
[I-D.ietf-grow-route-leak-detection-mitigation] uses large-
communities to perform detection and mitigation of route leaks.
While signaling using communities is easy to implement and deploy
quickly, it normally relies on operator-maintained policy
configuration, which is often vulnerable to misconfiguration. There
is also the vulnerability that the community signal may be stripped,
accidentally or maliciously.
This document provides configuration automation using 'BGP roles',
which are negotiated using a new BGP Capability Code in OPEN message
(see Section 4 in [RFC5492]). Either or both BGP speakers MAY be
configured to require that this capability be agreed for the BGP OPEN
to succeed.
A new BGP Path Attribute is specified that SHOULD be automatically
configured using BGP roles. This attribute prevents networks from
creating leaks, and detects leaks created by third parties.
2. Peering Relationships
Despite the use of terms such as "customer", "peer", etc. in this
document, these are not necessarily 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 various roles in BGP peering and
the corresponding rules for route propagation:
Provider: MAY send to a customer all available prefixes.
Customer: MAY send to a provider their own prefixes and prefixes
learned from any of their customers. A customer MUST NOT send to
a provider prefixes learned from its peers, from other providers,
or from Route Servers.
Route Server (RS): MAY send to an RS Client all available prefixes.
Route Server Client (RS-client): MAY send to an RS its own prefixes
and prefixes learned from its customers. An RS-client MUST NOT
send to an RS prefixes learned from its peers or providers, or
from another RS.
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Peer: MAY send to a peer its own prefixes and prefixes learned from
its customers. A peer MUST NOT send to a peer prefixes learned
from other peers, from its providers, or from RS(s).
Of course, any 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 and MUST not be allowed. Automatic enforcement of these rules
should significantly reduce route leaks that may otherwise occur due
to manual configuration mistakes. While enforcing the above rules
will address most BGP peering scenarios, their configuration is not
part of BGP itself; therefore, additionally requiring configuration
of ingress and egress prefix filters is still strongly advised.
3. BGP Role
BGP Role is new configuration option that SHOULD be configured on
each BGP session. It reflects the real-world agreement between two
BGP speakers about their relationship.
Allowed Role values for eBGP sessions are:
o Provider - sender is a transit provider to neighbor;
o Customer - sender is a transit customer of neighbor;
o RS - sender is a Route Server, usually at an Internet exchange
point (IX);
o RS-Client - sender is client of an RS;
o Peer - sender and neighbor are peers.
Since BGP Role reflects the relationship between two BGP speakers, it
could also be used for other purposes besides route leak mitigation.
4. BGP Role Capability
The TLV (type, length, value) of the BGP Role capability are:
o Type - <TBD1>;
o Length - 1 (octet);
o Value - integer corresponding to speaker's BGP Role.
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+-------+---------------------+
| Value | Role name |
+-------+---------------------+
| 0 | Sender is Provider |
| 1 | Sender is RS |
| 2 | Sender is RS-Client |
| 3 | Sender is Customer |
| 4 | Sender is Peer |
+-------+---------------------+
Table 1: Predefined BGP Role Values
5. Role correctness
Section 3 described how BGP Role encodes the relationship between two
BGP speakers. But the mere presence of BGP Role doesn't
automatically guarantee role agreement between two BGP peers.
To enforce correctness, the BGP Role check is applied with a set of
constraints on how speakers' BGP Roles MUST correspond. Of course,
each speaker MUST announce and accept the BGP Role capability in the
BGP OPEN message exchange.
If a speaker receives a BGP Role capability, it MUST check the value
of the received capability with its own BGP Role (if it is set). The
allowed pairings are (first a sender's Role, second the receiver's
Role):
+-------------+---------------+
| Sender Role | Receiver Role |
+-------------+---------------+
| Provider | Customer |
| Customer | Provider |
| RS | RS-Client |
| RS-Client | RS |
| Peer | Peer |
+-------------+---------------+
Table 2: Allowed Role Capabilities
If the observed Role pair is not in the above table, then the
receiving speaker MUST send a Role Mismatch Notification (code 2,
subcode <TBD2>).
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5.1. Strict mode
A new BGP configuration option "strict mode" is defined with values
of true or false. If set to true, then the speaker MUST refuse to
establish a BGP session with a neighbor which does not announce the
BGP Role capability in the OPEN message. If a speaker rejects a
connection, it MUST send a Connection Rejected Notification [RFC4486]
(Notification with error code 6, subcode 5). By default, strict mode
SHOULD be set to false for backward compatibility with BGP speakers
that do not yet support this mechanism.
6. BGP Only to Customer (OTC) Attribute
Newly defined here, the Only to Customer (OTC) is an optional,
transitive BGP Path attribute with the Type Code <TBD3>. The OTC
attribute is four bytes long and its value equals an AS number. The
semantics and usage of the OTC attribute are made clear by the
ingress and egress policies described below.
The following ingress policy applies to the OTC attribute:
1. If a route with OTC attribute is received from a Customer or RS-
client, then it is a route leak and MUST be rejected.
2. If a route with OTC attribute is received from a Peer and its
value is not equal to the neighbor's ASN, then it is a route leak
and MUST be rejected.
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 value equal
to the neighbor's AS number.
The egress policy MUST be:
1. A route with the OTC attribute set MUST NOT be sent to providers,
peers, or RS(s).
2. If route is sent to a customer or peer, or an RS clien and the
OTC attribute is not present, then it MUST be added with value
equal to AS number of the sender.
Once the OTC attribute has been set, it MUST be preserved unchanged.
7. Enforcement
Having the relationship unequivocally agreed between the two peers in
BGP OPEN is critical; BGP implementations MUST enforce the
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relationship/role establishment rules (see Section 5) in order to
overcome operator policy configuration errors (if any).
Similarly, the application of that relationship on prefix propagation
using OTC MUST BE enforced by the BGP implementations, and not
exposed to user mis-configuration.
As opposed to communities, BGP attributes may not be generally
modified or filtered by the operator; BGP router implementations
enforce such treatment. This is the desired property for the OTC
marking. Hence, this document specifies OTC as an attribute.
8. Additional Considerations
There are peering relationships that are 'complex', i.e., both
parties are intentionally sending prefixes received from each other
to their non-transit peers and/or transit providers. If multiple BGP
peerings can segregate the 'complex' parts of the relationship, the
complex peering roles can be segregated into different normal BGP
sessions, and BGP Roles MUST be used on each of the resulting normal
(non-complex) BGP sessions.
No Roles SHOULD be configured on a 'complex' BGP session (assuming it
is not segregated) and in that case, OTC MUST be set by configuration
on a per-prefix basis. However, there can be no measures to check
correctness of OTC use if BGP Role is not configured.
As the BGP Role reflects the peering relationship between neighbors,
it might have other uses beyond the route leaks solution discussed so
far. For example, BGP Role might affect route priority, or be used
to distinguish borders of a network if a network consists of multiple
ASs. Though such uses may be worthwhile, they are not the goal of
this document. Note that such uses would require local policy
control.
As BGP role configuration results in automatic creation of inbound/
outbound filters, existence of roles should be treated as existence
of Import and Export policy [RFC8212].
9. IANA Considerations
This document defines a new Capability Codes option [to be removed
upon publication: http://www.iana.org/assignments/capability-codes/
capability-codes.xhtml] [RFC5492], named "BGP Role" with an assigned
value <TBD1>. The length of this capability is 1.
The BGP Role capability includes a Value field, for which IANA is
requested to create and maintain a new sub-registry called "BGP Role
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Value". Assignments consist of Value and corresponding Role name.
Initially this registry is to be populated with the data in Table 1.
Future assignments may be made by a standard action procedure
[RFC5226].
This document defines a new subcode, "Role Mismatch" with an assigned
value <TBD2> in the OPEN Message Error subcodes registry [to be
removed upon publication: http://www.iana.org/assignments/bgp-
parameters/bgp-parameters.xhtml#bgp-parameters-6] [RFC4271].
This document defines a new optional, transitive BGP Path Attributes
option, named "Only to Customer (OTC)" with an assigned value <TBD3>
[To be removed upon publication: http://www.iana.org/assignments/bgp-
parameters/bgp-parameters.xhtml#bgp-parameters-2] [RFC4271]. The
length of this attribute is four bytes.
10. Security Considerations
This document proposes a mechanism for prevention of route leaks that
are the result of BGP policy mis-configuration.
Deliberate sending of a known conflicting BGP Role could be used to
sabotage a BGP connection. This is easily detectable.
A misconfiguration in OTC setup may affect prefix propagation. But
the automation that is provided by BGP roles should make such
misconfiguration unlikely.
11. Acknowledgments
The authors wish to thank Douglas Montgomery, Brian Dickson, Andrei
Robachevsky, and Daniel Ginsburg for their contributions to a variant
of this work.
12. References
12.1. 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>.
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[RFC4486] Chen, E. and V. Gillet, "Subcodes for BGP Cease
Notification Message", RFC 4486, DOI 10.17487/RFC4486,
April 2006, <https://www.rfc-editor.org/info/rfc4486>.
[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>.
[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>.
12.2. 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>.
[I-D.ietf-grow-route-leak-detection-mitigation]
Sriram, K. and A. Azimov, "Methods for Detection and
Mitigation of BGP Route Leaks", draft-ietf-grow-route-
leak-detection-mitigation-01 (work in progress), July
2019.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<https://www.rfc-editor.org/info/rfc5226>.
[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>.
[RFC8212] Mauch, J., Snijders, J., and G. Hankins, "Default External
BGP (EBGP) Route Propagation Behavior without Policies",
RFC 8212, DOI 10.17487/RFC8212, July 2017,
<https://www.rfc-editor.org/info/rfc8212>.
Authors' Addresses
Alexander Azimov
Qrator Labs
Email: a.e.azimov@gmail.com
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Eugene Bogomazov
Qrator Labs
Email: eb@qrator.net
Randy Bush
Internet Initiative Japan & Arrcus
Email: randy@psg.com
Keyur Patel
Arrcus, Inc.
Email: keyur@arrcus.com
Kotikalapudi Sriram
US NIST
Email: ksriram@nist.gov
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