Internet DRAFT - draft-white-linklocalnh
draft-white-linklocalnh
Interdomain Routing R.W. White
Internet-Draft Juniper Networks
Intended status: Standards Track D.S. Sharp
Expires: 11 May 2022 Cumulus Networks
D.D. Dutt
Stardust Consulting
B.S. Sadhu
VMWare
J.T. Tantsura
Apstra, Inc.
D.A. Abraitis
Hostinger
November 2021
Link Local Next Hop Handling for BGP
draft-white-linklocalnh-02
Abstract
BGP, described in [RFC4271], was originally designed to provide
reachability between domains and between the edges of a domain. As
such, BGP assumes the next hop towards any reachable destination may
not reside on the advertising speaker, but rather may either be
through a router connected to the same subnet as the speaker, or
through a router only reachable by traversing multiple hops through
the network. Because of this, BGP does not recognize the use of IPv6
link local addresses, as described in [RFC4291], as a valid next hop
for forwarding purposes.
However, BGP speakers are now often deployed on point-to-point links
in networks where multihop reachability of any kind is not assumed or
desired (all next hops are assumed to be the speaker reachable
through a directly connected point-to-point link). This is common,
for instance, in data center fabrics. In these situations, a global
IPv6 address is not required for the advertisement of reachability
information; in fact, providing global IPv6 addresses in these kinds
of networks can be detrimental to Zero Touch Provisioning (ZTP).
This draft standardizes the operation of BGP over a point-to-point
link using link local IPv6 addressing only.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
White, et al. Expires 11 May 2022 [Page 1]
Internet-Draft Link Local Next Hop Handling for BGP November 2021
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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 5 May 2022.
Copyright Notice
Copyright (c) 2021 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Changes to BGP Next Hop Attribute to Support Link Local on
Point-to-Point . . . . . . . . . . . . . . . . . . . . . 3
3. Receiver Processing of IPv6 Link Local Forwarding
Addresses . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Error handling . . . . . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
White, et al. Expires 11 May 2022 [Page 2]
Internet-Draft Link Local Next Hop Handling for BGP November 2021
1. Introduction
BGP, described in [RFC4271], was originally designed to provide
reachability between domains and between the edges of a domain. As
such, BGP assumes the next hop towards any reachable destination may
not reside on the advertising speaker, but rather may either be
through a router connected to the same subnet as the speaker, or
through a router only reachable by traversing multiple hops through
the network. Because of this, BGP does not recognize the use of IPv6
link local addresses, as described in [RFC4271], as a valid next hop
for forwarding purposes.
However, BGP speakers are now often deployed on point-to-point links
in networks where multihop reachability of any kind is not assumed or
desired (all next hops are assumed to be the speaker reachable
through a directly connected point-to-point link). This is common,
for instance, in data center fabrics. In these situations, a global
IPv6 address is not required for the advertisement of reachability
information; in fact, providing global IPv6 addresses in these kinds
of networks can be detrimental to Zero Touch Provisioning (ZTP).
2. Changes to BGP Next Hop Attribute to Support Link Local on Point-to-
Point
[RFC2545], section 2, notes link local IPv6 addresses are not
generally suitable for use in the Next Hop field of the
MP_REACH_NLRI. In order to support the many uses of link local
addresses, however, [RFC2545] constructs the Next Hop field in IPv6
route advertisements by setting the length of the field to 32, and
including both a link local and global IPv6 address in the resulting
enlarged field. In this way, the receiving BGP speaker can use the
global IPv6 address to build local forwarding information, and the
link local address for ICMPv6 redirects, etc. [RFC2545] does not,
however, provide an explanation for situations where there is only a
link local IPv6 address in the Next Hop field of the MP_REACH_NLRI.
The result is each implementation that supports link local peering
along with forwarding to a link local address has implemented the
construction of the Next Hop field in the MP_REACH_NLRI when there is
only a link local address available in slightly different ways.
If an implementation intends to send a single link local forwarding
address in the Next Hop field of the MP_REACH_NLRI, it MUST set the
length of the Next Hop field to 16 and include only the IPv6 link
local address in the Next Hop field.
If an implementation intends to send both a link local and global
IPv6 forwarding address in the Next Hop field of the MP_REACH_NLRI,
it MUST set the length of the Next Hop field to 32 and include both
White, et al. Expires 11 May 2022 [Page 3]
Internet-Draft Link Local Next Hop Handling for BGP November 2021
the IPv6 link local and global IPv6 forwarding addresses in the Next
Hop field. If both link local and global IPv6 forwarding addresses
are carried in the Next Hop Field, the speaker SHOULD provide a local
configuration option to determine which address is preferred for
forwarding.
3. Receiver Processing of IPv6 Link Local Forwarding Addresses
On receiving an MP_REACH_NLRI with a Next Hop length of 16,
implementations SHOULD form the forwarding information using the IPv6
next hop contained in the Next Hop field, regardless of whether it is
a link local or globally reachable IPv6 address.
Implementations MAY check the validity of any IPv6 link local address
used to calculate forwarding information by insuring the address is
in the local neighbor table for the interface on which the BGP update
was received (or through which the BGP speaker from which the update
was received is reachable). There MUST be a configuration option to
enable/disable this check.
Note: It is possible that checking the IPv6 neighbor table for the
existence or validity of a link local next hop may make instances
where a link is being overwhelmed through some form of Denial of
Service (DoS) attack worse than they would otherwise be. If the IPv6
neighbor cache is overrun in a way that causes the link local address
being used for BGP peering to be removed from the table, which is
possible through an on-link DoS attack, any fresh BGP update will
cause the entire peering session to fail if the implementation is
checking the validity of link local next hops as described above.
Operators should carefully assess the use of validation against the
local IPv6 neighbor table to determine if it is appropriate for any
particular peering session.
4. Error handling
A BGP speaker receiving an MP_REACH_NLRI with the length of the Next
Hop Field set to 32, where the update contains anything other than a
link local IPv6 address and a global IPv6 address, SHOULD consider
this a malformed UPDATE message, and proceed as described in the
following paragraphs. In order to support backwards compatibility
with existing implementations, an implementation MAY ignore a second
link local IPv6 address or 0::0/0 included with an IPv6 link local
address when the length of the Next Hop Field is set to 32; in this
case, the implementation SHOULD report the existence of this
additional information so the operator can correct the sending BGP
implementation.
White, et al. Expires 11 May 2022 [Page 4]
Internet-Draft Link Local Next Hop Handling for BGP November 2021
If the Next Hop field is malformed, the implementation MUST handle
the mailformed UPDATE message using the approach of "treat-as-
withdraw", as described in section 7.3 of [RFC7606]. It MAY send a
NOTIFICATION message as described in section 4 of [RFC4271], using
the UPDATE error message code (8 - Invalid NEXT_HOP Attribute)
indicating there is an invalid NEXT_HOP field
If the Next Hop field is properly formed, but the link local next hop
is not reachable (as determined by an examination of the IPv6
neighbor table), the implementation MAY handle the mailformed UPDATE
message using the approach of "treat-as-withdraw", as described in
section 7.3 of [RFC7606] (see note above on checking the local
neighbor table for the correctness of the next hop). The
implementation MAY send a NOTIFICATION message as described in
section 4 of [RFC4271] using the UPDATE error message code (TBA),
indicating a link local address was included in the MP_REACH_NLRI,
but the link local address included cannot be reached. As this could
indicate a security breach of some type (see the security
considerations section below), the operator SHOULD have a local
configuration option to terminate the peering session until manual
intervention is initiated.
5. Acknowledgements
The authors would like to thank Don Slice, Jeff Haas, and John
Scudder for their contributions to this draft.
6. IANA Considerations
This memo requests IANA assign a number from the "Error Subcodes"
registry defined in the IANA Considerations section in [RFC4271].
This allocation will be for a new UPDATE error subcode, code (TBA),
with a value of "Unreachable Link Local Address."
7. Security Considerations
The mechanism described in this draft can be used as a component of
ZTP for building BGP adjacencies across point-to-point links. This
method, then, can be used by an attacker to form a peering session
with a BGP speaker, ultimately advertising incorrect routing
information into a routing domain in order to misdirect traffic or
cause a denial of service. By using link local IPv6 addresses, the
attacker would be able to forego the use of a valid IPv6 address
within the domain, making such an attack easier.
White, et al. Expires 11 May 2022 [Page 5]
Internet-Draft Link Local Next Hop Handling for BGP November 2021
Operators SHOULD carefully consider security when deploying link
local addresses for BGP peering. Operators SHOULD filter traffic on
links where BGP peering is not intended to occur to prevent speakers
from accepting BGP session requests, as well as other mechanisms
described in [RFC7454].
Operators MAY also use some form of cryptographic validation on links
within the network to prevent unauthorized devices from forming BGP
peering sessions. Authentication, such as the TCP authentication
described in [RFC5925], may provide some relief, if it is present and
correctly configured. However, the distribution and management of
keys in an environment where global addresses are not present on BGP
speakers may be challenging.
Operators also MAY instruct a BGP peer which has received an UPDATE
with an unreachable NEXT_HOP to disable the peering session over
which the invalid NEXT_HOP was received pending manual intervention.
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>.
[RFC2545] Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
Extensions for IPv6 Inter-Domain Routing", RFC 2545,
DOI 10.17487/RFC2545, March 1999,
<https://www.rfc-editor.org/info/rfc2545>.
[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>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
February 2015, <https://www.rfc-editor.org/info/rfc7454>.
White, et al. Expires 11 May 2022 [Page 6]
Internet-Draft Link Local Next Hop Handling for BGP November 2021
[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>.
8.2. Informative References
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
DOI 10.17487/RFC2629, June 1999,
<https://www.rfc-editor.org/info/rfc2629>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
Authors' Addresses
Russ White
Juniper Networks
Email: russ@riw.us
Donald Sharp
Cumulus Networks
Email: sharpd@cumulusnetworks.com
Dinesh Dutt
Stardust Consulting
Email: ddutt@hobbesdutt.com
Biswajit Sadhu
VMWare
Email: biswajit.sadhu@gmail.com
Jeff Tantsura
Apstra, Inc.
Email: jefftant.ietf@gmail.com
White, et al. Expires 11 May 2022 [Page 7]
Internet-Draft Link Local Next Hop Handling for BGP November 2021
Donatas Abraitis
Hostinger
Email: donatas.abraitis@gmail.com
White, et al. Expires 11 May 2022 [Page 8]
