draft-white-linklocal-capability.xml

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<rfc category="std" docName="draft-white-linklocal-capability-00" ipr="trust200902">

<front>

<title>Link-Local Next Hop Capability for BGP</title>

<author fullname="Russ White" initials="R.W." surname="White">
<organization>Juniper Networks</organization>
<address>
<postal><street></street></postal>
<email>russ@riw.us</email>
</address>
</author>

<author fullname="Donatas Abraitis" initials="D.A." surname="Abraitis">
<organization>Hostinger</organization>
<address>
<postal><street></street></postal>
<email>donatas.abraitis@gmail.com</email>
</address>
</author>

<date month="November" year="2021" />

<area>Routing</area>
<workgroup>Interdomain Routing</workgroup>

<abstract>
<t>BGP, described in <xref target="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 <xref target="RFC4291" />, as a valid next hop for forwarding
purposes.</t>

<t>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).</t>

<t>This draft standardizes the operation of BGP over a point-to-point
link using link-local IPv6 addressing only.</t>
</abstract>
</front>

<middle>

<section title="Introduction">
<t>BGP, described in <xref target="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 <xref target="RFC4271" />, as a valid next hop for forwarding purposes.</t>

<t>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).</t>

<t>Such BGP deployment models require BGP to run on each
link, and any ease or simplification of BGP configuration can result
in simplifying orchestration and configuration management.  This
proposal is a step in that direction.</t>

<t>With the requirement of any global interface address being removed by
this new capability, BGP neighbor configuration can be further
simplified by making it (look) address-family independent.  E.g BGP
can just take the interface name for the peer config and link-local IPv6
address of the peer can be learned via a discovery protocol running
on the link or by an out-of-band tool.  In essence, link-local next
hop in combination with <xref target="RFC5549" /> makes it possible to
achieve an unnumbered interface-like solution <xref target="RFC5309" />
in BGP.</t>
</section>
<!-- end of Introduction section -->

<section title="Link-Local Next Hop Capability">
<t>The Link-Local Next Hop capability is a new BGP capability. A
BGP speaker that supports capabilities advertisement <xref target="RFC5492" />
in an OPEN message should send this capability only when:
     <list style="numbers">
          <t>It is capable of sending link-local IPv6 address as the only next
              hop address for a route.</t>

          <t>The implementation is capable of processing link-local address
               next hops with the help of peer interface binding to come up with
               interface-specific next hops for its routing table.</t>
     </list>
The presence of this capability does not affect the support of global
IPv6 only (16 bytes next hop) and global IPv6 combined with link-local
IPv6 (32 bytes next hop), which should continue to be supported as before.

The Capability Code for this capability is specified in the IANA
Considerations section of this document.  The Capability Length field
of this capability is 0.</t>

<t>The advantage of using this capability is that it can let two conforming
implementations know that they aren't talking to
<xref target="buggy" /> implementations with <xref target="unknown" /> behaviors.
Implementations are inconsistently broken, and can't readily change their
default behavior.</t>

<t>By announcing the capability and having the capability negotiated,
implementations don't need to set interoperational knobs to get a particular
set of fields populated a particular way or worry about how they are
received.</t>

<t>Implementations that don't support this new capability would still require
an additional knob for whatever variety of inconsistent implementations they
want to deal with.</t>

<t>A BGP speaker that is willing to use (send and receive) only link-local
addresses as next hops with a peer SHOULD advertise the Link-Local Next Hop
Capability to the peer using BGP Capabilities advertisement.</t>

<t>The peers have the flexibility to include both link-local and global
next hops or link-local only next hop.</t>
</section>
<!-- end of Link-Local Next Hop Capability section -->

<section title="Changes to BGP Next Hop Attribute to Support Link-Local on Point-to-Point">
<t><xref target="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, <xref target="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. <xref target="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.</t>

<t>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.</t>

<t>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 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.</t>

<t>For internal BGP peers configured as a route-reflector, when route-reflector
isn't configured to be in the data-path, the proposed link-local (only)
next hops MUST not be reflected.</t>

<t>Single (only) link-local next hop address needs to always be reset as
next hop self when passed to another link.</t>
</section>
<!-- end of Changes to BGP Next Hop Attribute to Support Link-Local on Point-to-Point section -->

<section title="Receiver Processing of IPv6 Link-Local Forwarding Addresses">
<t>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.</t>

<t>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.</t>

<t>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.</t>
</section>
<!-- end of Receiver Processing of IPv6 Link-Local Forwarding Addresses section -->

<section title="Error handling">
<t>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 backward 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.</t>

<t>If the Next Hop field is malformed, the implementation MUST handle the
malformed UPDATE message using the approach of "treat-as-withdraw", as
described in section 7.3 of <xref target="RFC7606" />. It MAY send a
NOTIFICATION message as described in section 4 of <xref target="RFC4271" />,
using the UPDATE error message code (8 - Invalid NEXT_HOP Attribute) indicating
there is an invalid NEXT_HOP field</t>

<t>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 malformed UPDATE message using the approach
of "treat-as-withdraw", as described in section 7.3 of <xref target="RFC7606" />
(see the 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 <xref target="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.</t>
</section>
<!-- end of Error handling section -->

<section title="Acknowledgements">
<t>The authors would like to thank Vipin Kumar, Dinesh Dutt, Jeff Haas, and
for their contributions to this draft.</t>
</section>
<!-- end of Acknowledgements section -->

<section title="IANA Considerations">
<t>This memo requests IANA assign a number from the "Error Subcodes"
registry defined in the IANA Considerations section in
<xref target="RFC4271" />. This allocation will be for a new UPDATE
error subcode, code (TBA), with a value of
"Unreachable Link-Local Address."</t>

<t>Also, IANA is requested to assign a capability number to the same.</t>
</section>
<!-- end of IANA Considerations section -->

<section title="Security Considerations">
<t>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.</t>

<t>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
<xref target="RFC7454" />.</t>

<t>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 <xref target="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.</t>

<t>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.</t>
</section>
<!-- end of Security Considerations section -->

</middle>

<back>
<!-- References split into informative and normative -->

<references title="Normative References">
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&RFC2545;
&RFC4271;
&RFC4291;
&RFC5309;
&RFC5492;
&RFC5549;
&RFC5925;
&RFC7454;
&RFC7606;
</references>

<references title="Informative References">
     <reference anchor="buggy" target="https://github.com/frrouting/frr/commit/606fdbb1fab98bac305dca3d19eb38b140b7c3e6">
          <front>
               <title>FRRouting - An example of buggy interoperational implementation</title>
               <author initials="D.A." surname="Abraitis" fullname="Donatas Abraitis">
                    <organization abbrev="Hostinger">Hostinger</organization>
               </author>
               <date year="2020" />
        </front>
     </reference>
     <reference anchor="unknown" target="https://gitlab.nic.cz/labs/bird/-/commit/17de3a023f7bde293892b41bfafe5740c8553fc8">
          <front>
               <title>Bird - An example of buggy interoperational implementation</title>
               <author initials="O.Z." surname="Zajicek" fullname="Ondrej Zajicek">
                    <organization abbrev="CZ.NIC Labs Projects">CZ.NIC Labs Projects</organization>
               </author>
               <date year="2020" />
        </front>
     </reference>
</references>

</back>
</rfc>