BIER Z. Zhang
Internet-Draft Juniper Networks
Intended status: Standards Track N. Warnke
Expires: January 7, 2020 Deutsche Telekom
I. Wijnands
Cisco Systems
D. Awduche
Verizon
July 6, 2019
Tethering A BIER Router To A BIER-incapable Router
draft-zzhang-bier-tether-02
Abstract
This document specifies optional procedures to optimize the handling
of Bit Index Explicit Replication (BIER) incapable routers, by
tethering a BIER router to a BIER incapable router.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119.
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
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 January 7, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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Table of Contents
1. Terminologies . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Additional Considerations . . . . . . . . . . . . . . . . . . 4
4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Advertising from Helped Node . . . . . . . . . . . . . . 6
4.2. Advertising from Helper Node . . . . . . . . . . . . . . 7
4.3. Procedures for BGP Signaling . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Terminologies
Familiarity with BIER architecture, protocols and procedures is
assumed. Some terminologies are listed below for convenience.
[To be added].
2. Introduction
Consider the following scenario where router X does not support BIER.
------ BFR2 ------- BFER2
/
BFER1 --- BFR1 ---- X ------- BFR3 ------- BFER3
.........
\
------ BFRn ------- BFERn
For BFR1 to forward BIER traffic towards BFR2...BFRn, it needs to
tunnel individual copies through X. This degrades to "ingress"
replication to those BFRs. If X's connections to BFRs are long
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distance or bandwidth limited, and n is large, it becomes very
inefficient.
A solution to the inefficient tunneling from BFRs is to tether a BFRx
to X:
------ BFR2 ------- BFER2
/
BFER1 --- BFR1 ---- X ------- BFR3 ------- BFER3
/ \ .........
/ \
BFRx ------ BFRn ------- BFERn
Instead of BFR1 tunneling to BFR2, ..., BFRn directly, BFR1 will get
BIER packets to BFRx, who will then tunnel to BFR2, ..., BFRn. There
could be fat and local pipes between the tethered BFRx and X, so
ingress replication from BFRx is acceptable.
For BFR1 to tunnel BIER packets to BFRx, the BFR1-BFRx tunnel need to
be announced in IGP as a forwarding adjacency so that BFRx will
appear on the SPF tree. This need to happen in a BIER specific
topology so that unicast traffic would not be tunneled to BFRx.
Obviously this is operationally cumbersome.
Section 6.9 of BIER architecture specification [RFC8279] describes a
method that tunnels BIER packets through incapable routers without
the need to announce tunnels. However that does not work here,
because BFRx will not appear on the SPF tree of BFR1.
There is a simple solution to the problem though. Even though X does
not support BIER forwarding, it could advertises BIER information as
if it supported BIER so BFRs will send BIER packets to it. The BIER
packets have a BIER label in front of the BIER header and X will use
the BIER label to label switch to BFRx, who will in turn do BIER
forwarding to other BFRs but via tunneling as described in section
6.9 of BIER architecture spec.
Even though X advertises as if it supported BIER, BFRx needs to know
that X does not really support BIER so it will tunnel to other BFRs
through X. The knowledge is through static provisioning or through
additional signaling. In the latter case, X could advertise that
BFRx is its helper node, so that other BFRs could optionally use the
Section 6.9 method to tunnel to BFRx, instead of sending native BIER
packets to X and rely on X label switching to BFRx. This also allows
it to work in the non-MPLS case.
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Alternatively, instead of for X to advertise that it supports BIER
but relies on helper BFRx, BFRx could advertise that it is X's helper
and other BFRs will use BFRx (instead of X's children on the SPF
tree) to replace X during its post-SPF processing as described in
section 6.9 of BIER architecture spec. That way, X does not need any
special knowledge, provisioning or procedure.
The two options both have pros and cons - the first option only needs
X and BFRx to support the new procedure while the second option does
not require anything to be done to the BIER incapable X.
BFRx could also be connected to other routers in the network so that
it could send BIER packets through other routers as well, not
necessarily tunneling through X. To prevent routing loops, smallest
metric, which is 1, must be announced for links between X and BFRx in
both directions.
3. Additional Considerations
While the example shows a local connection between BFRx and X, it
does not have to be like that. As long as packets can arrive at BFRx
without requiring X to do BIER forwarding, it should work. For
example, X could label switch incoming BIER packets through a multi-
hop tunnel to BFRx, or other BFRs could tunnel BIER packets to BFRx
based on X's advertisement that BFRx is its helper. However, BFRx
must make sure that if X appears in its SPF paths to some BFERs, then
it must tunnel BIER packets for those BFERs directly to X's BFR
children on BFRx's SPF tree.
Additionally, the helper BRFx can be a transit helper, i.e., it has
other connections (instead of being a stub helper that is only
connected to X), as long as BFRx won't send BIER packets tunneled to
it back towards the tunnel ingress:
------ BFR2 ------- BFER2
/
BFER1 --- BFR1 ---- X ------- BFR3 ------- BFER3
|
|
BFRx ------ BFR4 ------- BFER4
\
------ BFR5 ------- BFER5
In the following example, there is a connection between BFR1 and
BFRx. If the link metrics are all 1 on the three sides of
BFR1-X-BFRx triangle, loop won't happen but if the BFRx-X metric is 3
while other two sides of the triangle has metric 1 then BFRx will
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send BIER packets tunneled to it from BFR1 back to BFR1, causing a
loop.
------ BFR2 ------- BFER2
/
BFER1 --- BFR1 ---- X ------- BFR3 ------- BFER3
\ / \ .........
\ / \
BFRx ------ BFRn ------- BFERn
This can easily be prevented if BFR1 does an SPF calculation with the
helper BFRx as the root. For any BFERn reached via X from BFR1, if
BFRx's SPF path to BFERn includes BFR1 then BFR1 must not use the
helper. Instead, BFR1 must directly tunnel packets for BFERn to X's
BFR (grand-)child on BFR1's SPF path to BFERn, per section 6.9 of
[RFC8279].
Notice that this SPF calculation on BFR1 with BFRx as the root is no
different from the SPF done for a neighbor as part of LFA
calculation. In fact, BFR1 tunneling packets to X's helper is no
different from sending packets to a LFA backup.
Also notice that, instead of a dedicated helper BFRx, any one or
multiple ones of BFR2..N can also be the helper (as long as the
connection between that BFR and X has enough bandwidth for
replication to multiple helpers through X). To allow multiple
helpers to help the same non-BFR, the "I am X's helper" advertisement
carries a priority. BFR1 will choose the helper advertising the
highest priority among those satisfying the loop-free condition
described above. When there are multiple helpers advertising the
same priority and satisfying the loop-free condition, any one or
multiple ones could be used solely at the discretion of BFR1.
However, if multiple ones are used, it means that multiple copies may
be tunneled through X.
The following situation where a helper BFRxy helps two different non-
BFRs X and Y also works. It's just a special situation of a transit
helper.
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----- BFR2 ------- BFER2
/
X ------- BFR3 ------- BFER3
/ | \
/ \ ----- BFR4 ------- BFER4
/ \
BFIR1 -- BFR1 BFRxy ------------- BFERxy
\ /
\ / ----- BFR5 ------- BFER5
\ | /
Y ------- BFR6 ------- BFER6
\
----- BFRn ------- BFERn
4. Specification
The procedures in this document apply when a BFRx is tethered to a
BIER incapable router X as X's helper for BIER forwarding.
BFRx MUST not send BIER packets natively to X even if X advertises
BIER information. BFRx knows that X does not really support BIER
either from provisioning or from the BIER Helper Node sub-sub-TLV
advertised by X.
Procedures for BGP signaling is described in Section 4.3.
Either of the following two methods may be used for ISIS [RFC8401]
and OSPF [RFC8444]. The sub-sub-TLVs for both methods have the same
format: the value is BIER prefix of the helper/helped node followed
by a one-octet priority field, and one-octet reserved field. The
length is 6 for IPv4 and 18 for IPv6 respectively.
4.1. Advertising from Helped Node
For non-MPLS encapsulation, X MUST advertise a BIER Helper Node sub-
sub-TLV that specifies the BIER prefix of the helper BFRx. Other
BFRs MUST use the Section 6.9 procedure modified as following: X is
treated as BIER incapable (because of the BIER Helper Node sub-sub-
TLV), and is replaced with the BFRx (instead of X's children on the
SPF tree) during the post-SPF processing.
This requires other BFRs to recognize the BIER Helper Node sub-sub-
TLV. The same procedure MAY be used For MPLS encapsulation, though
with the following alternative for MPLS encapsulation, tethering is
transparent to other BFRs (except the helper node BFRx) - they do not
need to be aware that X does not support BIER at all.
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For MPLS encapsulation, X MAY advertises BIER information as if it
supported BIER forwarding, including the MPLS Encapsulation sub-sub-
TLV with a label range. X MUST set up its forwarding state such that
incoming packets with a BIER label in its advertised label range are
label switched to BFRx, either over a direct link or through a
tunnel. The incoming label is swapped to a BIER label advertised by
BFRx for the <sub-domain, bsl, set> that the incoming label
corresponds to.
Notice that both methods can be used for MPLS encapsulation at the
same time. In that case another BFR may send BIER packets to X
natively, or tunnel to BFRx directly.
4.2. Advertising from Helper Node
With this method, the helper node (BFRx) MUST advertise a BIER Helped
Node sub-sub-TLV that specifies the BIER incapable node (X) that this
node helps. When other BFRs follow the post-SPF processing
procedures as specified in section 6.9 of the BIER architecture spec
[RFC8279], they replace the helped node on the SPF tree with the
helper node (instead of the children of the helped node).
4.3. Procedures for BGP Signaling
Suppose that the BIER domain uses BGP signaling
[I-D.ietf-bier-idr-extensions] instead of IGP. BFR1..N advertises
BIER prefixes that are reachable through them, with BIER Path
Attributes (BPA) attached. There are three situations regarding X's
involvement:
(1) X does not participate in BGP peering at all
(2) X re-advertises the BIER prefixes but does not do next-hop-self
(3) X re-advertises the BIER prefixes and does next-hop-self
With (1) and (2), the BFR1..N will tunnel BIER packets directly to
each other. It works but not efficiently as explained earlier. With
(3), BIER forwarding will not work, because BFR1..N would try to send
BIER packets to X though X does not advertise any BIER information.
If Tunnel Encapsulation Attribute (TEA) [I-D.ietf-idr-tunnel-encaps]
is used as specified in [I-D.zzhang-bier-multicast-as-a-service] with
(3), then it becomes similar to (2) - works but still not
efficiently.
To make tethering work well with BGP signaling, the following can be
done:
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o Configure a BGP session between X and its helper BFRx. X re-
advertises BIER prefixes (with BPA) to BFRx without changing the
tunnel destination address in the TEA.
o BFRx advertises its own BIER prefix with BPA to X, and sets the
tunnel destination address in the TEA to itself. X then re-
advertises BFRx's BIER prefix to BFR1..N, without changing the
tunnel destination address in the TEA.
o For BIER prefixes (with BIER Path Attribute) that X re-advertises
to other BFRs, the tunnel destination in the TEA is changed to the
helper BFRx.
With the above, BFR1..N will tunnel BIER packets to BFRx (following
the tunnel destination address in the TEA), who will then tunnel
packets to other BFRs (again following the tunnel destination address
in the TEA). Notice that what X does is not specific to BIER at all.
5. Security Considerations
This specification does not introduce additional security concerns
beyond those already discussed in BIER architecture and OSPF/ISIS/BGP
extensions for BIER signaling.
6. IANA Considerations
This document requests two new sub-sub-TLV type values from the "Sub-
sub-TLVs for BIER Info Sub-TLV" registry in the "IS-IS TLV
Codepoints" registry:
Type Name
---- ----
TBD1 BIER Helper Node
TBD2 BIER Helped Node
This document also requests two new sub-TLV type values from the
OSPFv2 Extended Prefix TLV Sub-TLV registry:
Type Name
---- ----
TBD3 BIER Helper Node
TBD4 BIER Helped Node
7. Acknowledgements
The author wants to thank Eric Rosen and Antonie Przygienda for their
review, comments and suggestions.
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8. Normative References
[I-D.ietf-bier-idr-extensions]
Xu, X., Chen, M., Patel, K., Wijnands, I., and T.
Przygienda, "BGP Extensions for BIER", draft-ietf-bier-
idr-extensions-06 (work in progress), January 2019.
[I-D.ietf-idr-tunnel-encaps]
Patel, K., Velde, G., Ramachandra, S., and E. Rosen, "The
BGP Tunnel Encapsulation Attribute", draft-ietf-idr-
tunnel-encaps-12 (work in progress), May 2019.
[I-D.zzhang-bier-multicast-as-a-service]
Zhang, Z., Rosen, E., and L. Geng, "Multicast/BIER As A
Service", draft-zzhang-bier-multicast-as-a-service-00
(work in progress), October 2018.
[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>.
[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
[RFC8401] Ginsberg, L., Ed., Przygienda, T., Aldrin, S., and Z.
Zhang, "Bit Index Explicit Replication (BIER) Support via
IS-IS", RFC 8401, DOI 10.17487/RFC8401, June 2018,
<https://www.rfc-editor.org/info/rfc8401>.
[RFC8444] Psenak, P., Ed., Kumar, N., Wijnands, IJ., Dolganow, A.,
Przygienda, T., Zhang, J., and S. Aldrin, "OSPFv2
Extensions for Bit Index Explicit Replication (BIER)",
RFC 8444, DOI 10.17487/RFC8444, November 2018,
<https://www.rfc-editor.org/info/rfc8444>.
Authors' Addresses
Zhaohui Zhang
Juniper Networks
EMail: zzhang@juniper.net
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Nils Warnke
Deutsche Telekom
EMail: Nils.Warnke@telekom.de
IJsbrand Wijnands
Cisco Systems
EMail: ice@cisco.com
Daniel Awduche
Verizon
EMail: daniel.awduche@verizon.com
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