Network Working Group S. Venaas
Internet-Draft IJ. Wijnands
Intended status: Experimental L. Ginsberg
Expires: April 19, 2019 Cisco Systems, Inc.
M. Sivakumar
Juniper Networks
October 16, 2018
BIER MTU Discovery
draft-venaas-bier-mtud-02
Abstract
This document defines an IGP based mechanism for discovering the MTU
of a BIER sub-domain. This document defines extensions to OSPF and
IS-IS, but other protocols could potentially be extended. MTU
discovery is usually done for a given path, while this document
defines it for a sub-domain. This allows the computed MTU to be
independent of the set of receivers. Also, the MTU is independent of
rerouting events within the sub-domain.
Status of This Memo
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This Internet-Draft will expire on April 19, 2019.
Copyright Notice
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document authors. All rights reserved.
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. MTU discovery procedure . . . . . . . . . . . . . . . . . . . 3
4. IS-IS BIER Sub-Domain MTU Sub-sub-TLV . . . . . . . . . . . . 4
5. OSPF BIER Sub-Domain MTU Sub-TLV . . . . . . . . . . . . . . 5
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
This document defines an IGP based mechanism for discovering the MTU
of a BIER sub-domain. The discovered MTU indicates the largest
possible BIER packet that can be sent across any link in a BIER sub-
domain. This is different from [I-D.ietf-bier-path-mtu-discovery]
which performs Path MTU Discovery (PMTUD) for a set of receivers.
PMTUD is based on probing, and when there are routing changes, e.g.,
a link going down, the actual MTU for a path may become less than was
previously discovered, and there will be some delay until the next
probe is performed. Also, the set of receivers for a flow may change
at any time, which may cause the MTU to change. This document
instead discovers a BIER sub-domain MTU, which is independent of
paths and receivers within the sub-domain.
Discovering the sub-domain MTU is much simpler than discovering the
multicast path MTU, and is more robust with regards to path changes
as discussed above. However, the sub-domain MTU may be a lot smaller
than the path MTU would have been for a given flow. The discovery
mechanisms may be combined, allowing the discovery of the path MTU
for certain flows as needed.
The BIER sub-domain MTU defined here provides the maximum size of a
BIER packet that can be forwarded through the sub-domain regardless
of path. A BIER router that performs BIER encapsulation will need to
subtract the encapsulation overhead to find the largest size packet
that can be encapsulated. This would give the IP MTU, and may be
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used for IP PMTUD by for instance sending an ICMP Packet too big
message if an IP packet will be too large after BIER encapsulation.
2. Conventions used in this document
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.
3. MTU discovery procedure
An interface on a router is said to be a BIER interface if the router
has a BIER neighbor on the interface. That is, there is a directly
connected router on that interface that is announcing a BIER prefix.
Further, the BIER interface is said to belong to a given sub-domain
if the router itself announces a prefix tagged with the sub-domain,
and there is BIER neighbor on the interface also announcing a prefix
tagged with the sub-domain.
The BIER MTU of an interface is the largest BIER packet that can be
sent out of the interface. Further, the local sub-domain MTU of a
router is the minimum of all the BIER MTUs of the BIER interfaces in
the sub-domain. Note that the local sub-domain MTU of a router is
only defined if it has at least one BIER interface in the sub-domain.
A BIER router announces a BIER prefix in either IS-IS or OSPF as
specified in [RFC8401] and [I-D.ietf-bier-ospf-bier-extensions].
They both define a BIER Sub-TLV to be included with the prefix.
There is one BIER Sub-TLV included for each sub-domain. This
document defines how a router includes its local sub-domain MTU in
each of the BIER Sub-TLVs it advertizes.
A router can discover the MTU of a BIER sub-domain by identifying all
the prefixes that have a BIER Sub-TLV for the sub-domain. It then
computes the minimum of the advertised MTU values for that sub-
domain. This includes its own local sub-domain MTU. This allows all
the routers in the sub-domain to discover the same sub-domain wide
MTU.
Note that a router should announce a new local MTU for a sub-domain
immediately if the value becomes smaller than what it currently
announces. This would happen if the MTU of an interface is
configured to a smaller value, or the first BIER neighbor for a sub-
domain is detected on an interface, and the MTU of the interface is
less than all the other local BIER interfaces in the sub-domain.
However, if BIER neighbors go away, or if an interface goes down, so
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that the local MTU becomes larger, a router SHOULD NOT immediately
announce the larger value. A router MAY after some delay announce
the new larger MTU. The intention is that dynamic events such as a
quick link flap should not cause the announced MTU to be increased.
It is a concern that the sub-domain MTU will be based on the link
with the smallest MTU. This means that if for instance a single link
is accidentally configured with an extra small MTU, it will impact
the sub-domain MTU and potentially all the flows through the sub-
domain. As an example, an administrator might decide to use jumbo
frames and has configured that on all the links. But accidentally
forget to configure it on a new link before it is brought up. To
provide some protection against this, an implementation SHOULD
provide a configurable minimum BIER sub-domain MTU. When this is
configured, the MTU discovery is still done according to the above
procedure, but if the resulting MTU value is less than the configured
minimum, the configured minimum MUST be used instead. If the
discovery procedure later should provide an MTU larger than the
minimum, then the discovered MTU MUST be used. An implementation
SHOULD provide notification to the administrator when the discovered
MTU is less than the minimum, as this is likely a configuration
mistake that should be corrected.
4. IS-IS BIER Sub-Domain MTU Sub-sub-TLV
A router uses the BIER Sub-Domain MTU Sub-sub-TLV to announce the
minimum BIER MTU of all its BIER enabled interfaces in a sub-domain.
The BIER Sub-Domain MTU is the largest BIER packet that can be sent
out of all the interfaces in a sub-domain. The Sub-sub-TLV MUST be
ignored if it is included multiple times.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: 2
MTU: MTU in octets
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5. OSPF BIER Sub-Domain MTU Sub-TLV
A router uses the BIER Sub-Domain MTU Sub-TLV to announce the minimum
BIER MTU of all its BIER enabled interfaces in a sub-domain. The
BIER Sub-Domain MTU is the largest BIER packet that can be sent out
of all the interfaces in a sub-domain. The Sub-TLV MUST be ignored
if it is included multiple times.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD2
Length: 4
MTU: MTU in octets
6. IANA considerations
An allocation from the "sub-sub-TLVs for BIER Info sub-TLV" registry
as defined in [RFC8401] is requested for the IS-IS BIER Sub-Domain
MTU Sub-sub-TLV. Please replace the string TBD in this document with
the appropriate value.
An allocation from the "OSPF Extended Prefix sub-TLV" registry as
defined in [RFC7684] is requested for the OSPF BIER Sub-Domain MTU
Sub-TLV. Please replace the string TBD2 in this document with the
appropriate value.
7. Acknowledgments
The authors would like to thank Greg Mirsky in particular for
fruitful discussions and input. Valuable comments were also provided
by Alia Atlas, Eric C Rosen, Toerless Eckert, Tony Przygienda and Xie
Jingrong.
8. References
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8.1. Normative References
[I-D.ietf-bier-ospf-bier-extensions]
Psenak, P., Kumar, N., Wijnands, I., Dolganow, A.,
Przygienda, T., Zhang, Z., and S. Aldrin, "OSPFv2
Extensions for BIER", draft-ietf-bier-ospf-bier-
extensions-18 (work in progress), June 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>.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.
[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>.
[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>.
8.2. Informative References
[I-D.ietf-bier-path-mtu-discovery]
Mirsky, G., Przygienda, T., and A. Dolganow, "Path Maximum
Transmission Unit Discovery (PMTUD) for Bit Index Explicit
Replication (BIER) Layer", draft-ietf-bier-path-mtu-
discovery-04 (work in progress), June 2018.
Authors' Addresses
Stig Venaas
Cisco Systems, Inc.
Tasman Drive
San Jose CA 95134
USA
Email: stig@cisco.com
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IJsbrand Wijnands
Cisco Systems, Inc.
De kleetlaan 6a
Diegem 1831
Belgium
Email: ice@cisco.com
Les Ginsberg
Cisco Systems, Inc.
Tasman Drive
San Jose CA 95134
USA
Email: ginsberg@cisco.com
Mahesh Sivakumar
Juniper Networks
1133 Innovation Way
Sunnyvale CA 94089
USA
Email: sivakumar.mahesh@gmail.com
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