BIER MTU Discovery
draft-venaas-bier-mtud-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Expired".
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|---|---|---|---|
| Authors | Stig Venaas , Mahesh Sivakumar , IJsbrand Wijnands , Les Ginsberg | ||
| Last updated | 2018-03-05 | ||
| RFC stream | (None) | ||
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draft-venaas-bier-mtud-00
Network Working Group S. Venaas
Internet-Draft M. Sivakumar
Intended status: Experimental IJ. Wijnands
Expires: September 6, 2018 L. Ginsberg
Cisco Systems, Inc.
March 5, 2018
BIER MTU Discovery
draft-venaas-bier-mtud-00
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
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on September 6, 2018.
Copyright Notice
Copyright (c) 2018 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
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. IS-IS BIER MTU Sub-sub-TLV . . . . . . . . . . . . . . . . . 3
4. OSPF BIER MTU Sub-TLV . . . . . . . . . . . . . . . . . . . . 4
5. IANA considerations . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
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 payload, such as an IP 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.
For convenience we will refer to an interface on a router as 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. We say that it is a BIER interface in 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.
In order to allow MTU discovery in a BIER sub-domain, the procedure
is as follows. Every BIER router, for each sub-domain with at least
one local BIER interface in the sub-domain, per the above definition
of a BIER interface, determines the largest payload that can be sent
BIER encapsulated out of any of its BIER interfaces in the sub-
domain. That is, for each local BIER interface in the sub-domain, it
needs to determine the size of the largest BIER encapsulated payload
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that can be sent out of that interface. We define the local sub-
domain MTU of a router to be the minimum of the per BIER interface
maximum payload size.
A BIER router announces a BIER prefix in either IS-IS or OSPF as
specified in [I-D.ietf-bier-isis-extensions] 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 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
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.
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. IS-IS BIER MTU Sub-sub-TLV
A router uses the BIER MTU Sub-sub-TLV to announce the minimum BIER
MTU of all its BIER enabled interfaces. The Sub-sub-TLV MUST be
ignored if it is included multiple times.
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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
4. OSPF BIER MTU Sub-TLV
A router uses the BIER MTU Sub-TLV to annonce the minimum BIER MTU of
all its BIER enabled interfaces. It is a Sub-TLV of the BIER Sub-
TLV, and SHOULD be included exactly once within each of the
advertised BIER Sub-TLVs. 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
5. IANA considerations
An allocation from the "sub-sub-TLVs for BIER Info sub-TLV" registry
as defined in [I-D.ietf-bier-isis-extensions] is requested for the
IS-IS BIER 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 MTU Sub-TLV.
Please replace the string TBD2 in this document with the appropriate
value.
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6. References
6.1. Normative References
[I-D.ietf-bier-isis-extensions]
Ginsberg, L., Przygienda, T., Aldrin, S., and Z. Zhang,
"BIER support via ISIS", draft-ietf-bier-isis-
extensions-09 (work in progress), February 2018.
[I-D.ietf-bier-ospf-bier-extensions]
Psenak, P., Kumar, N., Wijnands, I., Dolganow, A.,
Przygienda, T., Zhang, Z., and S. Aldrin, "OSPF Extensions
for BIER", draft-ietf-bier-ospf-bier-extensions-15 (work
in progress), February 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>.
6.2. 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-03 (work in progress), January 2018.
Authors' Addresses
Stig Venaas
Cisco Systems, Inc.
Tasman Drive
San Jose CA 95134
USA
Email: stig@cisco.com
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Mahesh Sivakumar
Cisco Systems, Inc.
Tasman Drive
San Jose CA 95134
USA
Email: masivaku@cisco.com
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
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