Internet DRAFT - draft-kline-6man-pmo
draft-kline-6man-pmo
Internet Engineering Task Force E. Kline
Internet-Draft Loon LLC
Intended status: Standards Track July 8, 2019
Expires: January 9, 2020
IPv6 Path MTU Option
draft-kline-6man-pmo-00
Abstract
This document describes an IPv6 Neighbor Discovery (ND) Path MTU
Option (PMO) for inclusion in Router Advertisements (RAs). This
allows some environments greater flexibility to support, for example,
a higher MTU for on-link or intra-administrative-domain
communications than for broader Internet communications.
Status of This Memo
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1. Introduction
This document describes an IPv6 Neighbor Discovery (ND) Path MTU
Option (PMO) for inclusion in Router Advertisements (RAs). This
allows some environments greater flexibility to support, for example,
a higher MTU for on-link or intra-administrative-domain
communications than for broader Internet communications.
TBD: Explain why extending RFC4191 RIOs didn't look easy.
TBD: more discussion
2. Terminology
2.1. Requirements Language
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.
2.2. Summary of key terms
For the purposes of this document the following terms are used as
described here.
2.2.1. Link MTU
The MTU of the link ([RFC4861]); alternatively, the MTU as it would
be determined were no Path MTU Option (Section 2.2.5) present. This
may be:
the value specified in an MTU Option (Section 2.2.4),
a value specified in a separate document that covers operating IP
over a particular link type (e.g., [RFC2464]), or
a value derived by other means (e.g. administrative or a hint from
a sub-IP layer mechanism).
The Link MTU MUST be the initial Path MTU used when transmitting to
any link-local destination.
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2.2.2. Path MTU
TBD: Path MTU
2.2.3. Path MTU Discovery
TBD: Path MTU Discovery (cite [RFC8201])
2.2.4. MTU Option
The MTU Option is defined in [RFC4861] section 4.6.4. In this
documented it may also be referred to as the Link MTU Option, in
order to disambiguate it from this the Path MTU Option
(Section 2.2.5).
2.2.5. Path MTU Option
The IPv6 ND Path MTU Option is described in this document. It
provides more explicit signaling of the best initial Path MTU value
for a given set of destinations when sending via the advertising
router.
3. Path MTU Option Format
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 #1 (upper 16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU #1 (lower 16 bits) | num prefixes | prefix len #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| prefix #1 ... |
. .
. .
. .
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Fields:
Type TBD
Length The length of the option in units of 8 octets.
When not set, the receiving node MUST NOT make any
assumptions of exclusive use of the specified Prefix, i.e.
processing is unchanged from previous standards behavior.
Option The contents of the option as described below.
contents
The Path MTU Option contents are encoded as a repeated sequence of:
4-octet MTU value
1-octet number of prefixes
sequence of prefixes to which this MTU applies
where each prefix is encoded as:
1-octet prefix length
variable length leading bits of prefix or IP address
Each sequence of octets representing a prefix uses only as many
octets as required to by the prefix length (e.g. for a prefix length
of 0: 0 octets are required, for prefix lengths 1 through 8: 1 octet
is required, and so on).
The option is padded with zero-valued octets to the 8 octet boundary
as given by the option length.
4. Option Processing Rules
Nodes compliant with this specification do not change their
processing of RAs that contain no Path MTU Options. Additionally,
for all Path MTU determination, an effective Path MTU learned via a
Path MTU Discovery mechanism ([RFC8201]) MUST take precedence.
Any IPv6 link-local prefixes listed within a Path MTU Option MUST be
ignored by the receiver and SHOULD be logged for review by an
administrator.
Any MTU value lower than the IPv6 minimum MTU (i.e. 1280, [RFC8200]
section 5), SHOULD be logged for administrator review as a
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configuration error and MUST be treated by the receiver as though the
IPv6 minimum MTU had been the encoded value.
The Link MTU MUST also be used for all on-link destinations, to
maintain compatibility with existing behavior and expectations. For
the same reason, the Link MTU SHOULD be used for destinations within
any PIO prefix in the RA, even if the L bit is not set. As noted in
[RFC5942], a destination may at some time be learned to be on-link,
and this information may expire or be changed.
For all other destinations considered reachable via the option's
advertising router, an initial Path MTU SHOULD be determined by first
looking for a prefix that includes the destination in a Path MTU
Option and using the corresponding MTU value. If no such prefix
exists, the Link MTU SHOULD be assumed to be the default.
Note that as a matter of convenience a Path MTU Option may contain an
entry for ::/0 even when the router lifetime is zero. This in no way
indicates that the router will function as a default gateway.
Rather, it may be used, for example, to apply a Path MTU to all
prefixes listed in a set of RIOs.
5. Examples
TBD
6. Security Considerations
TBD
7. References
7.1. 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>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007, <https://www.rfc-
editor.org/info/rfc4861>.
[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>.
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[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017, <https://www.rfc-
editor.org/info/rfc8200>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017, <https://www.rfc-
editor.org/info/rfc8201>.
7.2. Informative References
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
<https://www.rfc-editor.org/info/rfc2464>.
[RFC5942] Singh, H., Beebee, W., and E. Nordmark, "IPv6 Subnet
Model: The Relationship between Links and Subnet
Prefixes", RFC 5942, DOI 10.17487/RFC5942, July 2010,
<https://www.rfc-editor.org/info/rfc5942>.
Author's Address
Erik Kline
Loon LLC
1600 Amphitheatre Parkway
Mountain View, California 94043
US
Email: ek@loon.com
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