Internet DRAFT - draft-templin-ndiscmtu
draft-templin-ndiscmtu
Network Working Group F. Templin
Internet-Draft Nokia
Expires: August 1, 2003 January 31, 2003
MTU Issues in IPv6 Neighbor Discovery
draft-templin-ndiscmtu-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document discusses Maximum Transmission Unit (MTU) issues in
IPv6 Neighbor Discovery and suggests minor augmentations to the
existing specification to rectify the issues.
1. Introduction
This document discusses Maximum Transmission Unit (MTU) issues in
Neighbor Discovery for IP Version 6 (IPv6) [1]. It argues that the
current specification is too restrictive in the use of MTU options,
and that per-neighbor MTU values should be maintained in the
conceptual Neighbor Cache. It finally proposes minor augmentations
to the existing specification to rectify the issues.
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2. Problem Statement
([1], section 4.6.4) defines a Maximum Transmission Unit (MTU) option
type. In the current specification, the MTU option is sent only in
Router Advertisement messages ([1], section 4.2) and interpreted by
receivers as the MTU value for all nodes on the link to use. While
this specification provides sufficient mechanism for many of the
supported link types in ([1], section 2.2), it may lead to
inefficiencies for other types, e.g., for certain variable MTU links.
In particular, it may be desirable in some cases for a node to track
independent MTU values for different neighbors on a link. For
example, on certain multiple-access wireless links the optimal MTU
for a particular neighbor may be proportional to the signal to noise
ratio sensed at the receiver's MAC layer. In another example,
constrained nodes on a link with large MTU may wish to receive
smaller packets than more robust nodes. In these and other cases,
maintaining independent per-neighbor MTUs for nodes on a link would
yield significant efficiency advantages.
3. Proposed Changes
In order to support per-neighbor MTUs, the following changes/
augmentations to RFC 2461 are proposed:
1. In ([1], section 4.6.4), add new text allowing the encoding of
MTU options in Neighbor Acknowledgement messages.
2. In ([1], section 5.1), add a per-neighbor MTU field (NBR_MTU) in
the Neighbor Cache data structure.
3. In ([1], section 6.3.2), add a new host variable, defined as
follows:
MaxLinkMTU
The maximum MTU supported on the link.
Default: LinkMTU.
4. In ([1], section 7.2.4), add new text allowing the encoding of
MTU options in solicited Neighbor Advertisements."
5. In ([1], section 7.2.5), add new text requiring the value in MTU
options received in Neighbor Advertisement messages be written
into the NBR_MTU field in the Neighbor Cache entry.
6. In (([1], section 7.2.6), add new text allowing Unsolicited
Neighbor Advertisements sent to the unicast address of a
neighbor.
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4. Operational Details
When nodes implement the changes proposed above, Neighbor
Advertisement messages containing MTU options provide a dynamic
mechanism for receivers to inform senders of MTU changes. Documents
that specify the operation of IPv6 over specific link layers (e.g.,
Ethernet, FDDI, etc.) provide details for the encoding of MTU options
in Neighbor Advertisement messages.
Receipt of an MTU option in the initial solicited Neighbor
Advertisement provides an indication to the sender that the receiver
implements the dynamic MTU mechanism. (Lack thereof conversely
provides an indication that the receiver does not implement the
dynamic MTU mechanism.)
Using these proposed changes, MTU options received in Router
Advertisements affect LinkMTU exactly as in the current
specification. However, packetization and forwarding layers see an
MTU of MaxLinkMTU when they examine the link. The relationship
between the three MTU parameters is as follows:
LinkMTU <= NbrMTU <= MaxLinkMTU
Multicast destinations, and unicast destinations that use a next hop
with no MTU support indicated in the Neighbor Cache, see an MTU of
LinkMTU.
5. IANA Considerations
TBD
6. Security considerations
TBD
7. Acknowledgements
TBD
Normative References
[1] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
Informative References
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Author's Address
Fred L. Templin
Nokia
313 Fairchild Drive
Mountain View, CA 94110
US
Phone: +1 650 625 2331
EMail: ftemplin@iprg.nokia.com
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