An example use case is that on certain multiple-access wireless links the optimal MTU for a particular neighbor may be proportional to certain QoS metrics, e.g., the signal to noise ratio sensed at the receiver's MAC layer.
I submitted a draft proposal back in January 2003 (attached below) that suggests one possible approach by allowing NA messages to include an MTU option (should I submit an update?).
Another approach would be to allow different semantics for the interpretation of MTU options in RA messages based on the link type.
Comments?
Fred [EMAIL PROTECTED]
Iljitsch van Beijnum wrote:
I'm not sure this should go into a replacement specification for RFC 2461, but I'll bring it up anyway:
Currently, routers can advertise an MTU for a link. That's nice. But what we really need is a way for hosts to find out the MTU each individual neighbor can handle. 100 Mbps and slower ethernet interfaces can typically handle only the standard 1500 byte ethernet MTU, while gigabit ethernet interfaces usually support a much larger MTU.
However, in most cases hosts with different MTUs are present on the same subnet, so simply advertising a larger MTU wouldn't solve this. (Not that this would work anyway as hosts are instructed to only listen to MTU advertisements where the MTU is between 1280 and 1500 (for ethernet).)
But if hosts can tell each other the MTU they support, each set of two hosts is always able to use the largest possible MTU between them. (This would also require a new link MTU option that conveys the maximum MTU the lower layer equipment supports. Switches have their own MTU and even some hubs start doing strange things when a larger than expected MTU is used.)
BTW, some duplication seems to have crept into the document:
variable MTU - a link that does not have a well-defined MTU (e.g., IEEE 802.5 token rings). Many links (e.g., Ethernet) have a standard MTU defined by the link- layer protocol or by the specific document describing how to run IP over the link layer.
variable MTU - Neighbor Discovery allows routers to specify a MTU for the link, which all nodes then use. All nodes on a link must use the same MTU (or Maximum Receive Unit) in order for multicast to work properly. Otherwise when multicasting a sender, which can not know which nodes will receive the packet, could not determine a minimum packet size all receivers can process.
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Network Working Group F. Templin
Internet-Draft Nokia
Expires: August 1, 2003 January 31, 2003
MTU Issues in IPv6 Neighbor Discovery
draft-ietf-templin-ndiscmtu-00.txt
Status of this Memo
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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: [EMAIL PROTECTED]
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