IPv6 defines a minimum MTU of 1280 bytes. Many link layers are more
limited in the maximum size of packets they can communicate. In
order to enable the transport of IP packets that are too large for
these link layers, typically their IP adaptation layers define a
segmentation or fragmentation scheme to transport an IP packet in a
sequence of multiple link layer packets.
Often, adaption layer fragmentation schemes reduce some performance
metric, such as the packet delivery probability. Application or
transport protocols may be able to reduce the maximum size of packets
they send, e.g. by transport layer segmentation or choice of
application layer data object size, which may have less of a
performance impact. It would therefore be desirable for them to know
about any adaptation layer fragmentation that is going on, so they
can choose packet sizes that minimize adaptation layer fragmentation.
At the IP layer, fragmentation can be detected using a number of
mechanisms used in Packetization Layer Path MTU Discovery [RFC4821].
However, adaptation layer fragmentation schemes are often designed to
be "transparent", i.e. there is no way at higher layers to find out
whether they had to be employed (except maybe by elaborate
measurement schemes targeting one of the impacted performance
metrics; this approach does not appear to be viable) [WEI].
The present specification defines two alternative mechanisms for IPv6
adaptation layers to indicate the presence of adaptation layer
fragmentation on one or more hops on the path from an IP sender to an
IP receiver, and to provide an indication of preferred (smaller)
packet sizes on these hops.
One design is based on the the IPv6 design and probably doesn't work
on the Internet. The other design goes strictly against the IPv6
design and probably works well on the Internet.
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