I-D Action: draft-bormann-intarea-alfi-04.txt

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	Title           : Adaptation Layer Fragmentation Indication
	Author(s)       : Carsten Bormann
	Filename        : draft-bormann-intarea-alfi-04.txt
	Pages           : 10
	Date            : 2013-09-09

Abstract:
   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.

   Comments are appreciated and should go to the [e-mail]
   mailing list.


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