Intarea Working Group                                         C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                      September 09, 2013
Expires: March 13, 2014

               Adaptation Layer Fragmentation Indication


   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
   mailing list.

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Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on March 13, 2014.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Objectives and Considerations . . . . . . . . . . . . . . . .   3
   3.  The ALFI option . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Should this be done at all? . . . . . . . . . . . . . . .   6
     4.2.  Is this the right way to do it? . . . . . . . . . . . . .   6
   5.  Another way of doing it . . . . . . . . . . . . . . . . . . .   6
     5.1.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8

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     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   (To be written - for now please read the Abstract.)

1.1.  Terminology

   The following terms are used in this specification:

   ALF:  Adaptation Layer Fragmentation.

   MUALTU:  Maximum Unfragmented Adaptation Layer Transmission Unit,
      i.e.  the largest piece of IPv6 packet (measured in bytes) that
      can be transferred by the adaptation layers on the path without
      invoking ALF.

   IFMUALTU:  Initial-Fragment MUALTU, the MUALTU for the initial
      adaptation layer fragment of an IP packet.

   FFMUALTU:  Following-Fragment MUALTU, the estimated minimum MUALTU
      for all but the initial adaptation layer fragments of an IP

   ALFI:  Adaptation Layer Fragmentation Indication, i.e.  indication
      that ALF was performed on a packet.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119] when they
   appear in ALL CAPS.  These words may also appear in this document in
   lower case as plain English words, absent their normative meanings.

   The term "byte" is used in its now customary sense as a synonym for

2.  Objectives and Considerations

   This draft is shaped by the requirements of 6LoWPAN networks
   [I-D.bormann-6lowpan-roadmap], including variants such as Bluetooth/
   Low Energy [I-D.ietf-6lowpan-btle] or DECT/ULE
   [I-D.mariager-6lowpan-v6over-dect-ule].  However, it should be
   beneficial with any adaptation layer that requires the use of ALF.

   One important consideration for ALFI is that the ALF scheme may not
   be able to provide a consistent MUALTU.  E.g., header compression may
   cause variable overheads, and initial and following fragments are

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   likely to cause different MUALTUs.  Header compression may be
   dependent on the specific characteristics of the packets employed, so
   indications will be most accurate if they can be made on the basis of
   actual packets as they are intended to be transferred.  (E.g., for a
   6LoWPAN with a physical layer maximum packet size of 127 bytes, the
   specific combination of MAC layer overheads, adaptation layer
   overheads, and header compression gains can turn the actual initial
   fragment size number into anything roughly between 70 and 160 bytes.
   Note that this is more than a factor of two, making it difficult to
   just guess a good MUALTU.)

   Therefore, ALFI provides the ability to equip packets with a probe
   that collects any information for adaptation layer fragmentation that
   may be available on the path.

   Note that probing for MUALTUs is likely to change the MUALTU.
   Implementations SHOULD attempt to indicate a MUALTU for an equivalent
   non-probe packet, i.e.  the packet under consideration with the ALFI
   option (and its hop-by-hop header, if applicable) removed.  If that
   is not possible, implementations SHOULD err towards indicating
   smaller MUALTUs, within reason.

   Obviously, not all nodes will immediately implement ALFI.  ALFI just
   "fails ignorant" (but see below).

   An adaptation layer instance may want to manipulate ALFI for other
   reasons than to indicate ALF (which would be somewhat comparable to
   the widespread practive of TCP "MSS clamping").  (In particular, as
   long as it can be expected that some other nodes on the path don't
   have ALFI yet, a border router such as a 6LBR [RFC6775] may want to
   provide some ALFI guessing.)

   Generally speaking, ALFI can be used as a mechanism to indicate any
   significant, step function degradation of some performance metric
   based on packet size.  However, as the mechanism can only collect a
   single value for the entire path (i.e., one IFMUALTU and one
   FFMUALTU), the performance degradation indicated SHOULD be
   significant.  In other words, ALFI indications SHOULD NOT be set for
   segmentation implementations where segmentation causes limited
   performance impact.  E.g., AAL5 implementations SHOULD NOT set ALFI.

3.  The ALFI option

   The ALFI option is an IPv6 option in the sense of section 4.2 of
   [RFC2460].  It is only used in the hop-by-hop header.

   The option type identifier is chosen to select the following behavior
   as detailed in section 4.2 of [RFC2460]:

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   o  00 - skip over this option and continue processing the header
      (this enables the "fail-ignorant" backwards compatibility

   o  1 - Option Data may change en-route (the option is used to record
      information en-route)

   .                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                               |0 0 1 x x x x x|       4       |
   |    Initial-Fragment MUALTU    |   Following-Fragment MUALTU   |

                    Figure 1: A hop-by-hoption for ALFI

   In IFMUALTU and FFMUALTU, the value zero represents infinity.  All
   other values are unsigned integers in network byte order,
   representing a MUALTU in bytes.

   The originator of a packet MAY, for occasional probing, insert an
   ALFI option into packets where it can choose the packet size and the
   performance metrics of which are important to the application.

   When generating the IP packet, the originator sets Initial-Fragment
   MUALTU (IFMUALTU) and Following-Fragment MUALTU (FFMUALTU) to zero.
   (Its own adaptation layer can then already update them as described
   in the following paragraphs before the packet even leaves the

   Each instance of an adaptation layer that employs ALF and that
   implements this specification computes its own estimate of IFMUALTU
   and FFMUALTU for the type of packet that has this option, ignoring
   the option itself and, if the option was the only option in the hop-
   by-hop header, the hop-by-hop header.  For each estimate, if it is
   below the value of the respective field encoded in the option (where
   zero represents infinity), the instance updates the field to the

   The receiver of the packet relays the information in the ALFI option
   to the transport layer and/or application.

   (TBD: How to ship this information through the IPv6 socket interface
   [RFC3493] [RFC3542].  Constrained implementations won't have this
   specific problem.)

   The receiving transport layer and/or application can then make this
   information available back to the peer instance, which enables the
   latter to choose IPv6 packet sizes of IFMUALTU or lower, or, if this

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   cannot be achieved, at least below IFMUALTU+n*FFMUALTU for a small n.
   For instance, in CoAP [I-D.ietf-core-coap], the receiver of an ALFI
   probe from a server can use the Block2 option [I-D.ietf-core-block]
   to negotiate a block size for further messages in a block-wise
   transfer accordingly.

4.  Discussion

   The discussion of this proposal should center around two questions.

4.1.  Should this be done at all?

   I.e., can't we just live with the inefficiency?

   In ATM or DVB, the inefficiency caused by not knowing about
   adaptation layer fragmentation was limited.  In constrained node
   networks [I-D.ietf-lwig-terminology], the issue becomes more urgent.
   One reason is that there the packet delivery rate is lower.  There
   also is more variability in the actual values of the MUALTUs.

   On the other hand, it is likely that one of the ends of a path that
   traverses a constrained node network is embedded in this constrained
   node network, so it may be able to make an educated guess for the
   path MUALTUs.

4.2.  Is this the right way to do it?

   IPv6 hop-by-hop options seem to be seriously challenged in the
   greater Internet.  Unfortunately, they are the only way to collect
   this kind of information on every hop.  Quick-Start [RFC4782] uses
   them in a similar way, but within what is mostly a controlled
   environment, where it is easier to make sure the option is indeed
   processed and not damaged.  Many constrained node networks can also
   be considered relatively controlled networks.  However, the
   corresponding node may be in the greater Internet, and it may neither
   be able to emit a hop-by-hop option in such a way that it arrives at
   the edge of the constrained node network nor be able to receive
   information that has been collected on the way out of a constrained
   node network.

   This may be remedied by assigning the constrained node network border
   router (e.g., a 6LBR) a specific role in this protocol.  This needs
   further discussion.

5.  Another way of doing it

   Given the questionable usability of the IPv6 hop-by-hop option,
   another approach is to rely on the increasing on-path inspection

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   capabilities of modern routers.  ALFI probes are sent as UDP packets,
   with a 20-byte payload as depicted in Figure 2:

   |                           0x24680000                          |
   |                           0x2112A442                          |
   |                             parity                            |
   |                             random                            |
   |    Initial-Fragment MUALTU    |   Following-Fragment MUALTU   |

                Figure 2: A STUN-like UDP payload for ALFI

   The parity field is computed to make the 32-bit XOR of all five words
   zero (even parity).  The random field is filled with a suitable
   pseudorandom number (with no requirement for cryptographic quality).
   All parameters of the IP and UDP header are chosen as for the data
   flow for which the MUALTU values are needed.

   A router that implements ALFI needs to detect transiting UDP packets
   that match the structure of the ALFI probe.  When updating the
   values, both the parity field in the ALFI payload and the UDP
   checksum need to be updated as well.

5.1.  Discussion

   Carrying ALFI probes in UDP data packets enables correspondent nodes
   on general purpose operating systems to send and receive these probes
   without any special interface such as that defined in [RFC3542].

   For best quality of the header compression performance prediction,
   ALFI probes are required to look very similar to the actual data
   packets.  This means this approach only works with protocols using
   UDP payloads.  For use with CoAP [I-D.ietf-core-coap], this is not a

   The application protocol must be able to ignore packets that look
   like ALFI probes.  Again, for use with CoAP [I-D.ietf-core-coap],
   this is not a problem.  (The design might be refined to enable
   interoperability with other protocols as desired.)

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   This approach routes around the damage done to the hop-by-hop header
   by creating damage to the data transparency of the Internet.  The
   conditions under which this is an acceptable trade-off must be
   carefully evaluated.

6.  IANA Considerations

   For the hop-by-hop option approach described in Section 3, IANA needs
   to allocate an IPv6 option number for the ALFI option, "Destination
   Options and Hop-by-Hop Options" registry in "Internet Protocol
   Version 6 (IPv6) Parameters", with act=00 and chg=1 (i.e., similar to
   the Quick-Start option [RFC4782]).

   For the STUN-related approach described in Section 5, the message
   type 0b10_010_011_1000 = 0x938 should be reserved in the STUN Methods

7.  Security Considerations

   It is hard to like hop-by-hop options from a security point of view.

   (This section will certainly grow as additional security
   considerations beyond those listed in the base specifications become

8.  Acknowledgements

   Peter van der Stok prompted the author to finally write up this
   protocol, a couple of years after the need for it had been shown in
   [WEI].  He then also provided a number of editorial comments that
   improved the document.

   Toerless Eckert pointed out that routers are today able to do on-path
   inspection, the approach that makes Section 5 possible.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S.E. and R.M. Hinden, "Internet Protocol, Version
              6 (IPv6) Specification", RFC 2460, December 1998.

9.2.  Informative References


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              Bormann, C., "6LoWPAN Roadmap and Implementation Guide",
              draft-bormann-6lowpan-roadmap-04 (work in progress), April

              Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets
              over BLUETOOTH Low Energy", draft-ietf-6lowpan-btle-12
              (work in progress), February 2013.

              Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-12 (work in progress), June 2013.

              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

              Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained Node Networks", draft-ietf-lwig-terminology-05
              (work in progress), July 2013.

              Mariager, P., Petersen, J., and Z. Shelby, "Transmission
              of IPv6 Packets over DECT Ultra Low Energy", draft-
              mariager-6lowpan-v6over-dect-ule-03 (work in progress),
              July 2013.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6", RFC
              3493, February 2003.

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, May 2003.

   [RFC4782]  Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
              Start for TCP and IP", RFC 4782, January 2007.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [RFC6775]  Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann,
              "Neighbor Discovery Optimization for IPv6 over Low-Power
              Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
              November 2012.

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   [WEI]      Shelby, Z. and C. Bormann, "6LoWPAN: the Wireless Embedded
              Internet", ISBN 9780470747995, 2009.

Author's Address

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Phone: +49-421-218-63921

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