Intarea Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track July 14, 2012
Expires: January 15, 2013
Adaptation Layer Fragmentation Indication
draft-bormann-intarea-alfi-01
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 a mechanism 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.
The main objective of this version of the draft is to present a
complete design in order to be able to gauge the complexity of the
approach against the gains to be expected from implementing it.
Comments are appreciated and should go to the intarea@ietf.org
mailing list.
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Status of this Memo
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This Internet-Draft will expire on January 15, 2013.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Objectives and Considerations . . . . . . . . . . . . . . . . 5
3. The ALFI option . . . . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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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
packet.
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
"octet".
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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
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.
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 [I-D.ietf-6lowpan-nd]
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.
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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]:
o 00 - skip over this option and continue processing the header
(this enables the "fail-ignorant" backwards compatibility
behavior)
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
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
originator.)
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
estimate.
The receiver of the packet relays the information in the ALFI option
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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
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.
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4. IANA Considerations
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]).
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5. 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
known.)
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6. 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.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
7.2. Informative References
[I-D.bormann-6lowpan-roadmap]
Bormann, C., "6LoWPAN Roadmap and Implementation Guide",
draft-bormann-6lowpan-roadmap-01 (work in progress),
March 2012.
[I-D.ietf-6lowpan-btle]
Nieminen, J., Patil, B., Savolainen, T., Isomaki, M.,
Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets
over Bluetooth Low Energy", draft-ietf-6lowpan-btle-08
(work in progress), June 2012.
[I-D.ietf-6lowpan-nd]
Shelby, Z., Chakrabarti, S., and E. Nordmark, "Neighbor
Discovery Optimization for Low Power and Lossy Networks
(6LoWPAN)", draft-ietf-6lowpan-nd-18 (work in progress),
October 2011.
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-08 (work in progress),
February 2012.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-10 (work in progress), June 2012.
[I-D.mariager-6lowpan-v6over-dect-ule]
Mariager, P. and J. Petersen, "Transmission of IPv6
Packets over DECT Ultra Low Energy",
draft-mariager-6lowpan-v6over-dect-ule-02 (work in
progress), May 2012.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, February 2003.
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[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.
[WEI] Shelby, Z. and C. Bormann, "6LoWPAN: the Wireless Embedded
Internet", ISBN 9780470747995, 2009.
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Author's Address
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Fax: +49-421-218-7000
Email: cabo@tzi.org
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