One document matched: draft-gomez-lpwan-fragmentation-header-01.txt
Differences from draft-gomez-lpwan-fragmentation-header-00.txt
Network Working Group C. Gomez
Internet-Draft J. Paradells
Intended status: Standards Track UPC/i2CAT
Expires: September 21, 2016 J. Crowcroft
University of Cambridge
March 20, 2016
Optimized 6LoWPAN Fragmentation Header for LPWAN
draft-gomez-lpwan-fragmentation-header-01
Abstract
LPWAN technologies are characterized by a very limited data unit and/
or payload size, often one order of magnitude below the one in IEEE
802.15.4. However, many such technologies do not support layer 2
fragmentation. The 6LoWPAN fragmentation header defined in RFC 4944
represents very high overhead for LPWAN technologies, and it even
does not support transporting IPv6 datagrams that require
fragmentation over layer 2 technologies of a payload size below 13
bytes. This specification defines an optimized 6LoWPAN fragmentation
header for LPWAN.
Status of This Memo
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This Internet-Draft will expire on September 21, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
2. 6LoFHL rules and format . . . . . . . . . . . . . . . . . . . 3
3. Changes from RFC 4944 fragmentation header and rationale . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
7. Annex A. Quantitative comparison of RFC 4944 fragmentation
header with 6LoFHL . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Low Power Wide Area Network (LPWAN) technologies are characterized,
among others, by a very reduced data unit and/or payload size
[I-D.minaburo-lp-wan-gap-analysis]. However, many such technologies
do not support layer two fragmentation, therefore the only option for
these to support IPv6 (and, in particular, its MTU requirement of
1280 bytes [RFC2460]) is the use of a fragmentation header at the
adaptation layer below IPv6, such as the 6LoWPAN fragmentation header
defined in [RFC4944].
While the aforementioned 6LoWPAN fragmentation header is appropriate
for IEEE 802.15.4-2003 (which has a frame payload size of 81 to 102
bytes), it is not suitable for several LPWAN technologies, many of
which have a maximum payload size that is one order of magnitude
below that of IEEE 802.15.4-2003. The overhead of the 6LoWPAN
fragmentation header is high, considering the reduced payload size of
LPWAN technologies and the limited energy availability of the devices
using such technologies. Furthermore, its datagram offset field is
expressed in increments of eight octets. In some LPWAN technologies,
the 6LoWPAN fragmentation header plus eight octets from the original
datagram exceeds the available space in the layer 2 (L2) payload.
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This specification defines an optimized 6LoWPAN Fragmentation Header
for LPWAN (6LoFHL). Nevertheless, other L2 technologies beyond the
LPWAN category may benefit from using 6LoFHL.
It is expected that this specification will be used jointly with
other 6Lo(WPAN) mechanisms such as [RFC6282] based header
compression.
The benefits of using 6LoFHL are the following:
-- While the 6LoWPAN fragmentation header defined in RFC 4944 has a
size of 4 bytes (first fragment) or 5 bytes (subsequent fragments),
6LoFHL has a size of 3 bytes (any fragment). This reduces
significantly both the L2 overhead and the adaptation layer overhead
for transporting an IPv6 packet that requires fragmentation (see
Annex A).
-- Because the datagram offset can be expressed in increments of a
single octet, 6LoFHL enables the transport of IPv6 packets over L2
data units with a maximum payload size as small as only 4 bytes in
the most extreme case.
1.1. Conventions used in this document
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]
2. 6LoFHL rules and format
If an entire payload (e.g., IPv6) datagram fits within a single L2
data unit, it is unfragmented and a fragmentation header is not
needed. If the datagram does not fit within a single L2 data unit,
it SHALL be broken into fragments. The first fragment SHALL contain
the first fragment header as defined in Figure 1.
1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 0 0 1| datagram_size | datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: First Fragment
The second and subsequent fragments (up to and including the last)
SHALL contain a fragmentation header that conforms to the format
shown in Figure 2.
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1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 0 1 0| datagram_offset | datagram_tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Subsequent Fragments
datagram_size: This 11-bit field encodes the size of the entire IP
packet before link-layer fragmentation (but after IP layer
fragmentation). For IPv6, the datagram size SHALL be 40 octets (the
size of the uncompressed IPv6 header) more than the value of Payload
Length in the IPv6 header [RFC4944] of the packet. Note that this
packet may already be fragmented by hosts involved in the
communication, i.e., this field needs to encode a maximum length of
1280 octets (the required by IPv6).
datagram_tag: The value of datagram_tag (datagram tag) SHALL be the
same for all fragments of a payload (e.g., IPv6) datagram. The
sender SHALL increment datagram_tag for successive, fragmented
datagrams. The incremented value of datagram_tag SHALL wrap from 255
back to zero. This field is 8 bits long, and its initial value is
not defined.
datagram_offset: This field is present only in the second and
subsequent fragments and SHALL specify the offset, in increments of 1
octet, of the fragment from the beginning of the payload datagram.
The first octet of the datagram (e.g., the start of the IPv6 header)
has an offset of zero; the implicit value of datagram_offset in the
first fragment is zero. This field is 11 bits long.
The recipient of link fragments SHALL use (1) the sender's L2 source
address, (2) the destination's L2 address, (3) datagram_size, and (4)
datagram_tag to identify all the fragments that belong to a given
datagram.
Upon receipt of a link fragment, the recipient starts constructing
the original unfragmented packet whose size is datagram_size. It
uses the datagram_offset field to determine the location of the
individual fragments within the original unfragmented packet. For
example, it may place the data payload (except the encapsulation
header) within a payload datagram reassembly buffer at the location
specified by datagram_offset. The size of the reassembly buffer
SHALL be determined from datagram_size.
If a fragment recipient disassociates from its L2 network, the
recipient MUST discard all link fragments of all partially
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reassembled payload datagrams, and fragment senders MUST discard all
not yet transmitted link fragments of all partially transmitted
payload (e.g., IPv6) datagrams. Similarly, when a node first
receives a fragment with a given datagram_tag, it starts a reassembly
timer. When this time expires, if the entire packet has not been
reassembled, the existing fragments MUST be discarded and the
reassembly state MUST be flushed. The reassembly timeout MUST be set
to a maximum of TBD seconds).
3. Changes from RFC 4944 fragmentation header and rationale
The main changes introduced in this specification to the
fragmentation header format defined in RFC 4944 are listed below,
together with their rationale:
-- The datagram size field is only included in the first fragment.
Rationale: In the RFC 4944 fragmentation header, the datagram size
was included in all fragments to ease the task of reassembly at the
receiver, since in an IEEE 802.15.4 mesh network, the fragment that
arrives earliest to a destination is not necessarily the first
fragment transmitted by the source. However, in LPWAN, such
reordering effects are not expected. LPWAN technologies typically
define star topology networks, peripheral to peripheral
communications are not expected, and the central device is not
expected to perform priority queuing operations. Nevertheless, the
fragmentation format defined in this document supports limited
reordering.
-- The datagram tag size is reduced from 2 bytes to 1 byte.
Rationale: Given the low bit rate, as well as the low message rate of
LPWAN technologies, ambiguities due to datagram tag wrapping events
are not expected to occur despite the reduced tag space.
-- The datagram offset size is increased from 8 bits to 11 bits.
Rationale: This allows to express the datagram offset in single-octet
increments.
4. IANA Considerations
This document allocates the following sixteen RFC 4944 Dispatch type
values:
11001 000
through
11001 111
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and
11010 000
through
11010 111
5. Security Considerations
TBD
6. Acknowledgments
In section 2, the authors have reused extensive parts of text
available in section 5.3 of RFC 4944, and would like to thank the
authors of RFC 4944.
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grant CAS15/00336. His contribution to this work has been
carried out during his stay as a visiting scholar at the Computer
Laboratory of the University of Cambridge.
7. Annex A. Quantitative comparison of RFC 4944 fragmentation header
with 6LoFHL
+-------------------------------------------------------+
| IPv6 datagram size (bytes) |
+-------------+-------------+-------------+-------------+
| 11 | 40 | 100 | 1280 |
+------------------+-------------+-------------+-------------+-------------+
|L2 payload (bytes)| 4944 |6LoFHL| 4944 |6LoFHL| 4944 |6LoFHL| 4944 |6LoFHL|
+------------------+-------------+-------------+-------------+-------------+
| 10 | ---- | 2 | ---- | 6 | ---- | 15 | ---- | 183 |
+------------------+-------------+------+------+------+------+-------------+
| 15 | 1 | 1 | 5 | 4 | 13 | 9 | 160 | 107 |
+------------------+-------------+------+------+------+------+-------------+
| 20 | 1 | 1 | 4 | 3 | 12 | 6 | 159 | 76 |
+------------------+-------------+------+------+------+------+-------------+
| 25 | 1 | 1 | 3 | 2 | 7 | 5 | 80 | 59 |
+------------------+-------------+------+------+------+------+-------------+
| 30 | 1 | 1 | 2 | 2 | 5 | 4 | 54 | 48 |
+------------------+-------------+------+------+------+------+-------------+
Figure 3: L2 overhead (in terms of L2 data units) required to
transport an IPv6 datagram
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+-------------------------------------------------------+
| IPv6 datagram size (bytes) |
+-------------+-------------+-------------+-------------+
| 11 | 40 | 100 | 1280 |
+------------------+-------------+-------------+-------------+-------------+
|L2 payload (bytes)| 4944 |6LoFHL| 4944 |6LoFHL| 4944 |6LoFHL| 4944 |6LoFHL|
+------------------+-------------+-------------+-------------+-------------+
| 10 | ---- | 6 | ---- | 18 | ---- | 45 | ---- | 768 |
+------------------+-------------+------+------+------+------+-------------+
| 15 | 0 | 0 | 24 | 12 | 64 | 27 | 799 | 321 |
+------------------+-------------+------+------+------+------+-------------+
| 20 | 0 | 0 | 19 | 9 | 59 | 18 | 794 | 228 |
+------------------+-------------+------+------+------+------+-------------+
| 25 | 0 | 0 | 14 | 6 | 34 | 15 | 399 | 177 |
+------------------+-------------+------+------+------+------+-------------+
| 30 | 0 | 0 | 9 | 6 | 24 | 12 | 269 | 144 |
+------------------+-------------+------+------+------+------+-------------+
Figure 4: Adaptation layer fragmentation overhead (in bytes) required
to transport an IPv6 datagram
Note 1: with the RFC 4944 fragmentation header it is not possible to
transport IPv6 datagrams of the considered sizes over a 10-byte
payload L2 technology.
Note 2: 11 bytes is the size of an IPv6 datagram with a 3-byte RFC
6282 compressed header (the shortest possible IPv6 header that uses
global addresses), a 4-byte RFC 6282 UDP compressed header, and a
CoAP message without header options and without payload.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<http://www.rfc-editor.org/info/rfc4944>.
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[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<http://www.rfc-editor.org/info/rfc6282>.
8.2. Informative References
[I-D.minaburo-lp-wan-gap-analysis]
Minaburo, A., Pelov, A., and L. Toutain, "LP-WAN GAP
Analysis", draft-minaburo-lp-wan-gap-analysis-01 (work in
progress), February 2016.
Authors' Addresses
Carles Gomez
UPC/i2CAT
C/Esteve Terradas, 7
Castelldefels 08860
Spain
Email: carlesgo@entel.upc.edu
Josep Paradells
UPC/i2CAT
C/Jordi Girona, 1-3
Barcelona 08034
Spain
Email: josep.paradells@entel.upc.edu
Jon Crowcroft
University of Cambridge
JJ Thomson Avenue
Cambridge, CB3 0FD
United Kingdom
Email: jon.crowcroft@cl.cam.ac.uk
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