Internet DRAFT - draft-gomez-lpwan-fragmentation-header
draft-gomez-lpwan-fragmentation-header
lpwan Working Group C. Gomez
Internet-Draft J. Paradells
Intended status: Standards Track UPC/i2CAT
Expires: April 26, 2017 J. Crowcroft
University of Cambridge
October 23, 2016
LPWAN Fragmentation Header
draft-gomez-lpwan-fragmentation-header-03
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 maximum payload size
below 13 bytes. This specification defines an optimized
fragmentation header for LPWAN.
Status of This Memo
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This Internet-Draft will expire on April 26, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
2. FHL rules and format . . . . . . . . . . . . . . . . . . . . 3
3. Changes from RFC 4944 fragmentation header and rationale . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
7. Annex A. Quantitative comparison of RFC 4944 fragmentation
header with LFH . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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-lpwan-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 mechanism at the
adaptation layer below IPv6.
The 6LoWPAN fragmentation mechanism [RFC4944] is appropriate for IEEE
802.15.4-2003 (which has a frame payload size of 81 to 102 bytes).
However, 6LoWPAN fragmentation it is not suitable for several LPWAN
technologies. Overhead of the 6LoWPAN fragmentation header is high,
considering the reduced payload size of LPWAN technologies (many of
which have a maximum payload size that is one order of magnitude
below that of IEEE 802.15.4-2003) and the limited energy availability
of the devices using such technologies. Furthermore, the datagram
offset field of the 6LoWPAN fragmentation header is expressed in
increments of eight octets. The 6LoWPAN fragmentation header plus
eight octets from the original datagram exceeds the available space
in the layer 2 (L2) payload of some LPWAN technologies, thus 6LoWPAN
fragmentation cannot be used to carry IPv6 packets over these.
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This specification defines the LPWAN Fragmentation Header (LFH).
While LFH has been designed for LPWAN technologies, other L2
technologies beyond the LPWAN category may benefit from using LFH.
It is expected that this specification will be used jointly with
other mechanisms such as header compression.
The benefits of using LFH 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),
LFH has a size of 2 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, LFH enables the transport of IPv6 packets over L2 data
units with a maximum payload size as small as only 3 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. FHL 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
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0| datagram_size | 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
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1| datagram_offset | 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), expressed in octets. 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).
tag: The value of 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 tag SHALL wrap from 7 back to zero. This field
is 3 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.
Note: the first bit of the LFH formats defined above could be used to
identify an LFH header (when set to 1) or another header (when set to
0). This will need to be aligned with work-in-progress header
compression specifications for LPWAN. The second bit in an LFH
format determines whether a fragment is the first one or a subsequent
one.
The recipient of link fragments SHALL use (1) the sender's L2 source
address (if present), (2) the destination's L2 address (if present),
(3) datagram_size, and (4) 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
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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
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 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).
Implementers need to be aware that in some LPWAN technologies, the
MTU in use may vary over time.
3. Changes from RFC 4944 fragmentation header and rationale
This specification has used RFC 4944 fragmentation header format as a
basis. 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 tag size is reduced from 2 bytes to 3 bits. Rationale: Given
the low bit rate, as well as the low message rate of LPWAN
technologies, ambiguities due to datagram tag wrapping events are
expected to occur with low probability despite the reduced tag space.
The reduced tag size provides significant overhead decrease.
-- The original 1-byte RFC 4944 6LoWPAN Dispatch field is not used.
Instead, two bits are used to signal an LFH header and whether a
fragment is the first one or not (this, to be aligned with on-going
work on header compression specifications).
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-- 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
TBD
5. Security Considerations
6LoWPAN fragmentation attacks have been analyzed in the literature.
Countermeasures to these have been proposed as well [HHWH].
A node can perform a buffer reservation attack by sending a first
fragment to a target. Then, the receiver will reserve buffer space
for the whole packet on the basis of the datagram size announced in
that first fragment. Other incoming fragmented packets will be
dropped while the reassembly buffer is occupied during the reassembly
timeout. Once that timeout expires, the attacker can repeat the same
procedure, and iterate, thus creating a denial of service attack.
The (low) cost to mount this attack is linear with the number of
buffers at the target node. However, the cost for an attacker can be
increased if individual fragments of multiple packets can be stored
in the reassembly buffer. To further increase the attack cost, the
reassembly buffer can be split into fragment-sized buffer slots.
Once a packet is complete, it is processed normally. If buffer
overload occurs, a receiver can discard packets based on the sender
behavior, which may help identify which fragments have been sent by
an attacker.
In another type of attack, the malicious node is required to have
overhearing capabilities. If an attacker can overhear a fragment, it
can send a spoofed duplicate (e.g. with random payload) to the
destination. A receiver cannot distinguish legitimate from spoofed
fragments. Therefore, the original IPv6 packet will be considered
corrupt and will be dropped. To protect resource-constrained nodes
from this attack, it has been proposed to establish a binding among
the fragments to be transmitted by a node, by applying content-
chaining to the different fragments, based on cryptographic hash
functionality. The aim of this technique is to allow a receiver to
identify illegitimate fragments.
Further attacks may involve sending overlapped fragments (i.e.
comprising some overlapping parts of the original datagram) or
announcing a datagram size in the first fragment that does not
reflect the actual amount of data carried by the fragments.
Implementers should make sure that correct operation is not affected
by such events.
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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.
The authors would like to thank Carsten Bormann, Tom Phinney, Ana
Minaburo and Laurent Toutain for valuable comments that helped
improve the document.
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grant CAS15/00336. Part of 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 LFH
+-------------------------------------------------------+
| IPv6 datagram size (bytes) |
+-------------+-------------+-------------+-------------+
| 11 | 40 | 100 | 1280 |
+------------------+-------------+-------------+-------------+-------------+
|L2 payload (bytes)| 4944 | LFH | 4944 | LFH | 4944 | LFH | 4944 | LFH |
+------------------+-------------+-------------+-------------+-------------+
| 10 | ---- | 2 | ---- | 5 | ---- | 13 | ---- | 160 |
+------------------+-------------+------+------+------+------+-------------+
| 15 | 1 | 1 | 5 | 4 | 13 | 8 | 160 | 99 |
+------------------+-------------+------+------+------+------+-------------+
| 20 | 1 | 1 | 4 | 3 | 12 | 6 | 159 | 62 |
+------------------+-------------+------+------+------+------+-------------+
| 25 | 1 | 1 | 3 | 2 | 7 | 5 | 80 | 56 |
+------------------+-------------+------+------+------+------+-------------+
| 30 | 1 | 1 | 2 | 2 | 5 | 4 | 54 | 46 |
+------------------+-------------+------+------+------+------+-------------+
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 | LFH | 4944 | LFH | 4944 | LFH | 4944 | LFH |
+------------------+-------------+-------------+-------------+-------------+
| 10 | ---- | 4 | ---- | 10 | ---- | 26 | ---- | 320 |
+------------------+-------------+------+------+------+------+-------------+
| 15 | 0 | 0 | 24 | 8 | 64 | 16 | 799 | 198 |
+------------------+-------------+------+------+------+------+-------------+
| 20 | 0 | 0 | 19 | 6 | 59 | 12 | 794 | 144 |
+------------------+-------------+------+------+------+------+-------------+
| 25 | 0 | 0 | 14 | 4 | 34 | 10 | 399 | 112 |
+------------------+-------------+------+------+------+------+-------------+
| 30 | 0 | 0 | 9 | 4 | 24 | 8 | 269 | 92 |
+------------------+-------------+------+------+------+------+-------------+
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
[HHWH] Hummen et al, R., "6LoWPAN fragmentation attacks and
mitigation mechanisms", 2013.
[I-D.minaburo-lpwan-gap-analysis]
Minaburo, A., Gomez, C., Toutain, L., Paradells, J., and
J. Crowcroft, "LPWAN Survey and GAP Analysis", draft-
minaburo-lpwan-gap-analysis-02 (work in progress), October
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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