Network Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Informational L. Toutain
Expires: December 23, 2016 Institut MINES TELECOM ; TELECOM Bretagne
June 21, 2016
6LPWA Static Context Header Compression (SCHC) for IPV6 and UDP
draft-toutain-6lpwa-ipv6-static-context-hc-01
Abstract
This document describes a header compression scheme for IPv6, IPv6/
UDP based on static contexts. This technique is especially tailored
for LPWA networks and could be extended to other protocol stacks.
During the IETF history several compression mechanisms have been
proposed. First mechanisms, such as RoHC, are using a context to
store header field values and send smaller incremental differences on
the link. Values in the context evolve dynamically with information
contained in the compressed header. The challenge is to maintain
sender's and receiver's contexts synchronized even with packet
losses. Based on the fact that IPv6 contains only static fields,
6LoWPAN developed an efficient context-free compression mechanisms,
allowing better flexibility and performance.
The Static Context Header Compression (SCHC) combines the advantages
of RoHC context which offers a great level of flexibility in the
processing of fields, and 6LoWPAN behavior to elide fields that are
known from the other side. Static context means that values in the
context field do not change during the transmission, avoiding complex
resynchronization mechanisms, incompatible with LPWA characteristics.
In most of the cases, IPv6/UDP headers are reduced to a small
identifier.
This document focuses on IPv6/UDP headers compression, but the
mechanism can be applied to other protocols such as CoAP. It will be
described in a separate document.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on December 23, 2016.
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described in the Simplified BSD License.
1. Introduction
Headers compression is mandatory to bring the internet protocols to
the node within a LPWA network [I-D.minaburo-lp-wan-gap-analysis].
Nevertheless, LPWA networks offer good properties for an efficient
header compression:
o Topology is star oriented, therefore all the packets follows the
same path. For the needs of this draft, the architecture can be
summarized to End-Systems (ES) exchanging information with a
single LPWA Compressor (LC). In most of the cases, End Systems
and LC form a star topology. ESs and LC maintain a context for
compression.
o Traffic flows are mostly deterministic, since End-Systems embed
built-in applications. Contrary to computers or smartphones, new
applications cannot be easily installed.
First mechanisms such as RoHC use a context to store header field
values and send smaller incremental differences on the link. The
first version of RoHC targeted IP/UDP/RTP stack. RoHCv2 extends the
principle to any protocol and introduces a formal notation [RFC4997]
describing the header and associating compression functions to each
field. To be efficient the sender and the receiver must check that
the context remains synchronized (i.e. contains the same values).
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Context synchronization imposes to periodically send a full header or
at least dynamic fields. If fully compressed, the header can be
compatible with LPWA constraints. However, the first exchanges or
context resynchronisations impose to send uncompressed headers, which
may be bigger than the original one. This will force the use of
inefficient fragmentation mechanisms. For some LPWA technologies,
duty cycle limits can also delay the resynchronization. Figure 1
illustrates this behavior.
sync
^ +-+ sync sync ^
| IPv6 | | +-+ +-+ | IPv6
v | | | | | | v
+------------+ | +-+-+ | | | | +------------+
| +--+ | | | | | | | | | | +--+ |
| | c| | | | | +-+-+-+ +-+-+-+-+ | | | c| |
| | t| | | | | | | | | | | | | | | | | t| |
| | x| | +-+-+-+-+-+-+-+-+-+-+-+-+ | | x| |
| | t| | <----------------------------> | | t| |
| +--+ | | +--+ |
+------------+ +------------+
Figure 1: RoHC Compressed Header size evolution.
On the other hand, 6LoWPAN [RFC4944] is context-free based on the
fact that IPv6, its extensions or UDP headers do not contain
incremental fields. The compression mechanism described in [RFC6282]
is based on sending a 2-byte bitmap, which describes how the header
should be decompressed, either using some standard values or sending
information after this bitmap. [RFC6282] also allows for UDP
compression.
In the best case, when Hop limit is a standard value, flow label,
DiffServ fields are set to 0 and Link Local addresses are used over a
single hop network, the 6LoWPAN compressed header is reduced to 4
bytes. This compression ratio is possible because the IID are
derived from the MAC addresses and the link local prefix is known
from both sides. In that case, the IPv6 compression is 4 bytes and
UDP compression is 2 bytes, which fills half of the payload of a
SIGFOX frame, or more than 10% of a LoRaWAN payload (with spreading
factor 12).
The Static Context Header Compression (SCHC) combines the advantages
of RoHC context, which offers a great level of flexibility in the
processing of fields, and 6LoWPAN behavior to elide fields that are
known from the other side. Static context means that values in the
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context field do not change during the transmission, avoiding complex
resynchronization mechanisms, incompatible with LPWA characteristics.
In most of the cases, IPv6/UDP headers are reduced to a small context
identifier.
2. Static Context Header Compression
Static Context Header Compression (SCHC) avoids context
synchronization, which is the most bandwidth-consuming operation in
RoHC. Based on the fact that the nature of data flows is highly
predictable in LPWA networks, a static context may be stored on the
End-System (ES). The other end, the LPWA Compressor (LC) can learn
the context through a provisionning protocol during the
identification phase (for instance, as it learns the encryption key).
The context contains an ordered list of rules. Each rule is a vector
of entries. Each entry is composed of a field descriptor, a
prescribed matching value, a matching rule for the compression side,
a matching rule for the decompression side and a compression/
decompression action. Contexts in the compressor and decompressor
are the same. A rule is identified by a rule identifier. If the
layer 2 allows it, the rule id can be carried in the layer 2 header.
Otherwise the rule id is located in the first byte of the L2 payload.
Being at the boundary between Layer 2 and Layer 3, the rule id will
also be called a shim id. Different ES will use the same shim id to
identify their own context. An LC may also use the ES device id to
identify the appropriate rule.
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+---------------------------------------------------------------------+
| Rule N |
+---------------------------------------------------------------------+ |
| Rule i | |
+---------------------------------------------------------------------+ | |
| Rule 1 | | |
| +---------+-------+------------+--------------+-----------------+ | | |
| | Field 1 | Value |match. comp.| match decomp | Action function | | | |
| +---------+-------+------------+--------------+-----------------+ | | |
| | Field 2 | Value |match. comp.| match decomp | Action function | | | |
| +---------+-------+------------+--------------+-----------------+ | | |
| | ... | ... |... | ... | ... | | | |
| +---------+-------+------------+--------------+-----------------+ | |----+
| | Field N | Value |match. comp.| match decomp | Action function | | |
| +---------+-------+------------+--------------+-----------------+ |------+
| |
+---------------------------------------------------------------------+
Figure 2: Context in LC
The compression/decompression process follows several steps:
o compression rule selection: the goal is to identify which rule
will be used to compress the headers. To each field is associated
a matching rule for compression. Each header field's value is
compared to the corresponding value stored in the rule for that
field using the matching operator. If all the fields match, the
packet is processed using this rule action functions and the rule
list exploration is aborted. Otherwise the next rule is tested.
If no rule is found, then the packet is dropped.
o compression: the action function indicates is the field is send on
the link or not. A field can also be partially sent regarding the
matching operator. The resulting compressed header must be
aligned on byte boundaries.
o decompression rule selection, as for compression, a rule has to be
selected to uncompress incoming packets. A matching operator is
defined on the compress header and works as for compression.
o decompression: the same action function indicates how the field
value can be rebuilt, either from bits received on the link, a
value stored in the rule or by using a specific algorithm.
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3. Matching operators
Matching a field with a value and header compression are related
operations; If a field matches a rule containing the value, it is not
necessary to send it on the link. Since context are synchronized,
reading the rule's value is enough to reconstruct the field's value
at the other end.
On some other cases, the value need to be sent on the link to inform
the other end. The field value may vary from one packet to another,
therefore the field cannot be used to select the rule id.
It may exist some intermediary cases, where part of the value may be
used to select a field and a variable part has to be sent on the
link. This is true for Least Significant Bits (LSB) where the most
significant bit can be used to select a rule id and the least
significant bits has to be sent on the link.
Several matching operators are defined:
o = : a field value in a packet matches with a field value in a rule
if they are equal.
o no : no check is done between a field value in a packet matches
with a field value in the rule
o lbs(L) : a field value of length T in a packet matches with a
field value in a rule if the most significant T-L bits are equal.
4. Action functions
The action functions describe the action taken by the compression and
inversely the action taken by the decompressor to restore the
original value.
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/--------------------+-------------+--------------------------\
| Function | Compression | Decompression |
| | | |
+--------------------+-------------+--------------------------+
|elided |not sent |use value stored in ctxt |
|send-value |send |build field from value |
|compute-IPv6-length |elided |compute IPv6 length |
|compute-UDP-length |elided |compute UDP length |
|compute-UDP-checksum|elided |compute UDP checksum |
|ESiid-DID |elided |build IID from L2 ES addr |
|LCiid-DID |elided |build IID from L2 LA addr |
\--------------------+-------------+--------------------------/
Figure 3: Simplified Protocol Stack for LP-WAN
Figure 3 lists all the functions defined to compress and decompress a
field. The first column gives the function's name. The second and
third columns outlines the compression/decompression process.
As with 6LoWPAN, the compression process may produce some data, where
fields that were not compressed (or were partially compressed) will
be sent in the order of the original packet. Information added by
the compression phase must be aligned on byte boundaries, but each
individual compression function may generate any size.
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/-----------------+---------------------+----------------------------------------\
| Field |Function | Behavior |
+-----------------+---------------------+----------------------------------------+
|IPv6 version |elided |The value is not sent, but each end |
|IPv6 DiffServ | |agrees on a value, which can be |
|IPv6 Flow Label | |different from 0. |
|IPv6 Next Header |send-value |Depending on the matching operator, the |
| | |entire field value is sent or an |
| | |adjustment to the context value |
+-----------------+---------------------+----------------------------------------+
|IPv6 Length |compute-IPv6-length |Dedicated function to reconstruct value |
+-----------------+---------------------+----------------------------------------+
|IPv6 Hop Limit |elided+no matching |The receiver will put a value stored in |
| | |the context. It may be different from |
| | |one originally sent, but in a star |
| | |topology, there is not risk of loops |
| |elided+matching |Receiver and sender agree on the value. |
| | |If the value is not correct the packet |
| | |the rule is not selected |
| |send-value |Explicitly sent |
+-----------------+---------------------+----------------------------------------+
|IPv6 ESPrefix |elided |The 64 bit prefix is stored on the ctxt |
|IPv6 LCPrefix |send-value |Explicitly send 64 bits on the link |
+-----------------+---------------------+----------------------------------------+
|IPv6 ESiid |elided |IID is not sent, but stored in the ctxt |
|IPv6 LCiid |ESiid-DID | LCiid-DID|IID is built from the ES Device ID |
| |send-value |IID is explicitly sent on the link. The |
| | |size depends of the L2 technology |
+-----------------+---------------------+----------------------------------------+
|UDP ESport |elided |In the context |
|UDP LCport |send-value |Send the 2 bytes of the port number |
| | |or less if lsb matching is specified in |
| | |the matching operator. |
+-----------------+---------------------+----------------------------------------+
|UDP length |compute-UDP-length |Dedicated function to reconstruct value |
+-----------------+---------------------+----------------------------------------+
|UDP Checksum |compute-UDP-checksum |Dedicated function to reconstruct value |
+-----------------+---------------------+----------------------------------------+
Figure 4: SCHC functions' example assignment for IPv6 and UDP
Figure 4 gives an example of function assignment to IPv6/UDP fields.
4.1. Action functions
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4.1.1. Elided
The compressor do not sent the field value on the link. The
decompressor restore the field value with the one stored in the
matched rule.
4.1.2. Send-value
The compressor send the field value on the link, if the matching
operator is "=". Otherwise the matching operator indicates the
information that will be sent on the link. For a LSB operator only
the Least Significant Bits are sent.
4.1.3. ESiid-DID, LCiid-DID
These functions are used to process respectively the End System and
the LC Device Identifier (DID). The IID value is computed from
device ID present in the Layer 2 header. The computation depends of
the technology and the device ID size.
5. Examples
This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the SCHC behaviour.
5.1. IPv6/UDP compression in a star topology
The most common case will be a LPWA end-system embeds some
applications running over CoAP. In this example, the first flow is
for instance for the device management based on CoAP using Link Local
addresses and UDP ports 123 and 124. The second flow will be a CoAP
server for measurements done by the end-system (using ports 5683) and
Global Addresses alpha::IID/64 to beta::1/64. The last flow is for
legacy applications using different ports numbers, the destination is
gamma::1/64.
Figure 5 presents the protocol stack for this end-system. IPv6 and
UDP are represented with dotted lines since these protocols are
compressed on the radio link. The rule ID is represented by a shim
id (respectively 0, 1 and 2).
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Managment Data
+----------+---------+---------+
| CoAP | CoAP | legacy |
+----||----+---||----+---||----+
. UDP . UDP | UDP |
................................
. IPv6 . IPv6 . IPv6 .
+--SHIM0------SHIM1-----SHIM2--+
| 6LPWA L2 technologies |
+------------------------------+
End System or LPWA GW
Figure 5: Simplified Protocol Stack for LP-WAN
Note that in some LPWA technologies, only End Systems have a device
ID . Therefore it is necessary to define statically an IID for the
Link Local address for the LPWA Compressor.
+----------------+---------+--------+--------+-------------++------+
| Field | Value | Match | Match | Function || Sent |
+----------------+---------+-----------------+-------------++------+
|LPWA SHIM |0 | No | = | send-value || 0 |
|ESDevice-ID |dev-id | No | = | elided || |
+================+=========+========+========+=============++======+
|IPv6 version |6 | = | No | elided || |
|IPv6 DiffServ |0 | = | No | elided || |
|IPv6 Flow Label |0 | = | No | elided || |
|IPv6 Length |XXXXXXXXX| No | No | comp-IPv6-l || |
|IPv6 Next Header|17 | = | No | elided || |
|IPv6 Hop Limit |255 | No | No | elided || |
|IPv6 ESprefix |FE80::/64| = | No | elided || |
|IPv6 ESiid | | No | No | ESiid-DID || |
|IPv6 LCprefix |FE80::/64| = | No | elided || |
|IPv6 LCiid |::1 | = | No | elided || |
+================+=========+========+========+=============++======+
|UDP ESport |123 | = | No | elided || |
|UDP LCport |124 | = | No | elided || |
|UDP Length |XXXXXXXXX| No | No | comp-UDP-l || |
|UDP checksum |XXXXXXXXX| No | No | comp-UDP-c || |
+================+=========+========+========+=============++======+
+----------------+---------+--------+--------+-------------++------+
| Field | Value | Match | Match | Function || Sent |
+----------------+---------+-----------------+-------------++------+
|LPWA SHIM |1 | No | = | send-value || 1 |
|ESDevice-ID |dev-id | No | = | elided || |
+================+=========+========+========+=============++======+
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|IPv6 version |6 | = | No | elided || |
|IPv6 DiffServ |0 | = | No | elided || |
|IPv6 Flow Label |0 | = | No | elided || |
|IPv6 Length |XXXXXXXXX| No | No | comp-IPv6-l || |
|IPv6 Next Header|17 | = | No | elided || |
|IPv6 Hop Limit |255 | No | No | elided || |
|IPv6 ESprefix |alpha/64 | = | No | elided || |
|IPv6 ESiid | | No | No | ESiid-DID || |
|IPv6 LCprefix |beta/64 | = | No | elided || |
|IPv6 LCiid |::1000 | = | No | elided || |
+================+=========+========+========+=============++======+
|UDP ESport |5683 | = | No | elided || |
|UDP LCport |5683 | = | No | elided || |
|UDP Length |XXXXXXXXX| No | No | comp-UDP-l || |
|UDP checksum |XXXXXXXXX| No | No | comp-UDP-c || |
+================+=========+========+========+=============++======+
+----------------+---------+--------+--------+-------------++------+
| Field | Value | Match | Match | Function || Sent |
+----------------+---------+-----------------+-------------++------+
|LPWA SHIM |2 | No | = | send-value || 2 |
|ESDevice-ID |dev-id | No | = | elided || |
+================+=========+========+========+=============++======+
|IPv6 version |6 | = | No | elided || |
|IPv6 DiffServ |0 | = | No | elided || |
|IPv6 Flow Label |0 | = | No | elided || |
|IPv6 Length |XXXXXXXXX| No | No | comp-IPv6-l || |
|IPv6 Next Header|17 | = | No | elided || |
|IPv6 Hop Limit |255 | No | No | elided || |
|IPv6 ESprefix |alpha/64 | = | No | elided || |
|IPv6 ESiid | | No | No | ESiid-DID || |
|IPv6 LCprefix |gamma/64 | = | No | elided || |
|IPv6 LCiid |::1000 | = | No | elided || |
+================+=========+========+========+=============++======+
|UDP ESport |8720 | lsb(4) | No | elided || lsb |
|UDP LCport |8720 | lsb(4) | No | elided || lsb |
|UDP Length |XXXXXXXXX| No | No | comp-UDP-l || |
|UDP checksum |XXXXXXXXX| No | No | comp-UDP-c || |
+================+=========+========+========+=============++======+
Figure 6: Simplified Protocol Stack for LP-WAN
All the fields described in the three rules Figure 6 are present in
the IPv6 and UDP headers. Two fields have been added at the begin,
they are used to identify the rule id for decompression when the
other end receives the compressed header. The shim id is read either
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from the L2 header or from the first bit in the payload depending on
the technology. The ESDevice-ID value is found in the L2 header.
The second and third rules use global addresses. The way the ES
learn the prefix is not in the scope of the document. One possible
way is to use a management protocol to set up in both end rules the
prefix used on the LPWA network.
The third rule compresses port numbers on 4 bits. This value is
selected to maintain alignment on byte boundaries for the compressed
header.
6. Acknowledgements
Thanks to Dominique Barthel, Alexander Pelov, Juan Carlos Zuniga for
useful design consideration.
7. Normative 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.
[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>.
[RFC4997] Finking, R. and G. Pelletier, "Formal Notation for RObust
Header Compression (ROHC-FN)", RFC 4997,
DOI 10.17487/RFC4997, July 2007,
<http://www.rfc-editor.org/info/rfc4997>.
[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>.
Authors' Addresses
Ana Minaburo
Acklio
2bis rue de la Chataigneraie
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
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Laurent Toutain
Institut MINES TELECOM ; TELECOM Bretagne
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@telecom-bretagne.eu
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