6lo Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Standards Track L. Toutain
Expires: May 7, 2020 Institut MINES TELECOM; IMT Atlantique
November 04, 2019
Comparison of 6lo and SCHC
draft-toutain-6lo-6lo-and-schc-00
Abstract
6lo and 6lowpan have standardized a stateless IPv6 and UDP
compression method for mesh networks. SCHC proposes a generic
compression mechanism that can be applied to any protocol stack. The
lpwan working group is focusing on star topologies for IPv6, UDP and
CoAP header compression and fragmentation.
This document summarizes the differences between 6lo and SCHC and
possible combination of SCHC and 6lo.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Comparison . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Stateless compression. . . . . . . . . . . . . . . . . . 2
2.2. Meshed vs Star . . . . . . . . . . . . . . . . . . . . . 2
2.3. Alignment . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Uniform vs specific compression rules. . . . . . . . . . . . 3
3.1. Bitmap vs Rule ID . . . . . . . . . . . . . . . . . . . . 4
3.2. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 4
4. Applicability of SCHC in a 6lo network. . . . . . . . . . . . 5
5. Normative References . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
6lo and 6lowpan have standardized a stateless IPv6 and UDP
compression method for mesh networks. SCHC proposes a generic
compression mechanism that can be applied to any protocol stack. The
lpwan working group is focusing on star topologies for IPv6, UDP and
CoAP header compression and fragmentation.
This document summarizes the differences between 6lo and SCHC and
possible combination of SCHC and 6lo.
2. Comparison
2.1. Stateless compression.
Both compression protocols are stateless regarding the compression/
decompression process. Each packet is compressed and decompressed
independently of the others and no information is stored during
compression or decompression.
The SCHC name comes from the fact that it is a generic mechanism and
the context tells how to compress a specific packet.
2.2. Meshed vs Star
6lo is defined for meshed network therefore all the node must be able
to manipulate any 6lo packet.
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SCHC is defined for star network and compression is done at both
ends. SCHC offers the possibility to have different compression
scheme for each branch of the star. This scheme is described though
a context.
If SCHC had to be used in a mesh network, all the intermediary nodes
will have to know the rules used in the network.
2.3. Alignment
6lo preserves alignment on bye boundary when sending header fields.
SCHC is bit oriented and padding can be added when the packet is
sent.
3. Uniform vs specific compression rules.
6lo focuses mainly on IPv6 header and predefine a compression scheme
known by all the nodes in a 6lo network.
SCHC defines a generic compression mechanism based on fields. A
field is an abstract notion. A field has several properties:
o An ID identifying a specific field.
o A position when a field is repeated several times in a header.
o A length with can either be a size in bit or a function indicating
how the size is computed.
o A direction which makes sense in a star topology since traffic is
originating from a node or is for a node.
The rule contains some functions:
o Matching Operator: this information is used to select candidate
rules for compression. A rule is selected if all the fields in
the packets matches all the fields in the rule. Current MO are:
* "ignore" (any value is possible),
* "equal",
* "MSB" (Most Significant Bits) or
* "Matching".
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o Compression Decompression Action: if a compression rule is
selected, then compression action tells how to compress header
fields into residues. Current CDA are:
* Not-sent: the field is elided. This behavior is found also in
6lo as elided.
* Value-sent: the field is sent. If the field was defined as
variable, the length can be sent before the residue. This
behavior is found also in 6lo, but only for well-known length
fields.
* LSB (Less Significant Bit): the less significant bits are sent.
* Matching-sent: an index is sent instead of the value. 6lo has
something similar for hop-limit. 3 well-known values are
defined.
3.1. Bitmap vs Rule ID
6lo defines a dispatch indicating the nature of the 6lo packet and
for IPHC defines a bitmap to indicates the nature of the header
compression.
SCHC uses a rule ID to identify the nature of the SCHC packet. Rule
ID have a variable length, most frequent rules may use shorter
values. The rule ID space is split between compression and
decompression rules. The rule ID refers to a context which contains
the nature of the rule and associated parameters.
In a sense the combination dispatch and bitmap for compression are
equivalent to the rule ID, the main difference is that the rules are
implicit in 6lo and the same rules are shared by all the node and
explicit in SCHC. Context synchronization is needed between both
ends.
3.2. Fragmentation
SCHC implement a fragmentation mechanisms dedicaded to LPWAN
networks. 3 modes exists:
o NoAck mode is an optimisitc mode, a RCS (rassembly Check Sequence)
is added in the last fragment. unvalid received messages are
discarded. No retransmission is done.
o Ack Always is base on a "jumping window", sender must received a
acknowledgement to jump to the next window.
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o Ack on Error is more efficient. The message is cut into tiles of
a specific length. Tiles are regrouped into windows. Tiles are
sent into fragments. Fragment size may vary during transmission.
Receiver generate
4. Applicability of SCHC in a 6lo network.
To apply SCHC in a 6lo meshed network, the following requirements are
needed:
o A SCHC dispatch to indicate that a SCHC rule ID follows,
o A context synchronization among all the 6lo nodes to share the
context, or predefined rules,
o The rule should not contain a direction indicator.
5. Normative References
[I-D.ietf-lpwan-ipv6-static-context-hc]
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
Zuniga, "Static Context Header Compression (SCHC) and
fragmentation for LPWAN, application to UDP/IPv6", draft-
ietf-lpwan-ipv6-static-context-hc-22 (work in progress),
October 2019.
[rfc2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
Authors' Addresses
Ana Minaburo
Acklio
1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
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Laurent Toutain
Institut MINES TELECOM; IMT Atlantique
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@imt-atlantique.fr
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