Hi,
Carsten Bormann wrote:
- Separate the Prefix Information and Context Information into separate
options. This clarifies what is a prefix and what is context.
- Use either the standard RFC4861 Prefix Information Option or an
optimized one to save overhead.
The 4861 PIO is about twice the size of the 6LoWPAN one, as 4861 always
sends all 16 bytes of a prefix and then needs to add padding (and a
reserved field).
Yep. It needs to be optimized.
Combining 6LoWPAN-PIO and CID-IO into one option makes it unnecessary to
send all the global prefix(es) of the LoWPAN twice.
There is still enough space in the option to indicate whether the CID
number is valid or not (the A bit already tells us whether this is a
valid subnet prefix).
>
When we went CBHC, I heard some noises that the context info might get
somewhat large for distribution in RAs.
With this changes, yes, they will.
You are correct, this will take more space using multiple options.
Already this is an issue when having lots of contexts.
I still haven't heard a rationale for this change. If there is indeed a
need to clarify, do so in the text, not the encoding.
If I understand it, the rational for separating the prefix information
from context information, was to allow for context information to be
distributed in other ways as well in the future. E.g. including a
Context Information Option also in other messages.
What if we would do something like this.
1. Rename the current option to "6LoWPAN Information Option" making it
more neutral to whether it is carrying prefix or context information.
2. Add the extra bits needed to tell if the CID is valid or not.
3. Make it clear that the option can carry a prefix, a context or a
prefix which is also a context.
This option would be generic enough that it could also potentially be
carried in messages other than RAs.
Here is a shot at a proposal:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Info Length |L|A|C|V| CID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Information .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6LoWPAN Information Option format
Type: TBD4
Length: 8-bit unsigned integer. The length of the option (including
the type and length fields) in units of 8 octets. May be 1, 2 or
3 depending on the length of the Prefix field.
Info Length: 8-bit unsigned integer. The number of leading bits
in the Information field that are valid. The value ranges from 0
to 128.
The info length field provides necessary information for on-link
determination (when combined with the L flag in the prefix
information option). It also assists with address
autoconfiguration as specified in [RFC4862], for which there may
be more restrictions on the info length.
L: 1-bit on-link flag. This flag MUST be unset for Route Over
LoWPANs and Extended LoWPANs, and MAY be set for Mesh Under Simple
LoWPANs.
A: 1-bit autonomous address-configuration flag. When set indicates
that this prefix can be used for stateless address configuration
as specified in [RFC4861].
C: 1-bit context flag. This flag indicates that this information
option also serves as a context on the LoWPAN and is identified
by the CID field.
V: 1-bit context validity flag. This flag indicates if the context
is valid, and is used only in combination with the C flag.
CID: 4-bit Context Identifier for this information. CID is
used by context based header compression specified in
[I-D.ietf-6lowpan-hc]. The list of CIDs for a LoWPAN is
configured on Edge Routers, who distribute the prefix list to all
nodes in the LoWPAN. CID = 0 identifies the default context to be
used.
Valid Lifetime: 32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is sent) that the prefix
is valid for the purpose of on-link determination. A value of all
one bits (0xffffffff) represents infinity.
Information: The IPv6 prefix or context information indicated. This
may be a
partial prefix, a partial context or even an entire IPv6 address
for use as a context for compression.
Zach
Gruesse, Carsten
--
http://www.sensinode.com
http://zachshelby.org - My blog “On the Internet of Things”
Mobile: +358 40 7796297
Zach Shelby
Head of Research
Sensinode Ltd.
Kidekuja 2
88610 Vuokatti, FINLAND
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