Re: [6tisch] Proposed improvement in RH3-6LoRH

Tengfei Chang <tengfei.chang@gmail.com> Tue, 19 January 2016 12:59 UTC

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Date: Tue, 19 Jan 2016 13:59:44 +0100
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From: Tengfei Chang <tengfei.chang@gmail.com>
To: "Pascal Thubert (pthubert)" <pthubert@cisco.com>
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Cc: "6tisch@ietf.org" <6tisch@ietf.org>, Routing Over Low power and Lossy networks <roll@ietf.org>, "6lo@ietf.org" <6lo@ietf.org>
Subject: Re: [6tisch] Proposed improvement in RH3-6LoRH
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Hello Pascal,

Thanks a lot for the detailed answer! I think it solves all my concern. I
will vote for it!

Tengfei



On Tue, Jan 19, 2016 at 11:00 AM, Pascal Thubert (pthubert) <
pthubert@cisco.com> wrote:

> Hello Tengfei:
>
>
>
> Basically the change proposal accounts for the assumption that the last
> segment may be different, which comes with the idea of supporting non RPL
> leaves. If you look at it the original RH4 (RFC 6554) makes a difference
> for the last hop.
>
>
>
> Also, the root is the one computing the 6LoRH so it is easier for it to
> take its own address as reference.
>
>
>
> Finally the text is unclear about which bits from the destination are to
> be used.
>
>
>
> All in all, using the root as reference makes more sense than using the
> final destination as I had initially proposed. So the new proposal is not a
> merge but an instead, with clarifying text about the process I showed in
> the picture.
>
>
>
> More answers  inline:
>
> Dear all
>
>
>
> The picture below illustrates how the RH3 6LoRH works with draft 03 in a
> case like Root -> A -> B -> C -> leaf
>
>
>
> [image: cid:image001.png@01D152A0.1C780E10]
>
>
>
> The first 6LoRH is expected to be a full address (128 bits) to set up a
> reference and the next 6LoRH are expected to be smaller and just override
> the rightmost bits which form the delta from the reference.
>
>
>
> What if the B and C address can be compressed to one byte(RH3-6LoRH type
> 0)?
>
> When leaving the root the format would like:
>
> *RH3-6LoRH type 4    IPv6 Prefix (64bits)   A's suffix (64bits)
>  RH3-6LoRH type 0      B's suffix (8bits)    C's suffix (8bits)*
>
>
>
> Ø  Pascal If A’s address is the same as B and C for the first 120 bits
> then this format is correct : )> Pascal
>
>
>
>
>
>
>
> should node A need to extend the address of B address when the packet
> leaving A? Like:
>
> *RH3-6LoRH type 4    IPv6 Prefix (64bits)   B's suffix (64bits)
>  RH3-6LoRH type 0      C's suffix (8bits)*
>
>
>
> Ø  Pascal: That is correct. Note that there is no separation between
> prefix in suffix in the type 4. So the drawing applies for all types if you
> consider that the prefix length is 64, 96, 112 or 120 depending on the type
> of the next RH. With the proposal, if the root has the same /120 prefix as
> A, B and C then we have a single type 0 with A, B and C in it.
>
> Proposal: we could consider that the 128bits source of the IP header
> before the RH3 is the reference to start with.
>
> I have no comments with the proposal. Just want to mention in the
> currently version of draft (version 3) , it says : (in section 5)
>
> ...
>
>    If some bits of the first address in the RH3-6LoRH header can be
>
>    derived from the final destination in the LoWPAN_IPHC, then that
>
>    address may be compressed; otherwise it is expressed as a full IPv6
>
>    address of 128 bits.  Next addresses only need to express the delta
>
>    from the previous address.
>
> ...
>
> How to understand this combining with the proposal? (Or this is the point
> we are going to replace by the proposal? Let me know)
>
>
>
> Ø  Pascal: proposal is to replace. That text was unclear and more complex
> to implement.  And fails to optimize for leaves that are not configured
> like the mesh.
>
> With that even the first hop could be compressed the same way as the other
> hops. With RPL, the root is the encapsulator if IP in IP in used. Good
> thing, in that case the root is totally elided with the IP-in-IP 6LoRH.
>
> That's great! Could I confirm when the packet is inside a RPL domain where
> IP in IP is elided, the source destination in IPHC is the reference?
>
> Ø  Pascal: Yes. The only case where IP in IP is elided is when the source
> is the root. The root would still be the reference. The text I proposed
> says the Source of the header that is placed before the RH3, and I meant in
> the non-compressed form. With RPL as it stands today this is always the
> root.
>
> So this simple proposal saves up to 16 octets (that’s in the case with a
> single subnet and all addresses differ only by the last 2 bytes). I’m
> willing to add it in the next revision.
>
> I don't know how to calculate the saved bytes. For this case, will the
> format looks like this?
>
> RH3-6LoRH type 1    A's suffix (16bits)     B's suffix (16bits)   C's
> suffix (16bits)
>
> Ø  Pascal: Yes. In that case you save 14 bytes in A’s address plus the
> RH3-6LoRH type 4 which is 2 additional bytes, 16 total.
>
> Any opposition?
>
>
>
> Pascal
>
>
>
> Thanks for the proposal!
>
>
>
>
>
> Ø  Pascal: Do you vote for it? I’m asking because the plugtest is coming.
>
>
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>
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