Re: [6lowpan] Mesh Delivery Field

gabriel montenegro Sat, 30 Sep 2006 09:40:48 -0700

Ok, it seems like folks would like to have more flexibility in the header. I'm just having a hard time translating that

to actual bits and bytes within the header formats, and how much flexibility is fine.

The first packet format has 5 bits of reserved field:

                           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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LF| prot_type    |M| rsv     |Payload (or Mesh Delivery Hdr)...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 1: LoWPAN unfragmented encapsulation header format

One bit out of that reserved field is what Mario Mao suggests we use to distinguish between the two current flavors

of mesh delivery header (unicast vs bcast/mcast). Is this sufficient flexibility?

The issue is that there is no such flexibility in the two other header formats, but we can certainly add such a

reserved field and make all 3 header formats the same up to the first 16 bits. Perhaps along these lines

(notice the new "reserved" field and the corresponding 5 bit shift for datagram_size and datagram_tag) :

Figure 2: LoWPAN first fragment encapsulation header format

                           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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LF|datagram_offset|M|reserved |   datagram_size     |dgram_tag|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |dgram_tag| Payload (or Mesh Delivery Hdr)...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 3: LoWPAN subsequent fragment(s) encapsulation header format

Would this be enough? If not, how many bits should be allocated? Should there be a version field in addition

(as suggested by Vipul a while back)? What changes in the format are suggested? Folks, please be specific

as to the suggested changes by sending header formats with clearly proposed fields, etc.

tnx,

-gabriel

----- Original Message ----
From: David Culler <[EMAIL PROTECTED]>
To: Ki-Hyung Kim <[EMAIL PROTECTED]>
Cc: gabriel montenegro <[EMAIL PROTECTED]>; Geoff Mulligan <[EMAIL PROTECTED]>; Soohong Daniel Park <[EMAIL PROTECTED]>; [email protected]
Sent: Friday, September 29, 2006 8:38:56 AM
Subject: Re: [6lowpan] Mesh Delivery Field

It certainly would be a shame to get into a position where the encoding is so so constrained that there was not a means of accomodating additional capabilities that are revealed through deeper implementation and usage experience. The IP community has historically been very good at planning for change. Options fields and leaving a little room in the initial encodings are essential to that. As much as we try, we seldom get every aspect of the protocol exactly right the first time. It is understandable that with so much focus being place on header compression that it is hard to leave any space unclaimed. However, that contingency planning and flexibility may be more critical to eventual adoption than squeezing out every unneccessary bit. As we look at the desired "rush to finish" it is important to verify that the proposed framework permits enough expressive power to address the issues that have not yet been fully vetted. You cannot expand the format, only work within it.

On 9/26/06, Ki-Hyung Kim <[EMAIL PROTECTED]> wrote:

Gabriel,

I am sorry for this late reply.

Basically, I agree that current format document is rather tightly scheduled and not well prepared for future enhancements or revisions, in terms of routing, diagnoses, management, etc. Based on the assumption that we are not yet in the stage of pursuing future rechartering items, it might not be the right time to consider all the cases at one time. Also, we should be in rush to finish the current format document ASAP for moving forward into next stage before next IETF meeting.

Regarding the mesh delivery field, Mario Mao's approach should be fine, and other approaches (like option 2,3, and 4) might be OK. However, it seems like we are trying too much to make the format compact while not considering the future expansions.

After all, my opinion is option 1. Let's have one byte for the multicast bit and the reserved fields for future.

having additional one byte for this is not a big deal in terms of packet size and won't add much complexity for handling it.

Examples of utilizing some reserved fields might be follows:

1. Based on the assumption that 6lowpan supports multiple routing protocols such as LOAD, DYMO-low, HiLow, DSR, or whatever, etc., the field might be used for indicating which specific routing protocol should be used for routing this particular packet. It might be argued whether the format document should support multiple protocols at a time. However, those kind of argue should be debated later while considering routing protocols without eliminating future possible necessity.

2. Routing protocols could be optimzed by having some option fields, while it might not be right time to specifically mention them.

I remember that the issue of the mesh underneath or above IP has been raised several times in the WG. I think it might not be right time considering it again right now.

If you have any other opinions, please let me know.

On 9/23/06, gabriel montenegro < [EMAIL PROTECTED]> wrote:

First of all, Thanks David for your thoughtful message. It's great to see you participate in this forum and I hope

you'll stick around!

From the point of view of laying the technology foundation, everything in your message is quite relevant.

From the point of view of WG management, I'm trying to figure out if we need to act on all of it right now

or if there are things we can address independently of the format draft under discussion. For example,

you bring up the topic of traceroute type functionality. In IP, this is carried over ICMP, a separate

management control plane. Since what we have in the format draft is a multiplexing layer, we can

define a new protocol value to carry out the management protocol packets. The question is if the

current format provides the required mechanism to go do work on such a separate protocol

in another draft. Intuitively, it seems like this would work. Someone (your group, David?) could

define such a protocol with functionality similar to ICMP for pings, traceroute, etc and provide

the route gathering functionality, the echo response, etc in that separate draft. Conceivably, it

would not necessarily use the mesh delivery header, as it would provide its own facility to

record hop-by-hop route information within its own protocol fields. It could use loose source

routing in which case some use of the mesh delivery header might be appropriate as well.

Would such an approach work, David? This would actually play into one of the items slated for

the next round of 6lowpan (if we ever recharter) to strengthen diagnosability or management of

these networks.

As for your point about us not knowing exactly what the header format must look like at this point in

time, this is perhaps something we need to take into consideration right now. For example, this

input would push us into reserving at least a couple of bits in the header for future expansion. A while

back Vipul was arguing for a "version" field. Not sure if such a field is required or if the current reserved

fields are enough. Mario Mao's proposal uses one of those reserved bits to solve the issue under

discussion regarding the mesh delivery header variants for unicast vs mcast/bcast. There has been

one group expressing support for this, so perhaps we already have the necessary mechanism in place

in the header.

David, you also mention the need to support different routing protocols side-by-side for comparison.

This is one of the design goals of the format document. As I mentioned, the protocol field allows one

to carry different protocols. We currently have proposals for a 6lowpan adaptations of AODV, a DyMO

and HiLow. I know Nandu was thinking of doing one for OLSR. Perhaps BVR is next. The point is that

the format document does provide the mechanism to define any such protocol, or is there a limitation you

can see in the current document? If so, what needs to change?

As for the claim that sub-IP benefits from simpler structures, my interpretation is that you're suggesting it

is better not to use the mesh delivery header and let IP deal with the underlying individual collection of links.

If so, one can ignore the mesh delivery header (not use it) and use the other parts of the format document to

simply layer IP on top. This approach would then make use of other activities like AUTOCONF or MANET

to create meshes if so desired. Personally, I see that *if* one wishes to do a mesh, not doing it in the sub-IP

layer just pushes to problem upwards into the IP routing realm, and this just makes things even more complex

than doing it under sub-IP. Complexity will be there no matter what, it's just a question of where one wants to

address it. I do note that sub-IP complex structures are very popular and deployed in some cases (802.1d bridged

networks). Perhaps likely to become even more so with 802.11s within a few years.

I would not be interested in pursuing an AUTOCONF path, but think it should

be possible to do so for those so inclined. Does this sound doable? Again, my immediate concern is that the

format document not prevent this approach.

-gabriel

----- Original Message ----
From: Geoff Mulligan < [EMAIL PROTECTED]>
To: Soohong Daniel Park <[EMAIL PROTECTED]>
Cc: [email protected]; David Culler <[EMAIL PROTECTED]>
Sent: Friday, September 22, 2006 6:41:21 AM
Subject: Re: [6lowpan] Mesh Delivery Field

Daniel,
  While the WG last call did expire, I'm not sure if David is too late.
I think that he raises some important concerns and I'm not sure that we
really have consensus on the mesh delivery field.

I would like to hear from the list on this.  I think that Gabriel has
put forth 4 alternatives.  Carsten indicated that he thought he might
have a different alternative and would post to the list either the
alternative or his consideration of the 4 alternatives - Carsten, please
send your thoughts.

I feel that the issue David raises should be dealt with by the WG
because I do not want to send the draft on to the IESG only to have
these issues brought up in the IETF last call.  These seem rather
fundamental to me and we might look rather foolish not having resolved
this in the working group!

We need some significant discussion from the members of the WG.  This WG
has been, my self included, rather quiet and as a result we have moved
at a snails pace getting anything done.  I implore everyone to take time
and read the thread on the mesh field and send your thoughts, pro, con
or otherwise to this list.  If everyone continues to be apathetic about
this work, I hold little hope for this WG being rechartered to do any
other work!

    geoff

On Fri, 2006-09-22 at 19:23 +1000, Soohong Daniel Park wrote:
> David,
>
> First off, your issue raising in this thread seems too late since our
> official WGLC was expired: "This Last Call will end on Wednesday 02
> August 2006 at 2359 UTC."
>
> Anyway, my sense is not to take Mesh issue at this stage. As long as
> we are looking at the 6lowpan solution format, mesh is out of scope of
> that. In fact, as far as I am concern, mesh belongs to 15.4 technology
> based on MAC layer. Instead, as described in the rechartering, we will
> go through adhoc issue based on IP layer. That's why we are here in
> 6lowpan wg of ietf.
>
> But, some staffs described in your mail seem helpful from the 6lowpan
> perspective, especially IPv6 related stuffs. If allowed, please bring
> your concern to the next 6lowpan face-to-face meeting in San Diego.
> Then, we will have further details in there I believe...
>
> --
>
>
> Daniel (Soohong Daniel Park)
> Mobile Convergence Laboratory, SAMSUNG Electronics.
>
>
>
> On 9/21/06, David Culler <[EMAIL PROTECTED]> wrote:
> > Dear All,
> >
> > I would like to raise a concern about the Mesh Delivery Field
> > definition in Section 11 of the format document.  While the actual
> > mesh routing protocol is appropriately outside the scope of the
> > 6lowpan Format document, that format needs provide the raw structure
> > upon which rough consensus and running code can be flourish.  We
> > know that low power wireless mesh routing is technically tricky and
> > strongly influenced by target traffic load, power management profile,
> > network design, and so on.  Thus, it is reasonable to expect that the
> > consensus building and technical refinement process will require
> > on-going experience with protocol and implementation alternatives.
> > The 6lowpan format document should support this process, although it
> > should not dictate its conclusion.
> >
> > Based on past experience with IP layer 3 routing and with past
> > internally routed links, we can establish a few primary goals for the
> > expressiveness of the 6lowpan Mesh Delivery Field.
> >    (1) It should support the visibility into the routed 15.4 cloud
> > necessary to develop, debug, diagnose, and manage the 15.4 cloud.
> >    (2) It should support the ability to define, develop and compare
> > routing protocols and possibly support the presence of multiple
> > protocols for widely differing usage cases.
> >    (3) It needs to be possible for nodes to communicate with other nodes
> > in the cloud in order to discover and negotiate joining the cloud
> > without joining the cloud a priori through some mechanism that is
> > opaque to IP and above.
> >
> > (1) has historically been addressed through various forms of source
> > routing.  It should be possible to do the equivalent of traceroute
> > through the 15.4 mesh and this should be accessible in some form through IP
> > so
> > higher layers can be built to perform the management.  The current
> > Mesh Delivery Field definition seems to prohibit any form of source routing.
> > It assumes that the routing tables are all set up and correct so that
> > the forwarder can "swizzle" the destination address field of the 15.4
> > header with a table entry.  (The experience of IP over ATM and the
> > thousands of pages of the Q83b signalling protocol required to set up
> > all the tables in advance ought to give us a little pause here.)  The
> > idea of supporting table-driven routing and thereby separating the
> > route formation from the forwarding engine is a good one, but there
> > needs to be enough expressive power to support the protocols that will
> > establish and manage those tables (rather than relying on an opaque
> > out-of-band signaling mechanism) and there needs to be the ability to
> > query and validate those tables without relying on out-of-band
> > mechanisms.
> >
> > (2) has historically be addressed with protocol identifiers and
> > options.  As we went through the various RIP, OSPF, etc. we didn't
> > need to go back and redefine the format documents.  There was enough
> > expressiveness for topology formation and routing to evolve.  Not all
> > networks need to support all protocols.  That has been the network
> > designers' determination.  The underlying structure provided the
> > primitives, allowing solutions (and consensus) to evolve.  We want to
> > avoid creating a "winner take all" structure where there is no room
> > for evolution and development.  It is possible that AODV, or TinyAODV,
> > or DSR, or GPSR or possibly one of several other alternatives is the right
> > solution.  However, there is little evidence of one having clear superiority
> > at
> > this time, despite numerous high profile efforts.  We should be defining the
> > canvas upon which competing alternatives can be defined and vetted.
> >
> > (3) has historically been addressed by a limited ability to
> > communicate on the link to a crudely defined set of nodes in order to
> > bootstrap the discovery, join, address assignment, negotiation, etc.
> > Most successful links under IP look either like a broadcast medium or
> > a point-to-point link (a degenerative broadcast).  This allows ARP,
> > RARP, BOOTP, DHCP, etc. to gain higher level accessibility and
> > addressibility through the link.  Links that require complex
> > signaling, discovery, joining and coordnation below the link layer,
> > including ATM, Bluetooth (piconets and scatter nets), and IRDA have
> > proved onerous for IP and have been relegated to far less general
> > usage scenarios - such as the desktop USB replacement.  Even though
> > the radio is a broadcast medium, the challenge in a routed link is
> > that the link is not by default a link-wide broadcast phy.  There
> > are two natural notions of "broadcast" in a lowpan.  (i) The physical
> > neighbors that happen to receive a transmission because they are close
> > enough to pick up the signal and have their radios turned on to
> > receive.  (ii) The entire collection of nodes that can be reached by a
> > multihop dissemination (flood).  Both of these are extremely useful in
> > bootstrapping higher levels, including routing, and should be
> > addressed in the format document.  Additionally, it is not
> > neccessary to use the 802.15.4 beaconing mechanism to bring up IP over
> > a 15.4 link.  Providing some forms of broadcast using data frames
> > makes the 802.15.4 format much more like 802.11 and other links that
> > have proved very suitable for IP.  Few, if any, of the public 15.4 routing
> > layers developed
> > by the TinyOS community utilize beacon packets.
> >
> > The recognition that the current Mesh Delivery field may be to restrictive
> > is intimated by the need to define a second form of the header for
> > multicast (and link local broadcast) that includes a sequence number.
> > The mesh header field type is defined by whether the destination
> > address is unicast or multicast. The document states that the use of
> > such a sequence number is out of scope. However, the observation that
> > that extra information may need to be included depending on how
> > packets are routed further suggests that more careful format
> > definition is needed to support future development of mesh routing.
> >
> > Have folks who have been developing mesh layers for lowpan6 felt that
> > the existing Mesh Delivery Field is too restrictive?
> >
> >
> > David E. Culler
> > UC Berkeley
> > [EMAIL PROTECTED]
> >
> >
> >
> >
> > _______________________________________________
> > 6lowpan mailing list
> > [email protected]
> > https://www1.ietf.org/mailman/listinfo/6lowpan
> >
> >
> >
>
> _______________________________________________
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--
Ki-Hyung Kim (김기형, 金起亨)
Associate Professor
Division of Information and Computer Eng., Ajou University, Suwon, Korea 442-749
Tel: +82-31-219-2433, Cel: +82-17-760-2551,  Fax: +82-31-219-2433 http://www.6lowpan.org

--
David E. Culler
Arched Rock Corporation
2168 Shattuck Ave
Berkeley, CA 94704
www.archedrock.com

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Re: [6lowpan] Mesh Delivery Field

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