[Roll] Comments on draft-dt-roll-p2p-rpl-01

[JP Vasseur <[e-mail]>]

Dear authors,

Since I had a number of comments I inserted them in the ID itself. As  
you will notice:
1) Some of them are purely editorial
2) Others are more fundamental and may require just clarifications,  
more discussion on the mailing list, ... In this case, feel free to  
start a separate thread (note that ticket cannot be assigned to non WG  
document).
3) I also suggested new sections


Internet Engineering Task Force                            M. Goyal, Ed.
Internet-Draft                         University of Wisconsin Milwaukee
Intended status: Informational                          E. Baccelli, Ed.
JP> Are you sure that you want to make it informational ?

Expires: December 16, 2010                                         INRIA
                                                                P2P.  
Team
                                                            June 14,  
2010


    Reactive Discovery of Point-to-Point Routes in Low Power and Lossy
                                 Networks
                         draft-dt-roll-p2p-rpl-01

Abstract

    This document describes a mechanism to discover and establish "on
    demand" one or more routes between two routers in a low power and
    lossy network.

JP> Needs a bit more details + explicitly refer to IPv6 in the abstract
and introduction.


Status of this Memo

    This Internet-Draft is submitted to IETF in full conformance with  
the
    provisions of BCP 78 and BCP 79.

    Internet-Drafts are working documents of the Internet Engineering
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    Internet-Drafts are draft documents valid for a maximum of six  
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    This Internet-Draft will expire on December 16, 2010.

Copyright Notice

    Copyright (c) 2010 IETF Trust and the persons identified as the
    document authors.  All rights reserved.

    This document is subject to BCP 78 and the IETF Trust's Legal
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    the Trust Legal Provisions and are provided without warranty as



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    described in the Simplified BSD License.


Table of Contents

    1.   
Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
    2.  Targeted Use  
Cases . . . . . . . . . . . . . . . . . . . . . .  4
    3.   
Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
    4.  Functional  
Overview  . . . . . . . . . . . . . . . . . . . . .  5
    5.  Propagation of Discovery  
Messages  . . . . . . . . . . . . . .  7
      5.1.  The Route Discovery  
Option . . . . . . . . . . . . . . . .  7
      5.2.  Setting a DIO Carrying a Route Discovery  
Option  . . . . .  9
      5.3.  Processing of a DIO Carrying a Route Discovery Option
            At An Intermediate Node  . . . . . . . . . . . . . . . . .  
10
      5.4.  Processing of a DIO Carrying a Route Discovery Option
            At The Target Router . . . . . . . . . . . . . . . . . . .  
12
    6.  Propagation of Discovery Reply Messages  . . . . . . . . . . .  
13
      6.1.  The Discovery Reply Object (DRO) . . . . . . . . . . . . .  
13
        6.1.1.  The Source Route Option  . . . . . . . . . . . . . . .  
15
      6.2.  DRO as Acknowledgement for Backward Source Routes  . . . .  
16
      6.3.  DRO as Carrier of Forward/Bidirectional Source Routes  . .  
17
      6.4.  Establishing Hop-by-hop Routes Via DRO . . . . . . . . . .  
17
    7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  
18
    8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  
18
    9.  Authors and Contributors . . . . . . . . . . . . . . . . . . .  
18
    10. References . . . . . . . . . . . . . . . . . . . . . . . . . .  
18
      10.1. Normative References . . . . . . . . . . . . . . . . . . .  
18
      10.2. Informative References . . . . . . . . . . . . . . . . . .  
18
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  
19






















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1.  Introduction

    RPL [I-D.ietf-roll-rpl] provides multipoint-to-point (MP2P) routes
    from routers in a low power and lossy network (LLN) to a sink router
    by organizing the routers along a Directed Acyclic Graph (DAG)  
rooted
    at the sink.
JP> low power and lossy .... Low Power and Lossy
JP> RPL provides P2MP, MP2P and P2P routes too.

The DAG root initiates the DAG formation by originating
    a DODAG Information Object (DIO) message.  The DIO message is sent
    via link-local multicast and carries information about the
    originating router's position (the "rank") in the DAG.  On receiving
    DIO messages sent by its neighbors, a router determines its own
    "rank" in the DAG and originates its own DIO message.
JP> Without going too much in the details, you may just want to add
that a node decides to join a DODAG according to a number of parameters
(identity of the DODAG, OF, routing/constraints a specified in the  
metric-ID,...).
    RPL enables point-to-multipoint (P2MP) routing from a router to its
    descendants in the DAG by allowing a router to send a Destination
    Advertisement Object (DAO) upwards along the DAG.  The DAO carries
    the aggregated
JP> Potentially "aggregated"
information regarding the descendants (and other local
    prefixes) reachable through the originating router.

    RPL also provides mechanisms for point-to-point (P2P) routing  
between
    any two routers in the DAG.
JP> Yes.
If the destination is within the
    source's "range", the source may directly send packets to the
    destination.  Otherwise, if the destination is a DAG descendant and
    the source maintains "downwards" routing state about this  
descendant,
    it can forward the packet along this route.  Otherwise, the source
    sends the packet to a DAG parent, which then applies the same set of
    rules to forward the packet further.  Thus, a packet travels up the
    DAG until it reaches a router that knows of the downwards route to
    the destination and then it travels down the DAG towards its
    destination.  A router may or may not maintain routing state about a
    descendant depending on whether its immediate children send it such
    information in their DAOs and whether the router can store this
    information.  Thus, in the best case scenario, the "upwards" segment
    of the P2P route between a source and a destination ends at the  
first
    common ancestor of the source and the destination.  In the worst
    case, the "upwards" segment would extend all the way to the DAG's
    root.  If the destination did not originate a DAO, the packet will
    travel all the way to the DAG's root, where it will be dropped.

JP> This may require to be a bit more accurate here. If there is no DAO,
there RPL is used for MP2P routing only (that is not a protocol design
characteristic though).
    The P2P routing functionality available in RPL suffers from several
    shortcomings [I-D.brandt-roll-rpl-applicability-home-building]:

JP> "suffering from several shortcomings" is a bit strong though. You  
may
want to say that additional mechanisms may be required to address  
specific
situation and list them.
    o  The need to maintain routes "proactively", i.e. every possible
       destination in the DAG must originate a DAO.

    o  The constraint to route only along a DAG potentialy leading to
       significantly suboptimal P2P routes and traffic congestion near
       the DAG root.
JP>
First bullet: you can mention that this requires to pro-actively  
advertise the
route and to keep refreshing the state accordingly
Second bullet: just to be very accurate, mention that the route may be  
sub-optimal.
This is highly network topology / OF dependent.





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    These shortcomings are not compatible with the home and commercial
    building domain application requirements described in [RFC5826] and
    [I-D.ietf-roll-building-routing-reqs].
JP> Again, to be accurate, there are no MUST requirements listed in
these ID that are not satisfied by RPL (with regards to the  
aforementioned
issues). I am NOT saying that the proposed mechanism is not useful
(by all means), just that the statement above is not correct.

Such applications require a
    mechanism providing source-initiated discovery of P2P routes that  
are
    not along a DAG.
JP> In some cases, it may be desirable to make use of source- 
initiated ...

This document thus describes such a mechanism,
    complementary to the basic RPL specification.

    The specified scheme is based on a reactive on-demand approach,  
which
    enables a router to discover one or more "good-enough" routes in
    either direction between itself and another router in the LLN  
without
    any constraints regarding the existing DAG-membership of the links
    that such routes may use.
JP> This will be an aspect triggering some discussion. How "good" is
"good enough" ? You will need some text to elaborate here or point
to another section explaining what is meant by "good enough"
Such routes may be source-routes or hop-
    by-hop ones.
JP> Just terminology: instead of referring to hop-by-hop routes, you may
want to say that once a route has been computed between two nodes in
the network, IPv6 packet may be forward using a source routing mechanism
as specified in [XX] or in a hop by hop fashion.
A complementary functionality, necessary to help decide
    whether to initiate a route discovery, is a mechanism to measure the
    end-to-end cost of an existing route.
JP> s/cost/path cost
Such functionality will be
    described in a separate document.

JP> But its use is described in this document ?


2.  Targeted Use Cases

    The mechanisms described in this document are intended to be  
employed
    as complementary to RPL in specific scenarios that need point-to-
    point (P2P) routes between arbitrary routers.

    One target use case, common in a home environment, involves a remote
    control (or a motion sensor) that suddenly needs to communicate with
    a lamp module, whose network address it knows apriori.  In this  
case,
    the source of data (the remote control or the motion sensor) must be
    able to discover a route to the destination (the lamp module) "on
    demand".

    Another target use case, common in a large commercial building
    environment, involves a large LLN deployment where P2P communication
    along a particular DAG among hundreds (or thousands) of routers
    creates severe traffic congestion near that DAG's root, and thus
    routes across this DAG are desirable.

    Targeted use cases also include scenarios where energy or delay
    constraints are not satisfied by the P2P routes along a DAG because
    they involve traversing many more intermediate routers than  
necessary
    to reach the destination.
JP> What I would suggest is to summarize it, put in the introduction,
and detail the use case in the applicability statement.

3.  Terminology

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
    "OPTIONAL" in this document are to be interpreted as described in



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    [RFC2119].

    Additionally, this document uses terminology from
    [I-D.ietf-roll-terminology] and [I-D.ietf-roll-rpl], and introduces
    the following terminology:

    Origin Router: The router initiating the route discovery.  The  
origin
    router acts as one end point of the routes to be discovered.

    Target Router: The other end point of the routes to be discovered.

    Intermediate Router: A router that is neither the origin nor the
    target.

    Forward Route: A route from the origin router to the target router.

    Backward Route: A route from the target router to the origin router.

    Bidirectional Route: A route that can be used in both directions:
    from the origin router to the target router and vice versa.

    Source Route: A complete and ordered list of routers that can be  
used
    by a packet to travel from a source to a destination.  Such source
    routes can be carried by a packet in a proposed Type 4 Routing  
Header
    [I-D.hui-6man-rpl-routing-header].

    Hop-by-hop Route: A route from a source to a destination where each
    router in the route knows only the address of the next hop on the
    route.
JP> I would suggest to change the definition for "routing paradigm
whereby packets are routed from their source to destination by
on a hop by hop basis where the packets is routed by a set of routers
along the path according to the routing table stored by each router
along the path".

    In this document, the term 'router' refers to any LLN device that  
can
    forward packets generated at other devices.


4.  Functional Overview

    Router A originates a "Discovery" message listing router B as the
    target.  Node A also indicates in the message:

    o  The nature of the routing cost, the method for accumulating the
       end-to-end cost
JP> This may de detailed hereafter but I guess that you will point to
the metric-ID for the metric used?

and the criteria used to determine if a route is
       "good enough";

    o  The direction (forward: router A to router B; backward: router B
       to router A; or both) of the route being discovered;

    o  The desired number of routes;




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    o  Whether the route is a source-route or a hop-by-hop one; and

    o  A limit on the "distance" the Discovery message may travel.

JP> Need to define the term "distance": is it a max path cost, number of
hops, ... ?


    The Discovery message propagates via link-local multicast
JP> Indicate "IPv6"
with each
    receiving router making the decision regarding whether to forward  
the
    message further based on the "distance" (from router A) that this
    particular copy of the message has already traveled.  Note that this
    document does not require a receiving router to use the "good  
enough"
    criteria to make the forwarding decision.  This is because the
    evaluation of such criteria may be too complex for a constrained
    intermediate router to perform for each received copy of the
    Discovery message.
JP> So I guess that you define later what to do in both cases (the node
cannot process the "good enough" criteria or it can process it) ?
The calculation of the "distance" that a copy of
    the Discovery message has already travelled should be simple enough
    for a constrained router to perform.
JP> Again it is hard to tell from what you wrote if this can  
reasonably be
achieved; it depends on the definition of the term "distance".
A router may optionally decide
    not to forward a copy of the Discovery message further because the
    aggregated cost of the route so far fails the "good enough"  
criteria.
JP> You will nee to be more specific here. Is it a "may" or a "MAY".
Define what happens in this case. You may provide more details later
in the document.

    The underlying mechanism being used to propagate the Discovery
    message may put further restrictions on its propagation.  A router
    should not propagate the Discovery message further if hop-by-hop
    routes are desired and the router can not store state for this  
route.

    As a copy of the Discovery message travels towards router B, it
    accumulates the relevant forward/backward costs as well as the route
    it takes.  When router B receives a copy of the Discovery message,  
it
    determines whether the route traveled by the message meets the "good
    enough" criteria.
JP> I think that you already mentioned this.

    If router A had requested the discovery of backward source-routes,
JP> Didn't you mean "forward" ?

    router B caches one or more good enough source-routes it identifies.
    Additionally, router B sends one or more "Discovery Reply" message  
to
    router A to acknowledge the discovery of these routes.  These
    acknowledgements allow router A to judge the success of the route
    discovery.  The Discovery Reply messages can travel towards router A
    in any manner chosen by router B. For example, router B may source-
    route the messages or send them towards router A along a DAG.

    If router A had requested the discovery of "n" forward source- 
routes,
    router B sends the "n" good enough source-routes it identifies to
    router A in one or more Discovery Reply messages.  Again, these
    Discovery Reply messages can travel towards router A in any manner
    chosen by router B.
JP> What if the DAG is "broken": retry from A after expiration of a  
local
timer ?

    If router A had requested the discovery of "n" bidirectional source-
    routes, router B caches the "n" good enough source-routes it
    identifies and also sends these routes to router A in one or more
    Discovery Reply messages.  These Discovery Reply messages can travel
    towards router A in any manner chosen by router B.



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    If router A had requested the discovery of "n" forward/backward/
    bidirectional hop-by-hop routes, router B sends out a Discovery  
Reply
    message to router A for each one of the "n" good enough routes it
    identifies.
JP> Do you need one "Discovery Reply message" per route or next-hop
in this case?
The Discovery Reply message travels towards router A
    using the source-route accumulated by the Discovery message.
JP> In the case of hop-by-hop routes, source routes, both ?
As this
    message travels towards router A, it establishes appropriate  
forward/
    backward routing state in the en-route routers.  This "non DAG"
    routing state should be specially marked to associate it with the
    routing metrics
JP> Refer to metric ID ?
used to discover the route and to distinguish such
    state from other DAG-specific routing state the router may have.
JP> Not sure that you need to distinguish the state. Suppose that you
discover a hop-by-hop route (not a big fan of that terms as pointed out
above but let's continue to use it for the time being), a storing node
may indeed have two routes for the same destination: do you want to
state that it should always use the P2P routes discovered by this
mechanism or let it choose the best one according to the metric in
use?
JP> What if there are two routes using different metrics?


5.  Propagation of Discovery Messages

    RPL uses DIO message propagation to build a DAG.  The DIO message
    travels via link-local multicast.
JP> Add everywhere "IPv6 link-local multicast addresses"

Each router joining the DAG
    determines a rank for itself and ignores the subsequent DIO messages
    received from lower (higher in numerical value) ranked neighbors.
    Thus, the DIO messages propagate outwards rather than return inwards
    towards the DAG root.
JP> I would suggest to align to the RPL terminology. Inward/outward
are not used anymore.

The DIO message generation at a router is
    further controlled by a trickle timer that allows a router to avoid
    generating unnecessary messages.
JP> Add a reference to the Trickle ID

The link-local multicast based
    propagation, trickle-controlled generation and the rank-based
    poisoning of messages traveling in the wrong direction (towards the
    DAG root) provide powerful incentives to use the DIO message as the
    Discovery message and propagate the DIO/Discovery message by  
creating
    a "temporary" DAG.

5.1.  The Route Discovery Option

        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 = 9    | Option Length | D |S|  N  | L |O|  Max  
Rank   |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 
+-+
         
|                                                               |
        |                       Target  
Address                          |
         
|                                                               |
         
|                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 
+-+
        |              OCP              |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


               Figure 1: Format of the Route Discovery Option

    In order to be used as a Discovery message, a DIO MUST carry a  
"Route
    Discovery" option illustrated in Figure 1.  A DIO MUST NOT carry  
more



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    than one Route Discovery options.
JP> Explain what you do if more than one is present:
	* Ignore the second one?
	* Ignore the entire message?

A Route Discovery option consists
    of the following fields:

    o  Option Type = 0x09 (to be confirmed by IANA).

    o  Option Length = 20 or 22 octets depending on whether the OCP  
field
       is included or not.

    o  D: A 2-bit field that indicates the direction of the desired
       routes:

       *  D = 0x00: Forward;

       *  D = 0x01: Backward;

       *  D = 0x02: Bidirectional.

    o  S: A flag that indicates whether source routes are desired.  The
       flag is set if the source routes are desired.  The flag is clear
       if hop-by-hop routes are desired.

    o  N: A 3-bit unsigned integer indicating the number of routes
       desired.

    o  L: A 2-bit field indicating the minimum "Life Time" of the
       temporary DAG, i.e., the minimum duration a router joining the
       temporary DAG must maintain its membership in the DAG:

       *  L = 0x00: Minimum life time is 5 seconds;

       *  L = 0x01: Minimum life time is 10 seconds;

       *  L = 0x02: Minimum life time is 1 minute;

       *  L = 0x03: Minimum life time is 10 minutes.

JP> Placeholder ... indicate what happens if a node cannot store that
additional state (add a manageability section).

    o  O: A flag that indicates whether the same OCP is used for route
       discovery as well as temporary DAG formation.  If this flag is
       clear, the OCP contained in the DODAG Configuration option in the
       DIO is used for route discovery as well.  Otherwise, the OCP
       specified in the Route Discovery option is used for route
       selection
JP> s/route selection/route discovery

and the metrics/constraints used for route selection are
       carried in a Metrics Container option immediately following the
       Route Discovery option.
JP> Since OF is decoupled from metrics, you always need to indicate
the metric in use. That said, I do not see your point here. Indeed, the
temporary DAG is built according to some OF and metrics so the
resulting routes will inherit these metrics?

    o  Max Rank: A 7-bit unsigned integer that indicates the maximum  
rank
       allowed in the temporary DAG advertised by the DIO message.  This
       upper limit on the DAG rank serves as the "distance" referred to



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       in Section 4 and provides a control over how far the Route
       Discovery option contained in the DODAG Configuration option in
       the DIO message may travel.
JP> You may discuss it later in this document ... but the distance is  
tied
to how you compute the rank.
A router MUST not
JP> s/MUST not/MUST NOT
join the temporary
       DAG if its rank in the DAG will exceed this limit.  Later  
versions
       of this draft will map the 128 value space available in this  
field
       to 16-bit long limits on the DAG rank.

    o  Target Address: The IPv6 address of the target router.

    o  OCP: 16 bit unsigned integer.  An optional field, present only if
       the O flag is set, that indicates the objective code point (OCP)
       to be used for route selection.

5.2.  Setting a DIO Carrying a Route Discovery Option

    A DIO message that carries a Route Discovery option MUST set the  
Base
    Object, described in [I-D.ietf-roll-rpl], in the following manner:

    o  RPLInstanceID: RPLInstanceID MUST be a local value as described  
in
       Section 4.1 of [I-D.ietf-roll-rpl].

    o  Grounded (G) Flag: MUST be clear since the objective of DAG
       formation is the propagation of Route Discovery option.  This DAG
       is temporary in nature and is not used for routing purpose.

    o  Destination Advertisement Supported (A) Flag: MUST be clear for
       same reasons as described above.

    o  Destination Advertisement Trigger (T) Flag: MUST be clear.

    o  Mode of Operation (MOP): Since the temporary DAG is not to be  
used
       for routing purposes, the value of this field is immaterial.  To
       allow constrained routers to join the DAG, the MOP field SHOULD  
be
       set to 00 (non-storing).

JP> I'd rather suggest to specify a new MOP value (0x02)

    o  DODAGPreference (Prf): TBD

    o  Destination Advertisement Trigger Sequence Number (DTSN): TBD

JP> To be discussed on the mailing list ?

    o  DODAGID: IPv6 address of the router initiating the route
       discovery.

JP> Origin router.

    The other fields in the Base Object are set as per the rules
    described in [I-D.ietf-roll-rpl].

    The DODAG Configuration option, carried in the DIO message,  
specifies
    the parameters for the trickle timer operation that governs the
    generation of DIO messages by the routers joining the temporary DAG.

JP> Also according to RPL ID or would you recommend different
default values ? Default values for RPL are TBD but we could easily
think of different values in this case. To be revisited in the future.



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    The other fields defined in the DODAG Configuration option are set  
as
    follows:

    o  The MaxRankIncrease field MUST be set to 0 to disable local  
repair
       of the temporary DAG.

    o  This document RECOMMENDS a value 1 for the MinHopRankInc field.

    o  Objective Code Point (OCP): The OCP to be used for temporary DAG
       formation.  This document RECOMMENDS RPL Objective Function 0, as
       defined in [I-D.ietf-roll-of0], for use as the objective function
       for the formation of the temporary DAG.

    A DIO, that contains a Route Discovery option with O flag set, MUST
    also contain a Metric Container option immediately following the
    Route Discovery option.  This Metric Container option carries the
    values for the routing metrics as well as the constraints
    (constituting the "good enough" criteria) used for route selection.
JP> This is confusing: the metric and constraint carried within the  
Metric
Container are used for the DODAG formation. Do you want to use them
for a different purpose ? At this point, you have not yet defined the  
use of
the good enough criteria but one can guess that you would use the good
enough metrics to control the flooding and the metric/OF to effectively
build the temporary DAG and discover the routes. If so, the good enough
metrics are of different use.

    Note that if O flag in the Route Discovery option is clear, the OCP
    and Metric Container used for temporary DAG formation are used for
    route selection as well.

    A DIO, carrying a Route Discovery option, MUST not carry any Route
JP> s/MUST not/MUST NOT

    Information or Prefix Information options described in
    [I-D.ietf-roll-rpl].

5.3.  Processing of a DIO Carrying a Route Discovery Option At An
       Intermediate Node

    The rules for DIO processing and transmission, described in  
Section 7
    of RPL [I-D.ietf-roll-rpl], apply to DIOs carrying a Route Discovery
    option as well except as modified in this document.

    When an intermediate router joins a temporary DAG advertized by a  
DIO
    carrying the Route Discovery option, it MUST maintain its membership
    in the DAG for the Minimum Life Time duration listed in the Route
    Discovery option.
JP> What if it cannot ? Just not join the DAG and log an error ?
Maintaining membership in the DAG implies
    remembering:

    o  The RPLInstanceID, the DODAGID and the DODAGVersionNumber for the
       temporary DAG;

    o  The router's rank in the temporary DAG;

    o  The best Metric Container,
JP> you mean the best path cost?
along with the associated source route
       from the initiator of route discovery till this router (carried  
in
       a Record Route IPv6 Extension Header proposed in
       [I-D.thubert-6man-reverse-routing-header]), for the Route



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       Discovery option being propagated by the temporary DAG.  If the
       router can not compare Metric Containers, it MUST remember the
       latest Metric Container it has received, along with the  
associated
       source route.
JP> Requires clarification on what you mean by "compare"

    A router belonging to a temporary DAG need not remember the identity
    of its DAG parents since the temporary DAG is not used for routing.
JP> Are you sure ? What is a hop-by-hop route is needed ? Don't you
need to remember your parent, or do you still record the route ?

    The main purpose of a temporary DAG's existence is to facilitate the
    propagation of the Route Discovery option.

    The router does not process a Route Discovery option, contained in
    the received DIO, any further if any of the following conditions are
    true:

    o  The router does not support the objective function
JP> and/or metrics
being used for
       route discovery

    o  The router does not have sufficient information to calculate the
       relevant routing metrics/constraints in the right direction (as
       specified by the D field).

    o  The S field is clear, i.e. hop-by-hop routes are desired, but the
       router can not participate in a hop-by-hop route.

    o  The router is an intermediate router and the router's rank in the
       temporary DAG, calculated on the basis of the rank and objective
       function carried in the Base Object in the DIO, exceeds the Max
       Rank value specified in the Route Discovery option.

    A router MUST discard the DIO if the Route Discovery option  
contained
    in the message does not need further processing.
JP> Log an error ?
Otherwise, the
    Route Discovery option is processed as follows.

    The router updates the Metrics Container associated with the  
received
    Route Discovery option as per the specified objective function.
JP> And routing metrics

The
    router may optionally check the Metric Container to determine if the
    aggregated values for the routing metrics meet all the constraints
    listed in the Metric Container.
JP> The aggregated values are for the metrics. Constraints are used to
prune potential path.
If not, the Route Discovery option
    is discarded
JP> You mean the DIO message.
without further processing.  Otherwise, the router
    updates its in-memory copy of the latest/best Metric Container for
    this Route Discovery option along with the associated source route
    updated in the correct direction (based on the D field of the Route
    Discovery option).  Any change in the in-memory copy also requires
    resetting the trickle timer and generating a new DIO carrying the
    Route Discovery option, the updated latest/best Metric Container and
    the associated source route (in a Record Route IPv6 extension header
    proposed in [I-D.thubert-6man-reverse-routing-header]) when the  
timer
    fires.
JP> If the changes occur during the lifetime window.
Note that the Metric Container MUST immediately follow the



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    Route Discovery option if the O flag in the Route Discovery option  
is
    set.

5.4.  Processing of a DIO Carrying a Route Discovery Option At The
       Target Router

    When the target router joins the temporary DAG advertized by a DIO
    carrying the Route Discovery option, it MUST maintain its membership
    in the DAG for the Minimum Life Time duration listed in the Route
    Discovery option.  Maintaining membership in the DAG implies
    remembering:

    o  The RPLInstanceID, the DODAGID and the DODAGVersionNumber for the
       temporary DAG;

    o  The router's rank in the temporary DAG;

    The target router does not process a Route Discovery option,
    contained in the received DIO, any further if any of the following
    conditions are true:

    o  The router does not support the objective function used for route
       discovery

    o  The router does not have sufficient information to calculate the
       relevant routing metrics/constraints in the right direction (as
       specified by the D field).

JP> or does not support the metric.

    The target router MUST discard the DIO if the Route Discovery option
    contained in the message does not need further processing.
JP> ? How does it knows (simply because it reaches the destination) ?

    Otherwise, the Route Discovery option is processed as follows.

    The target router updates the Metrics Container associated with the
    received Route Discovery option as per the specified objective
    function.
JP> and routing metrics (remember they are decoupled).

The target router then checks the Metric Container to
    determine if the aggregated values for the routing metrics meet all
    the constraints listed in the Metric Container.
JP> See previous comments.

If not, the Route
    Discovery option is discarded without further processing.   
Otherwise,
    the router MAY select the source route accumulated by the received
    DIO as one of the discovered routes.
JP> "MAY" ? When ?
The target router MUST send one
    or more RPL Control Messages carrying a Discovery Reply Object
    (defined in the next section) back to the origin router (identified
    by the DODAGID field in the DIO Base Object) as discussed in the
    following sections.

    The target router MUST NOT forward a DIO carrying a Route Discovery
    option that lists one of its own addresses as the Target Address.




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6.  Propagation of Discovery Reply Messages

6.1.  The Discovery Reply Object (DRO)

        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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 
+-+
        | RPLInstanceID |    Version    | D |S|  N  |      
Reserved      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 
+-+
         
|                                                               |
        |                         DODAGID                           |
         
|                                                               |
         
|                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 
+-+
         
|                                                               |
        |                       Target  
Address                          |
         
|                                                               |
         
|                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 
+-+
        | Option(s)...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...


            Figure 2: Format of the Discovery Reply Object (DRO)

    This document defines a new RPL Control Message type, the Discovery
    Reply Object (DRO) with code 0x04 (to be confirmed by IANA), that
    serves the following functions:

    o  An acknowledgement from the target router to the origin router
       regarding the successful discovery of backward source routes;
JP> ACK of routes from the origin to the target: you may not have routes
in the opposite direction.

    o  Carries one or more forward/bidirectional source routes from the
       target to the origin router;

    o  Establishes a hop-by-hop forward/backward/bidirectional route as
       it travels from the target to the origin router.

    The format for a Discovery Reply Object (DRO) is shown in Figure 2.
    A DRO consists of the following fields:

    o  RPLInstanceID: The RPLInstanceID of the temporary DAG used for
       route discovery.

    o  Version: The Version of the temporary DAG used for route
       discovery.





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    o  DODAGID: The DODAGID of the temporary DAG used for route
       discovery.  The DODAGID also identifies the origin router.  The
       RPLInstanceID, the Version and the DODAGID together uniquely
       identify the temporary DAG used for route discovery and can be
       copied from the Base Object of the DIO advertizing the temporary
       DAG.

    o  D: A 2-bit field that indicates the direction of the discovered
       routes:

       *  D = 0x00: Forward;

       *  D = 0x01: Backward;

       *  D = 0x02: Bidirectional.

       This field has the same value as the corresponding field in the
       Route Discovery option.

    o  S: A flag that is clear if the Discovery Reply Object is
       establishing an hop-by-hop route.  The flag is set if the
       Discovery Reply Object carries (or is an acknowledgement for the
       discovery of) one or more source routes.

    o  N: A 3-bit field that indicates the number of source routes
       carried or acknowledged in the Discovery Reply Object.

    o  Target Address: The IPv6 address of the target router originating
       the Discovery Reply Object.

    o  Reserved: These bits are reserved for future use.  These bits  
MUST
       be set to zero on transmission and MUST be ignored on reception.

    o  Options: The Discovery Reply Object MAY carry up to 7 Source  
Route
       options (defined in the next section) with each such option
       carrying a complete forward/bidirectional source route and
       optionally followed by a Metric Container option that lists the
       aggregated values for the routing metrics for the source route.
JP> This may not be an aggregated value but recorded metric too.













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6.1.1.  The Source Route Option

        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 = 10   | Option Length | Compr |  Pad  |      
Resvd     |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 
+-+
         
|                                                               |
        .                       
Addresses[1..n]                          .
        .                                                               .
         
|                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 
+-+


                 Figure 3: Format of the Source Route Option

    The Source Route option, illustrated in Figure 3, carries a complete
    forward/bidirectional source route from the target router to the
    origin router.  A Source Route option can only be a part of the
    Discovery Reply Object and MAY be immediately followed by a Metric
    Container option that contains the aggregated values of the routing
    metrics for this source route.

    A Route Discovery option consists of the following fields:

    o  Option Type = 0x0A (to be confirmed by IANA).

    o  Option Length = Variable, depending on the size of the Addresses
       vector.

    o  Compr: 4-bit unsigned interger indicating the number of prefix
       octets that are elided from each address.  For example, Compr
       value will be 0 if full IPv6 addresses are carried in the
       Addresses vector.

    o  Pad: 4-bit unsigned integer.  Number of octets that are used for
       padding between Address[n] and the end of the Source Route  
option.

    o  Address[1..n]: Vector of addresses, numbered 1 to n.  Each vector
       element has size (16 - Compr) octets.

    A common network configuration for an RPL domain is that all routers
    within an LLN share a common prefix.  The Source Route option uses
    the Compr field to allow compaction of the Addresses[1..n] vector
    when all entries share the same prefix as the DODAGID or the Target
    Address of the encapsulating Discovery Reply Object.  The shared
    prefix octets are not carried within the Source Route option and  
each
    entry in Address[1..n] has size (16 - Compr) octets.  When Compr is



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    non-zero, there may exist unused octets between the last entry,
    Address[n], and the end of the Source Route option.  The Pad field
    indicates the number of unused octets that are used for padding.
    Note that when Compr is 0, Pad MUST be null and carry a value 0.

    The Source Route option MUST NOT specify a path that visits a router
    more than once.  When generating a Source Route option, the target
    router may not know the mapping between IPv6 addresses and routers.
    Minimally, the target router MUST ensure that:

    o  The IPv6 Addresses do not appear more than once;

    o  The IPv6 addresses of the origin and the target routers do not
       appear in the Address vector;

    o  The Address vector represents a source route in forward direction
       with Address[1] being the next hop for the origin router.

    Multicast addresses MUST NOT appear in a Source Route option.

JP> you may want to add a ref to RH4 since the format is very similar.

6.2.  DRO as Acknowledgement for Backward Source Routes

    When a target router discovers a backward source route,
JP> I have an issue with your terminology. When the target receives the
DIO with route discovery option, what is discovered is a forward route ?

it sends a
    DRO message to the origin router as an acknowlegement for the
    discovered route.  Optionally, a target router MAY send a DRO
    acknowledgement to the origin router after discovering multiple
    backward source routes.  A DRO, serving as an acknowledgement for
    backward source route discovery, has its D field set to 0x01
    (indicating backward) while the S flag is set to 1 (indicating  
source
    route).  The N field is set to indicate the number of discovered
    backward source routes being acknowledged.  Such a DRO message MUST
    NOT contain any option.

    This document does not require a particular method for sending  
such a
    DRO message to the origin router.  The target router MAY send the  
DRO
    message to the origin router in any fashion, including:

    o  Using a source route carried in a Type 4 Routing header
       [I-D.hui-6man-rpl-routing-header];

    o  Along any DAG connecting the target and the origin routers;

    o  Along the temporary DAG created for route discovery, provided  
that
       the target router is reasonably sure that the DAG's life time is
       sufficiently long and the routers in the DAG do remember one or
       more of their parents in the DAG.
JP> You may want to explain a bit more why you the origin router
would be interested in receiving an ACK without the route itself (case
6.3): hop-by-hop routes.





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6.3.  DRO as Carrier of Forward/Bidirectional Source Routes

    When a target router discovers a forward source route,
JP> Here, I agree that this is a forward route.
it sends a DRO
    message to the origin router carrying the discovered source route
    inside a Source Route option.  Similarly, when a target router
    discovers a bidirectional source route, it caches the route for its
    own use and then sends a DRO message to the origin router carrying
    the discovered route inside a Source Route option.  Rather than
    immediately sending a discovered route back to the origin router, a
    target router MAY optionally send one DRO message after discovering
    multiple forward/bidirectional source routes with the sent DRO
    carrying the discovered source routes (in multiple Source Route
    options).
JP> Note that the algorithm for "best route" selection is outside the  
scope
of this document, so does the use of a timer on the target side to  
determine
how much time to wait before answering, ...


    A DRO message carrying one or more Source Route options MUST have  
its
    D field set to 0x00 (for forward routes) or 0x02 (for bidirectional
    routes).  The S flag MUST be set to 1 and the N field MUST indicate
    the number of Source Route options in the message.  A Source Route
    option MAY immediately be followed by a Metric Container option that
    carries the aggregated
JP> or recorded
values of the routing metrics for this source
    route.  The target router may send this DRO message to the origin
    router in any fashion it desires including the options listed in
    Section 6.2.

6.4.  Establishing Hop-by-hop Routes Via DRO

    In order to establish a hop-by-hop route, the target router sends a
    DRO message along the reverse of the discovered route (via a Type 4
    Routing Header specified in [I-D.hui-6man-rpl-routing-header]).  The
    D field of the message is set to convey the route's direction
    (forward/backward/bidirectional) and the S flag MUST be clear
    (indicating hop-by-hop).  The N field MUST be set to 1.  Such a DRO
    MUST NOT contain any option.

    A router receiving such a DRO message SHOULD establish hop-by-hop
    state in the right direction corresponding to the route carried in
    the Type 4 Routing Header of the DRO message.  If a router does not
    establish such state, it MUST drop the DRO message.  Otherwise, it
    MUST forward the DRO message along the source route contained in  
Type
    4 Routing Header of the received message.

JP> Any error log if it does not ? (optional) ICMP sent to the target  
node?

    A router SHOULD associate the hop-by-hop routing state, thus
    established, with the objective function and metrics used for route
    discovery.







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JP> New section to indicate how to clean-up the temporary DAG ?

7.  Security Considerations

    TBA


8.  IANA Considerations

    TBA


9.  Authors and Contributors

    In addition to the editor, the authors of this document include the
    following individuals (listed in alphabetical order).

    Anders Brandt, Sigma Designs, Emdrupvej 26A, 1., Copenhagen,  
Dk-2100,
    Denmark.  Phone: [phone]; Email: [e-mail]

    Robert Cragie, Gridmerge Ltd, 89 Greenfield Crescent, Wakefieldm WF4
    4WA, UK.  Phone: [phone]; Email: [e-mail]

    Jerald Martocci, Johnson Controls, Milwaukee, WI 53202, USA.  Phone:
    [phone]; Email:[e-mail]

    Charles Perkins, Tellabs Inc., USA.  Email:[e-mail]

    Authors gratefully acknowledge the contributions of Richard Kelsey
    and Zach Shelby in the development of this document.


10.  References

10.1.  Normative References

    [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

10.2.  Informative References

    [I-D.brandt-roll-rpl-applicability-home-building]
               Brandt, A., Baccelli, E., and R. Cragie, "Applicability
               Statement: The use of RPL in Building and Home
               Environments",
               draft-brandt-roll-rpl-applicability-home-building-00  
(work
               in progress), April 2010.

    [I-D.hui-6man-rpl-routing-header]
               Hui, J., Vasseur, J., and D. Culler, "A Source Routing



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               Header for RPL", draft-hui-6man-rpl-routing-header-00
               (work in progress), May 2010.

    [I-D.ietf-roll-building-routing-reqs]
               Martocci, J., Riou, N., Mil, P., and W. Vermeylen,
               "Building Automation Routing Requirements in Low Power  
and
               Lossy Networks", draft-ietf-roll-building-routing-reqs-09
               (work in progress), January 2010.

    [I-D.ietf-roll-of0]
               Thubert, P., "RPL Objective Function 0",
               draft-ietf-roll-of0-02 (work in progress), June 2010.

    [I-D.ietf-roll-routing-metrics]
               Vasseur, J., Kim, M., Networks, D., and H. Chong,  
"Routing
               Metrics used for Path Calculation in Low Power and Lossy
               Networks", draft-ietf-roll-routing-metrics-07 (work in
               progress), June 2010.

    [I-D.ietf-roll-rpl]
               Winter, T., Thubert, P., and R. Team, "RPL: IPv6 Routing
               Protocol for Low power and Lossy Networks",
               draft-ietf-roll-rpl-09 (work in progress), June 2010.

    [I-D.ietf-roll-terminology]
               Vasseur, J., "Terminology in Low power And Lossy
               Networks", draft-ietf-roll-terminology-03 (work in
               progress), March 2010.

    [I-D.thubert-6man-reverse-routing-header]
               Thubert, P., "Reverse Routing Header",
               draft-thubert-6man-reverse-routing-header-00 (work in
               progress), June 2010.

    [RFC5826]  Brandt, A., Buron, J., and G. Porcu, "Home Automation
               Routing Requirements in Low-Power and Lossy Networks",
               RFC 5826, April 2010.














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Authors' Addresses

    Mukul Goyal (editor)
    University of Wisconsin Milwaukee
    3200 N Cramer St
    Milwaukee, WI  53211
    USA

    Phone: [phone]
    Email: [e-mail]


    Emmanuel Baccelli (editor)
    INRIA

    Phone: [phone]
    Email: [e-mail]
    URI:   http://www.emmanuelbaccelli.org/


    P2P Team


JP> Remove P2P Team unless you list the people.





























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