Subject: AD review of draft-ietf-6man-rpl-option

From: Jari Arkko <[email-masked]>
Date: Sat, 18 Jun 2011 00:22:15 +0300

I have reviewed this draft. Some of the issues from the other draft are 
visible in this one as well, and I saw two additional substantive issues:

- we need to specify the behavior with regards to the data in this 
option better
- the text about processing packets in RPL border routers should be 
written in a different manner

Both of these should be easily addressed, however. Please revise the 
draft and we can send the draft to IETF Last Call.

Here are my comments in more detail:

> Datagrams being forwarded within a RPL domain MUST include a RPL
> Option.  For datagrams sourced within a RPL domain, the RPL Option
> MAY be included in the datagram itself.

I'm not sure I understand the difference or its motivation. Do you 
really mean that a packet might not have the option until it hits the 
first router? Or are you just talking about something that happens 
internally on a host, but on the wire all packets would still have the 
option? Also, since the tunnel (or something else) is used to include 
the option for datagrams sourced outside the RPL domain, wouldn't it be 
easier to just say this:

"Datagrams sent between nodes within an RPL domain MUST include an RPL 
Option."

>   For datagrams sourced
>    outside a RPL domain, IPv6-in-IPv6 tunneling, as specified in
>    [RFC2473 <http://tools.ietf.org/html/rfc2473>] SHOULD be used to include a RPL Option.

This text should be aligned with whatever conclusion we will have for 
the issue that I raised with the other document.

> To help avoid IP-layer fragmentation, the RPL Option has a maximum
> size of RPL_OPTION_MAX_SIZE octets and links within a RPL domain
> SHOULD have a MTU of at least 1280 + 44 (outer IP header, Hop-by-Hop
> Option header, Option header) + RPL_OPTION_MAX_SIZE + (additional
> extension headers or options needed within RPL domain).
>   

There's a same MTU issue here as in the other document.

> The action taken by using the RPL Option and the potential set of
> sub-TLVs carried within the RPL Option MUST be specified by the RFC
> of the protocol that use that option.  No TLVs are defined in this
> document.
>   

I think you should define the behavior when a node encounters a sub-TLV 
that it does not recognize. E.g., ignore and move on to the next 
sub-TLV. Or do you want a stricter policy? In any case, for future 
extensions it will be necessary to know how they are treated by legacy 
RPL nodes.

> In very specific cases, IPv6-in-IPv6 tunneling may be undesirable due
> to the added cost and complexity required to process and carry a
> datagram with two IPv6 headers.  [I-D.hui-6man-rpl-headers <http://tools.ietf.org/html/draft-ietf-6man-rpl-option-03#ref-I-D.hui-6man-rpl-headers>] describes
> how to avoid using IPv6-in-IPv6 tunneling in such specific cases and
> the risks involved.
>   

Again, the same comments as in the other draft. Please delete this 
paragraph.

> For datagrams exiting the RPL domain, RPL Border Routers MUST remove
> the RPL Option from the datagram.  If the RPL Option was included
> using tunneled mode and the RPL Border Router serves as the tunnel
> end-point, removing the outer IPv6 header serves to remove the RPL
> Option as well.  Otherwise, the RPL Border Router assumes that the
> RPL Option was included using transport mode and MUST remove the RPL
> Option from the IPv6 Hop-by-Hop Option header.
>   

The part about removing the RPL option even in a non-tunneled case 
relates to the issue of supporting that particular mode of operation.

But in addition, I wonder if you should write the above text not in 
terms of packet modification operations but rather in terms of 
forwarding decision outcomes. Like this, for instance:

"For datagrams destined to the RPL Border Router the router processes 
the packet in the usual way. For instance, if the RPL Option was 
included using tunneled mode and the RPL Border Router serves as the 
tunnel end-point, the router removes the outer IPv6 header, at the same 
removing the RPL Option as well. Datagrams destined elsewhere within the 
same RPL domain are forwarded to the right interface. Datagrams destined 
outside the RPL domain are dropped."

> 6. Usage of the RPL Option
>
>    The RPL Option is only for use within a RPL domain.  RPL routers MUST
>    process and include the RPL Option when forwarding datagrams to other
>    nodes within the RPL domain.  Routers on the edge of a RPL domain
>    MUST remove the RPL Option when forwarding datagrams to nodes outside
>    the RPL domain.

What is it that this section says that is not already covered by 
sections 2 and 5:

Sect 2: Datagrams being forwarded within a RPL domain MUST include a RPL 
Option.

Sect 5: ... serves as the tunnel end-point, removing the outer IPv6 
header serves to remove the RPL Option as well.  Otherwise, the RPL 
Border Router assumes that the RPL Option was included using transport 
mode and MUST remove the RPL Option from the IPv6 Hop-by-Hop Option header.

> This option may be used to mount several potential attacks since
> routers may be flooded by bogus datagram containing the RPL option.
> It is thus RECOMMENDED for routers to implement a rate limiter for
>    datagrams using the RPL Option.
>   

Please open this up a bit. What specific danger does flooding by bogus 
datagrams and RPL options cause? What would be the default settings for 
the rate limiter?

>    Opt Data Len:  8-bit field indicating the length of the option, in
>          octets, excluding the Option Type and Opt Data Len fields.
>
>    Down 'O':  1-bit flag as defined in Section 11 of
>          [I-D.ietf-roll-rpl <http://tools.ietf.org/html/draft-ietf-6man-rpl-option-03#ref-I-D.ietf-roll-rpl>].
>
>    Rank-Error 'R':  1-bit flag as defined in Section 11 of
>          [I-D.ietf-roll-rpl <http://tools.ietf.org/html/draft-ietf-6man-rpl-option-03#ref-I-D.ietf-roll-rpl>].
>
>    Forwarding-Error 'F':  1-bit flag as defined in Section 11 of
>          [I-D.ietf-roll-rpl <http://tools.ietf.org/html/draft-ietf-6man-rpl-option-03#ref-I-D.ietf-roll-rpl>].
>
>    RPLInstanceID:  8-bit field as defined in Section 11 of
>          [I-D.ietf-roll-rpl <http://tools.ietf.org/html/draft-ietf-6man-rpl-option-03#ref-I-D.ietf-roll-rpl>].
>
>    SenderRank:  16-bit field as defined in Section 11 of
>          [I-D.ietf-roll-rpl <http://tools.ietf.org/html/draft-ietf-6man-rpl-option-03#ref-I-D.ietf-roll-rpl>].
>
>    Values within the RPL Option are expected to change en-route. 

This specification needs to describe what the behavior of a router is 
with the content of the option. I think this is easy, you should just 
add to the end: "The processing shall follow the rules described in 
Section 11.2 of [roll-rpl].

As an aside, the entire Section 11 is marked in roll-rpl as 
non-normative. I don't think that's actually right as far as 11.2 goes, 
because it contains tons of MUSTs and SHOULDs. Perhaps you want to fix 
that in AUTH48...

Jari