[DNA] Comments on Combined DNA Solution draft

[Subba Reddy <subbareddy@samsung.com>]

Hi Brett,

        It is good to see combined solution.
Overall draft is good.

Here are few comments/questions.

1. Is the fast RA mechanism is applicable for all the RS's (not only
for DNA RS's) that a router receives
(with source address as unicast link local address)?

2. Consider the following case.
Router R1 (of Link1) is sending Prefix P1 whose life time is 15 seconds.
A host H came to Link1 and included P1 in DNAHostPrefixList.
P1 expired gracefully and assigned to Link2. If H moves to Link2 and
receives complete RA, it will find the match in DNAHostPrefixList and
will assume same link. Host will not delete P1 since expiry time for entries
in DNAHostPrefixList is 1.5 hours.

3. It may be better to mention, how to populate DNAHostPrefixList.
Only reference to this is, last paragraph of section 6.2.5

4. This comment is regarding "whether a host needs to know whether
a particulat prefix is common prefix (LinkID) or not?"
If a host stores all the prefixes of a link, it does not matter whether RA
has included LinkID or not. If a host cannot store all the prefixes
( may be because of maximum bound), there is a probability that host
may not store LinkID. In that case there may not be any use in including
LinkID without mentioning it as LinkID.  If the prefix is mentioned as
LinkID and if we mandate a host to store that common prefix,
this problem may not come.

Am I missing something?

5. From Section 2, Section 4.2 LinkID and Landmark are optional

Section 2,
" ... Aspects of prefix-based LinkID and
   Requested Landmark are optionally included to allow for a decrease in
   the packet sizes associated with Complete RA."

Section 4.2,
"   Frequently, all routers on a link will advertise the same set of
   prefixes and so this technique will incur no space penalty.  However,
   on links where routers advertise different sets of prefixes, packets
   may be larger.  To combat this, two optional mechanisms are defined."

But in some places, LinkID and Landmark are mentioned as MUST.

Section 6.1.5
"   In order to respond to a Router Solicitation, the router SHOULD
   generate a Complete RA as specified in Section 6.1.6.  The Router
   Advertisement MUST include the LinkID, as described in Section 6.1.7."

Section 6.1.6
"   Note that although it may not be possible to fit all of the prefixes
   into an RA, the LinkID prefix MUST be included."

Section 6.1.9
"      They MAY be Complete RAs or MAY include only a subset of the
      configured prefixes, but MUST include the LinkID prefix."

Section 6.2.1
"   Hosts MUST also maintain a "Landmark Prefix" as described in
   Section 6.2.3."

6. LinkID is sent in unsolicited RAs as per section 4.2  We may need to send
LinkID in solicited RAs also in case of incomplete RAs.

From Section 4.2
"  A second mechanism for reducing packet sizes applies to unsolicited
   Router Advertisements. By selecting one prefix on the link to be the
   "link identifier", and making sure that it is included in every
   advertisement, it is possible to omit some prefixes. "

LinkID is sent is solicited RAs as per section 6.1.5
From Section 6.1.5
"  In order to respond to a Router Solicitation, the router SHOULD
   generate a Complete RA as specified in Section 6.1.6.  The Router
   Advertisement MUST include the LinkID, as described in Section 6.1.7."

7. This is comment is raised by others also.
Do we need two bits Y and N in Landmark Option?
I think, one bit may serve the purpose.

8. From Section 6.1.5
"   In order to respond to a Router Solicitation, the router SHOULD
   generate a Complete RA as specified in Section 6.1.6.  The Router
   Advertisement MUST include the LinkID, as described in Section 6.1.7."

For RAs with Landmark option with "Y" bit set, we may not include
LinkId.

9. Since LinkID and Landmark are optional (Section 2),
different implementations are possible.
Router side:-
    a) Complete RA
    b) Complete RA + Landmark
    c) Complete RA + LinkID
    d) Complete RA + Landmark + LinkID

Host side:-
    a) Complete RA
    b) Complete RA + Landmark

As per my understanding all the combinations of router and host
implementations will co-work fine.
If Landmark & LinkID are also part of the solution
(instead of optional), these many possibilities may not be there. It may
be easy to understand and implement.  If this is already discussed by
design team, please ignore this comment.

Editorial:

1. From Section 6.1.5,
"   If this is not the case AND there is no multicast RA scheduled for
   transmission in the next MulticastRADelay, then the RA MUST be sent
   to the link-scoped *all-hosts multicast address* at the current time
   plus MulticastRADelay."

I think, generally we use the term "all-nodes multicast address" instead
of "all-hosts multicast address"

2. From Section 6.1.4
"   Regardless of the state of the DNA flag, each PIO in the RA is
   examined.  If the prefix is not in the router's DNARouterPrefixList
   *or* AdvPrefixList [3], it is added to the DNARouterPrefixList, and its
   expiry time is set."

From Section 6.1.6

"   If it is not possible to generate a Complete RA but the Router
   Solicitation that this Router Advertisement is in response to, if
   any, includes a Landmark Option containing a prefix that is not in
   DNARouterPrefixList *or* AdvPrefixList then the router SHOULD include a
   Landmark Option with the "N" flag set."

I think, "or" may need to be replaced with *and"

3. From Section 6.2.4
"   Hosts SHOULD include a Landmark Option (LO) in the RS message with
   the landmark prefix chosen based on the rules in *section*
   *Section* 6.2.3."

"section" is repeated.

Thanks & Regards,
Subba Reddy