comments on mech-v2-01

["Chirayu Patel" <chirayu@chirayu.org>]

Hello,

Few comments on the draft.

1) A race is triggered when an IPv6 router that has a configured tunnel
   with another router is doing dynamic mtu detection. The outcome of the
   race is that one or more ICMPv6 "packet too big" message might not be
   sent out to an host.

Assume an IPv6 network like:

[H1]-------->[R1]===========>[R2]--------->[H2]

H1, and H2 are IPv6 hosts R1, and R2 are IPv6 routers with a configured
tunnel between them.

and the message flow is:

  1. H1->R1 TCP packet of size 1400.
  2. R1->R2 encapsulated packet of size 1400+20. (R1 is doing PMTU here)
     (Assume 1500 is the link MTU).
  3. Assume one of the IPv4 routers between R1, and R2 send an ICMPv4
     "fragmentation needed" with only 8 bytes of payload.
  4. R1 records the new MTU for the tunnel but does *not* send an ICMPv6
     "packet too big" message with MTU as 1300.
  5. H1->R1 TCP packet of size 1400 (this is a retry)
  6. R1->H1 ICMPv6 "packet too big" with size 1300
  7. H1->R1 TCP packet of size 1400 (this is a retry)
  8. ... (the above cycle might repeat if there are more IPv4 routers
     that send ICMPv4 "fragmentation needed" with 8 bytes of payload)

IMHO, this one can be clarified in the draft so that it does not become a
puzzle for implementors.

9) Link-layer address (which is an IPv4 address) is not meant to be used
   with ND. Is there any reason that the sending (of link-layer address)
   is a "SHOULD NOT", and the receiving is a ?MUST ignore?. The sending,
   and the receiving parts should be made consistent with respect to
   link-
   layer address. i.e. sending should be a "SHOULD NOT", and receiving
   should be a "SHOULD ignore?, or sending should be a "MUST NOT", and
   receiving should be a "MUST ignore?. Btw, "NOT" is not a keyword.
   Hence "s/NOT/not".

   "For the purposes of Neighbor Discovery the configured tunnels
   specified in this document are assumed to NOT have a link-layer
   address, even though the link-layer (IPv4) does have address.  This
   means that:

    -   the sender of Neighbor Discovery packets SHOULD NOT include
        Source Link Layer Address options or Target Link Layer Address
        options on the tunnel link.

    -   the receiver MUST, while otherwise processing the neighbor
        discovery packet, silently ignore the content of any Source Link"


Editorial
---------

1) s/For that reason/For that reason,/