Subject: [mpls] Questions about L-ARP draft

From: Igor Malyushkin <[email-masked]>
Date: Wed, 23 Feb 2022 02:58:34 +0200

Dear Authors,

I read "draft-kompella-mpls-larp-11" and have a couple of questions.

Sections 3.3 & 3.4 are not clear for me in the context of label operations:

Section 3.3.

 2.  if it does, T1 allocates a label TL3 to reach H3 (if it doesn't
       already have such a label) and installs an L-FIB entry to swap L1
       with the label (stack) to reach H3.

Here we allocate a label (TL3) to reach H3.

 3.  sends a (proxy) L-ARP reply to H1 with the Source Hardware
       Address (SHA) set to (L, M), where M is T1's metric to H3.  T1
       may also set some attribute bits in the SHA.

I think it would be better to specify either that L is TL3 for clarity:
"... set to (L, M), where M is T1's metric to H3 and L is the label
allocated by T1 to reach H3 (TL3).".

Section 3.4.

 To send a packet to H3 over an MPLS tunnel, H1 pushes L1 onto the
   packet, sets the destination MAC address to M1 and sends it to T1.

Here H1 has to push L1 instead of TL3. Probably it`s better to preserve the
same label among all paragraphs. Also, I think it`s better to specify that
M1 belongs to T1 to avoid any discrepancies.

   5.  Finally, H1 pushes the label pair (TL3, VL3) onto the packet from
       VM1 and sends this to T1.  This packet will then end up at VM3 on
       H3.

The next reference of the T1`s label is again TL3. Somebody can think that
T1 allocated two labels (L1, TL3) for different cases.

Section 4.

May I suggest propagating the MTU value for the tunnel here? I believe MTU
is important because an end host may consider it to influence the state of
the tunnel. If a host knows that tunnel`s MTU doesn't fit (for example a
layer-2 service requires the MTU greater than the tunnel`s MTU) it can
consider/query another tunnel or even disable the service avoiding sending
traffic to the server in vain. From my point of view, a sender (a server)
also should check the possibility to receive the largest frame (not bigger
than the tunnel's MTU for sure) on an interface/port to a client it
allocates a label.

Hope these comments are helpful. Thank you!

---

From: Erik Auerswald <[email-masked]>
Date: Wed, 23 Feb 2022 08:47:57 +0100

Hi,

> Sections 3.3 & 3.4 are not clear for me in the context of label operations:

I too think that section 3 is not really clear an consistent.  I have
sent a suggestion on how to improve it in July last year:

https://mailarchive.ietf.org/arch/msg/mpls/ekPKzVVgwUoxmzPd-yximgVrrLw/

Best regards,
Erik
-- 
Dipl.-Inform. Erik Auerswald
Gesellschaft für Fundamental Generic Networking mbH
Geschäftsführung: Volker Bauer, Jörg Mayer
Gerichtsstand: Amtsgericht Kaiserslautern - HRB: 3630