Re: [Softwires] [Int-area] Errata on RFC 6333, "Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion"

["Eric Vyncke (evyncke)" <[e-mail]>]

Thank you Ian, Joe, and Med for your feedback on this errata. I will propose the text fix as proposed by Ian (Med’s suggestion is a paraphrase)

Med, would you mind opening a new errata for the section 6.3, which has the same problem as noticed by Mikael ? Thank you

-éric


From: "[e-mail]" <[e-mail]>
Date: Tuesday, 11 May 2021 at 08:49
To: "[e-mail]" <[e-mail]>, Eric Vyncke <[e-mail]>
Cc: "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>
Subject: RE: [Int-area] [Softwires] Errata on RFC 6333, "Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion"

Hi all,

I agree with Ian.

Actually, the document should use a similar wording in both the section mentioned in the errata report and the one in of 6.3. I’m afraid editorial changes are also needed for 6.3 to avoid any confusion.


(1) 5.3:

OLD:

   However, as not all service providers will be able to increase their

   link MTU, the B4 element MUST perform fragmentation and reassembly if

   the outgoing link MTU cannot accommodate the extra IPv6 header.  The

   original IPv4 packet is not oversized.  The packet is oversized after

   the IPv6 encapsulation.  The inner IPv4 packet MUST NOT be

   fragmented.  Fragmentation MUST happen after the encapsulation of the

                                                                  ^^^^^^

   IPv6 packet.  Reassembly MUST happen before the decapsulation of the

   ^^^^^^^^^^

   IPv4 packet.  A detailed procedure has been specified in [RFC2473]

   ^^^^^^^^^^^^

   Section 7.2.

NEW:

   However, as not all service providers will be able to increase their

   link MTU, the B4 element MUST perform fragmentation and reassembly if

   the outgoing link MTU cannot accommodate the extra IPv6 header.  The

   original IPv4 packet is not oversized.  The packet is oversized after

   the IPv6 encapsulation.  The inner IPv4 packet MUST NOT be

   fragmented.  Fragmentation MUST happen after the IPv6 encapsulation.

                                                    ^^^^

   Reassembly MUST happen before the decapsulation of the

   of the IPv6 header.  A detailed procedure has been specified in [RFC2473]

   ^^^^^^^^^^^^^^^^^^

   Section 7.2.


(2) 6.3



OLD:

   As noted previously, fragmentation and reassembly need to be taken

   care of by the tunnel endpoints.  As such, the AFTR MUST perform

   fragmentation and reassembly if the underlying link MTU cannot

   accommodate the encapsulation overhead.  Fragmentation MUST happen

   after the encapsulation on the IPv6 packet.  Reassembly MUST happen

                           ^^^^^^^^^^^^^^^^^^

   before the decapsulation of the IPv6 header.  A detailed procedure

   has been specified in [RFC2473] Section 7.2<https://datatracker.ietf.org/doc/html/rfc2473#section-7.2>.

NEW:

   As noted previously, fragmentation and reassembly need to be taken

   care of by the tunnel endpoints.  As such, the AFTR MUST perform

   fragmentation and reassembly if the underlying link MTU cannot

   accommodate the encapsulation overhead.  Fragmentation MUST happen

   after the IPv6 encapsulation.  Reassembly MUST happen^

             ^^^^

   before the decapsulation of the IPv6 header.  A detailed procedure

   has been specified in [RFC2473] Section 7.2<https://datatracker.ietf.org/doc/html/rfc2473#section-7.2>.

Cheers,
Med


De : Int-area [mailto:[e-mail]] De la part de [e-mail]
Envoyé : lundi 10 mai 2021 16:40
À : Eric Vyncke (evyncke) <evyncke=[e-mail]>
Cc : [e-mail]; [e-mail]
Objet : Re: [Int-area] [Softwires] Errata on RFC 6333, "Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion"

Hi Éric,

My reading of the current RFC6333 text is that it is trying to highlight the importance of not fragmenting the IPv4 packet before encapsulation as this will break things. This, combined with ‘… after the encapsulation of the IPv6 packet.’ which should certainly be ‘… of the IPv4 packet.’ Is where the confusion is from.

So, as a minimum, the errata is correct regarding the encapsulated IP version.

Beyond that, I don’t find the remaining text to conflict with RFC2743 section 7.2. The text is only covering how to deal with packets that you will encapsulate and forward (DF=0) - case (b) in the RFC2473 text. Case (a) - DF=1 for received packet, so drop and send ICMP error isn’t discussed as these packets will not be encapsulated and need to be fragmented.

I do agree that this is open to misreading. How about the following wording to preserve (what I think is) the authors intention:

   However, as not all service providers will be able to increase their
   link MTU, the B4 element MUST perform fragmentation and reassembly if
   the outgoing link MTU cannot accommodate the extra IPv6 header.  The
   original IPv4 packet is not oversized.  The packet is oversized after
   the IPv6 encapsulation.  The inner IPv4 packet MUST NOT be
   fragmented.  For packets forwarded by the B4 element, fragmentation
          MUST happen after the encapsulation of the IPv4 packet.  Reassembly
    MUST happen before the decapsulation of the IPv4 packet.  A detailed
    procedure has been specified in [RFC2473]
   Section 7.2.


From Mikael’s comment:
"RFC2473 7.2 says to use the DF bit and decide whether to inner fragment or drop+send ICMP error."

I can’t find anything in the Section 7.2 text that would result in inner fragmentation.

Thanks,
Ian


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