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

2021-05-10 Thread ianfarrer 
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


> On 10. May 2021, at 15:36, Eric Vyncke (evyncke) 
>  wrote:
> 
> Dear ex-softwires WG, dear int-area WG,
>  
> Mikael Abrahamsson filed an erratum https://www.rfc-editor.org/errata/eid5847 
>  in August 2019 (after the 
> softwires WG closure) and, as the past responsible AD for softwires, I would 
> like to fix this erratum but as Mikael I have no idea about the correction.
>  
> My own take is that the normative text in RFC 6333 indeed violates RFC 2473 
> for when the IPv4 DF is set... RFC 2473 appears to me as sensible and I would 
> prefer to keep this behavior, i.e., see my suggestion below.
>  
> Than you in advance for your comments and suggestions,
>  
> Regards
>  
> -éric
>  
>  
> -- Existing text in RFC 6333 –
> Section 5.3 says:
> 
>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.
>  
>  
> -- Comment by erratum submitter –
> I do not have a corrected text. The above text doesn't say what RFC2473 
> section 7.2 says, so... what should it be? RFC2473 7.2 says to use the DF bit 
> and decide whether to inner fragment or drop+send ICMP error. The above text 
> seems to make normative statements that counter at least the DF=1 case in 
> RFC2473 7.2. Also the text above says "Fragmentation MUST happen after the 
> encapsulation of the IPv6 packet.". The IPv6 packet isn't encapsulated, so 
> that sentence should be changed?
>  
> -- Section 7.2 of RFC 2473 ---
>When an IPv4 original packet enters a tunnel, if the original packet
>size exceeds the tunnel MTU (i.e., the Path MTU between the tunnel
>entry-point and the tunnel exit-point, minus the size of the tunnel
>header(s)), it is handled as follows:
>  
> (a)  if in the original IPv4 packet header the Don't Fragment  -
>  DF - bit flag is SET, the entry-point node discards the
>  packet and returns an ICMP message.  The ICMP message has
>  the type = "unreachable", the code = "packet too big", and
>  the recommended MTU size field set to the size of the
>  tunnel MTU - see sections 6.7 
>  and 8.3 
> .
>  
> (b)  if in the original packet header the Don't Fragment - DF  -
>  bit flag is CLEAR, the tunnel entry-point node encapsulates
>  the original packet, and subsequently fragments the
>  resulting IPv6 tunnel packet into IPv6 fragments that do
>  not exceed the Path MTU to the tunnel exit-point.
>  
>  
> -- Suggested new text by Éric Vyncke –

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

2021-05-10 Thread Eric Vyncke (evyncke) 
Dear ex-softwires WG, dear int-area WG,

Mikael Abrahamsson filed an erratum https://www.rfc-editor.org/errata/eid5847 
in August 2019 (after the softwires WG closure) and, as the past responsible AD 
for softwires, I would like to fix this erratum but as Mikael I have no idea 
about the correction.

My own take is that the normative text in RFC 6333 indeed violates RFC 2473 for 
when the IPv4 DF is set... RFC 2473 appears to me as sensible and I would 
prefer to keep this behavior, i.e., see my suggestion below.

Than you in advance for your comments and suggestions,

Regards

-éric


-- Existing text in RFC 6333 –
Section 5.3 says:
   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.


-- Comment by erratum submitter –
I do not have a corrected text. The above text doesn't say what RFC2473 section 
7.2 says, so... what should it be? RFC2473 7.2 says to use the DF bit and 
decide whether to inner fragment or drop+send ICMP error. The above text seems 
to make normative statements that counter at least the DF=1 case in RFC2473 
7.2. Also the text above says "Fragmentation MUST happen after the 
encapsulation of the IPv6 packet.". The IPv6 packet isn't encapsulated, so that 
sentence should be changed?

-- Section 7.2 of RFC 2473 ---
   When an IPv4 original packet enters a tunnel, if the original packet
   size exceeds the tunnel MTU (i.e., the Path MTU between the tunnel
   entry-point and the tunnel exit-point, minus the size of the tunnel
   header(s)), it is handled as follows:

(a)  if in the original IPv4 packet header the Don't Fragment  -
 DF - bit flag is SET, the entry-point node discards the
 packet and returns an ICMP message.  The ICMP message has
 the type = "unreachable", the code = "packet too big", and
 the recommended MTU size field set to the size of the
 tunnel MTU - see sections 
6.7 and 
8.3.

(b)  if in the original packet header the Don't Fragment - DF  -
 bit flag is CLEAR, the tunnel entry-point node encapsulates
 the original packet, and subsequently fragments the
 resulting IPv6 tunnel packet into IPv6 fragments that do
 not exceed the Path MTU to the tunnel exit-point.


-- Suggested new text by Éric Vyncke –
   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 detailed procedure specified in [RFC2473]
   Section 7.2 MUST be executed by the B4 element.

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