Subject: fragmentation in IPv6

From: Sridhar Gurivireddy <[email-masked]>
Date: Mon, 04 Jun 2001 17:04:30 -0500

Hai,
  I have a small doubt regarding fragmentation in IPv6.
In Ipv6, even header can be fragmented.
(with exception that fixed part of header should not be fragmented).
..................
If a packet's header is fragmented, a new packet header is added to each
fragment.But, once agin this new packet header should be fragmented..And
this goes on recursively...
.....
Can this problem occur in IPng?
Thanks in advance,
Sridhar.

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From: Jun-ichiro itojun Hagino <[email-masked]>
Date: Tue, 05 Jun 2001 07:08:18 +0900

>(with exception that fixed part of header should not be fragmented).

	packet fragmentation occurs only once, at the originating node.
	so there's no need to worry about the above scenario.

itojun

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From: Sridhar Gurivireddy <[email-masked]>
Date: Mon, 04 Jun 2001 17:20:13 -0500

even if fragmentation done only at the source,
 it may have this problem because,.....
 say IP header(with extensions) is of size 4k!!!!!
it's framented for the first time.....
you get three pieces..
yu add header to each piece...
each piece is now again >4k...
so, fragmentation again....
Does this problem occur..
Thanks for the sharp response

Stig Venaas wrote:

> On Tue, Jun 05, 2001 at 07:08:18AM +0900, Jun-ichiro itojun Hagino wrote:
> Also note that the minimum MTU for IPv6 is 1280 bytes.
>
> Stig

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From: Stig Venaas <[email-masked]>
Date: Tue, 5 Jun 2001 00:13:42 +0200

Stig

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From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Tue, 05 Jun 2001 10:35:18 +0900

>>>>> On Mon, 04 Jun 2001 17:20:13 -0500, 
>>>>> Sridhar Gurivireddy <[email-masked]> said:

> Does this problem occur..

Yes, when the unfragmentable part of a packet is larger than the path
MTU, fragmentation fails and you cannot send the packet.  However,
(at least through my operation experiences) we've never cared about
the problem, since we've not seen any practical usage of such a pakcet
with a long unfragmentale part.

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[email-masked]
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From: Stig Venaas <[email-masked]>
Date: Tue, 5 Jun 2001 00:48:46 +0200

> Thanks for the sharp response

It's not exactly recursion. The only problem I see, is if the size of the
IPv6 header plus hop-by-hop options and routing header (the headers that
must be inspected by routers) plus fragmentation header is >= path MTU.
This is explained in RFC 2460. The path MTU is guaranteed to be at least
1280.

Stig