IPSec transport mode, mtu, fragmentation...

[Andrey V. Elsukov]

On 16.01.2020 19:07, Victor Sudakov wrote:
> Eugene Grosbein wrote:
>>
>>> What beats me is that I cannot reproduce this problem in bhyve. In this
>>> packet dump: http://admin.sibptus.ru/~vas/ipsec1.pcap.gz I'm scp-ing a
>>> 50M file from 192.168.246.10 (bhyve guest) to 192.168.246.1 (bhyve
>>> host), and I see no fragments, and the largets packet is 1466 bytes, and
>>> the scp never stalls nor fails.
>>>
>>> Why is it NOT broken this time?
>>>
>>> Both hosts are 12.1-RELEASE-p1.
>>
>> I could not reproduce the problem with unpatched recent stable/11, either :-)
> 
> Is there a way to view the MSS in the TCP segments before encryption or
> after decryption? I want to compare them in situations with IPSec
> enabled and disabled.
> 
> I've never been able to see anything in "tcpdump -i enc0", probably it
> cannot do transport mode IPSec because the man page talks about "outer
> and inner header."

For transport mode inner and outer headers will be the same.
I guess the problem can be reproduced in the lab using the following config:

    [Host A] <--> [Router] <--> [Host B]

IPsec should be configured between hosts A and B. Then you need to
reduce MTU on the router. This should lead to ICMP NEEDFRAG messages
from the router, and then host should correctly handle them.

-- 
WBR, Andrey V. Elsukov

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