[dns-operations] DNS flag day 2020: Recommended EDNS buffer size discussion

[Lee]

On 9/2/19, Paul Vixie <paul at redbarn.org> wrote:
> On Monday, 2 September 2019 17:31:50 UTC Lee wrote:
>> On 9/2/19, Paul Vixie <paul at redbarn.org> wrote:
>> > ...
>> >>
>> >> we should not needlessly invent something that's different from TCP
>> >> MSS.
>> >> just use what works.
>>
>> I think tcp mss working as well as it does is because the mss is
>> negotiated at connection setup time (syn, syn+ack option tcp mss
>> size).
>
> no.

Are we arguing definitions?
Source sends a SYN with the tcp mss option set to 1460, destination
responds with a SYN+ACK with the tcp mss option set to 1280, both
sides use an mss of 1280.  Which sounds like a negotiation to me..

>> Your machine can figure out the mss & middle boxes that know
>> better can reduce the mss value to something they know will work.
>>   eg. 'ip tcp adjust-mss ' in ciscoland
>>
>> https://www.cisco.com/en/US/docs/ios-xml/ios/ipapp/command/ip_tcp_adjust-ms
>> s_through_ip_wccp_web-cache_accelerated.html#GUID-68044D35-A53E-42C1-A7AB-92
>> 36333DA8C4
>>
>> How do you do that with udp?
>
> TCP MSS is an offer not an acknowledgement,

agreed

> and it proceeds from local
> knowledge of the path MTU like the routing table or the sysctl definitions
> set
> by a sysadmin. generally speaking path MTU discovery does not work

Right - generally speaking path MTU discovery does not work.  So when
setting up ipsec tunnels between routers I'd also configure ip tcp
adjust-mss to lower the offered mss on traffic going through the
tunnel & have everything work.

I never did figure out how to automatically fix udp traffic.  So I'm
not understanding how your suggestion to use what works for tcp is
applicable to udp.

Regards,
Lee