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dksoft
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Joined: Thu Dec 06, 2012 8:56 am
Location: Germany

PPP Profile: TCP MSS works only on IPv4?

Wed Sep 07, 2022 11:03 am

My configuration uses a PPPoE client "dialin" to Deutsche Telekom FTTH with a MTU of 1492.
If I tick "Change TCP MSS" in the PPP profile, things work much better. E.g. YouTube videos start and skip faster.
But loading of Telegram Media like Images is very slow, e.g. take 10-15 seconds.

By adding a mangle rule into IPv6 things work well:
/interface list
add comment="Clamp TCP MSS on these interfaces" name=CLAMP

/interface list member
add interface=WG1 list=CLAMP
add interface=TELEKOM list=CLAMP

/ipv6 firewall mangle
add action=change-mss chain=forward comment="Clamp TCP MSS to interface MTU" log-prefix="CLAMP FWD6" new-mss=clamp-to-pmtu out-interface-list=CLAMP passthrough=yes \
    protocol=tcp tcp-flags=syn
add action=change-mss chain=forward in-interface-list=CLAMP log-prefix="CLAMP FWD6" new-mss=clamp-to-pmtu passthrough=yes protocol=tcp tcp-flags=syn

Finally, I have the feeling that the "Change TCP MSS" option in the PPP profiles only works on IPv4.
Can somebody confirm this?
 
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aaronk6
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Posts: 4
Joined: Tue Mar 14, 2023 11:06 am
Location: Germany

Re: PPP Profile: TCP MSS works only on IPv4?

Tue Mar 14, 2023 11:16 am

Hi,

My ISP is also Deutsche Telekom. I was encountering the same issue and found this post:

Also, when you are using IPv6, make sure you set the MTU of your link (1492) in the MTU field in IPv6->ND. That tells ND to broadcast that MTU value to the clients and the clients will put this in their tables as the MTU to be used on all outgoing traffic, with a similar effect as that mangle rule in IPv4.
Many consumer routers do this automatically but RouterOS does not, you have to configure that yourself.

This fixed certain sites for me that would time out without this setting (i.e. https://www.hetzner.com/).

So I support your theory that the “Change TCP MSS” option in the PPP profiles only works for IPv4. I could imagine that setting the MTU in the ND is considered to be the cleaner solution and therefore no MSS clamping is done on IPv6.

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