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It is often useful to enable TCP MSS clamping on PPPoE interfaces. This
When needed, this will usually present as TCP conections being established, but ‘freezing’ before much data has been transfered. That is because the MTU of a PPPoE link is 8 bytes shorter than that of the underlying ethernet size of 1500 bytes, and path MTU discovery often does not work between various hosts; in large part due to NATs the path between the hosts dropping the ICMP messages necessary for it, or firewall/ACL filters doing the sameused for classic PMTUd - usually at NATs, firewalls or ACLs. Also many hosts do not support the newer PLMTUD mechanism, and not all protocols define how to use it (TCP does).
This can be enabled with the following command, usually in the ‘mtu’ variant:
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vyatta@vyatta# set interfaces pppoe pppoe0 ip tcp-mss mtu vyatta@vyatta# set interfaces pppoe pppoe0 ipv6 tcp-mss |
It may also be possible to force the use of baby jumbo-grams for PPPoE (i.e. using a 1508 payload packet) by setting the PPPoE interface MTU to 1500. However support for this between various vendors is not universal, nor is ISP support, and so it used one should experiment to ensure it is operating properly. If that scheme works, then there is no need to enable MSS clamping.
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