The mssfix/fragment lines appear to have solved the issue.
Thanks to everyone who helped.
-Stuart
On Mon, Sep 4, 2017 at 10:24 PM wang yu <wangyu...@gmail.com> wrote:
> >I don't have any authoritative thoughts on why TCP would work when UDP
> doesn't, but one thing that comes to mind is MTU size. If the MTU size was
> too large, UDP will simply fail. TCP *should* [but may not always] get a
> fragmentation notification telling the sender to fragment the packets down
> to fit through a smaller than expected MTU in the connection.
>
> Make sense.
>
> Try to add following options to both sides:
>
> mssfix 1200 ##### tell up-level tcp to not send a packet larger than
> 1200,this only works for tcpfragment 1200 ##### fragment a packet if it
> is larger than 1200,this works for udp/icmp(also works for tcp if 'mssfix' is
> not set,but not as efficient as 'mssfix')
>
>
> On Mon, Sep 4, 2017 at 9:13 AM, Gregory Sloop <gr...@sloop.net> wrote:
>
>> Top Posting:
>>
>> UDP is preferred vs TCP because of TCP inside TCP issues - e.g. TCP
>> sliding window ACK inside another TCP sliding window ACK. As packet loss
>> increases, this becomes a huge problem. Essentailly, you have TCP stream
>> inside the OVPN tunnel, and it's being ferried to the remote site/client.
>> And the outside tunnel is also a TCP connection. All the "benefits" of TCP
>> connections, essentially can become problems with having a TCP inside TCP
>> conntection.
>>
>> Rather than rehash it all - here's a decent link that describes some of
>> the problems:
>> http://sites.inka.de/bigred/devel/tcp-tcp.html
>>
>> I don't have any authoritative thoughts on why TCP would work when UDP
>> doesn't, but one thing that comes to mind is MTU size. If the MTU size was
>> too large, UDP will simply fail. TCP *should* [but may not always] get a
>> fragmentation notification telling the sender to fragment the packets down
>> to fit through a smaller than expected MTU in the connection.
>>
>> Do some searches on MTU/MSS and see if you can determine if your pathway
>> has a smaller than expected MTU. PPPoE is one specific case where MTU's are
>> smaller because of the PPPoE overhead.
>>
>> Some good reading re: MTU/MSS. [Not carefully vetted, just quick perusal
>> seems to indicate they might be useful reading.]
>>
>> https://supportforums.cisco.com/t5/lan-switching-and-routing/mtu-mss-and-tcp-window-size/td-p/2966099
>> https://learningnetwork.cisco.com/thread/91863
>>
>> -Greg
>>
>>
>>
>> We’ve just tried TCP and the issue has gone away.
>>
>> Can anyone tell me why this happens? Also, what’s the reason for UDP
>> being preferred over TCP?
>>
>> Thanks.
>>
>> -Stuart
>>
>> [SNIP]
>>
>>
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