Re: [tcpm] Exceeding value in MSS option?

> On 26. Oct 2020, at 08:52, Frode Kileng <frodek@tele.no> wrote:
> 
> 
> 
> On 23/10/2020 17:11, Joseph Touch wrote:
>>> On Oct 23, 2020, at 1:37 AM, Frode Kileng <frodek@tele.no <mailto:frodek@tele.no>> wrote:
>>> 
>>> Although maybe not  a "general problem",  a study of MSS middlebox manipluation in  23 European and Asian mobile networks did reveal some issues:
>>> 
>>> - 20 of 23 networks always inserts MSS.
>> 
>> That IS a general problem, FWIW.
>> 
>> Besides being published, it should also be reported to the operators of those networks as violating TCP specs.
> 
> Do you mean as in "layering violation" or is it explicitly stated in an RFC that MSS clamping is not allowed.
I think we should differentiate two issues:
(a) a middlebox which reduces the MSS already in an SYN/SYN-ACK segement.
(b) a middlebox which adds an MSS option, which wasn't there in the SYN/SYN-ACK segment.

(a) is the typical MSS clamping you are referring to and it is used to mitigate MTU issues.
(b) is a problem in my view and referred to in the statement 'always inserts'.

Best regards
Michael
> 
> Anyway, since stopping this practice is likely to cause extensive fragmentation of TCP segments, mobile networks will continue this despite "violating TCP specs". At least until the networks fully support jumbo frames.
> 
> (The motivation for MSS clamping is the tunneling overhead in the mobile core of the networks. Within the core, the MSS is clamped to take into consideration the size of the  GTP-U header and potential additional ipsec headers. The GTP-U header is variable size and is at least 40 bytes for IPv4 and 60 for IPv6. In addition it may include vendor or mobile operator specific extension headers. I do not now why networks don't reduce the MTU size instead. My understanding of the 3GPP specs is that this is suggested but could not find out how this is provisioned to the terminals for IPv4. I've only seen one mobile network that provisioned the terminals with a MTU lower than 1500 bytes. But their MTU size of 1464 for for IPv4 and IPv6 did not seem reflect the GTP-U tunnel.)
> 
> 
> Regards
> 
> Frode Kileng
> 
> 
> 
> 
> 
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