Re: [RFC] tcp: Replace TCP buffer structure by an iovec array

[Laurent Vivier <lvivier@redhat.com>]

On 3/13/24 12:37, Stefano Brivio wrote:
> On Mon, 11 Mar 2024 14:33:56 +0100
> Laurent Vivier <lvivier@redhat.com> wrote:
> 
...
>> diff --git a/tcp.c b/tcp.c
>> index d5eedf4d0138..5c8488108ef7 100644
>> --- a/tcp.c
>> +++ b/tcp.c
>> @@ -318,39 +318,7 @@
...
>> -#define MSS4	ROUND_DOWN(USHRT_MAX - sizeof(struct tcp4_l2_head), 4)
>> -#define MSS6	ROUND_DOWN(USHRT_MAX - sizeof(struct tcp6_l2_head), 4)
>>
>> +#define MSS				(USHRT_MAX - sizeof(struct tcphdr))
> 
> We can't exceed USHRT_MAX at Layer-2 level, though, so the MSS for IPv4
> should now be:
> 
> #define MSS4	ROUND_DOWN(USHRT_MAX - sizeof(struct ethhdr) -
> 			   	       sizeof(struct iphdr) -
> 				       sizeof(struct tcphdr),
> 			   4)
> 
> ...and similar for IPv6.

Is there a specification that limits the MSS?
Or is it only a compatibility problem with the network stack implementation?

At headers level the only limitation we have is the length field size in the IP header 
that is a 16bit (it's why I put "USHRT_MAX - sizeof(struct tcphdr)").

Thanks,
Laurent