[net-next,1/3] ipv4: Raise tcp PMTU probe mss base size

Quotes from RFC4821 7.2.  Selecting Initial Values

   It is RECOMMENDED that search_low be initially set to an MTU size
   that is likely to work over a very wide range of environments.  Given
   today's technologies, a value of 1024 bytes is probably safe enough.
   The initial value for search_low SHOULD be configurable.

Moreover, set a small value will introduce extra time for the search
to converge. So set the initial probe base mss size to 1024 Bytes.

Signed-off-by: Fan Du <fan.du@intel.com>
---
 include/net/tcp.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

backward compatible? :-D

Zhu Yanjun
On 02/13/2015 04:16 PM, Fan Du wrote:
> Quotes from RFC4821 7.2.  Selecting Initial Values
>
>     It is RECOMMENDED that search_low be initially set to an MTU size
>     that is likely to work over a very wide range of environments.  Given
>     today's technologies, a value of 1024 bytes is probably safe enough.
>     The initial value for search_low SHOULD be configurable.
>
> Moreover, set a small value will introduce extra time for the search
> to converge. So set the initial probe base mss size to 1024 Bytes.
>
> Signed-off-by: Fan Du <fan.du@intel.com>
> ---
>   include/net/tcp.h |    2 +-
>   1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 8d6b983..7b57e5b 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -65,7 +65,7 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
>   #define TCP_MIN_MSS		88U
>   
>   /* The least MTU to use for probing */
> -#define TCP_BASE_MSS		512
> +#define TCP_BASE_MSS		1024
>   
>   /* After receiving this amount of duplicate ACKs fast retransmit starts. */
>   #define TCP_FASTRETRANS_THRESH 3

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于 2015年02月13日 17:49, yzhu1 写道:
> backward compatible? :-D

yes, it will be auto tuned back in black hole detecting path.
Also user could adjust the base size through /proc/sys/net/ipv4/tcp_base_mss
for optimal configuration.

> Zhu Yanjun
> On 02/13/2015 04:16 PM, Fan Du wrote:
>> Quotes from RFC4821 7.2.  Selecting Initial Values
>>
>>     It is RECOMMENDED that search_low be initially set to an MTU size
>>     that is likely to work over a very wide range of environments.  Given
>>     today's technologies, a value of 1024 bytes is probably safe enough.
>>     The initial value for search_low SHOULD be configurable.
>>
>> Moreover, set a small value will introduce extra time for the search
>> to converge. So set the initial probe base mss size to 1024 Bytes.
>>
>> Signed-off-by: Fan Du <fan.du@intel.com>
>> ---

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