From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net-next] tcp: tcp_make_synack() minor changes
Date: Wed, 26 Mar 2014 15:11:37 -0400 (EDT)
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From: Joe Perches <joe@perches.com>
Date: Wed, 26 Mar 2014 11:52:48 -0700

> On Wed, 2014-03-26 at 14:25 -0400, David Miller wrote:
>> From: Eric Dumazet <eric.dumazet@gmail.com>
>> Date: Wed, 26 Mar 2014 09:57:19 -0700
>> 
>> > From: Eric Dumazet <edumazet@google.com>
>> > 
>> > There is no need to allocate 15 bytes in excess for a SYNACK packet,
>> > as it contains no data, only headers.
>> > 
>> > SYNACK are always generated in softirq context, and contain a single
>> > segment, we can use TCP_INC_STATS_BH()
>> > 
>> > Signed-off-by: Eric Dumazet <edumazet@google.com>
>>  ...
>> > -	skb = sock_wmalloc(sk, MAX_TCP_HEADER + 15, 1, GFP_ATOMIC);
>> > +	skb = sock_wmalloc(sk, MAX_TCP_HEADER, 1, GFP_ATOMIC);
>> 
>> Do you know where this "+ 15" comes from? :-)
>> It is a kind of Linux networking trivia question.
> 
> It's really old, pre v1 days.
> I presume it was some kind of ethernet minimum packet size padding.

I'm pretty sure it was to accomodate the hard header cache.

HH_DATA_MOD is 16