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