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 14:25:51 -0400 (EDT)
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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.