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 15:12:09 -0400 (EDT)
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From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Wed, 26 Mar 2014 12:08:47 -0700

> On Wed, 2014-03-26 at 14:25 -0400, David Miller wrote:
>> Do you know where this "+ 15" comes from? :-)
>> 
>> It is a kind of Linux networking trivia question.
> 
> My random guess would be that it is related to a copy/paste
> thing, as some tx paths used to allocate skb with a very precise byte
> count, and they used HH_DATA_ALIGN().

Yes, this is exactly it.