netdev - Re: [PATCH net-next] tcp: tcp_make

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Message-ID: <1395860927.12610.226.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 26 Mar 2014 12:08:47 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: tcp_make_synack() minor changes

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().

MAX_TCP_HEADER already includes all needed extra room.

Note tcp_connect() also uses this +15, but not tcp_send_ack() or
tcp_send_active_reset()...

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