linux-kernel - Question about more headroom in skb in outgoing packets

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Message-ID: <689426.33806.qm@web112505.mail.gq1.yahoo.com>
Date:	Mon, 10 May 2010 16:45:48 -0700 (PDT)
From:	Sharat Masetty <sharat_masetty@...oo.com>
To:	linux-kernel@...r.kernel.org
Subject: Question about more headroom in skb in outgoing packets 

Hello All,

For my project I need 3 words of headroom in the skb in the network driver level, to add a  custom header to the ethernet/ip packets. I looked into the tcp code and figured out tcp uses sk->sk_prot->max_header for header allocation size. But I was not able to confirm that all other transport protocol use the same mechanism(?) For example in UDP/ICMP I was not able to figure out from the code where the skb allocation and header reservation happens(Any light here would be really helpful.). Is there a parameter I can tune to allocate 3 words for any outgoing skb's, or what is the best way to do it?

I want to figure out a good way(may be tune some parameters) to allocate extra 3 words for any skbuff independent of the transport protocol being used. I have also looked at an API in skbuff skb_pad() which does what I want(add either headroom or tailroom), but I want to avoid that for performance reasons(skb_pad does kmalloc and memcpy).  Any light here would be very much appreciated.

Thanks,
Sharat.

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