netdev - Re: pskb_copy() in tcp_transmit

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Message-ID: <4E6F45D3.50306@gmail.com>
Date:	Tue, 13 Sep 2011 20:00:19 +0800
From:	Gao feng <omarapazanadi@...il.com>
To:	christoph.paasch@...ouvain.be
CC:	netdev@...r.kernel.org
Subject: Re: pskb_copy() in tcp_transmit_skb()

于 11-9-13 下午6:55, Christoph Paasch 写道:
> Hi,
>
> On Tuesday 13 September 2011 wrote Gaofeng:
>>> I'm trying to understand the reason for the possible call to pskb_copy()
>>> in tcp_transmit_skb().
>>> I don't find, where we may have a cloned skb entering tcp_transmit_skb().
>>>
>>> The original pskb_copy() came from tcp_retransmit_skb() (commit
>>> dfb4b9dceb35c567a595ae5e9d035cfda044a103).
>>> But from that point, the git-history does not show where the pskb_copy()
>>> is coming from.
>>>
>>>
>>> Does somebody has an idea, how a cloned skb can be passed to
>>> tcp_transmit_skb() and thus trigger this call to pskb_copy().
>>
>> maybe nat,nat can change the tcp packet header
>
> although NAT may change the TCP header, it is not doing an skb_clone() - (at
> least I don't find it).
> Thus, we will not call pskb_copy() in tcp_transmit_skb() due to NAT.
>
> Christoph
>
> --
> Christoph Paasch
> PhD Student
>
> IP Networking Lab --- http://inl.info.ucl.ac.be
> MultiPath TCP in the Linux Kernel --- http://inl.info.ucl.ac.be/mptcp
> Université Catholique de Louvain
>
> www.rollerbulls.be
> --
> --
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>
maybe this.
if the nic driver has not transmit the cloned skb to the network yet.
then tcp_retransmit_skb is called by some logical.
if dont use pskb_copy here,it will make the nat or some header operation 
error.
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