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Date: Wed, 10 Feb 2016 10:28:47 +0100
From: Samuel Thibault <samuel.thibault@gnu.org>
Message-ID: <20160210092847.GC3423@var.bordeaux.inria.fr>
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Subject: Re: [Qemu-devel] [PATCHv7 4/9] slirp: Factorizing tcpiphdr
	structure with an union
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To: Thomas Huth <thuth@redhat.com>
Cc: zhanghailiang <zhang.zhanghailiang@huawei.com>, Li Zhijian <lizhijian@cn.fujitsu.com>, Stefan Hajnoczi <stefanha@gmail.com>, Jason Wang <jasowang@redhat.com>, qemu-devel@nongnu.org, Vasiliy Tolstov <v.tolstov@selfip.ru>, Dave Gilbert <dgilbert@redhat.com>, Gonglei <arei.gonglei@huawei.com>, Jan Kiszka <jan.kiszka@siemens.com>, Huangpeng <peter.huangpeng@huawei.com>, Guillaume Subiron <maethor@subiron.org>

That one is tricky, yes :)

Thomas Huth, on Wed 10 Feb 2016 09:05:32 +0100, wrote:
> > -#define SLIRP_MSIZE (IF_MTU + IF_MAXLINKHDR + offsetof(struct mbuf, m_dat) + 6)
> > +#define SLIRP_MSIZE\
> > +    (offsetof(struct mbuf, m_dat) + IF_MAXLINKHDR + TCPIPHDR_DELTA + IF_MTU)
> 
> I'm somehow having a hard time to understand why TCPIPHDR_DELTA is used
> here. As far as I understand, TCPIPHDR_DELTA is the difference between
> the size of struct tcpiphdr and the size of the IPv4 + TCP header. But
> if it's just the difference, where does the base size of the headers
> come from in this define, since the headers are stored in the mbuf, too,
> aren't they? ... I've got the feeling that I miss something here, could
> you enlighten me?

TCP/IP headers are within IF_MTU.

Samuel