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Date:   Sun, 13 Sep 2020 23:18:46 -0700
From:   Xie He <>
To:     Krzysztof Hałasa <>
Cc:     Jakub Kicinski <>, Krzysztof Halasa <>,
        "David S. Miller" <>,
        Linux Kernel Network Developers <>,
        LKML <>,
        Martin Schiller <>,
        Willem de Bruijn <>
Subject: Re: [PATCH net v2] drivers/net/wan/hdlc_fr: Add needed_headroom for
 PVC devices

On Sun, Sep 13, 2020 at 10:26 PM Krzysztof Hałasa <> wrote:
> Xie He <> writes:
> > The HDLC device is not actually prepending any header when it is used
> > with this driver. When the PVC device has prepended its header and
> > handed over the skb to the HDLC device, the HDLC device just hands it
> > over to the hardware driver for transmission without prepending any
> > header.
> That's correct. IIRC:
> - Cisco and PPP modes add 4 bytes
> - Frame Relay adds 4 (specific protocols - mostly IPv4) or 10 (general
>   case) bytes. There is that pvcX->hdlcX transition which adds nothing
>   (the header is already in place when the packet leaves pvcX device).
> - Raw mode adds nothing (IPv4 only, though it could be modified for
>   both IPv4/v6 easily)
> - Ethernet (hdlc_raw_eth.c) adds normal Ethernet header.

Thank you for the nice summary!

I just realized that we can indeed support both IPv4/v6 in hdlc_raw.
Both IPv4 and v6 packets have a version field in the header, so we can
use it to distinguish v4 and v6 packets on receiving. We can introduce
it as a new feature some time.

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