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Date:   Mon, 14 Sep 2020 07:26:12 +0200
From:   Krzysztof Hałasa <>
To:     Xie He <>
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

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.

(I had been "unplugged" for some time).
Krzysztof Halasa

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Przemysłowy Instytut Automatyki i Pomiarów PIAP
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