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Merge branch 'net-segmentation-offload-doc-fixes'

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Daniel Axtens says:

====================
Updates to segmentation-offloads.txt

I've been trying to wrap my head around GSO for a while now. This is a
set of small changes to the docs that would probably have been helpful
when I was starting out.

I realise that GSO_DODGY is still a notable omission - I'm hesitant to
write too much on it just yet as I don't understand it well and I
think it's in the process of changing.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
David S. Miller 2018-02-14 14:52:39 -05:00
commit 8ace02073e
1 changed files with 34 additions and 4 deletions
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38
Documentation/networking/segmentation-offloads.txt
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@ -13,6 +13,7 @@ The following technologies are described:
* Generic Segmentation Offload - GSO
* Generic Receive Offload - GRO
* Partial Generic Segmentation Offload - GSO_PARTIAL
* SCTP accelleration with GSO - GSO_BY_FRAGS
TCP Segmentation Offload
========================
@ -49,6 +50,10 @@ datagram into multiple IPv4 fragments. Many of the requirements for UDP
fragmentation offload are the same as TSO. However the IPv4 ID for
fragments should not increment as a single IPv4 datagram is fragmented.
UFO is deprecated: modern kernels will no longer generate UFO skbs, but can
still receive them from tuntap and similar devices. Offload of UDP-based
tunnel protocols is still supported.
IPIP, SIT, GRE, UDP Tunnel, and Remote Checksum Offloads
========================================================
@ -83,10 +88,10 @@ SKB_GSO_UDP_TUNNEL_CSUM. These two additional tunnel types reflect the
fact that the outer header also requests to have a non-zero checksum
included in the outer header.
Finally there is SKB_GSO_REMCSUM which indicates that a given tunnel header
has requested a remote checksum offload. In this case the inner headers
will be left with a partial checksum and only the outer header checksum
will be computed.
Finally there is SKB_GSO_TUNNEL_REMCSUM which indicates that a given tunnel
header has requested a remote checksum offload. In this case the inner
headers will be left with a partial checksum and only the outer header
checksum will be computed.
Generic Segmentation Offload
============================
@ -128,3 +133,28 @@ values for if the header was simply duplicated. The one exception to this
is the outer IPv4 ID field. It is up to the device drivers to guarantee
that the IPv4 ID field is incremented in the case that a given header does
not have the DF bit set.
SCTP accelleration with GSO
===========================
SCTP - despite the lack of hardware support - can still take advantage of
GSO to pass one large packet through the network stack, rather than
multiple small packets.
This requires a different approach to other offloads, as SCTP packets
cannot be just segmented to (P)MTU. Rather, the chunks must be contained in
IP segments, padding respected. So unlike regular GSO, SCTP can't just
generate a big skb, set gso_size to the fragmentation point and deliver it
to IP layer.
Instead, the SCTP protocol layer builds an skb with the segments correctly
padded and stored as chained skbs, and skb_segment() splits based on those.
To signal this, gso_size is set to the special value GSO_BY_FRAGS.
Therefore, any code in the core networking stack must be aware of the
possibility that gso_size will be GSO_BY_FRAGS and handle that case
appropriately. (For size checks, the skb_gso_validate_*_len family of
helpers do this automatically.)
This also affects drivers with the NETIF_F_FRAGLIST & NETIF_F_GSO_SCTP bits
set. Note also that NETIF_F_GSO_SCTP is included in NETIF_F_GSO_SOFTWARE.