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docs: segmentation-offloads.txt: add SCTP info

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Most of this is extracted from 90017accff ("sctp: Add GSO support"),
with some extra text about GSO_BY_FRAGS and the need to check for it.

Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Daniel Axtens 2018-02-14 18:05:33 +11:00 committed by David S. Miller
parent bc3c2431d4
commit a677088922
1 changed files with 26 additions and 0 deletions
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26
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
========================
@ -132,3 +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.