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docs: net: fix minor issues with segmentation offloads

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Update the segmentation offload documentation to match current GSO types:

- clarify csum_start for encapsulated TSO
- document TCP AccECN GSO and NETIF_F_GSO_ACCECN
- distinguish legacy UFO from UDP L4 GSO
- add ESP and fraglist GSO entries

Link: https://patch.msgid.link/20260526160151.2793354-11-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jakub Kicinski
2026-05-26 09:01:51 -07:00
parent 25bfb3a8ed
commit 1255733f2f
1 changed files with 36 additions and 1 deletions
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37
Documentation/networking/segmentation-offloads.rst
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@@ -14,10 +14,13 @@ to take advantage of segmentation offload capabilities of various NICs.
The following technologies are described:
* TCP Segmentation Offload - TSO
* UDP Fragmentation Offload - UFO
* UDP Segmentation Offload - USO
* IPIP, SIT, GRE, and UDP Tunnel Offloads
* Generic Segmentation Offload - GSO
* Generic Receive Offload - GRO
* Partial Generic Segmentation Offload - GSO_PARTIAL
* ESP Segmentation Offload
* Fraglist Generic Segmentation Offload - GSO_FRAGLIST
* SCTP acceleration with GSO - GSO_BY_FRAGS
@@ -38,7 +41,8 @@ In order to support TCP segmentation offload it is necessary to populate
the network and transport header offsets of the skbuff so that the device
drivers will be able determine the offsets of the IP or IPv6 header and the
TCP header. In addition as CHECKSUM_PARTIAL is required csum_start should
also point to the TCP header of the packet.
also point to the TCP header of the packet, or to the inner transport header
for encapsulated TSO.
For IPv4 segmentation we support one of two types in terms of the IP ID.
The default behavior is to increment the IP ID with every segment. If the
@@ -57,6 +61,10 @@ DF bit is not set on the outer header, in which case the device driver must
guarantee that the IP ID field is incremented in the outer header with every
segment.
SKB_GSO_TCP_ACCECN is a modifier used with TCP segmentation offload for
AccECN packets where the CWR bit must not be cleared during segmentation.
Devices advertise support for this using NETIF_F_GSO_ACCECN.
UDP Fragmentation Offload
=========================
@@ -71,6 +79,16 @@ still receive them from tuntap and similar devices. Offload of UDP-based
tunnel protocols is still supported.
UDP Segmentation Offload
========================
UDP segmentation offload allows a device to segment a large UDP packet into
multiple UDP datagrams. Unlike UFO, these are not IP fragments. The payload
size of each datagram is specified in skb_shinfo()->gso_size and the GSO type
is SKB_GSO_UDP_L4. Devices advertise support for this using
NETIF_F_GSO_UDP_L4.
IPIP, SIT, GRE, UDP Tunnel, and Remote Checksum Offloads
========================================================
@@ -154,6 +172,23 @@ that the IPv4 ID field is incremented in the case that a given header does
not have the DF bit set.
ESP Segmentation Offload
========================
ESP segmentation offload uses SKB_GSO_ESP to mark packets that require
IPsec ESP segmentation. This type is set by the XFRM output path for GSO
packets handled by ESP hardware offload.
Fraglist Generic Segmentation Offload
=====================================
Fraglist GSO uses SKB_GSO_FRAGLIST to mark packets whose segments are
already arranged as a list of skbs. The segmentation path splits the skb
based on that list rather than by creating segments of skb_shinfo()->gso_size
bytes from the linear and page-fragment data.
SCTP acceleration with GSO
===========================