aboutsummaryrefslogtreecommitdiffstats
path: root/Documentation/networking/segmentation-offloads.txt
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/networking/segmentation-offloads.txt')
-rw-r--r--Documentation/networking/segmentation-offloads.txt170
1 files changed, 0 insertions, 170 deletions
diff --git a/Documentation/networking/segmentation-offloads.txt b/Documentation/networking/segmentation-offloads.txt
deleted file mode 100644
index aca542ec125c..000000000000
--- a/Documentation/networking/segmentation-offloads.txt
+++ /dev/null
@@ -1,170 +0,0 @@
-Segmentation Offloads in the Linux Networking Stack
-
-Introduction
-============
-
-This document describes a set of techniques in the Linux networking stack
-to take advantage of segmentation offload capabilities of various NICs.
-
-The following technologies are described:
- * TCP Segmentation Offload - TSO
- * UDP Fragmentation Offload - UFO
- * IPIP, SIT, GRE, and UDP Tunnel Offloads
- * Generic Segmentation Offload - GSO
- * Generic Receive Offload - GRO
- * Partial Generic Segmentation Offload - GSO_PARTIAL
- * SCTP accelleration with GSO - GSO_BY_FRAGS
-
-TCP Segmentation Offload
-========================
-
-TCP segmentation allows a device to segment a single frame into multiple
-frames with a data payload size specified in skb_shinfo()->gso_size.
-When TCP segmentation requested the bit for either SKB_GSO_TCPV4 or
-SKB_GSO_TCPV6 should be set in skb_shinfo()->gso_type and
-skb_shinfo()->gso_size should be set to a non-zero value.
-
-TCP segmentation is dependent on support for the use of partial checksum
-offload. For this reason TSO is normally disabled if the Tx checksum
-offload for a given device is disabled.
-
-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.
-
-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
-GSO type SKB_GSO_TCP_FIXEDID is specified then we will not increment the IP
-ID and all segments will use the same IP ID. If a device has
-NETIF_F_TSO_MANGLEID set then the IP ID can be ignored when performing TSO
-and we will either increment the IP ID for all frames, or leave it at a
-static value based on driver preference.
-
-UDP Fragmentation Offload
-=========================
-
-UDP fragmentation offload allows a device to fragment an oversized UDP
-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
-========================================================
-
-In addition to the offloads described above it is possible for a frame to
-contain additional headers such as an outer tunnel. In order to account
-for such instances an additional set of segmentation offload types were
-introduced including SKB_GSO_IPXIP4, SKB_GSO_IPXIP6, SKB_GSO_GRE, and
-SKB_GSO_UDP_TUNNEL. These extra segmentation types are used to identify
-cases where there are more than just 1 set of headers. For example in the
-case of IPIP and SIT we should have the network and transport headers moved
-from the standard list of headers to "inner" header offsets.
-
-Currently only two levels of headers are supported. The convention is to
-refer to the tunnel headers as the outer headers, while the encapsulated
-data is normally referred to as the inner headers. Below is the list of
-calls to access the given headers:
-
-IPIP/SIT Tunnel:
- Outer Inner
-MAC skb_mac_header
-Network skb_network_header skb_inner_network_header
-Transport skb_transport_header
-
-UDP/GRE Tunnel:
- Outer Inner
-MAC skb_mac_header skb_inner_mac_header
-Network skb_network_header skb_inner_network_header
-Transport skb_transport_header skb_inner_transport_header
-
-In addition to the above tunnel types there are also SKB_GSO_GRE_CSUM and
-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_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
-============================
-
-Generic segmentation offload is a pure software offload that is meant to
-deal with cases where device drivers cannot perform the offloads described
-above. What occurs in GSO is that a given skbuff will have its data broken
-out over multiple skbuffs that have been resized to match the MSS provided
-via skb_shinfo()->gso_size.
-
-Before enabling any hardware segmentation offload a corresponding software
-offload is required in GSO. Otherwise it becomes possible for a frame to
-be re-routed between devices and end up being unable to be transmitted.
-
-Generic Receive Offload
-=======================
-
-Generic receive offload is the complement to GSO. Ideally any frame
-assembled by GRO should be segmented to create an identical sequence of
-frames using GSO, and any sequence of frames segmented by GSO should be
-able to be reassembled back to the original by GRO. The only exception to
-this is IPv4 ID in the case that the DF bit is set for a given IP header.
-If the value of the IPv4 ID is not sequentially incrementing it will be
-altered so that it is when a frame assembled via GRO is segmented via GSO.
-
-Partial Generic Segmentation Offload
-====================================
-
-Partial generic segmentation offload is a hybrid between TSO and GSO. What
-it effectively does is take advantage of certain traits of TCP and tunnels
-so that instead of having to rewrite the packet headers for each segment
-only the inner-most transport header and possibly the outer-most network
-header need to be updated. This allows devices that do not support tunnel
-offloads or tunnel offloads with checksum to still make use of segmentation.
-
-With the partial offload what occurs is that all headers excluding the
-inner transport header are updated such that they will contain the correct
-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.
-
-There are some helpers to make this easier:
-
- - skb_is_gso(skb) && skb_is_gso_sctp(skb) is the best way to see if
- an skb is an SCTP GSO skb.
-
- - For size checks, the skb_gso_validate_*_len family of helpers correctly
- considers GSO_BY_FRAGS.
-
- - For manipulating packets, skb_increase_gso_size and skb_decrease_gso_size
- will check for GSO_BY_FRAGS and WARN if asked to manipulate these skbs.
-
-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.