185 lines
7.7 KiB
ReStructuredText
185 lines
7.7 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=====================
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Segmentation Offloads
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=====================
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Introduction
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============
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This document describes a set of techniques in the Linux networking stack
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to take advantage of segmentation offload capabilities of various NICs.
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The following technologies are described:
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* TCP Segmentation Offload - TSO
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* UDP Fragmentation Offload - UFO
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* IPIP, SIT, GRE, and UDP Tunnel Offloads
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* Generic Segmentation Offload - GSO
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* Generic Receive Offload - GRO
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* Partial Generic Segmentation Offload - GSO_PARTIAL
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* SCTP acceleration with GSO - GSO_BY_FRAGS
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TCP Segmentation Offload
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========================
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TCP segmentation allows a device to segment a single frame into multiple
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frames with a data payload size specified in skb_shinfo()->gso_size.
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When TCP segmentation requested the bit for either SKB_GSO_TCPV4 or
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SKB_GSO_TCPV6 should be set in skb_shinfo()->gso_type and
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skb_shinfo()->gso_size should be set to a non-zero value.
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TCP segmentation is dependent on support for the use of partial checksum
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offload. For this reason TSO is normally disabled if the Tx checksum
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offload for a given device is disabled.
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In order to support TCP segmentation offload it is necessary to populate
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the network and transport header offsets of the skbuff so that the device
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drivers will be able determine the offsets of the IP or IPv6 header and the
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TCP header. In addition as CHECKSUM_PARTIAL is required csum_start should
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also point to the TCP header of the packet.
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For IPv4 segmentation we support one of two types in terms of the IP ID.
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The default behavior is to increment the IP ID with every segment. If the
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GSO type SKB_GSO_TCP_FIXEDID is specified then we will not increment the IP
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ID and all segments will use the same IP ID. If a device has
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NETIF_F_TSO_MANGLEID set then the IP ID can be ignored when performing TSO
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and we will either increment the IP ID for all frames, or leave it at a
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static value based on driver preference.
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UDP Fragmentation Offload
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=========================
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UDP fragmentation offload allows a device to fragment an oversized UDP
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datagram into multiple IPv4 fragments. Many of the requirements for UDP
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fragmentation offload are the same as TSO. However the IPv4 ID for
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fragments should not increment as a single IPv4 datagram is fragmented.
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UFO is deprecated: modern kernels will no longer generate UFO skbs, but can
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still receive them from tuntap and similar devices. Offload of UDP-based
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tunnel protocols is still supported.
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IPIP, SIT, GRE, UDP Tunnel, and Remote Checksum Offloads
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========================================================
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In addition to the offloads described above it is possible for a frame to
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contain additional headers such as an outer tunnel. In order to account
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for such instances an additional set of segmentation offload types were
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introduced including SKB_GSO_IPXIP4, SKB_GSO_IPXIP6, SKB_GSO_GRE, and
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SKB_GSO_UDP_TUNNEL. These extra segmentation types are used to identify
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cases where there are more than just 1 set of headers. For example in the
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case of IPIP and SIT we should have the network and transport headers moved
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from the standard list of headers to "inner" header offsets.
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Currently only two levels of headers are supported. The convention is to
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refer to the tunnel headers as the outer headers, while the encapsulated
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data is normally referred to as the inner headers. Below is the list of
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calls to access the given headers:
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IPIP/SIT Tunnel::
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Outer Inner
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MAC skb_mac_header
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Network skb_network_header skb_inner_network_header
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Transport skb_transport_header
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UDP/GRE Tunnel::
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Outer Inner
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MAC skb_mac_header skb_inner_mac_header
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Network skb_network_header skb_inner_network_header
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Transport skb_transport_header skb_inner_transport_header
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In addition to the above tunnel types there are also SKB_GSO_GRE_CSUM and
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SKB_GSO_UDP_TUNNEL_CSUM. These two additional tunnel types reflect the
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fact that the outer header also requests to have a non-zero checksum
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included in the outer header.
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Finally there is SKB_GSO_TUNNEL_REMCSUM which indicates that a given tunnel
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header has requested a remote checksum offload. In this case the inner
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headers will be left with a partial checksum and only the outer header
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checksum will be computed.
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Generic Segmentation Offload
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============================
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Generic segmentation offload is a pure software offload that is meant to
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deal with cases where device drivers cannot perform the offloads described
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above. What occurs in GSO is that a given skbuff will have its data broken
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out over multiple skbuffs that have been resized to match the MSS provided
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via skb_shinfo()->gso_size.
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Before enabling any hardware segmentation offload a corresponding software
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offload is required in GSO. Otherwise it becomes possible for a frame to
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be re-routed between devices and end up being unable to be transmitted.
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Generic Receive Offload
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=======================
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Generic receive offload is the complement to GSO. Ideally any frame
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assembled by GRO should be segmented to create an identical sequence of
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frames using GSO, and any sequence of frames segmented by GSO should be
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able to be reassembled back to the original by GRO. The only exception to
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this is IPv4 ID in the case that the DF bit is set for a given IP header.
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If the value of the IPv4 ID is not sequentially incrementing it will be
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altered so that it is when a frame assembled via GRO is segmented via GSO.
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Partial Generic Segmentation Offload
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====================================
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Partial generic segmentation offload is a hybrid between TSO and GSO. What
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it effectively does is take advantage of certain traits of TCP and tunnels
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so that instead of having to rewrite the packet headers for each segment
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only the inner-most transport header and possibly the outer-most network
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header need to be updated. This allows devices that do not support tunnel
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offloads or tunnel offloads with checksum to still make use of segmentation.
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With the partial offload what occurs is that all headers excluding the
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inner transport header are updated such that they will contain the correct
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values for if the header was simply duplicated. The one exception to this
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is the outer IPv4 ID field. It is up to the device drivers to guarantee
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that the IPv4 ID field is incremented in the case that a given header does
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not have the DF bit set.
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SCTP acceleration with GSO
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===========================
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SCTP - despite the lack of hardware support - can still take advantage of
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GSO to pass one large packet through the network stack, rather than
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multiple small packets.
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This requires a different approach to other offloads, as SCTP packets
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cannot be just segmented to (P)MTU. Rather, the chunks must be contained in
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IP segments, padding respected. So unlike regular GSO, SCTP can't just
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generate a big skb, set gso_size to the fragmentation point and deliver it
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to IP layer.
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Instead, the SCTP protocol layer builds an skb with the segments correctly
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padded and stored as chained skbs, and skb_segment() splits based on those.
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To signal this, gso_size is set to the special value GSO_BY_FRAGS.
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Therefore, any code in the core networking stack must be aware of the
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possibility that gso_size will be GSO_BY_FRAGS and handle that case
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appropriately.
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There are some helpers to make this easier:
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- skb_is_gso(skb) && skb_is_gso_sctp(skb) is the best way to see if
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an skb is an SCTP GSO skb.
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- For size checks, the skb_gso_validate_*_len family of helpers correctly
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considers GSO_BY_FRAGS.
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- For manipulating packets, skb_increase_gso_size and skb_decrease_gso_size
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will check for GSO_BY_FRAGS and WARN if asked to manipulate these skbs.
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This also affects drivers with the NETIF_F_FRAGLIST & NETIF_F_GSO_SCTP bits
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set. Note also that NETIF_F_GSO_SCTP is included in NETIF_F_GSO_SOFTWARE.
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