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diff --git a/Documentation/networking/segmentation-offloads.txt b/Documentation/networking/segmentation-offloads.txt deleted file mode 100644 index 2f09455a993a..000000000000 --- a/Documentation/networking/segmentation-offloads.txt +++ /dev/null @@ -1,130 +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 - -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_TCP or -SKB_GSO_TCP6 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. - -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_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. |