diff options
Diffstat (limited to 'Documentation/networking/segmentation-offloads.txt')
| -rw-r--r-- | Documentation/networking/segmentation-offloads.txt | 170 |
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. |