1.. SPDX-License-Identifier: BSD-3-Clause 2 Copyright(c) 2017 Intel Corporation. 3 4Generic Segmentation Offload Library 5==================================== 6 7Overview 8-------- 9Generic Segmentation Offload (GSO) is a widely used software implementation of 10TCP Segmentation Offload (TSO), which reduces per-packet processing overhead. 11Much like TSO, GSO gains performance by enabling upper layer applications to 12process a smaller number of large packets (e.g. MTU size of 64KB), instead of 13processing higher numbers of small packets (e.g. MTU size of 1500B), thus 14reducing per-packet overhead. 15 16For example, GSO allows guest kernel stacks to transmit over-sized TCP segments 17that far exceed the kernel interface's MTU; this eliminates the need to segment 18packets within the guest, and improves the data-to-overhead ratio of both the 19guest-host link, and PCI bus. The expectation of the guest network stack in this 20scenario is that segmentation of egress frames will take place either in the NIC 21HW, or where that hardware capability is unavailable, either in the host 22application, or network stack. 23 24Bearing that in mind, the GSO library enables DPDK applications to segment 25packets in software. Note however, that GSO is implemented as a standalone 26library, and not via a 'fallback' mechanism (i.e. for when TSO is unsupported 27in the underlying hardware); that is, applications must explicitly invoke the 28GSO library to segment packets. The size of GSO segments ``(segsz)`` is 29configurable by the application. 30 31Limitations 32----------- 33 34#. The GSO library doesn't check if input packets have correct checksums. 35 36#. In addition, the GSO library doesn't re-calculate checksums for segmented 37 packets (that task is left to the application). 38 39#. IP fragments are unsupported by the GSO library. 40 41#. The egress interface's driver must support multi-segment packets. 42 43#. Currently, the GSO library supports the following IPv4 packet types: 44 45 - TCP 46 - VxLAN 47 - GRE 48 49 See `Supported GSO Packet Types`_ for further details. 50 51Packet Segmentation 52------------------- 53 54The ``rte_gso_segment()`` function is the GSO library's primary 55segmentation API. 56 57Before performing segmentation, an application must create a GSO context object 58``(struct rte_gso_ctx)``, which provides the library with some of the 59information required to understand how the packet should be segmented. Refer to 60`How to Segment a Packet`_ for additional details on same. Once the GSO context 61has been created, and populated, the application can then use the 62``rte_gso_segment()`` function to segment packets. 63 64The GSO library typically stores each segment that it creates in two parts: the 65first part contains a copy of the original packet's headers, while the second 66part contains a pointer to an offset within the original packet. This mechanism 67is explained in more detail in `GSO Output Segment Format`_. 68 69The GSO library supports both single- and multi-segment input mbufs. 70 71GSO Output Segment Format 72~~~~~~~~~~~~~~~~~~~~~~~~~ 73To reduce the number of expensive memcpy operations required when segmenting a 74packet, the GSO library typically stores each segment that it creates as a 75two-part mbuf (technically, this is termed a 'two-segment' mbuf; however, since 76the elements produced by the API are also called 'segments', for clarity the 77term 'part' is used here instead). 78 79The first part of each output segment is a direct mbuf and contains a copy of 80the original packet's headers, which must be prepended to each output segment. 81These headers are copied from the original packet into each output segment. 82 83The second part of each output segment, represents a section of data from the 84original packet, i.e. a data segment. Rather than copy the data directly from 85the original packet into the output segment (which would impact performance 86considerably), the second part of each output segment is an indirect mbuf, 87which contains no actual data, but simply points to an offset within the 88original packet. 89 90The combination of the 'header' segment and the 'data' segment constitutes a 91single logical output GSO segment of the original packet. This is illustrated 92in :numref:`figure_gso-output-segment-format`. 93 94.. _figure_gso-output-segment-format: 95 96.. figure:: img/gso-output-segment-format.* 97 :align: center 98 99 Two-part GSO output segment 100 101In one situation, the output segment may contain additional 'data' segments. 102This only occurs when: 103 104- the input packet on which GSO is to be performed is represented by a 105 multi-segment mbuf. 106 107- the output segment is required to contain data that spans the boundaries 108 between segments of the input multi-segment mbuf. 109 110The GSO library traverses each segment of the input packet, and produces 111numerous output segments; for optimal performance, the number of output 112segments is kept to a minimum. Consequently, the GSO library maximizes the 113amount of data contained within each output segment; i.e. each output segment 114``segsz`` bytes of data. The only exception to this is in the case of the very 115final output segment; if ``pkt_len`` % ``segsz``, then the final segment is 116smaller than the rest. 117 118In order for an output segment to meet its MSS, it may need to include data from 119multiple input segments. Due to the nature of indirect mbufs (each indirect mbuf 120can point to only one direct mbuf), the solution here is to add another indirect 121mbuf to the output segment; this additional segment then points to the next 122input segment. If necessary, this chaining process is repeated, until the sum of 123all of the data 'contained' in the output segment reaches ``segsz``. This 124ensures that the amount of data contained within each output segment is uniform, 125with the possible exception of the last segment, as previously described. 126 127:numref:`figure_gso-three-seg-mbuf` illustrates an example of a three-part 128output segment. In this example, the output segment needs to include data from 129the end of one input segment, and the beginning of another. To achieve this, 130an additional indirect mbuf is chained to the second part of the output segment, 131and is attached to the next input segment (i.e. it points to the data in the 132next input segment). 133 134.. _figure_gso-three-seg-mbuf: 135 136.. figure:: img/gso-three-seg-mbuf.* 137 :align: center 138 139 Three-part GSO output segment 140 141Supported GSO Packet Types 142-------------------------- 143 144TCP/IPv4 GSO 145~~~~~~~~~~~~ 146TCP/IPv4 GSO supports segmentation of suitably large TCP/IPv4 packets, which 147may also contain an optional VLAN tag. 148 149VxLAN GSO 150~~~~~~~~~ 151VxLAN packets GSO supports segmentation of suitably large VxLAN packets, 152which contain an outer IPv4 header, inner TCP/IPv4 headers, and optional 153inner and/or outer VLAN tag(s). 154 155GRE GSO 156~~~~~~~ 157GRE GSO supports segmentation of suitably large GRE packets, which contain 158an outer IPv4 header, inner TCP/IPv4 headers, and an optional VLAN tag. 159 160How to Segment a Packet 161----------------------- 162 163To segment an outgoing packet, an application must: 164 165#. First create a GSO context ``(struct rte_gso_ctx)``; this contains: 166 167 - a pointer to the mbuf pool for allocating the direct buffers, which are 168 used to store the GSO segments' packet headers. 169 170 - a pointer to the mbuf pool for allocating indirect buffers, which are 171 used to locate GSO segments' packet payloads. 172 173 .. note:: 174 175 An application may use the same pool for both direct and indirect 176 buffers. However, since indirect mbufs simply store a pointer, the 177 application may reduce its memory consumption by creating a separate memory 178 pool, containing smaller elements, for the indirect pool. 179 180 181 - the size of each output segment, including packet headers and payload, 182 measured in bytes. 183 184 - the bit mask of required GSO types. The GSO library uses the same macros as 185 those that describe a physical device's TX offloading capabilities (i.e. 186 ``DEV_TX_OFFLOAD_*_TSO``) for gso_types. For example, if an application 187 wants to segment TCP/IPv4 packets, it should set gso_types to 188 ``DEV_TX_OFFLOAD_TCP_TSO``. The only other supported values currently 189 supported for gso_types are ``DEV_TX_OFFLOAD_VXLAN_TNL_TSO``, and 190 ``DEV_TX_OFFLOAD_GRE_TNL_TSO``; a combination of these macros is also 191 allowed. 192 193 - a flag, that indicates whether the IPv4 headers of output segments should 194 contain fixed or incremental ID values. 195 1962. Set the appropriate ol_flags in the mbuf. 197 198 - The GSO library use the value of an mbuf's ``ol_flags`` attribute to 199 to determine how a packet should be segmented. It is the application's 200 responsibility to ensure that these flags are set. 201 202 - For example, in order to segment TCP/IPv4 packets, the application should 203 add the ``PKT_TX_IPV4`` and ``PKT_TX_TCP_SEG`` flags to the mbuf's 204 ol_flags. 205 206 - If checksum calculation in hardware is required, the application should 207 also add the ``PKT_TX_TCP_CKSUM`` and ``PKT_TX_IP_CKSUM`` flags. 208 209#. Check if the packet should be processed. Packets with one of the 210 following properties are not processed and are returned immediately: 211 212 - Packet length is less than ``segsz`` (i.e. GSO is not required). 213 214 - Packet type is not supported by GSO library (see 215 `Supported GSO Packet Types`_). 216 217 - Application has not enabled GSO support for the packet type. 218 219 - Packet's ol_flags have been incorrectly set. 220 221#. Allocate space in which to store the output GSO segments. If the amount of 222 space allocated by the application is insufficient, segmentation will fail. 223 224#. Invoke the GSO segmentation API, ``rte_gso_segment()``. 225 226#. If required, update the L3 and L4 checksums of the newly-created segments. 227 For tunneled packets, the outer IPv4 headers' checksums should also be 228 updated. Alternatively, the application may offload checksum calculation 229 to HW. 230 231