30. Generic Segmentation Offload Library

30.1. Overview

Generic Segmentation Offload (GSO) is a widely used software implementation of TCP Segmentation Offload (TSO), which reduces per-packet processing overhead. Much like TSO, GSO gains performance by enabling upper layer applications to process a smaller number of large packets (e.g. MTU size of 64KB), instead of processing higher numbers of small packets (e.g. MTU size of 1500B), thus reducing per-packet overhead.

For example, GSO allows guest kernel stacks to transmit over-sized TCP segments that far exceed the kernel interface’s MTU; this eliminates the need to segment packets within the guest, and improves the data-to-overhead ratio of both the guest-host link, and PCI bus. The expectation of the guest network stack in this scenario is that segmentation of egress frames will take place either in the NIC HW, or where that hardware capability is unavailable, either in the host application, or network stack.

Bearing that in mind, the GSO library enables DPDK applications to segment packets in software. Note however, that GSO is implemented as a standalone library, and not via a ‘fallback’ mechanism (i.e. for when TSO is unsupported in the underlying hardware); that is, applications must explicitly invoke the GSO library to segment packets. The size of GSO segments (segsz) is configurable by the application.

30.2. Limitations

1. The GSO library doesn’t check if input packets have correct checksums.
2. In addition, the GSO library doesn’t re-calculate checksums for segmented packets (that task is left to the application).
3. IP fragments are unsupported by the GSO library.
4. The egress interface’s driver must support multi-segment packets.
5. Currently, the GSO library supports the following IPv4 packet types:

• TCP
• UDP
• VxLAN
• GRE

See Supported GSO Packet Types for further details.

30.3. Packet Segmentation

The rte_gso_segment() function is the GSO library’s primary segmentation API.

Before performing segmentation, an application must create a GSO context object (struct rte_gso_ctx), which provides the library with some of the information required to understand how the packet should be segmented. Refer to How to Segment a Packet for additional details on same. Once the GSO context has been created, and populated, the application can then use the rte_gso_segment() function to segment packets.

The GSO library typically stores each segment that it creates in two parts: the first part contains a copy of the original packet’s headers, while the second part contains a pointer to an offset within the original packet. This mechanism is explained in more detail in GSO Output Segment Format.

The GSO library supports both single- and multi-segment input mbufs.

30.3.1. GSO Output Segment Format

To reduce the number of expensive memcpy operations required when segmenting a packet, the GSO library typically stores each segment that it creates as a two-part mbuf (technically, this is termed a ‘two-segment’ mbuf; however, since the elements produced by the API are also called ‘segments’, for clarity the term ‘part’ is used here instead).

The first part of each output segment is a direct mbuf and contains a copy of the original packet’s headers, which must be prepended to each output segment. These headers are copied from the original packet into each output segment.

The second part of each output segment, represents a section of data from the original packet, i.e. a data segment. Rather than copy the data directly from the original packet into the output segment (which would impact performance considerably), the second part of each output segment is an indirect mbuf, which contains no actual data, but simply points to an offset within the original packet.

The combination of the ‘header’ segment and the ‘data’ segment constitutes a single logical output GSO segment of the original packet. This is illustrated in Fig. 30.2.

Fig. 30.2 Two-part GSO output segment

In one situation, the output segment may contain additional ‘data’ segments. This only occurs when:

• the input packet on which GSO is to be performed is represented by a multi-segment mbuf.
• the output segment is required to contain data that spans the boundaries between segments of the input multi-segment mbuf.

The GSO library traverses each segment of the input packet, and produces numerous output segments; for optimal performance, the number of output segments is kept to a minimum. Consequently, the GSO library maximizes the amount of data contained within each output segment; i.e. each output segment segsz bytes of data. The only exception to this is in the case of the very final output segment; if pkt_len % segsz, then the final segment is smaller than the rest.

In order for an output segment to meet its MSS, it may need to include data from multiple input segments. Due to the nature of indirect mbufs (each indirect mbuf can point to only one direct mbuf), the solution here is to add another indirect mbuf to the output segment; this additional segment then points to the next input segment. If necessary, this chaining process is repeated, until the sum of all of the data ‘contained’ in the output segment reaches segsz. This ensures that the amount of data contained within each output segment is uniform, with the possible exception of the last segment, as previously described.

Fig. 30.3 illustrates an example of a three-part output segment. In this example, the output segment needs to include data from the end of one input segment, and the beginning of another. To achieve this, an additional indirect mbuf is chained to the second part of the output segment, and is attached to the next input segment (i.e. it points to the data in the next input segment).

Fig. 30.3 Three-part GSO output segment

30.4. Supported GSO Packet Types

30.5. How to Segment a Packet

To segment an outgoing packet, an application must:

1. First create a GSO context (struct rte_gso_ctx); this contains:

   • a pointer to the mbuf pool for allocating the direct buffers, which are used to store the GSO segments’ packet headers.

   • a pointer to the mbuf pool for allocating indirect buffers, which are used to locate GSO segments’ packet payloads.

     Note

     An application may use the same pool for both direct and indirect buffers. However, since indirect mbufs simply store a pointer, the application may reduce its memory consumption by creating a separate memory pool, containing smaller elements, for the indirect pool.

   • the size of each output segment, including packet headers and payload, measured in bytes.

   • the bit mask of required GSO types. The GSO library uses the same macros as those that describe a physical device’s TX offloading capabilities (i.e. DEV_TX_OFFLOAD_*_TSO) for gso_types. For example, if an application wants to segment TCP/IPv4 packets, it should set gso_types to DEV_TX_OFFLOAD_TCP_TSO. The only other supported values currently supported for gso_types are DEV_TX_OFFLOAD_VXLAN_TNL_TSO, and DEV_TX_OFFLOAD_GRE_TNL_TSO; a combination of these macros is also allowed.

   • a flag, that indicates whether the IPv4 headers of output segments should contain fixed or incremental ID values.

2. Set the appropriate ol_flags in the mbuf.
   • The GSO library use the value of an mbuf’s ol_flags attribute to to determine how a packet should be segmented. It is the application’s responsibility to ensure that these flags are set.
   • For example, in order to segment TCP/IPv4 packets, the application should add the PKT_TX_IPV4 and PKT_TX_TCP_SEG flags to the mbuf’s ol_flags.
   • If checksum calculation in hardware is required, the application should also add the PKT_TX_TCP_CKSUM and PKT_TX_IP_CKSUM flags.
3. Check if the packet should be processed. Packets with one of the following properties are not processed and are returned immediately:
   • Packet length is less than segsz (i.e. GSO is not required).
   • Packet type is not supported by GSO library (see Supported GSO Packet Types).
   • Application has not enabled GSO support for the packet type.
   • Packet’s ol_flags have been incorrectly set.
4. Allocate space in which to store the output GSO segments. If the amount of space allocated by the application is insufficient, segmentation will fail.
5. Invoke the GSO segmentation API, rte_gso_segment().
6. If required, update the L3 and L4 checksums of the newly-created segments. For tunneled packets, the outer IPv4 headers’ checksums should also be updated. Alternatively, the application may offload checksum calculation to HW.