IP Fragmentation

Link-Inline™ Software Features
Platform
Napatech SmartNIC
Content Type
Feature Description
Capture Software Version
Link-Inline™ Software 3.2

The maximum size of a frame that is transmitted without fragmentation can be configured using DPDK API, rte_eth_dev_set_mtu, and fragmentation settings can be adjusted in the flow_api_profile_inline_config.h file.

Maximum transmission unit (MTU)

The rte_eth_dev_set_mtu function configures the maximum transmission unit (MTU) in the SmartNIC. The MTU does not include the preamble/SFD, Ethernet header and CRC. The supported range of values is 512 to 9982 bytes. By default, the MTU is set to 1500 bytes.

General operation

  • By default, fragmentation is disabled.
  • Fragmentation can be configured per port.
  • Fragmentation is only applied to flows offloaded by the SmartNIC.
  • Frames to or from the application in the host are not fragmented by the SmartNIC.
  • Fragmentation is the last process in the SmartNIC. This means that encapsulation/decapsulation impacts the decision of whether a frame needs fragmentation or not.

Configuring fragmentation

Fragmentation settings can be adjusted in the flow_api_profile_inline_config.h file. The following shows the default configuration for IPv4 traffic.
#define PORT_0_IPV4_FRAGMENTATION DISABLE_FRAGMENTATION
#define PORT_0_IPV4_DF_ACTION     IPV4_DF_DROP

#define PORT_1_IPV4_FRAGMENTATION DISABLE_FRAGMENTATION
#define PORT_1_IPV4_DF_ACTION     IPV4_DF_DROP
When IPV4_FRAGMENTATION is set to ENABLE, the following actions are taken.
  • If a received frame exceeds the MTU, and the Don't Fragment flag (DF) bit is not set, the frame gets fragmented.
  • If a received frame exceeds the MTU, the Don't Fragment flag (DF) bit is set, and IPV4_DF_ACTION is set to DF_DROP, the frame is discarded.
  • If a received frame exceeds the MTU, the Don't Fragment flag (DF) bit is set, and IPV4_DF_ACTION is set to DF_Forward, the frame is forwarded.
The following shows the default configuration for IPv6 traffic.
#define PORT_0_IPV6_FRAGMENTATION DISABLE_FRAGMENTATION
#define PORT_0_IPV6_ACTION        IPV6_DROP

#define PORT_1_IPV6_FRAGMENTATION DISABLE_FRAGMENTATION
#define PORT_1_IPV6_ACTION        IPV6_DROP
When IPV6_FRAGMENTATION is set to ENABLE, the following actions are taken.
  • If a received frame exceeds the MTU, and IPV6_ACTION is set to FRAGMENT, the frame gets fragmented.
  • If a received frame exceeds the MTU, and IPV6_ACTION is set to DROP, the frame is discarded.
Note: It is not recommended to set the action to DROP, as an ICMP frame is not sent when a frame is discarded.

The identification field of fragmented frames

The SmartNIC uses the upper half of the ID range for the identification field in the IP header as follows.
  • 16-bit identification field in the IPv4 header: 215 to 216 − 1 (32768 to 65535 in decimal)
  • 32-bit identification field in the IPv6 header: 231 to 232 − 1 (2147483648 to 4294967295 in decimal)
Consequently, the most significant bit of the ID field is set to 1 for frames fragmented by the SmartNIC. To avoid ID collisions, the application must ensure that the most significant bit of the ID field is set to 0 for frames fragmented by the application in the host.

Related links

See the DPDK API documentation for further details: