# 3.2.dpdk nic offload:2

* vlan strip offloading

make sure the nic supports `DEV_RX_OFFLOAD_VLAN_STRIP` feature by checking device information .

there are two ways to configure vlan striping in DPDK,one is to set the `.hw_vlan_strip` bit of rte\_eth\_conf during device configuration phase,another way is to configure at runtime,`rte_eth_dev_set_vlan_offload(port_id,rte_eth_dev_get_vlan_offload()|ETH_VLAN_STRIP_OFFLOAD)` .

upon receiving a packet,we see if it's a vlan packet ,'PKT\_RX\_VLAN\_PKT' will be set,and then we could also find `PKT_RX_VLAN_STRIPPED` bit is set ,and `rte-mbuf.vlan_tci` is the tci of real vlan header .

-vlan filter offloading

vlan filtering offloading allows nic to filter VLAN packets basing on the vlan white list,still ,we need to configure by doing so:`rte_eth_dev_set_vlan_offload(port_id,rte_eth_dev_get_vlan_offload()|ETH_VLAN_FILTER_OFFLOAD)` ,and then we could call `rte_eth_dev_vlan_filter()` to add a vlan to white list. there is no feature bit indicating the packet is allowed after vlan filtering in rte\_mbuf.

-vlan insertion offloading

if NIC supports `DEV_TX_OFFLOAD_VLAN_INSERT` ,then we could use the following code to make NIC insert a vlan header during transmission:

```c
mbufs[idx]->vlan_tci=0xef00;
mbufs[idx]->ol_flags|=PKT_TX_VLAN_PKT;
```

-tx checksum offloading

first check TX checksum features the NIC supports . for example ,the NIC supports IP checksum tx offloading ,then before transmission ,we set setup offloading flags as follows:

```c
buffer=rte_pktmbuf_mtod(mbufs[idx],char*);
ip4=(struct ipv4_hdr*)(buffer+14);
ip4->hdr_checksum=0;
mbufs[idx]->l2_len=14;
mbufs[idx]->l3_len=20;
mbufs[idx]->ol_flags=PKT_TX_IP_CKSUM|PKT_TX_IPV4;
```

different checksum types require different pre-setups , please refer to `rte_mbuf.h`.


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