Hash Infra


Modern physical NICs uses packet flow hash for different purposes, i.e. Receive Side Scaling, flow steering and interface bonding etc. NICs can also provide packet flow hash prepended to data packet as metadata which can be used by applications without recomputing the packet flow hash.

As more and more services are deployed in virtualized environment, making use of virtual interfaces to interconnect those services.

The Hash Infrastructure

VPP implements software based hashing functionality which can be used for different purposes. It also provides users a centralized way to registry custom hash functions based on traffic profile to be used in different vpp features i.e. Multi-TXQ, software RSS or bonding driver.

Data structures

Hashing infra provides two types of hashing functions: VNET_HASH_FN_TYPE_ETHERNET and VNET_HASH_FN_TYPE_IP for ethernet traffic and IP traffic respectively. Hashing infra provides uniform signature to the functions to be implemented:

void (*vnet_hash_fn_t) (void **p, u32 *h, u32 n_packets);

Here **p is the array of pointers pointing to the beginning of packet headers (either ethernet or ip). *h is an empty array of size n_packets. On return, it will contain hashes. n_packets is the number of packets pass to this function.

Custom hashing functions can be registered through VNET_REGISTER_HASH_FUNCTION. Users need to provide a name, description, priority and hashing functions for registration.

Default hashing function is selected based on the highest priority among the registered hashing functions.

typedef struct vnet_hash_function_registration
  const char *name;
  const char *description;
  int priority;
  vnet_hash_fn_t function[VNET_HASH_FN_TYPE_N];

  struct vnet_hash_function_registration *next;
} vnet_hash_function_registration_t;

For example, crc32c_5tuple provides two hashing functions: for IP traffic and for ethernet traffic. It uses 5 tuples from the flow to compute the crc32 hash on it.

void vnet_crc32c_5tuple_ip_func (void **p, u32 *hash, u32 n_packets);
void vnet_crc32c_5tuple_ethernet_func (void **p, u32 *hash, u32 n_packets);

VNET_REGISTER_HASH_FUNCTION (crc32c_5tuple, static) = {
  .name = "crc32c-5tuple",
  .description = "IPv4/IPv6 header and TCP/UDP ports",
  .priority = 50,
  .function[VNET_HASH_FN_TYPE_ETHERNET] = vnet_crc32c_5tuple_ethernet_func,
  .function[VNET_HASH_FN_TYPE_IP] = vnet_crc32c_5tuple_ip_func,

Users can see all the registered hash functions along with priority and description.

Hash API

There is no Hash API at the moment.

Hash CLI

show hash