VNET (VPP Network Stack)
========================
The files associated with the VPP network stack layer are located in the
*./src/vnet* folder. The Network Stack Layer is basically an
instantiation of the code in the other layers. This layer has a vnet
library that provides vectorized layer-2 and 3 networking graph nodes, a
packet generator, and a packet tracer.
In terms of building a packet processing application, vnet provides a
platform-independent subgraph to which one connects a couple of
device-driver nodes.
Typical RX connections include "ethernet-input" \[full software
classification, feeds ipv4-input, ipv6-input, arp-input etc.\] and
"ipv4-input-no-checksum" \[if hardware can classify, perform ipv4 header
checksum\].
Effective graph dispatch function coding
----------------------------------------
Over the 15 years, multiple coding styles have emerged: a
single/dual/quad loop coding model (with variations) and a
fully-pipelined coding model.
Single/dual loops
-----------------
The single/dual/quad loop model variations conveniently solve problems
where the number of items to process is not known in advance: typical
hardware RX-ring processing. This coding style is also very effective
when a given node will not need to cover a complex set of dependent
reads.
Here is an quad/single loop which can leverage up-to-avx512 SIMD vector
units to convert buffer indices to buffer pointers:
```c
static uword
simulated_ethernet_interface_tx (vlib_main_t * vm,
vlib_node_runtime_t *
node, vlib_frame_t * frame)
{
u32 n_left_from, *from;
u32 next_index = 0;
u32 n_bytes;
u32 thread_index = vm->thread_index;
vnet_main_t *vnm = vnet_get_main ();
vnet_interface_main_t *im = &vnm->interface_main;
vlib_buffer_t *bufs[VLIB_FRAME_SIZE], **b;
u16 nexts[VLIB_FRAME_SIZE], *next;
n_left_from = frame->n_vectors;
from = vlib_frame_vector_args (frame);
/*
* Convert up to VLIB_FRAME_SIZE indices in "from" to
* buffer pointers in bufs[]
*/
vlib_get_buffers (vm, from, bufs, n_left_from);
b = bufs;
next = nexts;
/*
* While we have at least 4 vector elements (pkts) to process..
*/
while (n_left_from >= 4)
{
/* Prefetch next quad-loop iteration. */
if (PREDICT_TRUE (n_left_from >= 8))
{
vlib_prefetch_buffer_header (b[4], STORE);
vlib_prefetch_buffer_header (b[5], STORE);
vlib_prefetch_buffer_header (b[6], STORE);
vlib_prefetch_buffer_header (b[7], STORE);
}
/*
* $$$ Process 4x packets right here...
* set next[0..3] to send the packets where they need to go
*/
do_something_to (b[0]);
do_something_to (b[1]);
do_something_to (b[2]);
do_something_to (b[3]);
/* Process the next 0..4 packets */
b += 4;
next += 4;
n_left_from -= 4;
}
/*
* Clean up 0...3 remaining packets at the end of the incoming frame
*/
while (n_left_from > 0)
{
/*
* $$$ Process one packet right here...
* set next[0..3] to send the packets where they need to go
*/
do_something_to (b[0]);
/* Process the next packet */
b += 1;
next += 1;
n_left_from -= 1;
}
/*
* Send the packets along their respective next-node graph arcs
* Considerable locality of reference is expected, most if not all
* packets in the inbound vector will traverse the same next-node
* arc
*/
vlib_buffer_enqueue_to_next (vm, node, from, nexts, frame->n_vectors);
return frame->n_vectors;
}
```
Given a packet processing task to implement, it pays to scout around
looking for similar tasks, and think about using the same coding
pattern. It is not uncommon to recode a given graph node dispatch function
several times during performance optimization.
Creating Packets from Scratch
-----------------------------
At times, it's necessary to create packets from scratch and send
them. Tasks like sending keepalives or actively opening connections
come to mind. Its not difficult, but accurate buffer metadata setup is
required.
### Allocating Buffers
Use vlib_buffer_alloc, which allocates a set of buffer indices. For
low-performance applications, it's OK to allocate one buffer at a
time. Note that vlib_buffer_alloc(...) does NOT initialize buffer
metadata. See below.
In high-performance cases, allocate a vector of buffer indices,
and hand them out from the end of the vector; decrement _vec_len(..)
as buffer indices are allocated. See tcp_alloc_tx_buffers(...) and
tcp_get_free_buffer_index(...) for an example.
### Buffer Initialization Example
The following example shows the **main points**, but is not to be
blindly cut-'n-pasted.
```c
u32 bi0;
vlib_buffer_t *b0;
ip4_header_t *ip;
udp_header_t *udp;
vlib_buffer_free_list_t *fl;
/* Allocate a buffer */
if (vlib_buffer_alloc (vm, &bi0, 1) != 1)
return -1;
b0 = vlib_get_buffer (vm, bi0);
/* Initialize the buffer */
fl = vlib_buffer_get_free_list (vm, VLIB_BUFFER_DEFAULT_FREE_LIST_INDEX);
vlib_buffer_init_for_free_list (b0, fl);
VLIB_BUFFER_TRACE_TRAJECTORY_INIT (b0);
/* At this point b0->current_data = 0, b0->current_length = 0 */
/*
* Copy data into the buffer. This example ASSUMES that data will fit
* in a single buffer, and is e.g. an ip4 packet.
*/
if (have_packet_rewrite)
{
clib_memcpy (b0->data, data, vec_len (data));
b0->current_length = vec_len (data);
}
else
{
/* OR, build a udp-ip packet (for example) */
ip = vlib_buffer_get_current (b0);
udp = (udp_header_t *) (ip + 1);
data_dst = (u8 *) (udp + 1);
ip->ip_version_and_header_length = 0x45;
ip->ttl = 254;
ip->protocol = IP_PROTOCOL_UDP;
ip->length = clib_host_to_net_u16 (sizeof (*ip) + sizeof (*udp) +
vec_len(udp_data));
ip->src_address.as_u32 = src_address->as_u32;
ip->dst_address.as_u32 = dst_address->as_u32;
udp->src_port = clib_host_to_net_u16 (src_port);
udp->dst_port = clib_host_to_net_u16 (dst_port);
udp->length = clib_host_to_net_u16 (vec_len (udp_data));
clib_memcpy (data_dst, udp_data, vec_len(udp_data));
if (compute_udp_checksum)
{
/* RFC 7011 section 10.3.2. */
udp->checksum = ip4_tcp_udp_compute_checksum (vm, b0, ip);
if (udp->checksum == 0)
udp->checksum = 0xffff;
}
b0->current_length = vec_len (sizeof (*ip) + sizeof (*udp) +
vec_len (udp_data));
}
b0->flags |= (VLIB_BUFFER_TOTAL_LENGTH_VALID;
/* sw_if_index 0 is the "local" interface, which always exists */
vnet_buffer (b0)->sw_if_index[VLIB_RX] = 0;
/* Use the default FIB index for tx lookup. Set non-zero to use another fib */
vnet_buffer (b0)->sw_if_index[VLIB_TX] = 0;
```
If your use-case calls for large packet transmission, use
vlib_buffer_chain_append_data_with_alloc(...) to create the requisite
buffer chain.
### Enqueueing packets for lookup and transmission
The simplest way to send a set of packets is to use
vlib_get_frame_to_node(...) to allocate fresh frame(s) to
ip4_lookup_node or ip6_lookup_node, add the constructed buffer
indices, and dispatch the frame using vlib_put_frame_to_node(...).
```c
vlib_frame_t *f;
f = vlib_get_frame_to_node (vm, ip4_lookup_node.index);
f->n_vectors = vec_len(buffer_indices_to_send);
to_next = vlib_frame_vector_args (f);
for (i = 0; i < vec_len (buffer_indices_to_send); i++)
to_next[i] = buffer_indices_to_send[i];
vlib_put_frame_to_node (vm, ip4_lookup_node_index, f);
```
It is inefficient to allocate and schedule single packet frames.
That's typical in case you need to send one packet per second, but
should **not** occur in a for-loop!
Packet tracer
-------------
Vlib includes a frame element \[packet\] trace facility, with a simple
vlib cli interface. The cli is straightforward: "trace add
input-node-name count".
To trace 100 packets on a typical x86\_64 system running the dpdk
plugin: "trace add dpdk-input 100". When using the packet generator:
"trace add pg-input 100"
Each graph node has the opportunity to capture its own trace data. It is
almost always a good idea to do so. The trace capture APIs are simple.
The packet capture APIs snapshoot binary data, to minimize processing at
capture time. Each participating graph node initialization provides a
vppinfra format-style user function to pretty-print data when required
by the VLIB "show trace" command.
Set the VLIB node registration ".format\_trace" member to the name of
the per-graph node format function.
Here's a simple example:
```c
u8 * my_node_format_trace (u8 * s, va_list * args)
{
vlib_main_t * vm = va_arg (*args, vlib_main_t *);
vlib_node_t * node = va_arg (*args, vlib_node_t *);
my_node_trace_t * t = va_arg (*args, my_trace_t *);
s = format (s, "My trace data was: %d", t-><whatever>);
return s;
}
```
The trace framework hands the per-node format function the data it
captured as the packet whizzed by. The format function pretty-prints the
data as desired.