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].
Over the 15 years, multiple coding styles have emerged: a single/dual/quad loop coding model (with variations) and a fully-pipelined coding model.
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:
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_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.
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.
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.
The following example shows the main points, but is not to be blindly cut-'n-pasted.
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.
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(...).
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!
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:
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.