path: root/docs/gettingstarted/developers/vnet.md

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;

  /* Allocate a buffer */
  if (vlib_buffer_alloc (vm, &bi0, 1) != 1)
    return -1;

  b0 = vlib_get_buffer (vm, bi0);

  /* Initialize the buffer */
  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
debug CLI interface. The cli is straightforward: "trace add
input-node-name count" to start capturing packet traces.

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"

To display the packet trace: "show trace"

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.

Graph Dispatcher Pcap Tracing
-----------------------------

The vpp graph dispatcher knows how to capture vectors of packets in pcap
format as they're dispatched. The pcap captures are as follows:

```
    VPP graph dispatch trace record description:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Major Version | Minor Version | NStrings      | ProtoHint     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Buffer index (big endian)                                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       + VPP graph node name ...     ...               | NULL octet    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Buffer Metadata ... ...                       | NULL octet    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Buffer Opaque ... ...                         | NULL octet    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Buffer Opaque 2 ... ...                       | NULL octet    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | VPP ASCII packet trace (if NStrings > 4)      | NULL octet    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Packet data (up to 16K)                                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
```

Graph dispatch records comprise a version stamp, an indication of how
many NULL-terminated strings will follow the record header and preceed
packet data, and a protocol hint.

The buffer index is an opaque 32-bit cookie which allows consumers of
these data to easily filter/track single packets as they traverse the
forwarding graph.

Multiple records per packet are normal, and to be expected. Packets
will appear multipe times as they traverse the vpp forwarding
graph. In this way, vpp graph dispatch traces are significantly
different from regular network packet captures from an end-station.
This property complicates stateful packet analysis.

Restricting stateful analysis to records from a single vpp graph node
such as "ethernet-input" seems likely to improve the situation.

As of this writing: major version = 1, minor version = 0. Nstrings
SHOULD be 4 or 5. Consumers SHOULD be wary values less than 4 or
greater than 5. They MAY attempt to display the claimed number of
strings, or they MAY treat the condition as an error.

Here is the current set of protocol hints:

```c
    typedef enum
      {
        VLIB_NODE_PROTO_HINT_NONE = 0,
        VLIB_NODE_PROTO_HINT_ETHERNET,
        VLIB_NODE_PROTO_HINT_IP4,
        VLIB_NODE_PROTO_HINT_IP6,
        VLIB_NODE_PROTO_HINT_TCP,
        VLIB_NODE_PROTO_HINT_UDP,
        VLIB_NODE_N_PROTO_HINTS,
      } vlib_node_proto_hint_t;
```

Example: VLIB_NODE_PROTO_HINT_IP6 means that the first octet of packet
data SHOULD be 0x60, and should begin an ipv6 packet header.

Downstream consumers of these data SHOULD pay attention to the
protocol hint. They MUST tolerate inaccurate hints, which MAY occur
from time to time.

### Dispatch Pcap Trace Debug CLI

To start a dispatch trace capture of up to 10,000 trace records:

```
     pcap dispatch trace on max 10000 file dispatch.pcap
```

To start a dispatch trace which will also include standard vpp packet
tracing for packets which originate in dpdk-input:

```
     pcap dispatch trace on max 10000 file dispatch.pcap buffer-trace dpdk-input 1000
```
To save the pcap trace, e.g. in /tmp/dispatch.pcap:

```
    pcap dispatch trace off
```

### Wireshark dissection of dispatch pcap traces

It almost goes without saying that we built a companion wireshark
dissector to display these traces. As of this writing, we have
upstreamed the wireshark dissector.

Since it will be a while before wireshark/master/latest makes it into
all of the popular Linux distros, please see the "How to build a vpp
dispatch trace aware Wireshark" page for build info.

Here is a sample packet dissection, with some fields omitted for
clarity.  The point is that the wireshark dissector accurately
displays **all** of the vpp buffer metadata, and the name of the graph
node in question.

```
    Frame 1: 2216 bytes on wire (17728 bits), 2216 bytes captured (17728 bits)
        Encapsulation type: USER 13 (58)
        [Protocols in frame: vpp:vpp-metadata:vpp-opaque:vpp-opaque2:eth:ethertype:ip:tcp:data]
    VPP Dispatch Trace
        BufferIndex: 0x00036663
    NodeName: ethernet-input
    VPP Buffer Metadata
        Metadata: flags:
        Metadata: current_data: 0, current_length: 102
        Metadata: current_config_index: 0, flow_id: 0, next_buffer: 0
        Metadata: error: 0, n_add_refs: 0, buffer_pool_index: 0
        Metadata: trace_index: 0, recycle_count: 0, len_not_first_buf: 0
        Metadata: free_list_index: 0
        Metadata:
    VPP Buffer Opaque
        Opaque: raw: 00000007 ffffffff 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
        Opaque: sw_if_index[VLIB_RX]: 7, sw_if_index[VLIB_TX]: -1
        Opaque: L2 offset 0, L3 offset 0, L4 offset 0, feature arc index 0
        Opaque: ip.adj_index[VLIB_RX]: 0, ip.adj_index[VLIB_TX]: 0
        Opaque: ip.flow_hash: 0x0, ip.save_protocol: 0x0, ip.fib_index: 0
        Opaque: ip.save_rewrite_length: 0, ip.rpf_id: 0
        Opaque: ip.icmp.type: 0 ip.icmp.code: 0, ip.icmp.data: 0x0
        Opaque: ip.reass.next_index: 0, ip.reass.estimated_mtu: 0
        Opaque: ip.reass.fragment_first: 0 ip.reass.fragment_last: 0
        Opaque: ip.reass.range_first: 0 ip.reass.range_last: 0
        Opaque: ip.reass.next_range_bi: 0x0, ip.reass.ip6_frag_hdr_offset: 0
        Opaque: mpls.ttl: 0, mpls.exp: 0, mpls.first: 0, mpls.save_rewrite_length: 0, mpls.bier.n_bytes: 0
        Opaque: l2.feature_bitmap: 00000000, l2.bd_index: 0, l2.l2_len: 0, l2.shg: 0, l2.l2fib_sn: 0, l2.bd_age: 0
        Opaque: l2.feature_bitmap_input:   none configured, L2.feature_bitmap_output:   none configured
        Opaque: l2t.next_index: 0, l2t.session_index: 0
        Opaque: l2_classify.table_index: 0, l2_classify.opaque_index: 0, l2_classify.hash: 0x0
        Opaque: policer.index: 0
        Opaque: ipsec.flags: 0x0, ipsec.sad_index: 0
        Opaque: map.mtu: 0
        Opaque: map_t.v6.saddr: 0x0, map_t.v6.daddr: 0x0, map_t.v6.frag_offset: 0, map_t.v6.l4_offset: 0
        Opaque: map_t.v6.l4_protocol: 0, map_t.checksum_offset: 0, map_t.mtu: 0
        Opaque: ip_frag.mtu: 0, ip_frag.next_index: 0, ip_frag.flags: 0x0
        Opaque: cop.current_config_index: 0
        Opaque: lisp.overlay_afi: 0
        Opaque: tcp.connection_index: 0, tcp.seq_number: 0, tcp.seq_end: 0, tcp.ack_number: 0, tcp.hdr_offset: 0, tcp.data_offset: 0
        Opaque: tcp.data_len: 0, tcp.flags: 0x0
        Opaque: sctp.connection_index: 0, sctp.sid: 0, sctp.ssn: 0, sctp.tsn: 0, sctp.hdr_offset: 0
        Opaque: sctp.data_offset: 0, sctp.data_len: 0, sctp.subconn_idx: 0, sctp.flags: 0x0
        Opaque: snat.flags: 0x0
        Opaque:
    VPP Buffer Opaque2
        Opaque2: raw: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
        Opaque2: qos.bits: 0, qos.source: 0
        Opaque2: loop_counter: 0
        Opaque2: gbp.flags: 0, gbp.src_epg: 0
        Opaque2: pg_replay_timestamp: 0
        Opaque2:
    Ethernet II, Src: 06:d6:01:41:3b:92 (06:d6:01:41:3b:92), Dst: IntelCor_3d:f6    Transmission Control Protocol, Src Port: 22432, Dst Port: 54084, Seq: 1, Ack: 1, Len: 36
        Source Port: 22432
        Destination Port: 54084
        TCP payload (36 bytes)
    Data (36 bytes)

    0000  cf aa 8b f5 53 14 d4 c7 29 75 3e 56 63 93 9d 11   ....S...)u>Vc...
    0010  e5 f2 92 27 86 56 4c 21 ce c5 23 46 d7 eb ec 0d   ...'.VL!..#F....
    0020  a8 98 36 5a                                       ..6Z
        Data: cfaa8bf55314d4c729753e5663939d11e5f2922786564c21…
        [Length: 36]
```

It's a matter of a couple of mouse-clicks in Wireshark to filter the
trace to a specific buffer index. With that specific kind of filtration,
one can watch a packet walk through the forwarding graph; noting any/all
metadata changes, header checksum changes, and so forth.

This should be of significant value when developing new vpp graph
nodes. If new code mispositions b->current_data, it will be completely
obvious from looking at the dispatch trace in wireshark.