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:

   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.

  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 */

  /* 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);
       /* 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) +
       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));


  /* 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(…).

    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:

    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:

    typedef enum
      } 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
    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, 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
    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
    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.