Hi Dharmaray,
The short answer is that ip4-lookup performs the lookup in a FIB table of the
packet’s destination address, the result of which is an adjacency. The
ip4-rewrite node applies the ‘L2’ encap of an adjacency and transmits the
packet on the output port*. So typically packets will go first to ip4-lookup,
get the adjacency, then proceed to ip4-rewrite.
When choosing between which of the two nodes to send your packet to, you should
consider which of the pre-conditions for the required data you meet. In order
to perform the IP lookup, the node needs to know which FIB table to use. In
order to perform the rewrite, the node needs to know which adjacency to use.
So if you want to send your packet to the ip4-rewrite node, then you must have
already chosen which adjacency to use. For single-hop BFD this is a simple
matter of getting the adjacency from the adjacency-DB based on the peer and
interface – you can think of this as caching the result of the lookup. The
advantage of this is that we bypass the ip4-lookup node and so don’t pay its
performance cost.
If you know the next-hop to send to, but not the interface (and so can’t use
the adjacency-DB directly) then you can register with the FIB to track that
next-hop and be informed of the changing adjacency to use.
Hth,
neale
*typically. The exception being tunnels.
-----Original Message-----
From: <vpp-dev-boun...@lists.fd.io> on behalf of Dharmaray Kundargi
<dharmaray.kunda...@mavenir.com>
Date: Wednesday, 21 June 2017 at 10:48
To: "Klement Sekera -X (ksekera - PANTHEON TECHNOLOGIES at Cisco)"
<ksek...@cisco.com>, Alessio Silvestro <ale.silver...@gmail.com>
Cc: "vpp-dev@lists.fd.io" <vpp-dev@lists.fd.io>
Subject: Re: [vpp-dev] VPP: Answer UDP Packets
BFD UDP code is interesting example.
I was looking at bfd_udp_input rather and one question in relation to this.
Though both ' bfd_udp_input ()' and ' bfd_send_periodic()' form the IP and
UDP headers using ' bfd_add_transport_layer', they pass the packet to different
next nodes.
bfd_udp_input uses "ip4-lookup" as the next node whereas '
bfd_send_periodic()' uses "ip4-rewrite".
So what's the difference between these two nodes? (ip4-rewrite vs ip4-
lookup) and when to use what ?
Regards
Dharmaray
-----Original Message-----
From: vpp-dev-boun...@lists.fd.io [mailto:vpp-dev-boun...@lists.fd.io] On
Behalf Of Klement Sekera -X (ksekera - PANTHEON TECHNOLOGIES at Cisco)
Sent: Monday, June 19, 2017 12:09 PM
To: Alessio Silvestro <ale.silver...@gmail.com>
Cc: vpp-dev@lists.fd.io
Subject: Re: [vpp-dev] VPP: Answer UDP Packets
Couple of potential issues which I see:
1.) if this buffer is generated by vpp, it should be flagged as such
(b->flags |= VNET_BUFFER_LOCALLY_ORIGINATED;)
2.) I don't see it passed to another node - you need to create a frame and
associate the buffer with it, then pass the frame to the next node, which would
be either ip4-arp or ip4-rewrite for ipv4 case depending on whether the arp
resolution has taken place or not...
take a look at functions
bfd_udp_init() - where the next nodes are resolved
bfd_transport_udp4() - where the next node is decided and frame created
You can take a look at show error and show trace to see what happens to the
packet..
HTH,
Klement
Quoting Alessio Silvestro (2017-06-16 17:20:30)
> Dear Klement,
>
> thanks for the hint. I had a look at the code bfd_main.c and in
particular
> at the function bfd_send_periodic().
>
> I wrote the function send_udp_packet() -- see below -- in order to
send
> out UDP packets.
>
> The function is executed (I can see from the VPP terminal) and no
> exception is raised, however I do not see any packet out.
>
> 1 -- is the function correct or I am missing something?
>
> 2 -- if it is not correct, how I can see where the problem is?
>
> Thanks,
>
> Alessio
>
> static void send_udp_packet(vlib_main_t * vm, vlib_node_runtime_t *
rt){
> u32 bi;
> vlib_buffer_alloc (vm, &bi, 1)
> vlib_buffer_t *b = vlib_get_buffer (vm, bi);
> ASSERT (b->current_data == 0);
> memset (vnet_buffer (b), 0, sizeof (*vnet_buffer (b)));
> VLIB_BUFFER_TRACE_TRAJECTORY_INIT (b);
> add_udp4_transport (vm,b);
> }
>
> int add_udp4_transport (vlib_main_t * vm, vlib_buffer_t * b)
> {
> vnet_buffer (b)->ip.adj_index[VLIB_RX] = ~0;
> vnet_buffer (b)->ip.adj_index[VLIB_TX]= ~0;
> vnet_buffer (b)->sw_if_index[VLIB_RX] = 0;
> vnet_buffer (b)->sw_if_index[VLIB_TX] = ~0;
> u8 *data;
> udp_header_t *udp;
> ip4_header_t *ip;
> data = vlib_buffer_get_current (b);
> udp = (udp_header_t *) (data - sizeof (udp_header_t));
> ip = (ip4_header_t *) ((u8 *) udp - sizeof (ip4_header_t));
> // Build packet header
> u32 src_address = 167772161; //[1]10.0.0.1 u32 version
> u32 dst_address = 167772162; // 10.0.0.2 u32 version
> ip->src_address.as_u32 = src_address;
> ip->dst_address.as_u32 = dst_address;
> ip->ip_version_and_header_length = 0x45;
> ip->ttl = 254;
> ip->protocol = IP_PROTOCOL_UDP; //0x45 == IP_PROTOCOL_UDP
> ip->length = clib_host_to_net_u16 (b->current_length + sizeof
(*udp));
> ip->checksum = ip4_header_checksum (ip);
> udp->src_port = DNS_SERVER_PORT;
> udp->dst_port = DNS_DST_PORT;
> udp->length = clib_host_to_net_u16 (b->current_length);
> udp->checksum = 0;
> b->current_length = sizeof (*ip) + sizeof (*udp);
> return 1;
> }
>
> On Wed, Jun 14, 2017 at 6:56 PM, Klement Sekera -X (ksekera - PANTHEON
> TECHNOLOGIES at Cisco) <[2]ksek...@cisco.com> wrote:
>
> Hi Alessio,
>
> you can take a look at BFD code which
>
> a.) creates and sends its own UDP packets - bfd_main.c -
> bfd_send_periodic() creates, encapsulates (UDP) and sends a packet
out
> b.) loops back packets received - bfd_udp.c -
> bfd_udp_echo_input()
>
> I'm not sure what's your use case, whether you are trying to reuse
> existing buffer, but one of these should fit it.
>
> Regards,
> Klement
>
> Quoting Alessio Silvestro (2017-06-14 17:22:14)
> > Dear all,
> > I implemented a new VPP node that receives UDP traffic using the
> following
> > function:
> >
> > udp_register_dst_port (vm, PORT, my_node.index , 1 /* is_ip4
*/);
> >
> > I am able to parse the packet and I would like to be able to
send
> back an
> > UDP packet.
> >
> > Looking at the source code, the only function that seems fit my
> scope is
> > the following (in ~/vpp/src/vnet/udp/udp.h)
> >
> > ip_udp_encap_one (vlib_main_t * vm, vlib_buffer_t * b0, u8 *
ec0,
> word
> > ec_len,
> >
> > u8 is_ip4)
> >
> > Is that correct or there is another function for this purpose?
> >
> > Thanks in advance for any help.
> >
> > Best regards,
> >
> > Alessio
>
> References
>
> Visible links
> 1. http://10.0.0.1/
> 2. mailto:ksek...@cisco.com
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