Create sdk-vpp chain element for creating loopback interface attached to the fib table

[denis-tingaikin]

Depends on

#341

Context

networkservicemesh/cmd-nse-vl3-vpp#1

Description

We need to

1. create a loopback interface attached to the fib table
2. assign its IP address to the loopback interface.
3. program a route in the fib table to the SrcIP for the NSC connecting and point out the interface.

[glazychev-art]

I would like to show my understanding of vl3 and loopback interface.

Chain (modification of this):

...
point2pointipam.NewServer(ipnet),
iptable.NewServer(),   // - #1
loopback.NewServer(),  // - #2
mechanisms.NewServer(map[string]networkservice.NetworkServiceServer{
	memif.MECHANISM: chain.NewNetworkServiceServer(
		sendfd.NewServer(),
		up.NewServer(ctx, vppConn),
		mtu.NewServer(vppConn),        //
		routes.NewServer(vppConn),     //
		unnumbered.NewServer(vppConn), // - #3
		tag.NewServer(ctx, vppConn),
		memif.NewServer(vppConn),
	),
}),
...

1. iptable
Creates an ip-table (if doesn't exist) for a given NetworkService.
Does:
-Stores tableId in metadata.
-Checks - if NetworkService usage counter is 0 - deletes ip-table

2. loopback
Creates an loopback interface (if doesn't exist) for a given tableID (from iptable metadata).
Does:
-Stores loopback swIfIndex in metadata.
-Adds itself to ip-table (from iptable metadata).
-Assigns IP-address to itself (from connection.GetContext().GetIpContext())
-Checks - if ip-table usage counter is 0 - deletes loopback interface.

3. unnumbered
New chain element. Sets unnumbered interface.
These elements:

		mtu.NewServer(vppConn),       
		routes.NewServer(vppConn),  
		ipaddress.NewServer(vppConn),

constitute connectioncontext chain element.
I suggest to replace ipaddress -> unnumbered in vl3 endpoint.
Does:
Sets unnumbered interface (gets loopback swIfIndex and ifindex from metadata).

Addition
I think, it would be better to use ipaddress chain in step 2 to set ip-address for the loopback. But we need to change chain element for that, because it uses ifindex.Load(...) inside. For example, as workaround, add flag like this:

...
ipaddress.NewServer(..., isUsedForLoopback)
loopback.NewServer(), 
...

[edwarnicke]

1. Could we rename 'iptable' to 'vrf' so as not to confuse it with the linux iptables. Other than naming you are completely right :)
2. Your loopback sounds right :)
3. unnumbered is also good :)

One 'Additions' I'd suggest simply giving ipaddress.NewServer the Options pattern with an Option like:

func WithLoadSwIfIndex(func(ctx context.Context, isClient bool) (value interface_types.InterfaceIndex, ok bool)) Option {...}

Default to using the ifindex.Load function ... but then you can optionall override it with another Load.

One thing to keep in mind though... do we want to keep resetting the IP on the loopback with each new connection (even if its the same value)?

[edwarnicke]

Also... how are you getting ipam for the dst IP for the loopback since you only need it once?

[glazychev-art]

I don't quite understand. Why do we need to reset IP address with new connection?

[glazychev-art]

#364 (comment)
That's a good question