feat: Add TTL connection param to multicast

[jmonticelli]

Hi, I am using Zenoh for a project and I use a multicast listen endpoint in peer mode.

Using a multicast endpoint for multicast communication works fine on flat networks, but once a router is involved, the multicast traffic is lost, due to the default TTL of 1 in multicast connections in the Tokio library.

See also: https://docs.rs/tokio/latest/tokio/net/struct.UdpSocket.html#method.set_multicast_ttl_v4

Note I have minimal rust programming experience, and used online resources and the code surrounding this addition to help me craft this PR, so feel free to make changes if I did something suboptimally or wrong.

I was able to build zenoh and run the zenoh pub client like so:

./target/release/examples/z_pub -m peer -l 'udp/239.1.3.37:9001#iface=myinterface;ttl=32' -k 'testkey' -p 'Hello from beyond the router' --no-multicast-scouting

I confirmed that the outgoing multicast TTL is set to what I specify it to, and that the traffic is no longer dropped on the other end of the network.

This is a critical feature when handling multicast traffic that is routed.

P.S., very nice library :)

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PR missing one of the required labels: {'breaking-change', 'enhancement', 'new feature', 'documentation', 'dependencies', 'bug', 'internal'}

[github-actions]

PR missing one of the required labels: {'enhancement', 'documentation', 'internal', 'breaking-change', 'new feature', 'bug', 'dependencies'}

[github-actions]

PR missing one of the required labels: {'bug', 'breaking-change', 'documentation', 'dependencies', 'internal', 'new feature', 'enhancement'}

[github-actions]

PR missing one of the required labels: {'enhancement', 'breaking-change', 'internal', 'new feature', 'bug', 'dependencies', 'documentation'}

[codecov]

Codecov Report

Attention: Patch coverage is 18.18182% with 9 lines in your changes missing coverage. Please review.

Project coverage is 70.84%. Comparing base (a89b501) to head (fa3aa05).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
io/zenoh-links/zenoh-link-udp/src/multicast.rs	18.18%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1842      +/-   ##
==========================================
- Coverage   70.99%   70.84%   -0.16%     
==========================================
  Files         360      360              
  Lines       65175    65186      +11     
==========================================
- Hits        46272    46180      -92     
- Misses      18903    19006     +103     

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[Mallets]

IPv6 hop limit is not yet supported in the Rust library.
I've opened rust-lang/rust#138744 to address the issue.

In the meanwhile, I suggest to simply log a warning in the case of an IPv6 socket.

        // If TTL is specified, add set the socket's TTL
        if let Some(ttl_str) = config.get(UDP_MULTICAST_TTL) {
            match &local_addr {
                IpAddr::V4(_) => {
                    let ttl = match ttl_str.parse::<u32>() {
                        Ok(ttl) => ttl,
                        Err(e) => bail!("Can not parse TTL '{}' to a u32: {}", ttl_str, e),
                    };

                    ucast_sock.set_multicast_ttl_v4(ttl).map_err(|e| {
                        zerror!("Can not set multicast TTL {} on {}: {}", ttl, mcast_addr, e)
                    })?;
                }
                IpAddr::V6(_) => {
                    tracing::warn!(
                            "UDP multicast hop limit not supported for v6 socket: {}. See https://github.com/rust-lang/rust/pull/138744.",
                            mcast_addr
                        );
                }
            }
        }

[jmonticelli]

Wasn't sure how tokio handled this under the hood, I just saw is said it "may not have any effect" and figured it was a shot in the dark to attempt to set the TTL.

Sounds great, I will make this change. Thanks!

[jmonticelli]

I have taken your changes, feel free to re-review that they were taken correctly.
It passed a local build for me.

[Mallets]

All good, thanks!