add links messages

Introduce two new wire messages, LINKS and LINKS_REPLY (swarm::links).
A member answers with its links: for each peer, the transport, whether
it’s relayed, the RTT, and whether it’s dormant.  It answers members
only, at most every 5 s.  This bumps wire::VERSION to 7.  Every build
answers, with or without the feature, so every node needs this patch
before the map shows anything beyond its own node.

3 files changed, 86 insertions, 8 deletions

diff --git a/src/transport/mod.rs b/src/transport/mod.rs
index fd3651b..a15a1f7 100644
--- a/src/transport/mod.rs
+++ b/src/transport/mod.rs
@@ -218,6 +218,11 @@ pub(crate) enum ParsedMessage {
     Ping(u64),
     /// Answers a PING.
     Pong(u64),
+    /// Asks a member how it is linked to its peers, for a map of the swarm
+    /// (`swarm::links`).
+    Links(LinksRequest),
+    /// Answers a LINKS.
+    LinksReply(LinksReply),
 }
 
 /// A node's proof that it holds the key of its node ID (`identity`), sending from its
@@ -343,6 +348,8 @@ pub(crate) enum MessageKind {
     Me,
     Ping,
     Pong,
+    Links,
+    LinksReply,
 }
 
 impl ParsedMessage {
@@ -358,6 +365,8 @@ impl ParsedMessage {
             ParsedMessage::Me(_) => MessageKind::Me,
             ParsedMessage::Ping(_) => MessageKind::Ping,
             ParsedMessage::Pong(_) => MessageKind::Pong,
+            ParsedMessage::Links(_) => MessageKind::Links,
+            ParsedMessage::LinksReply(_) => MessageKind::LinksReply,
         }
     }
 
@@ -372,6 +381,8 @@ impl ParsedMessage {
                 Some((status.epoch, status.epoch_seq))
             }
             ParsedMessage::Blob(blob) => Some((blob.epoch, blob.epoch_seq)),
+            ParsedMessage::Links(request) => Some((request.epoch, request.epoch_seq)),
+            ParsedMessage::LinksReply(reply) => Some((reply.epoch, reply.epoch_seq)),
             ParsedMessage::Who(_)
             | ParsedMessage::Me(_)
             | ParsedMessage::Ping(_)
@@ -380,6 +391,45 @@ impl ParsedMessage {
     }
 }
 
+/// LINKS: what a member is asked for a map of the swarm (`swarm::links`).
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub(crate) struct LinksRequest {
+    #[serde(with = "crate::transport::wire::codec::epoch")]
+    pub(crate) epoch: Epoch,
+    /// See `SyncEnvelope::epoch_seq`.
+    pub(crate) epoch_seq: u32,
+    /// Repeated in the reply: which request it answers.
+    pub(crate) nonce: u64,
+}
+
+/// LINKS_REPLY: how the sender is linked to its neighbours, as it measures them.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub(crate) struct LinksReply {
+    #[serde(with = "crate::transport::wire::codec::epoch")]
+    pub(crate) epoch: Epoch,
+    /// See `SyncEnvelope::epoch_seq`.
+    pub(crate) epoch_seq: u32,
+    /// The request's.
+    pub(crate) nonce: u64,
+    /// At most `swarm::links::MAX_LINKS`.
+    pub(crate) links: Vec<Link>,
+}
+
+/// One of a member's links, in a LINKS_REPLY.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub(crate) struct Link {
+    /// The peer's node ID.
+    pub(crate) peer: String,
+    /// The transport its messages go over now (the scheme of the path in use), if any.
+    pub(crate) transport: Option<String>,
+    /// Only through a relay (libp2p).
+    pub(crate) relayed: bool,
+    /// The round trip on that path, in microseconds, if measured.
+    pub(crate) rtt_us: Option<u32>,
+    /// Silent for long (`peers.md`, dormancy).
+    pub(crate) dormant: bool,
+}
+
 #[derive(Clone, Serialize, Deserialize)]
 pub(crate) struct HelloRequest {
     /// The sender's swarm, if it knows it (a new node doesn't yet). A receiver in
diff --git a/src/transport/traffic.rs b/src/transport/traffic.rs
index b237db2..f8ca586 100644
--- a/src/transport/traffic.rs
+++ b/src/transport/traffic.rs
@@ -45,6 +45,8 @@ pub(crate) struct ByKind {
     pub(crate) identity: Counter,
     /// PING and PONG (`transport::paths`), counted by the paths layer under the address.
     pub(crate) paths: Counter,
+    /// LINKS and LINKS_REPLY (`swarm::links`).
+    pub(crate) links: Counter,
 }
 
 impl ByKind {
@@ -58,6 +60,7 @@ impl ByKind {
             MessageKind::Blob => &mut self.blob,
             MessageKind::Who | MessageKind::Me => &mut self.identity,
             MessageKind::Ping | MessageKind::Pong => &mut self.paths,
+            MessageKind::Links | MessageKind::LinksReply => &mut self.links,
         }
     }
 }
diff --git a/src/transport/wire.rs b/src/transport/wire.rs
index 53fd460..3e1ab10 100644
--- a/src/transport/wire.rs
+++ b/src/transport/wire.rs
@@ -14,7 +14,7 @@ use super::{MessageKind, ParsedMessage};
 
 /// The wire format's version, the first byte of every payload. Bump it with every change
 /// to a message type or a codec (`golden_bytes` fails then).
-pub(crate) const VERSION: u8 = 6;
+pub(crate) const VERSION: u8 = 7;
 
 /// What the second byte of a payload says.
 fn tag(kind: MessageKind) -> u8 {
@@ -29,6 +29,8 @@ fn tag(kind: MessageKind) -> u8 {
         MessageKind::Me => 7,
         MessageKind::Ping => 8,
         MessageKind::Pong => 9,
+        MessageKind::Links => 10,
+        MessageKind::LinksReply => 11,
     }
 }
 
@@ -67,6 +69,8 @@ pub(crate) fn encode(message: &ParsedMessage) -> Result<Vec<u8>, WireError> {
         ParsedMessage::Ping(nonce) | ParsedMessage::Pong(nonce) => {
             rmp_serde::encode::write(&mut payload, nonce)?
         }
+        ParsedMessage::Links(request) => rmp_serde::encode::write(&mut payload, request)?,
+        ParsedMessage::LinksReply(reply) => rmp_serde::encode::write(&mut payload, reply)?,
     }
     Ok(payload)
 }
@@ -99,6 +103,10 @@ pub(crate) fn decode(payload: &[u8]) -> Result<ParsedMessage, WireError> {
         ParsedMessage::Ping(rmp_serde::from_slice(body)?)
     } else if is(MessageKind::Pong) {
         ParsedMessage::Pong(rmp_serde::from_slice(body)?)
+    } else if is(MessageKind::Links) {
+        ParsedMessage::Links(rmp_serde::from_slice(body)?)
+    } else if is(MessageKind::LinksReply) {
+        ParsedMessage::LinksReply(rmp_serde::from_slice(body)?)
     } else {
         return Err(WireError::UnknownKind(kind));
     })
@@ -396,6 +404,23 @@ pub(crate) fn samples() -> Vec<ParsedMessage> {
         ParsedMessage::Me(proof),
         ParsedMessage::Ping(7),
         ParsedMessage::Pong(u64::MAX),
+        ParsedMessage::Links(crate::transport::LinksRequest {
+            epoch,
+            epoch_seq: 1,
+            nonce: 3,
+        }),
+        ParsedMessage::LinksReply(crate::transport::LinksReply {
+            epoch,
+            epoch_seq: 1,
+            nonce: 3,
+            links: vec![crate::transport::Link {
+                peer: "p".into(),
+                transport: Some("libp2p".into()),
+                relayed: true,
+                rtt_us: Some(1500),
+                dormant: false,
+            }],
+        }),
     ]
 }
 
@@ -413,7 +438,7 @@ mod tests {
     #[test]
     fn every_kind_round_trips() {
         let samples = samples();
-        assert_eq!(samples.len(), 10, "a kind is missing from `samples`");
+        assert_eq!(samples.len(), 12, "a kind is missing from `samples`");
         for message in samples {
             let bytes = encode(&message).unwrap();
             let decoded = decode(&bytes).unwrap();
@@ -566,7 +591,7 @@ mod tests {
         });
         assert_eq!(
             hex(&encode(&hello).unwrap()),
-            format!("060097a5737761726d{epoch_hex}01c403010203c3{}07", run("04"))
+            format!("070097a5737761726d{epoch_hex}01c403010203c3{}07", run("04"))
         );
 
         let reply = ParsedMessage::HelloReply(HelloReply {
@@ -586,7 +611,7 @@ mod tests {
         assert_eq!(
             hex(&encode(&reply).unwrap()),
             format!(
-                "060199a5737761726d9192a17091a161{epoch_hex}01c400c2{}05{}",
+                "070199a5737761726d9192a17091a161{epoch_hex}01c400c2{}05{}",
                 run("04"),
                 run("06")
             )
@@ -601,7 +626,7 @@ mod tests {
         });
         assert_eq!(
             hex(&encode(&status).unwrap()),
-            format!("0603959290c400{epoch_hex}01c3c2")
+            format!("0703959290c400{epoch_hex}01c3c2")
         );
 
         let who = ParsedMessage::Who(IdentityProof {
@@ -610,7 +635,7 @@ mod tests {
         });
         assert_eq!(
             hex(&encode(&who).unwrap()),
-            format!("060692c420{}c440{}", "08".repeat(32), "09".repeat(64))
+            format!("070692c420{}c440{}", "08".repeat(32), "09".repeat(64))
         );
         let ParsedMessage::Me(proof) = decode(
             &encode(&ParsedMessage::Me(IdentityProof {
@@ -624,10 +649,10 @@ mod tests {
         };
         assert_eq!((proof.key, proof.signature), ([8; 32], [9; 64]));
 
-        assert_eq!(hex(&encode(&ParsedMessage::Ping(7)).unwrap()), "060807");
+        assert_eq!(hex(&encode(&ParsedMessage::Ping(7)).unwrap()), "070807");
         assert_eq!(
             hex(&encode(&ParsedMessage::Pong(u64::MAX)).unwrap()),
-            "0609cfffffffffffffffff"
+            "0709cfffffffffffffffff"
         );
         let ParsedMessage::Pong(nonce) = decode(&[VERSION, 9, 0x2a]).unwrap() else {
             panic!("not a PONG");