Use fork digest in gossipsub topic strings

CLAUDE.md

@@ -265,7 +265,8 @@ actual_slot = finalized_slot + 1 + relative_index
 ### Protocols
 - **Transport**: QUIC over UDP (TLS 1.3)
 - **Gossipsub**: Blocks + Attestations (snappy raw compression)
-  - Topic: `/leanconsensus/{network}/{block|attestation}/ssz_snappy`
+  - Topic: `/leanconsensus/{fork_digest}/{block|aggregation|attestation_N}/ssz_snappy`
+  - `fork_digest` is a 4-byte hex string (no `0x` prefix); currently the dummy `12345678` agreed across clients
   - Mesh size: 8 (6-12 bounds), heartbeat: 700ms
 - **Req/Resp**: Status, BlocksByRoot (snappy frame compression + varint length)
 

crates/net/p2p/src/gossipsub/messages.rs

@@ -1,20 +1,40 @@
+/// Fork digest embedded in every gossipsub topic string, as lowercase hex
+/// without a `0x` prefix.
+///
+/// The [leanSpec](https://github.com/leanEthereum/leanSpec/pull/622)
+/// currently mandates a dummy value shared across all clients; this will
+/// eventually be derived from the fork version and genesis validators root.
+// TODO: derive dynamically once the spec defines fork identification.
+pub const FORK_DIGEST: &str = "12345678";
+
 /// Topic kind for block gossip
 pub const BLOCK_TOPIC_KIND: &str = "block";
 /// Topic kind prefix for per-committee attestation subnets.
 ///
-/// Full topic format: `/leanconsensus/{network}/attestation_{subnet_id}/ssz_snappy`
+/// Full topic format: `/leanconsensus/{FORK_DIGEST}/attestation_{subnet_id}/ssz_snappy`
 pub const ATTESTATION_SUBNET_TOPIC_PREFIX: &str = "attestation";
 /// Topic kind for aggregated attestation gossip.
 ///
-/// Full topic format: `/leanconsensus/{network}/aggregation/ssz_snappy`
+/// Full topic format: `/leanconsensus/{FORK_DIGEST}/aggregation/ssz_snappy`
 pub const AGGREGATION_TOPIC_KIND: &str = "aggregation";
 
-// TODO: make this configurable (e.g., via GenesisConfig or CLI)
-pub const NETWORK_NAME: &str = "devnet0";
+/// Build the block gossipsub topic.
+pub fn block_topic() -> libp2p::gossipsub::IdentTopic {
+    libp2p::gossipsub::IdentTopic::new(format!(
+        "/leanconsensus/{FORK_DIGEST}/{BLOCK_TOPIC_KIND}/ssz_snappy"
+    ))
+}
+
+/// Build the aggregated-attestation gossipsub topic.
+pub fn aggregation_topic() -> libp2p::gossipsub::IdentTopic {
+    libp2p::gossipsub::IdentTopic::new(format!(
+        "/leanconsensus/{FORK_DIGEST}/{AGGREGATION_TOPIC_KIND}/ssz_snappy"
+    ))
+}
 
-/// Build an attestation subnet topic for the given subnet ID.
+/// Build an attestation subnet gossipsub topic for the given subnet.
 pub fn attestation_subnet_topic(subnet_id: u64) -> libp2p::gossipsub::IdentTopic {
     libp2p::gossipsub::IdentTopic::new(format!(
-        "/leanconsensus/{NETWORK_NAME}/{ATTESTATION_SUBNET_TOPIC_PREFIX}_{subnet_id}/ssz_snappy"
+        "/leanconsensus/{FORK_DIGEST}/{ATTESTATION_SUBNET_TOPIC_PREFIX}_{subnet_id}/ssz_snappy"
     ))
 }

crates/net/p2p/src/gossipsub/mod.rs

@@ -6,6 +6,4 @@ pub use encoding::decompress_message;
 pub use handler::{
     handle_gossipsub_message, publish_aggregated_attestation, publish_attestation, publish_block,
 };
-pub use messages::{
-    AGGREGATION_TOPIC_KIND, BLOCK_TOPIC_KIND, NETWORK_NAME, attestation_subnet_topic,
-};
+pub use messages::{aggregation_topic, attestation_subnet_topic, block_topic};

crates/net/p2p/src/lib.rs

@@ -36,8 +36,8 @@ use tracing::{info, trace, warn};
 
 use crate::{
     gossipsub::{
-        AGGREGATION_TOPIC_KIND, BLOCK_TOPIC_KIND, NETWORK_NAME, attestation_subnet_topic,
-        publish_aggregated_attestation, publish_attestation, publish_block,
+        aggregation_topic, attestation_subnet_topic, block_topic, publish_aggregated_attestation,
+        publish_attestation, publish_block,
     },
     req_resp::{
         BLOCKS_BY_ROOT_PROTOCOL_V1, Codec, MAX_COMPRESSED_PAYLOAD_SIZE, Request,
@@ -188,18 +188,15 @@ pub fn build_swarm(
         .expect("failed to bind gossipsub listening address");
 
     // Subscribe to block topic (all nodes)
-    let block_topic_str = format!("/leanconsensus/{NETWORK_NAME}/{BLOCK_TOPIC_KIND}/ssz_snappy");
-    let block_topic = libp2p::gossipsub::IdentTopic::new(block_topic_str);
+    let block_topic = block_topic();
     swarm
         .behaviour_mut()
         .gossipsub
         .subscribe(&block_topic)
         .unwrap();
 
     // Subscribe to aggregation topic (all validators)
-    let aggregation_topic_str =
-        format!("/leanconsensus/{NETWORK_NAME}/{AGGREGATION_TOPIC_KIND}/ssz_snappy");
-    let aggregation_topic = libp2p::gossipsub::IdentTopic::new(aggregation_topic_str);
+    let aggregation_topic = aggregation_topic();
     swarm
         .behaviour_mut()
         .gossipsub