libp2p is a JavaScript Implementation of libp2p networking stack. Prior to 16.0.0, @libp2p/gossipsub defaultDecodeRpcLimits set maxIhaveMessageIDs and maxIwantMessageIDs to Infinity, allowing oversized IHAVE and IWANT control message arrays in message/decodeRpc.ts and gossipsub.ts to synchronously iterate roughly 180,000 message IDs per 4 MB frame and block the Node.js event loop. This issue is fixed in version 16.0.0.

Affected: npm: @libp2p/gossipsub

### Summary gossipsub processes IHAVE and IWANT control messages by iterating every received message ID synchronously before doing anything with the results. There is no cap on how many IDs a single frame may contain. The default LP frame limit is 4MB, which fits roughly 180,000 message IDs. Iterating that many IDs blocks the Node.js event loop for around 200ms per call.

The two variants have different severity. For IHAVE there is a per-peer per-heartbeat counter that limits each peer to one full iteration per heartbeat, so causing a total stall requires around 10 Sybil peers. For IWANT there is no equivalent counter at all, so a single peer continuously streaming 4MB frames can hold the event loop above 80% utilisation indefinitely.

### Details ### No decode-time cap on message ID count (`message/decodeRpc.ts:11-19`) ```typescript export const defaultDecodeRpcLimits: DecodeRPCLimits = { maxSubscriptions: Infinity, maxMessages: Infinity, maxIhaveMessageIDs: Infinity, maxIwantMessageIDs: Infinity, maxIdontwantMessageIDs: Infinity, maxControlMessages: Infinity, maxPeerInfos: Infinity } ```

These are the defaults unless the operator explicitly overrides `opts.decodeRpcLimits`. A `TODO` at `gossipsub.ts:857` already notes the gap: `// TODO: Check max gossip message size, before decodeRpc()`.

### IHAVE iterates all IDs before truncating (`gossipsub.ts:1311-1327`)

```typ…