Debugging Peer Connections In Polkadot SDK

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Debugging Peer Connections in Polkadot SDK

Hey guys! Let's dive into a common headache when working with the Polkadot SDK: those pesky PeerConnected and PeerDisconnected events. Specifically, we'll talk about how to make debugging these events a whole lot easier. This is super important because, as we'll see, these events are key to understanding why collators might fail to do their job – like, say, advertising collations to validators. This is crucial for the network's health.

The Problem: Collator Failures and Network Instability

One of the main reasons collators might fall short is their inability to successfully negotiate a substream on the /collation/2 protocol. This protocol is vital for collators and validators to communicate. When things go wrong, the collator-validator relationship suffers, leading to potential network instability. This manifests as a flurry of PeerConnected followed by immediate PeerDisconnected messages. You'll often see this pattern repeating at regular intervals, like every five seconds. It's like a rapid-fire connection attempt, followed by an immediate rejection.

Let's look at an example:

2025-11-28 13:33:40.870 DEBUG tokio-runtime-worker parachain::network-bridge-rx: [Parachain] action="PeerConnected" peer_set=Collation version=2 peer=PeerId("12D3KooWFUuGagDZ5AkSHHpphVxnEn2fHNhYDvoVdx4ZmDNq88kH") role=Authority
2025-11-28 13:33:40.949 DEBUG tokio-runtime-worker parachain::network-bridge-rx: [Parachain] action="PeerDisconnected" peer_set=Collation peer=PeerId("12D3KooWFUuGagDZ5AkSHHpphVxnEn2fHNhYDvoVdx4ZmDNq88kH")
2025-11-28 13:33:46.399 DEBUG tokio-runtime-worker parachain::network-bridge-rx: [Parachain] action="PeerConnected" peer_set=Collation version=2 peer=PeerId("12D3KooWFUuGagDZ5AkSHHpphVxnEn2fHNhYDvoVdx4ZmDNq88kH") role=Authority
2025-11-28 13:33:46.478 DEBUG tokio-runtime-worker parachain::network-bridge-rx: [Parachain] action="PeerDisconnected" peer_set=Collation peer=PeerId("12D3KooWFUuGagDZ5AkSHHpphVxnEn2fHNhYDvoVdx4ZmDNq88kH")
2025-11-28 13:33:52.154 DEBUG tokio-runtime-worker parachain::network-bridge-rx: [Parachain] action="PeerConnected" peer_set=Collation version=2 peer=PeerId("12D3KooWFUuGagDZ5AkSHHpphVxnEn2fHNhYDvoVdx4ZmDNq88kH") role=Authority
2025-11-28 13:33:52.232 DEBUG tokio-runtime-worker parachain::network-bridge-rx: [Parachain] action="PeerDisconnected" peer_set=Collation peer=PeerId("12D3KooWFUuGagDZ5AkSHHpphVdx4ZmDNq88kH")

See that? Connect, disconnect, connect, disconnect. This cycle suggests an issue with establishing a stable connection for the collation protocol.

Deep Dive: Substreams and the /collation/2 Protocol

Digging a bit deeper, it becomes clear that the peerset isn't receiving inbound substreams on the /collation/2 protocol. This means that the two-way communication required for the protocol isn't happening. Specifically, it can't negotiate the two substreams, which are essentially the channels of communication needed for the collation protocol to function correctly. Without these substreams, the collator and validator can't exchange the necessary information.

Here's what that looks like in the logs:

2025-11-28 13:33:40.870 TRACE tokio-runtime-worker sub-libp2p::peerset: [Parachain] /77afd6190f1554ad45fd0d31aee62aacc33c6db0ea801129acb813f913e0764f/collation/2: substream opened to PeerId("12D3KooWFUuGagDZ5AkSHHpphVxnEn2fHNhYDvoVdx4ZmDNq88kH"), direction Outbound, reserved peer true
2025-11-28 13:33:46.399 TRACE tokio-runtime-worker sub-libp2p::peerset: [Parachain] /77afd6190f1554ad45fd0d31aee62aacc33c6db0ea801129acb813f913e0764f/collation/2: substream opened to PeerId("12D3KooWFUuGagDZ5AkSHHpphVxnEn2fHNhYDvoVdx4ZmDNq88kH"), direction Outbound, reserved peer true
2025-11-28 13:33:52.154 TRACE tokio-runtime-worker sub-libp2p::peerset: [Parachain] /77afd6190f1554ad45fd0d31aee62aacc33c6db0ea801129acb813f913e0764f/collation/2: substream opened to PeerId("12D3KooWFUuGagDZ5AkSHHpphVxnEn2fHNhYDvoVdx4ZmDNq88kH"), direction Outbound, reserved peer true

Notice that the logs only show outbound substreams being opened. There's no corresponding inbound substream, which means the connection isn't fully established, thus the repeated connection attempts.

Why Are Substreams Failing? Common Causes

So, what's causing these substream failures? There are a few likely culprits:

  • The Collator is Banned: The validator might have banned the collator from connecting. This could be due to various reasons, such as misbehavior or network issues. When a collator is banned, the validator won't accept its connection attempts, leading to the rapid connect-disconnect cycle.
  • Validator Overload: The validator might be at its connection limit. If the validator is already handling a maximum number of peers, it won't be able to accept any new connections, including those from the collator. Think of it like a busy phone line.
  • Race Conditions: There could be a timing issue where both peers try to open substreams simultaneously. This can lead to a conflict, and one or both might fail to establish a connection. It's like two people trying to use the same door at the same time.

Let's look at some code snippets that support the above explanation:

  • Banned Collator: The code in substrate/client/network/src/litep2p/shim/notification/peerset.rs (lines 611-618 in the provided link) shows how a collator can be rejected. This code checks for the validity of the connection, and if it's not valid, the connection is rejected.
  • Validator Overload: In the same file, lines 739-746, the code checks if the validator has available protocol slots. If all slots are occupied, the validator won't accept any new connections, explaining why collators can't connect.
  • Race Condition: Lines 674-684 in the same file demonstrate how a race condition can occur when peers try to open substreams at the same time. The code handles the logic for opening the substreams, and if there's a conflict, the connection might fail.

The Debugging Bottleneck: Lack of Information

The real challenge is that without detailed TRACE logs from the validator, it's almost impossible to nail down the exact reason for the failure. Validators often don't run with these trace levels enabled for peersets or litep2p, which means we miss valuable information about why substreams are being dropped. It's like trying to solve a puzzle with half the pieces missing!

A Proposed Solution: Enhanced Error Reporting

To make this debugging process easier, we need better error reporting. The current system doesn't provide enough information on why a substream is being rejected. This is where we can make some improvements. The notification protocol can be enhanced to provide more context when a connection is rejected.

Here's the suggestion:

  • Extend ValidationResult::Reject: The ValidationResult::Reject should be enhanced to include the reason for the rejection. This will provide valuable context on why the connection was refused.
  • Propagate the Reason: The rejection reason should be propagated through litep2p and encoded as part of the substream handshake. This ensures that the information is transmitted over the network.
  • Decode and Drop: When the collator receives the handshake, it will first attempt to decode its role. If it fails, then it can decode the reason for the rejection. This would allow the collator to receive a specific error message and drop the substream appropriately.

This approach will provide collators with immediate feedback on the reasons for connection failures. The collator can then use this information to diagnose and resolve any issues. This will also help in identifying and fixing underlying network issues that might be affecting connections.

By providing this additional context, we can significantly improve the debuggability of PeerConnected and PeerDisconnected events, leading to a more robust and reliable Polkadot network. With more information, we can more effectively identify and fix issues that cause collator failures. This in turn will lead to better network performance and improve the overall user experience. This also applies to other roles, as other roles such as full nodes and light clients can benefit from this enhancement, improving the robustness of the entire network.

This would vastly improve the debugging experience and make it easier to pinpoint the root cause of these connection issues. It's all about making the network more transparent and easier to manage!

Conclusion: Making the Network More Transparent

Improving the debuggability of PeerConnected and PeerDisconnected events is a key step in building a more resilient and efficient Polkadot network. By adding more detailed error reporting, we can empower developers and operators to quickly diagnose and resolve connection issues. This will ultimately contribute to a more stable and user-friendly experience for everyone involved in the Polkadot ecosystem.

By implementing the proposed enhancements, we are essentially adding more transparency. This allows us to gain a better understanding of what's happening under the hood. More information is always better when it comes to debugging, and this approach provides just that. Ultimately, this leads to a better experience for collators, validators, and everyone else who relies on the Polkadot network.