Can Websocket Messages Arrive Out-Of-Order

Can websocket messages arrive out-of-order?

Short answer: No.

Long answer:

WebSocket runs over TCP, so on that level @EJP 's answer applies. WebSocket can be "intercepted" by intermediaries (like WS proxies): those are allowed to reorder WebSocket control frames (i.e. WS pings/pongs), but not message frames when no WebSocket extension is in place. If there is a neogiated extension in place that in principle allows reordering, then an intermediary may only do so if it understands the extension and the reordering rules that apply.

If websocket messages arrive in order, why does 'sequence number' is needed?

The sequence number allows you to map your requests to responses even if the responses don't come in the order you make them.

HTTP and other relevant protocols support pipelining. Also there is no need for the request responses to be sent back to you in any specific order. Each one may be processed according to its individual cost or dispatched across a server farm and reassembled in an order that is not predetermined. Either way, if they are out of order you will need a key to map the response back to your request.

Will websocket messages always arrive entirely, at once?

Yes, each message is received as a whole. From the RFC 6455:

1.2. Protocol Overview

After a successful handshake, clients and servers transfer data back and forth in conceptual units referred to in this specification as "messages". On the wire, a message is composed of one or more frames. The WebSocket message does not necessarily correspond to a particular network layer framing, as a fragmented message may be coalesced or split by an intermediary.

6.2. Receiving Data

If the frame comprises an unfragmented message (Section 5.4), it is said that A WebSocket Message Has Been Received with type /type/ and data /data/. If the frame is part of a fragmented message, the "Application data" of the subsequent data frames is concatenated to form the /data/. When the last fragment is received as indicated by the FIN bit (frame-fin), it is said that A WebSocket Message Has Been Received with data /data/ (comprised of the concatenation of the "Application data" of the fragments) and type /type/ (noted from the first frame of the fragmented message). Subsequent data frames MUST be interpreted as belonging to a new WebSocket message.

https://www.rfc-editor.org/rfc/rfc6455

The confusion might come from the term 'socket' which is a low-level raw OS channel that yields data in chunks. However, WebSocket is a higher level protocol.

Can websocket messages get lost or not?

It can happen. TCP guarantees the order of packets, but it does not mean that all packets sent from a server reach a client even when an unrecoverable trouble happens in an underlying network. Imagine someone pulls out your LAN cable or switches off your WiFi access point at the worst timing while your application is communicating with your server. TCP does not overcome such a trouble.

To ensure that every WebSocket message sent from your server reaches your client, you have to implement some kind of SYN/ACK in the application layer.

Does Socket.io garantee that broadcasted events are received in order client side?

No, you have to do that at the application level if you need to do that. The internet doesn't guarantee that two different packets will take the same route, so timing can vary. Maybe add a timestamp to each message so you can sort by that timestamp to keep things in order.

---

Related Topics