Asynchronous communication is communication between two endpoints that doesn’t have to happen in real time or near-real-time.
Compared to synchronous communication, asynchronous communication lets you trade latency for reliability.
Asynchronous communication is closely related to the concept of store-and-forward networking and delay/disruption-tolerant networking. In fact, most asynchronous implementations are also store-and-forward and delay-tolerant, so these terms are often used interchangably.
Let’s examine the difference with synchronous communication, which we are probably more familiar with:
With synchronous communication – such as web sites – the web browser and the web server must be in live communication. It make take 20 hops across Internet routers to reach the other end, but those routers don’t take responsibility for the onward delivery of packets; it’s up to the two endpoints to detect if packets were dropped. If the delay from one end to the other exceeds even one second, things can start to get difficult. An interruption at any point along the path from source to destination will cause the communication to fail entirely.
Asynchronous communication, by contrast, only insists that a packet be able to advance to the next hop in real time. The next hop could be anything from another node to a USB drive. Once the packet is delivered to the next hop, the next hop takes responsibility to advance it onward to its destination. This lets the overall communication path be more reliable, because an interruption at any point simply causes traffic to be delayed, not lost. The tradeoff is that it has higher latency than synchronous communication.
Examples of asynchronous communication
Email – or more particularly, the way SMTP is used – is a widely-known system. With email, SMTP is the hop-to-hop protocol. Each mail server is only responsible for delivering the message to the next hop. Once delivered to the next hop, that server takes responsibility to advance it to its destination.
NNCP and the older UUCP are ground-up asynchronous protocols. NNCP in particular is a modern asynchronous system, supporting a variety of transports from the Internet to USB drives, and using encryption and onion routing.
Syncthing isn’t usually thought of as asynchronous, but its syncing protocol certainly can and does operate in that way.
Links to this note
Email is one of the most common examples of Asynchronous Communication people are familiar with today. It is a store-and-forward approach which is tolerant of temporary disruptions to services and supports multiple hops.
UUCP is a system for exchanging data and requesting remote execution. It dates back to 1979, and was primarily used over modems using telephone landlines for most of its days of popularity. It is an Asynchronous Communication system, which transmits data from one machine to the next on the way to its destination. Each intermediate node may store the data before passing it on to the next.
Here are some (potentially) interesting topics you can find here:
When things are difficult – maybe there’s been a disaster, or an invasion (this page is being written in 2022 just after Russia invaded Ukraine), or maybe you’re just backpacking off the grid – there are tools that can help you keep in touch, or move your data around. This page aims to survey some of them, roughly in order from easiest to more complex.
This page describes some basic concepts of NNCP.
According to the NNCP documentation, NNCP is intended to help build up small size ad-hoc friend-to-friend (F2F) statically routed darknet delay-tolerant networks for fire-and-forget secure reliable files, file requests, Internet Email and commands transmission. All packets are integrity checked, end-to-end Encrypted, explicitly authenticated by known participants public keys. Onion encryption is applied to relayed packets. Each node acts both as a client and server, can use push and poll behaviour model. Also there is multicasting area support.
NNCP lets you securely send files, or request remote execution, between systems. It uses asynchronous communication, so the source and destination need never be online simultaneously. NNCP can route requests via intermediate devices – other NNCP nodes, USB sticks, tapes, radios, phones, cloud services, whatever – leading to a network that is highly resilient and flexible. NNCP makes it much easier to communicate with devices that lack Internet connectivity, or have poor Internet.
I sometimes see people read about NNCP and wonder “This sounds great! But… what can I do with it?” This page aims to answer those questions.