nsurlsessiond

Something large is transferring: a macOS or app update, an iCloud Photos upload, a podcast back catalog, or an app syncing its data. The traffic belongs to that app, and nsurlsessiond is carrying it.

To see where it is connecting, run nettop -p nsurlsessiond in Terminal. The remote addresses usually make the source obvious: Apple's content servers for updates and iCloud, or a third-party app's own domain.

Stop the transfer at its source: pause the App Store download, check System Settings → General → Software Update, pause iCloud Photos in the Photos settings, or turn off automatic downloads in the app that asked for them.

Quitting nsurlsessiond in Activity Monitor does not help for long. macOS starts it again and the transfers resume, because the apps still want them.

There are two nsurlsessiond processes, and which one is busy tells you where to look.

1. Open Activity Monitor, choose View → All Processes, click the Network tab and type nsurl in the search field.
2. Look at the User column. The instance under the _nsurlsessiond account carries downloads for macOS itself, such as software updates. The one under your name carries downloads for your apps and iCloud.
3. Watch the Rcvd Bytes and Sent Bytes columns. Rcvd Bytes growing means a download, and Sent Bytes growing means an upload, such as photos going to iCloud.
4. If the system instance is the busy one, open Terminal and run softwareupdate --list to see whether a macOS update is on offer. You can also check System Settings → General → Software Update.
5. If your own instance is busy, think about which apps download in the background: the App Store, Music, Podcasts, Photos and cloud storage apps are the usual ones. Pause them one at a time and see whether the traffic stops.

Normal: heavy traffic for a while, then nothing. An update or an upload has a size, and once it has moved, nsurlsessiond goes quiet.

Worth looking into: it moves data for days, or the same amount of data again and again. The usual causes:

• A download that fails near the end and starts over, often a large update on an unstable connection.
• iCloud Photos uploading a large library for the first time, which can take days on a slow upload link.
• Podcasts set to download every episode of many shows.
• An app that syncs a large folder in the background and never quite catches up.
• A slow network, which turns a short download into one that runs all day.

On an iPhone hotspot or any network with a data cap, turn on Low Data Mode for that network: open System Settings → Wi-Fi, click Details next to the network, and turn on Low Data Mode. Apple describes it as a way to reduce your data use over that network. Features that respect it hold back their automatic transfers there, iCloud Photos syncing among them.

The setting belongs to that one network, so your home Wi-Fi keeps downloading as usual.