What is Google Cloud Print?

Driverless printing

Most print systems require specific drivers for each printer. Finding drivers can be a chore, and one driver doesn't work for all operating systems. Google Cloud Print can be described concisely as "driverless printing".

• GCP printers describe themselves with JSON-encoded data structures.
• Clients submit print jobs with JSON-encoded data structures; payloads are PDF, Postscript or PWG Raster.

Printers and clients can communicate in two different ways, each with pros and cons.

Cloud

Printers that communicate via the GCP backend service require an Internet connection to function:

• The GCP cloud service manages printers, print jobs and sharing.
• Clients
  • must have explicit permission granted by the printer owner, in order to discover printers and submit jobs.
  • submit print jobs with an HTTPS request.
  • submit print jobs from anywhere that has an Internet connection.
• Printers
  • listen for new print jobs on an XMPP channel and download payloads via HTTPS.
  • must have reliable Internet access.
  • are not affected by access points in isolation mode because client and printer never communicate directly.

Three network connections are created by the printer:

server DNS name	port	protocol	function
accounts.google.com	443	HTTPS	authenticate with OAuth 2.0
www.google.com	443	HTTPS	register with, receive print jobs from GCP backend
talk.google.com	443	TLS-encapsulated XMPP	receive print job notifications

Clients create one network connection to https://www.google.com/cloudprint to discover printers and create print jobs.

Local

Local mode is a lot like AirPrint. Printers that communicate locally are great for homes and small companies:

• Clients
  • discover printers when they are on the same subnet (connect to an access point where the printer lives)
  • forget printers when they are not on the same subnet (disconnect from the access point)
  • print by communicating directly with the printer
• Printers
  • manage their own print queues, pushing back on clients as needed
  • announce their presence to all clients within the local subnet
  • have an HTTP server to provide detailed self-description, and to accept print jobs
  • requires that the network policy, or WiFi access point, not isolate the printer from the client. Some access points call this "isolation mode".
  • requires that the network policy allow multicast for mDNS discovery
  • most depend on the GCP backend to handle print job processing, like format conversion (PDF to PWG Raster) and content adjustments (margin adjustments, fit-to-page, collation, ordering). These printers may require Internet access, or may provide more features when an Internet connection is available.

Two network connections are created by the printer:

mode	port	function
mDNS + DNS-SD	5353, multicast	announce presence to the local subnet
HTTP	arbitrary port, listen	HTTP server provides detailed self-description, receives print jobs