<pre>
  BIP: 73
  Layer: Applications
  Title: Use "Accept" header for response type negotiation with Payment Request URLs
  Authors: Stephen Pair <stephen@bitpay.com>
  Status: Deployed
  Type: Specification
  Assigned: 2013-08-27
</pre>

==Abstract==

This BIP describes an enhancement to the payment protocol ([[bip-0070.mediawiki|BIP 70]])
that addresses the need for short URLs when scanning from QR codes.  It
generalizes the specification for the behavior of a payment request URL in a
way that allows the client and server to negotiate the content of the
response using the HTTP Accept: header field.  Specifically, the client
can indicate to the server whether it prefers to receive a bitcoin URI or
a payment request.

Implementation of this BIP does not require full payment request ([[bip-0070.mediawiki|BIP 70]]) support.

==Motivation==

The payment protocol augments the bitcoin: uri scheme with an additional
"payment" parameter that specifies a URL where a payment request can be
downloaded.  This creates long URIs that, when rendered as a QR code, have
a high information density.  Dense QR codes can be difficult to scan resulting
in a more frustrating user experience.  The goal is to create a standard that
would allow QR scanning wallets to use less dense QR codes.  It also makes
general purpose QR code scanners more usable with bitcoin accepting
websites.

==Specification==

QR scanning wallets will consider a non bitcoin URI scanned from a QR code to
be an end point where either a bitcoin URI or a payment request can be obtained.

A wallet client uses the Accept: HTTP header to specify whether it can accept
a payment request, a URI, or both.  A media type of text/uri-list specifies that
the client accepts a bitcoin URI. A media type of application/bitcoin-paymentrequest
specifies that the client can process a payment request.  In the absence of an
Accept: header, the server is expected to respond with text/html suitable for
rendering in a browser.  An HTML response will ensure that QR codes scanned
by non Bitcoin wallet QR scanners are useful (they could render an HTML page
with a payment link that when clicked would open a wallet on the device).

It is not required that the client and server support the full semantics of an
HTTP Accept header.  If application/bitcoin-paymentrequest is specified in the
header, the server should send a payment request regardless of anything else
specified in the Accept header.  If text/uri-list is specified (but not
application/bitcoin-paymentrequest), a valid Bitcoin URI should be returned.  If
neither is specified, the server can return an HTML page.  When a uri-list is returned
only the first item in the list is used (and expected to be a bitcoin URI), any additional
URIs should be ignored.

==Compatibility==

Only QR scanning wallets that implement this BIP will be able to process QR
codes containing payment request URLs.  There are two possible workarounds for QR
scanning wallets that do not implement this BIP: 1) the server gives the user an
option to change the QR code to a bitcoin: URI or 2) the user scans the code with
a generic QR code scanner.

In the second scenario, if the server responds with a webpage containing a link
to a bitcoin URI, the user can complete the payment by clicking that link provided
the user has a wallet installed on their device and it supports bitcoin URIs.  If the
wallet/device does not have support for bitcoin URIs, the user can fall back on
address copy/paste.

This BIP should be fully compatible with BIP 70 assuming it is required that wallets
implementing BIP 70 make use of the Accept: HTTP header when retrieving a
payment request.

==Examples==
The first image below is of a bitcoin URI with an amount and payment request
specified (note, this is a fairly minimal URI as it does not contain a
label and the request URL is of moderate size).  The second image is a QR
code with only the payment request url specified.

<img src=bip-0073/a.png></img><img src=bip-0073/b.png></img>
