Tag Archives: Edge

One URL standard please

Following up on the problem with our current lack of a universal URL standard that I blogged about in May 2016: My URL isn’t your URL. I want a single, unified URL standard that we would all stand behind, support and adhere to.

What triggers me this time, is yet another issue. A friendly curl user sent me this URL:


… and pasting this URL into different tools and browsers show that there’s not a wide agreement on how this should work. Is the URL legal in the first place and if so, which host should a client contact?

  • curl treats the ‘@’-character as a separator between userinfo and host name so ‘example.com’ becomes the host name, the port number is 80 followed by rubbish that curl ignores. (wget2, the next-gen wget that’s in development works identically)
  • wget extracts the example.com host name but rejects the port number due to the rubbish after the zero.
  • Edge and Safari say the URL is invalid and don’t go anywhere
  • Firefox and Chrome allow ‘@’ as part of the userinfo, take the ’80’ as a password and the host name then becomes ‘daniel.haxx.se’

The only somewhat modern “spec” for URLs is the WHATWG URL specification. The other major, but now somewhat aged, URL spec is RFC 3986, made by the IETF and published in 2005.

In 2015, URL problem statement and directions was published as an Internet-draft by Masinter and Ruby and it brings up most of the current URL spec problems. Some of them are also discussed in Ruby’s WHATWG URL vs IETF URI post from 2014.

What I would like to see happen…

Which group? A group!

Friends I know in the WHATWG suggest that I should dig in there and help them improve their spec. That would be a good idea if fixing the WHATWG spec would be the ultimate goal. I don’t think it is enough.

The WHATWG is highly browser focused and my interactions with members of that group that I have had in the past, have shown that there is little sympathy there for non-browsers who want to deal with URLs and there is even less sympathy or interest for URL schemes that the popular browsers don’t even support or care about. URLs cover much more than HTTP(S).

I have the feeling that WHATWG people would not like this work to be done within the IETF and vice versa. Since I’d like buy-in from both camps, and any other camps that might have an interest in URLs, this would need to be handled somehow.

It would also be great to get other major URL “consumers” on board, like authors of popular URL parsing libraries, tools and components.

Such a URL group would of course have to agree on the goal and how to get there, but I’ll still provide some additional things I want to see.

Update: I want to emphasize that I do not consider the WHATWG’s job bad, wrong or lost. I think they’ve done a great job at unifying browsers’ treatment of URLs. I don’t mean to belittle that. I just know that this group is only a small subset of the people who probably should be involved in a unified URL standard.

A single fixed spec

I can’t see any compelling reasons why a URL specification couldn’t reach a stable state and get published as *the* URL standard. The “living standard” approach may be fine for certain things (and in particular browsers that update every six weeks), but URLs are supposed to be long-lived and inter-operate far into the future so they really really should not change. Therefore, I think the IETF documentation model could work well for this.

The WHATWG spec documents what browsers do, and browsers do what is documented. At least that’s the theory I’ve been told, and it causes a spinning and never-ending loop that goes against my wish.

Document the format

The WHATWG specification is written in a pseudo code style, describing how a parser would “walk” over the string with a state machine and all. I know some people like that, I find it utterly annoying and really hard to figure out what’s allowed or not. I much more prefer the regular RFC style of describing protocol syntax.


Can we please just say that host names in URLs should be handled according to IDNA2008 (RFC 5895)? WHATWG URL doesn’t state any IDNA spec number at all.

Move out irrelevant sections

“Irrelevant” when it comes to documenting the URL format that is. The WHATWG details several things that are related to URL for browsers but are mostly irrelevant to other URL consumers or producers. Like section “5. application/x-www-form-urlencoded” and “6. API”.

They would be better placed in a “URL considerations for browsers” companion document.

Working doesn’t imply sensible

So browsers accept URLs written with thousands of forward slashes instead of two. That is not a good reason for the spec to say that a URL may legitimately contain a thousand slashes. I’m totally convinced there’s no critical content anywhere using such formatted URLs and no soul will be sad if we’d restricted the number to a single-digit. So we should. And yeah, then browsers should reject URLs using more.

The slashes are only an example. The browsers have used a “liberal in what you accept” policy for a lot of things since forever, but we must resist to use that as a basis when nailing down a standard.

The odds of this happening soon?

I know there are individuals interested in seeing the URL situation getting worked on. We’ve seen articles and internet-drafts posted on the issue several times the last few years. Any year now I think we will see some movement for real trying to fix this. I hope I will manage to participate and contribute a little from my end.

HTTP/2 connection coalescing

Section 9.1.1 in RFC7540 explains how HTTP/2 clients can reuse connections. This is my lengthy way of explaining how this works in reality.

Many connections in HTTP/1

With HTTP/1.1, browsers are typically using 6 connections per origin (host name + port). They do this to overcome the problems in HTTP/1 and how it uses TCP – as each connection will do a fair amount of waiting. Plus each connection is slow at start and therefore limited to how much data you can get and send quickly, you multiply that data amount with each additional connection. This makes the browser get more data faster (than just using one connection).

6 connections

Add sharding

Web sites with many objects also regularly invent new host names to trigger browsers to use even more connections. A practice known as “sharding”. 6 connections for each name. So if you instead make your site use 4 host names you suddenly get 4 x 6 = 24 connections instead. Mostly all those host names resolve to the same IP address in the end anyway, or the same set of IP addresses. In reality, some sites use many more than just 4 host names.

24 connections

The sad reality is that a very large percentage of connections used for HTTP/1.1 are only ever used for a single HTTP request, and a very large share of the connections made for HTTP/1 are so short-lived they actually never leave the slow start period before they’re killed off again. Not really ideal.

One connection in HTTP/2

With the introduction of HTTP/2, the HTTP clients of the world are going toward using a single TCP connection for each origin. The idea being that one connection is better in packet loss scenarios, it makes priorities/dependencies work and reusing that single connections for many more requests will be a net gain. And as you remember, HTTP/2 allows many logical streams in parallel over that single connection so the single connection doesn’t limit what the browsers can ask for.


The sites that created all those additional host names to make the HTTP/1 browsers use many connections now work against the HTTP/2 browsers’ desire to decrease the number of connections to a single one. Sites don’t want to switch back to using a single host name because that would be a significant architectural change and there are still a fair number of HTTP/1-only browsers still in use.

Enter “connection coalescing”, or “unsharding” as we sometimes like to call it. You won’t find either term used in RFC7540, as it merely describes this concept in terms of connection reuse.

Connection coalescing means that the browser tries to determine which of the remote hosts that it can reach over the same TCP connection. The different browsers have slightly different heuristics here and some don’t do it at all, but let me try to explain how they work – as far as I know and at this point in time.

Coalescing by example

Let’s say that this cool imaginary site “example.com” has two name entries in DNS: A.example.com and B.example.com. When resolving those names over DNS, the client gets a list of IP address back for each name. A list that very well may contain a mix of IPv4 and IPv6 addresses. One list for each name.

You must also remember that HTTP/2 is also only ever used over HTTPS by browsers, so for each origin speaking HTTP/2 there’s also a corresponding server certificate with a list of names or a wildcard pattern for which that server is authorized to respond for.

In our example we start out by connecting the browser to A. Let’s say resolving A returns the IPs and from DNS, so the browser goes on and connects to the first of those addresses, the one ending with “1”. The browser gets the server cert back in the TLS handshake and as a result of that, it also gets a list of host names the server can deal with: A.example.com and B.example.com. (it could also be a wildcard like “*.example.com”)

If the browser then wants to connect to B, it’ll resolve that host name too to a list of IPs. Let’s say and here.

Host A: and
Host B: and

Now hold it. Here it comes.

The Firefox way

Host A has two addresses, host B has two addresses. The lists of addresses are not the same, but there is an overlap – both lists contain And the host A has already stated that it is authoritative for B as well. In this situation, Firefox will not make a second connect to host B. It will reuse the connection to host A and ask for host B’s content over that single shared connection. This is the most aggressive coalescing method in use.

one connection

The Chrome way

Chrome features a slightly less aggressive coalescing. In the example above, when the browser has connected to for the first host name, Chrome will require that the IPs for host B contains that specific IP for it to reuse that connection.  If the returned IPs for host B really are and, it clearly doesn’t contain and so Chrome will create a new connection to host B.

Chrome will reuse the connection to host A if resolving host B returns a list that contains the specific IP of the connection host A is already using.

The Edge and Safari ways

They don’t do coalescing at all, so each host name will get its own single connection. Better than the 6 connections from HTTP/1 but for very sharded sites that means a lot of connections even in the HTTP/2 case.

curl also doesn’t coalesce anything (yet).

Surprises and a way to mitigate them

Given some comments in the Firefox bugzilla, the aggressive coalescing sometimes causes some surprises. Especially when you have for example one IPv6-only host A and a second host B with both IPv4 and IPv4 addresses. Asking for data on host A can then still use IPv4 when it reuses a connection to B (assuming that host A covers host B in its cert).

In the rare case where a server gets a resource request for an authority (or scheme) it can’t serve, there’s a dedicated error code 421 in HTTP/2 that it can respond with and the browser can then  go back and retry that request on another connection.

Starts out with 6 anyway

Before the browser knows that the server speaks HTTP/2, it may fire up 6 connection attempts so that it is prepared to get the remote site at full speed. Once it figures out that it doesn’t need all those connections, it will kill off the unnecessary unused ones and over time trickle down to one. Of course, on subsequent connections to the same origin the client may have the version information cached so that it doesn’t have to start off presuming HTTP/1.