Avalanche Now Supports ETag and If-None-Match

With the release of version 4.37 this week, Avalanche now supports the ETag and If-None-Match concept. This will be highly useful for Content Delivery Network (CDN) testing as well as caching devices implementing this concept.

Let’s start with a little recap on the said concept.

What are Etag and If-None-Match?

Modern browsers help speeding up the loading of previously visited pages by caching as much static content as possible. If the header image, or the CSS template, or the JavaScript resources don’t change between two visits, it would be pretty dumb to download them each time you visit. It makes pages longer to load, generates more traffic on the network and more load on the servers.

Initially the browsers would “remember” the first time they downloaded a resource (like an image), and let the server know. They send an “If-Modified-Since: <some date>” header (which Avalanche also supports). The server would then compare with the local resource’s modified date (this is an attribute of the file on the file system). If the file was modified later than the client’s version (implying it’s a newer version of the same file), the server would return the resource, otherwise it would return a message saying the resource was not modified (and the browser would use the version it has in its cache).

If-None-Match (INM) does something similar, but in a smarter way. For INM to work, the server needs to “tag” the resources it sends. The first time you visit a website, the browser will retrieve the resources from the server. The server will return the content with an “ETag: <some value>” header. The browser stores the resource in its cache, along with the ETag value. The next time the resource is needed (for instance, the next time you visit the website), the browser will check against the server if the file is still the same: it will send the request to retrieve the resource along with an “If-None-Match” header with the same value as the server sent initially. When the server receives the request, it checks that value against its own. If it matches, it means the files are the same, so it will return a response “304 Not Modified” to let the browser know it can load the file from the cache. If the tags don’t match, it will return a “200 OK” and the new version of the file.

For instance, Bob requests the resource “spirent.png.” The server will return the image with an ETag header, for instance ETag: “123456789”. The next time Bob visits the page, the browser will send a request to the server to download “spirent.png”, with the header “If-None-Match: 123456789”. If the file hasn’t changed on the server, the server will return “304 Not Modified” and Bob’s browser will load the file from cache. Otherwise, the server will return the whole new image, with a different ETag, that the browser will save in its cache. Etc. ad nauseam.

Note: There’s no defined standard describing how to generate ETags. Each implementation can have its own method of doing so. Usually people will implement a collision-resistant hashing algorithm but it’s not required.

How does it work in Avalanche?

First, a caveat, to get that out of the way. Being a test tool for very high performance, Avalanche doesn’t “remember” ETags. Each Simulated User (SimUser) is like a fresh visit, there’s no simulated local browser cache. You can pre-define the ETags in your Action List, as I will show below, but it’s not automatic. It’s also more practical for CDN and caching device providers like this.

Let’s take a look at the server-side under “Transactions.” This is where we’ll define how we want to send the ETags. There are two ways (besides “None”): Incremental and Circulatory. The documentation is pretty clear on what they do, so allow me to quote it:

  • Incremental ETag: The server includes an ETag header in HTTP response messages, which starts with 1 and increments every number of milliseconds that you specify in the Interval field.
  • Circulatory ETag List: The server includes an ETag header in HTTP response messages, which gets assigned from the circulatory ETag List of strings every number of milliseconds that you specify in the Interval field.

In other words, when you choose “Incremental” Avalanche will generate the ETag value itself. Starting at 1 (i.e.: first header would be ETag: “1”) and incrementing by 1 every 1000 milliseconds (1 second). Note that this is since the start of the test, so if you have some delay at the beginning of your test (for ARPing, for instance), you won’t get a value of “1” in the first response (in my back-to-back tests it starts at 3).

When you choose “Circulatory” you can choose your own ETag values, and Avalanche will cycle through them. Using the default 1000 ms value, the returned header value will change every second.

Some Examples

Let’s create a few requests (with no If-None-Match yet) and see how the server behaves. I configured the clients to make 3 requests, at a 2 seconds interval.

Avalanche server-side Transaction configuration

Avalanche server-side Transaction configuration

This is the resulting PCAP:

Avalanche HTTP server returns the ETag values as configured.

Avalanche HTTP server returns the ETag values as configured.

It’s incrementing as expected! Now let’s see what happen when we do match. For this I need to add an “If-None-Match” header. To keep it simple I’ll make this static and the same for all requests, but it could be a variable or random, and different for each request of course. Server side stays the same, and this is the action list:

1 get <ADDITIONAL_HEADER="If-None-Match: \"7\"">
THINK <2000>
1 get <ADDITIONAL_HEADER="If-None-Match: \"7\"">
THINK <2000>
1 get <ADDITIONAL_HEADER="If-None-Match: \"7\"">

Note the escaping backslash characters. This is required because INM values are enclosed in double quotes, but the “ADDITIONAL_HEADER” as well, so we need to tell the backend parser we don’t want to close the quote at that point.

According to the previous PCAP, only the 3rd GET should match since the first response’s ETag is “3”, the second is “5” and the third one is “7.” Let’s see:

If-None-Match headers are sent, and the one we expected to match returns the correct 304 status code.

If-None-Match headers are sent, and the one we expected to match returns the correct 304 status code.

Looks fine! We can see (in blue) that the clients are now sending the “If-None-Match” value we configured in the Action List. The first two responses are a 200 OK since the ETag don’t match. However the third request sends a 304 Not Modified since the ETag match (and we can see there’s no content return, so we saved bandwidth and time).

There could be many more examples, so I’ll stop here. If you have specific questions, as usual feel free to contact me!


About acastaner

I'm the EMEA Technical Lead for Application & Security at Spirent. I specialize in layer 4-7 technologies, Cloud, Programming and CyberSecurity.
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