When I'm living in my RV, wireless service providers are my primary source of connectivity. So when either AT&T or Verizon make major changes, I take notice.
I recently noticed that multiple websites are quite slow when browsing with my AT&T business plan, listed in AT&T Premier (business account management UI) as "Wireless Broadband Ultra for Router or Hotspt (sic)". This is an "unlimited" **100Mbit** plan with 50GB for Business Fast Track (prioritized) data. Being that I was far below the 50GB of monthly Fast Track data, my data should have had top priority, so I became suspicious. To be honest, I've never noticed a discernible difference between Fast Track and non-Fast Track data rates. This is all to say that I have no reason to believe that I'm being deprioritized due to usage.
Naturally, the first hing I did was conduct a speed test. I already knew from previous experience that for some reason, AT&T traffic to [fast.com](https://fast.com) is throttled. Why AT&T wants bandwidth to appear lower than reality is a mystery to me, but I digress. Linode.com has speed tests that AT&T has no special treatment for, and the [nearest one to me](http://speedtest.fremont.linode.com/) was in Fremont, CA.
The speedtest revealed **21Mbps** down and **4.5Mbps** up -- pretty reasonable in a relatively rural area like Durango, CO. Latency was ~**130ms**. That speed certainly wouldn't explain why it took anywhere between 15 seconds and 2 minutes to load `strava.com`.
So I opened up the "Network" tab in Firefox and could clearly see that dozens of resources from `cloudfront.com` were taking multiple seconds to load. The problem clearly has _something_ to do with Cloudfront.
Is Cloudfront having problems? That's easy enough to verify; my [Sierra Wireless RV55 CAT-12 LTE-A router](https://www.sierrawireless.com/router-solutions/rv55/) also contains an unlimited Verizon Business SIM card that I can use to conduct tests on Cloudfront, independent of AT&T.
I noticed that one of Strava's javascript resources clocked in at **1.68MB**, making it a nice test subject for speed tests ([https://web-assets.strava.com/assets/federated/find-and-invite-friends/827.js](https://web-assets.strava.com/assets/federated/find-and-invite-friends/827.js)). At the time of writing `web-assets.strava.com` resolves to `dgpcy4fyk1eox.cloudfront.net`, so rest assured, we are dealing with Cloudfront.
After switching to Verizon, I could see that Cloudfront was having no problems. Our friend `827.js` downloaded in just over 1 second at **1.4MB/s**. I clearly saw earlier in the Firefox network tab that this resource took nearly 1 minute to load on AT&T.
> While `wget` is not my goto for command line HTTP fetching, it displays transfer rates in a human friendly format by default, so I used the following as my test case: `wget -O /dev/null -q --show-progress https://web-assets.strava.com/assets/federated/find-and-invite-friends/827.js`
So the problem isn't Cloudfront, because Verizon was fast enough. It wasn't blazing fast by any means, but I also didn't have to wait 2 minutes to learn whether Strava awarded me King of the Moutain on a local trail (I wasn't).
Maybe this is a global AT&T problem. That's easy to test as well -- my iPhone is also on the same AT&T business account as my data-only plan, so I turned on the iPhone hotspot and made it the router's WAN device to make sure we're changing a single variable at a time. I conducted another speed test, revealing **23Mbps** down and **3Mbps** up. Nothing surprising there -- normal bandwidth fluctuations for a wireless device. How about our `wget` test? **1.7MB/s**. The problem is clearly not _all_ of AT&T wireless.
Let's go back to the original configuration: connect directly to my AT&T data-only plan with my router and re-run the `wget` test. Maybe I was imagining things. I'm somewhat surprised to see the `wget` test with a transfer rate of **~30KB/s**. I believe this rules out AT&T with a souring peering agreement somewhere between me and Cloudfront. My phone traffic to Cloudfront is unaffected; it's only my data-only plan that is affected.
I now have a pretty clear picture of what is likely happening. Good old-fashion traffic shaping. Now that the router is connected directly to AT&T, the true test of traffic shaping is transfer rates while connected to a VPN. I'll let the image speak for itself.
---
As of the time of writing, I'm unsure what is causing such a significant slowdown. It has rendered some websites effectively useless. Everything in this writeup indicates, to me, that AT&T is engaged in extremely aggressive traffic shaping for some plans, rendering many websites nearly unusable.
Do you have any ideas how to diagnose this problem further? Do you know the best way engage AT&T's technical folks to take this seriously? Write me at `att-traffic-shaping @ this domain`. I'll add updates here if anything changes, or I get a response on https://bizcommunity.att.com.
Here is a pcap file captured with `sudo tcpdump host 'dgpcy4fyk1eox.cloudfront.net' -w iphone-capture.pcap` while tethered to my AT&T iPhone and fetching `827.js` with `wget -O /dev/null -q --show-progress https://web-assets.strava.com/assets/federated/find-and-invite-friends/827.js` : [iphone-capture.pcap](/files/iphone-capture.pcap)
As my initial post mentioned, I had hoped to get some sort of resolution by posting to http://bizcommunity.att.com. As of right now, that post [https://bizcommunity.att.com/conversations/the-break-room/im-apparently-experiencing-extremely-aggressive-att-traffic-shaping/643c3d81aba4c269accb1a2b](https://bizcommunity.att.com/conversations/the-break-room/im-apparently-experiencing-extremely-aggressive-att-traffic-shaping/643c3d81aba4c269accb1a2b) has been marked as "Private". This URL also 404s, unless I'm logged in under the account with which I posted it.
I can't say whether this is normal practice on the bizcommunity forum, but I posted a followup post with a much more benign title, "Are all new posts here marked as private?" [https://bizcommunity.att.com/conversations/the-break-room/are-all-new-posts-here-marked-as-private/643c866bb67b2d067eb0985c](https://bizcommunity.att.com/conversations/the-break-room/are-all-new-posts-here-marked-as-private/643c866bb67b2d067eb0985c), which as of right now is not private and still accessible to the public.
Hello reader, I'm another HN member trying to help get to the bottom of this! I've analyzed all the data provided and feel confident there is some kind of manipulation / throttling happening with Adriano's AT&T business plan, but if anyone wants to correct my interpretation or provide other insights feel free to reach out to me on my [blog](https://rasbora.dev/)!
We start by analyzing the packet capture from the throttled connection. There are a few takeaways from this:
- TCP segment & window sizes have normal values.
- There are almost no retransmissions or lost packets.
- There appears to be ample room for the TCP stream to scale up to a much faster speed.
- The TCP stream starts scaling up to 800Kbps before dramatically going down.
- The TCP stream regularly blocks for 50-200ms intervals waiting for new data to arrive from cloudfront.
- These blocks coincide with throughput scaling up to ~350Kbps.
All of the spikes in round trip time correspond to a segment of the stream where the client acknowladges receipt of new data and is forced to wait 50-200ms for any new data to arrive. This happens at a regular cadence that appears artificial in nature (i.e not regular network congestion) because the throughput chart stays within a well defined boundary.
If this were a problem with AT&T's network or a side effect of LTE networks being inherently high latency we would expect to observe similair patterns over the iphone connection:
- The iphone AT&T connection doesn't exhibit the same RTT or throughput pattern despite having similair latency & network routing.
- The throughput chart is indicative of a healthy TCP stream that slowly scales up the total bandwidth to make the most out of available network resources.
- We are never waiting very long for fresh data to arrive.
In my opinion these patterns rule out any extraneous factors affecting the network and are indicative of intentional manipulation of the TCP stream. If AT&T were to occasionaly drop an incoming packet than the client would never send an acknowledgement to cloudfront and after a little while cloudfront would automatically retransmit that packet, on our client side packet capture we would observe this as a packet coming in with a big delay, if we were being rate limited we would expect to see these delayed packets coming in regularly around the time when we were starting to hit a threshold, it seems likely AT&T is dropping certain inbound packets once the incoming rate reaches around ~350Kbps.
Refer to Update #4 above: I'm getting shut down on the AT&T bizcommunity site. My posts are being marked private, so I'll save them here for posterity.
### Forum Post 1: I'm apparently experiencing extremely aggressive AT&T traffic shaping
The short of it is that I'm experiencing what appears to be aggressive traffic shaping by AT&T. Many sites are affected, because Cloudflare is one content delivery network for which AT&T is apparently shaping my traffic.
This is a business account. I have both cell phone devices and a data-only plan "Wireless Broadband Ultra for Router or Hotspt (sic)" on the account. Only the data-only plan is affected.
Because Cloudflare is such a large content delivery network, this affects millions of websites. A large percentage of the websites I've accessed over the last few days have been affected, forcing me to use a VPN to subvert the problem.
I've added more concrete data in a writeup on my personal website: https://adriano.fyi/post/2023/2023-04-16-att-traffic-shaping-makes-websites-unusable
The data paint a fairly clear picture of traffic shaping.
[ GIF IMAGE FROM ABOVE HERE ]
[ NOTE: web-assets.strava.com resolves to Cloudflare, see the writeup on my site for details ]
I'd love to get a response from AT&T on the matter because this is a major performance degradation to the point that many websites are unusable. Is anyone else experiencing similar issues?
Cheers,
AC
### Forum Post 2: Are all new posts here marked as private?
I posted earlier today regarding a traffic shaping issue that I'm experiencing. The title of the post is "I'm apparently experiencing extremely aggressive AT&T traffic shaping". When I came back to check for any responses, I see that my previous post is marked as "Private", which I don't believe was under my control, and certainly wasn't my intention.
This post is a test to see whether posts are automatically flagged as private, or if someone took action on my last post to mark it as private.
[edit] As of 2023-04-16 23:37 UTC, this post is not Private like my previous post.
### Response and followup to Forum Post 2
Notice the `Private` label on my response. So it looks to others like I impolitely never respond to their polite response.
