Post Syndicated from Tom Strickx original https://blog.cloudflare.com/cloudflare-outage-on-june-21-2022/

Introduction

Today, June 21, 2022, Cloudflare suffered an outage that affected traffic in 19 of our data centers. Unfortunately, these 19 locations handle a significant proportion of our global traffic. This outage was caused by a change that was part of a long-running project to increase resilience in our busiest locations. A change to the network configuration in those locations caused an outage which started at 06:27 UTC. At 06:58 UTC the first data center was brought back online and by 07:42 UTC all data centers were online and working correctly.

Depending on your location in the world you may have been unable to access websites and services that rely on Cloudflare. In other locations, Cloudflare continued to operate normally.

We are very sorry for this outage. This was our error and not the result of an attack or malicious activity.

Background

Over the last 18 months, Cloudflare has been working to convert all of our busiest locations to a more flexible and resilient architecture. In this time, we’ve converted 19 of our data centers to this architecture, internally called Multi-Colo PoP (MCP): Amsterdam, Atlanta, Ashburn, Chicago, Frankfurt, London, Los Angeles, Madrid, Manchester, Miami, Milan, Mumbai, Newark, Osaka, São Paulo, San Jose, Singapore, Sydney, Tokyo.

A critical part of this new architecture, which is designed as a Clos network, is an added layer of routing that creates a mesh of connections. This mesh allows us to easily disable and enable parts of the internal network in a data center for maintenance or to deal with a problem. This layer is represented by the spines in the following diagram.

This new architecture has provided us with significant reliability improvements, as well as allowing us to run maintenance in these locations without disrupting customer traffic. As these locations also carry a significant proportion of the Cloudflare traffic, any problem here can have a very wide impact, and unfortunately, that’s what happened today.

Incident timeline and impact

In order to be reachable on the Internet, networks like Cloudflare make use of a protocol called BGP. As part of this protocol, operators define policies which decide which prefixes (a collection of adjacent IP addresses) are advertised to peers (the other networks they connect to), or accepted from peers.

These policies have individual components, which are evaluated sequentially. The end result is that any given prefixes will either be advertised or not advertised. A change in policy can mean a previously advertised prefix is no longer advertised, known as being “withdrawn”, and those IP addresses will no longer be reachable on the Internet.

While deploying a change to our prefix advertisement policies, a re-ordering of terms caused us to withdraw a critical subset of prefixes.

Due to this withdrawal, Cloudflare engineers experienced added difficulty in reaching the affected locations to revert the problematic change. We have backup procedures for handling such an event and used them to take control of the affected locations.

03:56 UTC: We deploy the change to our first location. None of our locations are impacted by the change, as these are using our older architecture.
06:17: The change is deployed to our busiest locations, but not the locations with the MCP architecture.
06:27: The rollout reached the MCP-enabled locations, and the change is deployed to our spines. This is when the incident started, as this swiftly took these 19 locations offline.
06:32: Internal Cloudflare incident declared.
06:51: First change made on a router to verify the root cause.
06:58: Root cause found and understood. Work begins to revert the problematic change.
07:42: The last of the reverts has been completed. This was delayed as network engineers walked over each other’s changes, reverting the previous reverts, causing the problem to re-appear sporadically.
09:00: Incident closed.

The criticality of these data centers can clearly be seen in the volume of successful HTTP requests we handled globally:

Even though these locations are only 4% of our total network, the outage impacted 50% of total requests. The same can be seen in our egress bandwidth:

Technical description of the error and how it happened

As part of our continued effort to standardize our infrastructure configuration, we were rolling out a change to standardize the BGP communities we attach to a subset of the prefixes we advertise. Specifically, we were adding informational communities to our site-local prefixes.

These prefixes allow our metals to communicate with each other, as well as connect to customer origins. As part of the change procedure at Cloudflare, a Change Request ticket was created, which includes a dry-run of the change, as well as a stepped rollout procedure. Before it was allowed to go out, it was also peer reviewed by multiple engineers. Unfortunately, in this case, the steps weren’t small enough to catch the error before it hit all of our spines.

The change looked like this on one of the routers:

[edit policy-options policy-statement 4-COGENT-TRANSIT-OUT term ADV-SITELOCAL then]
+      community add STATIC-ROUTE;
+      community add SITE-LOCAL-ROUTE;
+      community add TLL01;
+      community add EUROPE;
[edit policy-options policy-statement 4-PUBLIC-PEER-ANYCAST-OUT term ADV-SITELOCAL then]
+      community add STATIC-ROUTE;
+      community add SITE-LOCAL-ROUTE;
+      community add TLL01;
+      community add EUROPE;
[edit policy-options policy-statement 6-COGENT-TRANSIT-OUT term ADV-SITELOCAL then]
+      community add STATIC-ROUTE;
+      community add SITE-LOCAL-ROUTE;
+      community add TLL01;
+      community add EUROPE;
[edit policy-options policy-statement 6-PUBLIC-PEER-ANYCAST-OUT term ADV-SITELOCAL then]
+      community add STATIC-ROUTE;
+      community add SITE-LOCAL-ROUTE;
+      community add TLL01;
+      community add EUROPE;

This was harmless, and just added some additional information to these prefix advertisements. The change on the spines was the following:

[edit policy-options policy-statement AGGREGATES-OUT]
term 6-DISABLED_PREFIXES { ... }
!    term 6-ADV-TRAFFIC-PREDICTOR { ... }
!    term 4-ADV-TRAFFIC-PREDICTOR { ... }
!    term ADV-FREE { ... }
!    term ADV-PRO { ... }
!    term ADV-BIZ { ... }
!    term ADV-ENT { ... }
!    term ADV-DNS { ... }
!    term REJECT-THE-REST { ... }
!    term 4-ADV-SITE-LOCALS { ... }
!    term 6-ADV-SITE-LOCALS { ... }
[edit policy-options policy-statement AGGREGATES-OUT term 4-ADV-SITE-LOCALS then]
community delete NO-EXPORT { ... }
+      community add STATIC-ROUTE;
+      community add SITE-LOCAL-ROUTE;
+      community add AMS07;
+      community add EUROPE;
[edit policy-options policy-statement AGGREGATES-OUT term 6-ADV-SITE-LOCALS then]
community delete NO-EXPORT { ... }
+      community add STATIC-ROUTE;
+      community add SITE-LOCAL-ROUTE;
+      community add AMS07;
+      community add EUROPE;

An initial glance at this diff might give the impression that this change is identical, but unfortunately, that’s not the case. If we focus on one part of the diff, it might become clear why:

!    term REJECT-THE-REST { ... }
!    term 4-ADV-SITE-LOCALS { ... }
!    term 6-ADV-SITE-LOCALS { ... }

In this diff format, the exclamation marks in front of the terms indicate a re-ordering of the terms. In this case, multiple terms moved up, and two terms were added to the bottom. Specifically, the 4-ADV-SITE-LOCALS and 6-ADV-SITE-LOCALS terms moved from the top to the bottom. These terms were now behind the REJECT-THE-REST term, and as might be clear from the name, this term is an explicit reject:

term REJECT-THE-REST {
    then reject;
} 

As this term is now before the site-local terms, we immediately stopped advertising our site-local prefixes, removing our direct access to all the impacted locations, as well as removing the ability of our servers to reach origin servers.

On top of the inability to contact origins, the removal of these site-local prefixes also caused our internal load balancing system Multimog (a variation of our Unimog load-balancer) to stop working, as it could no longer forward requests between the servers in our MCPs. This meant that our smaller compute clusters in an MCP received the same amount of traffic as our largest clusters, causing the smaller ones to overload.

Remediation and follow-up steps

This incident had widespread impact, and we take availability very seriously. We have identified several areas of improvement and will continue to work on uncovering any other gaps that could cause a recurrence.

Here is what we are working on immediately:

Process: While the MCP program was designed to improve availability, a procedural gap in how we updated these data centers ultimately caused a broader impact in MCP locations specifically. While we did use a stagger procedure for this change, the stagger policy did not include an MCP data center until the final step. Change procedures and automation need to include MCP-specific test and deploy procedures to ensure there are no unintended consequences.

Architecture: The incorrect router configuration prevented the proper routes from being announced, preventing traffic from flowing properly to our infrastructure. Ultimately the policy statement that caused the incorrect routing advertisement will be redesigned to prevent an unintentional incorrect ordering.

Automation: There are several opportunities in our automation suite that would mitigate some or all of the impact seen from this event. Primarily, we will be concentrating on automation improvements that enforce an improved stagger policy for rollouts of network configuration and provide an automated “commit-confirm” rollback. The former enhancement would have significantly lessened the overall impact, and the latter would have greatly reduced the Time-to-Resolve during the incident.

Conclusion

Although Cloudflare has invested significantly in our MCP design to improve service availability, we clearly fell short of our customer expectations with this very painful incident. We are deeply sorry for the disruption to our customers and to all the users who were unable to access Internet properties during the outage. We have already started working on the changes outlined above and will continue our diligence to ensure this cannot happen again.

Post Syndicated from Matthew Prince original https://blog.cloudflare.com/analysis-of-todays-centurylink-level-3-outage/

Today CenturyLink/Level(3), a major ISP and Internet bandwidth provider, experienced a significant outage that impacted some of Cloudflare’s customers as well as a significant number of other services and providers across the Internet. While we’re waiting for a post mortem from CenturyLink/Level(3), I wanted to write up the timeline of what we saw, how Cloudflare’s systems routed around the problem, why some of our customers were still impacted in spite of our mitigations, and what  appears to be the likely root cause of the issue.

Increase In Errors

At 10:03 UTC our monitoring systems started to observe an increased number of errors reaching our customers’ origin servers. These show up as “522 Errors” and indicate that there is an issue connecting from Cloudflare’s network to wherever our customers’ applications are hosted.

Cloudflare is connected to CenturyLink/Level(3) among a large and diverse set of network providers. When we see an increase in errors from one network provider, our systems automatically attempt to reach customers’ applications across alternative providers. Given the number of providers we have access to, we are generally able to continue to route traffic even when one provider has an issue.

Automatic Mitigations

In this case, beginning within seconds of the increase in 522 errors, our systems automatically rerouted traffic from CenturyLink/Level(3) to alternate network providers we connect to including Cogent, NTT, GTT, Telia, and Tata.

Our Network Operations Center was also alerted and our team began taking additional steps to mitigate any issues our automated systems weren’t automatically able to address beginning at 10:09 UTC. We were successful in keeping traffic flowing across our network for most customers and end users even with the loss of CenturyLink/Level(3) as one of our network providers.

The graph below shows traffic between Cloudflare’s network and six major tier-1 networks that are among the network providers we connect to. The red portion shows CenturyLink/Level(3) traffic, which dropped to near-zero during the incident. You can also see how we automatically shifted traffic to other network providers during the incident to mitigate the impact and ensure traffic continued to flow.

The following graph shows 522 errors (indicating our inability to reach customers’ applications) across our network during the time of the incident.

The sharp spike up at 10:03 UTC was the CenturyLink/Level(3) network failing. Our automated systems immediately kicked in to attempt to reroute and rebalance traffic across alternative network providers, causing the errors to drop in half immediately and then fall to approximately 25 percent of the peak as those paths were automatically optimized.

Between 10:03 UTC and 10:11 UTC our systems automatically disabled CenturyLink/Level(3) in the 48 cities where we’re connected to them and rerouted traffic across alternate network providers. Our systems take into account capacity on other providers before shifting out traffic in order to prevent cascading failures. This is why the failover, while automatic, isn’t instantaneous in all locations. Our team was able to apply additional manual mitigations to reduce the number of errors another 5 percent.

Why Did the Errors Not Drop to Zero?

Unfortunately, there were still an elevated number of errors indicating we were still unable to reach some customers. CenturyLink/Level(3) is among the largest network providers in the world. As a result, many hosting providers only have single-homed connectivity to the Internet through their network.

To use the old Internet as a “superhighway” analogy, that’s like only having a single offramp to a town. If the offramp is blocked, then there’s no way to reach the town. This was exacerbated in some cases because CenturyLink/Level(3)’s network was not honoring route withdrawals and continued to advertise routes to networks like Cloudflare’s even after they’d been withdrawn. In the case of customers whose only connectivity to the Internet is via CenturyLink/Level(3), or if CenturyLink/Leve(3) continued to announce bad routes after they’d been withdrawn, there was no way for us to reach their applications and they continued to see 522 errors until CenturyLink/Level(3) resolved their issue around 14:30 UTC.

The same was a problem on the other (“eyeball”) side of the network. Individuals need to have an onramp onto the Internet’s superhighway. An onramp to the Internet is essentially what your ISP provides. CenturyLink is one of the largest ISPs in the United States.

Because this outage appeared to take all of the CenturyLink/Level(3) network offline, individuals who are CenturyLink customers would not have been able to reach Cloudflare or any other Internet provider until the issue was resolved. Globally, we saw a 3.5% drop in global traffic during the outage, nearly all of which was due to a nearly complete outage of CenturyLink’s ISP service across the United States.

So What Likely Happened Here?

While we will not know exactly what happened until CenturyLink/Level(3) issue a post mortem, we can see clues from BGP announcements and how they propagated across the Internet during the outage. BGP is the Border Gateway Protocol. It is how routers on the Internet announce to each other what IPs sit behind them and therefore what traffic they should receive.

Starting at 10:04 UTC, there were a significant number of BGP updates. A BGP update is the signal a router makes to say that a route has changed or is no longer available. Under normal conditions, the Internet sees about 1.5MBs – 2MBs of BGP updates every 15 minutes. At the start of the incident, the number of BGP updates spiked to more than 26MBs of BGP updates per 15 minute period and stayed elevated throughout the incident.

These updates show the instability of BGP routes inside the CenturyLink/Level(3) backbone. The question is what would have caused this instability. The CenturyLink/Level(3) status update offers some hints and points at a flowspec update as the root cause.

What’s Flowspec?

In CenturyLink/Level(3)’s update they mention that a bad Flowspec rule caused the issue. So what is Flowspec? Flowspec is an extension to BGP, which allows firewall rules to be easily distributed across a network, or even between networks, using BGP. Flowspec is a powerful tool. It allows you to efficiently push rules across an entire network almost instantly. It is great when you are trying to quickly respond to something like an attack, but it can be dangerous if you make a mistake.

At Cloudflare, early in our history, we used to use Flowspec ourselves to push out firewall rules in order to, for instance, mitigate large network-layer DDoS attacks. We suffered our own Flowspec-induced outage more than 7 years ago. We no longer use Flowspec ourselves, but it remains a common protocol for pushing out network firewall rules.

We can only speculate what happened at CenturyLink/Level(3), but one plausible scenario is that they issued a Flowspec command to try to block an attack or other abuse directed at their network. The status report indicates that the Flowspec rule prevented BGP itself from being announced. We have no way of knowing what that Flowspec rule was, but here’s one in Juniper’s format that would have blocked all BGP communications across their network.

route DISCARD-BGP {
   match {
      protocol tcp;
      destination-port 179;
   }
 then discard;
}

Why So Many Updates?

A mystery remains, however, why global BGP updates stayed elevated throughout the incident. If the rule blocked BGP then you would expect to see an increase in BGP announcements initially and then they would fall back to normal.

One possible explanation is that the offending Flowspec rule came near the end of a long list of BGP updates. If that were the case, what may have happened is that every router in CenturyLink/Level(3)’s network would receive the Flowspec rule. They would then block BGP. That would cause them to stop receiving the rule. They would start back up again, working their way through all the BGP rules until they got to the offending Flowspec rule again. BGP would be dropped again. The Flowspec rule would no longer be received. And the loop would continue, over and over.

One challenge of this is that on every cycle, the queue of BGP updates would continue to increase within CenturyLink/Level(3)’s network. This may have gotten to a point where the memory and CPU of their routers was overloaded, causing an additional set of challenges to getting their network back online.

Why Did It Take So Long to Fix?

This was a significant global Internet outage and, undoubtedly, the CenturyLink/Level(3) team received immediate alerts. They are a very sophisticated network operator with a world class Network Operations Center (NOC). So why did it take more than four hours to resolve?

Again, we can only speculate. First, it may have been that the Flowspec rule and the significant load that large number of BGP updates imposed on their routers made it difficult for them to login to their own interfaces. Several of the other tier-1 providers took action, it appears at CenturyLink/Level(3)’s request, to de-peer their networks. This would have limited the number of BGP announcements being received by the CenturyLink/Level(3) network and helped give it time to catch up.

Second, it also may have been that the Flowspec rule was not issued by CenturyLink/Level(3) themselves but rather by one of their customers. Many network providers will allow Flowspec peering. This can be a powerful tool for downstream customers wishing to block attack traffic, but can make it much more difficult to track down an offending Flowspec rule when something goes wrong.

Finally, it never helps when these issues occur early on a Sunday morning. Networks the size and scale of CenturyLink/Level(3)’s are extremely complicated. Incidents happen. We appreciate their team keeping us informed with what was going on throughout the incident. #hugops