Papers We Love Sept. 2018: 007: Democratically Finding The Cause of Packet Drops
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Network failures continue to plague datacenter operators as their symptoms may not have direct correlation with where or why they occur. We introduce 007, a lightweight, always-on diagnosis application that can find problematic links and also pinpoint problems for each TCP connection. 007 is completely contained within the end host. During its two month deployment in a tier-1 datacenter, it detected every problem found by previously deployed monitoring tools while also finding the sources of other problems previously undetected.
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Papers We Love Sept. 2018: 007: Democratically Finding The Cause of Packet Drops
- 1. Papers We Love Sept. 2018 007: Democratically Finding The Cause of Packet Drops Michael Kehoe Staff Site Reliability Engineer NDSI - https://www.usenix.org/conference/nsdi18/presentation/arzani
- 2. 007: Democratically Finding The Cause of Packet Drops Behnaz Arzani Selim Ciraci Luiz Chamon Yibo Zhu Hongqiang Liu Jitu Padhye Boon Thau Loo Geoff Outhred
- 3. Today’s agenda 1 Introduction & Motivation 2 TCP Monitoring Agent 3 Path Discovery Agent 4 Analysis Agent 5 Evaluations: Simulations 6 Evaluations: Production 7 Discussion
- 5. Introduction & Motivation “Even a small network outage or a few lossy links can cause the VM to “panic” and reboot. In fact, 17% of our VM reboots are due to network issues and in over 70% of these none of our monitoring tools were able to find the links that caused the problem.”
- 6. Introduction & Motivation • Pingmesh [1] • Leaves gaps • Overhead • Out-of-band
- 7. Introduction & Motivation • Roy et al [2] • Requires modifications to routers • Requires additional features on switches
- 8. Introduction & Motivation • Everflow [3] • Requires all traffic to be captured
- 9. “In a network of ≥ 106 links it’s a reasonable assumption that there is a non-zero chance that a number (> 10) of these links are bad (due to device, port, or cable, etc.)…However, currently we do not have a direct way to correlate customer impact with bad links". Introduction & Motivation
- 10. “007 records the path of TCP connections (flows) suffering from one or more retransmissions and assigns proportional “blame” to each link on the path. It then provides a ranking of links that represents their relative drop rates.” Introduction & Motivation
- 11. Introduction & Motivation 1. Does not require any changes to network infrastructure 2. Does not require any changes to client software 3. Detects in-band failures 4. Resilient to noise 5. Negligible overhead
- 12. Assumptions DISCUSSION 1. L2 networks are not viable unless; 1. Support path discovery methods 2. Supports EverFlow 2. No use of Source NATs (SNATs) 3. Assumes ECMP (L3) Clos network design 4. Don’t try to reverse-engineer ECMP
- 14. Design Overview
- 15. Design Overview • TCP monitoring agent: detects retransmissions at each end-host. • Path discovery agent: which identifies the flow’s path to the Destination IP (DIP) • At the end-hosts, a voting scheme is used based on the paths of flows that had retransmissions. At regular intervals of 30s the votes are tallied by a centralized analysis agent to find the top- voted links.
- 16. Design Overview • 6000 lines of C++ code • 600KB memory usage • 1-3% CPU Usage • 200 KBs bandwidth utilization
- 18. TCP Monitoring Agent • TCP Monitoring agent notifies Path Discovery Agent immediately after any retransmit • Use of ‘Event Tracing for Windows’ (ETW) • Could use BPF in Linux
- 20. Path Discovery Agent “The path discovery agent uses traceroute packets to find the path of flows that suffer retransmissions. These packets are used solely to identify the path of a flow. They do not need to be dropped for 007 to operate”
- 21. Path Discovery Agent “Once the TCP monitoring agent notifies the path discovery agent that a flow has suffered a retransmission, the path discovery agent checks its cache of discovered path for that epoch…It then sends 15 appropriately crafted TCP packets with TTL values ranging from 1–15.”
- 22. Path Discovery Agent ENGINEERING CHALLENGES – ECMP • ECMP algorithms are unknown • All packets of a given flow, defined by the five-tuple, follow the same path
- 23. Path Discovery Agent ENGINEERING CHALLENGES – RE-ROUTING & PACKET DROPS • Traceroute itself may fail • A lossy link may cause one or more BGP sessions to fail, triggering rerouting
- 24. Path Discovery Agent ENGINEERING CHALLENGES – ROUTER ALIASING • Have a pre-mapped topology of: • Switch/Router names • Router/ Interface IP addresses
- 25. Analysis Agent
- 26. Analysis Agent VOTING BASED SCHEME • Good votes are 0 • Bad votes are 1 ℎ where h is the number of hops on the path • Each link on the path is given a vote
- 27. Analysis Agent 4 2 1 3 0 0+ 1/2 1/2 + 1/2 + 1/2
- 28. Analysis Agent VOTING BASED SCHEME • Congestion & single drops are akin to noise • Single flow is unlikely to go through more than one failed link • Probability of errors in results diminishes exponentially with the number of flows
- 29. Simulations
- 30. Simulations PERFORMANCE • Accuracy: Proportion if correctly identified drop causes • Recall: How many of the failures are detected (false negatives) • Precision: How trusted are the results (false positives)
- 31. Evaluation: Simulations PERFORMANCE: OPTIMAL CASE • 0.05 -1% drop rate • Accuracy is > 96% • Recall/ Precision is almost always 100% https://github.com/behnazak/Vigil-007SourceCode
- 32. Evaluation: Simulations PERFORMANCE: VARYING DROP RATES • Maintains accuracy for both single and multiple failures https://github.com/behnazak/Vigil-007SourceCode
- 33. Evaluation: Simulations PERFORMANCE: IMPACT OF NOISE • Almost no impact https://github.com/behnazak/Vigil-007SourceCode
- 34. Evaluation: Simulations PERFORMANCE: NUMBER OF CONNECTIONS • Almost no impact https://github.com/behnazak/Vigil-007SourceCode
- 35. Evaluation: Simulations PERFORMANCE: TRAFFIC SKEWS • Can tolerate 50% skew • When TOR traffic >50% & >10 failures, accuracy suffers https://github.com/behnazak/Vigil-007SourceCode
- 36. Evaluation: Simulations PERFORMANCE: BAD LINKS • 007 can detect up to 7 failures with accuracy > 90% https://github.com/behnazak/Vigil-007SourceCode
- 37. Evaluation: Simulations PERFORMANCE: NETWORK SIZE • Single failure: • Accuracy >98% for up to 6 pods • Multiple failures: • Accuracy >98.01% for 30 failed links https://github.com/behnazak/Vigil-007SourceCode
- 39. Evaluation: Production • 007 located bad link correctly in 281 cases of VM reboot in Microsoft DCN • Identifies average 0.45 ± 0.12 as bad per epoch • Of links dropping packets: • 48%: Server to TOR • 24%: T1 – TOR
- 40. Discussion
- 41. Discussion • Congestion detection • Ranking with bias • Finding the cause of other problems • 007 can also be used for: • Detection of switch failures
- 42. Questions?
Editor's Notes
- List citations (count) Particular background on authors
- So from the beginning of the paper, they very precisely state the problem they are trying to solve: Find the link that dropped the packet and do so with negligible overhead and no changes to the network infrastructure “Even a small network outage or a few lossy links can cause the VM to “panic” and reboot. In fact, 17% of our VM reboots are due to network issues and in over 70% of these none of our monitoring tools were able to find the links that caused the problem.”
- This problem of course isn’t new at all. There has been a reasonable amount of research into link & forwarding failure detection. The paper does at the beginning highlight some very similar research and in section 10, there’s a deep-dive on related work and how this solution meets the stated goals. Firstly they mention Pingmesh, which is a Microsoft project. Pingmesh does have gaps, it doesn’t guarantee that it will cover all links There is a reasonable overhead on both CPU & Network It also does out-of-band link testing, so you don’t necessarily create the same conditions the application sees
- Roy et al Monitors all paths Requires modification to routers Special features in the switch
- Everflow Requires all traffic to be captured Not scalable
- They also dive a little deeper into the motivation and the reason why you need a reliable faulty link detection: So if you have a network of 10^6, or more than 1 million links, there is a non-zero chance you’ll have more than 10 of these links bad for various reasons Generally it’s hard determine customer impact and then prioritize the remediation of these links
- So how does 007 do this? “007 records the path of TCP connections (flows) suffering from one or more retransmissions and assigns proportional “blame” to each link on the path. It then provides a ranking of links that represents their relative drop rates.”
- Some direct benefits are: Does not require any changes to network infrastructure Does not require any changes to client software Detects in-band failures Resilient to noise (ability to filter out congestion vs faulty link) Negligible overhead
- So the assumptions part of the paper is actually at the back, but I think these are worth mentioning now: L2 networks are not supported unless there are path discovery methods or support everflow It’s assumed that there is no source-natting 007 assumes a Clos topology (which is a L3 network) Do not try to reverse-engineer ECMP
- So lets look at the design of 007
- TCP Monitoring agent that detects retransmissions on each end-host The Path Discovery Agent identifies the flow to the destination IP Finally an off-site analysis agent gathers votes every 30 seconds (a) we only track the path of those flows that have retransmissions, (b) assign each link on the path of such a flow a vote of 1/h, where h is the path length, and (c) sum up the votes during a given period, then the top-voted links are almost always the ones dropping packets
- If you recall, one of the key objectives of the project is to have low resource utilization. As you can see, this is all pretty lightweight
- So first is the TCP Monitoring Agent This code simply notifies the path discovery agent that there is a TCP retransmission. It will pass on a 5-tuple address In Microsoft’s case, they use the Event Tracing for Windows (or ETW) to handle this process. In Linux, it’s possible to use BPF to do this and you can find similar sample code from Brendan Gregg
- So the Path Discovery Agent gets the five-tuple address and then runs a traceroute between the destination end-host to the source. These packets are ONLY used to identify the path of the flow, they do not need to be dropped to be of use.
- There is a cache of discovered paths that help lower the amount of traceroutes we need to do If the cache doesn’t have a hit, 15 TCP packets are sent, with increasing TTL’s 1-15 to discover the path.
- The paper assumes that it’s possible to translate link IP’s to router and switch names and understand the network topology
- 007 has been designed for a specific use case, namely finding the cause of packet drops on individual connections in order to provide application context. This resulted in a number of design choices: Congestion usually comes in the form of very small packet-loss. In this paper’s case, 92% of congestion was between 10^-5 and 10^-8. These are treated as noise The ranking approach will become biased for highly traffic’ed links. This is generally ok as these links are the most impacted by faults. Can be used to find issues with switches, not just links