I am currently a Postdoctoral Researcher at CMU advised by Prof. Vyas Sekar. My research focuses on Internet routing and its implications on security and performance. I am particularly interested in the impact of the network on Blockchain systems. My most recent work is on cryptocurrencies' anonymity. I also enjoy working with programmable data planes.
During my studies, I have been a visiting student at MIT (2019), working on buffer management. I have also worked on analyzing data center traffic as a research intern at Microsoft Research, Redmond (2018), and Google, Mountain View (2017).
Before joining ETH, I earned my diploma in Electrical and Computer Engineering at the National Technical University of Athens, Greece.
ACM SIGCOMM 2022. Amsterdam, Netherlands (August 2022).
Today’s network devices share buffer across queues to avoid drops during transient congestion and absorb bursts. As the buffer-perbandwidth-unit in datacenter decreases, the need for optimal buffer utilization becomes more pressing. Typical devices use a hierarchical packet admission control scheme: First, a Buffer Management (BM) scheme decides the maximum length per queue at the device level and then an Active Queue Management (AQM) scheme decides which packets will be admitted at the queue level. Unfortunately, the lack of cooperation between the two control schemes leads to (i) harmful interference across queues, due to the lack of isolation; (ii) increased queueing delay, due to the obliviousness to the per-queue drain time; and (iii) thus unpredictable burst tolerance. To overcome these limitations, we propose ABM, Active Buffer Management which incorporates insights from both BM and AQM. Concretely, ABM accounts for both total buffer occupancy (typically used by BM) and queue drain time (typically used by AQM). We analytically prove that ABM provides isolation, bounded buffer drain time and achieves predictable burst tolerance without sacrificing throughput. We empirically find that ABM improves the 99th percentile FCT for short flows by up to 94% compared to the state-of-the-art buffer management. We further show that ABM improves the performance of advanced datacenter transport protocols in terms of FCT by up to 76% compared to DCTCP, TIMELY and PowerTCP under bursty workloads even at moderate load conditions.
ACM SIGCOMM Symposium on SDN Research 2021. Online (October 2021).
Internet routing can often be sub-optimal, with the chosen routes providing worse performance than other available policy-compliant routes. This stems from the lack of visibility into route performance at the network layer. While this is an old problem, we argue that recent advances in programmable hardware finally open up the possibility of performance-aware routing in a deployable, BGP-compatible manner. We introduce ROUTESCOUT, a hybrid hardware/software system supporting performance-based routing at ISP scale. In the data plane, ROUTESCOUT leverages P4-enabled hardware to monitor performance across policy-compliant route choices for each destination, at line-rate and with a small memory footprint. ROUTESCOUT’s control plane then asynchronously pulls aggregated performance metrics to synthesize a performance-aware forwarding policy. We show that ROUTESCOUT can monitor performance across most of an ISP’s traffic, using only 4 MB of memory. Further, its control can flexibly satisfy a variety of operator objectives, with sub-second operating times.
Doctoral dissertation. ETH Zurich. July 2021.
Cryptocurrencies are digital money operated by a set of nodes. The oldest and most widely-used cryptocurrency today, namely Bitcoin, gained its popularity thanks to its security properties: its openness, immutability, and anonymity. Indeed, Bitcoin offers all users an open platform to perform immutable transactions while hiding their real-world identity. Instead of relying on a central authority, Bitcoin nodes build an extensive overlay network between them and use consensus to agree on a set of transactions that are recorded within Bitcoin’s core data structure: the blockchain.
Bitcoin nodes communicate over the Internet infrastructure, which is composed of multiple networks called Autonomous Systems (ASes). In effect, any AS on the Internet forwarding path between two nodes can access the messages they exchange. The Bitcoin protocol does not specify how the Bitcoin peer-to- peer overlay network should be mapped to the Internet to maintain its security properties. As a result, Bitcoin’s security properties are in practice at risk. In this thesis, we aim at shedding light on the interactions between the application and the network layer by answering two questions.
Attack surface: What could be the impact of an AS-level adversary on the security properties of cryptocurrencies and Bitcoin in particular?
Defense: How can we shield such systems from an AS-level adversary?
In response to the first question, we prove that a single AS is able to compromise Bitcoin’s immutability, anonymity, and openness. In response to the second question, we show that we can protect cryptocurrencies from AS-level adversaries by leveraging Internet policies, state-of-the-art networking hardware and cross- layer awareness.
We start these efforts by analyzing the Bitcoin network from the routing perspective. Our findings contradict a core assumption about the Bitcoin network, namely its decentralization. From the application perspective, Bitcoin is indeed decentralized, as it is an open network of thousands of independent nodes that establish random connections. From the routing perspective, though, the Bitcoin network is highly centralized, as few ASes intercept a disproportionately large amount of Bitcoin traffic. Driven by this insight, we introduce a new attack vector, namely routing attacks. We uncover and ethically performed in the wild three novel routing attacks against Bitcoin: the partition, the delay, and the perimeter attack. These attacks generalize to other cryptocurrencies such as Ethereum. Other than the attacker’s goal, the three attacks differ in terms of their effectiveness and detectability. On the one hand, the partition attack is extremely powerful – even against the Bitcoin network as a whole – but it can be easily detected. On the other hand, the delay and the perimeter attacks are only effective against a targeted set of nodes, yet they are very hard to be detected.
To shield Bitcoin against the partition attack we design SABRE, a relay network that keeps the Bitcoin network connected even in the presence of an AS- level attacker. SABRE achieves this by leveraging Internet routing policies and emerging networking hardware. Notably, SABRE’s design is general and can be used to protect any blockchain system from such attacks.
Finally, to shield Bitcoin from harder-to-detect attacks such as the delay and the perimeter attack, we suggest a set of cross-layer countermeasures. In particular, we revisit the design and deployment choices of the Bitcoin (and the Ethereum) system to account for the risk of an AS-level adversary.
Communications of the ACM. (June 2021).
Attacks on Internet routing are typically viewed through the lens of availability and confidentiality, assuming an adversary that either discards traffic or performs eavesdropping. Yet, a strategic adversary can use routing attacks to compromise the security of critical Internet applications like Tor, certificate authorities, and the bitcoin network. In this paper, we survey such application-specific routing attacks and argue that both application-layer and network-layer defenses are essential and urgently needed. The good news is that, while deployment challenges have hindered the adoption of network-layer defenses (i.e. secure routing protocols) thus far, application-layer defenses are much easier to deploy in the short term.
Financial Cryptography and Data Security 2021. Grenada (March 2021).
Cryptocurrencies are widely used today for anonymous transactions. Such currencies rely on a peer-to-peer network where users can broadcast transactions containing their pseudonyms and ask for approval. Previous research has shown that application-level eavesdroppers, namely nodes connected to a large portion of the Bitcoin peer-to-peer network are able to deanonymize multiple users by tracing back the source of transactions. Yet, such attacks are highly visible as the attacker needs to maintain thousands of outbound connections. Moreover, they can be mitigated by purely application-layer countermeasures. This paper presents a stealthier and harder-to-mitigate attack exploiting the interactions between the networking and application layers. Particularly, the adversary combines her access over Internet infrastructure with application-layer information to deanonymize transactions. We show that PERIMETER is practical in today’s Internet, achieves high accuracy in Bitcoin, and generalizes to encrypted cryptocurrencies.
Today, network devices share buffer across priority queues to avoid drops during transient congestion. While cost-effective most of the time, this sharing can cause undesired interference among seemingly independent traffic. As a result, low-priority traffic can cause increased packet loss to high-priority traffic. Similarly, long flows can prevent the buffer from absorbing incoming bursts even if they do not share the same queue. The cause of this perhaps unintuitive outcome is that today’s buffer sharing techniques are unable to guarantee isolation across (priority) queues without statically allocating buffer space. To address this issue, we designed FB, a novel buffer sharing scheme that offers strict isolation guarantees to high-priority traffic without sacrificing link utilizations. Thus, FB outperforms conventional buffer sharing algorithms in absorbing bursts while achieving on-par throughput. We show that FB is practical and runs at line-rate on existing hardware (Barefoot Tofino). Significantly, FB’s operations can be approximated in non-programmable devices.
ACM HotNets 2020. Chicago, Illinois, USA (November 2020).
Programmable devices allow the operator to specify the data-plane behavior of a network device in a high-level language such as P4. The compiler then maps the P4 program to the hardware after applying a set of optimizations to minimize resource utilization. Yet, the lack of context restricts the compiler to conservatively account for all possible inputs -- including unrealistic or infrequent ones -- leading to sub-optimal use of the resources or even compilation failures. To address this inefficiency, we propose that the compiler leverages insights from actual traffic traces, effectively unlocking a broader spectrum of possible optimizations.
We present a system working alongside the compiler that uses traffic-awareness to reduce the allocated resources of a P4 program by: (i) removing dependencies that do not manifest; (ii) adjusting table and register sizes to reduce the pipeline length; and (iii) offloading parts of the program that are rarely used to the controller. Our prototype implementation on the Tofino switch automatically profiles the P4 program, detects opportunities and performs optimizations to improve the pipeline efficiency.
Our work showcases the potential benefit of applying profiling techniques used to compile general-purpose languages to compiling P4 programs.
ACM Workshop on Buffer Sizing. Stanford, CA, USA (December 2019).
Conventional buffer sizing techniques consider an output port with multiple queues in isolation and provide guidelines for the size of the queue. In practice, however, switches consist of several ports that share a buffering chip. Hence, chip manufacturers, such as Broadcom, are left to devise a set of proprietary resource sharing algorithms to allocate buffers across ports. This algorithm dynamically adjusts the buffer size for output queues and directly impacts the packet loss and latency of individual queues. We show that the problem of allocating buffers across ports, although less known, is indeed responsible for fundamental inefficiencies in today's devices. In particular, the per-port buffer allocation is an ad-hoc decision that (at best) depends on the remaining buffer cells on the chip instead of the type of traffic. In this work, we advocate for a flow-aware and device-wide buffer sharing scheme (FAB), which is practical today in programmable devices. We tested FAB on two specific workloads and showed that it can improve the tail flow completion time by an order of magnitude compared to conventional buffer management techniques.
USENIX NSDI 2019. Boston, Massachusetts, USA (February 2019).
We present Blink, a data-driven system that leverages TCP-induced signals to detect failures directly in the data plane. The key intuition behind Blink is that a TCP flow exhibits a predictable behavior upon disruption: retransmitting the same packet over and over, at epochs exponentially spaced in time. When compounded over multiple flows, this behavior creates a strong and characteristic failure signal. Blink efficiently analyzes TCP flows to: (i) select which ones to track; (ii) reliably and quickly detect major traffic disruptions; and (iii) recover connectivity---all this, completely in the data plane. We present an implementation of Blink in P4 together with an extensive evaluation on real and synthetic traffic traces. Our results indicate that Blink: (i) achieves sub-second rerouting for large fractions of Internet traffic; and (ii) prevents unnecessary traffic shifts even in the presence of noise. We further show the feasibility of Blink by running it on an actual Tofino switch.
NDSS Symposium 2019. San Diego, CA, USA (February 2019).
Nowadays Internet routing attacks remain practically effective as existing countermeasures either fail to provide protection guarantees or are not easily deployable. Blockchain systems are particularly vulnerable to such attacks as they rely on Internet-wide communications to reach consensus. In particular, Bitcoin---the most widely-used cryptocurrency---can be split in half by any AS-level adversary using BGP hijacking.
In this paper, we present SABRE, a secure and scalable Bitcoin relay network which relays blocks worldwide through a set of connections that are resilient to routing attacks. SABRE runs alongside the existing peer-to-peer network and is easily deployable. As a critical system, SABRE design is highly resilient and can efficiently handle high bandwidth loads, including Denial of Service attacks.
We built SABRE around two key technical insights. First, we leverage fundamental properties of inter-domain routing (BGP) policies to host relay nodes: (i) in networks that are inherently protected against routing attacks; and (ii) on paths that are economically-preferred by the majority of Bitcoin clients. These properties are generic and can be used to protect other Blockchain-based systems. Second, we leverage the fact that relaying blocks is communication-heavy, not computation-heavy. This enables us to offload most of the relay operations to programmable network hardware (using the P4 programming language). Thanks to this hardware/software co-design, SABRE nodes operate seamlessly under high load while mitigating the effects of malicious clients.
We present a complete implementation of SABRE together with an extensive evaluation. Our results demonstrate that SABRE is effective at securing Bitcoin against routing attacks, even with deployments of as few as 6 nodes.
IEEE Symposium on Security and Privacy 2017. San Jose, CA, USA (May 2017).
As the most successful cryptocurrency to date, Bitcoin constitutes a target of choice for attackers. While many attack vectors have already been uncovered, one important vector has been left out though: attacking the currency via the Internet routing infrastructure itself. Indeed, by manipulating routing advertisements (BGP hijacks) or by naturally intercepting traffic, Autonomous Systems (ASes) can intercept and manipulate a large fraction of Bitcoin traffic.
This paper presents the first taxonomy of routing attacks and their impact on Bitcoin, considering both small-scale attacks, targeting individual nodes, and large-scale attacks, targeting the network as a whole. While challenging, we show that two key properties make routing attacks practical: (i) the efficiency of routing manipulation; and (ii) the significant centralization of Bitcoin in terms of mining and routing. Specifically, we find that any network attacker can hijack few (<100) BGP prefixes to isolate 50% of the mining power—even when considering that mining pools are heavily multi-homed. We also show that on-path network attackers can considerably slow down block propagation by interfering with few key Bitcoin messages.
We demonstrate the feasibility of each attack against the deployed Bitcoin software. We also quantify their effectiveness on the current Bitcoin topology using data collected from a Bitcoin supernode combined with BGP routing data.
The potential damage to Bitcoin is worrying. By isolating parts of the network or delaying block propagation, attackers can cause a significant amount of mining power to be wasted, leading to revenue losses and enabling a wide range of exploits such as double spending. To prevent such effects in practice, we provide both short and long-term countermeasures, some of which can be deployed immediately.