---
title: '2SYN: Dual Use in TCP and Quantum Memory'
url: https://www.emergentmind.com/topics/2syn
type: topic
---

# 2SYN: Dual Use in TCP and Quantum Memory

The label **2SYN** appears in current arXiv literature in two distinct technical senses. In computer networking, it denotes a congestion-aware multihoming algorithm for TCP that selects a WAN path for each new connection by racing duplicate SYN packets and committing the flow to the path that returns the first SYN-ACK [2508.20044]. In quantum and neuromorphic-device research, it denotes a **second-order synaptic memory** realized in twisted double bilayer graphene, where the programmable state is read out through a second-order nonlinear electrical response rather than only through linear conductance [2606.02931]. These usages are unrelated and belong to different research communities.

## 1. Name, scope, and domain-specific meanings

The networking usage is the only one that appears as an explicit paper title, namely **"2SYN: Congestion-Aware Multihoming"** [2508.20044]. The condensed-matter usage appears as an acronymic shorthand for **second-order synaptic memory**, introduced for a dual-gated, hBN-encapsulated AB–AB stacked twisted double bilayer graphene device [2606.02931].

| Usage of “2SYN” | Domain | Defining meaning |
|---|---|---|
| 2SYN | Networking | TCP multihoming via duplicate SYN transmission and first-SYN-ACK path selection |
| 2SYN | Neuromorphic quantum materials | Memory encoded in second-order nonlinear electrical response in tDBLG |

In the networking sense, 2SYN addresses enterprise multihoming in which a branch router has multiple WAN links such as LTE, DSL, cable, or fiber and must choose a path for outbound TCP flows to arbitrary public destinations, including previously unseen servers [2508.20044]. In the neuromorphic sense, 2SYN addresses the realization of synaptic functionality in a single moiré quantum material without charge-trapping layers, ferroelectric gate stacks, ionic motion, or phase-change media [2606.02931].

## 2. 2SYN as congestion-aware multihoming

In networking, 2SYN is defined as a **congestion-aware multihoming algorithm for TCP** that works for **any destination**, including previously unseen ones [2508.20044]. Its central design constraint is that the router cannot assume remote cooperation, cannot require MPTCP support, and cannot rely on an SD-WAN endpoint, tunnel, or cooperating gateway at the far side. The destination may be any public Internet server.

The mechanism exploits a universally available TCP signal: the **3-way handshake**. For a new TCP flow, identified by its **5-tuple**, the multihoming router duplicates the outbound SYN across all available WAN links. Because each WAN link has its own public-facing source address, the router performs **NAT** so that each duplicate SYN uses the IP address of the corresponding outgoing WAN interface. The destination therefore sees multiple ordinary connection attempts, one per WAN path [2508.20044].

The router then waits for the **first SYN-ACK**. The path on which that SYN-ACK arrives is taken to be the preferred path for the connection, corresponding to the path with the **shortest initial RTT**. Once the winner is known, the router installs per-flow routing state, records the choice in an **Active Flow (AF) table**, and pins the connection to that WAN link for its lifetime. Duplicate connection attempts on losing path(s) are cancelled by sending a **RST** to the destination [2508.20044].

The workflow is therefore connection-establishment-centric rather than throughput-estimation-centric. It uses only locally visible handshake packets—**SYN**, **SYN-ACK**, **FIN**, and **RST**—and it does not depend on historical measurements, active probes, or destination-specific training. This is what allows it to operate for arbitrary TCP destinations, including those with no prior traffic history [2508.20044].

## 3. Implementation model, evaluation, and operational behavior

The Linux proof-of-concept combines **iptables** for packet classification and SYN duplication, **Linux routing tables** for per-flow path selection, **NAT** for source rewriting, and **Python Scapy** with **BPF filters** for monitoring only control packets of interest [2508.20044]. On SYN arrival, the router marks the flow as awaiting its first SYN-ACK; on first SYN-ACK arrival, it adds the flow to the AF table and installs the corresponding routing-table entry; on **FIN** or **RST**, it removes the state from both tables.

The reported overhead is modest in the prototype. Adding or removing a route takes about **4 ms on average**; AF-table operations are effectively \(O(1)\); and when processing a burst of **100 parallel SYN arrivals**, peak CPU overhead was about **3%** [2508.20044]. The traffic overhead per new TCP flow is one additional SYN, one additional SYN-ACK, and one RST for the non-selected path. The destination-side half-open-connection overhead is estimated as roughly
\[
RTT / FCT_D
\]
and, under the paper’s assumption that median \(FCT_D\) is about **100 RTTs** based on a median flow duration around **6 s**, this is estimated around **1%** [2508.20044].

The evaluation uses both a controlled lab and real LTE-versus-wired experiments. The lab testbed employs five Ubuntu 20.04 Linux servers, **iPerf3 3.9**, and **NetEm**. Baselines are: always choose path 1, always choose path 2, choose randomly with equal probabilities, and **2SYN** [2508.20044]. In one experiment with both paths at **300 Mbps** but RTTs of **120 ms** and **80 ms**, 2SYN always selected the lower-delay path for **1 MB** downloads repeated **20 times**. In another, both paths had RTT **120 ms** and bandwidth **300 Mbps**, but one path carried background TCP traffic of **five flows**, each limited to **100 Mbps**; 2SYN chose the less congested path and achieved flow-completion times close to the better fixed-path baseline [2508.20044].

The real wide-area study places the source at the Technion in Israel and uses public SpeedTest servers in England. The two links are **path 1**, a standard **4G LTE** link via cellphone, and **path 2**, a wired link throttled to **100 Mbps download / 10 Mbps upload** [2508.20044]. Workloads include **30 MB files**, sent **20 times**, and a **web-search traffic mix** consisting of **62%** small files \(<100\text{ KB}\), **18%** medium files \(100\text{ KB} - 1\text{ MB}\), and **20%** large files \(>1\text{ MB}\). Across “no issues,” bandwidth-drop, and congestion-increase scenarios, 2SYN switched paths when the DSL-like link degraded to **5 Mbps** or when **ten TCP flows** were injected as cross-traffic, and it outperformed fixed-path baselines in the representative cases reported [2508.20044].

## 4. Formal properties, comparative position, and limitations

2SYN is positioned against three classes of alternatives: **static failover**, **static load balancing**, and **history-based learning** [2508.20044]. Static failover prefers one link until failure; static load balancing distributes flows randomly, round-robin, uniformly, or by weights; and lightweight machine-learning baselines are modeled as **multi-armed bandits**, specifically \(\epsilon\)-greedy, UCB, and Thompson sampling. Under fixed path conditions, the bandit methods eventually converge to near-optimal behavior, but when conditions change—such as a path-2 bandwidth drop from **300 Mbps** to **100 Mbps** after **40%** of files—2SYN adapts immediately at connection setup and clearly outperforms the MAB approaches [2508.20044].

The paper also states a limited optimality theorem. If one path has strictly lower RTT for every packet/ACK pair than all other paths, and packets and ACKs experience no reordering on any path, then always using that path is optimal and **2SYN is also optimal**, achieving the same lowest flow completion time [2508.20044]. A corollary extends this to the case where one path has strictly lower round-trip propagation time and all paths have the same bottleneck capacities with negligible queueing delay.

The main limitations are explicit. First, 2SYN optimizes **only at connection establishment**. Because WAN links typically imply different source IP addresses, ordinary TCP semantics forbid path changes mid-connection; current flows cannot be migrated if the chosen path degrades after the handshake [2508.20044]. Second, the first SYN-ACK is only a **proxy** for eventual flow completion time. The paper notes that this may hurt when a higher-RTT path later provides higher throughput, especially for large flows. Third, the design is **TCP-specific**. The paper explicitly discusses difficulties for **QUIC** and notes that for **UDP** or other non-handshake protocols, duplicating the first packet would duplicate application data [2508.20044]. Fourth, scalability to large \(k\) is conceptually straightforward but incurs one more duplicate SYN and one more half-open attempt per additional path; the experimental focus is therefore on the common case \(k=2\).

A common misconception is to equate 2SYN with MPTCP or SD-WAN overlays. The paper is explicit that it requires **no changes at the end hosts or applications**, no destination cooperation, and no remote tunnel endpoint [2508.20044]. Its novelty lies precisely in remaining compatible with ordinary TCP and arbitrary Internet destinations.

## 5. 2SYN as second-order synaptic memory

A separate usage of **2SYN** denotes **second-order synaptic memory** in a single moiré quantum material, **twisted double bilayer graphene (tDBLG)** [2606.02931]. Here the device is a **dual-gated, hBN-encapsulated AB–AB stacked tDBLG** in which two properties coexist: **electronic plasticity / hysteretic memory** under vertical displacement field and a **second-order nonlinear electrical response** whose sign and magnitude are strongly tunable by carrier concentration and displacement field.

The defining distinction from conventional electronic synapses is that the memory state is encoded in the **second-order electrical output**, not only in first-order conductance. The nonlinear transport expansion is written as
\[
j_i = \sigma_{ij} E_j + \chi_{ijk} E_j E_k + \cdots,
\]
with the second-order current density
\[
j_a^{2\omega} = \sigma_{abc}^{2\omega} E_b^{\omega} E_c^{\omega}.
\]
Experimentally, the device is read out through longitudinal and transverse second-harmonic voltages,
\[
V_{xx}^{2\omega}, \qquad V_{xy}^{2\omega},
\]
with the measured relation
\[
V_{xy(xx)}^{2\omega} \propto \left(V_{xx}^{\omega}\right)^2
\]
verified for drive currents from **30 nA to 300 nA** [2606.02931].

The memory arises from **\(D\)-dependent hysteresis** in sheet resistance and nonlinear response, especially near integer moiré fillings. The authors argue against interfacial charge traps and conventional ferroelectricity and instead propose a mechanism based on **intrinsic strain-disorder-related metastability**, with **flexoelectricity associated with strain and strain gradients** in low-angle tDBLG providing the more compelling explanation [2606.02931]. Hysteresis appears in low-angle devices, is absent in the high-angle control sample at \(\theta \approx 10^\circ\), and persists up to **100 K**.

Programming is performed using **triangular displacement-field pulses**. A fixed negative reset pulse at
\[
D/\epsilon_0 = -0.675\,\mathrm{V\,nm^{-1}}
\]
is followed by a positive set pulse varied typically from
\[
0.2 \text{ to } 0.675\,\mathrm{V\,nm^{-1}}
\]
at a sweep rate of
\[
0.05\,\mathrm{V\,nm^{-1}\,s^{-1}}.
\]
At a read point around
\[
D/\epsilon_0 = 0.1\,\mathrm{V\,nm^{-1}},
\]
different pulse histories yield different metastable values of \(V_{xx}^{2\omega}\) and \(V_{xy}^{2\omega}\), which the authors interpret as **potentiation** and **depression** in the second-order signal [2606.02931].

The reported figures of merit are unusual for a nonlinear synaptic element: **more than 16 multilevel synaptic states**, endurance over **\(\gg 100\)** cycles, retention with decay as low as **\(\sim 0.2\%\) per day**, and an estimated synaptic-event energy
\[
E \approx 0.5 - 0.8\,\mathrm{pJ}
\]
at
\[
I_{ds}=200\,\mathrm{nA}.
\]
The broader significance is that 2SYN in this sense is a programmable **nonlinear susceptibility element**, not merely a programmable resistor [2606.02931].

## 6. Terminological ambiguity and adjacent labels

Because the two explicit uses of **2SYN** are unrelated, the term is intrinsically ambiguous outside context. In networking, it refers to a **two-SYN** handshake-racing procedure for multihomed TCP routing [2508.20044]. In moiré-device physics, it abbreviates **second-order synaptic memory** [2606.02931]. The overlap is lexical rather than conceptual.

There are also nearby labels that can be confused with 2SYN in search or OCR pipelines. The astronomy paper **"The Synchronous Network of distant Telescopes"** uses the acronym **SNT**, not 2SYN, even though the underlying concept is a synchronized multi-site observational system with GPS-based UTC alignment to **1 microsecond and better** [1112.3999]. The paper explicitly notes that an apparent “2SYN” query may arise from a terminology mismatch or acronym confusion rather than from the actual system name. This makes contextual disambiguation essential: a networking query centered on TCP, SYN packets, NAT, or WAN links almost certainly concerns the multihoming algorithm, whereas a materials query involving tDBLG, \(V^{2\omega}\), hysteresis, or moiré superlattices concerns the second-order synaptic memory.

In present arXiv usage, then, **2SYN** is not a single unified technical doctrine. It is a homonymous label spanning at least a router-level path-selection mechanism and a moiré-material synaptic device, each defined by a distinct higher-order selection or readout principle and each anchored in its own specialized literature [2508.20044] [2606.02931].

Source: https://www.emergentmind.com/topics/2syn