Network.framework Throughput Regression Repro (macOS 26 / Apple Silicon M4)

Product / Component

macOS -> Networking -> Network.framework (TCP/TLS performance)

Type

Performance

Summary

On macOS 26, server-side file transfer throughput is dramatically lower when using Network.framework (nw_listener + nw_connection_send) versus legacy CFStream (CFWriteStreamWrite) in the same application and environment.

Issue reproduces with both 64 KB and 512 KB application chunk sizes on Apple Silicon M4 systems (Mac mini M4 Pro and Mac Studio M4 Max).

The issue does not reproduce on Apple Silicon M1 running the same macOS version.

The legacy path (CFStream) does not show this slowdown on any tested computers.

Environment

• OS: macOS 26.4.1
• Hardware where issue reproduces: Apple Silicon M4
  • Mac mini M4 Pro
  • Mac Studio M4 Max
• Hardware where issue does not reproduce: Apple Silicon M1 (same OS)
• Network setup: client and server on same LAN
• Note: not reproducible on same machine via localhost

Expected Result

Throughput with Network.framework should be in the same range as CFStream for equivalent workload and environment (or at least not regress by an order of magnitude).

Actual Result

Network.framework path shows severe throughput degradation on macOS 26 (M4), independent of application chunk size (64 KB or 512 KB), while CFStream path is fast.

Notes

• Chunk-size tuning did not resolve the issue.
• Same application logic, same content, same machine/network.

Regression

Appears specific to macOS 26 behavior on M4.

Workaround

sudo sysctl net.inet.tcp.tso=0

After this change, throughput returns to normal levels.

---

This Repository: Minimal Repro Project

This Swift command-line project provides two server modes for A/B comparison in one binary:

• Default mode: Network.framework (NWListener + NWConnection.send)
• --legacyFramework mode: legacy CFStream (CFWriteStreamWrite)

Behavior in both modes:

• listens on port 9090
• accepts HTTP GET
• streams random bytes indefinitely

Build

cd NetworkFrameworkBug
swift build -c release --build-path NetworkFrameworkBug/build-artifacts/swift-build

Run (Network.framework)

./NetworkFrameworkBug/build-artifacts/swift-build/release/NetworkFrameworkThroughputRepro

Run (Legacy CFStream)

./NetworkFrameworkBug/build-artifacts/swift-build/release/NetworkFrameworkThroughputRepro --legacyFramework

Run (Network.framework) with TSO disabled

sudo sysctl net.inet.tcp.tso=0
./NetworkFrameworkBug/build-artifacts/swift-build/release/NetworkFrameworkThroughputRepro

Quick Client Check

curl http://<server-ip>:9090/ -o /dev/null

Open in Xcode

Open Package.swift in Xcode.

Results

##Test A : Network Framework

Server side: ./NetworkFrameworkThroughputRepro
Client side:
admin@iMac ~ % curl http://10.17.18.119:9090/ -o /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 7682k    0 7682k    0     0  1011k      0 --:--:--  0:00:07 --:--:--  989k

--> 989 kB/s : very low throughput, not saturating the 1 Gbps link (theoretical max ~125 MB/s)

##Test B : CFStream (legacy framework)

Server side: ./NetworkFrameworkThroughputRepro --legacyFramework
Client side:
admin@iMac ~ % curl http://10.17.18.119:9090/ -o /dev/null --legacyframework                                                                               
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  905M    0  905M    0     0   103M      0 --:--:--  0:00:08 --:--:--  106M

--> 106 MB/s : much higher throughput, saturating the 1 Gbps link (theoretical max ~125 MB/s)

##Test C : Network Framework with TSO disabled

Server side: sudo sysctl net.inet.tcp.tso=0, then ./NetworkFrameworkThroughputRepro
Client side:
admin@iMac ~ % curl http://10.17.18.119:9090/ -o /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  623M    0  623M    0     0   101M      0 --:--:--  0:00:06 --:--:--  105M

--> 105 MB/s : much higher throughput, saturating the 1 Gbps link (theoretical max ~125 MB/s)