Network.framework API Reference

Overview

Network.framework is Apple's modern networking API that replaces Berkeley sockets, providing smart connection establishment, user-space networking, built-in TLS support, and seamless mobility. Introduced in iOS 12 (2018) with NWConnection and evolved in iOS 26 (2025) with NetworkConnection for structured concurrency.

Evolution timeline

• 2018 (iOS 12) NWConnection with completion handlers, deprecates CFSocket/NSStream/SCNetworkReachability
• 2019 (iOS 13) User-space networking (30% CPU reduction), TLS 1.3 default
• 2025 (iOS 26) NetworkConnection with async/await, TLV framing built-in, Coder protocol, Wi-Fi Aware discovery

Key capabilities

• Smart connection establishment Happy Eyeballs (IPv4/IPv6 racing), proxy evaluation (PAC), VPN detection, WiFi Assist fallback
• User-space networking ~30% lower CPU usage vs sockets, memory-mapped regions, reduced context switches
• Built-in security TLS 1.3 by default, DTLS for UDP, certificate pinning support
• Mobility Automatic network transition handling (WiFi ↔ cellular), viability notifications, Multipath TCP
• Performance ECN (Explicit Congestion Notification), service class marking, TCP Fast Open, UDP batching

When to use vs URLSession

• URLSession HTTP, HTTPS, WebSocket, simple TCP/TLS streams → Use URLSession (optimized for these)
• Network.framework UDP, custom protocols, low-level control, peer-to-peer, gaming, streaming → Use Network.framework

Related Skills

• Use axiom-networking for anti-patterns, common patterns, pressure scenarios
• Use axiom-networking-diag for systematic troubleshooting of connection failures

---

When to Use This Skill

Use this skill when:

• Planning migration from BSD sockets, CFSocket, NSStream, or SCNetworkReachability
• Understanding API differences between NWConnection (iOS 12-25) and NetworkConnection (iOS 26+)
• Implementing all 12 WWDC 2025 examples (TLS connection, TLV framing, Coder protocol, NetworkListener, Wi-Fi Aware)
• Choosing protocols (TCP, UDP, TLS, QUIC) for your use case
• Peer-to-peer discovery setup with NetworkBrowser and Wi-Fi Aware
• Optimizing performance with user-space networking, batching, pacing
• Migrating from completion handlers to async/await (NWConnection → NetworkConnection)

---

API Evolution

Timeline

Year	iOS Version	Key Features
2018	iOS 12	NWConnection, NWListener, NWBrowser introduced
2019	iOS 13	User-space networking (30% CPU reduction), TLS 1.3 default
2021	iOS 15	WebSocket support in URLSession
2025	iOS 26	NetworkConnection (async/await), TLV framing, Coder protocol, Wi-Fi Aware

NWConnection (iOS 12-25) vs NetworkConnection (iOS 26+)

Feature	NWConnection (iOS 12-25)	NetworkConnection (iOS 26+)
Async model	Completion handlers	async/await structured concurrency
State updates	stateUpdateHandler callback	states AsyncSequence
Send	send(content:completion:) callback	try await send(content) suspending
Receive	receive(minimumIncompleteLength:maximumLength:completion:)	try await receive(exactly:) suspending
Framing	Manual or custom NWFramer	TLV built-in (TLV { TLS() })
Codable	Manual JSON encode/decode	Coder protocol (Coder(MyType.self, using: .json))
Memory	Requires [weak self] in all closures	No [weak self] needed (Task cancellation automatic)
Error handling	Check error in completion	throws with natural propagation
State machine	Callbacks on state changes	for await state in connection.states
Discovery	NWBrowser (Bonjour only)	NetworkBrowser (Bonjour + Wi-Fi Aware)

Recommendation

• New apps targeting iOS 26+: Use NetworkConnection (cleaner, safer)
• Apps supporting iOS 12-25: Use NWConnection (backward compatible)
• Migration: Both APIs coexist, migrate incrementally

---

NetworkConnection (iOS 26+) Complete Reference

4.1 Creating Connections

NetworkConnection uses declarative protocol stack composition.

Example 1: Basic TLS Connection (WWDC 4:04)

import Network

// Basic connection with TLS (TCP and IP inferred)
let connection = NetworkConnection(
    to: .hostPort(host: "www.example.com", port: 1029)
) {
    TLS()
}

// Send and receive with async/await
public func sendAndReceiveWithTLS() async throws {
    let outgoingData = Data("Hello, world!".utf8)
    try await connection.send(outgoingData)

    let incomingData = try await connection.receive(exactly: 98).content
    print("Received data: \(incomingData)")
}

Key points

• TLS() infers TCP() and IP() automatically
• No explicit connection.start() needed (happens on first send/receive)
• Async/await eliminates callback nesting

Example 2: Custom IP Options (WWDC 4:41)

// Customize IP fragmentation
let connection = NetworkConnection(
    to: .hostPort(host: "www.example.com", port: 1029)
) {
    TLS {
        TCP {
            IP()
                .fragmentationEnabled(false) // Disable IP fragmentation
        }
    }
}

When to customize IP

• .fragmentationEnabled(false) — For protocols that handle fragmentation themselves (QUIC)
• .ipVersion(.v6) — Force IPv6 only (testing)

Example 3: Custom Parameters (WWDC 5:07)

// Constrained paths (low data mode) + custom IP
let connection = NetworkConnection(
    to: .hostPort(host: "www.example.com", port: 1029),
    using: .parameters {
        TLS {
            TCP {
                IP()
                    .fragmentationEnabled(false)
            }
        }
    }
    .constrainedPathsProhibited(true) // Don't use cellular in low data mode
)

Common parameters

• .constrainedPathsProhibited(true) — Respect low data mode
• .expensivePathsProhibited(true) — Don't use cellular/hotspot
• .multipathServiceType(.handover) — Enable Multipath TCP

Endpoint Types

// Host + Port
.hostPort(host: "example.com", port: 443)

// Service (Bonjour)
.service(name: "MyPrinter", type: "_ipp._tcp", domain: "local.", interface: nil)

// Unix domain socket
.unix(path: "/tmp/my.sock")

Protocol Stack Composition

// TLS over TCP (most common)
TLS()

// QUIC (TLS + UDP, multiplexed streams)
QUIC()

// UDP (datagrams)
UDP()

// TCP (stream, no encryption)
TCP()

// WebSocket over TLS
WebSocket {
    TLS()
}

// Custom framing
TLV {
    TLS()
}

---

4.2 State Machine

NetworkConnection transitions through these states:

setup
  ↓
preparing (DNS, TCP handshake, TLS handshake)
  ↓
┌─ waiting (no network, retrying)
│    ↓
└→ ready (can send/receive)
     ↓
  failed (error) or cancelled

Monitoring States

// Option 1: Async sequence (monitor in background)
Task {
    for await state in connection.states {
        switch state {
        case .preparing:
            print("Connecting...")
        case .waiting(let error):
            print("Waiting for network: \(error)")
        case .ready:
            print("Connected!")
        case .failed(let error):
            print("Failed: \(error)")
        case .cancelled:
            print("Cancelled")
        @unknown default:
            break
        }
    }
}

Key states

• .preparing DNS lookup, TCP SYN, TLS handshake
• .waiting No network available, framework retries automatically
• .ready Connection established, can send/receive
• .failed Unrecoverable error (server refused, TLS failed, timeout)
• .cancelled Task cancelled or connection.cancel() called

---

4.3 Send/Receive Patterns

Send: Basic