Scaling HTTP/3 QUIC Header Compression (8623)

Technical Overview

Engineering breakdown of Scaling HTTP/3 QUIC Header Compression (8623). Bare-metal hardware performance requires isolated kernel parameters, zero-copy packet processing, and deterministic memory allocation. At Votion Cloud, we've observed that QUIC's QPACK dynamic table synchronization becomes the primary bottleneck at 100Gbps+ edge throughput due to head-of-line blocking in the decoder stream.

Root Cause Analysis

• Encoder/Decoder State Divergence: Concurrent streams cause dynamic table updates to arrive out-of-order, forcing decoder blocking.
• Memory Pressure: Each connection maintains a 4KB dynamic table; 1M concurrent connections = 4GB resident memory.
• CPU Cache Thrashing: Huffman decoding and integer representation parsing dominate cycles.

Our solution combines stream-local static dictionaries, lock-free ring buffers for header blocks, and eBPF-assisted packet steering to isolate compression contexts per CPU core.

Architecture: Per-Core QPACK Contexts

We partition the QUIC connection space across CPU cores using RSS (Receive Side Scaling) with a custom Toeplitz hash on the 4-tuple. Each core owns a dedicated QPACK encoder/decoder pair with a pre-warmed static dictionary derived from the top 10,000 header fields observed in production traffic.

// Per-core context initialization
struct qpack_ctx {
    uint8_t static_table[STATIC_TABLE_SIZE];
    uint8_t dynamic_table[DYNAMIC_TABLE_SIZE];
    uint32_t insert_count;
    uint32_t drop_count;
    spinlock_t lock; // only for dynamic table eviction
} __attribute__((aligned(64)));

static __thread struct qpack_ctx *core_ctx;

void qpack_init_per_core(void) {
    core_ctx = aligned_alloc(64, sizeof(*core_ctx));
    memcpy(core_ctx->static_table, global_static_table, STATIC_TABLE_SIZE);
    core_ctx->insert_count = 0;
    core_ctx->drop_count = 0;
}

This eliminates cross-core synchronization for 95% of header operations. The remaining 5% (dynamic table inserts) use a lock-free MPMC queue to a dedicated compression thread per NUMA node.

Benchmark Results: 400Gbps Edge Node

Deployed on dual-socket AMD EPYC 9654 (192 cores) with 200Gbps NICs. Tested with 2M concurrent QUIC connections, 50% header compression ratio.

Key optimizations: batch header processing (vectorized Huffman decode), connection migration awareness (preserve dynamic table across path changes), and adaptive dynamic table sizing based on RTT and loss signals.