How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Window Clamp VXLAN-GPE Encap Drops in Linux Using eBPF

High-throughput carrier-grade cloud infrastructures and financial backbones rely heavily on RFC 5925 TCP Authentication Option (TCP-AO) to secure Border Gateway Protocol (BGP) peering and critical control-plane sockets. When these cryptographic transport sessions are encapsulated across multi-tenant VXLAN Generic Protocol Extension (VXLAN-GPE) overlay fabrics, asymmetric network paths can induce silent drop anomalies. In particular, when hardware-accelerated cryptographic keys undergo asymmetric rollover while the kernel enforces TCP window clamping, SYN-ACK retransmissions (tracked via tcpi_retrans_synack in tcp_info) spike dramatically. This operational guide demonstrates how to build and deploy an eBPF tracing program to isolate, measure, and rectify these encapsulated drops in the Linux kernel.

Mechanisms of TCP-AO Key Rollovers and VXLAN-GPE Encapsulation

TCP-AO replaces obsolete MD5 signature options (RFC 2385) by decoupling master key tuples (MKTs) from static session configurations. Each MKT identifies Send Key ID (KeyID) and Receive Key ID (RNextKeyID) attributes. During an orchestrated key rollover, asymmetric routing can cause ingress packets bearing a newly updated KeyID to reach an endpoint before the hardware cryptographic co-processor finishes programming the matching security association.

When coupled with VXLAN-GPE overlays, inner TCP segments carry outer UDP encapsulation headers, including Next Protocol identifiers (such as IPv4, IPv6, or Ethernet). If an inner TCP segment experiences window clamping due to transient buffer exhaustion along an intermediate switch port, the inner TCP window advertisement contracts. An asymmetric key discrepancy during a handshake causes the kernel TCP stack to discard the incoming segment before acknowledging the clamped window, triggering rapid SYN-ACK retransmission loops visible in tcpi_retrans_synack counters.

Targeting Kernel Tracepoints with eBPF

Standard utilities such as ss and netstat sample connection statistics at coarse intervals, making transient millisecond-level key rollover drops invisible. eBPF provides direct, safe, in-kernel observability by attaching kprobes and tracepoints to tcp_retransmit_skb, tcp_rcv_established, and tcp_ao_verify_hash.

// File: tcp_ao_vxlan_trace.bpf.c
#include <vmlinux.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_core_read.h>
#include <bpf/bpf_tracing.h>

struct drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 snd_wnd;
    u32 rcv_wnd;
    u8  key_id;
    u8  rnext_key_id;
};

struct {
    __uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
    __uint(key_size, sizeof(u32));
    __uint(value_size, sizeof(u32));
} events SEC(".maps");

SEC("kprobe/tcp_ao_verify_hash")
int BPF_KPROBE(trace_ao_verify, struct sock *sk, struct sk_buff *skb, struct tcp_ao_key *key) {
    struct tcp_sock *tp = (struct tcp_sock *)sk;
    u32 retrans = BPF_CORE_READ(tp, total_retrans);
    
    // Check if synack retransmissions are escalating under clamped window
    if (retrans > 0) {
        struct drop_event_t event = {};
        event.saddr = BPF_CORE_READ(sk, __sk_common.skc_rcv_saddr);
        event.daddr = BPF_CORE_READ(sk, __sk_common.skc_daddr);
        event.sport = BPF_CORE_READ(sk, __sk_common.skc_num);
        event.dport = bpf_ntohs(BPF_CORE_READ(sk, __sk_common.skc_dport));
        event.retrans_synack = BPF_CORE_READ(tp, tcpi_retrans_synack);
        event.snd_wnd = BPF_CORE_READ(tp, snd_wnd);
        event.rcv_wnd = BPF_CORE_READ(tp, rcv_wnd);

        bpf_perf_event_output(ctx, &events, BPF_F_CURRENT_CPU, &event, sizeof(event));
    }
    return 0;
}

char LICENSE[] SEC("license") = "GPL";

Compiling and Attaching the eBPF Program

Compile the eBPF bytecode using modern Clang/LLVM toolchains against your running distribution’s vmlinux header:

# Generate BPF bytecode
clang -g -O2 -target bpf -D__TARGET_ARCH_x86 -c tcp_ao_vxlan_trace.bpf.c -o tcp_ao_vxlan_trace.bpf.o

# Load and attach via bpftool
bpftool prog load tcp_ao_vxlan_trace.bpf.o /sys/fs/bpf/tcp_ao_vxlan_trace type kprobe
bpftool attach kprobe name tcp_ao_verify_hash pinned /sys/fs/bpf/tcp_ao_vxlan_trace

Interpreting Output and Mitigating Asymmetric Drops

When the userland consumer reads from the perf buffer, inspect the correlation between retrans_synack increments and snd_wnd clamp figures:

• Key ID Synchronization Mismatch: If key_id transitions while the remote peer continues emitting segments with the prior rnext_key_id, adjust your BGP router daemon’s AO key rollover grace timer (e.g., in FRRouting or BIRD) to allow concurrent validation windows of at least 60 seconds.
• VXLAN-GPE MTU/MSS Clamping: Encapsulation adds a minimum of 50 bytes of overhead (Outer Ethernet + IP + UDP + VXLAN-GPE). Ensure your outer physical interface MTU is configured to at least 1600 bytes (Jumbo frames) or enforce path MTU clamping via iptables/nftables:

  nft add rule ip filter FORWARD tcp flags syn tcp option maxseg size set rt mtu - 50

• Kernel Socket Buffer Tuning: Prevent artificial receive window clamps by optimizing kernel socket memory ceilings in /etc/sysctl.conf:

  net.ipv4.tcp_rmem = 4096 87380 16777216
  net.ipv4.tcp_wmem = 4096 65536 16777216
  net.core.rmem_max = 16777216

Validating Socket Stability

Validate the resolution by querying the kernel’s native socket diagnostics using ss -tiepm. Observe that tcpi_retrans_synack stabilises at zero and cryptographic verification errors cease across automated key rotation cycles.