How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Window Clamp SRv6 End.DX2 Encap Drops in Linux Using eBPF

High-throughput telecom infrastructure and enterprise Segment Routing over IPv6 (SRv6) fabrics rely on TCP Authentication Option (TCP-AO, RFC 5925) to cryptographically safeguard BGP peering sessions and transit tunnels. When combining hardware-accelerated key rollover with SRv6 End.DX2 (Endpoint with decapsulation and Layer 2 cross-connect) forwarding, subtle kernel edge conditions can trigger retransmitted SYN-ACK segments and silent packet drops. Tracing socket-level metrics through TCP_INFO and correlating them with SRv6 packet processing requires low-overhead extended Berkeley Packet Filter (eBPF) telemetry. This guide demonstrates how to author, compile, and execute an eBPF tracing tool in Linux to diagnose TCP-AO asymmetric key rollover window clamp drops during SRv6 End.DX2 encapsulation.

Kernel Architecture: TCP-AO and SRv6 End.DX2 Forwarding

Under Linux kernel 6.x and later, TCP-AO provides cryptographic Message Authentication Codes (MACs) per segment using Current Key ID and Required Key ID pairs stored in the kernel socket structure struct tcp_ao_info. During an asynchronous key rollover, asymmetric key configurations between ingress routers and transit switches can cause initial SYN-ACK verification mismatches, prompting repeated retransmissions tracked in tcpi_retrans_synack within struct tcp_info.

Simultaneously, if the receiving interface encapsulates or decapsulates Layer 2 Ethernet payloads via an SRv6 End.DX2 behaviour (seg6_lookup_nexthop calling seg6_end_dx2_action), MTU boundary constraints and receive window clamping can trigger silent drops in ipv6_sr_input() or tcp_v6_rcv(). Observing these simultaneous phenomena in real time requires dynamic tracing at the socket layer and netfilter ingress hooks.

Writing the eBPF Telemetry Program

Save the following eBPF C program as trace_srv6_dx2_ao.bpf.c to attach kprobes to TCP SYN-ACK retransmission and SRv6 End.DX2 execution paths:

#include <vmlinux.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_core_read.h>
#include <bpf/bpf_tracing.h>

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

struct drop_event {
    __u32 saddr[4];
    __u32 daddr[4];
    __u16 sport;
    __u16 dport;
    __u8  retrans_synack;
    __u32 snd_wnd;
    __u32 rcv_wnd;
    __u32 drop_reason;
};

struct {
    __uint(type, BPF_MAP_TYPE_RINGBUF);
    __uint(max_entries, 1 << 24);
} events SEC(".maps");

SEC("kprobe/tcp_rtx_synack")
int BPF_KPROBE(trace_tcp_rtx_synack, struct sock *sk, struct request_sock *req)
{
    struct tcp_sock *tp = (struct tcp_sock *)sk;
    struct drop_event *e;

    e = bpf_ringbuf_reserve(&events, sizeof(*e), 0);
    if (!e)
        return 0;

    e->sport = BPF_CORE_READ(sk, __sk_common.skc_num);
    e->dport = BPF_CORE_READ(sk, __sk_common.skc_dport);
    e->retrans_synack = BPF_CORE_READ(tp, total_retrans);
    e->snd_wnd = BPF_CORE_READ(tp, snd_wnd);
    e->rcv_wnd = BPF_CORE_READ(tp, rcv_wnd);
    e->drop_reason = 101; // Key rollover mismatch or clamp

    bpf_ringbuf_submit(e, 0);
    return 0;
}

SEC("kprobe/seg6_end_dx2_action")
int BPF_KPROBE(trace_seg6_end_dx2, struct sk_buff *skb)
{
    __u32 len = BPF_CORE_READ(skb, len);
    if (len > 1500) {
        struct drop_event *e = bpf_ringbuf_reserve(&events, sizeof(*e), 0);
        if (e) {
            e->drop_reason = 202; // MTU / Encap overflow
            bpf_ringbuf_submit(e, 0);
        }
    }
    return 0;
}

Compiling and Loading via bpftool and Clang

Compile the eBPF bytecode targeting the BPF architecture and load it using modern Linux tooling:

# Install required compiler toolchain
sudo apt-get install -y clang llvm libbpf-dev linux-tools-common linux-tools-$(uname -r)

# Compile BPF target object
clang -g -O2 -target bpf -D__TARGET_ARCH_x86 -c trace_srv6_dx2_ao.bpf.c -o trace_srv6_dx2_ao.bpf.o

# Verify and inspect BPF bytecode
bpftool prog load trace_srv6_dx2_ao.bpf.o /sys/fs/bpf/trace_srv6_dx2_ao type kprobe

Monitoring Live Ring Buffer Events

Author a lightweight user-space consumer or use bpftool ringbuffer dump to monitor TCP-AO key rollover events:

# Stream ringbuffer events directly from kernel space
sudo bpftool map dump name events -j | jq '.[] | {sport: .value.sport, dport: .value.dport, retrans: .value.retrans_synack, reason: .value.drop_reason}'

Troubleshooting TCP-AO Key Rollover and Clamp Issues

Employing this eBPF tracing workflow enables systems engineers to isolate and resolve complex crypto-tunnel interaction failures without service interruption.