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

Segment Routing over IPv6 (SRv6) is an advanced transport paradigm powering modern telecommunications backbones and ultra-low-latency 5G core networks. When mission-critical control-plane connections—such as BGP-EVPN or telemetry streams—are secured using RFC 5925 TCP Authentication Option (TCP-AO) over SRv6 tunnel encapsulation, complex edge-case failure modes can arise. In particular, when cryptographic master key tuples (MKTs) undergo automated hardware key rollover while downstream segment routers enforce TCP receive window clamping, SYN-ACK retransmissions (tracked via tcpi_retrans_synack in the kernel’s tcp_info structure) can surge, causing silent packet drops. This engineering guide demonstrates how to build and deploy an eBPF tracing program to isolate, measure, and eliminate these encapsulated drops in Linux.

SRv6 Encapsulation Dynamics and TCP-AO Key Rollovers

In an SRv6 network, transit packets are encapsulated with an outer IPv6 header followed by a Segment Routing Header (SRH) containing a list of 128-bit Segment Identifiers (SIDs). Under RFC 5925, TCP-AO authenticates segments using an asymmetric key pair composed of Send Key ID (KeyID) and Receive Next Key ID (RNextKeyID). During an automated key rotation event, ingress segments bearing a newly assigned KeyID may encounter an endpoint whose hardware offload engine has not yet finished updating its crypto tables.

If an intermediate node experiences transient queue congestion, it can signal TCP window clamping to throttle sender buffers. When inner TCP segments encounter both asymmetric key validation stalls and dynamic window constriction inside the SRv6 decap path, the kernel drops the packet before completing the handshake, resulting in elevated tcpi_retrans_synack counters and connection timeouts.

Authoring the eBPF Kernel Probe

To detect transient drops with sub-millisecond precision, we attach eBPF kprobes directly to tcp_ao_verify_hash and tcp_retransmit_skb within the Linux networking subsystem:

// File: srv6_tcp_ao_trace.bpf.c
#include <vmlinux.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_core_read.h>
#include <bpf/bpf_tracing.h>

struct drop_metric_t {
    u32 saddr_v4;
    u32 daddr_v4;
    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));
} srv6_events SEC(".maps");

SEC("kprobe/tcp_ao_verify_hash")
int BPF_KPROBE(trace_srv6_tcp_ao, 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);

    if (retrans > 0) {
        struct drop_metric_t metric = {};
        metric.sport = BPF_CORE_READ(sk, __sk_common.skc_num);
        metric.dport = bpf_ntohs(BPF_CORE_READ(sk, __sk_common.skc_dport));
        metric.retrans_synack = BPF_CORE_READ(tp, tcpi_retrans_synack);
        metric.snd_wnd = BPF_CORE_READ(tp, snd_wnd);
        metric.rcv_wnd = BPF_CORE_READ(tp, rcv_wnd);

        bpf_perf_event_output(ctx, &srv6_events, BPF_F_CURRENT_CPU, &metric, sizeof(metric));
    }
    return 0;
}

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

Compiling and Loading via bpftool

Compile the eBPF object file and attach it to the kernel kprobe subsystem using standard Linux networking utilities:

# Compile eBPF program against vmlinux
clang -g -O2 -target bpf -D__TARGET_ARCH_x86 -c srv6_tcp_ao_trace.bpf.c -o srv6_tcp_ao_trace.bpf.o

# Pin and attach via bpftool
bpftool prog load srv6_tcp_ao_trace.bpf.o /sys/fs/bpf/srv6_tcp_ao_trace type kprobe
bpftool attach kprobe name tcp_ao_verify_hash pinned /sys/fs/bpf/srv6_tcp_ao_trace

Diagnostic Analysis and Kernel Remediation

When the perf buffer indicates repeated SYN-ACK retransmissions during key transitions, implement the following kernel and network adjustments:

• SRv6 Outer MTU Headroom: The SRH header introduces variable extension lengths (typically 24 to 64 bytes). Ensure the outer IPv6 path MTU is set to at least 1600 bytes, or enforce TCP MSS clamping at the ingress decap interface:

  ip6tables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

• TCP-AO Key Grace Windows: Extend the overlap grace window on BGP peer configurations to allow both the retiring key and the succeeding key to remain valid for a minimum of 90 seconds during automated rotation.
• Window Scale Buffer Allocation: Prevent aggressive TCP window clamping under transient burst traffic by tuning core socket memory buffers in /etc/sysctl.conf:

  net.ipv4.tcp_window_scaling = 1
  net.ipv4.tcp_adv_win_scale = 1
  net.core.rmem_default = 262144
  net.core.rmem_max = 33554432

Validating Kernel Connection Health

Execute ss -tiepm state syn-recv to inspect active connection states. Verify that tcpi_retrans_synack stabilizes at zero and that TCP-AO cryptographic validations complete seamlessly across all SRv6 encapsulation boundaries.