Add LLDP - dispatched by ethertype, no IP layer at all

Real switches broadcast this every ~30s, but this project had zero
treatment for it (0x88CC was previously the test suite's own example
of an "unhandled ethertype"). Dispatched by ethertype the same way
ARP is, since LLDP sits directly on Ethernet. TLV-encoded; only the
three mandatory TLVs (Chassis ID, Port ID, TTL) plus System Name are
rendered, while every other TLV is still walked over correctly so
nothing after it is lost.

Live-verified two ways, since this machine is on WiFi (LLDP isn't
relayed to wireless clients even when a real switch sends it) with no
LLDP daemon installed to generate traffic locally either: a 15-second
passive capture confirmed no organic LLDP traffic exists to
accidentally rely on, then a real 802.1AB frame was sent via a raw
AF_PACKET socket onto the actual NIC (not fed directly to parse_lldp()
in a unit test) and captured through the full pipeline, decoding
correctly.

2 files changed, 98 insertions, 0 deletions

diff --git a/include/packeteer/net/lldp.hpp b/include/packeteer/net/lldp.hpp
new file mode 100644
index 0000000..ccf9f13
--- /dev/null
+++ b/include/packeteer/net/lldp.hpp
@@ -0,0 +1,92 @@
+#pragma once
+
+#include <cstdint>
+#include <cstdio>
+#include <optional>
+#include <span>
+#include <string>
+
+#include "packeteer/byteio.hpp"
+
+// IEEE 802.1AB LLDP. Sent directly on Ethernet (ethertype 0x88CC), no
+// IP layer at all - the same reasoning ARP is dispatched by ethertype
+// in summarize.hpp rather than through anything IP-based. TLV-encoded:
+// each TLV is a 2-byte header (7-bit type, 9-bit length) followed by
+// that many bytes of value, terminated by an End of LLDPDU TLV (type
+// 0). Only the three mandatory TLVs (Chassis ID, Port ID, TTL) plus
+// System Name - usually the single most useful, human-readable field
+// in the whole frame - are decoded; the rest (Port/System
+// Description, Capabilities, Management Address, and any
+// organizationally-specific TLVs) are walked over correctly (so
+// nothing after them is missed) but not rendered.
+namespace packeteer::net {
+
+inline constexpr std::uint16_t kEthertypeLldp = 0x88CC;
+
+struct LldpInfo {
+    std::optional<std::string> chassis_id;
+    std::optional<std::string> port_id;
+    std::optional<std::uint16_t> ttl;
+    std::optional<std::string> system_name;
+};
+
+// Chassis ID and Port ID share the same subtype+value shape. Subtype 4
+// (MAC address) is rendered as hex-colon; everything else (interface
+// name/alias, component, locally-assigned string, etc.) is treated as
+// ASCII text - true for every subtype except "network address"
+// (subtype 5 for Chassis ID), which has its own AFI-prefixed encoding
+// this doesn't attempt to decode specially and would render as
+// mangled text instead. Uncommon enough in practice not to be worth a
+// separate code path for a one-line summary.
+inline std::string format_lldp_id(std::uint8_t subtype, std::span<const unsigned char> value) {
+    if (subtype == 4 && value.size() == 6) {
+        char buf[18];
+        std::snprintf(buf, sizeof(buf), "%02x:%02x:%02x:%02x:%02x:%02x", value[0], value[1],
+                      value[2], value[3], value[4], value[5]);
+        return buf;
+    }
+    return std::string(reinterpret_cast<const char*>(value.data()), value.size());
+}
+
+inline std::optional<LldpInfo> parse_lldp(std::span<const unsigned char> bytes) {
+    LldpInfo info{};
+    std::size_t pos = 0;
+    constexpr int kMaxTlvs = 32;  // real LLDPDUs rarely carry more than a handful
+
+    for (int i = 0; i < kMaxTlvs; ++i) {
+        if (pos + 2 > bytes.size()) break;
+        std::uint16_t tlv_header = read_be16(bytes, pos);
+        std::uint8_t type = static_cast<std::uint8_t>(tlv_header >> 9);
+        std::uint16_t length = tlv_header & 0x01FF;
+        pos += 2;
+        if (pos + length > bytes.size()) break;  // truncated: stop, keep what was decoded
+        std::span<const unsigned char> value = bytes.subspan(pos, length);
+
+        if (type == 0) break;  // End of LLDPDU
+        if (type == 1 && length >= 1) {
+            info.chassis_id = format_lldp_id(value[0], value.subspan(1));
+        } else if (type == 2 && length >= 1) {
+            info.port_id = format_lldp_id(value[0], value.subspan(1));
+        } else if (type == 3 && length == 2) {
+            info.ttl = read_be16(value, 0);
+        } else if (type == 5 && length >= 1) {
+            info.system_name = std::string(reinterpret_cast<const char*>(value.data()), value.size());
+        }
+
+        pos += length;
+    }
+
+    // The three mandatory TLVs (802.1AB 9.2) - anything missing one
+    // of these isn't really a well-formed LLDPDU.
+    if (!info.chassis_id || !info.port_id || !info.ttl) return std::nullopt;
+    return info;
+}
+
+inline std::string lldp_summary(const LldpInfo& info) {
+    std::string out = "LLDP chassis=" + *info.chassis_id + " port=" + *info.port_id +
+                       " ttl=" + std::to_string(*info.ttl);
+    if (info.system_name) out += " name=" + *info.system_name;
+    return out;
+}
+
+}  // namespace packeteer::net
diff --git a/include/packeteer/summarize.hpp b/include/packeteer/summarize.hpp
index 4bb2d24..c261083 100644
--- a/include/packeteer/summarize.hpp
+++ b/include/packeteer/summarize.hpp
@@ -28,6 +28,7 @@
 #include "packeteer/net/igmp.hpp"
 #include "packeteer/net/ipv4.hpp"
 #include "packeteer/net/ipv6.hpp"
+#include "packeteer/net/lldp.hpp"
 #include "packeteer/net/rtcp.hpp"
 #include "packeteer/net/rtp.hpp"
 #include "packeteer/net/tcp.hpp"
@@ -301,6 +302,11 @@ inline std::string summarize_packet(std::span<const unsigned char> bytes, int da
             return out;
         }
 
+        if (vlan.ethertype == net::kEthertypeLldp) {
+            if (auto lldp = net::parse_lldp(vlan.payload)) out += " | " + net::lldp_summary(*lldp);
+            return out;
+        }
+
         if (vlan.ethertype != net::kEthertypeIPv4 && vlan.ethertype != net::kEthertypeIPv6) {
             return out;
         }