net: l2: ppp: Add support for LCP MRU negotiation

Previously, the LCP MRU option sent by the peer was ignored.
This could result in the interface MTU remaining at the default (1500),
even if the peer requested a smaller MRU, potentially leading to packet
loss. This commit adds parsing for the peer's MRU option and updates the
network interface MTU accordingly.

Signed-off-by: Nik Schewtschuk <[email protected]>

Author: FacepalmMute

subsys/net/l2/ppp/Kconfig

@@ -60,6 +60,13 @@ config NET_L2_PPP_OPTION_MRU
 	help
 	  Enable support for LCP MRU option.
 
+config NET_L2_PPP_OPTION_MAX_MRU
+	int "LCP MRU maximum size"
+	default 1500
+	help
+	  Set the maximal MRU size which is allowed while negotiation with peer over LCP.
+	  The default value is defined by RFC 1661.
+
 config NET_L2_PPP_OPTION_SERVE_IP
 	bool "Serve IP address to peer"
 	help

subsys/net/l2/ppp/lcp.c

@@ -61,6 +61,7 @@ struct lcp_option_data {
 	bool auth_proto_present;
 	uint32_t async_ctrl_char_map;
 	uint16_t auth_proto;
+	uint16_t mru;
 };
 
 static const enum ppp_protocol_type lcp_supported_auth_protos[] = {
@@ -121,11 +122,49 @@ static int lcp_async_ctrl_char_map_parse(struct ppp_fsm *fsm, struct net_pkt *pk
 	return 0;
 }
 
+static int lcp_peer_mru_parse(struct ppp_fsm *fsm, struct net_pkt *pkt,
+			       void *user_data)
+{
+	struct lcp_option_data *data = user_data;
+	uint16_t peer_mru;
+	int ret;
+
+	ret = net_pkt_read_be16(pkt, &peer_mru);
+	if (ret < 0) {
+		/* Should not happen, is the pkt corrupt? */
+		return -EMSGSIZE;
+	}
+
+	NET_DBG("[LCP] Received peer MRU %u", peer_mru);
+
+	if (peer_mru > CONFIG_NET_L2_PPP_OPTION_MAX_MRU) {
+		LOG_WRN("[LCP] Received peer MRU is too big. %u > %u.",
+			peer_mru, CONFIG_NET_L2_PPP_OPTION_MAX_MRU);
+		return -EINVAL;
+	}
+
+	data->mru = peer_mru;
+
+	return 0;
+}
+
+static int lcp_peer_mru_nack(struct ppp_fsm *fsm, struct net_pkt *ret_pkt,
+			       void *user_data)
+{
+	struct ppp_context *ctx = ppp_fsm_ctx(fsm);
+
+	(void)net_pkt_write_u8(ret_pkt, LCP_OPTION_MRU);
+	(void)net_pkt_write_u8(ret_pkt, 4);
+	return net_pkt_write_be16(ret_pkt, ctx->lcp.my_options.mru);
+}
+
 static const struct ppp_peer_option_info lcp_peer_options[] = {
 	PPP_PEER_OPTION(LCP_OPTION_AUTH_PROTO, lcp_auth_proto_parse,
 			lcp_auth_proto_nack),
 	PPP_PEER_OPTION(LCP_OPTION_ASYNC_CTRL_CHAR_MAP, lcp_async_ctrl_char_map_parse,
 			NULL),
+	PPP_PEER_OPTION(LCP_OPTION_MRU, lcp_peer_mru_parse,
+			lcp_peer_mru_nack),
 };
 
 static int lcp_config_info_req(struct ppp_fsm *fsm,
@@ -220,7 +259,11 @@ static void lcp_up(struct ppp_fsm *fsm)
 	struct ppp_context *ctx = CONTAINER_OF(fsm, struct ppp_context,
 					       lcp.fsm);
 
-	/* TODO: Set MRU/MTU of the network interface here */
+	if (ctx->lcp.peer_options.mru > 0) {
+		NET_DBG("Set MTU size from peer options: %u -> %u",
+			net_if_get_mtu(ctx->iface), ctx->lcp.peer_options.mru);
+		net_if_set_mtu(ctx->iface, ctx->lcp.peer_options.mru);
+	}
 
 	ppp_link_established(ctx, fsm);
 }