fix: send echo reply through correct interface (#285)

This PR fixes an issue where echo replies were being sent through the
same interface the matching echo request was received. That results in
dropped replies in cases a router is connected to more than one viable
next-hop.

Example:

![example with
fix](https://github.com/user-attachments/assets/aedcccc3-43f0-4d77-b9fa-522b37457356)

Without the fix, the response shown on the example was being sent to
device 2 with the MAC address of device 3.

---------

Co-authored-by: Pedro Gallino <[email protected]>

Author: MegaRedHand

src/types/packet.ts

@@ -365,7 +365,7 @@ export function sendViewPacket(
   viewgraph: ViewGraph,
   srcId: DeviceId,
   rawPacket: EthernetFrame,
-  sendingIface?: number,
+  sendingIface: number,
 ) {
   const srcMac = rawPacket.source;
   const dstMac = rawPacket.destination;

src/types/view-devices/vNetworkDevice.ts

@@ -168,6 +168,7 @@ export abstract class ViewNetworkDevice extends ViewDevice {
       : undefined;
   }
 
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
   handleDatagram(datagram: IPv4Packet, iface: number) {
     console.debug("Packet has reach its destination!");
     const dstDevice = this.viewgraph.getDeviceByIP(datagram.sourceAddress);
@@ -181,16 +182,17 @@ export abstract class ViewNetworkDevice extends ViewDevice {
       case ICMP_PROTOCOL_NUMBER: {
         const request: EchoRequest = datagram.payload as EchoRequest;
         if (dstDevice && request.type === ICMP_REQUEST_TYPE_NUMBER) {
-          const { src, nextHop, dst } = ViewNetworkDevice.getForwardingData(
-            this.id,
-            dstDevice.id,
-            this.viewgraph,
-          );
+          const { src, nextHop, dst, sendingIface } =
+            ViewNetworkDevice.getForwardingData(
+              this.id,
+              dstDevice.id,
+              this.viewgraph,
+            );
           const echoReply = new EchoReply(0);
           const ipPacket = new IPv4Packet(src.ip, dst.ip, echoReply);
           const frame = new EthernetFrame(src.mac, nextHop.mac, ipPacket);
           console.debug(`Sending EchoReply to ${dstDevice}`);
-          sendViewPacket(this.viewgraph, this.id, frame, iface);
+          sendViewPacket(this.viewgraph, this.id, frame, sendingIface);
         }
         break;
       }