path: root/pkg/ipstack/ipstack.go

package ipstack

import (
	"encoding/binary"
	"fmt"
	ipv4header "github.com/brown-csci1680/iptcp-headers"
	"github.com/google/netstack/tcpip/header"
	"github.com/pkg/errors"
	"iptcp/pkg/lnxconfig"
	"log"
	"net"
	"net/netip"
	"sync"
	"time"
)

const (
	MAX_IP_PACKET_SIZE         = 1400
	LOCAL_COST          uint32 = 0
	STATIC_COST         uint32 = 4294967295 // 2^32 - 1
	MaxEntries                 = 64
	INFINITY                   = 16
	SIZE_OF_RIP_ENTRY          = 12
	RIP_PROTOCOL               = 200
	TEST_PROTOCOL              = 0
	SIZE_OF_RIP_HEADER		   = 4
)

// STRUCTS ---------------------------------------------------------------------
type Interface struct {
	Name     string
	IpPrefix netip.Prefix
	UdpAddr  netip.AddrPort

	RecvSocket    net.UDPConn
	SocketChannel chan bool
	State         bool
}

type Neighbor struct {
	Name    string
	VipAddr netip.Addr
	UdpAddr netip.AddrPort
}

type RIPHeader struct {
	command    uint16
	numEntries uint16
}

type RIPEntry struct {
	prefix netip.Prefix
	cost   uint32
}

type Hop struct {
	Cost uint32
	Type string

	Interface *Interface
	VIP       netip.Addr
}

// GLOBAL VARIABLES (data structures) ------------------------------------------
var myInterfaces []*Interface

var myNeighbors = make(map[string][]*Neighbor)

var myRIPNeighbors = make(map[string]*Neighbor)

type HandlerFunc func(src *Interface, dest *Neighbor, message []byte, hdr *ipv4header.IPv4Header) error

var protocolHandlers = make(map[int]HandlerFunc)

var routingTable = make(map[netip.Prefix]Hop)

// ************************************** INIT FUNCTIONS **********************************************************
// reference: https://github.com/brown-csci1680/lecture-examples/blob/main/ip-demo/cmd/udp-ip-recv/main.go

// createUDPListener creates a UDP listener.
func createUDPListener(UdpAddr netip.AddrPort, conn *net.UDPConn) error {
	listenString := UdpAddr.String()
	listenAddr, err := net.ResolveUDPAddr("udp4", listenString)
	if err != nil {
		return errors.WithMessage(err, "Error resolving address->\t"+listenString)
	}
	tmpConn, err := net.ListenUDP("udp4", listenAddr)
	if err != nil {
		return errors.WithMessage(err, "Could not bind to UDP port->\t"+listenString)
	}
	*conn = *tmpConn

	return nil
}

// initialize parse the lnxfile and initializes the data structures and listener routines
func Initialize(lnxFilePath string) error {
	// Parse the file
	lnxConfig, err := lnxconfig.ParseConfig(lnxFilePath)
	if err != nil {
		return errors.WithMessage(err, "Error parsing config file->\t"+lnxFilePath)
	}

	// 1) add each local "if" to the routing table, as dictated by its subnet
	for _, iface := range lnxConfig.Interfaces {
		prefix := netip.PrefixFrom(iface.AssignedIP, iface.AssignedPrefix.Bits())
		i := &Interface{
			Name:          iface.Name,
			IpPrefix:      prefix,
			UdpAddr:       iface.UDPAddr,
			RecvSocket:    net.UDPConn{},
			SocketChannel: make(chan bool),
			State:         true,
		}

		// create the UDP listener
		err := createUDPListener(iface.UDPAddr, &i.RecvSocket)
		if err != nil {
			return errors.WithMessage(err, "Error creating UDP socket for interface->\t"+iface.Name)
		}

		// start the listener routine
		go InterfaceListenerRoutine(i)

		// add to the list of interfaces
		myInterfaces = append(myInterfaces, i)

		// add to the routing table
		routingTable[prefix.Masked()] = Hop{LOCAL_COST, "L", i, prefix.Addr()}
	}

	// 2) add neighbors to if->neighbors map
	for _, neighbor := range lnxConfig.Neighbors {
		n := &Neighbor{
			Name:    neighbor.InterfaceName,
			VipAddr: neighbor.DestAddr,
			UdpAddr: neighbor.UDPAddr,
		}

		myNeighbors[neighbor.InterfaceName] = append(myNeighbors[neighbor.InterfaceName], n)
	}

	// 3) add RIP neighbors to RIP neighbor list
	for _, route := range lnxConfig.RipNeighbors {
		// add to RIP neighbors
		for _, iface := range myInterfaces {
			for _, neighbor := range myNeighbors[iface.Name] {
				if neighbor.VipAddr == route {
					myRIPNeighbors[neighbor.VipAddr.String()] = neighbor
					break
				}
			}
		}
	}

	// 4) add static routes to routing table
	for prefix, addr := range lnxConfig.StaticRoutes {
		// need loops to find the interface that matches the neighbor to send static to
		// hops needs this interface
		for _, iface := range myInterfaces {
			for _, neighbor := range myNeighbors[iface.Name] {
				if neighbor.VipAddr == addr {
					routingTable[prefix] = Hop{STATIC_COST, "S", iface, addr}
					break
				}
			}
		}
	}

	return nil
}

// defines the go routine that listens on the UDP socket
func InterfaceListenerRoutine(i *Interface) {
	// decompose the interface
	socket := i.RecvSocket
	signal := i.SocketChannel

	// booleans to control listening routine
	isUp := true
	closed := false

	// fmt.Println("MAKING GO ROUTINE TO LISTEN:\t", socket.LocalAddr().String())

	// go routine that hangs on the recv
	go func() {
		defer func() {
			fmt.Println("exiting go routine that listens on ", socket.LocalAddr().String())
		}()

		for {
			if closed { // stop this go routine if channel is closed
				return
			}
			//if !isUp { // don't call the listeners if interface is down
			//	continue
			//}
			// TODO: remove these "training wheels"
			// time.Sleep(1 * time.Millisecond)
			err := RecvIP(i, &isUp)
			if err != nil {
				continue
			}
		}
	}()

	for {
		select {
		// if the channel is closed, exit
		case sig, ok := <-signal:
			if !ok {
				fmt.Println("channel closed, exiting")
				closed = true
				return
			}
			// fmt.Println("received isUP SIGNAL with value", sig)
			isUp = sig
		// if the channel is not closed, continue
		default:
			continue
		}
	}
}

// ************************************** DOWN/UP FUNCTIONS ******************************************************

// sets the interface to be up and sends a triggered update
func InterfaceUp(iface *Interface) {
	// set the state to up and send the signal
	iface.State = true
	iface.SocketChannel <- true

	// if were a router, send triggered updates on up
	if _, ok := protocolHandlers[RIP_PROTOCOL]; ok {
		ripEntries := make([]RIPEntry, 0)
		ripEntries = append(ripEntries, RIPEntry{iface.IpPrefix.Masked(), LOCAL_COST})
		sendTriggeredUpdates(ripEntries)

		// send a request to all neighbors of this iface to get info ASAP
		for _, neighbor := range myNeighbors[iface.Name] {
			message := makeRipMessage(1, nil)
			addr := iface.IpPrefix.Addr()
			_, err := SendIP(&addr, neighbor, RIP_PROTOCOL, message, neighbor.VipAddr.String(), nil)
			if err != nil {
				fmt.Println("Error sending RIP request to neighbor on interfaceup", err)
			}
		}
	}

}

func InterfaceUpREPL(ifaceName string) {
	iface, err := GetInterfaceByName(ifaceName)
	if err != nil {
		fmt.Println("Error getting interface by name", err)
		return
	}
	// set the state to up and send the signal
	InterfaceUp(iface)
}

// sets the interface to be down and sends a triggered update
func InterfaceDown(iface *Interface) {
	// set the state to down and send the signal
	iface.SocketChannel <- false
	iface.State = false

	// if were a router, send triggered updates on down
	if _, ok := protocolHandlers[RIP_PROTOCOL]; ok {
		ripEntries := make([]RIPEntry, 0)
		ripEntries = append(ripEntries, RIPEntry{iface.IpPrefix.Masked(), INFINITY})
		sendTriggeredUpdates(ripEntries)
	}
}

func InterfaceDownREPL(ifaceName string) {
	iface, err := GetInterfaceByName(ifaceName)
	if err != nil {
		fmt.Println("Error getting interface by name", err)
		return
	}
	// set the state to down and send the signal
	InterfaceDown(iface)
}

// ************************************** GETTER FUNCTIONS **********************************************************
func GetInterfaceByName(ifaceName string) (*Interface, error) {
	// iterate through the interfaces and return the one with the same name
	for _, iface := range myInterfaces {
		if iface.Name == ifaceName {
			return iface, nil
		}
	}
	return nil, errors.Errorf("No interface with name %s", ifaceName)
}

func GetInterfaces() []*Interface {
	return myInterfaces
}

func GetNeighbors() map[string][]*Neighbor {
	return myNeighbors
}

func GetRoutes() map[netip.Prefix]Hop {
	return routingTable
}

// ************************************** PRINT FUNCTIONS **********************************************************

// returns a string representation of the myInterfaces data structure
func SprintInterfaces() string {
	tmp := ""
	for _, iface := range myInterfaces {
		if iface.State {
			// if the state is up, print UP
			tmp += fmt.Sprintf("%s\t%s\t%s\n", iface.Name, iface.IpPrefix.String(), "UP")
		} else {
			// if the state is down, print DOWN
			tmp += fmt.Sprintf("%s\t%s\t%s\n", iface.Name, iface.IpPrefix.String(), "DOWN")
		}
	}
	return tmp
}

// returns a string representation of the myNeighbors data structure
func SprintNeighbors() string {
	tmp := ""
	for _, iface := range myInterfaces {
		if !iface.State {
			// if the interface is down, skip it
			continue
		}
		for _, n := range myNeighbors[iface.Name] {
			tmp += fmt.Sprintf("%s\t%s\t%s\n", iface.Name, n.VipAddr.String(), n.UdpAddr.String())
		}
	}
	return tmp
}

// returns a string representation of the routingTable data structure
func SprintRoutingTable() string {
	tmp := ""
	for prefix, hop := range routingTable {
		if hop.Type == "L" {
			// if the hop is local, print LOCAL
			tmp += fmt.Sprintf("%s\t%s\tLOCAL:%s\t%d\n", hop.Type, prefix.String(), hop.Interface.Name, hop.Cost)
		} else if hop.Type == "S" {
			// if the hop is static, don't print the cost
			tmp += fmt.Sprintf("%s\t%s\t%s\t%s\n", hop.Type, prefix.String(), hop.VIP.String(), "-")
		} else {
			tmp += fmt.Sprintf("%s\t%s\t%s\t%d\n", hop.Type, prefix.String(), hop.VIP.String(), hop.Cost)
		}
	}
	return tmp
}

// ************************************** BASIC FUNCTIONS **********************************************************

// cleans up the data structures and closes the UDP sockets
func CleanUp() {
	fmt.Print("Cleaning up...\n")

	// go through the interfaces, pop thread & close the UDP FDs
	for _, iface := range myInterfaces {
		// close the channel
		if iface.SocketChannel != nil {
			close(iface.SocketChannel)
		}
		// close the UDP FD
		err := iface.RecvSocket.Close()
		if err != nil {
			continue
		}
	}

	// delete all the neighbors
	myNeighbors = make(map[string][]*Neighbor)
	// delete all the interfaces
	myInterfaces = nil
	// delete the routing table
	routingTable = make(map[netip.Prefix]Hop)

	time.Sleep(5 * time.Millisecond)
}

// SendIP sends an IP packet to a destination
func SendIP(src *netip.Addr, dest *Neighbor, protocolNum int, message []byte, destIP string, hdr *ipv4header.IPv4Header) (int, error) {
	// check if the interface is up
	iface, err := GetInterfaceByName(dest.Name)
	if !iface.State {
		return -1, errors.Errorf("error SEND: %s is down", iface.Name)
	}
	// if the header is nil, create a new one
	if hdr == nil {
		hdr = &ipv4header.IPv4Header{
			Version:  4,
			Len:      20, // Header length is always 20 when no IP options
			TOS:      0,
			TotalLen: ipv4header.HeaderLen + len(message),
			ID:       0,
			Flags:    0,
			FragOff:  0,
			TTL:      32,
			Protocol: protocolNum,
			Checksum: 0, // Should be 0 until checksum is computed
			Src:      *src,
			Dst:      netip.MustParseAddr(destIP),
			Options:  []byte{},
		}
	} else {
		// if the header is not nil, decrement the TTL
		hdr = &ipv4header.IPv4Header{
			Version:  4,
			Len:      20, // Header length is always 20 when no IP options
			TOS:      0,
			TotalLen: ipv4header.HeaderLen + len(message),
			ID:       0,
			Flags:    0,
			FragOff:  0,
			TTL:      hdr.TTL - 1,
			Protocol: protocolNum,
			Checksum: 0, // Should be 0 until checksum is computed
			Src:      *src,
			Dst:      netip.MustParseAddr(destIP),
			Options:  []byte{},
		}
	}

	// Assemble the header into a byte array
	headerBytes, err := hdr.Marshal()
	if err != nil {
		return -1, err
	}

	// Compute the checksum (see below)
	// Cast back to an int, which is what the Header structure expects
	hdr.Checksum = int(ComputeChecksum(headerBytes))

	headerBytes, err = hdr.Marshal()
	if err != nil {
		log.Fatalln("Error marshalling header:  ", err)
	}

	// Combine the header and the message into a single byte array
	bytesToSend := make([]byte, 0, len(headerBytes)+len(message))
	bytesToSend = append(bytesToSend, headerBytes...)
	bytesToSend = append(bytesToSend, []byte(message)...)

	sendAddr, err := net.ResolveUDPAddr("udp4", dest.UdpAddr.String())
	if err != nil {
		return -1, errors.WithMessage(err, "Could not bind to UDP port->\t"+dest.UdpAddr.String())
	}

	// TODO: make this faster by removing call
	bytesWritten, err := iface.RecvSocket.WriteToUDP(bytesToSend, sendAddr)
	if err != nil {
		fmt.Println("Error writing to UDP socket")
		return -1, errors.WithMessage(err, "Error writing to UDP socket")
	}

	return bytesWritten, nil
}


// RecvIP receives an IP packet from a source
func RecvIP(iface *Interface, isOpen *bool) error {
	buffer := make([]byte, MAX_IP_PACKET_SIZE) // TODO: fix wording

	// Read on the UDP port
	// fmt.Println("wating to read from UDP socket")
	_, _, err := iface.RecvSocket.ReadFromUDP(buffer)
	if err != nil {
		return err
	}

	// check if the interface is up
	if !*isOpen {
		return errors.Errorf("error RECV: %s is down", iface.Name)
	}

	// Marshal the received byte array into a UDP header
	hdr, err := ipv4header.ParseHeader(buffer)
	if err != nil {
		fmt.Println("Error parsing header", err)
		return err
	}

	// checksum validation
	headerSize := hdr.Len
	headerBytes := buffer[:headerSize]
	checksumFromHeader := uint16(hdr.Checksum)
	computedChecksum := ValidateChecksum(headerBytes, checksumFromHeader)

	var checksumState string
	if computedChecksum == checksumFromHeader {
		checksumState = "OK"
	} else {
		checksumState = "FAIL"
	}

	// Next, get the message, which starts after the header
	messageLen := hdr.TotalLen - hdr.Len
	message := buffer[headerSize : messageLen+headerSize]

	// 1) check if the TTL & checksum is valid
	TTL := hdr.TTL
	if TTL == 0 {
		// drop the packet
		return nil
	}

	// check if the checksum is valid
	if checksumState == "FAIL" {
		// drop the packet
		// fmt.Println("checksum failed, dropping packet")
		return nil
	}

	//if hdr.Protocol != RIP_PROTOCOL {
	//	fmt.Println("I see a non-rip packet")
	//}

	// at this point, the packet is valid. next steps consider the forwarding of the packet

	// 2) check if the message is for me, if so, sendUP (aka call the correct handler)
	for _, myIface := range myInterfaces {
		if hdr.Dst == myIface.IpPrefix.Addr() {
			// see if there is a handler for this protocol
			if handler, ok := protocolHandlers[hdr.Protocol]; ok {
				if hdr.Protocol != RIP_PROTOCOL {
					// fmt.Println("this test packet is exactly for me")
				}
				err := handler(myIface, nil, message, hdr)
				if err != nil {
					fmt.Println(err)
				}
			}
			return nil
		}
	}

	// 4) check forwarding table.
	// if it's a local hop, send to that iface
	// if it's a RIP hop, send to the neighbor with that VIP
	// fmt.Println("checking routing table")
	hop, err := LongestPrefix(hdr.Dst)
	if err == nil { // on no err, found a match
		// fmt.Println("found route", hop.VIP)
		if hop.Type == "S" {
			// default, static route
			// drop in this case
			return nil
		}

		// local hop
		if hop.Type == "L" {
			// if it's a local route, then the name is the interface name
			for _, neighbor := range myNeighbors[hop.Interface.Name] {
				if neighbor.VipAddr == hdr.Dst {
					_, err2 := SendIP(&hdr.Src, neighbor, hdr.Protocol, message, hdr.Dst.String(), hdr)
					if err2 != nil {
						return err2
					}
				}
			}
		}

		// rip hop
		if hop.Type == "R" {
			// if it's a rip route, then the check is against the hop vip
			for _, neighbor := range myNeighbors[hop.Interface.Name] {
				if neighbor.VipAddr == hop.VIP {
					_, err2 := SendIP(&hdr.Src, neighbor, hdr.Protocol, message, hdr.Dst.String(), hdr)
					if err2 != nil {
						return err2
					}
				}
			}
		}
	}

	// if not in table, drop packet
	return nil
}

// ************************************** RIP Routines *******************************************************

// creates a byte array that represents a RIP message
func makeRipMessage(command uint16, entries []RIPEntry) []byte {
	if command == 1 {           // request message
		buf := make([]byte, SIZE_OF_RIP_HEADER)
		binary.BigEndian.PutUint16(buf[0:2], command)
		binary.BigEndian.PutUint16(buf[2:4], uint16(0))
		return buf
	}
	// else, command == 2, response message

	// create the buffer
	bufLen := SIZE_OF_RIP_HEADER + // sizeof uint16 is 2, we have two of them
		len(entries)*SIZE_OF_RIP_ENTRY // each entry is 12

	buf := make([]byte, bufLen)

	// fill in the header
	binary.BigEndian.PutUint16(buf[0:2], command)
	binary.BigEndian.PutUint16(buf[2:4], uint16(len(entries)))

	// fill in the entries
	for i, entry := range entries {
		offset := SIZE_OF_RIP_HEADER + i*SIZE_OF_RIP_ENTRY
		binary.BigEndian.PutUint32(buf[offset:offset+4], entry.cost) // 0-3 = 4 bytes
		copy(buf[offset+4:offset+8], entry.prefix.Addr().AsSlice())  // 4-7 = 4 bytes

		// convert the prefix to a uint32
		ipv4Netmask := uint32(0xffffffff)
		ipv4Netmask <<= 32 - entry.prefix.Bits()
		binary.BigEndian.PutUint32(buf[offset+8:offset+12], ipv4Netmask)
	}

	return buf
}


func periodicUpdateRoutine() {
	for {
		// for each periodic update, we want to send our nodes in the table
		for _, iface := range myInterfaces {
			for _, n := range myNeighbors[iface.Name] {
				_, in := myRIPNeighbors[n.VipAddr.String()]
				if !in {
					continue
				}
				// TODO: consider making this multithreaded and loops above more efficient

				// if we're here, we are sending this to a rip neighbor
				entries := make([]RIPEntry, 0)
				for prefix, hop := range routingTable {
					// implement split horizon + poison reverse at entry level
					// fmt.Println("prefix: ", prefix)
					var cost uint32
					if hop.VIP == n.VipAddr {
						cost = INFINITY
					} else {
						cost = hop.Cost
					}
					entries = append(entries,
						RIPEntry{
							prefix: prefix,
							cost:   cost,
						})
				}

				message := makeRipMessage(2, entries)
				addr := iface.IpPrefix.Addr()
				_, err := SendIP(&addr, n, RIP_PROTOCOL, message, n.VipAddr.String(), nil)
				if err != nil {
					// fmt.Printf("Error sending RIP message to %s\n", n.VipAddr.String())
					continue
				}
			}
		}

		// wait 5 sec
		time.Sleep(5 * time.Second)
	}
}

var mu sync.Mutex
var timeoutTable = make(map[netip.Prefix]int)
var MAX_TIMEOUT = 12

func sendTriggeredUpdates(newEntries []RIPEntry) {
	for _, iface := range myInterfaces {
		for _, n := range myNeighbors[iface.Name] {
			_, in := myRIPNeighbors[n.VipAddr.String()]
			if !in {
				continue
			}

			message := makeRipMessage(2, newEntries)
			addr := iface.IpPrefix.Addr()
			_, err := SendIP(&addr, n, RIP_PROTOCOL, message, n.VipAddr.String(), nil)
			if err != nil {
				// fmt.Printf("Error sending RIP triggered update to %s\n", n.VipAddr.String())
				continue
			}
		}
	}
}

func timeoutKey(prefix netip.Prefix, addr netip.Addr) string {
	return prefix.String() + "-" + addr.String()
}

func manageTimeoutsRoutine() {
	for {
		time.Sleep(time.Second)

		// note: waitgroup causes deadlock then crashing
		//wg := &sync.WaitGroup{}
		//wg.Add(len(timeoutTable))
		//mu.Lock()
		//for prefix, _ := range timeoutTable {
		//	go func(p netip.Prefix) {
		//		timeoutTable[p]++
		//		if timeoutTable[p] == MAX_TIMEOUT {
		//			delete(routingTable, p)
		//			delete(timeoutTable, p)
		//			// TODO: send triggered update
		//		}
		//
		//		wg.Done()
		//	}(prefix)
		//}
		//wg.Wait()
		//mu.Unlock()

		mu.Lock()
		for key, _ := range timeoutTable {
			timeoutTable[key]++
			if timeoutTable[key] == MAX_TIMEOUT {
				delete(timeoutTable, key)

				newEntries := make([]RIPEntry, 0)
				delete(routingTable, key)
				newEntries = append(newEntries, RIPEntry{key, INFINITY})
				// send triggered update on timeout
				if len(newEntries) > 0 {
					sendTriggeredUpdates(newEntries)
				}
			}
		}
		// fmt.Println("timeout table: ", timeoutTable)
		mu.Unlock()

		//fmt.Println("Timeout table: ", timeoutTable)
	}
}

func startRipRoutines() {
	// send a request to every neighbor
	go func() {
		for _, iface := range myInterfaces {
			for _, neighbor := range myNeighbors[iface.Name] {
				_, in := myRIPNeighbors[neighbor.VipAddr.String()]
				if !in {
					continue
				}
				// send a request
				message := makeRipMessage(1, nil)
				addr := iface.IpPrefix.Addr()
				_, err := SendIP(&addr, neighbor, RIP_PROTOCOL, message, neighbor.VipAddr.String(), nil)
				if err != nil {
					return
				}
			}
		}
	}()

	go periodicUpdateRoutine()

	// make a "timeout" table, for each response we add to the table via rip
	go manageTimeoutsRoutine()

	// start a routine that sends updates every 10 seconds
}

// ************************************** Protocol Handlers *******************************************************

func RegisterProtocolHandler(protocolNum int) bool {
	if protocolNum == RIP_PROTOCOL {
		protocolHandlers[protocolNum] = handleRIP
		go startRipRoutines()
		return true
	}
	if protocolNum == TEST_PROTOCOL {
		protocolHandlers[protocolNum] = handleTestPackets
		return true
	}
	return false
}

func handleRIP(src *Interface, dest *Neighbor, message []byte, hdr *ipv4header.IPv4Header) error {
	// parse the RIP message
	// SIZE_OF_RIP_HEADER := 2 * 2
	command := int(binary.BigEndian.Uint16(message[0:2]))
	switch command {
	case 1:
		//fmt.Println("Received RIP command for specific info")
		// only send if the person asking is a RIP neighbor
		neighbor, in := myRIPNeighbors[hdr.Src.String()]
		if !in {
			break
		}

		// fmt.Println("he is my rip neighbor ", hdr.Src.String())

		// build the entries
		entries := make([]RIPEntry, 0)
		for prefix, hop := range routingTable {
			// implement split horizon + poison reverse at entry level
			// fmt.Println("prefix: ", prefix)
			var cost uint32
			if hop.VIP == hdr.Src {
				cost = INFINITY
			} else {
				cost = hop.Cost
			}
			entries = append(entries,
				RIPEntry{
					prefix: prefix,
					cost:   cost,
				})
		}
		res := makeRipMessage(2, entries)
		_, err := SendIP(&hdr.Dst, neighbor, RIP_PROTOCOL, res, hdr.Src.String(), nil)
		if err != nil {
			return err
		}
		break
	case 2:
		numEntries := int(binary.BigEndian.Uint16(message[2:4]))
		// fmt.Println("Received RIP response with", numEntries, "entries")

		// parse the entries
		entries := make([]RIPEntry, 0)
		for i := 0; i < numEntries; i++ {
			offset := SIZE_OF_RIP_HEADER + i*SIZE_OF_RIP_ENTRY

			// each field is 4 bytes
			cost := binary.BigEndian.Uint32(message[offset : offset+4])
			address, _ := netip.AddrFromSlice(message[offset+4 : offset+8])
			mask := net.IPv4Mask(message[offset+8], message[offset+9], message[offset+10], message[offset+11])

			// make the prefix
			bits, _ := mask.Size()
			prefix := netip.PrefixFrom(address, bits)

			entries = append(entries, RIPEntry{prefix, cost})
		}

		// update the routing table
		triggeredEntries := make([]RIPEntry, 0)
		for _, entry := range entries {
			destination := entry.prefix.Masked()

			// make upperbound for cost infinity
			var newCost uint32
			if entry.cost == INFINITY {
				newCost = INFINITY
			} else {
				newCost = entry.cost + 1
			}

			hop, isin := routingTable[destination]
			// if prefix not in table, add it (as long as it's not infinity)
			if !isin {
				if newCost != INFINITY {
					// given an update to table, this is now a triggeredUpdate
					// triggeredEntries = append(triggeredEntries, RIPEntry{destination, entry.cost + 1})

					routingTable[destination] = Hop{newCost, "R", src, hdr.Src}
					timeoutTable[destination] = 0
				}
				continue
			}

			// if the entry is in the table, only two cases affect the table:
			// 1) the entry SRC is updating (or confirming) the hop to itself
			//		in this case, only update if the cost is different
			//		if it's infinity, then the route has expired.
			//		we must set the cost to INF then delete the entry after 12 seconds
			//
			// 2) a different entry SRC reveals a shorter path to the destination
			//		in this case, update the routing table to use this new path
			//
			// all other cases don't meaningfully change the route

			// first, upon an update from this prefix, reset its timeout
			if hop.Type == "R" {
				mu.Lock()
				_, in := timeoutTable[destination]
				if in {
					if routingTable[destination].VIP == hdr.Src {
						timeoutTable[destination] = 0
					}
				}
				mu.Unlock()
			}

			// case 1) the entry SRC == the hop to itself
			if hop.VIP == hdr.Src &&
				newCost != hop.Cost {
				// given an update to table, this is now a triggeredUpdate
				triggeredEntries = append(triggeredEntries, RIPEntry{destination, newCost})
				routingTable[destination] = Hop{newCost, "R", src, hop.VIP}

				// if we receive infinity from the same neighbor, then delete the route after 12 sec
				if entry.cost == INFINITY {
					// remove after GC time if the COST is still INFINITY
					go func() {
						time.Sleep(time.Second * time.Duration(MAX_TIMEOUT))
						if routingTable[destination].Cost == INFINITY {
							delete(routingTable, destination)
							mu.Lock()
							delete(timeoutTable, destination)
							mu.Unlock()
						}
					}()
				}
				continue
			}

			// case 2) a shorter route for this destination is revealed from a different neighbor
			if newCost < hop.Cost && newCost != INFINITY {
				triggeredEntries = append(triggeredEntries, RIPEntry{destination, entry.cost + 1})
				routingTable[destination] = Hop{entry.cost + 1, "R", src, hdr.Src}
				continue
			}
		}

		// send out triggered updates
		if len(triggeredEntries) > 0 {
			sendTriggeredUpdates(triggeredEntries)
		}
	}

	return nil
}

func handleTestPackets(src *Interface, dest *Neighbor, message []byte, hdr *ipv4header.IPv4Header) error {
	fmt.Printf("Received test packet:  Src: %s, Dst: %s, TTL: %d, Data: %s\n",
		hdr.Src.String(), hdr.Dst.String(), hdr.TTL, string(message))
	return nil
}

// ************************************** CHECKSUM FUNCTIONS ******************************************************

func ComputeChecksum(b []byte) uint16 {
	checksum := header.Checksum(b, 0)
	checksumInv := checksum ^ 0xffff

	return checksumInv
}

func ValidateChecksum(b []byte, fromHeader uint16) uint16 {
	checksum := header.Checksum(b, fromHeader)

	return checksum
}

// ************************************** RIP FUNCTIONS **********************************************************

// TODO @ MICHAEL: LONGEST PREFIX MATCHING
func LongestPrefix(src netip.Addr) (Hop, error) {
	possibleBits := [2]int{32, 24}
	for _, bits := range possibleBits {
		cmpPrefix := netip.PrefixFrom(src, bits)
		for prefix, hop := range routingTable {
			if cmpPrefix.Overlaps(prefix) {
				return hop, nil
			}
		}
	}
	return Hop{}, errors.Errorf("No route to ip %s on table.", src)
}