path: root/pkg/ipstack/ipstack.go

package ipstack

// code begins on line 97 after imports, constants, and structs definitions
// This class is divided as follows:
// 1) INIT FUNCTIONS
// 2) DOWN/UP FUNCTIONS
// 3) GETTER FUNCTIONS
// 4) PRINT FUNCTIONS
// 5) ROUTE FUNCTIONS
// 6) RIP FUNCTIONS
// 7) PROTOCOL HANDLERS
// 8) CHECKSUM FUNCTIONS
// 9) HELPERS

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
	INFINITY                  = 16
	SIZE_OF_RIP_ENTRY         = 12
	RIP_PROTOCOL              = 200
	TEST_PROTOCOL             = 0
	SIZE_OF_RIP_HEADER        = 4
	MAX_TIMEOUT               = 12
)

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

	Socket        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, message []byte, hdr *ipv4header.IPv4Header) error

var protocolHandlers = make(map[int]HandlerFunc)

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

var timeoutTableMu sync.Mutex
var timeoutTable = make(map[netip.Prefix]int)

// ************************************** 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 on the given UDP address.
// It SETS the conn parameter to the created UDP socket.
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 initializes the data structures and creates the UDP sockets.
//
//	It will return an error if the lnx file is not valid or if a socket fails to be created.
//
//		After parsing the lnx file, it does the following:
//	 1. adds each local interface to the routing table, as dictated by its subnet
//	 2. adds neighbors to interface->neighbors[] map
//	 3. adds RIP neighbors to RIP neighbor list
//	 4. adds static routes to routing table
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,
			Socket:        net.UDPConn{},
			SocketChannel: make(chan bool),
			State:         true,
		}

		// create the UDP listener
		err := createUDPListener(iface.UDPAddr, &i.Socket)
		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
}

// InterfaceListenerRoutine is a go routine for interfaces to listen on a UDP port.
//
//		It is composed two go routines:
//	 1. a go routine that hangs on the recv and calls RecvIP() when a packet is received
//	 2. a go routine that listens on the channel for a signal to start/stop listening
//
// TODO: (performance) remove isUp and use the interface's value instead
func InterfaceListenerRoutine(i *Interface) {
	// decompose the interface
	socket := i.Socket
	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
			}
			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 ******************************************************

// InterfaceUp brings up the link layer
//
//		It does the following:
//	 1. tells the listener (through a channel) to start listening
//	 2. updates the interface state to up
//	 3. sends RIP request to all neighbors of this iface to quickly update the routing table
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)
}

// InterfaceDown cuts off the link layer.
//
//		It does the following:
//	 1. tells the listener (through a channel) to stop listening
//	 2. updates the interface state to down
//	 3. updates the routing table by removing the routes those neighbors connected to, sending triggered updates.
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 **********************************************************

// SprintInterfaces returns a string representation of the interfaces 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
}

// SprintNeighbors returns a string representation of the neighbors 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
}

// SprintRoutingTable returns a string representation of the routing table
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 **********************************************************

// CleanUp 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.Socket.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)
}

// ************************************** ROUTE FUNCTIONS **********************************************************

// SendIP sends an IP packet to a destination
//
//	If the header is nil, then a new header is created
//	If the header is not nil, then it will use that header after decrementing TTL & recomputing checksum
//
//	TODO: (performance) have this take in an interface instead of src for performance
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())
	}

	// send the packet
	bytesWritten, err := iface.Socket.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 the interface
// To be called by the listener routine, representing one interface
// Upon receiving a packet, this function:
//  1. determines if packet is valid (checksum, TTL)
//  2. determines if the packet is for me. if so, SENDUP (call correct handler)
//  3. the packet is not SENTUP, then checks the routing table
//  4. if there is no route in the routing table, then prints an error and DROPS the packet
func RecvIP(iface *Interface, isOpen *bool) error {
	buffer := make([]byte, MAX_IP_PACKET_SIZE)

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

	// 3) 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 := Route(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 FUNCTIONS *******************************************************

// MakeRipMessage returns the byte array to be used in SendIp for a RIP packet
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
	}

	// 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
}

// PeriodicUpdateRoutine sends RIP updates to neighbors every 5 seconds
// TODO: (performace) consider making this multithreaded and loops above more efficient
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 the neighbor is not a RIP neighbor, skip it
				if !in {
					continue
				}

				// Sending to a rip neighbor
				// create the entries
				entries := make([]RIPEntry, 0)
				for prefix, hop := range routingTable {
					// implement split horizon + poison reverse at entry level
					var cost uint32
					if hop.VIP == n.VipAddr {
						cost = INFINITY
					} else {
						cost = hop.Cost
					}
					entries = append(entries,
						RIPEntry{
							prefix: prefix,
							cost:   cost,
						})
				}

				// make the message and send it
				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 and repeat
		time.Sleep(5 * time.Second)
	}
}

// SendTriggeredUpdates sends the entries consumed to ALL neighbors
func SendTriggeredUpdates(newEntries []RIPEntry) {
	for _, iface := range myInterfaces {
		for _, n := range myNeighbors[iface.Name] {
			// only send to RIP neighbors, else skip
			_, in := myRIPNeighbors[n.VipAddr.String()]
			if !in {
				continue
			}

			// send the made entries to the neighbor
			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
			}
		}
	}
}

// ManageTimeoutsRoutine manages the timeout table by incrementing the timeouts every second.
// If a timeout reaches MAX_TIMEOUT, then the entry is deleted from the routing table and a triggered update is sent.
func ManageTimeoutsRoutine() {
	for {
		time.Sleep(time.Second)

		timeoutTableMu.Lock()
		// check if any timeouts have occurred
		for key, _ := range timeoutTable {
			timeoutTable[key]++
			// if the timeout is MAX_TIMEOUT, delete the entry
			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)
				}
			}
		}
		timeoutTableMu.Unlock()
		//fmt.Println("Timeout table: ", timeoutTable)
	}
}

// StartRipRoutines handles all the routines for RIP
//  1. sends a RIP request to every neighbor
//  2. starts the routine that sends periodic updates every 5 seconds
//  3. starts the routine that manages the timeout table
func StartRipRoutines() {
	// send a request to every neighbor
	go func() {
		for _, iface := range myInterfaces {
			for _, neighbor := range myNeighbors[iface.Name] {
				// only send to RIP neighbors, else skip
				_, 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
				}
			}
		}
	}()

	// start a routine that sends updates every 5 seconds
	go PeriodicUpdateRoutine()

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

// ************************************** PROTOCOL HANDLERS *******************************************************

// RegisterProtocolHandler registers a protocol handler for a given protocol number
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
}

// HandleRIP handles incoming RIP packets in the following way:
//  1. if the command is a request, send a RIP response only to that requestor
//  2. if the command is a response, parse the entries, update the routing table from them,
//     and send applicable triggered updates (see implementation for how to update)
func HandleRIP(src *Interface, message []byte, hdr *ipv4header.IPv4Header) error {
	// parse the RIP message
	command := int(binary.BigEndian.Uint16(message[0:2]))
	switch command {
	// request message
	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
		}

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

		// 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" {
				timeoutTableMu.Lock()
				_, in := timeoutTable[destination]
				if in {
					if routingTable[destination].VIP == hdr.Src {
						timeoutTable[destination] = 0
					}
				}
				timeoutTableMu.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)
							timeoutTableMu.Lock()
							delete(timeoutTable, destination)
							timeoutTableMu.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
}

// prints the test packet as per the spec
func HandleTestPackets(src *Interface, 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 ******************************************************
// reference: https://github.com/brown-csci1680/lecture-examples/blob/main/ip-demo/cmd/udp-ip-recv/main.go
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
}

// *********************************************** HELPERS **********************************************************

// Route returns the next HOP, based on longest prefix match for a given ip
// TODO: revisit how to do this at the bit level, not hardcoded for 32 & 24
func Route(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("error ROUTE: destination %s does not exist on routing table.", src)
}