CS计算机代考程序代写 python compiler cache assembly FTP algorithm Hive 44100113-0: COMPUTER NETWORKS

44100113-0: COMPUTER NETWORKS
FALL 2021

Build Your Own Router

Assigned: November 10, 2021
Due Date: 23:59, December 5, 2021

1 ASSIGNMENT

You must work on this assignment individually.

This project is designed for you to:

• Learn to build a simple router.
• To be familiar with IPv4, ICMP and ARP.

In this project, you will be writing a simple router with a static routing table. Your router will receive raw
Ethernet frames and process them just like a real router: forward them to the correct outgoing interface,
create new frames, etc. The starter code will provide the framework to receive Ethernet frames; your job
is to create the forwarding logic.

You are allowed to use some high-level abstractions, including C++11 extensions, for parts that are not
directly related to networking, such as string parsing, multi-threading, etc.

In the following contents, you will see

• Detailed description of this porject in Section 2

• Guidelines to setup the environment in Section 3

• Overview of the starter code in Section 4

• Some helpful hints in Sections 5

• How to submit your code in Section 6

• Grading criteria in Section 7

This assignment is considerably hard, so get started early, not as some of you did for FTP Project, to
avoid missing the deadline.

2 PROJECT DESCRIPTION

There are four main parts in this assignment:

• Handle Ethernet frames

• Handle ARP packets

• Handle IPv4 packets

• Handle ICMP packets

This assignment runs on top of Mininet which was built at Stanford. Mininet allows you to emulate a
network topology on a single machine. It provides the needed isolation between the emulated nodes so
that your router node can process and forward real Ethernet frames between the hosts like a real router.
You dont have to know how Mininet works to complete this assignment, but if youre curious, you can
learn more information about Mininet on its official website (http://mininet.org/).

Your router will route real packets between emulated hosts in a single-router topology. The project envi-
ronment and the starter code has the following default topology:

+−−−−−−−−−−−−−−−−+ server1 −eth0
| 192.168.2.2/24
+ +−−−−−−−−−−−+

192.168.2.1/24 | |
sw0−eth1 | server1 |

+−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−−+ | |
| | | | +−−−−−−−−−−−+
| c l i e n t | | SimpleRouter |
| | | (sw0) |
+−−−−−−−−−−+ | |

c l i en t −eth0 +−−−−−−−−−−−−−−−−−−+ +−−−−−−−−−−−+
10.0.1 .100/8 sw0−eth3 sw0−eth2 | |

+ 1 0 . 0 . 1 . 1 / 8 172.64.3.1/16 | server2 |
| + + | |
| | | +−−−−−−−−−−−+
| | | server2 −eth0
+−−−−−−−−−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−+ 172.64.3.10/16

The corresponding routing table for the SimpleRouter sw0 in this default topology:

Destination Gateway Mask Iface
−−−−−−−−−−−− −−−−−−−−−−−− −−−−−−−−−−−−−−−− −−−−−−−−
0 . 0 . 0 . 0 10.0 .1 .10 0 0 . 0 . 0 . 0 sw0−eth3
192.168.2.2 192.168.2.2 255.255.255.0 sw0−eth1
172.64.3.10 172.64.3.10 255.255.0.0 sw0−eth2

NOTE: Do not hardcode any IP addresses, network, or interface information. We will be testing your
code on other single-router topologies with different number of servers and clients, and different IP and
network addresses. So it is STRONGLY RECOMMENDED that you test your implementations under dif-
ferent IP addresses and routing tables (by modifying the IP_CONFIG and RTABLE file respectively).

If your router is functioning correctly, all of the following operations should work:

http://mininet.org/

• ping from the client to any of the routers interfaces:

mininet> c l i e n t ping 192.168.2.1
. . .
mininet> c l i e n t ping 1 72.64 .3 .1
. . .
mininet> c l i e n t ping 1 0 . 0 . 1 . 1
. . .

• ping from the client to any of the app servers:

mininet> c l i e n t ping server1 # or c l i e n t ping 192.168.2.2
. . .
mininet> c l i e n t ping server2 # or c l i e n t ping 172.64.3.10
. . .

• traceroute from the client to any of the routers interfaces:

mininet> c l i e n t traceroute 192.168.2.1
. . .
mininet> c l i e n t traceroute 172 .64.3 .1
. . .
mininet> c l i e n t traceroute 1 0 . 0 . 1 . 1
. . .

• Tracerouting from the client to any of the app servers:

mininet> c l i e n t traceroute server1
. . .
mininet> c l i e n t traceroute server2
. . .

• Downloading files from http-server by wget command

mininet> c l i e n t wget http ://192.168.2.2/ < filename >
. . .
mininet> c l i e n t wget http ://172.64.3.10/ < filename >
. . .

2.1 ETHERNET FRAMES

A data packet on a physical Ethernet link is called an Ethernet packet, which transports an Ethernet frame
as its payload.

The starter code will provide you with a raw Ethernet frame. Your implementation should understand
source and destination MAC addresses and properly dispatch the frame based on the protocol.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| Destination Address |
+ (48 b i t s ) +−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+

https://en.wikipedia.org/wiki/Ethernet_frame

| | Source Address |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+ (48 b i t s ) +
| |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| Type (16 b i t s ) | |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+ +
| |
| |
~ Payload ~
| |
| |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+

Note that actual Ethernet frame also includes a 32-bit Cyclical Redundancy Check (CRC). In this project,
you will not need it, as it will be added automatically.

• Type : Payload types

– 0x0806 (ARP)

– 0x0800 (IPv4)

For your convenience, the starter code defines Ethernet header as an ethernet_hdr structure in core/protocol.hpp :

struct ethernet_hdr
{

uint8_t ether_dhost [ETHER_ADDR_LEN ] ; / * destination ethernet address * /
uint8_t ether_shost [ETHER_ADDR_LEN ] ; / * source ethernet address * /
uint16_t ether_type ; / * packet type ID * /

} __attr ibute__ ( ( packed ) ) ;

Requirements

• Your router should ignore Ethernet frames other than ARP and IPv4.

• Your router must ignore Ethernet frames not destined to the router, i.e., when destination hard-
ware address is neither the corresponding MAC address of the interface nor a broadcast address
( FF:FF:FF:FF:FF:FF ).

• Your router must appropriately dispatch Ethernet frames (their payload) carrying ARP and IPv4
packets.

2.2 ARP PACKETS

The Address Resolution Protocol (ARP) (RFC 826) is a telecommunication protocol used for resolution
of Internet layer addresses (e.g., IPv4) into link layer addresses (e.g., Ethernet). In particular, before your
router can forward an IP packet to the next-hop specified in the routing table, it needs to use ARP re-
quest/reply to discover a MAC address of the next-hop. Similarly, other hosts in the network need to use
ARP request/replies in order to send IP packets to your router.

Note that ARP requests are sent to the broadcast MAC address ( FF:FF:FF:FF:FF:FF ). ARP replies are sent
directly to the requesters MAC address.

https://en.wikipedia.org/wiki/Address_Resolution_Protocol
https://tools.ietf.org/html/rfc826

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| Hardware Type | Protocol Type |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| HW addr len | Prot addr len | Opcode |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| |
~ Source hardware address ~
| |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| |
~ Source protocol address ~
| |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| |
~ Destination hardware address ~
| |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| |
~ Destination protocol address ~
| |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+

• Hardware Type : 0x0001 (Ethernet)

• Protocol Type : 0x0800 (IPv4)

• Opcode :

– 1 (ARP request)

– 2 (ARP reply)

• HW addr len : number of octets in the specified hardware address. Ethernet has 6-octet addresses,
so 0x06.

• Prot addr len : number of octets in the requested network address. IPv4 has 4-octet addresses, so
0x04.

For your convenience, the starter code defines the ARP header as an arp_hdr structure in core/protocol.hpp :

struct arp_hdr
{

unsigned short arp_hrd ; / * format of hardware address * /
unsigned short arp_pro ; / * format of protocol address * /
unsigned char arp_hln ; / * length of hardware address * /
unsigned char arp_pln ; / * length of protocol address * /
unsigned short arp_op ; / * ARP opcode (command) * /
unsigned char arp_sha [ETHER_ADDR_LEN ] ; / * sender hardware address * /
uint32_t arp_sip ; / * sender IP address * /
unsigned char arp_tha [ETHER_ADDR_LEN ] ; / * t a r g e t hardware address * /
uint32_t arp_tip ; / * t a r g e t IP address * /

} __attr ibute__ ( ( packed ) ) ;

Requirements

• Your router must properly process incoming ARP requests and replies:

– Must properly respond to ARP requests for MAC address for the IP address of the correspond-
ing network interface

– Must ignore other ARP requests

• When your router receives an IP packet to be forwarded to a next-hop IP address, it should check
ARP cache if it contains the corresponding MAC address:

– If a valid entry found, the router should proceed with handling the IP packet

– Otherwise, the router should queue the received packet and start sending ARP request to dis-
cover the IP-MAC mapping.

• When router receives an ARP reply, it should record IP-MAC mapping information in ARP cache
(Source IP/Source hardware address in the ARP reply). Afterwards, the router should send out all
corresponding enqueued packets.

NOTE: Your implementation should not save IP-MAC mapping based on any other messages, only
from ARP replies!

• To reduce staleness of the ARP information, entries in ARP cache should time out after 30 seconds .
The starter code ( ArpCache class) already includes the facility to mark ARP entries invalid. Your
task is to remove such entries. If there is an ongoing traffic (e.g., client still pinging the server), then
the router should go through the standard mechanism to send ARP request and then cache the
response. If there is no ongoing traffic, then ARP cache should eventually become empty.

• The router should send an ARP request about once a second until an ARP reply comes back or the
request has been sent out at least 5 times .

If your router didnt receive ARP reply after re-transmitting an ARP request 5 times, it should stop
re-transmitting, remove the pending request, and any packets that are queued for the transmission
that are associated with the request.

Your router should also send an ICMP Destination Host Unreachable message to the source IP.

2.3 IPV4 PACKETS

Internet Protocol version 4 (IPv4) (RFC 791) is the dominant communication protocol for relaying data-
grams across network boundaries. Its routing function enables internetworking, and essentially estab-
lishes the Internet. IP has the task of delivering packets from the source host to the destination host
solely based on the IP addresses in the packet headers. For this purpose, IP defines packet structures
that encapsulate the data to be delivered. It also defines addressing methods that are used to label the
datagram with source and destination information.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| Version | IHL | Type of Service | Total Length |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+

https://en.wikipedia.org/wiki/IPv4
https://tools.ietf.org/html/rfc791

For your convenience, the starter code defines the IPv4 header as an ip_hdr structure in core/protocol.hpp :

struct ip_hdr
{

unsigned int ip_hl : 4 ; / * header length * /
unsigned int ip_v : 4 ; / * version * /
uint8_t ip_tos ; / * type of s e r v i c e * /
uint16_t ip_len ; / * t o t a l length * /
uint16_t ip_id ; / * i d e n t i f i c a t i o n * /
uint16_t i p _ o f f ; / * fragment o f f s e t f i e l d * /
uint8_t i p _ t t l ; / * time to l i v e * /
uint8_t ip_p ; / * protocol * /
uint16_t ip_sum ; / * checksum * /
uint32_t ip_src , ip_dst ; / * source and dest address * /

} __attr ibute__ ( ( packed ) ) ;

Requirements

• For each incoming IPv4 packet, your router should verify its checksum and the minimum length of
an IP packet

– Invalid packets must be discarded (a proper ICMP error response is NOT required for this
project).

• Your router should classify datagrams into (1) destined to the router (to one of the IP addresses of
the router), and (2) datagrams to be forwarded:

– For (1), if packet carries ICMP payload, it should be properly dispatched. Otherwise, discarded
(a proper ICMP error response is NOT required for this project).

– For (2), your router should use the longest prefix match algorithm to find a next-hop IP ad-
dress in the routing table and attempt to forward it there

• For each forwarded IPv4 packet, your router should correctly decrement TTL and recompute the
checksum.

2.4 ICMP PACKETS

Internet Control Message Protocol (ICMP) (RFC 792) is a simple protocol that can send control informa-
tion to a host.

In this assignment, your router will use ICMP to send messages back to a sending host.

• Echo or Echo Reply message

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

– Type

* 8 : echo message

* 0 : echo reply message

• Time Exceeded message

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| Type = 11 | Code | Checksum |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| unused |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| Internet Header + 64 b i t s of Original Data Datagram |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+

– Code

* 0 : time to live exceeded in transit

* 1 : fragment reassembly time exceeded (NOT required to implement)

• Destination Unreachable message

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| Type = 3 | Code | Checksum |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| unused |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+
| Internet Header + 64 b i t s of Original Data Datagram |
+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+−+

https://en.wikipedia.org/wiki/Internet_Control_Message_Protocol
https://tools.ietf.org/html/rfc792

– Code

* 1 : Destination host unreachable

* 3 : Destination port unreachable

When an ICMP message is composed by a router, the source address field of the internet header can be
the IP address of any of the routers interfaces, as specified in RFC 792.

Note that Time Exceeded message is needed for traceroute to work properly.

For your convenience, the starter code defines the ICMP header as an icmp_hdr structure in core/protocol.hpp :

struct icmp_hdr {
uint8_t icmp_type ;
uint8_t icmp_code ;
uint16_t icmp_sum ;

} __attr ibute__ ( ( packed ) ) ;

You may want to create additional structs for ICMP messages, but make sure to use the packed attribute
so that the compiler doesnt try to align the fields in the struct to word boundaries (i.e., must use
__attribute__ ((packed)) annotation).

Requirements

Your router should properly generate the following ICMP messages, including proper ICMP header check-
sums:

• Echo Reply message ( type 0 ):

Sent in response to an incoming Echo Request message (ping) to one of the routers interfaces.
Echo requests sent to other IP addresses should be forwarded to the next hop address as usual.

In this project, Echo Reply message’s initial TTL field in its IPv4 header should be 64 .

• Time Exceeded message ( type 11 , code 0 ):

Sent if an IP packet is discarded during processing because the TTL field is 0. This is needed for
traceroute to work.

• Port Unreachable message ( type 3 , code 3 ):

Sent if an IP packet containing a UDP or TCP payload is sent to one of the routers interfaces. This
is needed for traceroute to work.

3 ENVIRONMENT SETUP

You are suggested to finish this project on Ubuntu 16.04.7. If your system is Windows or Mac OS, you can
install Ubuntu 16.04.7 Virtual Machine in VMware. (Do not use Windows Subsystem for Linux (WSL), as
Mininet is currently not supported by WSL.)

We provide setup.sh to setup the environment. Please run it in superuser mode:

sudo bash setup . sh

https://tools.ietf.org/html/rfc792

To run or test your code, please change work path to the project directory and open three terminals here,
then

1. in the first terminal, start pox

/opt/pox/pox . py −−verbose ucla_cs118

2. in the second terminal, start Mininet

chmod +x run . py
sudo . / run . py

To exit Mininet, type exit command in this terminal.

3. in the third terminal, start your router

make
. / router

Besides, you can use the show-arp.py script to print out your router’s current arp cache or routing table
in another terminal.

• To show arp cahce, run:

. / show−arp . py arp

• To show routing table, run:

. / show−arp . py routing

Here are some tips:

• If you get the following outputs after running router

Got packet of s i z e 42 on i n t e r f a c e sw0−eth1
Received packet , but i n t e r f a c e i s unknown, ignoring

Resetting SimpleRouter with 0 ports
Interface l i s t empty

Please restart Mininet.

• If you get the following outputs after running run.py

/ usr /bin/env : ’ python\ r ’ : No such f i l e or directory

Maybe converting the format of run.py is helpful

sudo apt−get i n s t a l l dos2unix
dos2unix run . py

• When POX controller is restrated, the simpler router needs to be manually stopped and started
again.

4 STARTER CODE OVERVIEW

Here is the overal structure of the starter code:

m_arp 1 | 1 | | 1 m_ifaces
+−−−−−−−−−−−−−−−−+ | +−−−−−−−−−−−−−−−−−−+
| | |
| | m_routingTable |
| | |
| 1 | 1 | N ( std : : set )
v v v

• SimpleRouter

Main class for your simple router, encapsulating ArpCache , RoutingTable , and as set of Interface
objects.

• Interface

Class containing information about routers interface, including router interface name ( name ),
hardware address ( addr ), and IPv4 address ( ip ).

• RoutingTable ( routing-table.hpp|cpp )

Class implementing a simple routing table for your router. The content is automatically loaded
from a text file with default filename is RTABLE (name can be changed using RoutingTable option

in router.config config file)

• ArpCache ( arp-cache.hpp|cpp )

Class for handling ARP entries and pending ARP requests.

4.1 KEY METHODS

Your router receives a raw Ethernet frame and sends raw Ethernet frames when sending a reply to the
sending host or forwarding the frame to the next hop. The basic functions to handle these functions are:

• Need to implement Method that receives a raw Ethernet frame ( simple-router.hpp|cpp ):

/ * *
* This method i s c al l e d each time the router r e c e i v e s a packet on

* the i n t e r f a c e . The packet buffer \p packet and the r e c e i v i n g

* i n t e r f a c e \p i n I f a c e are passed in as parameters .

* /
void
SimpleRouter : : handlePacket ( const Buffer& packet , const std : : s t r i n g& i nI f a c e ) ;

• Implemented Method to send raw Ethernet frames ( simple-router.hpp|cpp ):

/ * *
* Call t h i s method to send packet \p packt from the router on

* i n t e r f a c e \p outI face

* /
void
SimpleRouter : : sendPacket ( const Buffer& packet , const std : : s t r i n g& outIface ) ;

• Need to implement Method to handle ARP cache events ( arp-cache.hpp|cpp ):

/ * *
* This method g e t s c a l l e d every second . For each request sent out ,

* you should keep checking whether to resend a request or remove i t .

* /
void
ArpCache : : periodicCheckArpRequestsAndCacheEntries ( ) ;

• Need to implement Method to lookup entry in the routing table ( routing-table.hpp|cpp ):

/ * *
* This method should lookup a proper entry in the routing table

* using ” longest − p r e f i x match” algorithm

* /
RoutingTableEntry
RoutingTable : : lookup ( uint32_t ip ) const ;

4.2 DEBUGGING FUNCTIONS

We have provided you with some basic debugging functions in core/utils.hpp ( core/utils.cpp ). Feel
free to use them to print out network header information from your packets. Below are some functions
you may find useful:

• print_hdrs(const uint8_t *buf, uint32_t length) , print_hdrs(const Buffer& packet)

Print out all possible headers starting from the Ethernet header in the packet

• ipToString(uint32_t ip) , ipToString(const in_addr& address)

Print out a formatted IP address from a uint32_t or in_addr . Make sure you are passing the IP
address in the correct byte ordering

• macToString(const Buffer& macAddr)

Print out a formatted MAC address from a Buffer of MAC address

4.3 LOGGING PACKETS

You can use Mininet to monitor traffic that goes in and out of the emulated nodes, i.e., router, server1
and server2. For example, to see the packets in and out of server1 node, use the following command in
Mininet command-line interface (CLI):

mininet> server1 sudo tcpdump −n − i server1 −eth0

Alternatively, you can bring up a terminal inside server1 using the following command

mininet> xterm server1

then inside the newly opened xterm :

$ sudo tcpdump −n − i server1 −eth0

4.4 GRADING SCRIPT

To ease debugging, we make a simplified version of grading script public with autograde.py . It contains
all the public test cases (details in Section 7), which make up 45/85 of the total test score. To run the
script, first start pox and your router, then use the following command:

chmod +x autograde . py
sudo . / autograde . py

The grading result and related information will be shown in standard output as well as the details.log
file. You can also change the logging level to see more detailed information:

log . setLevel ( logging .DEBUG)

If you get the following outputs from router:

Got packet of s i z e 42 on i n t e r f a c e sw0−eth1
Received packet , but i n t e r f a c e i s unknown, ignoring

Just turn off your router, grading script, and then start and exit Mininet with run.py . Afterwards, the
grading script may work as expected.

Notice that the private, comprehensive grading script will be run on other single-router topology with
different interfaces, IP/MAC addresses. Thus, the output of provided script may be different from your
final score, even on public test cases.

5 HELPFUL HINTS

Given a raw Ethernet frame, if the frame contains an IP packet that is not destined towards one of our
interfaces:

• Sanity-check the packet (meets minimum length and has correct checksum).

• Decrement the TTL by 1, and recompute the packet checksum over the modified header.

• Find out which entry in the routing table has the longest prefix match with the destination IP ad-
dress.

• Check the ARP cache for the next-hop MAC address corresponding to the next-hop IP. If its there,
send it. Otherwise, send an ARP request for the next-hop IP (if one hasnt been sent within the last
second), and add the packet to the queue of packets waiting on this ARP request.

If an incoming IP packet is destined towards one of your routers IP addresses, you should take the follow-
ing actions, consistent with the section on protocols above:

• If the packet is an ICMP echo request and its checksum is valid, send an ICMP echo reply to the
sending host.

• If the packet contains a TCP or UDP payload, send an ICMP port unreachable to the sending host.
Otherwise, ignore the packet. Packets destined elsewhere should be forwarded using your normal
forwarding logic.

Obviously, this is a very simplified version of the forwarding process, and the low-level details follow. For
example, if an error occurs in any of the above steps, you will have to send an ICMP message back to the
sender notifying them of an error. You may also get an ARP request or reply, which has to interact with
the ARP cache correctly.

6 SUBMISSION REQUIREMENTS

To submit your project, you need to prepare:

1. A report.pdf file no more than THREE pages. (See details in section 7)

2. All your source code, Makefile and report.pdf as a .tar.gz archive (and any files from extra credit
part).

To create the submission, use the provided Makefile in the starter code. Just update Makefile to
include your student ID and then just type

make t a r b a l l

Then submit the resulting archive to WEB LEARNING.

Before submission, please make sure:

• Your code compiles

• Your implementation conforms to the specification

• .tar.gz archive does not contain temporary or other unnecessary files. We will automatically deduct
points otherwise.

7 GRADING GUIDELINES

Your project will be graded based on the following parts (private tests are not provided in autograde.py ):

1. Ping tests

a) (5 pts, public) Pings from client to all other hosts (all pings expected to succeed), including
non-existing host (error expected)

b) (5 pts, public) Pings from server1 to all other hosts (all pings expected to succeed), including
non-existing host (error expected)

c) (5 pts, public) Pings from server2 to all other hosts (all pings expected to succeed), including
non-existing host (error expected)

d) (10 pts, private) Ping responses (from client) have proper TTLs

e) (5 pts, public) Ping between selected hosts, check ARP cache, there should be a proper num-
ber of entries

f) (5 pts, public) Ping from client to server1, after 40 seconds, the ARP cache should be empty (+
no segfaults)

g) (5 pts, private) Ping from client a non-existing IP, router sends proper ARP requests (+ no
segfaults)

h) (5 pts, private) Ping from client, receive host unreachable message

2. Traceroute tests

a) (5 pts, public) Traceroute from client to all other hosts, including a non-existing host

b) (5 pts, public) Traceroute from server1 to all other hosts, including a non-existing host

c) (5 pts, public) Traceroute from server2 to all other hosts, including a non-existing host

d) (10 pts, private) Traceroute from client to routers interfaces (get 1 line)

3. File downloading tests

a) (5 pts, public) Download a small file (≈ 1 KB) from any server through http

b) (10 pts, private) Download a large file (≈ 10 MB) from any server through http

4. (20 pts) Code quality and Project Report ( report.pdf )

The project report could include:

a) Your name and student ID

b) The problems you have met in this project and how you solved them.

c) List of any additional libraries used.

d) Any advice on this project.

Note that poor design, code structure, or report will probably reduce the credits you gained in this
part.

Suppose you get A credits according to the above criteria, your final grade should be

• A, if you submit before or on Dec 5;

• A−20, if you submit after Dec 5 and before or on Dec 12;

• 0, if you submit after Dec 12.

8 ACKNOWLEDGEMENT

This project is based on the CS118 class project by Alexander Afanasyev, UCLA.

Assignment
Project Description
Ethernet Frames
ARP Packets
IPv4 Packets
ICMP Packets

Environment Setup
Starter Code Overview
Key Methods
Debugging Functions
Logging Packets
Grading Script

Helpful Hints
Submission Requirements
Grading Guidelines
Acknowledgement

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