辅导TCP/IP Stack、讲解Link Layer Ethernet、辅导java/c/c++，Python编程语言调试C/C++编程|辅导W

Part C (50 marks)
In this part of the assignment you will be required to implement the network layer for a host running on a virtual
IP network. You should be familiar with the typical TCP/IP stack (A.K.A. the DoD model or internet protocol
suite), where Ethernet is used as a link-layer protocol and IPv4 used as a network layer protocol. The protocols we
will be using will be virtual - that is, network layer addresses won’t actually correspond to physical interfaces. To
do this, the network stack will be redefined to that shown in the table below.
TCP/IP Stack Protocols Virtual Stack
Network Layer
Transport layer UDP
Network layer IPv4 Link layer
Link Layer Ethernet
As described in the above figure, UDP will be used as your virtual network’s link layer, and you will be required
to implement a virtualisation of IPv4 ontop of this UDP-based link layer. Each UDP socket will correspond to a
link layer interface, and localhost UDP port numbers will be used as the link layer addressing system of this virtual
network.
By the end of this assignment, your implemented host program should be able to:
Accept simple user commands through a basic command line interface (CLI)
Send and receive messages across this virtual network layer
Handle fragmentation of virtual IP packets
Program Invocation
Your program should be able to be invoked from a UNIX or UNIX-like command line as follows. It is expected
that any Python programs can run with version 3.6, and any Java programs can run with version 8. The ip-addr
and ll-addr parameters correspond to the IPv4 address in CIDR notation (indicating the client’s subnet) and link
layer address (UDP port number) of your host program respectively.
Python
python3 assign3.py ip-addr ll-addr
C/C++ make./assign2 ip-addr ll-addr Java
make java Assign2 ip-addr ll-addr
4Your Task
Command Line Interface (5 marks)
To start with, you should implement a basic command line interface that will allow the user to supply basic
information about the network. Your CLI should prompt users with a single > character, followed by a space.
For the rest of the assignment we define anything wrapped in square brackets as a parameter or field that needs
replacing (NOT as an optional parameter). The CLI should persistently prompt the user for another command until
the program is terminated. For full marks in this section your CLI needs to accommodate the following commands:
gw set [ip-addr] : set the gateway IP address of the LAN the client is a part of to [ip-addr] (overriding
any existing gateway address)
gw get : print the currently stored gateway IP address to stdout, or None if no gateway address has been
specified
arp set [ip-addr] [ll-addr] : insert a mapping from [ip-addr] to [ll-addr] in the host’s ARP table
(overriding any existing entries for [ip-addr])
arp get [ip-addr] : print the currently stored link layer address mapped to [ip-addr] to stdout, or None
if no mapping exists
exit : terminate the program
Error-handling of user input is optional - you won’t receive any marks for this but you can choose to implement it
provided it doesn’t impact on the rest of the assignment.
Sending Messages (15 marks)
To receive marks in this section, your CLI should be able to handle the following additional command:
msg [ip-addr] "[payload]" : send a virtual IPv4 packet to [ip-addr] with the given payload (which will
be supplied as a string)
Any packets sent should have a meaningful identifier and a protocol number of 0. You may assume the that all
payloads will be less than or equal to the MTU of the network’s links, but the DF flag should still be set to 0. Your
program should support any IP address, regardless of whether it is a part of your subnet or not. If your program
needs to send a packet to the gateway address and no gateway address has been specified has been specified, your
program should print No gateway found to stdout. If no required address mapping can be found in the host’s
ARP table, your program should print No ARP entry found to stdout.
Receiving Messages (15 marks)
In addition to sending packets, your program should be able to receive them from other hosts. When your program
receives an IPv4 packet with the protocol indicator set to 0, it should print the payload of that packet to stdout, in
the below format ([ip-addr] should be replaced with the sender’s IPv4 address and [message] should be replaced
with the string encoding of the payload):
Message received from [ip-addr]: "[message]"
When your program receives an IPv4 packet with a non-zero protocol number, it should print the following message
to stdout ([proto-num] should be the hexadecimal representation of the protocol formatted as 0x??):
Message received from [ip-addr] with protocol [proto-num]
You can assume all packets sent to your program are valid IPv4 packets. You should be able to receive messages
at any time without blocking the CLI, and any messages should be printed cleanly (without any CLI prompts or
responses disrupting the message contents). Remember to backspace the current prompt (the > character) before
printing the output.
5IP Fragmentation (15 marks)
To receive marks in this section your program should be able to handle IP fragmentation. Your CLI should support
the following extra commands:
mtu set [value] : set the MTU of the network’s links as the specified [value]
mtu get : print the currently stored MTU (the default MTU should be 1500)
Virtual packets that are longer than the specified MTU (or the default MTU if none has been specified) should be
fragmented before transmission. The length of a virtual packet is equal to the length of the IPv4 header added to
the length of the IPv4 payload (i.e. you don’t need to consider the length of the non-virtual headers). You can
assume the value of the MTU will never be smaller than 100.
Your program should also be able to receive packets that have been fragmented (and display them as a single
message).
Example CLI output
python3 assign2.py 192.168.1.1/24 1024
> gw get
None
> gw set 192.168.1.30
> gw get
192.168.1.30
> msg 192.168.1.2 "hello"
No ARP entry found
> arp get 192.168.1.2
None
> arp set 192.168.1.2 2222
> arp get 192.168.1.2
2222
> msg 192.168.1.2 "hello"
> mtu get
1500
> mtu set 1600
> mtu get
1600
Message received from 192.168.1.2: "hello there, thankyou for your message"
Message received from 192.168.1.3 with protocol 0x06
> exit
Packet Formatting
Your IPv4 packets don’t need to contain meaningful values for the DCSP/ECN fields, nor are you required to
compute a correct checksum. You can assume packets will never contain any options and subsequently that the
IHL will always be 5. The TTL field should contain some meaningful value (i.e. to allow the packet to reach its
destination).
6

辅导TCP/IP Stack、讲解Link Layer Ethernet、辅导java/c/c++，Python编程语言 调试C/C++编程|辅导W

辅导TCP/IP Stack、讲解Link Layer Ethernet、辅导java/c/c++，Python编程语言调试C/C++编程|辅导W