##
## SAMPLE SOLUTION --- ALTERNATING-BIT PROTOCOL --- MARCH 18
##
## Python version of:
##
## ****************************************************************************
## ALTERNATING BIT AND GO-BACK-N NETWORK SIMULATOR: VERSION 1.1 J.F.Kurose
##
## This code should be used for PA2, unidirectional data-transfer protocols
## from A to B.
## Network properties:
## - one-way network delay averages 5.0 time units (longer if there
## are other messages in the channel for GBN), but can be larger
## - packets can be corrupted (either the header or the data portion)
## or lost, according to user-defined probabilities
## - packets will be delivered in the order in which they were sent
## (although some can be lost).
## ****************************************************************************
##
## Python version by:
## Eric Eide <eeide@cs.utah.edu>
## University of Utah
## March 2022
##
## ==> STUDENTS: Implement the methods for EntityA and EntityB.
##
## Run `python3 <thisfile.py> -h` in a terminal window to see the various
## parameters that you can set for a run of this simulator.
##
## This program defines a variable `TRACE` that you can use to conditionally
## print messages from your EntityA and EntityB methods. For example:
##
## if TRACE>0:
## print('A very important event just happened!')
##
## The value of `TRACE` is set by the `-v` command line option. The default
## value of `TRACE` is 0 (meaning "no tracing").
##
import argparse
from copy import deepcopy
from enum import Enum, auto
import random
import sys
import time
from binascii import crc32
###############################################################################
## ************************* BASIC DATA STRUCTURES ****************************
##
## STUDENTS: Do not modify these definitions.
##
## ****************************************************************************
# A Msg is the data unit passed from layer 5 (teacher's code) to layer 4
# (student's code). It contains the data (bytes) to be delivered to layer 5
# via the student's transport-level protocol entities.
#
class Msg:
MSG_SIZE = 20
def __init__(self, data):
self.data = data # type: bytes[MSG_SIZE]
def __str__(self):
return 'Msg(data=%s)' % (self.data)
# A Pkt is the data unit passed from layer 4 (student's code) to layer 3
# (teacher's code). Note the pre-defined packet structure, which all students
# must follow.
#
class Pkt:
def __init__(self, seqnum, acknum, checksum, payload):
self.seqnum = seqnum # type: integer
self.acknum = acknum # type: integer
self.checksum = checksum # type: integer
self.payload = payload # type: bytes[Msg.MSG_SIZE]
def __str__(self):
return ('Pkt(seqnum=%s, acknum=%s, checksum=%s, payload=%s)'
% (self.seqnum, self.acknum, self.checksum, self.payload))
###############################################################################
## ***************** STUDENTS WRITE THE NEXT SEVEN METHODS ********************
##
## STUDENTS: When you implement these methods, use instance variables only!
## I.e., variables that you access through `self' like `self.x`. Do NOT use
## global variables (a.k.a. module-scoped variables) or class variables.
##
## The reason for this restriction is the autograder, which may run several
## simulations within a single Python process. For each simulation, the
## autograder will create a new instance of EntityA and a new instance of
## EntityB. If you use global variables and/or class variables in your
## implmentations of EntityA and EntityB, then your code may not work properly
## when run by the autograder, and you may LOSE POINTS!
##
## If you have any questions about this, please ask the teaching staff.
##
## ****************************************************************************
class EntityA:
# The following method will be called once (only) before any other
# EntityA methods are called. You can use it to do any initialization.
#
# seqnum_limit is "the number of distinct seqnum values that your protocol
# may use." The seqnums and acknums in all layer3 Pkts must be between
# zero and seqnum_limit-1, inclusive. E.g., if seqnum_limit is 16, then
# all seqnums must be in the range 0-15.
def __init__(self, seqnum_limit):
self.OUTPUT = 0
self.INPUT = 1
self.TIMER = 2
# How long to wait for ack?
self.WAIT_TIME = 10.0
# State
self.layer5_msgs = []
self.bit = 0
self.sent_pkt = None
self.handle_event = self.handle_event_wait_for_call
# Called from layer 5, passed the data to be sent to other side.
# The argument `message` is a Msg containing the data to be sent.
def output(self, message):
self.layer5_msgs.append(message)
self.handle_event(self.OUTPUT)
# Called from layer 3, when a packet arrives for layer 4 at EntityA.
# The argument `packet` is a Pkt containing the newly arrived packet.
def input(self, packet):
self.handle_event(self.INPUT, packet)
# Called when A's timer goes off.
def timer_interrupt(self):
self.handle_event(self.TIMER)
pass
#####
def handle_event_wait_for_call(self, e, arg=None):
if e==self.OUTPUT:
if not self.layer5_msgs:
return
m = self.layer5_msgs.pop(0)
p = Pkt(self.bit, 0, 0, m.data)
pkt_insert_checksum(p)
to_layer3(self, p)
self.sent_pkt = p
start_timer(self, self.WAIT_TIME)
self.handle_event = self.handle_event_wait_for_ack
elif e==self.INPUT:
pass
elif e==self.TIMER:
if TRACE>0:
print('EntityA: ignoring unexpected timeout.')
else:
self.unknown_event(e)
def handle_event_wait_for_ack(self, e, arg=None):
if e==self.OUTPUT:
pass
elif e==self.INPUT:
p = arg
if (pkt_is_corrupt(p)
or p.acknum != self.bit):
return
stop_timer(self)
#
self.bit = 1 - self.bit
self.handle_event = self.handle_event_wait_for_call
self.handle_event(self.OUTPUT)
elif e==self.TIMER:
to_layer3(self, self.sent_pkt)
start_timer(self, self.WAIT_TIME)
else:
self.unknown_event(e)
#####
def self_unknown_event(self, e):
print(f'EntityA: ignoring unknown event {e}.')
#####
class EntityB:
# The following method will be called once (only) before any other
# EntityB methods are called. You can use it to do any initialization.
#
# See comment for the meaning of seqnum_limit.
def __init__(self, seqnum_limit):
self.expecting_bit = 0
pass
# Called from layer 3, when a packet arrives for layer 4 at EntityB.
# The argument `packet` is a Pkt containing the newly arrived packet.
def input(self, packet):
# print(f'B received: {packet}')
if (packet.seqnum != self.expecting_bit
or pkt_is_corrupt(packet)):
p = Pkt(0, 1-self.expecting_bit, 0, packet.payload)
pkt_insert_checksum(p)
to_layer3(self, p)
else:
to_layer5(self, Msg(packet.payload))
# Ack.
p = Pkt(0, self.expecting_bit, 0, packet.payload)
pkt_insert_checksum(p)
to_layer3(self, p)
#
self.expecting_bit = 1 - self.expecting_bit
# Called when B's timer goes off.
def timer_interrupt(self):
pass
#####
def pkt_compute_checksum(packet):
crc = 0
crc = crc32(packet.seqnum.to_bytes(4, byteorder='big'), crc)
crc = crc32(packet.acknum.to_bytes(4, byteorder='big'), crc)
crc = crc32(packet.payload, crc)
return crc
def pkt_insert_checksum(packet):
packet.checksum = pkt_compute_checksum(packet)
def pkt_is_corrupt(packet):
return pkt_compute_checksum(packet) != packet.checksum
###############################################################################
## ********************** STUDENT-CALLABLE FUNCTIONS **************************
##
## STUDENTS: These are functions that should be called from your EntityA and
## EntityB methods.
##
## The first argument to each of these student-callable functions is the object
## that is invoking the function. Within an EntityA or EntityB method, that
## object is available as `self`. For example, to start a timer in one of your
## entity methods, you would do something like:
##
## start_timer(self, 10.0) # Start a timer that will go off in 10 time units.
##
## Or to send a packet to layer3, you would do something like:
##
## to_layer3(self, Pkt(...)) # Construct a Pkt and send it to layer3.
##
## ****************************************************************************
def start_timer(calling_entity, increment):
the_sim.start_timer(calling_entity, increment)
def stop_timer(calling_entity):
the_sim.stop_timer(calling_entity)
def to_layer3(calling_entity, packet):
the_sim.to_layer3(calling_entity, packet)
def to_layer5(calling_entity, message):
the_sim.to_layer5(calling_entity, message)
def get_time(calling_entity):
return the_sim.get_time(calling_entity)
###############################################################################
## ****************************************************************************
## ***************** NETWORK SIMULATION CODE STARTS BELOW *********************
##
## The code below simulates the layer 3 and below network environment:
## - simulates the tranmission and delivery (possibly with bit-level
## corruption and packet loss) of packets across the layer 3/4 interface
## - handles the starting/stopping of a timer, and generates timer
## interrupts (resulting in calling student's timer handler).
## - generates message to be sent (passed from later 5 to 4)
##
## THERE IS NO REASON THAT ANY STUDENT SHOULD HAVE TO READ OR UNDERSTAND
## THE CODE BELOW. YOU SHOULD NOT TOUCH OR REFERENCE (in your code) ANY
## OF THE DATA STRUCTURES BELOW. If you're interested in how I designed
## the simulator, you're welcome to look at the code - but again, you should
## not have to, and you definitely should not have to modify.
##
## ****************************************************************************
class EventType(Enum):
TIMER_INTERRUPT = auto()
FROM_LAYER5 = auto()
FROM_LAYER3 = auto()
class Event:
def __init__(self, ev_time, ev_type, ev_entity, packet=None):
self.ev_time = ev_time # float
self.ev_type = ev_type # EventType
self.ev_entity = ev_entity # EntityA or EntityB
self.packet = packet # Pkt or None
class Simulator:
def __init__(self, options, cbA=None, cbB=None):
self.n_sim = 0
self.n_sim_max = options.num_msgs
self.time = 0.000
self.interarrival_time = options.interarrival_time
self.loss_prob = options.loss_prob
self.corrupt_prob = options.corrupt_prob
self.seqnum_limit = options.seqnum_limit
self.n_to_layer3_A = 0
self.n_to_layer3_B = 0
self.n_lost = 0
self.n_corrupt = 0
self.n_to_layer5_A = 0
self.n_to_layer5_B = 0
if options.random_seed is None:
self.random_seed = time.time_ns()
else:
self.random_seed = options.random_seed
random.seed(self.random_seed)
if self.seqnum_limit < 2:
self.seqnum_limit_n_bits = 0
else:
# How many bits to represent integers in [0, seqnum_limit-1]?
self.seqnum_limit_n_bits = (self.seqnum_limit-1).bit_length()
self.trace = options.trace
self.to_layer5_callback_A = cbA
self.to_layer5_callback_B = cbB
self.entity_A = EntityA(self.seqnum_limit)
self.entity_B = EntityB(self.seqnum_limit)
self.event_list = []
def get_stats(self):
stats = {'n_sim' : self.n_sim,
'n_sim_max' : self.n_sim_max,
'time' : self.time,
'interarrival_time' : self.interarrival_time,
'loss_prob' : self.loss_prob,
'corrupt_prob' : self.corrupt_prob,
'seqnum_limit' : self.seqnum_limit,
'random_seed' : self.random_seed,
'n_to_layer3_A' : self.n_to_layer3_A,
'n_to_layer3_B' : self.n_to_layer3_B,
'n_lost' : self.n_lost,
'n_corrupt' : self.n_corrupt,
'n_to_layer5_A' : self.n_to_layer5_A,
'n_to_layer5_B' : self.n_to_layer5_B
}
return stats
def run(self):
if self.trace>0:
print('\n===== SIMULATION BEGINS')
self._generate_next_arrival()
while (self.event_list
and self.n_sim < self.n_sim_max):
ev = self.event_list.pop(0)
if self.trace>2:
print(f'\nEVENT time: {ev.ev_time}, ', end='')
if ev.ev_type == EventType.TIMER_INTERRUPT:
print(f'timer_interrupt, ', end='')
elif ev.ev_type == EventType.FROM_LAYER5:
print(f'from_layer5, ', end='')
elif ev.ev_type == EventType.FROM_LAYER3:
print(f'from_layer3, ', end='')
else:
print(f'unknown_type, ', end='')
print(f'entity: {ev.ev_entity}')
self.time = ev.ev_time
if ev.ev_type == EventType.FROM_LAYER5:
self._generate_next_arrival()
j = self.n_sim % 26
m = bytes([97+j for i in range(Msg.MSG_SIZE)])
if self.trace>2:
print(f' MAINLOOP: data given to student: {m}')
self.n_sim += 1
ev.ev_entity.output(Msg(m))
elif ev.ev_type == EventType.FROM_LAYER3:
ev.ev_entity.input(deepcopy(ev.packet))
elif ev.ev_type == EventType.TIMER_INTERRUPT:
ev.ev_entity.timer_interrupt()
else:
print('INTERNAL ERROR: unknown event type; event ignored.')
if self.trace>0:
print('===== SIMULATION ENDS')
def _insert_event(self, event):
if self.trace>2:
print(f' INSERTEVENT: time is {self.time}')
print(f' INSERTEVENT: future time will be {event.ev_time}')
# Python 3.10+: use the bisect module:
# insort(self.event_list, event, key=lambda e: e.ev_time)
i = 0
while (i < len(self.event_list)
and self.event_list[i].ev_time < event.ev_time):
i += 1
self.event_list.insert(i, event)
def _generate_next_arrival(self):
if self.trace>2:
print(' GENERATE NEXT ARRIVAL: creating new arrival')
x = self.interarrival_time * 2.0 * random.random()
ev = Event(self.time+x, EventType.FROM_LAYER5, self.entity_A)
self._insert_event(ev)
#####
def _valid_entity(self, e, method_name):
if (e is self.entity_A
or e is self.entity_B):
return True
print(f'''WARNING: entity in call to `{method_name}` is invalid!
Invalid entity: {e}
Call ignored.''')
return False
def _valid_increment(self, i, method_name):
if ((type(i) is int or type(i) is float)
and i >= 0.0):
return True
print(f'''WARNING: increment in call to `{method_name}` is invalid!
Invalid increment: {i}
Call ignored.''')
return False
def _valid_message(self, m, method_name):
if (type(m) is Msg
and type(m.data) is bytes
and len(m.data) == Msg.MSG_SIZE):
return True
print(f'''WARNING: message in call to `{method_name}` is invalid!
Invalid message: {m}
Call ignored.''')
return False
def _valid_packet(self, p, method_name):
if (type(p) is Pkt
and type(p.seqnum) is int
and 0 <= p.seqnum < self.seqnum_limit
and type(p.acknum) is int
and 0 <= p.acknum < self.seqnum_limit
and type(p.checksum) is int
and type(p.payload) is bytes
and len(p.payload) == Msg.MSG_SIZE):
return True
# Issue special warnings for invalid seqnums and acknums.
if (type(p.seqnum) is int
and not (0 <= p.seqnum < self.seqnum_limit)):
print(f'''WARNING: seqnum in call to `{method_name}` is invalid!
Invalid packet: {p}
Call ignored.''')
elif (type(p.acknum) is int
and not (0 <= p.acknum < self.seqnum_limit)):
print(f'''WARNING: acknum in call to `{method_name}` is invalid!
Invalid packet: {p}
Call ignored.''')
else:
print(f'''WARNING: packet in call to `{method_name}` is invalid!
Invalid packet: {p}
Call ignored.''')
return False
#####
def start_timer(self, entity, increment):
if not self._valid_entity(entity, 'start_timer'):
return
if not self._valid_increment(increment, 'start_timer'):
return
if self.trace>2:
print(f' START TIMER: starting timer at {self.time}')
for e in self.event_list:
if (e.ev_type == EventType.TIMER_INTERRUPT
and e.ev_entity is entity):
print('WARNING: attempt to start a timer that is already started!')
return
ev = Event(self.time+increment, EventType.TIMER_INTERRUPT, entity)
self._insert_event(ev)
def stop_timer(self, entity):
if not self._valid_entity(entity, 'stop_timer'):
return
if self.trace>2:
print(f' STOP TIMER: stopping timer at {self.time}')
i = 0
while i < len(self.event_list):
if (self.event_list[i].ev_type == EventType.TIMER_INTERRUPT
and self.event_list[i].ev_entity is entity):
break
i += 1
if i < len(self.event_list):
self.event_list.pop(i)
else:
print('WARNING: unable to stop timer; it was not running.')
def to_layer3(self, entity, packet):
if not self._valid_entity(entity, 'to_layer3'):
return
if not self._valid_packet(packet, 'to_layer3'):
return
if entity is self.entity_A:
receiver = self.entity_B
self.n_to_layer3_A += 1
else:
receiver = self.entity_A
self.n_to_layer3_B += 1
# Simulate losses.
if random.random() < self.loss_prob:
self.n_lost += 1
if self.trace>0:
print(' TO_LAYER3: packet being lost')
return
seqnum = packet.seqnum
acknum = packet.acknum
checksum = packet.checksum
payload = packet.payload
# Simulate corruption.
if random.random() < self.corrupt_prob:
self.n_corrupt += 1
x = random.random()
if (x < 0.75
or self.seqnum_limit_n_bits == 0):
payload = b'Z' + payload[1:]
elif x < 0.875:
# Flip a random bit in the seqnum.
# The result might be greater than seqnum_limit if seqnum_limit
# is not a power of two. This is OK.
# Recall that randrange(x) returns an int in [0, x).
seqnum ^= 2**random.randrange(self.seqnum_limit_n_bits)
# Kurose's simulator simply did:
# seqnum = 999999
else:
# Flip a random bit in the acknum.
acknum ^= 2**random.randrange(self.seqnum_limit_n_bits)
# Kurose's simulator simply did:
# acknum = 999999
if self.trace>0:
print(' TO_LAYER3: packet being corrupted')
# Compute the arrival time of packet at the other end.
# Medium cannot reorder, so make sure packet arrives between 1 and 9
# time units after the latest arrival time of packets
# currently in the medium on their way to the destination.
last_time = self.time
for e in self.event_list:
if (e.ev_type == EventType.FROM_LAYER3
and e.ev_entity is receiver):
last_time = e.ev_time
arrival_time = last_time + 1.0 + 8.0*random.random()
p = Pkt(seqnum, acknum, checksum, payload)
ev = Event(arrival_time, EventType.FROM_LAYER3, receiver, p)
if self.trace>2:
print(' TO_LAYER3: scheduling arrival on other side')
self._insert_event(ev)
def to_layer5(self, entity, message):
if not self._valid_entity(entity, 'to_layer5'):
return
if not self._valid_message(message, 'to_layer5'):
return
if entity is self.entity_A:
self.n_to_layer5_A += 1
callback = self.to_layer5_callback_A
else:
self.n_to_layer5_B += 1
callback = self.to_layer5_callback_B
if self.trace>2:
print(f' TO_LAYER5: data received: {message.data}')
if callback:
callback(message.data)
def get_time(self, entity):
if not self._valid_entity(entity, 'get_time'):
return
return self.time
###############################################################################
TRACE = 0
the_sim = None
def report_config():
stats = the_sim.get_stats()
print(f'''SIMULATION CONFIGURATION
--------------------------------------
(-n) # layer5 msgs to be provided: {stats['n_sim_max']}
(-d) avg layer5 msg interarrival time: {stats['interarrival_time']}
(-z) transport protocol seqnum limit: {stats['seqnum_limit']}
(-l) layer3 packet loss prob: {stats['loss_prob']}
(-c) layer3 packet corruption prob: {stats['corrupt_prob']}
(-s) simulation random seed: {stats['random_seed']}
--------------------------------------''')
def report_results():
stats = the_sim.get_stats()
time = stats['time']
if time > 0.0:
tput = stats['n_to_layer5_B']/time
else:
tput = 0.0
print(f'''\nSIMULATION SUMMARY
--------------------------------
# layer5 msgs provided to A: {stats['n_sim']}
# elapsed time units: {stats['time']}
# layer3 packets sent by A: {stats['n_to_layer3_A']}
# layer3 packets sent by B: {stats['n_to_layer3_B']}
# layer3 packets lost: {stats['n_lost']}
# layer3 packets corrupted: {stats['n_corrupt']}
# layer5 msgs delivered by A: {stats['n_to_layer5_A']}
# layer5 msgs delivered by B: {stats['n_to_layer5_B']}
# layer5 msgs by B/elapsed time: {tput}
--------------------------------''')
def main(options, cb_A=None, cb_B=None):
global TRACE
TRACE = options.trace
global the_sim
the_sim = Simulator(options, cb_A, cb_B)
report_config()
the_sim.run()
#####
if __name__ == '__main__':
desc = 'Run a simulation of a reliable data transport protocol.'
parser = argparse.ArgumentParser(description=desc)
parser.add_argument('-n', type=int, default=10,
dest='num_msgs',
help=('number of messages to simulate'
' [int, default: %(default)s]'))
parser.add_argument('-d', type=float, default=100.0,
dest='interarrival_time',
help=('average time between messages'
' [float, default: %(default)s]'))
parser.add_argument('-z', type=int, default=16,
dest='seqnum_limit',
help=('seqnum limit for data transport protocol; '
'all packet seqnums must be >=0 and <limit'
' [int, default: %(default)s]'))
parser.add_argument('-l', type=float, default=0.0,
dest='loss_prob',
help=('packet loss probability'
' [float, default: %(default)s]'))
parser.add_argument('-c', type=float, default=0.0,
dest='corrupt_prob',
help=('packet corruption probability'
' [float, default: %(default)s]'))
parser.add_argument('-s', type=int,
dest='random_seed',
help=('seed for random number generator'
' [int, default: %(default)s]'))
parser.add_argument('-v', type=int, default=0,
dest='trace',
help=('level of event tracing'
' [int, default: %(default)s]'))
options = parser.parse_args()
main(options)
report_results()
sys.exit(0)
###############################################################################
## End of program.
