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Network and Communications
Hongsik Choi
Department of Computer Science
Virginia Commonwealth University
Peer to Peer Networks
Large number of nodes, symmetric,no central database nor control
How to find the one that I want?
5bit id example
Chord System
Name in ASCII
IP to 160 bit number (N. ID)
Ring of 2^160 node
Store (name,My IP address) at
successor(hash(name)))
Find successor or prede.
Linear?
Finger table with m entries
start  k  2i (%2m )
Log n Key 3 14 16 at node 1
Congestion Control
Link management
Packet
streams
Buffer
Outgoing Link
• Admission (Drop Tail, Random Early Detection(RED))
• Scheduling FIFO
• Time Division Multiplexing (TDM) and (FDM)
• Rate proportional service
Drop Tail FIFO
Packet
streams
Buffer
Outgoing Link
• Statistical multiplexing: better average delay and
loss probabilities (due to buffer overflow). Better
utilization of resources -- Good
• Work conserving: as long as there is a packet then
the link works at full capacity
• Simple: Good
• Packet streams interfere with one another
Guaranteed QoS is harder to achieve
• Packets are lost in bursts -- not good for TCP
Multiplexing
Streams of packets
flow 0
flow 1
flow 2
flow 3
.
.
Buffer
Capacity C
How do we split up bandwidth to different flows?
TDM and FDM
C
r1
r2
r3
r4
r1 + r2 + r3 + r4 = C
TDM: slot time and organize into frames
each “channel” gets certain slots
in a frame.
FDM: bandwidth is split up using
analog techniques.
BW is reserved for each flow: if flow has
no packets then bw is unused
Processor Sharing: best of both worlds
CPU burst and single CPU
flow 1
flow 2
.
.
flow n
C
• Divide bandwidth of the link C equally among active
flows. Active means there are packets present.
• Each active flow gets C/A bandwidth, where A =
# active flows
In Network layer
Virtual Circuit versus datagram
1.
Admission control
2.
Careful route discovery
Packet queuing and service policy
Packet discard policy
Routing algorithm
Packet life time management
Which packet to drop?
New or old
Priority?
Random Early Detection
Virtual Clock Service
• Implementing processor sharing or GPS is difficult
because you have to split bandwidth dynamically
• We’ll look at virtual clock service
• It’s actually a packetized version of TDM
• We’ll look at packetized GPS (PGPS)
Generalized Processor Sharing (GPS)
Each flow k is assigned
a rate r(k).
Usually,

n
i 1
r (i)  C
Bandwidth for flow k
flow 1
flow 2
.
.
flow n
r (k )
C
iA r (i)
C
A = set of active
flows
Virtual Clock Service
Each flow k has
rate r(k)
flow 1
flow 2
.
.
flow n
Packets arrive into the link buffer:
• a(i) = arrival time of the ith packet of flow k
• L(i) = length of the packet
• d(i) = virtual departure time of the packet
= max{a(i), d(i-1)} + L(i)/r(k)
C
Virtual Clock Service
flow 1
flow 2
.
.
flow n
buffer
C
• Packets in the buffer are transmitted according
to the smallest virtual departure time
• What is going on?
• The scheduler is emulating a TDM system
The TDM system
flow 1
flow 2
.
.
flow n
r(1)
r(2)
r(n)
Consider flow k: The ith packet of flow k departs at
d(i) = the time the packet begins transmission + L(i)/r(k)
= max{d(i-1), a(i)} + L(i)/r(k)
= virtual departure time of virtual clock system
Packetized GPS (Weighted Fair Queueing)
arrival times
and packet
lengths
flow 1
flow 2
.
.
flow n
GPS emulator that
determines virtual
departure times (VDT) of
packets
VDTs of packets
C
buffer
Packets are transmitted in
order of their VDTs
Performance
For Virtual Clock Service and PGPS (or weighted fair
queueing), the departure time of a packet is upper
bounded by
virtual departure time + Lmax/C
Lmax = maximum packet size
Weighted Round Robin: A
Practical Scheduler
flow 1
flow 2
.
.
flow n
Buffers
The buffers are served in round robin fashion BUT
a buffer may be skipped under certain conditions
Weighted Round Robin: A
Practical Scheduler
Credit
r(k)
flow k
Buffer
• Each time flow is considered, it gets additional credit r(k)
• If a flow is considered and its HOL packet has length
at most Credit then its packet is served and
Credit = Credit - packet length
Random Early Detection
(RED)
prob
dropping
Fixed buffer size for B packets
Tail Drop Queueing
Dropping can be
bursty which can be
bad for TCP
1
Occupancy
Random Early Detection
(RED)
prob
dropping
Fixed buffer size for B packets
RED
Dropping some small
fraction early tells
TCP to back off
1
Occupancy
Congestion control in virtual Circuit subnet
Congestion happen -> send warnings to sources
The warning bit – any router along the path can set
Choke packet:
first packet reduce the flow rate by certain percent and
ignore for some interval,
if another choke packet, reduce the rate with certain
percent and so on
At high speed over long distance, it does not work well
Hop by hop choke
Buffers or loss?
Quality of Service
Jitter- delay variations
Buffering – reliability, bandwidth, delay
Can this smoothing work in server side? Traffic Shaping
tokens generated
at rate r
Leaky bucket flow control
s
Incoming traffic
token
bucket
Buffer
Require tokens
to launch data
(s,r) traffic
R(t)
Input
250k
500k
750k
500k w/
10MB/sec bucket
Token bucket allows burst!
C = 250KB
Burst length , S
M = 25MB/sec
Token bucket capacity, C bytes
r  2MB/ sec
Token arrival rate r bytes/sec
S =250KB/(25MB/sec – 2MB/sec)
Maximum output rate, M bytes/sec
Out put burst = C+ rS bytes
Number of bytes in S = MS
C+ rS = MS
S = C/(M- r)
=10.8msec
If all packets follow same route, we can reserve
1.
Bandwidth,
2.
buffer space,
3.
CPU cycles
If flow is well shaped and follows same route,
router have to decide accept new flow or not based
on Maximum packet size, minimum packet size,
peak data rate,token bucket size, token rate
What if there is one aggressive flow? Packet scheduling
Fair queueing
Weighted Fair queueing
How to model? GPS
How to implemented ? Virtual clock? Packetized GPS
Protocol for streaming Multimedia (flow based algorithm)
Multicast membership is dynamic
Can resource reservation scheme works?
RSVP(Resource reSerVation protocol):
Multi source multi receiver case
Flow based service
Disadvantage:
It is not scale well.
Require advanced set up
Marinating per flow information is too much
Too much overhead in frequent router code change
Class based? (differentiated services)
expedited forwarding
Assured Forwarding
•4 class
•low medium high
Label Switching and MPLS
Routing
Switching
Forward equivalence class
Data driven setup with colored thread
Internetworking
Service
How networks differ?
Protocol
Addressing
Packet Size
QoS
Error handling
Congestion control
Security
etc
How network can be connected?
Routers (multiprotocol routers) or switch
How to provide Internetwork routing?
Will be covered with IP
Next class we will discuss IP
A Network Calculus for
Performance
R(t): rate of traffic flow at time t
• Simple model: constant rate r, R(t) = r
• More complicated model: (s,r) traffic
For all x < y,

y
x
R(t )dt  s  r  y  x 
burstiness
parameter
(s,r) Traffic
Simple FIFO queue and link
R(t)
B(t)
C
backlog
B(t)
first packet
arriving s
t
(s,r) Traffic
Simple FIFO queue and link
R(t)
B(t)
C
backlog
B(t)
first packet
arriving s
t
For FIFO buffer with (s,r) traffic,
Buffer occupancy is at most s + L
Delay is at most (s + L)/C
Rin(t)
Dmax
Rout(t)
Bits may be arbitrarily delayed from 0 to Dmax

y
x
y
Rout (t )dt 
 Rin (t )dt  sin  rin( y  x  D max)
x  D max
 sin  rin ( D max)   rin ( y  x)
output is burstier
R1(t)
R2(t)
R3(t)
Rout
sout = s1 + s2 + s3
rout = r1 + r2 + r3
How do we get such traffic?
tokens generated
at rate r
Leaky bucket flow control
s
Incoming traffic
token
bucket
Buffer
Require tokens
to launch data
(s,r) traffic
R(t)