Download MAC, Scheduling, IP …

MAC, Scheduling, IP …
1
Announcements
Good news!


Only three PAs (5% each)
Exam 2 will now be worth 20%
Grouping

Please form groups of 3 and send TA Richelle email (one email per group)
by 1/18
Video students




No groups
Complete firstfeedbackform (available on class website) and send to TA
Richelle
Send from an email address that you will be reachable for the rest of the
semester
Mention whether or not you are taking the class from GT-Lorraine
Schedule change



Examination 1 : ON February 1st
ns2 tutorial: ON February 6th and 8th
Online schedule has been updated
2
Puzzle
Use TH and HT as the two events
with equal probability p(1-p). Ignore
events TT and HH
You have an unfair coin (prob(H) = p
!= ½)
How will you generate a fair toss?
3
Recap
Class goals and overview
Grading and other administrative stuff
Communication networks
OSI Protocol Stack
Introduction to TCP/IP
4
Medium Access Control
When multiple stations share a
common channel, the protocol that
determines which station gets access to
the shared channel
Key characteristics based on which
MAC protocols are evaluated: utilization
and fairness
5
ALOHA
ALOHA




When a station wants to transmit, it
transmits …
Collisions detected at a higher layer and
retransmissions done as required
Simple logic
Highly inefficient at large loads. Maximum
utilization of 18% at a mean load of 0.5
transmissions/slot
6
Slotted ALOHA
Stations can transmit only at the
beginning of pre-determined slots
Reduces the “vulnerable period” for
collisions
More efficient
Maximum utilization of 36% at a mean
load of 1 transmission/slot
7
Carrier Sense Multiple Access
Station wishing to transmit senses
channel. If channel idle, transmits. Else,
backs-off and tries later
Carrier sensing hardware required
More efficient than both versions of
ALOHA
3 flavors of CSMA
8
CSMA (contd.)
1-persistent CSMA

On finding channel busy, station continues
listening and transmits when channel becomes
idle
p-persistent CSMA

On finding channel idle, station transmits with a
probability of p, backs-off and tries again when
channel is busy
non-persistent CSMA

On finding channel busy, station backs-off for a
random amount of time and tries later
9
CSMA/CD
In CSMA, when there is a collision of two
transmissions, it is detected only after the
entire frames have been transmitted …
irrespective of when the collision occurs
CSMA/CD includes a collision detection
mechanism that can detect collisions even as
stations are transmitting
Transmitting stations terminate transmissions
upon collision detection and try later
10
CSMA/CD (Contd.)
When will the performance of
CSMA/CD be the same as that of CSMA?
IEEE 802.3 Ethernet LAN Standard



802.3u – fast ethernet
802.3ab – gigabit ethernet
802.3ae – 10G ethernet
Full-duplex Ethernet
11
Other MAC Schemes
Collision free protocols


Bit map protocol
Binary countdown protocol
Limited contention protocols

Adaptive tree walk protocol
12
Scheduling
When a station gets to transmit, the
protocol that determines which packet
gets to be transmitted
Fairness the primary consideration
Weighted fairness … an extension
Default scheduling in today’s Internet?
13
Scheduling Policies …
FCFS


Packets queued on a first-come-first-served
basis
No fairness
Priority


Multiple queues with different priorities
Packets belonging to queue k served only
when queues with higher priorities are
empty
14
Scheduling (Contd.)
Generalized Processor Sharing (GPS)


Ideal fair queuing approach based on a
fluid flow system
Complex, idealistic
Packetized GPS (WFQ)


Packetized version of GPS (practical)
Finish times in a correponding GPS system
used for prioritizing packets
15
Scheduling - WFQ
3 flows A, B, C
Weights: A(1), B(2), C(3)
Assume same packet sizes
8
6 5
2
7
4
3
1
• Complex overhead due to
computation of finishing times
• Simpler approach?
Tx Schedule: C B C C B A C B …
16
Scheduling (Contd.)
Weighted Round Robin



Simpler approximation of WFQ
Assumes constant packet sizes
Can cause unfair delay
A
B
Fixed Tx Schedule:
CCCBBAA
C
17
Scheduling (Contd.)
WRR with WFQ Spread



For each flow k with weight wk generate
wk elements of the form (1/wk,k),
(2/wk,k), …, (wk/wk,k)
Sort all such elements from all competing
flows in lexicographic order
Extracting the second element of each pair
from the resulting sorted list gives the
WRR with WFQ spread
18
WRR with WFQ Spread
A (1)
(1,A)
B (2)
(1/2,B), (1,B)
C (3)
(1/3,C), (2/3,C), (1,C)
Lexicographically Sorted: (1/3,C), (1/2,B), (2/3,C), (1,A), (1,B), (1,C)
Schedule: C B C A B C
19
Other scheduling policies…
Deficit Round Robin (DRR)



Handles varying size packets
Frame of N bits split among the competing flows
in the ratio of their weights
Flows get to transmit HOL packet only when they
have accumulated packet-length number of bits
Class based queuing (CBQ)


General scheduling and link-share scheduling
Greater flexibility to control scheduling policy
20
TCP/IP Protocol Suite
Physical layer
Data-link layer – ARP, RARP, SLIP
Network layer – IP, ICMP, IGMP, BootP
Transport layer _ TCP, UDP, RTP
Application layer – http, smtp, ftp
21
Internet Protocol (IP)
Addressing
Routing
Fragmentation and Reassembly
Quality of Service
Multiplexing and Demultiplexing
22
Addressing
Need unique identifier for every host in
the Internet (analogous to postal
address)
IP addresses are 32 bits long
Hierarchical addressing scheme
Conceptually …

IPaddress =(NetworkAddress,HostAddress)
23
Address Classes
Class A
0 netId
hostId
7 bits
24 bits
Class B
1 0
netId
14 bits
hostId
16 bits
Class C
11 0
netId
21 bits
hostId
8 bits
24
IP Address Classes (contd.)
Two more classes


1110 : multicast addressing
1111 : reserved
Significance of address classes?
Why this conceptual form?
25
Addresses and Hosts
Since netId is encoded into IP address,
each host will have a unique IP address
for each of its network connections
Hence, IP addresses refer to network
connections and not hosts
Why will hosts have multiple network
connections?
26
Special Addresses
hostId of 0
hostId of all
All 1’s
netId of 0
Loopback
:
1’s:
:
:
:
network address
directed broadcast
limited broadcast
this network
127.0.0.0
Dotted decimal notation: IP addresses are written as four
decimal integers separated by decimal points, where each
integer gives the value of one octet of the IP address.
27
Exceptions to Addressing
Subnetting



Splitting hostId into subnetId and hostId
Achieved using subnet masks
Useful for?
Supernetting (Classless Inter-domain Routing
or CIDR)



Combining multiple lower class address ranges
into one range
Achieved using 32 bit masks and max prefix
routing
Useful for?
28
Examples
Subnetting


192.168.1.0/24 – class C network
192.168.1.64/26 and 192.168.1.128/26 – 2
subnetworks with upto 62 stations each!
Supernetting


192.168.2.0/24 and 192.168.3.0/24 – 2
class C networks
192.168.2.0/23 – 1 super network with
upto 126 stations!!
29
Weaknesses
Mobility
Switching address classes
Notion of host vs. IP address
30
IP Routing
Direct


If source and destination hosts are connected
directly
Still need to perform IP address to physical
address translation. Why?
Indirect


Table driven routing
Each entry: (NetId, RouterId)
 Default router
 Host-specific routes
31
IP Routing Algorithm
RouteDatagram(Datagram, RoutingTable)
Extract destination IP address, D, from the
datagram and compute the netID N





If N matches any directly connected network address deliver
datagram to destination D over that network
Else if the table contains a host-specific route for D, send
datagram to next-hop specified in table
Else if the table contains a route for network N send
datagram to next-hop specified in table
Else if the table contains a default route send datagram to
the default router specified in table
Else declare a routing error
32
Routing Protocols
Interior Gateway Protocol (IGP)


Within an autonomous domain
RIP (distance vector protocol), OSPF (link
state protocol)
Exterior Gateway Protocol (EGP)


Across autonomous domains
BGP (border gateway protocol)
33
IP Fragmentation
The physical network layers of different
networks in the Internet might have
different maximum transmission units
The IP layer performs fragmentation
when the next network has a smaller
MTU than the current network
IP fragmentation
MTU = 1500
MTU=500
34
IP Reassembly
Fragmented packets need to be put
together
Where does reassembly occur?
What are the trade-offs?
35
Multiplexing
Web
Email
TCP
MP3
UDP
IP
IP datagrams
Web
Email
TCP
MP3
UDP
IP
IP datagrams
36
IP Header
Used for conveying information to peer
IP layers
Destn
Source
Application
Transport
IP
DataLink
Physical
Router
IP
DataLink
Router
IP
DataLink
Physical
Physical
Application
Transport
IP
DataLink
Physical
37
IP Header (contd.)
4 bit 4 bit hdr
version length
8 bit
TOS
16 bit identification
8 bit TTL
16 bit total length
3 bit
flags
8 bit protocol
13 bit fragment offset
16 bit header checksum
32 bit source IP address
32 bit destination IP address
Options (if any) (maximum 40 bytes)
data
38
Puzzle
10 bags with coins. 9 bags have coins
of equal weights (X units, X >> 1)
weights. 1 bag has defective coins
(heavier or lighter by 1 unit)
Using a spring balance, how many
times do you need to weigh in order to
find the defective bag?
39
Recap
MAC Protocols: ALOHA, slotted-ALOHA,
CSMA, CSMA/CD
Scheduling: FCFS, Priority, FQ, WFQ,
WRR, CBQ
Internet Protocol: Addressing, Routing,
Fragmentation & Reassembly,
Mux/Demux
40