1. MAC, Scheduling, IP …

2. Recap Class goals and overview Grading and other administrative stuff
Communication networks
OSI Protocol Stack
Introduction to TCP/IP

3. 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

4. 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

5. 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

6. 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

7. CSMA (contd.) 1-persistent CSMA p-persistent CSMA non-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

8. 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

9. CSMA/CD (Contd.)
When will the performance of CSMA/CD be the same as that of CSMA?
IEEE Ethernet LAN Standard
802.3u – fast ethernet
802.3ab – gigabit ethernet
802.3ae – 10G ethernet
Full-duplex Ethernet

10. Other MAC Schemes Collision free protocols
Bit map protocol
Binary countdown protocol
Limited contention protocols
Adaptive tree walk protocol

11. 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?

12. Scheduling Policies … FCFS Priority
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

13. 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

14. Scheduling - WFQ 3 flows A, B, C Weights: A(1), B(2), C(3)
Assume same packet sizes 8 7
Complex overhead due to
computation of finishing times
Simpler approach? 5 4 6 3 2 1
Tx Schedule: C B C C B A C B …

15. Scheduling (Contd.) Weighted Round Robin A Fixed Tx Schedule:
Simpler approximation of WFQ
Assumes constant packet sizes
Can cause unfair delay A
Fixed Tx Schedule:
C C C B B A A B C

16. 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

17. 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

18. 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

19. 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

20. Internet Protocol (IP)
Addressing
Routing
Fragmentation and Reassembly
Quality of Service
Multiplexing and Demultiplexing

21. 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)

22. Address Classes Class A Class B Class C 0 netId hostId
7 bits 24 bits
netId hostId
14 bits 16 bits
netId hostId
21 bits 8 bits

23. IP Address Classes (contd.)
Two more classes
1110 : multicast addressing
1111 : reserved
Significance of address classes?
Why this conceptual form?

24. 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?

25. Special Addresses hostId of 0 : network address
hostId of all 1’s: directed broadcast
All 1’s : limited broadcast
netId of 0 : this network
Loopback :
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.

26. Exceptions to Addressing (norm today)
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

27. Examples Subnetting Supernetting 192.168.1.0/24 – class C network
/26 and /26 – 2 subnetworks with upto 62 stations each!
Supernetting
/24 and /24 – 2 class C networks
/23 – 1 super network with upto 126 stations!!

28. Weaknesses Mobility Switching address classes
Notion of host vs. IP address

29. IP Routing Direct Indirect
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

30. 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

31. 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)

32. 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
MTU = MTU=500
IP fragmentation

33. IP Reassembly Fragmented packets need to be put together
Where does reassembly occur?
What are the trade-offs?

34. Multiplexing Web Email MP3 Web Email MP3 TCP UDP TCP UDP IP IP
IP datagrams
IP datagrams

35. IP Header Used for conveying information to peer IP layers Source
Destn
Application
Application
Transport
Router
Router
Transport IP IP IP IP
DataLink
DataLink
DataLink
DataLink
Physical
Physical
Physical
Physical

36. IP Header (contd.) 4 bit version 4 bit hdr length 16 bit total length
TOS
16 bit identification
3 bit
flags
13 bit fragment offset
8 bit TTL
8 bit protocol
16 bit header checksum
32 bit source IP address
32 bit destination IP address
Options (if any) (maximum 40 bytes)
data

37. Recap MAC Protocols: ALOHA, slotted-ALOHA, CSMA, CSMA/CD
Scheduling: FCFS, Priority, FQ, WFQ, WRR, CBQ
Internet Protocol: Addressing, Routing, Fragmentation & Reassembly, Mux/Demux

38. TCP/IP in relation to the OSI model
                                               
TCP/IP in relation to the OSI model