MAC, Scheduling, IP …

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

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

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

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

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

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

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

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

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

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?

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

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

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 …

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

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

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

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

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

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

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)

Address Classes Class A Class B Class C 0 netId hostId 7 bits 24 bits 1 0 netId hostId 14 bits 16 bits 1 1 0 netId hostId 21 bits 8 bits

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

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?

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 : 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.

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

Examples Subnetting Supernetting 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!!

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

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

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

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)

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 = 1500 MTU=500 IP fragmentation

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

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

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

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

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

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