Network Layer network layer services virtual circuit and datagram - PDF document

Overview: Network Layer � network layer services � virtual circuit and datagram Goals: networks � what’s inside a router? � understand principles behind network layer � IP: Internet Protocol services: � IPv4 datagram format � forwarding � IPv4 addressing � routing (path selection) � ICMP � dealing with scale � IPv6 � how a router works � routing algorithms � advanced topics: IPv6, � Link state multicast � Distance Vector � instantiation and � Hierarchical routing implementation in the � routing in the Internet Internet � RIP � OSPF � BGP � broadcast and multicast routing 26/4-07 Datakommunikation - Jonny Pettersson, UmU The Data Link Layer Today Our goals: � link layer services � understand principles � error detection, correction behind data link layer services: � multiple access protocols and LANs � error detection, � link layer addressing, ARP, correction DHCP � sharing a broadcast Next time channel: multiple access � link layer addressing � Ethernet � reliable data transfer, � hubs and switches flow control: done! � PPP � instantiation and implementation of various link layer technologies 26/4-07 Datakommunikation - Jonny Pettersson, UmU 1

Link Layer: setting the context 26/4-07 Datakommunikation - Jonny Pettersson, UmU Link Layer: setting the context � two physically connected devices: � host-router, router-router, host-host � unit of data: frame M application transport M H t network network M H n H t data link protocol link link M H l H n H t M H l H n H t physical physical frame phys. link adapter card 26/4-07 Datakommunikation - Jonny Pettersson, UmU 2

Link Layer Services � Framing, link access: � encapsulate datagram into frame, adding header, trailer � channel access if shared medium, � ‘physical addresses’ used in frame headers to identify source, dest • different from IP address! � Reliable delivery between two physically connected devices: � we learned how to do this already (chapter 3)! � seldom used on low bit error link (fiber, some twisted pair) � wireless links: high error rates • Q: why both link-level and end-end reliability? 26/4-07 Datakommunikation - Jonny Pettersson, UmU Link Layer Services (more) � Flow Control: � pacing between adjacent sending and receiving nodes � Error Detection : � errors caused by signal attenuation, noise. � receiver detects presence of errors: • signals sender for retransmission or drops frame � Error Correction: � receiver identifies and corrects bit error(s) without resorting to retransmission � Half-duplex and full-duplex � with half duplex, nodes at both ends of link can transmit, but not at same time 26/4-07 Datakommunikation - Jonny Pettersson, UmU 3

Adaptors Communicating datagram rcving link layer protocol node sending node frame frame adapter adapter � link layer implemented in � sending side: “adaptor” (aka NIC) � encapsulates datagram in a frame � Ethernet card, PCMCIA card, 802.11 card � adds error checking bits, rdt, flow control, etc. � typically includes: RAM, DSP (Digital Signal Processing) � receiving side chips, host bus interface, � looks for errors, rdt, flow and link interface control, etc � adapter is semi-autonomous � extracts datagram, passes to � link & physical layers rcving node 26/4-07 Datakommunikation - Jonny Pettersson, UmU Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable! • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction 26/4-07 Datakommunikation - Jonny Pettersson, UmU 4

Parity Checking Two Dimensional Bit Parity : Single Bit Parity: Detect and correct single bit errors Detect single bit errors 0 0 26/4-07 Datakommunikation - Jonny Pettersson, UmU Checksumming: Cyclic Redundancy Check � view data bits, D, as a binary number � choose r+1 bit pattern (generator), G � goal: choose r CRC bits, R, such that <D,R> exactly divisible by G (modulo 2) � � receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! � can detect all burst errors less than r+1 bits and any odd number of bit errors � a burst of length greater than r+1 bits is detected with probability 1-0.5 r � widely used in practice (ATM, HDCL) 26/4-07 Datakommunikation - Jonny Pettersson, UmU 5

Multiple Access protocols � single shared communication channel � two or more simultaneous transmissions by nodes: interference � only one node can send successfully at a time � multiple access protocol: � distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit � communication about channel sharing must use channel itself! � what to look for in multiple access protocols: • synchronous or asynchronous • information needed about other nodes • robustness (e.g., to channel errors) • performance 26/4-07 Datakommunikation - Jonny Pettersson, UmU Ideal Mulitple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: � no special node to coordinate transmissions � no synchronization of clocks, slots 4. Simple 26/4-07 Datakommunikation - Jonny Pettersson, UmU 6

MAC Protocols: a taxonomy (MAC – Media Access Control) Three broad classes: � Channel Partitioning � divide channel into smaller “pieces” (time slots, frequency, code) � allocate piece to node for exclusive use � Random Access � channel not divided, allow collisions � “recover” from collisions � “Taking turns” � tightly coordinate shared access to avoid collisions Goal: efficient, fair, simple, decentralized 26/4-07 Datakommunikation - Jonny Pettersson, UmU Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access � access to channel in "rounds" � each station gets fixed length slot (length = pkt trans time) in each round � unused slots go idle � example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 26/4-07 Datakommunikation - Jonny Pettersson, UmU 7

Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access � channel spectrum divided into frequency bands � each station assigned fixed frequency band � unused transmission time in frequency bands go idle � example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle time frequency bands 26/4-07 Datakommunikation - Jonny Pettersson, UmU Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) � unique “code” assigned to each user; ie, code set partitioning � used mostly in wireless broadcast channels (cellular, satellite,etc) � all users share same frequency, but each user has own “chipping” sequence (ie, code) to encode data � allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) � more later… 26/4-07 Datakommunikation - Jonny Pettersson, UmU 8

Random Access protocols � When node has packet to send � transmit at full channel data rate R � no a priori coordination among nodes � two or more transmitting nodes -> “collision”, � random access MAC protocol specifies: � how to detect collisions � how to recover from collisions (e.g., via delayed retransmissions) � Examples of random access MAC protocols: � slotted ALOHA � ALOHA � CSMA and CSMA/CD 26/4-07 Datakommunikation - Jonny Pettersson, UmU Slotted ALOHA Assumptions Operation � all frames same size � when node obtains fresh frame, it transmits in next � time is divided into slot equal size slots = time to transmit 1 frame � no collision, node can send new frame in next slot � nodes start to transmit frames only at � if collision, node beginning of slots retransmits frame in each subsequent slot with prob. � nodes are synchronized p until success � if 2 or more nodes transmit in slot, all nodes detect collision 26/4-07 Datakommunikation - Jonny Pettersson, UmU 9

Slotted ALOHA Cons Pros � collisions, wasting slots � single active node can � idle slots continuously transmit at full rate of channel � nodes may be able to detect collision in less � highly decentralized: than time to transmit only slots in nodes packet need to be in sync � clock synchronization � simple 26/4-07 Datakommunikation - Jonny Pettersson, UmU Slotted Aloha efficiency � For max efficiency Efficiency is the long-run with N nodes, find p* fraction of successful slots that maximizes when there are many nodes, Np(1-p) N-1 each with many frames to send � For many nodes, take � Suppose N nodes with limit of Np*(1-p*) N-1 many frames to send, as N goes to infinity, each transmits in slot gives 1/e = .37 with probability p � prob that node 1 has At best: channel success in a slot used for useful = p(1-p) N-1 transmissions 37% � prob that any node has of time! a success = Np(1-p) N-1 26/4-07 Datakommunikation - Jonny Pettersson, UmU 10