Lecture 7. Network Layer. Network Layer 1-1

Transcription

1 Lecture 7 Network Layer Network Layer 1-1

2 Agenda Introduction to the Network Layer Network layer functions Service models Network layer connection and connectionless services Introduction to data routing in datagram and virtual circuit s Network Layer 1-2

3 Quick Review on the Data Link Layer the layer in the IEEE standard is divided into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC) Devices Layer-2 devices switches, bridges Layer-1 devices; hubs, repeaters Network Layer 1-3

4 Quick Review on the Data Link Layer Logical Link Control (LLC) flow control, error control, and part of the framing duties provides one single control protocol for all IEEE LANs provide interconnectivity between different LANs because it makes the MAC sublayer transparent Media Access Control (MAC) provides different protocols for different LANs multiple access methods including random access, controlled access, and channelization handles part of the framing function Network Layer 1-4

5 Introduction to the Network Layer Goals of studying the layer understand principles behind layer services: layer service models forwarding versus routing how a router works routing (path selection) dealing with scale advanced topics: IPv6, mobility implementation in the Internet Network Layer 1-5

6 Introduction to the Network Layer transport segment from sending to receiving host on sending side encapsulates segments into datagrams on rcving side, delivers segments to transport layer layer protocols in every host, router router examines header fields in all IP datagrams passing through it application transport application transport Network Layer 5-6

7 Two Key Network-Layer Functions forwarding: move packets from router s input to appropriate router output routing: determine route taken by packets from source to dest. analogy: routing: process of planning trip from source to dest forwarding: process of getting through single interchange routing algorithms Network Layer 1-7

8 Interplay between Routing and Forwarding routing algorithm local forwarding table header value output link value in arriving packet s header Network Layer 1-8

9 Connection Setup before datagrams flow, two end hosts and intervening routers establish virtual connection (i.e., virtual dedicated path) important function in some architectures (e.g., ATM) routers get involved vs transport layer end-to-end connection service: : between two hosts transport: between two processes Network Layer 1-9

10 Network Service Model Q: What service model for channel transporting datagrams from sender to receiver? Example services for individual datagrams: guaranteed delivery guaranteed delivery with less than 40 msec delay Example services for a flow of datagrams: in-order datagram delivery guaranteed minimum bandwidth to flow Network Layer 1-10

11 Network Layer Service Models Network Architecture Service Model Bandwidth Guarantees? Loss Order Timing Congestion feedback Internet ATM ATM ATM ATM best effort CBR VBR ABR UBR none constant rate guaranteed rate guaranteed minimum none no yes yes no no no yes yes yes yes no yes yes no no no (inferred via loss) no congestion no congestion yes no Network Layer 1-11

12 Network Layer Connection and Connection-less Service Datagram provides -layer connectionless service Packet switched s Virtual Circuit (VC) provides layer connection service VC used for transporting data over a packet switched computer in such a way that it appears as though there is a dedicated layer link between the source and destination end systems of this data Network Layer 1-12

13 Virtual Circuits source-to-dest path behaves much like telephone circuit performance-wise actions along source-to-dest path call setup, before data can flow each packet carries VC identifier (not destination host address) every router on source-destination path maintains state for each passing connection link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service) Network Layer 4-13

14 VC Implementation a VC consists of: 1. path from source to destination 2. VC numbers, one number for each link along path 3. entries in forwarding tables in routers along path packet belonging to VC carries VC number (rather than destination address) VC number can be changed on each link. New VC number comes from forwarding table Network Layer 4-14

15 Forwarding Table VC number Forwarding table in northwest router: interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # Routers maintain connection state information! Network Layer 4-15

16 Virtual Circuits: Signaling Protocols used to setup, maintain, teardown VC used in ATM, frame-relay, X.25 (all are packet switched s) not used in today s Internet application transport 5. Data flow begins 6. Receive data 4. Call connected 3. Accept call 1. Initiate call 2. incoming call application transport Network Layer 4-16

17 Datagram Networks no call setup at layer routers: no state about end-to-end connections no -level concept of connection packets forwarded using destination host address packets between same source-dest pair may take different paths application transport 1. Send data 2. Receive data application transport Network Layer 4-17

18 Forwarding Table (IPv4) IPv4 addresses are of length 32 bits Destination Address Range 4 billion possible entries Link Interface through through through otherwise 3 Network Layer 4-18

19 Datagram or VC Network: Why? Internet (datagram) data exchange among computers elastic service, no strict timing req. smart end systems (computers) can adapt, perform control, error recovery simple inside, complexity at edge many link types different characteristics uniform service difficult ATM (VC) evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service dumb end systems telephones complexity inside Network Layer 4-19

20 Lecture Summary Covered material Introduction to the Network Layer Network layer functions Service models Network layer connection and connectionless services Data routing in datagram and virtual circuit s Material to be covered next lecture What is inside a router? IP Protocol Routing Algorithms Network Layer 1-20