Understanding Network Layer Forwarding and Routing in Computer Networks, Slides of Computer Networks

Download Understanding Network Layer Forwarding and Routing in Computer Networks and more Slides Computer Networks in PDF only on Docsity!

Network Layer

Computer Networks

Lect-

PREPARE YOUR PACKET

Source : Destination : Time Rcv : Time Sent :

223 250

223 : 33. 231 : 33.

---

231 : 33.

WHICH PATH TO FOLLOW?

Transport Layer 3-

THE NETWORK LAYER

»Outline » Introduction » Forwarding and Routing » Virtual Circuit and Datagram Networks » Inside a Router » The Internet Protocol » Routing Algorithms » Routing in the Internet » Broadcast and Multicast Routing

FORWARDING AND ROUTING

A router is placed between  Two Routing device  Host/Router and Router Forwarding  Packets arriving at a router are forwarded to the next node / router in the path to destination  Forwarding is done by using a pre-defined table  A forwarding table entries are pair of input-output port , different output port leads to different destination  Analogy: As traffic signal. Routing  Determine the path (actually best path) between a given source and destination  It is a coordinated function of more than one router.

FORWARDING AND ROUTING

NETWORK SERVICE MODEL

Network Architecture Internet ATM ATM ATM

Service Model best effort CBR VBR ABR

Bandwidth none constant rate guaranteed rate guaranteed Minimum

Loss no yes yes no

Order no yes yes yes

Timing no yes yes no

Congestion feedback no (inferred via loss) no congestion no congestion yes

Guarantees?

VIRTUAL CIRCUITS AND DATAGRAM NETWORKS

to some extentNetwork layer connection services are parallel to Transport Layer Services  Services are :Connection-Oriented , Connection-less Differences  Transport Layer: handshake, process to process, in end system  Network Layer : handshake, host to host, in network core and endsystem. Two fundamental Classes of Networks  VC Network: provides network-layer connection service  Datagram Network: provides network-layer connectionless service Or  Networks that provides connection service at network layer only arecalled virtual-circuit (VC) networks, others that provide connection less service are termed as Datagram Networks

Forwarding 13

VC IMPLEMENTATION

A VC consists of:

1. Path from source to destination (Routers and Links)
2. VC numbers, one number for each link along path
3. Entries in forwarding tables in routers along path » Packet belonging to VC carries a VC number. » VC number must be changed on each link. » New VC number comes from forwarding table

» Phases » VC-Setup (Reservation of path, all path nodes contacted) » Data Transfer (Flow of data using VC) » VC Teardown (Path nodes are updated, Forwarding tables are updated)

Q: Why same VC number is not used in a

Virtual Circuit?

Forwarding 16

FORWARDING TABLE

12 22 32 1 2 3

VC number

interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # (^12 6312 12 ) (^31 977 23 ) … … … …

Forwarding table in router:

Routers maintain connection state information!

Forwarding 17

VIRTUAL CIRCUITS: SIGNALING PROTOCOLS

»Control Signals » Messages passed during setup of connection and later to update the

»Protocol that define these messages are called signaling protocolforwarding table are called » » used to setup, maintain teardown VCused in ATM, frame-relay, X. » not used in today’s Internet

application transport network data link physical

application transport network data link physical

4. Call connected 1. Initiate call (^) 2. incoming call 3. Accept call
5. Data flow begins 6. Receive data

DATAGRAM FORWARDING TABLE

»Datagram networks also uses forwarding tables but not like VC as » They do not maintain state information » Decision to forward a packet is determined after looking at the destinationaddress in the header of the packet » Routing algorithms are used to update the forwarding tables, such algorithmsdepends upon the information about destination nodes and acquire that information from neighbors

»Longest Prefix Match » The forwarding table of datagram network has hierarchical entries like Destination Link interface 110101011001010010101010 0 1101010110010111101010100 1 11010101100101111011101000 2 Otherwise 3 Longest match is used to select the correct entry in the destination column

Term Router and Switch are used interchangeably

in literature,

Inside of a Router