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Forwarding 1
Network Layer: Routing & Forwarding
Instructor: Anirban Mahanti
Office: ICT 745
Class Location: ICT 122
Lectures: MWF 12:00 – 12:50 hours
Text Book: “ Computer Networking: A Top Down
Approach Featuring the Internet”, 3rd^ edition, Jim
Kurose and Keith Ross Addison-Wesley, 2005.
Slides are adapted from the companion web site of the book.
Forwarding 2
Key Network-Layer Functions
forwarding: move packets from router‟s input
to appropriate router output
routing: determine the path taken by packets
as they flow from a sender to a receiver
Routing algorithms – run at routers to determine
“paths”;
Routers have a forwarding table
- Destination address-based in Datagram networks
- Virtual circuit number-based in VC Networks
Forwarding 4
VC Networks: Connection setup
3 rd^ important function in some network
architectures:
ATM, frame relay, X.
Before datagrams flow, two hosts and
intervening routers establish virtual
connection
Routers get involved
Network and transport layer cnctn service:
Network: between two hosts
Transport: between two processes
Forwarding 5
Network service model
Q: What service model for “channel” transporting
datagrams from sender to rcvr?
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
Restrictions on
changes in inter-
packet spacing
Forwarding 7
Network layer connection and
connection-less service
Datagram network provides network-layer
connectionless service
VC network provides network-layer
connection service
Analogous to the transport-layer services,
but:
Service: host-to-host
No choice: network provides one or the other
Implementation: in the core
Forwarding 8
Virtual circuits
call setup, teardown for each call before data can flow
each packet carries VC identifier (not destination host address)
every router on source-dest path maintains “state” for
each passing connection link, router resources (bandwidth, buffers) may be
allocated to VC
“source-to-dest path behaves much like telephone
circuit”
performance-wise network actions along source-to-dest path
Forwarding 10
Forwarding table
12 22 32 1 2 3
VC number
interface number
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
1 12 2 22 2 63 1 18 3 7 2 17 1 97 3 87 … … … …
Forwarding table in
Northwest router:
Routers maintain connection state information!
Forwarding 11
Virtual circuits: signaling protocols
used to setup, maintain teardown VC
used in ATM, frame-relay, X.
not used in today‟s Internet
application transport network data link physical
application transport network data link physical
- Initiate call (^) 2. incoming call
- Call connected 3. Accept call
- Data flow begins 6. Receive data
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Datagram or VC network: why?
Internet
data exchange among computers “elastic” service, no strict timing req. “smart” end systems (computers) can adapt, perform control, error recovery simple inside network, complexity at “edge” many link types different characteristics uniform service difficult
ATM
evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service “dumb” end systems telephones complexity inside network
Forwarding 14
Inside a Router
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Input Port Functions
Decentralized switching :
given datagram dest., lookup output port using forwarding table in input port memory (caching of entries?) goal: complete input port processing at „line speed‟ queuing: if datagrams arrive faster than forwarding rate into switch fabric
Physical layer: bit-level reception
Data link layer: e.g., Ethernet
Forwarding 17
Three types of switching fabrics
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Switching Via a Bus
datagram from input port memory
to output port memory via a shared
bus
bus contention: switching speed
limited by bus bandwidth
1 Gbps bus, Cisco 1900: sufficient
speed for access and enterprise
routers (not regional or backbone)
Forwarding 20
Switching Via An Interconnection
Network
overcome bus bandwidth limitations
Banyan networks, other interconnection nets
initially developed to connect processors in
multiprocessor
Advanced design: fragmenting datagram into fixed
length cells, switch cells through the fabric.
Cisco 12000: switches Gbps through the
interconnection network
