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Network Layer
—— the core of networking
The Network Core
 mesh of interconnected
routers
 the fundamental question:
how is data transferred
through net?
circuit switching:
dedicated circuit per
call: telephone net
packet-switching: data
sent thru net in
discrete “chunks”
Network layer functions
 transport packet from
sending to receiving hosts
 network layer protocols in
every host, router
Three important functions:
 path determination: route
taken by packets from source
to dest. Routing algorithms
 switching: move packets from
router’s input to appropriate
router output
 call setup: some network
architectures require router
call setup along path before
data flows
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
Network Core: Circuit Switching
End-end resources
reserved for “call”
 link bandwidth, switch
capacity
 dedicated resources: no
sharing
 circuit-like (guaranteed)
performance
 call setup required
Network Core: Packet Switching
each end-end data stream
divided into packets
• user A, B packets share
network resources
• each packet uses full link
bandwidth
• resources used as needed,
Bandwidth division into “pieces”
Dedicated allocation
Resource reservation
resource contention:
• aggregate resource
demand can exceed
amount available
• congestion: packets
queue, wait for link use
• store and forward:
packets move one hop
at a time
– transmit over link
– wait turn at next
link
Network core: routing
Goal: move data among routers from source to dest.
datagram packet network:
circuit-switched network:
 destination address determines
 call allocated time slots
next hop
of bandwidth at each
 routes may change during session
link
 analogy: driving, asking directions
 fixed path (for call)
 No notion of call state
determined at call
setup
virtual circuit network:
 switches maintain lots
 packet carries tag, tag
of per call state
determines next hop
(what?): resource
 fixed path (for call) determined
allocation
at call setup time
 routers maintain little per-call
state; resources not allocated
Packet switching vs circuit switching: why?
 “reliability” – no congestion, in order data in
circuit-switching
 packet switching: better bandwidth use
 state, resources: packet switching has less
state
 good: less control-plane processing resources along
the way
 More dataplane (address lookup) processing
 failure modes (routers/links down):
 packet switching routing reconfigures sub-second
timescale;
 circuit-switching: more complex recovery – need to
involve all (downstream) switches on path
Virtual circuits
“source-to-dest path behaves much like telephone
circuit”
 performance-wise
 network actions along source-to-dest path
 call setup, teardown for each call before data can flow
 each packet carries VC identifier (not destination host ID)
 every router on source-dest path maintains “state” for each
passing connection
 transport-layer connection only involved two end systems
 link, router resources (bandwidth, buffers) may be allocated to
VC
 to get circuit-like performance.
Virtual circuits: signaling protocols
 used to setup, maintain teardown VC
 used in ATM, frame-relay, X.25
 not used in today’s Internet
application
transport 5. Data flow begins
network 4. Call connected
data link 1. Initiate call
physical
6. Receive data application
3. Accept call
2. incoming call
transport
network
data link
physical
Datagram networks:
the Internet model
 no call setup at network layer
 routers: no state about end-to-end connections
 no network-level concept of “connection”
 packets typically routed using destination host ID
 packets between same source-dest pair may take
different paths
application
transport
network
data link 1. Send data
physical
application
transport
network
2. Receive data
data link
physical
Datagram or VC network
Internet
ATM
• data exchange among
• evolved from telephony
computers
• human conversation:
– “elastic” service, no strict
– strict timing, reliability
timing req.
requirements
• “smart” end systems
– need for guaranteed
(computers)
service
– can adapt, perform
• “dumb” end systems
control, error recovery
– telephones
– simple inside network,
– complexity inside
complexity at “edge”
network
• many link types
– different characteristics
– uniform service difficult
Datagram or VC network
Routing: Store-and-Forward
• Store-and-Forward Packet Switching
A host with a packet to send transmits it to the nearest router,
either on its own LAN or over a point-to-point link to the carrier.
The packet is stored there until it has fully arrived so the
checksum can be verified. Then it is forwarded to the next
router along the path until it reaches the destination host,
where it is delivered.
Router Architecture Overview
Two key router functions:
• run routing algorithms/protocol
• forwarding datagrams from incoming to outgoing link
Input Port Functions
Physical layer:
bit-level reception
Data link layer:
e.g., Ethernet
Decentralized switching:
• given datagram dest., lookup output port
using forwarding table in input port
memory
• goal: complete input port processing at
‘line speed’
• queuing: if datagrams arrive faster than
forwarding rate into switch fabric
Three types of switching fabrics
Switching Via Memory
First generation routers:
• traditional computers with switching under direct control of
CPU
•packet copied to system’s memory
• speed limited by memory bandwidth (2 bus crossings per
datagram)
Input
Port
Memory
Output
Port
System Bus
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)
Switching Via An Interconnection
Network (CrossBar)
• 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
Output Ports
• Buffering required when datagrams arrive
from fabric faster than the transmission rate
• Scheduling discipline chooses among queued
datagrams for transmission
Output port queueing
• buffering when arrival rate via switch
exceeds output line speed
• queueing (delay) and loss due to output port
buffer overflow!
Routing Algorithms
• The main function of the network layer is routing
packets from the source machine to the destination
machine.
• The routing algorithm is that part of the network
layer software responsible for deciding which output
line an incoming packet should be transmitted on.
• If the subnet uses virtual circuits internally, routing
decisions are made only when a new virtual circuit is
being set up.
• One can think of a router as having two processes
inside it.
– One of them handles each packet as it arrives, looking up the
outgoing line to use for it in the routing tables. This process
is forwarding.
– The other process is responsible for filling in and updating
the routing tables.
Routing: Metrics of Algorithms
Routing protocol
Goal: determine “good” path
(sequence of routers) thru
network from source to dest.
• Graph abstraction for
routing algorithms:
• graph nodes are
routers
• graph edges are
physical links
– link cost: delay, $
cost, or congestion
level
5
2
A
B
2
1
D
3
C
3
1
5
F
1
E
2
• “good” path:
– typically means minimum
cost path
– other def’s possible
Routing: only two approaches
used in practice
Global:
• all routers have complete topology, link cost info
• “link state” algorithms: use Dijkstra’s algorithm to find
shortest path from given router to all destinations
Decentralized:
• router knows physically-connected neighbors, link costs
to neighbors
• iterative process of computation, exchange of info with
neighbors
• “distance vector” algorithms
• a ‘self-stabilizing algorithm’ (we’ll see these later)
Routing: Shortest Path Routing
• This is a “Global” routing algorithm
• The idea is to build a graph of the subnet,
with each node of the graph representing a
router and each arc of the graph representing
a communication line (often called a link).
• To choose a route between a given pair of
routers, the algorithm just finds the shortest
path between them on the graph.
Routing: Shortest Path Routing (Cont’d)
• Dijkstra's algorithm
Routing: Distance Vector Routing
• Distance vector routing algorithms operate by having each
router maintain a table (i.e, a vector) giving the best known
distance to each destination and which line to use to get there.
• These tables are updated by exchanging information with the
neighbors.
• It was the original ARPANET routing algorithm and was also
used in the Internet under the name RIP.
• each router maintains a routing table indexed by, and containing
one entry for, each router in the subnet. This entry contains two
parts: the preferred outgoing line to use for that destination
and an estimate of the time or distance to that destination. The
metric used might be number of hops, time delay in milliseconds,
total number of packets queued along the path, or something
similar.
Routing: Distance Vector Routing
(Cont’d)
• An example of Distance Vector Routing
(a) A subnet. (b) Input from A, I, H, K, and the new routing table for J.
Question: How J computes its new route to router G?
Routing: Distance Vector Routing
(Cont’d)
• The Count-to-Infinity Problem
– In particular, it reacts rapidly to good news,
but leisurely to bad news.
(a) A gets Up
(b) A turns Down
Routing: Hierarchical Routing
Our routing review thus far - idealization
• all routers identical
• network “flat”
… not true in practice
scale: with 200 million
destinations:
administrative autonomy
• internet = network of
networks
• can’t store all dest’s in
routing tables!
• each network admin may
want to control routing in its
• routing table exchange would
own network
swamp links!
Routing: Hierarchical Routing (Cont’d)
• aggregate routers into
regions, “autonomous
systems” (AS)
• routers in same AS run
same routing protocol
– “intra-AS” routing
protocol
– routers in different AS
can run different intraAS routing protocol
gateway routers
• special routers in AS
• run intra-AS routing
protocol with all other
routers in AS
• also responsible for
routing to destinations
outside AS
– run inter-AS routing
protocol with other
gateway routers
Routing: Hierarchical Routing (Cont’d)
• An example of Hierarchical Routing
(a) the Topology of network (b) the Full table of 1A (c) the Hierarchical table of 1A
Intra-AS and Inter-AS routing
C.b
a
C
Gateways:
B.a
A.a
b
A.c
d
A
a
b
c
a
c
B
b
•perform inter-AS
routing amongst
themselves
•perform intra-AS
routers with other
routers in their
AS
network layer
inter-AS, intra-AS
routing in
gateway A.c
link layer
physical layer
Intra-AS and Inter-AS routing
C.b
a
Host
h1
C
b
A.a
a
Inter-AS Internet: BGP
routing
between
B.a
A and B
Host
h2
c
A.c
a
b
B
d
c
b
A
Intra-AS routing
within AS A
Intra-AS routing
within AS B
Internet: OSPF, IS-IS, RIP
Addressing
• What’s an address?
– identifier that differentiates between me and
someone else, and also helps route data to/from
me
• Real world examples of addressing?
– mailing address
– office #, floor, etc
– different “levels of addressing”
– Phone#
Addressing: network layer
• IP address: 32-bit
identifier for host,
router interface
• interface: connection
between host, router
and physical link
– router’s typically have
multiple interfaces
– host may have multiple
interfaces
– IP addresses
associated with
interface, not host,
router
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.2
223.1.3.1
223.1.1.1 = 11011111 00000001 00000001 00000001
223
1
1
1
IP Addressing
• IP address:
– network part (high
order bits)
– host part (low order
bits)
• What’s a network ? (from
IP address perspective)
– device interfaces with
same network part of
IP address
– can physically reach
each other without
intervening router
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
LAN
223.1.3.1
223.1.3.2
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)