1 wide area networks and internet ct1403 lecture-8: internet network layer (part-3) by : najla...
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Wide Area Networks and InternetCT1403
Lecture-8: Internet Network Layer (Part-3)
By : Najla Al-Nabhan
Lecture goals:
understand principles behind:
Address Resolution Protocol (ARP) in the Internet
Network layer service models (Connection & Connectionless)
forwarding versus routing how a router works routing (path selection) Internet broadcast, multicast
Network layer: Recall! transport segment
from sending to receiving host
on sending side encapsulates segments into datagrams
on receiving side, delivers segments to transport layer
network layer protocols in every host, router
router examines header fields in all IP datagrams passing through it
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical network
data linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
Network Layer4-33
The Internet network layer
forwarding
table
host, router network layer functions:
routing protocols• path
selection• RIP, OSPF,
BGP
IP protocol• addressing
conventions• datagram format• packet handling
conventionsICMP protocol• error reporting• router
“signaling”
transport layer: TCP, UDP
link layer
physical layer
network
layer
IP Routing Process
When packet arrives, look up dest addr
local network? send immediately to destination
distant network? forward to next router on the interface given
in routing table not in the routing table?
forward to default gateway
Address Resolution Protocol (ARP)
Address Resolution Protocol (ARP)
Because there are both network -layer addresses (IP address) and link-layer addresses (that is MAC address), there is a need to translate between them
For Internet, this translation is the job of the Address Resolution Protocol (ARB)
MAC address allocation administered by IEEE. Manufacturer buys portion of MAC address space (to ensure uniqueness)
Analogy:MAC address: like Social Security NumberIP address: like postal address
ARP: address resolution protocol
ARP table: each IP node (host, router) on LAN has table
IP/MAC address mappings for some LAN nodes:
< IP address; MAC address; TTL>
TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)
Question: how to determineinterface’s MAC address, knowing its IP address?
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137.196.7.23
137.196.7.78
137.196.7.14
137.196.7.88
ARP protocol in the Internet: same LAN
1. A wants to send datagram to B
B’s MAC address not in A’s ARP table.
2. A broadcasts ARP query packet, containing B's IP address
dest MAC address = FF-FF-FF-FF-FF-FF
all nodes on LAN receive ARP query
3. B receives ARP packet, replies to A with its (B's) MAC address
frame sent to A’s MAC address (unicast)
4. A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
soft state: information that times out (goes away) unless refreshed
5. ARP is “plug-and-play”:
nodes create their ARP tables without intervention from net administrator
walkthrough: send datagram from A to B via R focus on addressing – at IP (datagram) and MAC layer (frame)
assume A knows B’s IP address assume A knows IP address of first hop router, R (how?)
assume A knows R’s MAC address (how?)
Addressing: routing to another LAN
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B
Addressing: routing to another LAN
IPEthPhy
IP src: 111.111.111.111 IP dest: 222.222.222.222
A creates IP datagram with IP source A, destination B
A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram
MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B
Addressing: routing to another LAN
IP src: 111.111.111.111 IP dest: 222.222.222.222
R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B
Addressing: routing to another LAN R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram
IP src: 111.111.111.111 IP dest: 222.222.222.222
MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
B
Addressing: routing to another LAN R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram
IP src: 111.111.111.111 IP dest: 222.222.222.222
MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A
IPEthPhy
Two key network-layer functions
network layer has three major functions:
1. forwarding: move packets from router’s input to appropriate router output
2. routing: determine route taken by packets from source to dest.
routing algorithms
analogy:routing: process of planning trip from source to dest
forwarding: process of getting through single interchange
1
23
0111
value in arrivingpacket’s header
routing algorithm
local forwarding tableheader
valueoutput
link0100
0101
0111
1001
3221
Interplay between routing and forwarding
routing algorithm determinesend-end-path through networkforwarding table determineslocal forwarding at this router
Connection setup Connection setup is the 3rd important function
in some network architectures; (such as ATM, frame relay, X.25)
Occurs before data transfer: 3-way handshake in TCP Connection All routers in the Virtual Circuit (VC) need to
handshake with each other in order to establish virtual connection
routers get involved network vs transport layer connection
service: network: between two hosts (may also involve
intervening routers in case of VCs) transport: between two processes
Network service modelQ: What service model for “channel” transporting datagrams from sender to receiver?example services for an individual datagram:
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
Network layer service models:
NetworkArchitectu
re
Internet
ATM
ATM
ServiceModel
best effort
CBR
ABR
Bandwidth
none
constantrateguaranteed minimum
No loss
no
yes
no
Order
no
yes
yes
Timing
no
yes
no
Congestionfeedback
no (inferredvia loss)nocongestionyes (indicated)
Guarantees ?
virtual circuit and datagram networks
Connection, connection-less service Transport layer provides connection &
connection-less services between two processes Network layer provides connection OR
connection-less services between two hosts datagram network provides network-layer
connectionless service virtual-circuit network provides network-layer
connection service analogous to TCP/UDP connection-oriented /
connectionless transport-layer services, but: service: host-to-host no choice: network provides one or the other
(not both) implementation: in network core
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 (dedicated resources = predictable service)
“source-to-dest path behaves much like telephone circuit”
performance-wise network actions along source-to-dest path
VC implementation
a VC consists of:1. path from source to destination2. VC numbers, one number for each link
along path3. entries in forwarding tables in routers
along path packet belonging to VC carries VC
number (rather than dest address) VC number can be changed on each
link. new VC number comes from forwarding
table
VC forwarding table12
22
32
1 23
VC numberinterfac
enumber
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …
forwarding table innorthwest router:
VC routers maintain connection state information!
applicationtransportnetworkdata linkphysical
Virtual circuits: signaling protocols
used to setup, maintain and terminate VC
used in ATM, frame-relay, X.25 not used in today’s Internet
1. initiate call
2. incoming call
3. accept call
4. call connected
5. data flow begins
6. receive data
applicationtransportnetworkdata linkphysical
Datagram networks no call setup at network layer routers: no state about end-to-end
connections no network-level concept of “connection”
packets forwarded using destination host address
1. send datagrams
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
2. receive datagrams
1
23
Datagram forwarding table
IP destination address in arriving packet’s header
routing algorithm
local forwarding tabledest address output
linkaddress-range 1
address-range 2
address-range 3
address-range 4
3221
4 billion IP addresses, so rather than list individual destination addresslist range of addresses(aggregate table entries)
Destination Address Range
11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111
11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111
11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111
otherwise
Link Interface
0
1
2
3
Q: but what happens if ranges don’t divide up so nicely?
Datagram forwarding table
Longest prefix matching
Destination Address Range
11001000 00010111 00010*** *********
11001000 00010111 00011000 *********
11001000 00010111 00011*** *********
otherwise
DA: 11001000 00010111 00011000 10101010
examples: DA: 11001000 00010111 00010110
10100001 which interface?which interface?
when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.
longest prefix matching
Link
interface
0
1
2
3
Datagram or VC network: why?Internet (datagram) data exchange among
computers “elastic” service, no
strict timing req. many link types
different characteristics uniform service difficult
“smart” end systems (computers) can adapt, perform
control, error recovery simple inside
network, complexity at “edge”
ATM (VC) evolved from
telephony human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems telephones complexity inside
network
what’s inside a router
Router architecture overviewtwo key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link
high-seed
switching
fabric
routing process
or
router input ports
router output ports
forwarding data plane (hardware)
routing, managementcontrol plane
(software)
forwarding tables computed,pushed to input ports
lineterminati
on
link layer
protocol(receive
)
lookup,forwardi
ng
queueing
Input port functions
decentralized switching: given datagram dest., lookup output
port using forwarding table in input port memory (“match plus action”)
goal: complete input port processing at ‘line speed’
queuing: if datagrams arrive faster than forwarding rate into switch fabric
physical layer:bit-level
receptiondata link layer:e.g.,
Ethernetsee chapter
5
switchfabric
Switching fabrics transfer packet from input buffer to
appropriate output buffer switching rate: rate at which packets
can be transfer from inputs to outputsoften measured as multiple of input/output line rateN inputs: switching rate N times line rate desirable
three types of switching fabrics
memory
memory
bus
crossbar
Switching via memoryfirst 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)
inputport(e.g.,
Ethernet)
memory
outputport(e.g.,
Ethernet)
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
32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers
bus
Switching via interconnection network
overcome bus bandwidth limitations
banyan networks, crossbar, 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 60 Gbps through the interconnection network
crossbar
Output ports
buffering required when datagrams arrive from fabric faster than the transmission rate
scheduling discipline chooses among queued datagrams for transmission
lineterminati
on
link layer
protocol(send)
switchfabric
datagram
buffer
queueing
Output port queueing
buffering when arrival rate via switch exceeds output line speed
queueing (delay) and loss due to output port buffer overflow!
at t, packets morefrom input to output
one packet time later
switchfabric
switchfabric
Input port queuing
fabric slower than input ports combined -> queueing may occur at input queues
queueing delay and loss due to input buffer overflow!
Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward
output port contention:only one red datagram can
be transferred.lower red packet is blocked
switchfabric
one packet time later: green
packet experiences HOL
blocking
switchfabric
Midterm Revision:
Your Questions: Please Ask!Difficult to Understand Topics?