Roadmap
1.1 What is the Internet?
1.2 Network edge
1.3 Network core
1.4 Network access and physical media
1.5 Internet structure and ISPs
1.6 Delay & loss in packet-switched
networks
1.7 Protocol layers, service models
1.8 History
Protocol “Layers”
Networks are
complex!
• many “pieces”: Question:
– hosts Is there any hope of
– routers organizing structure
– links of various of network?
media
– applications Or at least our
– protocols discussion of
networks?
– hardware,
software
Organization of air travel
ticket (purchase) ticket (complain)
baggage (check) baggage (claim)
gates (load) gates (unload)
runway takeoff runway landing
airplane routing airplane routing
airplane routing
• a series of steps
Layering of airline functionality
ticket (purchase) ticket (complain) ticket
baggage (check) baggage (claim baggage
gates (load) gates (unload) gate
runway (takeoff) runway (land) takeoff/landing
airplane routing airplane routing airplane routing airplane routing airplane routing
departure intermediate air-traffic arrival
airport control centers airport
Layers: each layer implements a service
– via its own internal-layer actions
– relying on services provided by layer below
Why layering?
Dealing with complex systems:
• explicit structure allows identification,
relationship of complex system’s pieces
– layered reference model for discussion
• modularization eases maintenance, updating
of system
– change of implementation of layer’s service
transparent to rest of system
– e.g., change in gate procedure doesn’t
affect rest of system
• layering considered harmful?
2. TCP/IP Reference Model
(Layers)
FTP TELNET MAIL …….. (4)
TCP UDP (3)
Gateway IP, ICMP and IGMP (2)
protocols
IEEE 802 Ethernet X.25 (1)
V.24 V.28 EIA-232 ISDN etc.
(1) Data link and physical layer: The protocols at this layer needed to manage a
specific physical medium, such as Ethernet or a point to point line
(2) Network layer: IP, which provides the basic service of getting datagrams to
their destination
(3) Transport layer: A protocol such as TCP that provides services need by many
applications
(4) Application layer: An application protocol such as mail
Internet protocol stack
• application: supporting network
applications application
– FTP, SMTP, HTTP
transport
• transport: process-process data
transfer network
– TCP, UDP
• network: routing of datagrams link
from source to destination
physical
– IP, routing protocols,
• link: data transfer between
neighboring network elements
– PPP, Ethernet
source
message M application
Encapsulation
segment Ht M transport
datagram Hn Ht M network
frame Hl Hn Ht M link
physical
link
physical
switch
destination Hn Ht M network
M application Hl Hn Ht M link Hn Ht M
Ht M transport physical
Hn Ht M network
Hl Hn Ht M link router
physical
Figure 3-1
OSI Model
Figure 3-2
OSI Layers
Figure 3-3
An Exchange Using the OSI Model
WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998
Figure 3-4
Physical Layer
Figure 3-14
Summary of Layer Functions
Introduction: Summary
Covered a “ton” of You now have:
material! • context, overview,
• Internet overview “feel” of networking
• what’s a protocol? • more depth, detail
• network edge, core, to follow!
access network
– packet-switching
versus circuit-
switching
• Internet/ISP structure
• performance: loss,
delay
MAC Addresses and ARP
• 32-bit IP address:
– network-layer address
– used to get datagram to destination IP subnet
• MAC (or LAN or physical or Ethernet)
address:
– used to get frame from one interface to
another physically-connected interface (same
network)
– 48 bit MAC address (for most LANs)
burned in the adapter ROM
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
1A-2F-BB-76-09-AD Broadcast address =
FF-FF-FF-FF-FF-FF
LAN
(wired or = adapter
wireless)
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
LAN Address (more)
• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address space
(to assure uniqueness)
• Analogy:
(a) MAC address: like Social Security Number
(b) IP address: like postal address
• MAC flat address ➜ portability
– can move LAN card from one LAN to another
• IP hierarchical address NOT portable
– depends on IP subnet to which node is attached
ARP: Address Resolution Protocol
Question: how to determine • Each IP node (Host,
MAC address of B Router) on LAN has
knowing B’s IP address? ARP table
137.196.7.78 • ARP Table: IP/MAC
1A-2F-BB-76-09-AD address mappings for
137.196.7.23 some LAN nodes
137.196.7.14
71-65-F7-2B-08-53
58-23-D7-FA-20-B0 – TTL (Time To Live):
time after which
0C-C4-11-6F-E3-98 address mapping will
137.196.7.88
be forgotten (typically
20 min)
ARP protocol: Same LAN
(network)
• A wants to send datagram to
B, and B’s MAC address not • A caches (saves) IP-to-MAC
in A’s ARP table. address pair in its ARP table
• A broadcasts ARP query until information becomes old
packet, containing B's IP (times out)
address – soft state: information that
– Dest MAC address = FF- times out (goes away)
FF-FF-FF-FF-FF unless refreshed
– all machines on LAN • ARP is “plug-and-play”:
receive ARP query – nodes create their ARP
• B receives ARP packet, tables without intervention
replies to A with its (B's) from net administrator
MAC address
– frame sent to A’s MAC
address (unicast)
Routing to another LAN
walkthrough: send datagram from A to B via R
assume A know’s B IP address
A
R
B
• Two ARP tables in router R, one for each IP network (LAN)
• A creates datagram with source A, destination B
• A uses ARP to get R’s MAC address for 111.111.111.110
• A creates link-layer frame with R's MAC address as dest, frame
contains A-to-B IP datagram
• A’s adapter sends frame
• R’s adapter receives frame
• R removes IP datagram from Ethernet frame, sees its destined to B
• R uses ARP to get B’s MAC address
• R creates frame containing A-to-B IP datagram sends to B
A
R
B
Link Layer
• 5.1 Introduction and • 5.6 Hubs and
services switches
• 5.2 Error detection • 5.7 PPP
and correction • 5.8 Link Virtualization:
• 5.3Multiple access ATM
protocols
• 5.4 Link-Layer
Addressing
• 5.5 Ethernet
Ethernet
“dominant” wired LAN technology:
• cheap $20 for 100Mbs!
• first widely used LAN technology
• Simpler, cheaper than token LANs and ATM
• Kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet
sketch
Star topology
• Bus topology popular through mid 90s
• Now star topology prevails
• Connection choices: hub or switch (more
later)
hub or
switch
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or
other network layer protocol packet) in
Ethernet frame
Preamble:
• 7 bytes with pattern 10101010 followed by
one byte with pattern 10101011
• used to synchronize receiver, sender clock
rates
Ethernet Frame Structure
(more)
• Addresses: 6 bytes
– if adapter receives frame with matching destination
address, or with broadcast address (eg ARP packet),
it passes data in frame to net-layer protocol
– otherwise, adapter discards frame
• Type: indicates the higher layer protocol (mostly
IP but others may be supported such as Novell
IPX and AppleTalk)
• CRC: checked at receiver, if error is detected,
the frame is simply dropped
Unreliable, connectionless
service
• Connectionless: No handshaking between
sending and receiving adapter.
• Unreliable: receiving adapter doesn’t send acks
or nacks to sending adapter
– stream of datagrams passed to network layer can
have gaps
– gaps will be filled if app is using TCP
– otherwise, app will see the gaps
10BaseT and 100BaseT
• 10/100 Mbps rate; latter called “fast ethernet”
• T stands for Twisted Pair
• Nodes connect to a hub: “star topology”; 100
m max distance between nodes and hub
twisted pair
hub
Hubs
Hubs are essentially physical-layer repeaters:
– bits coming from one link go out all other links
– at the same rate
– no frame buffering
– no CSMA/CD at hub: adapters detect collisions
– provides net management functionality
twisted pair
hub
Manchester encoding
• Used in 10BaseT
• Each bit has a transition
• Allows clocks in sending and receiving nodes to
synchronize to each other
– no need for a centralized, global clock among
nodes!
• Hey, this is physical-layer stuff!
Gbit Ethernet
• uses standard Ethernet frame format
• allows for point-to-point links and shared
broadcast channels
• in shared mode, CSMA/CD is used; short
distances between nodes required for efficiency
• uses hubs, called here “Buffered Distributors”
• Full-Duplex at 1 Gbps for point-to-point links
• 10 Gbps now !