Transcript Week 3 Protocols suite, Data Link Layer Protocols

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”:
– hosts
– routers
– links of various
media
– applications
– protocols
– hardware,
software
Question:
Is there any hope of
organizing structure
of network?
Or at least our
discussion of
networks?
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
departure
airport
airplane routing
airplane routing
intermediate air-traffic
control centers
arrival
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 ……..
TCP
Gateway
protocols
(1)
(2)
(3)
(4)
UDP (3)
IP, ICMP and IGMP
IEEE 802 Ethernet X.25
V.24 V.28 EIA-232 ISDN etc.
(4)
(2)
(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
Network layer: IP, which provides the basic service of getting datagrams to
their destination
Transport layer: A protocol such as TCP that provides services need by many
applications
Application layer: An application protocol such as mail
Internet protocol stack
• application: supporting network
applications
– FTP, SMTP, HTTP
• transport: process-process data
transfer
– TCP, UDP
• network: routing of datagrams
from source to destination
– IP, routing protocols,
• link: data transfer between
neighboring network elements
– PPP, Ethernet
application
transport
network
link
physical
source
message
segment
M
Ht
M
datagram Hn Ht
M
frame Hl Hn Ht
M
Encapsulation
application
transport
network
link
physical
link
physical
switch
destination
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
application
transport
network
link
physical
Hn Ht
Hl Hn Ht
M
M
network
link
physical
Hn Ht
M
router
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
material!
• Internet overview
• what’s a protocol?
• network edge, core,
access network
– packet-switching
versus circuitswitching
• Internet/ISP structure
• performance: loss,
delay
You now have:
• context, overview,
“feel” of networking
• more depth, detail
to follow!
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
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Broadcast address =
FF-FF-FF-FF-FF-FF
= adapter
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
MAC address of B
knowing B’s IP address?
137.196.7.78
1A-2F-BB-76-09-AD
137.196.7.23
137.196.7.14
• Each IP node (Host,
Router) on LAN has
ARP table
• ARP Table: IP/MAC
address mappings for
some LAN nodes
< IP address; MAC address;
TTL>
LAN
71-65-F7-2B-08-53
137.196.7.88
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
–
TTL (Time To Live):
time after which
address mapping will
be forgotten (typically
20 min)
ARP protocol: Same LAN
(network)
• A wants to send datagram to
B, and B’s MAC address not
in A’s ARP table.
• A broadcasts ARP query
packet, containing B's IP
address
– Dest MAC address = FFFF-FF-FF-FF-FF
– all machines on LAN
receive ARP query
• B receives ARP packet,
replies to A with its (B's) MAC
address
– frame sent to A’s MAC
address (unicast)
• 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
• ARP is “plug-and-play”:
– nodes create their ARP
tables without intervention
from net administrator
Routing to another LAN
walkthrough: send datagram from A to B via R
assume A know’s B IP address
A
R
• Two ARP tables in router R, one for each IP network (LAN)
B
•
•
•
•
•
•
•
•
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
services
• 5.2 Error detection
and correction
• 5.3Multiple access
protocols
• 5.4 Link-Layer
Addressing
• 5.5 Ethernet
• 5.6 Hubs and
switches
• 5.7 PPP
• 5.8 Link Virtualization:
ATM
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 !