CSE4471: Computer Network Review
 Network Layers
 TCP/UDP
 IP
 Ethernet
Internet Layers
 application: supporting network
applications

ftp, smtp, http
 transport: host-host data transfer
 tcp, udp
 network: routing of datagrams from
source to destination

ip, routing protocols
 link: data transfer between
neighboring network elements

application
transport
network
link
physical
ppp, ethernet
 physical: bits “on the wire”
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OSI Network Layers
Layering: logical communication
Each layer:
 distributed
 “entities”
implement
layer functions
at each node
 entities
perform
actions,
exchange
messages with
peers
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
application
transport
network
link
physical
4
Layering: physical communication
data
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
data
application
transport
network
link
physical
5
Protocol layering and data
Each layer takes data from above
 adds header information to create new data unit
 passes new data unit to layer below
source
M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
destination
application
transport
network
link
physical
M
message
Ht M
Hn Ht M
Hl Hn Ht M
segment
datagram
frame
6
Internet structure: network of networks
 roughly hierarchical
 national/international
local
ISP
backbone providers (NBPs)


e.g. BBN/GTE, Sprint,
AT&T, IBM, UUNet
interconnect (peer) with
each other privately, or at
public Network Access Point
(NAPs)
 regional ISPs
 connect into NBPs
 local ISP, company
 connect into regional ISPs
regional ISP
NBP B
NAP
NAP
NBP A
regional ISP
local
ISP
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National Backbone Provider
e.g. Sprint US backbone network
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TCP
 Transport Control Protocol
 Flow control and Responds to congestion
 Reliable In-order delivery
 “Nice” Protocol
TCP segment structure
32 bits
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
source port #
dest port #
sequence number
acknowledgement number
head not
UA P R S F
len used
checksum
rcvr window size
ptr urgent data
Options (variable length)
application
data
(variable length)
counting
by bytes
of data
(not segments!)
# bytes
rcvr willing
to accept
Reliable Delivery
 Sender, Receiver keep track of bytes sent and
bytes received.
 Acks have an indication of next byte expected.
 Three duplicate acks considered a packet loss sender retransmits
TCP seq. #’s and ACKs
Seq. #’s:
 byte stream
“number” of first
byte in segment’s
data
ACKs:
 seq # of next byte
expected from other
side
 cumulative ACK
Q: how receiver handles
out-of-order segments
 A: TCP spec doesn’t
say, - up to
implementer
Host A
User
types
‘C’
Host B
host ACKs
receipt of
‘C’, echoes
back ‘C’
host ACKs
receipt
of echoed
‘C’
simple telnet scenario
time
TCP Flow Control
 Window based
 Sender cannot send more data than a window
without acknowledgements.
 Window is a minimum of receiver’s buffer and
‘congestion window’.
 After a window of data is transmitted, in steady
state, acks control sending rate.
Flow Control
UDP
 No reliability, flow control, congestion control.
 Sends data in a burst.
 Provides multiplexing and demultiplexing of
sources.
 Most multimedia applications using UDP
UDP: User Datagram Protocol
[RFC 768]
 “no frills,” “bare bones”
Internet transport protocol
 “best effort” service, UDP
segments may be:
 lost
 delivered out of order to
app
 connectionless:
 no handshaking between
UDP sender, receiver
 each UDP segment handled
independently of others
Why is there a UDP?
 no connection establishment
(which can add delay)
 simple: no connection state at
sender, receiver
 small segment header
 no congestion control: UDP can
blast away as fast as desired
UDP segment structure
 often used for streaming
multimedia apps
 loss tolerant
 rate sensitive
Length, in
bytes of UDP
 other UDP uses (why?):
segment,
including
 DNS
header
 SNMP
 reliable transfer over UDP:
add reliability at application
layer
 application-specific error
recover!
32 bits
source port #
dest port #
length
checksum
Application
data
(message)
UDP segment format
IP datagram format
IP protocol version
number
header length
(bytes)
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
32 bits
type of
ver head.
len service
length
fragment
16-bit identifier flgs
offset
time to upper
Internet
layer
live
checksum
total datagram
length (bytes)
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
E.g. timestamp,
record route
taken, pecify
list of routers
to visit.
ICMP: Internet Control Message Protocol
 used by hosts, routers,
gateways to communication
network-level information
 error reporting:
unreachable host, network,
port, protocol
 echo request/reply (used
by ping)
 network-layer “above” IP:
 ICMP msgs carried in IP
datagrams
 ICMP message: type, code plus
first 8 bytes of IP datagram
causing error
Type
0
3
3
3
3
3
3
4
Code
0
0
1
2
3
6
7
0
8
9
10
11
12
0
0
0
0
0
description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header
Routing in the Internet
 The Global Internet consists of Autonomous Systems
(AS) interconnected with each other:



Stub AS: small corporation
Multihomed AS: large corporation (no transit)
Transit AS: provider
 Two-level routing:
 Intra-AS: administrator is responsible for choice: RIP,
OSPF
 Inter-AS: unique standard: BGP
Link Layer
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Link Layer: setting the context
 two physically connected devices:
 host-router, router-router, host-host
 unit of data: frame
M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
data link
protocol
phys. link
adapter card
network
link
physical
Hl Hn Ht M
frame
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Link Layer Services
 Framing, link access:
 encapsulate datagram into frame, adding header, trailer
 implement channel access if shared medium,
 ‘physical addresses’ used in frame headers to identify
source, dest
• different from IP address!
 Reliable delivery between two physically connected
devices:



we learned how to do this already (chapter 3)!
seldom used on low bit error link (fiber, some twisted
pair)
wireless links: high error rates
• Q: why both link-level and end-end reliability?
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Link Layer Services (more)
 Flow Control:
 pacing between sender and receivers
 Error Detection:
 errors caused by signal attenuation, noise.
 receiver detects presence of errors:
• signals sender for retransmission or drops frame
 Error Correction:
 receiver identifies and corrects bit error(s) without
resorting to retransmission
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Multiple Access Links and Protocols
Three types of “links”:
 point-to-point (single wire, e.g. PPP, SLIP)
 broadcast (shared wire or medium; e.g, Ethernet,
Wavelan, etc.)
 switched (e.g., switched Ethernet, ATM etc)
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Multiple Access protocols
 single shared communication channel
 two or more simultaneous transmissions by nodes:
interference

only one node can send successfully at a time
 multiple access protocol:
 distributed algorithm that determines how stations share
channel, i.e., determine when station can transmit
 communication about channel sharing must use channel itself!
 what to look for in multiple access protocols:
•
•
•
•
synchronous or asynchronous
information needed about other stations
robustness (e.g., to channel errors)
performance
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Ethernet: uses CSMA/CD
A: sense channel, if idle
then {
transmit and monitor the channel;
If detect another transmission
then {
abort and send jam signal;
update # collisions;
delay as required by exponential backoff algorithm;
goto A
}
else {done with the frame; set collisions to zero}
}
else {wait until ongoing transmission is over and goto A}
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A Summary on Network Layers
and Their Vulnerabilities
Network
Layer
Basic
Functions
Representative
Protocols
Security
Vulnerability
Examples
Application
Providing services such as
WWW to end-users
HTTP, SMTP, FTP
JavaScript-based malware,
Email spams
Transport
End-to-end message
transmission independent
of the underlying network
TCP, UDP
TCP SYN attack,
UDP flooding attack
Network
Routing
IP, ICMP, RIP, OSPF, BGP
IP spoofing,
Black hole attack to RIP
Data Link
Media access control
Ethernet, Wi-Fi
Eavesdropping attack
Physical
Transmitting raw bit
stream
Physical attack such as cut
to cable
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Acknowledgement
 Part of the slides are from Kurose and Ross’s book
“Computer Networking: A Top-Down Approach”.
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