1. CSE4471: Computer Network Review
Network Layers
TCP/UDP IP
Ethernet

2. 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
ppp, ethernet
physical: bits “on the wire”
application
transport
network
link
physical

3. OSI Network Layers

4. 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

5. Layering: physical communication
data
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
data
application
transport
network
link
physical
application
transport
network
link
physical

6. 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
destination
application
transport
network
link
physical
application
transport
network
link
physical M H t n l
message M H t n l
segment
datagram
frame

7. Internet structure: network of networks
roughly hierarchical
national/international 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
local
ISP
regional ISP
NBP B
NAP
NAP
NBP A
regional ISP
local
ISP

8. National Backbone Provider
e.g. Sprint US backbone network

9. TCP Transport Control Protocol Flow control and Responds to congestion
Reliable In-order delivery
“Nice” Protocol

10. TCP segment structure source port # dest port # application data
32 bits
application
data
(variable length)
sequence number
acknowledgement number
rcvr window size
ptr urgent data
checksum F S R P A U
head
len
not
used
Options (variable length)
URG: urgent data
(generally not used)
counting
by bytes
of data
(not segments!)
ACK: ACK #
valid
PSH: push data now
(generally not used)
# bytes
rcvr willing
to accept
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)

11. 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

12. simple telnet scenario
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
Host B
User
types
‘C’
Seq=42, ACK=79, data = ‘C’
host ACKs
receipt of
‘C’, echoes
back ‘C’
Seq=79, ACK=43, data = ‘C’
host ACKs
receipt
of echoed
‘C’
Seq=43, ACK=80
time
simple telnet scenario

13. 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.

14. Flow Control

15. UDP No reliability, flow control, congestion control.
Sends data in a burst.
Provides multiplexing and demultiplexing of sources.
Most multimedia applications using UDP

16. 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

17. UDP segment structure other UDP uses (why?): Application data
often used for streaming multimedia apps
loss tolerant
rate sensitive
other UDP uses (why?):
DNS
SNMP
reliable transfer over UDP: add reliability at application layer
application-specific error recover!
32 bits
source port #
dest port #
Length, in
bytes of UDP
segment,
including
header
length
checksum
Application
data
(message)
UDP segment format

18. 32 bit destination IP address
IP datagram format
IP protocol version
number
32 bits
total datagram
length (bytes)
header length
(bytes)
head.
len
type of
service
ver
length
for
fragmentation/
reassembly
“type” of data
fragment
offset
16-bit identifier
flgs
max number
remaining hops
(decremented at
each router)
time to
live
upper
layer
Internet
checksum
32 bit source IP address
32 bit destination IP address
upper layer protocol
to deliver payload to
Options (if any)
E.g. timestamp,
record route
taken, pecify
list of routers
to visit.
data
(variable length,
typically a TCP
or UDP segment)

19. 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 Code 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

20. 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

21. Link Layer

22. Link Layer: setting the context
two physically connected devices:
host-router, router-router, host-host
unit of data: frame
application
transport
network
link
physical
network
link
physical M H t n l
data link
protocol H l H t n M
frame
phys. link
adapter card

23. 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?

24. 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

25. 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)

26. 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

27. Ethernet: uses CSMA/CD
A: sense channel, if idle
then {
transmit and monitor the channel;
If detect another transmission
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}

28. A Summary on Network Layers and Their Vulnerabilities
Basic
Functions
Representative Protocols
Security Vulnerability Examples
Application
Providing services such as WWW to end-users
HTTP, SMTP, FTP
JavaScript-based malware, spams
Transport
End-to-end message transmission independent of the underlying network
TCP, UDP
TCP SYN attack,
UDP flooding attack
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

29. Acknowledgement
Part of the slides are from Kurose and Ross’s book “Computer Networking: A Top-Down Approach”.