CSE4471: Computer Network Review

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

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

5
Layering physical communication
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
message
segment
datagram
frame
7
Internet structure network of networks

• roughly hierarchical
• national/international backbone providers (NBPs)
• e.g. BBN/GTE, Sprint, ATT, 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
NBP A
regional 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
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
TCP seq. s and ACKs

• Seq. s
• byte stream number of first byte in segments
  data
• ACKs
• seq of next byte expected from other side
• cumulative ACK
• Q how receiver handles out-of-order segments
• A TCP spec doesnt say, - up to implementer

Host B
Host A
User types C
Seq42, ACK79, data C
host ACKs receipt of C, echoes back C
Seq79, ACK43, data C
host ACKs receipt of echoed C
Seq43, ACK80
simple telnet scenario
13
TCP Flow Control

• Window based
• Sender cannot send more data than a window
  without acknowledgements.
• Window is a minimum of receivers 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

• 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
checksum
length
Application data (message)
UDP segment format
18
IP datagram format
IP protocol version number
32 bits
total datagram length (bytes)
header length (bytes)
type of service
head. len
ver
length
for fragmentation/ reassembly
fragment offset
type of data
flgs
16-bit identifier
max number remaining hops (decremented at each
router)
upper layer
time to live
Internet checksum
32 bit source IP address
32 bit destination IP address
upper layer protocol to deliver payload to
E.g. timestamp, record route taken, pecify list
of routers to visit.
Options (if any)
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 0 0 echo
reply (ping) 3 0 dest. network
unreachable 3 1 dest host
unreachable 3 2 dest protocol
unreachable 3 3 dest port
unreachable 3 6 dest network
unknown 3 7 dest host unknown 4
0 source quench (congestion
control - not used) 8 0
echo request (ping) 9 0 route
advertisement 10 0 router
discovery 11 0 TTL expired 12 0
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

network link physical
data link protocol
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
• 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

28
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
29
Acknowledgement

• Part of the slides are from Kurose and Rosss
  book Computer Networking A Top-Down Approach.