Final Defense Talk

1
Network Protocols

• A network protocol defines the structure of
  messages sent over the network
• We will only talk about the Internet
• Network protocols need to address many
  complexities
• How is the physical hardware controlled?
• How is routing performed?
• How do you guarantee the message sequence?
• How do you guarantee message arrival?
• How do you manage high traffic, guarantee
  timing?
• Ex. US postal service

2
Network Protocol Stack

• Networking protocols are defined as a stack of
  related protocols
• Each protocol deals with some issues
• Each protocol adds its own header and trailer
  (footer) info

• Code (in libraries) is needed to pack and unpack
  each message

3
TCP/IP Protocol Stack

• TCP/IP protocol stack defines the Internet

Application Layer - Seen by the applications.
Services are application specific. Transport
Layer - Provides a reliable bitstream.
Guarantees reliability, message order, no
duplication, traffic control Network Layer -
Provides virtual network abstraction. Provides
abstract addressing and routing. Link Layer -
Describes how to control the physical hardware.
Deals with contention.
4
Link Layer

• Defines local communication when two machines are
  on the same physical network.
• Routing is typically not needed. Just broadcast
  or switch-based.

• Defines physical details (voltage levels, timing,
  etc.)
• Size of local network limited by bus/switch
  capacity (physical limits)

5
Link Layer Example

• Ethernet is a common link layer protocol, but
  there are others
• Contention is handled by random backoff

• Header contains
• MAC Destination (6 bytes)
• MAC Source (6 bytes)
• Ethernet Type (2 bytes)
• Footer contains
• Cyclic Redundancy Check (CRC) 4 bytes

6
Network Layer

• Creates a logical network on top of the physical
  network
• Nodes are associated with virtual addresses which
  are associated with physical addresses
• Communication is enabled between local networks
  via routers
• Routing must be addressed at this level
• Must find a path through routers which reaches
  the destination

router
router
router
router
router
7
Internet Protocol (IP)

• Internet Protocol (IP) is the network layer
  protocol used in the Internet
• Each node gets an IP address which is mapped to a
  MAC address
• IP addresses can change, physical addresses
  cannot
• Dynamic Host Connection Protocol (DHCP)
• MAC addresses are built into network cards

• IP Header includes
• IP Destination and Source Addresses
• Time To Live (TTL) - Needed to avoid infinite
  routing loops
• Header length and checksum (sum of header words
  in 1s comp)

• IP is unreliable

8
Transport Layer

• Takes care of several issues
• Reliability - May resend a message if no ACK is
  received
• Error checking - May resend if data is corrupted
  in transit
• Sequence - Messages are numbered so their order
  can be guaranteed

• Transmission Control Protocol (TCP)
• Connection-based (maintains sequence), reliable
• User Datagram Protocol (UDP)
• No connections, not reliable, not much more than
  IP

9
TCP and UDP Headers

• TCP header contains
• Source and Destination Ports - many connections
  to one IP address
• Sequence number - to maintain sequence
• Acknowledgement number - next ACK number expected
• Length and checksum

• UDP header contains
• Source and Destination ports
• Length and header checksum

10
Application Layer

• Application-specific protocols
• Hypertext Transfer Protocol (HTTP), Session
  Initiation Protocol (SIP), File Transfer
  Protocol (FTP), etc.
• Seen by the programmer, should be human-readable
• Headers contain application data

• Differentiation between client and server
• Client sends requests, server sends responses
• Requests and responses differ in headers

11
HTTP Request Example

• HTTP Request generated by going to www.mtv.com

GET / HTTP/1.1CRLF Host www.mtv.comCRLF Conne
ction closeCRLF User-Agent Web-sniffer/1.0.27
(http//web-sniffer.net/)CRLF Accept-Encoding
gzipCRLF Accept-Charset ISO-8859-1,UTF-8q0.7,
q0.7CRLF Cache-Control noCRLF Accept-Langu
age de,enq0.7,en-usq0.3CRLF Referer
http//web-sniffer.net/CRLF
12
HTTP Response Example

• HTTP Response received after GET request

HTTP Status Code HTTP/1.1 200 OK Server Sun-ONE-
Web-Server/6.1 Content-Type text/html Set-Cooki
e BrowserTypeNonMobile expiresFri, 20 Jul
2010 022922 GMT path/ ETag dd148f359cd88a
a7a1ed2e4d5802b1d Date Mon, 14 Apr 2008 162105
GMT Content-Length 12203 Content-Encoding gzi
p

• Message content is HTML of MTVs web page