Transcript Network Protocols

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
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
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.
Link Layer
•Defines local communication when two machines are on the
same physical network.
•Routing is typically not needed. Just broadcast or switch-based.
switch
node
node
node
node
node
node
•Defines physical details (voltage levels, timing, etc.)
•Size of local network limited by bus/switch capacity (physical limits)
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
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
router
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 1’s comp)
•IP is unreliable
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
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
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
HTTP Request Example
•HTTP Request generated by going to www.mtv.com
GET / HTTP/1.1[CRLF]
Host: www.mtv.com[CRLF]
Connection: close[CRLF]
User-Agent: Web-sniffer/1.0.27 (+http://web-sniffer.net/)[CRLF]
Accept-Encoding: gzip[CRLF]
Accept-Charset: ISO-8859-1,UTF-8;q=0.7,*;q=0.7[CRLF]
Cache-Control: no[CRLF]
Accept-Language: de,en;q=0.7,en-us;q=0.3[CRLF]
Referer: http://web-sniffer.net/[CRLF]
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-Cookie:
BrowserType=NonMobile; expires=Fri, 20 Jul 2010 02:29:22
GMT; path=/;
ETag:
dd148f359cd88aa7a1ed2e4d5802b1d
Date:
Mon, 14 Apr 2008 16:21:05 GMT
Content-Length:
12203
Content-Encoding: gzip
•Message content is HTML of MTV’s web page