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 complexitiesHow 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.

node node nodenode node node

switch

•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

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 Request generated by going to www.mtv.com

HTTP Response Example

•HTTP Response received after GET request

HTTP Status Code: HTTP/1.1 200 OKServer: Sun-ONE-Web-Server/6.1Content-Type: text/htmlSet-Cookie: BrowserType=NonMobile; expires=Fri, 20 Jul 2010 02:29:22

GMT; path=/;ETag: dd148f359cd88aa7a1ed2e4d5802b1dDate: Mon, 14 Apr 2008 16:21:05 GMTContent-Length: 12203Content-Encoding: gzip

•Message content is HTML of MTV’s web page