CSCE 313 Lecture 20
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Applications
Standard Applications are well-documented and adopted as official TCP/IP suite:
- telnet
- ftp
- ...
Sockets are file descriptors: open, close, read, and write.
HTTP
telnet www.tamu.edu 80 GET /
SMTP
- Mail User Agent (MUA) is ready to send message
- MUA connects to SMTP server
- MTA sends message to recipient
telnet servername 25 HELO <your domain name> MAIL FROM: <your email address> RCPT TO: <recipient address> DATA <type email here> . QUIT
Transport Layer
- TCP - reliable
- UDP - unreliable
Packets
Information is added incrementally at each layer
- Application Layer
- data (44 to 1460 bytes)
- Transport Layer
- TCP/UDP header (20 bytes)
- data
- Network Layer
- IP header (20 bytes)
- TCP/UDP header
- data
- Data Link Layer
- frame header (22 bytes)
- IP header
- TCP/UDP header
- data
- frame trailer (4 bytes)
Over ethernet data link layer, each computer has a unique 48-bit MAC address. Ethernet Packet structure over two computers on the same network:
- 8 byte preamble
- 6 byte destination MAC address
- 6 byte source MAC address
- 2 byte type
- 64-1500 bytes data
- 4 byte CRC
Address Resolution Protocol (ARP)
Maps 32-bit IP addresses to 48-bit MAC address. Destination responds with "here I am" (unreliable since anyone can respond). ARP results are cached.
Use arp command to view the ARP cache for that machine
Internet Protocol (IP)
Connectionless datagram delivery service. Routes data thorugh intermediate networks and computers
Wireshark can capture network packets. This is the best way to learn IP. tcpdump is an equivalent Linux tool.
Header contains protocol, source and destination IP addresses, TTL, etc.
Packets need to be routed (Router). Each route is a "hop". Use traceroute Linux command to record the hops between client and server.