1. Chabot College
ELEC 99.08
Ports (Layer 4)

2. TCP/IP Protocol Suite (simplified)

3. Layer 3 - Network Layer
In Chapters 10 & 11, we covered Layer 3 protocols:
Internet Protocol (IP)
Used for addressing
Address Resolution Protocol (ARP)
Used to relate IP and MAC addresses
Internet Control Message Protocol (ICMP)
Used for ping and traceroute commands

4. Layer 3 - IP, ARP, ICMP

5. A Key Limitation of Layer 3
No way to keep track of multiple conversations between between pairs of hosts running multiple applications. (e.g. mail & web)

6. Layer 4 - Transport Layer
Layer 4 uses port numbers to address this limitation. 4

7. Port Numbers
Layer 4 uses port numbers to direct data to/from correct upper layer application.

8. Port Numbers
Used to identify the application that Layer 4 should “hand off to.”
Like a “hole in the ceiling” that Layer 4 “throws” the data up into…
Applications “listen” on a particular port number.
IP address + port number = socket Each socket must be unique, and is another type of address.

9. TCP & UDP port numbers
TCP/UDP use 16 bits to represent a port number (65,536 possibilities).
Used for standard public applications
Used by companies for specific apps
1024 & up
Unregulated, but many have been “registered” by companies for specific uses. Also, this range is used as dynamically assigned “source port” numbers by TCP.

10. Well-Known Port Numbers
Well-known port numbers are defined in RFC 1700.
Port 80 is web/ www (TCP).

11. Non-Standard Port Numbers
A host can run its services on any port, not just on the well-known ones.
To direct traffic to a non-standard port, specify the port number after the IP address:

12. I want 207.35.52.4, but which service do I want?
Port Numbers in Action
Client
Web server
File server (FTP)
(SMTP) 25 21 80
Service (listening)
port
LAYER 4
I want , but which service do I want?
LAYER 3
LAYER 2
00-0c-4f-32-ab-41

13. Port Numbers in Action Client
TCP port 80
00-0c-4f-32-ab-41
The client addresses the message with the port # of the service it wants.

14. Destination Port 80 - HTTP

15. Port Numbers in Action Client
00-0c-4f-32-ab-41
TCP port 80
00-0c-4f-32-ab-41
TCP port 25
00-0c-4f-32-ab-41
TCP port 21
The client can maintain multiple conversations with the same host, but using different applications.

16. Port Numbers in Action Web Server Client SOURCE DESTINATION
00-4b c6-9f
TCP port 1250
00-0c-4f-32-ab-41
TCP port 80
SOURCE
DESTINATION
Note that client applications, like web browsers, dynamically “grab” a source port number above 1023 when needed.

17. Port Numbers in Action Web Server Client SOURCE DESTINATION
00-0c-4f-32-ab-41
TCP port 80
00-4b c6-9f
TCP port 1250
SOURCE
DESTINATION

18. An Example TCP Header : Sync Request
TCP - Transport Control Protocol
Source Port:
Destination Port: 80 World Wide Web HTTP
Sequence Number:
Ack Number:
Offset:
Reserved: %000000
Code: %000010
Sync Sequence
Window:
Checksum: x6711
Urgent Pointer: 0
TCP Options:

19. An Example TCP Header: Acknowlegement
TCP - Transport Control Protocol
Source Port: World Wide Web HTTP
Destination Port: 2352
Sequence Number:
Ack Number:
Offset:
Reserved: %000000
Code: %010010
Ack is valid
Sync Sequence
Window:
Checksum: x9640
Urgent Pointer: 0
TCP Options: