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Chabot College ELEC 99.08 Ports (Layer 4)
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TCP/IP Protocol Suite (simplified)
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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
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Layer 3 - IP, ARP, ICMP
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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)
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Layer 4 - Transport Layer
Layer 4 uses port numbers to address this limitation. 4
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Port Numbers Layer 4 uses port numbers to direct data to/from correct upper layer application.
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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.
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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.
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Well-Known Port Numbers
Well-known port numbers are defined in RFC 1700. Port 80 is web/ www (TCP).
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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:
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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
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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.
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Destination Port 80 - HTTP
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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.
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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.
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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
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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:
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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:
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