Chabot College

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)

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Layer 4 - Transport Layer

• Layer 4 uses port numbers to address this
  limitation.

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 socketEach 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).

1 - 255
Used for standard public applications
Used by companies for specific apps
256 - 1023
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

http//banner-web.clpccd.cc.ca.us7000
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Port Numbers in Action
Client
LAYER 4
I want 207.35.52.4, but which service do I want?
207.35.52.4
LAYER 3
00-0c-4f-32-ab-41
LAYER 2
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Port Numbers in Action
Client
TCP port 80
207.35.52.4
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
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
DESTINATION
SOURCE
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
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An Example TCP Header Sync Request

• TCP - Transport Control Protocol
• Source Port 2352
• Destination Port 80 World Wide Web HTTP
• Sequence Number 102274726
• Ack Number 0
• Offset 7
• Reserved 000000
• Code 000010
• Sync Sequence
• Window 8192
• Checksum 0x6711
• Urgent Pointer 0
• TCP Options

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An Example TCP Header Acknowlegement

• TCP - Transport Control Protocol
• Source Port 80 World Wide Web HTTP
• Destination Port 2352
• Sequence Number 2913145779
• Ack Number 102274727
• Offset 6
• Reserved 000000
• Code 010010
• Ack is valid
• Sync Sequence
• Window 17520
• Checksum 0x9640
• Urgent Pointer 0
• TCP Options