Multiplexing

1
Multiplexing
2
Multiplexing

• IP packets can carry different things in their
  data fields
• TCP segments
• UDP datagrams
• ICMP supervisory messages (later)
• RIP messages (later)

IP Data Field
IP Header
3
Multiplexing

• We say that IP can multiplex (mix) different
  types of traffic in a stream of IP packets

Single IP Packet Carrying UDP Datagram
UDP
IP-H
TCP
IP-H
UDP
IP-H
ICMP
IP-H
Stream of Arriving or Outgoing IP Packets
4
Multiplexing

• IP process must pass contents of arriving IP
  packets to the correct process for subsequent
  handling

TCP
UDP
UDP
IP-H
IP
ICMP
Arriving Packets
IP Process
5
Multiplexing

• IP process must also accept messages from
  multiple processes and multiplex them on an
  outgoing stream

TCP
UDP
UDP
IP-H
IP
ICMP
Outgoing Packets
IP Process
6
Multiplexing

• Need a Way for Receiving IP Process to Know What
  is in the Data Field
• So it can pass the contents to the appropriate
  process

IP Data Field
IP Header
7
Multiplexing

• IP Header has an 8-bit Protocol field
• Identifies the contents of the data field
• 1ICMP (later), 8TCP, 17UDP, etc.

Total Length in bytes (16)
Version (4)
Hdr Len (4)
TOS (8)
Indication (16 bits)
Flags (3)
Fragment Offset (13)
Time to Live (8)
Header Checksum (16)
Protocol (8)
Source IP Address
Destination IP Address

8
Multiplexing

• Other Messages have Analogous Fields
• Identify contents of data field
• TCP and UDP
• Have Port number fields
• Identify the application process (80HTTP)

Source Port (16)
Destination Port (16)
Sequence Number (32 bits)
Acknowledgement Number (32 bits)
Hdr Len (4)
Flags (6)
Window Size (16)
Reserved (6)
9
Multiplexing

• Other Messages have Analogous Fields
• Identify contents of data field
• PPP
• Protocol field identifies contents of information
  field as IP, IPX, a supervisory message, etc.

Flag
Addr
Ctrl
Prot
Info
CRC
Flag