User Datagram Protocol (UDP)
LESSON 14

Outline:
User Datagram Protocol (UDP)

Identifying Multiple Destinations within a Host:

• Software and communication protocols provide a mechanism that distinguishes among multiple destinations within a given host.
• Often, this mechanism is used to support transport services for multiple applications.
• The ultimate destination in a host is not a process, but a software Port.
• Software Ports:
  • A set of abstract destinations points called protocol ports are identified by positive integer values and controlled by the local operating system.
• Ports are usually allocated by an operating system to an application in the form of memory blocks, accessed by specific data structures..
• Packets arriving from the IP layer, destined for a particular port, are enqueued within a buffer.
• Protocol ports also use outgoing queues to store messages being sent from an application program.
• To send and receive, we need to know the source and destination port numbers. Both are always sent in each message.

User Datagram Protocol (UDP):

• User Datagram Protocol (UDP):
  • A connectionless, unreliable delivery service using the IP protocol for its message transport.
  • UDP distinguishes among multiple destinations in a host, but sends no acknowledgments (ACKs).
• Applications using UDP must handle all problems to ensure reliable transfer. They must deal with:
  • Duplicate messages.
  • Lost messages.
  • Delayed and Out-of-Order delivery.
• UDP is good for delay-sensitive traffic such as video and voice since no acknowledgment or retransmission features are used.

UDP Message Format:

• UDP messages (called user datagrams) have two distinct parts:
  • UDP Headers
  • UDP Data area
• The header is always a fixed size of 4 octets, which specifies (in part) the source and destination protocol ports.
  

• SOURCE PORT (16-bits):
  • UDP protocol port number which specifies where replies should be sent (optional = 0).
• DESTINATION PORT (16-bits):
  • Required protocol port number to demultiplex datagrams among waiting processes.
• MESSAGE LENGTH (16-bits):
  • The count of octets in the UDP datagram, which includes the header and data areas. Minimum length = 8 (length of only the header).
• CHECKSUM (16-bits):
  • The optional checksum is usually used since IP does not compute any checksum on carried data. Set to zero if not used.

UDP Pseudo-Header:

• UDP Pseudo-Header:
  • Used to verify that the UDP datagram has reached it’s correct destination (since UDP headers only use port numbers).
• A checksum is computed over the pseudo-header, UDP- header, and the data area before sending the user datagram.
  

• The twelve octets of the pseudo-header include the fields:
  • Source IP address (4-octets).
  • Destination IP address (4-octets).
  • Padding field filled with zeros (1-octet).
  • Protocol Type (1-octet), (UDP = 17).
  • UDP Length (2-octets)
    • Length of the UDP datagram without the pseudo-header.
• These values are extracted from the IP datagram header, then the checksum is recomputed (using this pseudo-header).
• ? Is there a problem with this scenario ?

Conceptual Layering:

UDP Encapsulation:

• UDP Encapsulation - [lowest layer to higher layers] involves stripping off the headers at each appropriate level.
• Outermost header- lowest layer protocol.
• Innermost header - highest protocol layer.
  

• Important distinction:
  • IP Layer - transferring of data between a pair of hosts on the Internet (only responsibility).
  • UDP Layer - differentiating among multiple sources or destinations within one host only (same for TCP as well).

UDP / IP Integration:

• UDP is tightly integrated with the IP protocol:
  • This is practical.
    • Produces greater efficiency.
• However, the tight integration violates our layering abstraction and the independents of functionality that each layer provides.
• ? How does it violate the layering abstraction?
  • The UDP layer encapsulates UDP messages within an IP datagram.
  • It fills in the source and destination IP header fields.
  • It computes the UDP checksum.
  • It then passes the IP datagram to the IP layer, which fills in the rest of the header fields.

Multiplexing, Demultiplexing, and Ports:

• Applications negotiate with the operating system to obtain a protocol port and the port number.
• UDP multiplexes application output from the various protocol ports.
• It also demultiplexes input from the network connection, to the appropriate receiving ports.
  

• Receiving packets to send to a port are:
  • Normally placed in the active port's queue.
  • Dropped if the port's queue is full.
• If the destination port is currently not in use, an ICMP port unreachable error message is sent and the packet is dropped.

UDP Input and Output:

• UDP does not have complex input and output modules since it is connectionless. It does however have separate modules to handle the processing required in each, as well as input and output queues.
  
  

UDP Port Numbers:

• ? How do we assign protocol port numbers ?
• Two fundamental approaches:
  • Dynamic Port Binding:
    • Large integer values are used to dynamically assign a protocol port number (network software performs the assignment).
  • Universal Assignment:
    • Everyone agrees on allowing a central authority to assign and publish port numbers (software is built around well-known port assignments).
• TCP/IP takes a hybrid approach to port assignment where it:
  • Assigns lower-valued port numbers to priori applications
  • Allows the higher-valued port numbersto be used by local sites or application programs.
• ? How do we know which port to use when requesting an application service?
  • Send a port-service request message to the destination.
  • It responds by providing the appropriate protocol port in which to send packets to.

TCP/IP Protocol Tracing: ARP

• Checking the ARP Cache on homer.njit.edu using:
  • ARP -a

  Device	IP Address	Mask	Physical Address
  be0	vienna	255.255.255.255	08:00:20:00:2b:97
  be0	empire	255.255.255.255	08:00:20:73:09:64
  be0	picard	255.255.255.255	08:00:20:72:a5:e6
  be0	ocean	255.255.255.255	08:00:20:73:35:c7
  le0	earth-gw	255.255.255.255	08:00:20:00:f5:4b
  be0	rtlab11	255.255.255.255	08:00:20:71:2a:a5
  le0	itc_cis-gw	255.255.255.255	00:00:81:11:2e:19
  be0	homer	255.255.255.255	08:00:20:71:3a:43
  le0	inis-gw	255.255.255.255	aa:00:04:00:45:04
  le0	newark-gw	255.255.255.255	08:00:20:01:6f:3b
  le0	BASE-ADDRESS.MCAST.NET	240.0.0.0	01:00:5e:00:00:00
  be0	BASE-ADDRESS.MCAST.NET	240.0.0.0	01:00:5e:00:00:00

TCP/IP Protocol Tracing: IP

• Connecting to CIS456-HTML from Daniel Tabor's home-page on hertz.njit.edu:
  
• Domain Name to IP Address Resolution:
  

  0	IP 127.0.0.1	->128.235.251.10	len 69	prot 17
  1	IP 128.235.251.10	->127.0.0.1	len 131	prot 17
  0	IP 127.0.0.1	->128.235.251.10	len 60	prot 17
  1	IP 128.235.251.10	->127.0.0.1	len 281	prot 17

• Document Transfer from CIS456-HTML:
  

  0	IP 127.0.0.1	->128.235.35.70	len 44	prot 6
  1	IP 128.235.35.70	->127.0.0.1	len 40	prot 6
  0	IP 127.0.0.1	->128.235.35.70	len 40	prot 6
  0	IP 127.0.0.1	->128.235.35.70	len 321	prot 6
  1	IP 128.235.35.70	->127.0.0.1	len 40	prot 6
  1	IP 128.235.35.70	->127.0.0.1	len 122	prot 6
  0	IP 127.0.0.1	->128.235.35.70	len 40	prot 6
  1	IP 128.235.35.70	->127.0.0.1	len 40	prot 6
  0	IP 127.0.0.1	->128.235.35.70	len 40	prot 6

• Copyright/Audit Image Loading and Resolution:
  

  0	IP 127.0.0.1	->204.217.174.42	len 44	prot 6
  1	IP 204.217.174.42	->127.0.0.1	len 40	prot 6
  0	IP 127.0.0.1	->204.217.174.42	len 40	prot 6
  0	IP 127.0.0.1	->204.217.174.42	len 319	prot 6
  1	IP 204.217.174.42	->127.0.0.1	len 40	prot 6
  1	IP 204.217.174.42	->127.0.0.1	len 315	prot 6
  0	IP 127.0.0.1	->204.217.174.42	len 40	prot 6
  1	IP 204.217.174.42	->127.0.0.1	len 40	prot 6
  0	IP 127.0.0.1	->204.217.174.42	len 40	prot 6

TCP/IP Protocol Tracing: DNS

• DNS to www.internet-audit.com:

  - id=0019	fl=8583
  - Questions
  - www.internet-audit.com.njit.edu	A	1
  - Authorities
  - njit.edu	SOA		1	86400	NJITGW.njit.edu
  - HOSTMASTER.NJITGW.njit.edu
  - ;size = 111
  - id=001A	fl=8180
  - Questions
  - www.internet-audit.com	A		1
  - Answers
  - www.internet-audit.com	CNAME		internet-audit.com
  - internet-audit.com		40980		204.217.174.42
  - Authorities
  - INTERNET-AUDIT.com		40128		MINET.COM
  - INTERNET-AUDIT.com		40128		INTERWORKS.COM
  - Additional records
  - MINET.COM			2592000		204.217.174.10
  - INTERWORKS.COM		36000		204.217.174.5
  - Hostent:
  - h_name = www.internet-audit.com
  - h_addrtype = 2
  - h_length  = 4
  - Address = 204.217.174.42
  - id=001B	fl=8583
  - Questions
  - www.minet.com.njit.edu	A	1
  - Authorities
  - njit.edu			86400	NJITGW.NJIT.EDU
  - HOSTMASTER.NJITGW.NJIT.EDU
  - ; size = 110
  - id=001C	fl=8580
  - Questions
  - www.minet.com		A		1
  - Answers
  - www.minet.com		CNAME		nt1.minet.com
  - nt1.minet.com		2592000		204.217.174.35
  - ; size = 74
  - Hostent:
  - h_name = www.minet.com
  - h_addrtype = 2
  - h_length  = 4
  - Address = 204.217.174.35
  

Last Modification: (Sunday, August 25, 1996)

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1996 Illusion Industries Inc.