User Datagram Protocol (UDP)







User Datagram Protocol 
(UDP)

 





UDP is defined in RFC 768. UDP is used by applications that only need
a connectionless, best effort transport service. There is not much to UDP
Figure 16 illustrates what is contained within a UDP packet.




Figure 16: UDP Packet


In order to use UDP, the application must supply the IP address and
port number of the destination application. A port is an abstraction to
allow transport protocols like UDP and TCP the capability of handling communications
between multiple hosts. It allows a communication to be uniquely identified.
Ports are identified by a positive integer. Figure 17 lists a few
of the commonly used UDP ports.
Port KEYWORD Description
53 DOMAIN Domain Name Server
43 NICNAME Whois
69 TFTP Trivial File Transfer
137 NETBIOS-NS NetBIOS Name Service
138 NETBIOS-DGM NetBIOS DataGrams
139 NETBIOS-SSN NetBIOS Session
Figure 17: A Few Commonly Used UDP Ports

UDP demultiplexes the ports, keeping track of which data is available
for or sent to a port.

Although the RFC allows implementations of UDP modules that allows for
fragmentation, the LM TCP/IP does not. Since LM TCP/IP does not allow fragmentation
of IP packets, only reassembly, the maximum allowable UDP packet is 1460
bytes. Therefore, although the LM TCP/IP will not allow outgoing UDP packets
to be greater than 1460 bytes, it can handle larger incoming UDP packets.
Figure 18 shows sniffer output of a UDP packet.


SUMMARY Delta T Destination Source Summary
3 1.8727 Broadcast U-B 0F71B1 DLC Ethertype=0800, size=119 bytes
IP D=[11.1.255.255] S=[11.1.1.69] LEN=85 ID=0
UDP D=137 S=137 LEN=85
DNS C ID=12802 OP=REGISTER NAME=WKST1054A<00>.NETX

DLC: DLC Header -
DLC:
DLC: Frame 3 arrived at 18:19:33.9606; frame size is 119 (0077
hex) bytes.
DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast
DLC: Source = Station U-B 0F71B1
DLC: Ethertype = 0800 (IP)
DLC:
IP: IP Header -
IP:
IP: Version = 4, header length = 20 bytes
IP: Type of service = 00
IP: 000. .... = routine
IP: ...0 .... = normal delay
IP: .... 0... = normal throughput
IP: .... .0.. = normal reliability
IP: Total length = 105 bytes
IP: Identification = 0
IP: Flags = 0X
IP: .0.. .... = may fragment
IP: ..0. .... = last fragment
IP: Fragment offset = 0 bytes
IP: Time to live = 30 seconds/hops
IP: Protocol = 17 (UDP)
IP: Header checksum = 853E (correct)
IP: Source address = [11.1.1.69]
IP: Destination address = [11.1.255.255]
IP: No options
IP:
UDP: UDP Header -
UDP:
UDP: Source port = 137 (NetBIOS)
UDP: Destination port = 137
UDP: Length = 85
UDP: Checksum = 608F (correct)
UDP:
DNS: Internet Domain Name Service header -
DNS:
DNS: ID = 12802
DNS: Flags = 29
DNS: 0... .... = Command
DNS: .010 1... = Registration
DNS: .... ..0. = Not truncated
DNS: .... ...1 = Recursion desired
DNS: Flags = 1X
DNS: ...1 .... = Broadcast packet
DNS: Question count = 1, Answer count = 0
DNS: Authority count = 0, Additional record count = 1
DNS:
DNS: Question section:
DNS: Name = WKST1054A<00>.NETX
DNS: Type = NetBIOS name service (NetBIOS name,32)
DNS: Class = Internet (IN,1)
DNS: Additional record section:
DNS: Name = WKST1054A<00>.NETX
DNS: Type = NetBIOS name service (NetBIOS name,32)
DNS: Class = Internet (IN,1)
DNS: Time-to-live = 0 (seconds)
DNS: Node flags = 00
DNS: 0... .... = Unique NetBIOS name
DNS: .00. .... = B-type node
DNS: Node address = [11.1.1.69]
DNS:
DNS: [Normal end of "Internet Domain Name Service header".]
DNS:

ADDR HEX ASCII
0000 FF FF FF FF FF FF 00 DD 01 0F 71 B1 08 00 45 00 ..........q...E.
0010 00 69 00 00 00 00 1E 11 85 3E 0B 01 01 45 0B 01 .i.......>...E..
0020 FF FF 00 89 00 89 00 55 60 8F 32 02 29 10 00 01 .......U`.2.)...
0030 00 00 00 00 00 01 20 46 48 45 4C 46 44 46 45 44 ...... FHELFDFED
0040 42 44 41 44 46 44 45 45 42 43 41 43 41 43 41 43 BDADFDEEBCACACAC
0050 41 43 41 43 41 41 41 08 4E 45 54 58 54 45 41 4D ACACAAA.NETXTEAM
0060 00 00 20 00 01 C0 0C 00 20 00 01 00 00 00 00 00 .. ..... .......
0070 06 00 00 0B 01 01 45 ......E

Figure 18: Sniffer Output of a UDP Packet

The UDP information shows that this packet is using the well known port
for the RFC NetBIOS name service.

UDP Parameters in PROTOCOL.INI

There are two parameters that can be set in the [tcp_xif] section
that deal with UDP. The udp_q_per_user parameter determines the
maximum number of UDP receive packets that will be queued per port. The
default is 3 for DOS machines and 16 for OS/2 machines. The range for DOS
machines is 1 to 10, for OS/2 machines it is 1 to 64. The udp_q_total
parameter defines the total number of UDP receive packets that can be queued
for a host. The default is 20 for DOS machines, 64 for OS/2 machines. The
range for DOS machines is 1 to 100, and for OS/2 machines 1 to 256.

The larger limits were placed into the OS/2 version to ensure the server
had sufficient space to handle NetBIOS name queries. NetBIOS name queries
use UDP as their transport mechanism in TCP/IP. The details of NetBIOS
over TCP/IP will be described later.

For NetBIOS applications like LAN Manager and the majority of socket
based applications that use UDP, the default settings for udp_q_per_user
and udp_q_total are sufficient. However, applications that make
heavy usage of UDP may require adjustment of these parameters.




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