User Datagram Protocol (UDP)
LESSON 14
(October 20, 1995)
Daniel Z. Tabor Jr.
New Jersey Institute of Technology
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:
| Application Layer |
| User Datagram Protocol (UDP) |
| Internet Protocol (IP) |
| Network Interface and Hardware |
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:
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 numbers to 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:
| 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
All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr.
Copyright ©1995 Illusion Industries Inc.
