why can't applications directly access IP and perform operation without UDP(no reliable service)?

Answer 1

Not sure what you're asking. Most apps use TCP to ensure packet delivery. It's more efficient for the app since the protocol stack provides for verification of delivery in order.

UDP is used where assured delivery isn't required. A good example is DNS. If the DNS client doesn't get a response from the server after the timeout interval it simply resends the request. Most DNS servers are too busy to be able to provide assured delivery without overloading. Many media apps use UDP and provide delivery assurance and packet sequencing within the app itself to take the load off of the protocol stack due to the volumes of data involved.

Answer 2

The TCP/IP protocol is considered a five layer model. IP is in the third layer, the Network/Internet layer, while UDP is the Transport layer. The five layers are:

5: Application layer: Which includes: DHCP, DNS, FTP, HTTP, IMAP4, IRC, NNTP, XXMP, SOAP, POP3, TELNET, and more.

4. Transport layer. Which includes: TCP, UDP, DCCP, SCTP, RTP, RSVP, and more.

3. Network/Internet layer. Which includes: IP, (IPv4, and IPv6), OSPF, IPsec, ARP, RIP, ICMP, and more.

2. Data Link Layer. Which includes: 802.11, 802.16, WiFi, WiMAX, ATM, DTM, Token Ring, Ethernet, Frame Relay, PPP, PPTP, ISDN, and more.

1. Physical layer. Which includes. Ethernet Physical Layer, Modems, PCL, SONET/CDH, G.709, Optical Fiber, Coaxial Cable, Twisted Pair, and more.

Each layer lays on top of the other, in other words, what occurs in one, relies on the items in each next layer to exist in order to create a full connection with data sent and received.

As you can see, the Transport layer, which included UDP, sits between the Application layer, the fifth layer, and the Network/Internet Layer, which includes IP addresses.

Due to this layering of the TCP/IP Model, the application MUST go through one of the Protocols in the layer below it before it can be router up through the other layers.

This is the reason why applications, including regular applications on a system, are incapable of accessing IP and performing an operation without UDP or another protocol in the Transport Layer.

UDP--Universal Datagram Protocol.

UDP is one of the core protocols of the Internet Protocol Suite. Using UDP, programs on networked computers can send short messages, known as "datagrams", (using Datagram Sockets) to one another.

You asked why application can't directly access IP and perform operations with UDP, not for a synopsis of what UDP is and what is is used for, so I won't go into that area.

But as you can see, each layer has its own distinct role, and each must be in effect for the others to complete a full network communication sequence.

Each layer has several components which can be used to complete a sequence of connection. UDP can use any of the Network Layer protocols, and the Network Protocol can use any of the Data Link Layer Protocols, and the Data Link Layer Protocol can use any of the Physical Layer Protocols, in completing the connection and sending data over the connection.

However, one layer can not skip a protocol in the layer directly above or below that layer. So, when an application attempts to initiate a connection using IP, which is in the Network/INternet Layer, it MUST use one of the protocols in the Transport Layer, which includes the option of using UDP.

Hope this helps you see why applications can't perform a connection operation without using UDP, (or another Transport Layer protocol).