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We know that UDP does supports multicasting and broadcasting. My question is which layer "UDP supports multicasting and broadcasting"? Is it transport layer or application layer? We know that in application layer multicasting possible and it's correspondence multicast address also exists. But how is it possible application layer broadcasting?

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  • Are you referring to the fundamental ideas of multicast/broadcast or asking about how they are handled in TCP/IP, i.e. IP multicast or IP broadcast? Apr 20 at 11:19

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Multicasting and broadcasting are at lower layers than UDP. UDP is used as a transport protocol for multicast and broadcast because it is connectionless. You could use any connectionless transport protocol, but NAPT has limited the IPv4 Internet to TCP and UDP, but TCP is connection-oriented, so it will not work with multicast or broadcast.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Ron Maupin
    Apr 20 at 12:31
  • "NAPT has limited the IPv4 Internet to TCP and UDP "---- why we use Nat router concepts here that restricted to UDP, but when we use ospf router don't use UDP? You mean when we use ospf application don't use NAPT as well as don't use UDP because it has personal l4 protocol. But we use other application that doesn't use ospf that uses NAPT that uses UDP as transport layer protocol?Am I correct?
    – Alok Maity
    Apr 20 at 15:15
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    Because of the way NAPT works, you must have a table for each transport protocol. TCP and UDP were chosen, and it forces programmers who want their applications to work on the public Internet to only use those protocols. RFC 3022, Traditional IP Network Address Translator (Traditional NAT) says "Sessions other than TCP, UDP and ICMP query type are simply not permitted from local nodes, serviced by a NAPT router." There are application, such as OSPF, that are not intended to run on the public Internet that use other protocols.
    – Ron Maupin
    Apr 20 at 15:23
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    @AlokMaity, please, please quit going so off course with questions in comments. If you have other questions, start a new question.
    – Ron Maupin
    Apr 20 at 15:25
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(a slightly different approach to an answer...)

UDP does supports multicasting and broadcasting.

UDP doesn't actually. UDP is compatible with multicast and broadcast addressing (group addressing) due to it being connectionless. There's nothing in UDP that actively supports multicast/broadcast.

In contrast, TCP is not compatible with group addressing in general: TCP requires a connection to be established before transmitting any data. That isn't possible with multicast or broadcast. Trying to do so with a group address fails because there can be no reply ever from the group address but from its individual members (if at all), not matching the initiator's connecting attempt.

Note that broadcast is only possible with IPv4, there's no broadcast functionality in IPv6.

which layer "UDP supports multicasting and broadcasting"?

Multicasting and broadcasting always happen at the network layer. You can see that by looking at the destination IP address that belongs to the network layer.

UDP actually has no idea that it is used with broad/multicast. It just sees some destination IP address, without understanding what that IP address means or how it works. An IP address is visible to higher layers, but it's only ever interpreted and used by the IP network layer.

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  • "Multicasting and broadcasting always happen at the network layer. "--- network layer and DLL isn't both involved?
    – Alok Maity
    Apr 21 at 7:46
  • Yes, you're correct - as with all actual data transport, the next lower layer needs to be used. An IP multicast or broadcast address needs to be translated to an appropriate data link layer (L2) address, and the actual frame multiplication (if required) is happening in L2. Mind you, the actual bit movement happens in the physical layer only.
    – Zac67
    Apr 21 at 9:19
  • frame multiplication means not understand?
    – Alok Maity
    Apr 21 at 9:51
  • I used frame multiplication to refer to the multiple forwarding process of broadcast/multicast frames across switches (one copy for each port but the one the frame was received from).
    – Zac67
    Apr 21 at 10:01

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