DNS-SD adds the ability to announce 'services' over mDNS or regular unicast DNS. It's also called Bonjour among other things, and it too uses multicast to let network devices discover each other.

mDNS with DNS-SD is an alternative system for doing pretty much the same thing as SSDP.

Any number of software running on your computer can be the source of the M-SEARCH, some OS'es like Windows will even send these themselves as a part of the built-in 'network discovery'. Only 'subscribers' to 239.255.255.250 will actually receive these messages (the OS network stack filters multicast messages), but as with all multicast packets, the packets themselves are sent to all network devices. SSDP is just the UPnP discovery mechanism that somebody thought should have its own name. 239.255.255.250 is a multicast address that is defined in the UPnP standard.

The SSDP traffic is just a general 'poll' for devices that matches the search.

Hence, they synchronize their 'view' of time by communicating numbers.You're observing two different things that are independent from each other. Then, the sender's view of time would be different from the receivers. Imagine if we didn't have a universal notion of time.

So your friend asks you to resend the letter at position 3 (this is the idea behind the expected acknowledgement number).Īs mentioned in the article, it may be just that ('c', 3) is taking longer to arrive and so in that case, the sender sends a duplicate message, but duplicates are typically dropped by the receiver.Ī helpful way to think about these numbers is that they synchronize the data so both parties have the same 'view' of it.

It's missing 'c' because it expects a continuous increase of numbers and 3 is missing. Now, suppose the friend gets ('b', 2), but then ('d', 4). The numbers are used in case the packets/messages arrive out of order. Imagine you want to send the letters of the alphabet to a friend over the Internet.

Amzn wplay tcp