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I am reading about SSDP from here and here. I understand that its purpose is to advertise and discover network services.

My question: Is the broadcast range of a SSDP packet local (e.g.: in a WLAN) or does it travel outside the local network?

For instance, if a device in a WLAN braodcasts a SSDP message, is it limited to the WLAN or does it go past the wireless access point?

Edit: I cannot seem to find the right tags, sorry about that!

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  • Unfortunately, questions about protocols above OSI layer-4 are explicitly off-topic here. You could try to ask this question on Server Fault for a business network. – Ron Maupin Jul 19 '19 at 14:36
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SSDP is multicast, being sent to address 239.255.255.250.

Multicast does not cross any subnet border without help from a router. If the given Wifi SSID is bridged into a wired network (or a VLAN thereof), these multicasts will reach the wired part of the (V)LAN.

A router must be explicitely enabled to forward multicast - but that's a huge topic in itself beyond this question.

On a sideline: The related protocol suite from zeroconf networking (a.k.a. Rendevous/Bonjour, Avahi) also uses multicast, but in a fashion which makes it ineffective to route. They set TTL 255 in the packet's IP header, which a router decrements to 254, but zeroconf libraries on the end systems refuse to accept these packets if TTL is not 255. Some routers and L3 switches support a proxy feature for zeroconf, acting as application layer relay/forwarder for zeroconf between subnets.

SSDP seems to be using TTL=4 in the IP header (although the Internet finds SSDP sample captures with TTL=1); so if you enabled routers to forward multicast between subnets, SSDP messages might be able cross 3 routing hops.

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  • I see. One more thing. In a WLAN, can all the other devices in the multicast range (or even a wireless snooping device that's in monitoring mode) see the SSDP packets (since it is multicasted and hence wouldn't be encrypted)? – Nht_e0 Jul 19 '19 at 14:00
  • I dont' think Multicast is unencrypted on Wifi - it just runs at the lowest rate on the given AP - but WiFi in deep detail isn't really my home turf. About Multicast on Wifi, you may want to look at networkengineering.stackexchange.com/questions/36450/… – Marc 'netztier' Luethi Jul 19 '19 at 14:07

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