Norton Internet Security 2009 - Firewall Actitities

I am using Norton Internet Security 2009. My computer is on a LAN. I realised that under the recent history, there is always at least one firewall activity alert in every few seconds. There are two common actitivities that I observed with different IP address. Here they are:

1. Rule "Default Block UPnP Discovery" stealthed (IP address, Port ssdp(1900)). Inbound packet.

2. Rule "Default Block Web Service Discovery" blocked (IP address, Port ssdp(3702)). Inbound packet.

 

Both always indicated as risk level "Low", status "Detected", and "No Action Required. What do these alert trying to say? Are these security threads or something? Thanks..