If i put the TTL as 0, will it never leave my broadcast domain? If I wanna send something to someone in my broadcast domain, can the ttl be 0 or will it still be dropped? Or should it be 1?
2 Answers
Packets with TTL 0 will be dropped at the interface.
Link-local protocols, like multicast (by default), use a TTL of 1 to be confined to the broadcast domain.
-
Multicast is not by default link-local. Some groups are, but the protocol itself is not. (multicast routing is a thing.)– RickyCommented Jan 15 at 12:24
-
Yes multicast routing is a thing, but part of that increases the TTL, which from the sender is normally
1. That is something that gets people when first trying to route multicast traffic. The multicast router sets the TTL to somethng larger than1.– Ron Maupin ♦Commented Jan 15 at 13:30 -
MR does not change (reset) the TTL. The user / application must set the TTL appropriately. A router is supposed to drop link-local group traffic with a TTL != 1. Eg. if you want multicast NTP to span your network, you have to set the TTL to something other than 1.– RickyCommented Jan 15 at 14:17
See RFC 791:
If the time to live reaches zero before the internet datagram reaches its destination, the internet datagram is destroyed.
Accordingly, a packet with TTL=0 can only be processed locally but cannot be sent anywhere any more. TTL=1 guarantees a packet cannot be forwarded outside the local subnet.
Most implementations decrease TTL on reception and drop the packet before it is sent from any interface.
