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A discussion on the security stackexchange https://security.stackexchange.com/a/145674/37973 has raised the question of unidirectional transmission over Ethernet with the return path physically removed. Unidirectional reception with the transmitting wires unconnected certainly is possible, e.g. in simple Ethernet wiretapping tools like https://greatscottgadgets.com/throwingstar/

Would transmitting raw Ethernet frames be possible in this manner?

An answer to an earlier question (Ethernet Frames on Unidirectional Link) states that a response link is required for Ethernet autonegotiation, however older standards such as 10BASE-T should work (with a proper crossover cable) even in absence of autonegotiation. It's clear that the sender can't know if the message was received, but that can be handled in various ways, the question is if standard Ethernet network cards will send the required data over the physical layer in absence of any response/negotiation/handshake/etc.

  • The most common ethernet standard at the moment is 1000Base-T and that uses all four pairs, so messing about with cables is not possible. Solutions using taps as discussed by the other guys are possible. – marctxk Dec 19 '16 at 11:37
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 15 '17 at 16:46
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Yes - it is possible and from a limited perspective it's not even uncommon in certain circumstances. A good example is the use-case with Ethernet taps where transmitted frames are passively copied to an analysis box. The breakout is certainly transmit only.

I recall seeing something similar to what you're describing in an ultra high volume Netflow collection situation where routers with optical interfaces would split a transmit from a common dummy source and drop it into the receive ports of multiple routers. It was enough to keep the link up while the corresponding transmit links (..carrying solely Netflow UDP packets) sent their payloads off into the ether.

There also used to be an accommodation in IOS for simplex Ethernet drops (usually from microwave) via the command "transmit-interface," which took an additional interface as a corresponding receive only. I think the point there was to have the transmit-only throttle based on flow control (..or collisions) on the corresponding receive port, but it's beyond obscure at this point.

There are a bunch of caveats - most fairly obvious. Any kind of link negotiation, flow control or QoS is completely out of the question. Speed and duplex need to be locked and only connectionless protocols (UDP, ICMP, etc) make any sense. Multicast and broadcast would be fine but unicast would likely require hard-coded MAC addresses (...this is how the Netflow situation mentioned above was managed) and if it's being dropped on a switch there very well could be some strange behaviors at both L2 and L3 as far as timers are concerned as the target address likely isn't sending any responses to build CAM and ARP tables.
This, again, could be mitigated through hard-coding if the platform supports it.

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Ethernet uses CSMA-CD, where the CD stands for Carrier Detection.

You can sometimes see this in a device, e.g. Cisco shows something like GigabitEthernet0/0 is up, line protocol is down because there is no carrier. The interface is unusable in this situation.

There are (expensive) devices sold for unidirectional ethernet as security devices.

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