Ethernet
Ethernet is a shared network, meaning that many different machines can connect to the same Ethernet and can communicate directly with each other. I’m going to assume that you’re using Ethernet as found in an average office or datacenter. Also, although Ethernet has been implemented over many different physical media, I’ll assume you’re working with CAT5 or better cable—today’s most popular choice. If you use some unusual media type, or your card supports multiple media, you might need to manually set your preferred media on your interface.
Protocol and Hardware
Ethernet is a broadcast protocol, which means that every packet you transmit can be sent to every host on the network (although most Ethernet hardware limits recipients). Either your network card or your device driver separates the data intended for your computer from the data meant for other computers. A section of Ethernet where all hosts can communicate directly with all other hosts, without involving a router, is called a collision domain or segment.
You connect Ethernet segments with hubs, which are hardware items that can physically connect many Ethernet hosts. Network hubs forward all received frames to all other network devices, and each host is responsible for filtering traffic. This is old-school Ethernet, which can be useful for debugging network issues.
Switches have largely supplanted hubs. Every Ethernet connection needs a unique identifier, called a MAC address (or sometimes an Ethernet address), which is a 48-bit number. Switches control the traffic sent to each host by filtering on the MAC and IP address of attached devices and (mostly) forwarding frames only to the devices they are meant for. Switching reduces the amount of traffic and load on each individual system by decreasing the amount of traffic each host must sort through.
On i386 and amd64 hardware, the MAC address is a property of the card. On some other platforms, such as SPARC, the MAC address is a property of the server itself. Both IPv4 and IPv6 use the MAC address to find other hosts on the local network.
IPv4 and ARP
When a system needs to transmit data to another IP-based host on the local Ethernet, it first broadcasts an Ethernet request asking, “Which MAC address is responsible for this IP address?” If a host responds, further data for that IP is transmitted to that MAC address. This process is handled by ARP.
Use arp(8) to view your system’s ARP table, which is the list of hosts that your system knows. Enter arp -a to show all of the MAC addresses and IPv4 hostnames your computer knows.
$ arp -a
fly.blackhelicopters.org (192.0.2.225) at 00:a0:c8:10:eb:82 on fxp0
caddis.blackhelicopters.org (192.0.2.226) at 00:16:36:c0:58:a5 on fxp0 static
treble.blackhelicopters.org (192.0.2.227) at 00:0c:42:5a:58:ae on fxp0
salmon.blackhelicopters.org (192.0.2.232) at (incomplete) on fxp0
Here, you see the three hosts on my Ethernet network that this host has communicated with. I have more hosts, but because this machine hasn’t spoken with them lately, they aren’t in the local ARP table.
If a MAC address shows up as incomplete, your machine has attempted to communicate with this host but cannot get its MAC address. In this example, I’ve tried to send data to the host salmon, but my computer can’t reach it. (Turning salmon back on would help.)
IPv6 and Neighbor Discovery
IPv6 hosts also use MAC addresses to find each other through ND, an IPv6 protocol introduced in the previous chapter. Interrogate your ND cache with ndp(8). The command-line flags used for ndp are intentionally similar to those for arp.
$ ndp -a
Neighbor Linklayer Address Netif Expire S Flags
2001:db8:0:12:20c:29ff:feb5:7565 0:c:29:b5:75:65 vic0 permanent R
2001:db8:0:12:5446:fbc:fca0:f2e9 0:c:29:b5:75:65 vic0 permanent R
…
fe80::20c:29ff:feb5:7565%vic0 0:c:29:b5:75:65 vic0 permanent R
fe80::20c:42ff:fe20:7f42%vic0 0:c:42:20:7f:42 vic0 11h20m47s S R
fe80::1%lo0 (incomplete) lo0 permanent R
Like the ARP cache, the ND cache shows an IPv6 address, a physical address, the interface, and other details for each host. You’ll see more ND entries than ARP entries because all of the link local addresses show up in the ND cache.
If you try to reach a host that is directly attached to your local network and it doesn’t respond, check the ND cache. If an ND cache entry shows up as (incomplete), as with ARP, there’s some sort of basic connectivity issue.
Speed and Duplex
Ethernet supports a variety of speeds. The slowest speed you’re likely to find today is 10 megabits per second (Mbps), but it’s quickly disappearing. Most people use either 10/100Mbps or 1 gigabit per second (Gbps), although you’ll see 10Gbps, 40Gbps, and 100Gbps Ethernet emerging.
The hosts and switch it’s connected to on your network must agree on the speed of their connection. If the OpenBSD host thinks that it’s connected at 100Mbps, but the switch thinks that the connection is 1Gbps, the connection will be flaky. While autonegotiation usually makes both sides agree on common settings (and is absolutely required for gigabit connections), you can manually set duplex and speed for 10/100Mbps connections. Although some switch vendors are notorious for poor autonegotiation, you should let your Ethernet configure itself whenever possible.
Duplex determines if a card can both transmit and receive data simultaneously. A half-duplex connection means that the Ethernet card is either transmitting or receiving at a given instant; it cannot do both. A full-duplex connection can both send and receive simultaneously. As with connection speed, if the switch and host disagree on the duplex setting, the connection will be flaky. Gigabit Ethernet connections involve much more than speed and duplex, and they must be autonegotiated.
Just because a device says that it can use the protocol defined as 10/100Mbps Ethernet doesn’t mean that it can use that protocol with any speed. Also, a card labeled “1Gbps” might not actually pass a gigabit per second. Some network cards will pass their stated amount of traffic, while others will stagger and stumble at a few percent of that. Switch quality varies widely, too.
This may make more sense if you think of an Ethernet’s stated speed as a language. For example, I could claim that I speak Russian and German, but I stopped studying foreign languages in 1985. When I went to Germany in 2007, I managed about three words a minute—with the aid of a translation card and phrase book. If I were an Ethernet card, the manufacturer would claim I spoke German and Russian, and ship me to Siberia.[33]
Get decent hardware. Don’t ask on the OpenBSD mailing list, though. Someone has asked about hardware recommendations in the past few months. Check the archives. The advice hasn’t changed.