Question

Can ip packets be lost between two hosts on the same subnet, or does a router need to be involved? And, I have the same question about packet reordering.

I should clarify that this is a subnet on a wired network in a data center with two web services communicating over http.

Answer 1

Yes, packets can be lost in a wired network in a data center.

At my house some sources of packet loss include:

• The cat peed on the switch
• The rabbit chewed through the Ethernet wire
• The wife moved the cable because it was ugly
• The power went out
• I knocked the switch off the shelf

While all of those are possible at a professionally-run data center, they seem unlikely as causes of your packet loss. However, even data centers have problems:

• The switch gets flaky when it overheats
• The endpoint computers get flaky when they overheat
• The packet volume overwhelms the switch’s fabric
• The packet volume overwhelms the endpoint computer
• Some 3rd-party computer poisons the ARP cache

If you are using TCP, then packet loss recovery almost certainly results in re-ordered packets.

Sender: Hey Bob, here's packet 78
Sender: Hey Bob, here's packet 79
Sender: Hey Bob, here's packet 80
Bob: Whoa, Sender, I'm missing 78!
Sender: Hey Bob, here's packet 78
Bob: Okay, I've got 78, 79, and 80.
Sender: Hey Bob, here's packet 81

You can see, the sender sent the packets 78, 79, 80, 78, and 81. To an outside observer, that might be perceived as out of order, since he sees the sequence 80, 78, 81.