ARP messages: Request, Reply, Probe, Announcement
ARP, as originally specified in RFC 826,
had two message types: REQUEST and REPLY.
But besides that one can hear of two "other" ARP messages:
PROBE ANNOUNCEMENT, the latter also called "gratuitous ARP".
They happen when a host send the reply message not in response to any
preceding request; this has been formalized in
RFC 5227.
One may find confusing that:
- The two "new" messages do not extend the protocol. In particular, they do not introduce new ARP message types. How can we speak about "new messages"?
- Source and destination MAC addresses are present in the MAC headers, but there are also "Source Hardware Address" and "Target Hardware Address" fields in the ARP body. Why this duplication?
These two questions are related. The table below shows the MAC headers and (relevant) ARP protocol fields for each of the four messages. The differences help to understand how each field is used.
| Message | Sent when the host... | MAC headers | ARP body | |||||
| src MAC | dst MAC | message type | Source Hardware Address | Source Protocol Address | Target Hardware Address | Target Protocol Address | ||
| request | wants to send an IP packet, but does not know the MAC address of the destination | own | broadcast | REQUEST | own | own | 0 | destination's |
| reply | receives an ARP request to an IP address this host owns | own | destination's, or broadcast1 | REPLY | own | own | requestor's | requestor's |
| probe (RFC 5227, 2.1) | configures a new IP address for an interface | own | broadcast | REQUEST2 | own | 03 | 0 | probed |
| announcement (RFC 5227, 2.3) | after a probe, concludes that it will use the probed address | own | broadcast | REQUEST2 | own | new own | 0 | new own |
Notes to the table
- RFC 5227, section 2.6 explains why "Broadcast replies are not universally recommended, but may be appropriate in some cases".
- RFC 5227, section 3 notes that the type here could be REPLY as well, then continues to give reasons why REQUEST is recommended.
- Zero is specified here in order not to pollute ARP caches of other hosts in case when the probed address is already taken by someone else.