CODEWITHSUNDEEP
wiresharkarptcp
Lunar Mushrooms
Lunar Mushrooms

Reputation: 8948

Why do we need sender MAC address in ARP request?

Here is a wireshark capture of an ARP request PNG image, I contains the sender MAC inside the ARP packet. The receiving station can derive the MAC from the Ethernet frame. It seems to be redundant. Is there any particular use of separately including the sender MAC address in ARP Request too ?.

Upvotes: 5

Views: 5011

Answers (3)

sunil
sunil

Reputation: 507

There is no rule that the ARP protocol field sender mac address to be same as ethernet source mac address. Eg: Its possible in few applications where multiple interfaces of same host are on network, but one only interface sends arp responses for all interfaces.

Upvotes: 0

l'L'l
l'L'l

Reputation: 47169

The "redundancy" was by design (RFC 826), and can be useful in targeting different layers. In RFC 3927 there's what is known as Gratuitous Address Resolution Protocol (GARP), and in certain circumstances the redundancy, or lack of, plays an important role, especially in troubleshooting and monitoring networking stacks.

Actually it's not rendunancy at all, the MAC (physical, layer 2) and IP (logical, layer 3) addresses are not the same thing. They serve different purposes on different network layers.

On large scale networks it's quite common to observe changes in the MAC/ARP/Source/Dest information, and at times can seem almost incorrect. For example, you might see a host send an ARP request with its own address as the target address. Depending on the exact situation, it might be telling us it's a link up/down event, maybe it's trying update other devices ARP tables, or possibly detecting an ip conflict and moving the ip to another NIC.

I could get into clustering, failovers — the list goes on, although I would end up writing a book trying to explain it all. Hopefully this gives you a bit of insight about the "redundancy" you were questioning. ;-)

More Info:

RFC 826 / RFC 3927 / Wireshark Gratuitous ARP

Upvotes: 6

mavam
mavam

Reputation: 12552

Although often used in conjunction with Ethernet, ARP by itself is an independent protocol. Imagine other link layer protocols that do not expose MAC addresses. ARP would not work in such circumstances if the sender field was not provided.

Upvotes: 2

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