2

Perhaps it's a novice question, but does a router send/receive frames? When I read about this subject, it seems that routers decapsulate the frame on arrival, and encapsulate the packet in a frame in order to send it.

But why do people talk about packets forwarding? Also, when you ping a domain name, you have "packets transmitted, received". Is it an abuse of language?

So, if I eavesdrop the link between two routers, do I see frames (with MAC addresses) or packets ?

Thanks !

  • 1
    Unless the packet is large enough to require fragmentation, there's usually a one-to-one correspondence between packets and frames. – Barmar Feb 5 at 20:52
  • A router (with an Ethernet interface) sends packets inside of frames. This is what "encapsulation" is about. – chrylis Feb 5 at 23:45
8

it seems that routers decapsulate the frame on arrival, and encapsulate the packet in a frame in order to send it.

Yes. A router must strip off the layer-2 frame in order to get to the layer-3 packet. The router then routes the packet to the next interface toward the destination, based on the layer-3 destination address. At the next interface, it must build a new frame for the packet for the layer-2 protocol on the next interface, which could be a completely different layer-2 protocol than the one used on the first interface.

But why people talk about packets forwarding ?

That is what the router is doing. It forwards packets from one interface to another interface (one network to another network).

Also, when you ping a domain name, you have "packets transmitted, received". Is it an abuse of language?

No. Ping is an application that uses ICMP echo requests and replies, and ICMP is an integral part of IP, which is a layer-3 protocol that uses packets.

So, if I eavesdrop the link between two routers, do I see frames (with mac addresses) or packets ?

Not all layer-2 protocols use MAC addresses, Only the IEEE LAN protocols use MAC addresses. Often, the layer-2 connection between routers will use something like PPP, which has no addressing on the layer-2 frames because there are only two possible endpoints. If the connection between the two routers is an IEEE protocol, then, yes, you will see frames with MAC addresses. If it is frame relay, you will see frames with DLCIs; with ATM, you will see frames with VPI/VCI; etc.

2

In the case of eavesdropping a web fetch over an ethernet-connected router, you could consider yourself to be looking at voltages, bits, bytes, frames, packets, segments, streams, or pages.

For an analogy, consider whether you hear words or sounds when someone speaks to you. Obviously, it's both, and which you focus on depends on what you're thinking about.

0

Perhaps it's a novice question, but does a router send/receive frames?

Yes

When I read about this subject, it seems that routers decapsulate the frame on arrival, and encapsulate the packet in a frame in order to send it.

That is correct. L2 router needs to decapsulate packet data from it's framing headers (and trailers) on source port, then reencapsulate it in another frame for sending on outgoing port. That's because frame headers will change (for example, router might receive Ethernet II frame on one port, decapsulate packet data, and reencapsulate with different source MAC and checksum before sending it on outgoing port).

Also, input and output ports could be of different type, so entirely different frame (like for example Frame relay could be constructed, but it will still hold the same payload ("packet data")

But why do people talk about packets forwarding? Also, when you ping a domain name, you have "packets transmitted, received". Is it an abuse of language?

More like ambiguity of language. You can view Network packet and Frame as synonyms. Or you can use "packet" to mean frame payload (that is just "packet data" from frame, as in for example commonly used "TCP/IP packet").

So, if I eavesdrop the link between two routers, do I see frames (with MAC addresses) or packets ?

You would see complete frames, including it's headers (like source/destination MAC on Ethernet)

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.