Preserving packet order

Question

In my understanding among the many responsabilities the iso/osi model link layer has, it has to make sure that the ordering of packets is preserved.

Can you please clarify to me what the point in doing that is ?

Why bother preserving packet ordering when this order will most probabily be screwed up in the forwarding process of the network layer ?

Am I missing something here ?

Are there any resources on this subject I should read ?

thanks.

Answer 1

Layered approach tries to minimize interdependencies between layers; in general, data link layer does not need knowledge of higher level protocol data/capabilities it carries.

PDU of a data link layer is a Frame. Data Frame will contain network layer data, which could, but does not need to, be a packet. Think about ISDN or Frame Relay, both of which are connection oriented (and use data link protocols Q.921 and Q.922 respectively).

I must also say that I rarely see this (ordering) emphasized, as data link transmission tend to be (logically) serialized so frame order is rather natural consequence of that.

There is also a specific example how this can be useful in a packet/connectionless scenario - 802.11 (http://en.wikipedia.org/wiki/IEEE_802.11) allows frame fragmentation effectively splitting packet into multiple frames for transmission. Obviously those frames need to be re-assembled in correct order otherwise packet they carry would get scrambled.