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In TCP/IP protocol there are some alike functions at the data link layer and transport layer such as:

  • flow control
  • numbering
  • reliability
  • errors control

Why there are the same functions in two layers?

http://www.slideshare.net/PeterREgli/tcp-6027334

http://www.slideshare.net/hemangkothari/flowcontrol-error-control-at-data-link-layer?qid=4f07bc84-1208-4d19-98f9-0cceff3fdc22&v=qf1&b=&from_search=1

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There aren't. TCP/IP doesn't really work on the (data) link layer. Instead, this is handled by things like Ethernet etc. and is only concerned with the integrity of e.g. Ethernet frames as they travel through a certain (physical) network. Higher level TCP/IP functions care about the actual IP packets and the data transported.

Think about it like this: It's absolutely possible that a router modifies a packet when it arrives. The link layer is concerned that this modified packet gets transported undamaged to the next physical network or the target host, but it's up to the upper layers to decide if this modified packet is still valid from their point of view.

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