Suppose a host on a PPP network receives a frame that contains a TCP segment in an IPv6 packet. Answer each of the below questions, being specific about field names wherever appropriate.

1. How does the link layer determine whether to pass the frame payload to IPv4, IPv6, or something else in the layer above?
2. How does the network layer determine that it should pass its payload to TCP?
3. How does TCP decide which process to pass the payload to?

Answers:

1. In the link layer, the PPP Protocol field would indicate IPv6 (text, p. 85 or RFC 1661).
   In class today someone asked whether answering the version field would be correct, and I said no. However, upon reflection, it's not entirely incorrect. The receiver can use IP's version field to determine whether the network-layer PDU is IPv4 or IPv6, but the receiver first needs to know that it has received IP to even know where to find the version field. So the part of the problem that this really doesn't answer is how the receiver knows the payload is IP. The receiver looks at the PPP protocol field to learn that it has received IP.
2. In the network layer, the IPv6 NextHeader field would indicate TCP (text, p. 325 or RFC 2460).
3. The transport layer would base its decision upon the destination port number, DestPort (text, p. 388) or Destination Port (RFC 761).