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unit 6: Journal

Q1

In the TFTP protocol:

Q1.1

If the client changes its port number on a subsequent connection, but the server does not, what prevents an old-duplicate data packet sent by the server from being accepted by the new client?

  • the old duplicate packets will still have the old port number; assuming that the port numbers are being issued correctly, then the new connection will -almost- never uses the same old port number;
  • the new connection will use different port number, that the old packet does not know about.

Q1.2

If the server changes its port number on a subsequent connection, but the client does not, what prevents an old-duplicate data packet sent by the server from being accepted by the new client?

  • once the new connection has been established on the new port number, the client will only accept packets sent from the server through that same new port number.
  • the packet with the old port number, will reach the client, but will be discarded.

Q2

In an RPC-like protocol in which multiple requests can be outstanding, and replies can be sent in any order: Assume that requests are numbered and that ACK[N] acknowledges reply[N]. Should ACKs be cumulative?If
not,what should happen if an ACK is lost?

  • since the replies can be sent in any order, then the acknowledges can not be cumulative.
  • the replies will arrive at any order, and ACKs will arrive at any order.
  • the server needs to cache all replies that have been sent.
  • once the transfer has been completed, the server should enter DALLY state, waiting for any late ACKs; then if there are missing ACKs, the server should consult its cache and retransmit requests with the missing ACKs, then re-enter DALLY state till all replies have been acknowledged.