TFTP: Trivial File Transfer Protocol

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TFTP Trivial File Transfer Protocol
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TFTP

• Used to transfer a file when bootstrapping
  dskless systems
• Goal Keep the code small so that it can fit in
  memory
• Uses Stop-and-wait approach

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TFTP Data Transmission

• Send a 512 byte data packet
• Receive a corresponding ACK
• Send the next packet
• The last packet could be less than 512 bytes
• Timeouts implemented by the sender

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TFTP Data/ACK Format
Opcode 1RRQ 2WRQ 3data 4ACK 5error
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Sorcerer's Apprentice Syndrome (SAS)a problem
with early implementations of TFTP

• S (source) sends data block X to computer D
  (destination)
• D receives block X, and sends an acknowledgement
  for X back to S
• The packet containing the acknowledgement for X
  is delayed in the internetwork
• S times out, and resends data block X to D
• S receives the delayed acknowledgement for X, and
  sends data block X1
• D receives the second copy of block X, and sends
  another acknowledgement for X back to S
• D receives block X1, and sends an
  acknowledgement for X1 back to S
• S receives the second acknowledgement for X, and
  sends a second copy of data block X1
• S receives the acknowledgement for X1, and sends
  data block X2
• D receives the second copy of block X1, and
  sends another acknowledgement for X1 back to S
• D receives block X2, and sends an
  acknowledgement for X2 back to S

Increasing number of duplicate packets can cause
increased number of timeouts leading to
congestion collapse
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RARP

• A Diskless system or an X terminal needs a way to
  learn its own IP address
• Broadcast a request and obtain a unicast reply

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Why is RARP not good enough?

• Limitations of RARP
• Only IP address is returned
• Uses Link layer broadcast which are not forwarded
  by routers
• So each network must have at least one RARP server

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BOOTP

• BOOTP
• Uses UDP
• Returns IP address and gateway routers address,
  if needed
• Can also return IP address of a router, subnet
  mask, IP address of name server
• A router can be configured to forward BOOTP
  requests

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BOOTP Request/Reply
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Questions

• If after a power outage 100 nodes uses BOOTP then
  there could be packet losses. How can you address
  this problem?
• What if we allow RARP messages to be forwarded by
  routers?