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Network Protocols

Network Protocols. Andy Wang Operating Systems COP 4610 / CGS 5765. Protocol. An agreement between two parties as to how information is to be transmitted A network protocol abstracts packets into messages . Physical Reality vs. Abstraction. Arbitrary-Size Messages.

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Network Protocols

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  1. Network Protocols Andy Wang Operating Systems COP 4610 / CGS 5765

  2. Protocol • An agreement between two parties as to how information is to be transmitted • A network protocol abstracts packets into messages

  3. Physical Reality vs. Abstraction

  4. Arbitrary-Size Messages • Can be built on top of limited-size ones • By splitting a message into fix-sized packets • Checksum can be computed on each fragment or the whole message

  5. Internet Protocol (IP) • Provides unreliable, unordered, machine-to-machine transmission of arbitrary-size messages

  6. Process-to-Process Communications • Built on top of machine-to-machine communications through the use of port addresses • Each message contains the destination port to talk to the correct process

  7. Unreliable Data Protocol (UDP) • Provides unreliable, unordered, user-to-user communication • Built on the top of IP

  8. Ordered Messages • Built on top of unordered ones • Use sequence numbers to indicate the order of arrival • Specific to a connection • If packet 3 arrives before packet 2, wait for packet 2. • Always deliver packets in order, to user applications

  9. Reliable Message Delivery • Built on top of unreliable delivery • Problem: Network infrastructure can garble messages • Packets can be dropped if network buffers are full

  10. Solution • Checksum each message • At a receiver, discard messages with mismatching checksums • A receiver acknowledges if a packet is received properly • A sender resends the same message after not hearing the acknowledgment for some time (a timeout period)

  11. A Minor Problem • A sender may send twice, if the first acknowledge is lost • The receiver needs to discard duplicate packets

  12. Implications • A sender needs to buffer messages that are not yet acknowledged • The receiver must track messages that could be duplicates

  13. Transmission Control Protocol (TCP) • Provides a reliable byte stream between two processes on different machines over the Internet sequence number: 1 checksum: fa73cd10

  14. Transmission Control Protocol • Fragments the byte stream into packets and hands them to IP

  15. TCP Message Categories • Sender • Sent and acknowledged • Sent and not acknowledged • Not yet sent • Receiver • Forwarded to application • Received and buffered • Not yet received

  16. More on the Sequence Number • Need a way to recycle sequence numbers • Each TCP packet has a time-to-live field • If the packet is not delivered in X seconds • The packet is dropped • Sequence numbers can be reused • An epoch number used to identify which set of sequence numbers is being used • Incremented at each boot • Stored on disk

  17. Congestion • Implications of timeout period at a sender • Too long  unnecessary waiting • Too short  a message is transmitted when an acknowledgement is in transit • Network congestion  delayed acknowledgement  timeout  data retransmission  more congestion

  18. TCP Solution • Slow start: TCP starts by sending a small amount of data • If no timeout, more data is sent • If timeout, TCP reduces the amount of data being sent

  19. The Byzantine Generals’ Problem • Two generals are on the tops of two mountains… • They communicate only through messengers… • They need to coordinate the attack… • If they attack at the same time, they win… • If they attack at different times, they will…die…

  20. The Byzantine Generals’ Problem • Question: can they guarantee a synchronized attack?

  21. General X 11am OK? So, 11am it is. General Y 11am sounds good Yeah, what if you don’t get this ack? The Byzantine Generals’ Problem Illustrated

  22. The Byzantine Generals’ Problem Over an unreliable network, we cannot guarantee that two computers will synchronize

  23. Distributed Transaction • Multiple machines agree to do something atomically, but not necessarily at exactly the same time • Mechanism: two-phase commit

  24. Two-Phase Commit

  25. Scenarios • If X crashes between 1 and 2 • Y will wake up and do nothing • X will timeout and abort the transaction • If X crashes before step 4 • X will wake up and abort the transaction • If X crashes between 4 and 5 • Y will timeout and ask X for the transaction

  26. Scenarios • If Y crashes between 2 and 5 • Y will wake up and check the log • When X sends Y the commit message, Y will commit • Y can also timeout and ask X the current status

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