Network Protocols Sarah Diesburg Operating Systems COP 4610

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 Physical reality: packets Limited size Unordered Unreliable Machine-to-machine Only on local area network Asynchronous Insecure Abstraction: messages Arbitrary size Ordered Reliable Process-to-process Routed anywhere Synchronous Secure

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

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

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

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

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

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

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)

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

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

Transmission Control Protocol (TCP) • Provides a reliable byte stream between two processes on different machines over the Internet sequence number: 1 checksum: fa 73 cd 10

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

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

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

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

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

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…

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

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

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

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

Two-Phase Commit Account X Account Y Phase 1: ask if each can commit 1. Begin transaction Ask Y for $1 Enough cash 2. Write “Y = Y - $1” Ready to commit Phase 2: commit 3. Write “X = X + $1” 4. Commit Ask Y to commit 5. Commit

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

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