Network Protocols Mark Stanovich Operating Systems COP 4610
- Slides: 27
Network Protocols Mark Stanovich 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 Abstraction: messages Limited size Arbitrary size Unordered Ordered Unreliable Reliable Machine-to-machine Process-to-process Only on local area network Routed anywhere Asynchronous Synchronous Insecure Secure
Physical Reality vs. Abstraction Physical reality: packets Abstraction: messages Limited size Arbitrary size Unordered Ordered Unreliable Reliable Machine-to-machine Process-to-process Only on local area network Routed anywhere Asynchronous Synchronous Insecure 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 addresses • Each message contains the destination address to talk to the correct machine
User Datagram Protocol (UDP) • Provides unreliable, unordered, user-to-user communication • Built on the top of IP • Generally lower latency at the cost of reliability • Sometimes referred to as Unreliable Datagram Protocol
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 Two 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 Two Generals’ Problem • Question: can they guarantee a synchronized attack?
The Two 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 Two Generals’ Problem Over an unreliable network, we cannot guarantee that two computers will coordinate an action
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 Y 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 status
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
- Good cop bad cop interrogation
- Cop 1 cop 2
- What is 224 in binary
- Network operating system example
- Type
- Network security protocols
- Playfair cipher
- Wireless sensor network protocols
- Whats my ip
- Network communication protocols map
- Chapter 3 network protocols and communications
- Distributed data store
- Ece 526
- Network operating system examples
- Networking background
- Cisco router operating system
- Different types of network operating system
- Types of nos
- Network operating system examples
- Configure a network operating system
- Pengenalan network operating system
- Configuring a network operating system
- Characteristics of network operating system
- Medium berwayar
- Network operating system administration
- System.net.peertopeer
- Operating system example
- Evolution of operating systems