Network Protocols

Sarah Diesburg

Operating Systems

CS 3430

Distributed Systems

Allow physically separate computers to work together

+ Easier and cheaper to mass-produce simple computers Off-the-shelf components

+ A company can incrementally increase the computing power

Promises of Distributed Systems Higher availability

If one machine goes down, use another Better reliability

A user is able to store data in multiple locations More security

Each simple component is easier to make secure

Reality of Distributed Systems Worse availability

A system may depend on many or all machines being up

Worse reliability One can lose data if any machine crashes

Worse security Security is as strong as the weakest component

Coordination is difficult because machines can only use the network medium

Network Technologies

Definitions Network: physical connection that allows two

computers to communicate Packet: a unit of transfer

A sequence of bits carried over the network Protocol: An agreement between two parties as

to how information is to be transmitted

Broadcast Networks

A broadcast network uses a shared communication medium e.g. wireless, Ethernet, cellular phone network The sender needs to specify the destination in the

packet header So the receiver knows which packet to receive

If a machine were not the intended destination Discard the packet

Arbitration

Concerns the way to share a given resource In Aloha network (1970s)

Packets were sent through radios on Hawaiian Islands

Aloha Network

Arbitration: blind broadcast, with a checksum at the end of a packet

Packets might become garbled in the case of simultaneous transmissions

Aloha Network

Arbitration: blind broadcast, with a checksum at the end of a packet

Packets might become garbled in the case of simultaneous transmissions

Aloha Network

Arbitration: blind broadcast, with a checksum at the end of a packet

Packets might become garbled in the case of simultaneous transmissions

Blind Broadcast

Receiver:If a packet is garbled

discardelse

sends an acknowledgement

Sender:If the acknowledgement does not arrive

resend the packet

Ethernet (introduced in the early ‘80s) By Xerox First practical local area network

Uses wire (as opposed to radio) Broadcast network Key advance: a new way for arbitration

Ethernet’s Arbitration Techniques Carrier sensing: Ethernet does not send

unless the network is idle Collision detection: sender checks if packet

is trampled If so, abort, wait, and retry

Adaptive randomized waiting: a sender picks a bigger wait time (plus some random duration) after a collision

The Internet

A generalization of interconnected local area networks

Uses machines to interconnect various networks Routers, gateways, bridges, repeaters Act like switches Packets are copied as they

transmitted across different networks

LAN 1

LAN 2

Routing

Concerns how a packet can reach its destination

Typically, a packet has to go through multiple hops before getting to a destination Each hop is a router, which directs a packet to

the next hop Routing is achieved through routing tables

Routing Table Updates

1. Each routing entry contains a cost <destination, next hop, # hops>

2. Neighbors periodically exchange routing table entries

3. If the neighbor has a cheaper route, use that one instead

Point-to-Point Networks

Instead of sharing a common network medium, all nodes in the network can be connected directly to a router/switch

Point-to-Point Networks

+ Higher link performance (no collisions)

+ Greater aggregate bandwidth than a single link

Point-to-Point Networks

+ Network capacity can be upgraded incrementally

+ Lower latency (no arbitration)

Issues in Point-to-Point Networks Congestion occurs when everyone sends to

the same output link on a switch

buffers buffers

Crossbar

Networking:Physical Reality vs. AbstractionPhysical 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-to-machine 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: 1checksum: fa73cd10

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

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