introduction to computer networks cs640 layering and protocols

Ming Liu [email protected]

Introduction to Computer Networks

CS640 https://pages.cs.wisc.edu/~mgliu/CS640/F21/

Layering and Protocols

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Wireless Network Outage During Weekend

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Today

Last lecture • Networks and applications • Network requirements: the hardware view

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Today • Network requirements: the software view

• Layering and protocols

Announcements • Form your lab group by the end of this week (September 17th 11:59pm)

• Lab1 and HW1 will be released today

Design Requirements

#1: Scalable connectivity

#2: Cost-efficient resource usage

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#3: Support for common services

What software functionalities the network should provide?

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What software functionalities the network should provide?

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• Addressing

• Routing or forwarding

• Sharing or multiplexing

• Security • …

Addressing and Routing

Address • Provide a way for hosts to identify and reach each other • Different network hardware elements use different addresses

Host A

Host B

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Host A

Host B

IP address: identifies end-point and encodes network location • 128.2.3.6 labels a node or host in a network numbered 128.2

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Just an example. No need to understand at this time. We’ll talk about IP in details later.


Host A

Host B

MAC address: identifies node or host alone • Does not carry any information about where the host is located in • Given by NIC vendors, e.g., 82:13:08:df:07:40 • When an endhost moves, IP could change but not MAC

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Just an example. No need to understand at this time. We’ll talk about MAC in details later.


Host A

Host B

Routing: determine how to forward messages • Switches use MAC addresses • Routers use IP addresses

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Host A

Host B

Routing: determine how to forward messages • Switches use MAC addresses • Routers use IP addresses

Destination address Forwarding port Metadata

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Host A

Host B

Routing: determine how to forward messages • A set of forwarding tables in each switch or router that collectively determine the path of a message through a network

Source

Destination10

Statistical Multiplexing

Problem: How to multiplex resources among many applications to architect a cost-effective network?

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Candidate solutions • TDM

• Allocate time slots to each application in a round-robin order • Apps send packets in its allocated time slots

• FDM

• Allocate capacity to each application in a round-robin order • Apps send as many packets per unit time as the capacity allows

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Candidate solutions • TDM

• Allocate time slots to each application in a round-robin order • Apps send packets in its allocated time slots

• FDM

• Allocate capacity to each application in a round-robin order • Apps send as many packets per unit time as the capacity allows

• Bandwidth is wasted if an application becomes idle • The number of application is unknown when allocation happens • Applications could enter/leave/move

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On-demand transmission with quantum throttling

Two ideas • MTU (maximum transmission unit) • Define an upper bound on the size of the block of data

• Hosts use the “Path MTU Discovery” service to find the MTU

• FIFO (first-in first-out) • Networking nodes use a buffer implementing FIFO the (first-in first-out) scheduling

• Packets from all the hosts are queued at the in-network buffer • But, congestion happens when the FIFO queues overflow

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Common communication services

Security • No eavesdropping, …

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Common communication services

Reliability • bit errors, packet drops, link/switch failures, in order or not, …

Communication patterns • unicast, multicast, broadcast, …

Channel types • request/reply, message streams, …

Security • No eavesdropping, …

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Functionalities Needed from the Network

Address and routing • Basic connectivity

Statistical multiplexing • Efficient resource sharing

Common communication services • Security, Reliability, communication patterns, channel types, …

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Functionalities Needed from the Network

Address and routing • Basic connectivity

Statistical multiplexing • Efficient resource sharing

Common communication services • Security, Reliability, communication patterns, channel types, …

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• How do you implement these functionalities? • Key: Layering and protocols

What is Layering?

A way to deal with complexity • Add multiple levels of abstraction

• Each level encapsulates some key functionalities • And exports an interface to other components

Layering: modular approach to implementing new functionality by introducing abstractions

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What is Layering?

A way to deal with complexity • Add multiple levels of abstraction

• Each level encapsulates some key functionalities • And exports an interface to other components

Layering: modular approach to implementing new functionality by introducing abstractions

Challenge: how to come up with the right abstractions?

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Example of Layering

Software and hardware for communication between two hosts

Advantages • Simplify design and implementation

• Easy to modify/evolve

Link hardware

Host-to-host connectivity

Application-to-application channelsApplication semantics

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What is a Protocol?

Could be multiple abstractions at a given level • Build on the same lower level • But provide different service to higher layers

Protocol: abstract object or module in layered structure

Link hardware

Host-to-host connectivity

Request-ReplyApplication

Message stream

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1. Protocols Offer Interfaces

Each protocol offers interfaces • One to higher-level protocols on the same end hosts

• Expects one from the layers on which it builds • Interface characteristics, e.g., IP service model

• A “peer interface” to a counterpart on destinations • Syntax and semantics of communications • (Assumptions about) data formats

Protocols build upon each other • Adds value, improves functionality overall, e.g., a reliable protocol running on top of IP

• Reuse, avoid re-writing, e.g., OS provides TCP, so apply don’t have to rewrite

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2. Protocols Necessary for Interoperability

Protocols are the key to interoperability • Networks are very heterogeneous • The HW/SW of communicating parties are often not build by the same vendor • Yet they can communicate because they use the same protocol • Actually implementations could be different • But must adhere to same specifications

Protocols exist at many levels • Application level protocols • Protocols at the hardware level

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Link optical fiber, coaxial cable

Endhost adapter Mellanox, Intel, Broadcom

Switch/Router Cisco, Arista, Juniper, …

App Email, Web, video, …




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OSI Model

One of the first standards for layering: OSI

Breaks up network functionality into seven layers

This is a reference model • For ease of thinking and implementation

A different model, TCP/IP, used in practice

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The OSI Standard: 7 layers

1. Physical: transmit bits

2. Data link: collect bits into frames and transmit frames (adaptor/device deriver)

3. Network: route packets in a packet switched network

4. Transport: send messages across processes end2end

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The OSI Standard: 7 layers (cont’d)

5. Session: tie related flows together

6. Presentation: format of app data (byte ordering, video format)

7. Application: application protocols (e.g., HTTP)

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The OSI Standard: 7 layers (cont’d)

5. Session: tie related flows together

6. Presentation: format of app data (byte ordering, video format)

7. Application: application protocols (e.g., HTTP)• OSI is very successfully at shaping thought

• TCP/IP standard has been amazingly successful, and it’s not based on a rigid OSI model

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OSI Layers and Locations

Application

Presentation

Session

Transport

Network

Data Link

Physical

Host Hub Switch Router Host

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The Reality: TCP/IP Model

FTP

TCP

HTTP TFTP

UDP

Kerberos App protocols

IP

NET1 NET2 NETn

Two major transport protocols: provide logical channels to apps

Interconnection of n/w technologies into a single logical n/w

Network protocols implemented

by a combination of hw and sw

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FTP

TCP

HTTP TFTP

UDP

Kerberos

IP

NET1 NET2 NETn

Waist

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FTP

TCP

HTTP TFTP

UDP

Kerberos

IP

NET1 NET2 NETn

Waist

• The waist: minimal, carefully chosen functions. Facilitates interoperability and rapid evolution

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TCP/IP v.s. OSI

Session

Transport

Presentation

Application

Network

Data link

Physical

Application (plus libraries)

IP

Data link

Physical

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TCP/UDP

TCP/IP Layering

Application

Transport

Network

Data Link

Physical

Host Hub Switch Router Host

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Layers & EncapsulationUser A

Get index.html

User B

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Get index.html

Connection ID

User B

Header

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Get index.html

Connection ID

Source/Destination

Link Address

User B

Header

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Protocol Demultiplexing

Multiple choice at each layer

How to know which one to pick?

FTP

TCP

HTTP TFTP

UDP

Kerberos

IP

NET1 NET2 NETn

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Multiplexing & Demultiplexing

Multiplex implementations of each layer • How does the receiver know what version/module of a layer to use

Packet header includes a demultiplexing field • Used to identify the right module for next layer • Filled in by sender • Used by the receiver

Multiplexing occurs at multiple layers • E.g., IP, TCP

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Multiplexing & Demultiplexing

Multiplex implementations of each layer • How does the receiver know what version/module of a layer to use

Packet header includes a demultiplexing field • Used to identify the right module for next layer • Filed in by sender • Used by the receiver

Multiplexing occurs at multiple layers • E.g., IP, TCP

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Layering v.s. Not

Layer N may duplicate layer N-1 functionality • E.g., error recovery

Layers may need same info (timestamp, MTU)

Some layers are not always cleanly separated • Inter-layer dependencies in implementation for performance reasons • Many cross-layer assumptions, e.g. buffer management

Layer interfaces are not really standardized • It would be hard to mix and match layers from independent implementations

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Summary

Today • Network requirements: the software view

• Layering and protocols

Next lecture • Evaluating a network -- performance

• Socket APIs • Lab1 overview

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introduction to computer networks cs640 layering and protocols

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