Slides for Chapter 3: Networking and Internetworking From Coulouris, Dollimore and Kindberg...

Slides for Chapter 3: Networking and Internetworking

From Coulouris, Dollimore and Kindberg

Distributed Systems: Concepts and Design

Edition 4, © Pearson Education 2005

Internet Architecture

The Design Philosophy of the DARPA Internet Protocols D. Clark, SIGCOMM

1998

Today’s Reading

Conceptual Lessons Design principles/priorities were designed for a

certain type of network. As the Internet evolves, we feel the sting of some of these choices.Examples: Commercialization

Engineering/Realization is key to testing an idea.

Technical Lessons Packet switching Fate Sharing/Soft state

Fundamental Goal

“technique for multiplexed utilization of existing interconnected networks”

Multiplexing (sharing) Shared use of a single communications channel

Existing networks (interconnection)

Fundamental Goal: Sharing

No connection setupForwarding based on destination address in packetEfficient sharing of resources

Tradeoff: Resource management potentially more difficult.

Packet Switching

Type of Packet Switching: Datagrams

Information for forwarding traffic is contained in destination address of packet

No state established ahead of time (helps fate sharing) Basic building block Minimal assumption about network service

AlternativesCircuit Switching: Signaling protocol sets up entire path out-of-band. (cf.

the phone network)Virtual Circuits: Hybrid approach. Packets carry “tags” to indicate path,

forwarding over IPSource routing: Complete route is contained in each data packet

An Age-Old Debate

Resource control, accounting, ability to “pin” paths, etc.

It is held that packet switching was one of the Internet’s greatest design choices.

Of course, there are constant attempts to shoehorn the best aspects of circuits into packet switching.

Examples: Capabilities, MPLS, ATM, IntServ QoS, etc.

Circuit Switching

Packet SwitchingSharing of resources, soft state (good resilience

properties), etc.

Stopping Unwanted Traffic is Hard

February 2000 March 2006

Research: Stopping Unwanted Traffic

Datagram networks: easy for anyone to send traffic to anyone else…even if they don’t want it!

Possible DefensesMonitoring + Filtering: Detect DoS attack and install filters to drop

traffic.Capabilities: Only accept traffic that carries a “capability”

cnn.com

“This set of goals might seem to be nothing more than a checklist of all the desirable network features. It is important to understand that these goals are in order of importance, and an entirely different network architecture would result if the order were changed.”

The Design Goals of Internet, v1

Interconnection/Multiplexing (packet switching) Resilience/Survivability (fate sharing) Heterogeneity

Different types of services Different types of networks

Distributed management Cost effectiveness Ease of attachment Accountability

These goals were prioritized for a military network. Should priorities change as the network evolves?

DecreasingPriority

Fundamental Goal: Interconnection

Need to interconnect many existing networks Hide underlying technology from applications Decisions:

Network provides minimal functionality “Narrow waist”

Tradeoff: No assumptions, no guarantees.

Technology

Applications email WWW phone...

SMTP HTTP RTP...

TCP UDP…

IP

ethernet PPP…

CSMA async sonet...

copper fiber radio...

The “Curse of the Narrow Waist”

IP over anything, anything over IP Has allowed for much innovation both above and

below the IP layer of the stack An IP stack gets a device on the Internet

Drawback: very difficult to make changes to IP But…people are trying NSF GENI project: http://www.geni.net/

Interconnection: “Gateways”

Interconnect heterogeneous networks No state about ongoing connections

Stateless packet switches

Generally, router == gateway But, we can think of your home router/NAT as also

performing the function of a gateway

Home Network Internet

192.168.1.51

192.168.1.52

68.211.6.120:50878

68.211.6.120:50879

Network Address Translation

For outbound traffic, the gateway: Creates a table entry for computer's local IP address

and port number Replaces the sending computer's non-routable IP

address with the gateway IP address. replaces the sending computer's source port

For inbound traffic, the gateway: checks the destination port on the packet rewrites the destination address and destination port

those in the table and forwards traffic to local machine

Goal #2: Survivability

Network should continue to work, even if some devices fail, are compromised, etc.

Failures on the Abilene (Internet 2) backbone network over the course of 6 months

How well does the current Internet support survivability?

Goal #2: Survivability

Replication Keep state at multiple places in the network, recover when nodes crash

Fate-sharing Acceptable to lose state information for some entity if the entity itself is lost

Two Options

Reasons for Fate Sharing Can support arbitrarily complex failure scenarios Engineering is easier

Some reversals of this trend: NAT, Routing Control Platform

Goal #3: Heterogeneous Services

TCP/IP designed as a monolithic transport TCP for flow control, reliable delivery IP for forwarding

Became clear that not every type of application would need reliable, in-order delivery Example: Voice and video over networks Example: DNS Why don’t these applications require reliable, in-order

delivery? Narrow waist: allowed proliferation of transport protocols

Goal #3b: Heterogeneous Networks

Build minimal functionality into the network No need to re-engineer for each type of network

“Best effort” service model. Lost packets Out-of-order packets No quality guarantees No information about failures, performance, etc.

Tradeoff: Network management more difficult

Goal #4: Distributed Management

Addressing (ARIN, RIPE, APNIC, etc.) Though this was recently threatened.

Naming (DNS)Routing (BGP)

Many examples:

No single entity in charge. Allows for organic growth, scalable management.

Tradeoff: No one party has visibility/control.

No Owner, No Responsible Party

Hard to figure out who/what’s causing a problemWorse yet, local actions have global effects…

“Some of the most significant problems with the Internet today relate to lack of sufficient tools for distributed

management, especially in the area of routing.”

Goal #5: Cost Effectiveness

Packet headers introduce high overheadEnd-to-end retransmission of lost packets

Potentially wasteful of bandwidth by placing burden on the edges of the network

Arguably a good tradeoff. Current trends are to exploit redundancy even more.

Goal #6: Ease of Attachment

IP is “plug and play” Anything with a working IP stack can connect to the Internet (hourglass model)

A huge success! Lesson: Lower the barrier to innovation/entry and people will

get creative (e.g., Cerf and Kahn probably did not think about IP stacks on phones, sensors, etc.)

But….

Tradeoff: Burden on end systems/programmers.

Goal #7: Accountability

Note: Accountability mentioned in early papers on TCP/IP, but not prioritized

Datagram networks make accounting tricky. The phone network has had an easier time figuring

out billing Payments/billing on the Internet is much less precise

Tradeoff: Broken payment models and incentives.

What’s Missing?

SecurityAvailabilityAccountability (the other kind)Support for disconnected/intermittent operationMobilityScaling…

Today’s Reading

Design Philosophy of the DARPA Internet Protocols. Dave Clark, 1988.


certain type of network. As the Internet evolves, we feel the sting of some of these choices.Examples: Commercialization,

Engineering/Realization is key to testing an idea.Technical Lessons

Packet switching Fate Sharing/Soft state

Design Goal Shakeup

Cost of bandwidth is dropping. IP networks are becoming a commodity.

Management == Human intervention Costly!! Human error a leading cause of downtime

More bandwidth: are 40-byte headers still “big”?

Today’s Reading

Design Philosophy of the DARPA Internet Protocols. Dave Clark, 1988.


certain type of network. As the Internet evolves, we feel the sting of some of these choices.Examples: Commercialization,

Engineering/Realization is key to testing an idea.Technical Lessons

Packet switching Fate Sharing/Soft state

Clark’s Paper and This Course

Flexible architectures (Good Thing) leave a lot of "wiggle room".

To determine whether something's going to work, it needs to be implemented/engineered.

Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Networking Issues (1)

Performance: Latency (time between send and start to receive) Data transfer rate (bits per second) Transmission time = latency + length / transfer rate System bandwidth, throughput: total volume of traffic

in a given amount of time Using different channels concurrently can make

bandwidth > data transfer rate traffic load can make bandwidth < data transfer rate network speed < memory speed (about 1000 times) network speed > disk speed (high-speed network file

servers can beat local disks)


Networking Issues (2)

scalability reliability

corruption is rare mechanisms in higher-layers to recover errors errors are usually timing failures, the receiver doesn't have

resources to handle the messages security

firewall on gateways (entry point to org's intranet) encryption is usually in higher-layers

mobility--communication is more challenging: locating, routing,...

quality of service--real-time services multicasting--one-to-many communication


Types of Networks (1)

Local Area Networks (LAN) floor/building-wide single communication medium no routing, broadcast segments connected by switches or hubs high bandwidth, low latency Ethernet - 10Mbps, 100Mbps, 1Gbps no latency guarantees (what could be the

consequences?) Personal area networks (PAN) [ad-hoc networks]:

blue tooth, infra-red for PDAs, cell phones, …



Metropolitan Area Networks (MAN) city-wide, up to 50 km Digital Subscriber Line (DSL): .25 - 8 Mbps, 5.5km

from switch Cable modem: 1.5 Mbps, longer range than DSL



Wide Area Networks (WAN) world-wide Different organizations Large distances routed, latency .1 - .5 seconds 1-10 Mbps (upto 600 Mbps)



Wireless local area networks (WLAN) IEEE 802.11 (WiFi) 10-100 Mbps, 1.5km

802.11 (1997): upto 2 Mbps, 2.4 GHz 802.11a (1999): upto 54 Mbps, 5 GHz, 60 feet 802.11b (1999): upto 11 Mbps, 2.4 GHz, 300 feet [most popular] 802.11g (2003): upto 54 Mbps, 2.4 GHz [backward compatible with

802.11b, becoming more popular]

Wireless metropolitan area networks (WMAN) IEEE 802.16 (WiMax) 1.5-20 Mbps, 5-50km



Wireless wide area networks (WWAN) worldwide GSM (Global System for Mobile communications) 9.6 – 33 kbps 3G (“third generation”): 128-384 kbps to 2Mbps



Internetworks connecting different kinds of networks routers, gateways


Network performance

Example Range Bandwidth(Mbps)

Latency(ms)

Wired:

LAN Ethernet 1-2 km 10-1000 1-10

MAN ATM 250 km 1-150 10

WAN IP routing worldwide .01-600 100-500

Internetwork Internet worldwide 0.5-600 100-500

Wireless:

WPAN Bluetooth (802.15.1) 10 - 30m 0.5-2 5-20

WLAN WiFi (IEEE 802.11) 0.15-1.5 km 2-54 5-20

WMAN WiMAX (802.16) 550 km 1.5-20 5-20

WWAN GSM, 3G phone nets worldwide 0.01-2 100-500


Network principles (1)

Packet transmission message: logical unit of informatio packet: transmission unit restricted length: sufficient buffer storage, reduce

hogging



Data Streaming audio/video Need 120 Mbps (1.5 Mbps compressed) play time: the time when a frame need to be

displayed for example, 24 frames per second, frame 48 must

be display after two seconds IP protocol provides no guaranteesIPv6 (new)

includes features for real-time streams, stream data are treated separately

Resource Reservation Protocol (RSVP), Real-time Transport Protocol (RTP)



Switching schemes (transmission between aribitrary nodes) Broadcast: ethernet, token ring, wireless Circuit switching: wires are connected Packet switching:

store-and-forward different routes “store-and-forward” needs to buffer the entire packet before

forwarding

Frame relay Small packets Looks only at the first few bits Don’t buffer/store the entire frame



Protocols Key components

Sequence of messages Format of messages



Protocol layers, why?

Layer n

Layer 2

Layer 1

Message sent Message received

Communicationmedium

Sender Recipient



Encapsulation in layered protocols

Presentation header

Application-layer message

Session header

Transport header

Network header



ISO Open Systems Interconnection (OSI) model

Application

Presentation

Session

Transport

Network

Data link

Physical

Message sent Message received

Sender Recipient

Layers

Communicationmedium

OSI Model

Open Systems Interconnection (OSI) is a set of internationally recognized, non-proprietary standards for networking and for operating system involved in networking functions.

7 Layers

7. Application Layer

6. Presentation Layer

7. Session Layer

8. Transport Layer

9. Network Layer

10. Data Link Layer

11. Physical Layer

All People Seem To Need Data Processing

Tasks involved in sending letter

LAYER 7 – The APPLICATION Layer

The top layer of the OSI model

Provides a set of interfaces for sending and receiving applications to gain access to and use network services, such as: networked file transfer, message handling and database query processing

The application layer is responsible for providing services to the user.

LAYER 6 – The PRESENTATION Layer

Manages data-format information for networked communications (the network’s translator)

For outgoing messages, it converts data into a generic format for network transmission; for incoming messages, it converts data from the generic network format to a format that the receiving application can understand

This layer is also responsible for certain protocol conversions, data encryption/decryption, or data compression/decompression

A special software facility called a “redirector” operates at this layer to determine if a request is network related on not and forward network-related requests to an appropriate network resource

The presentation layer is responsible for translation, compression, and encryption.

LAYER 5 – The SESSION Layer

Enables two networked resources to hold ongoing communications (called a session) across a network

Applications on either end of the session are able to ex hange data for the duration of the session

This layer is:

Responsible for initiating, maintaining and terminating sessions

Responsible for security and access control to session information (via session participant identification)

Responsible for synchronization services, and for checkpoint services

The session layer is responsible for dialog control and synchronization.

LAYER 4 – The TRANSPORT Layer

Manages the transmission of data across a network

Manages the flow of data between parties by segmenting long data streams into smaller data chunks (based on allowed “packet” size for a given transmission medium)

Reassembles chunks into their original sequence at the receiving end

Provides acknowledgements of successful transmissions and requests resends for packets which arrive with errors

The transport layer is responsible for the delivery of a message from one process to another.

LAYER 3 – The NETWORK Layer

Handles addressing messages for delivery, as well as translating logical network addresses and names into their physical counterparts

Responsible for deciding how to route transmissions between computers

This layer also handles the decisions needed to get data from one point to the next point along a network path

This layer also handles packet switching and network congestion control

The network layer is responsible for the delivery of individual packets from

the source host to the destination host.

LAYER 2 – The DATA LINK Layer

Handles special data frames (packets) between the Network layer and the Physical layer

At the receiving end, this layer packages raw data from the physical layer into data frames for delivery to the Network layer

At the sending end this layer handles conversion of data into raw formats that can be handled by the Physical Layer

The data link layer is responsible for moving frames from one hop (node) to the next.

LAYER 1 – The PHYSICAL Layer

Converts bits into electronic signals for outgoing messages

Converts electronic signals into bits for incoming messages

This layer manages the interface between the the computer and the network medium (coax, twisted pair, etc.)

This layer tells the driver software for the MAU (media attachment unit, ex. network interface cards (NICs, modems, etc.)) what needs to be sent across the medium

The bottom layer of the OSI model

The physical layer is responsible for movements of individual bits from one hop (node) to the next.

Remember

A convenient aid for remembering the OSI layer names is to use the first letter of each word in the phrase:

All People Seem To Need Data Processing



Internet layers Application = application + presentation Transport = transport + session

Underlying network

Application

Network interface

Transport

Internetwork

Internetwork packets

Network-specific packets

MessageLayers

Internetworkprotocols

Underlyingnetworkprotocols



Packet assembly header and data maximum transfer unit (MTU): 1500 for Ethernet 64K for IP (8K is common because of node storage)

ports: destination abstraction (application/service protocol)

addressing: transport address = network address + port Well-known ports (below 1023) Registered ports (1024 - 49151) Private (up to 65535)



Packet delivery (at the network layer) Datagram packet

one-shot, no initial set up different routes, out of order Ethernet, IP

Virtual circuit packet initial set up for resources virtual circuit # for addressing ATM

Similar but different pairs of protocols at the transport layer (connection-oriented and connectionless)



Routing LAN? Routing Algorithm

decide which out-going link to forward the packet

• for circuit switching, the route is determined during the circuit setup time

• for packet switching, each packet is routed independently update state of the out-going links

Routing Table a record for each destination fields: outgoing link, cost (e.g. hop count)



Router example

Hosts Linksor local networks

A

D E

B

C

12

5

43

6

Routers


Network principles (13): Routing tables

Routings from D Routings from E

To Link Cost To Link CostABCDE

336

local6

12201

ABCDE

4456

local

21110

Routings from A Routings from B Routings from C

To Link Cost To Link Cost To Link CostABCDE

local1131

01212

ABCDE

1local

214

10121

ABCDE

22

local55

21021



Router information protocol (RIP) "Bellman-Ford distance vector" algorithm Sender: send table summary periodically (30s) or changes to

neighbors Receiver: Consider A receives a table from B, A updates

1. A -> B -> … -> X: A updates--B has more up-to-date (authoritative) info

2. A -> not B -> … -> X: Does routing via B have a lower cost?

3. B -> … -> X: A does not know X

4. [B -> A -> … -> X]: A doesn’t update--A has more up-to-date info

5. Faulty link, cost is infinity

RIP-1 (RFC 1058) More recent algorithms

more information, not just neighbors link-state algorithms, each node responsible for finding the optimum routes


Network principles (15): Pseudocode for RIP routing algorithm

Tl is the table local table; Tr is the received remote table

Send: Each t seconds or when Tl changes, send Tl on each non-faulty outgoing link.Receive: Whenever a routing table Tr is received on link n:

for all rows Rr in Tr {if (Rr.link != n) { // destination not routed via the receiver

Rr.cost = Rr.cost + 1;Rr.link = n;if (Rr.destination is not in Tl) add Rr to Tl; // add new destination to Tlelse for all rows Rl in Tl {

if (Rr.destination = Rl.destination and (Rr.cost < Rl.cost or Rl.link = n)) Rl

= Rr;// Rr.cost < Rl.cost : remote node has better route// Rl.link = n : remote node is more authoritative

}}

}



Congestion control high traffic load, packets dropped due to limited

resources reducing transmission rate: "choke packets" from

sender to receiver


Networking principles (17)

Network connecting devices Hubs: extending a segment of LAN (broadcast) Switches: switching traffic at data-link level (different

segments of a LAN), making temporary hardware connections between two ports (or store and forward) [switches do not exchange info with each other]

Routers: routing traffic at IP level Bridges: linking networks of different types, could be

routers as well


Networking principles (18)

Tunneling communicate through an "alien" protocol “Hide” in the payload IPv6 traffic using IPv4 protocols

A BIPv6 IPv6

IPv6 encapsulated in IPv4 packets

Encapsulators

IPv4 network

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