INTERNETWORKING

A Client-Server Transaction• (Almost) every network application is based on client-server

model:– A server process and one or more client processes– Server manages some resource– Server provides service by manipulating resource for clients

Clientprocess

Serverprocess

1. Client sends request

2. Server handlesrequest

3. Server sends response4. Client handles

response

Resource

Note: clients and servers are processes running on hosts (can be same or different hosts)

Computer Networks• A network is a hierarchical system of boxes and wires organized by geographical

proximity– LAN (local area network) spans building or campus

• Ethernet is most prominent example• 802.11 (wireless) becoming more important

– WAN (wide-area network) spans country or world• Typically high-speed point-to-point copper or fiber lines• Also microwave and satellite links in some situations

• An internetwork (internet) is an interconnected set of networks– Global IP Internet (uppercase “I”) is most famous example of an internet

(lowercase “i”)• Let’s look at how to build an internet from ground up

Lowest Level: Ethernet Segment• Ethernet segment consists of collection of hosts connected by wires

(twisted pairs) to a hub - replaces common ‘wire’, ‘bus’ • Spans room or floor in a building

• Operation– Each Ethernet adapter has unique 48-bit address– Hosts send bits to any other host in chunks called frames– Hub slavishly copies each bit from each port to every other port

• Every adapter sees every bit; chooses which frames to hand to system– Alternative: switch copies bits only to proper destination port

host host host

hub100 Mb/s100 Mb/s

ports

Next Level: Bridged Ethernet Seg• Spans building or campus• Bridges cleverly learn which hosts are reachable from which ports and then

selectively copy frames from port to port. How? Frames have source and destination addresses….

host host host host host

hub

hub

bridge100 Mb/s 100 Mb/s

host host

hub

100 Mb/s 100 Mb/s

1 Gb/s

host host host

bridge

hosthost

hub

A B

C

X

Y

Conceptual View of LANs• For simplicity, hubs, bridges, and wires are often shown as collection of

hosts attached to a single wire:

host host host...

Next Level: internets• Multiple incompatible LANs can be physically connected by specialized

computers called routers• The connected networks are called an internet

host host host

LAN 1

... host host host

LAN 2

...

router router routerWAN WAN

LAN 1 and LAN 2 might be completely different, totally incompatible LANs (e.g., Ethernet and ATM)

Notion of an internet ProtocolHow is it possible to send bits across incompatible LANs and WANs?

Solution: protocol software running on each host and router smooths out differences between different networks

Implements an internet protocol (i.e., set of rules) that governs how hosts and routers should cooperate when they transfer data from network to network• TCP/IP is protocol (family) for global IP Internet

What Does an internet Protocol Do?• 1. Provides naming scheme

– Defines uniform format for host addresses– Each host (and router) is assigned at least one internet address that uniquely

identifies it• 2. Provides delivery mechanism

– An internet protocol defines a standard transfer unit (packet)– Packet consists of header and payload

• Header: contains info such as packet size, source and destination addresses• Payload: contains data bits sent from source host

– Encapsulation - key to network messages

Transferring Data via an internet

protocolsoftware

client

LAN1adapter

Host A

data

data PH FH1

data PH

data PH FH2

LAN1 LAN2

data

data PH data PH FH2

(1)

(2)

(3)

(4) (5)

(6)

(7)

(8)

internet packet

LAN2 frame

protocolsoftware

LAN1adapter

LAN2adapter

RouterFH1

data PH

protocolsoftware

server

LAN2adapter

Host B

Frame

Basic Internet Components• An Internet backbone is a collection of routers

(nationwide or worldwide) connected by high-speed point-to-point networks

• A Network Access Point (NAP) is a router that connects multiple backbones (sometimes referred to as peers)

• Regional networks are smaller backbones that cover smaller geographical areas (e.g., cities or states)

• A point of presence (POP) is a machine that is connected to the Internet

• Internet Service Providers (ISPs) provide dial-up or direct access to POPs

Internet Domain Names

mil edu gov com

hmc berkeleymit

cs math

mike1134.173.41.151

unnamed root

Knuth134.173.42.100

amazon

www208.216.181.15

First-level domain names

Second-level domain names

Third-level domain names

Internet Connections• Clients and servers communicate by sending streams of bytes over connections:

– Point-to-point, full-duplex (2-way communication), and reliable• Socket is endpoint of connection

– Socket address is IPaddress:port pair• Port is 16-bit integer that identifies a process:

– Ephemeral port: Assigned automatically on client when client makes connection request

– Well-known port: Associated with some service provided by a server (e.g., port 80 is associated with Web servers)

• Connection is uniquely identified by socket addresses of its endpoints (socket pair)– (clientaddr:clientport, serveraddr:serverport)

Network Layer: Protocols

forwarding

Routing protocols•path selection•e.g., RIP, OSPF, BGP

Network layer protocol (e.g., IP)•addressing conventions•packet format•packet handling conventions

Control protocols•error reporting e.g. ICMP

Transport layer

Link layer

physical layer

Networklayer

Control protocols- router “signaling” e.g. RSVP

15

Network layer

• transport segment from sending to receiving host

• on sending side encapsulates segments into datagrams

• on rcving side, delivers segments to transport layer

• network layer protocols in every host, router

• Router examines header fields in all IP datagrams passing through it

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

applicationtransportnetworkdata linkphysical

applicationtransportnetworkdata linkphysical

16

Key Network-Layer Functions

• forwarding: move packets from router’s input to appropriate router output

• routing: determine route taken by packets from source to dest.

– Routing algorithms

analogy:

routing: process of planning trip from source to dest

forwarding: process of getting through single interchange

17

A day in a life of a router

find pathforward packet, forward packet, forward packet,

forward packet...find alternate pathforward packet, forward packet, forward packet,

forward packet…repeat until powered off

IP Routing – finding the path

• Path derived from information received from a routing protocol

• Several alternative paths may exist– best path stored in forwarding table

• Decisions are updated periodically or as topology changes (event driven)

• Decisions are based on:– topology, policies and metrics (hop count, filtering,

delay, bandwidth, etc.)

18

Chapter 5: Network Layer: Routing #19

Routing Design Space• Routing has a large design space

– who decides routing?• source routing: end hosts make decision• network routing: networks make decision

– centralized control : one node builds tables for all routers– distributed control : each router builds own routing table

– how many paths from source s to destination d?• multi-path routing (several alternatives used)• single path routing

– will routing adapt to network traffic demand or to topology changes?

• dynamic routing : tables adapt to state changes• static routing : tables configured manually

– …

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Transport Gateway• Similar to application gateways but at the level of TCP connections

– independent of application code– requires client software to be aware of the gateway

Transport Gateway

(SOCKS Server)

1 GET xxx..

data

:1080 SYN

ACK

SYN ACK

SYN ACK

A

B

:80 SYN

connection relay requestto B :80

ACK

data relay

OK

1

2 3

4

21

The transport gateway is a layer 4 intermediate system. The example shows the SOCKS gateways. SOKCS is a standard being defined by the IETF.

1. A opens a TCP connection to the gateway. The destination port is the well known SOCKS server port 1080.

2. A requests from the SOCKS server the opening of a TCP connection to B. A indicates the destination port number (here, 80). The SOCKS server does various checks and accepts or rejects the connection request.

3. The SOCKS server opens a new TCP connection to B, port 80. A is informed that the connection is opened with success.

4. Data between A and B is relayed at the SOCKS server transparently. However, there are two distinct TCP connections with their own, distinct ack and sequence numbers.

Compared to an application layer gateway, the SOCKS server is simpler because it is not involved in application layer data units; after the connection setup phase, it acts on a packet by packet level. Its performance is thus higher.

However, it requires the client side to be aware of the gateway: it is not transparent. Netscape and Microsoft browsers support SOCKS gateways.

22

Application Layer Gateways• Application layer gateway is a layer 7 intermediate system

– normally not used according to the TCP/IP architecture– but mainly used for access control– also used for interworking issues

• Principle:– proxy principle: viewed by client as a server and by server as a client– supports access control restrictions, authentication, encryption, etc

HTTPserver

HTTPclient

gatewaylogic

TCP/IPTCP/IP

HTTPclient

TCP/IP

HTTPserver

HTTP Gateway

1 GET xxx.. 2 GET xxx..

3 data4 data

intranet Internet

AB

23

1. User at A sends an HTTP request. It is not sent to the final destination but to the application layer gateway. This results from the configuration at the client.

2. The gateway checks whether the transaction is authorized. Encryption may be performed. Then the HTTP request is issued again from the gateway to B as though it would be originating from A.

3. A response comes from B, probably under the form of a MIME header and data. The gateway may also check the data, possibly decrypt, or reject the data.

4. If it accepts to pass it further, it is sent to A as though it would be coming from B.

Application layer gateways can be made for all application level protocols. They can be used for access control, but also for interworking, for example between IPv4 and IPv6.

Time Division Multiplexing

• Data rate of medium exceeds data rate of digital signal to be transmitted

• Multiple digital signals interleaved in time• May be at bit level of blocks• Time slots preassigned to sources and fixed• Time slots allocated even if no data• Time slots do not have to be evenly

distributed amongst sources

Time Division Multiplexing

TDM System

TDM Link Control• No headers and trailers• Data link control protocols not needed• Flow control

– Data rate of multiplexed line is fixed– If one channel receiver can not receive data, the

others must carry on– The corresponding source must be quenched– This leaves empty slots

• Error control– Errors are detected and handled by individual

channel systems

Data Link Control on TDM

ATM Defined

• “ATM is a cell-switching and multiplexing technology that combines the benefits of circuit switching (guaranteed capacity and constant transmission delay) with those of packet switching (flexibility and efficiency for intermittent traffic)” –CISCO.

• It utilizes fixed length cells to carry the information

Areas of Application

Infrastucture BackbonesLAN

• LANs Asynchronous transfer mode (ATM) is a high-performance, cell-oriented switching and multiplexing technology that utilizes fixed-length packets to carry different types of traffic. ATM is a technology that will enable carriers to capitalize on a number of revenue opportunities through multiple ATM classes of services; high-speed local-area network (LAN) interconnection; voice, video, and future multimedia applications in business markets in the short term; and in community and residential markets in the longer term.

• Infrastructure – Backbones– LAN

• Application

Time Division Multiplexing

• Data rate of medium exceeds data rate of digital signal to be transmitted

• Multiple digital signals interleaved in time• May be at bit level of blocks• Time slots preassigned to sources and fixed• Time slots allocated even if no data• Time slots do not have to be evenly

distributed amongst sources

Time Division Multiplexing

TDM System

TDM Link Control

• No headers and trailers• Data link control protocols not needed• Flow control

– Data rate of multiplexed line is fixed– If one channel receiver can not receive data, the

others must carry on– The corresponding source must be quenched– This leaves empty slots

• Error control– Errors are detected and handled by individual

channel systems

Data Link Control on TDM

ATM Connections

Circuit Switching and Packet Switching

• ATM is circuit switched because it establishes virtual circuits for communication

• At the same time, the virtual circuits are established over packet switched networks

• As such, it combines the benefits of circuit switched and packet switched technologies

ATM Usage and Bandwidth• In theory, ATM can be deployed from small

LANs to very large WANs– At present, it is used mostly on backbones, but

this may change in the future with declining prices for ATM equipment

• ATM deployments can operate at speeds starting in the Mbps range scaling up to Gpbs range– Speed wise, it is very scalable

ATM’s Efficiency

• It is an asynchronous technology and it uses the links based on the need for information to be transmitted

• ATM is based on fixed length cells and the cells are small compared to many other forms of transmission such as frame relay etc.

ATM Cell Basics

• ATM carries information based on fixed length cells– Compare this to the other packet switching technologies

such as Frame Relay etc. where each packet may be of a different length

• The length of each cell is 53 Bytes– First 5 bytes are used as the cell header– Next 48 bytes are used as the payload carrying the data

ATM Cell Format

Header Payload (Data)

5 Bytes 48 Bytes

Fixed Length Cell Advantage• Delay or latency is significantly reduced

– ATM is therefore suited for voice and video transmission

• Fixed length cells make it easier to switch data across multiple networks– ATM networks are built based on switches and not

routers• Fixed length cell is similar to container based road

transportation– Some parallels can be drawn with respect to the

advantage of fixed length transportation based on the benefits of container transportation

ATM Cell Header Format

ATM Cell Header—UNI Format

ATM Cell Header—NNI Format

ATM Devices

• ATM networks are built around two categories of devices– ATM Switch– ATM end-point

• An ATM switch can be connected to either another ATM switch or and ATM end-point

ATM End-Points

• Will contain and ATM end-point adapter• Examples of ATM end-points

– Workstations– LAN switches– Routers– DSU/CSU Units– Video Coder-Decoders (CODECs)

Devices on Which ATM Has Been Implemented

• PC, workstation, and server network interface cards • switched-Ethernet and token-ring workgroup hubs • workgroup and campus ATM switches • ATM enterprise network switches • ATM multiplexers • ATM–edge switches • ATM–backbone switches

ATM Network Components

Source: CISCO

ATM Switches and Interfaces

• ATM switch supports two types of interfaces– User-Network Interface (UNI)

• Connects an ATM end-point to a switch– Network-Network Interface (NNI)

• Connects two ATM switches

50

Need for Multiprotocol Label Switching (MPLS)

• Forwarding function of a conventional router– a capacity demanding procedure– constitutes a bottle neck with increase in line

speed• MPLS simplifies forwarding function by

taking a totally different approach by introducing a connection oriented mechanism inside the connectionless IP networks

51

Label Switching

• Decomposition of network layer routing into control and forwarding components applicable

• Label switching forwarding component algorithm uses– forwarding table – label carried in the packet

• What is a Label ?– Short fixed length entity

MPLS Basics• A Label Switched Path (LSP) is set up for each route• A LSP for a particular packet P is a sequence of routers,

<R1,R2………..Rn>for all i, 1< i < n: Ri transmits P to R[i+1] by means of a label

• Edge routers – analyze the IP header to decide which LSP to use– add a corresponding local Label Switched Path Identifier, in the form of a label– forward the packet to the next hop

53

MPLS Basics contd..• Subsequent nodes

– just forward the packet along the LSP– simplify the forwarding function greatly– increase performance and scalability dramatically

• New advanced functionality for QoS, differentiated services can be introduced in the edge routers

• Backbone can focus on capacity and performance• Routing information obtained using a common intra domain routing protocol such as

OSPF

Basic Model for MPLS Network

MPLS

LSR = Label Switched RouterLER = Label Edge Router

LER

LER

LSR

LER

LSRLSR

IP

MPLS

IP

Internet

LSR

55

MPLS Benefits

Comparing MPLS with existing IP core and IP/ATM technologies, MPLS has many advantages and benefits:

• The performance characteristics of layer 2 networks

• The connectivity and network services of layer 3 networks

• Improves the price/performance of network layer routing

• Improved scalability

56

MPLS Benefits contd..

• Improves the possibilities for traffic engineering

• Supports the delivery of services with QoS guarantees

• Avoids need for coordination of IP and ATM address allocation and routing information