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Network Security

Data on the network is analogous to possessions of a person. It has to be kept secure from others with malicious intent. This intent ranges from bringing down servers on the network to using people's private information like credit card numbers to sabotage of major organizations with a presence on a network. To secure data, one has to ensure that it makes sense only to those for whom it is meant. This is the case for data transactions where we want to prevent eavesdroppers from listening to and stealing data. Other aspects of security involve protecting user data on a computer by providing password restricted access to the data and maybe some resources so that only authorized people get to use these, and identifying miscreants and thwarting their attempts to cause damage to the network among other things.

The various issues in Network security are as follows :

1. Authentication: We have to check that the person who has requested for something or has sent an e-mail is indeed allowed to do so. In this process we will also look at how the person authenticates his identity to a remote machine.
2. Integrity: We have to check that the message which we have received is indeed the message which was sent. Here CRC will not be enough because somebody may deliberately change the data. Nobody along the route should be able to change the data.
3. Confidentiality: Nobody should be able to read the data on the way so we need Encryption
4. Non-repudiation: Once we sent a message, there should be no way that we can deny sending it and we have to accept that we had sent it.
5. Authorization: This refers to the kind of service which is allowed for a particular client. Even though a user is authenticated we may decide not to authorize him to use a particular service.

For authentication, if two persons know a secret then we just need to prove that no third person could have generated the message. But for Non-repudiation we need to prove that even the sender could not have generated the message. So authentication is easier than Non-repudiation. To ensure all this, we take the help of cryptography. We can have two kinds of encryption :

1. Symmetric Key Encryption: There is a single key which is shared between the two users and the same key is used for encrypting and decrypting the message.
2. Public Key Encryption: There are two keys with each user : a public key and a private key. The public key of a user is known to all but the private key is not known to anyone except the owner of the key. If a user encrypts a message in his private key then it can be decrypted by anyone by using the sender's public key. To send a message securely, we encrypt the message in the public key of the receiver which can only be decrypted by the user with his private key.

Symmetric key encryption is much faster and efficient in terms of performance. But it does not give us Non-repudiation. And there is a problem of how do the two sides agree on the key to be used assuming that the channel is insecure ( others may snoop on our packet ). In symmetric key exchange, we need some amount of public key encryption for authentication. However, in public key encryption, we can send the public key in plain text and so key exchange is trivial. But this does not authenticate anybody. So along with the public key, there needs to be a certificate. Hence we would need a public key infrastructure to distribute such certificates in the world.

Key Exchange in Symmetric Key Schemes

We will first look at the case where we can use public key encryption for this key exchange. . The sender first encrypts the message using the symmetric key. Then the sender encrypts the symmetric key first using it's private key and then using the receiver's public key. So we are doing the encryption twice. If we send the certificate also along with this then we have authentication also. So what we finally send looks like this :

Z : Certificate_sender + Public_reciever ( Private_sender ( E_k ) ) + E_k ( M )

Here E_k stands for the symmetric key and E_k ( M ) for the message which has been encrypted in this symmetric key.

However this still does not ensure integrity. The reason is that if there is some change in the middle element, then we will not get the correct key and hence the message which we decrypt will be junk. So we need something similar to CRC but slightly more complicated. This is because somebody might change the CRC and the message consistently. This function is called Digital Signature.

Digital Signatures

Suppose A has to send a message to B. A computes a hash function of the message and then sends this after encrypting it using its own private key. This constitutes the signature produced by A. B can now decrypt it, recompute the hash function of the message it has received and compare the two. Obviously, we would need the hash functions to be such that the probability of two messages hashing to the same value is extremely low. Also, it should be difficult to compute a message with the same hash function as another given message. Otherwise any intruder could replace the message with another that has the same hash value and leave the signatures intact leading to loss of integrity. So the message along with the digital signature looks like this :

Z + Private_sender ( Hash ( M ) )

Digital Certificates

In addition to using the public key we would like to have a guarantee of talking to a known person. We assume that there is an entity who is entrusted by everyone and whose public key is known to everybody. This entity gives a certificate to the sender having the sender's name, some other information and the sender's public key. This whole information is encrypted in the private key of this trusted entity. A person can decrypt this message using the public key of the trusted authority. But how can we be sure that the public key of the authority is correct ? In this respect Digital signatures are like I-Cards. Let us ask ourselves the question : How safe are we with I-Cards? Consider a situation where you go to the bank and need to prove your identity. I-Card is used as a proof of your identity. It contains your signature. How does the bank know you did not make the I-Card yourselves? It needs some proof of that and in the case of I-Cards they contain a counter signature by the director for the purpose. Now how does the bank know the signature I claim to be of the director indeed belongs to him? Probably the director will also have an I-Card with a counter signature of a higher authority. Thus we will get a chain of signing authorities. Thus in addition to signing we need to prove that the signatures are genuine and for that purpose we would probably use multiple I-Cards each carrying a higher level of signature-counter signature pair.

So in order to distribute the public key of this authority we use certificates of higher authority and so on. Thus we get a tree structure where the each node needs the certificates of all nodes above it on the path to the root in order to be trusted. But at some level in the tree the public key needs to be known to everybody and should be trusted by everybody too.

Wireless Networks

Introduction

As the need of communication became more and more demanding, new technologies in the field of networks developed. One of them is the use of wireless networks. It is the transmission of data from source to destination without the use of wires as the physical media.

Why to use Wireless?

Three reasons may be stated for the over-growing use of wireless networks across the world:

1. They are ubiquitous networks. As the do not require messy wires as a medium of communication, they can be used to connect far-off places.
2. They are cheaper than wired networks specially in the case of long-distance communication.
3. They are pretty effective and fast, especially with the modern advancements in this field.

Some Terms and Technologies:

ATM-Asynchronous Transfer Mode:
ATM is a connection-oriented switching technology. It was built to support ISDN (Integrated Services Digital Network). ISDN required high speed cables for both its narrow band (64 Kbps) and broad band (155 Mbps) transmission. There were two technologies available for transmitting data-

1. Circuit Switching: In this technology, when a user makes a call, the resources are reserved for him. The advantage of this technology is that it prevents collisions among various users. But the disadvantage is that it leads to inefficient utilization of bandwidth-- if the user fails to send data or if the transmission speed is faster than the speed of sending data. then most of the bandwidth is wasted.
2. Packet Switching: In this technology, resources are never reserved for any particular user. The advantage of this technology is that it leads to efficient utilization of bandwidth i.e. the channel is never free until & unless there are no users, But the disadvantage is that it causes many collision.

ATM was built as a combination of the best features of these two. Also ATM provides QoS (Quality of Service) based on the following priority pattern:

1. CBR-Constant Bit Rate: Jobs that can tolerate no delay are assigned the CBR priority. These jobs are provided same number of bits every frame.. For example, viewing a video reel definitely requires some blocks in every frame.
2. VBR-Variable Bit Rate: Jobs that may produce different sized packets at different times are assigned VBR priority. They are provided with a variable number of bits varying between a maximum and a minimum in different frames. e.g.. a document may be compressed differently by different machines. Transmitting it will be a variable transmission.
3. ABR-Available Bit Rate: This is the same as VBR except that it has only the minimum fixed. If there are no CBR or VBR jobs left, it can use the entire frame,
4. UBR-Unavailable Bit Rate: These jobs are the least priority jobs. The network does not promise anything but simply tries its best to transmit it.

WLAN-Wireless LAN
This is currently being used as dictated by the standards of IEEE 802.11. It can be installed at the medium access layer and the data transmission can occur using a converter to reach the wired LAN network.( IEEE 802.x)

WATM-Wireless ATM
It is the wireless version of ATM. It provides QoS. It is not yet available in market. because installing it will require the simultaneous installation of ATM infrastructure. It is currently being tested thoroughly.

Coupling of Networks:
The alternatives are:

1. WLAN LAN
2. WATM LAN
3. WLAN ATM
4. WATM ATM

1. WLAN-LAN is the simplest of the above. According to the IEEE standards, the IEEE 802.11 (WLAN) can be used with IEEE 802.x (LAN) as follows:
2. WLAN-ATM- NOT FEASIBLE.
3. WATM-LAN- NOT FEASIBLE because WATM requires an infrastructure of the type ATM
4. WATM-ATM-this is also a simple scheme because WATM can run on ATM.

Issues involved in Wireless Networks

• Cost and Speed: As it is being considered as an alternative to wired networks, it should be faster and cheaper.
• Quality of Transmission: It gives a higher BER (Bit Error Rate). The BER is greater than 10 ^-6. This is caused because transmission quality depends highly on the physical media including landscape, weather etc.
• RayLeigh Fading: The data has to travel the distance through a medium like air. Several rays of the same stream cause Rayleigh fading due to interference. This causes poor transmission.
• Multipath Propagation: Similarly, due to multipath propagation, the signal received at the destination may be garbled.
• Hand-Offs: If hand-offs are used i.e., hexagonal cells each having a base station and many mobile terminals, two Mobile terminals that are far enough can use the same bandwidth. This reuse of bandwidth is helpful.
• Dynamic Physical Characteristics: The terminal may be mobile and constantly moving. Thus the distance between the base station and any active terminal may be constantly changing. This has to be taken into account while designing.
• Practical Implementation: The practical implementation of any wireless network requires CSMA/CD for proper transmission. The range of any terminal is fixed. So, there may be two terminals that are out of range of each other. These are called HIDDEN TERMINALS. Collisions may be caused due to simultaneous sending of data from any two hidden terminals. The HIDDEN TERMINAL PROBLEM should be overcome with the help of Base Station.
• .Mobility and Network Topologies: Wireless networks should be effective enough to overcome the problems caused by the topology of the area and the mobility of the terminals
• Frequency Allocation: Licensed & Unlicensed: For licensed networks, permission has to be taken from the authorities that grant you a fixed bandwidth which is not used by anybody else while unlicensed networking does not require any such permissions. It just provides with some unlicensed bands which can be used by anybody. Unlicensed bands may thus, cause collisions.
• Capture Effect: If there are more than one terminals requiring the attention of the Base Station, the one nearer to the base station may capture it. This unfair access to the base station should be prevented.
• Power Requirements and Battery: This problem arises for the Mobile Terminals that run battery or cells. Much dissipation of power is caused when switching from receiving mode to sending mode and vice versa.
• Human Safety: Not all bandwidths can be used . Also, the intensity should not be very high as it may lead to several complications in human body e.g.. cataract.

Wireless Physical Media

In the wireless physical media, three technologies are used:

1. Transmission at Infrared frequency: This is easier to build and set-up. It is mainly used for indoor purposes because the beam has to be focussed and can't cross opaque media like walls etc.
2. Transmission through Microwave: This is preferred as it requires low power consumption. (the bandwidth is fixed) But the basic problem is that it requires Line-of-Sight. Also, it requires license.
3. Transmission at Radio Frequency: This is the one that is most familiar to us. The bandwidth is pretty large.

Integrity and Security of the signal

Spread Spectrum: To reduce the effect of noise signals, the bandwidth of the signal is increased tremendously. This is costly but assures better transmission. This is called SPREAD-SPECTRUM. This is used in two ways:

• FHSS (Frequency hopping spread spectrum): The entire packet is not sent at the same bandwidth. Say, it is sent at frequency range A for time T1, frequency range B for time T2, A for T1, B for T2 and so on. The receiver also knows this sequence and so, looks at A for time T1, then at B for time T2 and so on. Thus this sort of understanding between the sender and receiver prevents the signal from being completely garbled .
• DSSS (Direct Sequence Spread Spectrum): This involves sending of coded data instead of the actual data. This code is known to the destination only which can decipher the data now.

The problem still left undealt is that of bursty errors. If there is lot of traffic, interference may hinder the Base Station from receiving data for a burst of time. This is called "Bursty Errors".
Such problem are looked at by MAC-Medium Access Control.

MEDIUM ACCESS CONTROL

To control the traffic, various techniques are used. MAC fulfills the following requirements:

1. QoS Requirements: It provides Quality of Service according to the priority of jobs.
2. Error Control: Error handling is done using some codes.
3. Frame Size: To transmit maximum data, we want the frame-size to be maximum but at the same time, large frame-size highly increases the probability of errors. So, MAC provides a tradeoff between the above two factors determining the size of the frame.
4. Secure Transmission: The data meant for a particular receiver is secured from others.
5. Reasonable Transmission: If the number of users increases, each should get reasonable service. MAC prevents unfair access to channel.
6. Efficient utilization of Power: If a transmitter is always on, it is continuously using power even if there is no data on the channel for it. This is reduced by sending the transmitter to "sleep mode" whenever the battery is going down. In this mode, the transmitter is unable to receive any data.

Architecture for Wireless Network

There are two types of architecture possible:

1. AD-HOC NETWORK
2. INFRASTRUCTURE NETWORK

The Ad-Hoc network can be set up anytime. It does not require a Base Station. It is generally used for indoor purposes.
The Infrastructure network involves Base Station and Mobile Terminals. It provides uplink facility ( link from MT to BS) and downlink facility (link from BS to MT).

THE MAC PROTOCOL

This protocol decides how to assign data slots to different users. The various policies it uses are:

1. Fixed Assignment Policy
2. Random Assignment Policy
3. Centrally Controlled Policy
4. Distributed Controlled Policy
5. Hybrid Controlled Policy

Fixed Assignment Policy:
In this policy, each terminal is assigned some sort of data slot to speak. It causes a fixed delay. It is done in 3 ways:

1. TDMA (TIME DIVISION MULTIPLE ACCESS) : Each user is given a fixed time to speak., after which the chance goes to another user. This cycle continues indefinitely.
2. FDMA (FREQUENCY DIVISION MULTIPLE ACCESS): Each user is given a fixed bandwidth in which he can speak at all times.
3. CDMA (CODIVISION MULTIPLE ACCESS): Each user is given different frequencies at different times. This ensures that each user gets a fair amount of channel each time.

Also, sometimes, statistical multiple access is used in which a slot is assigned to a user only if it has data to send.

Random Assignment Policy
In this policy, contention slots are provided to all the users. Problem may arise if the number of users increase drastically. The number of contention slots should be variable. This may cause some limiting of data slots but is necessary to prevent the derailment of the service .

Centrally Controlled Policy:
This is used in an infrastructure architecture. It involves the participation of a Base Station which may assign slots and priorities(CNBR,VBR etc.) to all the users.

Distributed Controlled Policy:
This is used in Ad-Hoc architecture. The control is among the terminals which decide among themselves about who is going to speak first.

Hybrid Controlled Policy:
This combines the best features of centrally controlled and distributed controlled policies.

KINDS OF MAC PROTOCOLS:

There are two kinds of Mac protocols:

1. FDD (Frequency Division Duplex) This provides two separate bandwidths for uplink and downlink transmission. This leads to inefficient utilization of bandwidth as there is more traffic on downlink than uplink
2. TDD (Time Division Duplex) This provides an adoptive boundary between the uplink and downlink frequency which depends on the what is being used at that particular time. It works as follows:

Any mobile terminal can be in 3 states : empty state, request state and ready-to-transmit state.

1. uplink-MT1 sends a random-access request to BS to communicate with MT2
2. downlink: BS sends a b-bit access id to MT2
3. uplink: MT1 sends the packet
4. downlink: BS sends the packet to MT2

The TDD is more in use now-a-days.

Firewalls

Introduction

This lecture discusses about security mechanisms in the Internet namely Firewall . In brief, It's a configuration of routers and networks placed between an organization's internal internet and a connection to an external internet to provide security. In other words, Firewall is a mechanism to provide limited access to machines either from the outside world to internal internet or from internal world to outside world. By, providing these security mechanisms, we are increasing the processing time before one can access a machine. So, there is a trade-off between security and ease of use. A firewall partitions an internet into two regions, referred to informally as the inside and outside.

__
| | _________ Firewall
______________________ | | ____________________
| | | | | |
| | | | | |
| Rest of Internet |________ | |_____ | Intranet |
| | | | | |
|_____________________ | | | |___________________|
|_|
Outside Inside

Security Lapses

• Vulnerable Services - NFS : A user should not be allowed to export certain files to the outside world and from the outside world also, someone should not be allowed to export our files.
• Routing based attacks : Some kind of ICMP message should not be allowed to enter my network. e.g.. Source routing and change route ICMP's.
• Controlled access to our systems : e.g.. Mail server and web pages should be accessible from outside but our individual PC's should not be accessible from the outside world.
• Authentication : Encryption can be used between hosts on different networks.
• Enhanced Privacy : Some applications should be blocked. e.g.. finger ...
• PING & SYN attack : Since these messages are send very frequently, therefore you won't be able to do anything except reply to these messages. So, I should not allow these messages to enter my network.

So. whatever I provide for my security is called Firewall. It is a mechanism and not just a hardware or software.

Firewall Mechanisms

1. Network Policy : Here, we take into consideration, what services are allowed for outside and inside users and the services which are allowed can have additional restrictions. e.g.. I might be allowed to download things from the net but not upload i.e.. some outside users cannot download the things from our net. Some exceptional cases might be there which have to be handled separately. And if some new application comes up then , we choose an appropriate network policy. 2. Authentication mechanism : An application can be designed which ask for a password for authentication.

3. Packet Filtering : Router have information about some particular packets which should not be allowed.

4. Application gateways : or proxy servers.

Certain Problems with Firewall

1. Complacency : There are lots of attacks on the firewall from internal users and therefore, it's limitations should be understood.

2. Encapsulated packets : An encapsulated packet is an IP packet within another IP packet. If we ask the router to drop encapsulated packets then, it will drop the multicast packets also.

3. Throughput :So, in order to check which packets are allowed and which are not, we are doing some processing which can be an overhead and thus affects throughput.

Authentication:

We can use the following mechanisms:

• One time passwords: passwords are used only once and then it changes. But only the user and the machine knows the changing passwords.
• password aging : User are forced to change passwords after some time on regular intervals.
• smart cards : swipe through the PC.
• biometrics : eyes or finger prints are used.

Packet Filtering :

Terms associated:

• Source IP address
• Destination IP address
• Source port #
• Destination port #
• protocol
• interface

Many commercial routers offer a mechanism that augments normal routing and permits a manager to further control packet processing. Informally called a packet filter, the mechanism requires the manager to specify how the router should dispose of each datagram. For example, the manager might choose to filter (i.e.. block) all datagrams that come from a particular source or those used by a particular application, while choosing to route other datagrams to their destination.

The term packet filter arises because the filtering mechanism does not keep a record of interaction or a history of previous datagrams. Instead, the filter considers each datagrams separately. When a datagram first arrives, the router passes the datagram through its packet filter before performing any other processing. If the filter rejects the datagram, the router drops it immediately.

For example, normally I won't allow TFTP, openwin, RPC, rlogin, rsh packets to pass through the router whether from inside or outside and router just discard these packets. But I might put some restrictions on telnet, ftp, http, and smtp packets in order to pass through the router and therefore some processing is to be done before discarding or allowing these packets.

Because TCP/IP does not dictate a standard for packet filters, each router vendor is free to choose the capabilities of their packet filter as well as the interface the manager uses to configure the filter. Some routers permit a manager to configure separate filter actions for each interface, while others have a single configuration for all interfaces. Usually, when specifying datagrams that the filter should block, a manager can list any combination of source IP address, destination IP address, protocol, source protocol port number, and destination protocol port number.
So, these filtering rules may become more tricky with complex network policies.

Since, Filtering rules are based on port numbers, there is a problem with RPC applications. First, the number of well-known ports is large and growing. Thus, a manager would need to update such a list continually because a simple error of omission could leave the firewall vulnerable. Second, much of the traffic on an internet does not travel to or from a well-known port. In addition to programmers who can choose port numbers for their private client-server applications, services like Remote Procedure Call (RPC) assigns port dynamically. Third, listing ports of well-known services leaves the firewall vulnerable to tunneling, a technique in which one datagram is temporarily encapsulated in another for transfer across part of an internet.

Relay Software (proxies) :

I can run multiple proxy on same machine. They may detect misuse by keeping loops. For example, some machine give login to Ph.D.. students. So, in this case it's better to keep proxy servers than to give login on those machines. But the disadvantage with this is that there are two connections for each process.

_________ __________
| | | |
| User |_______________| Proxy |___________ Outside
| ________| 1. |_________ | 2.

Various Firewall Considerations

1. Packet Filtering Firewall
This is the simplest design and it is considered when the network is small and user don't run many Intranet applications.
__________
| |
Intranet __________| Router |__________ Internet
|________ _ |
|
|
Filter

2. Dual home gateway
This gives least amount of flexibility. Instead of router, we have application gateways.
______________
| Application |
Inside ________ _ | level |___________ Outside
| gateway |
|____________ |
proxy

3. Sreened host Firewall
It's the combination of the above two schemes. Some applications are allowed uninterrupted while some have to be screened. For any reasonable size network, Screened host firewall can get loaded.

_________ ___________
| | | |
Inside _________| Router 1 |_______________________ | Router 2 |______ Outside
|_________| | |__________ |
____|______
| |
| Proxy |
|__________|

The problem with this is that there is only one proxy and thus, it may get overloaded. Therefore, to reduce load, we can use multiple screened host firewalls. And this is what normally used.

_________ __________
| | | |
Inside _____ | Router 1 |______________________________ | Router 2 |_____Outside
|_________| | |__________ |
____|____
| |
| Proxy 1 | Proxy2 .......
|________ |

Modem pool

User can dial and open only a terminal server but he has to give a password. But TELNET and FTP client does not understand proxy. Therefore, people come out with Transparent proxy which means that I have some memory which keeps track of whether this packet was allowed earlier or not and therefore, I need not check this time. Client does not know that there is somebody who is checking my authentication.
So, transparent proxy is used only for checking the IP packets whereas proxy is used when many IP addresses are not available.

Private IP (PIP address)
It is an extension of transparent proxy. Here we also change the IP address (source address) to one of the allocated IP address and send it. So, the client does not know that the IP address has been changed, only the proxy server knows it. The machine that changes the IP address is Network address translator (NAT) . NAT also changes other things like CRC, TCP header checksum ( this is calculated using pseudo IP header). NAT can also change the port number.

e.g.. Port address translation

____________
X -------| |
| NAT |
Y -------|___________ |

X₁ , P₁ ----> G₁ , P_a (IP address, port #)
X1 , P2 ----> G1 , Pb
Y , P3 ----> G1, Pc

I may not like to have global IP address because then, anybody can contact me inspite of these security measures. So, I work with Private IP. In that case, there has to be a one-to-one mapping between private IP and global IP.

---

Routing in Internet

The Origin of Internet

The response of Internet to the issue of choosing routing tables with complete/par tail information is shown by the following architecture. There are a few nodes having complete routing information and a large number of nodes with partial information. The nodes with complete information, called core gateways, are well connected by a Backbone Network. These nodes talk to each other to keep themselves updated. The non-core gateways are connected to the core gateways. (Historically, this architecture comes from the ARPANET.)

The original internet was structured around a backbone of ARPANET with several core gateways connected to it .These core gateways connected some Local Area Networks (LANs) to the rest of the network. These core gateways talked to themselves and exchanged routing information's. Every core gateway contained complete information about all possible destinations.

How do you do routing ?

The usual IP routing algorithm employs an internet routing table (some times called an IP routing table) on each machine that Stores the information about the possible destinations, and how to reach them.

Default Routes

This technique used to hide information and keep routing table size small consolidates multiple entries into a default case. If no route appears in the routing table, the routing routine sends the data gram to the default router.

Default routing is especially useful when a site has a small set of local addresses and only one connection to the rest of the internet.

Host-Specific Routes

Most IP routing software allows per-host routes to be specified as a special case. Having per-host routes gives the local network administrator more control over network use, permits testing, and can also be used to control access for security purposes. when debugging network connections or routing tables, the ability to specify a special route to one individual machine turns out to be especially useful.

Internet with Two Backbones

As long as there was just one single router connecting ARPANET with NSFNET there was no problem. The core gateways of ARPANET had information about all destinations and the routers inside NSFNET contained information about local destinations and used a default route to send all non-NSFNET traffic to between NSFNET and ARPANET as both of them used different matrices to measure costs. the core gateways through the router between ARPANET and NSFNET. However as multiple connections were made between the two backbones, problems arise. Which route should a packet from net1 to net2 take? Should it be R1 or R2 or R3 or R4 or R5? For this some exchange of routing information between the two backbones was necessary. But, this was again a problem as how should we compare information.

Gateway-To-Gateway Protocol (GGP)

This was the protocol used by the core-routers to exchange routing information among themselves. This is based on Distance Vector Algorithm and uses number of hops as the distance metric. This is a very poor metric as this does not take into account the load on the links and whether a link is slow or fast. A provision is made to manually increment the hop count in case a link is particularly slow.A protocol based on Shortest Path First Algorithm , known as SPREAD ,was also used for the same purpose.

Added Complexity To The Architecture Model

As the number of networks and routers increased, to reduce the load on the core gateways because of the enormous amount of calculations, routing was done with some core gateways keeping complete information and the non-core gateways keeping partial information.

In thisarchitecture, G₁ ,G₂ ,G₃ are all core gateways and G₄ and G₅ are non-core gateways. We must have a mechanism for someone to tell G₂ that it is connected to net2 , net3 and net4 , besides net1. Only G₅ can tell this to G₂ and so we must provide for a mechanism for G₂ to talk to G₅ . A concept of one backbone with core gateways connected to Autonomous Systems was developed. An Autonomous system is a group of networks controlled by a single administrative authority. Routers within an autonomous system are free to choose their own mechanisms for discovering , propagating ,validating , and checking the consistency of routes. Each autonomous system must agree to advertise network reachability information to other autonomous systems. Each advertisement propagates through a core router. The assumption made is that most of the routers in the autonomous system have complete information about the autonomous system. One such router will be assigned the task of talking to the core gateway.

Interior Gateway Protocols (IGP)

IGP is a type of protocols used by the routers in an autonomous system to exchange network reachability and routing information. Some of IGPs are given below.

Routing Information Protocol (RIP)

This is one of the most widely used IGP. It was developed at Berkeley. This is also known by the name of the program that implements it, routed .This implements Distance Vector algorithm.Features of RIP:

• RIP uses a hop count metric to measure the distance to a destination. To compensate for differences in technologies, many RIP implementations allow managers to configure artificially high hop counts when advertising connections to slow networks. All routinfg updates are broadcast. This allows all hosts on the network to know about the routes.
• To prevent routes from oscillating between two or more equal cost paths, RIP specifies that existing routes should be retained until a new route has strictly lower cost. Since RIP does not explicitly detect routing loops, RIP must either assume participants can be trusted (being part of one autonomous system) or take precautions to prevent such loops.
• To prevent instabilities, RIP must use a low value for the maximum possible distance.RIP uses 16 as the maximum hop count. This restricts the maximum network diameter of the system to 16.
• To solve the slow convergence problem arising due to slow propagation of routing information, RIP uses Hold Down. If a particular link is down , any new information about that link is not accepted till some time. This is because the router must wait till the information aboutthe link being down propagates to another router before accepting information from that router about that down link.
• RIP runs on top of TCP/IP. RIP allows addresses to be of a maximum size of 14 Bytes. The Distance varies from 1 to 16 (where 16 is used to signify infinity). RIP address 0.0.0.0 denotes a default route. There is no explicit size of the RIP message and any number of routes can be advertized.

The message format is as shown:

OSPF(Open Shortest Path First )

This is an Interior Gateway Protocol designed by the Internet Engineering Task Force ( IETF ). This algorithm scales better than the vector distance algorithms. This Protocol tackles several goals:

• OSPF includes type of service(ToS) routing. So, you can installmultiple routers to a given destination, one for each type of service. When routing a datagram, a router running OSPF uses both the destination address and type of service fields in the IP Header to choose a route.
• OSPF provides load balancing. If there are multiple routes to a given destination at the same cost, OSPF distributes traffic over all the routes equally.
• OSPF allows for creation of AREA HIERARCHIES. This makes the growth of the network easier and makes the network at a site easier to manage. Each area is self contained, so, multiple groups within a site can cooperate in the use of OSPF for routing.
• OSPF protocol specifies that all exchanges between the routers be authenticated. OSPF allows variety of authentication schemes, and even allows one area to choose a different scheme from the other areas.
• To accomodate multi-access networks like ethernet, OSPF allows every multi-access network to have a designated router( designated gateway).
• To permit maximum flexibility, OSPF allows the description of a virtual network topology that abstracts away from details of physical connections.
• OSPF also allows for routers to exchange routing information learned from other sites. The message format distinguishes between information acquired from external sources and information acquired from routers interior to the site, so there is no ambiguity about the source or reliability of routes.
• It hastoo much overhead of sending LSPs but is gradually becoming popular.

Exterior Gateway Protocol (EGP)

If two routers belonging to two different autonomous systems exchange routing information ,the protocol used is called EGP . EGP consists of:

• Acquisition Request: A router sends a request to another neighbour router saying 'I want to talk'.
• Acquisition Confirm: This is a positive reply to the Acquisition request.
• Acquisition Refuse: This is a negative response to the Acquisition request.
• Cease Request: This requests termination of neighbour relationship.
• Cease Confirm: This is a confirmation response to the Cease Request.
• Hello : This is used to find if the neighbour router is up or down.This requests router to respond if alive.
• I Heard You: This is a response to the Hello message confirming that the router is alive. Because it is possible for Hello or I Heard You messages to be lost in transit, EGP uses a k-out-of-n rule to determine whether a network is down.At least k of the last n messages must fail for the router to declare its neighbour down.
• Poll Request: This is a request for network routing update.
• Routing Update: This conveys routing information about reachable networks to its EGP neighbour. The routing information is the distance vector of the reachable networks.
• Error: This is a response to an incorrect message.

EGP is used only to find network reachability and not for differentiating between good and bad routes. We can only use distance metric to declare a route plausible and not for comparing it with some other route (unless the two route form part of a same autonomous system). Since there cannot be two different routes to the same network, EGP restricts the topology of any internet to a tree structure in which a core system forms the root. There are no loops among other autonomous systems connected to it. This leads to several problems:

• Univerasal connectivity fails if the core gateway system fails.
• EGP can advertise only one path to a given network.
• EGP does not support load sharing on routers between arbitrary autonomous systems.
• Multiple backbone networks with multiple connections between them cannot be handled by EGP.

Border Gateway Protocol(BGP)

BGP is a distance-vector protocol used to communicate between different ASes. Instead of maintaining just the cost to each destination,each BGP router keeps track of the exact path used.Similarly,instead of periodically giving each neighbour its estimated cost to each destination, each BGP router tells its neighbours the path it is using.Every BGP router contains a module that examines routes to a given destination and scores them returning a number for destination to each route. Any route violating a policy constraint automatically gets a score of infinity. The router adapts a route with shortest distance.The scoring function is not a part of the BGP protocol and can be any function that the system managers want.BGP easily solves the count to infinity problem that plagues other distance-vector algorithms as whole path is known.