Lecture – 33 DHCP and ICMP

Lecture – 33 DHCP and ICMP

Good day. Today we shall talk about some protocols which
are useful for controlling the network and also in keeping machines connected to the
network. Specifically under the broadcast we will talk
about DHCP and ICMP. There are some protocols associated with this
so we shall discuss about it. DHCP is Dynamic Host Configuration Protocol. It is about configuring a host, configuring
a machine may be a PC or some computers connected to the network. The chief motivation came from the dynamic
assignment of IP addresses. The dynamic assignment of IP addresses is
desirable for several reasons:IP addresses can be assigned on demand. For example, when you have a scarcity for
real IP addresses then you keep a central pool of IP addresses and
then as some computer comes on line it assigns an IP address from the pool and when it goes
out those IP addresses are withdrawn and are given to some other machines. Another place where it may be required is,
suppose you have some kind of a RAS or Remote Access Server to which a number of machines
should be connected via dial up connections then in that case you give them a temporary
IP address for the connection. Now if somebody wants to visit some network
with a laptop then they have to get a network address of that particular network therefore
that network address has to be dynamically assigned. IP addresses are assigned on demand.It avoids
manual IP configuration which is prone to errors. It also supports mobility of laptops. Dynamic assignment of IP addresses is done
using three different protocols. 1. RARP: It was widely used up to 1985 and even
beyond this period people kept using it. 2. BOOTP: Bootstrap protocol was used until 1993
3. DHCP: Is used after 1993 and currently this
is in wide usage. A Bootp client can also use DHCP server. RARP is actually the reverse ARP Address Resolution
Protocol. The problem is, given an IP address what is
the MAC address. This is for mapping between the IP addresses
and the MAC addresses. Finding the MAC address for the IP address
is useful when you want to communicate over a LAN. RARP is the reverse of this. Given a MAC address, RARP finds IP address. This would be necessary in case you have something
like disclosed work station which boosts the signal over the network. A disclosed work station has its own MAC address
and it wants to get an IP address assigned. This is where RARP is typically used. RARP is used to broadcast a request for IP
address associated with a given MAC address. RAP server responds with an IP address. It only assigns IP address and not the default
router, subnet masks etc that are required and they are not a part of this server. So IP address to MAC address is the ARP and
Ethernet MAC address to IP address is the RRAP.Let us see the improved version of RARP
i.e. BOOTP.BOOTP not only assigns IP addresses
dynamically but also has some more functions. Host can configure IP parameters at boot time. Basically there are three services: IP address
assignment Detection of the IP address for a serving machineThe name of the file to be
loaded and executed by the client machine i.e. the boot file name This is the source
from which it gets the name bootstrap protocol i.e. when the machine is booting up it not
only gets an IP address but also gets the name of the file which can be loaded and executed. This is the bootstrap protocol or BOOTP. BOOTP not only assigns IP address but also
default router, network mask, etc. Therefore whatever that particular machine
requires for communication namely the addresses such as network, subnet mask, gateway etc
are given by the BOOTP protocol. This is sent as an UDP message. So UDP port 67 is for server and UDP port
68 is for the host. Port 68 for host is required when you may
want to find out a machine from bootstrap protocol which is already available on the
network. And use limited broadcast address that is
255. 255. 255. 255. If you recall from our discussion about addresses
this is a broadcast address where the broadcast is limited to this particular subnet or network. BOOTP can be used for downloading memory image
for diskless workstations. So whatever was the motivation for RARP the
same thing can be done through BOOTP also. But assignment of IP address to hosts is static. This is one sort of drawback of BOOTP. To make it dynamic we go to the dynamic host
configuration protocol which is standard now and more versatile than RARP and BOOTP. It can do a lot of things apart from just
giving the IP address. This was designed in 1993 as an extension
of BOOTP with many similarities to BOOTP and same port numbers as BOOTP. That is why DHCP server can handle a few BOOTP
clients. Extensions: There are lots of extensions especially
with options. But these extensions support temporary allocation
or leases of IP addresses. Leasing of IP address, suppose when we have
a remote access server and when people are dialing what would happen is that, it would
be given a particular IP address for a fixed amount of time. When its lease expires then that IP address
may be withdrawn. And half way down the lease period if there
is no great demand for IP address then the lease may be automatically extended or if
there is a great demand the lease may be withdrawn also. This is for a temporary period of time and
that is how it is dynamic. DHCP client can acquire all IP configuration
parameters. Not only subnet mask and gateway addresses
which are there in BOOTP but also other kinds of parameters can be downloaded from a DHCP
server.So DHCP is the preferred mechanism for dynamic assignment of IP addresses and
DHCP can interoperate with BOOTP clients. DHCP has a number of options. It is not possible to mention all the available
options here. Other DHCP information is sent as an option
so the number of options is actually greater than 100 which include things like subnet
mask, name server, host name, domain name, forward on/off, default IP time to leave,
broadcast address, static route, Ethernet encapsulation, x window manager, x window
font, DHCP message type, DHCP renewal time, DHCP Rebinding, time server SMTP server, client
FQDN, printer name etc. As the number of services given over a network
grew it became important to give more information to the machines. Originally the machine was used just for communicating
between two computers. Suppose there may be a centralized print service
in the network and whenever you want to print something it can be done in the network. Similarly all other kinds of services became
available in the local network as well as over wider networks. So all these would require some kind of configuration
on the host end therefore such information can be transferred via this DHCP. There are a number of DHCP operations. Let us discuss a few of them. DHCP DISCOVER: At this time the DHCP client
can start to use the IP address.Renewing a lease: It is sent when 50% of the lease has
expired. If DHCP server sends DHCPNACK then the address
is released. Then you know your lease is not going to be
renewed.DHCP RELEASE: At this time the DHCP client has released the IP address, so the
client has given it up. DHCP message header fields: In some fields
there is an opcode. It may be a DHCP request from the client or
it may be DHCP reply from the server. The DHCP message type is sent as an option. The hardware type of message is 1 for Ethernet
and hardware address length is 6 for Ethernet. Hop count is set to 0 by client and transaction
ID is an integer used to match reply to response if there is more then one request. Seconds: It is the number of seconds since
the client started to boot. Client IP address, your IP address, server
IP address, gateway IP address, client hardware address, server host name, boot file name,
etc. All these fields are available so when the
client sends the request it would fill in whatever is known to it, maybe the MAC address
is known to it. So it puts in the MAC address and all other
fields are left blank. DHCP server will pick up the message that
we broadcast and then fill up all the other necessary fields and then broadcast it back. The following are the DHCP message types sent
as an option: DHCPDISCOVER
DHCP not acknowledge DHCPRELEASE
DHCPINFORM and so on Our next topic is ICMP Internet Control Message Protocol. Let us see IP protocol and its deficiencies
before that. The internet is of course based on the Internet
Protocol. IP protocol has some drawbacks. Though it is a best effort delivery service
it lacks error control and lack of assistance mechanisms. Since IP is a best effort delivery at some
point of time the effort may not be enough and routers ore other nodes on the network
may have to drop packets and packets may not reach its destination on time and in proper
order. First of all there is no error control and
secondly if such errors do occur there is no message to the sources. Secondly, if you want to control the network
for some reason, for example, may be the network is getting congested and so you want to do
something about it, but IP does not have the mechanism. So, for all these purposes ICMP was brought
in. Therefore what happens if a router must discard
a datagram because it cannot find a route to the final destination? What if the time to live field has zero value? What if it has to discard all the fragments
because not all were received in a predetermined time limit? In all these cases IP has to discard a packet. And similarly there are other situations. For example, may be it has reached the destination
but the port is not available. So IP protocol also lacks a mechanism for
host and management queries. So ICMP was designed to compensate for these
deficiencies. ICMP is a type field that indicates the type
of ICMP message being sent and the type may be queries or errors. Code field gives further information specific
to the ICMP message. For example, when an error occurs it tells
what kind of error it is. Checksum field is used to verify the integrity
of the ICMP data. So once again the checksum is included to
control the error. There are two types of ICMP messages. One is error reporting and the other is query
response. If there is some error then the error reporting
type of ICMP message would be generated and if there is a query another type of ICMP message
would be generated. There is no effort in ICMP to correct the
errors. This is the job of some other layer. So it does not really try to correct the errors
but nearly reports the errors. The error messages are sent to the source. Suppose the datagram has been sent and something
has happened to it and due to that there is an error, and now whoever drops that packet
send an ICMP message back to the source. It may be a router on the way or may even
be the final destination. These are the various types of errors in error
reporting. There may be a destination unreachable, there
may be a source quench sending to first. Some of the important ones may be time exceeding,
some may be parameters problem or redirection, etc. Please note that, no ICMP error message would
be generated in response to a datagram carrying an ICMP error message. That means, somebody has generated an ICMP
error message and it is traveling back to the source, and that error message itself
gets an error and may have to be dropped on the way, then in such cases we do not generate
another ICMP message. A little bit of problem happens due to congestion
of networks. So if the network is very congested many packets
may get dropped. And then if in response to dropping many packets
you generate more packets then the congestion is not going to go away. So, ICMP error messages do not trigger other
ICMP error messages for a fragment datagram that is not the first set of fragment. For example, the datagram may have been fragmented
into a number of parts, may be fifty parts, now for each of them you generate an ICMP
message. Then the ICMP message would be too many so
it is only generated for the first fragment. For a datagram having a multicast address,
once again we cannot send an ICMP messages to all members of the group and for a datagram
with a special address such as 127. 0000 or with some address like also
no ICMP error messages are generated for these. All error messages contain a data section
that includes the IP header of the original datagram plus the first 8 bytes of data in
that datagram. This information is required so that the source
can inform the protocols about the error. From the original packet that was dropped
the IP header of that original packet is sent back. First of all you need to know the source and
know where you want to send back this ICMP message. Secondly, even after this ICMP message gets
back to the specific machine from which the original packet was generated. At this point it may have some error messages
due to network intervening or this may have to do something with some process or application
which is running on the source machine. So, after getting the message the host must
know to which process it relates to. After the IP header
what comes is the transport layer header so, a part of the transport layer header also
goes back along with the ICMP message .This information is required so that the source
can inform the protocols about the error. Destination Unreachable: This is one type
of an error message. When a router cannot route a datagram or a
host cannot deliver a datagram, then in that case the destination is unreachable. A router cannot detect all problems that are
preventing the delivery of a packet. So it is not always possible to exactly know
why the destination is unreachable. But at least this information that the destination
is unreachable, goes back to the source. Source Quench: This is a crude attempt to
implement some kind of flow control. IP protocol has got no flow control. Routers and hosts have limited size queues. So what happens is that, may be in an intermediate
router and a number of packets have come up and certainly there is a flood of packets
into one intermediate router from various directions. So what would happen is that its buffer is
going to overflow and it will not be able to process because there is a limit depending
on the speed of the router etc, there is a limit as to how fast packets can be processed
and forwarded by an intermediate router and if other packets keep coming in, within that
time they are going to be stored in the buffer where in the buffer might overflow. In that case the router cannot do anything
else but to drop those packets. This router desperately wants to tell other
people in the network to slow down on sending packets and that it cannot handle it because
of overload. Basically it tries to slow down the flow of
packets into itself. So it sends the source quench ICMP message
towards the sources. If datagram is received faster than they can
be processed the queue may overflow and in that case it asks the network to slow down. If a router or host discards a datagram due
to congestion it sends a source quench message to the sender. The source must slow down the sending of datagram
until the congestion is relieved. This may be used when bottlenecks occur. For example, on a WAN link with too much congestion
it is used to reduce the amount of data lost. But a warning is, source quench message will
in turn generate network congestion. There were already too many packets in the
network but you have sent a source quench packet towards the source which is just one
hop towards the source was already getting packets from the source but will also get
an ICMP message from the router just one hop down so it is having more packets now. So by this way congestion might travel towards
the source but anyway finally it reaches the source and the source will hopefully slow
down and all these will die. Time Exceeded: Whenever a router receives
a datagram with a time-to-live value of zero that means it has been going round the network
it discards the datagram and sends a time exceeded message to the source. When the final destination does not receive
all of the fragments in a set time it discards the received fragments and sends a time exceeded
message to the source. These are two different cases: One is that,
in the destination all the fragments did not reach so there was a specified time after
which it has to drop all the fragments and send a time exceeded message to the original
source. The other thing is that, when a router receives
a datagram with the time-to-leave field which is zero. If you remember, keeping a time-to-leave field
and decrementing it at every hop is quite important because suppose there were some
packets which were floating around in the network and due to some trouble with the routing
tables a loop has been formed, so, if you do not have this time-to-leave field it will
go round and round at infinite term where they slowly burden the network. So, the solution to that was, after a certain
number of hops the packet is dropped and when a packet is dropped a time exceeded message
is sent to the source. There may be parameter problems. If an ambiguity is found in the header of
a datagram the datagram is discarded and a parameter problem message is sent back to
the source. Redirection: A host usually starts with a
small routing table that is gradually augmented and updated. One of the tools to accomplish this is the
redirection message, so, actually this helps in routing. Now let us come to queries. ICMP can also diagnose some network problems. For example, echo request and reply, time
stamp, address mask, router solicitation and advertisements, these are example of queries. We will just see a few of these also. Echo request and reply: Is used very often
when you want to find out whether the network is up and running or not. An echo request message can be sent by a host
or router. An echo reply message is sent by the host
or router which receives an echo request message. The echo request and echo reply message can
be used by network managers to check the operation of the IP protocol. Echo request and echo reply message can test
the reachability of a host. This is usually done by invoking the ping
command. Later on we will get into more details of
ping because that is one kind of command which even users quite often require. For example, if you are logged on and you
find that you cannot reach your destination anywhere in the network then you have to find
where the problem lies, is it in your local network or in the local subnet. Therefore, in the local subnet you might ping
that gateway to see whether you can reach up to the gateway. If your ping message goes up to the gateway
and you get an echo reply then you know that up to that much the network is ok. And if you are ok up to the router you may
want to ping the router in the entire network. Now the problem may be somewhere in the link
outside. The problem may even be in the destination
which you are trying to reach. So one way is to go probing the network, even
by users is to use ping. Timestamp Request and Reply: Timestamp request
and timestamp reply messages can be used to calculate the round-trip time between a source
and a destination machine even if their clocks are not synchronized. So, sending time is equal to the value of
receiving time stamp minus value of original time stamp. So this way you can get some idea about the
round-trip time. There are other ways also. So receiving time is equal to time the packet
returned minus the value of transmit timestamp. Round-trip time is equal to sending time plus
receiving time. So the timestamp request and timestamp reply
message can be used to synchronize two clocks in two machines if the exact one-way time
duration is known. Address-Mask Request and Reply: Enables a
host to request and receive the network or subnetwork mask. It is useful for diskless stations at start
up. But we have seen the DHCP is another way of
handling this. Router Solicitation and Advertisement: Allows
request of routing information and the reply of this information. Routers can periodically send router advertisements
without being solicited. Suppose a router has just been connected to
the network, anyway the routers have to run the routing protocol like the RIP or BGP,
OSP etc, this means it needs to communicate to the neighboring routers, but how do the
other routers know there is a new router in the group. So, one way is, as soon as the router gets
connected it does some router solicitation and it advertises itself so that other routers
get to know that and slowly the entire network becomes aware of this new router which is
connected. Similarly, a link may go down and all kinds
of other things may happen. So, the exchange of router information has
to happen through some mechanism. Router Discovery Message: Host can learn about
available gateways to other networks. Host send the router solicitation message
to begin the process using the multicast address of as the destination. It can also be a broadcast message in case
a router does not accept multicast messages. When a router receives the message it will
advertise its available gateway. The checksum of the ICMP message: In ICMP
the checksum is calculated over the entire message, that is the header and data combined. This is just to keep some control over errors. Clock Synchronization: Software may require
time synchronization. So ICMP time stamp message combats this problem. It allows local host to ask for the current
time from a remote host using ICMP timestamp request. So it is type 13. Remote host uses ICMP timestamp reply which
is type 14. So, the better way of synchronizing the clocks
is to use the network time protocol. The time is the UT Universal Time. Ping and Traceroute: This is an overview. This is part of the ICMP messages. Ping sends an ICMP message to a remote host
and lets you determine if that host is responding. Actually ping uses echo and echo reply for
the ICMP message. Traceroute uses TTL fields to query all hosts
enroute to a specific destination. You can use traceroute to map a network. That means, if you want to know which is the
route you are tracing then this helps you. Ping is named after sonar. In sonar if you want to probe some place you
send an ultra sound signal just like you do in radar and if it bounces of something you
get a ping, so that is where the name comes from. If you want to send an echo request you expect
an echo reply and that is your ping. So server normally implemented in kernel uses
ICMP echo and echo reply messages. On UNIX the identifier field is set to UNIX
PID or sending process. Sequence numbers starts at 0 incremented every
time a new echo message is sent. Actually, when you ping a machine not just
one request is sent. The machine you are trying to ping or the
channel may be noisy and if that happen then your echo request or the reply may get dropped
in between. So, sending one request is not sufficient
and may be three times or five times etc you can configure it, it sends echo request and
it expects all the three or all the five replies. And if it receives none of them, then in that
case it will say that hundred percent of packet loss or it may get two out of five so it will
say sixty percent of packet loss or forty percent. Let us see one example of ping. Suppose we ping a machine 144. 16.182.1, we have pinged this machine and
then give the IP address over here. By the way if you have a name server on the
network you could also put the name over there. Ping 144. 16.182.1, 56 data bytes is your data plus
it will have some thing. So you may get a result like this: 64 bytes,
this is what you are getting from the echo reply, 64 bytes from 144 16 182 1. ICMP sequence is equal to 0 time-to-leave
is 240 and time is equal to 37 milliseconds. So it gives you some idea about how much time
it takes. Then another packet has came back as an echo
reply 64 bytes from the same machine, sequence number 1 and time is so much. For each packet it receives back as reply
it is going to print a line like this and then finally it will give you a statistics
as something like this: 13 packets transmitted, 11 packets received which means that it had
originally sent 13 packets and it got only 11 packets back so 2 packets must have got
lost. Therefore it is a 15% packet loss. And the round-trip time you may calculate
the mean, average, max. So, from the ping you can get an idea about
the round-trip time. Some details on the output sequence number
are shown for each message. In our example message returned in order but
we lost some packets. They may be returned due to out of order. Also TTL field of return message is displayed
and round-trip time is calculated at the host based on the sequence number. We can estimate not only the round-trip time
but also the bandwidth using ping. But this works only for few hops. If it is beyond a number of hops your ping
will not work. The ping packet can estimate the bandwidth
in this way: 20 byte IP header, 8 byte ICMP header, 56 byte message this can be set by
the user, so the total datagram size is that plus 76 plus 8 is equal to 84 bytes so 84
bytes were sent. Now, if it was sent through PPP it will add
about 8 bytes so the total size will be 92 bytes. So this connection looks like 92/.180/2 that
is about 1069 bytes per second. What is this 0.180? It is 92 bytes so this is time. This gives you some idea about what kind of
bandwidth you have. In this particular case the bandwidth is not
that much but it is only about 1069 bytes per second. This is a very crude estimate but you can
get some kind of feel about your immediate locality. Record Route Option: Most ping implementations
provide record route which is R option on linux, r option on windows. Each router stores its address in the IP options
field, only 9 addresses are possible. Thus round-trip is only possible for 4 routing
hops. So you can take only 4 hops and within those
4 hops you can find out that how your message went and how it came back. That is, may be it came back through different
paths or it could have returned in the same path etc. You can actually trace the route and because
of the limitation on the size that is on the number of addresses you can store you can
only route or map the network in your immediate locality. But if you want to go beyond this then you
have to use something else called Traceroute. Traceroute uses a sequence of ICMP messages
to determine the current route to a particular destination. This is actually done in an iterative fashion. Suppose I want to traceroute to a distant
machine whose IP address is known. Then I will send a message to that machine
but with a very small number for the time-to live. Therefore what will happen is that my message
will take so many hops but then it has not reached the destination so may be it must
have just started and it will be somewhere in the beginning so its time-to-leave is going
to become 0. As soon as the time-to-live becomes 0 the
intermediate node may be that router will have to drop the packet and it sends an ICMP
message back to the source. Now my program gets this ICMP message and
now it sends the same dummy message to the destination after increasing the time-to-live
by one unit. Now it is going to pass that router that had
dropped the packet in the previous instant and so it will go one more hop and then the
packet will get dropped so that router is now going to send an ICMP message back to
the source. Now we will know which router is on the way. Therefore by this way iteratively you keep
on increasing the time-to-leave one by one and you trace the entire route, that is, you
map it out. But let us see what happens when it reaches
the destination? When it reaches the destination what happens
is that this message is sent to a very unlikely port, a randomly selected port. Most probably the destination machine will
not know about this port so it will say that the port is unreachable and then that ICMP
message will come back. Now we know that we have reached the destination. Hence this way we have traced the entire route
one by one from the source to the destination. Traceroute uses a sequence of ICMP messages
to determine the current route to a particular destination. The TTL specifies the number of hops a message
can travel. Trace route sends UDP datagrams while varying
the TTL. The router that drops the UDP packet now replies
with a time exceeded ICMP message. The end point will not reply with that ICMP
message because it has already reached there. So traceroute sends to an unlikely UDP port. Eventually get a no such port ICMP message. It knows that it has reached the end. So this is the reference about ICMP messages. Actually these are not the only internet control
message protocols but there are a number of others which we did not discuss. We just discussed a few of them. There are other protocols like DHCP, BOOTP,
RARP, ARP. For example, they help in running the network
in a better fashion. ARP protocol is a low level protocol. Then we have this RARP, BOOTP and DHCP for
assigning a network. This ICMP helps in controlling the network
operation and giving error messages. Then there is another side protocol which
we will discuss in the next class namely IGMP which is internet group management protocol. So, that is another part of routing that we
have not discussed as yet. so we will take it up in the next class. Good day. Today we will take up two topics, DNS and
directory. First let us talk about DNS. DNS is the short form for Domain Name System. We have seen two kinds of addresses till now:
one is that we have seen MAC address or the so-called hardware addresses. In case of Ethernet they are also called Ethernet
addresses which is used in the data link layer for direct communication. Then we have seen IP addresses which are used
for communication between two end points, two end points in the network so they may
be anywhere in the network. IP address includes information used for routing
so IP address is used for routing where there is a network part and post part etc but then
unfortunately these IP addresses are tough for humans to remember. You can remember only a few addresses which
you basically require for your own configuration etc may be you need to know your own IP address,
the address of your gateway etc etc but then may be also an address for your mail server. But then beyond that if you have to remember
IP addresses of other people it becomes very difficult for human beings to remember this
and of course they are impossible to guess. But then what humans find it much easier to
remember and use are the domain names other names, for example and the domain names you
must have come across because that is what people use most of the time surfing the web
and for a www site sometimes you can see that may be you do not know the name exactly but
you may make out three or four guesses may be make some combination as dot com, dot net
etc on this side and this side that is something which looks may be [intel..50:35] so I will
try intel dot com first thing and may be if it does not work may be I will try few other
things so we can guess. Not only we can guess the other thing is that,
the most important thing is that it is much easier to remember. So we will remember so many site names so
it is easier for humans to remember. Therefore, for this human interface we need. So this is once again what is the name after
all? It is supposed to map to some particular machine
or site or whatever so this is also in some sense this is also some kind of an address. The third layer which are the domain names
so today we will see how we will use these domain names and of course just as in the
local area network you require a mapping from IP addresses to the MAC addresses done by
the ARP protocol and the reverse MAC address to IP address RARP. Similarly, here you need a mechanism for mapping
the domain names to IP addresses. We have not discussed this, why not centralized
DNS? Single point of failure
Traffic volume Distant centralized database would not work
Maintenance would be a problem It does not scale So new server, it is distributed, no server
has all the names to IP address mappings. Local names server: each ISP company has local
name and default name server and host query first goes to the local name server. Authentitative name server, this you might
come across sometimes that something is being given by authentitative name server. For a host that hosts IP address names can
perform name address translation for that hosts name and anyway there are some more
things of the hierarchy. There are some route name servers. So the route name
servers are all in the U.S.A but of course that depends on what route it is so they could
be distributed also. Nslookup: nslookup is an interactive resolver
that allows the user to communicate directly with the DNS server. So from the OS you can use this nslookup and
give the name query. So this is actually the name server lookup
and that is how nslookup comes. Nslookup is usually available on Unix workstations. So servers handle requests for their domain
directory. Now let us talk about the servers and servers
handle requests for other domains by contacting remote DNS servers and servers cache external
mappings. If a server has no clue about where to find
the address for the host name it asks the root server. The root server will tell you what name server
to contact. A request may get forwarded a few times. For example, let us say that iitkgp has a
name server, now the iitkgp has a name server and a request for a particular translation
has come to it, it does not know the name server to whom to connect so it can always
transfer it to the next level the ernet, and if the ernet also does not know where this
e net is there then it can contact the in, the in will definitely know all the sub-domains
under it, it has to know because it is administering that domain. Similarly, if it is for some address which
is outside you can send it directly to the root of that particular domain. Suppose from India you are trying to contact
something for Japan so you can send it to the jp root name server and then jp root name
server would know which server to contact, that will come back. So in this way DNS queries will go back and
forth a few times and finally the name will be resolved. Now we come to LDAP which is the Lightweight
Directory Access Protocol. Actually since this was designed by the same
people who designed OSI so this X.500 actually tends to be a little complex, it is heavy
and it uses the OSI layer all the seven layers. Now, for the internet people, internet purpose
actually which uses the TCP/IP stack rather than the seven layer OSI stack there was a
lightweight directory access protocol which can inter-operate with this at least on one
side, the LDAP can use that X.500 directory service. But this is much simpler than X.500 but LDAP
is used in many places. Therefore, this is a Lightweight Directory
Access Protocol, supports X.500 interface, does not require the OSI protocol, it uses
the TCP/IP protocol, so this is X.500 for the internet crowd. This is useful as a generic addressing interface
like netscape, address book and so on. The LDAP or Lightweight Directory Access Protocol
is a networking protocol for querying and modifying directory services running over
TCP/IP. The LDAP directory usually follows the same
X.500 model which we have discussed. Now it is a tree of entries, each of which
consist a set of named attributes with values. An LDAP directory often reflects various political,
geographic, and/or organizational boundaries, depending on the model chosen and when you
do that you can also define your security policies based on this directory and based
on this boundary so you can do that, authentication service especially. So directory is a tree of entries that we
have seen and an entry consists of a set of attributes and attribute pairs and the attributes
are defined in a schema. This would be the protocol stack for LDAP
so you have a directory based application may be some authorization service or may be
some access to some information which may be there for the organization which uses LDAP. LDAP may use TLS, this TLS is transport level
security, transport level security otherwise you could use SSL also. Actually I am talking a lot about security
here; we have not discussed security so we will in future we will sort of give one lecture
to security because security becomes so important. Now, in an organizational context, a directory
may be an important component of the entire security arrangement, security is a complex
issue. But anyway for this we require that we communicate
securely in many cases and many LDAP implementations support this TLS that is the transport level
security or you can use SSL also or SASL.

Daniel Ostrander

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10 thoughts on “Lecture – 33 DHCP and ICMP

  1. Mrsumbli says:

    it seems that this guy has just come out of the sleep to deliver the lecture

  2. Mitch K says:

    @Cosmodot256 You mean "hear" not "here"

  3. patio87 says:

    So tiring to listen to a lecture when the presenter has a deep accent. I had an electronics class and it was physically tiring after 3 hours trying to listen to him.

  4. maryam wahab says:

    niceeee….. thanks

  5. Big Brand Deluxe says:

    Speak up!!

  6. subrata23 says:

    On DHCP Discover (12 Min 23 Sec) – not sure if the client has yet received its IP yet from the server. The client can use the IP after DHCP pack (DHCPPACK) has been issued by the server to the client acknoledging that the particular IP address has now been allocated to the client system (e.g. PC) for a specific lease period. Thanks.

  7. Anupama Lolage says:

    i cannot hear anything. can we please fix the audio. Thanks

  8. Project StarHubCity Live says:

    Excellent ….  But to difficult to remember …  Before Exam ..!

  9. Vikas Jarugumalli says:

    worst video i ever expect from this.audio is also too bad…

  10. Nishant Gupta says:

    Worst way of explaination…..just slides

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