Download TCP_IP_Part3_BigPicture_Part2

TCP/IP
Discussion Related to Essay Question on Final
Dr. V.T. Raja
Oregon State University
Minimum Information Needed by a
Computer in a TCP/IP Network

The computer’s IP address


A subnet mask address


With smask, the computer can determine which IP addresses are
part of its subnet
IP address of DNS Server


The minimum information needed is either provided by DHCP
Server or is obtained from the computer’s configuration file
DNS can help translate between AL addresses and IP addresses
IP address of Router/Gateway that connects the
computer outside its subnet (for static routing)

If dynamic routing is used, then appropriate routing software/
access to routing tables would be needed
Example for Essay Question
Example: Assume the following:
 All computers have been provided minimum
info specified in the previous slide
 Client A requests a web page from Web
Server D.
 Client A is connected to Router R, which is
connected to Server D.
 TCP/IP are the transport and network layer
protocols, and Ethernet is the data link layer
protocol.
Example

Client A knows IP address of Server D, and also AL
address of D (www.D.edu).


(Continued)
This implies that client A has already requested IP of D from
DNS in the past, and now client A has the IP of D in its
address table.
Assume A and D know the DL address of R, and that
R knows DL addresses of A and D.


This implies that both A and D have already issued ARP
messages to determine DL of R in the past, and now have
the DL address of R in their respective address tables.
Similarly, R has the DL address of A and D in its address
tables.
Example for Essay Question
(Continued)




IP of A is 16.32.64.33; Subnet mask of A is 255.255.255.128
IP of R is 16.32.64.128;
IP of D is 16.32.64.129;
Assume NIC address of A is 00-A0-B0-C0-A4-54-6A;
NIC address of R is 00-E0-B0-C0-E4-45-6E;
NIC address of D is 00-D0-B0-C0-D4-75-7D
Assume there are no errors in transmission
Question: Describe the data flow (across the
different layers in the 5-layer model) for the web
request made by Client A to Web Server D.
For Description of Data Flow
at Client A

See part 1 ‘big picture’ handout
Description of Data Flow
At R



(Continued)
Ethernet performs error detection (i.e., validates CRC32), based on info provided in DLH/DLT packets
obtained from Client A.
Ethernet performs error correction (Stop-and-WaitARQ); Since there are no errors in transmission, R
sends ACK for each packet it receives from A.
After sending ACK, DL removes DLH and DLT;
passes [IP|TCP|HTTP|User Data] to NL layer of R.
Description of Data Flow
At R

Network Layer of R:




(Continued)
IP looks at destination IP (16.32.64.129)
specified in IP header
Compares destination IP with its own IP
(16.32.64.128)
Concludes that it needs to re-route packet(s).
Since destination has not been reached, IP
does not invoke TCP.
Description of Data Flow
At R



(Continued)
IP modifies routing info and creates
modified IP header that reflects this
change.
IP determines and passes NIC address of
next hop device (D) to DLL of R.
IP passes [Modified-IP|TCP| HTTP|
UserData] to R’s DLL.
Description of Data Flow
At R


(Continued)
Data Link Layer creates new DL Header and Trailer,
which delineate each frame received from IP.
Sample contents of DLH/DLT include:







Source DL address (00-E0-B0-C0-E4-45-6E)
Destination DL address (00-D0-B0-C0-D4-75-7D)
MAC specified (i.e., CSMA/CD)
CRC-32 bit value that would help D detect transmission errors
Info on how D should perform EC (i.e., Stop-and-wait ARQ)
MD info (i.e., length of frame is specified in header)
DL passes [New DLH|Modified IP|TCP|HTTP|User Data|
NewDLT] to DL layer of R via physical layer.
Description of Data Flow
At D
(Continued)

Physical layer follows MAC as per CSMA/CD rules while
passing packets to DL layer of D.

DL layer performs error detection (e.g., CRC-32), based on
info provided in ‘New DLH/DLT’ obtained from R.


DL layer performs error correction/ ACK/NAK (e.g., Stopand-Wait-ARQ), based on info provided in ‘New DLH/DLT’
obtained from R.
After sending ACK, DL removes ‘New DLH/DLT’, and
passes [IP|TCP|HTTP|User Data] to NL layer of D.
Description of Data Flow
At D

(Continued)
Network Layer of D:





IP looks at destination IP (16.32.64.129)
specified in ‘modified IP header’
Compares destination IP with its own
(16.32.64.129) IP
Concludes that final destination (D) has
been reached
IP removes ‘modified IP header’
Passes [TCP|HTTP|User Data] to D’s TL
Description of Data Flow
At D

(Continued)
Transport Layer:

TCP reassembles packets based on packet #
specified in TCP Header. In this example, there is
no necessity for this due to the short size of
message.

TCP looks at destination port address (80), and
invokes the correct web application software in D

TCP removes TCP header

Passes [HTTP|User Data] to Application Layer of D
Description of Data Flow
At D

(Continued)
Application Layer:


Application layer removes HTTP Header
and
presents user data in expected userfriendly format
(Now the Web Server will have to pass its web page to Client A via R!!)