Indian Institute of Technology Kharagpur. TCP/IP Part II

Transcription

1 Indian Institute of Technology Kharagpur TCP/IP Part II Prof Indranil Sengupta Computer Science and Engineering Indian Institute of Technology Kharagpur Lecture 4: TCP/IP Part II On completion, the student will be able to: 1. Explain the differences between transparent and non-transparent fragmentation in IP packets. 2. Identify the IP header fields used in fragmentation and reassembly. 3. Illustrate how a data packet gets fragmented in the IP protocol with an example. 4. Interpret the various IP address classes, and their capacities. 1

2 Introduction Most of the fields in the header of an IP datagram have been explained. We now discuss the fields used for fragmentation and reassembly of packets. If the packet size exceeds a certain maximum value, it is split into two or more fragment packets. The fragments are reassembled at some later stage. Fragmentation Why needed? The IP layer injects a packet into the datalink layer, and hopes for the best. Not responsible for the reliable transport of these packets. Each layer imposes some maximum size of packets, due to various reasons. Called Maximum Transfer Unit (MTU). Suppose a large packet travels through a network whose MTU is too small. Fragmentation is required. 2

3 Fragmentation (contd.) What to do then? The different networks are connected among themselves through routers. Allow the routers to break the packets into fragments, if necessary. Each fragment is transmitted as a separate IP packet. The fragments need to be reassembled back. Fragmentation (contd.) When is reassembly of fragments carried out? Two alternatives: Transparent fragmentation Non-transparent fragmentation 3

4 Interconnection of Networks N2 N1 H N4 H N3 Transparent Fragmentation Fragmentation is made transparent to subsequent networks, through which the packet pass. Basic concept: An oversized packet reaches a router. outer breaks it up into fragments. All fragments sent to the same exit router (say, E ). E reassembles the fragments before forwarding to the next network. 4

5 Transparent Fragmentation (contd.) Why called transparent? Subsequent networks are not even aware that fragmentation had occurred. A packet may get fragmented several times on its way to the final destination. Transparent Fragmentation (contd.) N2 N1 H N4 H Packet N3 5

6 Transparent Fragmentation (contd.) Drawbacks: All packets must be routed via the same exit router. Exit router must know when all the pieces have been received. Either a count field or end-of-packet field must be stored in each packet. Lot of overhead. A large packet may be fragmented and reassembled repeatedly. Non-transparent Fragmentation Fragmentation is not transparent to subsequent networks. Basic concept: Packet fragments are not reassembled at any intermediate router. Each fragment is treated as an independent packet by the routers. The fragments are reassembled at the final destination host. 6

7 Non-transparent Frag. (contd.) N1 H P Packet P2 P1 N P1 P2 4 5 N N4 H Non-transparent Frag. (contd.) Advantage: Multiple exit routers may be used. Higher throughput. Drawback: When a large packet is fragmented, overhead increases. Each fragment must have a header (minimum 20 bytes). IP protocol uses non-transparent fragmentation. 7

8 IP Datagram VE HLEN Service type Total Length Identification Flags Fragment Offset Time to Live Protocol Header Checksum Source IP Address Destination IP Address Options HEADE DATA What does IP do? To allow fragment reassembly at the final destination, IP uses the following fields in the header: Identification (16 bits) A datagram id set by the source. Fragment offset (13 bits) Indicates where in the original datagram this fragment belongs to. Specified in multiple of 8 bytes. 8

9 What does IP do? (contd.) Flags (3 bits) Two flags are defined: D bit :: don t fragment; prevents fragmentation from taking place. M bit :: more fragment; specifies if this fragment is the last one in the original packet or not. Example :: IP Fragmentation N bytes data MTU = 620 bytes N2 MTU = 400 bytes 9

10 Example (contd.) ID=5,FO=0,M= By N1 ID=5,FO=0,M= ID=5,FO=75,M=0 ID=5,FO=47,M=1 ID=5,FO=75,M=1 By N ID=5,FO=0,M=1 ID=5,FO=122,M= bytes sent and 1080 bytes received Basic IP Addressing Each host connected to the Internet is identified by a unique IP address. An IP address is a 32-bit quantity. Expressed as a dotted-decimal notation W.X.Y.Z, where dots are used to separate each of the four octets of the address. Consists of two logical parts: A network number A host number This partition defines the IP address classes. 10

11 Dotted Decimal Notation IP address: (66) (134) (48) (126) Dotted Decimal Notation: Hierarchical Addressing A computer on the Internet is addressed using a two-tuple: The network number Assigned and managed by central authority. The host number Assigned and managed by local network administrator. When routing a packet to the destination network, only the network number is looked at. 11

12 IP Address Classes There are five defined IP address classes. Class A Class B Class C Class D Class E UNICAST UNICAST UNICAST MULTICAST ESEVED Identified by the first few bits in the IP address. There also exists some special-purpose IP addresses. IP Address Classes (contd.) The class-based addressing is also known as the classful model. Different network classes represent different network-to-hosts ratio. Lend themselves to different network configurations. 12

13 Class A Address 0 Network Host Host Host Network bits : 7 Number of networks = = 127 Host bits: 24 Number of hosts = = 16,777,214 Address range: to Class B Address 10 Network Network Host Host Network bits : 14 Number of networks = = 16,383 Host bits: 16 Number of hosts = = 65,534 Address range: to

14 Class C Address 110 Network Network Network Host Network bits : 21 Number of networks = = 2,097,151 Host bits: 8 Number of hosts = = 254 Address range: to Class D Address 1110 Multicast Address Address range: to

15 Address Distribution A 2 billion B C 1 billion 500 million Special-purpose IP Addresses eserved for private use 10.x.x.x (Class A) x.x x.x (Class B) x.x (Class C) Loopback/local address Default network Limited broadcast

16 Some Conventions Within a particular network (Class A, B or C), the first and last addresses serve special functions. The first address represents the network number. For example, The last address represents the directed broadcast address of the network. For example,

17 SOLUTIONS TO QUIZ QUESTIONS ON LECTUE 3 Quiz Solutions on Lecture 3 1. How many bits are there in the IP address? 32 bits 2. How many bits are there in the Ethernet address? 48 bits 17

18 Quiz Solutions on Lecture 3 3. What does the Ethernet address signify? It signifies the 48-bit physical address of the interface port. 4. What does the IP address signify? It signifies the 32-bit Internet address of a machine. Quiz Solutions on Lecture 3 5. What does the port number signify? It identifies an application/server running on the designated machine. It is an 8-bit number. 18

19 Quiz Solutions on Lecture 3 6. What does the various layers in the simplified TCP/IP protocol stack correspond to with respect to the OSI seven-layer model? Datalink Datalink, Physical IP Network TCP /UDP Transport Application Session, Presentation, Application Quiz Solutions on Lecture 3 7. Why is the transport layer called end-toend or host-to-host layer? Because it is at this layer two programs running on the two end hosts directly communicate with each other over a virtual channel. 19

20 Quiz Solutions on Lecture 3 8. IP is unreliable, and TCP uses IP. How does TCP provide reliable service to the application layer? TCP provides explicit error control. It checks to find out if all parts of a message has been transmitted successfully. If not, it arranges for a retransmission. Quiz Solutions on Lecture 3 9. List two common applications that use UDP. TFTP and DNS 10. Why is the IP protocol considered unreliable? This is because in the IP protocol a) a packet may be lost b) duplicate packets may be generated c) packets may arrive out of order. 20

21 Quiz Solutions on Lecture What does TCP do if the message to be sent is larger that what a single datagram can handle? It breaks the message into two or more datagrams. Quiz Solutions on Lecture If a 1000 byte data message is sent using TFTP, what will be the size in bytes of the corresponding Ethernet packet? 1050 bytes. This includes 50 bytes of header. 13. What are the minimum and maximum header size of an IP packet? 20 bytes, 60 bytes 21

22 Quiz Solutions on Lecture What is the purpose of the Time to live field in the IP header? It contains a counter that is decremented at each hop. The packet is discarded when counter value reaches zero. Prevents a packet from falling into a routing loop. Quiz Solutions on Lecture If the IP header is 192 bytes long, what will be the value of the HLEN field? 6, because 6x32= What is the maximum size of data that can be accommodated in an IP datagram? = bytes 22

23 QUIZ QUESTIONS ON LECTUE 4 Quiz Questions on Lecture 4 1. An IP packet arrives at a router with the first eight bits as The router discards the packet. Why? 2. An IP packet arrives at a router with the first eight bits as How many bytes of options are there in the packet? 3. In an IP packet, the value of HLEN is 5, and the value of the total length field is How many bytes of data the packet is carrying? 23

24 Quiz Questions on Lecture 4 4. A packet has arrived at the destination with the M bit as zero. What can you say about the packet? 5. A packet has arrived at the destination with the M bit as one. What can you say about the packet? 6. A packet has arrived at the destination with the M bit as one, and also the fragment offset field as zero. What can you say about the packet? 7. A packet has arrived at the destination with the fragment offset field as 500. What can you say about the packet? Quiz Questions on Lecture 4 8. A packet has arrived at the destination with the HLEN value as 5, the fragment offset field as 150, and the total length field as What can you say about the packet? 9. In an IP network, does the intermediate routers need to worry about fragmentation and reassembly? 10. Can an IP fragment contain any number of data bytes less than the maximum possible value? 24

25 Quiz Questions on Lecture 4 9. Change the following IP address from binary notation to dotted decimal notation Find the error if any in the following IP address: Find the class of the following IP address: Quiz Questions on Lecture Given the network address , find the class, the network id, and the range of the addresses. 13. Given the network address , find the class, the network id, and the range of the addresses. 14. What do the following IP address signify: An IP packet with 2500 bytes of data (plus header) passes through an IP network with MTU=500. How many additional bytes will be delivered at the destination? 25

26 Indian Institute of Technology Kharagpur 26