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Download IP : Internet Protocol
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IP : Internet Protocol Computer Network System Sirak Kaewjamnong 1 IP Layer • Defines a virtual network on top of difference kinds of hardware platform using IP address • function of IP – – – – route packet fragmentation handle type of service send and receive error and control message using ICMP 2 IP Attributes • • • • Handle data unit called IP datagrams connectionless protocol-doesn’t promise reliable delivery best effort delivery packets maybe lost, out of sequence, or duplicated due the various reasons 3 IP Encapsulation With Ethernet frame datagram Ethernet Header IP Header Data 4 IP Reframe • IP will reframe the packet when A send data to B IP 172.28.80.1 MAC 00:00:1a:2b:3c:4d MAC dest MAC src type IP src IP dest 00:00:1a:2b:3c:4d 00:50:ba:49:9d:b9 0x800 172.28.80.96 172.28.85.24 IP 172.28.85.1 MAC 00:00:2a:3b:4c:5d Router Packet to router A IP 172.28.80.96 MAC 00:50:ba:49:9d:b9 MAC dest MAC src type IP src IP dest 00:02:7a:d2:b3:00 00:00:2a:3b:4c:5d 0x800 172.28.80.96 172.28.85.24 Packet from router B IP 172.28.85.24 MAC 00:02:7a:d2:b3:00 Change MAC address, IP address be the same 5 The Internet Protocol (IP) Protocol Stack App Transport TCP / UDP IP Network Link Data Data Hdr Hdr TCP Segment IP Datagram 6 The Internet Protocol (IP) Characteristics of IP • CONNECTIONLESS: • UNRELIABLE: • BEST EFFORT: • DATAGRAM: Source D A D mis-sequencing may drop packets… … but only if necessary individually routed R2 H R1 R3 R4 H B Destination •Architecture •Links •Topology Transparent 7 The IP Datagram 15 16 0 vers : hlen: 4 4 TOS : 8 Total Length :16 Flags :3 Identification : 16 20 Bytes TTL : 8 31 Protocol :8 Offset within original packet FRAG Offset : 13 Header Checksum : 16 SRC IP Address : 32 <= 64 KBytes DST IP Address : 32 (OPTIONS) (PAD) Data . . 8 IP Header Details • Ver : version = 4 • hlen : header length in 32 bit words, hlen = 5 with no option • TOS : type of service , desired quality of services 0 1 2 3 Prec. Bits 0-2 3 4 5 6 7 4 D 5 T 6 R 7 C 0 if 0 if 1 Precedence Normal delay Low delay Normal throughputHigh throughput Normal reliability High reliability Normal cost Low cost Reserves 9 IP Header Details • Total Length : max size datagrams include header (64KB) • Identification , Flag, Fragmentation : use to segmentation and reassembly packet • TTL : Time to Live, maximum number of routers through with the datagrams may pass (hop count) – decrease at each router to prevent forever looping – normally set to 30 – if TTL == 0 discard and send ICMP TTL exceeded to source IP 10 IP Header Details • Protocol : higher level protocol that provide data – 1 = datagrams carries an ICMP messages – 6 = datagrams carries an TCP segments – 17 = datagrams carries an UDP datagrams • Header checksum : 16 bits one’s complement, note that there is no data checksum 11 IP Header Details • Source address : 32 bits IP source address • Destination address : 32 bits IP destination address • Option and Padding : (maximum 40 bytes) addition information to control functions such as routing and security 12 Type of Service in some Internet Protocol Application D T R C Meaning TELNET FTP control data TFTP SMTP command phase data phase DNS UDP query TCP query zone transfer SNMP 1 0 0 0 Minimize delay 1 0 1 0 1 0 0 0 0 0 0 0 Minimize delay Maximize throughput 1 0 0 1 0 0 0 0 Minimize delay Maximize throughput 1 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0 Minimize delay Maximize throughput Maximize reliability Minimize delay From RFC 1349 13 Routing • Process of choosing a path over which to send datagrams • IP routes packet by looking at the IP network number • Routing components – determine what path are available – selecting the best path for a particular purpose – using those paths to reach other network • devices which perform routing are routers (Historically call IP gateways) 14 Routing Table • Every router contains a routing table of the network numbers • The table record – Which connection can be used to reach a particular network – plus some indication of the performance or cost of using connection 15 Routing Table Form Routing table form • network , gateway, other 16 Routing Protocol • Routing protocol manages and updates routing table on each network node • UNIX often implemented using one of the two daemons – routed : basic routing daemon for interior routing, normally with RIP – gated : sophisticated daemon for interior and exterior routing, with additional protocol such as OSPF, BGP 17 Fragmentation • Fragmentation : processed used by IP to reduced size of datagram that are too long for network interface MTU e.g. fragment 2000 bytes to Ethernet MTU 1500 Bytes • fragmentation should be reassembled at final destination (expensive process) • how? – each fragment has it own header – each fragment carries the same 16 bits identification header – each fragment must be aligned with eight-octet boundary 18 Fragmentation Flag • Identification number – 16 bits integer value used to identify all fragments – This ID is not a sequence number • Flags : 3 bits control fragmentation 0 = may fragment 0 = last fragment 1 = don’t fragment 1 = more fragment R DF MF Reserve must be 0 • Fragment offset : indicate the distance of fragment data from the start of original datagram, measure in 8 octets unit 19 Fragmentation original Other header ident flags offset …….. 232 20 bytes Ethernet with MTU 1500 ... 232 1 0 0 0 data 0..2000 20 bytes 0 .. 1479 Identification number More fragment Post 0 ... 232 0 185 1480 .. 2000 Last fragment Post 185 * 8 = 1480 20 Problem in fragmentation • The end node has know way of knowing how many fragments there be. The end node has to manage enough buffer space to handle reassembly process. • If any fragments lost, all datagram must be discarded • End node starts a timer when received the first fragment, if any fragments fails to alive(usually 30 sec), all datagrams must be discarded • Since the IP service is connectionless. No attempt is made by IP to recover these situations, though ICMP error massage may be generated 21 Avoiding Fragmentation • For datagrams with in the same physical network, the MTU is known. TCP/UDP then use the MTU to limit the message size pass to IP, message will never be fragmented. • For datagram pass to difference network, unknown MTU. • Standard recommend that all networks supporting TCP/IP have an MTU of at least 576 bytes 512 bytes data + 20 bytes TCP header + 20 bytes IP header with options to guarantee that a packet of 576 bytes or less is never fragmented 22