Internet Protocols IP Header, Fragmentation / Forwarding / Encapsulation / IPv6IP Operation Go to Router X MAC address for Router X IP PDU Encapsulated with LAN protocol Encapsulated with X.25 protocolTCP/IP Stack Protocol p Bridge n IS used to connect two LANs using similar LAN protocols n Address filter passing on packets to the required network only n OSI layer 2 (Data Link) p Router n Connects two (possibly dissimilar) networks n Uses internet protocol present in each router and end system n OSI Layer 3 (Network)IP Header Format p Source address p Destination address p Protocol n Recipient e.g. TCP p Type of Service n Specify treatment of data unit during transmission through networks p Identification n Source, destination address and user protocol n Uniquely identifies PDU n Needed for re-assembly and error reporting n Send only + 0 - 3 4 - 7 8 - 15 16 - 18 19 - 31 0 Version Header length Type of Service Total Length 32 Identification Flags Fragment Offset 64 Time to Live Protocol Header Checksum 96 Source Address 128 Destination Address 160 Options + padding 192 Data HL=5 rowsà 20 octet (variable) / 8*20/32 1-64K Octets 20 BytesIP Header Format p VERS n Each datagram begins with a 4-bit protocol version number (the figure shows a version 4 header) p H.LEN (Header Length) n Number of 32-bit rows in the Header à Header Length n 4-bit header specifies the number of 32-bit quantities in the header (in the figure we have 5 32-bit rows) n If no options are present, the value is 5 p SERVICE TYPE n 8-bit field that carries a class of service for the datagram n Seldom used in practice n Chapter 28 explains the DiffServ interpretation of the service type field p TOTAL LENGTH n 16-bit integer that specifies the total number of bytes in the datagram n Includes both the header and the data HL=5 à 20 octet (variable), thus 8*20/32 1-64K Octets 20 Bytes bitIP Header Format p IDENTIFICATION n 16-bit number (usually sequential) assigned to the datagram p used to gather all fragments for reassembly of the datagram p FLAGS n 3-bit field with individual bits specifying whether the datagram is a fragment p If so, then whether the fragment corresponds to the rightmost piece of the original datagram p FRAGMENT OFFSET n 13-bit field that specifies where in the original datagram the data in this fragment belongs n the value of the field is multiplied by 8 to obtain an offsetIP Header Format p TIME TO LIVE n 8-bit integer initialized by the original sender n Represents the max. number of hops the packets can visit n it is decremented by each router that processes the datagram n if the value reaches zero (0) p the datagram is discarded and an error message is sent back to the source p TYPE n 8-bit field that specifies the type of the payload p HEADER CHECKSUM n 16-bit ones-complement checksum of header fields p SOURCE IP ADDRESS n 32-bit Internet address of the original sender n The addresses of intermediate routers do not appear in the headerExample: Encapsulated IP Packet in Ethernet Frame MAC and Associated IP address Ethernet Frame Carrying IP Packet Important! 90 BytesProtocol Analyzer Display: 0000 00 00 C0 A0 51 24 00 C0 93 21 88 A7 08 00 45 08 0010 00 5A DC 28 00 00 FF 01 88 08 C0 99 B8 01 C0 99 0020 B8 03 2a B4 DD ….. Example: Encapsulated IP Packet in Ethernet Frame Ethernet Frame Carrying IP Packet IP starting with 45 Hex indicates IPv4 with standard HED length of 20 bytes = 5 rows x 32/8 IP starting with 4F Hex indicates IPv4 with HED length of 60 bytes = 15 rows x 32/8 Remember: 24=16; 45= 0100 0101= One Byte An Ethernet frame containing IP information has 08 00 in its type field 99 is one byte 1001 1001 Example:Example of a Single IP Packet 20 byte was used for the header It is a single packet No fragmentation is used http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/ip/ipfragmentation.htmlForwarding p The Internet uses next-hop forwarding p To make the selection of a next hop efficient, an IP router uses a forwarding table p Mask field is used to direct the incoming packet p Number of entries in the table can be very large p A forwarding table is initialized when the router boots n Forwarding table must be updated if the topology changes or hardware fails Routing Table: e.g., If a packet with destination 30.0.0.0 arrives at R2 à The next hop will be 40.0.0.7Longest Prefix p Suppose a router's forwarding table contains entries for the following two network prefixes: 128.10.0.0/16 and 128.10.2.0/24 p What happens if a datagram arrives destined to 128.10.2.3? p Matching procedure succeeds for both of the entries n a Boolean and of a 16-bit mask will produce 128.10.0.0 n a Boolean and with a 24-bit mask will produce 128.10.2.0 p Which entry should be used? n To handle ambiguity that arises from overlapping address masks, Internet forwarding uses a longest prefix match p Instead of examining the entries in arbitrary order p forwarding software arranges to examine entries with the longest prefix first p In the example above, Internet forwarding will choose the entry that corresponds to 128.10.2.0/24Transmission Across the Internet Header can change – Going through WiFi or EthernetTransmission Across the Internet p Each hardware technology specifies the maximum amount of data that a frame can carry n The limit is known as a Maximum Transmission Unit (MTU) p There is no exception to the MTU limit n Network hardware is not designed to accept or transfer frames that carry more data than the MTU allows n A datagram must be smaller or equal to the network MTU p In an internet that contains heterogeneous networks, MTU restrictions create a problem p A router can connect networks with different MTU values n a datagram that a router receives over one network can be too large to send over another network Two networks with different MTU (a heterogeneous network)IP Fragmentation (1) p IP re-assembles at