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Transcript
IP Basics IP ICMP Network ARP Link Physical RoutingTables IP Basics • IP encapsulates TCP • IP packets travel through many different routers (hops) before reaching it’s destination • MTU variation at the physical layer requires IP to fragment the message into smaller units along the way • Reassembly is an option at each hop. • IP does NOT guarantee delivery! IP Fragmentation 1000 b R 500 b 500 b R 250 b 250 b 250 b 250 b Every link has the potential to dictate adjusting size of frames. It is possible to reassemble at any point. 1000 b R 500 b 500 b R 1000 b R R What if frames are lost? R 250 b4 250 b3 250 b2 250 b1 R Receive Computer Receive computer will hold the first 2 frames awaiting the 3rd. After a period of time, a timer expires and IP level passes the 500 bytes up and stops looking for the other pieces. TCP (NOT IP) then will acknowledge receipt of 500 more bytes to the sending TCP layer. If the first frame is lost, NONE are passed up to TCP. More on TCP later. IP Frame format Fragmentation Fields Version 32 bits Each line Header Length Type of Service Packet Length Identification Time To Live Flags Fragment Offset Checksum Protocol Source IP Address Destination IP Address Options DATA Fragmentation and the Frame Fig 7.34 FragId 345 FragOffset=0 MoreFragments Bit=1 Size->1400 byes 4000 byte frame router FragId 345 FragOffset=175 MoreFragments Bit=1 Size-> 1400 bytes FragId 345 FragOffset=350 MoreFragments Bit=0 Size->1200 bytes FragOffset should be multiplied by 8 (8x175=1400) IP Frame format Version 32 bits Each line Header Length Type of Service Packet Length Identification Time To Live Flags Fragment Offset Checksum Protocol Source IP Address Destination IP Address Options DATA IP Summary • Fragmentation results in delivery of frames which are potentially smaller than the original transmission. • Some of the frames can be lost • If a message is fragmented and frames are lost, all frames up to the first lost frame are passed up to the receiving TCP and all subsequent frames are dropped. • TCP views this as a stream and is unaware of the loss of frames. It just accepts the next “n” bytes, acks the receipt, and waits for subsequent data. TCP End To End Delivery Transport TCP UDP Network Link Physical TCP basics • Connection-oriented – Sets up the connection prior to data transmission • SYN and 3-way handshake – Guarantees delivery of data • Sender holds a copy of the data for retransmission if necessary • Receiver ACKS specific byte positions in the stream so sender can resend from any byte position • Encapsulated by IP • Receiver tells sender it’s receive window size to limit rate of data arrival (flow control) Consider How TCP and IP Work Together TCP handling of fragmentation (Send 2000 bytes) Transport 2000 Up to destination port Transport (ACK 500 bytes) 500 Network(IP) Network(IP) 1000 2501 2502 1000 Physical Network(IP) Physical 2503 2504 Physical TCP handling of fragmentation (Send 2000 bytes) Transport 2000 Up to destination port Transport (ACK 500 bytes) 500 Network(IP) Network(IP) 1000 2501 2502 1000 Physical Network(IP) Physical 2503 2504 Physical TCP handling of fragmentation (Send 2000 bytes) Transport 2000 Up to destination port Transport (ACK 500 bytes) 500 Network(IP) Network(IP) 1000 2501 2502 1000 Physical Network(IP) Physical 2503 2504 Physical TCP handling of fragmentation (Send 2000 bytes) Transport 2000 Up to destination port Transport (ACK 500 bytes) 500 Network(IP) Network(IP) 1000 2501 2502 1000 Physical Network(IP) Physical 2503 2504 Physical What does the TCP frame look like? Data Source Destination Length Checksum Port Port And after TCP is encapsulated in IP? IP Header IP Trailer TCP More TCP Issues TCP handshake/setup Host A Host B Ack 0, Syn 1 Ack 1, Syn 1 Ack 1, Syn 0 Ack 1, Syn 0 time . . . setup data TCP Frame Destination Port Source Port Sequence Number Acknowledgment Number HeaderLength Flags Window Checksum Urgent Pointer Options DATA Flow Control w/TCP A B Data … s=101, a=701 Data … s=201, a=701 B says STOP Data … s=701, a=301, c=0 Data … s=801, a=301, c=200 Data … s=301, a=901 Data … s=401, a=901 B says GO (up to 200) UDP Issues UDP basics • • • • • At transport layer as alternative to TCP No connection establishment No Guaranteed Delivery Practically adds nothing to IP Application must guarantee delivery if necessary • TCP and timing is hard for you to implement at the application layer • UDP runs faster if delivery is not required to be error-free. ICMP ICMP Basics TCP/UDP IP ICMP physical • • • • • Lower than IP A support protocol for routing info/problems Doesn’t use ports Used in ping Frequently used to deny service.. problematic – ping of death (too large message), and – denial of service (ping flood) ICMP Message types • • • • • Echo Request Echo Response Time Exceeded Destination Unreachable Redirect IP Tunnelling Transport Apple talk Transport (IP) Apple talk (IP) Network(IP) Physical Inside Network (AT) Router CAN do AT in IP Physical Intermediate Routers only See IP Physical Receiving Router Connected Network (AT) IP Tunnelling at one end Transport Apple talk (IP) Appletalk Appletalk IP AT Physical Physical Physical IP AT Appletalk to local Appletalk to non-local Route to Destination As IP Summary