IP Basics

Physical Link Network IP ARP ICMP 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 R R R 1000 b500 b 250 b Every link has the potential to dictate adjusting size of frames. It is possible to reassemble at any point. R R R 1000 b500 b 1000 b

What if frames are lost? R R 250 b Receive Computer Receive computer will hold the first 2 frames awaiting the 3 rd. 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 Header Length Type of Service Packet Length FlagsFragment OffsetIdentification Time To LiveProtocol Checksum Source IP Address Destination IP Address Options DATA 32 bits Each line

Fragmentation and the Frame Fig byte frame router FragId 345 FragOffset=0 MoreFragments Bit=1 Size->1400 byes 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 Header Length Type of Service Packet Length FlagsFragment OffsetIdentification Time To LiveProtocol Checksum Source IP Address Destination IP Address Options DATA 32 bits Each line

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

Physical Link Network Transport TCP UDP

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

Transport Network(IP) Physical Network(IP) Physical Transport Network(IP) Physical (Send 2000 bytes) (ACK 500 bytes) TCP handling of fragmentation Up to destination port

Transport Network(IP) Physical Network(IP) Physical Transport Network(IP) Physical (Send 2000 bytes) (ACK 500 bytes) TCP handling of fragmentation Up to destination port

Transport Network(IP) Physical Network(IP) Physical Transport Network(IP) Physical (Send 2000 bytes) (ACK 500 bytes) TCP handling of fragmentation Up to destination port

Transport Network(IP) Physical Network(IP) Physical Transport Network(IP) Physical (Send 2000 bytes) (ACK 500 bytes) TCP handling of fragmentation Up to destination port

What does the TCP frame look like? Source Port Destination Port LengthChecksum Data

And after TCP is encapsulated in IP? IP Header IP Trailer TCP

More TCP Issues

TCP handshake/setup time Host AHost B Ack 0, Syn 1 Ack 1, Syn 0 Ack 1, Syn 1 Ack 1, Syn setup data

TCP Frame Source Port Destination Port Sequence Number HeaderLengthFlags Acknowledgment Number Urgent Pointer Options DATA Checksum Window

Flow Control w/TCP Data … s=101, a=701 Data … s=201, a=701 Data … s=701, a=301, c=0 Data … s=801, a=301, c=200 Data … s=301, a=901 Data … s=401, a=901 AB B says STOP 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 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) physical IP TCP/UDP ICMP

ICMP Message types Echo Request Echo Response Time Exceeded Destination Unreachable Redirect

IP Tunnelling Transport (IP) Physical Network(IP) Physical Apple talk Intermediate Routers only See IP Router CAN do AT in IP Receiving Router Inside Network (AT) Connected Network (AT) Transport (IP) Physical Apple talk

Transport (IP) Physical Apple talk IP Tunnelling at one end Physical Appletalk Physical Appletalk Appletalk to local Appletalk to non-local AT IP AT IP Route to Destination As IP

Summary