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Communications Services• Connection Oriented Service
A connection is established Data is sent or received over this connection Connection may be terminated when session is
complete or may be permanent This paradigm is similar to telephone service
• Connectionless-Oriented Service There is no single connection established Data is organized in individual units called datagrams
or packets Each datagram is individually addressed and
individually processed This paradigm is similar to US Postal Service
The Internet
• The Internet protocols provide applications with both a connection and connectionless services
• The network layer, the data delivery service, provides only a connectionless service
• The basic unit of data processed by the network layer is the IP Datagram
• Each datagram is individually addressed and individually processed
• There is no guarantee that the datagrams that comprise a ‘session’ will follow the same path or that they will arrive in order
Position of IP in TCP/IP protocol suite
IP Datagram
• IP datagram is variable length (up to 64K bytes)
• Data length can be as little as 1 byte• Source and IP addresses in the datagram
used route the datagram to its destination
Header Data
Destination IP Address
Source IP Address
Configuration for routing example
/8
/27 /27
/24 /24
Routing Table for R1
Mask Dest. Next Hop I.
255.0.0.0 111.0.0.0 -- m0255.255.255.224 193.14.5.160 - m2255.255.255.224 193.14.5.192 - m1-------------------------------------------------------------------------------------255.255.255.255 194.17.21.16 111.20.18.14 m0-------------------------------------------------------------------------------------255.255.255.0 192.16.7.0 111.15.17.32 m0255.255.255.0 194.17.21.0 111.20.18.14 m0-------------------------------------------------------------------------------------0.0.0.0 0.0.0.0 111.30.31.18 m0
Router R1 receives a datagram for destination 192.16.7.14; the algorithm applies the masks row by row to the destination address until a match (with the value in the second column) is found:
Example 1
Direct delivery
192.16.7.14 & 255.0.0.0 192.0.0.0 no match
192.16.7.14 & 255.255.255.224 192.16.7.0 no match
192.16.7.14 & 255.255.255.224 192.16.7. no match
Host-specific
192.16.7.14 & 255.255.255.255 192.16.7.14 no match
Network-specific
192.16.7.14 & 255.255.255.0 192.16.7.0 match
Router R1 receives a datagram for destination 193.14.5.176; the algorithm applies the masks row by row to the destination address until a match is found:
Example 2
Direct delivery
193.14.5.176 & 255.0.0.0 193.0.0.0 no match
193.14.5.176 & 255.255.255.224 193.14.5.160 match
Router R1 receives a datagram for destination 200.34.12.34; the algorithm applies the masks row by row to the destination address until a match is found:
Example 3
Direct delivery
200.34.12.34 & 255.0.0.0 200.0.0.0 no match
200.34.12.34 & 255.255.255.224 200.34.12.32 no match
200.34.12.34 & 255.255.255.224 200.34.12.32 no match
Host-specific
200.34.12.34 & 255.255.255.255 200.34.12.34 no match
Network-specific
200.34.12.34 & 255.255.255.0 200.34.12.0 no match
200.34.12.34 & 255.255.255.0 200.34.12.0 no match
Default
200.34.12.34 & 0.0.0.0 0.0.0.0. match
IP Datagram Service
• Best-Effort delivery Datagrams could be lost Datagram delivery may be delayed Datagrams may arrive out of order Datagrams could arrive corrupted Datagrams might even be duplicated
• Upper layers must cope with these conditions
IP datagram
IP DatagramVERS Version number, now version 4
Version 6 slowly being deployed
HLEN Length of IP header in 32 –bit words
Service Type Intended as a service categoryMinimize delayMinimize costMaximize throughputMaximize reliability
Total Length Total length of datagramHeader + data
IP Datagram
Identification Used to break up large datagrams
Flags
Fragment Offset Discussed later
Time to Live Count, decremented at each hop
When zero, datagram discarded
IP Datagram
Protocol Specifies which upper layer protocol created datagram
TCP(6), UDP(17), ICMP(1)
Header Checksum Checksum applied to header only1-s complement sum of
16 bit words of headerSource IP addressDestination IP address
Network Byte Order
• Different processors store binary values differently
• Some store integers with most significant bits in low memory address positions
Big Endian• Others may store in reverse order
Little Endian• Standard for IP is Big Endian• Systems provide convenient conversion routines
Encapsulation
• An IP datagram inserted inside a frame at layer 2, such as Ethernet
Header Data
Ethernet Destination Address
Type = 0800
Ethernet Source AddressCRC
Demultiplexing
• A frame arrives at a host, say Ethernet
• Destination hardware address is compared
• Type field is checked• 0806 ARP frame• 0835 RARP frame• 0800 IP datagram
• If IP datagram, protocol field is checked• 1 ICMP• 6 TCP• 17 UDP
Maximum Transmission Unit(MTU)
• Most data link layers enforce a maximum frame size
• Maximum payload for Ethernet frames is 1500 bytes
• Other data link layers may impose even shorter frame lengths
• A datagram may travel over a path that includes different data link layers
• The smallest MTU in the path is called the path MTU
Fragmentation
• IP datagrams can be as much as 65,536 bytes• This cannot fit into most data link layer frames,
including Ethernet• The IP datagram must be broken down into
appropriate size fragments • The fragments will need to be reassembled at a
later point• Three fields in the IP datagram header control
fragmentation
IP Header
Identification - a counter incremented by 1 for each datagram
Flags D = do not fragment
M = More fragments
Fragmentation Offset - relative position of fragment in original datagram (divided by 8)
D M
Fragmentation example
Detailed example
Fragmentation
• The header of the original datagram is copied into each fragment with affected fields modified
• After fragmentation each fragment is treated as a separate datagram
• Reassembly of original datagram is done at final destination
• There is no guarantee that every fragment will reach its destination
• At destination, a reassembly timer is set• Either all fragments arrive in that time or all other
fragments discarded