Download - Network Layer Functions
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Network Layer Functions
• Key Layer in Internet Architecture
• End-to-end Packets
• Adapt to lower layers
1 12 2
3
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Network Layer
• Design Goals– ‘Independent’ of layer 1 & 2 implementations– Hide layer 1 & 2 details from upper layers
• Architecture– Connection oriented– Connectionless– (where should reliability be done?)
• Services– Routing (Path selection)– Adaptation to different lower layers
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Routing Algorithms• Goals
– Optimality– Fairness– Stablility– Robustness– Correctness– Simplicity
• Adaptive vs Static
• Congestion Control
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Adaptive Routing
• Centralized
• Isolated
• Distributed
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Distributed Routing• Metric - Vector Algorithms
– sometimes called shortest path– Bellman-Ford most famous– Knowledge of immediate neighbors– Result is “first step” in path to ultimate
destination
• Link State Algorithms– OSPF {Open Shortest Path First}– Knowledge of network layer map
(connectivity)
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Congestion Control
RESEARCH ISSUE!
• Resource Reservation
• Packet Discarding
• Flow Control
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Routing Information Protocol (RIP)
• Known as a routing table update protocol
• Developed by Xerox and gained widespread acceptance by the proliferation of TCP/IP’s implementation of it in UNIX.
• Other protocols (AppleTalk, NetWare) adopted RIP as their standard routing update protocol.
• Known as a distance vector protocol.
–Vector is an adjacent router and the distance is how far away (hops) the network is.
–One hop is considered one router traversed.
• Devised for relatively stable, small-to-medium size networks (less than 16 routers in diameter) .
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Calculating the Cost
Network 1
Network 2
Network 3
Network 4
Network Hops
2 1
3 1
1 2
Network Hops
1
1
1
2
Network Hops
1
1
2
3
3
4
2
1
3 2
Transmittedroutingtable
Transmittedroutingtable
Internal routingtable
Router A
Router B
Router C
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IP Routing
• Making Decisions
• Gathering Information
• ICMP
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Routing Table
Network Mask Router Hops Age Port ID 3 255 C 1 30 1 4 255 C 1 30 2 2 255 B 2 15 2 1 255 B 3 45 2
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Example: TCP/IP
• Not the only way to do things ...
• But well tested in the field
• Brief History
• TCP/IP vs ISO protocols
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References TCP/IP• Overall -- Douglas Comer
– Internetworking with TCP/IP, Vol I, 3d ed.– Internetworking with TCP/IP, Vol II– Internetworking with TCP/IP, Vol III (Sockets)– Internetworking with TCP/IP, Vol III (TLI)
• Unix Programming -- W. Richard Stevens– Unix Network Programming– Advanced Unix Network Programming
• Stevens & Wright– TCP/IP Illustrated Vol I– TCP/IP Illustrated Vol II– TCP/IP Illustrated Vol III
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TCP/IP Stack
1
2
3
4
5 -7
OSI
DIX Ethernet or ...
ARP
ICMPIP
TCP UDP
DNSTELNET
FTP
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Notes on TCP/IP Suite
• TCP/IP preceded the OSI Reference Model
• Layers 1 & 2 are not part of the standard
• Functions above the Transport Layer are consolidated
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Brief Definitions
• ARP -- Address Resolution Protocol
• IP -- Internet Protocol
• ICMP -- Internet Control Message Protocol
• UDP -- User Datagram Protocol
• TCP -- Transmission Control Protocol
• DNS -- Domain Name System
• FTP -- File Transfer Protocol
• TELNET -- remote terminal
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Other TCP/IP Protocols
• Routing Protocols -- RIP, OSPF, EGP, GGP• BOOTP - Bootstrap Protocol• RARP - Reverse ARP• TFTP - Trivial FTP• HTTP - Hyper Text Transfer Protocol• SMTP - Simple Mail Transfer Protocol• SNMP - Simple Network Management Protocol
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Layer ProtocolsDNSQuery DNS
Reply
SYN
SYN/ACK
ACKCaller Callee
{TCP Establishment}
ARPRequest ARP
Reply
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IP Functionality
• Presents single, virtual network to user
• Connectionless Delivery
• Packet Routing
• Interface to Lower Layers
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IP Addresses• “Dotted Decimal”
32 bit (4 byte) address, written by taking each byte as an unsigned number
• Address Classes [first octet]
– A - <net> < > <host> < > 1-126– B - <net> < > <host> < > 128-191– C -< > <net> < > <host> 192-223– D - special subset of C 224-239 multicast– E - reserved 240-254
• Netmask – a 32 bit value which, when ANDed with an address, selects only the
network part
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Netmask Usage“Do a bitwise AND then compare for
equality of results”
<src addr> & netmask =?= <dst addr> & netmask
if comparison is equal, then both src & dst are on same (sub)net.
Same netmask
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Netmask ExamplesS= 128.194.100.10M= 255.255.255.0result 128.194.100.0
D= 128.194.200.10M= 255.255.255.0result 128.194.200.0
Not Equal
S= 128.194.12.10M= 255.255.255.0result 128.194.12.0
S= 67.194.18.10M= 255.255.240.0result 67.194.16.0
D= 128.194.12.110M= 255.255.255.0result 128.194.12.0
D= 67.194.10.10M= 255.255.240.0result 67.194. 0.0
?
?
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Special Address Conventions
• This host
• Host on this net
• Limited broadcast
• Directed broadcast
• Loopback
all 0’s
127 anything (usually 1)
net
hostall 0’s
all 1’s
all 1’s
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IP Header0 16 31
VERS HLEN SERVICE TYPE TOTAL LENGTH
FLAGS FRAGMENT OFFSETIDENTIFICATION
TIME TO LIVE PROTOCOL HEADER CHECKSUM
SOURCE IP ADDRESS
DESTINATION IP ADDRESS
1984
IP OPTIONS (IF ANY) PADDING
DATA
...
24
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IP Packet Handling (rcv)
receiving
Transport Layer (TCP or UDP)
Sockets
IP
Link {DIX Type or 802.3 DSAP}
Other
Physical
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IP Packet Handling (xmit)
sending Local or Non-Local ?
ARP
Search RoutingTable
Found! Missing!
ICMP Error
Send locally...
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A
Bridges or Routers
B C
D
1
2
3
4
5
Address & Netmask1: 131.122.24.12 255.255.255.02: 131.122.31.253 255.255.255.03: 131.121.18.12 255.255.240.04: 131.121.24.18 255.255.240.05: 131.121.31.254 255.255.240.0
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ICMP• ping
– ECHO Request/Reply
• traceroutetraceroute to falcon.ece.utexas.edu (128.83.196.10),
30 hops max, 40 byte packets
1 exit_133 (128.194.133.254) 2 ms 2 ms 2 ms
2 exit_128 (128.194.128.254) 2 ms 3 ms 2 ms
3 FDDI-T3.TAMU.EDU (128.194.1.13) 3 ms 3 ms 3 ms
4 FDDI-WAN.TAMU.EDU (165.91.128.17) 5 ms 4 ms 6 ms
5 sprint-gw-h1-0.the.net (129.117.16.161) 20 ms 5 ms 5 ms
6 ut8-h1-0.the.net (129.117.16.241) 8 ms 9 ms 21 ms
7 129.117.20.12 (129.117.20.12) 7 ms 11 ms 7 ms
8 ens.gw.utexas.edu (128.83.7.132) 16 ms 23 ms 21 ms
9 ece-e0.gw.utexas.edu (128.83.249.251) 12 ms 9 ms 9 ms
10 * * *