network layer functions

Network Layer Functions

• Key Layer in Internet Architecture

• End-to-end Packets

• Adapt to lower layers

1 12 2

3

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

Routing Algorithms• Goals

– Optimality– Fairness– Stablility– Robustness– Correctness– Simplicity

• Adaptive vs Static

• Congestion Control

Adaptive Routing

• Centralized

• Isolated

• Distributed

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)

Congestion Control

RESEARCH ISSUE!

• Resource Reservation

• Packet Discarding

• Flow Control

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) .

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

IP Routing

• Making Decisions

• Gathering Information

• ICMP

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

Example: TCP/IP

• Not the only way to do things ...

• But well tested in the field

• Brief History

• TCP/IP vs ISO protocols

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

TCP/IP Stack

1

2

3

4

5 -7

OSI

DIX Ethernet or ...

ARP

ICMPIP

TCP UDP

DNSTELNET

FTP

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

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

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

Layer ProtocolsDNSQuery DNS

Reply

SYN

SYN/ACK

ACKCaller Callee

{TCP Establishment}

ARPRequest ARP

Reply

IP Functionality

• Presents single, virtual network to user

• Connectionless Delivery

• Packet Routing

• Interface to Lower Layers

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

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

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

?

?

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

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

IP Packet Handling (rcv)

receiving

Transport Layer (TCP or UDP)

Sockets

IP

Link {DIX Type or 802.3 DSAP}

Other

Physical

IP Packet Handling (xmit)

sending Local or Non-Local ?

ARP

Search RoutingTable

Found! Missing!

ICMP Error

Send locally...

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

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 * * *