Lecture 12

Introduction 1-1

Lecture 12

Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith RossAddison-WesleyMarch 2012

CS3516:

These slides are generated from those made available by the authors of our text.

Network Layer 4-2

BONUS: Useful commands4.4 IP: Internet Protocol

datagram format IPv4 addressing ICMP IPv6

4.5 routing algorithms link state distance vector hierarchical routing

4.6 routing in the Internet RIP OSPF BGP

4.7 broadcast and multicast routing

Lecture 12: outline

Network Layer 4-3

Get MAC Address (Getmac.exe) Discovers the Media Access Control (MAC) address and lists associated network protocols for all network cards in a computer, either locally or across a network.

C:\Users\jb>getmacPhysical Address Transport Name============ =============================60-36-DD-AA-13-69 Media disconnected60-36-DD-AA-13-65 \Device\Tcpip_{437F350E-DFD7-4A86-B063-0B9650BD4404}60-36-DD-AA-13-66 Media disconnected60-36-DD-AA-13-66 Media disconnectedB8-CA-3A-DC-C6-2B Media disconnected08-00-27-00-E4-38 \Device\Tcpip_{F551D578-DC71-4760-B91C-B349EAE4238F}

Useful Commands

Network Layer 4-4

IP Configuration Utility (Ipconfig.exe) Displays all current (TCP/IP) network configurations.

C:\Users\jb>ipconfig

Windows IP ConfigurationEthernet adapter Local Area Connection:

Connection-specific DNS Suffix . : WPI.EDU Link-local IPv6 Address . . . . . : fe80::e591:74d4:a495:7998%16 IPv4 Address. . . . . . . . . . . : 130.215.28.36 Subnet Mask . . . . . . . . . . . : 255.255.248.0 Default Gateway . . . . . . . . . : 130.215.24.1

C:\Users\jb>ipconfig /? Prints command line options

C:\Users\jb>ipconfig /displaydns gives dns info cached on nodecs.wpi.edu ---------------------------------------- Record Name . . . . . : cs.wpi.edu Record Type . . . . . : 1 Time To Live . . . . : 73497 Data Length . . . . . : 4 Section . . . . . . . : Answer A (Host) Record . . . : 130.215.28.181

Useful Commands

Network Layer 4-5

Name Server Lookup (Nslookup.exe) Displays information about Domain Name System records for specific IP addresses and/or host names so that you can troubleshoot DNS problems.

C:\Users\jb>nslookup www.google.comServer: a.resolvers.level3.net this is the name of the default serverAddress: 4.2.2.1

Non-authoritative answer:Name: www.google.comAddresses: 2607:f8b0:4000:804::1011 74.125.227.179 74.125.227.180 74.125.227.176 74.125.227.177 74.125.227.178

Useful Commands

Network Layer 4-6

Net services commands (Net.exe) Performs a broad range of network tasks. Type net with no parameters to see a full list of available command-line options.

C:\Users\jb>net helpThe syntax of this command is:

Commands available are:

NET ACCOUNTS NET HELPMSG NET STATISTICS NET COMPUTER NET LOCALGROUP NET STOP NET CONFIG NET PAUSE NET TIME NET CONTINUE NET SESSION NET USE NET FILE NET SHARE NET USER NET GROUP NET START NET VIEW NET HELP

NET HELP NAMES explains different types of names in NET HELP syntax lines.

NET HELP SERVICES lists some of the services you can start. NET HELP SYNTAX explains how to read NET HELP syntax lines. NET HELP command | MORE displays Help one screen at a time.

Useful Commands

Network Layer 4-7

Netstat (Netstat.exe) Displays active TCP connections, ports on which the computer is listening, Ethernet statistics, the IP routing table, and IPv4/IPv6 statistics.

C:\Users\jb>netstat

Proto Local Address Foreign Address State TCP 127.0.0.1:1029 jb-laptop:5354 ESTABLISHED TCP 127.0.0.1:1036 jb-laptop:27015 ESTABLISHED TCP 127.0.0.1:1047 jb-laptop:19872 ESTABLISHED TCP 127.0.0.1:39055 jb-laptop:39054 ESTABLISHED TCP 172.17.168.138:2492 blugro5relay:2492 ESTABLISHED

C:\Users\jb>netstat -sIPv4 Statistics Packets Received = 10158258 Received Header Errors = 2848 Received Address Errors = 2192434 Datagrams Forwarded = 0 Unknown Protocols Received = 170614 Received Packets Discarded = 4173788 Received Packets Delivered = 6692404

Useful Commands

Network Layer 4-8

Network Command Shell (Netsh.exe) Displays or modifies the network configuration of a local or remote computer that is currently running. This command-line scripting utility has a huge number of options, which are fully detailed in Help.

TCP/IP Route (Route.exe) Displays and modifies entries in the local IP routing table.

C:\Users\jb>route printInterface List13...60 36 dd aa 13 65 ......Intel(R) Centrino(R) Wireless-N 223012...60 36 dd aa 13 69 ......Bluetooth Device (Personal Area Network)31...08 00 27 00 e4 38 ......VirtualBox Host-Only Ethernet Adapter

IPv4 Route TableNetwork Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 172.17.1.1 172.17.168.138 25 127.0.0.0 255.0.0.0 On-link 127.0.0.1 306 127.0.0.1 255.255.255.255 On-link 127.0.0.1 306 127.255.255.255 255.255.255.255 On-link 127.0.0.1 306 169.254.0.0 255.255.0.0 On-link 169.254.40.182 276 169.254.40.182 255.255.255.255 On-link 169.254.40.182 276 169.254.255.255 255.255.255.255 On-link 169.254.40.182 276 172.17.0.0 255.255.0.0 On-link 172.17.168.138 281 172.17.168.138 255.255.255.255 On-link 172.17.168.138 281 172.17.255.255 255.255.255.255 On-link 172.17.168.138 281 224.0.0.0 240.0.0.0 On-link 169.254.40.182 276

Useful Commands

Network Layer 4-9

(Arp.exe) Displays current ARP entries by interrogating the current protocol data. If inet_addr is specified, the IP and Physical addresses for only the specified computer are displayed. If more than one network interface uses ARP, entries for each ARP table are displayed.

C:\Users\jb>arp -a

Interface: 130.215.28.36 --- 0x10 Internet Address Physical Address Type 130.215.24.1 00-00-5e-00-01-01 dynamic 130.215.24.2 00-23-9c-94-97-f0 dynamic 130.215.27.252 f0-1f-af-2f-e1-27 dynamic 130.215.28.63 00-16-3e-c5-01-25 dynamic 130.215.29.165 00-24-e8-32-32-1d dynamic 130.215.31.255 ff-ff-ff-ff-ff-ff static

Useful Commands

Network Layer 4-10

BONUS: Useful commands4.4 IP: Internet Protocol

datagram format IPv4 addressing ICMP IPv6




Lecture 12: outline

Network Layer 4-11

IP addressing: CIDR

CIDR: Classless InterDomain Routing subnet portion of address of arbitrary

length address format: a.b.c.d/x, where x is #

bits in subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Network Layer 4-12

IP addresses: how to get one?Q: How does a host get IP address?

hard-coded by system admin in a file Windows: control-panel->network-

>configuration->tcp/ip->properties UNIX: /etc/rc.config

DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server “plug-and-play”

Network Layer 4-13

DHCP: Dynamic Host Configuration Protocolgoal: allow host to dynamically obtain its IP address

from network server when it joins network can renew its lease on address in use allows reuse of addresses (only hold address while

connected/“on”) support for mobile users who want to join network

(more shortly)DHCP can return more than just allocated IP

address on subnet: address of first-hop router for client name and IP address of DNS sever network mask (indicating network versus host

portion of address)You don’t get any old IP address – it has to be one that can be services by the port on the router you’re connected to.

Network Layer 4-14

NAT: network address translation

192.168.0.1

192.168.0.2

192.168.0.3

192.168.0.4

138.76.29.7

local network(e.g., home network)

192.168/16rest of

Internet

datagrams with source or destination in this networkhave 192.168/16 address for source, destination (as usual)

all datagrams leaving local

network have same single source NAT IP

address: 138.76.29.7,different source port numbers

Network Layer 4-15

motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP:

just one IP address for all devices can change addresses of devices in local

network without notifying outside world can change ISP without changing

addresses of devices in local network devices inside local net not explicitly

addressable, visible by outside world (a security plus)


Network Layer 4-16

implementation: NAT router must:

outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #). . . remote clients/servers will respond using

(NAT IP address, new port #) as destination addr

remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table


Network Layer 4-17

NAT traversal problem client wants to connect to

server with address 192.168.0.1 server address 192.168.0.1

local to LAN (client can’t use it as destination addr)

only one externally visible NATed address: 138.76.29.7

solution1: statically configure NAT to forward incoming connection requests at given port to server e.g., (123.76.29.7, port 2500)

always forwarded to 192.168.0.1 port 25000

192.168.0.1

192.168.0.4

NAT router

138.76.29.7

client?

Network Layer 4-18

NAT traversal problem solution 2: relaying (used in Skype)

NATed client establishes connection to relay external client connects to relay relay bridges packets between to

connections

138.76.29.7client

1. connection torelay initiatedby NATed host

2. connection torelay initiatedby client

3. relaying established

NAT router

192.168.0.1

Network Layer 4-19

4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP




Lecture 12: outline

Network Layer 4-20

Hierarchical routing

scale: with 600 million destinations:

can’t store all dest’s in routing tables!

routing table exchange would swamp links!

administrative autonomy

internet = network of networks

each network admin may want to control routing in its own network

our routing study thus far - idealization

all routers identical network “flat”… not true in practice

Network Layer 4-21

3b

1d

3a

1c2aAS3

AS1AS21a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

forwarding table configured by both intra- and inter-AS routing algorithm intra-AS sets

entries for internal dests

inter-AS & intra-AS sets entries for external dests

Network Layer 4-22

Inter-AS tasks suppose router in

AS1 receives datagram destined outside of AS1: router should

forward packet to gateway router, but which one?

AS1 must:1. learn which dests

are reachable through AS2, which through AS3

2. propagate this reachability info to all routers in AS1

job of inter-AS routing!

AS3

AS2

3b

3c

3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

Network Layer 4-23

Example: setting forwarding table in router 1d

suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c), but not via AS2 inter-AS protocol propagates reachability info to all

internal routers router 1d determines from intra-AS routing info that its

interface I is on the least cost path to 1c installs forwarding table entry (x,I)

AS3

AS2

3b

3c

3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

x…

Network Layer 4-24

4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6




Lecture 12: outline

Network Layer 4-25

Intra-AS Routing

also known as interior gateway protocols (IGP)

most common intra-AS routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing

Protocol (Cisco proprietary)

Network Layer 4-26

RIP ( Routing Information Protocol)

included in BSD-UNIX distribution in 1982 distance vector algorithm

distance metric: # hops (max = 15 hops), each link has cost 1 DVs exchanged with neighbors every 30 sec in response

message (aka advertisement) each advertisement: list of up to 25 destination subnets (in IP

addressing sense)

DC

BAu v

w

x

yz

subnet hops u 1 v 2 w 2 x 3 y 3 z 2

from router A to destination subnets:

Network Layer 4-27

RIP: example

destination subnet next router # hops to dest

w A 2y B 2

z B 7x -- 1…. …. ....

routing table in router D

w x yz

A

C

D B

Network Layer 4-28

w x yz

A

C

D B

destination subnet next router # hops to dest

w A 2y B 2

z B 7x -- 1…. …. ....

routing table in router D

A 5

dest next hops w - 1 x - 1 z C 4 …. … ...

A-to-D advertisement

RIP: example

Network Layer 4-29

RIP table processing RIP routing tables managed by application-level

process called route-d (daemon) advertisements sent in UDP packets,

periodically repeated if no advertisement heard after 180 sec -->

neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors

physical

link

network forwarding (IP) table

transport (UDP)

routed

physical

link

network (IP)

transprt (UDP)

routed

forwardingtable

Network Layer 4-30

OSPF (Open Shortest Path First) “open”: publicly available

uses link state algorithm LS packet dissemination topology map at each node route computation using Dijkstra’s algorithm

OSPF advertisement carries one entry per neighbor

advertisements flooded to entire AS carried in OSPF messages directly over IP

(rather than TCP or UDP AS-AS routing protocol: nearly identical to

OSPF security: all OSPF messages authenticated

(to prevent malicious intrusion) multiple same-cost paths allowed (only

one path in RIP)

Network Layer 4-31

Hierarchical OSPF

boundary routerbackbone router

area 1area 2

area 3

backbone

areaborderrouters

internalrouters

Network Layer 4-32

two-level hierarchy: local area, backbone. link-state advertisements only in area each nodes has detailed area topology;

only know direction (shortest path) to nets in other areas.

area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.

backbone routers: run OSPF routing limited to backbone.

boundary routers: connect to other AS’s.

Hierarchical OSPF

Network Layer 4-33

Internet inter-AS routing: BGP BGP (Border Gateway Protocol): the de

facto inter-domain routing protocol “glue that holds the Internet together”

BGP provides each AS a means to: eBGP: obtain subnet reachability

information from neighboring ASs. iBGP: propagate reachability information to

all AS-internal routers. determine “good” routes to other networks

based on reachability information and policy.

allows subnet to advertise its existence to rest of Internet: “I am here”

Network Layer 4-34

BGP basics

when AS3 advertises a prefix to AS1: AS3 promises it will forward datagrams towards that prefix AS3 can aggregate prefixes in its advertisement

AS3

AS2

3b

3c

3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

BGP session: two BGP routers (“peers”) exchange BGP messages: advertising paths to different destination network prefixes

(“path vector” protocol) exchanged over semi-permanent TCP connections

BGP message

Network Layer 4-35

BGP basics: distributing path information

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2b

othernetworks

othernetworks

using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. 1c can then use iBGP do distribute new prefix info to

all routers in AS1 1b can then re-advertise new reachability info to AS2

over 1b-to-2a eBGP session when router learns of new prefix, it creates

entry for prefix in its forwarding table.

eBGP session

iBGP session

Network Layer 4-36

4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6




Lecture 12: outline

Network Layer 4-37

R1

R2

R3 R4

sourceduplication

R1

R2

R3 R4

in-networkduplication

duplicatecreation/transmissionduplicate

duplicate

Broadcast routing deliver packets from source to all other

nodes source duplication is inefficient:

source duplication: how does source determine recipient addresses?

Network Layer 4-38

In-network duplication

flooding: when node receives broadcast packet, sends copy to all neighbors problems: cycles & broadcast storm

controlled flooding: node only broadcasts pkt if it hasn’t broadcast same packet before node keeps track of packet ids already

broadacsted or reverse path forwarding (RPF): only

forward packet if it arrived on shortest path between node and source

spanning tree: no redundant packets received by any node

Network Layer 4-39

A

B

G

DE

c

F

A

B

G

DE

c

F

(a) broadcast initiated at A (b) broadcast initiated at D

Spanning tree

first construct a spanning tree nodes then forward/make copies only

along spanning tree

Network Layer 4-40

A

B

G

DE

c

F1

2

3

4

5

(a) stepwise construction of spanning tree (center: E)

A

B

G

DE

c

F

(b) constructed spanning tree

Spanning tree: creation center node each node sends unicast join message

to center node message forwarded until it arrives at a node

already belonging to spanning tree

The End is Near!

Lecture 12

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Transcript of Lecture 12