The Illustrated Network- P11 doc

We’ll investigate the frame types used in various technolo-gies and how they carry packets?. We’ll take a long look at Ethernet, and mention many other link types used primarily in priva

QUESTIONS FOR READERS

Figure 2.8 shows some of the concepts discussed in this chapter and can be used to help you answer the following questions

VLAN 1

Broadcast messages

sent only to the VLAN 1

broadcast domain

(and router).

Broadcast messages sent only to the VLAN 2 broadcast domain (and router).

VLAN 2

Server

Router

Transparent Bridge

Hub

Hub

ARP on LAN segement

before sending frame Use UDP for connectionless,TCP for connection-oriented

LAN Switch

Hub

FIGURE 2.8

Hubs, bridges, and routers can connect LAN segments to form an internetwork.

1 What is the main function of the ARP message on a LAN?

2 What is the difference between TCP and UDP terms of connection overhead and reliability?

3 What is a transparent bridge?

4 What is the difference between a bridge and a router in terms of broadcast domains?

5 What is the relationship between a broadcast domain and a VLAN?

69

What You Will Learn

In this chapter, you will learn more about the links used to connect the nodes of the Illustrated Network We’ll investigate the frame types used in various technolo-gies and how they carry packets We’ll take a long look at Ethernet, and mention many other link types used primarily in private networks

You will learn about SONET/SDH, DSL, and wireless technologies as well as Ethernet All four link types are used on the Illustrated Network

Network Link Technologies

3

This chapter explores the physical and data link layer technologies used in the Illus-trated Network We investigate the methods used to link hosts and intermediate nodes together over shorter LAN distances and longer WAN distances to make a complete network

For most of the rest of the book, we’ll deal with packets and their contents This is our only chance to take a detailed look at the frames employed on our network, and even peer inside them Because the Illustrated Network is a real network, we’ll empha-size the link types used on the network and take a more cursory look at link types that might be very important in the TCP/IP protocol suite, but are not used on our network We’ll look at Ethernet and the Synchronous Optical Network/Synchronous Digital Hier-archy (SONET/SDH) link technologies, and explore the variations on the access theme that digital subscriber line (DSL) and wireless technologies represent

We’ll look at public network services like frame relay and Asynchronous Transfer

Mode (ATM) in a later chapter In this book, the term private network is used to

char-acterize network links that are owned or directly leased by the user organization, while

a public network is characterized by shared user access to facilities controlled by a service provider The question of Who owns the intermediate nodes? is often used as

a rough distinguisher between private and public network elements

Because of the way the TCP/IP protocol stack is specifi ed, as seen in Chapter 1, we won’t talk much about physical layer elements such as modems, network interface cards (NICs), and connectors As important as these aspects of networking are, they

lo0: 192.168.0.1

fe-1/3/0: 10.10.11.1 MAC: 00:05:85:88:cc:db (Juniper_88:cc:db) IPv6: fe80:205:85ff:fe88:ccdb

P9

lo0: 192.168.9.1

PE5

lo0: 192.168.5.1

P4

lo0: 192.168.4.1

so-0/0/1 79.2

so-0/0/1 24.2

so-0/0/0 47.1

so-0/0/2 29.2 so-0/0/3

49.2

so-0/0/3 49.1

so-0/0/059.2

so-0/0/2 45.

1

so-0/0/2 45.2

so-0/0/059.1

ge-0/0/350.2

ge-0/0/350.1

Ethernet LAN Switch with Twisted-Pair Wiring

em0: 10.10.11.177

MAC: 00:0e:0c:3b:8f:94

(Intel_3b:8f:94)

IPv6: fe80::20e:

cff:fe3b:8f94

eth0: 10.10.11.66 MAC: 00:d0:b7:1f:fe:e6 (Intel_1f:fe:e6) IPv6: fe80::2d0:

b7ff:fe1f:fee6

LAN2: 10.10.11.51 MAC: 00:0e:0c:3b:88:3c (Intel_3b:88:3c) IPv6: fe80::20e:

cff:fe3b:883c

LAN2: 10.10.11.111 MAC: 00:0e:0c:3b:87:36 (Intel_3b:87:36) IPv6: fe80::20e:

cff:fe3b:8736

winsvr1

LAN1

Los Angeles

Office

Ace ISP

AS 65459

DSL Link

Wireless

in Home

Note: All links use 10.0.x.y

addressing only the last

two octets are shown.

FIGURE 3.1

Connections used on the Illustrated Network SONET/SDH links are indicated by heavy lines, Ethernet types by dashed lines, and DSL is shown as a dotted line The home wireless network

is not given a distinctive representation.

lo0: 192.168.6.1

fe-1/3/0: 10.10.12.1 MAC: 0:05:85:8b:bc:db (Juniper_8b:bc:db) IPv6: fe80:205:85ff:fe8b:bcdb Ethernet LAN Switch with Twisted-Pair Wiring

eth0: 10.10.12.77

MAC: 00:0e:0c:3b:87:32

(Intel_3b:87:32)

IPv6: fe80::20e:

cff:fe3b:8732

eth0: 10.10.12.166 MAC: 00:b0:d0:45:34:64 (Dell_45:34:64) IPv6: fe80::2b0:

d0ff:fe45:3464

LAN2: 10.10.12.52 MAC: 00:0e:0c:3b:88:56 (Intel_3b:88:56) IPv6: fe80::20e:

cff:fe3b:8856

LAN2: 10.10.12.222 MAC: 00:02:b3:27:fa:8c IPv6: fe80::202: b3ff:fe27:fa8c

LAN2

New York

Office

P7

lo0: 192.168.7.1

PE1

lo0: 192.168.1.1

P2

lo0: 192.168.2.1

so-0/0/1

79.1

so-0/0/1

24.1

so-0/0/0

47.2

so-0

/0/2 29.1

so-0/0/3 27.2

so-0/0/3 27.1

so-0/0/2 17.2

so-0/0/2 17.1

so-0/0/0 12.2

so-0/0/0 12.1

ge-0/0/3 16.2

ge-0/0/3 16.1

Best ISP

AS 65127

Global Public Internet

have little to do directly with how TCP/IP protocols or the Internet operates For exam-ple, a full exploration of all the connector types used with fi ber-optic cable would take many pages, and yet add little to anyone’s understanding of TCP/IP or the Internet Instead, we will concentrate on the structure of the frames sent on these link types,

which are often important to TCP/IP, and present some operational details as well.

ILLUSTRATED NETWORK CONNECTIONS

We will start by using Ethereal (Wireshark), the network protocol analyzer introduced

in the last chapter, to investigate the connections between systems on the Illustrated Network It runs on a variety of platforms, including all three used in the Illustrated Network: FreeBSD Unix, Linux, and Windows XP Ethereal can display real-time packet interpretations and, if desired, also save traffi c to fi les (with a variety of formats) for later analysis or transfer to another system Ethereal is most helpful when examining all types of Ethernet links The Ethernet links are shown as dashed lines in Figure 3.1 The service provider networks’ SONET links are shown as heavy solid lines, and the DSL link to the home offi ce is shown as a dotted line The wireless network inside the home is not given a distinctive representation in the fi gure Note that ISPs today typi-cally employ more variety in WAN link types

Displaying Ethernet Traffi c

On the Illustrated Network, all of the clients and servers with detailed information listed are attached to LANs Let’s start our exploration of the links used on the

Illus-trated Network by using Ethereal both ways to see what kind of frames are used on

these LANs

Here is a capture of a small frame to show what the output looks like using tethe-real, the text-based version of Ethereal The example uses the verbose mode (–V) to force tethereal to display all packet and frame details The example shows, highlighted

in bold, that Ethernet II frames are used on LAN1

[root@lnxserver admin]# /usr/sbin/tethereal –V

Frame 2 (60 bytes on wire, 60 bytes captured)

Arrival Time: Mar 25, 2008 12:14:36.383610000

Time delta from previous packet: 0.000443000 seconds

Time relative to first packet: 0.000591000 seconds

Frame Number: 2

Packet Length: 60 bytes

Capture Length: 60 bytes

Ethernet II, Src: 00:05:85:88:cc:db, Dst: 00:d0:b7:1f:fe:e6

Destination: 00:d0:b7:1f:fe:e6 (Intel_1f:fe:e6)

Source: 00:05:85:88:cc:db (Juniper 88:cc:db)

Type: ARP (0x0806)

Trailer: 00000000000000000000000000000000

Address Resolution Protocol (reply)

Hardware type: Ethernet (0x0001)

Protocol type: IP (0x0800)

Hardware size: 6

Protocol size: 4

Opcode: reply (0x0002)

Sender MAC address: 00:05:85:88:cc:db (Juniper 88:cc:db)

Sender IP address: 10.10.11.1 (10.10.11.1)

Target MAC address: 00:d0:b7:1f:fe:e6 (Intel_1f:fe:e6)

Target IP address: 10.10.11.66 (10.10.11.66)

Many details of the packet and frame structure and content will be discussed in later chapters However, we can see that the source and destination MAC addresses are present in the frame The source address is 00:05:85:88:cc:db (the router), and the destination (the Linux server) is 00:d0:b7:1f:fe:e6 Ethereal even knows which organizations have been assigned the fi rst 24 bits of the 48-bit MAC address (Intel and Juniper Networks) We’ll say more about MAC addresses later in this chapter

Figure 3.2 shows the same packet, and the same information, but in graphical for-mat Only a small section of the entire window is included Note how the presence of Ethernet II frames is indicated, parsed on the second line in the middle pane of the window

Why use text-based output when a graphical version is available? The graphical out-put shows the raw frame in hex, something the text-based version does not do, and the interpretation of the frame’s fi elds is more concise

However, the graphical output is not always clearer In most cases, the graphical rep-resentation can be more cluttered, especially when groups of packets are involved The graphical output only parses one packet at a time on the screen, while a whole string

of packets can be parsed with tethereal (but printouts of graphical information can be formatted like tethereal)

FIGURE 3.2

Graphical interface for Ethereal There are three main panes Top to bottom: (1) a digest of the packets header and information, (2) parsed details about frame and packet contents, and (3) the raw frame captured in hexadecimal notation and interpreted in ASCII.

In addition, many network administrators of Internet servers do not install or use

a graphical interface, and perform their tasks from a command prompt If you’re not sitting in front of the device, it’s more expedient to run the non-GUI version Tethereal

is the only realistic option in these cases We will use both types of Ethereal in the examples in this book

In our example network, what about LAN2? Is it also using Ethernet II frames? Let’s capture some packets on bsdserver to fi nd out

bsdserver# tethereal –V

Capturing on em0

Frame 1 (98 bytes on wire, 98 bytes captured)

Arrival Time: Mar 25, 2008 13:05:00.263240000

Time delta from previous packet: 0.000000000 seconds

Time since reference or first frame: 0.000000000 seconds

Frame Number: 1

Packet Length: 98 bytes

Capture Length: 98 bytes

Ethernet II, Src: 00:0e:0c:3b:87:32, Dst: 00:05:85:8b:bc:db

Destination: 00:05:85:8b:bc:db (Juniper 8b:bc:db)

Source: 00:0e:0c:3b:87:32 (Intel_3b:87:32)

Type: IP (0x0800)

Internet Protocol, Src Addr: 10.10.12.77 (10.10.12.77), Dst Addr: 10.10.12.1 (10.10.12.1)

Header length: 20 bytes

Yes, an Ethernet II frame is in use here as well Even though we’re running Ethereal (tethereal) on a different operating system (FreeBSD) instead of on Linux, the output is nearly identical (the differences are due to a slightly different version of Ethereal on the servers) However, LANs are not the only type of connections used on the Illustrated Network

Displaying SONET Links

What about link types other than Ethernet? ISPs in the United States often use SONET

fi ber links between routers separated by long distance In most other parts of the world, SDH is used SONET was defi ned initially in the United States, and the specifi cation was adapted, with some changes, for international use by the ITU-T as SDH

The Illustrated Network uses SONET, not SDH There are small but important differ-ences between SONET and SDH, but this book will only reference SONET Line moni-toring equipment that allows you to look directly at SONET/SDH frames is expensive and exotic, and not available to most network administrators So we’ll take a different approach: We’ll show you the information that’s available on a router with a SONET interface This will show the considerable bandwidth available even in the slowest of SONET links, which runs at 155 Mbps and is the same as the basic SDH speed

Admin>ssh ce0

adminCE6’s password: *********

- JUNOS 8.4R1.3 built 2007-08-06 06:58:15 UTC

admin@ce0> monitor interface so-0/0/1

Interface: so-0/0/1, Enabled, Link is Up

Encapsulation: PPP, Keepalives, Speed: OC3

Encapsulation statistics:

LCP state: Opened

Error statistics:

SONET and SDH

The SONET fi ber-optic link standard was developed in the United States and is

mainly used in places that follow the digital telephony system used in the United

States, such as Canada and the Philippines SDH, on the other hand, is used in

places that follow the international standards developed for the digital telephony

system in the rest of the world SDH must be used for all international links, even

those that link to SONET networks in the United States

The differences between SONET and SDH transmission frame structures,

nomenclature, alarms, and other details are relatively minor In most cases,

equip-ment can handle SONET/SDH with equal facility

Routers and Users

Usually, network administrators don’t let ordinary users casually log in to routers,

even edge routers, and poke around Even if they were allowed to, the ISP’s core

routers would still remain off limits But this is our network, and we can do as we

please, wherever we please

We can log in to router CE0 and monitor a SONET interface for a minute or so and

see what’s going on

Input runts: 0 [0]

Active alarms : None

Active defects: None

SONET error counts/seconds:

SONET statistics:

Not much is happening yet on our network in terms of traffi c, but the output is still informative The fi rst column shows cumulative values and the second column shows the change since the last monitor “snapshot” on the link “Live” traffi c during

these 59 seconds, in this case mostly a series of keepalive packets, is shown in

paren-theses, both in bytes per second and in packets per second (the example rounds the

39 packets in 59 seconds, or 0.66 packets per second, down to 0 packets per second) The frames carried on the link, listed as encapsulation, belong to a protocol called Point-to-Point Protocol (PPP) Six PPP keepalives have been sent in the 59-second window, and seven have been received (they are exchanged every 10 seconds), add-ing to the total of more than 477,000 since the link was initialized The cumulative errors also occurred as the link was initializing itself, and it is reassuring that there are

no new errors

Displaying DSL Links

The Illustrated Network also has a broadband DSL link from an ISP that is used to allow

a home offi ce to attach to the router network This link is shown in red in Figure 3.1

If the permissions are set up correctly, the home user will be able to access network resources on LAN1 and LAN2 DSL links are much faster than ordinary dial-up lines and are always available, just like a leased access line The DSL link terminates at home in a

DSL router (more properly, a residential gateway), and the distribution of information

to devices in the home can be by wired or wireless LAN