advanced networking hardware design lecture 2 wan and introduction to routing on the internet

Advanced Networking Hardware Design

Lecture 2 WAN and Introduction to Routing on

the Internet

Kocak 2.2

What is a WAN

• A WAN is a data communications network that covers a relatively broad geographic area

• WAN technologies function at the lower three layers of the OSI reference model

• A point-to-point link provides a single, pre-established WAN communication path from the customer premises through a carrier network to a remote network. Point-to-point lines are also called leased lines.

Kocak 2.3

Circuit Switching

• The mechanism used in the public switched telephone network (PSTN)

• It consists of explicitly establishing a physical end-to-end connection that is dedicated to each pair of communicating devices

Kocak 2.4

Packet Switching

• Packet switching is a WAN technology in which users share common carrier resources

• By dividing data communications into packets, a number of concurrent communicating applications are statistically multiplexed on a link

Kocak 2.5

Routing basics

• Routing is the act of moving information across an internetwork from a source to a destination.

• Routing occurs at the network layer• Two activities

– Optimal path determination– Packet transportation

Kocak 2.6

Path determination

• Routing protocols use metric to evaluate what path will be the best for a packet to travel. (e.g., metric: path bandwidth)

• Routing algorithms initialize and maintain routing tables (aka. forwarding tables), which contain route information.

Kocak 2.7

Routing algorithms

• Use network topology and link state information to determine the nodes through which paths should flow, and load this information into the forwarding table

• In connectionless datagram network, the routing protocol operates asynchronously with individual flows to keep the table loaded with entries

• In connection-oriented networks, the routing algorithm runs at each connection setup to load the appropriate information into the forwarding table

Kocak 2.8

Routing algorithms (cont.)

• Design goals:– Optimality: capability of selecting the best route– Simplicity: minimum software and utilization

overhead– Robustness: perform correctly in unforeseen

conditions– Convergence: process of agreement, by all routers,

on optimal routes– Flexibility: adapt to a variety of network

circumstances

• Types:– Static vs dynamic– Single path vs multipath– Flat vs hierarchical– Host-intelligent vs router intelligent– Intradomain vs interdomain– Link-state vs distance vector

Kocak 2.9

Routing metrics

• Path length – is the sum of the costs associated with each link traversed. Also defined as hop count (number of nodes a packet must pass)

• Reliability – dependability of each network link. Usually described in terms of the bit-error rate

• Delay – the length of time required to move a packet from source to destination thru the network

• Bandwidth – available traffic capacity of a link• Load – refers to the degree to which a network

resource is busy• Communication cost

(c) Stanford Univ. (adapted from Mckeown's notes) 2.10

The Internet Protocol (IP)

• Characteristics of IP

CONNECTIONLESS: mis-sequencing

UNRELIABLE: may drop packets…

BEST EFFORT: … but only if necessary

DATAGRAM: individually routed

A

R1

R2

R4

R3

BSource Destination

D H

D H

•Architecture•Links•Topology

Transparent


The IP Datagram

Flags

vers

TTL

TOS

checksum

HLen Total Length

ID FRAG Offset

Protocol

SRC IP Address

DST IP Address

(OPTIONS) (PAD)

<=64 KBytes

Offset within original packet

Hop count


Fragmentation

A

EthernetMTU=1500 bytes

MTU=1500 bytes

B

Source Destination

MTU<1500 bytes

Problem: A router may receive a packet larger than the maximum transmission unit (MTU) of the outgoing link.

R1 R2

Solution: R1 fragments the IP datagram into mutiple, self-contained datagrams.

Data HDR (ID=x)

Data HDR (ID=x) Data HDR (ID=x) Data HDR (ID=x)

Offset>0More Frag=0

Offset=0More Frag=1


Fragmentation

• Fragments are re-assembled by the destination host; not by intermediate routers.

• To avoid fragmentation, hosts commonly use path MTU discovery to find the smallest MTU along the path.

• Path MTU discovery involves sending various size datagrams until they do not require fragmentation along the path.

• Most links use MTU>=1500bytes today. • Try: traceroute –f www.mit.edu 1500 and

traceroute –f www.mit.edu 1501• (DF=1 set in IP header; routers send “ICMP” error

message, which is shown as “!F”).• Can you find a destination for which the path MTU

< 1500 bytes?

http://www.mit.edu/

http://www.mit.edu/


IP AddressesOriginally there were 5 classes:

CLASS “A” 1 7

0 Net ID Host-ID

CLASS “B” 10 Net ID Host-ID

24

2 14 16

CLASS “C” 110

Net ID Host-ID

3 21 8

CLASS “D”

1110

Multicast Group ID

4 28

CLASS “E” 11110 Reserved

5 27

A B C D0 232-1


IP Addressing

Problem: Address classes were too “rigid”. For most

organizations, Class C were too small and Class B too big. Led to very inefficient use of address space, and a shortage of addresses.

Organizations with internal routers needed to have a separate (Class C) network ID for each link.

And then every other router in the Internet had to know about every network ID in every organization, which led to large address tables.

Small organizations wanted Class B in case they grew to more than 255 hosts. But there were only about 16,000 Class B network IDs.


IP Addressing

Two solutions were introduced: Subnetting is used within an organization to subdivide

the organization’s network ID. Classless Interdomain Routing (CIDR) was introduced in

1993 to provide more efficient and flexible use of IP address space across the whole Internet.

CIDR is also known as “supernetting” because subnetting and CIDR are basically the same idea.


Subnetting

CLASS “B”e.g.

Company

10 Net ID Host-ID

2 14 16

10 Net ID Host-ID

2 14 16

0000

Subnet ID (20) SubnetHost ID (12)

10 Net ID Host-ID

2 14 16

1111


10 Net ID Host-ID

2 14 16

000000


10 Net ID Host-ID

2 14 16

1111011011


e.g. Site

e.g. Dept


Classless Interdomain Routing (CIDR)Addressing

The IP address space is broken into line segments. Each line segment is described by a prefix. A prefix is of the form x/y where x indicates the prefix of all

addresses in the line segment, and y indicates the length of the segment.

e.g. The prefix 128.9/16 represents the line segment containing addresses in the range: 128.9.0.0 … 128.9.255.255.

0 232-1

128.9/16

128.9.0.0

216

142.12/19

65/8

128.9.16.14



0 232-1

128.9/16

128.9.16.14

128.9.16/20128.9.176/20

128.9.19/24

128.9.25/24

Most specific route = “longest matching prefix”



Prefix aggregation: If a service provider serves two organizations

with prefixes, it can (sometimes) aggregate them to form a larger prefix. Other routers can refer to this larger prefix, and so reduce the size of their address table.

E.g. ISP serves 128.9.14.0/24 and 128.9.15.0/24, it can tell other routers to send it all packets belonging to the prefix 128.9.14.0/23.

ISP Choice: In principle, an organization can keep its

prefix if it changes service providers.


Size of the Routing Table at the core of the Internet

Source: http://www.telstra.net/ops/bgptable.html

0

20000

40000

60000

80000

100000

120000

140000

Aug-87 May-90 J an-93 Oct-95 J ul-98 Apr-01 J an-04

Num

ber

of

prefi

xes


Prefix Length Distribution

0

10000

20000

30000

40000

50000

60000

70000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Prefix Length

Num

ber

of P

refi

xes

Source: Geoff Huston, Oct 2001


Mapping Computer Names to IP addressesThe Domain Naming System (DNS)

Names are hierarchical and belong to a domain:– e.g. elaine17.stanford.edu– Common domain names: .com, .edu, .gov, .org, .net, .uk

(or other country-specific domain).– Top-level names are assigned by the Internet Corporation

for Assigned Names and Numbers (ICANN).– A unique name is assigned to each organization.

DNS Client-Server Model– DNS maintains a hierarchical, distributed database of

names.– Servers are arranged in a hierarchy.– Each domain has a “root” server.– An application needing an IP address is a DNS client.


Mapping Computer Names to IP addressesThe Domain Naming System (DNS)

A DNS Query1. Client asks local server.2. If local server does not have address, it asks the root

server for the requested domain.3. Addresses are cached in case they are requested

again.

E.g. www.eecs.berkeley.edu

.stanford.edu

.berkeley.edu .eecs.berkeley.edu.edu

“What is the IP address of www.eecs.berkeley.edu?”

e.g. gethostbyname()

Clientapplication

Example: On elaine machines, try “host www.mit.edu” or “nslookup www.mit.edu”Question: Why does “host gates-gateway” return multiple IP addresses?

http://www.eecs.berkeley.edu/

http://www.mit.edu/

http://www.mit.edu/


How a Router Forwards Datagrams

128.9/16128.9.16/20

128.9.176/20

128.9.19/24128.9.25/24

142.12/19

65/8

Prefix Port

3227213

128.17.14.1128.17.14.1

128.17.20.1

128.17.10.1128.17.14.1

128.17.16.1

128.17.16.1

Next-hop

R1

R2

R3

R4

12

3

128.17.20.1

128.17.16.1

e.g. 128.9.16.14 => Port 2

Forwarding/routing table


How a Router Forwards Datagrams

Every datagram contains a destination address.

The router determines the prefix to which the address belongs, and routes it to the“Network ID” uniquely identifies a physical network.

All hosts and routers sharing a Network ID share same physical network.


Forwarding Datagrams

Is the datagram for a host on directly attached network?

If no, consult forwarding table to find next-hop.


Inside a Router

ForwardingDecision

ForwardingDecision

ForwardingDecision

ForwardingTable

ForwardingTable

ForwardingTable

Interconnect

OutputScheduling

1.

2.

3.


Forwarding in an IP Router

• Lookup packet DA in forwarding table.– If known, forward to correct port.– If unknown, drop packet.

• Decrement TTL, update header Checksum.

• Forward packet to outgoing interface.• Transmit packet onto link.

Question: How is the address looked up in a real router?


Making a Forwarding DecisionClass-based addressing

Class A Class B Class C D

212.17.9.4

Class A

Class B

Class C212.17.9.0 Port 4

Exact Match

Routing Table:

IP Address Space

212.17.9.0

Exact Match: There are many well-known ways to find an exact match in a table.


Error Reporting (ICMP)

Internet Control Message Protocol:– Used by a router/end-host to report some types

of error:– E.g. Destination Unreachable: packet can’t be

forwarded to/towards its destination. – E.g. Time Exceeded: TTL reached zero, or

fragment didn’t arrive in time. Traceroute uses this error to its advantage.

– An ICMP message is an IP datagram, and is sent back to the source of the packet that caused the error.

advanced networking hardware design lecture 2 wan and introduction to routing on the internet

Documents

network link

carrier network

network resource

routing tables

remote network

routing basicsrouting

data communications

routing algorithms cont