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Advanced Networking Hardware Design Lecture 2 WAN and Introduction to Routing on the Internet. 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

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

Advanced Networking Hardware DesignLecture 2 WAN and Introduction to Routing on the Internet

what is a wan
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

circuit switching
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

packet switching
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

routing basics
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

path determination
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

routing algorithms
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

routing algorithms cont
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

routing metrics
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

Kocak

the internet protocol ip

B

A

D

H

The Internet Protocol (IP)
  • Characteristics of IP
    • CONNECTIONLESS: mis-sequencing
    • UNRELIABLE: may drop packets…
    • BEST EFFORT: … but only if necessary
    • DATAGRAM: individually routed

Source

Destination

R2

D

H

R1

R3

  • Architecture
  • Links
  • Topology

R4

Transparent

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

the ip datagram
The IP Datagram

vers

HLen

TOS

Total Length

Offset within original packet

ID

Flags

FRAG Offset

Hop count

TTL

Protocol

checksum

SRC IP Address

<=64 KBytes

DST IP Address

(OPTIONS)

(PAD)

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

fragmentation

A

B

Fragmentation

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

Source

Destination

MTU=1500 bytes

MTU=1500 bytes

Ethernet

MTU<1500 bytes

R1

R2

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

Data

HDR (ID=x)

Offset=0

More Frag=1

Offset>0

More Frag=0

Data

HDR (ID=x)

Data

HDR (ID=x)

Data

HDR (ID=x)

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

fragmentation1
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 andtraceroute –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?

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

ip addresses
IP Addresses

Originally there were 5 classes:

24

1

7

CLASS “A”

Host-ID

0

Net ID

16

2

14

CLASS “B”

Host-ID

10

Net ID

8

3

21

CLASS “C”

Host-ID

110

Net ID

4

28

CLASS “D”

1110

Multicast Group ID

5

27

CLASS “E”

11110

Reserved

A

B

C

D

0

232-1

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

ip addressing
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.

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

ip addressing1
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.

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

subnetting
Subnetting

16

2

14

CLASS “B”

e.g. Company

Host-ID

10

Net ID

16

16

2

14

2

14

e.g. Site

0000

1111

Host-ID

Host-ID

10

Net ID

10

Net ID

Subnet ID (20)

Subnet

Host ID (12)

Subnet ID (20)

Subnet

Host ID (12)

16

16

2

14

2

14

e.g. Dept

Host-ID

10

Net ID

1111011011

Host-ID

10

Net ID

000000

Subnet ID (26)

Subnet

Host ID (6)

Subnet ID (22)

Subnet

Host ID (10)

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

classless interdomain routing cidr addressing

142.12/19

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.

128.9.0.0

65/8

128.9/16

0

232-1

216

128.9.16.14

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

classless interdomain routing cidr addressing1

128.9.19/24

128.9.25/24

128.9.16/20

128.9.176/20

Most specific route = “longest matching prefix”

Classless Interdomain Routing (CIDR)Addressing

128.9/16

0

232-1

128.9.16.14

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

classless interdomain routing cidr addressing2
Classless Interdomain Routing (CIDR)Addressing

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.

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

size of the routing table at the core of the internet
Size of the Routing Table at the core of the Internet

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

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

slide22

Prefix Length Distribution

Source: Geoff Huston, Oct 2001

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

mapping computer names to ip addresses the domain naming system dns
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.

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

mapping computer names to ip addresses the domain naming system dns1
Mapping Computer Names to IP addressesThe Domain Naming System (DNS)

A DNS Query

  • Client asks local server.
  • If local server does not have address, it asks the root server for the requested domain.
  • Addresses are cached in case they are requested again.

E.g. www.eecs.berkeley.edu

.stanford.edu

“What is the IP address of

www.eecs.berkeley.edu?”

e.g. gethostbyname()

.edu

Client

application

.berkeley.edu

.eecs.berkeley.edu

Example: On elaine machines, try “host www.mit.edu” or “nslookup www.mit.edu”

Question: Why does “host gates-gateway” return multiple IP addresses?

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

how a router forwards datagrams
How a Router Forwards Datagrams

128.17.20.1

e.g. 128.9.16.14 => Port 2

R2

Prefix

Next-hop

Port

3

65/8

128.17.16.1

R1

R3

1

2

128.9/16

128.17.14.1

2

2

128.9.16/20

128.17.14.1

3

7

128.9.19/24

128.17.10.1

128.9.25/24

128.17.14.1

2

R4

128.9.176/20

128.17.20.1

1

142.12/19

128.17.16.1

3

128.17.16.1

Forwarding/routing table

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

how a router forwards datagrams1
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.

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

forwarding datagrams
Forwarding Datagrams
  • Is the datagram for a host on directly attached network?
  • If no, consult forwarding table to find next-hop.

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

inside a router
Inside a Router

3.

1.

Output

Scheduling

2.

Forwarding

Table

Interconnect

Forwarding

Decision

Forwarding

Table

Forwarding

Decision

Forwarding

Table

Forwarding

Decision

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

forwarding in an ip router
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?

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

making a forwarding decision class based addressing
Making a Forwarding DecisionClass-based addressing

IP Address Space

Class A

Class B

Class C

D

Class A

Routing Table:

Class B

212.17.9.4

Exact

Match

Class C

212.17.9.0

Port 4

212.17.9.0

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

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

error reporting icmp
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.

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