Networks - an overview fileNetworks anoverview KarstKoymans Informatics Institute University of...

Networksan overview

Karst Koymans

Informatics InstituteUniversity of Amsterdam

(version 1.5, 2011/02/03 12:07:08)

Monday, January 31, 2011

Karst Koymans (UvA) Networks Monday, January 31, 2011 1 / 52

1 Network modelingLayered networksNetworks as graphsAddressing in graphs

2 RoutingBasic RoutingLocations and EndpointsIP addressingSubnettingIPv6 addressing

3 Packet formatsIPv4 packetsIPv6 packets


Network modeling

Outline





Network modeling Layered networks

Outline






OSI model

Obligatory 7-layer view, in particular for TCP/IP

Link layer (2): switching

mostly connectionless (Ethernet)

Network layer (3): routing

connectionless (IP)

Transport layer (4)

connection-oriented on layer 4 (TCP)

also connectionless on layer 4 (UDP)



Interfaces and Protocols

InterfacesSame computer

Different layer

Vertical: SDU

(Service Data Unit)

via SAP

(Service Access Point)

to/from lower layer

ProtocolsDifferent computer

Same layer

Horizontal: PDU

(Protocol Data Unit)

to/from peer

mostly through a

lower layer


Interfaces and Protocols illustration


Headers, PDU encapsulation and multiplexing

Layer N adds control (header/trailer/...) to Layer N+1

Encapsulation

Multiplexing

Demultiplexing

Decapsulation


Network modeling Networks as graphs

Outline





Host

Host

Two hosts and a network

Host Host

Network

Two hosts, two networks and a router

Host

Network Network

HostRouter

Networks with a WAN-connection“The Internet”

Host

Network Network

HostRouter

Host

Network Network

HostRouter

LinkWAN

Network modeling Addressing in graphs

Outline





Network modeling Addressing in graphs

Addresses

Missing in the pictures are some fundamental concepts

AddressesLocators

Used in routing

Explicit network elementsAs nodes

Not as edges


Host with interface

Host

The host has an interface

The interface has an address

The host is the endpoint

Network as a node

Host Host

Host

Network

Host

Network

Router has multiple interfaces

Host Network

Host Network

Router

Duality of nodes and networksBipartite Graphs

Abstraction of layersRefinement

Routing

Outline





Routing Basic Routing

Outline





Routing Basic Routing

Basic “routing”

Delivering packets to an address

Using a number of hops through intermediate networks

(using wires, air, devices,. . . )

Can be done at all layers, not only the network layer

Based on table lookup of next hop

Examples

IP packet routing

Ethernet frame switching

MPLS label switching


Routing Locations and Endpoints

Outline






Locations versus endpoints

Addresses are the locators for locations

They are the names for the locations

Names for endpoints are missing

Hosts, processes, communication endpoints

Is this an architectural flaw?



Comparison: Overloading of A records in DNS

Host with multiple interfaces. . .

Overloaded A records

router.students.os3.nl. A 145.100.102.1

router.studlab.os3.nl. A 145.100.104.1

router.os3.nl. A 145.100.102.1

A 145.100.104.1

Corresponding PTR records

1.102.100.145.in-addr.arpa. PTR router.students.os3.nl.

1.104.100.145.in-addr.arpa. PTR router.studlab.os3.nl.


Routing IP addressing

Outline






IP (layer 3) addresses

IP version 432 bit addresses

Network part (first), host part (last)

Decimal dotted notation 145.100.102.1



Address classes

Definition (IP address)An IP address is a function f : 32 → 2(= {0, 1})

Class A: f (0) = 0; f (0), . . . , f (7) network part

Class B: f (0) = 1; f (1) = 0, f (0), . . . , f (15) network part

Class C: f (0) = f (1) = 1; f (2) = 0, f (0), . . . , f (23) network part

Class D: f (0) = f (1) = f (2) = 1; f (3) = 0 multicast

Class E: f (0) = f (1) = f (2) = f (3) = 1 reserved



Special addresses

Definition (Special addresses; see also RFC 5735)

0.0.0.0 IP address unknown

127.0.0.1 loopback address (anycast?)

Host part all 0 Network identification

Host part all 1 Directed broadcast

255.255.255.255 Local broadcast



Private addresses

Definition (Private addresses (RFC 1918, RFC 3927))

Class Address Range # of nets CIDR notation

A 10.0.0.0-10.255.255.255 1 10.0.0.0/8

B 172.16.0.0-172.31.255.255 16 172.16.0.0/12

C 192.168.0.0-192.168.255.255 256 192.168.0.0/16

B 169.254.0.0-169.254.255.255 1 169.254.0.0/16

(Link local dynamic configuration)


IntermezzoThe “Routing Tree”

λ

0

00

000 001

01

010 011

1

10

100 101

11

110 111

. . . and so on. . .

Routing Subnetting

Outline





Routing Subnetting

Subnetting

Original RFC 950 subnetting

Only one level of subnets

All subnets are the same size (RIPv1 friendly)

Subnets all-zeroes and all-ones are not allowed

This has been obsoleted in the all-CIDR era

Example subnet schemeDivide the class B net 145.18.0.0/16 into 254 “class C nets”

145.18.1.0/24,. . . , 145.18.254.0/24


Routing Subnetting

Masks

Not necessarily contiguous according to RFC 950

Silly subnetting exampleNetmask 255.255.255.88 (in a class C scenario)

88 = 01011000

6 (explain. . . ) subnets with 30 (explain. . . ) hosts each

This is really obsolete, because humans can’t handle it well

Nowadays masks are contiguous (33 options)

All-zeroes and all-ones subnets are allowed


Routing Subnetting

Possible Masks

Definition (Possible masks)

Prefix length Subnet mask CIDR notation (for first non-zero net)

0 0.0.0.0 0.0.0.0/0

1 128.0.0.0 128.0.0.0/1

2 192.0.0.0 64.0.0.0/2

. . . . . . . . .

31 255.255.255.254 0.0.0.2/31

32 255.255.255.255 0.0.0.1/32


Routing Subnetting

Variable Length Subnet Masks

Definition (VLSM)Variable length subnet masks (VLSM) is the idea to use subnets that are

not necessarily all the same size.

Not possible with RIPv1

Subnets can be subnetted again, and again, and again

Used to organize and subdivide larger organizations

(universities, enterprises)


Routing Subnetting

CIDR

Definition (CIDR)Classless Inter-Domain Routing (CIDR) is the idea to put the division

between network part and host part of addresses on arbitrary bit positions,

not only on 8, 16 or 24 bit positions as in classful A/B/C-networks.

This idea comes from the need for supernetting

It is also used to aggregate routes

Routing is based on longest match (which is most specific)

A-, B-, C-classes are abandoned

Together with VLSM, the difference between

network and subnetwork disappears.Karst Koymans (UvA) Networks Monday, January 31, 2011 38 / 52

Routing Subnetting

Point to Point links

Usually the interfaces at both ends each have an IP address

Often a /30 is used for that

Point to Point links may be unnumbered

Or may use a /31 (RFC 3021)

Or may use two /32’s (within PPP, RFC 1332)


Routing IPv6 addressing

Outline





Routing IPv6 addressing

IPv6 addresses

IP version 6

128 bit addresses

4 times as many bits as IPv4

296 times as many addresses

Much more hierarchical addressing

See also the IPv6_intro presentation


Packet formats

Outline





Packet formats IPv4 packets

Outline





IPv4 header

0 3 4 7 8 15 16 18 19 31

Version IHL Type of Service Total Length

Identification Flags Fragment offset

Time to Live Protocol Header Checksum

Source IP

Destination IP

Options (optional; padded)


IPv4 header fields

IPv4 header fields

Version 4

IHL Total header length in 32-bit words (5-15)

Type of Service Now used for DiffServ and ECN

Total Length Including header, in bytes (max 65535)

Identification Used with fragmentation

Flags DF (Don’t Fragment) and MF (More Fragments)

Fragment offset First packet has offset 0



IPv4 header fields (continued)

IPv4 header fields (continued)

Time to Live Really a hop count

Protocol Upper layer protocol differentiation

Header Checksum One’s complement of one’s complement sum

Source Address IP address of sender

Destination Address IP address of receiver

Options Source routing, timestamp, . . .



Protocol

Common protocols

1 ICMP

2 IGMP

4 IP in IP

6 TCP

17 UDP

89 OSPF

Less common protocols

41 IPv6 in IP

46 RSVP (IntServ)

47 GRE

111 IPX in IP

115 L2TP

132 SCTP



Outline






IPv6 header0 3 4 11 12 15 16 23 24 31

Vers. Traffic Class Flow Label

Payload Length Next Header Hop Limit

Source Address

Destination Address



IPv6 header fields

IPv6 header fields

Version 6

Traffic Class Type of Service management

Flow Label Identify flows with special requirements

Payload Length Including extension headers

Next Header Type of following header

Hop Limit Forwarding count and loop protection

Source Address IPv6 address of sender

Destination Address IPv6 address of recipient



Next HeaderProtocol type

Header types

Number Name Meaning

0 HOPOPT Hop-by-hop Option

6 TCP Upper layer Transmission Control

17 UDP Upper layer User Datagram

41 IPv6 IPv6 (in IPv6)

43 IPv6-Route Routing Header

44 IPv6-Frag Fragment Header



Next Headerprotocol type

Header types (continued)

Number Name Meaning

50 ESP Encap Security Payload

51 AH Authentication Header

58 IPv6-ICMP ICMP for IPv6

59 IPv6-NoNxt No Next Header

60 IPv6-Opts Destination Options

See http://www.iana.org/assignments/protocol-numbers


http://www.iana.org/assignments/protocol-numbers

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