1

The Internet Protocol (IP)

Part 2: Related Topics

Jean-Yves Le Boudec

Fall 2007

ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE

2

Contents

! IP Part 1. IP basics

1. Principles

2. Addressing

3. Packet Delivery and Forwarding

4. IP header

! IP Part 2. Related Topics (this module)

5. ICMP

6. Fragmentation

7. IPv6

8. DHCP

9. Static Configuration of Unix Host

10. Terminology

! IP Part 3. Multicast

! Related Modules: Routing

3

5. ICMP: Internet Control Message Protocol

used by router or host to send error or control messages to other hosts

or routers

error or control messages relate to layer 3 only

carried in IP datagrams (protocol type = 1)

! ICMP message types

echo request ( reply) -> used by ping

destination unreachable

time exceeded (TTL = 0) -> used for traceroute responses

address mask request/reply

source quench

redirect - router discovery

timestamps

ICMP messages never sent in response to

ICMP error message - datagram sent or multicast or broadcast IP or layer 2

address - fragment other than first

4

! Sent by router R1 to source host A when R1 receives a packet from A with

destination = B, and R1 finds that the next hop is R2, A is on-link with R2

(thus A should not have sent to R1, but directly to R2)

R1 sends ICMP redirect to A saying next hop for destination B is R2

A updates its routing table with a host route

! General routing principle of the TCP/IP architecture:

host have minimal routing information

learn host routes from ICMP redirects

routers have extensive knowledge of routes

ICMP Redirect

/ /| IP datagram header (prot = ICMP) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Type=5 | code | checksum |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Router IP address that should be preferred |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| IP header plus 8 bytes of original datagram data |/ /

ICMP Redirect Format

5

ICMP Redirect Example

lrcsuns

in-inr156.1182.5

156.24 156.100

1

4

4

dest IP addr srce IP addr prot data part1: 128.178.29.9 128.178.156.24 udp xxxxxxx2: 128.178.29.9 128.178.156.24 udp xxxxxxx3: 128.178.156.24 128.178.156.1 icmp type=redir code=host cksum

128.178.156.100 xxxxxxx (28 bytes of 1)

4: 128.178.29.9 128.178.156.24 udp .........

3

2

2lemas3

29.1

ed2-el

inr-el29.929.200

ed2-in

6

ICMP Redirect Example (cont’d)

lrcsuns:/export/home1/leboudec$ netstat -nrRouting Table: Destination Gateway Flags Ref Use Interface-------------------- -------------------- ----- ----- ------ ---------127.0.0.1 127.0.0.1 UH 0 11239 lo0128.178.29.9 128.178.156.100 UGHD 0 19 128.178.156.0 128.178.156.24 U 3 38896 le0224.0.0.0 128.178.156.24 U 3 0 le0default 128.178.156.1 UG 0 85883

After 4

7

6. MTU

Link-layer networks havedifferent maximum framelength

! MTU (maximumtransmission unit) =maximum frame sizeusable for an IP packet

! value of short MTU!? oflong MTU!?

solution

Link-layer Network MTU

Ethernet, WiFi802.3 with LLC/SNAPToken Ring 4 Mb/s 16 Mb/s

FDDIX.25

Frame RelayATM with AAL5Hyperchannel

PPP

1500149244641791443525761600918065535296 to1500

lrcsuns:/export/home1/leboudec$ ifconfig -alo0: flags=849<UP,LOOPBACK,RUNNING,MULTICAST> mtu 8232 inet 127.0.0.1 netmask ff000000le0: flags=863<UP,BROADCAST,NOTRAILERS,RUNNING,MULTICAST> mtu 1500 inet 128.178.156.24 netmask ffffff00 broadcast128.178.156.255 ether 8:0:20:71:d:d4

8

IP Fragmentation

IP hosts or routers may have IP datagrams larger than MTU

! Fragmentation is performed when IP datagram too large

! re-assembly is only at destination, never at intermediate points

! fragmentation is in principle avoided with TCP

R2R1

MTU = 1500 MTU = 620 MTU =1500

IPHeader

1400 BytesIP

Header600 B

IPHeader

600 B

IPHeader

200 B

IPHeader

600 B

IPHeader

600 B

IPHeader

200 B

1 2a

2b

2c

3a

3b

3c

9

IP Fragmentation (2)! IP datagram is fragmented if

MTU of interface < datagram total length

! all fragments are self-contained IP packets

! fragmentation controlled by fields: Identification, Flag and Fragment

Offset

! IP datagram = original ; IP packet = fragments or complete datagram

Fragment data size (here 600) is always a multiple of 8Identification given by source

LengthIdentification

More Fragment flagOffset

8 * Offset

1

1420567000

2a

620567100

2b

620567175600

2c

2205670

1501200

10

Fragmentation Algorithm

! Repeated fragmentations may occur

! Don’t fragment flag prevents fragmentation

! Fragmentation Algorithm:procedure sendIPp(P0):

if P0.totalLength > MTU then data1Length = (MTU-P0.HLEN rounded to multiple of 8) data1= first data1Length bytes of P0 data part data2= remainder of P0 data part header1 = P0.header with More bit set

totalLength = P0.HLEN + data1Length P1= new (IPPacket; header1; data1) send P1 on data link layer header2 = P0.header with

totalLength = P0.totalLength - data1Length fragmentOffset += data1Length/8 P2= new(IPPacket; header2; data2) sendIPp(P2)else send P0 on data link layer

11

IP packet arrival (P0) /* and packet is not a complete datagram */ -> if (P0.(identification, source address)) is new then if (new(fragmentList, P0.(identification, source address), fl)) then insert P0 in fl start reassemblyTimer(fl) else

fl = fragmentList(P0.(identification, source address)) insert(fl,P0)

if fl is complete then deliver IP datagram else start reassemblyTimer(fl)

reassemblyTimer(fl) expires -> send ICMP error message to source delete(fl)

IP packets are sorted in fragment listsone fragment list per (Identification, source IP @)sorted by increasing Fragment Offset

Fragments F1 and F2 are contiguous iff F1.moreBit = 1 F1.fragmentOffset + F1.dataLength/8 = F2.fragmentOffsetFragment List F0…Fn is complete iff

F0.fragmentOffset = 0Fi and Fi+1 are contiguous for i=0…(n-1)Fn.moreBit = 0

Comments: new(fragment list) may fail if there is no buffer left; in that case the datagram is lost

insert may fail; if insert fails, then the fragment is discarded

12

Issues with Fragmentation

! Fragmentation requires re-assembly; issues are

deadlocks

identification wrapping problem

unit of loss is smaller than unit of re-transmission: can worsen

congestion

Q. explain why

! Solution = avoid fragmentation

Path MTU = minimum MTU for all links of one path

Discovery of path MTU

heuristics: local -> 1500; other : 576 (subnetsarelocal variable)

Path MTU discovery avoids fragmentation

solution

13

Path MTU Discovery

! Method for Path MTU (PMTU) discovery

1. host sets Don’t Fragment bit on all datagrams and estimate PMTU to

local MTU

2. routers send an ICMP message: “destination unreachable/

fragmentation needed”

3. host reduces PMTU estimate to next smallest value

4. after timeout, host increases PMTU estimate

route changes may cause 2

14

TCP, UDP and Fragmentation

! The UDP service interface accepts a datagram up to 64 KB

UDP datagram passed to the IP service interface as one SDU

is fragmented at the source if resulting IP datagram is too large

! The TCP service interface is stream oriented

packetization is done by TCP

several calls to the TCP service interface may be grouped into one TCP

segment (many small pieces)

or: one call may cause several segments to be created (one large piece)

TCP always creates a segment that fits in one IP packet: no

fragmentation at source

fragmentation may occur in a router, if IPv4 is used, and if PMTU

discovery is not implemented

Q. If all sources use PMTU discovery, in which cases has a router to

fragment a packet ?

solution

15

7. IPv6

! The current IP is IPv4. IPv6 is the next version of IP

! Why a new version ?

IPv4 address space is too small (32 bits). It will be exhausted some day.

IP over cellular, UMTS

! What does IPv6 do ?

Redefine packet format with a larger address: 128 bits

Otherwise essentially the same as IPv4, but with minor improvementson header format

Facilitate hardware implementation – not seen in this module

! We now review how the IPv6 addresses are made and what newfacilities this allows

Why IPv6 and not IPv5 ? Because the version number 5 is already used by an experimental

Protocol called ST2, used to provide quality of service for example in military networks.

16

IPv6 Addresses

45b

prefix by prov.001 subnet interface Id

3b 64b16b

allocated by customerallocated by IANA

and org / provider

Why IPv6 and not IPv5 ? Because the version number 5 is already used by an experimental

Protocol called ST2, used to provide quality of service for example in military networks.

Address type

17

! IPv6 address is 16B = 128 bits

! Notations: 1 piece = 16 bits = [0-4 ]hexa digits; pieces separated by“:”:: replaces any number of 0s; appears only once in address

! ExamplesA698:6:0945:100:FFFF:96A:A:980:0:0:0:0:0:128.178.156.38

IPv4 comp IPv6 address (dual stack host,,obsolete)2002:80b2:9c26:0:800:2078:30f9

6to4 IPv6 address of dual stack host with IPv4 address 128.178.156.38 and MAC address 08:00:20:78:30:f90:0:0:0:0:FFFF:128.178.156.38

IPv4 mapped address (IPv4 only host)::FFFF:80b2:9c26

same as previous FF02::43

all NTP servers on this LAN0:0:0:0:0:0:0:0 = :: = unspecified address (absence of address)

! hosts may have several addresses

! addresses are: unicast, anycast or multicastno broadcast addresses

IPv6 Addresses: Notation

18

Address type Binary prefix IPv6 notation

------------ ------------- -------------

Unspecified 00...0 (128 bits) ::/128

Loopback 00...1 (128 bits) ::1/128

Multicast 11111111 FF00::/8

Link-Local unicast 1111111010 FE80::/10

Global Unicast (everything else)

From RFC4291, Feb 2006

19

INTERNET PROTOCOL VERSION 6 ADDRESS SPACE

(IANA)

[last updated 27 February 2006]

IPv6 Prefix Allocation Reference Note

----------- ---------- --------- ----

0000::/8 Reserved by IETF [RFC3513] [1] [5]

0100::/8 Reserved by IETF [RFC3513]

0200::/7 Reserved by IETF [RFC4048] [2]

0400::/6 Reserved by IETF [RFC3513]

0800::/5 Reserved by IETF [RFC3513]

1000::/4 Reserved by IETF [RFC3513]

2000::/3 Global Unicast [RFC3513] [3]

4000::/3 Reserved by IETF [RFC3513]

6000::/3 Reserved by IETF [RFC3513]

8000::/3 Reserved by IETF [RFC3513]

A000::/3 Reserved by IETF [RFC3513]

C000::/3 Reserved by IETF [RFC3513]

E000::/4 Reserved by IETF [RFC3513]

F000::/5 Reserved by IETF [RFC3513]

F800::/6 Reserved by IETF [RFC3513]

FC00::/7 Unique Local Unicast [RFC4193]

FE00::/9 Reserved by IETF [RFC3513]

FE80::/10 Link Local Unicast [RFC3513]

FEC0::/10 Reserved by IETF [RFC3879] [4]

FF00::/8 Multicast [RFC3513]

[0] The IPv6 address management function was formally delegated to

IANA in December 1995 [RFC1881].

[1] The "unspecified address", the "loopback address", and the IPv6

Addresses with Embedded IPv4 Addresses are assigned out of the

0000::/8 address block.

[2] 0200::/7 was previously defined as an OSI NSAP-mapped prefix set

[RFC-gray-rfc1888bis-03.txt]. This definition has been deprecated as of December

2004 [RFC4048].

[3] The IPv6 Unicast space encompasses the entire IPv6 address range

with the exception of FF00::/8. [RFC3513] IANA unicast address

assignments are currently limited to the IPv6 unicast address

range of 2000::/3. IANA assignments from this block are registered

in the IANA registry: iana-ipv6-unicast-address-assignments.

[4] FEC0::/10 was previously defined as a Site-Local scoped address

prefix. This definition has been deprecated as of September 2004

[RFC3879].

[5] 0000::/96 was previously defined as the "IPv4-compatible IPv6

address" prefix. This definition has been deprecated by [RFC4291].

20

IPv6 Multicast Addresses

11111111 flgs scpe group Id

8b 4b 4b 112 bits

flgs: (flags)=000T T=0: well-known T=1: transientscpe: (scope) 0: reserved 1: node local 2:link local 5: site local 8: org local E: global F: reserved examples: FF01::43 = all NTP servers on this node FF02::43 = all NTP servers on this link

FF05::43 = all NTP servers on this site

FF0E::43 = all NTP servers in the Internet

reserved addresses: FF0x::1 all nodes in the scope (x=1, 2) FF0x::2 all routers in the scope (x=1, 2) FF02::1:0 all DHCP servers/relay on this link

solicited node multicast: FF02::1:XXXX:XXXX where XXXX:XXXX= lowest order 32 bits of unicast addr.

21

! Automatic assignment of addresses in hosts is possible, using MAC

address

This is called “stateless” autoconfiguration

! The next slide shows how it works:"#Host creates a link local unicast address from its MAC address (cannot be used outside a

LAN, but can be used to reach a router). Validity of address is verified by sending a packet

to a special multicast address that only nodes with the same MAC address can have.

$#Host asks for a router present and gets a prefix.

The New Address Format Allows

Plug and Play

22

Stateless Autoconfiguration Overviewhost A other host on-link router on-link

A accepts its linklocal unicast address:FE80::0800:2072:8CFC

router response with prefix4001:41:1234:156:128(if M flag set : use DHCP instead)

A accepts its globalunicast address:4001:41:1234:156:128:0800:2072:8CFC

2. RS, multicast to FF02::2

1. NS, multicast to FF02::1:2072:8CFC (dupl test)

A attempts to acquire its link localunicast address:FE80::0800:2072:8CFC

23

IPv6 Host Configuration Example! Output of "netstat -q" at lrcsun12;

Interface Destination/Mask Phys Addr Ref State

--------- ------------------- ---------- ------- ---------------

le0#v6 ff02::2/128 33:33:00:00:00:02 1 REACHABLE

le0#v6 ff02::1:80b2:9c26/128 33:33:80:b2:9c:26 1 REACHABLE

le0#v6 fe80::1:0:800:2078:30f9/128 08:00:20:78:30:f9 1 REACHABLE

le0#v6 ff02::1:2078:30f9/128 33:33:20:78:30:f9 1 REACHABLE

Q. analyze the addresses on the four lines;

given that lrcsun13’s IPv4 address is 128.178.156.38

and lrcsun13’s MAC address is 08-00-20-78-30-F9

solution

24

Compatibility of IPv4 and IPv6

! IPv6 is incompatible with IPv6

Packet format is different – address size does not fit

Software is different – socket programs are differentTCP code for IPv6 need to be different, DNS code etc. because they all contain data

structures for IP addresses that are fixed size

Q. How does a host know, when receiving a packet from Ethernet,

whether it is an IPv4 or IPv6 packet ?

! How can IPv6 be deployed while most systems are IPv4 ?

Main solution: interworking at application layer

solution

25

Interworking at Application Layer

! A dual stack host implement both IPv4 and IPv6; it is configured

with an IPv4 address and an IPv6 address

! Dual stack host uses DNS to know whether to use IPv4 or IPv6 send

packets

hostname2addr(AF_INET6, hostName) returns IPv6 address (read

from AAAA record) if available, else IPv4 mapped address read from A

record

Application

TCP

IPv6

Application

TCP/ IP

IPv4

Application

TCP/ IP

IPv6

TCP

IPv4

Joe’s PC Mail Server Elaine’s PC

IPv6 IPv4

26

Problems solved by Interworking at Application

Layer

! Q. Review the problems posed by the deployment of IPv6 and

discuss whether this dual stack approach solves them.

solution

27

Deployment of IPv6

! IPv6 is implemented in Unix, Windows, Cisco but… is not deployed.

Why ?

Q. Give possible explanations.

solution

28

8. DHCP

! Why invented ?

Allocation of IP addresses is painful and error prone – wrong address =

system does not work

Renumbering is difficult, but once in while is needed

! What does it do ?

Dynamic Host Configuration Protocol = DHCP: Allocate an IP address

and network mask to host when it boots (or on user’s demand)

! How does it do its job ?

DHCP servers maintain lists of addresses and prefixes that are

available for allocation

MAC address used to identify a host to DHCP server

DHCP was initially developed for IPv6, so we show it in this context

29

DHCPv6

! For IPv6, this is an alternatively to stateless address allocation

Provides more control about who is allowed to insert itself in the

network

The next slides show how DHCPv6 (i.e. DHCP for IPv6) works

2: sent to IPv6 multicast address: well known, link scope address transId =

set by client; token = depends on type of network (MAC@ on Ethernet)UDP

destination port shown

4: sent to multicast address to inform other servers

5 is the commit flow; commitment done by server when sending message;

done by client on reception option field contains: printer addr, DNS server

address, name of a file to retrieve from server with for example config info

(such as name)

30

DHCPv6 Address Acquisition

assignment of link local address

DISCOVER(IP DA=FE02::1:0, SA=lla, netHdr=UDP;udp dport=DHCPv6s; transId, interface token=MACaddr,client link addr=lla,client addr=::)

1

2

3

4

5

CONF-RESP(IP DA=lla, SA=dsa, netHdr=UDP; udp dport=DHCPv6c; transId, interface token=MACaddr,client link addr=lla; client addr=ca)

ACCEPT(IP DA=FE02::1:0, SA=lla, netHdr=UDP; udp dport=DHCPv6s; transId, interface token=MACaddr,client link addr=lla,client addr=ca)

SERVER-ACK(IP DA=lla, SA=dsa, netHdr=UDP; udp dport=DHCPv6s; transId, interface token=MACaddr,client link addr=lla; client addr=ca)

commitcommit

DHCPv6client(host)

DHCPv6server

31

DHCP with Remote DHCP Server

DISCOVER(IP DA=?, SA=?,…gateway addr=?,…)

assignment oflink local address

1

2

3 CONF-RESP(IP DA=?, SA=?,…gateway addr=?,…)

DHCPv6client(host)

DHCPv6server

DHCPv6relay

(router)

DISCOVER(IP DA=?, SA=?,…gateway addr=?,…)

IPv6address=ra

IPv6address=dsa

CONF-RESP(IP DA=?, SA=?,…client link addr=?,…)

Q1. replace ‘?’ by plausible values

Q2. does DHCP relay keep state information ?

Solutions

32

DHCP for IPv4

! Originally, DHCP was intended for IPv6

! Q: How would one map the concepts of DHCP used with IPv6 to

IPv4 ?

! Q: is DHCP relay a router function ?

! Q: should the DHCP server be colocated on router or not ?

solution

33

Functions Developed for IPv6 Can Often be

Retrofitted to IPv4

! Example: DHCP

! Other functions such as quality of service, mobility, security are

now supported equally well by IPv6 and IPv4.

! Example: can you do stateless address allocation in IPv4 as in IPv6

?

Q. Explain how you would do it using private IP addresses instead

of link local unicast address.

solution34

9. Terminology

! Architecture IP router

a system that forwards packets based on IP addresses

performs packet forwarding + control method

! Implementation:

any UNIX machine can be configured as IP router

normally, dedicated box with specialized hardware called router

35

What is a “Multiprotocol Router” ?

! Multiprotocol router

a system that forwards packets based on layer 3 addresses for various

protocol architectures (ex: IP, Appletalk)

CISCO, IBM, etc…

most multiprotocol routers perform both bridging and routing

architecture: bridge + router

implementation: one CISCO

IP router boxes also perform other functions: port filtering, DHCP relay,

…

! Q. In a pure IP world (if all machines run TCP/IP) do we need

multiprotocol routers ?

A. Yes if both IPv4 and IPv6 are used.

solution

36

Example of Combined Functions in One Product! Put a bridge + Ethernet concentrator + router in the same box

! The resulting product is called “switching router”

Avoids ARP broadcastsThe words switches and routers are normally used in many different ways. For us, a switch is an

intermediate system for connection oriented network layers such as ATM or Frame Relay. For the

commercial literature, it usually means a fast packet forwarder, usually implemented in hardware. In reality,

routers can be implemented exactly in the same way and with the same performance as “switches”. The

main difference is for multiprotocol routers that need to understand not just one network layer, but many. In

such cases, only software implementations are available. In contrast, IP only routers are emerging with a

performance similar to that of switches.

The “switching router” concept is an example of product, which is new as a product, but from an

architecture viewpoint is nothing new. Since the router is in the same box as the Ethernet concentrator, it

can know (by software) the MAC address of directly attached systems. Thus, the ARP broadcasts are

avoided.

Router

SwitchingRouter

SwitchingRouter

M1p.h1

M2p.h2

M3q.h1

M8q.1

M4q.h3

M9p.1

1

2

H1 H2

37

Why are Bridges called “Multiprotocol” ?

! Some network protocols (ex:

Appletalk, IPX, IPv6) are not

compatible with IPv4

routers must be multiprotocol

but bridges work independently

of which network layer protocol

is used -- they are called

“multiprotocol” in the

commercial literature!

LLC

PHY

MAC

Appletalk

LLC

PHY

MAC

Ap

ple

talk

IP

TCP

PHY

MAC

IP

TCP

Bridge

A B

C

LLC

PHY

MAC

Appletalk

LLC

PHY

MAC

Appletalk

LLC

PHY

MAC

Ap

ple

talk

IP

TCP

LLC

PHY

MAC

Ap

ple

talk

IP

TCP

PHY

MAC

IP

TCP

PHY

MAC

IP

TCP

BridgeBridge

A B

C

B (an old Macintosh file server) runs only

Appletalk. Only applications using the Appletalk

protocols can be used (MacOS file sharing,

printing). TCP/IP applications such as the web

cannot be used on B.

C (a modern PC) runs only TCP/IP. All TCP/IP

applications can be used, but not native MacOS

file sharing.

A (a windows server) runs both in parallel. It can

talk to both C and B.

A bridge can be used to interconnect A, B and C;

there is nothing special to do. If a router is used

instead, it must run in parallel Appletalk and IP.

The protocol stacks shown are all implemented in

software. They use the standard Ethernet adapters.

38

What is a “Non Routable Protocol” ?

! NetBIOS was originally developed to work only in one bridged LAN

uses LLC-2, similar to TCP but located in layer 2 (also called NETBEUI)

in that form, it is not “routable”: can only be bridgedNetBIOS is an interface for distributed applications that is commonly used with

IBM and Microsoft systems. Only MAC addresses are used. In addition, NetBIOS

offers a naming service. This version of NetBIOS works only in a bridged

environment.

! NetBIOS today is offered as a TCP/IP application

uses the NBT reserved port

Windows machines at EPFL use TCP/IP only

LLC2

PHY

MAC

NetBIOS

LLC2

PHY

MAC

NetBIOS

Layer 2

R1

MACMAC

PHY R2

Bridge

App App

LLC2

PHY

MAC

NetBIOS

LLC2

PHY

MAC

NetBIOS

Layer 2

R1R1

MACMAC

PHY R2R2

Bridge

App App

39

Virtual LANs and Subnets

! IP requires machines to be organized by subnets

-- This is a problem when machines (and people) move

! One solution is provided by layer 2: virtual LANs

What is does : define LANs independent from location

How: associate (by configuration rules) hosts with virtual LAN labels.

The picture shows two virtual LANs: (ACLNV) and (BDMPU). The concentrators perform bridging between

the different collision domains of the same virtual LAN.

Between two virtual LANs, a router must be used. The figure shows one router that belongs to both VLANs

Between X1 and X2, the two virtual LANs use the same physical link. This is made possible by adding a label

to the Ethernet packet header, that identifies the virtual LAN.

Q. How many spanning trees are there in this network ?

solution

! Q. Can you think of another solution to the same problem ?

solution

40

Virtual

LAN

Concen-

trator

Virtual

LAN

Concen-

trator

Virtual

LAN

Concen-

trator

A

B

C

D

VU

L

M

N

P

X1 X2

X3

Virtual

LAN

Concen-

trator

Virtual

LAN

Concen-

trator

Virtual

LAN

Concen-

trator

A

B

C

D

VU

L

M

N

P

X1 X2

X3

Router

41

Solutions

42

MTU

! value of short MTU ?

reduces queue lengths and delays

on lossy links (radio) reduces proba of packet error

! of long MTU ?

reduces per packet processing

back

43

Issues with Fragmentation

! Fragmentation requires re-assembly; issues are

deadlocks

identification wrapping problem

unit of loss is smaller than unit of re-transmission: can worsencongestion

Q. explain whyA. when a network is congested, packets get lost. Assume everydatagram is fragmented in 10, and a single loss causesretransmission. The losses of a n packets (belonging to differentdatagrams) causes 10n retransmissions, which increases the offeredtraffic and makes congestion worse.

! Solution = avoid fragmentation

Path MTU = minimum MTU for all links of one path

Discovery of path MTU

heuristics: local -> 1500; other : 576 (subnetsarelocal variable)

Path MTU discovery avoids fragmentation

back

44

Fragmentation (sol)

! The UDP service interface accepts a datagram up to 64 KB

UDP datagram passed to the IP service interface as one SDU

is fragmented at the source if resulting IP datagram is too large

! The TCP service interface is stream oriented

packetization is done by TCP

several calls to the TCP service interface may be grouped into one TCP segment

(many small pieces)

or: one call may cause several segments to be created (one large piece)

TCP always creates a segment that fits in one IP packet: no fragmentation at

source

fragmentation may occur in a router, if IPv4 is used, and if PMTU discovery is not

implemented

Q. If all sources use PMTU discovery, in which cases has a router to

fragment a packet ?

A. 1. UDP packets sent by sources that have a larger local MTU than the

path MTU

2. TCP packets where PMTU estimation failed (due to path changes)

back

45

IPv6 Host Configuration Example! Output of "netstat -q" at lrcsun12;

Interface Destination/Mask Phys Addr Ref State

--------- ------------------- ---------- ------- ---------------

le0#v6 ff02::2/128 33:33:00:00:00:02 1 REACHABLE

le0#v6 ff02::1:80b2:9c26/128 33:33:80:b2:9c:26 1 REACHABLE

le0#v6 fe80::1:0:800:2078:30f9/128 08:00:20:78:30:f9 1 REACHABLE

le0#v6 ff02::1:2078:30f9/128 33:33:20:78:30:f9 1 REACHABLE

Q. analyze the addresses on the four lines;

given that lrcsun13’s IPv4 address is 128.178.156.38

and lrcsun13’s MAC address is 08-00-20-78-30-F9

A.

ff02::2/128 33:33:00:00:00:02 all routers on link

ff02::1:80b2:9c26/128 33:33:80:b2:9c:26 snmc addr of ::128.178.156.38 (specialmulticast address)

fe80::1:0:800:2078:30f9/128 08:00:20:78:30:f9 link local of lrcsun13

ff02::1:2078:30f9/128 33:33:20:78:30:f9 snmc addr of above

Comment: could have been present:

4800::1:0:800:2078:30f9/128 08:00:20:78:30:f9 configured addr of lrcsun13

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Compatibility of IPv4 and IPv6

! IPv6 is incompatible with IPv6

Packet format is different – address size does not fit

Software is different – socket programs are differentTCP code for IPv6 need to be different, DNS code etc. because they all contain data

structures for IP addresses that are fixed size

Q. How does a host know, when receiving a packet from Ethernet,

whether it is an IPv4 or IPv6 packet ?

A. The protocol type in the Ethernet header is different

! How can IPv6 be deployed while most systems are IPv4 ?

Main solution: interworking at application layer

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Problems solved by Interworking at Application

Layer

! Q. Review the problems posed by the deployment of IPv6 and

discuss whether this dual stack approach solves them.

A. 1. PCs deployed with only IPv6 addresses (IPv4 address

exhaustion). They can access the IPv6 services directly. For

services provided by IPv4 servers, they have no access, except if

the server is dual stack. This is OK for email, as the PC connects to

its local server, which we assume runs both IPv6 and IPv4. In

contrast, web access requires something else: web proxies that

run both IPv6 and IPv4.

2. This solution does not solve the problem of interconnecting IPv6

devices over a network of IPv4 only routers, and vice-versa. See

module “mixed.ppt” for how this can be done.

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Deployment of IPv6

! IPv6 is implemented in Unix, Windows, Cisco but… is not deployed. Why ?Q. Give possible explanations.A.

1. IPv6 is incompatible, so a smooth deployment is not easy. If I installIPv6 in my PC and remove IPv4, I cannot access the existing base of IPv4services.

2. Address space exhaustion is not critical in the US, which is themain source of product development. This is because many networks usenetwork address translation or HTTP proxies that allow one to use privateaddresses for hosts.

3. The benefit of introducing IPv6 is for others (those who do not haveenough addresses). There is no incentive for a company to move to IPv6(but there are many associated costs).

So the move to IPv6 is likely to occur under pressure of serious problems –it is like moving to green power sources…

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DHCP with Remote DHCP Server

DISCOVER(IP DA=FE02::1:0, SA=lla,…gateway addr=::,…)

assignment oflink local address

1

2

3CONF-RESP(IP DA=lla, SA=dsa,…gateway addr=ra,…)

DHCPv6client(host)

DHCPv6server

DHCPv6relay

(router)

DISCOVER(IP DA=dsa, SA=ra,…gateway addr=ra,…)

IPv6address=ra

IPv6address=dsa

CONF-RESP(IP DA=ra, SA=dsa,…client link addr=lla,…)

Q2. no; DHCP relay puts all needed info in request and so

does the DHCPv6 server

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DHCP for IPv4

! Originally, DHCP was intended for IPv6

! Q: How would one map the concepts of DHCP used with IPv6 to

IPv4 ?

! A: one needs to replace the IPv6 multicast address and the link

local address;

client sends DHCPDISCOVER to broadcast IP address; source IP

address =0; UDP is used (ports 67 on server, 68 on client); message

contains the MAC address of client

DHCP server or relay (colocated in router) receives it and answers;

sends it to the MAC address of client, to IP address = broadcast or the

address allocated to client

! Q: is DHCP relay a router function ?

no, it can be colocated in a router but is not a layer-3 IS function

! Q: should the DHCP server be colocated on router or not ?

DHCP server requires permanent storage (disk) usually better placed

on a server than on a router.

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Functions Developed for IPv6 Can Often be

Retrofitted to IPv4

! Example: DHCP

! Other functions such as quality of service, mobility, security arenow supported equally well by IPv6 and IPv4.

! Example: can you do stateless address allocation in IPv4 as in IPv6?Q. Explain how you would do it using private IP addresses insteadof link local unicast address.A. 1. when booting, host uses 192.168.x.y where x and y are drawnat random. An ARP packet is broadcast to resolve this address tocheck if it is use. If not, host keeps this address.

However, this works only for hosts on the same LAN, and theaddress obtained in this way is private, so we need for example aNetwork Address Translator between this host and the rest of theinternet. So we have an example where IPv6 brings more (the IPv6address allocated in this way is globally unique and is validworldwide).

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What is a “Multiprotocol Router” ?

! Multiprotocol router

a system that forwards packets based on layer 3 addresses for various

protocol architectures (ex: IP, Appletalk)

CISCO, IBM, etc…

most multiprotocol routers perform both bridging and routing

architecture: bridge + router

implementation: one CISCO

IP router boxes also perform other functions: port filtering, DHCP relay,

…

! Q. In a pure IP world (if all machines run TCP/IP) do we need

multiprotocol routers ?

A. Yes if both IPv4 and IPv6 are used.

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Virtual LANs and Subnets

! IP requires machines to be organized by subnets

-- This is a problem when machines (and people) move

! One solution is provided by layer 2: virtual LANs

What is does : define LANs independent from location

How: associate (by configuration rules) hosts with virtual LAN labels.

The picture shows two virtual LANs: (ACLNV) and (BDMPU). The concentrators perform

bridging between the different collision domains of the same virtual LAN.

Between two virtual LANs, a router must be used. The figure shows one router that belongs

to both VLANs

Between X1 and X2, the two virtual LANs use the same physical link. This is made possible

by adding a label to the Ethernet packet header, that identifies the virtual LAN.

Q. How many spanning trees are there in this network ?

A. 2 (one per virtual LAN)

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Virtual LANs and Subnets

! Q. Can you think of another solution to the same problem ?

A. DHCP

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