Network Layer5-1 2010 session 1 TELE3118: Network Technologies Week 5: Network Layer Forwarding, Features Some slides have been taken from: r Computer.

Network Layer 5-1

2010 session 1TELE3118: Network

Technologies

Week 5: Network LayerForwarding, Features

Some slides have been taken from:Computer Networking: A Top Down Approach Featuring the Internet, 4th edition. Jim Kurose, Keith Ross. Addison-Wesley, July 2007. All material copyright 1996-2004. J.F Kurose and K.W. Ross, All Rights Reserved.Computer Networks, 4th edition. Andrew S. Tanenbaum. Prentice-Hall, 2003.

Network Layer 5-2

IP Forwarding

Case I: hosts in same LAN (A B)

Case II: hosts in different LANs (A E)

A note on terminology:Switch (bridge) vs. Router

Hardware vs. software? layer-2 vs. layer-3?

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B

E

IP datagram:

miscfields

sourceIP addr

destIP addr data

Network Layer 5-3

Case I: hosts in same LAN

Starting at A, dest. B: look up dest-IP in routing

table dest is in LAN on interface

223.1.1.1 send datagram directly to B

in Ethernet frame how to determine B’s

Ethernet MAC address?

routing table at ADest Mask Next-hop

223.1.1.0 24 L: 223.1.1.1

0.0.0.0 0 223.1.1.4

B’s MACaddr

A’s MACaddr

A’s IPaddr

B’s IPaddr

IP payload

datagramframe

frame source,dest address

datagram source,dest address

miscfields223.1.1.1223.1.1.3data

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B

E

Network Layer 5-4

ARP: Address Resolution Protocol

Each IP node (Host, Router) on LAN has ARP table

ARP Table: IP/MAC address mappings for same LAN nodes

< IP address; MAC address; TTL>

TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

Network Layer 5-5

ARP protocol

A wants to send datagram to B, and A knows B’s IP address.

Suppose B’s MAC address is not in A’s ARP table.

A broadcasts ARP query packet, containing B's IP address all machines on LAN

receive ARP query B receives ARP packet,

replies to A with its (B's) MAC address frame sent to A’s MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state: information

that times out (goes away) unless refreshed

ARP is “plug-and-play”: nodes create their ARP

tables without intervention from net administrator

Network Layer 5-6

Case II: hosts in different LANs

Starting at A, dest. E: look up network address of

E in routing table E on different network

A, E not directly attached

routing table: next hop router to E is 223.1.1.4

link layer sends datagram to router 223.1.1.4 in Ethernet frame (ARP)

datagram arrives at 223.1.1.4

continued…..

miscfields223.1.1.1223.1.2.2 data

routing table at ADest Mask Next-hop

223.1.1.0 24 L: 223.1.1.1

0.0.0.0 0 223.1.1.4

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B

E

Network Layer 5-7

Case II (contd.)

Arriving at 223.1.4, destined for 223.1.2.2

look up network address of E in router’s routing table

E on same network as router’s interface 223.1.2.9 router, E directly

attached link layer sends datagram

to 223.1.2.2 in Ethernet frame via interface 223.1.2.9 (ARP)

datagram arrives at 223.1.2.2!!! (hooray!)

miscfields223.1.1.1223.1.2.2 data

routing table in router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B

E

Dest Mask Next-hop

223.1.1.0 24 L: 223.1.1.4

223.1.2.0 24 L: 223.1.2.9

223.1.3.0 24 L: 223.1.3.27

Network Layer 5-8

Packet walk-throughA (111.111.111.111) B (222.222.222.222)

A

RB

Each node (host/router) has Route table: dest/mask next-hop ARP table: LAN IP addr MAC address

Network Layer 5-9

A creates datagram with source A, destination B A uses ARP to get R’s MAC address for 111.111.111.110 A creates link-layer frame with R's MAC address as dest,

frame contains A-to-B IP datagram A’s data link layer sends frame R’s data link layer receives frame R removes IP datagram from Ethernet frame, sees its

destined to B R uses ARP to get B’s physical layer address R creates frame containing A-to-B IP datagram sends to B B receives the frame and extracts IP datagram

A

RB

Network Layer 5-10

To switch or route?

vlan 100 vlan 200

1

2

3

4

sMAC dMAC sIP dIP ----Data---- ??

Assume unicast traffic Lookup dMAC in MAC-table If (dMAC ≠ interface MAC) switch

switch (bridge) the frame as is onto learnt port

Else frame is for upper layer (IP) route Lookup dest-IP in routing table (discard if no match) Determine next hop MAC addr (ARP table) Send datagram with new Ethernet header

Network Layer 5-11

Switch-Router MAC table

VLAN MAC address port

3 08-00-60-00-09 10

3 08-00-60-00-17 6

3 00-4E-3A-02-08 Self

3 08-00-60-00-51 4

76 08-00-60-00-A3 7

76 08-00-60-00-46 8

76 08-00-60-00-1B 10

2018 08-00-60-00-51 3

2018 08-00-60-00-92 10

2018 00-4E-3A-02-10 Self

router IP interfaceson VLANs 3 and 2018

No router IP interfaceon VLAN 76

Network Layer 5-12

Switch-Router routing table

0.0.0.0 0 192.168.1.1

10.0.0.0 8 172.20.4.1

200.23.16.0 20 199.31.18.4

200.23.18.0 23 172.20.4.1

10.20.0.0 24 199.31.18.4

192.168.1.0 24 L 192.168.1.18

172.20.4.0 24 L 172.20.4.253

199.31.18.0 24 L 199.31.18.52

destination mask loca

l

next-hop

LAN

inte

rface

s

172.20.4.253/24

192.168.1.18/24199.31.18.52/24

default route

Network Layer 5-13

Unicast forwarding algorithm

Determinemost specific

match in routingtable

foundone?

droppacket

dest onsame LAN.

nh-IP = dest-IP

localintf?

next-hopis router.

nh-IP = gway-IP

nh-IP inARP table?

send ARPrequest and

wait for response

get ARPresponse and

fill in ARP table

constructEthernet

header andsend frame

no yes

yes no

no

yes

dest-MACaddressmine?

discard

extract IPdatagram

dest-IPaddressmine?

pass datagramdata to upper layer

yes

no

no

yes

Send:Host receive:

switchEthernet

frame

dest-MACaddressmine?

Route IPdatagram

extract IPdatagram

dest-IPaddressmine?

pass datagramdata to

upper layer

yes

no

no

yes

Switch/routerreceive:

Network Layer 5-14

IP/Ethernet configuration

1.1.1.1/24 1.1.1.2/24 1.1.1.1/24 1.1.2.2/24

1.1.1.1/24 1.1.2.2/24

what’s going on ??

1.1.1.3/16

A B

C

Dest Mask Gateway

1.1.1.0 24 L: 1.1.1.1

0.0.0.0 0 1.1.1.100

IP interface: 1.1.1.1/24Interface route

Network Layer 5-15

IP/Ethernet configuration

1.1.1.1/24default route: 1.1.1.100

1.1.2.2/24default route: 1.1.2.100

1.1

.1.1

00

/24

1.1

.2.1

00

/24

Dest Mask

Gateway

1.1.1.0 24 L:1.1.1.1

0.0.0.0 0 1.1.1.100

Internet

Network Layer 5-16

IP/Ethernet configuration192.168.1.1/16

default route: 192.168.1.100

192.168.1.3/24default route: 192.168.1.100

19

2.1

68

.1.1

00

/24

19

2.1

68

.2.1

00

/24

192.168.2.4/24default route: 192.168.2.100

A

C

D

IP reachability:

switch

---

---

---

---

from

ABCD

A B C Dto

192.168.2.2/24default route: 192.168.2.100

B

router

Network Layer 5-17

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

20 bytes of TCP 20 bytes of IP = 40 bytes +

app layer overhead

Network Layer 5-18

IP Fragmentation & Reassembly network links have MTU

(max.transfer size) - largest possible link-level frame. different link types,

different MTUs large IP datagram divided

(“fragmented”) within net one datagram becomes

several datagrams “reassembled” only at

final destination IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

Network Layer 5-19

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte

datagram MTU = 1500

bytes

1480 bytes in data field

offset =1480/8

Network Layer 5-20

ICMP: Internet Control Message Protocol

used by hosts & routers to communicate network-level information error reporting:

unreachable host, network, port, protocol

echo request/reply (used by ping)

network-layer “above” IP: ICMP msgs carried in IP

datagrams ICMP message: type, code

plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

Network Layer 5-21

Traceroute and ICMP

Source sends series of UDP segments to dest First has TTL =1 Second has TTL=2, etc. Unlikely port number

When nth datagram arrives to nth router: Router discards

datagram And sends to source an

ICMP message (type 11, code 0)

Message includes name of router& IP address

When ICMP message arrives, source calculates RTT

Traceroute does this 3 times

Stopping criterion UDP segment eventually

arrives at destination host

Destination returns ICMP “host unreachable” packet (type 3, code 3)

When source gets this ICMP, stops.

Network Layer 5-22

IPv6 Initial motivation: 32-bit address space

soon to be completely allocated. Additional motivation:

header format helps speed processing/forwarding

header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed

Network Layer 5-23

IPv6 Header (Cont)Priority: identify priority among datagrams in flowFlow Label: identify datagrams in same “flow.” (concept of“flow” not well defined).Next header: identify upper layer protocol for data

Network Layer 5-24

Other Changes from IPv4

Checksum: removed entirely to reduce processing time at each hop

Options: allowed, but outside of header, indicated by “Next Header” field

ICMPv6: new version of ICMP additional message types, e.g. “Packet Too

Big” multicast group management functions

Network Layer 5-25

Transition From IPv4 To IPv6

Not all routers can be upgraded simultaneous no “flag days” How will the network operate with mixed IPv4

and IPv6 routers? Tunneling: IPv6 carried as payload in IPv4

datagram among IPv4 routers

Network Layer 5-26

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: XSrc: ADest: F

data

Flow: XSrc: ADest: F

data

Flow: XSrc: ADest: F

data

Src:BDest: E

Flow: XSrc: ADest: F

data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6

B-to-C:IPv6 inside

IPv4

B-to-C:IPv6 inside

IPv4

Network Layer 5-27

Future of IPv6?

hourglass wineglass?