Inter-Networking

Chong-Kwon Kim

SNU SCONE lab. 2

Routing Within a LAN MAC Address

– Each station (or network interface) should be uniquely

identified

– Use 6 byte long address

Broadcast & Filter

– Broadcast medium

• Signals are transmitted to all stations

A DB C YX

Suppose B X

B specifies X’s address in a frame

All stations receive the frame, but only X filters in the frame

Interconnection Devices

H HB Router

H HHHRouter

Extended LAN =

Broadcast domain

LAN = MAC domain

Application

Transport

Physical

Data Link

Network

Application

Transport

Physical

Data Link

Network

Repeater

Bridge/Switch

Router

Gateway

The Internet Provides world-wide end-to-end

connections

Need to Inter-connect many small

networks

Inter-connection

devices

SNU SCONE lab.

Interconnection Device - Repeater

Repeater

– Physical layer device that restores and

relays signals

Hub

– Multiport repeater + Fault detection & recovery

Limitations of repeater

– Collision domain

– Physical limitations

• Ethernet – 2500 meter max. distance

Single Collision Domain

Bridge

– Store and forward: relay frames if needed obeying MAC protocol

– Extended LAN

– Propagate MAC multicast/broadcast frames

Switch (layer 2)

– Multiport bridge with parallel

paths

– Full duplex link

Interconnection Device - Bridge

a b c d e f g h i

Bridge manages filtering DB

SNU SCONE lab.

Bridge - Routing

SNU SCONE lab. 6

Bridge maintains routing

information

Questions

1. How to obtain the routing Information?

2. What if a node migrate to other network?

3. What mechanism to use to purge old information?

STP

Bridge floods frames

– Unknown destinations

– Multicast/Broadcast

Infinite packet circulation if

a network has physical loops

Build a virtual tree on top of non-tree network

STP (Spanning Tree Protocol)

SNU SCONE lab. 7

SNU SCONE lab. 8

Interconnection Devices – Router &

Others

Router

– Network layer device

– Does not propagate MAC multicast

Layer N switch

Gateway

– Transport & upper layer devices

Data Link Protocols

Protocols

– HDLC

– LAP-b

– LLC

Functions

– Framing

– Error control

– Flow control

– MAC

Design

SNU INC lab. 9

SNU SCONE lab.

Network Layer

Functions

– Construction of a logical network connecting multiple

physical networks

• internetwork, internet

– End-to-end (host) packet delivery

Physical Network

Logical Network

NetworkRouter (Gateway)

- Routing

- Arbitrate difference between

two physical networks

(internetwork, internet)

Layer 4~N

Layer 1/2

SNU SCONE lab.

IP Packet Delivery Model

Datagram (not Virtual Circuit)

– No connection setup (Read Textbook, Section 3.1)

Best-effort (no guarantee)

– Lost packets

– Out-of-order delivery

– Packet duplication

– Delayed delivery

IP works on any hardware (Phy. Network) technologies

– IP over X

Design Goal: Be FLEXIBLE enough to support any

underlying network technologies.

SNU SCONE lab.

IP Address

Routing ≈ Addressing

Each Internet host has a universally unique IP address

Format

– 4 bytes

– Hierarchical

• Network ID & Host ID

Each (physical) network has a unique network ID

– Assign unique host IDs to the hosts within the same (physical)

network

Net ID Host ID

IP address NotationBinary

- 11000000 00000101 00110000 00000011

Dotted decimal

- 192.5.48.3

SNU SCONE lab.

IP Address Classes

0 Net ID Host ID

1 0 Net ID Host ID

1 1 0 Net ID Host ID

1 1 1 0 Multicast Address

Class A

Class B

Class C

Class D

Class Range (First Byte)

A 0 - 127B 128 - 191C 192 - 223D 224 - 239E 240 - 255

One network ID is allocated to

an physical network

In reality: A Class A or B network ID is

allocated to an organization or to an

AS with many physical networks

SNU SCONE lab.

Special IP Addresses

All-0s– This host

All-1s– All hosts on this net

– Limited broadcast

All-0 host suffix– Network address

All-1 host suffix– All hosts in the specified net

– Directed broadcast

127.*.*.*– Loopback through IP layer

IP Addressing All hosts on a same physical network have the same

network ID(prefix)

147.46.0.0147.47.0.0

192.5.48.0

10.0.0.0

147.46.0.3

147.46.214.5

10.0.64.1

10.10.0.7

192.5.48.24

147.47.0.3

How about the host(router) w/

more than one interfaces?

147.46.a.b 147.47.c.d

SNU SCONE lab.

Internetworking with Routers

Ethernet

A

Z B

X Y Point-to-

point

Ethernet

FDDI

TCP

IP

ETH

IP IPIPIP

ETH ETH ETHP2PP2PFDDIFDDI

TCP

Flight of a packet from A to B

X Y ZA B

SNU SCONE lab.

IP Routing (Forwarding)

Direct and Indirect delivery

– Direct: Source & destination are in the same physical network

– Indirect: Source & destination are on different physical

networks

Case 1: Host a --> Host b

Host a knows that host b is in the same physical network

How?

Case 2: Host a --> Host c

Host a relay datagram to router A or B

C

d

b

a

F

E

D

B

A

c

50.0.0.1

40.0.0.5

10.0.0.5

10.0.0.1

30.0.0.0

20.0.0.0

10.0.0.0

40.0.0.0

50.0.0.0

Routers (Hosts also) manage directives called

Forwarding Table that shows best routes to

destinations

To reduce the forwarding table size (scalability), route

based on networks, not hosts

Hop by hop

forwarding

– A forwarding table

indicates the very next

hop router on the path

to destination

(compare to Source

Routing)

SNU SCONE lab.

Forwarding Table - 1

19

Forwarding Table - 2

Format

– <Destination ID, next hop>

– Usually, destination ID is the network ID

Forwarding table

at host a

Dest. Next hop

10.0.0.0

20.0.0.0

30.0.0.0

40.0.0.0

50.0.0.0

Direct Delivery

Router B

Router A

Router B

Router A

C

d

b

a

F

E

D

B

A

c

50.0.0.1

40.0.0.5

10.0.0.5

10.0.0.1

30.0.0.0

20.0.0.0

10.0.0.0

40.0.0.0

50.0.0.0

IP address of Router B’s

interface to a network

10.0.0.0

SNU SCONE lab. 20

Forwarding Table - 3

Further size reduction

– Default route

Search sequence is important

– List specific routes first

– Search from top to bottom

Forwarding table

at host a

Dest. Next hop

10.0.0.0

20.0.0.0

40.0.0.0

Default

Direct Delivery

Router B

Router B

Router A

Dest. Next hop

10.0.0.0

20.0.0.0

30.0.0.0

40.0.0.0

50.0.0.0

Direct Delivery

Router B

Router A

Router B

Router A

How to look at your forwarding table?

“netstat” command

Forwarding Table - 4 Host’s forwarding table is simple

– Default route (Most hosts are connected to an edge network)

Routers have more entries

– Some have > 105 entries

SNU INC lab.21

C

d

b

a

F

E

D

B

A

c

50.0.0.1

40.0.0.5

10.0.0.5

10.0.0.1

30.0.0.0

20.0.0.0

10.0.0.0

40.0.0.0

50.0.0.0

Forwarding table

at router B

Dest. Next hop

10.0.0.0

20.0.0.0

30.0.0.0

40.0.0.0

50.0.0.0

Direct Delivery

Direct Delivery

Router C

Router E

Router C

SNU SCONE lab.

Physical/Logical Network interaction

= 2A:33:5E:21:76:3A =C4:6E:1F:7A:1D:E1

20.0.0.3

c10.0.0.0 40.0.0.0B E

10.0.0.1 10.0.0.4

=88:36:6C:43:73:5A40.0.0.1 40.0.0.5

20.0.0.0

=88:36:6C:43:54:AB

= C4:6E:1F:ED:47:A1

End-to-end delivery over a logical network is realized by

- Successive hardware-level forwarding within a series of

physical networks

- Network layer forwarding at intermediate routers

a

DA SA DA SA PayloadFrom host a to router B

MAC DA = C4:6E:1F:ED:47:A1

MAC SA = C4:6E:1F:7A:1D:E1

IP DA = 40.0.0.5

IP SA = 10.0.01

Host a sends a datagram to Host c using the following path

20.0.0.5

=2A:33:5E:21:C4:6E

SNU INC lab.

IP Header Format

SNU SCONE lab.

IP Datagram Format

TTL (Time To Live)

– In hop count

– Remove bad packets

Header checksum– 1’s complement sum of all 16-bit words in the header

As an IP datagram moves around the Internet, TTL is counted

down by one at each router.

How do you update the checksum field?

Should we check error at each router?

The link speed of today’s fast routers: Tbps

Should process > 𝟏𝟎𝟔 pkts/sec

Fast path & slow path

SNU SCONE lab.

Fragmentation & Reassembly

MTU (Maximum Transfer Unit)

– Maximum IP datagram size that a physical network can

transmit

– Different physical networks have different MTUs

• Ethernet - 1500 Byte

• 802.11g – 2300 Byte, FDDI - 4500 Byte

Ethernet

Router Router

8000 Byte

S R

SNU SCONE lab.

Fragmentation & Reassembly

Fragmentation

– Partitioning of a datagram into multiple smaller fragments

– Sizes <= MTU of the next physical network

Reassembly

– Concatenation of fragments into the original datagram

– Protocol principle

SNU SCONE lab.

Fragmentation & Reassembly

Original =

2000 Byte

Fragments =

820 Byte

Fragments = 400 Byte

Information for reassembly

ID

Offset

Total length

Flag

R1 R2S R

MTU = 2000 MTU = 820 MTU = 2000

Where to perform reassembly? Router OR Destination?

Any security issues??

SNU SCONE lab.

ARP (Address Resolution protocol)

Problem

– Each host has two different addresses

– Physical address (Hardware address, MAC address)

– Logical address (Protocol address, IP address)

A

C EF

DB Assumption: Every host knows its own logical &

Physical addresses

Suppose A wants to send a packet to C

Same physical network

How to know C’s physical address?

SNU SCONE lab.

ARP – Basic

Use an ARP table that maps IP address – MAC address

Who manages the table ?

Note that IP address and MAC address bindings may

change dynamically

IP address MAC address

197.15.3.1 0A:4B:00:00:07:08

197.15.3.2 0B:4B:00:00:07:00

197.15.3.3 0A:5B:00:01:01:03

197.15.3.4 04:06:07:08:09:10

197.15.3.5 06:07:09:08:03:01

SNU SCONE lab.

ARP – Two Methods

Two types of network

– Broadcast network: LANs (Ethernet, Token ring, …)

– NBMA (Non-Broadcast Multiple Access)

• Example: ATM, X.25

Two AR approaches

– Distributed

• Each host builds the mapping table

• Collect mapping information asking to targets

– Centralized

• A specialized server maintains the table

• Usually, each host periodically reports its own mapping information

to the servers

SNU SCONE lab.

ARP - Distributed

• Suppose host A wants to resolve host C’s address

• Host A broadcasts a request packet

• How would you assure C receives the request?

Physical broadcast

• All hosts receive the request, but only C will respond. How?

• How to design the protocol?

A B C D E

Broadcast

Network

SNU SCONE lab.

ARP Packet Format

IP-Ethernet

HW Type Protocol Type

HLEN PLEN Operation

Sender HA (Octets 0-3)

Sender HA (octets 4,5) Sender IP (Octet 0,1)

Sender IP (octets 2,3) Target HA (Octet 0,1)

Target HA (Octets 2-5)

Target IP (Octets 0-3)

DA SA SIP SHA TIP THARequest

packet

DA SA SIP SHA TIP THAResponse

packet

SNU SCONE lab.

ARP Enhancements

ARP cache– Store mapping information in an ARP cache for later uses

When to remove cache entries?– After timeout

• e.g. 5 min

Improvements– Request packets are delivered to all hosts

– A host refreshes its cache if the sender is already in the cache

– The target adds the sender’s mapping in its cache

How to look at your ARP table?

“arp” command

SNU SCONE lab. 34

ARP Variations

Proxy ARP

– A server (usually a router) may act as a proxy for others’ IP

addresses

Gratuitous ARP (GARP)

Reverse ARP (RARP) & DHCP

A host may not know its IP address

– Knows its hardware address

Problem in general

– What is the IP address of a host with the given h/w address?

– RARP server

But, RARP has been evolved to RARP BOOTP

DHCP(Dynamic Host Configuration Protocol)

Static IP address & Dynamic IP

– Ease of management: Automatic configuration

– Efficient use of addresses: Assign address only when needed

SNU SCONE lab.

RARP, designed for diskless clients, is seldom used now

SNU SCONE lab. 36

DHCP To join the Internet, a host needs

– Unique IP address + subnet mask

– Forwarding table – Default router

– DNS server

DHCP

– A protocol to auto-configure hosts

– DHCP server has

• A pool of available IP addresses

• Default routers & DNS server info.

IP Addresses?

MAC addresses?

SNU INC lab. 37

DHCP Packet Format

Operation HType HLen Hops

Xid

ciaddr

yiaddr

siaddr

giaddr

chaddr (16 bytes)

sname (64 bytes)

file (128 bytes)

options

Secs Flags

Refer to:http://support.microsoft.com/kb/169289/ko

http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol

SNU SCONE lab. 38

DHCP Relay

DHCP server for each network

– Management overhead

Relay

DHCP relay

DHCP serverOther networks

Unicast to server

Broadcast

Host