Addressing

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27 Οκτ 2013 (πριν από 4 χρόνια και 17 μέρες)

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10/27/2013

Digital Switching

1

LANs

(addresses and standardization)

Digital Switching

By

Kashif Hesham Khan


10/27/2013

Digital Switching

2

An Internet Connection


End stations are connected to LANs


LANs are connected through Bridges to form
extended LANs


Extended LANs are connected through
gateways/routers/switches


Layered architecture


Connection is between “peers”


Service Models (Fig. 1.3 of Perlman)


PDUs (between peers) and SDUs (from up layers)

10/27/2013

Digital Switching

3

Local Area Networks


IEEE 802 Committee


LAN Standardization


Physical and Data Link Layers of OSI Model


Data Link layer subdivided by them


MAC (Dependent on the type of LAN)


LLC (allows sharing data link resources)


Several LANs were standardized

10/27/2013

Digital Switching

4

IEEE 802 Subcommittees


802.1
---

common issues


802.2
---

LLC


Does not deal with PHY and MAC




802.3
---

CSMA/CD


802.4
---

Token Bus


802.5
---

Token Ring

Type 1, 2, …

LLC

MAC

PHY

Data Link

10/27/2013

Digital Switching

5

IEEE 802 Subcommittees


802.11


Wireless LAN


802.15


WPAN (PHY and MAC layer)


802.16


fixed broadband wireless access
systems


802.17


resilient packet ring


802.20


air interface for mobile
broadband wireless access systems


802.21


media independent handover for
MAN and LAN

10/27/2013

Digital Switching

6

IETF
-

RFCs


STD 64 (RFC3550) RTP: A Transport Protocol for Real
-
Time
Applications


STD 56 (RFC2453) RIP Version 2


STD 54 (RFC2328) OSPF Version 2 J.


STD 53 (RFC1939) Post Office Protocol
-

Version 3


STD 51 (RFC1661) Point
-
to
-
Point Protocol (PPP)


STD 44 (RFC0891) DCN Local
-
Network Protocols


STD 38 (RFC0903) A Reverse Address Resolution Protocol


STD 37 (RFC0826) Ethernet Address Resolution Protocol


STD 26 (RFC0868) Time Protocol


STD 23 (RFC0865) Quote of the Day Protocol


STD 22 (RFC0864) Character Generator Protocol


STD 9 (RFC0959) File Transfer Protocol


STD 7 (RFC0793) Transmission Control Protocol


STD 6 (RFC0768) User Datagram Protocol


STD 5 (RFC0792) Internet Control Message Protocol


STD 5 (RFC0791) Internet Protocol

10/27/2013

Digital Switching

7

LAN Addresses


Most LANs are “broadcast” type


LAN addresses solve two problems on shared
(or broadcast) LANs


Who is the sender?


Who is the receiver?


IEEE 802 standardized the address length


Two different lengths were chosen


16 bit (unique on the network)
---

obsolete


48 bit (unique globally
---

plug and play)

10/27/2013

Digital Switching

8

48 bit LAN Addresses


Globally unique


Assigned by IEEE


Cost is $1650 for a “block” of addresses


A “block” includes 2
24

addresses

1st octet

2nd octet

3rd octet

4th octet

5th octet

6th octet

Vendor code (OUI)

Vendor
-
assigned values

10/27/2013

Digital Switching

9

48 bit LAN Addresses


OUI = Organizationally unique identifier


Fixed value assigned by IEEE


2
24

different possibilities


Not all of them are used!!!


Vendor
-
assigned Values


A total of 2
24

unique addresses are available by
purchasing one block


A block may be shared


A vendor can buy more blocks with different OUIs

10/27/2013

Digital Switching

10

Group/Individual bit in OUI


In fact, One block


2
25

addresses


2
24

of the addresses are unicast


2
24

of the addresses are multicast


G/I bit decides if the address is multicast


G/I = 0 means unicast or individual station


G/I = 1 means a (LAN) multicast address


10111101

G/I (group/individual)
---

first bit on the wire

G/L (global/local)

10/27/2013

Digital Switching

11

Global/Local bit in OUI


Another bit in the OUI is designated by the
IEEE as G/L bit


IEEE sets G/L = 0 when giving out the blocks
of addresses


Addresses with G/L = 1 can be used without
paying IEEE but the network administrator is
responsible to assign addresses such that
there is no collision


This leaves with 2
22

unique OUIs

10/27/2013

Digital Switching

12

Why multicast addresses?


In most LANs (e.g., CSMA/CD LANs), every entity
receives all the data on the LAN segment it is
connected to


Looking for appropriate neighbors


Hardware filtering is desirable because
promiscuous

listening is expensive


Some entities (e.g., bridges and LAN monitors) have to
listen promiscuously


One station will be interested in one unicast address
and multiple multicast addresses


Unicast address is hardwired


Multicast addresses fall into hardwired hash buckets

10/27/2013

Digital Switching

13

Protocol Type Multiplexing


One station, many higher layer protocols


Which protocol is the desired recipient?


Which protocol constructed the packet?





This information is also included in the LAN
header
---

just like LAN addresses are!


IP

IPX

ARP

MAC Layer

XNS

10/27/2013

Digital Switching

14

Protocol Type Multiplexing


Original Ethernet design


2 octet long field included in LAN header






Previously administered by Xerox, currently by
IEEE


Protocol vendors need to negotiate for getting a
protocol type added


http://standards.ieee.org/regauth/ethertype/index.html

6 octets

6 octets

2 octets

variable

Destination

Address

Source

Address

Protocol

Type

Data

10/27/2013

Digital Switching

15

SAP Multiplexing


More flexible to have separate source
and destination protocol type fields


Can assign different numbers to the same
protocol on different machines


Service Access Points (SAPs)


Included in 802 LAN header


SSAP and DSAP


1 octet each but only 6 bits are used

10/27/2013

Digital Switching

16

SAP Multiplexing





All 1’s


ALL SAPs


All 0’s (except G/L)


data link layer itself


6
-
bit globally assigned SAP numbers (by IEEE)


6 octets

6 octets

2 octets

variable

Destination

Address

Source

Address

Protocol

Type

Data

10111101

G/I (group/individual)

G/L (global/local)

DSAP SSAP

length

2 octets

CTL

10/27/2013

Digital Switching

17

SAP Multiplexing


G/L bit is similar to the one used in LAN
addresses


G/I bit
---

perhaps to keep compatibility with
the LAN addresses???


G/I bit in LAN addresses was used to make
hardware filtering convenient


Hardware filtering is meaningless in SAP
multiplexing


Only 64 unique SAP protocols are supported


Strict rules for assigning a SAP number


Protocol must be designed by standard bodies

10/27/2013

Digital Switching

18

SAP Multiplexing


Local SAP protocols can be used


Network/Protocol manager’s responsibility
to ensure unique SAPs to protocols


Conversation startup is difficult


SAP number at the destination machine is not
known at the source machine!

10/27/2013

Digital Switching

19

SNAP SAP


Subnetwork Access Protocol


Single globally assigned SAP value


AA hex (10101010)
---

SNAP SAP


When DSAP = SSAP = SNAP SAP


Header is expanded to include a “protocol type”
field


A “longer” protocol type field can then be
used


Standardized to 5 octets (see book for reason!)

10/27/2013

Digital Switching

20

Addresses and Protocol Types


By using 5 octets to indicate protocol
type, LAN address administration is tied
to protocol type administration


1st octet

2nd octet

3rd octet

4th octet

5th octet

6th octet

Vendor code (IEEE
-
assigned)

Vendor
-
assigned values

1st octet

2nd octet

3rd octet

4th octet

5th octet

LAN

Addresses

Protocol

Type

10/27/2013

Digital Switching

21

Transmission Bit Order


802.1 defines a canonical format for LAN
addresses



00
-
60
-
1D
-
23
-
20
-
A9


802.3 and 802.4


LSB is transmitted first


802.5 and FDDI


MSB is transmitted first


Internetworking different topologies


Bit order should be shuffled if forwarding frames
between incompatible LAN topologies

10/27/2013

Digital Switching

22

Frame Formats


Ethernet (Ethernet II)




802.3 Frame Format





Formats are compatible (Max length: 1536)


Protocols are assigned values > 0600 hex (=1536)

6 octets

6 octets

2 octets

Destination

Address

Source

Address

Protocol

Type

Data

6 octets

6 octets

2 octets

Destination

Address

Source

Address

Protocol

Type

Data

DSAP SSAP

length

2 octets

CTL