Transcript Addressing - Suraj @ LUMS

LAN Addressing
Network Protocols and Standards
Autumn 2004-2005
Dec 14, 2006
CS573: Network Protocols and Standards
1
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)
Dec 14, 2006
CS573: Network Protocols and Standards
2
Local Area Networks


First part of the course
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
Dec 14, 2006
CS573: Network Protocols and Standards
3
IEEE 802 Subcommittees


802.1 --- common issues
802.2 --- LLC

Does not deal with PHY and MAC
Data Link



802.3 --- CSMA/CD
802.4 --- Token Bus
802.5 --- Token Ring
Dec 14, 2006
CS573: Network Protocols and Standards
Type 1, 2, …
LLC
MAC
PHY
4
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)
Dec 14, 2006
CS573: Network Protocols and Standards
5
48 bit LAN Addresses



Globally unique
Assigned by IEEE
Cost is $1250 for a “block” of addresses

A “block” includes 224 addresses
1st octet
2nd octet
3rd octet
Vendor code (OUI)
Dec 14, 2006
4th octet
5th octet
6th octet
Vendor-assigned values
CS573: Network Protocols and Standards
6
48 bit LAN Addresses

OUI = Organizationally unique identifier


Fixed value assigned by IEEE
224 different possibilities


Not all of them are used!!!
Vendor-assigned Values



A total of 224 unique addresses are available by
purchasing one block
A block may be shared
A vendor can buy more blocks with different OUIs
Dec 14, 2006
CS573: Network Protocols and Standards
7
Group/Individual bit in OUI

In fact, One block  225 addresses



224 of the addresses are unicast
224 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)
Dec 14, 2006
CS573: Network Protocols and Standards
8
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 222 unique OUIs
Dec 14, 2006
CS573: Network Protocols and Standards
9
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
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
Dec 14, 2006
CS573: Network Protocols and Standards
10
Protocol Type Multiplexing

One station, many higher layer protocols


Which protocol is the desired recipient?
Which protocol constructed the packet?
IP
IPX
ARP
XNS
MAC Layer

This information is also included in the LAN
header --- just like LAN addresses are!
Dec 14, 2006
CS573: Network Protocols and Standards
11
Protocol Type Multiplexing

Original Ethernet design




2 octet long field included in LAN header
6 octets
6 octets
Destination
Address
Source
Address
2 octets
Protocol
Type
variable
Data
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
Dec 14, 2006
CS573: Network Protocols and Standards
12
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

Dec 14, 2006
1 octet each but only 6 bits are used
CS573: Network Protocols and Standards
13
SAP Multiplexing
6 octets
6 octets
2 octets
length
Destination
Address



Source
Address
2 octets
variable
DSAP SSAP CTL
Protocol
Type
Data
All 1’s  ALL SAPs
All 0’s (except G/L)  data link layer itself
6-bit globally assigned SAP numbers (by IEEE)
10111101
G/I (group/individual)
G/L (global/local)
Dec 14, 2006
CS573: Network Protocols and Standards
14
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
Dec 14, 2006
CS573: Network Protocols and Standards
15
SAP Multiplexing

Local SAP protocols can be used


Network/Protocol manager’s responsibility
to ensure unique SAPs to protocols
Conversation startup is difficult

Dec 14, 2006
SAP number at the destination machine is not
known at the source machine!
CS573: Network Protocols and Standards
16
SNAP SAP

Subnetwork Access Protocol



When DSAP = SSAP = SNAP SAP


Single globally assigned SAP value
AA hex (10101010) --- 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!)
Dec 14, 2006
CS573: Network Protocols and Standards
17
Addresses and Protocol Types

LAN
Addresses
By using 5 octets to indicate protocol
type, LAN address administration is tied
to protocol type administration
1st octet
2nd octet
3rd octet
Vendor code (IEEE-assigned)
4th octet
5th octet
6th octet
Vendor-assigned values
Protocol
Type
1st octet
Dec 14, 2006
2nd octet
3rd octet
4th octet
5th octet
CS573: Network Protocols and Standards
18
Transmission Bit Order

802.1 defines a canonical format for LAN
addresses
00-60-1D-23-20-A9


802.3 and 802.4


802.5 and FDDI


LSB is transmitted first
MSB is transmitted first
Internetworking different topologies

Bit order should be shuffled if forwarding frames
between incompatible LAN topologies
Dec 14, 2006
CS573: Network Protocols and Standards
19
Frame Formats


Ethernet
6 octets
6 octets
2 octets
Destination
Address
Source
Address
Protocol
Type
802.3 Frame Format
6 octets
6 octets
2 octets
length
Destination
Address

Data
Source
Address
2 octets
DSAP SSAP CTL
Protocol
Type
Data
Formats are compatible (Max length: 1536)

Protocols are assigned values > 0600 hex (=1536)
Dec 14, 2006
CS573: Network Protocols and Standards
20