LAN Addressing

1
LAN Addressing

• Network Protocols and Standards
• Winter 2007-2008

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)

3
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

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

LLC
Type 1, 2,
Data Link
MAC
PHY
5
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)

6
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
4th octet
5th octet
6th octet
Vendor code (OUI)
Vendor-assigned values
7
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

8
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)
9
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

10
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

11
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
XNS
MAC Layer
12
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
13
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

14
SAP Multiplexing

• All 1s ? ALL SAPs
• All 0s (except G/L) ? data link layer itself
• 6-bit globally assigned SAP numbers (by IEEE)

6 octets
6 octets
2 octets
variable
2 octets
DSAP SSAP
length
CTL
Destination Address
Source Address
Protocol Type
Data
10111101
G/I (group/individual)
G/L (global/local)
15
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

16
SAP Multiplexing

• Local SAP protocols can be used
• Network/Protocol managers responsibility to
  ensure unique SAPs to protocols
• Conversation startup is difficult
• SAP number at the destination machine is not
  known at the source machine!

17
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!)

18
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
LAN Addresses
Vendor code (IEEE-assigned)
Vendor-assigned values
Protocol Type
1st octet
2nd octet
3rd octet
4th octet
5th octet
19
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

20
Frame Formats

• Ethernet
• 802.3 Frame Format
• Formats are compatible (Max length 1536)
• Protocols are assigned values gt 0600 hex (1536)

6 octets
6 octets
2 octets
Destination Address
Source Address
Protocol Type
Data
6 octets
6 octets
2 octets
2 octets
DSAP SSAP
length
CTL
Destination Address
Source Address
Protocol Type
Data