Transcript Data/Link Layer Issues

Data/Link Layer Issues
• Protocol & Services
• Topology
• Error Detection & Recovery
Topology vs Geography
Logical Layout
Physical Layout
"How devices talk to
each other" -or"How devices hear
each other"
How the signal
actually travels
Topologies
Bus
Star
Ring
Mesh
BUS
• Every
node hears every other node's transmission
directly.
Ring
• Series of unidirectional point-to-point links
without "store & forward", usually with a bypass
ability.
Star
• Switching functions all in central node
Mesh
• Each node independently routes over
(bi-directional) point-to-point links.
Data Link Layer
Uses 'bit pipe' Physical Layer to send packets
Packet Formats - Generic: Framing (Layer 1), Addresses and
control information (layer 2), and data (info from layer 3 and up)
Point-to-Point vs Broadcast - Key idea is that not all
packet formats are alike. One needs to look at particluar technologies
to see what is needed.
Data Link Services
• Unacknowledged Connectionless Service
– Most LANs
– Upper layers handle error recovery
• Acknowledged Connectionless Service
– Odd duck. Example?
• Connection-oriented Service
– Reliable Delivery ...
Link Protocols
Used to provide reliability. Basic idea can be
used at any layer
Don't need to know details at this time, but know general operation
and that they provide assured delivery.
ABP
SRP
GoBack N
Windowing & Flow Control
Performance
• Overhead vs Frame Length
• Error rate (bit error vs block error)
• Physical Layer
– distance
– propagation delay
Error Control
Error Detection - Methods: Parity, Checksum, CRC -generically Frame Check Sequences
Error Correction - The basic idea is to add redundant information
so that the receiver can deocde the message even if some (specified)
number of bits are damaged (e.g., Hamming codes)
Error Recovery includes error correction but also includes actions taken
to get a message retransmitted
Connection Oriented Services
• Two modes of operation:
–Operational
–Non-operational
• Operational mode incorporates three functions:
–Link establishment.
• A source station sends a frame to a destination station
requesting a connection.
• The destination station may accept or reject the connection
request.
–Information transfer.
• Allows information to be transferred after a connection is set
up and the required handshaking has taken place.
• Reliable information is transferred between the two stations.
IEEE & OSI {again}
LLC
2
MAC
PHY
LLC = Logical Link Control
MAC = Media Access Control
PHY = Physical
1
Bit 0
IEEE 802.2 Fields
I/G D D D D D D D D
C/R S S S S S S S
Length of the Information field
is access method dependent
Destination
address
Source
address
DSAP
address
SSAP
address
1 byte
1 byte
Length
field
Control
Information
1 or 2 bytes
IEEE 802.2 field
CRC
SAP Types
•
E0 - Novell NetWare
•
F0 - NetBIOS
•
06 - TCP/IP
•
42 - Spanning Tree BPDU
•
FF - Global SAP
•
F4 - IBM Network Management
•
7F - ISO 802.2
•
00 - NULL LSAP
•
F8, FC - Remote Program Load
•
04, 05, 08, 0C - SNA
•
AA - SNAP
•
80 - XNS
•
FE - OSI
SubNetwork Access Protocol (SNAP)
• Most common implementation of LLC1 is from a subsection of the IEEE
802.2 standard known as SNAP.
• At the time of IEEE 802.2’s introduction, most network protocols were
designed to use the Ethernet packet format.
• SNAP allows for the migration of the standard network protocols to the
IEEE 802.2 format.
• Supported by TCP/IP, NetWare, OSI, AppleTalk, and many other protocols.
• The second purpose for the SNAP protocol is to allow those protocols that
do not support the IEEE 802 standard to be able to traverse IEEE 802
LANs.
• SNAP uses a reserved SAP: AA (for both the DSAP and SSAP).
– It uses the unnumbered frame format: control field equal to 03.
– Actual SNAP header consumes 5 bytes:
Protocol Discriminator
Destination
address
Source
address
Length
field
DSAP
SSAP
Control
AA
AA
03
SNAP
header
Data
OUI
Type
field
00-00-00
08-00
3 bytes
2 bytes
Pad
CRC-32
Protocol discriminator
Verification
• Finite State Machines
• Estelle & Other Languages
• Petri Nets
• Blind Faith (or, code it in C...)
Naming
Conventions
{and Confusion}
Segment
Segment
Repeater
Link Layer Subnet
Bridge
Segment
Link Layer Network
Network Layer Subnet
Segment
Router
Network Layer Network
Naming Conventions {cont}
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Router
Data/Link
Data/Link
Bridge
Physical
Physical
Repeater