Lectures 24 & 25
Higher Layer Protocols:
TCP/IP and ATM
Eytan Modiano
Massachusetts Institute of Technology
Laboratory for Information and Decision Systems
Eytan Modiano
Slide 1
Outline
? Network Layer and Internetworking
? The TCP/IP protocol suit
? ATM
? MPLS
Eytan Modiano
Slide 2
Higher Layers
Virtual link for
reliable packets
Application
Presentation
Session
Transport
Network
Data link
Control
Application
Presentation
Session
Transport
Network
Data link
Control
Network Network
DLC DLC DLC DLC
Virtual bit pipe
Virtual link for end to end packets
Virtual link for end to end messages
Virtual session
Virtual network service
physical
interface
phys. int. phys. int. phys. int. phys. int.
physical
interface
TCP, UDP
IP, ATM
Physical link
External subnet subnet External
Site node node site
Eytan Modiano
Slide 3
Packet Switching
? Datagram packet switching
– Route chosen on packet-by-packet basis
– Different packets may follow different routes
– Packets may arrive out of order at the destination
– E.g., IP (The Internet Protocol)
? Virtual Circuit packet switching
– All packets associated with a session follow the same path
– Route is chosen at start of session
– Packets are labeled with a VC# designating the route
– The VC number must be unique on a given link but can change from
link to link
Imagine having to set up connections between 1000 nodes in a mesh
Unique VC numbers imply 1 Million VC numbers that must be represented
and stored at each node
– E.g., ATM (Asynchronous transfer mode)
Eytan Modiano
Slide 4
Virtual Circuits Packet Switching
? For datagrams, addressing information must uniquely distinguish
each network node and session
– Need unique source and destination addresses
? For virtual circuits, only the virtual circuits on a link need be
distinguished by addressing
– Global address needed to set-up virtual circuit
– Once established, local virtual circuit numbers can then be used to
represent the virtual circuits on a given link: VC number changes from
link to link
? Merits of virtual circuits
– Save on route computation
Need only be done once
at start of session
– Save on header size
– More complex
– Less flexible
3
6
5
8
2
9
VC3
VC13
VC7
VC4
VC3
VC7
Node 5 table
(3,5) VC13 -> (5,8) VC3
(3,5) VC7 -> (5,8) VC4
(6,5) VC3 -> (5,8) VC7
Eytan Modiano
Slide 5
The TCP/IP Protocol Suite
? Transmission Control Protocol / Internet Protocol
? Developed by DARPA to connect Universities and Research Labs
Four Layer model
Telnet, FTP, email, etc.
TCP, UDP
IP, ICMP, IGMP
?Device drivers, interface cards
TCP - Transmission Control Protocol
UDP - User Datagram Protocol
IP - Internet Protocol
Applications
Transport
Network
Link
Eytan Modiano
Slide 6
Internetworking with TCP/IP
FTP
FTP Protocol
FTP
client
server
TCP Protocol
TCP
TCP
IP IP Protocol IP Protocol
Ethernet
Ethernet
Protocol
token
driver
token ring
Protocol
Ethernet
driver
IP
ROUTER
IP
Ethernet
driver
token
driver
token
ring
ring
ring
Eytan Modiano
Slide 7
Encapsulation
14 20 20
4
Ethernet frame
46 to 1500 bytes
Ethernet
Ethernet
Application
user data
Appl
user data
header
TCP
header
application
header
IP
TCP
header
application
IP datagram
TCP
header
application
header
IP
Ethernet
header
Ethernet
trailer
driver
IP
TCP
TCP segment
data
data
data
Eytan Modiano
Slide 8
Bridges, Routers and Gateways
? A Bridge is used to connect multiple LAN segments
– Layer 2 routing (Ethernet)
– Does not know IP address
– Varying levels of sophistication
Simple bridges just forward packets
smart bridges start looking like routers
? A Router is used to route connect between different networks
using network layer address
– Within or between Autonomous Systems
– Using same protocol (e.g., IP, ATM)
? A Gateway connects between networks using different protocols
– Protocol conversion
– Address resolution
? These definitions are often mixed and seem to evolve!
Eytan Modiano
Slide 9
Bridges, routers and gateways
Ethernet A
Ethernet B
Bridge
IP
Router
Small company
Gateway
Service
provider’s
ATM
backbone
ATM switches
(routers)
Gateway
Another provider’s
Frame Relay
Backbone
Eytan Modiano
Slide 10
IP addresses
? 32 bit address written as four decimal numbers
– One per byte of address (e.g., 155.34.60.112)
? Hierarchical address structure
– Network ID/ Host ID/ Port ID
– Complete address called a socket
– Network and host ID carried in IP Header
– Port ID (sending process) carried in TCP header
? IP Address classes:
8
32
Net ID Host ID
Net ID
Net ID
Host ID
Host ID
0
10
110
16
32
24 32
Class A Nets
Class B Nets
Class C Nets
Class D is for multicast traffic
Eytan Modiano
Slide 11
Host Names
? Each machine also has a unique name
? Domain name System: A distributed database that provides a
mapping between IP addresses and Host names
? E.g., 155.34.50.112 => plymouth.ll.mit.edu
Eytan Modiano
Slide 12
Internet Standards
? Internet Engineering Task Force (IETF)
– Development on near term internet standards
– Open body
– Meets 3 times a year
? Request for Comments (RFCs)
– Official internet standards
– Available from IETF web page: http://www.ietf.org
Eytan Modiano
Slide 13
The Internet Protocol (IP)
? Routing of packet across the network
? Unreliable service
– Best effort delivery
– Recovery from lost packets must be done at higher layers
? Connectionless
– Packets are delivered (routed) independently
– Can be delivered out of order
– Re-sequencing must be done at higher layers
? Current version V4
? Future V6
– Add more addresses (40 byte header!)
– Ability to provide QoS
Eytan Modiano
Slide 14
Header Fields in IP
1 4 8 16 32
Protocol
Note that the minimum size header is 20 bytes; TCP
also has 20 byte header
Ver
Header
length
type of service Total length (bytes)
16 - bit identification
Flags 13 - bit fragment offset
TTL Header Checksum
Source IP Address
Destination IP Address
Options (if any)
Data
Eytan Modiano
Slide 15
IP HEADER FIELDS
? Vers: Version # of IP (current version is 4)
? HL: Header Length in 32-bit words
? Service: Mostly Ignored
? Total length Length of IP datagram
? ID Unique datagram ID
? Flags: NoFrag, More
? FragOffset: Fragment offset in units of 8 Octets
? TTL: Time to Live in "seconds” or Hops
? Protocol: Higher Layer Protocol ID #
? HDR Cksum: 16 bit 1's complement checksum (on header only!)
? SA & DA: Network Addresses
? Options: Record Route,Source Route,TimeStamp
Eytan Modiano
Slide 16
FRAGMENTATION
ethernet
mtu=1500
X.25
G
GMTU = 512
ethernet
mtu=1500
? A gateway fragments a datagram if length is too great for next
network (fragmentation required because of unknown paths).
? Each fragment needs a unique identifier for datagram plus
identifier for position within datagram
? In IP, the datagram ID is a 16 bit field counting datagram from
given host
Eytan Modiano
Slide 17
POSITION OF FRAGMENT
? Fragment offset field gives starting position of fragment within
datagram in 8 byte increments (13 bit field)
? Length field in header gives the total length in bytes (16 bit field)
– Maximum size of IP packet 64K bytes
? A flag bit denotes last fragment in datagram
? IP reassembles fragments at destination and throws them away if
one or more is too late in arriving
Eytan Modiano
Slide 18
IP Routing
? Routing table at each node contains for each destination the next
hop router to which the packet should be sent
– Not all destination addresses are in the routing table
Look for net ID of the destination “Prefix match”
Use default router
? Routers do not compute the complete route to the destination but
only the next hop router
? IP uses distributed routing algorithms: RIP, OSPF
? In a LAN, the “host” computer sends the packet to the default
router which provides a gateway to the outside world
Eytan Modiano
Slide 19
Subnet addressing
? Class A and B addresses allocate too many hosts to a given net
? Subnet addressing allows us to divide the host ID space into
smaller “sub networks”
– Simplify routing within an organization
– Smaller routing tables
– Potentially allows the allocation of the same class B address to more
than one organization
? 32 bit Subnet “Mask” is used to divide the host ID field into
subnets
– “1” denotes a network address field
– “0” denotes a host ID field
16 bit net ID 16 bit host ID
Class B
Address
140.252
Subnet ID Host ID
Mask 111111 111 1111111 11111111 00000000
Eytan Modiano
Slide 20
Classless inter-domain routing (CIDR)
? Class A and B addresses allocate too many hosts to an
organization while class C addresses don’t allocate enough
– This leads to inefficient assignment of address space
? Classless routing allows the allocation of addresses outside of
class boundaries (within the class C pool of addresses)
– Allocate a block of contiguous addresses
E.g., 192.4.16.1 - 192.4.32.155
Bundles 16 class C addresses
The first 20 bits of the address field are the same and are essentially the
network ID
– Network numbers must now be described using their length and
value (I.e., length of network prefix)
– Routing table lookup using longest prefix match
? Notice similarity to subnetting - “supernetting”
Eytan Modiano
Slide 21
Dynamic Host Configuration (DHCP)
? Automated method for assigning network numbers
– IP addresses, default routers
? Computers contact DHCP server at Boot-up time
? Server assigns IP address
? Allows sharing of address space
– More efficient use of address space
– Adds scalability
? Addresses are “least” for some time
– Not permanently assigned
Eytan Modiano
Slide 22
Address Resolution Protocol
? IP addresses only make sense within IP suite
? Local area networks, such as Ethernet, have their own addressing
scheme
– To talk to a node on LAN one must have its physical address
(physical interface cards don’t recognize their IP addresses)
? ARP provides a mapping between IP addresses and LAN
addresses
? RARP provides mapping from LAN addresses to IP addresses
? This is accomplished by sending a “broadcast” packet requesting
the owner of the IP address to respond with their physical address
– All nodes on the LAN recognize the broadcast message
– The owner of the IP address responds with its physical address
? An ARP cache is maintained at each node with recent mappings
ARP RARP
IP
Ethernet
Eytan Modiano
Slide 23
Routing in the Internet
? The internet is divided into sub-networks, each under the control
of a single authority known as an Autonomous System (AS)
? Routing algorithms are divided into two categories:
– Interior protocols (within an AS)
– Exterior protocols (between AS’s)
? Interior Protocols use shortest path algorithms (more later)
– Distance vector protocols based on Bellman-ford algorithm
Nodes exchange routing tables with each other
E.g., Routing Information Protocol (RIP)
– Link state protocols based on Dijkstra’s algorithm
Nodes monitor the state of their links (e.g., delay)
Nodes broadcast this information to all of the network
E.g., Open Shortest Path First (OSPF)
? Exterior protocols route packets across AS’s
– Issues: no single cost metric, policy routing, etc..
– Routes often are pre-computed
– Example protocols: Exterior Gateway protocol (EGP) and Border
Gateway protocol (BGP)
Eytan Modiano
Slide 24
IPv6
? Effort started in 1991 as IPng
? Motivation
– Need to increase IP address space
– Support for real time application - “QoS”
– Security, Mobility, Auto-configuration
? Major changes
– Increased address space (6 bytes)
1500 IP addresses per sq. ft. of earth!
Address partition similar to CIDR
– Support for QoS via Flow Label field
– Simplified header
? Most of the reasons for IPv6 have been
taken care of in IPv4
– Is IPv6 really needed?
– Complex transition from V4 to V6
0
ver class Flow label
length Hop limitnexthd
Source address
Destination address
Eytan Modiano
Slide 25
31
Resource Reservation (RSVP)
? Service classes (defined by IETF)
– Best effort
– Guaranteed service
Max packet delay
– Controlled load
emulate lightly loaded network via priority queueing mechanism
? Need to reserve resources at routers along the path
? RSVP mechanism
– Packet classification
Associate packets with sessions (use flow field in IPv6)
– Receiver initiated reservations to support multicast
– “soft state” - temporary reservation that expires after 30 seconds
Simplify the management of connections
Requires refresh messages
– Packet scheduling to guarantee service
Proprietary mechanisms (e.g., Weighted fair queueing)
? Scalability Issues
– Each router needs to keep track of large number of flows that grows
with the size (capacity) of the router
Eytan Modiano
Slide 26
Differentiated Services (Diffserv)
? Unlike RSVP Diffserv does not need to keep track of individual
flows
– Allocate resources to a small number of classes of traffic
Queue packets of the same class together
– E.g., two classes of traffic - premium and regular
Use one bit to differential between premium and regular packets
– Issues
Who sets the premium bit?
How is premium service different from regular?
? IETF propose to use TOS field in IP header to identify
traffic classes
– Potentially more than just two classes
Eytan Modiano
Slide 27
User Datagram Protocol (UDP)
? Transport layer protocol
– Delivery of messages across network
? Datagram oriented
– Unreliable
No error control mechanism
– Connectionless
– Not a “stream” protocol
? Max packet length 65K bytes
? UDP checksum
– Covers header and data
– Optional
Can be used by applications
? UDP allows applications to interface directly to IP with minimal
additional processing or protocol overhead
Eytan Modiano
Slide 28
UDP header format
IP Datagram
IP header UDP header data
16 bit source port number 16 bit destination port number
16 bit UDP length
16 bit checksum
Data
? The port numbers identifie the sending and receiving processes
– I.e., FTP, email, etc..
– Allow UDP to multiplex the data onto a single stream
? UDP length = length of packet in bytes
– Minimum of 8 and maximum of 2^16 - 1 = 65,535 bytes
? Checksum covers header and data
– Optional, UDP does not do anything with the checksum
Eytan Modiano
Slide 29
Transmission Control Protocol (TCP)
? Transport layer protocol
– Reliable transmission of messages
? Connection oriented
– Stream traffic
– Must re-sequence out of order IP packets
? Reliable
– ARQ mechanism
– Notice that packets have a sequence number and an ack number
– Notice that packet header has a window size (for Go Back N)
? Flow control mechanism
– Slow start
Limits the size of the window in response to congestion
Eytan Modiano
Slide 30
Basic TCP operation
? At sender
– Application data is broken into TCP segments
– TCP uses a timer while waiting for an ACK of every packet
– Un-ACK’d packets are retransmitted
? At receiver
– Errors are detected using a checksum
– Correctly received data is acknowledged
– Segments are reassembled into their proper order
– Duplicate segments are discarded
? Window based retransmission and flow control
Eytan Modiano
Slide 31
TCP header fields
16 32
Source port Destination port
Sequence number
Request number
Data
Offset
Reserved Control Window
Check sum Urgent pointer
Options (if any)
Data
Eytan Modiano
Slide 32
TCP header fields
? Ports number are the same as for UDP
? 32 bit SN uniquely identify the application data contained in the
TCP segment
– SN is in bytes!
– It identify the first byte of data
? 32 bit RN is used for piggybacking ACK’s
– RN indicates the next byte that the received is expecting
– Implicit ACK for all of the bytes up to that point
? Data offset is a header length in 32 bit words (minimum 20 bytes)
? Window size
– Used for error recovery (ARQ) and as a flow control mechanism
Sender cannot have more than a window of packets in the network
simultaneously
– Specified in bytes
Window scaling used to increase the window size in high speed networks
? Checksum covers the header and data
Eytan Modiano
Slide 33
TCP error recovery
? Error recovery is done at multiple layers
– Link, transport, application
? Transport layer error recovery is needed because
– Packet losses can occur at network layer
E.g., buffer overflow
– Some link layers may not be reliable
? SN and RN are used for error recovery in a similar way to Go Back
N at the link layer
– Large SN needed for re-sequencing out of order packets
? TCP uses a timeout mechanism for packet retransmission
– Timeout calculation
– Fast retransmission
Eytan Modiano
Slide 34
TCP timeout calculation
? Based on round trip time measurement (RTT)
– Weighted average
RTT_AVE = a*(RTT_measured) + (1-a)*RTT_AVE
? Timeout is a multiple of RTT_AVE (usually two)
– Short Timeout would lead to too many retransmissions
– Long Timeout would lead to large delays and inefficiency
? In order to make Timeout be more tolerant of delay variations it
has been proposed (Jacobson) to set the timeout value based on
the standard deviation of RTT
Timeout = RTT_AVE + 4*RTT_SD
? In many TCP implementations the minimum value of Timeout is
500 ms due to the clock granularity
Eytan Modiano
Slide 35
Fast Retransmit
? When TCP receives a packet with a SN that is greater than the
expected SN, it sends an ACK packet with a request number of the
expected packet SN
– This could be due to out-of-order delivery or packet loss
? If a packet is lost then duplicate RNs will be sent by TCP until the
packet it correctly received
– But the packet will not be retransmitted until a Timeout occurs
– This leads to added delay and inefficiency
? Fast retransmit assumes that if 3 duplicate RNs are received by
the sending module that the packet was lost
– After 3 duplicate RNs are received the packet is retransmitted
– After retransmission, continue to send new data
? Fast retransmit allows TCP retransmission to behave more like
Selective repeat ARQ
– Future option for selective ACKs (SACK)
Eytan Modiano
Slide 36
TCP congestion control
? TCP uses its window size to perform end-to-end congestion
control
– More on window flow control later
? Basic idea
– With window based ARQ the number of packets in the network
cannot exceed the window size (CW)
Last_byte_sent (SN) - last_byte_ACK’d (RN) <= CW
? Transmission rate when using window flow control is equal to one
window of packets every round trip time
R = CW/RTT
? By controlling the window size TCP effectively controls the rate
Eytan Modiano
Slide 37
Effect Of Window Size
? The window size is the number of bytes that are allowed to be in
transport simultaneously
WASTED BW
WINDOW WINDOW
? Too small a window prevents continuous transmission
? To allow continuous transmission window size must exceed round-trip
delay time
Eytan Modiano
Slide 38
Length of a bit (traveling at 2/3C)
At 300 bps 1 bit = 415 miles 3000 miles = 7 bits
At 3.3 kbps 1 bit = 38 miles 3000 miles = 79 bits
At 56 kbps 1 bit = 2 miles 3000 miles = 1.5 kbits
At 1.5 Mbps 1 bit = 438 ft. 3000 miles = 36 kbits
At 150 Mbps 1 bit = 4.4 ft. 3000 miles = 3.6 Mbits
At 1 Gbps 1 bit = 8 inches 3000 miles = 240 Mbits
Eytan Modiano
Slide 39
Dynamic adjustment of window size
? TCP starts with CW = 1 packet and increases the window size
slowly as ACK’s are received
– Slow start phase
– Congestion avoidance phase
? Slow start phase
– During slow start TCP increases the window by one packet for every
ACK that is received
– When CW = Threshold TCP goes to Congestion avoidance phase
– Notice: during slow start CW doubles every round trip time
Exponential increase!
? Congestion avoidance phase
– During congestion avoidance TCP increases the window by one
packet for every window of ACKs that it receives
– Notice that during congestion avoidance CW increases by 1 every
round trip time - Linear increase!
? TCP continues to increase CW until congestion occurs
Eytan Modiano
Slide 40
Reaction to congestion
? Many variations: Tahoe, Reno, Vegas
? Basic idea: when congestion occurs decrease the window size
? There are two congestion indication mechanisms
– Duplicate ACKs - could be due to temporary congestion
– Timeout - more likely due to significant congstion
? TCP Reno - most common implementation
– If Timeout occurs, CW = 1 and go back to slow start phase
– If duplicate ACKs occur CW = CW/2 stay in congestion avoidance
phase
Eytan Modiano
Slide 41
Understanding TCP dynamics
? Slow start phase is actually fast
? TCP spends most of its time in Congestion avoidance phase
? While in Congestion avoidance
– CW increases by 1 every RTT
– CW decreases by a factor of two with every loss
“Additive Increase / Multiplicative decrease”
CW
“Saw-tooth Behavior”
Time
Eytan Modiano
Slide 42
Random Early Detection (RED)
? Instead of dropping packet on queue overflow, drop them probabilistically earlier
? Motivation
– Dropped packets are used as a mechanism to force the source to slow down
If we wait for buffer overflow it is in fact too late and we may have to drop many packets
Leads to TCP synchronization problem where all sources slow down simultaneously
– RED provides an early indication of congestion
Randomization reduces the TCP synchronization problem
? Mechanism
– Use weighted average queue size
If AVE_Q > T
min
drop with prob. P
If AVE_Q > T
max
drop with prob. 1
– RED can be used with explicit congestion
notification rather than packet dropping
– RED has a fairness property
Large flows more likely to be dropped
– Threshold and drop probability values
are an area of active research
1
P
max
T
min
T
max
Ave queue length
Eytan Modiano
Slide 43
TCP Error Control
EFFICIENCY VS. BER
CHANNEL BER
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1E-07 1E-06 1E-05 1E-04 1E-03 1E-02
SRP
1 SEC R/T DELAY
T-1 RATE
1000 BIT PACKETS
GO BACK N
WITH TCP
WINDOW CONSTRAINT
? Original TCP designed for low BER, low delay links
? Future versions (RFC 1323) will allow for larger windows and selective
retransmissions
Eytan Modiano
Slide 44
Impact of transmission errors on
TCP congestion control
EFFICIENCY VS BER FOR TCP'S
CONGESTION CONTROL
BER
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03
1,544 KBPS
64 KBPS
16 KBPS
2.4 KBPS
? TCP assumes dropped packets are due to congestion and responds
by reducing the transmission rate
? Over a high BER link dropped packets are more likely to be due to
errors than to congestion
? TCP extensions (RFC 1323)
– Fast retransmit mechanism, fast recovery, window scaling
Eytan Modiano
Slide 45
TCP releases
? TCP standards are published as RFC’s
? TCP implementations sometimes differ from one another
– May not implement the latest extensions, bugs, etc.
? The de facto standard implementation is BSD
– Computer system Research group at UC-Berkeley
– Most implementations of TCP are based on the BSD implementations
SUN, MS, etc.
? BSD releases
– 4.2BSD - 1983
First widely available release
– 4.3BSD Tahoe - 1988
Slow start and congestion avoidance
– 4.3BSD Reno - 1990
Header compression
– 4.4BSD - 1993
Multicast support, RFC 1323 for high performance
Eytan Modiano
Slide 46
The TCP/IP Suite
UDP
Telnet&
Rlogin
FTP SMTP
X
Trace
route
ping
DNS
TFTP
BOOTP SNMP
NFS
TPC
ICMP
ARP
IP
Data Link
RARP
IGMP
RPC
media
Eytan Modiano
Slide 47
Asynchronous Transfer Mode (ATM)
? 1980’s effort by the phone companies to develop an integrated
network standard (BISDN) that can support voice, data, video, etc.
? ATM uses small (53 Bytes) fixed size packets called “cells”
– Why cells?
Cell switching has properties of both packet and circuit switching
Easier to implement high speed switches
– Why 53 bytes?
– Small cells are good for voice traffic (limit sampling delays)
For 64Kbps voice it takes 6 ms to fill a cell with data
? ATM networks are connection oriented
– Virtual circuits
Eytan Modiano
Slide 48
ATM Reference Architecture
? Upper layers
– Applications
– TCP/IP
? ATM adaptation layer
– Similar to transport layer
– Provides interface between
upper layers and ATM
Break messages into cells and
reassemble
? ATM layer
– Cell switching
– Congestion control
? Physical layer
– ATM designed for SONET
Synchronous optical network
TDMA transmission scheme with
125 μs frames
Upper Layers
AT M A daptation
Layer (A A L)
AT M
Physical
Eytan Modiano
Slide 49
ATM Cell format
5 Bytes
48 Bytes
ATM Cell Header
Data
? Virtual circuit numbers
(notice relatively small address
space!)
– Virtual channel ID
– Virtual path ID
? PTI - payload type
? CLP - cell loss priority (1 bit!)
– Mark cells that can be dropped
? HEC - CRC on header
ATM Header (NNI)
1
2
3
4
5
HEC
PTI
VPI
C
L
P
VCI
VPI
VCI
VCI
Eytan Modiano
Slide 50
VPI/VCI
? VPI identifies a physical path between the source and destination
? VCI identifies a logical connection (session) within that path
– Approach allows for smaller routing tables
and simplifies route computation
ATM Backbone
Use VPI for switching
in backbone
Private
network
Private
network
Private
network
Use VCI to ID connection
Within private network
Eytan Modiano
Slide 51
ATM HEADER CRC
? ATM uses an 8 bit CRC that is able to correct 1 error
? It checks only on the header of the cell, and alternates between
two modes
– In detection mode it does not correct any errors but is able to detect
more errors
– In correction mode it can correct up to one error reliably but is less
able to detect errors
? When the channel is relatively good it makes sense to be in
correction mode, however when the channel is bad you want to be
in detection mode to maximize the detection capability
No detected
Detect double error
Correct
errors
Detect
errors
errors
Correct single
error
No detected errors
Detected
errors
Eytan Modiano
Slide 52
ATM Service Categories
? Constant Bit Rate (CBR) - e.g. uncompressed voice
– Circuit emulation
? Variable Bit Rate (rt-VBR) - e.g. compressed video
– Real-time and non-real-time
? Available Bit Rate (ABR) - e.g. LAN interconnect
– For bursty traffic with limited BW guarantees and congestion control
? Unspecified Bit Rate (UBR) - e.g. Internet
– ABR without BW guarantees and congestion control
Eytan Modiano
Slide 53
ATM service parameters
(examples)
? Peak cell rate (PCR)
? Sustained cell rate (SCR)
? Maximum Burst Size (MBS)
? Minimum cell rate (MCR)
? Cell loss rate (CLR)
? Cell transmission delay (CTD)
? Cell delay variation (CDV)
? Not all parameters apply to all service categories
– E.g., CBR specifies PCR and CDV
– VBR specifies MBR and SCR
? Network guarantees QoS provided that the user conforms to his
contract as specified by above parameters
– When users exceed their rate network can drop those packets
– Cell rate can be controlled using rate control scheme (leaky bucket)
Eytan Modiano
Slide 54
Flow control in ATM networks (ABR)
? ATM uses resource management cells to control rate parameters
– Forward resource management (FRM)
– Backward resource management (BRM)
? RM cells contain
– Congestion indicator (CI)
– No increase Indicator (NI)
– Explicit cell rate (ER)
– Current cell rate (CCR)
– Min cell rate (MCR)
? Source generates RM cells regularly
– As RM cells pass through the networked they can be marked with
CI=1 to indicate congestion
– RM cells are returned back to the source where
CI = 1 => decrease rate by some fraction
CI = 1 => Increase rate by some fraction
– ER can be used to set explicit rate
Eytan Modiano
Slide 55
End-to-End RM-Cell Flow
ABR
Switch
BRM
D D
ABR
Switch
D FRM D
ABR
Destin
ation
BRM
D FRM
ABR
Source
= data cell
= forward RM cell
= backward RM cell
At the destination the RM cell is “turned around”
and sent back to the source
D
FRM
BRM
Eytan Modiano
Slide 56
ATM Adaptation Layers
? Interface between ATM layer and higher layer packets
? Four adaptation layers that closely correspond
to ATM’s service classes
– AAL-1 to support CBR traffic
– AAL-2 to support VBR traffic
– AAL-3/4 to support bursty data traffic
– AAL-5 to support IP with minimal overhead
? The functions and format of the adaptation layer depend on the
class of service.
– For example, stream type traffic requires sequence numbers to
identify which cells have been dropped.
Each class of service has
A different header format
(in addition to the 5 byte
ATM header)
USER PDU
ATM CELL
ATM CELL
(DLC or NL)
Eytan Modiano
Slide 57
Example: AAL 3/4
ATM CELL PAYLOAD (48 Bytes)
ST SEQ MID
LEN CRC
2 4 10 6 10
44 Byte User Payload
? ST: Segment Type (1st, Middle, Last)
? SEQ:4-bit sequence number (detect lost cells)
? MID: Message ID (reassembly of multiple msgs)
? 44 Byte user payload (~84% efficient)
? LEN: Length of data in this segment
? CRC: 10 bit segment CRC
? AAL 3/4 allows multiplexing, reliability, & error detection but is
fairly complex to process and adds much overhead
? AAL 5 was introduced to support IP traffic
– Fewer functions but much less overhead and complexity
Eytan Modiano
Slide 58
ATM cell switches
Input
1
Input
Q's
Output
Q's
S/W
Control
Cell
Processing
Cell
Processing
Cell
Processing
Switch
Fabric
m
Output
1
Input
2
Output
2
Input
Output
m
? Design issues
– Input vs. output queueing
– Head of line blocking
– Fabric speed
Eytan Modiano
Slide 59
ATM summary
? ATM is mostly used as a “core” network technology
? ATM Advantages
– Ability to provide QoS
– Ability to do traffic management
– Fast cell switching using relatively short VC numbers
? ATM disadvantages
– It not IP - most everything was design for TCP/IP
– It’s not naturally an end-to-end protocol
Does not work well in heterogeneous environment
Was not design to inter-operate with other protocols
Not a good match for certain physical media (e.g., wireless)
– Many of the benefits of ATM can be “borrowed” by IP
Cell switching core routers
Label switching mechanisms
Eytan Modiano
Slide 60
Multi-Protocol Label Switching (MPLS)
“As more services with fixed throughput and
delay requirements become more common, IP will
need virtual circuits (although it will probably call
them something else)”
RG, April 28, 1994
Eytan Modiano
Slide 61
Label Switching
? Router makers realize that in order to increase the speed and
capacity they need to adopt a mechanism similar to ATM
– Switch based on a simple tag not requiring complex routing table
look-ups
– Use virtual circuits to manage the traffic (QoS)
– Use cell switching at the core of the router
? First attempt: IP-switching
– Routers attempt to identify flows
Define a flow based on observing a number of packets between a given
source and destination (e.g., 5 packets within a second)
– Map IP source-destination pairs to ATM VC’s
Distributed algorithm where each router makes its own decision
? Multi-protocol label switching (MPLS)
– Also known as Tag switching
– Does not depend on ATM
– Add a tag to each packet to serve as a VC number
Tags can be assigned permanently to certain paths
Eytan Modiano
Slide 62
Label switching can be used to create a virtual
mesh with the core network
? Routers at the edge of the core
network can be connected to
each other using labels
? Packets arriving at an edge router
can be tagged with the label to
the destination edge router
– “Tunneling”
– Significantly simplifies routing
in the core
– Interior routers need not
remember all IP prefixes of
outside world
– Allows for traffic engineering
Assign capacity to labels based
on demand
Core network
Label switched routes
D
D
Eytan Modiano
Slide 63
References
? TCP/IP Illustrated (Vols. 1&2), Stevens
? Computer Networks, Peterson and Davie
? High performance communication networks, Walrand and Varaiya
Eytan Modiano
Slide 64
Eytan Modiano
Slide 65
Class A
Class B
Class C
Class D
Class E
7 bits
24 bits
21 bits
8 bits
28 bits
netid hostid 011
netid
hostid 01
netid
hostid0111
27 bits
(reserved for future use)
01111
netid
hostid 0
14 bits
16 bits