Download Limitations of Layer2 switching

Layer 3
The Network Layer
Slides adapted from Tanenbaum
Purpose of Internetworking
•
As we have seen, Layer 1 and Layer 2
networks are satisfactory for LAN level
networks
–
–
Bridges (layer2 switches) are used to match
different Layer 2 protocols
Bridges can also create Virtual LANs
(VLANS) by grouping Hosts on different
LAN segments through MAC level
addressing.
Limitations of Layer2 switching
•
Even with complex bridges, NIC cards (hosts) never know
the MAC address of all their possible destinations.
–
–
•
•
Frames get broadcast to everyone on the segment
Bridges filter somewhat, but may not know the exact host that
should receive the Ethernet Frame (ends up wasting lots of
bandwidth)
Layer 2 “Virtual LANs” (VLANs) only partially solve this.
Layer 2 networks don’t scale.
–
Complexity increases faster than number of nodes
There’s got to be a better way.
Network Layer Design Issues
•
•
•
•
•
Store-and-Forward Packet Switching
Services Provided to the Transport Layer
Implementation of Connectionless Service
Implementation of Connection-Oriented Service
Comparison of Virtual-Circuit and Datagram Subnets
Store-and-Forward Packet Switching
The environment of the network layer protocols.
fig 5-1
Implementation of Connectionless Service
Routing within a diagram subnet.
Key point: Packet’s destination address is used to determine the required
output port. Routing tables can change over time and packets can
take different routes between the same two routers.
Implementation of Connection-Oriented
Service
Routing within a virtual-circuit subnet. Each switch looks at VCI and
input port to determine the output port. Does not look at destination.
This is like X.25, Frame and ATM VC switching that we saw
before.
Comparison of Virtual-Circuit and
Datagram Subnets
5-4
Routing Algorithms
•
•
•
•
•
•
•
•
•
•
The Optimality Principle
Shortest Path Routing
Flooding
Distance Vector Routing
Link State Routing
Hierarchical Routing
Broadcast Routing
Multicast Routing
Routing for Mobile Hosts
Routing in Ad Hoc Networks
Routing Algorithms (2)
Conflict between fairness and optimality.
If traffic flows between A and A’, B and B’, and C and C’, the
utilization of the network will be good, but flow between X and X’
will be severely restricted. Fairness must often be imposed at the
expense of overall utilization.
Optimality Principle

Optimality principle:
–

This means:
–
Given the final destination, routers only need to know the
optimal route to the next router.
This entails a lookup-table correlating destination address and
output port.
–

If router J is on the optimal path from router I to router K, then
the optimal path from J to K also falls along the same route.
The following figure:
–
–
–
Shows the sink tree for destination “B”
There would be different sink trees for each final destination.
Note there are no loops in this tree.
The Optimality Principle
(a) A subnet. (b) A sink tree for router B.
Shortest Path Routing
The first 5 steps used in computing the shortest path from A to D.
The arrows indicate the working node.
Shortest path routing

Algorithms that efficiently compute the shortest
path
–
–

Dijkstra
Bellman
Another approach -- flooding may be used to send
packets to all adjacent routers.
–
Packets that arrive at desired destination first are
considered to have taken the “best”route. (this route is
then chosen for subsequent packets.)
Distance Vector Routing

In a network of routers, the trick is to Adapt these
algorithms to run in a distributed fashion.
–
–

Each router only needs to know who it’s connected to.
Doesn’t know the complete topology of the entire network.
Defining the “cost” of each link is an important
consideration
–
–
–
–
Delay??
Bandwidth??
Real-cost??
Queue-depth at input port of next router??
Distance Vector Routing

Each node builds up route tables with best output port and cost for
all destinations of the network
–
–



Looks at each possible route to every destination and selects lowest
Creates and stores the table locally
The “Count to Infinity” problem exists with distance vector routing
–

In following slide J receives vectors from each neighbor
Also receives cost to those immediate neighbors
Each node is slow to realize a node has failed.
Split-horizon is a modification to alleviate this problem
–
Still doesn’t work perfectly
RIP is an example of Distance Vector Routing Protocol
Distance Vector Routing
(a) A subnet. (b) Input from A, I, H, K, and the new
routing table for J.
Distance Vector Routing (2)
The count-to-infinity problem, these vectors are the hops to A
In (a), the router A has just come up, B learns then C, etc.
(b) Shows what happens when A fails, each node is very slow
to realize the cost to A is INFINITY.
Link State Routing
Each router must do the following:
 Discover its neighbors, learn their network address.
 Measure the delay or cost to each of its neighbors.
 Construct a packet telling all it has just learned.
 Send this packet to all other routers.
 Compute the shortest path to every other router.
Learning about the Neighbors
(a) Nine routers and a LAN. (b) A graph model of (a).
Measuring Line Cost
A subnet in which the East and West parts are connected by two lines.
If link status is reported based congestion and distance,
oscillations can occur. CF may be the best route so all traffic is
moved over there. Then EI becomes a much better route, etc.,
etc., etc.
Building Link State Packets

With link-state routing, packets are created by each node that
describe its adjacent links.
–
–

The AGE of the update and the sequence number are also included.
This allows changes in the network to be considered.
(a) A subnet. (b) The link state packets for this subnet.
Distributing the Link State Packets
The packet buffer for router B in the previous slide
Flooding is used to distribute link-state packets. As the linkstate packets are flowing around the network, they must be
managed. New ones are used to update and also sent on to
adjacent nodes. Old ones are ignored and not forwarded.
Using both Age and Sequence # prevents problems when
routers reboot and lose sequence.
Hierarchical Routing
Hierarchical routing.
This type of architecture allows the routing protocols to scale. For
example, all the flooded Link-state messages don’t have to
propagate outside the areas.
Congestion Control Algorithms
•
•
•
•
•
•
General Principles of Congestion Control
Congestion Prevention Policies
Congestion Control in Virtual-Circuit Subnets
Congestion Control in Datagram Subnets
Load Shedding
Jitter Control
Basic Queuing Theory
Arrivals
Servicing

In a queue, things stack up as they are waiting to be serviced
–
–

Infinite length queues where the service rate is greater than the arrival rate
produce a stable system.
The average waiting time for a M/M/1 queue is well known
In reality, things are different
–
Arrivals are not exponentially distributed
•
–

Data tends to be more bursty and “self-similar” (what is this?)
Real Queues can’t hold infinite number of messages (not enough memory, too
much delay)
Multiple queues and priorities are implemented for QoS
Congestion

When queues in the Routers begin to be congested,
measures must be taken to manage this.
Why not just let the queues overflow??

–
–
–
–

Often this makes the problem worse
Flow control algorithms will try to resend further compounding
the problem
Delay in the network would be maximum
Longer queues introduce more chance for Jitter
Lots of work has been done on congestion management
Congestion
When too much traffic is offered, congestion sets in and
performance degrades sharply.
General Principles of Congestion Control

Monitor the system .
–


detect when and where congestion occurs.
Pass information to where action can be taken.
Adjust system operation to correct the problem.
Congestion Prevention Policies
Policies that affect congestion.
5-26
From “Congestion control in Computer Networks, Issues and
Trends”, Raj Jain, IEEE Network Magazine May/June 1990.
Congestion prevention policies

Layers 2-4 are all involved in congestion control and prevention
–
–

Bits errors at layer 1 can even become an issue
Interaction between the layers can be a problem
Data Link Layer
–
–
–

Can request info too quick
Can send too many (or not enough) ACKs
Etc.
Network Layer
–
–

Responsible for Queueing and routing the data through large network
Lots of potential to introduce delay
Transport Layer
–
–
TCP Window size can be important
Flow Control
Congestion Control in Virtual-Circuit
Subnets
(a) A congested subnet. (b) A redrawn subnet, eliminates
congestion and a virtual circuit from A to B.
With VC based networks, it may be possible to dynamically set up a
new
VC that takes an uncongested route.
Hop-by-Hop
Choke Packets
Choke packets are send in
the reverse direction to slow
down the transmitter
(a) A choke packet that affects only
the source.
(b) A choke packet that affects
each hop it passes through.
Jitter Control
(a) High jitter.
(b) Low jitter.
Quality of Service
•
•
•
•
•
Requirements
Techniques for Achieving Good Quality of Service
Integrated Services
Differentiated Services
Label Switching and MPLS
Requirements
How stringent the quality-of-service requirements are.
5-30
Different applications need different QoS
characteristics to function well.
Buffering
Smoothing the output stream by buffering packets.
A major factor in packet networks is variance in the time each
packet takes to traverse the network. This jitter can be removed
by playout buffers but they introduce additional delay.
The Leaky Bucket Algorithm
(a) A leaky bucket with water. (b) a leaky bucket with packets.
Variants of the the leaky bucket are used in protocols besides
Frame Relay – Often combined with the TOKEN BUCKET
The Leaky
Bucket
Algorithm
(a) Input to a leaky bucket.
(b) Output from a leaky
bucket. Output from a token
bucket with capacities of (c)
250 KB, (d) 500 KB, (e)
750 KB, (f) Output from a
500KB token bucket feeding
a 10-MB/sec leaky bucket.
The Token Bucket Algorithm
5-34
(a) Before.
(b) After.
Admission Control
An example of flow specification.
5-34
Packet Scheduling
Fair queuing is used so that longer frames don’t take an
unfair amount of capacity. Here, A is 6 units long and
Frame C is only 2 units long. C should get completely sent
first.
(a) A router with five packets queued for line O.
(b) Finishing times for the five packets.
Reserving capacity in Layer 3 networks

For real-time streaming media applications, the required
bandwidth can be significant
Packet networks may eventually be used to distribute
movies and things cable is providing today
Lots of these applications are Multicasting


–

There are sources sending the same content to many receivers
RSVP, Resource reSerVation Protocol is a well-known
protocol for this scenario
–
–
–
RSVP support the ability to RESERVE bandwidth in a packet
network
It’s important that the network have a way to enforce that
reservation
How does know whether it can Admit the request? (Admission
Control?)
RSVP-The ReSerVation Protocol
(a) A network, (b) The multicast spanning tree for host 1.
(c) The multicast spanning tree for host 2.
RSVP



Note that when two sources subscribe to the same source,
links that they share in common don’t need to reserve 2X
the amount of bandwidth.
RSVP has evolved into a popular mechanism for receivers
to request resources in a network.
It is an entire Protocol and will be discussed in more detail
later during the MPLS discussion.
RSVP-The ReSerVation Protocol (2)
(a) Host 3 requests a channel to host 1. (b) Host 3 then requests a
second channel, to host 2. (c) Host 5 requests a channel to host 1.
Expedited Forwarding
Expedited packets experience a traffic-free network.
Expedited Forwarding is a concept where Packet networks can
achieve very high performance because the network is lightly
loaded. This can be accomplished by building a separate
network or carving a “virtually” separate network within existing
switches
Just like the HOV lane on 635
Assured Forwarding
A possible implementation of the data flow for assured forwarding.
Often, different applications are classified before they enter
the network. An edge router than breaks them out to
separate queuing systems based on their class. It may also
shape the traffic to reduce congestion
VOIP may get high priority
Email could get the lowest
Internetworking
•
•
•
•
•
•
•
How Networks Differ
How Networks Can Be Connected
Concatenated Virtual Circuits
Connectionless Internetworking
Tunneling
Internetwork Routing
Fragmentation
Connecting Networks
A collection of interconnected networks.
How Networks Differ
Some of the many ways networks can differ.
5-43
How Networks Can Be Connected
(a) Two Ethernets connected by a switch.
(b) Two Ethernets connected by routers.
Concatenated Virtual Circuits
Internetworking using concatenated virtual circuits.
Connectionless Internetworking
A connectionless internet.
Here, the internetworking is done by just
forwarding Datagrams. Packets might
take different routes.
Tunneling
Tunneling a packet from Paris to London.
Tunneling makes the network seem to be all one
Layer 3 cloud. The Multiprotocol routers likely have
either Frame or ATM interfaces
Tunneling (2)
Tunneling a car from France to England.
Internetwork Routing
(a) An internetwork. (b) A graph of the internetwork.
Note that 1,2,3,4 and 5 are
actually networks themselves
Internetwork routing (cont)


Note in previous slide
This is a hierarchical network i.e.
–


Within the subnetworks there is separate routing occurring
An ‘exterior’ routing protocol is used at the coarse level.
Within each subnetwork, an ‘interior’ routing protocol is
used. (RIP-- distance vector or OSFP-- link-state)
–

These subnetworks could be service providers, enterprises, or
regions.
Essentially, this can be considered a rough model of the
Internet.
–
–
The internet is a loose coalition of several different networks.
Not really any centralized control
Typical Internet node
Layer3
Layer 2
Layer 1 or 2
Layer 2
Frame Relay or ATM
connections to POPs or
large customers
Layer 3
Layer 2
ATM switch
Layer 1
Backbon
e
Router
Layer 0
SONET LTE
OXC
WDM transponder
ATM switch
Backbon
e
Router
Backbon
e
Router
FDDI ring
Backbon
e
Router
ATM switch
Frame Relay or ATM
connections to POPs or
large customers
Source: RHK
Fragmentation
(a) Transparent fragmentation. (b) Nontransparent fragmentation.
Fragmentation (2)
Fragmentation when the elementary data size is 1 byte.
(a) Original packet, containing 10 data bytes.
(b) Fragments after passing through a network with maximum
packet size of 8 payload bytes plus header.
(c) Fragments after passing through a size 5 gateway.
The Network Layer in the Internet
•
•
•
•
•
•
•
•
The IP Protocol
IP Addresses
Internet Control Protocols
OSPF – The Interior Gateway Routing Protocol
BGP – The Exterior Gateway Routing Protocol
Internet Multicasting
Mobile IP
IPv6
Design Principles for Internet










Make sure it works.
Keep it simple.
Make clear choices.
Exploit modularity.
Expect heterogeneity.
Avoid static options and parameters.
Look for a good design; it need not be perfect.
Be strict when sending and tolerant when receiving.
Think about scalability.
Consider performance and cost.
Collection of Subnetworks
The Internet is an interconnected collection of many networks.
The IP Protocol
The IPv4 (Internet Protocol) header.
The IP Protocol (2)
Some of the IP options.
5-54
IP Addresses
IP address formats.
IP Addresses (2)
Special IP addresses.
Subnets
A campus network consisting of LANs for various departments.
Subnets (2)
A class B network subnetted into 64 subnets.
CDR – Classless InterDomain Routing
A set of IP address assignments.
5-59
NAT – Network Address Translation
Placement and operation of a NAT box.
Internet Control Message Protocol
The principal ICMP message types.
5-61
ARP– The Address Resolution Protocol
Three interconnected /24 networks: two Ethernets and an FDDI ring.
Dynamic Host Configuration Protocol
Operation of DHCP.
OSPF – The Interior Gateway Routing
Protocol
(a) An autonomous system. (b) A graph representation of (a).
OSPF (2)
The relation between ASes, backbones, and areas in OSPF.
OSPF (3)
The five types of OSPF messages.
5-66
Problems with Distance-Vector and
Link-State Routing
 Neither distance-vector (RIP) nor link state (OSPF)
protocols effective for exterior routing
 Distance vector and link state protocols assume all
routers share common metric
 Priorities and restrictions may differ between ASs
 Flooding of link state information may become
unmanageable
Path Vector Routing
 Dispense with routing metrics
 Provide information about:
– Which networks can be reached by given router
– Which ASs must be crossed to get there
 No distance or cost element
 Routing information includes all Ass visited to reach
destination
– Allows policy routing
BGP – The Exterior Gateway Routing
Protocol
(a) A set of BGP routers.
(b) Information sent to F.
Boarder Gateway Protocol (BGP)

Allows routers (gateways) in different ASs to exchange
routing information
Messages sent over TCP

–

See next slide
Three functional procedures
–
–
–
Neighbor acquisition
Neighbor reachability
Network reachability
BGP v4 Messages
 Open
– Start neighbor relationship with another router
 Update
– Transmit information about single route
– List multiple routes to be withdrawn
 Keepalive
– Acknowledge open message
– Periodically confirm neighbor relationship
 Notification
– Send when error condition detected
Neighbor Acquisition
 Neighbors attach to same subnetwork
 If in different ASs routers may wish to exchange
information
 Neighbor acquisitionis when two neighboring routers agree
to exchange routing information regularly
– Needed because one router may not wish to take part
 One router sends request, the other acknowledges
– Knowledge of existence of other routers and need to exchange
information established at configuration time or by active
intervention
Neighbor Reachability


Periodic issue of keepalive messages
Between all routers that are neighbors
Network Reachability



Each router keeps database of subnetworks it can reach and
preferred route
When change made, router issues update message
All BGP routers build up and maintain routing information
BGP Message
Formats
 Marker:
– Reserved for authentication
 Length:
– In octets
 Type:
– Open, Update, Keepalive,
Notification
Neighbor Acquisition Detail
 Router opens TCP connection with neighbor
 Sends open message
– Identifies sender’s AS and gives IP address
– Includes Hold Time
• As proposed by sender
 If recipient prepared to open neighbor relationship
– Calculate hold time
• min [own hold time, received hold time]
• Max time between keepalive/update messages
– Reply with keepalive
Keepalive Detail


Header only
Often enough to prevent hold time expiring
Update Detail
 Information about single route through internet
– Information to be added to database of any recipient router
– Network layer reachability information (NLRI)
• List of network portions of IP addresses of subnets reached by
this route
– Total path attributes length field
– Path attributes field (next slide)
 List of previously advertised routes being withdrawn
 May contain both
Path Attributes Field
 Origin
– Interior (e.g. OSPF) or exterior (BGP) protocol
 AS_Path
– ASs traversed for this route
 Next_Hop
– IP address of boarder router for next hop
 Multi_Exit_disc
– Information about routers internal to AS
 Local_Pref
– Tell other routers within AS degree of preference
 Atomic_Aggregate, Aggregator
– Uses subnet addresses in tree view of network to reduce information needed in NLRI
Withdrawal of Route(s)

Route identified by IP address of destination subnetwork(s)
Notification Message
 Error notification
 Message header error
– Includes authentication and syntax errors
 Open message error
– Syntax errors and option not recognised
– Proposed hold time unacceptable
 Update message error
– Syntax and validity errors
 Hold time expired
 Finite state machine error
 Cease
– Close connection in absence of any other error
Diagram for BGP Routing Information
Exchange
BGP Routing Information Exchange
 R1 constructs routing table for AS1 using OSPF
 R1 issues update message to R5 (in AS2)
– AS_Path: identity of AS1
– Next_Hop: IP address of R1
– NLRI: List of all subnets in AS1
 Suppose R5 has neighbor relationship with R9 in AS3
 R9 forwards information from R1 to R9 in update message
– AS_Path: list of ids {AS2,AS1}
– Next_Hop: IP address of R5
– NLRI: All subnets in AS1
 R9 decides if this is prefered route and forwards to neighbors
The Main IPv6 Header
The IPv6 fixed header (required).