Computer Networks, Jan-May 20041 Computer Networks (Lecture 5: Network Layer Protocols ) Arzad...

Computer Networks, Jan-May 2004

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Computer NetworksComputer Networks(Lecture 5: Network Layer (Lecture 5: Network Layer

Protocols )Protocols )

Arzad Kherani([email protected])

Dept. of Computer Sc. And Engg.

Indian Institute of Technology Delhi


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OutlineOutline

Connection-less vs. connection-oriented data transfer

Routing protocol Congestion control IP protocol ICMP protocol


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The Network LayerThe Network Layer End-to-end data transfer

– Addressing– Store-and-forward packet

switching– Routing– Congestion control– Interconnection between networks


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Packet switchingPacket switching Store-and-forwarding at intermediate nodes using connection-

less or connection-oriented data transfer Service-provider vs. customer-premise equipment


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Datagram RoutingDatagram Routing No connection is established Each packet is forwarded independent of others

– Every packet carries a destination address Intermediate routers maintain (and update) “routing tables”


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Routing connectionsRouting connections A connection is established before data transfer can take place Route is fixed at the tie connection is established

– And resources allocated Connections are also known as virtual circuits


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Datagrams vs. virtual circuitsDatagrams vs. virtual circuits


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Routing: the problemRouting: the problem

Largely concerned with routing datagrams through a subnet Between a pair of source-destination devices, packets may

have to traverse several “subnets” Routing tables are updated every T seconds

Router2Router1LAN

H1

H2

H3 Router1

LAN

H4

H6

H5


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Routing: the problem (2)Routing: the problem (2)

Correct Simple Robust

– Address the problems of changing traffic conditions, changes to topology, failures (both transient and permanent)

Stable– In several cases route computation is an iterative process– In such cases the process must converge– Incremental changes in traffic/topology must result in increment

changes in routes (I.e. there are no large swings in routes due to increnetal changes)

Fair Optimal


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Fairness vs. Optimality


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Performance metrics:– Transit delay– Throughput– Number of hops– Security

Delay vs. throughput


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Routing protocols: classificationRouting protocols: classification Static routes

– Computed off-line– based on certain topology, traffic,

performance metric– Not change, unless there is a

major network overhaul Adaptive routing

– Routes adapt to changes in topology, traffic

– On-line based on current measurements

– Based on complete or partial knowledge

– Distributed computation vs. centralized computation

Other algorithms– Flooding– Ho-potato

static adaptive

Centralized (based on all info)

Decentralized (on incomplete info)

Routing algorithms

others


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FloodingFlooding

An incoming packet is sent on all incoming links Limit the number of hops to avoid infinite loops

– Or, forward packets only once using a packet ID Or only on selected links (in the right direction)

Useful in case some data is to be “broadcasted” Terribly expensive in terms of resource utilization But, results in minimum delay


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Static routing Static routing

Shortest path routing using Dijkstra algorithm– Where “distance” is either delay, drop rate, or simply number of

hops Results in “rooted” tree with destination as the root


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Static routing: Dijkstra algorithmStatic routing: Dijkstra algorithm


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Adaptive routingAdaptive routing

Distance-vector routing Link-state routing Others

– Hierarchical routing Standards

– OSPF– BGP– MPLS and “traffic engineering”


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Distance-vector routingDistance-vector routing

Also known as Bellman-Ford routing– Used in Arpanet, till 1979

Each router maintains a routing table, with estimated “distance” to each destination (and updates it periodically)

Each router periodically exchanges this table with its neighbors

At node J


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Each router measures “distance” on each outgoing link– Using e.g. queue length, round-trip delay

It re-computes the routes as follows:

At node J


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Several problems with Distance Vector routing:– Poor estimate of delays along each link– Count-to-infinity problem:

Good news spreads fast Bad news travels slow, very slow


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Link State RoutingLink State Routing Every few seconds (or minutes), each router:

– Re-discovers the neighborhood (and their addresses)– Estimate delays (or distances) to each of its neighbors– Construct a packet with above information– Send it to all routers in the network – Collate similar information from all routers in the network– Re-compute the “shortest” routes

Possibly using Dijkstra’s algorithm


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Two fundamental pointsTwo fundamental points

Routing schemes discussed thus far– Belong to “ routes for all source-destination pairs”

As opposed to “on-demand routing”, where a route is determined only if and when needed (as in wire-less networks, MPLS networks)

– Belong to schemes where “routing tables” are used to route packets

As opposed to “source-routing”, where each packet carries the route that it must follow


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Link State Routing:Link State Routing: Neighborhood discoveryNeighborhood discovery

Use “hello” packets on each outgoing links– Neighbors respond with an “ack”


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Link State Routing:Link State Routing: Measuring Distances over LinksMeasuring Distances over Links

Use hello packets, and timers, to estimate delay– Start timer when the “hello” packet is put in the queue

Takes into account “load”

– Or, when its transmission is started Does not take into account “load”


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Link State RoutingLink State Routing Format of the link-state packet:

– “seq no” helps with flooding the packet to all routers– Age, so that the information can be discarded after a while


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Link State RoutingLink State Routing Packet processing:

– Re-sequencing of link-state info packets Ignore packets with “lower” sequence numbers (as “stale”)

– What if a packet is lost? No big deal

– Other problems What if a sequence number is corrupted by noise? And this fact goes

undetected What if a router re-boots?

– Each packet has an associated “age” in seconds (say 60 sec) “age” is decremented every second by intermediate routers, and by

the router that caches it processing starts afresh if age 0


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Link State RoutingLink State Routing Route computation:

– Note every router has identical information– Use Dijkstra’s shortest path algorithm

Problems:– Stale information– Incorrect information– Incomplete information– Inconsistent routes loops


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Link State RoutingLink State Routing Standards

– IS-IS Used with variety of protocols, including IP, IPX

– OSPF An Internet RFC


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Hierarchical RoutingHierarchical Routing

Essentially solves “scalability” problem for large networks Considers a network to consist of a connected network of regional

networks Routing is either within the local region, or across regions Multiple levels of hierarchy ( 2 or more)


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Hierarchical RoutingHierarchical Routing

Significant saving in size of routing tables– In example below, entries in table at 1A:

for local destination: 3 (size of local network) For other regions: 4 (one for every other region)

– For a network with say 720 routers organized as 8 regional networks, each consisting of 9 sub-nets, each of which contains 10 routers:

10 entries, one for each router in its sub-net 8 entries, one for every other sub-net 7 entries, one for every other regional network


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Broadcast routing: multi-destination routingBroadcast routing: multi-destination routing

Send n-1 copies, one for every other router Multi-destination routing (a smarter of sending one copy to every other

router)– The source sends a packet, containing list of all n-1 destinations addresses– When a packet arrives at an intermediate router, the router identifies for

each destination the “best” route, and then sends a packet on an outgoing line with the packet containing a list of sub-set of destination addresses

– Both distance-vector and link-state routing algorithms will provide the necessary information

source


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Broadcast routing: spanning-tree basedBroadcast routing: spanning-tree based

Intelligent form of broadcast, based on a spanning tree rooted-at-source

– The problem with this router is: does each router know the spanning tree– Works with link-state routing, but not distance-vector routing

source


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Broadcast routing: reverse path forwardingBroadcast routing: reverse path forwarding

Essentially spanning tree based packet broadcast, except that the spanning tree is determined on-the-fly

Simple, efficient


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Multicast routingMulticast routing

Spanning tree vs. multicast tree– The latter includes nodes that are required to forward packets to all member nodes

Multi-destination routing to all K-1members– When a packet arrives at an intermediate router, the router identifies for each destination the “best”

route, and then sends a packet on an outgoing line with the packet containing a list of sub-set of destination addresses

– Both distance-vector and link-state routing algorithms will provide the necessary information


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Routing in peer-to-peer ad hoc networksRouting in peer-to-peer ad hoc networks

What is different about routing in ad hoc networks– Routing environment

Wireless, mobile hosts resulting in:– Greater probability of link, node failure

– Changing topology

– Frequent route changes Every device is a potential router

Potentially different goals:– Stability of routes– Power consumption


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Classification of routing protocolsClassification of routing protocols

Multicast routing Unicast routing

– Proactive protocols Where routes between every pair of nodes are computed a-priori Examples: distance-vector, link-state rouitng in IP networks Advantage: reduced latency Dis-advantage: excessive overhead due to route computation

– Reactive protocols Routes are determined between a pair of devices only when required As in MPLS networks Advantage: overhead is minimized Dis-advantage: Increased latency

– Example routing protocols for ad hoc networks Flooding Dynamic source routing AODV …


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Dynamic Source Routing (DSR)Dynamic Source Routing (DSR)

If source S does not have a route to destination D:– It initiates “route discovery”– Or, broadcasts (floods) Route Request (RREQ)– RREQ includes address S as “source address”

B

A

S EF

H

J

D

C

G

IK

M

N

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[S]


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Dynamic Source Routing (contd.)Dynamic Source Routing (contd.)

Each node appends own identifier when forwarding RREQ Issues concerning hidden terminal and of collisions again arise

B

A

S E

F

H

J

D

C

G

IK

M

N

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[S,E]

[S,C]

[S,B]


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DSR effectively uses flooding to discover a route to destination

B

A

S E

F

H

J

D

C

G

IK

M

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[S,C,G]

[S,E,F]


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Route discovery continues till node M has also attempted to find a route to destination D

Final route is say [S, E, F, J, D]

B

A

S E

F

H

J

D

C

G

IK

M

N

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[S,C,G,K]

[S,E,F,J]


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Destination sends a RREP (Route Reply) back to source S, together with route [S, E, F, J, D]

RREP is sent along route obtained by reversing discovered route, viz. [D, J,F, E, S]

B

A

S EF

H

J

D

C

G

IK

M

N

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RREP [S,E,F,J,D]


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For DSR to succeed, links must be bi-directional:– RREP is sent along route obtained by reversing discovered route– Ensure:

Intermediate node forwards RREQ if it is received on a bi-directional link

Intermediate node forwards RREQ on links that are known to be bi-directional

– If links (in general) are not bi-directional then RREP is sent on a (new) discovered route from D to S RREP is piggybacked onto RREQ packets for D to S

Links are bi-directional in IEEE 802.11 and in Bluetooth


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Processing of RREP:– Source S caches the discovered route for subsequent packets– The route is included in each packet as “source route”

B

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S EF

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J

D

C

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DATA [S,E,F,J,D]


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– Intermediate nodes also cache relevant portions of the route for their use

B

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S EF

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D

C

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DATA [E,F,J,D]


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Route Caching in DSRRoute Caching in DSR

All nodes along the discovered route deduce and cache a route by any means:

– Given fact that RREP contains route [S, E, F, J, D]: S has a route to E, F, J as well E has a route [E, F, J, D] to D So does E to F and to J, and F to J

– Given fact that intermediate nodes need to forward RREP Nodes D, J, F, E all have a route to S, and to intermediate nodes Etc.

S EF

J

DRREP [S,E,F,J,D]


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Route Caching in DSR (contd.)Route Caching in DSR (contd.)

Other nodes not on the discovered path also discover routes:– For example, when node K receives RREQ [S,C,G] for node D, node K

learns route [K,G,C,S] to node S

B

A

S E

F

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J

D

C

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IK

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N

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[S,C,G]

[S,E,F]


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Cached routes are used:– To route packets– To obtain alternate routes when a route in use is broken

speed up recovery

– To respond to RREQ if a route is cached speed up route discovery limit propagation of RREQ


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Route Recovery in DSRRoute Recovery in DSR

Speed up recovery:– If node E fails

S initiates route discovery Node C responds immediately with RREP [S,C,G,K,D] S routes data with source route [S,C,G,K,D]

B

A

S EF

H

J

D

C

G

IK

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N

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route [S,E,F,J,D]

route [C, G, K, D]


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Route Recovery in DSR (contd.)Route Recovery in DSR (contd.)

Link failure is detected when node is unable to forward source-routed packet – notification is sent up-stream

Source S and intermediate nodes remove all routes with broken link as one of the links

B

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D

C

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RERR [J-D]


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Cached routes may become invalid due to changes in topology (or mobility)– Stale, invalid cache pollute neighboring caches– Impact on performance

No route is available Route is poor

Need to implement policy to “purge” stale/invalid cache entries


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DSR: pros and consDSR: pros and cons

Pros:– On demand routing– Caching speeds up route discovery– Route discovery uses flooding discovers minimum delay routes– Routing tables are not maintained

Cons:– Requires entire route to be included in packet header– Requires symmetric links– Inherits all problems associated with flooding (too many RREQs,

collisions, hidden terminals)– Stale, invalid cache


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Congestion, and its controlCongestion, and its control

Congestion == when a network is unable to move packets because there are too many packets in the network

It occurs because of:– Slow links– Slow routers/switches– Burst of packets are injected into the network– Small number of buffers

Congestion feeds upon itself Congestion can spread


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Congestion, and its controlCongestion, and its control

Difference between “flow control” and “congestion control”– Congestion has to do with networks carrying capacity

Flow control has to do with a destination node having to receive and process incoming packets

– Congestion is a global issue Flow control is an issue pertaining to communication between a pair of devices

Yet, methods used for flow control and congestion control CAN be similar


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Congestion controlCongestion control

Open loop control– Good design

Accept new traffic carefully Discard traffic Schedule packet transmission Allocate buffers

– Attempted in all protocol layers Closed loop control

– Closely monitor congestion Queue lengths packets dropped due to unavailability of buffers Link utilization Transit delay, and jitter

– Exchange information with other nodes (particularly those responsible for taking actions)

adds to congestion

– Adjust network operation (re-schedule, re-route, drop packets, block traffic, …)


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Congestion controlCongestion control


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Congestion control in virtual-circuit based networksCongestion control in virtual-circuit based networks

Admission control– Works only with virtual circuits-based networks– A new connection is accepted only if adequate resources are available to

support it– Different routes may be used to circumvent congestion– Comes with its own issues with reservations

Under-utilization When required, excess capacity is unavailable


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Congestion control in datagram-based networksCongestion control in datagram-based networks

Each node is responsible for monitoring, communicating status, and controlling it

Monitor congestion by measuring:– Queue length, channel utilization, delays, etc.– Usually work with averages

Averaging interval? Averaging process? E.g.

Signaling congestion to source– Implicitly:

Set a “congestion bit” in packet sent to destination, which in turn sets the bit in an ACK

Simply drop the packet, and let source discover that fact, as with– Drop-tail– RED

– Explicitly: Send a “choke” packet to source


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Random Early Detection (RED) algorithm-based Random Early Detection (RED) algorithm-based congestion avoidancecongestion avoidance

RED algorithm– Developed by Sally Floyd and Van Jacobson, 1993– Used extensively in Internet

Design goals:– Avoid congestion, rather than remove congestion early detect

Do so by ensuring that the queue does not overflow– Also ensures that the queuing delay is small

– Avoid global and synchronous pull-back of traffic Thus ensures that throughput remains high

– Not be biased against bursty traffic Basic idea

– Act upon when average queue length begins to grow– Randomly “mark” a packet, in the hope that TCP connection will slow down

In the present context “mark” == ”drop”


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Random Early Detection (RED) algorithmRandom Early Detection (RED) algorithm


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Computation of average queue length


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RED algorithmRED algorithm

Packets dropped


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Quality of Service (QoS)Quality of Service (QoS)

Low delay/jitter == not sensitive to delay/jitter

Two ways to characterize Q0S requirements of end applications– As was done in ATM networks:

Constant bit rate, CBR (e.g. telephony) Variable bit-rate, VBR (e.g. video conferencing) Available bit rate, ABR (e.g. file transfer)

– Based on performance parameters: reliability, delay, jitter etc.


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QoSQoS

Techniques:– Over-provision of resources

Comes with its own limitation (hogging, …)

– Buffering Basically counters the effect of large jitter

– Traffic shaping Comes with traffic policing, marking (or dropping)


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QoS: traffic shapingQoS: traffic shaping

Leaky bucket– Useful when a host generate bursty traffic, but at a higher rate

MUX

H_1

H_n

H_2

…

25MBps link

2 MBps link


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QoS: traffic shaping using “leaky bucket”QoS: traffic shaping using “leaky bucket”

A leaky bucket is essential a finite buffer

But does not permit host to accumulate “credits”

1 MB buffer


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QoS: traffic shaping using “token bucket”QoS: traffic shaping using “token bucket”

Token Bucket scheme for traffic shaping permits host to accumulate “credits”

– Tokens are generated at a fixed rate, and saved in a bucket– One packet may be sent for every available token in bucket– If the “token bucket” overflows, token are lost– The packet buffer may be very large, independent of size of token bucket

packets are not discarded when large burst arrives


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Assume:rate tokens are generated: /secbucket size: C bytesmaximum output rate: M Bpslength of burst: S sec.

Then:C + S = M S

OrS = C/ (M- ), burst size in Bytes is MS

500 KB token bucket

250 KB token bucket

Burst= 1 MB


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The output rate need not be the same as that at which the host produces data use a leaky bucket following the token buffer

– I.e. just put a buffer for data packets, and pull packets at the rate dictated by availability of token, but at the reduced TX rate

500 KB token bucket

250 KB token bucket

750 KB token bucket

Burst= 1 MB


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Resource ReservationResource Reservation

Resources need to be reserved in order to guarantee committed QoS– Bandwidth

Easy enough to determine Need to keep some spare bandwidth to handle bursts and “best-effort” traffic

– Buffer space somewhat difficult, unless “burst length” is specified else estimate using avg_q_length = /(-), where and are respectively

arrival and service rates – Service rate is determined by available bandwidth & CPU capacity

– CPU cycles Even more difficult Router characterized by routing capacity, X packets/sec Need to specify required processing capacity in terms of Y packets/sec

– May be calculated using peak & avg data rate, burst size, min and max packet size


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Admission controlAdmission control

If resources are to be reserved, each “flow” needs to be “admitted” using an “admission control” scheme

– QoS requirements for each flow is a must (e.g.

– Control based on available resources, viz.-a-viz. resources required by a flow or the aggregate of flows

– Yet there must be spare capacity


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RoutingRouting

Ensure that each “flow” or an “aggregate” is routed suitably, so that QoS constraints can be met

MPLS is one way to route


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Has been around for decades Uses “maximal prefix match” to route packets

– Slows down routing Routing is based on destination IP address

– But, one may prefer routes based on QoS, security, etc.

Routing (using MPLS)Routing (using MPLS)


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Provides for a tunnel for each “equivalence class” through a public network– Provide secure communication– Provide QoS guarantees (throughput, delay, drop rates, …)

128.1.47.1

128.1.47.2128.1.47.3

Ingress router

IP network

IP network

Egress router

IP network

IP networkMPLS network

MPLSMPLS


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TunnelsTunnels


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128.1.47.1

128.1.47.3

123

12

1

2

3

3

IP I/F DestAddr MPLS I/F Label In

… … … …

1 128.1.47.1 3 99

… … … …

IP I/F DestAddr MPLS I/F LabelOut

… … … …

3 128.1.47.1 1 50

… … … …

MPLS I/F Label In MPLS I/F LabelOut

… … … …

3 50 1 99

… … … …

MPLSMPLS


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Each LSP is routed independent of others– Uses “traffic engineering” to identify routes– Protection from node/link failure is on a per-LSP basis

Uses faster label swapping in place of routing Provides for a stack of labels, to allow tunnels to be built within tunnels

IP routing

Ingress router

Egress router

LSP

IP routing

MPLSMPLS


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Packet SchedulingPacket Scheduling

Queuing– Hogging, no way to ensure QoS

Fair queuing– Everyone gets the same share

Weighted fair queuing– Fair queuing with priority


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Differentiated servicesDifferentiated services

A simpler approach– No initial set up– No per-flow information– Defines several “types of services”

Expedited forwarding Assured forwarding Etc.

Classification of packets, based on– SRC, DST addresses and port nos.– “type of service” byte in IP packet header (actually 6 bits)

Once classified, traffic may still be subject to policing, marking Once classified, packets are treated differently


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Expedited forwarding– May be implemented using two separate queues, with say 20%

bandwidth reserved for expedired traffic


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Assured forwarding– Different levels of priority– Different drop probability for each “class”


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Interconnected NetworksInterconnected Networks

Internet is the prime example Enterprise networks, that connect into the Internet


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Interconnected NetworksInterconnected Networks

Interconnected networks differ from each other several different ways:


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IP ProtocolIP Protocol

Internet Protocol (IP) is the glue– It facilitates packets to be transported across different types of

networks, from source host to destination host


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IP addressingIP addressing

32 bit IP address == network address + host address– This is so in IPv4

Different classes of of networks– Classes A, B, C


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Several IP addresses are reserved, and have specific meaning, pre-assigned to them


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Subnets split a network into subnets for two different departments/labs, or 10.20.3.0 and 10.20.4.0

– or


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The notion of “Mask”

Consider IP address 194.24.17.4 in Oxford:

it is AND-ed with mask of Cambridge, Edinburgh and of Oxford it matches only with Oxford base address. Longer matches are also tried.

Mask for Cambridge = 255.255.248.0

Mask for Oxford = 255.255.240.0

Mask for Edinburgh = 255.255.252.0

11100 0010 0001 1000 0001 0000 0000 0000,

Or 194.24.16.0


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IP packet formatIP packet format

version of the IP protocol

IP header length in 32 bit words

used for DiffServ length of header + payload

Unique packet id “do not fragment” “more fragments”Specified in terms of “no of 8 bytes”


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IP packet fragmentationIP packet fragmentation

Basic principle


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IP packet formatIP packet format

Helps to limit the no. of hops or time spent in the

network

Protocol used to generate the payload (TCP, UDP etc.)

16 bit checksum, covers header only

Source IP address Destination IP addressOptional information, such as source route


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Internet control protocolsInternet control protocols

Several protocols:– ARP, RARP (these are discussed later)– ICMP

Several messages, including “echo” and “echo-reply” used to “ping” hosts

These are encapsulated inside an IP packet


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ARP protocolARP protocol

ARP protocol: “address resolution protocol”– IP address Data-link (or physical) address– This is distinct from”domain-name” IP address problem


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ARP protocol:– ARP-REQ ARP-REPLY packets

ARP-REQ is broadcast over local subnet only

– Destination IP address Ethernet address is cached by source, once a reply is received

– The destination also caches similar info about the source


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Consider H1 to H4 communication– H1 issues an ARP-REQ, to which CS router responds with its E3

address– CS router issues an ARP-REQ on FDDI ring, to which EE router

responds with its F3 address– EE router issues an ARP-REQ on the Ethernet, to which H4

responds with its E6 address


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ARP protocol: packet formatARP protocol: packet format


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RARP protocolRARP protocol

ARP gives IP-addr Physical-addr RARP solves the problem of “what is my IP address”?

– A problem that occurs in disk-less workstations, that have no disk resident OS

RARP-REQ issued by client, while RARP-REPLY is sent by RARP server

Need a RARP server for each network separated by a router Need to have entries for each IP-addr IP address Both problems solved using DHCP protocol


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Computer Networks, Jan-May 20041 Computer Networks (Lecture 5: Network Layer Protocols ) Arzad...

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