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COMPUTER NETWORKS

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MODULE-3

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Dept. of ISE

The Network Layer: Network Layer Design Issues, Routing Algorithms, Congestion Control Algorithms, QoS. ��Textbook 1: Ch 5.1 to 5.4 ��

CO achieved from this module: Identify and organize the communication system network components ���

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THE NETWORK LAYER

The main function is to transfer network packets from the source to the destination.

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Network Layer Design Issues

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  1. Store and Forward Packet Switching
  2. Services Provided to the Transport Layer.
  3. Implementation of Connectionless Service.
  4. Implementation of Connection oriented Service.
  5. Comparison of virtual circuit and Datagram Networks.

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Store and Forward Packet Switching

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  • The major components of the network are the ISP’s equipment (routers connected by transmission lines)
  • A host transmits a packet it to the nearest router, either on its own LAN or over a point-to-point link to the ISP.
  • The packet is stored there until it has fully arrived and the link has finished its processing by verifying the checksum.
  • Then it is forwarded to the next router along the path until it reaches the destination host, where it is delivered.
  • This mechanism is store-and-forward packet switching.���

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Services Provided to the Transport Layer

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1. The services should be independent of the router technology.�2. The transport layer should be shielded from the number, type, and topology of the routers present.�3. The network addresses made available to the transport layer should use a uniform numbering plan, even across LANs and WANs. �

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  • One camp (represented by the Internet community) argues that the routers’ job is moving packets around and nothing else.
  • This viewpoint leads to the conclusion that the network service should be connectionless, with primitives SEND PACKET and RECEIVE PACKET and little else end-to-end argument, a design principle that has been very influential in shaping the Internet.

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  • The other camp (represented by the telephone companies) argues that the network should provide a reliable, connection-oriented service.

Services Provided to the Transport Layer

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Implementation of Connectionless Service

  • If connectionless service is offered,
    • packets are injected into the network individually
    • routed independently of each other.
    • No advance setup is needed.
    • In this context, the packets are referred as datagrams
    • the network is called a datagram network.
  • If connection-oriented service is used,
    • First a path must be established from source router to destination router . This connection is called a VC.
    • Data packets can be sent over VC.

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  • ���

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  • The algorithm that manages the tables and makes the routing decisions is called the routing algorithm.
  • IP (Internet Protocol), is the dominant example of a connectionless network service.
  • Each packet carries a destination IP address that routers use to individually forward each packet.
  • The addresses are 32 bits in IPv4 packets and 128 bits in IPv6 packets.

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Implementation of Connection oriented service

For connection-oriented service, we need a virtual-circuit network.

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Label Switching

  • EG of a connection-oriented network service is MPLS (MultiProtocol Label Switching).
  • It is used within ISP networks in the Internet, with IP packets wrapped in an MPLS header having a 20-bit connection identifier or label.
  • MPLS is often hidden from customers, with the ISP establishing long-term connections for large amounts of traffic, but it is increasingly being used to help when quality of service is important but also with other ISP traffic management tasks.

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Routing Algorithms

  • Function of network layer - route packets from the source to destination machine.
  • The routing algorithm is part of network layer software that is responsible for deciding which output line an incoming packet should be transmitted on.
  • Session Routing: Route remains in force for an entire session.
  • Forwarding: One of them handles each packet as it arrives, looking up the outgoing line to use for it in the routing tables. This process is forwarding.

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Properties of routing algorithm : correctness, simplicity, robustness, stability, fairness, and efficiency.

Goal: Stability.

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Routing Types

  • Non Adaptive Algorithms
    • routing decisions are not based on any measurements or estimates of the current topology and traffic.
    • Static Routing: Route is computed in advance and downloaded to the routers when network is booted.
  • Adaptive Algorithms
    • change routing decisions to reflect changes in the topology and changes in the traffic as well.
    • Dynamic Routing: Differ in where they get their information when they change the route and what metric is used for optimization

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The Optimality Principle

  • It states that 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. ��� � �

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  • Sink tree: the set of optimal routes from all sources to a given destination form a tree rooted at the�destination. Such a tree is called a sink tree.
  • It does not contain any loops, so each packet will be delivered within a finite and bounded number of hops.
  • DAG (Directed Acyclic Graph): DAGs have no loops. ��� � � � �

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Shortest Path Algorithm

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Flooding

  • When a routing algorithm is implemented, each router must make decisions based on local knowledge, not the complete picture of the network. A simple local technique is flooding, in which every incoming packet is sent out on every outgoing line except the one it arrived on.
  • Flooding with a hop count can produce an exponential number of duplicate packets as the hop count grows and routers duplicate packets they have seen before. ��

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  • Flooding is not practical for sending most packets.
  • Important Uses
  • First, it ensures that a packet is delivered to every node in the network.
  • Second, flooding is tremendously robust. � �

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Distance Vector Routing

A distance vector routing algorithm operates by having each router maintain a table (i.e., a vector) giving the best known distance to each destination and which link to use to get there.

The distance vector routing algorithm is sometimes called by other names, most commonly the distributed Bellman-Ford routing algorithm, � �

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The Count-to Infinity Problem

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Link State Routing

  • Variants of link state routing called IS-IS and OSPF are the routing algorithms that are most widely�used inside large networks and the Internet today.
  • The idea behind link state routing is fairly simple and can be stated as five parts.

1. Discover its neighbors and learn their network addresses.�2. Set the distance or cost metric to each of its neighbors.�3. Construct a packet telling all it has just learned.�4. Send this packet to and receive packets from all other routers.�5. Compute the shortest path to every other router. ���

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Learning about the Neighbors

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Setting the Link Costs:

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  • The link state routing algorithm requires each link to have a distance or cost metric for finding shortest paths.
  • The cost to reach neighbors can be set automatically, or configured by the network operator. �

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Building Link State Packets

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Distributing the Link State Packets

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Computing the New Routes

  • Link state routing is widely used in actual networks, so a few words about some example protocols are in order.
  • Many ISPs use the IS-IS (Intermediate System-Intermediate System) link state protocol (Oran, 1990).
  • It was designed for an early network called DECnet, later adopted by ISO for use with the OSI�protocols and then modified to handle other protocols as well, most notably, IP.
  • OSPF (Open Shortest Path First) is the other main link state protocol. �

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Hierarchical Routing

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Broadcast Routing

Sending a packet to all destinations simultaneously is called broadcasting.

An improvement is multidestination routing, in which each packet contains either a list of destinations or a bit map indicating the desired destinations.

Reverse Path Forwarding��

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Spanning Tree:

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A spanning tree is a subset of the network that includes all the routers but contains no loops. Sink trees are spanning trees. If each router knows which of its lines belong to the spanning tree, it can copy an incoming broadcast packet onto all the spanning tree lines except the one�it arrived on. �

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Multicast Routing

Sending a message to such a group is called multicasting, and the routing algorithm used is called multicast routing �

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  • An alternative design uses core-based trees to compute a single spanning tree for the group
  • All of the routers agree on a root (called the core or rendezvous point) and build the tree by sending a packet from each member to the root. �

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Anycast Routing

  • In anycast, a packet is delivered to the nearest member of a group.
  • Schemes that find these paths are called anycast routing. �

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Routing for Mobile Hosts

  • Mobile hosts to mean either category, as distinct from stationary hosts that never move.
  • All hosts are assumed to have a permanent home location that never changes.
  • Home Agents: The basic idea used for mobile routing in the Internet and cellular networks is for the mobile host to tell a host at the home location where it is now. This host, which acts on behalf of the mobile host, is called the home agent.
  • Care of Address: The local address is called care of address�� �

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  • Tunneling

encapsulates the packet with a new header and sends this bundle to the care of address (step 3). This mechanism is called tunneling.

When the encapsulated packet arrives at the care of address, the mobile host unwraps it and retrieves the packet from the sender. The mobile host then sends its reply packet directly to the sender (step 4). The overall route is called triangle routing because it may be circuitous if the remote location is far from the home location. ��

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Routing in AdHoc Networks

Networks of nodes that just happen to be near each other are called ad hoc networks or MANETs (Mobile Ad hoc NETworks).

AODV- Adhoc On Demand Distance Vector Routing

Route Discovery

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Congestion Control Algorithms

Too many packets present in (a part of) the network causes packet delay and loss that degrades performance. This situation is called congestion.

The network and transport layers share the responsibility for handling congestion.

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Congestion Collapse:

Performance plummets as the offered load increases beyond the capacity. This can happen because packets can be sufficiently delayed inside the network that they are no longer useful when they leave the network.

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Goodput:

The rate at which useful packets are delivered by the network. ��

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Approaches to Congestion Control

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Provisioning

Links and routers that are regularly heavily utilized are upgraded at the earliest opportunity. This is called provisioning.

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Some local radio stations have helicopters flying around their cities to report on road congestion to�make it possible for their mobile listeners to route their packets (cars) around hotspots. This is called traffic-aware routing. ��

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In a virtual-circuit network, new connections can be refused if they would cause the network to become congested. This is called admission control.

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When all else fails, the network is forced to discard packets that it cannot deliver. The general name for this is load shedding. ��

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Traffic- Aware Routing

  • The goal in taking load into account when computing routes is to shift traffic away from hotspots that will be the first places in the network to experience congestion.� �

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  • Traffic-aware routing was used in the early Internet.
  • In the Internet routing protocols do not generally adjust their routes depending on the load. Instead, adjustments are made outside the routing protocol by slowly changing its inputs. This is called traffic engineering. � �

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Admission Control

  • One technique that is widely used in virtual-circuit networks to keep congestion at bay is admission control.
  • The idea is simple: do not set up a new virtual circuit unless the network can carry the added traffic without becoming congested.
  • Traffic is often described in terms of its rate and shape.
  • Leaky Bucket: A leaky bucket has two parameters that bound the average rate and the instantaneous burst size of traffic. �Admission control can also be combined with traffic-aware routing by considering routes around traffic hotspots as part of the setup procedure. �������

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Traffic Throttling

  • The term congestion avoidance is sometimes used to contrast this operating point with the one in which the network has become (overly) congested.
  • The queueing delay inside routers directly captures any congestion experienced by packets. It should be low most of time, but will jump when there is a
  • dnew = αd old + (1 - α)s�where the constant α determines how fast the router forgets recent history. This is called an EWMA (Exponentially Weighted Moving Average). It smoothes out fluctuations and is equivalent to a low-pass filter.�������

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Choke packets

  • The router selects a congested packet and sends a choke packet back to the source host, giving it the destination found in the packet.
  • The original packet may be tagged (a header bit is turned on) so that it will not generate any more choke packets farther along the path and then forwarded in the usual way.
  • When the source host gets the choke packet, it is required to reduce the traffic sent to the specified destination � ��

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Explicit Congestion Notification

  • When the network delivers the packet, the destination can note that there is congestion and inform the sender when it sends a reply packet. The sender can then throttle its transmissions as before.�This design is called ECN (Explicit Congestion Notification) and is used in the Internet.

​

�

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  • Hop By Hop BackPressure

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Load Shedding� is a fancy way of saying that when routers are being inundated by packets that they cannot handle, they just throw them away.

The former policy (old is better than new) is often called wine and the latter (new is better than old) is often called milk because most people would rather drink new milk and old wine than the alternative. ��

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��

Random Early Detection

  • To determine when to start discarding, routers maintain a running average of their queue lengths.
  • When the average queue length on some link exceeds a threshold, the link is said to be congested and a small fraction of the packets are dropped at random.
  • Picking packets at random makes it more likely that the fastest senders will see a packet drop; this is the best option since the router cannot tell which source is causing the most trouble in a datagram network.
  • RED routers improve performance compared to routers that drop packets only when their buffers are full, though they may require tuning to work well. ��

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Quality of Service

  • An easy solution to provide good quality of service is to build a network with enough capacity for whatever traffic will be thrown at it. The name for this solution is overprovisioning.
  • Four issues must be addressed to ensure quality of service:�1. What applications need from the network.�2. How to regulate the traffic that enters the network.�3. How to reserve resources at routers to guarantee performance.�4. Whether the network can safely accept more traffic. � �

​

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Application Requirements

  • A stream of packets from a source to a destination is called a flow.
  • A flow might be all the packets of a connection in a connection-oriented network, or all the packets sent from one process to another process in a connectionless network.
  • Each flow can be characterized by 4 primary parameters: bandwidth, delay , jitter and loss.
  • The applications differ in their bandwidth needs, with email, audio in all forms, and remote login not needing much, but file sharing and video in all forms needing a great deal. �� �

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Delay: If all packets are delayed uniformly by a few seconds, no harm is done. �Interactive applications, such as Web surfing and remote login, are more delay sensitive. �The variation (i.e., standard deviation) in the delay or packet arrival times is called jitter.

​

QoS supports:

1. Constant bit rate (e.g., telephony).�2. Real-time variable bit rate (e.g., compressed videoconferencing).�3. Non-real-time variable bit rate (e.g., watching a movie on demand).�4. Available bit rate (e.g., file transfer). � ��

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Traffic Shaping

Traffic shaping is a technique for regulating the average rate and burstiness of a flow of data that enters the network. The goal is to allow applications to transmit a wide variety of traffic that suits their needs, including some bursts, yet have a simple and useful way to describe the possible traffic patterns to the network.

SLA (Service Level Agreement), especially when it is made over aggregate flows and long periods of time, such as all of the traffic for a given customer. ��

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Leaky and Token Buckets

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  • Leaky Bucket Algorithm

​

  • Each host is connected to the network by an interface containing a leaky bucket.
  • To send a packet into the network, it must be possible to put more water into the bucket.
  • If a packet arrives when the bucket is full, the packet must either be queued until enough water leaks out to hold it or be discarded.
  • The former might happen at a host shaping its traffic for the network as part of the operating system.
  • The latter might happen in hardware at a provider network interface that is policing traffic entering the network .�

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Token Bucket Algorithm

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  • The tap is running at rate R and the bucket has a capacity of B, as before.
  • Now, to send a packet we must be able to take water, or tokens, as the contents are commonly called, out of the bucket (rather than putting water into the bucket).
  • No more than a fixed number of tokens, B, can accumulate in the bucket, and if the bucket is empty, we must wait until more tokens arrive before we can send another packet �

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Token Bucket Algorithm

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Packet Scheduling

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Algorithms that allocate router resources among the packets of a flow and between competing flows are called packet scheduling algorithms.

Three different kinds of resources can potentially be reserved for different flows:�1. Bandwidth.�2. Buffer space.�3. CPU cycles. �

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Packet Scheduling

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Bandwidth:

If a flow requires 1 Mbps and the outgoing line has a capacity of 2 Mbps, trying to direct three flows through that line is not going to work. Thus, reserving bandwidth means not oversubscribing any output line.

Buffer Space:

When a packet arrives, it is buffered inside the router until it can be transmitted on the chosen�outgoing line. The purpose of the buffer is to absorb small bursts of traffic as the flows contend with each other. If no buffer is available, the packet has to be discarded since there is no place to put it. ���

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Packet Scheduling

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CPU Cycles:

It takes router CPU time to process a packet, so a router can process only a certain number of packets per second.

​

Packet scheduling algorithms allocate bandwidth and other router resources by determining which of the buffered packets to send on the output line next.

Each router buffers packets in a queue for each output line until they can be sent, and they are sent in the same order that they arrived. This algorithm is known as FIFO (First-In First-Out), or equivalently FCFS (First-Come First-Serve). ���

��

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Packet Scheduling

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FIFO routers usually drop newly arriving packets when the queue is full. Since the newly arrived packet would have been placed at the end of the queue, this behavior is called tail drop.

Fair Queuing Algorithm

  • The essence of this algorithm is that routers have separate queues, one for each flow for a given output line. When the line becomes idle, the router scans the queues round-robin.
  • It then takes the first packet on the next queue.
  • In this way, with n hosts competing for the output line, each host gets to send one out of every n packets.
  • It is fair in the sense that all flows get to send packets at the same rate. Sending more packets will not improve this rate. ������

��

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Packet Scheduling

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Round Robin Fair Queuing

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������

��

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Packet Scheduling

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Weighted Fair Queuing

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Packet Scheduling

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Weighted Fair Queuing

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  • If a packet has length L, the round at which it will finish is simply L rounds after the start time.
  • The start time is either the finish time of the previous packet, or the arrival time of the packet, if the queue is empty when it arrives. �

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Packet Scheduling

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Weighted Fair Queuing

​

From the table in Fig. 5-32(b), and looking only at the first two packets in the top two queues, packets arrive in the order A, B, D, and F. Packet A arrives at round 0 and is 8 bytes long, so its finish time is round 8. Similarly the finish time for packet B is 11. Packet D arrives while B is being sent. Its finish time is 9 byte-rounds after it starts when B finishes, or 20. Similarly, the finish time for F is 16. In the absence of new arrivals, the relative sending order is A, B, F, D, even though F arrived after D. It is possible that another small packet will arrive on the top flow and obtain a finish time before D. It will only jump ahead of D if the transmission of that packet has not started. ��

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Packet Scheduling

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Weighted Fair Queuing

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  • WFQ requires that packets be inserted by their finish time into a sorted queue.
  • With N flows, this is at best an O(logN) operation per packet, which is difficult to achieve for many flows in high-speed routers. �

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Admission Control

​

  • QoS guarantees are established through the process of admission control.
  • The guarantees we are considering now are stronger, but the model is the same.
  • The user offers a flow with an accompanying QoS requirement to the network.
  • The network then decides whether to accept or reject the flow based on its capacity and the commitments it has made to other flows.
  • If it accepts, the network reserves capacity in advance at routers to guarantee QoS when traffic is sent on the new flow. � �

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Admission Control

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  • QoS Routing
  • The reservations must be made at all of the routers along the route that the packets take through the network.
  • Any routers on the path without reservations might become congested, and a single congested router can break the QoS guarantee.
  • Many routing algorithms find the single best path between each source and each destination and send all traffic over the best path.
  • This may cause some flows to be rejected if there is not enough spare capacity along the best path. QoS guarantees for new flows may still be accommodated by choosing a different route for the flow that has excess capacity. This is called QoS routing. �

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Admission Control

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  • Flow Specification

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Admission Control

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  • Poisson Distribution

ρ=λ/μ is the CPU utilization.

The first factor, 1/μ, is what the service time would be in the absence of competition.

The second factor is the slowdown due to competition with other flows

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Admission Control

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Relation of flow Specification to the Router

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Integrated Services

​

Architecture for streaming multimedia.

It was aimed at both unicast and multicast applications.

​

RSVP- The Resource reservation Protocol

The main part of the integrated services architecture that is visible to the users of the network is RSVP.

It is described in RFCs 2205–2210.

This protocol is used for making the reservations; other protocols are used for sending the data.

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RSVP

RSVP allows multiple senders to transmit to multiple groups of receivers, permits individual receivers to switch channels freely, and optimizes bandwidth use while at the same time eliminating congestion.

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RSVP

​

​

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Differentiated Services

  • Flow-based algorithms have the potential to offer good quality of service to one or more flows because they reserve whatever resources are needed along the route.
  • IETF has also devised a simpler approach to quality of service, one that can be largely implemented locally in each router without advance setup and without having the whole path involved. This approach is known as class-based (as opposed to flow-based) quality of service. IETF has standardized an architecture for it, called differentiated services,

​

​

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Differentiated Services

  • Differentiated services can be offered by a set of routers forming an administrative domain (e.g., an ISP or a telco). The administration defines a set of service classes with corresponding forwarding rules.
  • If a customer subscribes to differentiated services, customer packets entering the domain are marked with the class to which they belong. This information is carried in the Differentiated services field of IPv4 and IPv6 packets.
  • The classes are defined as per hop behaviors because they correspond to the treatment the packet will receive at each router, not a guarantee across the network.

​

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Differentiated Services

  • Class-based service also occurs in other industries.
  • For example, package delivery companies often offer overnight, two-day, and three-day service. Airlines offer first class, business class, and cattle-class service. Long-distance trains often have multiple service classes

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Sri Krishna Institute of Technology

(Approved by AICTE, Accredited by NAAC, Affiliated to VTU, Karnataka)

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Expedited Forwarding

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Sri Krishna Institute of Technology

(Approved by AICTE, Accredited by NAAC, Affiliated to VTU, Karnataka)

/skit.org.in

/skit.org.in

/skit.org.in

Sri Krishna Institute of Technology

(Approved by AICTE, Accredited by NAAC, Affiliated to VTU, Karnataka)

1/30/2024

84

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Expedited Forwarding

The idea behind expedited forwarding is very simple.

Two classes of service are available: regular and expedited.

The vast majority of the traffic is expected to be regular, but a limited fraction of the packets are expedited.

The expedited packets should be able to transit the network as though no other packets were present

Sri Krishna Institute of Technology

(Approved by AICTE, Accredited by NAAC, Affiliated to VTU, Karnataka)

/skit.org.in

/skit.org.in

/skit.org.in

Sri Krishna Institute of Technology

(Approved by AICTE, Accredited by NAAC, Affiliated to VTU, Karnataka)

1/30/2024

85

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Assured Forwarding

  • Assured forwarding specifies that there shall be four priority classes, each class having its own resources.
  • The top three classes might be called gold, silver, and bronze.
  • In addition, it defines three discard classes for packets that are experiencing congestion: low, medium, and high.
  • Taken together, these two factors define 12 service classes.

Sri Krishna Institute of Technology

(Approved by AICTE, Accredited by NAAC, Affiliated to VTU, Karnataka)

/skit.org.in

/skit.org.in

/skit.org.in

Sri Krishna Institute of Technology

(Approved by AICTE, Accredited by NAAC, Affiliated to VTU, Karnataka)

1/30/2024

86

87 of 87

87

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Assured Forwarding

​

Sri Krishna Institute of Technology

(Approved by AICTE, Accredited by NAAC, Affiliated to VTU, Karnataka)

/skit.org.in

/skit.org.in

/skit.org.in

Sri Krishna Institute of Technology

(Approved by AICTE, Accredited by NAAC, Affiliated to VTU, Karnataka)

1/30/2024

87