CSCD 330 Network Programming Winter 2019 Lecture 13 Network Layer Network Superhighway Lecture 13 Network Layer Reading: Chapter 4 Some slides provided courtesy of J.F Kurose and K.W. Ross, All Rights Reserved, copyright 1996-2007 1 1

Introduction So far, have looked at Now, move down protocol stack to Application Layer Transport Layer Now, move down protocol stack to Network Layer What services are offered? How does this layer fit with other layers? Hardware is used at this layer? 2

Network Layer Goals Understand principles behind network layer Network layer service models Forwarding versus routing, difference How a router works - Internally Routing itself Addresses, paths, algorithms 3

TCP/IP Model Appliction Transport We are Here Network Data Link 4

Routing in the Internet 5

Hierarchical Routing - Solution Internet is huge, distributed system managed by mostly private, possibly competing corporations Each corporate entity is responsible for their own routing within their IP space So, they aggregate routers into regions ... Autonomous Systems (AS) Routers in same AS run same routing protocol 6

Autonomous System (AS)‏ What is an Autonomous System? Within Internet, an AS is Unit of router policy, either single network or group of networks controlled by common network administrator On behalf of single administrative entity An autonomous system is assigned globally unique number, sometimes called an Autonomous System Number (ASN) Report of current numbers http://www.cidr-report.org/as2.0/autnums.html 7

Routing Within and Between AS's

Autonomous Systems Example AS’s AS # Provider 701 UUnet (U.S. ) (AS 701-705)‏ 1239 Sprintlink U.S. Domestic 3356 Level 3 7018 AT&T WorldNet 209 Qwest 3935 Eastern Washington University . . . . 9

Autonomous Systems Have Routing policy for each AS Can decide what routing algorithms to use Typical to have different routing algorithms Interior to the AS Interior Gateway Protocols (IGP's)‏ Exterior networks between AS's Exterior Gateway Protocols (EGP's)‏

Network Layer Diagram shows at end points entire protocol stack implemented Need transport and applications layers for processes Routers only need layers up through network layer Data Link Network application transport network data link physical network data link physical application transport network data link physical Diagram of Differences Network vs. Transport 11

Two Key Network-Layer Functions Forwarding Move packets from router’s input queues to router's output queues Movement contained within one router Routing Determine routing for packets from source to destination Routing algorithms important in efficiently routing packets Movement of packets is distributed 12

Routing vs. Forwarding Analogy You take a road trip across country, go through many highway interchanges Routing: Process of planning trip from source to destination Forwarding: Process of getting through single interchange

How Routing Works … Every router has Forwarding Table Also called a Routing Table Router forwards packets by examining value in arriving packet header Destination IP Address Uses value to index into router’s forwarding table See following slide … 14

Interplay between Routing and Forwarding 1 2 3 0111 Destination value in packet’s header routing algorithm local forwarding table header value output link 0100 0101 1001 All routers have routing tables Specify next hop in route Create Routing Table Built by routing algorithms 15

IP Datagram Delivery View at Data link Layer Ed Ted IP Internetwork collection of LANs or point-to-point links or switched networks that are connected by routers Ed Ted IP

IP Datagram Delivery View at IP Layer IP An IP network is a logical entity with a network number We represent an IP network as a “cloud” The IP delivery service takes the view of clouds, and ignores data link layer view IP

Routing tables Routing Table Example Each router keeps a routing table tells the router how to process an outgoing packet Main columns Destination address: Where is IP datagram going to? Next hop: How to send the IP datagram? Interface: What is the output port? Routing tables help datagrams gets closer to their destination Routing table unique for each router Routing Table Example direct direct R4 direct R4 R4 Next Hop eth0 serial0 eth1 eth0 eth0 interface 10.1.0.0/24 10.1.2.0/24 10.2.1.0/24 10.3.1.0/24 20.1.0.0/16 20.2.1.0/28 Destination IP datagrams can be directly delivered (“direct”) to connected networks or sent to another router (“R4”)‏

Delivery with routing tables to: 20.2.1.2

Two Service Models at Network Layer What services could network layer provide? Guarantee delivery, Guarantee bandwidth, Order packets, Time delay guarantee … Given what you know of the Network layer, are any of these services implemented? No! Decided that network only does best effort delivery!! 20

Two Service Models at Network Layers Recall, we discussed Circuit Switched Network and Datagram Network However, you can try to emulate these services in network layer virtually Datagram service is what we typically have 1. Datagram service – best effort, no previous connection setup Virtual service model lets you obtain services other than best effort 2. Virtual Service – tries to compensate for lack of guaranteed service, sets up connections ahead of time Examples: ATM, Asynchronous Transfer Mode 21

Datagram Service Connectionless Unacknowledged Network Service Attitude to packets Characterized by “Send and forget!” Does not guarantee actual delivery Does not guarantee data is undamaged Does not guarantee data delivered in order Does not guarantee that only one copy of the data will be delivered 22

Virtual Service Sets Up ... Source-to-destination path ahead of time Uses that route to send all packets Behaves much like telephone circuit Call setup, teardown for each call before data can flow Each packet carries Virtual Circuit (VC) identifier, VC25, VC601 Not same as destination host address‏ Every router on source-destination path maintains “state” for each passing connection 23

Virtual Service Book discusses mechanics of a generic virtual service , Section 4.2 Have virtual identifiers instead of IP addresses Can run a virtual service over the Internet Routers must recognize the virtual identifiers Two Examples: ATM and MPLS Asynchronous Transfer Mode http://www2.rad.com/networks/infrastructure/atm/main.htm Multiprotocol Label Switching http://www2.rad.com/networks/infrastructure/ipmpls/main.htm

Datagram Networks Basics No call setup at network layer Routers: No state about end-to-end connections No network-level concept of “connection” Packets forwarded using destination host address Packets between same source-dest pair may take different paths application transport network data link physical application transport network data link physical 1. Send data 2. Receive data 25

Datagram Networks No advance setup of paths, link to link connections Need a global number recognized by all network components, hosts and routers How would you set up routing tables in order to accommodate … currently, over 4 billion entries? Question: Whats the best way to do this? 26

Routing Table Organization One dumb idea, global table 4 billion entries 232 But … didn't do that IP addresses assigned contiguous address blocks by region So, can use a matching algorithm and match the network prefix to route packets Have distributed address space More on addressing later ... 27

Routers 28

Routers Internals Routers are the “glue” that holds the Internet together Router speeds greatly affect how well traffic gets moved around the network Performance of routers turns out to be critical Today, look at evolution of routers and the speedups that have occurred

Routers Commercial Realities A router is sold as one big box Cisco, Juniper, Redback, Avici, … No standard interfaces between components Cisco switch, Juniper cards, and Avici software Vendors vs. Service providers Vendors: Build the routers and obey standards Providers: Buy the routers and configure them EWU is a provider

Routing Architectures A Router Consists of Ports Connections to wires to other network entities Switching fabric A “network” inside the router that transfers packets between ports Routing processor Brain of the router ...Maintains lookup tables ports Routing Processor Switching Fabric

Router Architecture Overview Routers do two important things Build and Maintain Routing Tables Performs Packet Switching and Updating Several input ports Several output ports 32

Generic Router Architecture Header Processing Data Hdr Lookup IP Address Update Header Data Hdr Queue Packet IP Address Next Hop Address Table Buffer Memory 1M prefixes Off-chip DRAM 1M packets Off-chip DRAM 33 33

Input Port Functions Input Port Function: Physical layer: bit-level reception Input Port Function: Given datagram destination lookup output port using forwarding table in input port memory Data link layer: Ethernet Process packet up to network layer 34

Input Port Functions Needs to perform lookup at line speed Example: Gbps in most networks Example: OC-48 link – runs at 2.5 Gbps (OC) Optical Carrier – SONET fiber optic network, different sizes If packets are 256 bytes – small packet Must lookup speeds of 1 Million lookups/sec Binary search typically done to speed things up Uses special tree structures And other methods ... more later 35

Switching Fabrics Switching fabric is heart of a router Through switching that datagrams are actually moved from an input port to an output port Switching can be accomplished in a number of ways ...

Three types of switching fabrics 1. 2. Relate to Router Evolution 3.

First Generation, Switching Via Memory First Generation Routers Traditional computers with switching under direct control of CPU, act as router Packet copied to system’s memory Speed limited by Memory Bandwidth 2 bus crossings per datagram, looks like shared memory multi-processors Input Port Output Memory System Bus CISCO Catalyst 8500's

First Generation Routers One route table Off-chip Buffer Shared Bus Route Table CPU Buffer Memory Line Interface MAC Typically <0.5Gb/s aggregate capacity CPU Memory Line Interface

First Generation Switching via Memory Comment Modern routers also switch via memory Difference from early routers Address lookup and packet switching in memory Performed by processors on input line cards Greatly speeds things up !!!!

2nd Generation Switching Via a Bus Use shared bus No intervention by routing processor Since bus is shared Only one packet at a time can be transferred over bus And since every packet must cross bus, switching bandwidth of router limited to bus speed Bus contention: Switching speed limited by bus bandwidth, one packet at a time Another speedup have buffer memory on input cards! Example: 1 Gbps bus, Cisco 1900: OK … for access and enterprise routers Not regional or backbone

Second Generation Routers CPU Route Table Buffer Memory Shared bus Line Card Line Card Line Card Buffer Memory Buffer Memory Buffer Memory Buffer Memory Fwding Cache Fwding Cache Fwding Cache MAC MAC MAC Typically <5Gb/s aggregate capacity

Third Generation Crossbar Switch A crossbar switch is a matrix of switches between inputs and outputs Overcomes bandwidth limitation of single, shared bus More sophisticated interconnection network Used in past to interconnect processors in multiprocessor computer architectures

Crossbar Switch Improvements Crossbar switch enables high performance for two reasons: First, connections from line cards to central switch are now simple point-to-point links Operate at very high speed. Semiconductor companies have developed chip-to-chip serial links operating at over 1 Gbps Second, can support multiple bus transactions simultaneously – Parallelizing the process This greatly increases aggregate bandwidth of the system

Crossbar Switch How it works Consists of 2N buses Connect - N Input ports to N Output ports Packet arrives at input port travels along horizontal bus attached to input port until it intersects with vertical bus leading to desired output port Key idea – parallel processing of packets If vertical bus leading to output port is free Packet is transferred to output port Else If vertical bus being used to transfer packet from another input port to same output port, Arriving packet is blocked and must be queued at the input port … More on queuing later … Paper on routers http://www.cs.cmu.edu/~srini/15-744/F02/readings/McK97.html

Third Generation Routers “Crossbar”: Switched Backplane Line Card CPU Card Line Card Local Buffer Memory Local Buffer Memory CPU Line Interface Routing Table Memory Fwding Table Fwding Table MAC MAC Typically <50Gb/s aggregate capacity

Output Ports Output port processing Datagrams then stored in output port's memory, transmits them over outgoing link Queuing and buffer management needed when switch fabric delivers packets to output port at a rate that exceeds output link rate Cover output port queuing below

Output Ports Here is where many packets get dropped encapsulation Buffering required when datagrams arrive from fabric faster than transmission rate Scheduling algorithm chooses among queued datagrams for transmission Can be simple FIFO or some fairness algorithm based on destination packet distribution 48

Where Does Packet Queue Occur? Consider That ... Packet queues can form at both input ports and output ports As queues grow large, router's buffer space will eventually be exhausted Packet loss will occur!!!

Input and Output Queues Question of Speeds Suppose that input line speeds and output line speeds are all identical, With n input ports and n output ports If switching fabric speed is at least n times as fast as the input line speed, Will queues form at input ports? NO.

Input and Output Queues What about output ports? Worst case, packets arrive at each n input ports will be destined to same output port In time it takes to receive (or send) a packet, n packets will arrive at this output port Since output port can only transmit a single packet in a unit of time (the packet transmission time), n arriving packets will have to queue (wait) for transmission over the outgoing link

Input and Output Queues If switch fabric is not fast enough to transfer all arriving packets through fabric without delay, then packet queuing will also occur at input ports If two packets at front of two input queues are destined to same output queue, then one packet will be blocked and must wait at input queue -

Input and Output Queues This phenomenon is known as Head-of-the-line (HOL) blocking On an input-queued switch Queued packet in an input queue must wait for transfer through fabric due to blocking of another packet at head-of-the-line What would be one solution?

Solution: Virtual Output Queues Maintain N virtual queues at each input one per output Input 1 Output 1 Output 2 Input 2 Output 3 Input 3

Summary Multiple things can be done to enhance router performance Input cards Have own CPU processors Use caches for address lookup Have copies of routing tables Switch fabric Cross Switches faster - u2013-22013-2018- 2013-2018-2013-2018-2013-2018-se parallelism to switch packets

Reading: Chapter 4 - Network Layer 56