CSCD 330 Network Programming Network Superhighway Spring 2018 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
Introduction So far, have looked at 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 Network We are Here 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 AS 45 AS 123 AS 678
Autonomous Systems Example AS s AS # 701 1239 3356 7018 209 3935... Provider UUnet (U.S. ) (AS 701-705) Sprintlink U.S. Domestic Level 3 AT&T WorldNet Qwest 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 Diagram of Differences Network vs. Transport application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network network data link data link physical physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical 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 routing algorithm Create Routing Table All routers have routing tables local forwarding table header value output link 0100 0101 0111 1001 Specify next hop in route 3 2 2 1 Destination value in packet s header 0111 1 3 2 Built by routing algorithms 15
IP Datagram Delivery View at Data link Layer Internetwork collection of LANs or point-to-point links or switched networks that are connected by routers R1 R2 Point-to-point link Ted Ed H2 Network of Ethernet switches Ethernet IP R3 H1 Ethernet R4 Token Ring LAN 16
IP Datagram Delivery View at IP Layer 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 R2 R1 10.2.1.0/24 20.2.1.0/28 H2 10.1.2.0/24 IP 10.1.0.0/24 H1 20.1.0.0/16 10.3.0.0/16 R3 R4 17
Routing tables 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 IP datagrams can be directly delivered ( direct ) to connected networks or sent to another router ( R4 ) Routing Table Example Destination Next Hop interfac e 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 direct direct R4 direct R4 R4 eth0 eth0 serial0 eth1 eth0 eth0 18
Delivery with routing tables D e s tin a tio n 1 0.1.0.0 /2 4 1 0.1.2.0 /2 4 1 0.2.1.0 /2 4 1 0.3.1.0 /2 4 2 0.2.0.0 /1 6 3 0.1.1.0 /2 8 N ext H op R3 d ire c t d ire c t R3 R2 R2 D e s tin a tio n 1 0.1.0.0 /2 4 1 0.1.2.0 /2 4 1 0.2.1.0 /2 4 1 0.3.1.0 /2 4 2 0.1.0.0 /1 6 2 0.2.1.0 /2 8 N ext H op R1 R1 d ire c t R4 d ire c t d ire c t D e s tin a tio n 1 0.1.0.0 /2 4 1 0.1.2.0 /2 4 1 0.2.1.0 /2 4 1 0.3.1.0 /2 4 2 0.1.0.0 /1 6 2 0.2.1.0 /2 8 N ext H op R2 R2 R2 R2 R2 d ire c t R2 R1 1 0.2.1.0 /2 4 2 0.2.1.0 /2 8 H2 1 0.1.2.0 /2 4 to: 20.2.1.2 1 0.1.0.0 /2 4 H1 D e s tin a tio n 1 0.1.0.0 /2 4 1 0.1.2.0 /2 4 1 0.2.1.0 /2 4 1 0.3.1.0 /2 4 2 0.1.0.0 /1 6 2 0.2.1.0 /2 8 N ext H op d ire c t R3 R3 R3 R3 R3 2 0.2.1.2 /2 8 2 0.1.0.0 /1 6 1 0.3.0.0 /1 6 R3 D e s tin a tio n 1 0.1.0.0 /2 4 1 0.1.2.0 /2 4 1 0.2.1.0 /2 4 1 0.3.1.0 /2 4 2 0.1.0.0 /1 6 2 0.2.1.0 /2 8 R4 N ext H op d ire c t d ire c t R4 d ire c t R4 R4 D e s tin a tio n 1 0.1.0.0 /2 4 1 0.1.2.0 /2 4 1 0.2.1.0 /2 4 1 0.3.1.0 /2 4 2 0.1.0.0 /1 6 2 0.2.1.0 /2 8 N ext H op R3 R3 R2 d ire c t d ire c t R2 19
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 1. Send data physical application transport 2. Receive data network data link physical 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 ports Switching fabric A network inside the router that transfers packets between ports Routing processor Brain of the router...maintains lookup tables Switching Fabric Routing Processor 31
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 IP Address 1M prefixes Off-chip DRAM Update Header Queue Packet Data Hdr Next Hop Address Address Table Table Buffer Buffer Memory Memory 1M packets Off-chip DRAM 33
Input Port Functions Physical layer: bit-level reception Data link layer: Ethernet Process packet up to network layer Input Port Function: Given datagram destination lookup output port using forwarding table in input port memory 34
Input Port Functions Needs to perform lookup at line speed 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... 36
Three types of switching fabrics 2. 1. 3. Relate to Router Evolution 37
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 Memory Output Port System Bus CISCO Catalyst 8500's 38
First Generation Routers One route table Off-chip Buffer Shared Bus CPU L CP I ine U nte rfa ce M em or y Route Table Buffer Memory Line Interface Line Interface Line Interface MAC MAC MAC Typically <0.5Gb/s aggregate capacity 39
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!!!! 40
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 41
Second Generation Routers CPU Route Table Buffer Memory Shared bus Line Card Line Card Line Card Buffer Memory Buffer Memory Buffer Memory Fwding Cache Fwding Cache Fwding Cache MAC MAC MAC Typically <5Gb/s aggregate capacity Buffer Memory 42
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 43
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 44
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 45
Third Generation Routers Crossbar : Switched Backplane Li I CPnt ne Uerf ac e M em or y Line Card CPU Card Line Card Local Buffer Memory Routing Table Local Buffer Memory Fwding Table Fwding Table MAC MAC Typically <50Gb/s aggregate capacity 46
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 47
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 Input 2 Output 1 Output 2 Output 3 Input 3 54
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 - use parallelism to switch packets 55
Reading: Chapter 4 - Network Layer 56