Topics for Today. Network Layer. Readings. Introduction Addressing Address Resolution. Sections 5.1,

Transcription

1 Topics for Today Network Layer Introduction Addressing Address Resolution Readings Sections 5.1,

2 Network Layer: Introduction A network-wide concern! Transport layer Between two end hosts Data link layer Between two physically connected hosts Network layer Involves every router, host in the network 2

3 Network Layer Functions Addressing Globally unique address for each routable device Logical address, unlike MAC address Assigned by network operator Need to map to MAC address Forwarding From input port to appropriate output port in a router Routing Which path to use to forward packets from src to dest 3

4 Logical View of Router Architecture 4

5 Router Input/Output Processing 5

6 Internet Protocol (IP) Universal service in a heterogeneous world IP over everything Virtual overlay network Globally unique logical address for a host Address resolution logical to physical address mapping 6

7 IP Addressing A 32-bit number that uniquely identifies a location Written using dotted decimal notation Two-level hierarchy: network id and host id Network IDs administered by Internet Assigned Number Authority Host IDs administered locally 7

8 IP Addressing IP address is assigned to each network interface Routers connect two or more physical networks Each interface has its own address Multi-homed host A host having multiple connections to Internet Multiple addresses identify the same host Does not forward packets between its interfaces 8

9 IP Classful Addressing Scheme class Three unicast address classes: A, B, and C One multicast: class D A B C D 0 network host 10 network host 110 network host 1110 multicast address to to to to bits 9

10 Classless Inter-Domain Routing Classful addressing scheme wasteful IP address space exhaustion A class B net allocated enough for 65K hosts Even if only 2K hosts in that network Solution: CIDR Eliminate class distinction No A,B,C Keep multicast class D 10

11 Classless Addressing Addresses allocated in contiguous blocks Number of addresses assigned always power of 2 Network portion of address is of arbitrary length Address format: a.b.c.d/x x is number of bits in network portion of address network part /23 host part 11

12 IP Addressing first 24 bits are network address LAN

13 IP Addressing Interconnected system consisting of six networks

14 Special IP Addresses Network address: host id = all 0 s Directed broadcast address: host id = all 1 s Local broadcast address: all 1 s Local host address (this computer): all 0 s Loopback address network id = 127, any host id (e.g ) 14

15 Components of a Generic Router 15

16 Per Packet Processing in an IP router 1. Accept packet arriving on an incoming link. 2. Lookup packet destination address in the forwarding table, to identify outgoing port(s). 3. Manipulate packet header: e.g., decrement TTL, update header checksum. 4. Send packet to the outgoing port(s). 5. Buffer packet in the queue. 6. Transmit packet onto outgoing link. 16

17 Another View of an IP Router Routing Protocols Routing Control Plane Forwarding Switching Datapath per-packet processing 17

18 Basic Architectural Components 1. Ingress Datapath: per-packet processing Forwarding 2. Interconnect 3. Egress Forwarding Decision Forwarding Forwarding Decision Forwarding Forwarding Decision 18

19 Forwarding Engine payload Packet header Router Destination Address Routing Lookup Data Structure Outgoing Port Forwarding Dest-network / /16 Port /

20 Routing Lookup is Not an Easy Task Search operation is not an exact match Direct lookup needs 4G entries (32 bits IP address) Longest prefix match Hashing table Balanced tree 20

21 Number of Prefixes Size of the Forwarding ,000/year Year Source: 21

22 Lookup Rate Required Year Line Line-rate (Gbps) 40B packets (Mpps) OC12c OC48c OC192c OC768c Mpps 33 ns 22

23 Basic Architectural Components Admission Control Routing Protocols Routing Reservation Control Plane Packet Classification Policing & Access Control Forwarding Switching Output Scheduling Datapath per-packet processing Ingress Interconnect Egress

24 Basic Architectural Components 1. Ingress Datapath: per-packet processing 2. Interconnect 3. Egress Classifier Forwarding Policing & Access Control Forwarding Decision Classifier Forwarding Policing & Access Control Forwarding Decision Classifier Forwarding Policing & Access Control Forwarding Decision Limitation: Memory b/w Limitation: Interconnect b/w Power & Arbitration Limitation: Memory 24 b/w

25 First Generation Routers Shared Backplane Fixed length DMA blocks or cells. Reassembled on egress linecard CPU Route Buffer Memory Line Interface MAC Line Interface MAC Line Interface MAC Fixed length cells or variable length packets Typically <0.5Gb/s aggregate capacity 25

26 First Generation Routers Queueing Structure: Shared Memory Input 1 Output 1 Input 2 Numerous work has proven and made possible: Fairness Delay Guarantees Delay Variation Control Loss Guarantees Output 2 Statistical Guarantees Input N Large, single dynamically allocated memory buffer: N writes per cell time N reads per cell time. Limited by memory bandwidth. Output N 26

27 Second Generation Routers CPU Route Buffer Memory Slow Path Drop Policy Line Card Buffer Memory Line Card Buffer Memory Line Card Buffer Memory Drop Policy Or Backpressure Fwding Cache MAC Fwding Cache MAC Fwding Cache MAC Output Link Scheduling Typically <5Gb/s aggregate capacity 27

28 Second Generation Routers As caching became ineffective Exception Processor CPU Route Line Card Buffer Memory Fwding MAC Line Card Buffer Memory Fwding MAC Line Card Buffer Memory Fwding MAC 28

29 Second Generation Routers Queueing Structure: Combined Input and Output Queueing 1 write per cell time Rate of writes/reads 1 read per cell time determined by bus speed Bus 29

30 Third Generation Routers Switched Backplane Line Card Local Buffer Memory Fwding MAC CPU Card Routing Line Card Local Buffer Memory Fwding MAC Typically <50Gb/s aggregate capacity 30

31 Third Generation Routers Queueing Structure 1 write per cell time Rate of writes/reads 1 read per cell time determined by switch fabric speedup Switch Arbiter 31

32 Third Generation Routers Queueing Structure 1 write per cell time Rate of writes/reads 1 read per cell time determined by switch fabric speedup Switch Arbiter Flow-control backpressure Per-flow/class or peroutput queues (VOQs) Per-flow/class or perinput queues 32

33 Fourth Generation Routers/Switches Optical links 100 s of feet Switch Core Linecards 33

34 Physically Separating Switch Core and Linecards Distributes power over multiple racks. Allows all buffering to be placed on the linecard: Reduces power. Places complex scheduling, buffer mgmt, drop policy etc. on linecard. 34

35 Fourth Generation Routers/Switches The LCS Protocol Optical links 100 s of feet Switch Core Linecards 35

36 Fourth Generation Routers/Switches Queueing Structure Virtual Output Queues 1 write per cell time 1 read per cell time Rate of writes/reads determined by switch Lookup & Drop Policy fabric speedup Output Scheduling Lookup & Drop Policy Switch Fabric Switch Arbitration Output Scheduling Lookup & Drop Policy Switch Core (Bufferless) Output Scheduling Linecard Typically <5Tb/s aggregate capacity Linecard 36

37 Problems Facing Router Builders Fast forwarding (route lookup) Flow identification Fast forwarding path QoS support Core routers, enterprise routers, access routers Simpler is winner Reliability (stability) of high-speed routers Dual power Duplicate data paths Ease of configuration Misconfiguration is another major problem Accountability 37