1-1. Switching Networks (Fall 2010) EE 586 Communication and. October 25, Lecture 24

Transcription

1 EE 586 Communication and Switching Networks (Fall 2010) Lecture 24 October 25,

2 Announcements Midterm 1: Mean = 92.2 Stdev = 8 Still grading your programs (sorry about the delay) Network Layer 4-2

3 Router Architecture Overview Two key router functions: run routing algorithms/protocol forwarding datagrams from incoming to outgoing link Linksys Broadband Wireless Router $59.99 Cisco Terabit Router $210,

4 Three types of switching fabrics 1-4

5 Switching Via Memory First generation routers: traditional computers with switching under direct control of CPU packet copied to system s memory speed limited by memory bandwidth (2 bus crossings per datagram) Input Port Memory Output Port System Bus 1-5

6 Switching Via a Bus datagram from input port memory to output port memory via a shared bus bus contention: switching speed limited by bus bandwidth 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers 1-6

7 Switching Via An Interconnection Network overcome bus bandwidth limitations Cross-bar consisting of 2n buses connected n input ports to n output ports advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. Cisco 12000: switches 60 Gbps through the interconnection network 1-7

8 Output Ports Buffering required when datagrams arrive from fabric faster than the transmission rate Expensive input rate = switching rate >> line rate Scheduling discipline chooses among queued datagrams for transmission First-In First-Out Weighted Queuing based on flow label (more later) 1-8

9 Output port queueing buffering when arrival rate via switch exceeds output line speed queueing (delay) and loss due to output port buffer overflow! 1-9

10 Input Port Functions Physical layer: bit-level reception Data link layer: e.g., Ethernet see chapter 5 Decentralized switching: given datagram dest., lookup output port using forwarding table in input port memory goal: complete input port processing at line speed queuing: if datagrams arrive faster than forwarding rate into switch fabric 1-10

11 Input Port Queuing Fabric slower than input ports combined -> queueing may occur at input queues Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward -> loss of efficiency queueing delay and loss due to input buffer overflow! 1-11

12 The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP Network layer Routing protocols path selection RIP, OSPF, BGP forwarding table IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router signaling Link layer physical layer Network Layer 4-12

13 IPv4 datagram format IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each router) 32 bits head. type of ver length len service 16-bit identifier flgs time to live upper layer fragment offset header checksum 32 bit source IP address total datagram length (bytes) for fragmentation/ reassembly upper layer protocol to deliver payload to how much overhead with TCP? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) E.g. timestamp, record route taken, specify list of routers to visit. Network Layer 4-13

14 IP Fragmentation & Reassembly (IPv4 only) network links have MTU (max.transfer size) - largest possible link-level frame. different link types, different MTUs fragmentation: in: one large datagram out: 3 smaller datagrams large IP datagram divided ( fragmented ) within net reassembly one datagram becomes several datagrams reassembled only at final destination IP header bits used to identify, order related fragments Network Layer 4-14

15 IP Fragmentation and Reassembly Example 4000 byte datagram MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 (always in 8- byte chunk) length =4000 ID =x length =1500 length =1500 fragflag =0 ID =x ID =x ID =x offset =0 One large datagram becomes several smaller datagrams length =1040 fragflag =1 fragflag =1 fragflag =0 offset =0 offset =185 offset =370 Network Layer 4-15

16 IP Addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router s typically have multiple interfaces host typically has one interface IP addresses associated with each interface = Network Layer 4-16

17 Subnets IP address: subnet part (high order bits) host part (low order bits) What s a subnet? device interfaces with same subnet part of IP address can physically reach each other without intervening router subnet network consisting of 3 subnets Network Layer 4-17

18 Subnets Recipe To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet / / /24 Subnet mask: /24 Network Layer 4-18

19 Subnets How many? Network Layer 4-19

20 IP addressing: CIDR CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet part host part /23 Network Layer 4-20

21 IP addresses: how to get one? Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes Q: How does a host get IP address? hard-coded by system admin in a file Windows: control-panel->network->configuration->tcp/ip- >properties UNIX: /etc/rc.config : Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play Network Layer 4-21

22 : Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network server when it joins network Can renew its lease on address in use Allows reuse of addresses (only hold address while connected an on ) Support for mobile users who want to join network (more shortly) Beside an IP address, the client may also request: address of first-hop router for client name and IP address of DNS sever network mask (indicating network versus host portion of address) Network Layer 4-22

23 client-server scenario A server B E arriving client needs address in this network Network Layer 4-23

24 : example UDP IP Eth Phy UDP IP Eth Phy router (runs ) connecting laptop needs its IP address, addr of firsthop router, addr of DNS server: use request encapsulated in UDP, encapsulated in IP, encapsulated in Ethernet Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running server Ethernet demux ed to IP demux ed, UDP demux ed to Network Layer 4-24