CS 356: Computer Network Architectures. Lecture 14: Switching hardware, IP auxiliary functions, and midterm review. [PD] chapter 3.4.1, 3.2.

Transcription

1 CS 356: Computer Network Architectures Lecture 14: Switching hardware, IP auxiliary functions, and midterm review [PD] chapter 3.4.1, Xiaowei Yang

2 Switching hardware

3 Software switch Packets cross the bus twice Half of the memory bus speed 133Mhz, 64-bit wide I/O bus à 4Gpbs Short packets reduce throughput 1Mpps, 64 bytes packet Throughput = 512 Mbps Shared by 10 ports: 51.2Mbps

4 Hardware switches Ports communicate with the outside world Eg, maintains VC tables Switching fabric is simple and fast

5 Performance bottlenecks Input port Line speed: 2.48 Gbps 2.48x10 9 /(64x8) = 4.83 Mpps Buffering Head of line blocking May limit throughput to only 59% Use output buffers or sophisticated buffer management algorithms to improve performance

6 Fabrics Shared bus The workstation switch Shared memory Input ports read packets to shared memory Output ports read them out to links

7 Fabrics Cross bar Each output ports need to accept from all input ports

8 Fabrics Self routing a self-routing header added by the input port Most scalable Often built from 2x2 switching units

9 An example of self-routing 3-bit numbers are self-routing headers Multiple 2x2 switching elements 0: upper output; 1: lower output

10 Midterm Policy Up to March 1 s lecture Closed book/notes One page of your own note (letter-size) No Internet Calculator is allowed 75 mins

11 What s in the test Mastery of networking knowledge Application of networking knowledge

12 Network architectures Layering Encapsulation / decapsulation Multiplexing vs demultiplexing Connectionless vs connection oriented Internet architecture Statistical multiplexing Protocols Link, network, transport, and application layers Functions of layers Best effort service

13 Physical Layer Transmission delay Propagation delay Bandwidth Delay / bandwidth product Throughput How to keep a pipe full?

14 Link layer Coding/encoding NRZ, NRZI, Manchester, 4B/5B Framing Byte-oriented: sentinel, byte-counting Bit-oriented: bit-stuffing Clock-based framing Error detection Parity, checksum, CRC Reliable transmission Forward Error Correcion Stop and wait Sliding window Sequence number space vs window size Concurrent logical channels

15 Multi-access links Ethernet Link layer (cont.) Multi-access carrier sense with collision detection How to detect collision How to avoid collision Maximum segment length Minimum packet size WIFI How to avoid collision (MACA) Bluetooth Cellular Switching, bridges, ATM

16 Link layer (cont.) Switching technologies Datagram Virtual circuits How to set up Virtual circuit identifier Switching table ATM switches Source routing Learning bridges Address learning Spanning tree algorithm

17 Internet Protocol Classful vs classless IP addressing CIDR IP forwarding How to determine a host is on the same subnet Longest prefix lookup Fragmentation and assembly Path MTU discovery ARP What s used for? ICMP What s used for?

18 Dynamic routing protocols Routing information protocol (RIP) Distance vector algorithm Count to infinity How to alleviate to count to infinity Split horizon Reverse poisoning Path vector Open shortest path first Dijkstra Reliable flooding Forward search algorithm for efficiency

19 Dynamic Host Configuration Protocol (DHCP)

20 Dynamic Assignment of IP addresses Dynamic assignment of IP addresses is desirable IP addresses are assigned on-demand Avoid manual IP configuration Inconvenient, error prone ifconfig Support mobile devices

21 DHCP Dynamic Host Configuration Protocol (DHCP) Designed in 1993 Supports temporary allocation ( leases ) of IP addresses DHCP client can acquire all IP configuration parameters Default router, network mask, DNS resolver Sent as UDP packets A client-server protocol Server port: 67 Client port: 68 Most client-server protocols do not have unique client ports

22 OpCode DHCP Message Format Hardware Type Number of Seconds Transaction ID Client IP address Your IP address Server IP address Gateway IP address Client hardware address (16 bytes) Server host name (64 bytes) Boot file name (128 bytes) Options (There are >100 different options) Hardware Address Hop Count Length Unused (in BOOTP) Flags (in DHCP)

23 DHCP OpCode: 1 (Request), 2(Reply) Note: DHCP message type is sent in an option Hardware Type: 1 (for Ethernet) Hardware address length: 6 (for Ethernet) Hop count: set to 0 by client Transaction ID: Integer (used to match reply to response) Seconds: number of seconds since the client started to boot Client IP address, Your IP address, server IP address, Gateway IP address, client hardware address, server host name, boot file name: client fills in the information that it has, leaves rest blank

24 DHCP Message Type Message type is sent as an option. Value Message Type 1 DHCPDISCOVER 2 DHCPOFFER 3 DHCPREQUEST 4 DHCPDECLINE 5 DHCPACK 6 DHCPNAK 7 DHCPRELEASE 8 DHCPINFORM

25 DHCP operations Src: , 68 Dest: , 67 DHCPDISCOVERY Yiaddr: Transaction ID: 654 Src: , 67 Dest: , 68 DHCPOFFER Yiaddr: Transaction ID: 654 Server ID: Lifetime: 3600 secs

26 DHCP operations Src: , 68 Dest: , 67 DHCPREQUEST Yiaddr: Transaction ID: 655 server ID: Lifetime: 3600 secs Src: , 67 Dest: , 68 DHCPACK Yiaddr: Transaction ID: 655 Server ID: Lifetime: 3600 secs

27 More on DHCP operations A client may receive DCHP offers from multiple servers The DHCPREQUEST message accepts offers from one server Other servers who receive this message considers it as a decline A client can use its address after receiving DHCPACK DHCP replies can be unicast, depending on implementation Client hardware address as MAC destination Yiaddr as IP destination

28 Scalability How many DHCP servers do we need? Routers do not forward broadcast IP addresses One per subnetwork! Too many Solution: relay agent Configured with the DHCP server s IP address One relay agent per subnetwork Unicast to the DHCP server

29 DHCP relay agent Src: , 68 Dest: , 67 Giaddr: DHCPDISCOVER Src: , 68 Dest: , 67 Giaddr: 0 DHCPDISCOVER Src: , 67 Src: , 67 Dest: , 67 Dest: , 68 Giaddr: Giaddr: DHCPOFFER DHCPOFFER

30 Well-known client port Why does DHCP choose well-known client port? A: For relay purpose. Otherwise, the relay agent has to remember the port of the original DHCP discovery message.

31 History of DHCP Three Protocols: RARP (until 1985, no longer used) BOOTP ( ) DHCP (since 1993) Only DHCP is widely used today

32 Network Address Translation

33 Network address translation A fix to the IP address depletion problem. NAT is a router function where IP addresses (and possibly port numbers) of IP datagrams are replaced at the boundary of a private network We ll discuss another solution: IPv6

34 Basic operation of NAT Private network Internet Source = Destination = Source = Destination = private address: public address: NAT device public address: H1 Source = Destination = Source = Destination = H5 Private Address Public Address NAT device has address translation table

35 Private Network Private IP network is an IP network that is not directly connected to the Internet IP addresses in a private network can be assigned arbitrarily. Not registered and not guaranteed to be globally unique Public IP address are assigned via Internet registries Generally, private networks use addresses from the following experimental address ranges (non-routable addresses):

36 Main uses of NAT Pooling of IP addresses Supporting migration between network service providers IP masquerading Load balancing of servers

37 Pooling of IP addresses Scenario: Corporate network has many hosts but only a small number of public IP addresses NAT solution: Corporate network is managed with a private address space NAT device manages a pool of public IP addresses

38 Pooling of IP addresses Private network Internet H1 Source = Destination = private address: public address: NAT device Source = Destination = public address: H5 Private Address Public Address Pool of addresses:

39 Supporting migration between network service providers Scenario: In CIDR, the IP addresses in a corporate network are obtained from the service provider. Changing the service provider requires changing all IP addresses in the network. NAT solution: Assign private addresses to the hosts of the corporate network NAT device has address translation entries which bind the private address of a host to the public address. Migration to a new network service provider merely requires an update of the NAT device. The migration is not noticeable to the hosts on the network.

40 Supporting migration between network service providers

41 IP masquerading Also called: Network address and port translation (NAPT), port address translation (PAT). Scenario: Single public IP address is mapped to multiple hosts in a private network. NAT solution: Assign private addresses to the hosts of the corporate network NAT device modifies the port numbers for outgoing traffic

42 IP masquerading Source = Source port = 2001 Source = Source port = 2100 private address: H1 Private network NAT device Internet private address: H2 Source = Source port = 3020 Source = Source port = 4444 Private Address Public Address / / / /4444

43 Load balancing of servers Scenario: Balance the load on a set of identical servers, which are accessible from a single IP address Used by many distributed service providers such as Google NAT solution: Here, the servers are assigned private addresses NAT device acts as a proxy for requests to the server from the public network The NAT device changes the destination IP address of arriving packets to one of the private addresses for a server A sensible strategy for balancing the load of the servers is to assign the addresses of the servers in a round-robin fashion. Or hashing

44 Load balancing of servers Private network Source: Dest: Source: Dest: NAT device Inside network Source = Destination = Source = Destination = Public Address Outside network Public Address Internet

45 Concerns about NAT Performance: Modifying the IP header by changing the IP address requires that NAT boxes recalculate the IP header checksum Modifying port number requires that NAT boxes recalculate TCP checksum Fragmentation Care must be taken not to assign a fragment different IP or port number

46 Concerns about NAT End-to-end connectivity: NAT destroys universal end-to-end reachability of hosts on the Internet. A host in the public Internet often cannot initiate communication to a host in a private network. The problem is worse, when two hosts that are in a private network need to communicate with each other. Difficult to deploy peer-to-peer applications such as Skype

47 NAT and FTP FTP client public address: public address: FTP server H1 PORT /1027 H2 200 PORT command successful RETR myfile 150 Opening data connection establish data connection Normal FTP operation What problem will FTP run into if using unmodified NAT?

48 NAT and FTP Private network Internet FTP client private address: public address: NAT device FTP server H1 PORT /1027 PORT /1027 H2 200 PORT command successful 200 PORT command successful RETR myfile RETR myfile 150 Opening data connection 150 Opening data connection establish data connection establish data connection NAT device with FTP support

49 NAT and FTP Private network Internet FTP client private address: public address: NAT device public address: FTP server H1 PASV PASV H2 Entering Passive Mode /10001 Entering Passive Mode /10001 Establish data connection Establish data connection FTP in passive mode and NAT.

50 Midterm Policy Up to March 1 s lecture Closed book/notes One page of your own note (letter-size) No Internet Calculator is allowed 75 mins

51 What we ve learned Network architectures Basic concepts, Internet architecture, Physical layer Delay, bandwidth, and throughput Link layer Coding/encoding, framing, error detection, reliable transmission Multi-access links Switching, bridges, ATM

52 What we ve learned (cont.) Internetworking Challenges, solutions Classful vs classless IP addressing IP forwarding, longest prefix lookup, ARP DHCP Dynamic routing protocols Distance vector (RIP) Link state (OSPF)

53 Midterm Policy Up to March 1 s lecture Closed book/notes One page of your own note (letter-size) No Internet Calculator is allowed 75 mins

54 What we ve learned Network architectures Basic concepts, Internet architecture, Physical layer Delay, bandwidth, and throughput Link layer Coding/encoding, framing, error detection, reliable transmission Multi-access links Switching, bridges, ATM

55 What we ve learned (cont.) Internetworking Challenges, solutions Classful vs classless IP addressing IP forwarding, longest prefix lookup, ARP DHCP Dynamic routing protocols Distance vector (RIP) Link state (OSPF)