CSC 401 Data and Computer Communications Networks

Transcription

1 CSC 401 Data and Computer Communications Networks Network Layer IPv4, Format and Addressing,, IPv6 Prof. Lina Battestilli Fall 2017

2 Chapter 4 Outline Network Layer: Data Plane 4.1 Overview of Network layer data plane control plane 4.2 What s inside a router 4.3 Internet Protocol (IP) datagram format, IPv4 addressing,, IPv6, NAT 4.4 Generalized Forward and SDN

3 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

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

5 IP fragmentation, reassembly network links have MTU (max transmission unit) - largest possible link-level frame different link types, different MTUs large IP datagram divided ( fragmented ) within net one datagram becomes several datagrams reassembled only at final destination IP header bits used to identify, order related fragments reassembly fragmentation: in: one large datagram out: 3 smaller datagrams

6 example: IP fragmentation, reassembly 4000 byte datagram 3980 bytes of payload MTU = 1500 bytes length =4000 ID =x fragflag =0 offset = bytes of payload one large datagram becomes several smaller datagrams length =1500 ID =x fragflag =1 offset =0 Offset is specified in units of 8 bytes chunks offset = 1480/8 length =1500 length =1040 ID =x ID =x fragflag =1 fragflag =0 offset =185 offset =370 Does fragmentation have a cost?

7 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 or two interfaces (e.g., wired Ethernet, wireless ) IP addresses associated with each interface dotted-decimal notation = Each interface must have a unique IP address

8 IP addressing: introduction Q: how are interfaces actually connected? A: we ll learn about that in chapter 6 and A: wired Ethernet interfaces connected by Ethernet switches For now: don t need to worry about how one interface is connected to another A: wireless WiFi interfaces connected by WiFi base station

9 Subnets no router IP address: subnet part - high order bits host part - low order bits netmask specifies subnet and host part bits. What s a subnet? device interfaces with same subnet part of IP address hosts on the same subnet can communicate with each other without intervening router subnet network consisting of 3 subnets

10 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 / / subnet /24 Let s look at the the interfaces on my laptop subnet mask: /24

11 Subnets how many? / / / / / /

12 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 Broadcast Address all hosts in a subnet get the those datagrams

13 Chapter 4 Outline Network Layer: Data Plane 4.1 Overview of Network layer data plane control plane 4.2 What s inside a router 4.3 Internet Protocol (IP) datagram format, IPv4 addressing,, IPv6, NAT 4.4 Generalized Forward and SDN

14 IP addresses: how to get one? 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 manually configure IP addresses routers by network admins : Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play, zeroconf protocol

15 : Dynamic Host Configuration Protocol goal: allow host to dynamically obtain its IP address from network when it joins the network can renew its lease on address in use allows reuse of addresses (only hold address while connected/ on ) support for mobile users who want to join network (more shortly) overview: host broadcasts discover msg [optional] server responds with offer msg [optional] host requests IP address: request msg server sends address: ack msg

16 client-server scenario / server arriving client needs address in this network Relay agent / /24

17 client-server scenario server: request src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 lifetime: 3600 secs discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 offer src: , 67 dest: , 68 yiaddrr: transaction ID: 654 lifetime: 3600 secs ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 lifetime: 3600 secs arriving client server runs over UDP, port 67 clients can renew lease Q: Why 4 steps? Only need an IP address?

18 : more than IP addresses can return more than just allocated IP address on subnet: address of first-hop router for client name and IP address of DNS sever network mask (indicating network versus host portion of address)

19 : example UDP IP Eth Phy UDP IP Eth Phy router with server built into router connecting laptop needs its IP address, addr of first-hop 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 demuxed to IP demuxed, UDP demuxed to

20 : example UDP IP Eth Phy server formulates ACK containing client s IP address, IP address of first-hop router for client, name & IP address of DNS server UDP IP Eth Phy router with server built into router encapsulation of server, frame forwarded to client, demuxing up to at client client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router

21 Example What is the a.b.c.d/x notation for this example? 248 in binary is

22 IP addresses: how to get one? Q: how does network get subnet part of IP addr? A: gets allocated portion of its provider ISP s address space ISP's block /20 Organization /23 Organization /23 Organization / Organization /23

23 Hierarchical addressing: route aggregation hierarchical addressing allows efficient advertisement of routing info: Organization /23 Organization /23 Organization /23 Organization / Fly-By-Night-ISP ISPs-R-Us Send me anything with addresses beginning /20 Send me anything with addresses beginning /16 Internet

24 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization /23 Organization /23 Organization /23 Organization / Fly-By-Night-ISP ISPs-R-Us Send me anything with addresses beginning /20 Internet Send me anything with addresses beginning /16 or /23 Why does that still work?

25 IP addressing: the last word... 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

26 Chapter 4 Outline Network Layer: Data Plane 4.1 Overview of Network layer data plane control plane 4.2 What s inside a router 4.3 Internet Protocol (IP) datagram format, IPv4 addressing,, IPv6, NAT 4.4 Generalized Forward and SDN

27 IPv6: motivation initial motivation: 32-bit address space soon to be completely allocated. additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header NO fragmentation allowed Work started in mid 1990s Feb 2011 IANA allocated the last pool of unassigned IPv4 addresses

28 IPv6 Address Structure IPv6 address has 128bits (3.4x10 38 ) - 21 addresses/in 2 of the Earth surface Separated into subnet & interface portions: subnet prefix (n) interface ID (128-n) Addresses written in hexidecimal as 8 blocks of 16 bits, separated by : 2001:470:806d:1::9 prefixlen :0470:806d:0001:0000:0000:0000:0009

29 IPv6 datagram format ver pri flow label payload len next hdr hop limit source address (128 bits) destination address (128 bits) data priority: identify priority among datagrams in flow flow Label: identify datagrams in same flow. next header: identify upper layer protocol for data 32 bits Other Changes from IPv4 checksum: removed entirely to reduce processing time at each hop options: allowed, but outside of header, indicated by Next Header field ICMPv6: new version of ICMP (e.g. Packet Too Big )

30 Transition from IPv4 to IPv6 not all routers can be upgraded simultaneously no flag days how will network operate with mixed IPv4 and IPv6 routers? tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers IPv4 header fields IPv4 source, dest addr IPv6 header fields IPv6 source dest addr UDP/TCP payload IPv4 payload IPv6 datagram IPv4 datagram

31 Tunneling logical view: A IPv6 B IPv6 IPv4 tunnel connecting IPv6 routers E IPv6 F IPv6 physical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 flow: X src: A dest: F data src:b dest: E Flow: X Src: A Dest: F src:b dest: E Flow: X Src: A Dest: F flow: X src: A dest: F data data data A-to-B: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 E-to-F: IPv6

32 IPv6: adoption Google: 8% of clients access services via IPv6 NIST: 1/3 of all US government domains are IPv6 capable Long (long!) time for deployment, use 20 years and counting! think of application-level changes in last 20 years: WWW, Facebook, streaming media, Skype, Why?

33 References Some of the slides are identical or derived from 1. Slides for the 7 th edition of the book Kurose & Ross, Computer Networking: A Top-Down Approach, 2. Computer Networking, Nick McKeown and Philip Levis, 2014 Stanford University