CS 348 Computer Networks. IP and Routing. Indian Institute of Technology, Bombay

Transcription

1 Computer Networks IP and Routing

2 Network Interconnections Data Link Layer Delivery of frames on the same LAN Extend reach using switches/bridges and hubs Limitations Solution? Cannot address heterogeniety Ethernet, Token ring, FDDI, PPP, HDLC, ATM... Broadcast food Managing all nodes as single network is not scalable Internet Interconnect diferent networks

3 Internet Protocol (IP) Layer 3 of TCP/IP stack (Internet Layer) Heterogenity Common network layer (IP) irrespective of layer 2 Best-efort service model Scalability Packets can get lost, delayed, duplicated, dropped Assumes any DLL underneath (homework: check RFC 1149) Hierarchical addressing Efcient Routing between routing elements

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5 The Internet Network Layer Image source: Kameswari, CS348 Spring 2009

6 IP Datagram Format Image source: Kameswari, CS348 Spring 2009

7 IP Fragmentation and Reassembly MTUs of diferent network links are diferent MTU: maximum transmission unit Ethernet 1500 bytes Token Ring 4500 bytes Solution for Layer 3 datagrams?... Fragmentation + Reassembly

8 IP Fragmentation and Reassembly One to many Reassembly at destination Use of bits in header

9 Fragmentation Example Ofset is mod 8 Fragmentation on 8-byte boundaries

10 Addressing Each host has a layer 3 (IP) address Why? Hosts already have layer 2 addresses bit Ethernet MAC address Address space Flat addressing N nodes N addresses Hierarchical addressing advantage?

11 IP Addressing 32-bit address Consists of two parts Network Host Network on which a host is connected Unique id of host on a network Mask Unique bits that determine a network

12 Address Classes Diferent classes of networks (addresses) Class A: 0, network (7), host (24) Class B: 10, network (14), host (16) Class C: 110, network (21), host (8) Class D: 1110, 28 bits (group) Class E: 1111, 28 bits (reserved) Private addresses Class A: through Class B: to Class C: to : loopback address

13 Address Classes Diferent classes of networks (addresses) Class A: 0, network (7), host (24) Class B: 10, network (14), host (16) Class C: 110, network (21), host (8) Class D: 1110, 28 bits (group) Class E: 1111, 28 bits (reserved)

14 Example: Class C Networks

15 Datagram Forwarding/Routing Every datagram consists of src, dest IP addr. With network and host info Network part uniquely identifes on Internet All hosts that share network part are connected to same physical network Router: on the edge of the network Connects at least two networks

16 Action at Router If (matching network address) on any interface Deliver packet to destination on network If network address in routing table Deliver packet to NextHop router Else Deliver packet to default router

17 Action at Host If src and dest have same network address Deliver packet directly Both hosts on same network Else Deliver packet to default router

18 Addressing+Packet Delivery At Layer 3 <src, dest, data> Sent to lower layer (DLL) What does DLL do next?...

19 ARP ARP: Address Resolution Protocol DLL service IP to MAC address mapping

20 ARP ARP: Address Resolution Protocol DLL service IP to MAC address mapping Protocol (broadcast based) Broadcast query with target IP address Host with IP address responds Host maintains ARP cache IP:Mac cache Times out after a fnite time (15 secs)

21 ARP Protocol Details ARP frame format Transmitted as Ethernet Payload Ethernet type set to ARP

22 How to get an IP address?

23 IP Address Confguration Manual Ad-hoc Issues? DHCP Dynamic Host Confguration Protocol Auto on-demand IP lease Plug-n-play Application-layer protocol

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25 DHCP Advantages Central repository of IP addresses Can reuse Lease time adds efciency Use MAC-flter to allocate IPs No IP for unverifed hosts (users)

26 ICMP Internet Control Message Protocol Layer 3 communication for Error reporting Echo request/reply (router heartbeats) ICMP above IP layer ICMP packets carried as IP Payload Service feld in IP set to ICMP ICMP packets contain Type, code 8 bytes of IP packet responsible for ICMP message

27 ICMP Types and Codes

28 ICMP Usage Examples Ping Send an ICMP message with type=8 and code=0 ICMP echo request Destination host (if up) Responds to echo with response Type=0, Code=0 (for icmp message) Can also measure time for echo request-response Gives estimate of delay on network

29 Traceroute Program to determine route from source to destination Exploits the TTL feld On TTL expiry, router sends an ICMP message to host

30 Re-look at IP Address Space Is 32-bit enough? 4 billion hosts, how many networks? Are class categories optimal? Network with 2 hosts (Class C) Network with 256 hosts (Class B)

31 Re-look at IP Address Space One network: one network id + host range At router: one entry per network If same ISP has 200 Class C addresses?

32 Subnetting Subnet: sub-network in a network 3-level hierarchy instead of 2 Subnet number/mask for each subnet is same Host: IP address + Subnet mask

33 CIDR Classless Inter-domain Routing Abolishes class-based categorization

34 CIDR Classless Inter-domain Routing Abolishes class-based categorization Balance address usage efciency and routing table entries /X notation specify number of bits for network 2 to 32 bits /21 ISP can subdivide Longest-prefx match rule

35 Network Address Translation (NAT)

36 NAT in Action

37 NAT Debats +ves -ves

38 NAT Debate +ves Do not need address range from ISP Can change local IP distribution scheme easily Can change ISP easily No direct IP for nodes behind frewall (security) -ves No direct IP for nodes behind frewall Breaks end-to-end argument Port is for process, not for host IPv6