Chapter 4 Internetworking Spring 2010 CS 332 1

Chapter 4: Internetworking Spring 2010 CS 332 1

Assumptions • Data pipe from every machine to every other machine. – Need not be single link. – Pipe can lose or corrupt messages. • Sender/receiver may be on different physical networks, using different technology • So what info do we need to build a single “logical” network (either reliable or unreliable)? Spring 2010 CS 332 2

Issues • Getting various technologies to work with one another (i. e. creating a single “network” from many heterogeneous systems). – Problem magnified since packet may need to traverse several different networks (and network technologies), each with their own addressing schemes, service models, media access protocols, etc. • Scale: It’s the big issue – How can you find an efficient path through a network with millions (and perhaps billions eventually) of nodes? – How do you provide addressing for a network with this many nodes? Spring 2010 CS 332 3

Internetwork • Arbitrary collection of possibly heterogeneous networks interconnected to provide host-to-host packet delivery service. • Network: Directly connected or switched network that uses a single technology (i. e. ATM, 802. 5, Ethernet). – Could be many physical networks creating a single logical network. Spring 2010 CS 332 4

Internetwork • Internet—THE internetwork. – – Runs the Internet Protocol (IP or Kahn-Cerf) Interesting because it has faced the problems of scale Experimental versions 1977 – 1981 IPv 4 first deployed in 1981 • internet—abstract internetwork Spring 2010 CS 332 5

IP is a big deal White House News & Policies photo • Vint Cerf and Bob Kahn with Pres. Bush at 2006 ceremony where they received the Presidential Medal of Freedom for their work on IP. Spring 2010 CS 332 6

IP Internet • Concatenation of Networks Spring 2010 CS 332 Note Hn denotes host, Rn denotes router. 7

IP Internet • Protocol Stack H 1 H 8 TCP R 1 IP IP ETH Spring 2010 R 2 ETH R 3 IP FDDI IP PPP CS 332 PPP TCP IP ETH 8

The Internet Outline Best Effort Service Model Global Addressing Scheme Spring 2010 CS 332 9

Service Model • Connectionless (datagram-based) – So each packet must be “self-contained” • Best-effort delivery (unreliable service) – – Spring 2010 packets are lost packets are delivered out of order duplicate copies of a packet are delivered (? !) packets can be delayed for a long time CS 332 10

Why? ! • Best Effort service model is simple as it gets – intentionally! – If you provide best effort service over a network technology that provides reliable delivery, you’re fine – Providing reliable delivery over an unreliable network means extra functionality in the routers – Keeping the routers as simple as possible was an IP design goal. (Why? ) • Note: IP today runs over many technologies that were not in existence when IP was invented! Spring 2010 CS 332 11

Note: fields aligned on 32 bit boundaries 0 IP Datagram Format In 32 bit words 4 V ersion 8 HLen 16 TOS 31 In bytes Length Ident TTL 19 Flags Protocol Offset Checksum Source. Addr Destination. Addr Pad (variable) Options (variable) Data Spring 2010 CS 332 12

Fields • Version: note placement at front of packet (why? ) • Header Length: in 32 bit words (20 bytes when no options) • Type of service: later • Length: of entire packet in bytes (note max of 65, 535 bytes because of 16 bit length field) • Ident, flags, offset all deal with fragmentation • Time to live: first seconds, but evolved to be hop count Spring 2010 CS 332 13

Fields • Protocol: demux key specifying higher level protocol that gets datagram • Checksum: take IP header as sequence of 16 bit words, add them using ones complement, take ones complement of result. – Relatively easy to calculate in software – Not as strong error detection as CRC – Bad packets discarded by router (potential bad dest. addr. ) • Src, dest address: pretty clear (and these are unique!) • Options: rare, but complete IP implementation must handle them all! Presence determined by header length field Spring 2010 CS 332 14

Fragmentation and Reassembly • Each network has some MTU (why not uniform? ) – Why not some uniform standard? – What is a reasonable choice for a given host? • Strategy – – – fragment when necessary (MTU < Datagram length) try to avoid fragmentation at source host re-fragmentation is possible fragments are self-contained datagrams delay reassembly until destination host do not recover from lost fragments Spring 2010 CS 332 15

Fragmentation and Reassembly Header fields used in F &R (bits in parens) • Ident field (16): chosen by sending host, intended to be unique among all datagrams that might be received at this dest from this source over reasonable time period. – All fragments keep this same ident value • Offset (13): specifies 8 byte chunks of data (Why? And why not fragment #? ) • Flags: M is “more” flag Spring 2010 CS 332 16

Example Start of header Ident= x 0 Offset= 0 Rest of header 1400 data bytes MTU 532 bytes Start of header Ident= x 1 Offset= 0 Rest of header 512 data bytes Start of header Ident= x 1 Offset= 512 Rest of header 512 data bytes Start of header Ident= x Note: fragmentation can occur at multiple hops! Spring 2010 0 Offset= 1024 Rest of header 376 data bytes CS 332 17

Global Addresses • Properties – globally unique (don’t want anyone with my phone #) • Why not just use Ethernet address? ! – hierarchical: network + host (really interface) 7 • Dot Notation – 10. 3. 2. 4 – 128. 96. 33. 81 – 192. 12. 69. 77 A: Network Host 14 B: 1 0 16 Network Host 21 C: Spring 2010 0 24 1 1 0 CS 332 8 Network Host 18

IP Internet Note Hn denotes host, Rn denotes router. Routers need two IP addresses. All hosts on same network have same network part of IP address Spring 2010 CS 332 19

Terminology • • Routing Mechanism: How a router selects the link over which to forward a packet Routing Protocol: Policies that determine what is placed in the routing tables. These are not the same thing! Spring 2010 CS 332 20

Datagram Forwarding • Strategy – every datagram contains destination’s address – if directly connected to destination network, then forward to host – if not directly connected to destination network, then forward to some router – forwarding table maps network number into next hop – each host has a default router – each router maintains a forwarding table • Example (R 2) Spring 2010 Network Number 1 2 3 4 CS 332 Next Hop R 3 R 1 interface 0 21

Recall: Network 1 (Ethernet) H 7 H 2 H 1 R 3 H 8 H 3 Network 4 (point-to-point) Network 2 (Ethernet) R 1 R 2 H 4 Network 3 (FDDI) H 5 Spring 2010 H 6 CS 332 22

Pseudocode if (networknum dest = networknum my interface) deliver packet over that interface else if (networknum in my routing table) deliver packet to next hop router else deliver packet to default router Spring 2010 CS 332 23

Address Translation • Map IP addresses into physical addresses – destination host – next hop router – Why not just broadcast it? (E. g. if physical network is Ethernet). • Techniques – encode physical address in host part of IP address – table-based • ARP – – table of IP to physical address bindings broadcast request if IP address not in table target machine responds with its physical address table entries are discarded if not refreshed Spring 2010 CS 332 24

ARP Details • Request Format – Hardware. Type: type of physical network (e. g. , Ethernet) – Protocol. Type: type of higher layer protocol (e. g. , IP) – HLEN & PLEN: length of physical and protocol addresses • Stands for “hardware address length” and “protocol address length” – Operation: request or response – Source/Target-Physical/Protocol addresses • Notes – – table entries timeout in about 15 minutes (why? ) update table with source when you are the target (why? ) "refresh" table if already have an entry (i. e. reset timeout) do not refresh table entries upon reference Spring 2010 CS 332 25

ARP Packet Format 0 8 16 Hardware type = 1 HLen = 48 31 Protocol. T ype = 0 x 0800 PLen = 32 Operation Source. Hardware. Addr (bytes 0 – 3) Source. Hardware. Addr (bytes 4 – 5) Source. Protocol. Addr (bytes 0 – 1) Source. Protocol. Addr (bytes 2 – 3) Target. Hardware. Addr (bytes 0 – 1) Target. Hardware. Addr (bytes 2 – 5) Target. Protocol. Addr (bytes 0 – 3) Spring 2010 CS 332 26

Dynamic Host Configuration Protocol (DHCP) • Manually configuring IP information can be hard – Large networks – Configuration process error prone • Every host needs correct network number • No two hosts can have same IP address • DHCP automates process – Network management has to scale, too – not just network operation Spring 2010 CS 332 27

DHCP (continued) • At least one DHCP server per administrative domain – Centralized repository for host configuration info • Info can be sent to hosts at boot or connection time. • Can also be used to maintain pool of available addresses assigned on demand • Method – – – Send DHCPDISCOVER msg to 255. Response is DHCP Offer message – also broadcast (why? ) Host chooses one of offers and sends Reply and gets Ack Host "leases" IP address for a period of time – can renew Relay agents Spring 2010 CS 332 28

Internet Control Message Protocol (ICMP) • Communicates error messages and other conditions that require attention • ICMP messages are acted on by either the IP layer or higher layers (TCP or UDP). • Transmitted within IP datagrams 0 7 8 type 15 16 code 31 checksum Contents depends on type and code Spring 2010 CS 332 29

ICMP (cont. ) • 15 different values for the type field, then several codes for each of the types • Checksum computed same as for IP packet • Contains first 8 bytes of IP datagram that generated the message so sender can ID • Complete specification of protocol is RFC 792 (Postel) • Another good source is TCP/IP Illustrated, Vol. 1, Ch. 6 Spring 2010 CS 332 30

Types of ICMP Messages • • Echo (ping) Redirect (from router to source host) Destination unreachable (protocol, port, or host) TTL exceeded (so datagrams don’t cycle forever) Checksum failed Reassembly failed Cannot fragment Spring 2010 CS 332 31

Virtual Private Networks (VPNs) • Goal: simulate private network of dedicated links on a public (shared) network • Easy on circuit-switched infrastructure • Not so easy on IP-based internetwork • Strategy: routers create an IP "tunnel" for VPN traffic. Spring 2010 CS 332 32

IP Tunneling • Router R 1 at one end of tunnel is provided with IP address of router R 2 at the other end. • R 1 is directly connected to the network where the host that requested the tunnel lives. • R 2 is directly connected to the network where the destination host lives. • All VPN traffic is encapsulated as IP packets from R 1 to R 2 (this is not the norm) • R 2 strips tunnel header and forwards packet to ultimate recipient. Spring 2010 CS 332 33

Tunnel implementation Routing table for router R 1: Spring 2010 Network. Num 1 2 Default CS 332 Next. Hop Interface 0 Virtual interface 0 Interface 1 34

What is it good for? • Security – tunnel plus encryption • Using routers that have enhanced capabilities, e. g. multicast • Tunneling other protocols across IP • Short-circuiting normal routing – useful for mobility Spring 2010 CS 332 35