Extending Networks

Three Levels of Extension Physical Layer –Repeaters Link Layer –Bridges –Switches Network –Routers: “Connecting networks”

(Physical Layer) Rule For IEEE LANs, there is a limit on the length of a segment, how to extend the limit? –Adding repeaters (Hubs) while respecting the rule: Any path should traverse at most 5 segments Any path should traverse at most 4 repeaters (hubs) Any path should traverse at most 3 populated segments

(Link Layer) Bridges Filter network component –Back learning Internetworking component (between LANs of different standards)

DIX and Frames PreambleDest.Source.Type CRC DIX PreambleDest.Source.LengthTypeCRC IEEE Up to How to make the difference?

Connecting Networks LANs are connected with point-to-point links Packets are “routed” using another level of addresses other than MAC addresses Paths may be multihop

Network Layer Read Tanenbaum (Chapter 5)

Where in the OSI Reference Model ? Transport Layer Network Layer Link Layer Physical Layer Session Layer

Fundamental Functions of a Network Layer Addressing Routing Congestion control –Note that not all network protocols provide congestion control

Network Layer Protocols : two families Connectionless –Each piece of information is sent as an independent entity. No state information is kept in hosts or routers Connection oriented –There exists a virtual circuit over which all pieces of information will transit.

Connectionless vs Connection-oriented Tanenbaum Figure 5.4 Circuit setup Addressing State information Routing Effect of routing failures Congestion control

Routing Routing policy: –updates the routing table Routing mechanism: – decides how to route a packet depending on some policy (i.e, chooses the output line) Routing structure modifies reads Quite elementary

Routing Policy: Updating the Routing Tables Manual Using routing Daemons such as: –RIP –OSPF –EGP –BGP

Routing Mechanism Takes the decision how to route For example, IP protocol implements in each Internet host a mechanism to route

Desirable Properties of Routing Correct Simple Fair Robust STABLE Optimal

Routing Mechanisms Flooding Hot-potatoe Shortest path Fixed routing Dynamic routing

Overview Three big classes of routing policies –Distance vector routing (DVR) –Link state routing (LSR) –Hierarchical Routing (HR) Application to Internet –RIP is a DVR –OSPF is an LSR –EGP, BGP (DVR)

Distance Vector A node –tells its neighbors only –its distance to EVERY NODE in the network Example: –Initial A(0,1,4,inf), – B(1,0,1,1) – C(4,1,0,2) – D(inf,1,2,0) A B D C A receives update from B ….

Distance Vector (Problems!!!) When ? –Links go down What kind of problem ? –Count-to-infinity –Example Solution –path vector ABC 1 1

Link-State Routing A node –tells its EVERY node –its distance to its NEIGHBORS How ? –Send link-state packets (LSP) – using controlled flooding –Use Dijkstra’s algorithm A B D C A receives update from B ….

LANs Internal and External Protocols Internet Backbone Autonomous Systems Autonomous Systems Autonomous Systems Autonomous Systems Exterior Gateway Protocol Interior Gateway Protocol

RIP (RFC 1058) (Routing Information Protocol) Interior gateway protocol Distance vector protocol Uses split horizon to avoid count-to-infinity (Does not advertise a cost to a neighbor if it is a next hop for that destination) Exchange each 30s Time-out after 180 s.

OSPF (RFC 1247) (Open Shortest Path First) Interior gateway protocol Link state protocol Uses directly IP (while RIP uses UDP)

BGP (RFC 1267) (Exterior Gateway Protocol) Exterior gateway protocol (Exception: BGP uses TCP !!!!) Path Vector protocol (+ policy attributes) Topology may be loop-free (BGP guarantees loop-freeness)

The Internet Protocol (IP) RFC 791 Read 5.5

A Connectionless Network Layer: the Internet Protocol (IP) The Internet Protocol is found on every: –host that is connected to Internet –router on a LAN connected to the Internet –router on the backbone Application Transport Network IP Link Layer

Fundamental Idea of IP Routes between Networks, not between hosts This allows shorter routing tables

IP Header (Figure 3.1, p. 34) VHLTOSTotal Length 16-bit identificationFlgs13-bit frag. offset TTLProtocol16-bit Hdr Checksum 32-bit source IP address 32-bit destination IP address Options (Variable 0 ---> ??) Data (TCP segment, or UDP Dtg, or ICMP ….)

Internet Addresses Class A Class B Class C Class D Class E 0 netidhostid 7bits24 bits 1 netid hostid 14 bits16 bits 0 1 netid hostid 21 bits8 bits Multicast group ID 28 bits Future use 27 bits

Special Internet Addresses This host Local host Local broadcast Remote Loopback 127.X.X.X … bits 00000… hostid … … NetID

Internet Addresses (Cont’d) Unicast addresses (Classes A,B, and C) Multicast addresses (Class D) Dotted notation : the 32-address is divided in 4 groups of 8 bits (byte, octet). Each byte is expressed in base 10 separated by dots

Subnetting Why to do subnetting ? How is it done ? Notion of subnet mask

Subnetting (RFC 950) 1 netid hostid 14 bits16 bits 0 Class B netid hostid n bits m bits Example : netid hostid 5 bits 11 bits

Subnetmask Idea : IP1 IP2 & (Bitwise AND) & Subnetmask = R1 = R2 R1 = R2 if IP1 and IP2 are on the same subnet R1 # R2 if IP1 and IP2 are on different subnets

IP Routing Based on a routing table with entries having –Destination IP addr (host ID or net ID) –Next hop router IP address –flags (hostid or netid, next hop or connected interface…) –network interface

IP Routing Cont’d 1) Try to find a complete IP address match in the routing table 2) If 1) fails, then try to find a match with network id if 1) and 2) fail search for default router

Some IP Helpers ARP (Address Resolution Protocol) : RFC 826 RARP : Reverse Address Protocol (RFC 903) BOOTP (RFC 951, 1048, 1084) replaced by DHCP (RFC 1541) ICMP (RFC 792)

Extensions to IP IPv6 Mobile IP

Conclusion You must know: –The functions of the network layer –the difference between connectionless and connection oriented network protocols (what they can do and cannot do) –IP protocol and helpers (ICMP, DHCP, ARP) –the general features introduced by IPv6 –globally what is Mobile IP.