CSCE 515: Computer Network Programming Chin-Tser Huang University of South Carolina

2/15/20052 Open Shortest Path First (OSPF) A link-state routing protocol Each router tests status of all its links and broadcasts its link status to all routers on network Use IP to carry its message Provide features superior to RIP

2/15/20053 OSPF Message Format ver- sion type packet length router IDarea ID checksum auth type authenticationdata

2/15/20054 Autonomous Systems A collection of networks administered by single entity, e.g. a corporation or a campus Three categories of AS Stub AS Multihomed AS Transit AS

2/15/20055 Example of Autonomous System Source: Internetworking Technologies Handbook by Cisco Press

2/15/20056 IGP and EGP Each AS selects its interior gateway protocol (IGP) for communications between routers in this AS E.g. RIP, OSPF Multiple AS’s use exterior gateway protocol (EGP) for communications between routers in different AS’s E.g. EGP, BGP

2/15/20057 Border Gateway Protocol (BGP) An exterior gateway protocol Distance-vector protocol but enumerates route to each destination Allow policy-based routing Use TCP to transport its messages

2/15/20058 BGP Message Types BGP uses four message types Open message Keepalive message Update message Notification message

2/15/20059 Classless Interdomain Routing (CIDR) Solve two problems Shortage of class B network IDs Explosion in routing table size due to allocation of multiple class C network IDs Summarize multiple IP addresses into a smaller number of routing table entries

2/15/ Requirements of CIDR Multiple IP addresses to be summarized need to share same high-order bits Routing decisions need to be based on 32-bit IP address and 32-bit mask Routing protocols need to carry 32-bit mask in addition to 32-bit address

2/15/ Example of CIDR CIDR Block Prefix# Equivalent Class C# of Host Addresses /271/8th of a Class C32 hosts /261/4th of a Class C64 hosts /251/2 of a Class C128 hosts /241 Class C256 hosts /232 Class C512 hosts … /15512 Class C131,072 hosts /141,024 Class C262,144 hosts /132,048 Class C524,288 hosts

2/15/ User Datagram Protocol (UDP) A datagram-oriented transport layer protocol Each output operation by a process produces exactly one UDP datagram Provide no reliability

2/15/ UDP Header bytes destination port number UDP checksumUDP length data (if any) source port number

2/15/ UDP Checksum Cover UDP header and UDP data End-to-end checksum calculated by sender and verified by receiver Should always be enabled 16-bit one’s complement sum of header (including a pseudo header) and data is calculated

2/15/ Computation of UDP Checksum destination port number UDP checksumUDP length data (if any) UDP header source port number UDP lengthzero protocol(17) destination IP address source IP address pad byte(0) UDP pseudo header

2/15/ Fragmentation Need to fragment if size of resulting IP datagram exceeds MTU Can take place at source or at an intermediate router Fragments are reassembled at destination

2/15/ Example of UDP Fragmentation IP header UDP header UDP data IP header UDP header IP header 201

2/15/ ICMP Unreachable Error (Fragmentation Required) type(3)code(0-15) checksum IP header + first 8 bytes of original datagram data unused (must be 0)MTU of next-hop network

2/15/ Next Class UDP Broadcasting and multicasting Class MulticastSocket Read TI Ch. 11, 12, JNP Ch. 22