Lecture 5 TCP/IP Network Layer (3)

Lecture 5TCP/IP Network Layer (3) Khaled Mahbub, IICT, BUET, 2008

Outline (Network Layer) • Principles behind network layer services: • Virtual circuit and datagram networks • Routing algorithms • Link State • Distance Vector • Hierarchical • Routing Tables • What’s inside a router • Internet Protocol • Datagram format • IPv4 addressing • CIDR • IPv6 • Routing in the Internet • RIP • OSPF • BGP • Broadcast and multicast routing • Miscellaneous protocols and programs Khaled Mahbub, IICT, BUET, 2008

Routing Protocols in the Internet • Internet consists of inter connected Autonomous Systems (ASs). • ASs typically consists of many networks. • Datagrams are routed within each AS using Intra-AS routing protocols, also known as Interior Gateway Protocols (IGP). • Datagrams among ASs are routed using inter-AS routing protocoal, also known as Exterior Gateway Protocols (EGP). • Most common Intra-AS routing protocols: • RIP: Routing Information Protocol • OSPF: Open Shortest Path First • Most common Inter-AS routing protocols: • BGP: Border Gateway Protocol Khaled Mahbub, IICT, BUET, 2008

u v destinationhops u 1 v 2 w 2 x 3 y 3 z 2 w x z y C A D B RIP ( Routing Information Protocol) • Distance vector algorithm • Included in BSD-UNIX Distribution in 1982 • Distance metric: # of hops, each hop has cost of 1, (max = 15 hops) • Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement) • Each advertisement: list of up to 25 destination nets within AS Khaled Mahbub, IICT, BUET, 2008

z w x y A D B C Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B 7 x -- 1 …. …. .... Routing table in D RIP: Example Khaled Mahbub, IICT, BUET, 2008

Dest Next hops w - - x - - z C 4 …. … ... Advertisement from A to D z w x y A D B C Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B A 7 5 x -- 1 …. …. .... Routing table in D RIP: Example Khaled Mahbub, IICT, BUET, 2008

RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead • routes via neighbor invalidated • new advertisements sent to neighbors • neighbors in turn send out new advertisements (if tables changed) • link failure info quickly propagates to entire net • poison reverse used to prevent count to infinity problem (infinite distance = 16 hops) Khaled Mahbub, IICT, BUET, 2008

routed routed Transprt (UDP) Transprt (UDP) network forwarding (IP) table network (IP) forwarding table link link physical physical RIP Table processing • RIP routing tables managed by application-level process. (e.g. in UNIX route-d (daemon)) • advertisements sent in UDP packets, periodically repeated Khaled Mahbub, IICT, BUET, 2008

RIP Message Format Khaled Mahbub, IICT, BUET, 2008

RIP Message Format • Command—Indicates whether the packet is a request or a response. The request asks that a router send all or part of its routing table. The response can be an unsolicited regular routing update or a reply to a request. Responses contain routing table entries. Multiple RIP packets are used to convey information from large routing tables. • Version Number—Specifies the RIP version used. This field can signal different potentially incompatible versions. • Zero—Not used. • Address-Family Identifier (AFI)—Specifies the address family used. RIP is designed to carry routing information for several different protocols. Each entry has an address-family identifier to indicate the type of address being specified. The AFI for IP is 2. • Address—Specifies the IP address for the entry. • Metric—Indicates how many internetwork hops (routers) have been traversed in the trip to the destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route. • Up to 25 occurrences of the AFI, address, and metric fields are permitted in a single IP RIP packet. Khaled Mahbub, IICT, BUET, 2008

RIP Table example (netstat command) C:\Documents and Settings\Administrator>netstat -r Route Table =========================================================================== Interface List 0x1 ........................... MS TCP Loopback interface 0x2 ...00 12 79 db e6 94 ...... Broadcom NetXtreme Gigabit Ethernet - Packet Scheduler Miniport =============================================================== =============================================================== Active Routes: Network Destination Netmask Gateway Interface Metric 0.0.0.0 0.0.0.0 172.16.20.1 172.16.10.29 30 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1 169.254.0.0 255.255.0.0 172.16.10.29 172.16.10.29 30 172.16.0.0 255.255.0.0 172.16.10.29 172.16.10.29 30 224.0.0.0 240.0.0.0 172.16.10.29 172.16.10.29 30 Default Gateway: 172.16.20.1 =============================================================== Persistent Routes: None Khaled Mahbub, IICT, BUET, 2008

RIP Message Format (v2) Khaled Mahbub, IICT, BUET, 2008

RIP Message Format (v2) • Command—Indicates whether the packet is a request or a response. The request asks that a router send all or a part of its routing table. The response can be an unsolicited regular routing update or a reply to a request. Responses contain routing-table entries. Multiple RIP packets are used to convey information from large routing tables. • Version—Specifies the RIP version used. In a RIP packet implementing any of the RIP 2 fields or using authentication, this value is set to 2. • Routing Domain: An identifier of the routing daemon, to which this packet belongs. • Address-Family Identifier (AFI)—Specifies the address family used. RIP is designed to carry routing information for several different protocols. Each entry has an address-family identifier to indicate the type of address specified. The address-family identifier for IP is 2. If the AFI for the first entry in the message is 0xFFFF, the remainder of the entry contains authentication information. Currently, the only authentication type is simple password. • Route Tag—Provides a method for distinguishing between internal routes (learned by RIP) and external routes (learned from other protocols). • IP Address—Specifies the IP address for the entry. • Subnet Mask—Contains the subnet mask for the entry. If this field is zero, no subnet mask has been specified for the entry. • Next Hop—Indicates the IP address of the next hop to which packets for the entry should be forwarded. • Metric—Indicates how many internetwork hops (routers) have been traversed in the trip to the destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route. Khaled Mahbub, IICT, BUET, 2008

OSPF (Open Shortest Path First) • “open”: publicly available • Uses Link State algorithm • LS packet dissemination • Topology map at each node • Route computation using Dijkstra’s algorithm • OSPF advertisement carries one entry per neighbor router • Advertisements disseminated to entire AS (via flooding) • Carried in OSPF messages directly over IP (rather than TCP or UDP Khaled Mahbub, IICT, BUET, 2008

OSPF “advanced” features (not in RIP) • Security: all OSPF messages authenticated (to prevent malicious intrusion) • Multiple same-cost paths allowed (only one path in RIP) • For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time) • Integrated uni- and multicast support: • Multicast OSPF (MOSPF) uses same topology data base as OSPF • Hierarchical OSPF in large domains. Khaled Mahbub, IICT, BUET, 2008

Hierarchical OSPF Khaled Mahbub, IICT, BUET, 2008

Hierarchical OSPF • Two-level hierarchy: local area, backbone. • Link-state advertisements only in area • each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. • Area border routers:“summarize” distances to nets in own area, advertise to other Area Border routers. • Backbone routers: run OSPF routing limited to backbone. • Boundary routers: connect to other AS’s. Khaled Mahbub, IICT, BUET, 2008

Internet inter-AS routing: BGP • BGP (Border Gateway Protocol):the de facto standard • Path Vector protocol: • similar to Distance Vector protocol • each Border Gateway broadcast to neighbors (peers) entire path (i.e., sequence of AS’s) to destination • E.g., Gateway X may send its path to destination. Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z • BGP routes to networks (ASs), not individual hosts Khaled Mahbub, IICT, BUET, 2008

BGP • Suppose: gateway X sends its path, Path (X,Z) = X,Y1,Y2,Y3,Z, to peer gateway W • W may or may not select path offered by X • cost, policy (don’t route via competitors AS), loop prevention reasons. • If W selects path advertised by X, then: • Path (W,Z) = W, Path (X,Z)= W,X,Y1,Y2,Y3,Z • Note: X can control incoming traffic by controlling it route advertisements to peers: • e.g., don’t want to route traffic to Z -> don’t advertise any routes to Z Khaled Mahbub, IICT, BUET, 2008

BGP Routing Policy • A,B,C are provider networks • X,W,Y are customer (of provider networks) • X is dual-homed: attached to two networks • X does not want to have a route from B to C via X • .. so X will not advertise to B a route to C Khaled Mahbub, IICT, BUET, 2008

BGP Routing Policy • A advertises to B the path AW • B advertises to X the path BAW • Should B advertise to C the path BAW? • No. B gets no “revenue” for routing CBAW since neither W nor C are B’s customers • B wants to force C to route to w via A • B wants to route only to/from its customers! Khaled Mahbub, IICT, BUET, 2008

BGP • Routers’ responsibility • Receiving and filtering route advertisements from directly attached neighbor(s). • Route selection. • To route to destination X, which path (of several advertised) will be taken? • Sending route advertisements to neighbors. • BGP Messages: • BGP messages exchanged using TCP. • OPEN: opens TCP connection to peer and authenticates sender • UPDATE: advertises new path (or withdraws old) • KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request • NOTIFICATION: reports errors in previous message; also used to close connection Khaled Mahbub, IICT, BUET, 2008

Why different Intra- and Inter-AS routing ? Policy: • Inter-AS: admin wants control over how its traffic routed, who routes through its net. • Intra-AS: single admin, so no policy decisions needed Scale: • hierarchical routing saves table size, reduced update traffic Performance: • Intra-AS: can focus on performance • Inter-AS: policy may dominate over performance Khaled Mahbub, IICT, BUET, 2008

Reading Material • Chapter 4 – text3 (Kurose) • Chapter 5 – text2 (Tanenbaum) • Chapter 10 – text1 (Stevens) Khaled Mahbub, IICT, BUET, 2008

Lecture 5 TCP/IP Network Layer (3)