Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms link state distance vector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer

Intra-AS Routing also known as interior gateway protocols (IGP) most common intra-AS routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco proprietary) Network Layer

RIP ( Routing Information Protocol) included in BSD-UNIX distribution in 1982 distance vector algorithm distance metric: # hops (max = 15 hops), each link has cost 1 DVs exchanged with neighbors every 30 sec via advertisement message each advertisement: list of up to 25 destination subnets within AS from router A to destination subnets: D C B A u v w x y z subnet hops u 1 v 2 w 2 x 3 y 3 z 2 Network Layer

RIP: example z y w x D B A C 2 y B 2 3 z B 7 x -- 1 … 1 2 3 Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B 7 x -- 1 …. …. .... Interface 2 3 … Routing/Forwarding table in D Network Layer

RIP: example z w y x A D B C 2 3 y B 2 3 2 z B A 7 5 x -- 1 … dest next hops w - 1 x - 1 z C 4 …. … ... A-to-D advertisement z w y x 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 …. …. .... Interface 2 3 3 2 … Routing/Forwarding table in D Network Layer

RIP: link failure, recovery if no advertisement heard after 180 sec --> neighbor/link declared dead “routes via the dead 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 ping-pong loops (infinite distance = 16 hops) Network Layer

RIP table processing RIP routing tables managed by application-level process called route-d (daemon) advertisements sent in UDP packets, periodically repeated routed routed transport (UDP) transprt (UDP) network forwarding (IP) table network (IP) forwarding table link link physical physical Network Layer

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 advertisements flooded to entire AS carried in OSPF messages directly over IP (rather than TCP or UDP IS-IS routing protocol: nearly identical to OSPF Network Layer

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 integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology data base as OSPF hierarchical OSPF in large domains. Network Layer

Hierarchical OSPF backbone boundary router backbone router area border routers area 3 internal routers area 1 area 2 Network Layer

Hierarchical OSPF two-level hierarchy: local area, backbone. link-state advertisements only in area each nodes has detailed area topology It only know direction (shortest path) to nets in other areas. area border routers: “summarize” net info. in own area, advertise to other Area Border routers. backbone routers: run OSPF routing limited to backbone. boundary routers: connect to other AS’s. Network Layer

Internet inter-AS routing: BGP BGP (Border Gateway Protocol): the de facto inter-domain routing protocol “glue that holds the Internet together” BGP provides each AS a means to: eBGP: obtain subnet reachability information from neighboring ASs. iBGP: propagate reachability information to all AS-internal routers. determine “good” routes to other networks based on reachability information and policy. allows subnet to advertise its existence to rest of Internet: “I am here” Network Layer

BGP basics BGP session: two BGP routers (“peers”) exchange BGP messages: advertising paths to different destination network prefixes exchanged over semi-permanent TCP connections when AS3 advertises a prefix to AS1: AS3 promises it will forward datagrams towards that prefix AS3 can aggregate prefixes in its advertisement 3c 3a BGP message 3b 2c AS3 other networks AS1 1c 1a 1d 1b 2a 2b other networks AS2 Network Layer

BGP basics: distributing path information using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. 1c can then use iBGP do distribute new prefix info to all routers in AS1 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session when router learns of new prefix, it creates entry for prefix in its forwarding table. eBGP session 3a 3b iBGP session 2c AS3 other networks 1c 2a 2b other networks 1a 1b AS2 1d AS1 Network Layer

Path attributes and BGP routes advertised prefix includes BGP attributes prefix + attributes = “route” two important attributes: AS-PATH: contains ASs through which prefix advertisement has passed: e.g., AS 67, AS 17 NEXT-HOP: indicates specific internal-AS router to next-hop AS gateway router receiving route advertisement uses import policy to accept/decline e.g., never route through AS x policy-based routing Network Layer

BGP route selection router may learn about more than 1 route to destination AS, selects route based on: local preference value attribute: policy decision shortest AS-PATH closest NEXT-HOP router: hot potato routing additional criteria Network Layer

BGP messages BGP messages exchanged between peers over TCP connection 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 msg; also used to close connection Network Layer

BGP routing policy A,B,C are provider networks X Y legend: customer network: provider network 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 route from B via X to C .. so X will not advertise to B a route to C Network Layer

BGP routing policy (2) A advertises path AW to B X Y legend: customer network: provider network A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C? No way! 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! BGP에 관련한 중요 사건: http://openweb.or.kr/?p=5620&utm_source=twitterfeed&utm_medium=twitter&utm_campaign=Feed%3A+open-web+%28Open+Web%29&utm_content=Google+International export policy Network Layer

Why different Intra-, 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 Network Layer

Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what’s inside a router 4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms link state distance vector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Layer

Unicasting, Broadcasting, Multicasting 인터넷 전송 방식 유니캐스팅(Unicasting): 하나의 송신자가 다른 하나의 수신자로 데이터를 전송하는 방식. 점대점 통신(one-to-one) 브로드 캐스팅(Broadcasting): 하나의 송신자가 같은 서브네트웍 상의 모든 수신자에게 데이터를 전송하는 방식. 멀티캐스팅(Multicasting): 동일한 메시지를 하나이상의 수신자에게 동시에 보내는 방식(one-to-many). VOD, 인터넷 화상 회의 등의 응용에서 사용. Network Layer

creation/transmission Broadcast routing 복습문제 R32와 관련 deliver packets from source to all other nodes source duplication is inefficient: R1 R2 R3 R4 source duplication in-network duplication duplicate creation/transmission source duplication: network is overloaded how does source determine recipient addresses? Network Layer

Broadcast Routing N-way Unicast Broadcast (and Multicast) Network Layer

Broadcast Address Definition Local Limited Broadcast Address An IP address that allows information to be sent to all machines on a given subnet rather than a specific machine. The standard is laid out in RFC 919. Local Limited Broadcast Address 255.255.255.255 : Can be used, during the DHCP (or BOOTP) process, when a host might not know its IP address and subnet mask, and discover DHCP (BOOTP) server A datagram destined for the limited broadcast address is never forwarded by a router.

Broadcast Address Net-directed broadcast Address net Id (netid) + host ID (all one) : Router must forward a net-directed broadcast by default but it must also have an option to disable this forwarding. Class A net-directed broadcast address: netid.255.255.255 Class B net-directed broadcast address: netid.netid.255.255 Class C net-directed broadcast address: netid.netid.netid.255 Subnet-directed Broadcast net ID (netid) + subnet ID (subnetid) + All host IDs (all one) Configuration Method subnet mask - 255.255.192.0 IP address - 172.16.148.196 the bit complement of the subnet mask is 0.0.63.255 172.16.148.196 || 0.0.63.255 = 172.16.191.255 Network Layer

In-network duplication flooding: when node receives broadcast packet, sends copy to all neighbors problems: cycles & broadcast storm controlled flooding: node only broadcasts pkt if it hasn’t broadcast same packet before node keeps track of packet ids already broadacsted or reverse path forwarding (RPF): only forward packet if it arrived on shortest path between node and source spanning tree: no redundant packets received by any node Network Layer

Spanning tree first construct a spanning tree nodes then forward/make copies only along spanning tree A B G D E c F (a) broadcast initiated at A (b) broadcast initiated at D Network Layer

Spanning tree: creation center node each node sends unicast join message to center node message forwarded until it arrives at a node already belonging to spanning tree A A 3 B B c c 4 2 D D F E F E 1 5 G G stepwise construction of spanning tree (center: E) (b) constructed spanning tree Network Layer

Broadcasting vs. Multicasting Broadcasting and Multicasting only apply to UDP The problem with IP broadcasting : If there are 50 hosts on the cable, but only 20 are participating in the application (Application is designed to use UDP broadcasts), 30 hosts have to process the broadcast, all the way through the UDP layer, before UDP datagram is discarded. UDP datagram is discarded by these 30 hosts because the destination port number is not in use. IP broadcasting relies on the underlying router’s setting. Network Layer

Multicasting Multicast group address Class D Multicast addresses are identified by the pattern “1110” in the first four bits 224.0.0.0 – 239.255.255.255 Host group can locate in multiple networks. Membership in a host group is dynamic. Permanent host groups : well-known addresses by IANA Well-Known Address : 224.0.0.1 ~ 224.0.0.225

Group Management Multicast Group Management IGMP (Internet Group Management Protocol) The IGMP operates between a host and its directly attached router. It provides the means for a host to inform its attached router that an application running on the hosts wants to join a specific multicast group. Network Layer

Four situation of IGMP operation Network Layer

IGMP in Internet-Wide Net A host in Internet can join a multicast group Join Join Join Network Layer

Multicast routing: problem statement goal: find a tree (or trees) connecting routers having local multicast group members tree: not all paths between routers used shared-tree: same tree used by all group members group member not group router with a without legend source-based: different tree from each sender to rcvrs source-based trees Notes: 3.3 Network Layer: Multicast Routing Algorithms 3-9 shared tree Network Layer

Approaches for building mcast trees group-shared tree: group uses one tree center-based trees source-based tree: one tree per source shortest path trees reverse path forwarding Notes: 3.3 Network Layer: Multicast Routing Algorithms 3-11 Network Layer

Center-based trees (group-shared tree) single delivery tree shared by all one router identified as “center” of tree to join: edge router sends unicast join-msg addressed to center router join-msg “processed” by intermediate routers and forwarded towards center join-msg either hits existing tree branch for this center, or arrives at center path taken by join-msg becomes new branch of tree for this router Notes: 1. It’s always nice to see a PhD dissertation with impact. The earliest discussion of center-based trees for multicast appears to be D. Wall, “Mechanisms for Broadcast and Selective Broadcast,” PhD dissertation, Stanford U., June 1980. 3.3 Network Layer: Multicast Routing Algorithms 3-17 Network Layer

Center-based trees: example suppose R6 chosen as center: LEGEND R1 router with attached group member R4 3 router with no attached group member R2 2 1 R5 path order in which join messages generated R3 Notes: 3.3 Network Layer: Multicast Routing Algorithms 3-18 1 R6 R7 Network Layer

Shortest path tree (source-based tree) mcast forwarding tree: tree of shortest path routes from source to all receivers Dijkstra’s algorithm R1 R2 R3 R4 R5 R6 R7 2 1 6 3 4 5 s: source LEGEND router with attached group member router with no attached group member Notes: 3.3 Network Layer: Multicast Routing Algorithms 3-12 link used for forwarding, i indicates order link added by algorithm i Network Layer

Reverse path forwarding (source-based tree) rely on router’s knowledge of unicast shortest path from it to sender each router has simple forwarding behavior: if (mcast datagram received on incoming link on shortest path back to center) then flood datagram onto all outgoing links else ignore datagram Notes: 3.3 Network Layer: Multicast Routing Algorithms 3-13 Network Layer

Reverse path forwarding: example s: source LEGEND router with attached group member router with no attached group member datagram will be forwarded datagram will not be forwarded result is a source-specific reverse SPT Notes: 3.3 Network Layer: Multicast Routing Algorithms 3-14 Network Layer

Reverse path forwarding: pruning forwarding tree contains subtrees with no mcast group members no need to forward datagrams down subtree “prune” msgs sent upstream by router with no downstream group members s: source LEGEND R1 R4 router with attached group member R2 P router with no attached group member R5 Notes: 3.3 Network Layer: Multicast Routing Algorithms 3-15 P prune message R3 P R6 links with multicast forwarding R7 Network Layer

Multicast Routing Protocols IETF Protocols Distance-Vector Multicast Routing Protocol (DVMRP) – RFC 1075 Source-based Reverse Path Forwarding, Pruning Protocol Independent Multicast (PIM) – RFC 2362 Dense Mode (RFC 3973, PIM-DM) - Source-based Sparse Mode (RFC 3569, RFC 4607, PIM-SM) – Both Shard and Source-based Network Layer