M U LT I C A S T C O M M U N I C AT I O N S. Tarik Cicic

Size: px

Start display at page:

Download "M U LT I C A S T C O M M U N I C AT I O N S. Tarik Cicic"

Annabella Barton
6 years ago
Views:

1 M U LT I C A S T C O M M U N I C AT I O N S Tarik Cicic 9..08

2 O V E R V I E W One-to-many communication, why and how Algorithmic approach: Steiner trees Practical algorithms Multicast tree types Basic concepts in multicast routing and protocols Recent development: BIER (RFC879, Nov. 07) Next week: Presentations on protocols; network- and applicationlevel

3 C A S T S Unicast: one-to-one Broadcast: one-to-all Multicast: one-to-group The group is a subset of all Most general notion Anycast: one-to-one-from-a-group

4 O N E - TO- M A N Y C O M M U N I C AT I O N Sender needs to send the same information to all three receivers Sender Router Receiver Receiver transmissions over the same link! Receiver 4

5 O N E - TO- M A N Y C O M M U N I C AT I O N Sender needs to send the same information to all three receivers Sender Router Receiver Receiver Only one transmission Receiver 5

6 I P M U LT I C A S T C H A L L E N G E S Internet is designed for one-to-one communication IP packet header contains one source and one destination address How to Identify a group of users wishing to receive same data How the host tells the network it wishes to join a group Route packets efficiently Route between the networks 6

55.55 Sessions are associated to these addresses or

Host-network interface Internet Group Management 7

7 E N D - TO- E N D S U P P O R T. API Multicast uses Class D addresses Sessions are associated to these addresses or groups. Host-network interface Internet Group Management 7 Protocol (IGMP). Network (routing) Main focus of this lecture Application OS Application OS Router Router Router Application OS

8 G R A P H - A L G O R I T H M I C B A C K G R O U N D Network topology mapped to a graph G=(V, E) Problem: Given a node set S V, Create a spanning tree that interconnects S 8

9 S T E I N E R T R E E S Each link in the tree has an associated cost The minimal spanning tree we call Steiner tree Constructing this tree is a wellknown NP-complete problem 4 4 9

10 A P P R O X I M AT E A L G O R I T H M S

11 A P P R O X I M AT E A L G O R I T H M S ( ) Cost: 7 Optimal cost: 4

12 A P P R O X I M AT E A L G O R I T H M S ( ) Optimal tree (cost 4) Heuristic tree (cost 7)

13 S PA N N I N G T R E E S I N P R A C T I C E Creating a minimal spanning tree is an NP-complete problem More than one metric adds to the complexity (cost/delay optimization) Research shows that best effort trees are often quite good (0% costlier on average)

14 S PA N N I N G T R E E S I N P R A C T I C E ( ) Practical algorithm: Use communication delay or # hops as metric Find the (unicast) shortest path between a special node and all other nodes in the group G This special root node can be: An administratively assigned node: Core Based Tree (CBT) Data source: Shortest Path Tree (SPT) 4

15 C O R E B A S E D A N D S H O R T E S T PAT H T R E E S Core

16 B U I L D I N G M U LT I C A S T T R E E S Traffic flooding ( Flood-and-prune ) Include receivers by extending the tree until all reached Explicit messaging Receivers join the tree by sending control messages 6

17 F L O O D - A N D - P R U N E Senders start transmitting packets on all adjacent links Receivers Sender 7

18 F L O O D - A N D - P R U N E Senders start transmitting packets on all adjacent links Receivers Sender Candidate branch 8

19 F L O O D - A N D - P R U N E Routers forward data on all links except where the data arrived Receivers Sender Candidate branch 9

20 F L O O D - A N D - P R U N E Routing loops must be avoided! Receivers Sender Candidate branch 0

21 F L O O D - A N D - P R U N E Prune control messages are sent downstream on inactive links Receivers Sender Candidate branch

22 F L O O D - A N D - P R U N E Prunes are also sent downstream if prunes are received on all upstream interfaces Receivers Sender Candidate branch

23 F L O O D - A N D - P R U N E Receivers Sender Candidate branch Tree branch

24 F L O O D - A N D - P R U N E Receivers Sender Candidate branch Tree branch 4

25 F L O O D - A N D - P R U N E What if a new receiver wants to join? - Flooding is periodically repeated! Receivers Sender Candidate branch Tree branch 5

26 F L O O D - A N D - P R U N E P R O B L E M Can we flood the Internet in order to reach 5 receivers? NO! Flood-and-prune was used in the first Internet-wide multicast overlay (Mbone) This was the main reason why Mbone did not scale, collapsed under its own weight, and was abandoned 6

27 R E V E R S E PAT H F O R WA R D I N G Basis for explicit messaging protocols Utilizes the unicast routing tables Send control messages toward the known source (core) Reject data packets coming from directions other than the source R OK, add (S, G): 4 Join (S, G) Discard multicast data from S at any interface except Multicast data is forwarded only in opposite direction of unicast data Prevents looping R (S, (S, G): G): {, {4} 7 S

28 E X P L I C I T M E S S A G I N G Receivers send a Join message to the group core (or source in SPT) Receivers Sender Core 8

29 E X P L I C I T M E S S A G I N G Senders use unicast to reach the core in CBT (*, G): {, } Receivers Sender Core Tree branch pp stream 9

30 TA X O N O M Y O F M U LT I C A S T S O L U T I O N S Different categorizations: Tree type (CBT / SPT) Service model Unidirectional vs. bidirectional forwarding Intra- and inter-domain Protocol stack position 0

31 C B T V S. S P T Core Based Tree Multiple sources use the same tree, but single point of failure Fewer routers involved More complex operation, less optimal paths Shortest Path Tree Only one source Optimal path Scalability challenge due to increased amount of state information

32 UNI- O R B I D I R E C T I O N A L F O R WA R D I N G Should the data flow along the multicast tree in one or both directions? 4 Unidirectional: for each branch in the multicast tree, data is flowing in one direction only Simple Most used (PIM-SM)

33 B I D I R E C T I O N A L F O R WA R D I N G Only, and 4 in the tree, do not send on and the incoming In multicast protocols with bidirectional forwarding, data flowing in both directions 4 More complex, not widely used in practice

34 S E R V I C E M O D E L S Single source (one-to-many) simple, one-tree approach (SPT) sufficient for most applications E.g.: IPTV streaming Multiple sources (many-to-many) general, powerful, but... problematic in many ways, security in particular E.g.: Conferencing 4

35 M U LT I C A S T A D D R E S S I N G Class D address range in IPv Last 8 bits is the group ID reserved Never forwarded by routers E.g all hosts ( try to ping, UNINETT is multicast friendly) all routers 6

36 G L O B A L I P V 4 A D D R E S S A S S I G N M E N T Is problematic if no global rules GLOP: allocates 56 addresses to each AS 8 bits 8 bits 6 bits 0-55 AS number (GLOP was suggested before 4-byte AS numbers) PIM SSM introduces channel concept that prevents address collisions Requires knowledge of sender address Uses

37 I N T E R - N E T W O R K I S S U E S Multicast between network domains particularly challenging: scalability policing payments CBT core placement and communication heterogeneous multicast protocols Router Router Router? Router Router Router 8

38 G E N E R A L I P M U LT I C A S T I S S U E S Support only for unreliable datagram service Open service model Anyone can join, address collisions, prone to Denial of Service attacks Service billing Scalability Most approaches target only one application profile Patchy deployment 9

39 B I T I N D E X E X P L I C I T R E P L I C AT I O N ( B I E R ) Multicast state is a real issue, specially in large, transit networks. BIER does not require any state, nor explicit building of multicast distribution trees. Instead, multicast packets are encapsulated in BIER header, containing a bit string where each bit represents one egress router in BIER-domain. Each transit router in the BIER domain forwards the packets on one or more outgoing interfaces, shortest path to the domain egress, clearing the egress bits to prevent looping.

40 B I E R I N P R A C T I C E Multicast Flow Overlay used to inform ingress about egress routers for any given multicast group. [] 7 [, 6, 9, 5] [] 5 [5] 8 0 Egress Ingress 4 [6, 9] 6 [9] 4 [a, b, c] bits a, b and c set in the header, others

41 P R O T O C O L S TA C K P O S I T I O N Multicast can be implemented from physical to application level Network Layer (L) Multicast Very efficient Difficult to deploy Application Layer (L5-L7) Multicast Easy to deploy, no changes on lower layers Network system support not required, controlled by end-systems 4

42 S U M M A R Y: W H AT H AV E W E L E A R N E D? Algorithms: Optimal multicast is an NP complete problem Good behavior in practice using simple means SPT and CBT Tree construction: Flood-and-Prune Explicit messaging Reverse Path Forwarding Uni- and bidirectional forwarding Many challenges, some could be solved on application level (more on that next week) 47

Configuring the maximum number of external LSAs in LSDB 27 Configuring OSPF exit overflow interval 28 Enabling compatibility with RFC Logging

Configuring the maximum number of external LSAs in LSDB 27 Configuring OSPF exit overflow interval 28 Enabling compatibility with RFC Logging Contents Configuring OSPF 1 Overview 1 OSPF packets 1 LSA types 1 OSPF areas 2 Router types 4 Route types 5 Route calculation 6 OSPF network types 6 DR and BDR 6 Protocols and standards 8 OSPF configuration