multicasting© dr. ayman abdel-hamid, cs4254 spring 20061 cs4254 computer network architecture and...
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Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 1
CS4254
Computer Network Architecture and Programming
Dr. Ayman A. Abdel-Hamid
Computer Science Department
Virginia Tech
Multicasting
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 2
Outline
•Multicasting (Chapter 21)
Multipoint Communications
IP Multicast
IPv4 Multicast addresses
Sending and Receiving Messages
Multicasting on a LAN
Multicasting on a WAN
Multicast Issues
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 3
Multipoint Communications
• Multipoint communications support communications between more than two hosts
One-to-many
Many-to-many
•Unlike broadcast, allows a proper subset of hosts to participate
•Example standards
IP Multicast (RFC 1112, standard)
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 4
Logical Multipoint Communications•Two basic logical organizations
Rooted: hierarchy (perhaps just two levels) that structures communications
Non-rooted: peer-to-peer (no distinguished nodes)
•Different structure could apply to control and data “planes”
Control plane determines how multipoint session is created
Data plane determines how data is transferred between hosts in the multipoint session
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 5
Logical Multipoint CommunicationsControl Plane
•The control plane manages creation of a multipoint session
Rooted control plane
One member of the session is the root, c_root
Other members are the leafs, c_leafs
Normally c_root establishes a session
Root connects to one or more c_leafs
c_leafs join c_root after session established
Non-rooted control plane
All members are the same (c_leafs)
Each leaf adds itself to the session
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 6
Logical Multipoint CommunicationsData Plane
The data plane is concerned with data transfer
•Rooted data planeSpecial root member, d_rootOther members are leafs, d_leafsData transferred between d_leafs and d_roots
d_leaf to d_rootd_root to d_leaf
There is no direct communication between d_leafs
•Non-rooted data planeNo special members, all are d_leafsEvery d_leafs communicate with all d_leafs
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 7
Forms of Multipoint Communications
•Server-based -- rooted multipoint communications with server as d_root
Passive or inactive
Relay
Reflector
Active
Bridge or multipoint control unit (MCU)
•Strictly peer-to-peer multipoint – Non-rooted
Multicast
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 8
Multipoint ServersPassive Multipoint Server•a relay or reflector service•Provides no processing of the data•Minimum requirement is for transport-level semantics, so can operate at the transport or application level
Active Multipoint Server•Does application-level processing
transcoding•uses application-level semantics
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 9
Multicast Communication•Multicast abstraction is peer-to-peer
Application-level multicastNetwork-level multicast
Requires router support (multicast-enabled routers)Multicast provided at network protocol level IP multicast
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 10
Multicast Communication•Transport mechanism and network layer must support multicast
•Internet multicast limited to UDP (not TCP)
Unreliable: No acknowledgements or other error recovery schemes (perhaps at application level)
Connectionless: No connection setup (although there is routing information provided to multicast-enabled routers)
Datagram: Message-based multicast
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 11
IP Multicast•IP supports multicasting
Uses only UDP, not TCP
Special IP addresses (Class D) identify multicast groups
Internet Group Management Protocol (IGMP) to provide group routing information
Multicast-enabled routers selectively forward multicast datagrams
IP TTL field limits extent of multicast
•Requires underlying network and adapter to support broadcast or, preferably, multicast
Ethernet (IEEE 802.3) supports multicast
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 12
IP Multicast: Group Address•How to identify the receivers of a multicast datagram?
•How to address a datagram sent to these receivers?Each multicast datagram to carry the IP addresses of all recipients? Not scalable for large number of recipients Use address indirection
A single identifier used for a group of receivers
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 13
IP Multicast: IGMP Protocol•RFC 3376 (IGMP v3): operates between a host and its directly attached router
•host informs its attached router that an application running on the host wants to join or leave a specific multicast group
•another protocol is required to coordinate multicast routers throughout the Internet network layer multicast routing algorithms
•Network layer multicast IGMP and multicast routing protocols
•IGMP enables routers to populate multicast routing tables
•Carried within an IP datagram
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 14
IP Multicast: IGMP ProtocolIGMP v2 Message types
•membership query: general
•Sent by routers router query multicast groups joined by attached hosts
•membership query: specific
•Sent by routers query if specific multicast group joined by attached hosts
•membership report
•Sent by host report host wants to join or is joined to given multicast group
•leave group (optional)
•Sent by host report leaving given multicast group
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 15
IP Multicast: IGMP Protocol•Joining a group
Host sends group report when the first process joins a given group
Application requests join, service provider (end-host) sends report
•Maintaining table at the router
Multicast router periodically queries for group information
Host (service provider) replies with an IGMP report for each group
Host does not notify router when the last process leaves a group this is discovered through the lack of a report for a query
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 16
IP Multicast: Multicast Routing•Multicast routers do not maintain a list of individual members of each host group•Multicast routers do associate zero or more host group addresses with each interface
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 17
IP Multicast: Multicast Routing
•Multicast router maintains table of multicast groups that are active on its networks•Datagrams forwarded only to those networks with group members
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 18
IP Multicast: Multicast Routing•How multicast routers route traffic amongst themselves to ensure delivery of group traffic?
Find a tree of links that connects all of the routers that have attached hosts belonging to the multicast group
Group-shared treesSource-based trees
Shared Tree Source Trees
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 19
MBONE: Internet Multicast Backbone
•The MBone is a virtual network on top of the Internet (section B.2)
Routers that support IP multicast
IP tunnels between such routers and/or subnets
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 20
Unicast versus Broadcast versus Multicast
•A unicast address identifies a single IP interface
•A broadcast address identifies all IP interfaces on the subnet
•A multicast address identifies a set of IP interfaces
•A multicast datagram is received only by those interfaces interested in the datagram (applications wishing to participate in the multicast group)
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 21
IPv4 Multicast Addresses 1/3
•Class D addresses in the range 224.0.0.0 through 239.255.255.255
•Low order 28 bits of class D Naddress (see appendix A) form the multicast group ID (32-bit address is the group address)
•Mapping of IPv4 multicast address to Ethernet address
High-order 24 bits of Ethernet address are always 01:00:5E
Next bit always 0
Low-order 23 bits are copied from low-order 23 bits of multicast group address
High-order 5 bits of group address are ignored in the mapping
Mapping not one-to-one
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 22
IPv4 Multicast Addresses 2/3
224.0.1.88 mapped into an Ethernet address?
•Remember an Ethernet address is 48 bits
•The address 224 is E0 in hex, 0 is 00 in hex, 1 is 01 in hex, and 88 is 58 in hex. However, only the low-order 23 bits are used
•Therefore, the IP address of 224.0.1.88 converted to a MAC address is 01-00-5E-00-01-58.
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 23
IPv4 Multicast Addresses 3/3
Some special IPv4 multicast addresses
•224.0.0.0 reserved
•224.0.0.1 all-host group
•224.0.0.2 all-routers group
•224.0.0.1 through 224.0.0.255 reserved for routing-protocols
•Datagrams destined to any of theses addresses are never forwarded by a multicast router
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 24
Sending & Receiving Multicast Messages
Receiving Multicast Messages•Create a UDP socket•Bind it to a UDP port, e.g., 1234
All processes must bind to the same port in order to receive the multicast messages
•Join a multicast group address•Use recv or recvfrom to read the messages
Sending Multicast Messages•You may use the same socket (you used for receiving) for sending multicast messages or you can use any other UDP socket (it does not have to join any multicast group)
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 25
Multicast on a LAN 1/3
•Receiving application creates a UDP socket, binds to port 123 and joins multicast group 224.0.1.1
•IPv4 layers saves the information internally and tells appropriate datalink to receive Ethernet frames destined to 01:00:5E:00:01:01
•Sending applications creates a UDP socket and sends a datagram to 224.0.1.1, port 123
•Ethernet frame contains destination Ethernet address, destination IP address, and destination port
•A host on the LAN that did not express interest in receiving multicast from that group will ignore such datagram
•Destination Ethernet address does not match the interface address
•Destination Ethernet address is not the ethernet broadcast address
•The interface has not been told to receive any group addresses
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 26
Multicast on a LAN 2/3
•Ethernet frame received by datalink of receiver based on imperfect filtering (When interface told to receive frames destined to one specific Ethernet multicast address, it can receive frames destined to other Ethernet multicast addresses)
Ethernet interface cards apply a hash function to group address, calculating a value between 0 and 511. This information turns on a bit in a 512-bit arraySmall size bit-array implies receiving unwanted framesSome network cards provide perfect filteringSome network cards have no multicast filtering at all (multicast promiscuous mode)
•Packet passed to IP layer (IP layer compares group address against all multicast addresses that applications on this host have joined perfect filtering)
•Packet passed to UDP layer, which passes it to socket bound to port 123
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 27
Multicast on a LAN 3/3
Some Other scenarios
•A host running an application that has joined 225.0.1.1 Ethernet address 01:00:5E:00:01:01. Packet will be discarded by perfect filtering in IP layer
•A host running an application that has joined some multicast group which the Ethernet address produces the same hash value as 01:00:5E:00:01:01. Packet will be discarded by datalink layer or by IP layer
•A packet destined to the same group, but a different port. Accepted by IP layer, but discarded by UDP layer (no socket has bound the different port)
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 28
Multicast on a WAN•A program started on five hosts belonging to different LANs
•Multicast routers communicate with neighbor routers using a multicast routing protocol (MRP)
•When a process on a host joins a multicast group, that host sends an IGMP message to any attached multicast routers, which in turn exchange this information using MRP with neighbor routers
•When a sender sends a multicast message, mutlicast routing information is used to direct the message
MR1
MR2 MR3 MR4
MR5H1
H3H2
H4 H5
S
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 29
Some Multicast IssuesTime To Live
Set TTL for outgoing multicast datagrams (default is 1 local subnet)
Loopback mode
•Enable or disable local loopback of multicast datagrams
•By default loopback is enabled
•A copy of each multicast datagram sent by a process on the host will also be looped back and processed as a received datagram by that host
Port Reuse
•Allow the same multicast application to have several instances running on the same host
•In Java, Port reuse is enabled by default
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 30
Socket Options
•Various attributes that are used to determine the behavior of sockets (see chapter 7)#include <sys/socket.h>int getsockopt (int sockfd, int level, int optname, void * optval, socklen_t *optlen);int setsockopt (int sockfd, int level, int optname, const void * optval, socklen_t optlen);
Both return 0 if OK, -1 on error•sockfd: an open socket descriptor•level: code in the system that interprets the option (general socket code, or protocol-specific code) (SOL_SOCKET, IPPROTO_IP, IPPROTO_IPv6, IPPROTO_TCP are examples)•optname: see page 193
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 31
Socket Options
Some socket options examples (see table on page 193 and 194)For multicast socket options see section 21.6 on page 559
•Socket LevelSO_SNDBUF, SO_RCVBUF, SO_KEEPALIVE, SO_BROADCAST, SO_REUSEADDR, SO_RESUEPORT
•IP LevelIP_TTL, IPMULTICAST_IF, IPMUTLICAST_TTL, IP_MULTICAST_LOOP, IP_ADD_MEMBERSHIP, IP_DROP_MEMBERSHIP
•TCP LevelTCP_KEEPALIVE, TCP_MAXSEG, TCP_NODELAY
Multicasting © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 32
Sending and Receiving•Section 21.10 page 575
•A program to send and receive multicast datagramsSend datagram to a specific group every five seconds (datagram contains sender’s hostname and process ID)An infinite loop that joins the multicast group to which the sending part is sending and prints every received datagram
•Create a UDP socket then set multicast socket options for address reuse, joining the group, and setting loopback
•See mcast/main.c, mcast/send.c, and mcast/recv.c