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Routing multicast messages. How to adjust our Routing Table so we can examine the IGMP group membership messages. We need a ‘quiet’ LAN.

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routing multicast messages

Routing multicast messages

How to adjust our Routing Table so we can examine the IGMP group membership messages

we need a quiet lan
We need a ‘quiet’ LAN
  • If we want to examine the actual datagram exchanges that implement IP multicasting, then we should do it on the local network that doesn’t have hundreds of irrelevant packets going by which would distract us
  • By default our Linux systems will forward multicast packets to our ‘public’ network, so we’ll need to modify our routing tables
add route for ip multicasts
Add route for IP-multicasts
  • Here is the command you can use to add a routing-table entry which will direct all of the outgoing IP-multicast packets to ‘eth1’ (which is on a much less ‘noisy’ network)

$ sudo /sbin/route add –net netmask dev eth1

using nicwatch
Using ‘nicwatch’
  • Now we can use our ‘nicwatch’ utility to let us inspect multicast packets that are sent or received using our multicasting demos



$ ./multisend

$ ./multirecv

$ ./nicwatch eth1

$ ./nicwatch eth1

transparent management
‘transparent’ management
  • We discover that some lower levels of the TCP/IP protocol stack exchange various messages in support of multitasking which are ‘unseen’ by application-layer software
  • Some examples are:
    • When ‘ifconfig’ changes an interface’s state
    • When a host ‘joins’ a multicast group
    • When a host ‘leaves’ a multicast group
special multicast addresses
Special multicast addresses
  • Here are some multicast addresses which are dedicated to specific purposes:
    • All systems on this local subnet
    • All routers on this local subnet
    • IGMP membership messages
    • mDNS (multicast name lookup)
    • ‘Link-Local’ name resolution
effect of ifconfig
Effect of ‘ifconfig’

This UDP datagram was sent to the multicast-DNS group address (

by the ‘eth1’ interface on hrn23528 when the ‘/sbin/ifconfig’ command was used

to assign an IP-address to that interface, and simultaneously to bring it ‘UP’

reliance on routers
Reliance on routers
  • Multicasting works by relying on routers to make copies of multicast packets for hosts on a router’s local subnet that have asked to receive them


single copy of an arriving

multicast datagram

multiple copies are forwarded to

the multicast group’s ‘members’

hosts and routers
Hosts and Routers



IGMP Report

IGMP Query

Two types of IGMP Host Membership messages

are used in IGMP version 1

Type 1 = Query, Type 2 = Report





Multicast group-address (or zero)

igmp version 1
IGMP version 1
  • Whenever a host joins a multicast group, it sends an IGMP Host Membership Report to the specific group multicast address
  • A multicast router listens for all multicast messages, so it will process these IGMP Host Membership Reports upon receipt
  • And periodically a router will send a Host Membership Query, to refresh awareness
igmp version 2
IGMP version 2
  • A third type of Host Membership Message is added to the IGMP protocol, called the Leave Group message, resulting in more prompt awareness by the router that the group’s membership has been changed, so it can stop forwarding messages to a host that is no longer a group-member
igmp version 3
IGMP version 3
  • Some additional message-types added to allow the various routers on a network to stay informed about group memberships, and expansion of some former message-types to allow more member-information within each individual IGMP message, thus reducing the total ‘overhead’ traffic
tcp ip protocol stack
TCP/IP Protocol Stack





Application Layer



Transport Layer




Network Layer




Token Ring

Link Layer

igmp datagrams
IGMP datagrams

MAC header

IP header

IGMP packet

Type: 1=Host Membership Query, 2=Host Membership Reply,

3=DVMRP (Distance Vector Multicast Routing Protocol)



Max time



IGMP Checksum

Group Address in Reply (or zeroed in Query)

stream of tagged data

in DVMRP messages


Recall IP header format

32 bits





Type of


Total Length

(in bytes)




Fragment offset




Header Checksum

Source IP-address

Destination IP-address


when multirecv starts
When ‘multirecv’ starts…

These duplicate packets were sent to the IGMP multicast group (

from the ‘eth1’ interface on hrn23528 when our ‘multirecv’ program started,

with a brief, but noticible, time-delay in between these two transmissions

when multirecv quits
When ‘multirecv’ quits…

These duplicate packets were sent to the IGMP multicast group (

from the ‘eth1’ interface on hrn23528 when our ‘multirecv’ program ended,

with a brief, but noticible, time-delay in between these two transmissions

router alert
‘router alert’
  • Notice that the IGMP membership report messages include an IP option, known as the ‘router alert’ option
  • It’s a 2-byte option: 0x94 0x04 (but then it’s padded with NOPs to fill out 32-bits)
  • Notice also that the IP-header’s TOS field uses precedence code 6, signaling that it’s an ‘internetwork management’ datagram
subnet to subnet hops
Subnet-to-subnet ‘hops’
  • All of our classroom, CS Lab, and anchor-cluster machines, as well as the ‘stargate’ gateway-server, belong to the university’s subnetwork
  • The CS department’s machines named ‘steelhead’, ‘spaten’ and ‘stella’ belong to another university subnet:
  • So internetworking goes through a router
distinct subnets
Distinct subnets



‘stella’ subnet







‘stargate’ subnet

Note: Our router’s current firewall configuration blocks multicast packets sent

by hosts on the subnet -- but ‘stargate’ and ‘lectern01’ are setup

as ‘exceptions’ to that general firewall policy (for purposes of our class demo)

ttl 1
TTL == 1?
  • By default, the Time-to-Live field in the IP header for multicast packets is set to 1
  • This means routers won’t ‘forward’ these multicast packets beyond the local subnet
  • But we can ‘adjust’ this default TTL-value in our multicasting application programs if we use the ‘setsockopt()’ socket-function
  • Our ‘multihop.cpp’ demo-program does it
ip multicast ttl
  • Here is how our ‘multisend.cpp’ demo can be modified to allow its multicast packets to ‘hop’ from one subnet to the next one
  • Our ‘multihop.cpp’ demo lets a user enter a desired default-value for TTL multicasts

int oval = 2;

int olen = sizeof( oval );

if ( setsockopt( sock, SOL_IP, IP_MULTICAST_TTL, &oval, olen ) < 0 )

{ perror( “setsockopt IP_MULTICAST_TTL” ); exit(1); }