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					Routing and Forwarding


       4/17/2012




                         1
Recap

 Internet routing is divided into intra-AS
  (intradomain) and inter-AS (interdomain)
  routing
    Technical reasons
    Non-technical reasons


 BGP (Border Gateway Protocol), a path-
  vector protocol, is the de facto standard
  interdomain protocol
                                                   AS D

    Routing: Example                               d
                                                              d1

E                                                                      d2
                           AS A
                           (OSPF)

                              a2


F          i

                             a1                        AS C

                   How to specify?
                                     AS B
                                     (OSPF intra
                                      routing)


                                      b




           AS I
                                                                   3
IP Addressing Scheme: Requirements

 We need an address to uniquely identify
  each destination

 Routing scalability needs flexibility in
  aggregation of destination addresses
     we should be able to aggregate a set of
      destinations as a single routing unit

 Preview: the unit of routing in the Internet
  is a network---the destinations in the routing
  protocols are networks

                                                   4
IP Address: An IP Address Identifies an Interface

 IPv4 address: 32-bit                 223.1.1.1

    identifier for an                                            223.1.2.1
    interface                          223.1.1.2
                                              223.1.1.4    223.1.2.9


   interface:                        223.1.1.3     223.1.3.27
                                                                 223.1.2.2

       routers typically
        have multiple
        interfaces
       host may have                   223.1.3.1                223.1.3.2
        multiple interfaces

    %/sbin/ifconfig -a
                              223.1.3.2 = 11011111 00000001 00000011 00000010

                                          223         1          3           2


                                                                                 5
IP Addressing                                          223.1.1.2



 IP address:                        223.1.1.1                     223.1.1.4

    network part
                                                   223.1.1.3
    host part

                                           223.1.9.2         223.1.7.0
   What’s a network
    ? (from IP address
    perspective)
      is a unit of routing:
                               223.1.9.1                                   223.1.7.1
       can be routed                         223.1.8.1     223.1.8.0

       together (depend          223.1.2.6                             223.1.3.27
       on the routing
       protocol)

                               223.1.2.1 223.1.2.2                           223.1.3.2
                                                               223.1.3.1

                                                                                         6
 IP Addressing: Class-ful Addressing
given notion of “network”, let’s re-examine IP
   addresses:
“class-ful” addressing in the original IP design:

                                             1.0.0.0 to
                                             127.255.255.255

                                              128.0.0.0 to
                                              191.255.255.255
                                             192.0.0.0 to
                                             223.255.255.255
                                             224.0.0.0 to
                                             239.255.255.255

                                             240.0.0.0 to
                                             255.255.255.255

Problem of class-ful addressing?
                                                                7
   IP Addressing: CIDR
    (Static)classful addressing:
          inefficient use of address space, address space exhaustion
            • e.g., a class A net allocated enough addresses for 16 million
              hosts; a class B address may also be too big
          not flexible for aggregation

    CIDR: Classless InterDomain Routing
          network portion of address of arbitrary length
          address format: a.b.c.d/x, where x is # bits in network
           portion of address

                          network                          host
                            part                           part
           11001000 00010111 00010000 00000000
                            200.23.16.0/23
Some systems use mask (1’s to indicate network bits), instead of the /x format
                                                                              8
                                                                   AS D

CIDR Address Aggregation                                            d
                                                                          d1


                                              AS A
                                              (OSPF)

                                                a2
 130.132.1/24
                       i->a1: I can reach
                       130.132/22; my
                i      path: I
                                                a1




                                                       intradomain
                                                       routing uses /24
     130.132.2/24

                                            130.132.3/24
                AS I
                                                                               9
CIDR Address Aggregation

                                    B

                          x00/24: B


            A           x01/24: C       C



                         x10/24: E
                                            E
            x11/24: F
        G

                         F
                          F

                                                10
Routing Table Size of BGP (number of globally
advertised, aggregated networks)




 Active BGP Entries (http://bgp.potaroo.net/as1221/bgp-active.html)
 Internet Growth
 (http://www.caida.org/research/topology/as_core_network/historical.xml)
                                                                     11
Routing Table Prefix Length Distr.
   180000


   160000


   140000


   120000


   100000


   80000


   60000


   40000


   20000


       0
            8   13   18   23   28




                                     12
IP Addressing: How to Get One?

Q: How does an ISP get its block of
 addresses?

A: ICANN: Internet Corporation for Assigned
   Names and Numbers
    allocates addresses
    manages DNS
    assigns domain names, resolves disputes

 Use
 %whois –h whois.arin.net “n <org>”
 to check addresses allocated to <org>
                                               13
IP addresses: How to Get One?
Q: How does a host get an IP address?

 Static configured
    wintel: control-panel->network->configuration-
     >tcp/ip->properties
    unix:
     %/sbin/ifconfig eth0 inet 192.168.0.10 netmask
     255.255.255.0

 DHCP: Dynamic Host Configuration Protocol:
  dynamically get address from as server
    “plug-and-play”


                                                      14
DHCP: Dynamic Host Configuration Protocol

 Goal: allow host to dynamically obtain its IP address
   from network server when it joins network
       can renew its lease on address in use
       allows reuse of addresses (only hold address while
        connected)
       support for mobile users who want to join network

 DHCP msgs:
    host broadcasts “DHCP discover” msg
    DHCP server responds with “DHCP offer” msg
    host requests IP address: “DHCP request” msg
    DHCP server sends address: “DHCP ack” msg



                                                             15
 Network Address Translation: Motivation
  A local network uses just one public IP address as far as outside
 world is concerned
  Each device on the local network is assigned a private IP address

            rest of                          local network
           Internet                      (e.g., home network)
                                            192.168.1.0/24             192.168.1.2

                                    192.168.1.1
                                                                        192.168.1.3
                  138.76.29.7

                                                                       192.168.1.4


  All datagrams leaving local          Datagrams with source or
network have same single source        destination in this network
 NAT IP address: 138.76.29.7,        have 192.168.1/24 address for
 different source port numbers        source, destination (as usual)
                                                                             16
Private IP




             17
  NAT: Network Address Translation
Implementation: NAT router must:

     outgoing datagrams: replace (source IP address, port
      #) of every outgoing datagram to (NAT IP address,
      new port #)
       . . . remote clients/servers will respond using (NAT
          IP address, new port #) as destination addr.

     remember (in NAT translation table) every (source
      IP address, port #) to (NAT IP address, new port #)
      translation pair

     incoming datagrams: replace (NAT IP address, new
      port #) in dest fields of every incoming datagram
      with corresponding (source IP address, port #)
      stored in NAT table
                                                              18
     NAT: Network Address Translation
                              NAT translation table
2: NAT router                                                        1: host 192.168.1.2
                          WAN side addr    LAN side addr
changes datagram                                                     sends datagram to
                     138.76.29.7, 5001 192.168.1.2, 3345             128.119.40.186, 80
source addr from
                       ……                            ……
192.168.1.2, 3345 to
138.76.29.7, 5001,                               S: 192.168.1.2, 3345
updates table                                    D: 128.119.40.186, 80
                                                                                192.168.1.2
                                                                    1
                    S: 138.76.29.7, 5001
              2     D: 128.119.40.186, 80   192.168.1.1
                                                                               192.168.1.3
                           138.76.29.7          S: 128.119.40.186, 80
                                                D: 192.168.1.2, 3345    4
            S: 128.119.40.186, 80
            D: 138.76.29.7, 5001    3                                          192.168.1.4
                                                 4: NAT router
         3: Reply arrives                        changes datagram
         dest. address:                          dest addr from
         138.76.29.7, 5001                       138.76.29.7, 5001 to 192.168.1.2, 3345


                                                                                      19
Network Address Translation: Advantages
 No need to be allocated range of addresses
  from ISP: - just one public IP address is
  used for all devices
   16-bit port-number field allows 60,000
    simultaneous connections with a single LAN-side
    address !
   can change ISP without changing addresses of
    devices in local network
   can change addresses of devices in local network
    without notifying outside world
 Devices inside local net not explicitly
  addressable, visible by outside world (a
  security plus)
                                                       20
Network Address Translation: Problems

 If both hosts are behind NAT, they will have
  difficulty establishing connection

 NAT is controversial:
   routers should process up to only layer 3
   violates end-to-end argument
       • NAT possibility must be taken into account by app
         designers, e.g., P2P applications
     address shortage should instead be solved by
      having more addresses --- IPv6 !


                                                             21
Outline
 Admin. and recap
 BGP
 IP addressing
   IP forwarding




                     22
  IP Datagram Format
  IP protocol version
              number                    32 bits                total datagram
       header length           head. type of                   length (bytes)
              (bytes)     ver                     length
                                 len service                    for
      “type” of data                                fragment
                           16-bit identifier flgs               fragmentation/
                                                      offset    reassembly
          max number       time to upper          Internet
       remaining hops        live     layer       checksum
     (decremented at
         each router)          32 bit source IP address

  upper layer protocol       32 bit destination IP address
  to deliver payload to                                        E.g. timestamp,
                                    Options (if any)
                                                               record route
how much overhead                        data                  taken, specify
  with TCP?                        (variable length,           list of routers
 20 bytes of TCP                   typically a TCP            to visit.
                                   or UDP segment)
 20 bytes of IP
 = 40 bytes + app
  layer overhead                                                           23
Data Forwarding: Steps

 Error checking, e.g., check header checksum;
  if error, set up error flag

 Decrement TTL; if TTL == 0, set error flag


 If error, drop the packet, and generate
  ICMP report




                                                 24
   The Network Layer

   Host, router network layer functions:

                        Transport layer: TCP, UDP


           Routing protocols                The IP protocol
           •path selection                  •addressing
                                            •datagram format
Network    •RIP, OSPF, BGP

   layer                       forwarding
                                            ICMP protocol
                                            •error reporting
                                            •router “signaling”

                                   Link layer

                                 Physical layer

                                                                  25
ICMP: Internet Control Message Protocol
 communicate network-level        Type   Code   description
  information                      0      0      echo reply (ping)
    error reporting:              3      0      dest network unreachable
      unreachable host, network,   3      1      dest host unreachable
      port, protocol               3      2      dest protocol unreachable
    echo request/reply (used by   3      3      dest port unreachable
      ping)                        3      6      dest network unknown
 network-layer “above” IP:        3      7      dest host unknown
                                   4      0      source quench (congestion
    ICMP msgs carried in IP
                                                 control - not used)
      datagrams
                                   8      0      echo request (ping)
 ICMP message: type, code plus    9      0      route advertisement
  first 8 bytes of IP datagram     10     0      router discovery
  causing error                    11     0      TTL expired
                                   12     0      bad IP header
    type   code     checksum
                                   traceroute is developed by a clever use
      ICMP message body                            of ICMP
                                                                        26
Data Forwarding: Steps
 If no error, look up packet destination
  address in forwarding table:

      if datagram for a host on directly attached
       network, it is the job of the link layer now

      otherwise,
        • lookup: find next-hop router, and its outgoing interface

        • if needed, do fragmentation

        • forward packet to outgoing interface (to the next hop
          neighbor)
try %netstat –rn to see the forwarding table
                                                                     27
Forwarding Look up
                                                          default: -
  #    prefix   interface
  a)   00001                            0             1
  b)   00010                  a
  c)   00011
  d)   001
  e)   0101                                               g
  f)   011
                              d             f     h
  g)   10
  h)   100
  i)   1010                                           i
                                    e                          jk
  j)   1011
  k)   1100                       The networks are represented by
                            abc   a decision tree, e.g., a Patricia Trie
                                  to look for the longest match of
                                  the destination address
                                                                       28
Content Addressable Memory (CAM)

 Standard computer memory (Random Access
 Memory or RAM)
   Word  Read(addr)
   Write(addr, Word)

 CAM
   Word  Read(addr)
   Write(addr, Word)
   AddrList  Search(Word)




                                            29
Ternary CAM
 Binary CAM
     It searches words consisting entirely of 1s and
      0s.
 TCAM
   It allows a third matching state of “X” or “Don’t
    Care” for one or more bits in the stored data
    word.
   “10XX0” will match “10000”, “10010”, “10110”,
    “10100”.



                                                        30
Prototype of TCAM Based IP Table Lookup

Physical Address        IP Prefix (4 Bytes)       Next Hop (4 Bytes)
0x00000000              0x010203XX (1.2.3.0/24)   1
0x00000008              0x050607XX(5.6.7.0/24) 2
0x00000010              0x0102XXXX(1.2.0.0/16)    0




Keep an eye on hardware revolutions
        Solid State Drive (SDD)
        Optical Switching or Routing?




                                                                       31
Example 1 (same network): A->B
         src       dst                     forwarding table in A
  misc                                 Dest. Net. next router           Nhops
                            data
 fields 223.1.1.1 223.1.1.3
                                     223.1.1/24             1
                                     223.1.2/24 223.1.1.4   2
 Look up dest address
                                     223.1.3/24 223.1.1.4   2
 find dest is on same net           0.0.0.0/0 223.1.1.4     -
 link layer will send the                          To Internet
                                   A 223.1.1.1
   datagram directly inside a                              223.1.4.1
   link-layer frame                                             223.1.2.1
                                         223.1.1.2
                                               223.1.1.4    223.1.2.9
                                   B
                                                                  223.1.2.2
                                       223.1.1.3    223.1.3.27

                                        223.1.3.1                223.1.3.2




                                                                                32
Example 2 (Different Networks): A-> E
                                              forwarding table in A
  misc                                    Dest. Net. next router           Nhops
                            data
 fields 223.1.1.1 223.1.2.3
                                        223.1.1/24             1
                                        223.1.2/24 223.1.1.4   2
  look up dest address in
                                        223.1.3/24 223.1.1.4   2
   forwarding table
                                        0.0.0.0/0 223.1.1.4     -
  routing table: next hop
                                      A 223.1.1.1      To Internet
   router to dest is 223.1.1.4                                223.1.4.1
  link layer sends datagram to                                    223.1.2.1
   router 223.1.1.4 inside a link-          223.1.1.2
                                                               223.1.2.9
   layer frame
                                                  223.1.1.4
                                      B
       the dest. of the link layer                                  223.1.2.3
        frame is 223.1.1.4                223.1.1.3    223.1.3.27                E

                                           223.1.3.1                223.1.3.2




                                                                                   33
Example 2 (Different Networks): A-> E
                                     forwarding table in router
  misc                              Dest. Net router Nhops interface
                            data
 fields 223.1.1.1 223.1.2.3
                                    223.1.1/24        -       1    223.1.1.4

Arriving at 223.1.1.4,              223.1.2/24        -       1    223.1.2.9
                                    223.1.3/24        -       1
  destined for 223.1.2.2
                                                                   223.1.3.27
                                    0.0.0.0/0         -       -    223.1.4.1
 look up dest address in           A                       To Internet
                                         223.1.1.1
  router’s forwarding table                                 223.1.4.1
 E on same network as
                                                                 223.1.2.1
                                          223.1.1.2
  router’s interface 223.1.2.9                  223.1.1.4    223.1.2.9
     router, E directly attached   B
                                                                   223.1.2.3
 link layer sends datagram to          223.1.1.3    223.1.3.27                E
  223.1.2.2 inside link-layer
                                         223.1.3.1                223.1.3.2
  frame via interface 223.1.2.9
 datagram arrives at
  223.1.2.2!! (hooray!)
                                                                                34
What A Router Looks Like: Outside




                                    35
Look Inside a Router
Two key router functions:
 run routing algorithms/protocol (RIP, OSPF, BGP)
   switching datagrams from incoming to outgoing ports




                                                          36
Input Port Functions




     physical layer:
bit-level reception


   data link layer:
    e.g., Ethernet             network layer:
                           lookup output port
                       using forwarding table

                                                37
Switching: Low End




                     38
Switching Via An
Interconnection Network

 Overcome bus bandwidth limitations
 fragmenting datagram into fixed length
  cells, switch cells through the fabric.
 Crossbar, Banyan networks, and others




 Cisco 12416: switches 320 Gbps (upgradeable to 1.28
  Tbps) with 16 slots (each 10G full-duplex) through the
  crossbar interconnection network
                                                           39
New Potential Bottleneck: Output Ports

 Due to output port contention and head-of-the-Line
  (HOL) blocking (i.e., queued datagram at front of
  queue prevents others in queue from moving
  forward)




                                                       40
 Head-of-Line Blocking Limits Thrput

 Due to output-port contention and HOL blocking, the stable throughput
is only around 2 - sqrt(2) = 0.586 of line speed !




                                                                          41
  Avoiding Port Contention and HOB
   Virtual output queueing




   Input/output ports matching algorithm
   Switch fabric speedup, e.g., two cells to one
     output port
For more details: http://www.cisco.com/warp/public/63/arch12000-swfabric.html
                                                                        42
Output Ports
                  Buffering required when
                   datagrams arrive from
                   fabric faster than the
                   transmission rate

                  Queueing (delay) and
                   loss due to output port
                   buffer overflow !

                  Scheduling and
                   queue/buffer
                   management choose
                   among queued
                   datagrams for
                   transmission



                                             43
Backup Slides




                44
 Another Method for Checking
 Convergence: Dispute Wheels
  A dispute
      wheel                               u2             R2
       u1 prefers           R1
        R1Q2 over Q1
       u2 prefers
                                               Q2

        R2 Q3 over                  Q1              Q3
        Q2              u1
                                          d
                                                              u3

       etc
                  210                Qn
                  20    Rn
              2                                               R3
                               un

              0
130                 3    320
10
      1                  30
                                                               45
Patterns of Valid Routes
 Consider how a path is               Valid consecutive link types
  extended according to the             (starting from source to
  export policies                       destination, i.e., reserve)
      case 1 (format: Destination,         PC PC
       link type, …)                        CP PC
        • Dest, …CP  Dest, … CP CP         PP PC
        • Dest, …CP  Dest, … CP PC
                                            CP CP
        • Dest, …CP  Dest, … CP PP
                                            CP PP
      case 2
        • Dest, …PC  Dest, … PC PC
      case 3
        • Dest, …PP  Dest, … PP PC




                                                                       46
 Case 1: The first link of Q1 is a PC link
 Then the first link of R1Q2 must be a PC
  link
      because u1 chooses R1Q2 and C > E/P
 Thus all remaining links along R1Q2 are PC
  links                                                               PC    R1
      because only PC follows PC                               u1
                                                           PC
 Thus the first link of Q2 is                        Rn
  a PC link
                                                                 Q1
                                                                                 u2
 ……
                                         un          Qn

 All links along R1, …, Rn are                             d          Q2
  PC links                                    Rn-1
                                                                             R2
 The network has a PC loop.
  contradiction !
                                                                                 47
  Case 2: The first link of Q1 is a CP/PP link
 All links along Rn are CP links because all
  links before CP/PP are CP
 The first link of Qn must be a CP/PP link
       because C > E/P
 ……                                                    CP
                                                                     u1
                                                        Rn   CP/PP

                                                                      Q1
                                                       Qn                       u2
                                           un

                                            CP/PP                          Q2
 All links along R1, …, Rn are CP links                       d
                                                Rn-1
 The network has a PC loop.                                                    R2
   contradiction !

                                                                                48
Backup: IP Multicast




                       49
  IP Fragmentation & Reassembly
 Network links have MTU
  (max.transfer size) -
  largest possible link-level
  frame.                                     fragmentation:
    different link types,                   in: one large datagram
      different MTUs, e.g.                   out: 3 smaller datagrams
      Ethernet MTU is 1500
      bytes
 Large IP datagram divided
  (“fragmented”)                reassembly
    one datagram becomes
      several datagrams
    “reassembled” only at
      final destination
    IP header bits used to
      identify, order related
      fragments

                                                                50
 IP Fragmentation and Reassembly
                       length ID fragflag offset
Example                =4000 =x     =0      =0
 4000 byte
                     One large datagram becomes
  datagram           several smaller datagrams
 MTU = 1500 bytes
                           length ID fragflag offset
                           =1500 =x     =1      =0

                           length ID fragflag offset
                           =1500 =x     =1    =1480

                           length ID fragflag offset
                           =1040 =x     =0    =2960




                                                       51
IP Multicast: Service Model
                                           128.59.16.12


128.119.40.186

                                 multicast                    128.34.108.63
                                   group
                               226.17.30.197
                                                                  128.34.108.60


   Multicast group concept: use of indirection
        A group is identified by a location-independent
         logical address (class D IP address: prefix 1110)
   Open group model
        Anyone can send packets to the “logical” group address
        Anyone can join a group and receive packets
   Normal, best-effort delivery semantics of IP
    Needed: infrastructure to deliver mcast-addressed datagrams to
    all hosts that have joined that multicast group
                                                                                  52
Multicast Across LANs
   Goal: find a tree (or trees) connecting routers having
    local mcast group members
       source-based: different tree from sender to each receiver
             – Distance-vector multicast routing protocol (DVMRP)
             – Protocol-independent multicast-dense mode (PIM-DM)
       shared-tree: same tree used by all group members
             – Core-Based Tree (CBT)
             – Protocol-independent multicast-sparse mode (PIM-SM)




             shared tree                  source-based trees
                                                                     53
Source Tree:
Reverse Path Flooding (RPF)
 A router x forwards a packet from source (S)
  iff it arrives via neighbor y, and y is on the
  shortest path from x back to S
 A packet is replicated to all but the incoming
  interface
                                              S
                                          1       1

                                                          y
                                  x
                                                              1

                                      1                   z

                                                      1
                                              t

                                                                  a
                                                                  54
Reverse Path Forwarding:
Improvement
 Basic idea: forward a packet from S only on
  child links for S
 A child link of router x for source S
   a link that has x as parent on
    the shortest path from the
                                          S
    link to S
   a child x notifies its parent y   x
                                              y
    (through the routing protocol)
    that it has selected y as its             z
    parent
                                          t

                                                  a
                                                  55
Reverse Path Forwarding: Pruning
 No    need to forward datagrams down subtree
   with no mcast group members
 “prune” msgs sent upstream by router with
   no downstream group members

S: source                           LEGEND

            R1                           router with attached
                          R4
                                         group member

      R2                                 router with no attached
                      P                  group member
                                     P
                               R5         prune message
 R3                    P                 links with multicast
                 R6   R7                 forwarding


                                                                   56
Pruning
 Prune (Source, Group) at a leaf router if no members
    send No-Membership Report (NMR) up tree

 If all children of router R prune (S,G)
    propagate prune for (S,G) to its parent

 What do you do when a member of a group (re)joins?
   send a Graft message to upstream parent

 How to deal with failures?
   prune dropped
   flow is reinstated
   down stream routers re-prune

 Note: again a soft-state approach



                                                         57
Implementation of
Source Trees in the Internet
 Multicast OSFP (MOSFP)
   Membership is part of the link state distribution; calculate
    source specific, pre-pruned trees

 Reverse Path Forwarding
    Distance Vector Multicast Routing Protocol (DVMRP)
    Protocol Independent Multicast – Dense Mode (PIM-DM)
        • very similar to DVMRP

      Difference: PIM uses any unicast routing algorithm to
       determine the path from a router to the source; DVMRP
       uses distance vector

      Question: the state requirement of Reverse Path
       Forwarding
                                                                   58
Building a Shared Tree
 Steiner Tree: minimum cost
    tree connecting all routers
    with attached group members
   A Steiner tree is not a
    spanning tree because you
    do not need to connect all
    nodes in the network
   Problem is NP-hard
   Excellent heuristics exists
   Not used in practice:
       computational complexity
       information about entire network needed
       monolithic: rerun whenever a router needs to join/leave

                                                                  59
Center (Core) based Shared Tree
 Single delivery tree shared by all
 One router identified as      “center” of tree
 Tree construction is receiver-based
    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
 A sender unicasts a packet to center
    The packet is distributed on the tree when it hits the tree

                                                                   60
Example: M3 Joins
 Group members: M1, M2

                                                        core


                    M1




                            M2                             M3

      shared tree
     join message
                                           S1


Discussion: what is property of the constructed tree?
                                                                61
Example: M1 Sends Data
 Group members: M1, M2, M3
 M1 sends data
                                       core


               M1




                             M2           M3



   control (join) messages
   data                           S1



                                               62
Shared Tree Protocols in the Internet

 Core Based Tree
 Protocol Independent Multicast (PIM)
  Sparse mode
 The catch: how do you know the center?
     session announcement




                                           63
Mbone: Tunneling
Q: How to connect “islands” of multicast
 routers in a “sea” of unicast routers?




            physical topology        logical topology


 mcast datagram encapsulated inside “normal” (non-multicast-
  addressed) datagram
 normal IP datagram sent thru “tunnel” via regular IP unicast
  to receiving mcast router
 receiving mcast router unencapsulates to get mcast datagram

                                                                 64

				
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