Exterior Routing Protocols

1
Chapter 16

• Exterior Routing Protocols
• And Multicasting

2
Problems with Distance-Vector and Link-State
Routing

• Neither distance-vector (RIP) nor link state
  (OSPF) protocols effective for exterior routing
• Distance vector and link state protocols assume
  all routers share common metric
• Priorities and restrictions may differ between
  ASs
• Flooding of link state information may become
  unmanageable

3
Path Vector Routing

• Dispense with routing metrics
• Provide information about
• Which networks can be reached by given router
• Which ASs must be crossed to get there
• No distance or cost element
• Routing information includes all Ass visited to
  reach destination
• Allows policy routing

4
Boarder Gateway Protocol (BGP)

• Allows routers (gateways) in different ASs to
  exchange routing information
• Messages sent over TCP
• See next slide
• Three functional procedures
• Neighbor acquisition
• Neighbor reachability
• Network reachability

5
BGP v4 Messages

• Open
• Start neighbor relationship with another router
• Update
• Transmit information about single route
• List multiple routes to be withdrawn
• Keepalive
• Acknowledge open message
• Periodically confirm neighbor relationship
• Notification
• Send when error condition detected

6
Neighbor Acquisition

• Neighbors attach to same subnetwork
• If in different ASs routers may wish to exchange
  information
• Neighbor acquisitionis when two neighboring
  routers agree to exchange routing information
  regularly
• Needed because one router may not wish to take
  part
• One router sends request, the other acknowledges
• Knowledge of existence of other routers and need
  to exchange information established at
  configuration time or by active intervention

7
Neighbor Reachability

• Periodic issue of keepalive messages
• Between all routers that are neighbors

8
Network Reachability

• Each router keeps database of subnetworks it can
  reach and preferred route
• When change made, router issues update message
• All BGP routers build up and maintain routing
  information

9
BGP MessageFormats

• Marker
• Reserved for authentication
• Length
• In octets
• Type
• Open, Update, Keepalive, Notification

10
Neighbor Acquisition Detail

• Router opens TCP connection with neighbor
• Sends open message
• Identifies senders AS and gives IP address
• Includes Hold Time
• As proposed by sender
• If recipient prepared to open neighbor
  relationship
• Calculate hold time
• min own hold time, received hold time
• Max time between keepalive/update messages
• Reply with keepalive

11
Keepalive Detail

• Header only
• Often enough to prevent hold time expiring

12
Update Detail

• Information about single route through internet
• Information to be added to database of any
  recipient router
• Network layer reachability information (NLRI)
• List of network portions of IP addresses of
  subnets reached by this route
• Total path attributes length field
• Path attributes field (next slide)
• List of previously advertised routes being
  withdrawn
• May contain both

13
Path Attributes Field

• Origin
• Interior (e.g. OSPF) or exterior (BGP) protocol
• AS_Path
• ASs traversed for this route
• Next_Hop
• IP address of boarder router for next hop
• Multi_Exit_disc
• Information about routers internal to AS
• Local_Pref
• Tell other routers within AS degree of preference
• Atomic_Aggregate, Aggregator
• Uses subnet addresses in tree view of network to
  reduce information needed in NLRI

14
Withdrawal of Route(s)

• Route identified by IP address of destination
  subnetwork(s)

15
Notification Message

• Error notification
• Message header error
• Includes authentication and syntax errors
• Open message error
• Syntax errors and option not recognised
• Proposed hold time unacceptable
• Update message error
• Syntax and validity errors
• Hold time expired
• Finite state machine error
• Cease
• Close connection in absence of any other error

16
Diagram for BGP Routing Information Exchange
17
BGP Routing Information Exchange

• R1 constructs routing table for AS1 using OSPF
• R1 issues update message to R5 (in AS2)
• AS_Path identity of AS1
• Next_Hop IP address of R1
• NLRI List of all subnets in AS1
• Suppose R5 has neighbor relationship with R9 in
  AS3
• R9 forwards information from R1 to R9 in update
  message
• AS_Path list of ids AS2,AS1
• Next_Hop IP address of R5
• NLRI All subnets in AS1
• R9 decides if this is prefered route and forwards
  to neighbors

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Inter-Domain Routing Protocol (IDRP)

• Exterior routing protocol for IPv6
• ISO-OSI standard
• Path-vector routing
• Superset of BGP
• Operates over any internet protocol (not just
  TCP)
• Own handshaking for guaranteed delivery
• Variable length AS identifiers
• Handles multiple internet protocols and address
  schemes
• Aggregates path information using routing domain
  confederations

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Routing Domain Confederations

• Set of connected AS
• Appear to outside world as single AS
• Recursive
• Effective scaling

20
Multicasting

• Sending message to multicast address
• Multicast address refers to a group of hosts
• Multimedia
• Teleconferencing
• Databases
• Distributed computation
• Real-time workgroup

21
Multicasting within LAN

• MAC level multicast addresses
• IEEE 802 uses highest order bit 1
• All stations that recognise the multicast address
  accept the packet
• Works because of broadcast nature of LAN
• Packet only sent once
• Much harder on internet

22
Example Configuration for Multicast Internet
23
Broadcast

• Assume location of recipients not know
• Send packet to every network
• Packet addressed to N3 traverses N1, link L3, N3
• Router B translates IP multicast address to MAC
  multicast address
• Repeat for each network
• Generates lots of packets
• In example, 13

24
Multiple Unicast

• Location of each member of multicast group known
  to source
• Table maps multicast address to list of networks
• Only need to send to networks containing members
  of multicast group
• Reduced traffic (a bit)
• In example, 11

25
True Multicast

• Least cost path from source to each network
  containing member of group is determined
• Gives spanning tree configuration
• For networks containing group members only
• Source transmits packet along spanning tree
• Packet replicated by routers at branch points of
  spanning tree
• Reduced traffic
• In example, 8

26
Multicast Transmission Example
27
Requirements for Multicasting (1)

• Router must forward two or more copies of
  incoming packet
• Addressing
• IPv4 uses class D
• Start 1110 plus 28 bit group id
• IPv6 uses 8 bit prefix of all 1s, 4 bit flags
  field, 4 bit scope field 112 bit group id
• Node must translate between multicast address and
  list of networks containing members of group
• Router must translate between IP multicast
  address and subnet multicast address to deliver
  to destination network

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Requirements for Multicasting (2)

• Multicast addresses may be permanent or dynamic
• Individual hosts may join or leave dynamically
• Need mechanism to inform routers
• Routers exchange information on which subnets
  contain members of groups
• Routers exchange information to calculate
  shortest path to each network
• Need routing protocol and algorithm
• Routes determined based on source and destination
  addresses
• Avoids unnecessary duplication of packets

29
Internet Group Management Protocol (IGMP)

• TypeMembership query (general or group
  specific), membership report, leave group, max.
  response time
• Checksum uses IPv4 algorithm
• Group address zero for request, valid IP
  multicast for report or leave

30
IGMP Operation

• Host uses IGMP to make itself know as member of
  group to other hosts and routers
• To join, send IGMP membership report message
• Send to multicast destination of group being
  joined
• Routers periodically issue IGMP query
• To all-hosts multicast address
• Hosts respond with report message for each group
  to which it belongs
• Only one host in group needs to respond to keep
  group alive
• Host keeps timer and reponds if no other reply
  heard in time
• Host sends leave group message
• Group specific query from router determins if any
  members remain

31
Group Membership with IPv6

• Function incorporated in ICMPv6
• Includes all ICMPv4 plus IGMP
• Includes group membership query and report
• Addition of new group membership termination
  message

32
Multicast Extension to OSPF (MOSPF)

• Enables routing of IP multicast datagrams within
  single AS
• Each router uses MOSPF to maintain local group
  membership information
• Each router periodically floods this to all
  routers in area
• Routers build shortest path spanning tree from a
  source network to all networks containing members
  of group (Dijkstra)
• Takes time, so on demand only

33
Forwarding Multicast Packets

• If multicast address not recognised, discard
• If router attaches to a network containing a
  member of group, transmit copy to that network
• Consult spanning tree for this source-destination
  pair and forward to other routers if required

34
Equal Cost Multipath Ambiguities

• Dijkstra algorithm will include one of multiple
  equal cost paths
• Which depends on order of processing nodes
• For multicast, all routers must have same
  spanning tree for given source node
• MOSPF has tiebreaker rule

35
Interarea Multicasting

• Multicast groups amy contain members from more
  than one area
• Routers only know about multicast groups with
  members in its area
• Subset of areas border routers forward group
  membership information and multicast datagrams
  between areas
• Interarea multicast forwarders

36
Inter-AS Multicasting

• Certain boundary routers act as inter-AS
  multicast forwarders
• Run and inter-AS multicast routing protocol as
  well as MOSPF and OSPF
• MOSPF makes sure they receive all multicast
  datagrams from within AS
• Each such router forwards if required
• Use reverse path routing to determine source
• Assume datagram from X enters AS at point
  advertising shortest route back to X
• Use this to determine path of datagram through
  MOSPF AS

37
MOSPF Routing Illustration
38
Multicast Routing Protocol Characteristics

• Extension to existing protocol
• MOSPF v OSPF
• Designed to be efficient for high concentration
  of group members
• Appropriate with single AS
• Not for large internet

39
Protocol Independent Multicast (PIM)

• Independent of unicast routing protocols
• Extract required routing information from any
  unicast routing protocol
• Work across multiple AS with different unicast
  routing protocols

40
PIM Strategy

• Flooding is inefficient over large sparse
  internet
• Little opportunity for shared spanning trees
• Focus on providing multiple shortest path unicast
  routes
• Two operation modes
• Dense mode
• For intra-AS
• Alternative to MOSPF
• Sparse mode
• Inter-AS multicast routing

41
Spares Mode PIM

• A spare group
• Number of networks/domains with group members
  present significantly small than number of
  networks/domains in internet
• Internet spanned by group not sufficiently
  resource rich to ignore overhead of current
  multicast schemes

42
Group Destination Router Group Source Router

• Group Destination Router
• Has local group members
• Router becomes destination router for given group
  when at least one host joins group
• Using IGMP or similar
• Group source router
• Attaches to network with at least one host
  transmitting on multicast address via that router

43
PIM Approach

• For a group, one router designated rendezvous
  point (RP)
• Group destination router sends join message
  towards RP requesting its members be added to
  group
• Use unicast shortest path route to send
• Reverse path becomes part of distribution tree
  for this RP to listeners in this group
• Node sending to group sends towards RP using
  shortest path unicast route
• Destination router may replace group-shared tree
  with shortest path tree to any source
• By sending a join back to source router along
  unicast shortest path
• Selection of RP dynamic
• Not critical

44
Example of PIM Operation