1. Computer Networks Chapter 4: Advanced Internetworking

2. The University of Adelaide, School of Computer Science
11 April 2017
Problems
How do we build a routing system that can handle hundreds of thousands of networks and billions of end nodes?
How to handle address space exhaustion of IPv4?
How to enhance the functionalities of Internet?
Chapter 2 — Instructions: Language of the Computer

3. The University of Adelaide, School of Computer Science
11 April 2017
Chapter Outline
Global Internet
Multicast
Mobile IP
Chapter 2 — Instructions: Language of the Computer

4. The University of Adelaide, School of Computer Science
11 April 2017
Chapter Goal
Understanding the scalability of routing in the Internet
Discussing IPv6
Understanding the concept of multicasting
Discussing Mobile IP
Chapter 2 — Instructions: Language of the Computer

5. The University of Adelaide, School of Computer Science
11 April 2017
The Global Internet
The tree structure of the Internet in 1990
Chapter 2 — Instructions: Language of the Computer

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11 April 2017
The Global Internet
A simple multi-provider Internet
Chapter 2 — Instructions: Language of the Computer

7. Interdomain Routing (BGP)
Internet is organized as autonomous systems (AS) each of which is under the control of a single administrative entity
Autonomous System (AS)
corresponds to an administrative domain
examples: University, company, backbone network
A corporation’s internal network might be a single AS, as may the network of a single Internet service provider

8. A network with two autonomous system
Interdomain Routing
A network with two autonomous system

9. Route Propagation
Idea: Provide an additional way to hierarchically aggregate routing information is a large internet.
Improves scalability
Divide the routing problem in two parts:
Routing within a single autonomous system
Routing between autonomous systems
Another name for autonomous systems in the Internet is routing domains
Two-level route propagation hierarchy
Inter-domain routing protocol (Internet-wide standard)
Intra-domain routing protocol (each AS selects its own)

10. EGP and BGP Inter-domain Routing Protocols
Exterior Gateway Protocol (EGP)
Forced a tree-like topology onto the Internet
Did not allow for the topology to become general
Tree like structure: there is a single backbone and autonomous systems are connected only as parents and children and not as peers
Border Gateway Protocol (BGP)
Assumes that the Internet is an arbitrarily interconnected set of ASs.
Today’s Internet consists of an interconnection of multiple backbone networks (they are usually called service provider networks, and they are operated by private companies rather than the government)
Sites are connected to each other in arbitrary ways

11. BGP
Some large corporations connect directly to one or more of the backbone, while others connect to smaller, non-backbone service providers.
Many service providers exist mainly to provide service to “consumers” (individuals with PCs in their homes), and these providers must connect to the backbone providers
Often many providers arrange to interconnect with each other at a single “peering point”

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11 April 2017
The Global Internet
Multihomed AS
Transit AS
Stub AS
A simple multi-provider Internet
Chapter 2 — Instructions: Language of the Computer

13. BGP-4: Border Gateway Protocol
Assumes the Internet is an arbitrarily interconnected set of AS's.
Define local traffic as traffic that originates at or terminates on nodes within an AS, and transit traffic as traffic that passes through an AS.
We can classify AS's into three types:
Stub AS: an AS that has only a single connection to one other AS; such an AS will only carry local traffic (small corporation in the figure of the previous page).
Multihomed AS: an AS that has connections to more than one other AS, but refuses to carry transit traffic (large corporation at the top in the figure of the previous page).
Transit AS: an AS that has connections to more than one other AS, and is designed to carry both transit and local traffic (backbone providers in the figure of the previous page).

14. BGP
The goal of Inter-domain routing is to find any loop free path to the intended destination
Reachability is more concerned than optimality
Finding path close to optimal is considered to be a great achievement
Why?

15. BGP
Scalability: An Internet backbone router must be able to forward any packet destined anywhere
Having a routing table that will provide a match for any valid IP address
Autonomous nature of the domains
Not possible to calculate meaningful path costs for a path that crosses multiple ASs
A cost of 1000 across one provider might imply a great path but it might mean an unacceptable bad one from another provider
Issues of trust
Provider A might be unwilling to believe certain advertisements from provider B

16. BGP Each AS has: One BGP speaker that advertises:
local networks
other reachable networks (transit AS only)
gives path information
In addition to the BGP speakers, the AS has one or more border “gateways” which need not be the same as the speakers
The border gateways are the routers through which packets enter and leave the AS

17. BGP
BGP does not belong to either of the two main classes of routing protocols (distance vectors and link-state protocols)
BGP advertises complete paths as an enumerated lists of ASs to reach a particular network

18. Example of a network running BGP
BGP Example
Example of a network running BGP

19. BGP Example Speaker for AS 2 advertises reachability to P and Q
Network , , , and , can be reached directly from AS 2.
Speaker for backbone network then advertises
Networks , , , and can be reached along the path <AS 1, AS 2>.
Speaker can also cancel previously advertised paths

20. BGP Issues
It should be apparent that the AS numbers carried in BGP need to be unique
For example, AS 2 can only recognize itself in the AS path in the example if no other AS identifies itself in the same way
AS numbers are 16-bit numbers assigned by a central authority

21. Integrating Interdomain and Intradomain Routing
All routers run iBGP and an intradomain routing protocol. Border routers (A, D, E) also run eBGP to other ASs

22. Integrating Interdomain and Intradomain Routing
BGP routing table, IGP routing table, and combined table at router B

23. Routing Areas Backbone area Area border router (ABR)
A domain divided into areas

24. Next Generation IP (IPv6)

25. Major Features 128-bit addresses Multicast Real-time service
Authentication and security
Auto-configuration
End-to-end fragmentation
Enhanced routing functionality, including support for mobile hosts

26. IPv6 Addresses Classless addressing/routing (similar to CIDR)
Notation: x:x:x:x:x:x:x:x (x = 16-bit hex number)
contiguous 0s are compressed: 47CD::A456:0124
IPv6 compatible IPv4 address: ::
Address assignment
provider-based
geographic

27. IPv6 Header 40-byte “base” header
Extension headers (fixed order, mostly fixed length)
fragmentation
source routing
authentication and
security
other options

28. Internet Broadcast/ Multicast Routing

29. creation/transmission
Broadcast Routing
Deliver packets from source to all other nodes
Source duplication is inefficient: R1 R2 R3 R4
source duplication
in-network
duplication
duplicate
creation/transmission
source duplication: how does source determine recipient addresses?

30. In-network duplication
Flooding: when node receives broadcast packet, sends copy to all neighbors
Problems: cycles & broadcast storm
Controlled flooding: node only broadcasts packet if it hasn’t broadcast same packet before
Node keeps track of packet ids already broadcasted
Or reverse path forwarding (RPF): only forward packet if it arrived on shortest path between node and source
Spanning tree
No redundant packets received by any node

31. Spanning Tree First construct a spanning tree
Nodes forward copies only along spanning tree A B G D E c F
(a) Broadcast initiated at A
(b) Broadcast initiated at D

32. Spanning Tree: Creation
Center node
Each node sends unicast join message to center node
Message forwarded until it arrives at a node already belonging to spanning tree A A 3 B B c c 4 2 D D F E F E 1 5 G G
Stepwise construction of spanning tree
(b) Constructed spanning tree

33. Multicast Routing: Problem Statement
Goal: find a tree (or trees) connecting routers having local multicast group members
tree: not all paths between routers used
source-based: different tree from each sender to receivers
shared-tree: same tree used by all group members
Shared tree
Source-based trees
Notes:
3.3 Network Layer: Multicast Routing Algorithms

34. Approaches for building multicast trees
source-based tree: one tree per source
shortest path trees
reverse path forwarding
group-shared tree: group uses one tree
minimal spanning (Steiner)
center-based trees
Notes:
3.3 Network Layer: Multicast Routing Algorithms
…We first look at basic approaches, then specific protocols adopting these approaches

35. Shortest Path Tree
Multicast forwarding tree: tree of shortest path routes from source to all receivers
Dijkstra’s algorithm
LEGEND
S: source
router with attached
group member R1 2 R4 1
router with no attached
group member R2 5
Notes:
3.3 Network Layer: Multicast Routing Algorithms 3 4 R5
link used for forwarding,
i indicates order link
added by algorithm i 6 R3 R6 R7

36. Reverse Path Forwarding
Rely on router’s knowledge of unicast shortest path from it to sender
Each router has simple forwarding behavior:
if (multicast datagram received on incoming link on shortest path back to sender)
then flood datagram onto all outgoing links
else ignore datagram
Notes:
3.3 Network Layer: Multicast Routing Algorithms

37. Reverse Path Forwarding: example
S: source
LEGEND R1
router with attached
group member R4 R2
router with no attached
group member R5
datagram will be forwarded R3 R6 R7
datagram will not be
forwarded
Notes:
3.3 Network Layer: Multicast Routing Algorithms
Result is a source-specific reverse SPT
may be a bad choice with asymmetric links

38. Reverse Path Forwarding: pruning
Forwarding tree contains subtrees with no multicast group members
no need to forward datagrams down subtree
“prune” messages sent upstream by router with no downstream group members
LEGEND
S: source R1
router with attached
group member R4
router with no attached
group member R2 P
Notes:
3.3 Network Layer: Multicast Routing Algorithms R5 P
prune message P R3
links with multicast
forwarding R6 R7

39. Shared-Tree: Steiner Tree
Steiner Tree: minimum cost tree connecting all routers with attached group members
problem is NP-complete
excellent heuristics exists
not used in practice:
computational complexity
information about entire network needed
monolithic: rerun whenever a router needs to join/leave
Notes:
1. See L. Wei and D. Estrin, “A Comparison of multicast trees and algorithms,” TR USC-CD , Dept. Computer Science, University of California, Sept 1993 for a comparison of heuristic approaches.
3.3 Network Layer: Multicast Routing Algorithms

40. Center-based trees Single delivery tree shared by all
One router identified as “center” of tree
to join:
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
Notes:
1. It’s always nice to see a PhD dissertation with impact. The earliest discussion of center-based trees for multicast appears to be D. Wall, “Mechanisms for Broadcast and Selective Broadcast,” PhD dissertation, Stanford U., June 1980.
3.3 Network Layer: Multicast Routing Algorithms

41. Center-based trees: an example
Suppose R6 chosen as center: R1
LEGEND R4 3
router with attached
group member R2 2 R5
router with no attached
group member R3 1 1 R6 R7
path order in which join messages generated
Notes:
3.3 Network Layer: Multicast Routing Algorithms

42. Internet Multicasting Routing: DVMRP
DVMRP: Distance Vector Multicast Routing Protocol, RFC1075
Flood and prune: reverse path forwarding, source-based tree
RPF tree based on DVMRP’s own routing tables constructed by communicating DVMRP routers
no assumptions about underlying unicast routing protocols (RIP, OSPF, etc)
initial datagram to multicast group flooded everywhere via RPF
routers not wanting group: send upstream prune messages
Notes:
D. Waitzman, S. Deering, C. Partridge, “Distance Vector Multicast Routing Protocol,” RFC 1075, Nov The version of DVMRP in use today is considerably enhanced over the RFC1075 spec.
A more up-to-date “work-in-progress” defines a version 3 of DVMRP: T. Pusateri, “Distance Vector Multicast Routing Protocol,” work-in-progress, draft-ietf-idmr-v3-05.ps
3.4 Network Layer: Internet Multicast Routing Algorithms

43. DVMRP: continued…
Soft state: DVMRP router periodically (1 min.) “forgets” branches are pruned:
Multicast data again flows down unpruned branch
downstream router: reprune or else continue to receive data
routers can quickly regraft to tree
following IGMP join at leaf
odds and ends
commonly implemented in commercial routers
Mbone routing done using DVMRP
Notes:
1. See for a (slightly outdatet) list of multicast capable routers (supporting DVMPR as well as other protocols) from various vendors.
2. ftp://parcftp.xerox.com/pub/net-research/ipmulti for circa 1996 public copy “mrouted” v3.8 of DVMRP routing software for various workstation routing platforms.
3.4 Network Layer: Internet Multicast Routing Algorithms

44. Tunneling
Q: How to connect “islands” of multicast routers in a “sea” of unicast routers?
physical topology
logical topology
Multicast datagram encapsulated inside “normal” (non-multicast-addressed) datagram
Normal IP datagram sent thru “tunnel” via regular IP unicast to receiving multicast router
receiving multicast router unencapsulates to get multicast datagram
Notes:
For a general discussion of IP encapsulation, see C. Perkins, “IP Encapsulation within IP,” RFC 2003, Oct
The book S. Bradner, A Mankin, “Ipng: Internet protocol next generation,” Addison Wesley, 1995 has a very nice discussion of tunneling
Tunneling can also be used to connect islands of IPv6 capable routers in a sea IPv4 capable routers. The long term hope is that the sea evaporates leaving only lands of IPv6!
3.4 Network Layer: Internet Multicast Routing Algorithms

45. PIM: Protocol Independent Multicast
Not dependent on any specific underlying unicast routing algorithm (works with all)
Two different multicast distribution scenarios :
Dense (密):
group members densely packed, in “close” proximity.
bandwidth more plentiful
Sparse(疏):
Number of networks with group members small wrt number of interconnected networks
group members “widely dispersed”
bandwidth not plentiful
Notes:
a very readable discussion of the PIM architecture is S. Deering, D. Estrin, D. Faranacci, V. Jacobson, C. Liu, L. Wei, “The PIM Architecture for Wide Area Multicasting,” IEEE/ACM Transactions on Networking, Vol. 4, No. 2, April 1996.
D. Estrin et al, PIM-SM: Protocol Specification, RFC 2117, June 1997
S. Deering et al, PIM Version 2, Dense Mode Specification, work in progress, draft-ietf-idmr-pim-dm-05.txt
PIM is implemented in Cisco routers and has been deployed in UUnet as part of their streaming multimedia delivery effort. See S. LaPolla, “IP Multicast makes headway among ISPs,” PC Week On-Line,
3.4 Network Layer: Internet Multicast Routing Algorithms

46. Consequences of Sparse-Dense Dichotomy:
group membership by routers assumed until routers explicitly prune
data-driven construction on multicast tree (e.g., RPF)
bandwidth and non-group-router processing profligate
Sparse:
no membership until routers explicitly join
receiver- driven construction of multicast tree (e.g., center-based)
bandwidth and non-group-router processing conservative
Notes:
3.4 Network Layer: Internet Multicast Routing Algorithms

47. PIM- Dense Mode Flood-and-prune RPF, similar to DVMRP but
underlying unicast routing protocol provides RPF information for incoming datagram
less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm
has protocol mechanism for router to detect it is a leaf-node router
Notes:
3.4 Network Layer: Internet Multicast Routing Algorithms

48. PIM - Sparse Mode Center-based approach
Router sends join message to rendezvous point (RP)
intermediate routers update state and forward join
after joining via RP, router can switch to source-specific tree
increased performance: less concentration, shorter paths R1 R4
join R2
join R5
join R3 R7 R6
all data multicast
from rendezvous
point
rendezvous
point
Notes:
3.4 Network Layer: Internet Multicast Routing Algorithms

49. PIM - Sparse Mode Sender(s):
unicast data to RP, which distributes down RP-rooted tree
RP can extend multicast tree upstream to source
RP can send stop message if no attached receivers
“no one is listening!” R1 R4
join R2
join R5
join R7 R6
all data multicast
from rendezvous
point
rendezvous
point
Notes:
3.4 Network Layer: Internet Multicast Routing Algorithms

50. Routing for Mobile Hosts
Mobile IP
home agent: Router located on the home network of the mobile hosts
home address: The permanent IP address of the mobile host. Has a network number equal to that of the home network and thus of the home agent
foreign agent: Router located on a network to which the mobile node attaches itself when it is away from its home network

51. Routing for Mobile Hosts

52. Routing for Mobile Hosts
Problem of delivering a packet to the mobile node
How does the home agent intercept a packet that is destined for the mobile node ?
Proxy ARP
How does the home agent then deliver the packet to the foreign agent ?
IP tunnel
Care-of-address
How does the foreign agent deliver the packet to the mobile node ?

53. Routing for Mobile Hosts
Route optimization in Mobile IP
The route from the sending node to mobile node can be significantly sub-optimal
One extreme example
The mobile node and the sending node are on the same network, but the home network for the mobile node is on the far side of the Internet
Triangle Routing Problem

54. Routing for Mobile Hosts
Solution
Let the sending node know the care-of-address of the mobile node. The sending node can create its own tunnel to the foreign agent
Home agent sends binding update message
The sending node creates an entry in the binding cache
The binding cache may become out-of-date
The mobile node moved to a different network
Foreign agent sends a binding warning message

55. The University of Adelaide, School of Computer Science
11 April 2017
Summary
We have talked about the following issues
Scalability routing in the Internet
IPv6
Broadcast/Multicasting
Mobile IP
Chapter 2 — Instructions: Language of the Computer

56. End of Chapter 4