1. Chapter 4: outline 4.1 introduction
4.2 virtual circuit and datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol
datagram format
IPv4 addressing
ICMP
IPv6
4.5 routing algorithms
link state
distance vector
hierarchical routing
4.6 routing in the Internet
RIP
OSPF
BGP
4.7 broadcast and multicast routing
Network Layer

2. Intra-AS Routing also known as interior gateway protocols (IGP)
most common intra-AS routing protocols:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
IGRP: Interior Gateway Routing Protocol (Cisco proprietary)
Network Layer

3. RIP ( Routing Information Protocol)
included in BSD-UNIX distribution in 1982
distance vector algorithm
distance metric: # hops (max = 15 hops), each link has cost 1
DVs exchanged with neighbors every 30 sec via advertisement message
each advertisement: list of up to 25 destination subnets within AS
from router A to destination subnets: D C B A u v w x y z
subnet hops u v w x y z
Network Layer

4. RIP: example z y w x D B A C 2 y B 2 3 z B 7 x -- 1 … 1 2 3
Destination Network Next Router Num. of hops to dest.
w A
y B
z B x
… …
Interface 2 3 …
Routing/Forwarding table in D
Network Layer

5. RIP: example z w y x A D B C 2 3 y B 2 3 2 z B A 7 5 x -- 1 …
dest next hops w x
z C
…. …
A-to-D advertisement z w y x A D B C
Destination Network Next Router Num. of hops to dest.
w A
y B
z B A x
… …
Interface 2 3
3 2 …
Routing/Forwarding table in D
Network Layer

6. RIP: link failure, recovery
if no advertisement heard after 180 sec --> neighbor/link declared dead
“routes via the dead neighbor” invalidated
new advertisements sent to neighbors
neighbors in turn send out new advertisements (if tables changed)
link failure info quickly (?) propagates to entire net
poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)
Network Layer

7. RIP table processing
RIP routing tables managed by application-level process called route-d (daemon)
advertisements sent in UDP packets, periodically repeated
routed
routed
transport
(UDP)
transprt
(UDP)
network forwarding
(IP) table
network
(IP)
forwarding
table
link
link
physical
physical
Network Layer

8. OSPF (Open Shortest Path First)
“open”: publicly available
uses link state algorithm
LS packet dissemination
topology map at each node
route computation using Dijkstra’s algorithm
advertisements flooded to entire AS
carried in OSPF messages directly over IP (rather than TCP or UDP
IS-IS routing protocol: nearly identical to OSPF
Network Layer

9. OSPF “advanced” features (not in RIP)
security: all OSPF messages authenticated (to prevent malicious intrusion)
multiple same-cost paths allowed (only one path in RIP)
for each link, multiple cost metrics for different TOS
integrated uni- and multicast support:
Multicast OSPF (MOSPF) uses same topology data base as OSPF
hierarchical OSPF in large domains.
Network Layer

10. Hierarchical OSPF backbone boundary router backbone router area border
routers
area 3
internal
routers
area 1
area 2
Network Layer

11. Hierarchical OSPF two-level hierarchy: local area, backbone.
link-state advertisements only in area
each nodes has detailed area topology
It only know direction (shortest path) to nets in other areas.
area border routers: “summarize” net info. in own area, advertise to other Area Border routers.
backbone routers: run OSPF routing limited to backbone.
boundary routers: connect to other AS’s.
Network Layer

12. Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto inter-domain routing protocol
“glue that holds the Internet together”
BGP provides each AS a means to:
eBGP: obtain subnet reachability information from neighboring ASs.
iBGP: propagate reachability information to all AS-internal routers.
determine “good” routes to other networks based on reachability information and policy.
allows subnet to advertise its existence to rest of Internet: “I am here”
Network Layer

13. BGP basics
BGP session: two BGP routers (“peers”) exchange BGP messages:
advertising paths to different destination network prefixes
exchanged over semi-permanent TCP connections
when AS3 advertises a prefix to AS1:
AS3 promises it will forward datagrams towards that prefix
AS3 can aggregate prefixes in its advertisement 3c 3a
BGP
message 3b 2c
AS3
other
networks
AS1 1c 1a 1d 1b 2a 2b
other
networks
AS2
Network Layer

14. BGP basics: distributing path information
using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.
1c can then use iBGP do distribute new prefix info to all routers in AS1
1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session
when router learns of new prefix, it creates entry for prefix in its forwarding table.
eBGP session 3a 3b
iBGP session 2c
AS3
other
networks 1c 2a 2b
other
networks 1a 1b
AS2 1d
AS1
Network Layer

15. Path attributes and BGP routes
advertised prefix includes BGP attributes
prefix + attributes = “route”
two important attributes:
AS-PATH: contains ASs through which prefix advertisement has passed: e.g., AS 67, AS 17
NEXT-HOP: indicates specific internal-AS router to next-hop AS
gateway router receiving route advertisement uses import policy to accept/decline
e.g., never route through AS x
policy-based routing
Network Layer

16. BGP route selection
router may learn about more than 1 route to destination AS, selects route based on:
local preference value attribute: policy decision
shortest AS-PATH
closest NEXT-HOP router: hot potato routing
additional criteria
Network Layer

17. BGP messages BGP messages exchanged between peers over TCP connection
OPEN: opens TCP connection to peer and authenticates sender
UPDATE: advertises new path (or withdraws old)
KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request
NOTIFICATION: reports errors in previous msg; also used to close connection
Network Layer

18. BGP routing policy A,B,C are provider networksX Y
legend:
customer
network:
provider
network
A,B,C are provider networks
X,W,Y are customer (of provider networks)
X is dual-homed: attached to two networks
X does not want to route from B via X to C
.. so X will not advertise to B a route to C
Network Layer

19. BGP routing policy (2) A advertises path AW to BX Y
legend:
customer
network:
provider
network
A advertises path AW to B
B advertises path BAW to X
Should B advertise path BAW to C?
No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers
B wants to force C to route to w via A
B wants to route only to/from its customers!
BGP에 관련한 중요 사건:
export policy
Network Layer

20. Why different Intra-, Inter-AS routing ?
policy:
inter-AS: admin wants control over how its traffic routed, who routes through its net.
intra-AS: single admin, so no policy decisions needed
scale:
hierarchical routing saves table size, reduced update traffic
performance:
intra-AS: can focus on performance
inter-AS: policy may dominate over performance
Network Layer

21. Chapter 4: outline 4.1 introduction
4.2 virtual circuit and datagram networks
4.3 what’s inside a router
4.4 IP: Internet Protocol
datagram format
IPv4 addressing
ICMP
IPv6
4.5 routing algorithms
link state
distance vector
hierarchical routing
4.6 routing in the Internet
RIP
OSPF
BGP
4.7 broadcast and multicast routing
Network Layer

22. Unicasting, Broadcasting, Multicasting
인터넷 전송 방식
유니캐스팅(Unicasting): 하나의 송신자가 다른 하나의 수신자로 데이터를 전송하는 방식. 점대점 통신(one-to-one)
브로드 캐스팅(Broadcasting): 하나의 송신자가 같은 서브네트웍 상의 모든 수신자에게 데이터를 전송하는 방식.
멀티캐스팅(Multicasting): 동일한 메시지를 하나이상의 수신자에게 동시에 보내는 방식(one-to-many). VOD, 인터넷 화상 회의 등의 응용에서 사용.
Network Layer

23. creation/transmission
Broadcast routing
복습문제 R32와 관련
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:
network is overloaded
how does source determine recipient addresses?
Network Layer

24. Broadcast Routing N-way Unicast Broadcast (and Multicast)
Network Layer

25. Broadcast Address Definition Local Limited Broadcast Address
An IP address that allows information to be sent to all machines on a given subnet rather than a specific machine.
The standard is laid out in RFC 919.
Local Limited Broadcast Address :
Can be used, during the DHCP (or BOOTP) process, when a host might not know its IP address and subnet mask, and discover DHCP (BOOTP) server
A datagram destined for the limited broadcast address is never forwarded by a router.

26. Broadcast Address Net-directed broadcast Address
net Id (netid) + host ID (all one) :
Router must forward a net-directed broadcast by default
but it must also have an option to disable this forwarding.
Class A net-directed broadcast address: netid
Class B net-directed broadcast address: netid.netid
Class C net-directed broadcast address: netid.netid.netid.255
Subnet-directed Broadcast
net ID (netid) + subnet ID (subnetid) + All host IDs (all one)
Configuration Method
subnet mask
IP address
the bit complement of the subnet mask is
|| =
Network Layer

27. In-network duplication
flooding: when node receives broadcast packet, sends copy to all neighbors
problems: cycles & broadcast storm
controlled flooding: node only broadcasts pkt if it hasn’t broadcast same packet before
node keeps track of packet ids already broadacsted
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
Network Layer

28. Spanning tree first construct a spanning tree
nodes then forward/make copies only along spanning tree A B G D E c F
(a) broadcast initiated at A
(b) broadcast initiated at D
Network Layer

29. 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 (center: E)
(b) constructed spanning tree
Network Layer

30. Broadcasting vs. Multicasting
Broadcasting and Multicasting only apply to UDP
The problem with IP broadcasting :
If there are 50 hosts on the cable, but only 20 are participating in the application (Application is designed to use UDP broadcasts), 30 hosts have to process the broadcast, all the way through the UDP layer, before UDP datagram is discarded.
UDP datagram is discarded by these 30 hosts because the destination port number is not in use.
IP broadcasting relies on the underlying router’s setting.
Network Layer

31. Multicasting Multicast group address
Class D Multicast addresses are identified by the pattern “1110” in the first four bits –
Host group can locate in multiple networks.
Membership in a host group is dynamic.
Permanent host groups : well-known addresses by IANA
Well-Known Address : ~

32. Group Management Multicast Group Management
IGMP (Internet Group Management Protocol)
The IGMP operates between a host and its directly attached router.
It provides the means for a host to inform its attached router that an application running on the hosts wants to join a specific multicast group.
Network Layer

33. Four situation of IGMP operation
Network Layer

34. IGMP in Internet-Wide Net
A host in Internet can join a multicast group
Join
Join
Join
Network Layer

35. Multicast routing: problem statement
goal: find a tree (or trees) connecting routers having local multicast group members
tree: not all paths between routers used
shared-tree: same tree used by all group members
group
member
not group
router
with a
without
legend
source-based: different tree from each sender to rcvrs
source-based trees
Notes:
3.3 Network Layer: Multicast Routing Algorithms
shared tree
Network Layer

36. Approaches for building mcast trees
group-shared tree: group uses one tree
center-based trees
source-based tree: one tree per source
shortest path trees
reverse path forwarding
Notes:
3.3 Network Layer: Multicast Routing Algorithms
Network Layer

37. Center-based trees (group-shared tree)
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
Network Layer

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

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

40. Reverse path forwarding (source-based tree)
rely on router’s knowledge of unicast shortest path from it to sender
each router has simple forwarding behavior:
if (mcast datagram received on incoming link on shortest path back to center)
then flood datagram onto all outgoing links
else ignore datagram
Notes:
3.3 Network Layer: Multicast Routing Algorithms
Network Layer

41. Reverse path forwarding: example
s: source
LEGEND
router with attached
group member
router with no attached
group member
datagram will be forwarded
datagram will not be
forwarded
result is a source-specific reverse SPT
Notes:
3.3 Network Layer: Multicast Routing Algorithms
Network Layer

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

43. Multicast Routing Protocols
IETF Protocols
Distance-Vector Multicast Routing Protocol (DVMRP) – RFC 1075
Source-based
Reverse Path Forwarding, Pruning
Protocol Independent Multicast (PIM) – RFC 2362
Dense Mode (RFC 3973, PIM-DM) - Source-based
Sparse Mode (RFC 3569, RFC 4607, PIM-SM) – Both Shard and Source-based
Network Layer