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Review
 The Internet (IP) Protocol
Datagram format
 IP fragmentation
 ICMP: Internet Control Message Protocol
 NAT: Network Address Translation
 Routing in the Internet
 Intra-AS routing: RIP and OSPF
 Inter-AS routing: BGP

Some slides are in courtesy of J. Kurose and K. Ross
IP datagram format
IP protocol version
number
header length
(bytes)
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
how much overhead
with TCP?
 20 bytes of TCP
 20 bytes of IP
 = 40 bytes + app
layer overhead
32 bits
head. type of
length
ver
len service
fragment
16-bit identifier flgs
offset
upper
time to
Internet
layer
live
checksum
total datagram
length (bytes)
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
Overview
 BGP
 Multicast Routing
Some slides are in courtesy of J. Kurose and K. Ross
Internet inter-AS routing: BGP
 BGP (Border Gateway Protocol): the de
facto standard
 BGP provides each AS a means to:
1.
2.
3.
Obtain subnet reachability information from
neighboring ASs.
Propagate the reachability information to all
routers internal to the AS.
Determine “good” routes to subnets based on
reachability information and policy.
 Allows a subnet to advertise its existence
to rest of the Internet: “I am here”
BGP basics
 Pairs of routers (BGP peers) exchange routing info over semi-
permanent TCP conctns: BGP sessions
 Note that BGP sessions do not correspond to physical links.
 When AS2 advertises a prefix to AS1, AS2 is promising it will
forward any datagrams destined to that prefix towards the
prefix.

AS2 can aggregate prefixes in its advertisement
3c
3a
3b
AS3
1a
AS1
2a
1c
1d
1b
2c
AS2
2b
eBGP session
iBGP session
Distributing reachability info
 With eBGP session between 3a and 1c, AS3 sends prefix
reachability info to AS1.
 1c can then use iBGP do distribute this new prefix reach info
to all routers in AS1
 1b can then re-advertise the new reach info to AS2 over the
1b-to-2a eBGP session
 When router learns about a new prefix, it creates an entry
for the prefix in its forwarding table.
3c
3a
3b
AS3
1a
AS1
2a
1c
1d
1b
2c
AS2
2b
eBGP session
iBGP session
AS Numbers (ASNs)
ASNs are 16 bit values.
64512 through 65535 are “private”
•
•
•
•
•
•
•
•
Genuity: 1
MIT: 3
JANET: 786
UC San Diego: 7377
AT&T: 7018, 6341, 5074, …
UUNET: 701, 702, 284, 12199, …
Sprint: 1239, 1240, 6211, 6242, …
…
ASNs represent units of routing policy
How Many ASNs are there
today?
Thanks to Geoff Huston. http://bgp.potaroo.net on October 24, 2003
15,981
How Many ASNs are there
today?
18,217
Thanks to Geoff Huston. http://bgp.potaroo.net on October 26, 2004
How many prefixes today?
154,894
Note: numbers
actually depends
point of view…
Thanks to Geoff Huston. http://bgp.potaroo.net on October 24, 2003
How many prefixes today?
179,903
Note: numbers
actually depends
point of view…
Thanks to Geoff Huston. http://bgp.potaroo.net on October 26, 2004
Path attributes & BGP routes
 When advertising a prefix, advert includes BGP
attributes.

prefix + attributes = “route”
 Two important attributes:
 AS-PATH: contains the ASs through which the advert
for the prefix passed: AS 67 AS 17
 NEXT-HOP: Indicates the specific internal-AS router to
next-hop AS. (There may be multiple links from current
AS to next-hop-AS.)
 When gateway router receives route advert, uses
import policy to accept/decline.
ASPATH Attribute
AS 1129
135.207.0.0/16
AS Path = 1755 1239 7018 6341
135.207.0.0/16
AS Path = 1239 7018 6341
AS 1239
Sprint
AS 1755
135.207.0.0/16
AS Path = 1129 1755 1239 7018 6341
Ebone
AS 12654
AS 6341
AT&T Research
135.207.0.0/16
Prefix Originated
RIPE NCC
RIS project
135.207.0.0/16
AS Path = 7018 6341
AS7018
135.207.0.0/16
AS Path = 6341
Global Access
135.207.0.0/16
AS Path = 3549 7018 6341
AT&T
135.207.0.0/16
AS Path = 7018 6341
AS 3549
Global Crossing
BGP: controlling who routes to you
legend:
B
W
provider
network
X
A
customer
network:
C
Y
Figure 4.5-BGPnew: a simple BGP scenario
 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

BGP: controlling who routes to you
legend:
B
W
provider
network
X
A
customer
network:
C
Y
 A advertises to B the path AW
Figure 4.5-BGPnew: a simple BGP scenario
 B advertises to X the path BAW
 Should B advertise to C the path BAW?
 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!
Why different Intra- and 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
Why different Intra- and 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
Overview
 BGP
 Multicast Routing
Some slides are in courtesy of J. Kurose and K. Ross
Multicast: one sender to many receivers
 Multicast: act of sending datagram to multiple receivers
with single “transmit” operation
 Difference from Broadcast

Deliver packets from source to all other nodes
 Question: how to achieve multicast
Multicast via unicast
 source sends N
unicast datagrams,
one addressed to
each of N receivers
routers
forward unicast
datagrams
multicast receiver (red)
not a multicast receiver (red)
Multicast: one sender to many receivers
 Multicast: act of sending datagram to multiple
receivers with single “transmit” operation
 analogy: one teacher to many students
 Question: how to achieve multicast
Network multicast
 Router actively
Multicast
routers (red) duplicate and
forward multicast datagrams
participate in multicast,
making copies of packets
as needed and
forwarding towards
multicast receivers
Multicast: one sender to many receivers
 Multicast: act of sending datagram to multiple
receivers with single “transmit” operation
 analogy: one teacher to many students
 Question: how to achieve multicast
Application-layer
multicast
 end systems involved in
multicast copy and
forward unicast
datagrams among
themselves
Internet Multicast Service Model
128.59.16.12
128.119.40.186
multicast
group
226.17.30.197
128.34.108.63
128.34.108.60
multicast group concept: use of indirection
 hosts addresses IP datagram to multicast group
 routers forward multicast datagrams to hosts that
have “joined” that multicast group
Multicast groups
 class D Internet addresses reserved for multicast:
 host group semantics:
o anyone can “join” (receive) multicast group
o anyone can send to multicast group
o no network-layer identification to hosts of
members
 needed: infrastructure to deliver mcast-addressed
datagrams to all hosts that have joined that multicast
group
Joining a mcast group: two-step process
 local: host informs local mcast router of desire to join
group: IGMP (Internet Group Management Protocol)
 wide area: local router interacts with other routers to
receive mcast datagram flow
 many protocols (e.g., DVMRP, MOSPF, PIM)
IGMP
IGMP
wide-area
multicast
routing
IGMP
Multicast Routing: Problem Statement
 Goal: find a tree (or trees) connecting
routers having local mcast group members



tree: not all paths between routers used
source-based: different tree from each sender to rcvrs
shared-tree: same tree used by all group members
Shared tree
Source-based trees
Approaches for building mcast trees
Approaches:
 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
Shortest Path Tree
 mcast forwarding tree: tree of shortest
path routes from source to all receivers

Dijkstra’s algorithm
S: source
LEGEND
R1
1
2
R4
R2
3
R3
router with attached
group member
5
4
R6
router with no attached
group member
R5
6
R7
i
link used for forwarding,
i indicates order link
added by algorithm
Reverse Path Forwarding
 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 sender)
then flood datagram onto all outgoing links
else ignore datagram
Reverse Path Forwarding: example
S: source
LEGEND
R1
R4
router with attached
group member
R2
R5
R3
R6
R7
router with no attached
group member
datagram will be
forwarded
datagram will not be
forwarded
• result is a source-specific reverse SPT
– may be a bad choice with asymmetric links
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
LEGEND
S: source
R1
router with attached
group member
R4
R2
P
R5
R3
R6
P
R7
P
router with no attached
group member
prune message
links with multicast
forwarding
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

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

Center-based trees: an example
Suppose R6 chosen as center:
LEGEND
R1
3
R2
router with attached
group member
R4
2
R5
R3
1
R6
R7
1
router with no attached
group member
path order in which join
messages generated