CS 164: Global Internet
Slide Set -- 11
In this set ...
• More about subnets
• Classless Inter Domain Routing
(CIDR)
• Border Gateway Protocol (BGP)
• Areas with OSPF
Forwarding on Subnets
• Resolution of subnet address: Bitwise ANDing
Host IP address with Subnet Mask gives
subnet number.
• When a host wants to send an IP packet:
– Perform BITwise AND between subnet mask and
destination IP address
• If result == its subnet no. destination is on same
subnet (Send ARP etc.).
– If not, send packet to default router R.
Router Forwarding Tables
• Table holds entries like
<SubnetNumber,
SubnetMask, NextHop>
• Router ANDs dest addr
with subnet mask of each
entry.
• Find the right entry (Match
with subnet no.) and
forward to Next hop.
Subnet Number
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.0
128.96.34.15
128.96.34.1
R1
H1
Subnet mask: 255.255.255.128 28
Subnet number: 128.96.34.128 28
128.96.34.130
128.96.34.139
128.96.34.129
H3
R2
H2
128.96.33.1
128.96.33.14
Router R1’s
table
Subnet mask: 255.255.255.0
Subnet number: 128.96.33.0
Subnet Mask
Next Hop
128.96.34.0
255.255.255.128
Int 0
128.96.34.128
255.255.255.128
Int 1
128.96.33.0
255.255.255.0
R2
Other Issues
• Subnet Mask need not align with byte
boundaries (e.g. 255.255.255.128) -- 7
zeroes.
• Non contiguous masks are possible -255.255.1.0 -- however, this makes
administration difficult -- not recommended.
• One could have multiple subnets on the same
physical network ! However, now, hosts on the
same net would need to go through a router in
order to talk to each other.
Outside View
• Routers outside a group of subnets
see the group as a single network - e.g. 128.96
• However, once packet arrives to
the group, routers within the group
need to forward the packets to the
proper subnet.
Classless Interdomain Routing
• Abbreviated as CIDR.
• If a network grows to more than 255 hosts, it may
want a Class B address.
• One possible way of avoiding is to handle many Class C
routing addresses -- but then, for this one network,
each router has to maintain multiple routing entries.
• CIDR is an attempt to balance the desire to minimize
the number of routes that a router needs to know
versus the need to hand out addresses efficiently.
• Key property: CIDR enables aggregation of routes !
Removing rigid boundaries
• The idea is to break the rigid boundaries between
classes.
• As an example, if a network grows to about 16x255
hosts, assign a contiguous “block” of Class C addresses as
opposed to a Class B address.
– Example: 192.4.16 to 192.4.31
• Note -- 16 Class C addresses better than 1 Class B in
terms of address efficiency.
• In the above example -- the top 20 bits are the same
for all the addresses and so we have effectively created
a 20 bit network number !
The Prefix
• The 20 bit address in our previous example is called
the “common prefix” for the set of addresses that are
allocated.
• Observe -- what we did was that we allocated a block
of Class C addresses that shared a common prefix.
• Now, with this new representation, the network
numbers are represented by <length,value> -- the
length represents the number of bits in the prefix.
Subnets vs CIDR
• The concept is similar but:
– In a subnet, a single address is shared among
multiple physical networks.
– With CIDR, we collapse multiple network
addresses into a longer network address that is
typically assigned to an AS (the single AS would
have a network number or prefix that reflects
the block of addresses).
• Thus, when we want to route to “any” of
the networks or even subnets within the AS,
we route to the AS.
Route Aggregation
• Specifying simply the prefix associated
with an AS (as opposed to stating the
subnet number explicitly) is called route
aggregation.
• When sending route advertisements (we
will see how), it suffices to simply
advertise “common prefixes”.
• Note that for this, careful planning
would be needed.
An Example
• Border gateway advertises the common
prefix only!
Corporation X
(11000000000001000001)
Border gatew ay
(advertises path to
11000000000001)
Regional netw ork
Corporation Y
(11000000000001000000)
Longest Prefix Match
• Prefixes may overlap:
– Example 171.69 and 171.69.10 may be found in the
forwarding table of a single router.
– Now, if the destination is 179.69.10.7, both the prefixes
match !
– Policy -- Choose the longest prefix. why ?
• Choosing the longest prefix the right choice since an
organization may switch ISPs.
ISP 1: 223.1.1.0
223.1.1.240
ISP 2
ISP 2 would advertise
223.1.1.240
explicitly
Revisiting Autonomous Systems
• ASes provide an additional way of hierarchically
aggregating routing information in the Internet.
• AS --> also called domains and can run their own
protocols within their administrative regimes.
• In each AS, the amount of routing information
may be dramatically reduced by using a default
router.
• If the corporate network is connected to the
Internet by means of a single border router
nodes simply send messages to this router.
Internet View Revisited
Multihomed -- no
transit traffic.
Large corporation
“Consumer”ISP
Peering
point
Backbone service provider
“Consumer”ISP
Large corporation
Peering
point
“Consumer”ISP
Small
corporation
Stub AS -only local
traffic
Service Provider Networks
Some notation
• Peering Point: Points where different
providers interconnect.
• Local Traffic: Traffic that begins and
terminates on nodes within an AS.
• Transit traffic -- passes through various
Ases -- backbones carry transit traffic.
Exterior Gateway Protocol
• Abbreviated as EGP
–
–
–
–
Hierarchical routing
Reach higher level in the hierarchy
Tree structure for routing topology.
No peer-to-peer communications.
Border Gateway Protocol
• Abbreviated BGP -- currently
version 4 and is in use.
• Rather complex.
• Goals of BGP are modest:
– Any loop free path is to be found
between the source and destination
(not necessarily min cost or shortest).
Why the modest goal ?
• Each AS may have a different set of
nodes, so it is unclear which route is the
min-cost route!
– Route aggregation also difficult
• Lots of routing information is required in
order to guarantee optimality -- may be
infeasible.
• Trust -- misconfiguration may not yield
optimal.
BGP Details
• BGP supports flexibility -- paths could be chosen
by a provider based on a policy.
• To configure BGP, each AS admin picks at least
one node to be the “BGP” speaker -- a
spokesperson node for the entire AS.
–
The BGP speaker establishes a BGP session with other
BGP speakers in other ASes.
• In addition, there are border gateways using which
packets enter/leave ASes.
• Source advertises complete paths (unlike distance
vector or link state routing) -- thus loops are
prevented.
An Example
Customer P
(AS 4)
128.96
192.4.153
Customer Q
(AS 5)
192.4.32
192.4.3
Customer R
(AS 6)
192.12.69
Customer S
(AS 7)
192.4.54
192.4.23
Regional provider A
(AS 2)
Backbone netw ork
(AS 1)
Regional provider B
(AS 3)
• AS 2 says 128.96, 192.4.15, 192.4.32,
192.4.3 can be reached via AS 2.
• AS 1 advertises that these networks can be
reached via <AS1, AS2> --note full path
description.
• Loops are avoided.
AS Numbers
• Each AS is assigned a number -16 bits and is unique.
– The uniqueness requirement has been
relaxed to some extent --stub ASes
do not need to be unique.
• One could have up to 65 K AS
numbers.
BGP Messages
• BGP has four types of messages
– OPEN: Establish a connection with a BGP
peer
• Note: BGP connection is TCP based ! (Port no.
179).
– UPDATE -- advertise or withdraw routes to
a destination
• Note --BGP speaker needs to be able to cancel
previously advertised paths if nodes or links fail.
This form of negative advertisements are said to
advertise “withdrawn routes”.
BGP Messages (cont)
• KEEPALIVE: Inform a peer that
the sender is still alive but has no
information to send.
• NOTIFICATION: Notify that
errors are detected.
BGP Message Format
• 16 byte fields.
• For more detail look at book.
• Important thing --- BGP
updates are of the type
prefix/length
– 192.4.16/20
• Note that forwarding entries
can also be similarly
represented.
0
15
Unfeasible routes
length
Withdraw n routes
(variable)
Total path
attribute length
Path attributes
(variable)
Netw ork layer
reachability info
(variable)
Routing with BGP
• For stub AS -- border router injects a
default route into the intra-domain
routing protocol.
• If there are more than one border
router, each injects specific routes that
they have learned from outside the AS.
• IBGP or Interior BGP is used to
distribute the information to all other
routers in the domain (and the speaker).
Routing Areas
• Especially used with OSPF.
• Subdomains of larger domains.
• One special area called
backbone area. (Area 0).
• Within each area -- link state
routing.
• Link state advertisements of
non border routers do not leave
area.
• Packet goes from non-backbone
area to backbone area and
crosses the backbone into the
Internet.
Area 3
Area 1
Area 0
R9
R7
R8
R3
R1
R4
R2
Area 2
R6
R5
• A router that is a
member of both the
backbone and a nonbackbone area (R1) is
called a area router.
Areas (continued)
• Border routers “summarize” routing
information and make it available to other
areas -- act like proxies --reflect costs to
reach networks from an area.
• When there are many possible routes, routers
choose cost info to forward packets.
• Trade-offs -- Optimality versus scalability - All packet have to pass through the
backbone area (may not be optimal).
Next Time
• IPv6
• Introduction to the transport layer.