1. Selected Topics in Modern Networks
Amr El Mougy
C7.207

2. Internet Technologies

3. Introduction 2

4. Introduction 2

5. Internet Traffic
Global IP traffic has exceeded 1 ZB in 2016, and now reaching 122 exabytes per month
An increase of 5 –fold over the last 5 years
An increase of 3 folds is expected by 2019
Wireless and mobile traffic makes up 54% of global traffic
By 2019, two thirds of global traffic will be generated by non-PC devices (welcome to the IoT)

6. Internet Architecture
The core of the Internet is a transportation network. Consisting of a large number of high speed routers
Devices connect to edge networks
Ex:
WiFi
Mobile Networks
Institutional Networks
This is a simplified architecture
Core

7. Autonomous Systems
An Autonomous System (AS) is a network or group of networks controlled by a single administrative entity
Often referred to as a single routing domain (clear routing policies to the Internet)
Each AS is given an AS number (ASN), which is globally unique
An ASN is 16-bits or 32-bits and managed by regional Internet registries
AS 2
AS 1
AS 3
AS 4

9. ~ 1,000 servers/pod == IP subnet
What Does a Datacenter Look Like?
Internet CR CR
DC-Layer 3
. . . AR AR AR AR
DC-Layer 2 S S S S S S S S
. . . S S S S
Key
CR = Core Router (L3)
AR = Access Router (L3)
S = Ethernet Switch (L2)
A = Rack of app. servers A A … A A A … A
~ 1,000 servers/pod == IP subnet

10. Cisco Datacenter Model

11. What Does an ISP Look Like?
Countries are divided into metropolitan areas (metros)
Within each metro, the transmission and switching devices for intra-metro communications are placed in central offices (COs)
Customers connect directly to a CO
COs are interconnected by optical fibres
Packets destined outside a CO can be routed accordingly
Packets destined outside the metro are routed to a gateway called point of presence (POP)
Each metro can be organized into 3 layers: access (to customer), metro (between COs), and core (to other metros)

12. What Does an ISP Look Like?
Customer equipment connect to access routers (AR), which then connect to backbone routers (BR)
AR may be co-located with BR or not. If not, then it is called Remote AR (RAR)
IETF has an alternative terminology:
Customer-Edge Equipment (CE)
Provider-Edge Router (PE)
Provider Router (P)

13. Features of Different types of Routers
Access routers:
Their main task is aggregation
They aggregate low-rate interfaces from customers and transport them to backbone routers
They have high processing requirements and are customized for the services they support (ex: access routers for enterprise VPN, for residential access, etc.)
Backbone routers:
They provide transport services
Main requirement is high speed interfaces

14. Internet Routing Protocols
Interior Gateway Protocols (IGP)
Within an Autonomous System
Carries information about internal infrastructure prefixes
Autonomous neighbor discovery
Trust internal routers
Optimized routes
Two widely used IGPs are OSPF and ISIS
Exterior Gateway Protocol (EGP)
Used to convey routing information between ASes
De-coupled from the IGP
Specifically configured peers (Policy-based routing). Designed for reachability
Sets administrative boundaries
Current EGP is BGP

15. How are Routing Protocols Used?
IGP protocols are used to find routes within an AS.
BGP is used typically between gateways
Ex: packet arriving from CE in A.1 heading to A
Thus it needs to cross from the ingress PE, across several P routers, to reach the egress PE
Once the packet arrives at the ingress PE, BGP is used to discover which egress PE should be used
Then OSPF can be to find the route of P routers needed
Problem: this has to be repeated at every P router

16. Open Shortest Path First (OSPF)
One of the most prominent interior gateway protocols
It is a link-state protocol that finds optimized routes to destinations
Routers periodically send Link State Announcements (LSA) to advertise their information
Also capable of detecting topological changes within a short period of time
Link-state means that routers have to broadcast their neighbor information throughout the network

17. OSPF Areas To reduce the overhead, OSPF divides the network into areas
Area 0 is the backbone of the network. It connects other areas
Each area has an area router for inter-area connectivity
Optimized routes are found from each router to the area router
Routing metric include reliability, throughput, delay, etc.
This means that inter-area routing has to go through the backbone
Since this is restrictive, virtual links can be defined that connect routers directly

19. Shortest Path First Algorithm

20. BGP Routing How does an AS in Mexico reach an AS in Australia?
Soln 1: Purchase a direct connection
Costly solution. Does not scale
AS Mexico
AS Australia
Soln 2: announce routing information to neighbors in an aggregated fashion AS AS AS

21. Routing Flow Vs. Traffic Flow
Announce
Accept
AS 1
AS 2
Accept
Announce
Routing Flow
Packet Flow
For networks in AS1 and AS2 to communicate:
AS1 must announce to AS2
AS2 must accept from AS1
AS2 must announce to AS1
AS1 must accept from AS2
Traffic flow is always in the opposite direction of routing flow

22. Ingress Traffic Vs. Egress Traffic
How packets get to your network and your customers’ networks
Ingress traffic depends on:
what information you send and to whom
based on your addressing and AS’s
based on others’ policy (what they accept from you and what they do with it)
Egress Traffic
How packets leave your network
Egress traffic depends on:
route availability (what others send you)
route acceptance (what you accept from others)
policy and tuning (what you do with routes from others)
Peering and transit agreements

23. Routing Policy
Used to control traffic flow in and out of an ISP network
ISP makes decisions on what routing information to accept and discard from its neighbours
Individual routes
Routes originated by specific ASes
Routes traversing specific ASes
Routes belonging to other groupings
Groupings which you define as you see fit

24. Routing Policy Limitations
red
AS99
red
Internet
green
green
packet flow
AS99 uses red link for traffic to the red AS and the green link for remaining traffic
To implement this policy, AS99 has to:
Accept routes originating from the red AS on the red link
Accept all other routes on the green link

25. Routing Policy Limitations
red
Internet
AS99
red
AS22
green
green
packet flow
AS99 would like packets coming from the green AS to use the green link.
But unless AS22 cooperates in pushing traffic from the green AS down the green link, there is very little that AS99 can do to achieve this aim

26. Border Gateway Protocol (BGP)
The de facto EGP protocol
Current version is BGP 4. Has been widely running since 2006.
May run internally within an AS (IBGP), or more commonly between ASes.
Routers exchange routing information by establishing a TCP connection.
Routers will send a “keep alive” message every 60 seconds to keep the connection open
Peers have to be manually configured. BGP only chooses the best routes
Allows policy-based routing by selecting which information to advertise

27. I-BGP and E-BGP R3 can tell R1 and R2 prefixes from R4
R3 can tell R4 prefixes from R1 and R2
R3 cannot tell R2 prefixes from R1
R2 can only find these prefixes through a direct connection to R1
Result: I-BGP routers must be fully connected (via TCP)!
contrast with E-BGP sessions that map to physical links R1
E-BGP
AS1 R3 R4
AS2 R2
I-BGP

28. Link Failures Two types of link failures:
Failure on an E-BGP link
Failure on an I-BGP Link
These failures are treated completely different in BGP
Why?

29. Failure in E-BGP If the link R1-R2 goes down The TCP connection breaks
BGP routes are removed
This is the desired behavior
AS1
AS2
E-BGP session R1 R2
Physical link
/30
/30

30. Failure in I-BGP
If link R1-R2 goes down, R1 and R2 should still be able to exchange traffic
The indirect path through R3 must be used
Thus, E-BGP and I-BGP must use different conventions with respect to TCP endpoints
/30 R2
Physical link
/30 R1 R3
I-BGP connection

31. BGP Graph Each AS has a designated BGP router
This router communicates with other ASes using BGP and communicates internally using an IGP protocol
An AS may have several routes to the same IP
A BGP route is a promise (a set of attributes) to carry packets to a particular IP prefix
As routes are aggregated from AS to AS, the ASN gets stamped on the route

32. Advertising Routing Information
Each AS advertises what it can reach from each BGP router
Policies I: filter what you advertise
Policies II: filter from what you hear advertised
Hear advertisements: IP prefix, AS-path
Filter if desired (i.e. ignore)
Append yourself: IP prefix, myAS+AS-path
Forward to appropriate ASs
Build up a BGP routing table
Remember which prefix you hear from which link

33. BGP Relationships Customer – Provider Peer to Peer: mutual cooperation
Customer pays Provider for service
The Customer is always right
Peer to Peer: mutual cooperation
Ex. MCI and AT&T
Sibling-Sibling
Ex. AT&T research and AT&T wireless

34. Status: Its Complicated
BGP Relationships
Status: Its Complicated
Data flows between customers-providers
Top level providers are peers
They exchange information to ensure connectivity
What can possibly go wrong?
Thousands of ASs
Complicated relationships
Multiple providers for one AS!!
Traffic engineering
I want to use multiple paths and load balance

35. BGP Policy-Based Routing
Provider
Customer
Examples:
100
Peer
200
Peer 10 11 12 13 1 3 4 2
Transit traffic: traffic that does not go to my customers (or their customers)
A provider carries any traffic to or from a customer
Peers exchange traffic only if between their customers
Policies are not part of BGP: they are provided to BGP as configuration information
BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS’s

36. Transit Vs. Peer A multi-homed AS refuses to act as transit
Limit path advertisement
A multi-homed AS can become transit for some AS’s
Only advertise paths to some AS’s
An AS can favor or disfavor certain AS’s for traffic transit from itself

37. X Stub ASes $ $ Loses $ ISP 1 ISP Level 3 ISP stub Verizon Wireless
A stub is an AS that never transits traffic.
(Transit = carry traffic from one neighbor to another) $
ISP 1
ISP
Level 3
ISP
stub X
Loses $
Verizon
Wireless
ISP 2 $
22394
Stub ASes do not transit trafifc, even for their providers as they would lose money
For ISPs, transit traffic is a source of revenue
85% of ASes are stubs!

38. Multi-Protocol Label Switching (MPLS)
An encapsulation protocol between Layer 2 and Layer 3, designed to address several challenges:
Routing lookup has to be done at every hop (BGP lookup followed by OSPF lookup). Populating these tables is expensive (lookups are not)
Need to bind forwarding to path
Traffic engineering and quality of service (QoS). Need to dynamically modify paths to avoid congestion and prioritize packet types
Establishing VPNs

39. How MPLS Works
PE receives a packet heading outside the AS. It uses BGP to identify the proper gateway PE
The first PE becomes the ingress router for MPLS
It assigns a label to this flow and adds an MPLS header to the packet
All the routers supporting MPLS are called label switched routers (LSRs)
Label-based routing is pre-configured
Now, these LSRs no longer need to do double table lookups. They do forwarding based on the added labels
The egress PE removes the last label and forwards the packet

40. How MPLS Works
Determining the path is decoupled from the forwarding process
MPLS can be used for traffic engineering.
OSPF by itself will always choose the shortest path
Can be used to establish VPNs. Determine secure paths for certain flows

41. Supporting QoS
Can determine a forward equivalence class (FEC), where all packets within one group are treated the same by the routers
With MPLS, the lookup is only done at the ingress edge router
LER
LER
LSR
LSR
IP1
IP1
#L1
IP2
IP1
#L2
IP2
IP1
#L3
IP2
IP1
IP2
IP2
IP3
IP3
#L4
IP3
#L5
IP3
#L6
IP3
IP4
IP4
#L4
IP4
#L5
IP4
#L6
IP4

42. MPLS – Traffic Engineering
Overload !!
LER 4
LER 1 IP
Overload !! IP L IP L IP L IP
Forward to
LSR 2
LSR 3
LSR 4
LSR X
LSR 2
LSR 3
End-to-End forwarding decision determined by ingress node.
Enables Traffic Engineering