1. CS 5565 Network Architecture and Protocols
Lecture 20
Godmar Back

2. Announcements Project 2B due in 2 parts: Extra Credit Opportunities:
Apr 29 and May 6
Extra Credit Opportunities:
Expand simulator (and your implementation) to introduce multiple link failures and link resurrection
Additional, requiring reading posted
Andersen et al [SIGCOMM’08]: Accountable Internet Protocol (AIP)
Casado et al [HotNets’08]: Rethinking Packet Forwarding Hardware
CS 5565 Spring 2009
9/19/2018

3. Project 2B
Simulator Overview

4. Project 2B High-level View Suppose node 2 calls inbound
Simulated link in topology
RPC Outbound/ Inbound over TCP
High-level View
Suppose node 2 calls inbound
Simulator calls outbound to n3
write_msg(/*port=*/2, data)
Sim
read_msg(/*from*/2, /*port*/1, data)
CS 5565 Spring 2009
9/19/2018

5. Project 2B: Under the hood
Uses discrete event simulation
Events are: packet received, timer expired, cost change, link failure
Virtual time
cost change on link connecting n2 and n3
n3 receives msg from n2 on port 1
timer on n2 expires, n2 calls write_msg(2,..)
CS 5565 Spring 2009
9/19/2018

6. Routing Algorithms

7. Roadmap Done Next Discussed forwarding vs routing
Discussed theory behind two major routing algorithms:
Link-state routing
Distance Vector routing
Discuss theory behind hierarchical routing
Discuss application in Internet
IPv4 addressing
Next
Routing in the Internet
CS 5565 Spring 2009
9/19/2018

8. Addressing in IP IP address interfaces, not hosts
IP address interfaces, not hosts
Sets of interfaces form subnets
Subnets share common prefix
Route to CIDR-ized subnet addresses
a.b.c.d/x
Within subnet, reach destination directly
CS 5565 Spring 2009
9/19/2018

9. Internet Ethernet LAN 1 60 Machines R1 PPP Link 2 Subnet address: R1
PPP Link 2
Subnet address:
/26
Default gateway:
/30 R2
PPP Link 1
/30
Ethernet LAN 2
120 Machines R3
Subnet address:
/25
Default gateway:
CS 5565 Spring 2009
9/19/2018

10. Routing Tables in End Systems
Typical: local subnets + default gateway (“first-hop router”)
Example: “route print” on Windows XP
FastEthernet
g wireless
Active Routes:
Network Destination Netmask Gateway Interface Metric …
Default Gateway:
CS 5565 Spring 2009
9/19/2018

11. ICMP: Internet Control Message Protocol
used by hosts & routers to communicate network-level information
error reporting: unreachable host, network, port, protocol
echo request/reply (used by ping)
network-layer “above” IP:
ICMP msgs carried in IP datagrams
ICMP message: type, code plus first 8 bytes of IP datagram causing error
Type Code description
echo reply (ping)
dest. network unreachable
dest host unreachable
dest protocol unreachable
dest port unreachable
dest network unknown
dest host unknown
source quench (congestion
control - not used)
echo request (ping)
route advertisement
router discovery
TTL expired
bad IP header
CS 5565 Spring 2009
9/19/2018

12. Traceroute and ICMP Source sends series of UDP segments to dest
First has TTL =1
Second has TTL=2, etc.
Unlikely port number
When nth datagram arrives to nth router:
Router discards datagram
And sends to source an ICMP message (type 11, code 0)
Message includes name of router& IP address
When ICMP message arrives, source calculates RTT
Traceroute does this 3 times
Stopping criterion
UDP segment eventually arrives at destination host
Destination returns ICMP “port unreachable” packet (type 3, code 3)
When source gets this ICMP, stops.
See also [Heideman 2008]
CS 5565 Spring 2009
9/19/2018

13. IP addresses: how to get one?
Host gets IP address either hardcoded or via DHCP (Dynamic Host Configuration Protocol)
Network gets subnet part of IP address allocated from ISP’s address space
ISP gets address space assigned by ICANN (Internet Corporation for Assigned Names and Numbers)
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23
CS 5565 Spring 2009
9/19/2018

14. Hierarchical Addressing: Route Aggregation
Hierarchical addressing allows efficient advertisement of routing information:
Organization 0
/23
Organization 1
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning
/16”
ISPs-R-Us
CS 5565 Spring 2009
9/19/2018

15. Hierarchical Addressing: More Specific Routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning /16
or /23”
ISPs-R-Us
Organization 1
/23
CS 5565 Spring 2009
9/19/2018

16. Intra-AS vs Inter-AS Routing
In Internet:
Intra-AS known as Interior Gateway Protocols (IGP)
Most common Intra-AS routing protocols:
RIP: Routing Information Protocol (original protocol, now rarely used)
OSPF: Open Shortest Path First
IGRP/EIGRP: (Enhanced) Interior Gateway Routing Protocol
Inter-AS known as Border Gateway Protocols:
BGP4: Only protocol used
CS 5565 Spring 2009
9/19/2018

17. RIP (Routing Information Protocol)
Distance vector algorithm
Included in BSD-UNIX Distribution in 1982
Distance metric: # of hops (max = 15 hops)
Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)
Each advertisement: list of up to 25 destination nets within AS D C B A u v w x y z
destination hops u v w x y z
A’s routing table
CS 5565 Spring 2009
9/19/2018

18. RIP: Example z w x y A D B C Routing table in D y B 2 z B 7 x -- 1
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B 7 x
…. …
CS 5565 Spring 2009
9/19/2018

19. RIP: Example w x y z A C D B Routing table in D Advertisement
Dest Next hops w x
z C 4
…. …
Advertisement
from A to D w x y z A C D B
Routing table in D
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B A 7 5 x
…. …
CS 5565 Spring 2009
9/19/2018

20. RIP: Link Failure and Recovery
If no advertisement heard after 180 sec → neighbor/link declared dead
routes via neighbor invalidated
new advertisements sent to neighbors
neighbors in turn send out new advertisements (if tables changed)
poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)
CS 5565 Spring 2009
9/19/2018

21. RIP Table processing
RIP routing tables managed by application-level process called route-d (daemon)
advertisements sent in UDP packets, periodically repeated
routed
routed
Transprt
(UDP)
Transprt
(UDP)
network forwarding
(IP) table
network
(IP)
forwarding
table
link
link
physical
physical
CS 5565 Spring 2009
9/19/2018

22. EIGRP Cisco proprietary Distance Vector Protocol with enhancements
See [Cisco Whitepaper], [Malhotra 2002]
Distance Vector Protocol with enhancements
Explicit Signaling (HELLO packets)
DUAL “diffusing update algorithm”
“feasible successor” concept guarantees loop freedom
Intuition: rather than count to infinity, trigger route recomputation unless another loop-free path is known
Optimize this by keeping track of all advertised routes, not just best one
CS 5565 Spring 2009
9/19/2018

23. OSPF (Open Shortest Path First)
“open”: publicly available protocol (not proprietary)
Uses Link State algorithm
LS packet dissemination
Topology map at each node
Route computation using Dijkstra’s algorithm
OSPF advertisement carries one entry per neighbor router
Advertisements have age field to allow for expiration
Advertisements disseminated to entire AS (via flooding)
Carried in OSPF messages directly over IP (rather than TCP or UDP)
CS 5565 Spring 2009
9/19/2018

24. 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 (e.g., satellite link cost set “low” for best effort; high for real time)
Integrated uni- and multicast support:
Multicast OSPF (MOSPF) uses same topology data base as OSPF
Hierarchical OSPF in large domains.
CS 5565 Spring 2009
9/19/2018

25. Hierarchical OSPF
CS 5565 Spring 2009
9/19/2018

26. Hierarchical OSPF Two-level hierarchy: local area, backbone.
link-state advertisements only in same area
each nodes has detailed area topology; only know direction (shortest path) to nets in other areas.
Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.
Backbone routers: run OSPF routing limited to backbone.
Boundary routers: connect to other AS’s.
CS 5565 Spring 2009
9/19/2018

27. Internet Inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto standard
BGP provides each AS a means to:
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”
CS 5565 Spring 2009
9/19/2018

28. BGP Basics
Pairs of routers (BGP peers) exchange routing info over semi-permanent TCP conctns: BGP sessions
Note that BGP sessions do not always 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 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b 3c
eBGP session
iBGP session
CS 5565 Spring 2009
9/19/2018

29. 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. 3b 1d 3a 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b 3c
eBGP session
iBGP session
CS 5565 Spring 2009
9/19/2018

30. 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.
CS 5565 Spring 2009
9/19/2018

31. BGP Route Selection
Router may learn about more than 1 route to some prefix. Router must select route.
Elimination rules:
Local preference value attribute: policy decision
Shortest AS-PATH (like DV routing, except with more information!)
Closest NEXT-HOP router: hot potato routing
Additional criteria
CS 5565 Spring 2009
9/19/2018

32. Path Vector Routing in BGP
Accomplished via AS-PATH attributes
Each node is entire AS!
CS 5565 Spring 2009
9/19/2018

33. BGP routing policy 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
CS 5565 Spring 2009
9/19/2018

34. BGP routing policy (2) A advertises to B the path AW
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!
CS 5565 Spring 2009
9/19/2018

35. Relationship between OSPF&BGP
OSPF hierarchy is intra-AS
BGP connects ASs
CS 5565 Spring 2009
9/19/2018

36. Motivation for different Intra/Inter Protocols
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
CS 5565 Spring 2009
9/19/2018

37. Usage of Routing Protocols
EBGP
Sessions
IGP
OSPF
EIGRP
RIP
Total
Intra-
1,490
9,624
12,741
156
22,521
Inter-
13,830
1,161
1,342
161
2,664
Sample obtained by reverse-engineering router config files
Source David Maltz et al:
Routing Design in Operational Networks – A Look from the inside, [SIGCOMM 2004]
CS 5565 Spring 2009
9/19/2018

38. Summary IP ICMP RIP OSPF BGP Addressing, subnets CS 5565 Spring 2009
9/19/2018