1. Inter-domain Routing Outline Homework #3 solutions
Border Gateway Protocol
Quiz #2

2. Homework #3 Solutions 4:12 - Solutions A) B) C) Fall, 2001 CS 640 A BD E F
INF 3 8 2 1 6 B) A B C D E F
INF 3 8 4 9 2 1 6 7 C) A B C D E F 6 3 4 9 2 1 7
Fall, 2001
CS 640

3. Homework #3 Solutions Contd.
5:20 a) There are 4096 ports so we run out if the connect rate exceeds 4096/60 = 70/sec.
5:20 b) A is the host that initiated close and is in TimeWait, the other is B. If B does not receive an ACK of its FIN, it will retransmit FINs until A responds as long as it is in TimeWait. If we allow reopening before TimeWait expires then a given late arriving FIN might be part of any one of a number of prior connections. Thus, A would have to keep a list of all prior connects and if a FIN arrives A would have to check its list to decide whether to send an ACK or a RST. An ACK to all FINs would be fine as well.
5:25 With Dev = 1.0, it takes 20 iterations for RTO to fall below 4.0sec. With initial Dev = 0.1 it takes 19 iterations, with initial Dev = 2.0 it takes 21.
6:14 a) The window doubles every RTT thus it will take 10 RTTs for the send window to reach 210 KB = 1MB
6:14 b) After 10 RTTs, 1023KB have been transferred and the window size is now 1MB. Since the max. capacity of the network is not yet reached, slow start continues but now we’re receive window limited so it will take 9 more RTTs to transfer the remaining 9MB so it takes 19 RTTs to complete the file transfer.
Fall, 2001
CS 640

4. Homework #3 Solutions Contd.
6:14 c) It takes 1.4 seconds to transfer the file. The effective throughput is 10MB/1.4sec = 57.1Mbps which is only 5.7% of the available bandwidth
6:20 A)
Time
A rcvs
A snds
R snds
Cwnd
Data0 1
Ack0
Data1,2
Data1 2
Ack1
Data3,4
Data2 3
Ack2
Data5,6
Data3 4
Ack3
Data4
Data5 5 6
Ack5
Data6 7
Ack6
Data7,8
Data7 B)
Time
A rcvs
A snds
R snds
Cwnd
Data0 1
Ack0
Data1,2
Data1 2
Ack1
Data3,4
Data2 3
Ack2
Data5,6
Data3 4
Ack3
Data7,8
Data5 5
Data4
Data7 6 7
Ack5
Data6 8
Ack7
Data8 9
Ack8
Data9,10
Data9
Fall, 2001
CS 640

5. Internet Structure Original idea Backbone service provider “ Consumer”
ISP
Large corporation “
Consumer
ISP ”
Small “
Consumer ”
ISP “ ”
corporation
Consumer
ISP
Small
Small
Small
corporation
corporation
corporation
Fall, 2001
CS 640

6. Internet Structure Today Backbone service provider Peering point
Large corporation
Small
corporation “
Consumer ”
ISP
Fall, 2001
CS 640

7. Route Propagation in the Internet
Autonomous System (AS)
corresponds to an administrative domain
examples: University, company, backbone network
assign each AS a 16-bit number
Two-level route propagation hierarchy
interior gateway protocol (each AS selects its own)
exterior gateway protocol (Internet-wide standard)
Routes information is propagated at various levels
hosts know local router
local routers know site routers
site routers know core router
core routers know everything
Fall, 2001
CS 640

8. Popular Interior Gateway Protocols
RIP: Route Information Protocol
distributed with BSD Unix
distance-vector algorithm
based on hop-count (infinity set to 16)
OSPF: Open Shortest Path First
recent Internet standard
uses link-state algorithm
supports load balancing
supports authentication
Fall, 2001
CS 640

9. EGP: Exterior Gateway Protocol
Overview
Original standard for Internet routing protocol (c 1983)
designed for tree-structured Internet
Single backbone
concerned with reachability, not optimal routes
Protocol messages
neighbor acquisition: one router requests that another be its peer; peers exchange reachability information
neighbor reachability: one router periodically tests if the another is still reachable; exchange HELLO/ACK messages;
uses a k-out-of-n rule: ¼ to stay up, ¾ to establish
routing updates: peers periodically exchange their routing tables (including route weights) using a basic distance vector method
There can be multiple connections between ASs
Fall, 2001
CS 640

10. Limits of EGP
At first glance, EGP seems like a distance vector protocol since updates carry lists of destinations and distances – but distances are NOT reliable.
EGP was designed to support tree topologies, not meshes
False routes injected by accident can have really bad consequences (black holes) – there is no easy way for dealing with this problem
Loops can easily occur – all we are doing is forwarding routing tables
EGP was not designed to easily support fragmented IP packets – all data is assumed to fit in MTU.
Solutions to these and other EGP problems were all manual
Fall, 2001
CS 640

11. BGP-4: Border Gateway Protocol
BGP-1 developed in 1989 to address problems with EGP.
Assumes Internet is an arbitrarily interconnected set of ASs
AS traffic types
Local
starts or ends within an AS
Transit
passes through an AS
AS Types
stub AS: has a single connection to one other AS
carries local traffic only
multihomed AS: has connections to more than one AS
refuses to carry transit traffic
transit AS: has connections to more than one AS
carries both transit and local traffic
Fall, 2001
CS 640

12. BGP-4 contd. Each AS has: BGP goal: find loop free paths between ASs
one or more border routers
Handles inter-AS traffic
one BGP speaker for an AS that participates in routing
BGP speaker establishes BGP sessions with peers and advertises:
local network names
other reachable networks (transit AS only)
gives path information including path weights (MEDs)
withdrawn routes
BGP goal: find loop free paths between ASs
Optimality is secondary goal
It’s neither a distance-vector nor a link-state protocol
Hard problem
Internet’s size (~12K active ASs) means large tables in BGP routers
Autonomous domains mean different path metrics
Need for flexibility
Fall, 2001
CS 640

13. BGP Example Speaker for AS2 advertises reachability to P and Q
network , , , and , can be reached directly from AS2
Speaker for backbone advertises
networks , , , and can be reached along the path (AS1, AS2).
Speaker can cancel previously advertised paths
Backbone network
(AS 1)
Regional provider A
(AS 2)
Regional provider B
(AS 3)
Customer P
(AS 4)
Customer Q
(AS 5)
Customer R
(AS 6)
Customer S
(AS 7)
128.96
Fall, 2001
CS 640

14. Some BGP details Path vectors are most important innovation in BGP
Enables loop prevention in complex topologies
If AS sees itself in the path, it will not use that path
Routes can be aggregated
Based on CIDR (classless) addressing
Routes can be filtered
Runs over TCP
Most of the same messages as EGP
Open, Update, Notify, Keepalive
BGP session have only recently been made secure
Fall, 2001
CS 640

15. BGP in practice 10-20 “tier 1” ASs which are the Internet backbone
Clearly convergence is an issue – why?
Black holes are always a potential problem
There are lots of BGP updates every day!
BGP is really the heart of the Internet
BGP is a means by which network operators control congestion in the Internet.
BGP is really a big problem!
Fall, 2001
CS 640

16. Quiz #2
Consider a network that is running a link-state routing protocol. Suppose node A initially knows that it is directly connected to nodes B, C and D over unidirectional links of cost 2, 5, and 10 respectively. It then receives link-state packets from the other nodes in the network with the following information:
· B is directly connected to A, C, and D with link costs of 2, 3 and 7 respectively.
· C is directly connected with A, B, D, and E with link costs of 5, 3, 6 and 1 respectively.
· D is directly connected to A, B, C, and E with link costs of 10, 7, 6 and 3 respectively.
· E is directly connected to C, D and F with link costs of 1, 3, and 4 respectively.
· F is connected to E with link cost of 4.
1) Draw the graph of the network consistent with this information.
2) Compute the routing table that A would build with this information. For each destination (B – F) specify the path and distance. Show each step in the process.
3) Why would one want to use a link state instead of a distance vector protocol?