Interdomain Routing

1
Inter-domain Routing

• Outline
• Border Gateway Protocol

2
Homework 3 Solutions

• 412 - Solutions

A)
B)
C)
3
Homework 3 Solutions Contd.

• 520 a) There are 4096 ports so we run out if
  the connect rate exceeds 4096/60 70/sec.
• 520 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.
• 525 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.
• 614 a) The window doubles every RTT thus it
  will take 10 RTTs for the send window to reach
  210 KB 1MB
• 614 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 were
  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.

4
Homework 3 Solutions Contd.

• 614 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
• 620

B)
A)
5
Internet Structure

• Original idea

Backbone service provider
Consumer

ISP

Consumer

ISP
Large corporation

Small

Consumer

ISP


Consumer
ISP
corporation
Small
Small
Small
corporation
corporation
corporation
6
Internet Structure

• Today

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

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

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

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

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

12
BGP-4 contd.

• Each AS has
• 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
• Its neither a distance-vector nor a link-state
  protocol
• Hard problem
• Internets size (12K active ASs) means large
  tables in BGP routers
• Autonomous domains mean different path metrics
• Need for flexibility

13
BGP Example

• Speaker for AS2 advertises reachability to P and
  Q
• network 128.96, 192.4.153, 192.4.32, and 192.4.3,
  can be reached directly from AS2
• Speaker for backbone advertises
• networks 128.96, 192.4.153, 192.4.32, and 192.4.3
  can be reached along the path (AS1, AS2).
• Speaker can cancel previously advertised paths

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

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!