Routing: Exterior Gateway Protocols and Autonomous

Systems

Chapter 15

Adding Complexity to an internet

• We learned last time that we cannot continue to add routers to an already full backbone and continue to process efficiently. Why?– It takes a lot of bandwidth for many routers to

communicate routing information

– In a large internet, networks and routers may be owned or managed by different groups

• “…it is impractical for all routers in an arbitrarily large internet to particpate in a single routing update protocol.”

Issues on size of an internet

• Delay– How long does it take to notify all routers of changes in the

internet?

• Overhead– How much of the total traffic on an internet is made up of

routing data?

• What about combinations of:– low delay and high capacity?– high delay and low capacity?

Issues on size of an internet

• As the number of hosts on a network grows over time, more of the network traffic is consumed by the increased traffic

• Network managers usually implement a network monitoring scheme– a monitor listens passively to a network and records

statistics about the traffic, determining:• network utilization - bandwidth used

• percentage of packets containing routing messages

The Extra Hop Problem

• Usually, we have several routers connecting to a backbone and those routers agree on a single routing protocol– Another router may also be connected to that backbone, but

is considered a non-participating router, as in Figure 15.1

• If the non-participating router chooses one of the particpating routers as its default router, routes chosen may be suboptimal

• Router 3 wants to send to Router 2, but has Router 1 as its default; an extra hop is taken

Hidden Networks

• Local networks may be hidden from participating routers, as local network 4 in Figure 15.2

• Information must flow in two directions– Routing information flows from participating routers to

nonparticipating routers

– Nonparticipating routers pass information about hidden networks to the participating group of routers

• Which router should do this? R3 which is one hop from R1, or R4 which knows local network 4?

Autonomous Systems

• A group of networks and routers controlled by a single administrative authority is called an autonomous system (AS)– One router apprises outside world of the networks

inside this group• R3 might be chosen to serve that purpose

– The system is free to choose internal routing architectures and protocols

• discovering, propagating, validating and checking consistency of routes

From Core to Autonomous Systems• The natural evolution from the core system is

shown in Figure 15.3• Advertisement of local information is made

available to other AS’s through a designated router

• Each AS is given an AS number (ASN) to distinguish among the AS’s

An Exterior Gateway Protocol

• EGP is a general term for protocols used in passing routing information between AS’s

• TCP/IP uses the Border Gateway Protocol version 4 (BGP)

• When AS’s agree to exchange routing information, each designates a router to speak BGP on its behalf– the two routers are BGP peers of each other– routers chosen are “near the edge”, thus Border, as in

Figure 15.4

BGP Characteristics

• Allows AS’s to communicate with each other• Coordinates among multiple BGP speakers if there

is more than one• Advertises reachable destinations• Supplies next hop information (as distance vector)• Allows a router’s configuration to adapt to various

policies• Uses TCP for reliable transport

BGP Characteristics

• Advertises path information so receiver can learn a series of AS’s along path to destination

• Exchanges full information once, then updates with incremental changes

• Supports CIDR addressing and sending masks• Aggregates route information• Allows a receiver to verify the identity of a sender

(authentication)

BGP Functionality and Message Types• BGP performs 3 functions

– Initial peer acquisition and authentication• Two peers establish a TCP connection

– Each side sends positive or negative reachability information

• sender can advertise reachable destinations and next hop

• or sender can declare that previously available sites are no longer accessible

– Continual verification that peers and network connections are functioning correctly

BGP Functionality and Message Types• BGP defines 4 message types

– Open - initialize communication

– Update - advertise or withdraw routes

– Notification - response to an incorrect message

– Keepalive - continually check peer connectivity

BGP Message Header

• Marker field 16-octets - a value that both sides agree to use to mark the beginning of a message– initial message consists of all ones

– since TCP does not preserve message boundaries, this is necessary

• Length field 16-bits - total message length in octets, minimum is 19

• Type field 8 bits - indicates message type

OPEN Message

• Two BGP peers establish a TCP connection• They send each other an OPEN with

– their ASN

– a value for a hold timer for the maximum time to wait

– a BGP identifier IP address (a router must choose one of its IP addresses to use with all BGP peers

• See Figure 15.7

UPDATE Message

• Indicates:– Destinations to be withdrawn (variable, not required if

there are none)

– Advertisement of new destinations (also variable)

– Lengths for both of the above - size zero if none

• See Figure 15.8• Routers need to advertise a next hop that is

optimal from the outsider’s perspective, See Figure 15.12

Compressed Mask-Address Pairs

• To accommodate classless addressing, for each of the addresses in the UPDATE message, an IP address and a mask are compressed– See Figure 15.9

– Mask information is encoded into 8 bits, which represents the number of bits in the mask (0-32)

– The address is also compressed• If the value of the mask is < 8, one octet is covered and that is

what follows

• If the value is between 9 and 16, two octets follow, etc.

Path Attributes

• BGP is not a pure distance vector protocol because it advertises more than a next hop– Additional information is in the Path Attributes field of the

UPDATE message– Allows the receiver to:

• check for routing loops and sender tospecify an exact path through AS’s to destination

• implement policy constraints and accept/reject routes unsafe

• know the source of all routes

• Path attributes are factored, so that the attributes apply to all destinations advertised in one message

Path Attributes

• The Path Attributes field consists of a triple:

(type, length, value)• The two octets shown in Figure 15.10 precede

each item in the Path Attributes list of Figure 15.8

KEEPALIVE Message

• Verifies that two peers are continually functioning• Consists of the header and no data (19 octets)• Why?

– BGP uses TCP for transport and TCP does not have a mechanism to continue to check for endpoint reachability

– Saves bandwith as opposed to continually sending routing information (which generally changes infrequently)

• Standards recommend keepalive timer = 1/3 hold timer

A Restriction of Exterior Gateway Protocols• Exterior Gateway Protocols do not communicate

or interpret distance metrics• BGP can only specify whether a destination is

reachable, it cannot determine a best path• BGP does not know the cost of routes across

intermediate AS’s• We say that BGP is a reachability protocol rather

than a routing protocol• See consequences on pages 286 and 287

Internet Routing Arbiter System

• For an internet to work, routing information must be globally consistent

• The Routing Arbiter (RA) system consists of a replicated, authenticated database of reachability information– updates are authenticated

– generally, only the AS that owns a network is allowed to adverise reachability

Internet Routing Arbiter System

• Major ISPs interconnect at Network Access Points (NAPs)– Thus, an NAP represents the boundary between multiple

AS’s

– Each NAP has a computer called a route server (RS) that maintains a copy of the database and runs BGP

– Each ISP designates one router near a NAP to be a BGP border router

• This router maintains a connection to the route server

• The ISP advertises reachability to its networks and networks of its customers

NOTIFICATION Message

• This message type is used when errors occur• Once an error is detected, the TCP connection is

closed• Error codes are indicated in Figure 15.14

– Subcodes for each of the error codes are shown in Figure 15.15

Unanswered Questions

• How can we move from a centralized router system?

• Can we have trust between Autonomous Systems?

Summary

• Routers must be partitioned into groups or the amount of routing information exchanged is too large

• The Internet consists of many Autonomous Systems – consisting of routers and networks under one

administrative authority– the AS’s use an EGP to advertise reachability of its

networks from outside• TCP’s EGP is BGP

Summary

• Border Gateway Protocol is the most widely used EGP

• BGP message types– initiate communication (OPEN)

– send reachability information (UPDATE)

– report errors (NOTIFICATION)

– ensure that peers are connected (KEEPALIVE)

• Multiple ISP’s connect at NAP’s and each NAP includes a route server, which uses BGP

For Next Time

• Read Chapter 16