routing information protocol - os3 · routing information protocol ... rip version 1 and subnetting...

Routing Information ProtocolA simple distance vector scheme

Karst Koymans

Informatics InstituteUniversity of Amsterdam

(version 16.3, 2017/03/01 13:00:45)

Friday, March 3, 2017

Karst Koymans (UvA) Routing Information Protocol Friday, March 3, 2017 1 / 40

1 RIP version 1Origin and applicationBasic RIPv1 operationRIPv1 packetsRIP version 1 and subnetting

2 Protocol extensionsIGRPEIGRP

3 RIP version 2Extensions with respect to version 1Summary of changes with respect to version 1

4 RIP Next Generation and RIP variants


RIP version 1

Outline






RIP version 1 Origin and application

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RIP version 1

RFC 1058Charles Hedrick, Rutgers University, 1988

Based on the Bellman-Ford distance vector algorithm

Also used as the first ARPANET routing protocol

Many implementations were already presentdiffering in details (timers . . . )available before the specification itself

GWINFO from Xerox PARCrouted from BSD 4.2



RIP application

Inside homogeneous networkswith a moderate size

An interior routing protocol (IGP)

Maximum diameter is 15 hopsBased on a uniform cost (weight) of 1Hence in this case “16 = ∞”


RIP version 1 Basic RIPv1 operation

Outline







Basic operation

Keep a table of routes to destination networks withdistance (metric), gateway (next hop)

Periodically send out a complete table to all neighboursSend all destination networks with distancesDo not send the gateways

Update your table with newly collected informationThe distance can only decrease (become better)

unless the information comes from the gateway itself,thereby overriding the old informationwhich only occurs if the topology changes or links fail or come up



Split horizon (with poisoned reverse)

Split horizonDo not advertize back routes you learned fromyour neighbour to the network you both are on

Poisoned reverseDo advertise routes back but with distance ∞

which indicates unreachability explicitlyAdvantage is the faster convergence in case topology changes

for instance when routers point to each other

Disadvantage is the larger routing update packetsTherefore implementation is optional or configurable



Timers

Update timer: updates are sent every 30 seconds (default)

Invalid timer: routes time out after 180 seconds (default)Called “timeout” in RFC 1058

Flush timer: routes disappear after 240 seconds (default)Called “garbage-collection timer” in RFC 1058

with a default value of 120 seconds after timeout

Unreachable networks are included in updates until the flush timerexpires them and they are eliminated from the routing table

Infinity = ∞ = 16



Timers (later additions)

Triggered updatesAn update is being sent as soon as a route changes

Hold-down timer (default 180 seconds)This is a later (Cisco) addition, not present in RFCsIt exists to prevent incorporating possibly bad routing informationwhich might be present in a network that didn’t converge yetThe semantics is not completely clear (not standardized)

Essentially do not accept updates during hold-down on routesthat have been marked unreachable


RIP version 1 RIPv1 packets

Outline







RIP version 1 packets

Packets are broadcast to 255.255.255.255or to a directed broadcast addressor to an individual host or router as a response to a request

Works on top of UDP and uses 520 as its well-known port

The maximum size of this UDP packet is 512 bytes,including the 8 bytes long UDP header

There is room for 25 route updatesEach update needs 20 bytes



RIPv1 packet format

0 7 8 15 16 23 24 31

Command Version Reserved (must be 0)Address Family Identifier Reserved (must be 0)

IP Address (network)

Reserved (must be 0)

Metric

Repeatedmaximally 25times



RIPv1 packet fields

RIPv1 fields

Command Request(1) or Response(2)

Version 1

AFI IP(2)

IP Address Destination Network

Metric Distance (max 16=∞)


RIP version 1 RIP version 1 and subnetting

Outline






RIP version 1 RIP version 1 and subnetting

Subnets

RIPv1 “supports” fixed length, one level deep, subnet masks

RIPv1 uses heuristics to derive the uniform subnet mask

in use throughout the whole subnetted network

from its own directly connected subnets

RIPv1 differentiates between neighbours for updates about subnetsbeing part of that same subnetted networkbeing part of another network


Protocol extensions

Outline






Protocol extensions IGRP

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IGRP (1)

IGRP stands for “Interior Gateway Routing Protocol”

IGRP is a Cisco proprietary protocol

IGRP is just like RIP a distance vector protocolThe basics are the same as with RIPIt runs directly on top of IP (protocol 9)It has a larger notion of ∞, typically 100, maximum 255It accepts up to four parallel paths

which Cisco also used in its RIP implementation,deviating from the official specification



IGRP (2)

Division into independent domains is possiblewhich are called (confusingly) autonomous systems

Uses three types of network routesInterior route

with implicit subnet information, like in RIPv1System route

which is possibly summarizedExterior route

candidate default

Support for multiple default routesusing fictitious network numberscontrary to RIP that only (optionally) includes 0.0.0.0



IGRP (3)

RIPThe only metric is the hop count

IGRPThe metric is composite, based on

Delay, Bandwidth, Reliability and Load

The hop count is still communicated as a separate field

to see when ∞ has been reached

MTU information about the path is also communicated


Protocol extensions EIGRP

Outline







EIGRP (1)

EIGRP stands for “Enhanced Interior Gateway Routing Protocol”It runs directly on top of IP (protocol 88)Remembers all paths, not only the best pathIt carries explicit subnet information (VLSM)It keeps state about its neighboursIt can take the MTU into accountIt uses the DUAL (Diffusing Update Algorithm) algorithmfor loop free routing and fast convergence



EIGRP (2)

It uses no regular (periodic) routing table updatesUses partial and incremental updates when the routing table changesIt uses hello multicast packets to be discovered by neighboursIt forms adjacencies with neighbours

It uses ack(nowledgement)s for reliable communication

It can summarize on arbitrary bit boundaries (classless)



EIGRP (3)

Use of DUALComplicated algorithm, remotely similar to thepropose and agree mechanism for Rapid Spanning Tree

Suppose S-M-T is a path from S to T via neighbour MSuppose the link cost for S-M is cSuppose the path cost for M-T is p

When is it safe (with respect to loop avoidance) to switchto a backup path via neighbour N when the link S-M fails?

Say the link cost for S-N is d and the path cost for N-T is qAnswer: q ≤ c + p (feasible successor condition)


RIP version 2

Outline






RIP version 2 Extensions with respect to version 1

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RIP version 2

Specified in RFC 2453Gary Malkin, Bay Networks, 1998

It has extensions forSubnet masksAlternate next hopAuthenticationMulticastingRoute tags



RIPv2 packet format

0 7 8 15 16 23 24 31

Command Version Reserved (must be 0)Address Family Identifier Route Tag

IP Address (network)Subnet Mask

Next HopMetric

Repeatedmaximally 25times



RIPv2 packet fields

RIPv2 fields

Command Request(1) or Response(2)

Version 2

AFI IP(2)

Route Tag Identification of route origin

IP Address Destination Network

Subnet Mask CIDR support

Next Hop Gateway (if different from advertising router)

Metric Distance (max 16=∞)



First-routing-entry-as-authentication hack

0 7 8 15 16 23 24 31

Address Family Identifier (=0xFFFF) Authentication Type

Authentication

Authentication type 2 means use of a plaintext password

Authentication type 3 (RFC 4822) means use of

Keyed-MD5 or HMAC-SHA-{1,256,384,512} where extended

authentication data is added as a trailer to the RIP dataKarst Koymans (UvA) Routing Information Protocol Friday, March 3, 2017 32 / 40

RIP version 2 Summary of changes with respect to version 1

Outline







Summary of changes (1)

Authentication fieldsFirst AFI is 0xFFFFType 2: simple password of length 16 bytesType 3: key id, sequence number and length and offsetof a variable length authentication trailer

Route tag fieldThis is simply carried alongIt differentiates between internally and externally generated routes



Summary of changes (2)

Subnet mask fieldnet/subnet/host route differentiation

Next Hop fieldUsed when best next hop does not speak RIP itself

Multicast (not a field) replaces broadcastThe destination of updates is 224.0.0.9 (which is not forwarded)


Next Hop scenario

A B E F

Network

A and B speak RIP internally.E and F speak some other protocol over the external network.

E can speak RIP to A and B on behalf of F,setting the Next Hop to F as appropriate.

RIP Next Generation and RIP variants

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RIPng (for IPv6)

Specified in RFC 2080

It still runs over UDP

It uses port number 521

Packets can be any length that fits inside the network unfragmented

Supports IPv6 prefix, route tags, next hopNo authentication (which can be done at the IP level)

It uses FF02::9, the all-rip-routers multicast address



RIPng packet format

0 7 8 15 16 23 24 31

Command Version Reserved (must be 0)

IPv6 Prefix

Route Tag Prefix Length Metric

As many as canfit in a packetunfragmented



RTE-with-big-metric-as-next-hop hack

0 7 8 15 16 23 24 31

Next Hop

0x0000 0x00 Metric(=0xFF)

The next hop must be a link local address

Using :: as next hop means use originator as next hop

Next Hop remains valid until next “next-hop-RTE”Karst Koymans (UvA) Routing Information Protocol Friday, March 3, 2017 40 / 40

routing information protocol - os3 · routing information protocol ... rip version 1 and subnetting...

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