Intradomain Routing Protocols

By Behzad Akbari

These slides are based in part upon slides of Prof. Shivkumar (Rpi university) and Sanjay Rao (Purdue university)

Intradomain routing protocols Distance Vector

RIP, RIPv2, EIGRP Link State

OSPF, IS-IS Intradomain Traffic Engineering

Outline

RIP: Routing Information Protocol Uses hop count as metric (max: 16 is infinity) Tables (vectors) “advertised” to neighbors every 30 s.

Each advertisement: up to 25 entries No advertisement for 180 sec: neighbor/link declared dead

routes via neighbor invalidated new advertisements sent to neighbors (Triggered updates) neighbors in turn send out new advertisements (if tables changed) link failure info quickly propagates to entire net poison reverse used to prevent ping-pong loops (infinite distance = 16

hops) If Z routes through Y to get to X :

Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) will this completely solve count to infinity problem?

x z14

50

y60

RIPv1 Problems (Continued)

Split horizon/poison reverse does not guarantee to solve count-to-infinity problem 16 = infinity => RIP for small networks

only! Slow convergence

Broadcasts consume non-router resources It sends updates as broadcasts on

255.255.255.255 RIPv1 does not support subnet masks

(VLSMs) It does not send subnet mask

information in its updates. It does not support authentication

RIPv2 Why ? Installed base of RIP routers Provides:

VLSM support Authentication Multicasting

Uses reserved fields in RIPv1 header. First route entry replaced by authentication

info.

EIGRP (Interior Gateway Routing Protocol)

CISCO proprietary; successor of RIP (late 80s) Several metrics (delay, bandwidth, reliability, load

etc) Uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg.

(DUAL) based on diffused computation Freeze entry to particular destination Diffuse a request for updates Other nodes may freeze/propagate the diffusing

computation (tree formation) Unfreeze when updates received. Tradeoff: temporary un-reachability for some destinations

Link State vs. Distance Vector Link State (LS) advantages:

More stable (aka fewer routing loops) Faster convergence than distance vector Easier to discover network topology,

troubleshoot network. Can do better source-routing with link-state Type & Quality-of-service routing (multiple

route tables) possible

Link State Protocols

Key: Create a network “map” at each node.

1. Node collects the state of its connected links and forms a “Link State Packet” (LSP)

2. Flood LSP => reaches every other node in the network and everyone now has a network map.

3. Given map, run Dijkstra’s shortest path algorithm (SPF) => get paths to all destinations

4. Routing table = next-hops of these paths. 5. Hierarchical routing: organization of areas, and

filtered control plane information flooded.

Link State Issues

Reliable Flooding: sequence #s, age

LSA types, Neighbor discovery and maintenance (hello) Efficiency in Broadcast LANs,

NBMA, Pt-Mpt subnets: designated router (DR) concept

Areas and Hierarchy Area types: Normal, Stub, NSSA:

filtering External Routes (from other

ASs), interaction with inter-domain routing.

Sending Link States by Flooding X Wants to Send Information

Sends on all outgoing links When Node Y Receives

Information from Z Send on all links other than Z

Naïve Approach: Floods indefinitely. Prevent through sequence

numbers

X A

C B D

(a)

X A

C B D

(b)

X A

C B D

(c)

X A

C B D

(d)

OSPF Reliable Flooding

Transmit Link State Advertisements Originating Router List of directly connected

neighbors of that node with the cost of the link to each one

Sequence Number Incremented each time

sending new link information Link State Age

Packet expires when a threshold is reached,

OSPF Flooding Operation

Node X Receives LSA from Node Y With Sequence Number q Looks for entry with same origin/link ID

Cases No entry present

Add entry, propagate to all neighbors other than Y Entry present with sequence number p < q

Update entry, propagate to all neighbors other than Y Entry present with sequence number p > q

Send entry back to Y To tell Y that it has out-of-date information

Entry present with sequence number p = q Ignore it

Flooding Issues When Should it be Performed

Periodically When status of link changes

Detected by connected node

What Happens when Router Goes Down & Back Up Sequence number reset to 0

Other routers may have entries with higher sequence numbers

Router will send out LSAs with number 0 Will get back LSAs with last valid sequence number p Router sets sequence number to p+1 & resends

Flooding Issues (Cont.) What if Sequence Number Wraps Around

Use circular comparison OSPF v1

Force sequence number back to 0 OSPF v2 With 32-bit counter, doesn’t happen very often

0Max

a

b

a < b

0Max

ab

a < b

OSPF Load Balancing

Modification to Dijkstra’s algorithm Keep track of all links giving optimum cost d(v) Only get multiple routes when exactly same cost

Routing Alternate link used Tends to cause packets to arrive out of order

A

E

F

C

D

B

1

3

6

3

1

1

1

3

Table for B

Dst Cst Hop

A 3 A

B 0 B

C 2 F

D 3 D

E 4 F

F 1 F

Table for B

Dst Cst Hop

A 3 A

B 0 B

C 2 F

D 3 D,F

E 4 A,F

F 1 F

Type of Service (TOS) Metrics Link Characteristic Vary in Multiple Dimensions

Latency Throughput Cost Reliability

Example Satellite link

High throughput, long latency Fiber optic link

High throughput, low latency Routing Requirements Vary

Typing at terminal: minimize latency for short packet Sending video data: maximize throughput

Proposed OSPF Support for TOS Support up to Five Different Routing Metrics

Normal service Don’t do anything extreme

Minimize cost For networks that charge for traffic

Maximize reliability Maximize throughput Minimize delay

Link Can Have Different LSA for each TOS Expressed in units where lower value is better Path cost either sum or maximum of link costs

Designated Router (DR)

New Question: Who creates the network-LSA?

Dijkstra algo view Encoding of LSAs, Flooding/DB sync model

Designated Router (…) One router elected as a designated router (DR) on

LAN Each router maintains flooding adjacency with the DR, I.e.,

sends acks of LSAs to DR DR informs each router of other routers on LAN DR generates the network-LSA on subnet’s behalf after

synchronizing with all routers

Primary/Backup: DR, BDR (…) Backup DR (BDR) also syncs with all routers, and takes

over if DR dies (typically 5 s wait) Total: 2N – 1 adjacencies DR election:

First router on net = DR, second = BDR RouterPriority: [0, 127] indicated in Hello packet=> highest

priority router becomes DR If network is partitioned and healed, the two DRs are reduced

to one by looking at RouterPriority

Hierarchical Routing

Why Hierarchy? Information hiding (filtered) => computation,

bandwidth, storage saved => efficiency => scalability But filtering in control plane, not data plane

Address abstraction vs. Topology Abstraction Multiple paths possible between two adj. areas

Area

Configured area ID A set of address prefixes

Do not have to be contiguous So a prefix can be in only one area

A set of router IDs Router functions may be interior, inter-area, or

external

Hierarchical OSPF Two-level hierarchy: local area, backbone.

Link-state advertisements only in area each nodes has detailed area topology; only know direction

(shortest path) to nets in other areas. Two-level restriction avoids count-to-infinity issues in backbone

routing. Area border routers (ABR): “summarize” distances to nets in

own area, advertise to other Area Border routers. Backbone routers: uses a DV-style routing between backbone

routers

Boundary routers (AS-BRs): connect to other ASs (generate “external” records)

Hierarchical OSPF

Sample Area Configuration

10.2.0.0/24

IS-IS Overview The Intermediate Systems to Intermediate System

Routing Protocol (IS-IS) was originally designed to route the ISO Connectionless Network Protocol (CLNP) . (ISO10589 or RFC 1142)

Adapted for routing IP in addition to CLNP (RFC1195) as Integrated or Dual IS-IS (1990)

IS-IS is a Link State Protocol similar to the Open Shortest Path First (OSPF). OSPF supports only IP

IS-IS competed neck-to-neck with OSPF. OSPF deployed in large enterprise networks IS-IS deployed in several large ISPs

IS-IS TerminologyIntermediate system (IS) - RouterDesignated Intermediate System (DIS) - Designated RouterPseudonode - Broadcast link emulated as virtual node by DISEnd System (ES) - Network Host or workstationNetwork Service Access Point (NSAP) - Network Layer AddressSubnetwork Point of attachment (SNPA) - Datalink interfacePacket data Unit (PDU) - Analogous to IP PacketLink State PDU (LSP) - Routing information packetLevel 1 and Level 2 – Area 0 and lower areas

Functional Comparison Protocols are recognizably similar in function

and mechanism (common heritage) Link state algorithms Two level hierarchies Designated Router on LANs

Widely deployed (ISPs vs. enterprises) Multiple interoperable implementations OSPF more “optimized” by design (and

therefore significantly more complex)

Sample comparison points Encapsulation

OSPF runs on top of IP=> Relies on IP fragmentation for large LSAs

IS-IS runs directly over L2 (next to IP) => fragmentation done by IS-IS

Media support Both protocols support LANs and point-to-point links in

similar ways IS-IS supports NBMA in a manner similar to OSPF pt-mpt

model: as a set of point-to-point links OSPF NBMA mode is configuration-heavy and risky (all

routers must be able to reach DR; bad news if VC fails)

Packet Encoding

OSPF is “efficiently” encoded Positional fields, 32-bit alignment Only LSAs are extensible (not Hellos, etc.) Unrecognized types not flooded. Opaque-LSAs recently

introduced. IS-IS is mostly Type-Length-Value (TLV) encoded

No particular alignment Extensible from the start (unknown types ignored but still

flooded) All packet types are extensible Nested TLVs provide structure for more granular extension

IS-IS LS Database: Generic Packet Format

Intra-domain Routing Protocol Discriminator

Length Indicator

TLV Fields

Version/Protocol ID Extension

ID Length

R R R PDU Type

Version

Reserved

Maximum Area Addresses

Packet-Specific Header Fields

No. of Octets

1

1

1

1

1

1

1

1

Traffic Engineering: Motivation TE: “…that aspect of Internet network engineering

dealing with the issue of performance evaluation and performance optimization of operational IP networks …’’

90’s approach to TE was by changing link weights in IGP (OSPF, IS-IS) or EGP (BGP-4) Performance limited by the shortest/policy path nature Assumptions: Quasi-static traffic, knowledge of demand

matrix

A

B

C

D

1

1 2

1

E

2

Can not do this with OSPFA

B

C

D

1

1 2

1

E

2

Links AB and BD are overloaded

A

B

C

D

1

1 2

4

E

2

Links AC and CD are overloaded

Traffic Engineering What is traffic engineering?

Control and optimization of routing, to steer traffic through the network in the most effective way

Two fundamental approaches to adaptation Adaptive routing protocols

Distribute traffic and performance measurements Compute paths based on load, and requirements

Adaptive network-management system Collect measurements of traffic and topology Optimize the setting of the “static” parameters

Big debates still today about the right answer

QoS routing = optimization of user QoS objectivesTE = optimization of user AND network QoS objectives

Outline: Three Alternatives Load-sensitive routing at packet level

Routers receive feedback on load and delay Routers re-compute their forwarding tables Fundamental problems with oscillation

Load-sensitive routing at circuit (or aggregate) level Routers receive feedback on load and delay Router compute a path for the next circuit Less oscillation, as long as circuits last for a while

Traffic engineering as a management problem Routers compute paths based on “static” values Network management system sets the parameters to influence

the mapping of traffic to paths Acting on network-wide view of traffic and topology