Transcript IP: Addresses and Forwarding - ECSE

Routing II: Protocols (RIP,
EIGRP, OSPF, PNNI, IS-IS)
Shivkumar Kalyanaraman
Rensselaer Polytechnic Institute
[email protected]
Based in part upon slides of Prof. Raj Jain (OSU), S. Keshav (Cornell), J. Kurose (U Mass)
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Overview
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RIP, RIPv2, EIGRP
OSPF, PNNI, IS-IS: LS efficiency & robustness
 Link state distribution, DB synchronization, NBMAs etc
Refs: Chap 16,14
Books: “Interconnections” by Perlman, “OSPF” by John Moy, “Routing in
Internet” by Huitema.
Reference: RFC 2328: OSPF Version 2: In HTML
Reading: Notes for Protocol Design, E2e Principle, IP and Routing: In PDF
Reading: Routing 101: Notes on Routing: In PDF | In MS Word
Reference: Tsuchiya, "The Landmark Hierarchy: A New Hierarchy for Routing in Very
Large Networks"
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RIP: Routing Information Protocol
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Uses hop count as metric (max: 16 is infinity)
Tables (vectors) “advertised” to neighbors every 30 s.
 Each advertisement: upto 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)
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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
 RIPv1 does not support subnet masks (VLSMs)
 No authentication
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RIPv2
Why ? Installed base of RIP routers
 Provides:
 VLSM support
 Authentication
 Multicasting
 “Wire-sharing” by multiple routing domains,
 Tags to support EGP/BGP routes.
 Uses reserved fields in RIPv1 header.
 First route entry replaced by authentication info.
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E-IGRP (Interior Gateway Routing Protocol)
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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
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Link State vs. Distance Vector
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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
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Caveat: With path-vector-type (paths instead of
distances) DV routing, these differences blur…
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Link State Protocols
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Key: Create a network “map” at each node.
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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.
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Link State Issues
Reliable Flooding: sequence #s, age
 LSA types, Neighbor discovery and maintainence
(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.
 Advanced topics: incremental SPF algorithms
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Reliable Flooding…
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Topology Dissemination
A.k.a LSP distribution
 1. Flood LSPs on links except incoming link
 Require at most 2E transfers for n/w with E
edges
 2. Sequence numbers to detect duplicates
 Why? Routers/links may go down/up
 Issue: wrap-around, larger sequence number
is not the most recent!
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Sequence Number Space Organization
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Circular space: S1 > S2 > S3 > S1
 Accidental bit errors in switch memory caused this
problem in ARPANET
Lollipop sequence: Start with S0, increment till you
reach circle and then view it as a circular space
 No ambiguity in lollipop handle
Linear space: OSPFv2.
 If Smax reached, expicitly delete Smax LSA before
wrapping around
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Topology Dissemination (Continued)
Checksum field:
 Drop packet if in error, get retransmission from
neighbor
 Age field (similar to TTL)
 Number of seconds since LSA originated
 Periodically incremented after acceptance
 Originating router refreshes LSA after 30 min
 Delete if Age = MaxAge
 Low age field + large seq # => that LSA is
flapping or frequently changing …
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Recovering from a partition
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On partition, LSP databases can get out of synch
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Databases described by database descriptor records
Routers on each side of a newly restored link talk to each
other to update databases (determine missing and out-ofdate LSPs) => selective synchronization
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LSA-types, Neighbor & flooding
Adjacencies in Different Subnets
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OSPF Router-LSA: Scenario
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Neighbor Discovery & Relationship
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Every OSPF router sends out 'hello' packets
Hello packets used to determine if neighbor is up
Hello packets sent periodically (short intervals)
HelloInterval = 10s (in example)
Assumes neighbor dead if no response within
 RouterDeadInterval = 40s (in example)
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This is also called an “adjacency”
 Note that adjacency is a logical routing relationship
and is more than physical connection.
 It consumes bandwidth and computation resources
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Becomes an issue if large number of adj need to be
maintained
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Neighbor …
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Once an adjacency is established, trade information
 Neighbor relationship is bi-directional as a result of
OSPF hello packets
Local topology information is packaged in a "link state
announcement“ (LSA)
 Multiple types of LSAs: (detail later)
Initial DB synchronization
 New announcements are sent ONCE, and only
updated if there's a change
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Or every 45mins...
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Hello:
Packet Format
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Router-LSA:
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Database Synchronization
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LS Database (LSDB): collection of the Link State
Advertisements (LSAs) accepted at a node.
 This is the “map” for Dijkstra algorithm
When the connection between two neighbors comes up,
the routers must wait for their LS DBs to be synchronized.
 Else routing loops and black holes due to inconsistency
OSPF technique:
 Source sends only LSA headers, then
 Neighbor requests LSAs that are more recent.
 Those LSAs are sent over
 After sync, the neighbors are said to be “fully adjacent”
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Recap: IP Subnet Model
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Each subnet assigned one
or more address prefixes.
Each address prefix is
called an IP subnet
IP routes to subnets, not to
individual hosts
Two hosts on different IP
subnets have to go through
one or more routers.
 Even if they are on the
same “physical” network
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IP Subnet Model (Contd)
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Two hosts or routers on
a common subnet can
send packets “directly”
to one another
Two routers cannot
exchange routing
information directly
unless they have one or
more IP subnets in
common
All these issues will be
strained as we study
OSPF adjacency
operation over different
subnets
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Broadcast Media Issues
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Multiple (N) OSPF routers attached to a common subnet
 Problems:
 One “physical link” vs N*(N-1) “adjacencies”
 How many “links” to be counted for Dijkstra algo?
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Broadcast net: # links for DIjkstra
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Each router is assumed to be “linked” to every other
router for the purposes of Dijkstra.
Hello protocol optimization:
 Each node multicasts Hello to 224.0.0.5 (multicast
address “AllSPFRouters”)
 The Hello multicast message also indicates acks for
other routers’ Hellos by listing their RouterIDs
 “Link” relationship for purposes of Dijkstra maintained
by each node sending a single Hello packet, instead of
N packets.
What about “flooding adjacencies”, I.e.,
 Whom to send (flood) LSAs when a router generates
or learns a new LSA?
 Does it need to synchronize DBs with all nodes ?
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Flooding Adjacencies : option 1
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Using Router-LSAs …
 O(N) Router-LSAs, with O(N2) adjacency info
 Multicast of Router-LSAs does not solve O(N2) DB
synchronization issue
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Flooding Adjacencies: option 2
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New LSA-type: Network-LSA …
 O(N) Router-LSAs + 1 network-LSA+ O(N) adjacencies
 Converted O(N2) adjacency problem into O(N) problem
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Recap: O(N2) model  O(N) model
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Question: Who creates the network-LSA?
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Designated Router (DR)
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One router elected as a designated router (DR)
 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
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DR, BDR … continued
Backup DR (BDR) also syncs with all routers, and
takes over if DR dies (typically 5 s wait)
 Total: 2N – 1 adjacencies
 Multicast-based optimization:
 New LSAs, Hellos sent to AllSPFRouters avoids
DR re-advertising new information
 LSA acks sent to AllDRRouters avoids separate
copies to be sent to DR and BDR
 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
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Network-LSA Example: Summary
DR
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What if subnet does not support
broadcast?
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Non-Broadcast Multiple Access (NBMA) media
NBMA segments may support more than 2 routers, and
allow any two routers to communicate directly, but do not
support data-link broadcast/mcast capability
 Eg:X.25, SMDS, Frame-Relay, ATM etc
 Connection-oriented (VC-based) communication
 Each VC is costly => setting up full mesh for Hellos is
prohibitively expensive
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Two flooding adjacency models in OSPF:
 Non-Broadcast Multiple Access (NBMA) model
 Point-to-Multipoint (pt-mpt) Model
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Different tradeoffs…
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NBMA Subnet Model
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Neighbor discovery: manually configured
Dijkstra SPF views NBMA as a full mesh!
Most routers assigned a RouterPriority = 0
Other routers: eligible to become DRs =>
 ID of all routers in the NBMA configured
 Maintains VCs and Hellos with all routers eligible to
become DRs (RouterPriority > 0)
 Enables election of new DR if current one fails
DR and BDR only maintain VCs and Hellos with all
routers on NBMA
 DB synchronization works same as broadcast subnet
 Flooding in NBMA always goes through DR
 Multicast not available to optimize LSA flooding.
DR generates network-LSA just like broadcast subnet
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NBMA vs Pt-Mpt Subnet Model
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Key assumption in NBMA model:
 Each router on the subnet can communicate with
every other (same as IP model)
 But this requires a “full mesh” of expensive PVCs at
the lower layer!
 Many organizations have a hub-and-spoke PVC
setup, a.k.a. “partial mesh”
 Conversion into NBMA model requires multiple IP
subnets, and complex configuration (see fig on next
slide)
OSPF’s pt-mpt subnet model breaks the rule that two
routers on the same network must be able to talk directly
 Can turn partial PVC mesh into a single IP subnet
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Partial Mesh F-Relay: NBMA model
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Partial Mesh F-Relay: pt-mpt model
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Pt-Mpt Subnet Model
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Each router: single OSPF interface, but multiple neighbor
relationships
Note that neighbor relationships not formed to nodes to
which direct PVC does not exist.
Key differences:
 No DRs or BDRs! Just hellos over the PVCs. Make
sure that the communication is bi-directional.
 I.e. Partial mesh is viewed in Dijkstra as a partial
mesh. Full mesh view not forced like in NBMA model.
Sometimes auto-configuration is possible.
Loss in efficiency because the DB synchronization has to
be done between every peer.
O(n^2) if full mesh. So, in true full PVC mesh situations,
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Hierarchical Routing
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Why Hierarchy?
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Information hiding (filtered) => computation,
bandwidth, storage saved => efficiency => scalability
Address abstraction vs Topology Abstraction
 Multiple paths possible between two areas
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Hierarchical OSPF
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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
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Hierarchical OSPF
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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)
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Sample Area Configuration
10.2.0.0/24
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Summary-LSA Example
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Externals and Aggregation 1
A full ISP routing table has approximately 100K
routes!
 But will you do anything differently if you know
all of them and have a single ISP?
 Multiple ISP situations call for complex OSPF
and BGP design
 Never redistribute IGPs into BGP! (later…)
 Redistribute BGP into IGPs with extreme care
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Externals & Aggregation 2
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In an enterprise
 Limit externals from subordinate domains
(e.g., RIP) to be within area (area-scope)
 Flood only in area 0 and in area with ASBR
 Allow
externals from Internet, peer domains to
go outside Area 0…
 Only when there will be significant path
differences
 Do things with defaults where possible
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Type 1 and Type 2 externals
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Type 2:
 Default type for routes distributed into OSPF
 EGP costs very different from IGP costs
 Exit based on external (EGP) cost only
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Type 1
 Needs to be set explicitly: not default
 IGP costs can be compared and summed
 Selects exit based on internal + external costs
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Stubbiness: A Means of
Controlling Externals
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Normal Areas
Flood AS-external-LSAs (type 5) across areaboundaries (AS flooding scope)
 ASBR-summary-LSAs (type 4) advertises location of
ASBR (area flooding scope)
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Stub Areas
AS-external-LSAs (type 5) not flooded into stub areas
 Summary-LSA flooded only optionally
 Default route to ABR for all non-area prefixes
 Paths may be inefficient, cannot place an ASBR in
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stub areas
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Not-So-Stubby-Areas (NSSA)
A subset of external LSAs may be flooded
 Use Type-7 LSAs for such external routes
 Used to import RIP domain routes and flood it
externally, but keep default route for BGP routes
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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
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IS-IS Terminology
Intermediate system (IS) - Router
Designated Intermediate System (DIS) - Designated Router
Pseudonode - Broadcast link emulated as virtual node by DIS
End System (ES) - Network Host or workstation
Network Service Access Point (NSAP) - Network Layer Address
Subnetwork Point of attachment (SNPA) - Datalink interface
Packet data Unit (PDU) - Analogous to IP Packet
Link State PDU (LSP) - Routing information packet
Level 1 and Level 2 – Area 0 and lower areas
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Functional Comparison
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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)
IS-IS not designed from the start as an IP routing protocol
(and is therefore a bit clunky in places)
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Sample comparison points
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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 ptmpt 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)
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Packet Encoding
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OSPF is “efficiently” encoded
 Positional fields, 32-bit alignment
 Only LSAs are extensible (not Hellos, etc.)
 Unrecognized types not flooded. Opaque-LSAs
recently introduced.
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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
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IS-IS LS Database: Generic Packet Format
No. of Octets
R
Intra-domain Routing Protocol Discriminator
1
Length Indicator
1
Version/Protocol ID Extension
1
ID Length
1
R
R
PDU Type
1
Version
1
Reserved
1
Maximum Area Addresses
1
Packet-Specific Header Fields
TLV Fields
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More detailed comparison provided
as a reference (not covered in
class)…
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Private Network to Node Interface (PNNI)
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Link State Routing Protocol for ATM Networks
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“A hierarchy mechanism ensures that this
protocol scales well for large world-wide ATM
networks. A key feature of the PNNI hierarchy
mechanism is its ability to automatically configure
itself in networks in which the address structure
reflects the topology…”
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PNNI Features
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Scales to very large networks.
Supports hierarchical routing.
Supports QoS.
Supports multiple routing metrics and attributes.
Uses source routed connection setup.
Operates in the presence of partitioned areas.
Provides dynamic routing, responsive to changes in
resource availability.
Separates the routing protocol used within a peer group
from that used among peer groups.
Interoperates with external routing domains, not
necessarily using PNNI.
Supports both physical links and tunneling over VPCs.
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PNNI Terminology (partial)
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Peer group: A group of nodes at the same hierarchy
Border node: one link crosses the boundary
Logical group node: Representation of a group as a single point
Child node: Any node at the next lower hierarchy level
Parent node: LGN at the next higher hierarchy level
Logical links: links between logical nodes
Peer group leader (PGL): Represents a group at the next higher
level.
 Node with the highest "leadership priority" and highest
ATM address is elected as a leader.
 PGL acts as a logical group node.
 Uses same ATM address with a different selector value.
Peer group ID: Address prefixes up to 13 bytes
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PNNI Terminology
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Hierarchical Routing: PNNI
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Source Routing
Source specifies route as a list of all intermediate
systems in the route. Abstracts out area hops.
 Designated Transit List (DTL) Source route
across each level of hierarchy
 Entry switch of each peer group specifies
complete route through that group
 Set of DTLs and manipulations implemented
as a stack
 DTL example: next slide
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DTL Example
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Crank back and Alternate Path Routing
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If a call fails along a particular route:
 It is cranked back to the originator of the top DTL
 The originator finds another route or
 Cranks back to the generator of the higher level
source route
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Summary
DV Protocols: RIP, EIGRP
 LS Protocols: OSPF, IS-IS, PNNI
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