15-744: computer networking l-7 routing issues. l -7; 2-5-01© srinivasan seshan, 20012 routing...

15-744: Computer Networking

L-7 Routing Issues

L -7; 2-5-01© Srinivasan Seshan, 2001 2

Routing Alternatives

• Overlay routing techniques• Landmark hierarchies• Synchronization of periodic messages• Assigned reading

• [S+99] The End-to-End Effects of Internet Path Selection

• [Tsu88] The Landmark Hierarchy: A New Hierarchy for Routing in Very Large Networks


Outline

• Multi-homing

• Landmark Hierarchy

• Synchronization of Routing Messages

• Overlay Routing


Issues with Multi-homing

• Symmetric routing• While preference symmetric paths, many are

asymmetric

• Packet re-ordering• May trigger TCP’s fast retransmit algorithm

• Other concerns:• Address aggregation


Multi-homing to a Single Provider

ISP

Customer

R1

R2

• Easy solution:• Use IMUX or Multi-link

PPP

• Hard solution:• Use BGP• Makes assumptions

about traffic (same amount of prefixes can be reached from both links)



ISP

Customer

R1

R2

• If multiple prefixes, may use MED• Good if traffic load to

prefixes is equal

• If single prefix, load may be unequal• Break-down prefix and

advertise different prefixes over different links

R3

138.39/16 204.70/16



ISP

Customer

R1 R2

• For traffic to customer, same as before:• Use MED• Good if traffic load to

prefixes is equal

• For traffic to ISP:• R3 alternates links• Multiple default routes R3

138.39/16 204.70/16



ISP

Customer

R1 R2

• Most reliable approach• No equipment sharing

• Use MED

R3

138.39/16 204.70/16

R4


Outline

• Multi-homing



• Overlay Routing


Landmark Hierarchy

• Details about things nearby and less information about things far away

• Not defined by arbitrary boundaries• Thus, not well suited to the real world that does

have administrative boundaries


A Landmark

1

2

3

4

56

7

89

10

11

Router 1 is a landmarkof radius 2


Landmark Overview

• Landmark routers have “height” which determines how far away they can be seen (visibility)• Routers within radius n can see a landmark router LMn

• See means that those routers have LMn’s address and know next hop to reach it. • Router x as an entry for router y if x is within radius of y

• Distance vector style routing with simple metric• Routing table: Landmark (LM2(d)), Level(2), Next

hop


LM Hierarchy Definition

• Each LM i associated with level (i) and radius (ri)

• Every node is an LM0 landmark

• Recursion: some LMi are also LMi+1

• Every LMi sees at least one LMi+1

• Terminating state when all level j LMs are seen by entire network


LM Self-configuration

• Bottom-up hierarchy construction algorithm• Goal to bound number of children• Every router is L0 landmark• All Li landmarks run election to self-promote one or more L i+1

landmarks• LM level maps to radius (part of configuration), e.g.:

• LM level 0: radius 2• LM level 1: radius 4• LM level 2: radius 8

• Dynamic algorithm to adapt to topology changes – Efficient hierarchy


LM Addresses

• LM(2).LM(1).LM(0) (C.B.A)

• If destination is more than two hops away, will not have complete routing information, refer to LM(1) portion of address, if not known then refer to LM(2)

LM2C

LM1B

R2

R1

LM0A aka C.B.A

R0


LM Routing

• LM does not imply hierarchical forwarding• It is NOT a source route

• En route to LM1,packet may encounter router that is within LM0 radius of destination address (like longest match)

• Paths may be asymmetric


•Source wants to reach LM0[a], whose address is c.b.a:

•Source can see LM2[c], so sends packet towards c

•Entering LM1[b] area, first router diverts packet to b

•Entering LM0[a] area, packet delivered to a

•Not shortest path•Packet may not reach landmarks

Landmark Routing: Basic Idea

LM2[c]

LM1[b]r0[a]

LM0[a]

r2[c]

r1[b]

Network Node

Path

Landmark Radius


Landmark Routing: Example

d.d.a

d.d.b

d.d.c

d.d.e

d.d.d

d.d.f

d.i.kd.i.g

d.d.j

d.i.i

d.i.w

d.i.ud.d.kd.d.l

d.n.hd.n.x

d.n.n

d.n.o

d.n.p

d.n.q

d.n.t

d.n.s

d.n.r

d.i.v


Routing Table for Router g

Landmark Level Next hop

LM2[d]

LM0[e]

LM1[i]

LM0[k]

LM0[f]

2

1

0

0

0

f

k

f

k

f

Router g

Router t

r0 = 2, r1 = 4, r2 = 8 hops • How to go from d.i.g to d.n.t? g-f-e-d-u-t

• How does path length compare to shortest path? g-k-I-u-t

d.d.a

d.d.b

d.d.c

d.d.e

d.d.d

d.d.f

d.i.kd.i.g

d.d.j

d.i.i

d.i.w

d.i.ud.d.kd.d.l

d.n.hd.n.x

d.n.n

d.n.o

d.n.p

d.n.q

d.n.t

d.n.s

d.n.r


Final Questions

• Why should the routing algorithm be distance-vector?• Link state algorithms need a full topology map

• What modifications are needed to DV?• Must expire information about destination as it

propagates• Some support of administrative domains

• How about naming?• Addresses may change


Outline

• Multi-homing



• Overlay Routing


Routing Update Synchronization

• Another interesting robustness issue to consider...

• Even apparently independent processes can eventually synchronize• Intuitive assumption that independent

streams will not synchronize is not always valid

• Periodic routing protocol messages from different routers

• Abrupt transition from unsynchronized to synchronized system states


Examples/Sources of Synchronization

• TCP congestion windows• Cyclical behavior shared by flows through gateway

• Periodic transmission by audio/video applications• Periodic downloads• Synchronized client restart

• After a catastrophic failure

• Periodic routing messages• Manifests itself as periodic packet loss on pings

• Pendulum clocks on same wall• Automobile traffic patterns


How Synchronization Occurs

T

AMessage from B

Weak Coupling when A’s behavior is triggered off of B’smessage arrival!

A

T

No Coupling when A sends at a timethat is independentof B’s messagearrival

Weak couplingcan result in

eventual synchronization


Periodic Message Model

• Router prepares and sends update, resets timer Tc seconds after start time• Received by other routers Td seconds from start

• If router receives incoming routing update while preparing its own update• Router processes incoming update for Tc2 seconds

• After generating update, sets timer drawn uniformly from [Tp-Tr, Tp+Tr] seconds, Tp is avg period, Tr random component• When timer expires repeat• If update occurs reflecting topology event also repeat

Why is this a problem???


The Periodic Message Model

A

A’s routing updateOthers’

routing updates

Tc Tc2

Td

[Tp-Tr, Tp+Tr]Tc2

Triggered updates cause sending of a message before timerexpires


Routing Source of Synchronization• Router resets timer after processing its own

and incoming updates• Creates weak coupling among routers• There are solutions:

• Set timer based on clock event that is not a function of processing other routers’ updates, or

• Add randomization, or reset timer before processing update


What Happens?


Why Does it Happen?


Important Results

• With increasing Tr (randomization)• Takes longer to synchronize• May need Tr to be ten times Tc

• A robust choice of timer Tr = Tp/2• With increasing randomization, abrupt

transition• From predominantly synchronized to

predominantly unsynchronized


Other Thoughts

• Emergent large-scale structure likely to result from more complex system interactions

• Most protocols now incorporate some form of randomization


Outline

• Multi-homing



• Overlay Routing


Overlay Routing

• Basic idea:• Treat multiple hops through IP network as one hop in

overlay network• Run routing protocol on overlay nodes

• Why?• For performance – can run more clever protocol on

overlay• For efficiency – can make core routers very simple• For functionality – can provide new features such as

multicast, active processing, IPv6


Overlay for Performance [S+99]

• Why would IP routing not give good performance?• Policy routing – limits selection/advertisement

of routes• Early exit/hot-potato routing – local not global

incentives• Lack of performance based metrics – AS hop

count is the wide area metric• How bad is it really?

• Look at performance gain an overlay provides


Quantifying Performance Loss

• Measure round trip time (RTT) and loss rate between pairs of hosts• ICMP rate limiting

• Alternate path characteristics• 30-55% of hosts had lower latency• 10% of alternate routes have 50% lower

latency• 75-85% have lower loss rates


Bandwidth Estimation

• RTT & loss for multi-hop path• RTT by addition• Loss either worst or combine of hops – why?

• Large number of flows combination of probabilities• Small number of flows worst hop

• Bandwidth calculation• TCP bandwidth is based primarily on loss and

RTT• 70-80% paths have better bandwidth• 10-20% of paths have 3x improvement


Sources of Experimental Error

• Much of paper targeted at proving accuracy of measurements

• Use of mean vs. median – checked for one dataset

• Random sampling noise – compare 95% confidence intervals

• Time of day effects – alternate paths better during higher load

• Long-term averaging – looked a simultaneous measurements


Possible Sources of Alternate Paths

• A few really good or bad AS’s • No, benefit of top ten hosts not great

• Better congestion or better propagation delay?• How to measure?

• Propagation = 10th percentile of delays

• Both contribute to improvement of performance


Overlay for Efficiency

• Multi-path routing• More efficient use of links or QOS• Need to be able to direct packets based on

more than just destination address can be computationally expensive

• What granularity? Per source? Per connection? Per packet?

• Per packet re-ordering• Per source, per flow coarse grain vs. fine grain

• Take advantage of relative duration of flows• Most bytes on long flows


Edge vs. Core Routers

• Island of routers• Edges can perform complex computation to

classify flow• Cores do extremely simple forwarding• How to communicate computation results

• MPLS• Dynamic packet state


Overlay for Features

• How do we add new features to the network?• Does every router need to support new feature?• Choices

• Reprogram all routers active networks• Support new feature within an overlay

• Basic technique: tunnel packets

• Tunnels• IP-in-IP encapsulation• Poor interaction with firewalls, multi-path routers, etc.


Examples

• IP V6 & IP Multicast• Tunnels between routers supporting feature

• Mobile IP• Home agent tunnels packets to mobile host’s

location

• QOS• Needs some support from intermediate routers


Overlay Challenges

• “Routers” no longer have complete knowledge about link they are responsible for

• How do you build efficient overlay• Probably don’t want all N2 links – which links to

create?• Without direct knowledge of underlying

topology how to know what’s nearby and what is efficient?


Future of Overlay

• Application specific overlays• Why should overlay nodes only do routing?

• Caching• Intercept requests and create responses

• Transcoding• Changing content of packets to match available

bandwidth

• Peer-to-peer applications


Next Lecture: TCP Basics

• TCP connection setup/data transfer• TCP reliability• TCP options• Assigned reading

• [FF96] Simulation-based Comparisons of Tahoe, Reno, and SACK TCP

15-744: computer networking l-7 routing issues. l -7; 2-5-01© srinivasan seshan, 20012 routing...

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