Distance Vector Algorithm

Bellman-Ford Equation (dynamic programming)

Define
dx(y) := cost of least-cost path from x to y
Then
dx(y) = min
{c(x,v) + dv(y) }
v
where min is taken over all neighbors v of x
Network Layer

4-1

Bellman-Ford example
5
2

v
2

w
3

Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3

5
2

B-F equation says:

du(z) = min { c(u,v) + dv(z),
c(u,x) + dx(z),
c(u,w) + dw(z) }
= min {2 + 5,
1 + 3,
5 + 3} = 4

Node that achieves minimum is next

hop in shortest path forwarding table
Network Layer

4-2

Distance Vector Algorithm

Dx(y) = estimate of least cost from x to y
Node x knows cost to each neighbor v:

c(x,v)
Node x maintains distance vector Dx =
[Dx(y): y N ]
Node x also maintains its neighbors
distance vectors

For each neighbor v, x maintains

Dv = [Dv(y): y N ]
Network Layer

4-3

Distance vector algorithm (4)

Basic idea:
Each node periodically sends its own distance
vector estimate to neighbors
When a node x receives new DV estimate from
neighbor, it updates its own DV using B-F equation:

Dx(y) minv{c(x,v) + Dv(y)}

for each node y N

Under minor, natural conditions, the estimate

Dx(y) converge to the actual least cost dx(y)

Network Layer

4-4

Distance Vector Algorithm (5)

Iterative, asynchronous:

each local iteration caused

by:
local link cost change
DV update message from
neighbor

Distributed:

wait for (change in local link

cost or msg from neighbor)
recompute estimates

each node notifies

neighbors only when its DV

changes

Each node:

neighbors then notify

their neighbors if
necessary

if DV to any dest has

changed, notify neighbors

Network Layer

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}

= min{2+0 , 7+1} = 2

node x table
cost to
x y z

Dx(z) = min{c(x,y) +
Dy(z), c(x,z) + Dz(z)}
= min{2+1 , 7+0} = 3

cost to
x y z
from

from

x 0 2 7
y
z
node y table
cost to
x y z

x 0 2 3
y 2 0 1
z 7 1 0

x
y 2 0 1
z
node z table
cost to
x y z

from
from

x
y
z 71 0

4-5

time

Network Layer

4-6

Dx(z) = min{c(x,y) +
Dy(z), c(x,z) + Dz(z)}

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}

= min{2+0 , 7+1} = 2

node x table
cost to
x y z

x
y
z 71 0

from

from

from

from

x 0 2 7
y 2 0 1
z 7 1 0
cost to
x y z
x 0 2 7
y 2 0 1
z 3 1 0

x 0 2 3
y 2 0 1
z 3 1 0
cost to
x y z
x 0 2 3
y 2 0 1
z 3 1 0

cost to
x y z
from

from

from

x
y 2 0 1
z
node z table
cost to
x y z

x 0 2 3
y 2 0 1
z 7 1 0

= min{2+1 , 7+0} = 3

cost to
x y z

cost to
x y z

from

from

x 0 2 7
y
z
node y table
cost to
x y z

cost to
x y z

x 0 2 3
y 2 0 1
z 3 1 0
time

Network Layer

4-7

Distance Vector: link cost changes

Link cost changes:
node detects local link cost change
updates routing info, recalculates

distance vector
if DV changes, notify neighbors

good
news
travels
fast

y
50

At time t0, y detects the link-cost change, updates its DV,

and informs its neighbors.
At time t1, z receives the update from y and updates its table.
It computes a new least cost to x and sends its neighbors its DV.
At time t2, y receives zs update and updates its distance table.
ys least costs do not change and hence y does not send any
message to z.

Network Layer

4-8

Distance Vector: link cost changes

Link cost changes:

60

good news travels fast

bad news travels slow -

count to infinity problem!

44 iterations before
algorithm stabilizes: see
text

y
50

Poisoned reverse:
If Z routes through Y to

get to X :

Z tells Y its (Zs) distance

to X is infinite (so Y wont
route to X via Z)

will this completely solve

count to infinity problem?

Network Layer

4-9

Comparison of LS and DV algorithms

Message complexity
LS: with n nodes, E links,

O(nE) msgs sent

DV: exchange between

neighbors only
convergence time varies

Speed of Convergence
LS: O(n2) algorithm requires

O(nE) msgs
may have oscillations
DV: convergence time varies
may be routing loops
count-to-infinity problem

Robustness: what happens

if router malfunctions?
LS:

node can advertise

incorrect link cost
each node computes only
its own table

DV:

DV node can advertise

incorrect path cost
each nodes table used by
others
error propagate thru
network
Network Layer 4-10

Chapter 4: Network Layer

4. 1 Introduction
4.2 Virtual circuit and

datagram networks
4.3 Whats inside a
router
4.4 IP: Internet
Protocol

Datagram format
IPv4 addressing
ICMP
IPv6

4.5 Routing algorithms

Link state
Distance Vector
Hierarchical routing
4.6 Routing in the

Internet

RIP
OSPF
BGP

4.7 Broadcast and

multicast routing

Network Layer

4-11

Hierarchical Routing
Our routing study thus far - idealization
all routers identical
network flat
not true in practice
scale: with 200 million
destinations:
cant store all dests in

routing tables!

routing table exchange

would swamp links!

administrative autonomy
internet = network of

networks
each network admin may
want to control routing in its
own network

Network Layer 4-12

Hierarchical Routing
aggregate routers into

regions, autonomous
systems (AS)
routers in same AS run
same routing protocol

Gateway router
Direct link to router in
another AS

intra-AS routing
protocol
routers in different AS
can run different intraAS routing protocol

Network Layer 4-13

Interconnected ASes
3c
3b

3a
AS3
1a

2a

1c
1d

1b

Intra-AS
Routing
algorithm

2c
AS2

AS1

Inter-AS
Routing
algorithm

Forwarding
table

2b

forwarding table

configured by both
intra- and inter-AS
routing algorithm

intra-AS sets entries

for internal dests
inter-AS & Intra-As
sets entries for
external dests
Network Layer 4-14

Inter-AS tasks

AS1 must:
1. learn which dests
reachable through
AS2, which through
AS3
2. propagate this
reachability info to all
routers in AS1
Job of inter-AS routing!

suppose router in AS1

receives datagram
dest outside of AS1
router should
forward packet to
gateway router, but
which one?

3c
3b

3a
AS3
1a

2a

1c
1b

1d

2c
AS2

2b

AS1
Network Layer 4-15

Example: Setting forwarding table in router 1d

suppose AS1 learns (via inter-AS protocol) that subnet

x reachable via AS3 (gateway 1c) but not via AS2.

inter-AS protocol propagates reachability info to all

internal routers.
router 1d determines from intra-AS routing info that
its interface I is on the least cost path to 1c.
installs forwarding table entry (x,I)
3c
3b

3a
AS3
1a

x
2a

1c
1d

1b AS1

2c

2b
AS2
Network Layer 4-16

Example: Choosing among multiple ASes

now suppose AS1 learns from inter-AS protocol that

subnet x is reachable from AS3 and from AS2.

to configure forwarding table, router 1d must

determine towards which gateway it should forward

packets for dest x.
this is also job of inter-AS routing protocol!

3c
3b

3a
AS3

1a

2a

1c
1d

1b

2c
AS2

2b

AS1
Network Layer 4-17

Example: Choosing among multiple ASes

now suppose AS1 learns from inter-AS protocol that

subnet x is reachable from AS3 and from AS2.

to configure forwarding table, router 1d must

determine towards which gateway it should forward

packets for dest x.
this is also job of inter-AS routing protocol!
hot potato routing: send packet towards closest of
two routers.

Learn from inter-AS

protocol that subnet
x is reachable via
multiple gateways

Use routing info

from intra-AS
protocol to determine
costs of least-cost
paths to each
of the gateways

Hot potato routing:

Choose the gateway
that has the
smallest least cost

Determine from
forwarding table the
interface I that leads
to least-cost gateway.
Enter (x,I) in
forwarding table

Network Layer 4-18

Chapter 4: Network Layer

4. 1 Introduction
4.2 Virtual circuit and

datagram networks
4.3 Whats inside a
router
4.4 IP: Internet
Protocol

Datagram format
IPv4 addressing
ICMP
IPv6

4.5 Routing algorithms

Link state
Distance Vector
Hierarchical routing
4.6 Routing in the

Internet

RIP
OSPF
BGP

4.7 Broadcast and

multicast routing

Network Layer 4-19

Intra-AS Routing
also known as Interior Gateway Protocols (IGP)
most common Intra-AS routing protocols:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco

proprietary)

Network Layer 4-20

10

Chapter 4: Network Layer

4. 1 Introduction
4.2 Virtual circuit and

datagram networks
4.3 Whats inside a
router
4.4 IP: Internet
Protocol

4.5 Routing algorithms

Link state
Distance Vector
Hierarchical routing
4.6 Routing in the

Internet

Datagram format
IPv4 addressing
ICMP
IPv6

RIP
OSPF
BGP

4.7 Broadcast and

multicast routing

Network Layer 4-21

RIP ( Routing Information Protocol)

distance vector algorithm
included in BSD-UNIX Distribution in 1982
distance metric: # of hops (max = 15 hops)
From router A to subsets:
u

x
y

destination hops
u
1
v
2
w
2
x
3
y
3
z
2

Network Layer 4-22

11

RIP advertisements
distance

vectors: exchanged among

neighbors every 30 sec via Response

Message (also called advertisement)
each advertisement: list of up to 25
destination nets within AS

Network Layer 4-23

RIP: Example
z
w

C
Destination Network

w
y
z
x

Next Router

Num. of hops to dest.

....

A
B
B
--

2
2
7
1

Routing table in D
Network Layer 4-24

12

RIP: Example
Dest
w
x
z
.

Next
C

hops
1
1
4
...

Advertisement
from A to D

z
x

Destination Network

w
y
z
x

Next Router

Num. of hops to dest.

....

A
B
B A
--

Routing table in D

2
2
7 5
1

Network Layer 4-25

RIP: Link Failure and Recovery

If no advertisement heard after 180 sec -->
neighbor/link declared dead
routes via neighbor invalidated
new advertisements sent to neighbors
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)

Network Layer 4-26

13

RIP Table processing

RIP routing tables managed by application-level

process called route-d (daemon)

advertisements sent in UDP packets, periodically
repeated
routed

routed

Transprt
(UDP)
network
(IP)

Transprt
(UDP)
forwarding
table

forwarding
table

network
(IP)

link

link

physical

physical
Network Layer 4-27

Chapter 4: Network Layer

4. 1 Introduction
4.2 Virtual circuit and

datagram networks
4.3 Whats inside a
router
4.4 IP: Internet
Protocol

Datagram format
IPv4 addressing
ICMP
IPv6

4.5 Routing algorithms

Link state
Distance Vector
Hierarchical routing
4.6 Routing in the

Internet

RIP
OSPF
BGP

4.7 Broadcast and

multicast routing

Network Layer 4-28

14

OSPF (Open Shortest Path First)

open: publicly available
uses Link State algorithm
LS packet dissemination
topology map at each node
route computation using Dijkstras algorithm
OSPF advertisement carries one entry per neighbor

router
advertisements disseminated to entire AS (via
flooding)

carried in OSPF messages directly over IP (rather than TCP

or UDP
Network Layer 4-29

OSPF advanced features (not in RIP)

security: all OSPF messages authenticated (to

prevent malicious intrusion)

multiple same-cost paths allowed (only one path in
RIP)
For each link, multiple cost metrics for different
TOS (e.g., satellite link cost set low for best effort;
high for real time)
integrated uni- and multicast support:
Multicast OSPF (MOSPF) uses same topology data
base as OSPF
hierarchical OSPF in large domains.
Network Layer 4-30

15

Chapter 4: Network Layer

4. 1 Introduction
4.2 Virtual circuit and

datagram networks
4.3 Whats inside a
router
4.4 IP: Internet
Protocol

Datagram format
IPv4 addressing
ICMP
IPv6

4.5 Routing algorithms

Link state
Distance Vector
Hierarchical routing
4.6 Routing in the

Internet

RIP
OSPF
BGP

4.7 Broadcast and

multicast routing

Network Layer 4-31

Internet inter-AS routing: BGP

BGP (Border Gateway Protocol):

the de

facto standard
BGP provides each AS a means to:
1.
2.
3.

Obtain subnet reachability information from

neighboring ASs.
Propagate reachability information to all ASinternal routers.
Determine good routes to subnets based on
reachability information and policy.

allows subnet to advertise its existence to

rest of Internet: I am here

Network Layer 4-32

16

BGP basics
pairs of routers (BGP peers) exchange routing info

over semi-permanent TCP connections: BGP sessions

BGP sessions need not correspond to physical
links.
when AS2 advertises prefix to AS1:
AS2 promises it will forward any addresses
datagrams towards that prefix.
AS2 can aggregate prefixes in its advertisement
eBGP session

3c
3a
3b
AS3
1a
AS1

iBGP session

2a

1c
1d

2c
AS2

1b

2b

Network Layer 4-33

Distributing reachability info

using eBGP session between 3a and 1c, AS3 sends

prefix reachability info to AS1.

1c can then use iBGP do distribute new prefix
info to all routers in AS1
1b can then re-advertise new reachability info
to AS2 over 1b-to-2a eBGP session
when router learns of new prefix, creates entry
for prefix in its forwarding table.
eBGP session

3c
3a
3b
AS3
1a
AS1

iBGP session

2a

1c
1d

1b

2c
AS2

2b

Network Layer 4-34

17

Path attributes & BGP routes

advertised prefix includes BGP attributes.
prefix + attributes = route

two important attributes:

AS-PATH: contains ASs through which prefix
advertisement has passed: e.g, AS 67, AS 17
NEXT-HOP: indicates specific internal-AS router
to next-hop AS. (may be multiple links from
current AS to next-hop-AS)
when gateway router receives route

advertisement, uses import policy to

accept/decline.
Network Layer 4-35

BGP route selection

router may learn about more than 1 route

to some prefix. Router must select route.

elimination rules:
1.
2.
3.
4.

local preference value attribute: policy

decision
shortest AS-PATH
closest NEXT-HOP router: hot potato routing
additional criteria

Network Layer 4-36

18

BGP messages
BGP messages exchanged using TCP.
BGP messages:

OPEN: opens TCP connection to peer and

authenticates sender
UPDATE: advertises new path (or withdraws old)
KEEPALIVE keeps connection alive in absence of
UPDATES; also ACKs OPEN request
NOTIFICATION: reports errors in previous msg;
also used to close connection

Network Layer 4-37

BGP routing policy

legend:

B
W

provider
network
customer
network:

C
Y

A,B,C are provider networks

X,W,Y are customer (of provider networks)
X is dual-homed: attached to two networks

X does not want to route from B via X to C

.. so X will not advertise to B a route to C

Network Layer 4-38

19

BGP routing policy (2)

legend:

B
W

provider
network
customer
network:

C
Y

A advertises path AW to B
B advertises path BAW to X
Should B advertise path BAW to C?

No way! B gets no revenue for routing CBAW

since neither W nor C are Bs customers
B wants to force C to route to w via A
B wants to route only to/from its customers!

Network Layer 4-39

Why different Intra- and Inter-AS routing ?

Policy:
Inter-AS: admin wants control over how its traffic

routed, who routes through its net.

Intra-AS: single admin, so no policy decisions needed

Scale:
hierarchical routing saves table size, reduced update

traffic
Performance:
Intra-AS: can focus on performance
Inter-AS: policy may dominate over performance

Network Layer 4-40

20

Chapter 4: Network Layer

4. 1 Introduction

4.5 Routing algorithms

Link state
Distance Vector
Hierarchical routing

4.2 Virtual circuit and

datagram networks
4.3 Whats inside a
router
4.4 IP: Internet
Protocol

4.6 Routing in the

Internet

Datagram format
IPv4 addressing
ICMP
IPv6

RIP
OSPF
BGP

4.7 Broadcast and

multicast routing

Network Layer 4-41

Broadcast Routing
deliver packets from source to all other nodes
source duplication is inefficient:
duplicate

duplicate
creation/transmission

R1

R1
duplicate

R2

R2
R3

R4

source
duplication

R3

R4

in-network
duplication

source duplication: how does source

determine recipient addresses?

Network Layer 4-42

21

In-network duplication
flooding: when node receives brdcst pckt,

sends copy to all neighbors

Problems: cycles & broadcast storm

controlled flooding: node only brdcsts pkt

if it hasnt brdcst same packet before

Node keeps track of pckt ids already brdcsted

Or reverse path forwarding (RPF): only forward
pckt if it arrived on shortest path between
node and source

spanning tree
No redundant packets received by any node

Network Layer 4-43

Spanning Tree
First construct a spanning tree
Nodes forward copies only along spanning

tree

A
B

c
D
F
G

(a) Broadcast initiated at A

D
G

(b) Broadcast initiated at D

Network Layer 4-44

22

Spanning Tree: Creation

Center node
Each node sends unicast join message to center

node

Message forwarded until it arrives at a node already

belonging to spanning tree

A
3

c
4

F
1

c
D

(a) Stepwise construction

of spanning tree

(b) Constructed spanning

tree
Network Layer 4-45

Multicast Routing: Problem Statement

Goal: find a tree (or trees) connecting

routers having local mcast group members

tree: not all paths between routers used

source-based: different tree from each sender to rcvrs
shared-tree: same tree used by all group members

Shared tree

Source-based trees

23

Approaches for building mcast trees

Approaches:
source-based tree: one tree per source
shortest path trees
reverse path forwarding

group-shared tree: group uses one tree

minimal spanning (Steiner)
center-based trees
we first look at basic approaches, then specific
protocols adopting these approaches

Shortest Path Tree

mcast forwarding tree: tree of shortest

path routes from source to all receivers

Dijkstras algorithm

S: source

LEGEND

R1
1

R4

R2
3
R3

router with attached

group member

5
4
R6

router with no attached

group member

R5
6
R7

link used for forwarding,

i indicates order link
added by algorithm

24

Reverse Path Forwarding

rely on routers knowledge of unicast

shortest path from it to sender

each router has simple forwarding behavior:
if (mcast datagram received on incoming link

on shortest path back to center)

then flood datagram onto all outgoing links
else ignore datagram

Reverse Path Forwarding: example

S: source

LEGEND

R1

R4

router with attached

group member

R2
R5
R3

R6

R7

router with no attached

group member
datagram will be
forwarded
datagram will not be
forwarded

result is a source-specific reverse SPT

may be a bad choice with asymmetric links

25

Reverse Path Forwarding: pruning

forwarding tree contains subtrees with no mcast

group members
no need to forward datagrams down subtree
prune msgs sent upstream by router with no
downstream group members
LEGEND

S: source
R1

router with attached

group member

R4

R2

P
R5

R3

R6

P
R7

router with no attached

group member
prune message
links with multicast
forwarding

Shared-Tree: Steiner Tree

Steiner Tree: minimum cost tree

connecting all routers with attached group

members
problem is NP-complete
excellent heuristics exists
not used in practice:
computational complexity
information about entire network needed
monolithic: rerun whenever a router needs to
join/leave

26

Center-based trees
single delivery tree shared by all
one router identified as

center of tree

to join:
edge router sends unicast join-msg addressed
to center router
join-msg processed by intermediate routers
and forwarded towards center
join-msg either hits existing tree branch for
this center, or arrives at center
path taken by join-msg becomes new branch of
tree for this router

Center-based trees: an example

Suppose R6 chosen as center:
LEGEND
R1
R2

router with attached

group member

R4

3
2

R5
R3

R6

router with no attached

group member
path order in which join
messages generated

R7

27

Internet Multicasting Routing: DVMRP

DVMRP: distance vector multicast routing

protocol, RFC1075
flood and prune: reverse path forwarding,
source-based tree
RPF tree based on DVMRPs own routing tables
constructed by communicating DVMRP routers
no assumptions about underlying unicast
initial datagram to mcast group flooded
everywhere via RPF
routers not wanting group: send upstream prune
msgs

DVMRP: continued
soft

state: DVMRP router periodically (1 min.)

forgets branches are pruned:

mcast data again flows down unpruned branch

downstream router: reprune or else continue to
receive data

routers can quickly regraft to tree

following IGMP join at leaf
odds and ends
commonly implemented in commercial routers
Mbone routing done using DVMRP

28

Tunneling
Q: How to connect islands of multicast
routers in a sea of unicast routers?

physical topology

logical topology

mcast datagram encapsulated inside normal (non-multicast-

addressed) datagram
normal IP datagram sent thru tunnel via regular IP unicast to
receiving mcast router
receiving mcast router unencapsulates to get mcast datagram

PIM: Protocol Independent Multicast

not dependent on any specific underlying unicast

routing algorithm (works with all)

two different multicast distribution scenarios :

Dense:

Sparse:

group members

# networks with group

densely packed, in
close proximity.
bandwidth more
plentiful

members small wrt #

interconnected networks
group members widely
dispersed
bandwidth not plentiful

29

Consequences of Sparse-Dense Dichotomy:

Dense

group membership by

Sparse:

no membership until

routers assumed until

routers explicitly join
routers explicitly prune receiver- driven
data-driven construction
construction of mcast
on mcast tree (e.g., RPF)
tree (e.g., center-based)
bandwidth and non bandwidth and non-groupgroup-router processing
router processing

profligate

conservative

PIM- Dense Mode

flood-and-prune RPF, similar to DVMRP but
underlying unicast protocol provides RPF info

for incoming datagram

less complicated (less efficient) downstream
flood than DVMRP reduces reliance on
underlying routing algorithm
has protocol mechanism for router to detect it
is a leaf-node router

30

PIM - Sparse Mode

center-based approach
router sends

join msg

to rendezvous point
(RP)

router can switch to

source-specific tree
increased performance:
less concentration,
shorter paths

R4

join

intermediate routers
update state and
forward join

after joining via RP,

R1
R2

join
R5

join

R3

R7

R6

all data multicast

from rendezvous
point

rendezvous
point

PIM - Sparse Mode

sender(s):
unicast data to RP,
which distributes down
RP-rooted tree
RP can extend mcast
tree upstream to
source
RP can send stop msg
if no attached
receivers

no one is listening!

R1

R4

join
R2

R3

join
R5

join
R6

all data multicast

from rendezvous
point

R7
rendezvous
point

31

Router Architecture Overview

Two key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP)

forwarding datagrams from incoming to outgoing link

Network Layer 4-63

Input Port Functions

Physical layer:
bit-level reception
Data link layer:
e.g., Ethernet
see chapter 5

Decentralized switching:

given datagram dest., lookup output port

using forwarding table in input port

memory
goal: complete input port processing at
line speed
queuing: if datagrams arrive faster than
forwarding rate into switch fabric

Network Layer 4-64

32

Three types of switching fabrics

Network Layer 4-65

Switching Via Memory

First generation routers:
traditional computers with switching under direct
control of CPU
packet copied to systems memory
speed limited by memory bandwidth (2 bus
crossings per datagram)
Input
Port

Memory

Output
Port

System Bus

Network Layer 4-66

33

Switching Via a Bus

datagram from input port memory

to output port memory via a shared

bus
bus contention: switching speed
limited by bus bandwidth
32 Gbps bus, Cisco 5600: sufficient
speed for access and enterprise
routers
Network Layer 4-67

Switching Via An Interconnection

Network
overcome bus bandwidth limitations
Banyan networks, other interconnection nets

initially developed to connect processors in

multiprocessor
advanced design: fragmenting datagram into fixed
length cells, switch cells through the fabric.
Cisco 12000: switches 60 Gbps through the
interconnection network

Network Layer 4-68

34

Output Ports

Buffering required when datagrams arrive from

fabric faster than the transmission rate

Scheduling discipline chooses among queued
datagrams for transmission

Network Layer 4-69

Output port queueing

buffering when arrival rate via switch exceeds

output line speed

queueing (delay) and loss due to output port

buffer overflow!
Network Layer 4-70

35

How much buffering?

RFC 3439 rule of thumb: average buffering

equal to typical RTT (say 250 msec) times

link capacity C

e.g., C = 10 Gps link: 2.5 Gbit buffer

Recent recommendation: with

buffering equal to RTT. C

N flows,

Network Layer 4-71

Input Port Queuing

Fabric slower than input ports combined -> queueing

may occur at input queues

Head-of-the-Line (HOL) blocking: queued datagram
at front of queue prevents others in queue from
moving forward

queueing delay and loss due to input buffer overflow!

Network Layer 4-72

36