Lesson 4 © 2005 Cisco Systems, Inc. All rights reserved. SNPA v4.0—4-1 Understanding Translations...

Lesson 4

© 2005 Cisco Systems, Inc. All rights reserved. SNPA v4.0—4-1

Understanding Translations and Connections


Transport Protocols


Sessions in an IP World

In an IP world, a network session is a transaction between two end systems. It is carried out primarily over two transport layer protocols:• TCP • UDP


TCP

• TCP is a connection-oriented, reliable-delivery, robust, and high-performance transport layer protocol.

• TCP features:– Sequencing and acknowledgment of data– A defined state machine (open connection,

data flow, retransmit, close connection)– Congestion detection and avoidance mechanisms


TCP Initialization: Inside to Outside

Security Appliance

TCP HeaderIP Header

The security appliance checks for a translation slot. If one is not found, it creates one after verifying NAT, global, access control, and authentication or authorization, if any. If OK, a connection is created.

10.0.0.11

The security appliance utilizes the stateful packet inspection algorithm:• Source IP, source port, destination IP, destination port check• Sequence number check • Translation check

# 1 172.30.0.50# 2

# 3# 4

Start the embryonicconnection counter.No Data

Private Network

Source Port

Destination AddressSource Address

Initial Sequence #

Destination Port

Flag

ACK

172.30.0.50

10.0.0.11

1026

23

49091

Syn

10.0.0.11

172.30.0.50

23

1026

92513

Syn-Ack

49092

Public Network

172.30.0.50

192.168.0.20

49769

Syn

192.168.0.20

172.30.0.50

23

1026

92513

Syn-Ack

49770

1026

23


TCP Initialization: Inside to Outside (Cont.)

Private Network Public Network

Security Appliance

The security appliance Reset the embryonic counter for this client.. It then increases the connection counter for this host.

# 5 # 6The security appliance Strictly enforces the stateful packet inspection algorithm.

Data Flows

172.30.0.50

192.168.0.20

1026

23

49770

Ack

92514

Source Port


Initial Sequence #

Destination Port

Flag

ACK

172.30.0.50

10.0.0.11

1026

23

49092

Ack

92514

TCP HeaderIP Header

10.0.0.11 172.30.0.50


UDP

• Connectionless protocol• Efficient protocol for some services• Resourceful, but difficult to secure


UDP (Cont.)

Security Appliance

UDP HeaderIP Header

The security appliance checks for a translation slot. If one is not found, it creates one after verifying NAT, global, access control, and authentication or authorization, if any. If OK, a connection is created.

The security appliance follows the stateful packet inspection algorithm:• Source IP, source port, destination IP, destination Port check• Translation check

Private Network

Source Port


Destination Port

172.30.0.50

10.0.0.11

1028

45000

10.0.0.11

172.30.0.50

45000

1028

Public Network

172.30.0.50

192.168.0.20

192.168.0.20

172.30.0.50

45000

1028

1028

45000

All UDP responses arrive from outside and within UDP user-configurable timeout (default is 2 minutes).

10.0.0.11 # 1 172.30.0.50# 2

# 3# 4


Network Address Translation


Addressing Scenarios

• NAT was created to overcome several addressing problems that occurred with the expansion of the Internet:– To mitigate global address depletion– To use RFC 1918 addresses internally– To conserve internal address plan

• NAT also increases security by hiding the internal topology.

10.0.0.11

10.0.0.4

10.0.0.11192.168.6.9Internet

NAT


Access Through the Security Appliance

e0 outsideSecurity Level 0

e1 insideSecurity Level 100

Internet

More SecureLess Secure

e4 IntranetSecurity Level 70

e3 PartnernetSecurity Level 50

e2 DMZSecurity Level 30

Allowed unless explicitly denied

(NAT and global)

More SecureLess Secure

Deniedunless explicitly allowed(static and access list)


Inside Address Translation

10.0.0.4

10.0.0.11

10.0.0.4192.168.6.20

NAT

Outside GlobalIP Address

192.168.6.10

InsideIP Address10.0.0.11

Static Translation

Dynamic Translation 10.0.0.4

Outside globalIP address pool192.168.6.20-254

• Inside NAT translates addresses of hosts on higher security level to a less secure interface: – Dynamic translation– Static translation

Internet

WebServer


Dynamic Inside NAT

• Dynamic translationsfw1(config)# nat (inside) 1 10.0.0.0 255.255.255.0fw1(config)# global (outside) 1 192.168.0.20-192.168.0.254 netmask 255.255.255.0

10.0.0.11

10.0.0.4

10.0.0.11192.168.0.20

NAT

Internet


Two Interfaces with NAT

fw1(config)# nat (inside) 1 10.0.0.0 255.255.255.0fw1(config)# nat (inside) 2 10.2.0.0 255.255.255.0fw1(config)# global (outside) 1 192.168.0.3-192.168.0.16 netmask 255.255.255.0fw1(config)# global (outside) 2 192.168.0.17-192.168.0.32 netmask 255.255.255.0

• All hosts on the inside networks can start outbound connections.

• A separate global pool is used for each internal network.

10.2.0.0 /24

192.168.0.0

10.0.0.0/24

Internet

Global Pool192.168.0.17-32

Global Pool192.168.0.3-16


Three Interfaces with NAT

192.168.0.0 10.0.0.0

Global Pool172.16.0.20-254

fw1(config)# nat (inside) 1 10.0.0.0 255.255.255.0fw1(config)# nat (dmz) 1 172.16.0.0 255.255.255.0fw1(config)# global (outside) 1 192.168.0.20-192.168.0.254 netmask 255.255.255.0fw1(config)# global (dmz) 1 172.16.0.20-172.16.0.254 netmask 255.255.255.0

• Inside users can start outbound connections to both the DMZ and the Internet.

• The nat (dmz) command enables DMZ services to access the Internet.

• The global (dmz) command enables inside users to access the DMZ web server.

Internet

DMZ

InsideGlobal Pool192.168.0.20-254

Outside


Port Address Translation


Port Address Translation

10.0.0.11

10.0.0.4

10.0.0.11192.168.0.20

Port 1024

PAT

10.0.0.4192.168.0.20

Port 1025

• PAT is a combination of an IP address and a source port number.

• Many different sessions can be multiplexed over a single global IP address.

• Sessions are kept distinct by the use of different port numbers.

Internet


PAT Example

• Outside IP addresses are typically registered with InterNIC.

• Source addresses of hosts in network 10.0.0.0 are translated to 192.168.0.3 for outgoing access.

• A single IP address (192.168.0.3) is assigned to the global pool.

• The source port is dynamically changed to a unique number that is greater than 1023.

fw1(config)# route (outside) 0.0.0.0 0.0.0.0 192.168.0.1fw1(config)# nat (inside) 1 10.0.0.0 255.255.0.0fw1(config)# global (outside) 1 192.168.0.3 netmask

255.255.255.255

SalesEngineering

10.0.1.0 10.0.2.0

192.168.0.0

10.0.0.0

Global Address192.168.0.3

.2

.1

.1


PAT Using Outside Interface Address

• The outside interface is configured as a DHCP client.

• The interface option of the global command enables use of a DHCP address as the PAT address.

• The source addresses of hosts in network 10.0.0.0 are translated into a DHCP address for outgoing access, in this case, 192.168.0.2.

• The source port is changed to a unique number greater than 1023.

fw1(configs)# interface ethernet0 fw1(configs-if)# ip address inside 10.0.0.1 255.255.255.0fw1(configs-if)# ip address outside dhcpfw1(configs)# nat (inside) 1 10.0.0.0 255.255.0.0fw1(config)# global (outside) 1 interface

SalesEngineering

10.0.1.0 10.0.2.0

192.168.0.0

10.0.0.0

Global DHCP Address

(192.168.0.2) .2

.1

.1


Mapping Subnets to PAT Addresses

• Each internal subnet is mapped to a different PAT address.



• The source port is changed to a unique number greater than 1023.

fw1(config)# nat (inside) 1 10.0.1.0 255.255.255.0fw1(config)# nat (inside) 2 10.0.2.0 255.255.255.0fw1(config)# global (outside) 1 192.168.0.8 netmask

255.255.255.255fw1(config)# global (outside) 2 192.168.0.9 netmask

255.255.255.255

SalesEngineering

10.0.1.0 10.0.2.0

192.168.0.0

10.0.0.0

192 .168.0.8

.2

.1

.1

192 .168.0.9


Backing Up PAT Addresses by Using Multiple PATs


• Address 192.168.0.9 will be used only when the port pool from 192.168.0.8 is at maximum capacity.

fw1(config)# nat (inside) 1 10.0.0.0 255.255.252.0fw1(config)# global (outside) 1 192.168.0.8 netmask


255.255.255.255

SalesEngineering

10.0.1.0 10.0.2.0

192.168.0.0

10.0.0.0

192 .168.0.8

.2

.1

.1

192 .168.0.9


fw1(config)# nat (inside) 1 10.0.0.0 255.255.0.0fw1(config)# global (outside) 1 192.168.0.20-192.168.0.253 netmask


255.255.255.255

Augmenting a Global Pool with PAT

• When hosts on the 10.0.0.0 network access the outside network through the security appliance, they are assigned public addresses from the 192.168.0.20–192.168.0.253 range.

• When the addresses from the global pool are exhausted, PAT begins with the next available IP address, in this case, 192.168.0.254.

SalesEngineering

10.0.1.0 10.0.2.0

192.168.0.0

10.0.0.0

PAT192 .168.0.254

.2

.1

.1

NAT192 .168.0.20


Identity NAT

With NAT control enabled:• All packets traversing a security appliance require a translation rule.• Identity NAT is used to create a transparent mapping.• IP addresses on the high security interface translate to themselves

on all lower security interfaces.

Internet

InsideOutside

10.0.0.15

DMZ InternetServer

192.168.0.9192.168.0.9


Identity NAT: nat 0 Command

• NAT 0 ensures that the Internet server is translated to its own address on the outside.

• Security levels remain in effect with NAT 0.

fw1(config)# nat (dmz) 0 192.168.0.9 255.255.255.255

Internet

Inside

Outside

DMZInternetServer

192.168.0.9

192.168.0.9


Static Command


Global NAT and Static NAT

Global NAT• For dynamic NAT and PAT address assignments• Inside end user receives an address from a pool of available addresses • Used mostly for outbound end-user connections

Internet

Inside

Outside

Bob Smith10.0.0.11

Static• For NAT “permanent” address assignments• Used mostly for server connections

InternetInsideOutside

Sam Jones10.0.0.12

FTP Server 172.16.1.10

Web Server 172.16.1.9

GlobalPool

FixedFixed

Bob Smith10.0.0.11

Sam Jones10.0.0.12


static Command: Parameters

Interfaces• Real interface – DMZ• Mapped interface – OutsideIP Addresses• Real IP address – 172.16.1.9• Mapped IP address – 192.168.1.3




192.168.1.3192.168.1.4

Bob Smith10.0.0.11

Sam Jones10.0.0.12

DMZ


static Command: Web Server

pixfirewall(config)#

static (real_interface,mapped_interface) {mapped_address | interface} real_address [netmask mask]

fw1(config)# static (dmz,outside) 192.168.1.3 172.16.1.9 netmask 255.255.255.255

• Packets sent to 192.168.1.3 on the outside are translated to 172.16.1.9 on the DMZ.

• Permanently maps the Web server IP address.




192.168.1.3192.168.1.4

Bob Smith10.0.0.11

Sam Jones10.0.0.12

DMZ


static Command: FTP Server

• Packets sent to 192.168.1.4 on the outside are translated to 172.16.1.10 on the DMZ.

• Permanently maps the FTP server IP address.







192.168.1.3192.168.1.4

Bob Smith10.0.0.11

Sam Jones10.0.0.12

DMZ


Net Static



• Recommended when you want to translate multiple addresses with a single command

• Host address on 172.16.1.0 subnet is translated to host address on 192.168.10.0 subnet





192.168.10.9192.168.10.10

Bob Smith10.0.0.11

Sam Jones10.0.0.12

DMZ


Static PAT: Port Redirection

• Used to create a permanent translation between a mapped IP address and port number to a specific real IP address and port number– 192.168.0.9/www redirected to 172.16.1.9/www– 192.168.0.9/ftp redirected to 172.16.1.10/ftp

192.168.0.9/www




DMZ

192.168.0.9/ftp

ftp 192.168.0.9


static [(real_interface, mapped_interface)] {tcp | udp} {mapped_ip | interface} mapped_port {real_ip real_port [netmask mask]


static pat Command

fw1(config)# static (dmz,outside) tcp 192.168.0.9 ftp 172.16.1.9 ftp netmask 255.255.255.255

fw1(config)# static (dmz,outside) tcp 192.168.0.9 2121 172.16.1.10 ftp netmask 255.255.255.255

• Packet sent to 192.168.0.9/FTP translated by security appliance to 172.16.1.9 (first FTP server)

• Packet sent to 192.168.0.9/2121 translated by security appliance to 172.16.1.10 (second FTP server)

192.168.0.9/FTP


FTP2 Server 172.16.1.10

FTP1 Server 172.16.1.9

DMZ

192.168.0.9/2121

ftp 192.168.0.9


TCP Intercept and Connection Limits


Connection Limits

Administrator can set connection limits:• Emb_lin – Maximum number of embryonic connections per

host. An embryonic connection is a connection request that has not completed a TCP three-way handshake between the source and the destination.

• TCP_max_conns – Maximum number of simultaneous TCP connections that each real IP host is allowed to use. Idle connections are closed after the time specified by the timeout conn command.

• udp_max_conns – Maximum number of simultaneous UDP connections that each real IP host is allowed to use.


TCP Three-Way Handshake

172.26.26.45

Target10.0.0.2

Spoofed Host172.16.16.20

172.26.26.46

SYN, SRC: 172.26.26.45, DST: 10.0.0.2SYN-ACK

ACK

SYN, SRC: 172.16.16.20, DST: 10.0.0.3Target

10.0.0.3

DoSAttack

SYN, SRC: 172.16.16.20, DST: 10.0.0.3SYN, SRC: 172.16.16.20, DST: 10.0.0.3

Normal

EmbryonicConnection

?

Internet

SYN-ACK

SYN-ACK

SYN-ACK

??


TCP Intercept

Internet

EmbryonicConnection Count = 3

SYNSYN-ACK

ACK

SYNDoS

AttackSYNSYN

Normal

TCPIntercept

SYNSYN-ACK

ACK


SYN Cookies

Internet

SYNSYN-ACK (Cookie)

ACK (Cookie)

Normal

TCPIntercept

SYNSYN-ACK

ACK

The security appliance responds to the SYN itself, which includes a cookie in the TCP header of the SYN-ACK. The security appliance keeps no state information.

• The cookie is a hash of parts of the TCP header and a secret key.• A legitimate client completes the handshake by sending the ACK

back with the cookie.• If the cookie is authentic, the security appliance proxies the TCP

session.


Embryonic Connection Limit

• Setting the embryonic connections (emb_lim) enables TCP proxying using either TCP Intercept or SYN cookies.– A value of 0 disables protection (default). – When the embryonic connection limit is exceeded, all

connections are proxied.

fw1(config)# nat (inside) 1 0 0 0 25fw1(config)# static (inside,outside) 192.168.0.11

172.16.0.2 0 25

static (real_ifc,mapped_ifc) {mapped_ip | interface} {real_ip [netmask mask] | access-list access_list_name} [dns] [norandomseq] [[tcp] [max_conns [emb_lim]] [udp udp_max_conns]

firewall (config)#

nat (local_interface) nat_id local_ip [mask] [dns] [outside] [[tcp] tcp_max_conns [emb_limit] [norandomseq]]] [udp udp_max_conns]

firewall (config)#


UDP Maximum Connection Limit

• Maximum number of simultaneous UDP connections that the local IP hosts are allowed.– A value of 0 disables protection (default). – Idle connections are closed after the time specified in the udp

timeout command.

fw1(config)# nat (inside) 1 0.0.0.0 0.0.0.0 200 25fw1(config)# static (inside,outside) 192.168.0.11

172.16.0.2 0 0 udp 100

firewall (config)#

firewall (config)#

static (real_ifc,mapped_ifc) {mapped_ip | interface} {real_ip [netmask mask]} | {access-list access_list_name} [dns] [[tcp] [max_conns [emb_lim]] [norandomseq] ]] [udp udp_max_conns]

nat {local_interface} nat_id local_ip [mask [dns] [outside] [[tcp] tcp_max_conns [emb_limit] [norandomseq]]] [udp udp_max_conns]


Connections and Translations


Connections Versus Translations

• Translations: NAT – Mapped address to real address PAT – Mapped address and port to real address and port

• Connections: Host address and port to host address and port

Inside Local

Outside Mapped Pool

10.0.0.11192.168.0.20

10.0.0.11

10.0.0.4Translation

10.0.0.11192.168.0.20

192.168.10.5

Translation

Connections

Connection 192.168.10.11:23 10.0.0.11:1026

Connection 192.168.10.11:80 10.0.0.11:1027

192.168.10.11

Internet

Telnet

HTTP


show conn Command

show conn

fw1#show conn2 in use, 2 most usedfw1# show conn2 in use, 9 most usedTCP out 192.168.10.11:80 in 10.0.0.11:2824 idle 0:00:03 bytes 2320 flags UIOTCP out 192.168.10.11:80 in 10.0.0.11:2823 idle 0:00:03 bytes 3236 flags UIO

pixfirewall#

10.0.0.11

10.0.0.4

192.168.10.11

Connection

Internet

• Enables you to view all active connections


show conn detail Command

fw1# show conn2 in use, 9 most usedTCP out 192.168.10.11:80 in 10.0.0.11:2824 idle 0:00:03 bytes 2320 flags UIOTCP out 192.168.10.11:80 in 10.0.0.11:2823 idle 0:00:03 bytes 3236 flags UIOfw1# show conn detail2 in use, 9 most usedFlags: A - awaiting inside ACK to SYN, a - awaiting outside ACK to SYN, B - initial SYN from outside, C - CTIQBE media, D - DNS, d - dump, E - outside back connection, F - outside FIN, f - inside FIN, G - group, g - MGCP, H - H.323, h - H.225.0, I - inbound data, i - incomplete, J - GTP, j - GTP data, k - Skinny media, M - SMTP data, m - SIP media, O - outbound data, P - inside back conn, q - SQL*Net data, R - outside acknowledged FIN, R - UDP RPC, r - inside acknowledged FIN, S - awaiting inside SYN, s - awaiting outside SYN, T - SIP, t - SIP transient, U - upTCP outside:192.168.10.11/80 inside:10.0.0.11/2824 flags UIOTCP outside:192.168.10.11/80 inside:10.0.0.11/2823 flags UIO

10.0.0.11192.168.10.11

Connection

Internet


show local-host Command

fw1# show local-hostInterface dmz: 0 active, 0 maximum active, 0 deniedInterface inside: 1 active, 5 maximum active, 0 deniedlocal host: < 10.0.0.11 >, TCP flow count/limit = 2/300 TCP embryonic count to host = 0 TCP intercept watermark = 25 UDP flow count/limit = 0/unlimited

Conn: TCP out 192.168.10.11 :80 in 10.0.0.11 :2824 idle 0:00:05 bytes 466 flags UIO TCP out 192.168.10.11 :80 in 10.0.0.11 :2823 idle 0:00:05 bytes 1402 flags UIO

Interface outside: 1 active, 1 maximum active, 0 deniedlocal host: < 192.168.10.11 >, TCP flow count/limit = 2/unlimited TCP embryonic count to host = 0 TCP intercept watermark = unlimited UDP flow count/limit = 0/unlimited

Conn: TCP out 192.168.10.11 :80 in insidehost:2824 idle 0:00:05 bytes 466 flags UIO TCP out 192.168.10.11 :80 in insidehost:2823 idle 0:00:05 bytes 1402 flags UIO

10.0.0.11192.168.10.11

Connection

Internet


show xlate Command

show xlate

• Enables you to view translation slot information

fw1#show xlate1 in use, 2 most usedGlobal 192.168.0.20 Local 10.0.0.11

pixfirewall#

10.0.0.11

10.0.0.4

10.0.0.11192.168.0.20

192.168.10.11

Translation

Internet


show xlate detail Command

fw1#show xlate1 in use, 3 most usedGlobal 192.168.0.20 Local 10.0.0.11fw1# show xlate detail1 in use, 3 most usedFlags: D - DNS, d - dump, I - identity, i - dynamic, n - no random, r - portmap, s

- staticNAT from inside:10.0.0.11 to outside:192.168.0.20 flags i

10.0.0.11

10.0.0.11192.168.0.20

192.168.10.11

Translation

Internet


Security Appliance NAT Philosophy

• Security appliance translation rules are configured between pairs of interfaces.

• With NAT control enabled, a packet cannot be switched across the security appliance if it does not match a translation slot in the translation table except for NAT 0, which doesn’t create a translation entry.

• If there is no translation slot, the security appliance will try to create a translation slot from its translation rules.

• If no translation slot match is found, the packet is dropped.

10.0.0.11

10.0.0.4

10.0.0.11192.168.0.20192.168.10.11

Outside Inside

NAT

Internet


Matching Outbound Packet Addresses

• A packet arrives at an inside interface:- The security appliance consults the access rules first.- The security appliance makes a routing decision to determine

the outbound interface.• The source address is checked against the local addresses in the

translation table:- If found, the source address is translated according to the

translation slot.• Otherwise the security appliance looks for a match to the local

address in the following order:- nat0 access-list (NAT exemption) – In order, until first match- static (static NAT) – In order, until first match- static {tcp | udp} (static PAT) – In order, until first match- nat nat_id access-list (policy NAT) – In order, until first match- nat (regular NAT): Best match

• If no match is found, the packet is dropped.


Configuring Multiple Interfaces


Additional Interface Support

• Supports additional interfaces• Increases the security of publicly available services• Easily interconnects multiple extranets and partner

networks• Easily configured with standard security appliance

commands

e0 OutsideSecurity Level 0

e1 InsideSecurity Level 100

Internet

e4 IntranetSecurity Level 70

e3 Partner NetworkSecurity Level 50

e2 DMZSecurity Level 30


Configuring Three Interfaces

fw1(config)# interface ethernet0fw1(config-if)# nameif outsidefw1(config-if)# ip address 192.168.0.2

255.255.255.0fw1(config)# interface ethernet1fw1(config-if)# nameif insidefw1(config-if)# ip address 10.0.0.1 255.255.255.0

fw1(config)# interface ethernet2fw1(config-if)# nameif dmzfw1(config-if)# sec 50fw1(config-if)# ip address 172.16.0.1 255.255.255.0

fw1(config)# nat (inside) 1 10.0.0.0 255.255.255.0fw1(config)# global (outside) 1 192.168.0.20-

192.168.0.254 netmask 255.255.255.0


fw1(config)# static (inside,dmz) 10.0.0.0 10.0.0.0 netmask 255.255.255.0

.2

.1.1

10.0.0.0/24

Internet

172.16.0.2

192.168.0.11

Net Static

192.168.0.20

DMZ

Inside


Configuring Four Interfaces

fw1(config)# interface ethernet0fw1(config-if)# nameif outsidefw1(config-if)# ip address 192.168.0.2 255.255.255.0fw1(config)# interface ethernet1fw1(config-if)# nameif insidefw1(config-if)# ip address 10.0.0.1 255.255.255.0 fw1(config)# interface ethernet2fw1(config-if)# nameif dmzfw1(config-if)# sec 50fw1(config-if)# ip address 172.16.0.1 255.255.255.0fw1(config)# interface ethernet3fw1(config-if)# nameif partnernetfw1(config-if)# sec 40fw1(config-if)# ip address 172.18.0.1 255.255.255.0

fw1(config)# nat (inside) 1 10.0.0.0 255.255.255.0fw1(config)# global (outside) 1 192.168.0.20-

192.168.0.254 netmask 255.255.255.0

fw1(config)# static (inside,dmz) 10.0.0.0 10.0.0.0 netmask 255.255.255.0

fw1(config)# static (dmz,outside) 192.168.0.11 172.16.0.2

fw1(config)# static (dmz,partnernet) 172.18.0.11 172.16.0.2

Partner Network

172.16.0.2

DMZ

.1.1 Net Static

10.0.0.0/24

172.18.0.0/24

.1

InternetInternet

Inside

192.168.0.11

192.168.0.20

172.18.0.11


Summary

• The security appliance manages the TCP and UDP protocols through the use of a translation table (for NAT sessions) and a connection table (for TCP and UDP sessions).

• The static command creates a permanent translation.• Mapping between local and global address pools is

done dynamically with the nat command.• The nat and global commands work together to hide

internal IP addresses.• The security appliance supports PAT. • Configuring multiple interfaces requires a greater

attention to detail, but can be done with standard security appliance commands.

Lesson 4 © 2005 Cisco Systems, Inc. All rights reserved. SNPA v4.0—4-1 Understanding Translations...

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Transcript of Lesson 4 © 2005 Cisco Systems, Inc. All rights reserved. SNPA v4.0—4-1 Understanding Translations...