Ipsec rbe guide

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IPSec Router Based Encryption
Engineering Guide
Version 2.4
Document Revision and Review History
Version Date Compiled by Description
1.0 01/20/2003 George Lai First Draft
2.0 04/26/2004 Richard Hou Second Draft
2.1 03/10/2005 Corinne Narassiguin Yearly review updates
2.2 10/25/2005 Ali iloglu
RSA-sig with generated keys is added. GRE
tunneling section is updated to highlight the
importance.
2.3 3/8/2006 Ali iloglu
ISR throughputs and limitations added. Minor
changes made to make it consistent with IPSec
Standard.
2.4 5/10/2006 Jeevan Balasubramanian
ISR Throughput Numbers Updated, VAM2+
Throughput Numbers Included, Cheat Sheet for
ISR routers and authentication methods included
in the Appendix
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Table of Content
1 Introduction....................................................................................................................................................3
2 IPSec Overview.............................................................................................................................................4
3 Network Design Considerations....................................................................................................................7
4 Other Considerations...................................................................................................................................17
5 Configuration Tasks.....................................................................................................................................22
6 References...................................................................................................................................................37
7 Appendices..................................................................................................................................................38
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1 Introduction
Internet Protocol Security (IPSec) is a framework of open standard for transferring information
securely over IP networks. Based on IETF standards, IPSec provides a robust, flexible means to
ensure confidentiality, integrity and authenticity for data communication across public networks.
With the support of Internet Key Exchange protocol (IKE, formally known as ISAKMP/Oakley)
and the interoperability of Certificate Authority (CA), IPSec further becomes a highly scalable
solution for large-scale deployment.
This document is designed to provide design and engineering guidelines for deploying IPSec
RBE networks in Citigroup. The scope of the document is limited to Cisco routers only. IPSec
overview is given in Section 2 to familiarize the readers with the IPSec protocols and
terminology. Section 3 and 4 cover the design considerations and known issues at a higher level.
Configuration tasks are described in Section 5 to demonstrate how IPSec RBE is configured on
Cisco routers. New IPSec features will be included in the future revision of this document if they
prove to be useful.
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2 IPSec Overview
IPSec is an open standard defined by IETF for IP network layer security. It is a suite of protocols
that includes AH (Authentication Header), ESP (Encapsulating Security Payload) and IKE
(Internet Key Exchange) protocols. Collectively they provide the services for data integrity,
authentication, confidentiality and replay protection in secure communications across IP
networks. With the support of Internet Key Exchange protocol and the interoperability of
Certificate Authority (CA), IPSec further becomes a highly scalable solution for large-scale
deployment.
2.1 IKE (ISAKMP/Oakley) Protocol
Internet Key Exchange (IKE) protocol, also referred to as ISAKMP/Oakley, is a framework that
provides the functionality of automated negotiation of IPSec Security Associations (SAs) and the
management of cryptography key generation and refreshing. Protecting keying material is the
most crucial component in secure communications since no matter how strong the cryptography
algorithm is; there will be no security if the keying material is compromised.
IKE is designed to provide the protection against security exposures such as denial of services
and man in the middle attacks. Optionally, it provides the property of perfect forward secrecy
(PFS) so that compromise of past key material provides no useful clues for breaking current or
future keys.
IKE adopted a two-phase approach to ensure the robustness of the key exchange procedure. It
uses UDP port 500 for communication.
The term ISAKMP will be used interchangeably with IKE throughout this document.
2.1.1 Phase 1
Phase 1, also referred to as Oakley Main Mode (MM), is all about protecting the ISAKMP
messages themselves. During the negotiations, it uses public key cryptography to establish a
protecting suite known as ISAKMP Security Association (SA). Upon successful completion, the
peers will be mutually authenticated and a master secret is established. The following
authentication methods are defined for IKE:
• Pre-shared Key
• Digital Signature (Digital Certificate)
• Public Key Encryption (Encrypted Nonce)
The master secret will be used to further derive keys that protect the ISAKMP messages in
subsequent Phase 2 negotiations; during which the very master secret is used to derive
cryptography keys for protecting user’s data traffic. The cryptography operations performed in
Phase 1 negotiations are more processor intensive than those in Phase2 but only need to be
executed less frequently. Indeed, multiple Phase 2 exchanges can be executed during the lifetime
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of an ISAKMP SA established by a single Phase 1 exchange. In Phase 1, the peers agree on an
encryption algorithm, a hash algorithm and a Diffie-Hellman group identifier used for protecting
IKE messages. The peers also propose lifetime for ISAKMP SA during negotiations and agree on
the shorter one. Note that ISAKMP SA is bidirectional, a single SA protects the ISAKMP
messages flowing in both directions.
2.1.2 Phase 2
Phase 2, also referred to as Oakley Quick Mode (QM), is a set of negotiations to establish non-
ISAKMP Security Associations (SAs) and cryptography keys to protect user’s data traffic. Non-
ISAKMP SA will also be referred to as IPSec SA throughout this document. Phase 2 is all about
setting up the protecting suite for user’s data traffic. In Phase 2, the negotiating peers agree upon
the IPSec transforms they propose by selecting the first matched one, if multiple transform sets
are offered. An IPSec transform is the specific implementation of an algorithm for use by an
IPSec protocol. For example, the triple DES algorithm used in ESP is called ESP-3DES
transform and the HMAC variant of SHA hash algorithm used in AH is called AH-SHA-HMAC.
Note that IPSec SA is a unidirectional logical connection uniquely identified by:
<Security Parameter Index, IP Destination Address, Security Protocol>
Security Parameter Index (SPI) is used to differentiate multiple SAs with the same IP destinations
and security protocols. An IPSec SA is required for each traffic direction and for each security
protocol (ESP or AH). For example, if both AH and ESP are used to protect the data, a total of 4
IPSec SAs are required to establish a 2-way communication; 2 for AH, inbound and outbound
and the other 2 for ESP, inbound and outbound. It is logical to regard these 4 closely related
IPSec SAs, along with their protecting ISAKMP SA, as an “IPSec tunnel” which can be viewed
as an bidirectional, secure data flow.
It is important that the readers draw distinctions between the IPSec SA discussed in this section
and the ISAKMP SA discussed in the previous section.
2.2 Authentication Header (AH) Protocol
Authentication Header (AH) protocol is designed to provide integrity check and authentication to
IP packets. These services are combined and carried out via the authentication data (hash value)
in AH header. A monotonically increasing sequence number is used to provide anti-replay
protection. As a result, out of order AH packets will be rejected outside a sliding receive window.
AH processing is performed on a per-packet basis and it authenticates the whole IP packet except
the mutable fields in the outermost IP headers. Mutable fields in IPv4 header includes
• Type of Service (TOS)
• Flags
• Time to Live (TTL)
• Header Checksum
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AH can be operated in either transport mode or tunnel mode.
2.2.1 AH in Transport Mode
As shown in Figure 1, in transport mode AH header is inserted into the original packet, right after
the IP header. The mutable fields in the original IP header are not authenticated. Transport mode
is typically used by host to host IPSec tunnels.
2.2.2 AH in Tunnel Mode
In tunnel mode, the original IP packet is encapsulated as the payload of a new IP packet. AH
transport mode is then applied to the new packet. The mutable fields in the new IP header are not
authenticated but the original IP packet, including its mutable fields, are authenticated. This is
considered an advantage since it provides total protection to the original packet and it creates the
possibility of using private addresses. The disadvantage of using tunnel mode is the additional 20-
byte overhead introduced by the new IP header.
Figure 1 - Authentication Header (AH) Protocol
AH is identified by IP protocol number 51, assigned by IANA.
2.3 Encapsulating Security Payload (ESP) Protocol
Encapsulating Security Payload (ESP) protocol is designed to provide integrity check,
authentication and encryption to IP packets. Integrity check and authentication services are
combined and carried out via the authentication data in ESP Authentication Data field. A
monotonically increasing sequence number is used to provide anti-replay protection. As a result,
out of order ESP packets will be rejected outside a sliding receive window. ESP processing is
performed on per-packet basis.
Although both authentication (with integrity check) and encryption are optional, at least one of
them has to be selected. ESP processing includes the addition of an ESP header, an ESP trailer
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IP HDR IP Payload
Transport Mode
IP HDR AH HDR IP Payload
Authenticated except mutable fields
New IP HDR AH HDR IP HDR IP Payload
Tunnel Mode
Authenticated except mutable fields in new IP header

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and ESP authentication data to the original IP packet. ESP trailer contains padding since most
encryption algorithms require that input data be an integer number of blocks. DES, and thus triple
DES, cipher uses 64-bit blocks, for instance.
ESP can be operated in either transport mode or tunnel mode.
2.3.1 ESP in Transport Mode
As shown in Figure 2, in transport mode an ESP header is inserted into the original packet, right
after the IP header. The ESP trailer and ESP authentication data are then appended to the payload.
ESP encrypts the IP payload and ESP trailer. If authentication service is selected, ESP
authenticates ESP header, IP payload and ESP trailer. Note that the IP header is not protected by
authentication as in AH protocol.
2.3.2 ESP in Tunnel Mode
In tunnel mode, the original IP packet is entirely encapsulated in a new IP packet. ESP transport
mode is then applied to the new packet as illustrated in Figure 2. Note that the original IP packet
becomes the payload of the new packet and can be fully protected by encryption and
authentication. The disadvantage of using tunnel mode is the additional 20-byte or so overhead
introduced by new IP header and necessary padding.
Figure 2 - Encapsulating Security Payload (ESP) Protocol
ESP is identified by IP protocol number 50, assigned by IANA.
3 Network Design Considerations
3.1 Applications
The potential areas of deployment for IPSec RBE in Citigroup are:
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IP HDR IP Payload
Transport Mode
IP HDR ESP HDR IP Payload
ESP
Trailer
ESP
Auth
Encrypted
Authenticated
New IP HDR ESP HDR IP HDR IP Payload
ESP
Trailer
ESP
Auth
Tunnel Mode
Encrypted
Authenticated

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• Hub and Spoke Branch Networks
• Replacement for Layer-2 Link Encryptors
• Site to Site VPNs (B2B)
3.2 Platforms
IPSec processing is supported in Cisco IOS software. However, IPSec processing is highly CPU
intensive and therefore it is inefficient and impractical to perform software-only IPSec
processing. Cisco offers hardware IPSec accelerators, which offload the IPSec processing from
the router processors, on various platforms. Deployment of software-only IPSec RBE networks
should be avoided.
Bandwidth throughput and the maximum number of IPSec tunnels supported on Cisco platforms
are the typical measurements in evaluating IPSec performance. In general, the maximum number
of IPSec tunnels supported by the platforms is adequately sufficient in IPSec RBE scenarios. The
bandwidth throughput, on the other hand, is the key consideration for platform selection. The
following table contains a list of the Cisco platforms and the IPSec hardware accelerators (VPN
modules) they support. Throughput numbers, which are measured at approximately 50% CPU
utilization for 7206VXR and at 60-65% for others, are reported by Cisco Enterprise Solutions
Engineering VPN Team.
Router HW VPN Module Throughput # of Tunnels Deployed As
6509 VPN SM 850 Mbps 8000 Hub router
7206VXR ISA-VPN 30.0 Mbps 5000 Hub router
7206VXR VAM 45.0 Mbps 5000 Hub router
7206VXR VAM2+ 60.0 Mbps Not Tested Hub Router
38451
Embedded VPN
Module
25.0 Mbps Not Tested Branch or Hub Router
3845 VPN/HPII-PLUS 30.0 Mbps Not Tested Branch or Hub Router
3745 AIM-VPN/HP 16.0 Mbps 1800 Hub or Branch router
3662 AIM-VPN/HP 16.0 Mbps 1800 Hub or Branch router
3640 NM-VPN/MP 3.5 Mbps 800 Branch router
3620 NM-VPN/MP 1.8 Mbps 800 Branch router
28112
Embeded VPN
Module
6.5Mbps Not Tested Branch router
2651XM AIM-VPN/EP > 2.8 Mbps 800 Branch router
2651 AIM-VPN/EP > 2.8 Mbps 800 Branch router
2621 AIM-VPN/BP 2.4 Mbps 300 Branch router
2611 AIM-VPN/BP 2.0 Mbps 300 Branch router
1841* Embedded VPN
Module
6.0 Mbps Not Tested Branch router, Kiosk
1760 AIM-VPN TBA 100 Branch router, Kiosk
1751 AIM-VPN 2.6 Mbps 100 Branch router, Kiosk
1
Embedded VPN module does not support AH+ESP transform set, so AIM HPII plus is recommended,
which does not support rsa-encr. Number of actual GRE tunnel is not tested but that is not scale limiting
factor in our applications.
2
Throughput is measured with 99% CPU on 2811 and 1841 by Cisco. Some room should be left in actual
deployment.
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Cisco IPSec RBE can be deployed on almost any physical and logical interfaces capable of
carrying IP traffic. High-speed WAN links will use VPN-SM and VPN-SPA ( next generation
encryption card under test replacing VPN-SM) in 6500. The following interfaces are either
already deployed in Citigroup network or known to work with IPSec RBE:
• T1/E1, T3/E3 interfaces and Inverse MUX of them
• Asynchronous serial interfaces
• HSSI interfaces
• V.35 interfaces
• ATM point-to-point sub-interfaces
• Frame Relay point-to-point sub-interfaces
• Ethernet and Fast Ethernet interfaces
• 802.1Q and ISL sub-interfaces
• ISDN interfaces
• GRE tunnel interfaces
IPSec RBE also works with DLSW+ traffic.
There are certain IPSEC cards, which does not support rsa-encr.
• AIM-BP2
• AIM-EP2
• AIM-HP2
Also, currently there is bug (CSCsc65207) on ISR release, which limits the use of 1024 size key
length, which will be fixed 12.4(3d) and 12.3.(11)T10 codes for ISR.
3.3 High Availability and Resiliency
High availability and resiliency after fail-over is critically important for Citigroup network
design. IPSec RBE network should be designed to work in sync with the resilient design of the
underlying network. It should not introduce new limiting factors that would degrade the inherited
resiliency. The major concern on potential resiliency degradation regards the persistency of SAs
which may prevent the underlying fail-over mechanism from functioning. This sticky property
results from the fact that IPSec protocol provides no mechanism to detect the lost of connectivity.
SAs are stateful relationships between the two peers. The state information includes, but is not
limited to, secret keys, security parameters, and peer identity. This state information is established
during the MM negotiation for ISAKMP SAs and during QM negotiation for IPSec SAs.
ISAKMP SAs, once established, will live through its lifespan as specified in ISAKMP
configuration. There are only two conditions under which an ISAKMP SA be removed
automatically. The first condition is when the lifetime associated with the SA is reached. This is
considered the natural aging of the SA. If there is interesting traffic on the wire, within a small
grace period of time before the SA expires, a new SA will be negotiated so that the data traffic
will not be interrupted when the old SA expires. The second condition is when the local peer
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eventually realizes the lost of connectivity in failing to renew IPSec SAs in QM negotiation at the
expiration of the old IPSec SAs. This implies that a black hole situation may exist for as long as 1
hour, the IPSec SA lifetime mandated by GNAST IPSec RBE standard [1].
3.3.1 IKE Keepalive
IKE keepalive is a Cisco proprietary means against black hole situation described above. If
enabled, if the local peer misses 3 keepalive messages, it concludes that the connectivity is lost
and it proceeds to purge the IKE SA and the IPSec SAs associated with it. However, IKE
keepalive alone cannot satisfy all the requirements in a resilient network design. Consider the
following scenario that router X establishes dual peering relationships with router A (primary)
and router B (backup) by specifying both peers in a single crypto map.
A subtle point is that at any given time, there exists only one ISAKMP SA in this scenario. Under
normal circumstances, the primary tunnel (indicated by solid line) is active. The backup tunnel,
(indicated by dashed line) does not even exist, i.e., none of the SAs associated with the backup
tunnel exist, until the SAs associated with the primary tunnel are purged. When the fail-over takes
place, it requires fresh MM and QM negotiations to establish the backup tunnel. Then the roles of
the primary and backup tunnels simply reverse. IKE keepalive can contribute to better fail-over
behavior if the keepalive timers are carefully calibrated to account for the convergence time of
the underlying routing protocol plus the time for tunnel establishment.
However, as reported by Cisco [2], there are still open issues with IKE keepalive mechanism:
• IKE keepalive is not always available
• IPSec performance degradation has been witnessed with IKE keepalive
• The remote site does not switch back to its original peer once the peer is restored
So, the recommendation is to tie routing with IPSec by using GRE tunneling as discussed in
detail at RBE standard.
3.3.2 Independent IPSec Tunnels
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X
A
B
Primary IPSec tunnel
Backup IPSec tunnel

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It is evident at this point that more robust resiliency can be achieved by having two independent
IPSec tunnels associated with the primary and backup sites, instead of only one tunnel switching
back and forth. This involves creating multiple logical interfaces on router X so that each logical
interface can be applied a separate crypto map which results in independent IPSec tunnels as
illustrated by the diagram below.
GRE tunneling is the most common approach for this purpose. However, GRE tunnel introduces
15-20% performance degradation in addition to IPSec processing overhead to the network. Other
GRE issues will be discussed in later section. However, GRE tunneling allows IPSec to tie with
routing protocols like OSPF. So, when IPSec fails, routing protocol fails. So, routing protocol
keep lives detect IPSec level failures to move traffic to backup tunnel.
3.3.3 Loopback Address Considerations
IPSec RBE is essentially a point-to-point operation. Inherited from the fact that current IPSec
standard does not support multicast or broadcast traffic, the point-to-point relationship is built
upon the peers’ unicast IP addresses. The binding of the peers is further fortified by the need to
apply crypto maps, as indicated by the ellipses in the following diagram, on the interfaces that
IPSec tunnel traverses.
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X
A
B
Backup IPSec tunnel
XA
B Y
AlternativeIPSec tunnel
fa0/0 fa0/0
Backup IPSec tunnel

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The use of loopback as IPSec peer’s address indeed breaks the tight binding of the specific
interfaces to IPSec tunnel. In the example, if the serial interfaces are used as the peer’s addresses
between router A and X, an IPSec tunnel will be established as designed (gray line). Crypto maps
(gray ellipses) need to be applied on the serial interfaces on both router A and X. In this case, the
tunnel is in sync with the underlying serial link.
On the other hand, if loopback is used as the peer’s address, it creates an alternative path
(indicated by red line) for the IPSec tunnel, i.e., the path traversing A-B-Y-X. However, for that
alternative path to form a functional IPSec tunnel, crypto maps need to be applied on all possible
interfaces the alternative path traverses. In this case, additional crypto maps (red ellipses) are
required to be applied on the fa0/0 interfaces on both router A and X. This is an undesirable side
effect and it complicates the design. It also violates the next hop only rule for building IPSec
tunnels as described in GNAST IPSec RBE standard [1].
It is interesting to contrast IPSec tunnel with GRE tunnel. It may be desirable in some cases that
GRE tunnel is built upon loopback addresses. The advantage of doing so is that there may exist
alternative underlying paths that the GRE tunnel can actually traverse. This makes the GRE
tunnel more resistant to link failure occurred in the underlying network. The alternative paths
may be suboptimal though. Unfortunately, the analogy is not applicable to its IPSec tunnel
counterpart.
As a matter of fact, in IPSec RBE design, it is preferred that an IPSec tunnel is only associated
with an underlying path. Upon failure, the traffic quickly switches to the pre-provisioned backup
tunnel. This simple approach generally results in less complicated configuration and highly
predictable behavior which make the network more manageable.
3.4 Routing
IPSec RBE does not impose additional requirements on the routing of underlying network. IGP
(e.g. OSPF, EIGRP) routing traffic should be pushed inside the IPSec tunnel as best design
practice. This will not only provide security to routing protocol but also tie IPSec failures with
IPSec.
3.5 HSRP
HSRP provides the resiliency on the LAN side. In an IPSec RBE network scenario as shown
below, the worst impact of HSRP is the possibility of an extra hop. Highlights of the RBE
configurations are:
• First hop approach is used for IPSec peering
• Primary and backup IPSec tunnels are independently established
• Dynamic routing is configured on both WAN and LAN sides
• IKE keepalive is configured
• HSRP preemptive is enabled and priority is set higher on X
• HSRP tracks the WAN link
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It is logical to configure router X, which is associated with the primary IPSec tunnel, as the HSRP
active router. When the link between X and A is down or when router A crashes, the route that
traverses the primary IPSec tunnel will be removed from X’s routing table. However, the data
traffic coming out from the LAN side will still be using the active router, X, as the first hop.
Since the LAN segment participates in the routing, the traffic will be sent back to the LAN in
clear from X to Y and then traverses the backup IPSec tunnel. The returned traffic will take the
backup tunnel since router A no longer advertises the route to the LAN via the primary tunnel.
The inefficiency of the extra hop may be eliminated by tracking the WAN link on X in its HSRP
configuration. However, HSRP tracking may not always yield the desire result. For example, an
Ethernet WAN connection plugging into the carrier’s switch will be always up even if the remote
peer’s interface is down.
A potential problem exists if the network outage is quickly resolved and the connectivity between
router X and A is restored. If it was a link problem, the SAs associated with the primary IPSec
tunnel are still available on both A and X. Therefore the primary IPSec tunnel is restored in no
time. However, if the outage resulted in router A to reload, a black hole scenario will surface and
lasts until the next QM negotiation. With IKE keepalive enabled, the network is immune to this
black hole scenario.
3.6 GRE Tunneling
The need to use GRE tunneling in IPSec RBE network falls into three categories:
1. Multi-protocol support is needed to encapsulate legacy protocols in IP
2. Multiple logical paths are needed for resilient design
3. Complicated ACLs for encrypted traffic become infeasible or impractical
4. Encrypting IPSec Multicast traffic
5. The most importantly, GRE tunneling ties IPSec failures with routing protocol failures to
avoid traffic black holing.3
3
Multicast based routing like OSPF protocol requires GRE tunneling. Also, BGP can require GRE
tunneling over IP based transport networks. In future, we plan to use virtual IPSec interfaces to avoid GRE
tunneling overhead.
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XA
B Y
Backup IPSec tunnel
Active
Standby

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It is strongly recommended to use GRE tunneling to avoid routing protocol stay up and send
traffic over IPSec failed links.
The issues incurred by using GRE tunnel are discussed below.
Performance Degradation
GRE encapsulation, as illustrated in the above diagram, can introduce 5-25% performance
degradation, depending on the packet sizes. The performance degradation is observed even when
CEF or fast switching is in effect. It results from the encapsulation header which adds additional
24 bytes to the original packets. Additional sources of the performance degradation may come
from
1. Encapsulation processing that falls back to process switching in earlier IOS versions
2. The need for pre-fragmentation at GRE ingress routers
3. The need to reassemble GRE fragments at GRE tunnel egress routers
4. The need to reassemble IPSec fragments at IPSec tunnel egress routers
The following table demonstrates the performance degradation by throughput numbers (Mbps)
measured in a Lab environment using Ixia traffic generator. The device under test is a 7206VXR
running IOS 12.2(7a) with NPE400 processor board and VPN ISA card. IMIX traffic is a
repeated sequence of frames with size pattern 64-64-570-64-64-570-64-1400-570-64-64-570.
CEF switching is turned on. Note that the router CPU utilization is close to 100% when these
numbers were measured. The test data are Ethernet frames. The size of Ethernet frame is the IP
packet size plus 18 bytes (14-byte Ethernet header and 4-byte Ethernet FCS).
Frame Size 64 256 1400 IMIX
Theoretical Limit 76.19 92.75 98.59 94.50
Baseline 76.19 92.75 98.59 94.49
GRE 59.30 85.33 96.99 88.45
3DES 17.30 53.11 70.73 56.70
3DES + GRE 14.50 46.38 69.36 56.67
The following table contrasts the performance degradation with and without GRE reassembling.
Fragments are CEF switched (first column) yet reassembling can only be process switched
(second column).
Frame Size 1496 (w/o GRE reassem.) 1496 (w/ GRE reassem.)
14
New IP HDR GRE HDR IP HDR IP Payload
20 bytes 4 bytes
IP HDR IP Payload
or legacy protocol datagram

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GRE 92.26 20.86
When IPSec processing is involved, reassembling is inevitable for frames larger than 1494 bytes.
The fragmentation and reassembling take place at different levels though. The following table
contrasts the performance degradation with and without GRE reassembling, respectively, when
CEF switching is on.
3DES + GRE ~15.00 8.46
The following table illustrates the throughput results as that in the above table except fast
switching is used instead of CEF switching.
3DES + GRE 12.42 14.60
The impact of GRE overhead should be taken into account when IPSec is implemented
Fragmentation and Reassembling
The impact of fragmentation incurred by GRE and IPSec header/trailer overhead can be more far-
reaching than merely performance degradation when firewalls or similar devices are involved. In
the worst scenario, the end users will experience unpredictable connectivity problems over IPSec
RBE network that some applications work fine and others fail. The discussion of these problems
(which are MTU related), their causes and possible solutions will be deferred until later section.
Whether by design or not, once fragmentation occurs on one end, reassembling may take place at
the other end. Reassembling is a resource intensive process. Not only it cannot be fast or CEF
switched but it also requires the use of huge buffers (18,024 bytes in size) on the routers. This is
because IP fragments do not carry any information about the original IP packet size. Routers have
no way to know how much buffer space is needed for reassembling until the last fragment is
received. Therefore, routers have no choice but to request a huge buffer upon receiving the first
fragment. Furthermore, since the default permanent number of huge buffers is zero, each huge
buffer hit also results in a huge buffer miss; if there’s none already exists. Hence it may be
necessary to perform buffer tuning if fragments cannot be totally eliminated in the networks.
Interested readers should turn to Appendix A for how GRE tunneling incurs fragmentation.
Cisco Crypto map Caveat
A long-lived Cisco bug regarding how crypto map is applied on GRE tunnel interface is worth
mentioning. This bug case (CSCdk59089, CSCdw03713) describes the misbehavior of the need
to apply crypto map on both physical and GRE tunnel interfaces. The fix was later backed out
since it caused 7200 and RSP platforms to reload as described in bug case CSCuk27481. Under
the influence of this bug (most IOS versions are), a crypto map set is required to bypass the
problem when multiple GRE tunnels are configured on a single physical interface. Such crypto
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map set will contain multiple crypto map entries. All of the crypto map entries in the crypto map
set share the same map name yet with different map sequence numbers. Each of the crypto map
entry is associated with a single GRE tunnel. The router will evaluate the traffic passing through
the interface against the applied crypto map set. Inefficiency is introduced that each crypto map
entry is evaluated in sequence until the traffic is matched.
Since the bug status is in flux, it is advisable that the workaround be used so that functionality is
guaranteed regardless of the IOS versions. The correct configuration (applying independent
crypto map on each GRE tunnel) can be attempted, if desired, for IOS versions newer than
12.1(12)E, 12.2(10), and 12.2(8)T.
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4 Other Considerations
4.1 IPSec Modes
IPSec protocols (ESP and AH) can operate in either tunnel mode or transport mode. Tunnel mode
is required for use with security gateways, in which case IPSec peers are security proxies for the
protected end hosts. Transport mode is used for end to end security in which case IPSec peers
themselves are the end hosts. GRE tunneling creates a unique scenario that transport mode is
applicable. In theory, the use of transport mode increases bandwidth efficiency. However, Lab
tests showed statistically indifferent throughput numbers between these two modes when
hardware IPSec modules are in use. Cisco engineers speculated that the hardware modules are
optimized for tunnel mode. It is design engineers’ judgment to determine if transport mode is
critically necessary from bandwidth efficiency perspective.
4.2 NAT
Network Address Translation (NAT) is typically used at the firewalls for various purposes. AH
protocol, by its nature, will not work if the IPSec tunnel traverses a NAT device. This is because
AH authenticates the IP header, including the outermost one. When a NAT device translates the
addresses for an AH packet, the resulting packet will not pass the authentication check on the
other end of the tunnel and the NATed packet will be dropped. If an IPSec tunnel has to traverses
a NAT device, using ESP in tunnel mode is the only way.
4.3 Firewall
If an IPSec tunnel passes through a firewall, the existing filtering needs to be modified to permit
protocol 50 for ESP, protocol 51 for AH and UDP port 500 for ISAKMP negotiations. Since the
content of packets protected by ESP encryption cannot be examine by the firewalls, it is not
uncommon to deploy a dual-firewall topology that IPSec filtering is performed at the outer
firewall and the usual data packet filtering takes place at the inner firewall.
A common issue associated with firewall, even if IPSec RBE is not involved, is the permission
for ICMP type 3 code 4 packets to pass the firewall. ICMP type 3 code 4 packets signal a
situation that fragmentation needed but Don't Fragment (DF) bit was set in the committed
packet’s IP header. The use of ICMP type 3 code 4 packets is a crucial component for path MTU
Discovery (pMTU-D) protocol to work. Connectivity problems may occur if ICMP type 3 code 4
packets are blocked at the firewall for scenarios which meet the following conditions:
1. The link with the smallest MTU size is in the middle of an end to end path, rather than
the links directly connected to the end hosts
2. Path MTU discovery support is enabled on the end hosts.
In the example illustrated below, a GRE tunnel is configured outside of the firewall. Assume the
MTU sizes of the physical links in the end to end path are all 1500 bytes. To account for its
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18
overhead, routers (A and B) will set the MTU size of the GRE tunnel interfaces to be 24 bytes
less than its physical interface, i.e., 1476 bytes. Since pMTU-D support is enabled on the end
hosts, all TCP packets will be sent with DF bit set in the IP header. There will be no problems
until some applications (e.g. ftp) start to send large packets. When a TCP packet coming out from
GDN, traversing the firewall and arriving at router B, it will be dropped by router B if its size is
larger than the next hop MTU size, 1476 bytes. While dropping the packet, router B also sends
back an ICMP type 3 code 4 packet to the source of the packet, i.e., the end host. This ICMP
packet, unfortunately, is blocked by the firewall. As a result, the applications do not know that
packets have been dropped because they are too large and the router cannot notify the source host
about about the packets being dropped. This is a black hole situation, the TCP applications
eventually time out.
Citigroup’s corporate security policies do not permit ICMP type 3 code 4 packets to pass through
the firewalls. If it is critically important for the pMTU-D to work across the firewalls, CISO or
BISO should be contacted for risk analysis and approval for deviation. ICMP type 3 code 4
packets should not be filtered inside corporate firewalls.
There is also a known issue specific to the CyberGuard firewalls that out of order IP fragments
will be dropped immediately. Furthermore, this behavior is not controlled by explicit firewall
rules so nothing in the configuration can change the behavior. In the case of TCP traffic and the
fragment drop rate is not high, TCP retransmission mechanism will prevent the session from
failing. However, there have been incidents reported from production IPSec RBE networks that
solely relying on TCP retransmission does not guarantee the networks being immune from this
problem. Other firewalls buffer the out of order fragments for a certain period of time before
dropping them, therefore alleviate this problem greatly.
Evidently these firewall related issues are not specific to IPSec RBE networks only. However, the
encapsulation from both GRE tunneling and IPSec protocols makes the exposure to these
problems more likely.
4.4 Fragmentation and MTU
Fragmentation does exist in IP networks for various reasons. It consumes network resources and
slows the packet flow down. In some cases, it even results in connectivity problems. Therefore it
is highly desirable to avoid fragmentation from happening at all. Path MTU discovery protocol is
designed to address this problem from its root. Path MTU discovery is already supported in most
modern operating systems for end hosts and routers.
Unfortunately, pMTU-D is not a silver bullet for all fragmentation issues. As a matter of fact, if
failing to work end to end, pMTU-D itself can become the problem. The remedy, then, will be to
18
Firewall
GDN
A B
Partner
Network
GRE
tunnel

19
disable pMTU-D support. This can be done either on the end hosts, if it’s feasible, or it can be
done on the routers by using policy routing to strip the DF bit.
In cases when pMTU-D is not supported by the operating systems of the end hosts or when the
application is not TCP based, solutions do not necessarily exist. With CyberGuard firewall related
problems, forcing GRE egress to perform reassembling turns out to be a viable solution.
The following table summaries some known issues and potential solutions related to
fragmentation and MTU settings:
GRE
tunnel
outside
firewall
ICMP 3/4
allowed
TCP traffic
Host OS
pMTU-D
support
Issues & Solutions
Y N Y Y
Issue:
End-to-end pMTU-D is not possible,
fragments may occur. CyberGuard will drop
out of sequence fragments.
Solution:
1. Permit ICMP 3 /4 to pass the firewall on a
limited basis.
2. Set GRE MTU size to link MTU (1500
typically) to force reassembling. Be aware
of reassembling cost.
3. Manually adjust link MTU to the smallest
path MTU on end hosts. Not
recommended since side effect exists.
Y Y/N Y N
Issue:
TCP application determines MSS based on
MTU size of directly connected link.
Without host OS pMTU-D support, DF bit
will not be set therefore ICMP 3/4 will not
be sent back. The GRE ingress will perform
fragmentation. Out of sequence fragments
may be dropped by CyberGuard. If the
packets loss rate is high, even TCP
retransmission cannot recover the loss, the
connection eventually times out.
Solution:
Manually adjust link MTU to the smallest
path MTU on end hosts. Be aware of the
side effect.
Y Y/N N Y Issue:
Non-TCP protocols cannot take advantage
of pMTU-D. The first-hop router may be
overwhelmed by fragmentation, which in
turn may induce system buffer allocation
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issue. Out of sequence fragments may be
dropped by CyberGuard
Solution:
1. If the application provides a mechanism
to control maximum packet or segment
size, use that mechanism. Example:
Novell server 4.x with NWIP.
2. Otherwise, there is no robust solution.
N N Y Y
Issue:
Large packets, with DF bit set, are dropped
by GRE ingress but ICMP 3/4 cannot reach
the end host. This is a black hole situation.
Typical symptom is applications with small
packets (telnet) work fine but applications
with largest packets (ftp) fail.
Solution:
1. Remove the mechanism, e.g. ACLs,
which blocks ICMP 3/4.
2. Use policy routing to strip the DF bit at
GRE ingress.
Y/N Y Y Y
Issue:
When IPSec is configured on a GRE
interface, the next hop MTU size in the
ICMP 3/4 sent back from the routers does
not take into account of the IPSec overhead.
Fragments may still occur since pMTU-D
does not discover the smallest path MTU.
Solution:
Manually set the GRE interface’s IP MTU
size to 100 bytes below the link MTU to
account for IPSec overhead.
Due to certain application in the network, the packet size when accompanied with IPSec overhead
is sometimes exceeding the router MTU size; some packets have the DF bit set as well. Ideally
when this occurs the router would send an ICMP message back to the sender which would then
lower its MTU size. We discovered that many of the ICMP messages were being dropped due to
rate-limiting, thus the senders would never receive them and would continue sending packets
with too large of a MTU.
The ideal solution is to change the host MTU size. Alternatively decreasing the amount of rate-
limiting that will occur has been implemented to verify the problem and serves as a viable
workaround. At some point the number of ICMP messages may again lead to rate-limiting and
the number would have to be changed. Given information available for network traffic today we
agree the "ip icmp rate-limit unreachable df 10" to be sufficient. This may need to be revisited
should application traffic change.
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All BA routers with inter-area GRE tunnels will have to implement this command:
ip icmp rate-limit unreachable df 10
This requirement may be waived later with newer Cisco IOS code release that make this
command default.
4.5 Compression
IP payload compression (IPComp) is a protocol that reduces the size of IP packets. This protocol
increases the overall communication efficiency between a pair of communicating nodes by
compressing the payload of the packets. However, encrypting the IP payload increases the
randomness of the data in nature, rendering compression at lower protocol layers ineffective. If
both compression and encryption are required, compression should be applied before encryption.
IPComp is stateless, in other words, each IP packet is compressed and decompressed by itself
without any relation to other packets, as IP packets may arrive out of order or not arrive at all.
Cisco IOS supports IPComp with LZS compression algorithm. It has been verified from Lab tests
that when compression is used, the routers achieve higher throughput under the same CPU
utilization. Furthermore, after compression the need for fragmentation is largely reduced. This is
a welcome bonus side effect. However, compression is not supported when hardware IPSec
module is present. Compression should be used when software-only encryption is used on both
ends of the IPSec tunnel. However, hardware based encryption is strongly recommended
4.6 Rate-Limiting IPSec Traffic
IPSec processing imposes heavy load on router’s processor, even when hardware IPSec module is
present. It is possible that the encrypted traffic overloads the router with very high CPU
utilization since there is no built-in mechanism to prevent this from occurring. This scenario is
likely in Ethernet WAN, which becomes more prevalent recently. Rate-limiting the traffic to be
IPSec processed is thus a necessary maneuver to ensure router health.
Note that rate-limiting techniques that are applied on the outbound interfaces, such as traffic
shaping, are incapable of alleviating the CPU utilization. This is because the discarded traffic still
went through IPSec processing. Traffic policing in the inbound interfaces, before IPSec
processing takes place, is the only known technique for this purpose.
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5 Configuration Tasks
5.1 Create ISAKMP Policies
On Cisco routers, IKE phase 1 parameters are defined in “ISAKMP policy”. These parameters
are listed in Table 1 below.
Table 1 - Parameters defined in ISAKMP Policy
Parameter Accepted Values Keyword Default Value Citigroup
Compliant
Authentication
Method *
RSA signature
RSA encrypted nonces
Pre-shared keys
rsa-sig
rsa-encr
pre-share
rsa-sig rsa-sig
rsa-encr
pre-share4
Encryption Algorithm 56-bit DES
Triple DES
des
3des
des 3des
Hash Algorithm SHA-1 (HMAC variant)
MD5 (HMAC variant)
sha
md5
sha sha
Diffie-Hellman Group
Identifier
768-bit Diffie-Hellman
group 1
group 2
group 5
group 1 group 2
Security Association’s
Lifetime
Time in seconds - 86400 < 86400
* Please Note: For all new green field implementations, it is recommended that RSA-sig be used as the
authentication method. Having RSA-sig as the authentication method will also make the transition to
Certificate Authority (once certified) easier. RSA-encr is not widely supported on the ISR platforms. Please
refer to Appendix B for more information.
A Citigroup compliant configuration session is shown below:
router(config)#crypto isakmp policy 10
router(config-isakmp)# authentication rsa-encr
router(config-isakmp)# encryption 3des
router(config-isakmp)# group 2 ! This is the default value
router(config-isakmp)# hash sha ! This is the default value
router(config-isakmp)# lifetime 86400 ! This is the default value
router(config-isakmp)# exit
The number associated with “crypto isakmp policy” command indicates priority; it can range
from 1 to 10,000. The smaller this number is, the higher the priority. Multiple policies can be
configured on each peer with different settings of parameters. However, at least one of these
policies must contain exactly the same authentication, hash, encryption and Diffie-Hellman
parameter values as one of the policies at the remote peer. ISAKMP SA’s lifetime doesn’t have to
be the same. When they are different, the shorter lifetime proposed by the remote peer will be
used. This behavior, however, is not consistent among IOS versions. It has been changed after
4
Pre-shared keys widely used in external connections as well as internal, when we have issues supporting
rsa based authentication solution.
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IOS 12.2(6). A Cisco Bug ID, CSCdu70355, is created for this change. Therefore, the best
strategy is to ensure IPSec peers are configured with the same ISAKMP lifetimes.
ISAKMP SA lifetime (MM lifetime) must be set to 24 hours to be Citigroup compliant. Despite
the fact that this is the current default value, ISAKMP SA lifetime should be set explicitly to
avoid potential impact introduced by the change of default value between IOS versions.
An IPSec peer will always use a built-in (lowest priority) policy if it cannot find a matched policy
at the remote peer or if it doesn’t explicitly set any policy. This default policy, among other
policies, can be displayed by the “show crypto isakmp policy” command:
router# show crypto isakmp policy
Protection suite of priority 10
encryption algorithm: Three key triple DES
hash algorithm: Secure Hash Standard
authentication method: Rivest-Shamir-Adleman Encryption
Diffie-Hellman group: #2 (1024 bit)
lifetime: 86400 seconds, no volume limit
Default protection suite
encryption algorithm: DES - Data Encryption Standard (56 bit
keys).
hash algorithm: Secure Hash Standard
authentication method: Rivest-Shamir-Adleman Signature
Diffie-Hellman group: #1 (768 bit)
lifetime: 86400 seconds, no volume limit
Note that the built-in default policy is not Citigroup compliant.
Additionally, Cisco proprietary ISAKMP keepalive is useful for recovery from router crash. The
configuration command takes two arguments. The first argument defines keepalive poll interval
and the second argument defines retry interval. Citigroup compliant IKE keepalive configuration
is shown below.
router(config)# crypto isakmp keepalive 120 2
The keepalive interval can be set between 10 and 3600 seconds. The retry interval can be set
between 2 and 10 seconds, with the default being 2 seconds. Retry interval is the interval between
retries after a keepalive response has not been received.
5.2 Configure Authentication with RSA Keys
IKE can use one of the three methods to authenticate negotiating parties. The first method is pre-
shared key authentication, which is not Citigroup compliant and will not be discussed here. The
other two authentication methods, encrypted nonce authentication and digital signature
authentication, are both based on a pair of public/private keys, known as RSA keys.
5.2.1 Generate RSA Public/Private Key Pair
Step 1: Adjust system calendar or synchronize the system clock to an NTP server.
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System clock on the router plays a crucial role in digital signature authentication. Digital
certificate is only considered valid if the time indicated by the router’s system clock falls between
the start date and end date of the certificate. Clock synchronization with an NTP server should be
configured before attempting to generate RSA keys. This is especially true on lower end routers
such as the 1700, 2600 and 3600 series since apparently these routers are not equipped with
battery clocks and the system clock configured by “clock set” command does not survive a
reload. For example, with IOS 12.1(5)T, the system clock is always set back to 00:00:00 March 1,
1993 after reloading. For encrypted nonce authentication, although the system clock information
is not as critical as in the case of digital signature authentication, it serves documentary purpose
for when the keys were generated. RSA keys should be renewed every two years.
Changing pre-shared, rsa-nonce and rsa-sig keys in periodically in large size networks as outlined
by The ISTG Key Management Guidelines is challenging task. It requires manual changes in
network and has risk of impacting the network availability. We are working to get certificate
based authentication solution to address key management issue for RBE solution.
CTI’s Network Infrastructure is Class “C” certified which means there is no need to encrypt at
the Confidential level or below because our network is protected from “casual” users. So, not
changing IPSEC keys periodically over Class “C” infrastructure does not expose us any
additional risk.
Step 2: Set up domain name.
Domain name is required for RSA public/private keys generation. The FQDN of the router will
be used as the name of the RSA keys for both signing and encryption.
remote(config)# ip domain name citicorp.com
Step 3: Generate two RSA public/private key pairs, one for signing and the other for encryption.
For encrypted nonce authentication, only the encryption key is used. Key length of 1024 bits
should be used for both key pairs. The keyword “usage-keys” is required in order to generate two
RSA key pairs.
remote(config)# crypto key generate rsa usage-keys
The name for the keys will be: remote.citicorp.com
Choose the size of the key modulus in the range of 360 to 2048 for your
General Purpose Keys. Choosing a key modulus greater than 512 may take
a few minutes.
How many bits in the modulus [512]: 1024
Generating RSA keys ...
[OK]
a few minutes.
Generating RSA keys ...
[OK]
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25
remote(config)#
01:55:52: %SSH-5-ENABLED: SSH 1.5 has been enabled
Step 4: Check the public keys.
remote# show crypto key mypubkey rsa
% Key pair was generated at: 16:44:54 UTC Mar 21 2001
Key name: remote.citicorp.com
Usage: Signature Key
Key Data:
30819F30 0D06092A 864886F7 0D010101 05000381 8D003081 89028181 00C6AFA1
2D083334 2688C3CD 599604BE 1EEB66F4 8A6F6B1A D3787CF0 19E89B02 A73292AF
B171640B 670E457D 888A5BA1 B156A8B4 430017C5 FBA31DCD 7F5A94A4 B73542E3
954C93A4 B6B9C26D 79290480 4C66B9B9 BE7BD480 61DBE6DD 0546F440 1888159B
96A28DA8 B53277ED 9B7E11A8 0895122B 215C6637 4EB11D4E E5200632 C5020301 0001
Key name: remote.citicorp.com
Usage: Encryption Key ! This is the key to be used in encrypted nonce authentication
Key Data:
30819F30 0D06092A 864886F7 0D010101 05000381 8D003081 89028181 00CB2BF6
37B36563 93EF8AB7 9A0269AF 582337C0 AFCFAB56 097628F2 B5E89738 6BC2D777
6B81C7FD 251A6665 18E25E85 76C1F390 4E98E657 EEEB7A63 57653426 EBF9EB3D
1B595859 3D0A3D32 9FC6CCB8 C423AEB8 062CEC83 E2C10EA4 DD425A31 ACBC5D9C
799F97AB D85AA73B DAC92421 DCF6D1BE 5461F1F8 380E924E CAE01AAC 05020301 0001
Key name: remote.citicorp.com.server
Usage: Encryption Key
Key Data:
307C300D 06092A86 4886F70D 01010105 00036B00 30680261 00DD0B97 5954A8DD
8D0C444B 371C3BC4 E8F9C336 F1A620DC 46854A1F F553FB96 54DFC429 8BD855B4
6B166D37 39F4892F 63C9384A E6D55B00 CF3459E2 B5117C6C 0032564D 63E17B1F
C145C424 01679842 7096760B 05A44DA3 D84466FC 5FEA065F 57020301 0001
Note that the private key is stored in a secure area on the router and can never be displayed.
5.2.2 RSA Encrypted Nonce Authentication
Nonce is a fancy name for a value that is considered to be random according to some very strict
mathematical guidelines. RSA encrypted nonce authentication encrypts a random number with
the other party’s public key as the means for authentication since only the intended party can
reveal the random number with the corresponding private key. For the authentication to work, a
router needs to store its remote peer’s public key locally on its public key chain, and vice versa.
Step 1: Generate RSA public/private key pairs on both the local and remote peers following the
procedures described in the previous section. Display the public key on the local router.
local# show crypto key mypubkey rsa
Key name: local.citicorp.com
Usage: Signature Key
Key Data:
30819F30 0D06092A 864886F7 0D010101 05000381 8D003081 89028181 00C4B93D
840B5AFC 73C54C40 04F33225 C0163A7C 49DC6FE3 86F1BB3D 61AF6FE0 F208A005
4C700E07 422EAF0F 48062FEB 6C4CBDAD 573E06EB FF2CFA0F B66E4136 8A1D5939
EB441CDD 61C93735 F4BFA9E4 30A3BA0D D85CD3BB 6D82D973 0CFC58BB 371DD23C
CE2DC673 D4A3D213 9836F77B 997ECE9B 474B2063 396795D3 5B1E11A5 BD020301 0001
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26
Key name: local.citicorp.com
Usage: Encryption Key ! This is the key to be used in encrypted nonce authentication
Key Data:
30819F30 0D06092A 864886F7 0D010101 05000381 8D003081 89028181 00BE0D2D
4B15B9D8 3CDF8949 92A2D36D 405B34C1 9F4C6EDD 56FAC177 9402DE30 DFECE7AA
6B019AB6 0CACE6C7 7F05D3BB 218835A7 361B00DA CE544097 20B61202 1D0AC0CA
49A88D37 54AF8189 E09A1B3E 7DDBACBD FC6BBA49 0F3A6ED2 FB0D2A0E 8677D3C8
D757B17C 40B7436C E9EFAA0E 150664CE 4FA2AE33 5CD10F99 E9FAD019 21020301 0001
Key name: local.citicorp.com.server
Key Data:
307C300D 06092A86 4886F70D 01010105 00036B00 30680261 00D034DD CD17ACC4
6740F56C 3D66F33F D5C18014 C5C4EF91 395FF505 B566A763 5EF81030 832FE992
D5A75862 81F8479C 1F8AA726 14B66F05 5DA86B25 685AAB4B 41406AE7 36A0CB73
AA456D89 33BE594F D2FE00DC 0C7E870B 919B8633 505B4BCD 2F020301 0001
Step 2: Login to the remote router, cut and paste the local router’s public key onto its public key
chain.
Assume the IPSec tunnel end point address is 172.16.110.1 on the local router and 172.16.110.2
on the remote router.
remote(config)# crypto key pubkey-chain rsa
remote(config-pubkey-chain)# addressed-key 172.16.110.1 encryption ! peer’s IP address
remote(config-pubkey-key)# key-string ! paste peer’s public key below
Enter a public key as a hexidecimal number ....
remote(config-pubkey)# $ 886F7 0D010101 05000381 8D003081 89028181 00BE0D2D
remote(config-pubkey)# $ 2A2D36D 405B34C1 9F4C6EDD 56FAC177 9402DE30 DFECE7AA
remote(config-pubkey)# $ F05D3BB 218835A7 361B00DA CE544097 20B61202 1D0AC0CA
remote(config-pubkey)# $ 09A1B3E 7DDBACBD FC6BBA49 0F3A6ED2 FB0D2A0E 8677D3C8
remote(config-pubkey)# $ 0664CE 4FA2AE33 5CD10F99 E9FAD019 21020301 0001
remote(config-pubkey)# quit
remote(config-pubkey-key)# ^Z
Then verify whether the peer’s public key is put on the public key chain correctly and if the key is
the encryption key.
remote# show crypto key pubkey-chain rsa ! What’s on the key chain?
Codes: M - Manually configured, C - Extracted from certificate
Code Usage IP-Address Name
M Encrypt 172.16.110.1
remote# show crypto key pubkey-chain rsa address 172.16.110.1 ! Is the peer’s public key correct?
Key address: 172.16.110.1
Source: Manually entered
Data:
30819F30 0D06092A 864886F7 0D010101 05000381 8D003081 89028181 00BE0D2D
4B15B9D8 3CDF8949 92A2D36D 405B34C1 9F4C6EDD 56FAC177 9402DE30 DFECE7AA
6B019AB6 0CACE6C7 7F05D3BB 218835A7 361B00DA CE544097 20B61202 1D0AC0CA
49A88D37 54AF8189 E09A1B3E 7DDBACBD FC6BBA49 0F3A6ED2 FB0D2A0E 8677D3C8
D757B17C 40B7436C E9EFAA0E 150664CE 4FA2AE33 5CD10F99 E9FAD019 21020301 0001
Step 3: Repeat Step 2 on the local router by adding the remote router’s encryption key onto its
key chain (output not shown).
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27
Step 4: Ensure that RSA encrypted nonce (rsa-enc) authentication is configured in the ISAKMP
policies on both routers.
remote(config)# crypto isakmp policy 10
remote(config-isakmp)# auth rsa-enc
remote (config-isakmp)# encr 3des
remote(config-isakmp)# group 2
5.2.3 RSA-Sig Authentication with Generated Keys
RSA-Encr (with encrypted nonces) is not supported on certain hardware encrypiption based
platforms. Cisco has developed feature (based on CSCdv30620) to make sure RSA-sig to work
with generated keys. This feature is integrated into 12.3 mainline and 12.1(19.4)E.
Below is the process used for key generations and configuration for RSA-sig based
authentication.
Step 1: Configure RSA-sig as the authentication method on the first router.
(config)# crypto isakmp policy 10
(config-isakmp)# auth rsa-sig
(config-isakmp)# encr 3des
(config-isakmp)# group 2
Step 2: Generate Keys on the peer router
u02_6500_02(config)#crypto key generate rsa general-keys exportable
a few minutes.
% Generating 1024 bit RSA keys ...[OK]
u02_6500_02#show crypto key mypubkey rsa
% Key pair was generated at: 14:07:54 GMT Sep 26 2005
Key name: u02_6500_02.citigroup.com
Usage: General Purpose Key
Key is exportable.
Key Data: //below key is used
30819F30 0D06092A 864886F7 0D010101 05000381 8D003081 89028181 00C9B94E
B7910846 1B572A9B 65CA9114 7E5F68B9 A4484DAA A02CBBD4 883BCAEB 83701110
D1B559F0 0739383A 30E3D79E DE417FAE 652286C2 09B5FA55 7A475E48 18457C43
568A41C8 139C73D5 5D241C35 2D5B7F74 B1EB85CE DE1D6D63 27F1D0EC 7BBF605D
E5106474 1314EF91 0B0F6F13 C7DFBFDE 8B9BD34F 6C6C489D 9D92A246 05020301 0001
% Key pair was generated at: 14:07:55 GMT Sep 26 2005
Key name: u02_6500_02.citigroup.com.server
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28
Key is exportable.
Key Data:
307C300D 06092A86 4886F70D 01010105 00036B00 30680261 00DB6140 66D28268
27D6166D 61EC42AE 306A8CF0 1C150E07 57B69005 5C1B1507 9E1B28CA F10F13A6
E26919F0 1E03CCAE C51A0CC3 9B22761A C2C47C6D 33856A5D E5ECFA49 8B0BAA7A
3A5C6938 96D198B4 C1FE5901 BBB25403 D52EC2B1 357655F8 E1020301 0001
u02_6500_02#
Step 3: Configure the key (obtained from the peer) on the first router
crypto key pubkey-chain rsa
addressed-key 192.168.10.138
address 192.168.10.138
key-string
30819F30 0D06092A 864886F7 0D010101 05000381 8D003081 89028181 00C9B94E
B7910846 1B572A9B 65CA9114 7E5F68B9 A4484DAA A02CBBD4 883BCAEB 83701110
D1B559F0 0739383A 30E3D79E DE417FAE 652286C2 09B5FA55 7A475E48 18457C43
568A41C8 139C73D5 5D241C35 2D5B7F74 B1EB85CE DE1D6D63 27F1D0EC 7BBF605D
E5106474 1314EF91 0B0F6F13 C7DFBFDE 8B9BD34F 6C6C489D 9D92A246 05020301 0001
Quit
Repeat Steps 1 through 3 to configure the peer router.
5.2.4 RSA Digital Signature Authentication
To use RSA signature as the authentication method, each IPSec peer has to request a digital
certificate from a Certificate Authority (CA). Digital certificate, which binds the certificate
holder’s identity with its public key, is signed by a trusted third party CA. Certificates are
exchanged between peers as part of the IKE negotiation.
Note that since Citigroup’s PKI infrastructure has not been fully deployed for IPSec RBE, the
FQDN used in the configuration example below is for illustration purpose only.
The procedures for requesting digital certificates from the Entrust CA are described below.
Step 1: Declare the CA to be contacted for issuing certificates
rbe-a(config)# crypto ca identity citicorp.com
rbe-a(ca-identity)# enrollment mode ra
rbe-a(ca-identity)# enrollment url http://caserver.citicorp.com/
rbe-a(ca-identity)# exit
Note that the identity of the CA (in this case, citicorp.com) is supposed to serve as an internal
name for the CA, only known to the local router. However, the certificates acquired are
considered invalid in the authentication process unless this identity is an FQDN!!
For Entrust CA, the enrollment mode must be set to ra, meaning that the routers will not contact
CA directly. A Registration Authority (RA) will be contacted instead. The URL specifies the
28

29
location of the RA, in this case, http://caserver.citicorp.com/. Entrust VPN Connector is the
product name for the RA.
Step 2: Generate public/private key pairs on the router (described in previous section)
Step 3: Authenticate CA
A router must authenticate the CA before it submits its public keys to the RA for certificate
enrollment. The authentication is done by verifying the CA’s fingerprint and downloading the
CA’s certificate to the router. On the router, use the following command:
rbe-a(config)# crypto ca authenticate citigroup.com
Certificate has the following attributes:
Fingerprint: 1FF2A297 67B877F7 FADEC7D9 DD12738B
% Do you accept this certificate? [yes/no]: yes
Note that the name of the CA is the identity that was set by “crypto ca identity” command in
step 1. The authentication is a manual process that a human operator needs to have prior
knowledge of the CA’s fingerprint. The fingerprint shown in the output of this command should
match that known fingerprint. By accepting the offer, the router will receive the CA’s own
certificate.
Step 4: Certificate enrollment
Cisco routers perform certificate enrollment via Simple Certificate Enrollment Protocol (SCEP),
a proprietary protocol defined jointly by Cisco and VeriSign for requesting certificates from a
CA. SCEP requests are encoded as PKCS#7 messages and conveyed via HTTP to a CA. The
SCEP protocol also allows the CA to defer its response to a certificate request until sometime
later (usually to allow a human operator to ratify the request). Hence, SCEP clients must poll the
CA to retrieve certificates.
The actual certificate enrollment takes place when the following command is issued:
rbe-a(config)# crypto ca enroll citicorp.com
% Start certificate enrollment ..
% Create a challenge password. You will need to verbally provide this
password to the CA Administrator in order to revoke your certificate.
For security reasons your password will not be saved in the configuration.
Please make a note of it.
Password:
Re-enter password:
% The subject name in the certificate will be: rbe-a.citicorp.com
% Include the router serial number in the subject name? [yes/no]: yes
% The serial number in the certificate will be: 45DEC199
% Include an IP address in the subject name? [yes/no]: yes
Interface: Serial 1/0
Request certificate from CA? [yes/no]: yes
% Certificate request sent to Certificate Authority
% The certificate request fingerprint will be displayed.
% The 'show crypto ca certificate' command will also show the fingerprint.
rbe-a(config)#
29

30
Signing Certificate Reqeust Fingerprint:
20D89EF0 A8FA719E 4BAAC1B4 0E363178
Encryption Certificate Request Fingerprint:
2A166016 D99433A1 F78AAF01 02F706FA
Again, the CA is referenced by its internal name, which is citicorp.com in this case. A challenge
password is required to type in for certificate revocation purpose. The router serial number should
be included in the certificate.
The approval is a manual process that the human operator of the RA needs to verify the router
serial numbers matches the one submitted to the operator in advance. The router will poll the RA
every 2 minutes for the availability of the certificate. The following command can be used to
display certificate information for the router. CA and RA’s certificates information is displayed
as well.
rbe-a# show crypto ca certificates
Certificate
Status: Available
Certificate Serial Number: 397487AD
Key Usage: Encryption
Issuer:
OU = CA0 QA
OU = Certification Authority
O = Citigroup
C = US
Subject Name Contains:
Name: rbe-a.citicorp.com
Serial Number: 45DEC199
CRL Distribution Point:
CN = CRL1, OU = CA0 QA, OU = Certification Authority, O = Citigroup, C = US
Validity Date:
start date: 15:35:15 UTC Jan 4 2001
end date: 16:05:15 UTC Jan 4 2002
RA Signature Certificate
Status: Available
Certificate Serial Number: 397481FA
Key Usage: Signature
Issuer:
OU = CA0 QA
O = Citigroup
C = US
Subject:
CN = VPNTESTMD
OU = Device
L = USMD
L = NORAM
O = citigroup
C = us
Validity Date:
start date: 15:52:57 UTC Sep 29 2000
end date: 16:22:57 UTC Sep 29 2002
Certificate
Status: Available
Certificate Serial Number: 397487AC
Key Usage: Signature
Issuer:
30

31
OU = CA0 QA
O = Citigroup
C = US
Subject Name Contains:
Name: rbe-a.citicorp.com
Serial Number: 45DEC199
Validity Date:
start date: 15:35:14 UTC Jan 4 2001
end date: 16:05:14 UTC Jan 4 2002
CA Certificate
Status: Available
Certificate Serial Number: 3974804F
Key Usage: General Purpose
Issuer:
OU = CA0 QA
O = Citigroup
C = US
Subject:
OU = CA0 QA
O = Citigroup
C = US
Validity Date:
start date: 15:35:39 UTC Jul 18 2000
end date: 16:05:39 UTC Jul 18 2020
RA KeyEncipher Certificate
Status: Available
Certificate Serial Number: 397481F9
Key Usage: Encryption
Issuer:
OU = CA0 QA
O = Citigroup
C = US
Subject:
CN = VPNTESTMD
OU = Device
L = USMD
L = NORAM
O = citigroup
C = us
Validity Date:
start date: 15:52:57 UTC Sep 29 2000
end date: 16:22:57 UTC Sep 29 2002
Step 5: Ensure that RSA sigature (rsa-sig) authentication is configured in the ISAKMP policies
on both routers.
remote(config)# crypto isakmp policy 10
remote(config-isakmp)# auth rsa-sig
remote (config-isakmp)# encr 3des
remote(config-isakmp)# group 2
31

32
5.3 Configure IPSec Transforms
On Cisco routers, IKE phase 2 parameters are defined by IPSec transform sets. These parameters
determine how the user data will be encrypted and authenticated. Specifically, an IPSec transform
set defines combinations of ESP and AH protocols to provide data confidentiality and integrity.
Optionally, compression (IPComp) can be used with other IPSec transforms to improve
efficiency. However, IPComp transform cannot be used when hardware IPSec accelerator is
present. Available transforms on Cisco routers as well as the protocols and algorithms associated
with them are listed in the following table. Note that not all combinations are valid.
Transform Name Protocol and Algorithm
ah-md5-hmac AH-HMAC-MD5 transform
ah-sha-hmac AH-HMAC-SHA transform
comp-lzs IP payload compression using the LZS algorithm
esp-3des ESP transform using 3DES(EDE) cipher
esp-des ESP transform using DES cipher (56 bits)
esp-md5-hmac ESP transform using HMAC-MD5 authentication
esp-null ESP transform without cipher
esp-sha-hmac ESP transform using HMAC-SHA authentication
Citigroup compliant IPSec transform sets are listed in the following table. The first transform set,
citi-trans, uses AH to provide data integrity and ESP to provide data confidentiality with triple
DES encryption algorithm. This is considered the best practice and should be used whenever
possible. The second transform set, citi-trans-nat, uses ESP for both data confidentiality and data
integrity. This transform set, to be used in tunnel mode, is required when the IPSec tunnel needs
to pass through a NAT device. The third and the fourth transform sets are the counterparts of the
first and the second, respectively, except payload compression using LZS algorithm, is also used.
Transform-set Name Transform Combination
citi-trans as-sha-hmac esp-3des
citi-trans-nat esp-sha-hmac esp-3des
citi-trans-comp as-sha-hmac esp-3des comp-lzs
citi-trans-nat-comp esp-sha-hmac esp-3des comp-lzs
A configuration example is shown below:
router(config)# crypto ipsec transform-set citi-trans ah-sha-hmac esp-3des
riouter(cfg-crypto-trans)# exit
The following Cisco IOS command can be used to check the IPSec transforms currently
configured:
router(config)# show crypto ipsec transform-set
Transform set citi-trans: { ah-sha-hmac }
will negotiate = { Tunnel, },
{ esp-3des }
will negotiate = { Tunnel, },
32

33
IPSec SA lifetime (QM lifetime) must be set to 1 hour to be Citigroup compliant. Despite the fact
that this is the current default value, IPSec SA lifetime should be set explicitly to avoid potential
impact introduced by the change of default value between IOS versions, as shown below.
router(config)# crypto ipsec security-association lifetime seconds 3600 ! default value
5.4 Specify Access Lists
Access lists (ACLs) are used to filter the traffic to be protected by IPSec. The traffic patterns
associated with the “permit” keyword define the interesting traffic, i.e., traffic to be IPSec
processed. On the other hand, the keyword “deny” defines the traffic that will not be IPSec
processed. The ACLs configured on the IPSec peers have to be mirror images of each other.
Each ACL entry will build a separate tunnel, which is corresponding to two to four IPSec SAs,
depending on the IPSec transform sets. Increasing ACL entries slows performance, complicates
troubleshooting, and hinders the scalability. The naming convention adopted by GNAST IPSec
RBE standard [1] is to use the remote peer’s hostname as the ACL name.
Example 1: This example demonstrates a branch network scenario where 192.168.1.0/24 is the
only subnet in the branch office. Two independent IPSec tunnels are constructed on a pair of
redundant branch routers, branch-01-a-2621 and branch-01-b-2621, respectively. Router branch-
01-a-2621 is peering to a hub router, nj-03-e-7206 and router branch-01-b-2621 is peering to
another hub router, ny-02-f-7206. The ACLs configured on branch-01-a-2621 and nj-03-e-7206
should be mirror images to each other as shown below:
branch-01-a-2621(config)# ip access-list nj-03-e-7206
branch-01-a-2621(config-ext-nacl)# permit ip 192.168.1.0 0.0.0.255 any
branch-01-a-2621(config-ext-nacl)# exit
nj-03-e-7206(config)# ip access-list branch-01-a-2621
nj-03-e-7206 (config-ext-nacl)# permit ip any 192.168.1.0 0.0.0.255
nj-03-e-7206 (config-ext-nacl)# exit
Similarly, the ACLs configured on branch-01-b-2621 and ny-02-f-7206 are mirror images to each
other:
branch-01-b-2621(config)# ip access-list ny-02-f-7206
branch-01-b-2621(config-ext-nacl)# permit ip 192.168.1.0 0.0.0.255 any
branch-01-b-2621(config-ext-nacl)# exit
ny-02-f-7206(config)# ip access-list branch-01-b-2621
ny-02-f-7206 (config-ext-nacl)# permit ip any 192.168.1.0 0.0.0.255
ny-02-f-7206 (config-ext-nacl)# exit
Example 2: This example demonstrates a branch network scenario where independent IPSec
tunnels are constructed on two separate GRE tunnels created on the same physical interface of the
only branch router, chlprv-01-a-2621. The GRE tunnels are terminated at the hub routers, chlfor-
03-i-7206 (172.16.1.0/29) and chlpac-02-e-7206 (172.16.1.4/29), respectively. The ACLs
configured on the routers are shown below:
chlprv-01-a-2621(config)# ip access-list chlfor-03-i-7206
33

34
chlprv-01-a-2621(config-ext-nacl)# permit gre host 172.16.1.3 host 172.16.1.1
chlprv-01-a-2621(config-ext-nacl)# exit
chlprv-01-a-2621(config)# ip access-list chlpac-02-e-7206
chlprv-01-a-2621(config-ext-nacl)# permit gre host 172.16.1.3 host 172.16.1.2
chlprv-01-a-2621(config-ext-nacl)# exit
chlfor-03-i-2621(config)# ip access-list chlprv-01-a-2621
chlfor-03-i-2621(config-ext-nacl)# permit gre host 172.16.1.1 host 172.16.1.3
chlfor-03-i-2621(config-ext-nacl)# exit
chlpac-02-e-2621(config)# ip access-list chlprv-01-a-2621
chlpac-02-e-2621(config-ext-nacl)# permit gre host 172.16.1.2 host 172.16.1.3
chlpac-02-e-2621(config-ext-nacl)# exit
Care needs to be taken when “any” keyword is used in a permit statement that it does not
unintentionally match multicast or broadcast traffic. The use of “permit any any” statement is
strongly discouraged unless it becomes the desperate last resort to avoid GRE tunneling. In that
case, it should be use with care as illustrated below:
rbe-a(config)# ip access-list rbe-b
rbe-a(config-ext-nacl)# deny any 224.0.0.0 15.255.255.255
rbe-a(config-ext-nacl)# deny 224.0.0.0 15.255.255.255 any
rbe-a(config-ext-nacl)# deny any host 255.255.255.255
rbe-a(config-ext-nacl)# deny host 255.255.255.255 any
rbe-a(config-ext-nacl)# permit any any
rbe-a(config-ext-nacl)# exit
5.5 Create Crypto Maps
A crypto map set is a collection of settings for IPSec to establish security associations. Typically,
these settings define what IPSec transforms should be applied to the traffic (set transform),
where the protected traffic is sent to (set peer), and what traffic should be protected (match
address). Optionally, a crypto map set may specify whether perfect forward secrecy should be
used (set pfs). Furthermore, IPSec SA’s lifetime, as time or traffic volume, can also be specified
for this crypto map set (set security-association lifetime). The lifetime specified in a crypto map
set overwrites that defined in the global configuration. The naming convention adopted by
GNAST IPSec RBE standard [1] is to use the remote peer’s hostname as the crypto map name.
The only exception is when multiple GRE tunnels are configured on the same physical interface
as illustrated in Example 2 below.
Example 1: Crypto map set with a single crypto map entry.
branch-01-a-2621(config)# crypto map nj-03-e-7206 10 ipsec-isakmp ! Crypto map name and number
branch-01-a-2621(config-crypto-map)# set peer 172.16.3.10 ! Remote IPSec peer’s address
branch-01-a-2621(config-crypto-map)# set transform-set citi-trans ! Best practice transform set
branch-01-a-2621(config-crypto-map)# match address nj-03-e-7206 ! Named ACL
branch-01-a-2621(config-crypto-map)# set pfs group2 ! Optional
branch-01-a-2621(config-crypto-map)# exit
The above crypto map set was given a name “nj-03-e-7206” with map number 10. The keyword
“ipsec-isakmp” indicates that key exchange will be performed by IKE negotiation. The “set
34

35
peer“ subcommand specified the remote peer’s IP address at the interface where crypto map is
applied. The transform set “citi-trans” and the named ACL “nj-03-e-7206” are pre-defined as
discussed in the previous sections. Setting pfs to group 2 indicates that at each QM negotiation, a
fresh 1024-bit Diffie-Hellman key exchange will be performed to generate a new secret instead of
deriving keys from the existing secret determined at the last MM negotiation. Enabling pfs is
optional since the mandated MM lifetime already satisfies CISO requirement.
Only one crypto map set can be applied to a single interface, physical or logical. However, crypto
map entries with the same map name but different map numbers are considered a single crypto
map set. When negotiating IPSec SAs, crypto map with the lowest map number has the highest
priority.
Example 2: Crypto map set with multiple crypto map entries.
chlprv-01-a-2621(config)# crypto map chlfor-03-i-7206+chlpac-02-e-7206 10 ipsec-isakmp
chlprv-01-a-2621(config-crypto-map)# set peer 172.16.3.1 ! peer’s physical interface address
chlprv-01-a-2621(config-crypto-map)# set transform-set citi-trans
chlprv-01-a-2621(config-crypto-map)# match address chlfor-03-i-7206 ! Named ACL
chlprv-01-a-2621(config-crypto-map)# exit
chlprv-01-a-2621(config)# crypto map chlfor-03-i-7206+chlpac-02-e-7206 20 ipsec-isakmp
chlprv-01-a-2621(config-crypto-map)# set peer 172.16.3.2 ! peer’s physical interface address
chlprv-01-a-2621(config-crypto-map)# set transform-set citi-trans
chlprv-01-a-2621(config-crypto-map)# match address chlpac-02-e-7206 ! Named ACL
chlprv-01-a-2621(config-crypto-map)# exit
5.6 Apply Crypto Maps
Crypto maps have no effect until they are applied to an interface as illustrated below:
branch-01-a-2621(config)# interface Serial1/0
branch-01-a-2621(config-if)# ip address 192.168.17.6 255.255.255.252
branch-01-a-2621(config-if)# crypto map nj-03-e-7206
branch-01-a-2621(config-if)# exit
If multiple GRE tunnels are configured on the same physical interface, the same crypto map
needs to be applied on the GRE tunnel interfaces and the physical interface until Cisco fixes the
bug.
chlprv-01-a-2621(config)# interface Ethernet0/0
chlprv-01-a-2621(config-if)# ip address 172.16.3.3 255.255.255.252
chlprv-01-a-2621(config-if)# crypto map chlfor-03-i-7206+chlpac-02-e-7206
chlprv-01-a-2621(config-if)# exit
chlprv-01-a-2621(config)# interface Tunnel0
chlprv-01-a-2621(config-if)# tunnel source 172.16.3.3
chlprv-01-a-2621(config-if)# tunnel destination 172.16.3.1
chlprv-01-a-2621(config)# interface Tunnel1
35

36
chlprv-01-a-2621(config-if)# tunnel source 172.16.3.3
chlprv-01-a-2621(config-if)# tunnel destination 172.16.3.2
36

37
6 References
[1] GNAST-SEC-0004: IPSec Router-Based Encryption (RBE)
(http://www.citigroup.net/tie/neteng/core/branch_head_rmt/GNAST-SEC-
0004_IPsec_rbe.doc)
[2] GNAST-ENG-0010: Network Routing
(http://www.citigroup.net/tie/neteng/basestds/routing/GNAST-ENG-
0010_Network_Routing.doc)
[3] Application Note: IPSec Virtual Private Network Resilience Solutions
(http://www.cisco.com/warp/public/cc/so/neso/vpn/vpne /vpne_an.htm)
37

38
7 Appendices
7.1 Appendix A – Fragments in IPSec RBE Network
Cisco’s GRE Implementation
In order to understand why fragmentation occurs in the first place, it is interesting to examine
how GRE (more specifically, GRE-IP) tunnels [RFC1701, RFC2784] are implemented on Cisco
routers. Assume a GRE tunnel is created between router A and router. A 1500-byte packet is sent
from network X to network Y.
1. Arriving at the GRE tunnel ingress at router A, the packet contains 20 bytes of IP header
and 1480 bytes of payload.
2. Knowing the 24-byte overhead of GRE tunnels, router A assigns an MTU size 1476 bytes
to the tunnel interface. Analogous to IP packet entering an Ethernet interface, the original
1500-byte packet needs to be pre-fragmented appropriately before it enters the GRE
tunnel interface.
3. Analogous to Ethernet frame encapsulation, each of these two IP fragments is
encapsulated with a new header, which contains a new IP header and a 4-byte GRE
header. The source and destination addresses in the new IP headers are the physical
interface addresses on router A and router B, respectively
38
payloadIP hdr
20 1480
payloadfrag. IP hdr
20 1456
frag. IP hdr payload
20 24
payloadfrag. IP hdr
20 1456
20 24
tunnel IP hdr
tunnel IP hdr GRE
GRE
20
20 4
4
Network
Y
A B
Network
X
GRE
tunnel

39
4. These two GRE packets are transmitted via the physical interface, upon which the GRE
tunnel is build, on router A to the corresponding physical interface on router B. The
physical interface on router B hands the packets to the GRE interface, where GRE
headers are peeled off. The resulting fragmented packets will stay fragments all the way
to the ultimate destination in network Y.
It is worth noting that the GRE implementation is not only encapsulation processing, it is also a
logical interface. As an interface, the payload packet is subject to fragmentation before
encapsulation takes place. The layer 3 MTU of the tunnel interface is default to the layer 2 MTU
of the physical interface minus 24 bytes of GRE overhead. And this tunnel interface MTU is
configurable.
Also note that the effect of the fragmentation is not local to the GRE tunnel end points, it
propagates all the way to the destination. All routers in between the GRE tunnel egress and the
destination are impacted since they have to forward additional fragments.
IPSec Processing
The IPSec implementation is purely encapsulation processing. It doesn’t perform any
fragmentation; instead, it leaves the task to the outbound interface. Unless IPSec processing is
implemented as an interface, it doesn’t seem that there exists any mechanism for the routers to
take into account the IPSec overhead in MTU calculation. As a result, path MTU discovery will
not work and another round of fragmentation would occur inside the IPSec tunnels for large
packets. To demonstrate this, a combined AH/ESP IPSec tunnel is configured on top of the GRE
tunnel. The IPSec transform set consists of ah-sha-hmac and esp-3des and tunnel mode is used.
The processes are identical to that in the GRE example except there are a few more steps between
step 3 and 4.
3a. IPSec ESP processing (3DES encryption, no authentication) prepends an 8-byte ESP header
and an 8-byte DES/3DES initialization vector (IV) to the GRE packet. It then appends a
variable size padding field and a 2-byte ESP trailer at the end. The padding field, preceding
ESP trailer, is there to ensure that
• ESP trailer terminates at 32-bit boundary
• ESP payload (starting from DES/3DES IV field) through ESP trailer is a multiple of the
block size that is used in the CBC cipher algorithm. For DES/3DES, the block size is 64
bits.
When a CBC cipher algorithm such as 3DES is used, it is evident that satisfying the second
condition automatically satisfies the first. Note that it is a coincidence that the padding fields
for these two GRE packets are both 2 bytes. IPSec AH processing only prepends an AH
header to the ESP processed packet. The AH header consists of 12 bytes of fixed header
fields and a variable field containing authentication data (hash value). The size of the
authentication data field depends on the hash algorithm used. In this particular case, the
39
payloadfrag. IP hdr
20 1456
20 24

40
truncated HMAC variant of SHA-1 algorithm is used and the authentication data contains the
lower 96 bits of the 160-bit SHA-1 hash value. Next a new IP header is added to fulfill the
tunnel mode operation. The source and destination addresses are the Ethernet interface
addresses of the IPSec peers, router A and router B, respectively.
3b. Before the first IPSec packet enters the outbound Ethernet interface, router A discovers that
the size of this packet, now 1564 bytes, has exceeded the MTU of the physical interface,
which is 1500 bytes. Therefore fragmentation is performed to break the IPSec encapsulated
packet into two fragments – one of them 1500 bytes and the other 84 bytes in size. These two
fragmented packets are then encapsulated in layer 2 frames and sent out toward router B. The
second IPSec packet, of size 132 bytes, is simply encapsulated in layer 2 frame and sent out
since its size is much smaller than the physical interface MTU. Three fragments occurred on
the layer 2 segment between router A and B, as illustrated below.
3c. Upon arriving at the physical interface on router B, the two fragments are reassembled first
into a whole IPSec packet since the destination of the outmost IP header has reached.
Consequently the reassembled IPSec tunnel mode IP header is peeled off before the packet is
sent for IPSec decryption, after which both the AH and ESP add-ons are removed. The
second (non-fragmented) IPSec packet follows the same processing except the reassembling.
At this point, two GRE packets are recovered. The processing beyond this point is identical to
the step 4 in the GRE example.
40
payloadfrag. IP hdr
20 1396
20 24
tunnel IP hdr
tunnel IP hdr GRE
GRE
20
20 4
4
AHnew frag. IP hdr ESP
AH ESP trailerpadding
2
20 24 16
24 1620 2
payload
60
trailerpadding
220 2
new IP hdr
new frag. IP hdr
payloadfrag. IP hdr
20 1456
20 24
tunnel IP hdr
tunnel IP hdr GRE
GRE
20
20 4
4
AHnew IP hdr
new IP hdr
ESP
AH ESP
padding
trailer
trailer
padding
2
2
Sequence Number
SPI
Sequence Number
SPI
Reserved
HMAC-SHA-1-96
LgthNext hdr
20 24
Payload
ESP trailer
Padding
16
DES/3DES IV
32 bits32 bits
32 bits
24 1620 2
2

41
The additional fragmentation is considered part of the IPSec overhead. Even if path MTU
discovery is activated on these routers, there is no way to eliminate this round of fragmentation
for large packets. The effect, however, is local to the segment between IPSec peers.
41

42
7.2 Appendix B – IPSec Authentication Methods + ISR Routers
Assumptions
• Citigroup IPSec Standards will be followed (Please refer to the IPSec Design Guide for more information)
• Target Code: 12.4(x) (Release Date: TBD/2006)**
Device Onboard
Encryptor
AIM-VPN Card PSK RSA-encr RSA-sig
(General Keys)
RSA-sig
(CA)
1841 √ NOT USED √ √ √ U
N
D
E
R
T
E
S
T
2811 √ NOT USED √ √ √
3845
Citigroup Standard
IPSec Transform-
Set NOT
SUPPORTED,
hence not
recommended
√
(AIM-VPN/HPII-
PLUS)
√ NOT
SUPPORTED
√
** 12.4(x) will be tested in the Warren Lab once Cisco comes out with a code.
42

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