MACsec and IPsec – A (not so :-)) Deep Dive

8 minute read

“Security” has many meanings depending on the context and it has become a vital piece in many areas including, but not limited to, information, finance, homeland, people, food, network, social, health, etc.  

When it comes to ICT and Telecoms, security is used at many levels. These include securing the networks, systems, DCs, hardware, software, traffic, control planes, data planes, management planes etc. 

If the data plane and traffic are considered (to understand the difference between data plane and control plane, check this video), we can think of the popular ISO OSI stack or the TCP/IP stack, which are modular designs of the data plane with abstraction, and the different layers to see the implementations of security for communication channels established at these layers. 

As it is usually not practical to secure an entire network against physical access, preventing disruption and data loss arising from transmission and reception by unauthorized devices is a vital capability to have. What we achieve ultimately is the confidentiality, integrity and authenticity of traffic being communicated. In fact, it is mostly the application data that we need to make secure.

If the OSI stack is considered, security can be implemented for traffic/application data at different levels. These mostly include Physical, Data Link, Network and Application layers. 

If we consider a simple network link (point to point) working on OSI principles, our objective is to send some application data (aka payload) to and from with respective to the two ends. In packet networking, where the layer 2 (data link) is mostly Ethernet and layer 3 (network) is mostly IP, these communications happen through a series of encapsulations (adding headers (and sometimes footers/trailers)) and decapsualtions (removing headers (and sometimes footers/trailers)) from application layer to physical layer and then from physical layer to application layer respectively. These headers consists of different information, values, identifications etc. 

When we want to secure the data being transported over the communication channel, we usually use different encryption mechanisms to encrypt the data before being put on to the channel. The reverse - decryption, is performed at the other end to retrieve the original data. 

Encryption is the process of encoding a message or information (aka cypher text) so that only authorized party/ies can access it. Although encryption does not itself prevent interference, it denies the intelligible (aka clear text) content to a would-be interceptor. 

There are many ways to do encryption, usually using modern block cypher symmetric secret key algorithms (ex:- DES – Data Encryption Standard, 3DES – Triple DES, AES – Advanced Encryption Standard). Encryption can be performed at any layer, including layer 1 (physical), layer 2 (data link), layer 3 (network) and even layer 7 (application). Although the layer 1 encryption is not end-to-end with respect to the application, it can provide high throughput, low latency and high efficiency. 

As we move up in the stack, the bandwidth efficiency decreases, while both overheads and latency increase. However, if application level encryption is considered, it is end-to-end. 

Media Access Control Security (MACsec) and Internet Protocol Security (IPsec) are two such mechanisms used in layer 2 and layer 3 of the OSI stack. IPsec is commonly heard while MACsec is not. 

Both of these work by adding additional header information (and sometimes trailers) relevant to encryption, keys etc. to the existing headers. Let’s look at MACsec first.

MACsec

Like any other Ethernet related standard, MACsec is also defined under IEEE. IEEE 802.1AE is the relevant standard. Published in 2006, the standard is over 10 years old, although we don’t get much to hear about MACsec compared to IPsec. It is applicable to local and metropolitan area networks. 

Therefore, the main applications of MACsec is seen in DC environments. Carrier Ethernet and Metro Ethernet environments can also implement MACsec if necessary. 

The standard specifies provision of connection-less user data confidentiality, data integrity and data origin authenticity by media access independent protocols and entities that operate transparently to MAC Clients. The MACsec Key Agreement Protocol (MKA) specified in IEEE 802.1X discovers mutually authenticated MACsec peers, and elects one as a Key Server that distributes the Symmetric Secure Association Keys (SAKs) used by MACsec to protect frames. The current standard is IEEE 802.1AE-2018.

Let’s go little deep into understand how this is done at the Ethernet frame level. Before that, let’s look at how a standard Ethernet frame structure would look like. Ignoring the Preamble and Start of Frame Delimiter (SFD), a standard IEEE 802.3 Ethernet frame structure would look like below.

Fig. 01 - Original Ethernet frame

Here, Payload (or Data) includes everything starting from the IP header.

A MACsec frame structure would be similar to the Ethernet frame structure, but includes additional frames/headers. 802.1 AE header is the main one. These frames are encrypted and protected with Integrity Check Value (ICV), which is added before the Frame Check Sequence (FCS) of the original Ethernet frame. 

Fig. 02 - MACsec frame

As we can see, though most of the frame is authenticated, only few fields including the payload is encrypted. The reason for not encrypting the source and destination MAC is obvious as it would prevent finding the paths and forwarding of frames in the Ethernet environment, if done otherwise.   

IPsec

IPsec coming from IP side, the standards are defined in multiple IETF RFCs. Similar to MACsec, IPsec also authenticates and/or encrypts the data (packets) sent over an IP network, acting as a secure network protocol. It includes protocols for establishing authentication between the two ends at the beginning of a session and negotiation of cryptographic keys to use during the session. 

With respect to the application, IPsec is usually end-to-end. This makes Virtual Private Networks (VPNs) possible across the public Internet for example. It can also be used on many different devices - routers, firewalls, hosts, servers, etc. Anyway, first we need two IPsec peers that build the IPsec tunnel. To establish an IPsec tunnel, we use IKE (Internet Key Exchange) protocol.

Even though IKE builds the tunnels, it doesn’t authenticate or encrypt user data. Therefore, we have to use two other protocols. Thus there are two types of IPsec security suits.

1.   ESP – Encapsulating Security Payload

2.   AH – Authentication Header

Both AH and ESP offer authentication and integrity. But only ESP supports encryption. Therefore, ESP is the most popular choice.

Both AH and ESP support two different operation modes:

1.   Tunnel Mode

2.   Transport Mode

Main difference between the two is that transport mode uses the original IP header while tunnel mode uses a new IP header. 

Let’s look at the IP header first. For simplicity, let’s consider IPv4 (note that application of IPsec is different in IPv4 and IPv6). Being in Layer 3, IP packet only has a header (unlike the Layer 2 Ethernet frame which has a header and a trailer). In fact, the payload in the Ethernet frame is the IP packet.

Fig. 03 – Original IP packet

Since the IP header has many fields, for simplicity, let’s represent all fields as IP Header. Here Data includes everything starting from the TCP header.

Fig. 04 – Simplified IP packet

Let’s try to understand the two IPsec operation modes now, together with ESP and AH protocols.

Tunnel Mode

Tunnel mode encapsulates the whole IP packet by either encrypting, authenticating or doing both. Encapsulation will be done with IPsec headers and trailers using ESP. While both AH and ESP supports authentication, the way it is performed is different. If encapsulation is performed, a new IP header is prepended to the original IP packet so that the information about the communication endpoints are not lost. 

However, the fields in the new IP header are not protected for obvious reasons, similar to what is explained under MACsec.

Below figure shows IPsec tunnel mode that uses ESP header:

Fig. 05 – ESP Tunnel Mode

Note that the whole IP packet is encrypted and a new IP header is added. Otherwise, we will not be able to perform IP forwarding and routing.

Here’s how the same with AH header:

Fig. 06 – AH Tunnel Mode

Note that there’s no encryption in AH mode.

Transport Mode

Unlike the Tunnel mode, Transport mode only protects the payload (data). No headers involved in encrypting and only the payload is encapsulated by IPsec header and trailer. The original IP header remains the same, but IP Protocol field is changed to 50 for ESP or 51 in case of AH. 

Since it is required to restore the packet once decrypted, the original protocol value will always be saved in IPsec trailer.

Below figure shows IPsec transport mode that uses ESP header:
 

Fig. 07 – ESP Transport Mode

Here’s how the same with AH header:

Fig. 08 – AH Transport Mode

Note that there’s no encryption in AH mode.

Following high level comparison can be made between MACsec and IPsec;

	MACsec	IPsec
OSI layer	2	3
Addressing involves	MAC	IP
Data exchange entity authentication	✓	✓
Encryption	✓	✓ (only in ESP)
Bandwidth overhead	Low	high
Delay	Low	high

I hope it’s now clear to you what is MACsec, what is IPsec, their applications, comparison between two, different options for authentication and encryption,  and some frame/packet level understanding as to how it is actually done. 

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Anuradha Udunuwara, ARC Tube  |  @AnuradhaU