3. Lower Layer Behavior

3.1. Lower Layer Requirements

EAP makes the following assumptions about lower layers:

[1] Unreliable transport. In EAP, the authenticator retransmits

Requests that have not yet received Responses so that EAP does not assume that lower layers are reliable. Since EAP defines its own retransmission behavior, it is possible (though undesirable) for retransmission to occur both in the lower layer and the EAP layer when EAP is run over a reliable lower layer.

Note that EAP Success and Failure packets are not retransmitted. Without a reliable lower layer, and with a non-negligible error rate, these packets can be lost, resulting in timeouts. It is therefore desirable for implementations to improve their resilience to loss of EAP Success or Failure packets, as described in Section 4.2.

[2] Lower layer error detection. While EAP does not assume that the

lower layer is reliable, it does rely on lower layer error detection (e.g., CRC, Checksum, MIC, etc.). EAP methods may not include a MIC, or if they do, it may not be computed over all the fields in the EAP packet, such as the Code, Identifier, Length, or Type fields. As a result, without lower layer error detection, undetected errors could creep into the EAP layer or EAP method layer header fields, resulting in authentication failures.

For example, EAP TLS [RFC2716], which computes its MIC over the Type-Data field only, regards MIC validation failures as a fatal error. Without lower layer error detection, this method, and others like it, will not perform reliably.

[3] Lower layer security. EAP does not require lower layers to

provide security services such as per-packet confidentiality, authentication, integrity, and replay protection. However, where these security services are available, EAP methods supporting Key Derivation (see Section 7.2.1) can be used to provide dynamic keying material. This makes it possible to bind the EAP authentication to subsequent data and protect against data modification, spoofing, or replay. See Section 7.1 for details.

[4] Minimum MTU. EAP is capable of functioning on lower layers that

provide an EAP MTU size of 1020 octets or greater.

EAP does not support path MTU discovery, and fragmentation and reassembly is not supported by EAP, nor by the methods defined in this specification: Identity (1), Notification (2), Nak Response (3), MD5-Challenge (4), One Time Password (5), Generic Token Card (6), and expanded Nak Response (254) Types.

Typically, the EAP peer obtains information on the EAP MTU from the lower layers and sets the EAP frame size to an appropriate value. Where the authenticator operates in pass-through mode, the authentication server does not have a direct way of determining the EAP MTU, and therefore relies on the authenticator to provide it with this information, such as via the Framed-MTU attribute, as described in [RFC3579], Section 2.4.

While methods such as EAP-TLS [RFC2716] support fragmentation and reassembly, EAP methods originally designed for use within PPP where a 1500 octet MTU is guaranteed for control frames (see [RFC1661], Section 6.1) may lack fragmentation and reassembly features.

EAP methods can assume a minimum EAP MTU of 1020 octets in the absence of other information. EAP methods SHOULD include support for fragmentation and reassembly if their payloads can be larger than this minimum EAP MTU.

EAP is a lock-step protocol, which implies a certain inefficiency when handling fragmentation and reassembly. Therefore, if the lower layer supports fragmentation and reassembly (such as where EAP is transported over IP), it may be preferable for fragmentation and reassembly to occur in the lower layer rather than in EAP. This can be accomplished by providing an artificially large EAP MTU to EAP, causing fragmentation and reassembly to be handled within the lower layer.

[5] Possible duplication. Where the lower layer is reliable, it will

provide the EAP layer with a non-duplicated stream of packets. However, while it is desirable that lower layers provide for non-duplication, this is not a requirement. The Identifier field provides both the peer and authenticator with the ability to detect duplicates.

[6] Ordering guarantees. EAP does not require the Identifier to be

monotonically increasing, and so is reliant on lower layer ordering guarantees for correct operation. EAP was originally defined to run on PPP, and [RFC1661] Section 1 has an ordering requirement:

"The Point-to-Point Protocol is designed for simple links which transport packets between two peers. These links provide full-duplex simultaneous bi-directional operation, and are assumed to deliver packets in order."

Lower layer transports for EAP MUST preserve ordering between a source and destination at a given priority level (the ordering guarantee provided by [IEEE-802]).

Reordering, if it occurs, will typically result in an EAP authentication failure, causing EAP authentication to be re-run. In an environment in which reordering is likely, it is therefore expected that EAP authentication failures will be common. It is RECOMMENDED that EAP only be run over lower layers that provide ordering guarantees; running EAP over raw IP or UDP transport is

NOT RECOMMENDED. Encapsulation of EAP within RADIUS [RFC3579] satisfies ordering requirements, since RADIUS is a "lockstep" protocol that delivers packets in order.

3.2. EAP Usage Within PPP

In order to establish communications over a point-to-point link, each end of the PPP link first sends LCP packets to configure the data link during the Link Establishment phase. After the link has been established, PPP provides for an optional Authentication phase before proceeding to the Network-Layer Protocol phase.

By default, authentication is not mandatory. If authentication of the link is desired, an implementation MUST specify the Authentication Protocol Configuration Option during the Link Establishment phase.

If the identity of the peer has been established in the Authentication phase, the server can use that identity in the selection of options for the following network layer negotiations.

When implemented within PPP, EAP does not select a specific authentication mechanism at the PPP Link Control Phase, but rather postpones this until the Authentication Phase. This allows the authenticator to request more information before determining the specific authentication mechanism. This also permits the use of a "backend" server which actually implements the various mechanisms while the PPP authenticator merely passes through the authentication exchange. The PPP Link Establishment and Authentication phases, and the Authentication Protocol Configuration Option, are defined in The Point-to-Point Protocol (PPP) [RFC1661].

3.2.1. PPP Configuration Option Format

A summary of the PPP Authentication Protocol Configuration Option format to negotiate EAP follows. The fields are transmitted from left to right.

Exactly one EAP packet is encapsulated in the Information field of a PPP Data Link Layer frame where the protocol field indicates type hex C227 (PPP EAP).

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

Length

Authentication Protocol

C227 (Hex) for Extensible Authentication Protocol (EAP)

3.3. EAP Usage Within IEEE 802

The encapsulation of EAP over IEEE 802 is defined in [IEEE-802.1X]. The IEEE 802 encapsulation of EAP does not involve PPP, and IEEE 802.1X does not include support for link or network layer negotiations. As a result, within IEEE 802.1X, it is not possible to negotiate non-EAP authentication mechanisms, such as PAP or CHAP [RFC1994].

3.4. Lower Layer Indications

The reliability and security of lower layer indications is dependent on the lower layer. Since EAP is media independent, the presence or absence of lower layer security is not taken into account in the processing of EAP messages.

To improve reliability, if a peer receives a lower layer success indication as defined in Section 7.2, it MAY conclude that a Success packet has been lost, and behave as if it had actually received a Success packet. This includes choosing to ignore the Success in some circumstances as described in Section 4.2.

A discussion of some reliability and security issues with lower layer indications in PPP, IEEE 802 wired networks, and IEEE 802.11 wireless LANs can be found in the Security Considerations, Section 7.12.

After EAP authentication is complete, the peer will typically transmit and receive data via the authenticator. It is desirable to provide assurance that the entities transmitting data are the same ones that successfully completed EAP authentication. To accomplish

this, it is necessary for the lower layer to provide per-packet integrity, authentication and replay protection, and to bind these per-packet services to the keys derived during EAP authentication. Otherwise, it is possible for subsequent data traffic to be modified, spoofed, or replayed.

Where keying material for the lower layer ciphersuite is itself provided by EAP, ciphersuite negotiation and key activation are controlled by the lower layer. In PPP, ciphersuites are negotiated within ECP so that it is not possible to use keys derived from EAP authentication until the completion of ECP. Therefore, an initial EAP exchange cannot be protected by a PPP ciphersuite, although EAP re-authentication can be protected.

In IEEE 802 media, initial key activation also typically occurs after completion of EAP authentication. Therefore an initial EAP exchange typically cannot be protected by the lower layer ciphersuite, although an EAP re-authentication or pre-authentication exchange can be protected.