TCP/IP Illustrated Episode 19

802.11 Higher Throughput/802.11n

In late 2009, the IEEE standardized 802.11n [802.11n-2009] as an amendment to [802.11-2007]. It makes a number of important changes to 802.11. To support higher throughput, it incorporates support for multiple input, multiple output (MIMO) management of multiple simultaneously operating data streams carried on multiple antennas, called spatial streams. Up to four such spatial streams are supported on a given channel. 802.11n channels may be 40MHz wide (using two adjacent 20MHz channels), twice as wide as conventional channels in 802.11a/b/g/y. Thus, there is an immediate possibility of having up to eight times the maximum data rate of 802.11a/g (54Mb/s), for a total of 432Mb/s. However, 802.11n also improves the single-stream performance by using a more efficient modulation scheme (802.11n uses MIMO- orthogonal frequency division multiplexing (OFDM) with up to 52 data subcarriers per 20MHz channel and 108 per 40MHz channel, instead of 48 in 802.11a and 802.11g), plus a more efficient forward error-correcting code (rate 5/6 instead of 3/4), bringing the per-stream performance to 65Mb/s (20MHz channel) or 135Mb/s (40MHz channel). By also reducing the guard interval (GI, a forced idle time between symbols) duration to 400ns from the legacy 800ns, the maximum per-stream performance is raised to about 72.2Mb/s (20MHz channel) and 150Mb/s (40MHz channel). With four spatial streams operating in concert perfectly, this provides a maximum of about 600Mb/s.

Wi-Fi Security

There has been considerable evolution in the security model for 802.11 networks. In its early days, 802.11 used an encryption method known as wired equivalent privacy (WEP). WEP was later shown to be so weak that some replacement was required. Industry responded with Wi-Fi protected access (WPA), which replaced the way keys are used with encrypted blocks (see Chapter 18 for the basics of cryptography). In WPA, a scheme called the Temporal Key Integrity Protocol (TKIP) ensures, among other things, that each frame is encrypted with a different encryption key. It also includes a message integrity check, called Michael, that fixed one of the major weaknesses in WEP. WPA was created as a placeholder that could be used on fielded WEP-capable equipment by way of a firmware upgrade while the IEEE 802.11i standards group worked on a stronger standard that was ultimately absorbed into Clause 8 of [802.11-2007] and dubbed “WPA2” by industry. Both WEP and WPA use the RC4 encryption algorithm [S96]. WPA2 uses the Advanced Encryption Standard (AES) algorithm [AES01].

With the completion of the IEEE 802.11i group’s work, the RC4/TKIP combination in WPA was extended with a new algorithm called CCMP as part of WPA2. CCMP is based on using the counter mode (CCM [RFC3610]) of the AES for confidentiality with cipher block chaining message authentication code (CBC-MAC; note the “other” use of the term MAC here) for authentication and integrity. All AES processing is performed using a 128-bit block size and 128-bit keys. CCMP and TKIP form the basis for a Wi-Fi security architecture named the Robust Security Network (RSN), which supports Robust Security Network Access (RSNA). Earlier methods, such as WEP, are called pre-RSNA methods. RSNA compliance requires support for CCMP (TKIP is optional), and 802.11n does away with TKIP entirely. Table 3-4 provides a summary of this somewhat complicated situation.

In Key Reinstallation Attacks, the current technology of Wi-Fi is proven not situation for the security factors on current situation.

Wi-Fi Mesh (802.11s)

The IEEE is working on the 802.11s standard, which covers Wi-Fi mesh operation. With mesh operation, wireless stations can act as data-forwarding agents (like APs). The standard is not yet complete as of writing (mid-2011). The draft version of 802.11s defines the Hybrid Wireless Routing Protocol (HWRP), based in part on the IETF standards for Ad-Hoc On-Demand Distance Vector (AODV) routing [RFC3561] and the Optimized Link State Routing (OLSR) protocol [RFC3626]. Mesh stations (mesh STAs) are a type of QoS STA and may participate in HWRP or other routing protocols, but compliant nodes must include an implementation of HWRP and the associated airtime link metric. Mesh nodes coordinate using EDCA or may use an optional coordinating function called mesh deterministic access. Mesh points (MPs) are those nodes that form mesh links with neighbors. Those that also include AP functionality are called mesh APs (MAPs). Conventional 802.11 stations can use either APs or MAPs to access the rest of the wireless LAN.

Point-to-Point Protocol (PPP)

PPP stands for the Point-to-Point Protocol [RFC1661][RFC1662][RFC2153]. It is a popular method for carrying IP datagrams over serial links—from low-speed dial-up modems to high-speed optical links [RFC2615]. It is widely deployed by some DSL service providers, which also use it for assigning Internet system parameters (e.g., initial IP address and domain name server; see Chapter 6).

Link Control Protocol (LCP)

The LCP portion of PPP is used to establish and maintain a low-level two-party communication path over a point-to-point link. PPP’s operation therefore need be concerned only with two ends of a single link; it does not need to handle the problem of mediating access to a shared resource like the MAC-layer protocols of Ethernet and Wi-Fi.

Note

There has been considerable debate over the years as to how much reliability link-layer networks should provide, if any. With Ethernet, up to 16 retransmission attempts are made before giving up. Typically, PPP is configured to do no retransmission, although there do exist specifications for adding retransmission [RFC1663]. The trade-off can be subtle and is dependent on the types of traffic to be carried. A detailed discussion of the considerations is contained in [RFC3366].