Transmission of IPv4 Packets over the IP Convergence Sublayer of IEEE 802.16
Extracted elements (18)
IPv4 lacks a reliable mechanism analogous to IPv6 Neighbor Discovery MTU options for advertising link MTU to hosts. Legacy IPv4 clients do not determine link MTU before sending packets, and DHCP servers only provide MTU information when explicitly requested, making MTU misconfiguration a significant practical concern.
The default MTU is set to 1500 octets rather than the WiMAX Forum's 1400 octets because this document represents the IETF working group's consensus on the best value for IEEE 802.16 from the IETF perspective, despite the WiMAX Forum's deployment practice. The WiMAX MTU mismatch is a known interoperability concern acknowledged but accepted.
If an MS transmits 1500-octet packets in a deployment with a smaller MTU, packets may be dropped at the link layer silently because IPv4 (unlike IPv6 with ND MTU options) has no reliable automatic mechanism for an MS to discover the link MTU before sending.
The WiMAX Forum specifies an IP MTU of 1400 octets for IPv4 CS, whereas this specification defaults to 1500 octets. An IPv4 CS MS configured with 1500-octet MTU entering a WiMAX network may experience silent packet drops, and an IPv4 CS client cannot use ARP probing to discover the link MTU.
Two MSs using different Convergence Sublayers (e.g., one using Ethernet CS and another using IPv4 CS) cannot communicate at the data link layer and require interworking at the IP layer; they must be configured on different subnets (see RFC 4840).
DHCP (RFC 2131) SHOULD be used for assigning an IPv4 address for the MS. DHCP messages are transported over the IEEE 802.16 MAC connection to and from the BS and relayed to the AR.
Each MS shall be assigned an address with a 32-bit prefix length or subnet mask, reflecting the point-to-point link model between the MS and the AR.
If the DHCP server does not reside in the AR, the AR SHOULD implement a DHCP relay agent (RFC 1542).
Implementations of IPv4 and IPv4 CS clients SHOULD use the DHCP interface MTU option (RFC 2132) in order to configure the interface MTU accordingly.
The default MTU for IPv4 packets over an IEEE 802.16 link SHOULD be 1500 octets.
The DHCP relay agent in the AR MUST maintain the tunnel interface on which it receives DHCP requests so that it can relay the DHCP responses to the correct MS.
ARP (RFC 826) packets cannot be transmitted over the IPv4 CS. In the point-to-point link model, address resolution is not needed.
IPv4 CS classifies packets based on upper-layer header fields (source and destination IPv4 addresses, source and destination ports, Type-of-Service, and IP Protocol field) to place the packet on one of the available connections identified by the CID. The CS may employ Packet Header Suppression (PHS) after classification.
Link establishment for IPv4 CS follows RFC 5121 Section 6.2 steps 1–4, with step 5 indicating IPv4 support and step 6 creating a service flow for IP-layer signaling such as DHCP-based address assignment.
The IPv4 CS link is a point-to-point link between the MS and the AR. When the AR and BS are co-located, the collection of transport connections to an MS forms a single IPv4 link. When separated, a tunnel (e.g., GRE) with granularity no greater than per-MS or per-service-flow must be established between AR and BS.
The 'next hop' IP address for an IPv4 CS MS is always the IP address of the AR, because the MS and AR are attached via a point-to-point link.
This document does not introduce new vulnerabilities to IPv4 specifications or operation. Security of the IEEE 802.16 air interface is governed by IEEE Std 802.16. Security of network architectures beyond the BS is governed by architecture documents such as the WiMAX network architecture specification.
IPv4 packets are transmitted in Generic IEEE 802.16 MAC frames. The MAC header contains H (1 bit, Header Type), E (Encryption Control), TYPE (subheader indicators), R (Reserved), C (CRC Indicator), EKS (Encryption Key Sequence), LEN (11 bits, length of MAC PDU in bytes including header and CRC), CID (16 bits, Connection Identifier), HCS (8 bits, Header Check Sequence), followed by the IPv4 header and payload, and an optional 8-bit CRC appended after encryption.