Chapter 2. Network interfaces
This chapter provides an overview of the protocols and interfaces that allow TCP/IP traffic to flow over various kinds of physical networks. TCP/IP, as an internetwork protocol suite, can operate over a vast number of physical networks. The most common and widely used of these protocols is, of course, Ethernet. The number of network protocols that have provisions for natively supporting IP is clearly beyond the scope of this redbook. However, we provide a summary of some of the different networks most commonly used with TCP/IP.
2.1 Ethernet and IEEE 802.x Local Area Networks (LANs) Two frame formats can be used on the Ethernet coaxial cable: 1. The standard issued in 1978 by Xerox Corporation, Intel Corporation and Digital Equipment Corporation, usually called Ethernet (or DIX Ethernet). 2. The international IEEE 802.3 standard, a more recently defined standard.
See Figure 6 for more details.
The difference between the two standards is in the use of one of the header fields, which contains a protocol-type number for Ethernet and the length of the data in the frame for IEEE 802.3.
Dest Source Preamble Addr Addr Type Info FCS
8bytes 6bytes 6bytes 2bytes 46<=N<=1500 bytes 4 bytes
Ethernet IEEE 802.2 header Dest Source Preamble SFD Addr Addr Length DSAP SSAP Ctrl Info FCS 7 bytes1byte 6 bytes 6 bytes 2 bytes 1byte 1byte 1byte variable 4bytes
IEEE 802.3
3376\3376F2AE
Figure 6. ARP - Frame formats for Ethernet and IEEE 802.3
• The type field in Ethernet is used to distinguish between different protocols running on the coaxial cable, and allows their coexistence on the same physical cable.
© Copyright IBM Corp. 2001 29 • The maximum length of an Ethernet frame is 1526 bytes. This means a data field length of up to 1500 bytes. The length of the 802.3 data field is also limited to 1500 bytes for 10 Mbps networks, but is different for other transmission speeds. • In the 802.3 MAC frame, the length of the data field is indicated in the 802.3 header. The type of protocol it carries is then indicated in the 802.2 header (higher protocol level; see Figure 6). In practice, however, both frame formats can coexist on the same physical coax. This is done by using protocol type numbers (type field) greater than 1500 in the Ethernet frame. However, different device drivers are needed to handle each of these formats.
Thus, for all practical purposes, the Ethernet physical layer and the IEEE 802.3 physical layer are compatible. However, the Ethernet data link layer and the IEEE 802.3/802.2 data link layer are incompatible.
The 802.2 Logical Link Control (LLC) layer above IEEE 802.3 uses a concept known as link service access point (LSAP), which uses a 3-byte header, where DSAP and SSAP stand for destination and source service Access Point respectively. Numbers for these fields are assigned by an IEEE committee (see Figure 7).
1byte 1byte 1byte
DSAP SSAP Ctrl
Figure 7. ARP - IEEE 802.2 LSAP header
Due to a growing number of applications using IEEE 802 as lower protocol layers, an extension was made to the IEEE 802.2 protocol in the form of the Sub-Network Access Protocol (SNAP) (see Figure 8). It is an extension to the LSAP header in Figure 7, and its use is indicated by the value 170 in both the SSAP and DSAP fields of the LSAP frame Figure 7.
3bytes 2bytes
Prot.ID or org.code Ethertype
Figure 8. ARP - IEEE 802.2 SNAP header
In the evolution of TCP/IP, three standards were established that describe the encapsulation of IP and ARP frames on these networks:
30 TCP/IP Tutorial and Technical Overview 1. Introduced in 1984, RFC 894 – Standard for the Transmission of IP Datagrams over Ethernet Networks specifies only the use of Ethernet type of networks. The values assigned to the type field are: a. 2048 (hex 0800), for IP datagrams b. 2054 (hex 0806), for ARP datagrams 2. Introduced in 1985, RFC 948 – Two Methods for the Transmission of IP Datagrams over IEEE 802.3 Networks specifies two possibilities: a. The Ethernet compatible method: The frames are sent on a real IEEE 802.3 network in the same fashion as on an Ethernet network, that is, using the IEEE 802.3 data-length field as the Ethernet type field, thereby violating the IEEE 802.3 rules, but compatible with an Ethernet network. b. IEEE 802.2/802.3 LLC type 1 format: Using 802.2 LSAP header with IP using the value 6 for the SSAP and DSAP fields. The RFC indicates clearly that the IEEE 802.2/802.3 method is the preferred method, that is, that all future IP implementations on IEEE 802.3 networks are supposed to use the second method. 3. Introduced in 1987, RFC 1010 – Assigned Numbers (now obsoleted by RFC 1700, dated 1994) notes that as a result of IEEE 802.2 evolution and the need for more Internet protocol numbers, a new approach was developed based on practical experiences exchanged during the August 1986 TCP Vendors Workshop. It states, in an almost completely overlooked part of this RFC, that all IEEE 802.3, 802.4, and 802.5 implementations should use the Sub-Network Access Protocol (SNAP) form of the IEEE 802.2 LLC, with the DSAP and SSAP fields set to 170 (indicating the use of SNAP), with SNAP assigned as follows: a. 0 (zero) as organization code. b. EtherType field: 1. 2048 (hex 0800), for IP datagrams 2. 2054 (hex 0806), for ARP datagrams 3. 32821 (hex 8035), for RARP datagrams These are the same values used in the Ethernet type field. 4. In 1988, RFC 1042 – Standard for the Transmission of IP Datagrams over IEEE 802 Networks was introduced. As this new approach (very important for implementations) passed almost unnoticed in a little note of an unrelated RFC, it became quite confusing, and finally, in February 1988, it was repeated in an RFC on its own: RFC 1042, which obsoletes RFC 948.
Chapter 2. Network interfaces 31 The relevant IBM TCP/IP products implement RFC 894 for DIX Ethernet and RFC 1700 for IEEE 802.3 networks.
However, in practical situations, there are still TCP/IP implementations that use the older LSAP method (RFC 948 or 1042). Such implementations will not communicate with the more recent implementations (such as IBM's).
Also note that the last method covers not only the IEEE 802.3 networks, but also the IEEE 802.4 and 802.5 networks, such as the IBM Token-Ring LAN.
2.2 Fiber Distributed Data Interface (FDDI) The FDDI specifications define a family of standards for 100 Mbps fiber optic LANs that provides the physical layer and media access control sublayer of the data link layer, as defined by the ISO/OSI Model.
IP-FDDI is a draft-standard protocol. Its status is elective. It defines the encapsulating of IP datagrams and ARP requests and replies in FDDI frames. Figure 9 shows the related protocol layers.
It is defined in RFC 1188 – A Proposed Standard for the Transmission of IP Datagrams over FDDI Networks for single MAC stations. Operation on dual MAC stations will be described in a forthcoming RFC.
RFC 1188 states that all frames are transmitted in standard IEEE 802.2 LLC Type 1 Unnumbered Information format, with the DSAP and SSAP fields of the 802.2 header set to the assigned global SAP value for SNAP (decimal 170). The 24-bit Organization Code in the SNAP header is set to zero, and the remaining 16 bits are the EtherType from Assigned Numbers (see RFC 1700), that is: • 2048 for IP • 2054 for ARP
The mapping of 32-bit Internet addresses to 48-bit FDDI addresses is done via the ARP dynamic discovery procedure. The broadcast Internet addresses (whose
IP datagrams are transmitted as series of 8-bit bytes using the usual TCP/IP transmission order called big-endian or network byte order.
The FDDI MAC specification (ISO 9314-2 - ISO, Fiber Distributed Data Interface - Media Access Control) defines a maximum frame size of 4500 bytes for all frame fields. After taking the LLC/SNAP header into account, and
32 TCP/IP Tutorial and Technical Overview