• A learned database containing add resses indi cating on which side of the bridge each MAC is
located and special filtering status int()rmation assigned ro each address by network management The gate array bu rster (GAR) al lows the queue manager to access the
FDDI
packet memory. This a ppl ication-specific i ntegrated circuit (ASIC) is a specialized direct memory access(DMA)
device. It is capable of moving selected fields or large sec t ions of frames into or out ofFDDI
packet memory 'fhe objects may be moved e it her into i n ternal hold ing registe rs for examination by the queue manager engi ne or d irectly to destinati ons such as registers in the table lookup engi neClUJ).
Note that the GAB used in the queue manager subsystemis the same device used in the translat ion proces sor, wh ich is discussed later in this paper. These two subsystems have many simi lar requirements. but each also has u nique requ irements. Using one
c;AB design for both subsystems reduced the over a l l development effort.
58
The table address
RAJ\'1
and the TLU are key com ponents of the queue manager. The RAJ\'1 contains a table of up to !6K 4H -bit add resses. Each address also has statu s bits that dete r m i ne what action the bridge should take when a frame's source or dest i nation add ress matches a particular address. The TLU is an ASIC with a port that is a slave to the queue manager engine. The queue manager engine inputs an address to the TLU which scans thelv\..\1 for that add ress. If the address i s found, the
TLU presents that status to the queue manage r pro cessor. Otherwise, t he TLl l gives the queue man ager processor a programmable status indicating whether to forward or to discard the frame. A sec ond TLl l port a llows the Tl.l l and the table address
RAM
to serve as s laves to the A P . Thus, destination address fi ltering for t raffic received from the Ethernet/H02 . :) LAN and table mai ntenance can be performed.To k eep up with the packet arrival rate, the queue manager subsystem makes extensive use of pipcli n ing. The queue manager engi ne operates
concu rrent l y on six packets. The TLU u n i t per
forms three searches concu rren tly: one each for the sou rce and destination addresses o n FOOl packets and one source or destination search on Ethernet/802.3 packets.
Discarding and Keeping Frames The decision to
discard a frame is based principa l l y on the frame's address or its contents. The fol lowing are typical of frames t hat are discarded:
• Frames destined for nodes that the bridge rec ognizes as not on the Ethernet/802.3 side of the LAN . Also, network m anagement may specify addresses to be d iscarded regardless of location in the topology.
• Frames of either a reserved or u ndefi ned frame control type.
• Frames that are either too long or too short. When a frame is d iscarded, its buffers are returned
to the end of the receive queue by reassign i ng them in the page table.
Frames that are kept are placed on either the for ward or bridge queues. Frames ulti mately destined for the Ethernet/802.3 LAN are placed on the for ward queue. Frames placed on t he bridge queue, to be processed interna.l l y by the bridge, are of the following types:
• FDDI station management (SMT) frames
• D igital's extended LAN management software
(DECelms) frames or m a i ntenance operati o n protocol (MOP) frames
• Spanni ng tree frames, cont a i n i ng messages used to determ ine the network topology and turn incl i vidual b ridge ports on or off to eliminate path redundancy
• Frames conta i n i ng errors
Frames placed on the bridge queue not forwarded to the Ethernet/802.3 LAN. However, after receiving and processing these frames, the bridge may gener ate one or more frames on either or both LANs. For example, received SMT frames are never forwarded, but a given SMT frame may cause t he bridge to t ransm i t additional SMT frames on the FDDI LAN. Counters Each frame type is guaranteed a m ini mum a mount of processing time by the bridge. If at any time the bridge holds too many of any one frame type, it d iscards further frames of t hat type. The queue manager uses al location cou n ters to keep
Digital Teclmicaljournal vbl. 3 No. 2 Spring 1991
Deuelopment of the DECbridge 500 Product
track of the number of fonvarded, FDDJ SNIT, bridge management, spanning t ree, and e rror frames.
The queue manager also has counters that sum marize its activity. These counters are periodically dumped to the AP and are used to calculate LAN uti l ization statistics required by network management.
Translation
Bridges operate at and below the data l ink level i n the seven-layer I nternational Sta ndards Organiza tion (ISO)/Open System Interconnection (OS!) ref erence m odel shown i n Figure 6. The data l i nk layer is d ivided into a lower MAC sublayer and an upper logical l i nk control (LLC) sublayer. The LJ.C protocol is specified i n ANSI/IEEE standard 802.2.'
DATA L I N K
{
LAYER APPLICATION P R ESENTATION SESSION TRANSPORT N ETWORKLOGICAL LINK CONTROL - - - - M E DIA ACCESS CONTROL
PHYSICAL
Figm·e 6
150/051
Seven-layer Reference ModelWhen forwarding fra mes from o n e LAN to another, t he DECbridge 500 device converts the outgoing frame to the MAC frame format of that LAN . This process is called translation. Also, when a frame is generated by the DECbridge 500 product on either LAN, the data l i nk frame format of that IAN is employed.
By performi ng translation, the DECbridge 500 product complies with the IEEE 802 . l d require ments for t ransparent bridging. This enables end nodes to communicate across the extended LA N as if the nodes are d i rectly connected to the same LAt'\J . An a l ternative to t ranslat ion, cal led e ncapsu lation, is possible, but i t does not comply with the IEEE 802 . l d requirements. Further, u s i ng encap sulat i o n puts restrictions on the configuration of the network.
The Process Ethernet, 802.3, and FDDI have d if ferent MAC frame formats. When Ethernet or 802.3 frames are bridged to an FDDI LA N, they are refor matted to the FDDT MAC frame format. The original
F iber Distributed Data Interface
MAC type (Ethernet or 802.3) is i ndicated by setting i nformation in the LLC header. If the frame passes through a second FDDI-to-Ethernet/802.3 transla tion at another bridge, the LLC i nformation is used to determine if the bridge should translate the frame i n to Ethernet or 802.3 MAC protocol . IEEE standard 802.1 defines a mechanism for translating E thernet frames i nto an IEEE 802.2 format (as is u sed on FDDI LANs). Figure 7 i ll ustrates how Ethernet frames and two types of 802.3 I.LC frames are translated i n to three differen t types of FDDI data l i nk frames.
Maximum and m i nimum frame sizes of the LAJ"fs also impose requirements on the translation pro cess. Ethernet and 802.3 MAC protocols require a m i nimum data field length of 46 bytes. The FDDI MAC protocol supports zero-length data fields. When a bridge forwards frames that originated at nodes on an FDDI LAN to an Ethernet/802.3 LAN , the translation process m u st add padd i ng ( n u l l bytes) t o a n y short data fields to bring t hem up to the 46-byte m inimum size.
FDDI has a maximum frame s ize of 4500 bytes. The Ethernet/802.3 max i m u m frame s ize is 1518
ETHERN ET/802 3 FRAME FORMAT
bytes. Frames received from the FDDI ring that are longer than 1518 bytes after translation are d is carded with the exception of frames d iscussed i n t h e text that fol l ows.
The i nternet protocol (IP) is a widely used, network-layer protocol. Nodes o n FDDI rings may generate IP frames longer than the Ethernet/802.3 maxi mum size. The DECbridge 500 product per forms one function beyond the process of t rans parent bridgi ng. The bridge b reaks up large I P packets i nto s m a ller o nes. This function is sup ported by I P and is called fragmentation. Without fragmentation, the queue manager would discard these long IP frames, preventing com m u nication between nodes on separate FDDI rings that are l inked by Ethernet/802.3 LANs.
Since the translation process a lters frames, the original cyclic redundancy check (CRC) field is no longer valid. I n the DECbridge 500 device, the trans lation process concurrently verifies the received CRC, t ranslates the frame, and gen erates a new CRC. This concu rrent processing resul ts in a high degree of data i n tegrity.
FOOl FRAME FORMAT
ETHERNET V20 F D D ' SNmSAP
DATA SNAP SNAP CTRL O U I TYPE DATA
= AA = AA = 03 = 00,00,00
802.2 LLC DSAP/SSAP
DSAP SSAP CTRL DATA
?' AA ?' AA ?' 03 ?' 00,00,00
FDDI LLC
DSAP SSAP C T R L DATA ?' AA 7' AA 7' 03 ?' 00,00,00
802.2 LLC S N A P/SAP
= AA = AA = 03 ?' 00,00,00 = AA = AA = 03 7' 00.00,00
K E Y :
DA, DA' DESTINATION A D D R ESS ( ' = B IT REVERSED FORMAT) SA, SA' SOURCE ADDR ESS ( ' = B I T REVERSED FORMAT) TYPE/LEN TYPE F I E L D FOR E T H E R N ET, LENGTH F I ELDS FOR 802.3
FC FRAME CONTROL
E D E N D D E L I M ITER
C R C CYCLIC R E DU N DANCY C H E C K
O U I ORGANIZATIONAL ( V E N D O R ) U N IQUE I D E NTITY CODE
DATA DATA FIELD
FOR DSAP. SSAP. SNAP. SAP. CTRL. P I D . AA. A N D TYPE. R E F E R TO 802.2 STANDA R D 3
Figure 7 Translation of Ethernet/802.3 Frames to FDDJ Frames
A ddress Bit-ordering The bits of the desti nation and source addresses are transmined in the reverse order on FDDI from that on Ethernet/802.3 data l i nks. Digital's F D D I chip set performs a bit-reversal operation on receive and transmit for only the MAC frames' destination and source address fields. Since these MAC fields are stored i ns ide the bridge i n
IEEE
802 . 1 , canonical bit-ordering, only one ve rsion of each address needs to be kept in the forward ing data base. Also, when generat ing management messages, this method of frame storage al lows the bridge to move an address from the source or desti nation field of a received frame into the data field of the manage ment message without modification. When calcu lating the CRC on i ncom i ng packets, or generating a new CRC on forwarded packets, the translation process must take into acco unt the bit-ordering. Implementation The translation processor con sists of principa lly a GAB and a t ranslation engine (based on a 68020 subsystem). The GAB and t ransla tion engi ne i n teractivel y copy frames from FDD!frame memory to Ethernet/802.3 frame memory. Concurrently, the translation engi ne makes changes to the frame format, and the GAB calculates both
the CRC of the i ncomi ng frame, using old bit-order i ng, and generates the CRC of the t ranslated frame, using both new bit -ordering and new frame format.
Frames from the forward queue i n FDD! frame memory are, t hus, t ranslated and moved to the for ward queue i n E thernet frame memory. Frames from the bridge queue are separated i nto manage ment and spanning t ree queues i n Ethernet/802.3 frame memory. The translation processor returns buffers from the forward and bridge queues to the free queue in FOOl frame memory. The queue man ager returns buffers from the free queue to the receive queue, making them ava i lable to store newly received frames.