A 50-Gb/s IP Router Authors: Craig Partridge et al. IEEE/ACM TON June 1998 Presenter: Srinivas R. Avasarala CS Dept., Purdue University

S.R.AvasaralaCS Dept., Purdue University2 Why a Gigabit Router? Transmission link bandwidths are improving at very fast rates Network usage is expanding Host Adapters, OS, switches and MUX also need to get faster for improved network performance The goal of the work is to show routers can keep pace with other technologies

S.R.AvasaralaCS Dept., Purdue University3 Goals of a Multi-Gigabit Router (MGR) 1. Enough internal bandwidth to move packets between its interfaces at gigabit rates 2. Enough packet processing power to forward several million packets per second (MPPS) 3. Conformance to protocol standards MGR achieves up to 32 MPPS forwarding rates with 50 Gb/s of full duplex backplane capacity

S.R.AvasaralaCS Dept., Purdue University4 Router Architecture Multiple line cards, each with one or more network interfaces Forwarding Engine cards (FEs), to make packet forwarding decisions High speed switch Network processor

S.R.AvasaralaCS Dept., Purdue University5 Router Architecture

S.R.AvasaralaCS Dept., Purdue University6 Major Innovations 1. Each FE has a complete set of the routing tables 2. A switched fabric is used instead of the traditional shared bus 3. FEs are on boards distinct from the line cards 4. Use of an abstract link layer header 5. Include QoS processing in the router

S.R.AvasaralaCS Dept., Purdue University7 The Forwarding Engine Processor A 415-MHz DEC Alpha processor 64bit 32 register super scalar RISC processor 2 Integer logic units E0, E1 2 Floating point logic units FA, FM Each cycle schedules one instruction to each logic unit, processing 4 instructions (quad) in a group

S.R.AvasaralaCS Dept., Purdue University8 Forwarding Engine’s Caches 3 internal caches First level instruction cache (Icache) 8kB First-level data cache (Dcache) 8kB An on-chip secondary cache (Scache) 96kB used as a cache of recent routes. Can store routes approx. with 64b per route An external tertiary cache (Bcache) 16 MB Divided into two 8MB banks One bank stores entire forwarding table Other is updated by NW processor via PCI bus

S.R.AvasaralaCS Dept., Purdue University9 Forwarding Engine Hardware Headers are placed in a request FIFO queue Alpha reads from queue head, examines header, makes route decision and informs inbound card Header includes 24/56B of packet + 8B abstract link layer header and alpha reads a min of 32B Alpha writes out the updated header indicating the outbound interface to use (dispatching info) Updated header contains the outbound link layer address and a flow id used for packet scheduling Unique approach to ARP !!

S.R.AvasaralaCS Dept., Purdue University10 Forwarding Engine Software A few 100 lines of code 85 instructions in the common case taking a minimum of 42 cycles. This gives a peak forwarding rate of 9.8MPPS 415 MHz/42 cycles ~ 9.8MPPS Fast path of the code is in 3 stages, each with about instructions (10-15 cycles)

S.R.AvasaralaCS Dept., Purdue University11 Fast path of the code Stage 1: 1. Basic error checking to see if header is from a IP datagram 2. Confirm packet/header lengths are reasonable 3. Confirm that IP header has no options 4. Compute hash offset into route cache and load the route 5. Start loading of next header

S.R.AvasaralaCS Dept., Purdue University12 Fast path of the code Stage 2 1. Check if cached route matches destination of the datagram 2. If not then do an extended lookup in the route table in Bcache 3. Update TTL and CHECKSUM fields Stage 3 1. Put updated ttl, checksum and the route information into IP hdr along with link layer info from the forwarding table

S.R.AvasaralaCS Dept., Purdue University13 An exception !! IP HDR checksum is not verified but only updated The incremental update algorithm is safe because if the checksum is bad, it remains bad Reason: Checksum verification is expensive and is a large penalty to pay for a rare error that can be caught end-to-end Requires 17 instructions with min of 14 cycles, increasing forwarding time by 21% IPv6 does not include a header checksum too!!

S.R.AvasaralaCS Dept., Purdue University14 Some datagrams not handled in fast path 1. Headers whose destination misses in the cache 2. Headers with errors 3. Headers with IP options 4. Datagrams that require fragmentation 5. Multicast datagrams  Requires multicast routing which is based on source address and inbound link as well  Requires multiple copies of header to be sent to different line cards

S.R.AvasaralaCS Dept., Purdue University15 Instruction set 27% of them do bit, byte or word manipulation due to extraction of various fields from headers The above instructions can only be done in E0, resulting in contention (checksum verifying) Floating point instructions account for 12% but do not have any impact on performance as they only set SNMP values and can be interleaved There is a minimum of loads(6) and stores(4)

S.R.AvasaralaCS Dept., Purdue University16 Issues in forwarding design Why not use an ASIC in place of the engine ? Since IP protocol is stable, why not do it ? Answer depends on where the router will be deployed: corporate LAN or ISP’s backbone? How effective is a route cache ? A full route lookup is 5 times more expensive than a cache hit. So we need modest hit rates. And modest hit rates seem to be assured because of packet trains

S.R.AvasaralaCS Dept., Purdue University17 Abstract link layer header Designed to keep the forwarding engine and its code simple

S.R.AvasaralaCS Dept., Purdue University18 The Switched bus Instead of the conventional shared bus, MGR uses a 15-port point-to-point switch Limitation of a point-to-point switch is that it does not support one to many transfers The switch has 2 interfaces to each function card Data Interface: 75 input, 75 output pins clocked at MHz Allocation Interface: 2 request pins, 2 inhibit pins, 1 input status pin and 1 output status pin clocked at MHz

S.R.AvasaralaCS Dept., Purdue University19 Data transfer in the switch An epoch is 16 ticks of data clock (8 allocation clk) Up to 15 simultaneous transfers in an epoch Each transfer is 1024 bits of data auxiliary bits for parity and control Aggregate data bandwidth is Gb/s Gb/s including the auxiliary bits. 3.3 Gb/s per line card The 1024 bits are sent in two 64B blocks Function cards are expected to wait several epochs for another 64B block to fill the transfer

S.R.AvasaralaCS Dept., Purdue University20 Scheduling of the switch Minimum of 4 epochs to schedule and complete a transfer but scheduling is pipelined. Epoch1: source card signals that it has data to send to the destination card Epoch2: switch allocator schedules transfer Epoch3: source and destination cards are notified and told to configure themselves Epoch4: transfer takes place Flow control through inhibit pins

S.R.AvasaralaCS Dept., Purdue University21 The Switch Allocator card Takes connection requests from function cards Takes inhibit requests from destination cards Computes a transfer configuration for each epoch 15X15 = 225 possible pairings with 15! Patterns Disadvantages of the simple allocator Unfair: there is a preference for low-numbered sources Requires evaluating 225 positions per epoch, which is too fast for an FPGA

S.R.AvasaralaCS Dept., Purdue University22 The Switch Allocator Card

S.R.AvasaralaCS Dept., Purdue University23 The Switch Allocator Solution to unfairness problem: Random shuffling of sources and destinations Solution to timing problem: Parallel evaluation of multiple locations Priority to requests from forwarding engines over line cards to avoid header contention on line cards

S.R.AvasaralaCS Dept., Purdue University24 Line Card Design A line card in MGR can have up to 16 interfaces on it, all of the same type Total bandwidth of all interfaces on a card must not exceed 2.5 Gb/s. The difference between 2.5 and 3.3 Gb/s is to allow for transfer of headers to and from forwarding engines Can support: 1 OC-48c 2.4 Gb/s SONET interface 4 OC-12c 622 Mb/s SONET interfaces 3 HIPPI 800 Mb/s interfaces Mb/s Ethernet or FDDI interfaces

S.R.AvasaralaCS Dept., Purdue University25 Line card: Inbound packet processing Assigns a packet id and breaks data into a chain of 64B The first page is sent to the FE to get routing info 2 Complications Multicasting: FE sends multiple copies of updated first pages for a single packet ATM: Cells are 53b. So we need SAR. OAM cells between interfaces on the same card must be sent directly in a single page.

S.R.AvasaralaCS Dept., Purdue University26 Line card: Outbound packet processing Receives pages of a packet from the switch Assembles them in a list Creates a packet record pointing to the list Passes the packet record QoS processor (an FPGA) which does scheduling based on flow ids Any link layer scheduling is done separately later

S.R.AvasaralaCS Dept., Purdue University27 Network processor, Routing tables 233-MHz Alpha processor Access to line cards through a PCI bridge Runs 1.1 NetBSD UNIX All routing protocols run on the NW processor FEs have only small tables with minimal info NW processor periodically downloads new tables into FEs FEs then switch memory banks and invalidate the route cache

S.R.AvasaralaCS Dept., Purdue University28 Conclusions Makes 2 important contributions Emphasizes on examining every header: improves robustness and security Shows it is feasible to build routers that can serve in emerging high-speed networks In all, an excellent paper providing complete and intricate details about high speed router design