You configure the link services interface (ls-0/0/0) on a J-series device to support multilink services and Compressed Real-Time Transport Protocol (CRTP).
The link services interface on a J-series Services Router consists of services provided by the following interfaces on the Juniper M-series and T-series routing platforms: multilink services interface (ml-fpc/pic/port), link services interface (ls-fpc/pic/port), and link services intelligent queuing interface (lsq-fpc/pic/port). Although the multilink services, link services, and link services intelligent queuing (IQ) interfaces on M-series and T-series routing platforms are installed on Physical Interface Cards (PICs), the link services interface on a J-series Services Router is an internal interface only and is not associated with a physical medium or Physical Interface Module (PIM).
For information about interface names, see Network Interface Naming.
For more information about the link services interfaces, see the JUNOS Services Interfaces Configuration Guide.
This section contains the following topics.
On a J-series device, the link services interface is a logical interface available by default. Table 124 summarizes the services available on a J-series link services interface.
Table 124: Services Available on J-series Link Services Interface
The link and multilink services implementation on a J-series Services Router is similar to the implementation on the M-series and T-series routing platforms, with the following exceptions:
The J-series device supports MLPPP and MLFR multilink encapsulations. MLPPP enables you to bundle multiple PPP links into a single multilink bundle, and MLFR enables you to bundle multiple Frame Relay data-link connection identifiers (DLCIs) into a single multitlink bundle. Multilink bundles provide additional bandwidth, load balancing, and redundancy by aggregating low-speed links, such as T1, E1, and serial links.
You configure multilink bundles as logical units or channels on the link services interface ls-0/0/0:
After creating multilink bundles, you add constituent links to the bundle. The constituent links are the low-speed physical links that are to be aggregated. You can create 64 multilink bundles, and on each multilink bundle you can add up to 8 constituent links. The following rules apply when you add constituent links to a multilink bundle:
For information about configuring MLPPP bundles, see Configuring an MLPPP Bundle. For information about configuring MLFR bundles, see Configuring MLFR FRF.15 Bundles and Configuring MLFR FRF.16 Bundles.
As it does on any other interface, priority scheduling on a multilink bundle determines the order in which an output interface transmits traffic from an output queue. The queues are serviced in a weighted round-robin fashion. But when a queue containing large packets starts using the multilink bundle, small and delay-sensitive packets must wait their turn for transmission. Because of this delay, some slow links, such as T1 and E1, can become useless for delay-sensitive traffic.
Link fragmentation and interleaving (LFI) solves this problem. It reduces delay and jitter on links by fragmenting large packets and interleaving delay-sensitive packets with the resulting smaller packets for simultaneous transmission across multiple links of a multilink bundle.
Figure 41 illustrates how LFI works. In this figure, Device R0 and Device R1 have LFI enabled. When Device R0 receives large and small packets, such as data and voice packets, it divides them into two categories. All voice packets and any other packets configured to be treated as voice packets, such as CRTP packets, are categorized as LFI packets and transmitted without fragmentation or an MLPPP header. The remaining non-LFI (data) packets can be fragmented or unfragmented based on the configured fragmentation threshold. The packets larger than the fragmentation threshold are fragmented. An MLPPP header (containing a multilink sequence number) is added to all non-LFI packets, fragmented and unfragmented.
The fragmentation is performed according to the fragmentation threshold that you configure. For example, if you configure a fragmentation threshold of 128 bytes, all packets larger than 128 bytes are fragmented. When Device R1 receives the packets, it sends the unfragmented voice packets immediately but buffers the packet fragments until it receives the last fragment for a packet. In this example, when Device R1 receives fragment 5, it reassembles the fragments and transmits the whole packet.
The unfragmented data packets are treated as a single fragment. Thus Device R1 does not buffer the unfragmented data packets and transmits them as it receives them.
Figure 41: LFI on a Services Router
For information about configuring LFI, see Enabling Link Fragmentation and Interleaving.
Real-Time Transport Protocol (RTP) can help achieve interoperability among different implementations of network audio and video applications. However, in some cases, the header, which includes the IP, UDP, and RTP headers, can be too large (around 40 bytes) on networks using low-speed lines such as dial-up modems. Compressed Real-Time Transport Protocol (CRTP) can be configured to reduce network overhead on low-speed links. CRTP replaces the IP, UDP, and RTP headers with a 2-byte context ID (CID), reducing the header overhead considerably.
Figure 42 shows how CRTP compresses the RTP headers in a voice packet and reduces a 40-byte header to a 2-byte header.
Figure 42: CRTP
On J-series devices, you can configure CRTP with MLPPP or PPP logical interface encapsulation on link services interfaces. For more information about configuring MLPPP, see Configuring an MLPPP Bundle.
When you configure CRTP, link fragmentation and interleaving (LFI) is automatically enabled. Real-time and non-real-time data frames are carried together on lower-speed links without causing excessive delays to the real-time traffic. For more information about LFI, see Link Fragmentation and Interleaving Overview.
When LFI is enabled, all large packets are fragmented. These packet fragments have a multilink header that contains a multilink sequence number. The sequence numbers on the fragments must be preserved so that the remote device receiving these fragments can correctly reassemble them into a complete packet. To accommodate this requirement, the software queues all fragmented packets on constituent links of a multilink bundle to a single queue (Q0), by default.
Although they are not fragmented, data packets smaller than the fragmentation threshold are also queued to Q0.
When you configure CRTP with LFI, CRTP packets on a multilink bundle from queues other than Q2 are queued to Q2 (instead of Q0) on the constituent links. Because CRTP packets are compressed and do not require fragmentation, they are treated as LFI (voice) packets and are sent to Q2 on the constituent links.
Figure 43 shows how traffic is queued on an MLPPP or MLFR multilink bundle and its constituent links. Irrespective of the packet queuing on the multilink bundle, the packets on the constituent links are queued according to the default setting so that traffic from all queues except Q2 is mapped to Q0.
Figure 43: Queuing on Constituent Links
On a multilink bundle, packet fragments from all queues except Q2 are transmitted to Q0 on constituent links. On the Q0s of constituent links, the packets are queued in a weighted round-robin fashion to enable per-fragment load balancing.
Figure 44 shows how queuing is performed on the constituent links.
Figure 44: Queuing on Q0 of Constituent Links
Packet fragments from the multilink bundle are queued to constituent links one by one in a weighted round-robin fashion. Packet 1 from Q0 on the multilink bundle is queued to Q0 on Constituent Link 1, packet 2 from Q1 on the multilink bundle is queued to Q0 on Constituent Link 2, packet 3 from Q3 on the multilink bundle is queued to Q0 on Constituent Link 1,and so on.
On a multilink bundle, all Q2 traffic (LFI traffic) from the multilink bundle is queued to Q2 of constituent links based on a hash computed from the source address, destination address, and IP protocol of the packet. If the IP payload is TCP or UDP traffic, the hash also includes the source port and destination port. As a result of this hash algorithm, all traffic belonging to one traffic flow is queued to Q2 of one constituent link.
On link services interfaces, the traffic load is queued and balanced differently for LFI (voice and CRTP packets) and non-LFI packets (data packets) depending on the protocols configured.
Table 125 compares queuing and load balancing for LFI and non-LFI packets when MLPPP is configured with LFI and CRTP.
Table 125: LFI Queuing and Load Balancing for Different Protocols
If you configure CoS components with LFI on a J-series device, we recommend that you follow certain recommendations for shaping rate, scheduling priority, and buffer size. For configuration instructions, see Configuring MLPPP Bundles and LFI on Serial Links. For more information about other CoS components, see JUNOS CoS Components.
When you configure LFI, we recommend that you configure the shaping rate on each constituent link of the multilink bundle. Shaping rate configuration on the constituent links is required to limit the jitter on the LFI queue. If you anticipate no delay-sensitive or jitter-sensitive traffic on the LFI queue, or if there is no LFI traffic at all, shaping rate configuration is optional.
For information about how to configure a shaping rate, see Applying Shaping Rates to Interfaces.
J-series devices support per-unit scheduling that allows you to configure scheduler maps on each MLPPP or MLFR multilink bundle. You can also configure scheduler maps on constituent links, but you must maintain the same relative priority on the constituent links and on the multilink bundle.
Table 126 shows an example of correct and incorrect relative priorities on a multilink bundle and its constituent link. In this example, you have assigned a high priority to LFI packets and a low priority to data packets on the multilink bundle. To maintain the relative priority on the constituent links, you can assign a high priority to the LFI packets and a medium-high priority to the data packets, but you cannot assign a medium-high priority to LFI packets and a high priority to data packets.
Table 126: Relative Priorities on Multilink Bundles and Constituent Links
All non-LFI traffic from the multilink bundle (from different queues) is transmitted to Q0 on the constituent links. On the constituent links, you must configure a large buffer size for Q0. If the Q0 buffer size on a constituent link is insufficient, the scheduler might drop overflowing packets.
