Transport Layer: UDP

1. Transport Layer: UDP COMS W6998 Spring 2010 Erich Nahum

2. Outline • UDP Layer Architecture • Receive Path • Send Path

3. Recall what UDP Does • RFC 768 • IP Proto 17 • Connectionless • Unreliable • Datagram • Supports multicast • Optional checksum • Nice and simple. • Yet still 2187 lines of code! UDP packet format 0 15 31 3 7 Source Port (16) Destination Port (16) Length (16) Checksum (16) Data

4. UDP Header The udp header: include/linux/udp.h struct udphdr { __be16 source; __be16 dest; __be16 len; __sum16 check; };

5. Sidebar: UDP-Lite • RFC 3828 • Very similar to UDP • Difference is checksum covers part of packet rather than all • Checksum coverage says how many bytes (starting from header) are covered by checksum • Idea is certain apps would rather have a damaged packet than none • Examples are audio, video codecs • IP Protocol 136 • Linux UDP-Lite implementation shares most code with UDP UDP packet format 0 15 31 3 7 Source Port (16) Destination Port (16) Checksum Coverage (16) Checksum (16) Data

6. Sources of UDP Packets • Packets arrive on an interface and are passed to the udp_rcv() function. • UDP packets are packed into an IP packet and passed down to IP via ip_append_data() and ip_push_pending_frames()

7. UDP Implementation Design Higher Layers socket.c sock.c sock_sendmsg sock_queue_rcv_skb udp.c udp.c ICMP icmp_send __udp_queue_rcv_skb ROUTING udp_sendmsg ip_route_output_flow __udp4_lib_lookup_skb __udp4_lib_rcv MULTICAST udp_push_pending_frames __udp4_lib_mcast_deliver udp_rcv Ip_output.c Ip_input.c ip_push_pending_frames ip_append_data ip_local_deliver_finish

8. UDP Proto struct proto udp_prot = { .name = "UDP", .owner = THIS_MODULE, .close = udp_lib_close, .connect = ip4_datagram_connect, .disconnect = udp_disconnect, .ioctl = udp_ioctl, .destroy = udp_destroy_sock, .setsockopt = udp_setsockopt, .getsockopt = udp_getsockopt, .sendmsg = udp_sendmsg, .recvmsg = udp_recvmsg, .sendpage = udp_sendpage, .backlog_rcv = __udp_queue_rcv_skb, .hash = udp_lib_hash, .unhash = udp_lib_unhash, .get_port = udp_v4_get_port, .memory_allocated = &udp_memory_allocated, .sysctl_mem = sysctl_udp_mem, .sysctl_wmem = &sysctl_udp_wmem_min, .sysctl_rmem = &sysctl_udp_rmem_min, .obj_size = sizeof(struct udp_sock), .slab_flags = SLAB_DESTROY_BY_RCU, .h.udp_table = &udp_table, };

9. udp_table /** * struct udp_table - UDP table * * @hash: hash table, sockets are hashed on (local port) * @hash2: hash table, sockets are hashed on (local port, local address) * @mask: number of slots in hash tables, minus 1 * @log: log2(number of slots in hash table) */ struct udp_table { struct udp_hslot *hash; struct udp_hslot *hash2; unsigned int mask; unsigned int log; }; udp_table_init() allocates the hash tables, initializes them: for (i = 0; i <= table->mask; i++) { INIT_HLIST_NULLS_HEAD(&table->hash[i].head, i); table->hash[i].count = 0; spin_lock_init(&table->hash[i].lock); }

10. Outline • UDP Layer Architecture • Receive Path • Send Path

11. Receiving packets in UDP • From user space, you can receive udp traffic with three system calls: • recv() (when the socket is connected). • recvfrom() • recvmsg() • All three are handled by udp_rcv() in the kernel.

12. Recall IP’s inet_protos net_protocol udp_rcv() 0 handler inet_protos[MAX_INET_PROTOS] udp_err() err_handler gso_send_check gso_segment gro_receive gro_complete net_protocol igmp_rcv() 1 handler Null err_handler gso_send_check gso_segment gro_receive gro_complete net_protocol MAX_INET_PROTOS

13. Receive Path: udp_rcv Higher Layers sock.c • Calls __udp4_lib_rcv(skb, &udp_table, IPPROTO_UDP); • Function is used by both UDP and UDP-Lite sock_queue_rcv_skb udp.c ICMP icmp_send __udp_queue_rcv_skb __udp4_lib_lookup_skb __udp4_lib_rcv MULTICAST __udp4_lib_mcast_deliver udp_rcv Ip_input.c ip_local_deliver_finish

14. Receive: __udp4_lib_rcv Higher Layers sock.c sock_queue_rcv_skb • Looks up the route table from the skb • Checks that skb has a header • Checks that length is good • Calcs the checksum • Pulls out saddr, daddr • Checks if address is multicast • Calls __udp4_lib_mcast_deliver() udp.c ICMP icmp_send __udp_queue_rcv_skb __udp4_lib_lookup_skb __udp4_lib_rcv MULTICAST __udp4_lib_mcast_deliver udp_rcv Ip_input.c ip_local_deliver_finish

15. Receive: __udp4_lib_rcv (cont) Higher Layers sock.c • Looks up the socket in the udptable • Via __udp4_lib_lookup_skb() • Increases refcount on the sk (socket) • If socket is found • Calls __udp_queue_rcv_skb() • Decrements refcount with sock_put(sk) • If not, • Send ICMP_UNREACHABLE • Drop packet. sock_queue_rcv_skb udp.c ICMP icmp_send __udp_queue_rcv_skb __udp4_lib_lookup_skb __udp4_lib_rcv MULTICAST __udp4_lib_mcast_deliver udp_rcv Ip_input.c ip_local_deliver_finish

16. Recv: __udp_queue_rcv_skb Higher Layers sock.c • Calls sock_queue_rcv_skb • Increments some statistics sock_queue_rcv_skb udp.c ICMP icmp_send __udp_queue_rcv_skb __udp4_lib_lookup_skb __udp4_lib_rcv MULTICAST __udp4_lib_mcast_deliver udp_rcv Ip_input.c ip_local_deliver_finish

17. Outline • IP Layer Architecture • Receive Path • Send Path

18. Sending packets in UDP • From user space, you can send udp traffic with three system calls: • send() (when the socket is connected). • sendto() • sendmsg() • All three are handled by udp_sendmsg() in the kernel. • udp_sendmsg() is much simpler than the tcp parallel method , tcp_sendmsg(). • udp_sendpage() is called when user space calls sendfile() (to copy a file into a udp socket). • sendfile() can be used also to copy data between one file descriptor and another. • udp_sendpage() invokes udp_sendmsg().

19. UDP Socket Options • For IPPROTO_UDP/SOL_UDP level, there exists a socket option UDP_CORK • Added in Linux kernel 2.5.44. int state=1; setsockopt(s, IPPROTO_UDP, UDP_CORK, &state, sizeof(state)); for (j=1;j<1000;j++) sendto(s,buf1,...) state=0; setsockopt(s, IPPROTO_UDP, UDP_CORK, &state, sizeof(state));

20. UDP_CORK (cont) • The above code fragment will call udp_sendmsg() 1000 times without actually sending anything on the wire (in the usual case, when without setsockopt() with UDP_CORK, 1000 packets will be sent). • Only after the second setsockopt() is called, with UDP_CORK and state=0, one packet is sent on the wire. • Kernel implementation: when using UDP_CORK, udp_sendmsg() passes MSG_MORE to ip_append_data(). • UDP_CORK is not in glibc, you need to add it to your program: #define UDP_CORK 1

21. Send Path: udp_sendmsg() Higher Layers socket.c • Checks length, MSG_OOB • Checks if there are frames pending • If so, jump to do_append_data • Gets the address • Checks if socket is connected • If so, pull routing info out of sk • Otherwise, look up via ip_route_output_flow() • Calls ip_append_data() • Handles fragmentation • Calls udp_push_pending_frames() sock_sendmsg udp.c ROUTING udp_sendmsg ip_route_output_flow udp_push_pending_frames Ip_output.c ip_push_pending_frames ip_append_data

22. udp_push_pending_frames() Higher Layers socket.c • Checks that there is room in the skb via skb_peek() • If not, goto out and bail • Creates UDP header • Checksums if necessary (or partially for UDP-Lite) • Calls ip_push_pending_frames() • Combines all pending IP fragments on the socket as one IP datagram and sends it out sock_sendmsg udp.c ROUTING udp_sendmsg ip_route_output_flow udp_push_pending_frames Ip_output.c ip_push_pending_frames ip_append_data

23. UDP Backup

24. Recall the sk_buff structure sk_buff next sk_buff sk_buff_head prev sk tstamp net_device dev struct sock ...lots.. ...of.. Packetdata ...stuff.. ``headroom‘‘ transport_header network_header MAC-Header mac_header IP-Header head UDP-Header data UDP-Data tail ``tailroom‘‘ end dataref: 1 truesize nr_frags users skb_shared_info ... destructor_arg linux-2.6.31/include/linux/skbuff.h

25. Recall pkt_type in sk_buff • pkt_type: specifies the type of a packet • PACKET_HOST: a packet sent to the local host • PACKET_BROADCAST: a broadcast packet • PACKET_MULTICAST: a multicast packet • PACKET_OTHERHOST:a packet not destined for the local host, but received in the promiscuous mode. • PACKET_OUTGOING: a packet leaving the host • PACKET_LOOKBACK: a packet sent by the local host to itself.